Abstract
Membrane contact sites emerged in the last decade as key players in the integration, regulation and transmission of many signals within cells, with critical impact in multiple pathophysiological contexts. Numerous studies accordingly point to a role for mitochondria-endoplasmic reticulum contacts (MERCs) in modulating aging. Nonetheless, the driving cellular mechanisms behind this role remain unclear. Recent evidence unravelled that MERCs regulate cellular senescence, a state of permanent proliferation arrest associated with a pro-inflammatory secretome, which could mediate MERC impact on aging. Here we discuss this idea in light of recent advances supporting an interplay between MERCs, cellular senescence and aging.
Similar content being viewed by others
Introduction
From the second half of twentieth century, the growing use of subcellular imaging through electron microscopy has allowed the identification of local physical contacts between multiple organelles and/or plasma membrane (PM), called membrane contact sites (MCSs), to the point that the field of “contactology” has emerged in the last decade1,2,3,4. Indeed, the first structural observations were extended to point out the multiple functional roles of these MCSs in regulating cellular responses in various pathophysiological conditions4. Firstly, MCSs constitute physical bridges to exchange metabolites (e.g. ions or lipids) between organelles, thus regulating intracellular metabolic fluxes. Secondly and as dynamic molecular microdomains, MSCs create subcellular signalling platforms, allowing local protein modifications and interactions4.
Endoplasmic reticulum (ER) and mitochondria constitute two key organelles involved in macromolecules synthesis and bioenergetics, but also in stress sensing and integration of cell fate signalling. As they form two independent networks occupying up to 45% of cell volume in eukaryotic cells5,6, ER and mitochondria are involved in multiple MCSs, including contacts between ER and PM, Golgi or peroxysomes, and between mitochondria and PM, lysosomes or lipid droplets4. ER and mitochondria are also able to interact with each other through mitochondria–ER contacts (MERCs)7.
MERCs define tight contacts with a distance shorter than 50 nm specifically between ER and outer mitochondrial membranes7. MERC structures are dynamic structures known to be present in every eukaryotic cells, along approximately 10–15% of all mitochondrial membranes2. Highly heterogeneous among tissues and species, MERCs contain up to hundreds of proteins including tethering/structuring proteins, ion channels, transport binding proteins, enzymes and other signalling proteins2 (Fig. 1). Firstly described in the context of lipid biosynthesis/transfer, MERCs role has been largely extended to other metabolites fluxes, such as calcium8,9,10. MERCs are involved in lipid metabolism, calcium and redox signalling but also modulate mitochondrial dynamics11, autophagy12, inflammation13 and apoptosis9 (Fig. 1).
The use of genetic mouse models targeting MERC components has highlighted MERCs importance in controlling various pathophysiological situations, ranging from vascular remodelling to inflammation and metabolic disorders13,14,15,16,17. More recently, a role of MERCs in aging was suggested by evidences showing a modulation of their quantity and quality with specific age-related diseases or aging18,19,20. While autophagy and apoptosis are proposed to mediate some of MERCs age-associated effects18,19, whether other critical mechanisms are also involved remains unclear.
Senescent cells accumulate during aging in various animal models21,22. Cellular senescence can be induced by a myriad of stresses and defines a state of permanent cell proliferation arrest and the concomitant acquisition of a senescence-associated secretory phenotype (SASP), including pro-inflammatory factors, pro-fibrotic factors and metalloproteases23 (Fig. 2a). Mechanistically, proliferation arrest is mediated by the activation of cyclin-dependent kinase inhibitors, mainly p21 and p16 (Fig. 2a). Besides, the SASP is mostly driven by NF-κB and C/EBPβ, and can be also positively regulated either by Notch, mTOR or NLRP3 pathways24,25,26,27 (Fig. 2a). Although these factors regulate cell cycle or SASP, the upstream molecular and subcellular mechanisms regulating them are largely unknown. Notably, signalling platforms mediating the link between stresses sensing and molecular activation of senescence effectors are largely unknown (Fig. 2a).
Cellular senescence has been early linked to aging, but the functional demonstration of its involvement in it was recent. The accumulation of senescent cells during aging is the concomitant result of both increased intracellular damage and declined senescence immune surveillance28,29,30 (Fig. 2b). While cell autonomous effects lead to stem cell exhaustion31,32 and aberrant cellular function33, non-cell autonomous effects through SASP can mediate paracrine senescence and modify the surrounding microenvironment. Subsequently, the chronic and systemic accumulation of senescent cells favours tissue dyshomeostasis, fibrosis, inflammation or immunosenescence, ultimately leading to aging and age-associated pathologies, including chronic metabolic disorders, atherosclerosis, age-related muscular defects and neurodegenerative diseases (Fig. 2b)34,35,36. Finding strategies to target and specifically eliminate senescent cells or attenuate their pro-inflammatory SASP, namely with senolytics or senomorphics, has thus become a major challenge in aging research field.
In this perspective, we will present and discuss recent advances suggesting a potential interplay of MERCs and cellular senescence in regulating aging. Based on the fact that MERCs and cellular senescence are both involved in aging and age-related pathologies, we compiled evidence that either modulation of MERC components or increased MERCs formation are pro-senescent signals. Lastly, we propose future directions to elucidate molecular mechanisms behind this emerging role of MERCs in regulating cellular senescence.
MERCs and cellular senescence both control age-associated diseases and aging
While senescent cells accumulate during aging and participate in multiple age-related pathologies21,37,38, numerous clues indicate that MERCs could also play a role in age-associated diseases.
Chronic metabolic disorders
Chronic metabolic disorders such as obesity and type 2 diabetes (T2D) account for two main contemporary diseases linked to overnutrition and aging39. High-fat diet (HFD) and obese pathological contexts promote senescence in several cell types and tissues including adipose tissue, liver, pancreas or brain promoting insulin-resistance and T2D40,41,42,43. Of note, a chronic increase of MERCs was observed in HFD-fed and ob/ob mice, two models exhibiting altered glucose homeostasis, insulin resistance and steatosis44. More strikingly, the use of an artificial MERCs linker45 is able to induce this insulin resistance44. Conversely, reduced MERCs in mice depleted for inositol-triphosphate receptor 2 (ITPR2) are able to improve glucose homeostasis, alleviating age-dependent steatosis and fibrosis46. Accordingly, decreased MERCs through Pdk4 deletion ameliorates glucose homeostasis and insulin response47. Of note, decreased number of hepatic MERCs correlates with reduced mRNA and protein levels of p16 senescence marker46, known to be upregulated during aging37,46. Remarkably and as apparent contradictory result, decreased MERCs using genetic and pharmacological approaches targeting Cyclophilin D (CypD) also perturb insulin response in liver and muscle48,49. Taken together, these data demonstrate that an accurate balanced MERC number is necessary to maintain glucose homeostasis15.
Non-alcoholic fatty liver disease (NAFLD) is characterized by hepatic fat deposits, evolving from steatosis to fibrosis, cirrhosis and/or hepatocellular carcinoma50. As T2D, obesity and aging are three main factors of NAFLD, a detrimental role of cellular senescence has been proposed51. Nonetheless, recent functional studies highlighted the detrimental effects of some senolytics, namely Dasatinib and Quercetin, in the context of fully recapitulated NAFLD52, suggesting a complex role of cellular senescence in NAFLD. Furthermore, the elimination of p16High senescent cells induces liver fibrosis if these senescent cells are not replaced53. In addition, senescence of hepatic stellar cells can inhibit their pro-fibrotic roles28, together showing different roles of senescent cells in liver diseases probably depending on the type of senescent cells and on their dynamic54. The importance of MERCs in NAFLD remains unclear16. For instance, Mfn2 knockout was suggested to modulate MERCs quantity, though its precise role is still under debate55, and induces non-alcoholic steatohepatitis, one of the advanced stages of NAFLD, in mouse model. The importance of MERCs in the progression of NAFLD still needs to be further tested using specific MERC tools.
Altogether, all these data strongly indicate that MERCs disruption and modulation of cellular senescence are at the crossroads of age and obesity-related alterations of glucose metabolism and chronic liver dysfunction (Table 1).
Atherosclerosis
Atherosclerosis is also promoted by overnutrition and aging, constituting the major cause of complications of cardiovascular diseases, including stroke and ischaemic heart failure56. Chronic atheromatous plaques are formed by endothelium dysfunction, the accumulation of oxidized low-density lipoprotein (oxLDL) in blood vessels and the subsequent aggregation of macrophages foamy cells and vascular smooth muscle cells (VSMCs). Endothelial cells, foamy cells and VSMCs in atheromatous plaques display senescence features38,57. The secretome of these senescent cells inhibits promigratory phenotype switching of medial VSMCs and their lesion entry for fibrous cap assembly58. Senolytics treatment rescues this promigratory phenotype-limiting atherosclerosis38,58. Interestingly, recent evidences pointed out that oxLDL treatment of VSMCs or endothelial cells impacts MERCs, notably thanks to PACS-259,60, and mitochondrial calcium accumulation through increased MERCs is proposed to accentuate apoptosis of endothelial cells60. Nonetheless, sub-lethal elevation of mitochondrial calcium can also promote cellular senescence61,62. Taken together, and as correlations, these data suggest that early steps of atheromatous plaques formation involve MERCs modulation and cellular senescence (Table 1).
Age-related muscular defects
Muscular aging is characterized by reduced muscle fibre size and number, associated with loss of motoneurons, and terminally results in muscle dyshomeostasis. Muscular aging is driven at least partly by activation of senescence pathways in muscle stem cells through autophagy defects63,64 and mitochondrial dysfunction65. More importantly, the removal of senescent cells delays this age-associated muscular loss-of-function37. Noteworthy, aged muscle was reported to display a reduction of ER–mitochondria calcium fluxes coupling to higher mitochondrial oxidative stress66. Functionally, potential MERCs modulation through Mfn2 knockout55 has detrimental effects in muscle and leads to age-related sarcopenia67. Furthermore, CisD2 knockout mice also display premature muscle degeneration68. As age-related muscular defects may also come from neuromuscular synaptic loss, it is interesting to note that neuronal Mfn2 is also reduced during aging and its specific deletion is sufficient to trigger skeletal muscle atrophy69. Overall, MERCs integrity or MERC components appear to participate in proper neuromuscular function, as MERCs uncoupling drives premature muscular aging (Table 1).
Neurodegenerative diseases
Aging is considered as the main risk factor driving neurodegenerative diseases, which include among others Alzheimer disease (AD), Parkinson disease (PD), primary progressive multiple sclerosis (PPMS). While both neuronal and glial senescent cells accumulate during aging and lead to neural stem cells exhaustion, their elimination significantly ameliorates symptoms of AD, PD and PPMS70,71,72,73. MERCs were also studied in the context of neurodegenerative disorders. In AD patients, MERCs are enhanced and three enzymes involved in amyloid β generation (presenilin-1, presenilin-2 and γ-secretase) colocalize at MERCs74,75. Moreover, increased amyloid β induces expression of multiple MERC components including ITPR3 and voltage-dependent channel 1 (VDAC1)75, and presenilin-2 mutants display increased MERCs76. Importantly, pdzd8 deletion decreases MERCs and rescues AD-associated locomotor decline in fly77. In PD, MERCs structure is altered78 and most of PD-associated proteins, such as α-synuclein, Parkin or PINK1, are found in MERCs fraction79. Furthermore, α-synuclein enhances MERCs and mitochondrial calcium uptake80. Taken together, these later studies suggest a detrimental role of MERCs in AD and PD. Noteworthy, multiple studies point out also decreased MERCs in PD- and AD-associated conditions, underlying also a beneficial role of these MERCs. For example, recent live FRET imaging of rat neurons displays decreased average length of tight MERCs in an AD model81. The use of an artificial linker in vivo also rescues locomotor defects in AD model in fly82. In a PD context, α-synuclein displays opposite role in MERCs formation as it can also decrease VAPB–PTPIP51 interaction83. Moreover, loss of parkin-induced ubiquitination of MFN2 is responsible for an inefficient coupling of MERCs and restoration of MERCs rescues motricity defects in a PD model in fly84. While no consensus, all these results support that MERCs dyshomeostasis could contribute to AD and PD. Finally, reduced MERCs are found in a model of Friedreich’s ataxia (FA), accompanied by decreased mitochondrial calcium and cellular senescence85,86, according to the importance of physiological basal mitochondrial calcium in sustaining mitochondrial bioenergetics87. Collectively, these data indicate that MERCs dysregulation and cellular senescence could be linked to neurodegenerative disorders. Nonetheless, while cellular senescence displays detrimental roles, MERCs roles are still under debate because of a low number of functional studies (Table 1).
Systemic aging and lifespan
Senescent cells accumulate during aging in spite of multifactorial heterogeneity in the speed and level of their accumulation88. Removal of these senescent cells or reduction of their SASP result in a decrease of some age-related alterations as described above. Most importantly, this can also result in delayed aging and improve in both lifespan and healthspan37,38,89,90,91,92,93 (Table 1). Altogether, cellular senescence thus appears to be a key cellular phenotype driving tissue dysfunction and aging (Fig. 2b). Recent reviews have depicted how MERCs dyshomeostasis may be involved in some age-related pathologies and aging18,19. Among genetic mouse models studied in the context of MERCs dyshomeostasis, CypD knockout and Mfn2 knockout67,94,95,96 have been largely studied, while their specific role in MERCs is still under debate. Interestingly, lifespan of CypD haploinsufficient but not CypD knockout mice is increased compared to their control WT littermates94. Lifespan of Mfn2 knockout mice, although displaying premature muscular aging67, was not monitored. Noteworthy, C. elegans lifespan is extended by knock-in of Hsp9a, encoding the GRP75 (Glucose-Related Protein 75) scaffold protein binding to ER and mitochondria through ITPR and VDAC channels97. Though not clearly demonstrated, GRP75 may enhance MERCs in this model, and its muscle constitutive expression could counteract deleterious effects caused by MERCs disruption in muscle, as previously discussed97. More recently, the contribution of ER-mitochondrial calcium flux to aging in mice and worms was assessed. On one side, deletion of the murine ER-calcium release channel ITPR2 reduces MERCs and age-related alterations in males and females and it extends lifespan only in females46. On the other side, atf6 loss-of-function in worms results in sustained ITPR-mediated ER-mitochondria fluxes enhancing lifespan98. Altogether, these studies demonstrate that dyshomeostasis in both MERCs and ER-mitochondrial calcium fluxes may promote aging-associated phenotypes. Studies using genetic models targeting MERC components face major limits. For instance, whether Mfn2 knockout reduces or increases MERCs number is still controversial55,99,100,101 even if it does not question the ability of MFN2 to contribute to MERCs. Furthermore, phenotypes driven by deletion of MERC components might be in some cases MERC-independent. As two major examples, MFN2 is associated to mitochondrial fusion55 and GRP75 (mtHsp70/Mortalin/GRP75) acts as a mitochondrial chaperone determinant for the quality control of intra-mitochondria folding of matrix-directed precursor proteins, these functions being independent of their roles in MERCs102. For these reasons, future works led on MERC components would need to clarify MERC-dependent and MERC-independent roles. In order to avoid MERC-independent roles, an alternative consists in using artificial linker/uncoupler of MERCs2. Whether forcing or uncoupling MERCs in vivo through artificial linkers/uncouplers may promote/delay aging and previously established age-related pathologies remains so far unclear. Two recent studies led on Drosophila melanogaster reported contradictory results on the same MERC-associated age-related pathology, namely AD. Using a synthetic linker, the two studies suggest either an extension82 or a reduction77 of lifespan. According to these results, while one reports that increasing MERCs ameliorates cognitive functions82, the other demonstrates that reducing MERCs has a similar effect, heightening mitophagy77. Clarifying the functional role of MERCs in physiological or pathological premature aging using specific linker/uncoupler in vivo is thus one avenue that would need to be explored in the future (Table 1).
Overall, these studies indicate that MERCs and cellular senescence can control similar age-related pathologies and establish multiple correlations between MERCs and cellular senescence in regulating aging (Table 1). That is the reason why the hypothesis of a role of MERCs in contributing to cellular senescence, which could at least partially mediate the impact of MERCs on aging, has recently emerged. Interestingly, numerous studies have reported that key MERC components are able to regulate features of cellular senescence.
MERC components modulate key senescence features
MERCs are heterogeneous in terms of number and activity. MERCs composition is also highly variable according to recent proteomic data in different cell types2,103,104,105, challenging its specific study. Nonetheless, MERCs are regulated by abundance of scaffold proteins, defining MERCs quantity, and by other interface proteins, interacting with each other and including calcium channels, receptors and kinases, underlying MERCs activity and function. Noteworthy, quantity and activity/function of MERCs are interrelated as some enzymes and calcium channels, for example Pyruvate Dehydrogenase Kinase 4 (PDK4) and ITPRs, are able to regulate MERCs integrity46,47,106. Finally, numerous MERC components are regulated by protein interactions such as ITPR3-TOM70107, VDAC2-CDKAP4108, ITPR2-FUNDC1109, but also by post-translational modifications such as DRP1 SUMOylation by MAPL/MUL1110, which further adds a layer of complexity in the organisation and function of MERCs.
Role of MERCs scaffold/tethering proteins in cellular senescence
MERCs establishment is regulated by multiple scaffold and tethering factors that include among others MFN2, GRP75-ITPRs-VDACs, FIS1-BAP31, VAPB-PTPIP51, PDZD8, CYPD and SIGMAR12,111 (Fig. 1). MFN1 and MFN2 have been linked functionally to senescence112. MFN1 is a target of ubiquitin ligase MARCH5 and accumulates in MARCH5-deficient cells, which display hyperfused mitochondria and senescence features113. On the contrary, downregulation of Mfn1 extends replicative lifespan114. Concerning Mfn2, its depletion boosts cellular proliferation of both B cell lymphoma cell line and mouse embryonic fibroblasts (MEFs)115. MFN2 mediates hyperfused mitochondria and promotes senescence of mesenchymal stem cells116. Concerning haematopoietic stem cells, opposite results highlight that Mfn2 deletion induces defect in long-term lymphoid repopulation117, though no senescence markers were investigated. Overall, these data indicate that alterations of MFN1 and MFN2, key regulators of MERCs number, are able to regulate senescence-associated phenotypes. However, MFN1 and MFN2 are, independently of MERCs, also involved in mitochondrial fusion, and whether their effects on senescence depends on their function inside the MERCs is still unknown.
Concerning ITPR–GRP75–VDAC complex, GRP75 overexpression leads to increased replicative lifespan through downregulation of RAS and reduced phosphorylation of ERK2118,119 and ITPRs promote cellular senescence46,61,62, as it will be further discussed below.
BAP31 is an ER-resident chaperone transmembrane protein, found in MERCs fraction, forming a bridge with FIS1120 and known mainly to regulate apoptosis121. Although the functional implication of BAP31 has never been investigated in the context of cellular senescence, its deletion reduces cell proliferation of colon cancer cells122 and it is found to be upregulated in both replicative and X-ray induced senescence of fibroblasts123.
Overall, alterations of key MERC tethers are able to drive features of cellular senescence. Aside from these reports, further studies on additional tethers such as VAPB, PTPIP51 or PDZD8, or proteins participating in MERCs structure such as SIGMAR1 and CYPD, need to be conducted to determine whether they could also regulate cellular senescence.
Role of MERC components involved in ER–mitochondria exchanges in cellular senescence
MERCs constitute a privileged site for exchanges of metabolites, including calcium and lipids, which have been recently involved in the regulation of cellular senescence.
Calcium exchanges at MERCs interface are regulated by multiple calcium channels, namely ITPRs (ITPR1, 2 and 3) in ER, VDACs (1, 2 and 3) at the OMM and MCU at the IMM for the mitochondrial influx and SERCA (1, 2) for ER influx (Fig. 1). At the ER interface, a few studies evaluated the impact of ITPRs in functionally regulating cellular senescence. ITPR2 knockdown results in the escape from oncogene-induced senescence (OiS) in human mammary epithelial cells (hMECs) but also delays replicative senescence in normal human fibroblasts61. Itpr2 knockout MEFs display also a reduction of senescence markers associated with a reduced mitochondrial calcium accumulation over passages46. More strikingly, ITPR2 abrogation, which reduces some age-related alterations and increases mouse lifespan as described above, also lowers cellular senescence46. Concerning ITPR1 and ITPR3, their knockdown is also able to delay senescence61. Altogether these data point out a key role of ITPRs in the regulation of cellular senescence. Regarding VDAC1/2/3 or SERCA ATPases pumps, no studies reported their role in cellular senescence, in contrary to MCU, the IMM mitochondrial import calcium channel, which participates in OiS, probably through ITPR2-released calcium61. Importantly, decreased MERCs and deficient mitochondrial calcium uptake through depletion of frataxin lead to cellular senescence in neuroblastoma cells, highlighting also the importance of MERCs to ensure minimum ER–mitochondrial calcium fluxes85,86.
Besides calcium, MERCs constitute exchange sites for numerous other molecules such as phospholipid and cholesterol2 (Fig. 1). Oxysterol-Binding Protein-Related Protein 5 (ORP5) and 8 (ORP8) are ER-anchored proteins involved in phospholipids exchanges at MERCs. Strikingly, ORP5 administers phospholipid and calcium transfers, while ORP8 function is exclusively limited to phospholipid transfers. The constitutive expression of ORP5 boosts cell proliferation124, while its knockdown promotes senescence, through an increase of mitochondrial calcium uptake125. Of note, the sole downregulation of the exclusive phospholipid-exchanger ORP8 does not affect senescence125.
Taken together, the results suggest that MERCs calcium transfers could be predominant on phospholipid transfers in the regulation of senescence. Still, other functional studies on key MERCs proteins involved in the transfer of lipids between ER and mitochondria need to be performed in order to describe their potential contribution through MERCs in the regulation of cellular senescence.
Aside from tethers and transport proteins, multiple other MERCs proteins have been functionally involved in the regulation of cellular senescence, and especially of the SASP. Among SASP regulators, mTOR complex and inflammasome are respectively necessary for IL1-α translation and processing25,26. Interestingly, mTOR complex and NLRP3 are both found in MERCs126,127. Noteworthy, MERCs constitute hotspots for cytosolic calcium signalling and both mTOR and NLRP3 can be activated by calcium128,129. The hypothesis of a role for MERCs in calcium-mediated activation of mTOR and NLRP3, and subsequent SASP promotion, still needs to be tested. In addition to mTOR and NLRP3, two classical MERC resident proteins, namely PTIPT51 and PACS-2, are involved in the regulation of the NF-κB pathway which drives the pro-inflammatory SASP. While the MERCs tether PTPIP51 interacts with RelA and IKBα130, the induction of NF-κB programme upon irradiation in Pacs-2 −/− thymocytes is abrogated, through blunted phosphorylation of IκBα, IκBβ and RelA131. Taken together, these data suggest that the mTOR, NLRP3 and NF-κB pathway could regulate SASP through MERC-dependent activation, even if more precise investigations should be conducted in the future to decipher this potential role.
p66Shc was largely studied in the context of senescence and also in relation with aging. Decreased p66Shc delays replicative senescence of human diploid fibroblasts132, and its increase promotes hepatocyte senescence and subsequent senescence-driven steatosis133. In the context of aging, p66Shc is phosphorylated at Serine 36 in an age-dependent manner in old animals, and strongly correlates with enhanced ROS production134. Though p66Shc intracellular localization is still under debate, frequently found in mitochondria, MERCs or PM-associated membranes, it has been suggested that p66Shc accumulates in MERCs with age before re-localizing to the mitochondria134. Exogenous oxidative stress is able to relocate p66Shc to the nucleus, where the oxidative stress response is integrated to establish a senescence response135,136,137. Collectively, these studies highlight the role of p66Shc as a sensor and regulator of cellular senescence and lifespan135,136,137.
Promyelocytic Leukemia protein (PML), an essential component of the PML nuclear bodies (PML-NBs), critically regulates cellular senescence138. Not only present in PML-NBs, PML is also located in cytoplasm, in ER and in MERCs139. PML limits phosphorylation of ITPR3, leading to dampened ER-mitochondria calcium flux and subsequent failed apoptosis139. A possible role of PML localization at MERCs in its pro-senescence function could be considered as well.
Two other MERCs proteins, CisD2 and PACS-2, were found to also regulate cell proliferation arrest, one of the key features of cellular senescence. CisD2, an ER protein located at the MERCs interface through Bcl2–IP3R interactions140, is necessary for proliferation of induced-pluripotent stem cells141. The knockdown of PACS-2, mentioned above for its ability to regulate NF-κB activation, also delays cell proliferation arrest induced by DNA damage–p53–p21 axis in thymocytes142. Additional cell types and complementary senescence markers should be monitored to better evaluate the contribution of CisD2 and PACS-2 in the modulation of the senescence fate.
Altogether these data indicate that several MERCs components, including tethers, calcium channels and others, regulate key features of cellular senescence (Table 2), although it still has to be proven that this action is MERC-dependent. Of note, it seems particularly interesting that some key SASP regulators are found associated and activated at MERCs interface. Overall, these data strongly suggest that MERCs regulate the senescence fate, which is an idea that has been recently evaluated.
MERCs regulate cellular senescence
A pro-senescent role for MERCs: potential mechanistic
Increased evidence of the role of MERCs proteins during aging and in cellular senescence raised the question of the importance of MERCs in regulating cellular senescence. Expression of an artificial linker tightening ER and mitochondrial membranes45 led to premature cellular senescence in normal human fibroblasts, with a NF-κB-dependent SASP46. This cellular senescence was accompanied by increased mitochondrial calcium and functionally involved ROS production, as antioxidant treatment rescued the onset of this premature senescence46. Downstream, p53 was necessary to induce senescence46. The importance of MERCs in established senescence models (replicative senescence, OiS, oxidative stress-induced senescence) should be critically tested in further investigations.
Noteworthy, deletion of MERC-associated proteins (such as MFN2, Frataxin or ORP5) and potential MERCs uncoupling were also proposed to mediate a senescence phenotype85,86,113,125, whether these effects are dependent of MERCs is unknown. To properly assess whether MERCs decrease may also impact senescence, it will be necessary to evaluate the impact of a specific MERCs spacer, such as FATE1143, in senescence models. While forcing MERCs induces a ROS- and p53-dependent senescence through an increased mitochondrial calcium uptake, an opposite role of MERCs uncoupling is nonetheless still unknown (Fig. 3).
How MERCs dyshomeostasis mechanistically drives cellular senescence remains an open question, but involves necessarily at least one of the three MERCs sub-compartments, i.e., mitochondrion, ER and apposed cytosol.
Mitochondrion side—An important number of studies report that senescent cells display mitochondrial abnormalities and various mitochondrial dysfunctions promote cellular senescence112. These mitochondrial dysfunctions are largely driven by non-exclusive mechanisms which include among others defective ETC, excessive ROS production, abnormal dynamics and altered mitophagy112. Nonetheless, the upstream mechanisms driving these mitochondrial dysfunctions during cellular senescence remain so far elusive.
In line with increased evidence of the role of calcium fluxes within the cell and specifically between ER and mitochondria46,61,62,144,145, calcium signalling appears to be an interesting candidate to mediate the senescence phenotype. In human endothelial cells and MEFs, replicative senescence is characterized by an increased MERCs number and a subsequent accumulation of mitochondrial calcium46,146. Accordingly, replicative senescent neurons display also an increased transfer of calcium from the ER to mitochondria accompanied by an upregulation of MCU expression147. Reduction of this mitochondrial calcium accumulation by knocking down ITPR261,62 or MCU61 inhibits the establishment of cellular senescence in several cell models, including hMEC and fibroblasts61,62. When excessively accumulated, mitochondrial calcium leads to mitochondrial membrane depolarization, ROS generation and subsequent cellular senescence46,61,62,144 or apoptosis148. Of note, accumulation of mitochondrial calcium through the suppression of mitochondria calcium efflux by knocking down the mitochondrial Na+/Ca2+ exchanger triggers superoxide generation and neuronal apoptosis, driving AD-associated pathology149.
Mitochondrial calcium regulates mitochondrial bioenergetics mainly through its transfer through MERCs87,150,151. Indeed, mitochondrial dehydrogenases present EF-hand calcium binding necessary for enzymatic activity152. As mitochondrial calcium homeostasis is necessary in order to maintain mitochondrial bioenergetics85,87,151,153, massive MERCs uncoupling may also trigger mitochondrial dysfunction. For instance, reduced MERCs through CypD knockout48,154 leads to dysregulation of TCA cycle and fatty acid β-oxidation95, while no senescence markers were monitored in these studies. Interestingly, this dual role of the importance of a balanced ER-mitochondrial calcium influx has been recapitulated in the context of neural stem cell development in fly. Indeed, depletion of Miro, an OMM GTPase, reduces mitochondrial calcium and leads to mitochondrial metabolic impairment, whereas its constitutive expression triggers mitochondrial calcium overload and apoptosis155. Both conditions impaired neural stem cells lineage progression, though no senescence markers were investigated155.
To summarize the role of MERC-mediated mitochondrial calcium influx in cellular senescence, it appears that an altered calcium transfer from ER to mitochondria triggers mitochondrial dysfunction, eventually leading to cellular senescence. At this stage, most of the data support the hypothesis of an enhanced calcium influx into the mitochondria during cellular senescence, which could be mediated notably by increased MERCs, without excluding the hypothesis that reduced MERCs could also elicit pro-senescent signals (Fig. 3). Apart from calcium, whether lipid fluxes at MERCs interface are involved in mitochondrial alterations promoting cellular senescence remains an open question.
Beyond metabolites transfers, MERCs also promote early steps of mitochondrial fission through ER wrapping of mitochondria at future fission sites11. Reducing mitochondrial fission is able to induce senescence in normal cells112,156,157. Though no evidences were clearly established, MERC uncoupling could lead to accumulation of hyperfused damaged mitochondria by fission and subsequent mitophagy defects, eventually mediating a senescence phenotype. Nonetheless, whether the sole MERC uncoupling induces senescence features should be critically addressed (Fig. 3).
ER side—ER stress is a potent candidate to also mediate senescence phenotype158. This role is suggested by the fact that ER stress is observed in several models of senescence in vitro, such as OiS159,160,161,162 and UV- or X-ray-induced senescence159,163, but also in several senescence-associated pathological contexts, including therapy-induced senescence (TIS) in lymphomas164, diabetic nephropathy165, age-related sarcopenia166 and osteoarthritis167. ER stress may result from a persistent unfolded protein response (UPR) or from a luminal calcium depletion168 and includes three main sensors: PERK, ATF6 and IRE1168. Interestingly, MFN2 depletion in MEFs heightens the activity of the three ER stress branches169. Furthermore, PERK interacts with MFN2 and is found in MERCs fraction to transduce apoptosis triggered by ROS-mediated ER stress170. MFN2 is able to interact with PERK in normal conditions to restrain its activity. Remarkably, another ER-anchored MERCs tether, namely VAPB, interacts with ATF6, repressing transcription of its target genes171. Finally, IRE1 regulates lipid composition at MERCs and thus subsequent calcium fluxes151. Taken together, these data indicate cross-talks between MERC components and ER stress169,170. Concerning calcium fluxes, MERCs may decrease ER calcium content. ER calcium is determinant for chaperones involved in protein folding, such as Calreticulin or BiP/GRP78, and variations of ER calcium content subsequently lead to UPR and ER stress172. How ER stress is regulated upon modulation of MERCs and how it participates in cellular senescence should be addressed in the future.
Altogether, these data strongly suggest the importance of mitochondria in participating in the regulation of senescence phenotype, while ER contribution is still barely understood. Finally, some non-ER and non-mitochondria resident proteins modulating senescence features can be located in MERCs, as for example NLRP3, mTOR or PML. Therefore, MERCs could regulate cellular senescence by impacting signalling pathways in the cytosol through these proteins.
MERCs: a dynamic platform for integrating pro-senescence signals?
MERCs are constantly modified and some pro-senescence signals were shown to affect MERCs protein levels. For example, persistent DNA damage response at telomeres during replicative senescence and X-ray-induced DNA damage upregulate the MERC tether BAP31 at mRNA levels123. Noteworthy, expression of ITPR2, the most efficient channel for ER-to-mitochondria calcium transfer106, is upregulated by other pro-senescence stresses, including oncogenic stress61, oxidative stress173 or high-fat diet44. Aside from transcriptional and translational regulations, some MERCs proteins were found relocated, as previously mentioned for p66Shc, or activated upon stresses inducing cellular senescence. Taken together, these data indicate that pro-senescence signals may modify MERCs composition and function. Proteomic analysis of MERCs during cellular senescence induced by different stresses will be needed to appreciate MERCs rearrangement in composition in response to pro-senescence signals.
MERCs quantity is also regulated upon senescence-inducing stresses. Increased MERCs were found during replicative senescence of endothelial cells and MEFs46,146. MERCs quantity may be regulated by redox stress, as, for example, antioxidant treatment rescues MERCs deficiency in FA model85,86. Importantly, DNA damage promotes the formation of MERCs to promote intrinsic cell death through enhanced mitochondrial calcium uptake174. At sublethal doses, oxidative stress and DNA damage also mediate cellular senescence23.
Altogether, it seems that MERCs are highly modulated in response to pro-senescence signals. Further studies need to be performed in order to evaluate how MERCs evolve during senescence in quantity, composition and function. Upon senescence-inducing stresses, MERCs could behave as platforms integrating these signals and modulating signalling to other subcellular compartments such as mitochondria, cytosol and nucleus where senescence features are regulated.
Perspectives and conclusion
In conclusion, the observation that MERCs and cellular senescence play a crucial role in aging and age-associated diseases led to the hypothesis that MERCs may regulate aging at least partly through cellular senescence, and indeed, experimental data support that MERCs are key platforms controlling cellular senescence. This idea was first supported by studies showing a role for MERC components in cellular senescence and was strengthened by the observation that forcing MERCs with an artificial linker induces senescence in vitro. MERCs trigger this phenotype through ROS production and p53 activation. However how MERC-mediated mitochondrial calcium accumulation impacts mitochondrial functions and ROS generation and whether other calcium-independent processes are involved remains unknown.
Artificial linkers are powerful tools to investigate MERC functions but face some limitations. Some improvements have been obtained by the generation of inducible linker175,176 when compared to the constitutive artificial linker45,46. However, these linkers do not reflect many aspects of MERC complexity. Indeed, they do not allow to control the distance between ER and mitochondria (<50 nm but heterogeneous)177. This thickness is defined as the width of the cleft separating OMM from ER, and subdivides MERCs in tight (~10 nm) and loose (~25–40 nm) structures178. For instance, while loose MERCs were reported to promote ER–mitochondrial calcium transfers, tight MERCs were shown to limit them177. Beyond the distance between ER and mitochondria, the complexity of MERC composition and their plasticity critically orientate the effects of MERCs. There is then an urgent need to develop new tools to better manipulate MERCs to better understand the role of MERCs in controlling cellular senescence.
Whether MERC dysregulation is at the origin of cellular senescence which in turn results in pathological outcomes needs further investigations. For instance, the effects of linkers and spacers allowing to modulate MERCs need to be monitored in vivo, on cellular senescence and aging. Further studies will have to assess in these models and in in vivo models harbouring a knockout of MERC components (Mfn2 −/−, CisD2 −/−, CypD −/−, Pdk4 −/−, Pacs-2 −/−...) if the impact of these linkers, spacers and knockout on aging-associated alterations depends on cellular senescence. Crossing these models with models less prone to senescence or treating them with chemical compounds able to suppress features of senescent cells (senomorphics) or eliminate them (senolytics) would precise the importance of cellular senescence in the phenotypes induced by MERCs alterations.
We are then proposing that modulating MERCs could be a new avenue to modulate senescence, associated-pathological alterations and healthspan. Either the death of senescent cells could be promoted by increasing ER to mitochondria calcium flux or their accumulation could be reduced by lowering this flux. Chemical compounds targeting MERCs already exist179 and could be tested for these capacities.
In summary, we presented and discussed new insights into the potential role of MERCs in regulating cellular senescence which could participate in MERCs impact on aging. As it has been proposed for cellular senescence, we propose that the principle of antagonistic pleiotropy could be also applied to MERCs, which have a beneficial role for the cells when tightly regulated and which could be detrimental when dysregulated, leading to cellular senescence and associated consequences such as aging and age-related diseases. More broadly, this new field of research on the role of MERCs in cellular senescence and aging has highlighted the importance of communication between different compartments, sometimes mediated through contacts sites such as MERCs. This will pave the way to investigate the role of other MCSs, such as PM-ER, PM-MT or Lysosome-MT, in the context of cellular senescence and aging.
References
Helle, S. C. J. et al. Organization and function of membrane contact sites. Biochim. Biophys. Acta 1833, 2526–2541 (2013).
Csordás, G., Weaver, D. & Hajnóczky, G. Endoplasmic reticulum–mitochondrial contactology: structure and signaling functions. Trends Cell Biol. 28, 523–540 (2018).
Scorrano, L. et al. Coming together to define membrane contact sites. Nat. Commun. 10, 1287 (2019).
Prinz, W. A., Toulmay, A. & Balla, T. The functional universe of membrane contact sites. Nat. Rev. Mol. Cell Biol. 21, 7–24 (2020).
Schwarz, D. S. & Blower, M. D. The endoplasmic reticulum: structure, function and response to cellular signaling. Cell Mol. Life Sci. 73, 79–94 (2016).
Kaasik, A., Safiulina, D., Zharkovsky, A. & Veksler, V. Regulation of mitochondrial matrix volume. Am. J. Physiol. Cell Physiol. 292, C157–C163 (2007).
Rowland, A. A. & Voeltz, G. K. Endoplasmic reticulum–mitochondria contacts: function of the junction. Nat. Rev. Mol. Cell Biol. 13, 607–615 (2012).
Bell, R. M., Ballas, L. M. & Coleman, R. A. Lipid topogenesis. J. Lipid Res. 22, 391–403 (1981).
Marchi, S. et al. Mitochondrial and endoplasmic reticulum calcium homeostasis and cell death. Cell Calcium 69, 62–72 (2018).
Elustondo, P., Martin, L. A. & Karten, B. Mitochondrial cholesterol import. Biochim. Biophys. Acta Mol. Cell Biol. Lipids 1862, 90–101 (2017).
Friedman, J. R. et al. ER tubules mark sites of mitochondrial division. Science 334, 358–362 (2011). This study in yeast demonstrated the importance of MERCs in early steps of mitochondrial fission processing.
Yang, M. et al. Mitochondria-associated ER membranes—the origin site of autophagy. Front. Cell Dev. Biol. 8, 595 (2020).
Missiroli, S. et al. Mitochondria-associated membranes (MAMs) and inflammation. Cell Death Dis. 9, 1–14 (2018).
Annunziata, I., Sano, R. & d’Azzo, A. Mitochondria-associated ER membranes (MAMs) and lysosomal storage diseases. Cell Death Dis. 9, 328 (2018).
Rieusset, J. The role of endoplasmic reticulum-mitochondria contact sites in the control of glucose homeostasis: an update. Cell Death Dis. 9, 388 (2018).
Hernández-Alvarez, M. I. et al. Deficient endoplasmic reticulum-mitochondrial phosphatidylserine transfer causes liver disease. Cell 177, 881–895.e17 (2019).
Gӧbel, J. et al. Mitochondria-endoplasmic reticulum contacts in reactive astrocytes promote vascular remodeling. Cell Metab. https://doi.org/10.1016/j.cmet.2020.03.005 (2020).
Janikiewicz, J. et al. Mitochondria-associated membranes in aging and senescence: structure, function, and dynamics. Cell Death Dis. 9, 332 (2018).
Moltedo, O., Remondelli, P. & Amodio, G. The mitochondria-endoplasmic reticulum contacts and their critical role in aging and age-associated diseases. Front. Cell Dev. Biol. 7, 172 (2019).
Petkovic, M., O’Brien, C. E. & Jan, Y. N. Interorganelle communication, aging, and neurodegeneration. Genes Dev. 35, 449–469 (2021).
Herbig, U., Ferreira, M., Condel, L., Carey, D. & Sedivy, J. M. Cellular senescence in aging primates. Science 311, 1257–1257 (2006).
Wang, C. et al. DNA damage response and cellular senescence in tissues of aging mice. Aging Cell 8, 311–323 (2009).
Gorgoulis, V. et al. Cellular senescence: defining a path forward. Cell 179, 813–827 (2019).
Chien, Y. et al. Control of the senescence-associated secretory phenotype by NF-κB promotes senescence and enhances chemosensitivity. Genes Dev. 25, 2125–2136 (2011). This paper was the first to demonstrate the role of NF-κB pathway in the regulation of pro-inflammatory SASP.
Acosta, J. C. et al. A complex secretory program orchestrated by the inflammasome controls paracrine senescence. Nat. Cell Biol. 15, 978–990 (2013).
Laberge, R.-M. et al. MTOR regulates the pro-tumorigenic senescence-associated secretory phenotype by promoting IL1A translation. Nat. Cell Biol. 17, 1049–1061 (2015).
Marín-Aguilar, F. et al. NLRP3 inflammasome suppression improves longevity and prevents cardiac aging in male mice. Aging Cell 19, e13050 (2020).
Krizhanovsky, V. et al. Senescence of activated stellate cells limits liver fibrosis. Cell 134, 657–667 (2008).
Kang, T.-W. et al. Senescence surveillance of pre-malignant hepatocytes limits liver cancer development. Nature 479, 547–551 (2011). This original study was among the first in vivo clues demonstrating the role of cellular senescence as a tumour suppressing mechanism.
Lujambio, A. et al. Non-cell-autonomous tumor suppression by p53. Cell 153, 449–460 (2013).
Liu, H. et al. Augmented Wnt signaling in a mammalian model of accelerated aging. Science 317, 803–806 (2007).
Castilho, R. M., Squarize, C. H., Chodosh, L. A., Williams, B. O. & Gutkind, J. S. mTOR mediates Wnt-induced epidermal stem cell exhaustion and aging. Cell Stem Cell 5, 279–289 (2009).
Helman, A. et al. p16(Ink4a)-induced senescence of pancreatic beta cells enhances insulin secretion. Nat. Med. 22, 412–420 (2016).
Muñoz-Espín, D. & Serrano, M. Cellular senescence: from physiology to pathology. Nat. Rev. Mol. Cell Biol. 15, 482–496 (2014).
Schafer, M. J. et al. Cellular senescence mediates fibrotic pulmonary disease. Nat. Commun. 8, 14532 (2017).
Miao, J. et al. Wnt/β-catenin/RAS signaling mediates age-related renal fibrosis and is associated with mitochondrial dysfunction. Aging Cell 18, e13004 (2019).
Baker, D. J. et al. Clearance of p16Ink4a-positive senescent cells delays ageing-associated disorders. Nature 479, 232–236 (2011). Using the transgene INK-ATTAC to target senescent cells in progeroid BubR1 progeroid mouse background, this work shed light on the deleterious impact of cellular senescence during aging ang age-related alterations.
Childs, B. G. et al. Senescent intimal foam cells are deleterious at all stages of atherosclerosis. Science 354, 472–477 (2016).
Spinelli, R. et al. Molecular basis of ageing in chronic metabolic diseases. J. Endocrinol. Invest. 43, 1373–1389 (2020).
Ogrodnik, M. et al. Cellular senescence drives age-dependent hepatic steatosis. Nat. Commun. 8, 15691 (2017).
Ogrodnik, M. et al. Obesity-induced cellular senescence drives anxiety and impairs neurogenesis. Cell Metab. https://doi.org/10.1016/j.cmet.2018.12.008 (2019).
Aguayo-Mazzucato, C. et al. Acceleration of β cell aging determines diabetes and senolysis improves disease outcomes. Cell Metab. 30, 129–142.e4 (2019).
Palmer, A. K. et al. Targeting senescent cells alleviates obesity-induced metabolic dysfunction. Aging Cell 0, e12950 (2019).
Arruda, A. P. et al. Chronic enrichment of hepatic ER-mitochondria contact sites leads to calcium dependent mitochondrial dysfunction in obesity. Nat. Med 20, 1427–1435 (2014). Studying ob/ob mice and HFD-fed mice, this study firstly demonstrated a correlation between increased MERCs and glucose dyshomeostasis. Secondly and using a synthetic linker in vivo, this study functionally highlighted the role of MERCs and ITPR-dependent calcium fluxes in regulation of glucose homeostasis.
Csordás, G. et al. Structural and functional features and significance of the physical linkage between ER and mitochondria. J. Cell Biol. 174, 915–921 (2006).
Ziegler, D. V. et al. Calcium channel ITPR2 and mitochondria–ER contacts promote cellular senescence and aging. Nat. Commun. 12, 720 (2021).
Thoudam, T. et al. PDK4 augments ER–Mitochondria contact to dampen skeletal muscle insulin signaling during obesity. Diabetes 68, 571–586 (2019).
Tubbs, E. et al. Mitochondria-associated endoplasmic reticulum membrane (MAM) integrity is required for insulin signaling and is implicated in hepatic insulin resistance. Diabetes 63, 3279–3294 (2014).
Tubbs, E. et al. Disruption of mitochondria-associated endoplasmic reticulum membrane (MAM) integrity contributes to muscle insulin resistance in mice and humans. Diabetes 67, 636–650 (2018).
Musso, G., Gambino, R. & Cassader, M. Recent insights into hepatic lipid metabolism in non-alcoholic fatty liver disease (NAFLD). Prog. Lipid Res. 48, 1–26 (2009).
Papatheodoridi, A.-M., Chrysavgis, L., Koutsilieris, M. & Chatzigeorgiou, A. The role of senescence in the development of nonalcoholic fatty liver disease and progression to nonalcoholic steatohepatitis. Hepatology 71, 363–374 (2020).
Raffaele, M. et al. Mild exacerbation of obesity- and age-dependent liver disease progression by senolytic cocktail dasatinib + quercetin. Cell Commun. Signal. 19, 44 (2021). This study is the first to investigate the use of senolytics (Dasatinib + Quercetin) treatment in a fully recapitulated model of NAFLD-induced HCC. Questioning this treatment, the authors demonstrate no effects, even a detrimental effect of using senolytics in the context of age-associated NAFLD-induced HCC.
Grosse, L. et al. Defined p16High senescent cell types are indispensable for mouse healthspan. Cell Metab. 32, 87–99.e6 (2020).
Huda, N. et al. Hepatic senescence, the good and the bad. World J. Gastroenterol. 25, 5069–5081 (2019).
Filadi, R., Pendin, D. & Pizzo, P. Mitofusin 2: from functions to disease. Cell Death Dis. 9, 330 (2018).
Wang, J. C. & Bennett, M. Aging and atherosclerosis: mechanisms, functional consequences, and potential therapeutics for cellular senescence. Circ. Res. 111, 245–259 (2012).
Gorenne, I., Kavurma, M., Scott, S. & Bennett, M. Vascular smooth muscle cell senescence in atherosclerosis. Cardiovasc. Res. 72, 9–17 (2006).
Childs, B. G. et al. Senescent cells suppress innate smooth muscle cell repair functions in atherosclerosis. Nat. Aging 1, 698–714 (2021).
Moulis, M. et al. The multifunctional sorting protein PACS-2 controls mitophagosome formation in human vascular smooth muscle cells through mitochondria-ER contact sites. Cells 8, 638 (2019).
Yu, S. et al. PACS2 is required for ox-LDL-induced endothelial cell apoptosis by regulating mitochondria-associated ER membrane formation and mitochondrial Ca2+ elevation. Exp. Cell Res. 379, 191–202 (2019).
Wiel, C. et al. Endoplasmic reticulum calcium release through ITPR2 channels leads to mitochondrial calcium accumulation and senescence. Nat. Commun. 5, 3792 (2014).
Ma, X. et al. The nuclear receptor RXRA controls cellular senescence by regulating calcium signaling. Aging Cell 17, e12831 (2018).
Sousa-Victor, P. et al. Geriatric muscle stem cells switch reversible quiescence into senescence. Nature 506, 316–321 (2014).
García-Prat, L. et al. Autophagy maintains stemness by preventing senescence. Nature 529, 37–42 (2016).
Habiballa, L., Salmonowicz, H. & Passos, J. F. Mitochondria and cellular senescence: Implications for musculoskeletal ageing. Free Radic. Biol. Med. 132, 3–10 (2019).
Fernandez-Sanz, C. et al. Defective sarcoplasmic reticulum-mitochondria calcium exchange in aged mouse myocardium. Cell Death Dis. 5, e1573 (2014).
Sebastián, D. et al. Mfn2 deficiency links age-related sarcopenia and impaired autophagy to activation of an adaptive mitophagy pathway. EMBO J. 35, 1677–1693 (2016).
Chen, Y.-F. et al. Cisd2 deficiency drives premature aging and causes mitochondria-mediated defects in mice. Genes Dev. 23, 1183–1194 (2009).
Wang, L. et al. Mitofusin 2 regulates axonal transport of calpastatin to prevent neuromuscular synaptic elimination in skeletal muscles. Cell Metab. 28, 400–414.e8 (2018).
Bussian, T. J. et al. Clearance of senescent glial cells prevents tau-dependent pathology and cognitive decline. Nature 562, 578 (2018).
Chinta, S. J. et al. Cellular senescence is induced by the environmental neurotoxin Paraquat and contributes to neuropathology linked to Parkinson’s disease. Cell Rep. 22, 930–940 (2018).
Nicaise, A. M. et al. Cellular senescence in progenitor cells contributes to diminished remyelination potential in progressive multiple sclerosis. Proc. Natl Acad. Sci. USA 116, 9030–9039 (2019).
Zhang, P. et al. Senolytic therapy alleviates Aβ-associated oligodendrocyte progenitor cell senescence and cognitive deficits in an Alzheimer’s disease model. Nat. Neurosci. 22, 719 (2019).
Area-Gomez, E. et al. Upregulated function of mitochondria-associated ER membranes in Alzheimer disease. EMBO J. 31, 4106–4123 (2012). This original work was the first to focus on the importance of MERCs in neurodegenerative diseases and paved the way to multiple other studies.
Hedskog, L. et al. Modulation of the endoplasmic reticulum–mitochondria interface in Alzheimer’s disease and related models. Proc. Natl Acad. Sci. USA 110, 7916–7921 (2013).
Filadi, R. et al. Presenilin 2 modulates endoplasmic reticulum-mitochondria coupling by tuning the antagonistic effect of Mitofusin 2. Cell Rep. 15, 2226–2238 (2016).
Hewitt, V. L. et al. Decreasing pdzd8-mediated mitochondrial-ER contacts in neurons improves fitness by increasing mitophagy. Preprint at bioRxiv https://doi.org/10.1101/2020.11.14.382861 (2020).
Lee, K.-S. et al. Altered ER–mitochondria contact impacts mitochondria calcium homeostasis and contributes to neurodegeneration in vivo in disease models. Proc. Natl Acad. Sci. USA 115, E8844–E8853 (2018).
Gómez-Suaga, P., Pedro, J. M. B.-S., González-Polo, R. A., Fuentes, J. M. & Niso-Santano, M. ER–mitochondria signaling in Parkinson’s disease. Cell Death Dis. 9, 1–12 (2018).
Calì, T., Ottolini, D., Negro, A. & Brini, M. α-Synuclein controls mitochondrial calcium homeostasis by enhancing endoplasmic reticulum-mitochondria interactions. J. Biol. Chem. 287, 17914–17929 (2012).
Martino Adami, P. V. et al. Perturbed mitochondria–ER contacts in live neurons that model the amyloid pathology of Alzheimer’s disease. J. Cell Sci. 132 (2019).
Garrido-Maraver, J., Loh, S. H. Y. & Martins, L. M. Forcing contacts between mitochondria and the endoplasmic reticulum extends lifespan in a Drosophila model of Alzheimer’s disease. Biol. Open 9, bio047530 (2020). This in vivo work using synthetic linker in fly constituted the first evidence of the beneficial role of MERCs in lifespan in a model of Alzheimer’s disease.
Paillusson, S. et al. Synuclein binds to the ER–mitochondria tethering protein VAPB to disrupt Ca2+ homeostasis and mitochondrial ATP production. Acta Neuropathol. 134, 129–149 (2017).
Basso, V. et al. Regulation of ER-mitochondria contacts by Parkin via Mfn2. Pharmacol. Res. 138, 43–56 (2018).
Bolinches-Amorós, A., Mollá, B., Pla-Martín, D., Palau, F. & González-Cabo, P. Mitochondrial dysfunction induced by frataxin deficiency is associated with cellular senescence and abnormal calcium metabolism. Front Cell Neurosci 8, 124 (2014).
Rodríguez, L. R. et al. Oxidative stress modulates rearrangement of endoplasmic reticulum-mitochondria contacts and calcium dysregulation in a Friedreich’s ataxia model. Redox Biol. 37, 101762 (2020).
Cárdenas, C. et al. Essential regulation of cell bioenergetics by constitutive InsP3 receptor Ca2+ transfer to mitochondria. Cell 142, 270–283 (2010). This study highlighted the role of minimal ensured ER-mitochondrial calcium fluxes to sustain adequate mitochondrial bioenergetics.
Tuttle, C. S. L. et al. Cellular senescence and chronological age in various human tissues: a systematic review and meta-analysis. Aging Cell 19, e13083 (2020). This review constituted the first meta-analysis correlating age and senescence markers in various tissues underlying tissue-specificity of accumulation of senescent cells.
Farr, J. N. et al. Targeting cellular senescence prevents age-related bone loss in mice. Nat. Med 23, 1072–1079 (2017).
Deursen, J. M. van. Senolytic therapies for healthy longevity. Science 364, 636–637 (2019).
Griveau, A., Wiel, C., Ziegler, D. V., Bergo, M. O. & Bernard, D. The JAK1/2 inhibitor ruxolitinib delays premature aging phenotypes. Aging Cell 19, e13122 (2020).
Beaulieu, D. et al. Phospholipase A2-Receptor 1 promotes lung-cell senescence and emphysema in obstructive lung disease. Eur. Respir. J. 2000752 https://doi.org/10.1183/13993003.00752-2020 (2021).
Zhang, Y., Zhang, J. & Wang, S. The role of Rapamycin in healthspan extension via the delay of organ aging. Ageing Res. Rev. 70, 101376 (2021).
Vereczki, V. et al. Cyclophilin D regulates lifespan and protein expression of aging markers in the brain of mice. Mitochondrion 34, 115–126 (2017).
Tavecchio, M., Lisanti, S., Bennett, M. J., Languino, L. R. & Altieri, D. C. Deletion of Cyclophilin D impairs β-oxidation and promotes glucose metabolism. Sci. Rep. 5, 15981 (2015).
Shum, L. C. et al. Cyclophilin D knock-out mice show enhanced resistance to osteoporosis and to metabolic changes observed in aging bone. PLoS ONE 11, e0155709 (2016).
Yokoyama, K. et al. Extended longevity of Caenorhabditis elegans by knocking in extra copies of hsp70F, a homolog of mot-2 (mortalin)/mthsp70/Grp75. FEBS Lett. 516, 53–57 (2002).
Burkewitz, K. et al. Atf-6 regulates lifespan through ER-mitochondrial calcium homeostasis. Cell Rep. 32, 108125 (2020).
de Brito, O. M. & Scorrano, L. Mitofusin 2 tethers endoplasmic reticulum to mitochondria. Nature 456, 605–610 (2008).
Filadi, R. et al. Mitofusin 2 ablation increases endoplasmic reticulum–mitochondria coupling. Proc. Natl Acad. Sci. USA 112, E2174–E2181 (2015).
Leal, N. S. et al. Mitofusin-2 knockdown increases ER-mitochondria contact and decreases amyloid β-peptide production. J. Cell. Mol. Med. 20, 1686–1695 (2016).
Srivastava, S., Vishwanathan, V., Birje, A., Sinha, D. & D’Silva, P. Evolving paradigms on the interplay of mitochondrial Hsp70 chaperone system in cell survival and senescence. Crit. Rev. Biochem. Mol. Biol. 54, 517–536 (2019).
Cho, I.-T. et al. Ascorbate peroxidase proximity labeling coupled with biochemical fractionation identifies promoters of endoplasmic reticulum–mitochondrial contacts. J. Biol. Chem. 292, 16382–16392 (2017).
Cho, K. F. et al. Split-TurboID enables contact-dependent proximity labeling in cells. Proc. Natl Acad. Sci. USA 117, 12143–12154 (2020).
Kwak, C. et al. Contact-ID, a tool for profiling organelle contact sites, reveals regulatory proteins of mitochondrial-associated membrane formation. Proc. Natl Acad. Sci. USA 117, 12109–12120 (2020).
Bartok, A. et al. IP 3 receptor isoforms differently regulate ER-mitochondrial contacts and local calcium transfer. Nat. Commun. 10, 1–14 (2019).
Filadi, R. et al. TOM70 sustains cell bioenergetics by promoting IP3R3-mediated ER to mitochondria Ca2+ transfer. Curr. Biol. 28, 369–382.e6 (2018).
Harada, T. et al. Palmitoylated CKAP4 regulates mitochondrial functions through an interaction with VDAC2 at ER-mitochondria contact sites. J. Cell Sci. 133, jcs249045 (2020).
Wu, S. et al. Binding of FUN14 domain containing 1 with inositol 1,4,5-trisphosphate receptor in mitochondria-associated endoplasmic reticulum membranes maintains mitochondrial dynamics and function in hearts in vivo. Circulation 136, 2248–2266 (2017).
Prudent, J. et al. MAPL SUMOylation of Drp1 stabilizes an ER/mitochondrial platform required for cell death. Mol. Cell 59, 941–955 (2015).
Hirabayashi, Y. et al. ER-mitochondria tethering by PDZD8 regulates Ca2+ dynamics in mammalian neurons. Science 358, 623–630 (2017).
Ziegler, D. V., Wiley, C. D. & Velarde, M. C. Mitochondrial effectors of cellular senescence: beyond the free radical theory of aging. Aging Cell 14, 1–7 (2015).
Park, Y.-Y. et al. Loss of MARCH5 mitochondrial E3 ubiquitin ligase induces cellular senescence through dynamin-related protein 1 and mitofusin 1. J. Cell. Sci. 123, 619–626 (2010).
Son, J. M. et al. Mitofusin 1 and optic atrophy 1 shift metabolism to mitochondrial respiration during aging. Aging Cell 16, 1136–1145 (2017).
Chen, K.-H. et al. Role of mitofusin 2 (Mfn2) in controlling cellular proliferation. FASEB J. 28, 382–394 (2014).
Li, X. et al. FGF21 mediates mesenchymal stem cell senescence via regulation of mitochondrial dynamics. Oxid. Med. Cell Longev. 2019, 4915149 (2019).
Luchsinger, L. L., de Almeida, M. J., Corrigan, D. J., Mumau, M. & Snoeck, H.-W. Mitofusin 2 maintains haematopoietic stem cells with extensive lymphoid potential. Nature 529, 528–531 (2016).
Kaul, S. C., Yaguchi, T., Taira, K., Reddel, R. R. & Wadhwa, R. Overexpressed mortalin (mot-2)/mthsp70/GRP75 and hTERT cooperate to extend the in vitro lifespan of human fibroblasts. Exp. Cell Res. 286, 96–101 (2003).
Wadhwa, R., Yaguchi, T., Hasan, M. K., Taira, K. & Kaul, S. C. Mortalin-MPD (mevalonate pyrophosphate decarboxylase) interactions and their role in control of cellular proliferation. Biochem. Biophys. Res. Commun. 302, 735–742 (2003).
Iwasawa, R., Mahul-Mellier, A.-L., Datler, C., Pazarentzos, E. & Grimm, S. Fis1 and Bap31 bridge the mitochondria-ER interface to establish a platform for apoptosis induction. EMBO J. 30, 556–568 (2011).
Quistgaard, E. M. BAP31: Physiological functions and roles in disease. Biochimie 186, 105–129 (2021).
Xu, K. et al. MiR-451a suppressing BAP31 can inhibit proliferation and increase apoptosis through inducing ER stress in colorectal cancer. Cell Death Dis. 10, 1–16 (2019).
Tang, H. et al. Single senescent cell sequencing reveals heterogeneity in senescent cells induced by telomere erosion. Protein Cell 10, 370–375 (2019).
Du, X. et al. Oxysterol-binding protein-related protein 5 (ORP5) promotes cell proliferation by activation of mTORC1 signaling. J. Biol. Chem. 293, 3806–3818 (2018).
Rochin, L. et al. ORP5 regulates transport of lipids and calcium to mitochondria at endoplasmic reticulum-mitochondria membrane contact sites. Preprint at bioRxiv https://doi.org/10.1101/695577 (2019).
Zhou, R., Yazdi, A. S., Menu, P. & Tschopp, J. A role for mitochondria in NLRP3 inflammasome activation. Nature 469, 221–225 (2011).
Betz, C. et al. mTOR complex 2-Akt signaling at mitochondria-associated endoplasmic reticulum membranes (MAM) regulates mitochondrial physiology. Proc. Natl Acad. Sci. USA 110, 12526–12534 (2013). This study highlighted the MERCs-localisation of mTOR complex, and its role in modulating MERCs physiology through AKT-dependent phosphorylation of IP3R, Hexokinase 2, and phosphofurin acidic cluster sorting protein 2.
Murakami, T. et al. Critical role for calcium mobilization in activation of the NLRP3 inflammasome. Proc. Natl Acad. Sci. USA 109, 11282–11287 (2012).
Ito, N., Ruegg, U. T. & Takeda, S. ATP-induced increase in intracellular calcium levels and subsequent activation of mTOR as regulators of skeletal muscle hypertrophy. Int. J. Mol. Sci. 19, 2804 (2018).
Brobeil, A. et al. PTPIP51—a new relationship with the NFκB signaling pathway. Biomolecules 5, 485–504 (2015).
Barroso-González, J. et al. PACS-2 mediates the ATM and NF-κB-dependent induction of anti-apoptotic Bcl-xL in response to DNA damage. Cell Death Differ. 23, 1448–1457 (2016).
Xu, F. et al. let-7-repressesed Shc translation delays replicative senescence. Aging Cell 13, 185–192 (2014).
Zhang, J. et al. The p66shc-mediated regulation of hepatocyte senescence influences hepatic steatosis in nonalcoholic fatty liver disease. Med. Sci. Monit. 26, e921887 (2020).
Lebiedzinska, M., Duszynski, J., Rizzuto, R., Pinton, P. & Wieckowski, M. R. Age-related changes in levels of p66Shc and serine 36-phosphorylated p66Shc in organs and mouse tissues. Arch. Biochem. Biophys. 486, 73–80 (2009).
Migliaccio, E. et al. The p66shc adaptor protein controls oxidative stress response and life span in mammals. Nature 402, 309–313 (1999).
Trinei, M. et al. A p53-p66Shc signalling pathway controls intracellular redox status, levels of oxidation-damaged DNA and oxidative stress-induced apoptosis. Oncogene 21, 3872–3878 (2002).
Pinton, P. et al. Protein kinase C beta and prolyl isomerase 1 regulate mitochondrial effects of the life-span determinant p66Shc. Science 315, 659–663 (2007).
Pearson, M. et al. PML regulates p53 acetylation and premature senescence induced by oncogenic Ras. Nature 406, 207–210 (2000).
Giorgi, C. et al. PML regulates apoptosis at endoplasmic reticulum by modulating calcium release. Science 330, 1247–1251 (2010). This work demonstrated the importance of PML at MERCs to mediate IPR3R-phosphorylation and calcium-mediated apoptosis.
Chang, N. C., Nguyen, M., Germain, M. & Shore, G. C. Antagonism of Beclin 1-dependent autophagy by BCL-2 at the endoplasmic reticulum requires NAF-1. EMBO J. 29, 606–618 (2010).
Tsai, P.-H. et al. Dysregulation of mitochondrial functions and osteogenic differentiation in Cisd2-deficient murine induced pluripotent stem cells. Stem Cells Dev. 24, 2561–2576 (2015).
Atkins, K. M. et al. The multifunctional sorting protein PACS-2 regulates SIRT1-mediated deacetylation of p53 to modulate p21-dependent cell-cycle arrest. Cell Rep. 8, 1545–1557 (2014).
Doghman‐Bouguerra, M. et al. FATE1 antagonizes calcium‐ and drug‐induced apoptosis by uncoupling ER and mitochondria. EMBO Rep. 17, 1264–1280 (2016).
Martin, N. & Bernard, D. Calcium signaling and cellular senescence. Cell Calcium 70, 16–23 (2018).
Farfariello, V., Iamshanova, O., Germain, E., Fliniaux, I. & Prevarskaya, N. Calcium homeostasis in cancer: a focus on senescence. Biochim. Biophys. Acta 1853, 1974–1979 (2015).
Madreiter-Sokolowski, C. T. et al. Enhanced inter-compartmental Ca2+ flux modulates mitochondrial metabolism and apoptotic threshold during aging. Redox Biol. 20, 458–466 (2018).
Calvo-Rodríguez, M., García-Durillo, M., Villalobos, C. & Núñez, L. In vitro aging promotes endoplasmic reticulum (ER)-mitochondria Ca2+ cross talk and loss of store-operated Ca2+ entry (SOCE) in rat hippocampal neurons. Biochim. Biophys. Acta 1863, 2637–2649 (2016).
Kerkhofs, M. et al. Emerging molecular mechanisms in chemotherapy: Ca 2+ signaling at the mitochondria-associated endoplasmic reticulum membranes. Cell Death Dis. 9, 1–15 (2018).
Jadiya, P. et al. Impaired mitochondrial calcium efflux contributes to disease progression in models of Alzheimer’s disease. Nat. Commun. 10, 3885 (2019).
Rizzuto, R., De Stefani, D., Raffaello, A. & Mammucari, C. Mitochondria as sensors and regulators of calcium signalling. Nat. Rev. Mol. Cell Biol. 13, 566–578 (2012).
Carreras-Sureda, A. et al. Non-canonical function of IRE1α determines mitochondria-associated endoplasmic reticulum composition to control calcium transfer and bioenergetics. Nat. Cell Biol. 21, 755–767 (2019).
Denton, R. M. Regulation of mitochondrial dehydrogenases by calcium ions. Biochim. Biophys. Acta 1787, 1309–1316 (2009).
Bravo, R. et al. Increased ER-mitochondrial coupling promotes mitochondrial respiration and bioenergetics during early phases of ER stress. J. Cell. Sci. 124, 2143–2152 (2011).
Rieusset, J. et al. Disruption of calcium transfer from ER to mitochondria links alterations of mitochondria-associated ER membrane integrity to hepatic insulin resistance. Diabetologia 59, 614–623 (2016).
Lee, S. et al. Polo kinase phosphorylates Miro to control ER-Mitochondria contact sites and mitochondrial Ca(2+) homeostasis in neural stem cell development. Dev. Cell 37, 174–189 (2016).
Yoon, Y.-S. et al. Formation of elongated giant mitochondria in DFO-induced cellular senescence: involvement of enhanced fusion process through modulation of Fis1. J. Cell. Physiol. 209, 468–480 (2006).
Lee, S. et al. Mitochondrial fission and fusion mediators, hFis1 and OPA1, modulate cellular senescence. J. Biol. Chem. 282, 22977–22983 (2007).
Abbadie, C. & Pluquet, O. Unfolded protein response (UPR) controls major senescence hallmarks. Trends Biochem. Sci. 45, 371–374 (2020).
Kim, H. S. et al. The p38-activated ER stress-ATF6α axis mediates cellular senescence. FASEB J. 33, 2422–2434 (2019).
Chen, H. et al. MacroH2A1 and ATM play opposing roles in paracrine senescence and the senescence-associated secretory phenotype. Mol. Cell 59, 719–731 (2015).
Blazanin, N. et al. ER stress and distinct outputs of the IRE1α RNase control proliferation and senescence in response to oncogenic Ras. Proc. Natl Acad. Sci. USA 114, 9900–9905 (2017).
Drullion, C. et al. Pre-malignant transformation by senescence evasion is prevented by the PERK and ATF6alpha branches of the unfolded protein response. Cancer Lett. 438, 187–196 (2018).
Panganiban, R. A. M., Mungunsukh, O. & Day, R. M. X-irradiation induces ER stress, apoptosis, and senescence in pulmonary artery endothelial cells. Int. J. Radiat. Biol. 89, 656–667 (2013).
Dörr, J. R. et al. Synthetic lethal metabolic targeting of cellular senescence in cancer therapy. Nature 501, 421–425 (2013). This original work shed light on the importance of active changes in metabolism of senescent cells compared to non-senescent ones, and that could further be used as potential future therapeutic targets.
Liu, J. et al. Impact of ER stress-regulated ATF4/p16 signaling on the premature senescence of renal tubular epithelial cells in diabetic nephropathy. Am. J. Physiol. Cell Physiol. 308, C621–C630 (2015).
Tezze, C. et al. Age-associated loss of OPA1 in muscle impacts muscle mass, metabolic homeostasis, systemic inflammation, and epithelial senescence. Cell Metab. 25, 1374–1389.e6 (2017).
Liu, Y. et al. Endoplasmic reticulum stress participates in the progress of senescence and apoptosis of osteoarthritis chondrocytes. Biochem. Biophys. Res. Commun. 491, 368–373 (2017).
Hetz, C. The unfolded protein response: controlling cell fate decisions under ER stress and beyond. Nat. Rev. Mol. Cell Biol. 13, 89–102 (2012).
Muñoz, J. P. et al. Mfn2 modulates the UPR and mitochondrial function via repression of PERK. EMBO J. 32, 2348–2361 (2013).
Verfaillie, T. et al. PERK is required at the ER-mitochondrial contact sites to convey apoptosis after ROS-based ER stress. Cell Death Differ. 19, 1880–1891 (2012).
Gkogkas, C. et al. VAPB interacts with and modulates the activity of ATF6. Hum. Mol. Genet. 17, 1517–1526 (2008).
Krebs, J., Agellon, L. B. & Michalak, M. Ca(2+) homeostasis and endoplasmic reticulum (ER) stress: an integrated view of calcium signaling. Biochem. Biophys. Res. Commun. 460, 114–121 (2015).
Bánsághi, S. et al. Isoform- and species-specific control of inositol 1,4,5-trisphosphate (IP3) receptors by reactive oxygen species. J. Biol. Chem. 289, 8170–8181 (2014).
Zheng, P. et al. DNA damage triggers tubular endoplasmic reticulum extension to promote apoptosis by facilitating ER-mitochondria signaling. Cell Res. 28, 833–854 (2018).
Csordás, G. et al. Imaging interorganelle contacts and local calcium dynamics at the ER-mitochondrial interface. Mol. Cell 39, 121–132 (2010).
Booth, D. M., Enyedi, B., Geiszt, M., Várnai, P. & Hajnóczky, G. Redox nanodomains are induced by and control calcium signaling at the ER-mitochondrial interface. Mol. Cell 63, 240–248 (2016).
Giacomello, M. & Pellegrini, L. The coming of age of the mitochondria-ER contact: a matter of thickness. Cell Death Differ. 23, 1417–1427 (2016).
Cieri, D. et al. SPLICS: a split green fluorescent protein-based contact site sensor for narrow and wide heterotypic organelle juxtaposition. Cell Death Differ. 25, 1131–1145 (2018).
Magalhães Rebelo, A. P. et al. Chemical modulation of mitochondria-endoplasmic reticulum contact sites. Cells 9, 1637 (2020). This review focused on the importance of druggable targets to modulate MERCs function and opened new perspectives to extend investigations on MERCs importance.
Yousefzadeh, M. J. et al. Fisetin is a senotherapeutic that extends health and lifespan. EBioMedicine 36, 18–28 (2018).
Xu, M. et al. Senolytics improve physical function and increase lifespan in old age. Nat. Med. 24, 1246–1256 (2018).
Acknowledgements
We thank Fondation ARC and Fondation pour la Recherche Médicale (FRM) for their support.
Author information
Authors and Affiliations
Contributions
D.V.Z., N.M. and D.B. conceived the concepts described in this perspective. D.V.Z. drew the figures and wrote the introduction section and the main topic sections with the assistance of N.M. and D.B., and N.M. and D.B. wrote the Perspectives/conclusion section with the assistance of D.V.Z.
Corresponding authors
Ethics declarations
Competing interests
The authors declare no competing interests.
Additional information
Peer review Information Communications Biology thanks J. Cesar Cardenas and the other, anonymous, reviewers for their contribution to the peer review of this work. Primary Handling Editor: Eve Rogers.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Ziegler, D.V., Martin, N. & Bernard, D. Cellular senescence links mitochondria-ER contacts and aging. Commun Biol 4, 1323 (2021). https://doi.org/10.1038/s42003-021-02840-5
Received:
Accepted:
Published:
DOI: https://doi.org/10.1038/s42003-021-02840-5
Comments
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.