PERK Signaling Regulates Extracellular Proteostasis of an Amyloidogenic Protein During Endoplasmic Reticulum Stress

The PERK arm of the unfolded protein response (UPR) regulates cellular proteostasis and survival in response to endoplasmic reticulum (ER) stress. However, the impact of PERK signaling on extracellular proteostasis is poorly understood. We define how PERK signaling influences extracellular proteostasis during ER stress using a conformational reporter of the secreted amyloidogenic protein transthyretin (TTR). We show that inhibiting PERK signaling impairs secretion of destabilized TTR during thapsigargin (Tg)-induced ER stress by increasing its ER retention in chaperone-bound complexes. Interestingly, PERK inhibition increases the ER stress-dependent secretion of TTR in non-native conformations that accumulate extracellularly as soluble oligomers. Pharmacologic or genetic TTR stabilization partially restores secretion of native TTR tetramers. However, PERK inhibition still increases the ER stress-dependent secretion of TTR in non-native conformations under these conditions, indicating that the conformation of stable secreted proteins can also be affected by inhibiting PERK. Our results define a role for PERK in regulating extracellular proteostasis during ER stress and indicate that genetic or aging-related alterations in PERK signaling can exacerbate ER stress-related imbalances in extracellular proteostasis implicated in diverse diseases.

Scientific REPoRTS | (2019) 9:410 | DOI: 10.1038/s41598-018-37207-0 (PSP), suggesting that reduced PERK signaling promotes toxic tau aggregation 19,20 . Consistent with this, pharmacologic PERK activation attenuates aggregation and toxicity of PSP-related tau mutants in mouse models 21 . Pharmacologic or chemical genetic increases in PERK signaling also reduce the toxic aggregation of rhodopsin mutants associated with retinal degeneration 19,22,23 . Thus, while significant focus has been directed to the pathologic importance of overactive or chronic PERK signaling in disease, it is clear that deficiencies in PERK activity also promote pathogenesis, reflecting a protective role for this UPR signaling arm in regulating cellular physiology in response to ER stress. Interestingly, recent work has revealed PERK as a critical regulator of proteostasis within the ER -the first organelle of the secretory pathway. PERK-dependent translation attenuation regulates ER protein folding load in response to acute ER stress, freeing ER proteostasis factors to protect the secretory proteome from misfolding during the initial stages of toxic insult 2,24 . In addition, PERK regulates both ER-to-Golgi anterograde trafficking and ER-associated degradation 25,26 , the latter a primary mechanism by which cells degrade ER proteins 27 . As such, genetic or pharmacologic inhibition of PERK signaling disrupts secretory proteostasis to reduce the secretion and increase the intracellular accumulation of proteins such as collagen, insulin, or mutant rhodopsin as high molecular weight (HMW) aggregates [28][29][30][31] . These results define an important role for PERK in regulating ER proteostasis in response to pathologic insults. However, considering the importance of the ER in regulating proteostasis in downstream secretory environments such as the extracellular space, a critical question remains: "How does PERK signaling impact extracellular proteostasis in response to ER stress?' .
Imbalances in ER proteostasis can propagate to extracellular environments through the secretion of proteins in non-native conformations that accumulate as toxic oligomers and aggregates associated with proteotoxicity in etiologically-diverse protein aggregation diseases, including many amyloid diseases [32][33][34] . Thus, impaired secretory proteostasis afforded by genetic or aging-related imbalances in PERK activity could exacerbate ER stress-dependent increases in the secretion of non-native proteins, challenging the conformational integrity of the secreted proteome and increasing extracellular populations of toxic oligomers. Here, we use a conformational reporter of the model secreted amyloidogenic protein transthyretin (TTR) -a natively tetrameric protein that aggregates into toxic oligomers and amyloid fibrils associated with diverse TTR-related amyloid diseases 35,36 -to define how pharmacologic inhibition of PERK signaling impacts extracellular proteostasis in response to ER stress. We show that co-administration of PERK inhibitors and ER stress induced by the SERCA inhibitor thapsigargin (Tg) increases secretion of destabilized and wild-type TTR in non-native conformations that accumulate extracellularly as soluble oligomers, the TTR conformation commonly associated with proteotoxicity in disease 35 . These results demonstrate that alterations in PERK signaling can challenge extracellular proteostasis by reducing the secretion of proteins in native, functional conformations and increasing the extracellular accumulation of soluble oligomers associated with the pathogenesis of diverse protein aggregation diseases.

Results and Discussion
Pretreatment with Tg and PERK inhibitors disrupts secretion of destabilized FT TTR A25T . Previously, we showed that administration of Tg reduces the secretion of destabilized, aggregation-prone TTR variants such as TTR A25T33,37,38 . To define how PERK inhibition influences the Tg-dependent reduction in TTR A25T secretion, we pretreated HEK293T cells expressing FLAG-tagged TTR A25T ( FT TTR A25T ) with Tg and/or the PERK signaling inhibitor ISRIB for 16 h and then monitored FT TTR A25T secretion by [ 35 S] metabolic labeling (Fig. 1A). ISRIB is an antagonist of PERK signaling that binds to eIF2B and desensitizes cells to eIF2α phosphorylation 39,40 . Importantly, co-pretreatment with Tg and ISRIB increased metabolic labeling of FT TTR A25T relative to pretreatment with Tg alone (Fig. 1A,B). This is consistent with ISRIB blocking Tg-dependent translational attenuation downstream of PERK 39,40 . Interestingly, co-pretreatment with ISRIB and Tg enhanced the Tg-dependent reduction in the fraction of FT TTR A25T secreted (Fig. 1A,C). The enhanced reduction in FT TTR A25T fraction secretion observed upon Tg and ISRIB co-pretreatment did not correspond with an increased loss of total FT TTR A25T , indicating that this condition does not increase FT TTR A25T degradation (Fig. 1A,D). Instead, this pretreatment increased the accumulation of FT TTR A25T in lysate fractions, reflecting increased intracellular retention (Fig. 1A,E). Interestingly, while the fraction of FT TTR A25T secreted to the media is reduced in cells co-pretreated with Tg and ISRIB relative to pretreatment with Tg alone, the total amount of [ 35 S]-labeled FT TTR A25T that accumulates in media is similar between these conditions, albeit lower than that observed in controls (Fig. S1A). This reflects the higher expression of FT TTR A25T observed in cells co-pretreated with Tg and ISRIB (Fig. 1B). Identical results were obtained using the PERK inhibitor GSK2656157 (herein referred to as GSK), which blocks PERK signaling through direct binding to the PERK kinase active site ( Fig. S1A-E) 41 . The use of the two mechanistically distinct inhibitors of PERK signaling indicates that the enhanced reduction in FT TTR A25T secretion observed in cells co-pretreated with Tg and PERK inhibitors cannot be attributed to off-pathway activities of these compounds 42 . Thus, these results show that pharmacologic inhibition of PERK signaling impairs secretory proteostasis of FT TTR A25T during ER stress. A25T and ER chaperones. The intracellular retention of FT TTR A25T observed upon co-pretreatment with Tg and ISRIB likely reflects increased accumulation of non-native TTR conformations within the ER, as observed for other proteins 28,29,31 . To test this, we measured the intracellular interaction between FT TTR A25T and ER chaperones (which bind non-native protein conformations) in HEK293T cells pretreated with Tg and/or ISRIB for 16 h using an established immunopurification (IP)/immunoblotting (IB) assay 38 . Pretreatment with Tg increases the relative recovery of the ER ATP-dependent HSP70 chaperone BiP and the BiP co-chaperones ERdj3 and HYOU1 in FT TTR A25T IPs (Figs 1F and S1F). This indicates that Tg pretreatment increases the non-native population of intracellular TTR that can engage ER chaperones. Co-pretreatment with Tg and ISRIB further increased the relative interactions between FT TTR A25T and these ER chaperones, as compared to Tg pretreatment alone. Importantly, we do not observe statistically significant differences in the intracellular levels of BiP or HYOU1 in cells pretreated with Tg versus cells co-pretreated with Tg and ISRIB ( Fig. S1F-I). This indicates that ISRIB-dependent inhibition of PERK signaling does not globally disrupt ER stress-dependent upregulation of these UPR target genes. Instead, this result suggests that disruption of FT TTR A25T secretion afforded by PERK inhibition likely results from impaired translational attenuation. However, we cannot rule out the possibility that altered transcription of other ER stress-regulated proteins contribute to the altered secretion of FT TTR A25T observed in cells co-treated with Tg and ISRIB. Collectively, these results indicate that the increased accumulation of destabilized FT TTR A25T afforded by co-pretreatment with PERK inhibitors and Tg reflects increased ER retention of non-native TTR in chaperone-bound complexes. This suggests that PERK inhibition during ER stress disrupts the ability of cells to properly fold and assemble FT TTR A25T into its native tetrameric conformation.

PERK inhibition reduces the population of FT TTR A25T secreted as native tetramers during
Tg-induced ER stress. Imbalances in ER proteostasis can propagate to extracellular environments through the secretion of proteins in non-native conformations 32,33 . Thus, we used a previously established assay to define how co-treatment with PERK inhibitors and Tg-induced ER stress impacts the conformation of FT TTR A25T secreted from mammalian cells ( Fig. 2A) 33 . In this assay, we collect conditioned media for 16 h on cells expressing FT TTR A25T prepared in the presence of the cell impermeable compound tafamidis-sulfonate (Taf-S; Fig. S2A) -a compound that immediately binds and stabilizes native TTR tetramers once secreted to the extracellular space 33 . We then quantify total, tetrameric, and aggregate TTR in this conditioned media. Total TTR is quantified by SDS-PAGE/IB. Tetrameric TTR is measured by incubating conditioned media with compound 1 (Fig. S2A)a fluorogenic compound that readily displaces the Taf-S in the two small-molecule binding sites of the native TTR tetramer and becomes fluorescent upon binding 33,43,44 . Tetramers are then separated by anion exchange chromatography and quantified by compound 1 fluorescence (Fig. S2A). Finally, soluble TTR aggregates are monitored by Clear Native (CN)-PAGE/IB 33,34 . We previously used this approach to show that Tg treatment reduces the secretion of destabilized FT TTR A25T as native tetramers and increases accumulation of this destabilized, aggregation-prone protein as soluble oligomers in conditioned media 33 .
To define how PERK inhibition influences ER stress-dependent alterations in the conformation of secreted FT TTR A25T , we conditioned media in the presence of Taf-S for 16 h on HEK293T cells expressing FT TTR A25T and treated with Tg and/or ISRIB. We then measured total, tetrameric, and aggregate TTR using the assays described above ( Fig. 2A). Treatment with Tg reduced the accumulation of total FT TTR A25T in conditioned media, consistent with published results ( Fig. 2B) 33,38 . Surprisingly, co-treatment with ISRIB rescued the Tg-dependent reduction in total FT TTR A25T in conditioned media (Fig. 2B), likely reflecting the inhibition of PERK-regulated translational attenuation afforded by ISRIB over the duration of the media conditioning (Fig. 1B). Interestingly, these results are distinct from those observed using [ 35 S] metabolic labeling, where we show that co-pretreatment with Tg and ISRIB significantly impaired FT TTR A25T secretion (see Figs 1C and S1A). This likely reflects differences in ER proteostasis afforded by the distinct experimental setups. In the SDS-PAGE experiment, the accumulation of TTR in conditioned media is monitored during a 16 h treatment where the ER proteostasis environment is in the process of changing due to both the ER stress and UPR activation. In contrast, the [ 35 S] metabolic labeling experiments measure FT TTR A25T secretion following a 16 h pretreatment after the ER environment has been significantly altered. The fact that PERK inhibition influences TTR secretion under both paradigms highlights the dynamic impact ER stress has on ER function and underscores the critical role for PERK signaling in regulating TTR secretion in response to ER stress.
In order to define how PERK inhibition influences the conformation of FT TTR A25T secreted during Tg-induced ER stress, we measured tetrameric FT TTR A25T using our compound 1 fluorescence/anion exchange chromatography assay ( Fig. 2A) in conditioned media prepared on cells co-treated with Tg and/or ISRIB (the same media used to monitor total FT TTR A25T levels in Fig. 2B). As reported previously 33 , FT TTR A25T tetramers in conditioned media migrate as multiple peaks by anion exchange chromatography, reflecting the distribution of heterotetramers consisting of unmodified subunits and subunits containing posttranslational modifications such as sulfonylation and cysteinylation (Fig. 2C). Co-treatment with Tg and ISRIB significantly altered the distribution of these peaks, resulting in an increase of heterotetramers eluting earlier on the anion exchange column (Fig. 2C, green). This suggests that Tg and ISRIB co-treatment alters posttranslational modifications of FT TTR A25T secreted to the extracellular space. As expected, liquid chromatography (LC)-mass spectrometry (MS) analysis of FT TTR A25T IP' d from conditioned media prepared on cells co-treated with Tg and ISRIB shows increased relative populations of unmodified FT TTR A25T (Fig. S2A). This is consistent with the earlier migration of tetramers observed under these conditions by anion exchange chromatography and demonstrates that PERK inhibition disrupts the secretory proteostasis environment to influence posttranslational TTR modifications during ER stress.
To quantify the recovery of FT TTR A25T tetramers in conditioned media, we integrated the fluorescence across all of the tetramer peaks. We observed significant reductions in the recovery of FT TTR A25T tetramers in media prepared on Tg-treated cells (Fig. 2B), consistent with published results 33 . However, despite lower levels of total FT TTR A25T in media conditioned on cells treated with Tg relative to cells treated with Tg and ISRIB, identical amounts of tetramers were detected in conditioned media prepared under these two conditions (Fig. 2B). This indicates that PERK inhibition decreases the relative population of FT TTR A25T secreted as native tetramers. This can be demonstrated by normalizing the recovered FT TTR A25T tetramers by the total amount of FT TTR A25T in media prepared on cells treated with Tg and/or ISRIB 33 . This normalization confirms that PERK inhibition reduces the population of FT TTR A25T secreted as native tetramers during ER stress (Fig. 2D). Thus, our results show that PERK inhibition increases the ER-stress-dependent secretion of FT TTR A25T in non-native conformations.
Inhibiting PERK during Tg-induced ER stress increases accumulation of extracellular FT TTR A25T oligomers. Non-native FT TTR A25T accumulates as soluble oligomers in conditioned media owing to the low stability of the aggregation-prone FT TTR A25T variant 33,34 . Thus, the increased total (Fig. 2B) and non-native (Fig. 2D) TTR in media conditioned on cells co-treated with Tg and ISRIB should be reflected by increased accumulation of extracellular soluble oligomers in media prepared under these conditions. Tg treatment alone increased the accumulation of soluble FT TTR A25T aggregates (Fig. 2E,F). This is consistent with previous results and further demonstrates the ER stress-dependent increase in the secretion of non-native FT TTR A25T  media relative to treatment with Tg alone (Fig. 2E,F), consistent with the increase in non-native FT TTR A25T secreted under these conditions (Fig. 2B,D). These results demonstrate that inhibiting PERK signaling during Tg-induced ER stress disrupts extracellular proteostasis by exacerbating the Tg-dependent secretion of destabilized FT TTR A25T in non-native conformations that accumulate extracellularly as soluble oligomers. Pharmacologic stabilization of TTR tetramers partially rescues the secretion of native FT TTR A25T tetramers in cells treated with Tg and PERK inhibitors. TTR tetramers can be stabilized intracellularly by administration of the cell permeable kinetic stabilizer tafamidis (Taf) 33,37 . Taf-dependent intracellular stabilization prevents the dissociation of tetramers prior to secretion to the extracellular space. Thus, conditioning media in the presence of Taf provides an opportunity to determine whether the increased populations of FT TTR A25T secreted from cells co-treated with Tg and ISRIB reflects increased dissociation of tetramers during the secretion process (i.e., reduced tetramer stability) and/or increased secretion of FT TTR A25T prior to tetramer assembly (i.e., reduced tetramer assembly). To test this, we conditioned media on HEK293T cells expressing FT TTR A25T treated with Tg and/or ISRIB in the presence of either Taf-S or Taf. We then monitored total, tetrameric, and aggregate TTR using the assays shown in Fig. 2A.
Conditioning media in the presence of Taf significantly increased the recovery of FT TTR A25T tetramers in every condition, relative to media conditioned in the presence of Taf-S (Figs 3A,B and S3). This indicates that Taf-dependent intracellular stabilization of FT TTR A25T tetramers increases secretion of FT TTR A25T as native tetramers. This increase cannot be attributed to alterations in FT TTR A25T posttranslational modifications, as the addition of Taf does not alter the shift in tetramer distribution observed in cells co-treated with Tg and ISRIB (Fig. 3B). Importantly, the increased secretion of FT TTR A25T tetramers corresponds with reductions of soluble FT TTR A25T aggregates in conditioned media (Fig. 3C). This demonstrates that Taf-dependent intracellular stabilization of TTR reduces the secretion of FT TTR A25T in non-native conformations under all conditions. This also indicates that the accumulation of non-native TTR observed in media conditioned in the presence of Taf-S partially reflects dissociation of TTR tetramers (that can be stabilized intracellularly by Taf) within the secretory pathway. This reveals a specific advantage for employing kinetic stabilizing compounds such as Taf that are cell permeable, as these compounds can stabilize aggregation-prone proteins intracellularly and mitigate potential imbalances in secretory proteostasis associated with the aberrant secretion of non-native conformations.
Despite the increased secretion of FT TTR A25T tetramers afforded by Taf-dependent intracellular stabilization, in the presence of Taf, co-administration of Tg and ISRIB still increased secretion of FT TTR A25T in non-native conformations, relative to Tg treatment alone (Fig. 3A,D). Consistent with this, conditioned media prepared on cells co-treated with Tg and ISRIB in the presence of Taf show higher levels of soluble FT TTR A25T oligomers relative to Tg-treated cells, albeit significantly less than that observed for media conditioned in the presence of Taf-S (Fig. 3C). This demonstrates that despite the ability for Taf to stabilize a large population of FT TTR A25T tetramers in the secretory environment, PERK inhibition can still increase the secretion of FT TTR A25T in non-native conformations during Tg-induced ER stress.

Secretion of stable, wild-type TTR as native tetramers is reduced in cells co-treated with Tg and PERK inhibition.
The ability for PERK inhibition to promote Tg-dependent secretion of FT TTR A25T in non-native conformations even in the presence of the stabilizing ligand Taf suggests that stable, wild-type TTR (TTR WT ) could similarly be susceptible to reductions in tetramer secretion under these conditions. Initially, we confirmed that PERK inhibition disrupts secretory proteostasis for stable FT TTR WT by monitoring the interactions between FT TTR WT and ER chaperones in HEK293T cells treated with Tg and/or the PERK inhibitor GSK. Co-treatment with Tg and GSK significantly increased relative interactions between FT TTR WT and the ER chaperones BiP, ERdj3, and HYOU1 (Fig. 4A), mimicking the results observed with destabilized FT TTR A25T (Figs 1F and S1F). Furthermore, co-treatment with Tg and GSK increased intracellular levels of FT TTR WT , suggesting that this condition leads to intracellular accumulation of stable FT TTR WT (Fig. S4A). However, we did not observe significant increases in intracellular levels of BiP or HYOU1, further demonstrating that PERK inhibition does not impact Tg-dependent increases in these chaperones (Fig. S1G-I and S4A). Collectively, these results indicate that PERK inhibition during ER stress disrupts secretory proteostasis of stable, wild-type TTR by increasing its ER retention in chaperone-bound complexes -results analogous to that observed for destabilized FT TTR A25T (Figs 1F and S1F).
Next, we conditioned media in the presence of Taf-S on HEK293T cells expressing FT TTR WT treated with Tg and/or GSK and monitored the recovery of total, tetrameric, or aggregate TTR using the assays described in Fig. 2A. Tg and GSK co-treatment increased the population of FT TTR WT secreted in non-native conformations (Figs 4B,C and S4A). Identical results were observed in cells co-treated with Tg and ISRIB (Fig. 4D,E). Normalizing the recovered FT TTR WT tetramers by the total amount of FT TTR WT in conditioned media confirms that PERK inhibition reduces the population of stable FT TTR WT secreted as tetramers during ER stress (Fig. 4F). Unfortunately, the stability of FT TTR WT monomers (relative to FT TTR A25T monomers) combined with the relatively low concentrations of non-native FT TTR WT in conditioned media hinders the aggregation of this protein in the extracellular space, making it difficult to reproducibly visualize FT TTR WT soluble aggregates in conditioned media 45 . However, co-treatment with Tg and GSK does appear to increase the population of FT TTR WT soluble aggregates observed by CN-PAGE/IB, consistent with the increased secretion of this protein in non-native conformations (Fig. S4C). Collectively, these results show that PERK inhibition increases the Tg-dependent secretion of wild-type TTR in non-native conformations, indicating that PERK inhibition can disrupt extracellular proteostasis of stable secreted proteins during Tg-induced ER stress.
Here, we show that PERK inhibition during Tg-induced ER stress increases the secretion of the disease relevant amyloidogenic protein TTR in non-native conformations that accumulate extracellularly as soluble oligomers. This indicates that PERK signaling has an important role in dictating extracellular proteostasis by controlling the conformational integrity of secreted proteins such as TTR. As such, genetic, aging-related, or pharmacologic conditions that reduce PERK signaling could lead to pathologic imbalances in extracellular proteostasis that contribute to human disease. The increased secretion of proteins in non-native conformations would    reveal a new consideration when employing PERK inhibitors to attenuate chronic PERK activation in disease, as this approach could promote imbalances in extracellular proteostasis and function that could be detrimental for long-term organismal survival.

Materials and Methods
Plasmids, Antibodies and Reagents. The FT TTR A25T and FT TTR WT plasmids were prepared in the pcD-NAI vector as previously reported 38

Media Conditioning, SDS-PAGE, CN-PAGE and Immunoblotting. HEK293T cells transfected with
FT TTR A25T / FT TTR WT were incubated overnight. The following morning media was changed and cells were incubated for 1 hour. Cells were then replated into a poly-D-lysine coated 6-well plate. The next evening, cells were treated with the indicated treatment for 16 hours and conditioned in 1 mL of media. Conditioned media was then collected and cleared from large debris by centrifugation at 1000 × g for 10 min. For SDS-PAGE, samples were boiled in Lammeli Buffer containing 100 mM DTT for 5 minutes and resolved on a 12% acrylamide gel. Gels were transferred to 0.2 µm nitrocellulose membranes at 100 V for 60 minutes. For CN-PAGE, samples were added to 5x Clear Native Page Buffer (final concentration: 0.1% Ponceau Red, 10% glycerol, 50 mM 6-aminohexanoic acid, 10 mM Bis-Tris pH 7.0) and resolved at 4 °C on a Novex NativePage 4-16% Bis-Tris Protein Gel run at 150 V for 3 hours. Protein was transferred at 100 V for 90 minutes to a 0.2 µm nitrocellulose membrane. Membranes were incubated with the indicated primary antibody overnight and then incubated with the appropriate LICOR secondary antibody. Protein bands where then visualized and quantified using the LICOR Odyssey Infrared Imaging System.
Quantification of FT TTR tetramers in conditioned media. FT TTR tetramers in conditioned media were quantified as previously described 33 . Briefly, conditioned media was prepared on HEK293T cells expressing the indicated FT TTR variant treated with the indicated conditions. Conditioned media was then incubated with 5 µM compound 1 overnight. Media samples (60 µL) were separated over a Waters Protein-Pak Hi Res Q, 5 µm 4.6 × 100 mm anion exchange column in 25 mm Tris pH 8.0, 1 mM EDTA with a linear 1 M NaCl gradient using a ACQUITY UPLC H-Class Bio System (Waters). Fluorescence of TTR tetramers conjugated to compound 1 was observed by excitation at 328 nm and emission at 430 nm. Peaks were integrated and data was collected using Empower 3 software according to the manufacture's protocol.
Liquid chromatography (LC)-mass spectrometry of immunopurified FT TTR. Media was conditioned for 18 h on HEK293T cells transfected with FT TTR A25T treated with the indicated condition. Media was then collected and cleared from debris by centrifugation at 1000x for 10 min. FT TTR A25T was immunopurified from the conditioned media using anti-Flag M1 Agarose gel. Beads were than washed 3x with 10 mM Tris pH 8.0 100 mM NaCL containing 0.05% saponin and 2x with 10 mM Tris pH 8.0 100 mM NaCL. FT TTR A25T was then eluted at 4 degrees C overnight with 50 µL of 0.1 M triethylamine pH 11.5 with gentle shaking. Eluted TTR Scientific REPoRTS | (2019) 9:410 | DOI:10.1038/s41598-018-37207-0 molecular weights were analyzed by LC-MS, as previously described 33 . Briefly, eluates were separated over a 5 µm ID 300 A pore size C8 reverse-phase HPLC column (Agilent) using a 15%/min linear acetonitrile gradient and then analyzed on an 1100 MSD SL mass spectrometer (Agilent). Modified and unmodified FT TTR A25T was defined by the presence of different previously reported mass shifts 33 relative to the parent mass. The absolute abundance of each modified FT TTR A25T peak was defined for each sample then divided by the total abundance of FT TTR A25T to define the reported percent abundance for each species.
Statistical Methods. All p-values were calculated using a paired student's t-test.