Introduction

TRPP2 (transient receptor potential polycystic 2, previously known as polycystin-2, PC2, or PKD2) is a nonselective cation channel protein encoded by the PKD2 gene. It has a large single-channel conductance and high Ca2+ permeability1,2,3. The TRPP2 protein comprises 968 amino acids, which form six transmembrane segments1 with an endoplasmic reticulum (ER) retention signal in the C-terminal domain4,5, two cytosolic extremities including two EF-hands (residues 720–797)6, a coiled-coil domain (residues 828–895)6,7,8 and an acidic amino acid cluster (residues 798–827)9,10 (Figure 1). TRPP2 interacts with many proteins, primarily cytoskeletal components11. The acidic cluster can be recognized by phosphofurin acidic cluster sorting protein-1 (PACS-1) and PACS-2, and it regulates the trafficking of TRPP2 among the ER, the Golgi and the plasma membrane9. The coiled-coil domain contributes to homomeric assembly within TRPP2 or heteromeric assembly between TRPP2 and another TRPP family protein, polycystin-17,12,13. Four TRPP2 subunits assemble into a functional ion channel (Figure 1). N- and C-termini of the TRPP2 protein contain dimerization domains, which are important components of TRPP2 channel assembly14,15. In addition to TRPP2, PKD2-like 1 (PKD2L1) and PKD2-like 2 (PKD2L2) are two other members of the TRPP subfamily of ion channels sharing structural homology with TRPP216. TRPP2 has been implicated in various cellular processes including mechanosensation, polarity, apoptosis, cellular proliferation, mating behavior and directed sperm movement17.

Figure 1
figure 1

The structural features of a TRPP2 channel. A single TRPP2 subunit comprises six transmembrane segments (S1–S6) with a cation-permeable pore between S5 and S6. In the cytoplasmic C-terminus, there are EF-hands, a coiled-coil domain, and an acidic amino acid cluster. Four such units assemble into an ion channel.

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Mutation of the PKD2 gene is a major cause of autosomal dominant polycystic kidney disease (ADPKD), which is a common genetic disease of the kidney18. However, patients with ADPKD suffer from not only a defect in kidney function but also a systemic disorder associated with abnormalities in the vasculature, such as cerebral, intracranial, and aortic aneurysms and cardiac valvular dysfunction19,20,21. Therefore, cardiovascular complications are typically the leading cause of mortality and morbidity in ADPKD patients22,23,24,25,26. Evidence from the literature has shown that TRPP2 is widely expressed in the endothelial cells of mesenteric arteries27 and the smooth muscle cells of cerebral arteries28,29, the aorta30,31,32, and mesenteric arteries31, thus suggesting that TRPP2 may serve an important function in the cardiovascular system.

TRPP2's expression pattern and agonists and antagonists

TRPP2 is expressed in various tissues including epithelial cells, vascular smooth muscle cells, endothelial cells, cardiac myocytes, adrenal glands, and ovaries33. The subcellular localization of TRPP2 is dependent on its subsequent interaction with various trafficking proteins. TRPP2 contains an ER retention signal, which causes the protein to be retained in the ER compartment4,5. TRPP2 is most prominently localized to the ER, plasma membrane, primary cilia, mitotic spindles, and lamellipodia5,34,35,36,37,38,39,40,41. Numerous studies have indicated that transient receptor potential (TRP) isoforms have the ability to form homomultimeric or heteromultimeric channels with the same or other subfamilies. These new heteromultimeric channels typically have many novel biophysical properties, modes of activation, or simply more efficient trafficking of the ion conducting subunit to the plasma membrane42. To date, the literature has shown that the TRPP2 subunit might assemble into both homomeric and heteromeric complex channels with polycystin-143,44, TRPC129,45,46,47, TRPC348, TRPC449, TRPC550, TRPC748 and TRPV451,52,53.

The pharmacology associated with TRPP2 is poorly understood. So far, identification of a specific chemical antagonist acting on TRPP2 is still lacking. To examine the physiological function of TRPP2, Nauli et al have designed a specific antibody targeting the part of the first extracellular loop of mouse TRPP2 (residues 278–428) and thereby inhibiting pore function34. The authors found that inhibitory antibody completely prevented flow-induced Ca2+ influx into kidney cells, whereas a control antibody against the intracellular domain of TRPP2 (p96525) had no effect34. Amiloride, La3+, and Gd3+ have been proposed to be TRPP2 inhibitors, although they do not specifically inhibit TRPP2. In addition, TRPP2 is inhibited by cytoplasmic pH and is modulated by the holding potential due to its H+ sensitive and voltage-dependent properties54. Abdi et al have reported that triptolide, an agonist of TRPP2, evokes TRPP2-mediated Ca2+ release from internal Ca2+storage; however, the specificity of triptolide's action on TRPP2 is not well documented28. Therefore, the identification of the specific agonist and antagonist of TRPP2 should be a challenge for scientists in future studies.

The function of TRPP2 in the regulation of intracellular Ca2+ homeostasis

A debate exists with respect to the localization of TRPP2 in cells. Many studies have reported that TRPP2 located in the plasma membrane, along with polycystin-1, TRPV4 and TRPC1, bears a cationic current induced by mechanotransduction and is responsible for various physiological functions27,34,43,51,55,56,57. Other studies have shown that TRPP2 is expressed in the ER and either acts as a leak channel that lowers the Ca2+ concentration in the ER58,59 or acts in association with inositol 1,4,5-trisphosphate receptors (IP3R) and the ryanodine receptor (RyR) in modulating intracellular Ca2+ concentration ([Ca2+]i) homeostasis60,61,62. A portion of the TRPP2 N-terminus interacts with RyR and thus reduces RyR-mediated Ca2+ release60. Additionally, the acidic cluster in the TRPP2 C-terminal cytoplasmic tail may interact with a cluster of positively charged residues in the IP3R N-terminal ligand-binding domain. Through this protein-protein interaction, the local cytosolic Ca2+ concentration increases. This concentration increase is initiated by Ca2+ release via IP3R and may activate TRPP2 and hence evoke further Ca2+ release61,62. The decreased Ca2+ concentration in the ER then evokes the oligomerization of stromal interacting molecule 1 (STIM1), which is a Ca2+ sensor in the ER63,64. Subsequently, oligomerized STIM1 activates a Ca2+-permeable ion channel, Orai1, in the plasma membrane65 and other store-operated channels66, thus resulting in Ca2+ entry. This process is referred to as store-operated Ca2+ entry (SOCE). The purpose of SOCE is to mediate Ca2+ influx to refill Ca2+ stores after depletion.

Much like the Ca2+ release channels in the ER, TRPP2 channels may partially contribute to the transient [Ca2+]i increase and the intensity of SOCE. Ca2+ entry after depletion of intracellular Ca2+ stores by treatment with caffeine and thapsigargin is significantly lower in TRPP2+/- mice than in wild type mice32,58. Cytoplasmic and ER Ca2+ levels and SOCE have been found to be decreased in TRPP2-defective cholangiocytes compared with wild type cells, whereas the expression levels of STIM1 and Orai1 were unchanged67.

TRPP2's function in vascular smooth muscle cells

TRPP2 is broadly expressed in vascular smooth muscle cells, where it optimizes muscle contractility30,68. Qian et al have found that, in vascular smooth muscle cells, TRPP2 is primarily located in the sarcoplasmic reticulum (SR) and in intracellular compartments, but polycystin-1 is located in the plasma membrane32. Despite these different subcellular localizations, the interaction between TRPP2 and polycystin-1 occurs across closely apposed plasmalemmal and SR membranes near the cell surface32. The interaction of TRPP2 and polycystin-1 have also been found at dense plaques, which are sites of cytoskeletal/plasma membrane/extracellular matrix interaction that provide a mechanical and functional link to the regulation of the elastic and contractile tension of the stretched vessel32,69,70.

The SR Ca2+ store and basal [Ca2+]i are decreased in Pkd2+/− arterial smooth muscle cells58. Furthermore, consistently with this finding, Pkd2 mutations severely decrease the contractility of the visceral smooth muscle cells in Drosophila, which can be restored by expressing wild type Pkd2 cDNA through a muscle-specific Gal4 driver68. Ren et al have also found that TRPP2 shRNA effectively suppresses TRPP2 protein expression as well as α1-adrenergic receptor agonist (phenylephrine)-induced vasocontraction in the thoracic aorta and mesenteric arteries31. However, Pkd2+/− arteries exhibit exaggerated vasocontraction and increased sensitivity to phenylephrine, which is caused by enhanced Ca2+-independent force generation and increased contractile protein expression71. In high salt intake-induced hypertension, TRPP2 expression in vascular smooth muscle cells and phenylephrine-induced contractility of the aorta and mesenteric arteries are dramatically increased31. However, the weight of the aortic rings and the basal diameter of the mesenteric arteries is decreased31. Therefore, with the exception of blood vessel hypertrophy, the increase in the TRPP2 expression level in arterial smooth muscle cells appears to cause enhanced vasoconstriction induced by phenylephrine. One study has reported that in denuded aortic rings and mesenteric arteries, when Ca2+ stores are depleted through pretreatment with thapsigargin, no significant difference in the vasocontraction between TRPP2 and control shRNA-transduced rats was observed31. These findings indicate that TRPP2 located in the ER membrane may be more important than TRPP2 located in the plasma membrane in terms of agonist-induced vasocontraction. TRPP2 differentially regulates the myogenic response in different vascular beds. TRPP2 protein is primarily located in the plasma membrane of human and rat cerebral artery smooth muscle cells, in contrast to the expression observed in the ER or SR membrane in the mesenteric arteries and aorta29,31,58. TRPP2 knockdown by TRPP2 specific siRNA decreases swelling-activated cation currents in vascular smooth muscle cells and pressure-induced constriction in resistance-size cerebral arteries29. In addition, in vascular smooth muscle cells, an overabundance of free TRPP2 inhibits stretch-activated ion channels (SACs) through filamin A coupled to the actin cytoskeleton, but this inhibition can be reversed by coexpression with polycystin-157. This result indicates that the polycystin-1/TRPP2 ratio regulates SAC mechanosensitivity and pressure sensing.

TRPP2's function in endothelial cells

In the circulatory system, the shear stress generated from hemodynamic blood flow is a major physiological stimulus inducing endothelial cell-dependent vascular dilation72. In the process of shear stress-induced vessel dilation, a key early event is the flow-stimulated [Ca2+]i increase in the vascular endothelial cells. TRPP2 is expressed in endothelial cells along with polycystin-1, TRPC1 and TRPV4, and it plays an important role as a mechanosensing Ca2+ channel. We have used a lentiviral construct carrying TRPP2D511V (dead mutant) to explore the functional role of TRPP2 in flow-induced vascular dilation27. Dilation was significantly reduced in the arteries isolated from lenti-TRPP2D511V-treated rats compared with arteries from empty lentivector–treated rats, which suggests the participation of TRPP2 in flow-induced vascular dilation.

TRPP2 shows mechanosensitive properties when it associates with polycystin-1 (formerly TRPP1), thereby functioning as a mechano-sensor. However, TRPP2 may lose its mechanosensitive activity when it is expressed alone3,34,57. TRPP2 may owe its mechanosensitive properties to coupling with other TRP subunits, presumably TRPC1 and/or TRPV427,52,73. Knockdown of the TRPP2 protein significantly decreases the initial stretch-induced [Ca2+]i increase, whereas TRPC1 knockdown significantly decreases the later [Ca2+]i increase45. However, the combined knockdown of TRPP2 and TRPC1 is necessary to significantly decrease the stretch-induced [Ca2+]i changes45. These findings suggest that TRPP2 channels may substantially contribute to the [Ca2+]i increase in the first few minutes after stretch stimulus.

TRPP2 may associate with TRPV4 and TRPC1, thus forming heteromultimeric channels mediating flow-induced Ca2+ influx in TRPV4, TRPC1, and TRPP2 co-overexpressing HEK293 cells and rat mesenteric artery endothelial cells27. In this cross-subfamily assembly of TRPs, TRPV4 is essential to the flow-induced response. However, the heteromultimeric channels may display different properties from those of the homomultimeric channels. Flow-induced Ca2+ increases are transient in HEK293 cells overexpressing TRPV4 alone but become sustained when TRPV4 is co-expressed with TRPP2 or TRPC127. PKG-mediated inhibition of TRPV4-TRPC1-TRPP2 channels might contribute to the transient nature of the flow-induced [Ca2+]i increase in vascular endothelial cells27. Through this negative feedback mechanism, Ca2+ influx would stimulate a nitric oxide (NO)-cGMP-PKG signaling cascade and cause the inhibition of Ca2+ entry channels27.

Mechanical stress can quickly lead to the dysfunction of the microvessel endothelial cells, including the disruption of the blood–brain barrier (BBB)45. Mechanical stress and [Ca2+]i elevation can produce functional modification of the tight junction complex, which leads to barrier leakiness74,75. The mechanical stress delivered to the brain tissue during traumatic brain injury is associated with two ion channels: TRPP2 and TRPC145. These channels appear to be largely responsible for the prolonged dynamics45.

An abnormal response to hyperemia and endothelium-dependent vasorelaxation has been found in ADPKD patients and in different animal models of ADPKD20,21. ADPKD patients display a reduction in endothelium-dependent vessel dilatation in conduit arteries during the increase of sustained blood flow, which is associated with the complete loss of NO release76. Karen et al have found that the aorta in an autosomal recessive ARPKD animal model had abnormal endothelium-dependent vascular reactivity, likely due to a primary defect in the endothelial cells that occurred prior to the changes in mean arterial pressure or renal function. However, the endothelium-independent relaxation, for example, that in smooth muscle cells, was not impaired19.

TRPP2 in blood pressure regulation

A portion (10%–20%) of children with ADPKD presents with hypertension symptoms, and the majority of adults exhibit hypertension before any loss of kidney function has occurred77. Hypertension relates to progressive kidney enlargement and is a significant independent risk factor for the progression toward end stage renal disease78,79. The pathogenesis of hypertension in ADPKD is complex and dependent on many factors that influence each other. Defects in the primary cilia causing endothelial dysfunction and the activation of the renin-angiotensin-aldosterone system are central pathophysiologic explanations for the development of hypertension in ADPKD80. The treatment of hypertension effectively reduces cardiovascular mortality and may also slow the progression of kidney disease81,82.

Decreased or absent polycystin-1 or TRPP2 protein expression is associated with abnormal vascular structure and function. A Pkd1 (which encodes polycystin-1) /Pkd2 deficiency results in decreased NO levels and altered endothelial response to shear stress with the attenuation in vascular relaxation in ADPKD patients20,21.

Kocyigit et al have demonstrated that ADPKD patients with hypertension have low levels of endothelial nitric oxide synthase (eNOS) expression compared with patients without hypertension and that eNOS expression is an independent predictive factor of hypertension in the ADPKD population83. In addition, TRPP2 protein expression has been shown to be dramatically increased in the vascular smooth muscle cells of the thoracic aorta and mesenteric arteries in high salt intake-induced hypertensive rats compared to age-controlled rats31.

Conclusion and perspectives

TRPP2 is widely expressed in the endothelial cells and smooth muscle cells of diverse vascular beds. Even dysfunctional mutation of TRPP2 causes severe renal dysfunction, and vascular complications are the primary reason for mortality and morbidity. Therefore, TRPP2 is considered to be an important target for modulating vascular function. However, the physiological and pathological functions of TRPP2, as well as their underlying molecular mechanisms in the vascular system, are still not completely understood. The conditional TRPP2 knockout model of the vascular system might be a powerful approach to address this issue. However, specific chemical agonists and antagonists of TRPP2 are largely lacking, thus hindering the elucidation of TRPP2's functional role in various tissues. Collaboration between researchers in various fields is needed for identifying the TRPP2-specific agonists and antagonists. These compounds will benefit both the investigation of physiological function and the development of therapeutic drugs in the future.