bims-unfpre Biomed News
on Unfolded protein response
Issue of 2021‒05‒09
fourteen papers selected by
Susan Logue
University of Manitoba
  1. The Ca2+ -binding protein sorcin stimulates transcriptional activity of the unfolded protein response mediator ATF6.
  2. Kinetic and structural parameters governing Fic-mediated adenylylation/AMPylation of the Hsp70 chaperone, BiP/GRP78.
  3. Periplocin and cardiac glycosides suppress the unfolded protein response.
  4. Irisin ameliorates endoplasmic reticulum stress and liver fibrosis through inhibiting PERK-mediated destabilization of HNRNPA1 in hepatic stellate cells.
  5. The Unfolded Protein Response: An Overview.
  6. Activation of TREM-1 induces endoplasmic reticulum stress through IRE-1α/XBP-1s pathway in murine macrophages.
  7. The Role of Endoplasmic Reticulum Stress Response in Pollen Development and Heat Stress Tolerance.
  8. ER Stress, UPR Activation and the Inflammatory Response to Viral Infection.
  9. Endoplasmic Reticulum-Mitochondria Crosstalk and Beta-Cell Destruction in Type 1 Diabetes.
  10. Prohibitin depletion extends lifespan of a TORC2/SGK-1 mutant through autophagy and the mitochondrial UPR.
  11. Metformin Dysregulates the Unfolded Protein Response and the WNT/β-Catenin Pathway in Endometrial Cancer Cells through an AMPK-Independent Mechanism.
  12. FSHR-1/GPCR Regulates the Mitochondrial Unfolded Protein Response in Caenorhabditis elegans.
  13. The Prognostic Value of PERK in Cancer and Its Relationship With Immune Cell Infiltration.
  14. Preservation of endoplasmic reticulum (ER) Ca2+ stores by deletion of inositol-1,4,5-trisphosphate receptor type 1 promotes ER retrotranslocation, proteostasis, and protein outer segment localization in cyclic nucleotide-gated channel-deficient cone photoreceptors.
  1. FEBS Lett. 2021 May 07.
    The Ca2+ -binding protein sorcin stimulates transcriptional activity of the unfolded protein response mediator ATF6.
    Steven Z Parks, Tian Gao, Natalia Jimenez Awuapura, Joseph Ayathamattam, Pauline L Chabosseau, Dhananjaya V Kalvakolanu, Héctor H Valdivia, Guy A Rutter, Isabelle Leclerc.
      Sorcin is a calcium-binding protein involved in maintaining endoplasmic reticulum (ER) Ca2+ stores. We have previously shown that overexpressing sorcin under the rat insulin promoter was protective against high fat diet-induced pancreatic beta cell dysfunction in vivo. Activating Transcription Factor 6 (ATF6) is a key mediator of the unfolded protein response (UPR) that provides cellular protection during the progression of ER stress. Here, using non-excitable HEK293 cells, we show that sorcin overexpression increased ATF6 signalling, whereas sorcin knock out caused a reduction in ATF6 transcriptional activity and increased ER stress. Altogether, our data suggest that sorcin downregulation during lipotoxic stress may prevent full ATF6 activation and a normal UPR during the progression of obesity and insulin resistance.
    Keywords:  ATF6; ER stress; Sorcin; lipotoxicity
    DOI:  https://doi.org/10.1002/1873-3468.14101
  2. Cell Stress Chaperones. 2021 May 03.
    Kinetic and structural parameters governing Fic-mediated adenylylation/AMPylation of the Hsp70 chaperone, BiP/GRP78.
    Anwesha Sanyal, Erica A Zbornik, Ben G Watson, Charles Christoffer, Jia Ma, Daisuke Kihara, Seema Mattoo.
      Fic (filamentation induced by cAMP) proteins regulate diverse cell signaling events by post-translationally modifying their protein targets, predominantly by the addition of an AMP (adenosine monophosphate). This modification is called Fic-mediated adenylylation or AMPylation. We previously reported that the human Fic protein, HYPE/FicD, is a novel regulator of the unfolded protein response (UPR) that maintains homeostasis in the endoplasmic reticulum (ER) in response to stress from misfolded proteins. Specifically, HYPE regulates UPR by adenylylating the ER chaperone, BiP/GRP78, which serves as a sentinel for UPR activation. Maintaining ER homeostasis is critical for determining cell fate, thus highlighting the importance of the HYPE-BiP interaction. Here, we study the kinetic and structural parameters that determine the HYPE-BiP interaction. By measuring the binding and kinetic efficiencies of HYPE in its activated (Adenylylation-competent) and wild type (de-AMPylation-competent) forms for BiP in its wild type and ATP-bound conformations, we determine that HYPE displays a nearly identical preference for the wild type and ATP-bound forms of BiP in vitro and preferentially de-AMPylates the wild type form of adenylylated BiP. We also show that AMPylation at BiP's Thr366 versus Thr518 sites differentially affect its ATPase activity, and that HYPE does not adenylylate UPR accessory proteins like J-protein ERdJ6. Using molecular docking models, we explain how HYPE is able to adenylylate Thr366 and Thr518 sites in vitro. While a physiological role for AMPylation at both the Thr366 and Thr518 sites has been reported, our molecular docking model supports Thr518 as the structurally preferred modification site. This is the first such analysis of the HYPE-BiP interaction and offers critical insights into substrate specificity and target recognition.
    Keywords:  AMPylation; Adenylylation; BiP/GRP78; ER stress; FicD/HYPE; Hsp70; Post-translational modification; Unfolded protein response
    DOI:  https://doi.org/10.1007/s12192-021-01208-2
  3. Sci Rep. 2021 May 04. 11(1): 9528
    Periplocin and cardiac glycosides suppress the unfolded protein response.
    Muneshige Tokugawa, Yasumichi Inoue, Kan'ichiro Ishiuchi, Chisane Kujirai, Michiyo Matsuno, Masaki Ri, Yuka Itoh, Chiharu Miyajima, Daisuke Morishita, Nobumichi Ohoka, Shinsuke Iida, Hajime Mizukami, Toshiaki Makino, Hidetoshi Hayashi.
      The unfolded protein response (UPR) controls protein homeostasis through transcriptional and translational regulation. However, dysregulated UPR signaling has been associated with the pathogenesis of many human diseases. Therefore, the compounds modulating UPR may provide molecular insights for these pathologies in the context of UPR. Here, we screened small-molecule compounds that suppress UPR, using a library of Myanmar wild plant extracts. The screening system to track X-box binding protein 1 (XBP1) splicing activity revealed that the ethanol extract of the Periploca calophylla stem inhibited the inositol-requiring enzyme 1 (IRE1)-XBP1 pathway. We isolated and identified periplocin as a potent inhibitor of the IRE1-XBP1 axis. Periplocin also suppressed other UPR axes, protein kinase R-like endoplasmic reticulum kinase (PERK), and activating transcription factor 6 (ATF6). Examining the structure-activity relationship of periplocin revealed that cardiac glycosides also inhibited UPR. Moreover, periplocin suppressed the constitutive activation of XBP1 and exerted cytotoxic effects in the human multiple myeloma cell lines, AMO1 and RPMI8226. These results reveal a novel suppressive effect of periplocin or the other cardiac glycosides on UPR regulation, suggesting that these compounds will contribute to our understanding of the pathological or physiological importance of UPR.
    DOI:  https://doi.org/10.1038/s41598-021-89074-x
  4. Biol Chem. 2021 May 26. 402(6): 703-715
    Irisin ameliorates endoplasmic reticulum stress and liver fibrosis through inhibiting PERK-mediated destabilization of HNRNPA1 in hepatic stellate cells.
    Xin Liao, Wei Zhan, Rui Li, Tian Tian, Lei Yu, Qin Yang.
      Liver fibrosis is a common consequence of chronic liver diseases involved with the activation of hepatic stellate cells (HSCs) and endoplasmic reticulum (ER) stress. Irisin is a small polypeptide hormone that shows beneficial effects on metabolic disorders. The current study aimed to investigate the biological function of irisin on hepatic fibrosis. A mouse model of carbon tetrachloride (CCl4)-induced hepatic fibrosis was established. CCl4-treated mice showed elevated serum levels of AST and ALT, increased collagen accumulation, induced ER stress, and upregulated expressions of pro-fibrotic proteins in the liver compared to the controls. The administration of irisin, however, ameliorated CCl4-induced hepatic fibrosis in both cultured HSCs and mice. PKR-like ER kinase (PERK) is a key component of the ER stress-associated signaling pathway. We found that irisin treatment improved the stability of heterogeneous nuclear ribonucleoprotein A1 (HNRNPA1) via regulating the phosphorylation of PERK in mouse livers and isolated HSCs. Also, the knockdown of HNRNPA1 eliminated the hepatoprotective effects of irisin on hepatic fibrosis and ER stress. In summary, this study showed that irisin alleviated ER stress and hepatic fibrosis by inhibiting PERK-mediated HNRNPA1 destabilization, suggesting that irisin may represent a promising therapeutic strategy for patients with liver fibrosis.
    Keywords:  PKR-like ER kinase; endoplasmic reticulum stress; hepatic fibrosis; hepatic stellate cells; heterogeneous nuclear ribonucleoprotein A1; irisin
    DOI:  https://doi.org/10.1515/hsz-2020-0251
  5. Biology (Basel). 2021 Apr 29. pii: 384. [Epub ahead of print]10(5):
    The Unfolded Protein Response: An Overview.
    Adam Read, Martin Schröder.
      The unfolded protein response is the mechanism by which cells control endoplasmic reticulum (ER) protein homeostasis. Under normal conditions, the UPR is not activated; however, under certain stresses, such as hypoxia or altered glycosylation, the UPR can be activated due to an accumulation of unfolded proteins. The activation of the UPR involves three signaling pathways, IRE1, PERK and ATF6, which all play vital roles in returning protein homeostasis to levels seen in non-stressed cells. IRE1 is the best studied of the three pathways, as it is the only pathway present in Saccharomyces cerevisiae. This pathway involves spliceosome independent splicing of HAC1 or XBP1 in yeast and mammalians cells, respectively. PERK limits protein synthesis, therefore reducing the number of new proteins requiring folding. ATF6 is translocated and proteolytically cleaved, releasing a NH2 domain fragment which is transported to the nucleus and which affects gene expression. If the UPR is unsuccessful at reducing the load of unfolded proteins in the ER and the UPR signals remain activated, this can lead to programmed cell death.
    Keywords:  ATF6; ERAD; IRE1; PERK; RIDD; UPR; inactivation
    DOI:  https://doi.org/10.3390/biology10050384
  6. Mol Immunol. 2021 May 03. pii: S0161-5890(21)00140-1. [Epub ahead of print]135 294-303
    Activation of TREM-1 induces endoplasmic reticulum stress through IRE-1α/XBP-1s pathway in murine macrophages.
    Liang Dong, Cheng-Wei Tan, Peng-Jiu Feng, Fu-Bing Liu, De-Xing Liu, Jun-Jie Zhou, Yan Chen, Xin-Xin Yang, Yu-Hang Zhu, Zhao-Qiong Zhu.
      Increasing evidence suggests that endoplasmic reticulum (ER) stress activates several pro-inflammatory signaling pathways in many diseases, including acute lung injury (ALI). We have reported that blocking triggering receptor expressed on myeloid cells 1 (TREM-1) protects against ALI by suppressing pulmonary inflammation in mice with ALI induced by lipopolysaccharides (LPS). However, the molecular mechanism underlying the TREM-1-induced pro-inflammatory microenvironment in macrophages remains unclearly. Herein, we aimed to determine whether TREM-1 regulates the inflammatory responses induced by LPS associated with ER stress activation. We found that the activation of TREM-1 by a monoclonal agonist antibody (anti-TREM-1) increased the mRNA and protein levels of IL-1β, TNF-α, and IL-6 in primary macrophages. Treatment of the anti-TREM-1 antibody increased the expression of ER stress markers (ATF6, PERK, IRE-1α, and XBP-1s) in primary macrophages. While pretreatment with 4-PBA, an inhibitor of ER stress, significantly inhibited the expression of ER stress markers and pro-inflammatory cytokines and reduced LDH release. Furthermore, inhibiting the activity of the IRE-1α/XBP-1s pathway by STF-083010 significantly mitigated the increased levels of IL-1β, TNF-α, and IL-6 in macrophages treated by the anti-TREM-1 antibody. XBP-1 silencing attenuated pro-inflammatory microenvironment evoked by activation of TREM-1. Besides, we found that blockade of TREM-1 with LR12 ameliorated ER stress induced by LPS in vitro and in vivo. In conclusion, we conclude that TREM-1 activation induces ER stress through the IRE-1α/XBP-1s pathway in macrophages, contributing to the pro-inflammatory microenvironment.
    Keywords:  Endoplasmic reticulum stress; IRE-1α; Macrophages; TREM-1; XBP-1s
    DOI:  https://doi.org/10.1016/j.molimm.2021.04.023
  7. Front Plant Sci. 2021 ;12 661062
    The Role of Endoplasmic Reticulum Stress Response in Pollen Development and Heat Stress Tolerance.
    Mohan B Singh, Neeta Lohani, Prem L Bhalla.
      Endoplasmic reticulum (ER) stress is defined by a protracted disruption in protein folding and accumulation of unfolded or misfolded proteins in the ER. This accumulation of unfolded proteins can result from excessive demands on the protein folding machinery triggered by environmental and cellular stresses such as nutrient deficiencies, oxidative stress, pathogens, and heat. The cell responds to ER stress by activating a protective pathway termed unfolded protein response (UPR), which comprises cellular mechanisms targeted to maintain cellular homeostasis by increasing the ER's protein folding capacity. The UPR is especially significant for plants as being sessile requires them to adapt to multiple environmental stresses. While multiple stresses trigger the UPR at the vegetative stage, it appears to be active constitutively in the anthers of unstressed plants. Transcriptome analysis reveals significant upregulation of ER stress-related transcripts in diploid meiocytes and haploid microspores. Interestingly, several ER stress-related genes are specifically upregulated in the sperm cells. The analysis of gene knockout mutants in Arabidopsis has revealed that defects in ER stress response lead to the failure of normal pollen development and enhanced susceptibility of male gametophyte to heat stress conditions. In this mini-review, we provide an overview of the role of ER stress and UPR in pollen development and its protective roles in maintaining male fertility under heat stress conditions.
    Keywords:  endoplasmic reticulum stress; heat stress; male gametophyte; plant reproduction; pollen; pollen development; sperm cell; unfolded protein response
    DOI:  https://doi.org/10.3389/fpls.2021.661062
  8. Viruses. 2021 04 29. pii: 798. [Epub ahead of print]13(5):
    ER Stress, UPR Activation and the Inflammatory Response to Viral Infection.
    Mara Cirone.
      The response to invading pathogens such as viruses is orchestrated by pattern recognition receptor (PRR) and unfolded protein response (UPR) signaling, which intersects and converges in the activation of proinflammatory pathways and the release of cytokines and chemokines that harness the immune system in the attempt to clear microbial infection. Despite this protective intent, the inflammatory response, particularly during viral infection, may be too intense or last for too long, whereby it becomes the cause of organ or systemic diseases itself. This suggests that a better understanding of the mechanisms that regulate this complex process is needed in order to achieve better control of the side effects that inflammation may cause while potentiating its protective role. The use of specific inhibitors of the UPR sensors or PRRs or the downstream pathways activated by their signaling could offer the opportunity to reach this goal and improve the outcome of inflammation-based diseases associated with viral infections.
    Keywords:  PPRs; UPR; inflammation; viruses
    DOI:  https://doi.org/10.3390/v13050798
  9. Front Immunol. 2021 ;12 669492
    Endoplasmic Reticulum-Mitochondria Crosstalk and Beta-Cell Destruction in Type 1 Diabetes.
    Saurabh Vig, Joost M Lambooij, Arnaud Zaldumbide, Bruno Guigas.
      Beta-cell destruction in type 1 diabetes (T1D) results from the combined effect of inflammation and recurrent autoimmunity. In response to inflammatory signals, beta-cells engage adaptive mechanisms where the endoplasmic reticulum (ER) and mitochondria act in concert to restore cellular homeostasis. In the recent years it has become clear that this adaptive phase may trigger the development of autoimmunity by the generation of autoantigens recognized by autoreactive CD8 T cells. The participation of the ER stress and the unfolded protein response to the increased visibility of beta-cells to the immune system has been largely described. However, the role of the other cellular organelles, and in particular the mitochondria that are central mediator for beta-cell survival and function, remains poorly investigated. In this review we will dissect the crosstalk between the ER and mitochondria in the context of T1D, highlighting the key role played by this interaction in beta-cell dysfunctions and immune activation, especially through regulation of calcium homeostasis, oxidative stress and generation of mitochondrial-derived factors.
    Keywords:  ER stress; Type 1 diabetes (T1D); beta-cell; cytokines; endoplasmic reticulum; inflammation; mitochondria
    DOI:  https://doi.org/10.3389/fimmu.2021.669492
  10. Aging Cell. 2021 May 03. e13359
    Prohibitin depletion extends lifespan of a TORC2/SGK-1 mutant through autophagy and the mitochondrial UPR.
    Patricia de la Cruz-Ruiz, Blanca Hernando-Rodríguez, Mercedes M Pérez-Jiménez, María Jesús Rodríguez-Palero, Manuel D Martínez-Bueno, Antoni Pla, Roxani Gatsi, Marta Artal-Sanz.
      Mitochondrial prohibitins (PHB) are highly conserved proteins with a peculiar effect on lifespan. While PHB depletion shortens lifespan of wild-type animals, it enhances longevity of a plethora of metabolically compromised mutants, including target of rapamycin complex 2 (TORC2) mutants sgk-1 and rict-1. Here, we show that sgk-1 mutants have impaired mitochondrial homeostasis, lipogenesis and yolk formation, plausibly due to alterations in membrane lipid and sterol homeostasis. Remarkably, all these features are suppressed by PHB depletion. Our analysis shows the requirement of SRBP1/SBP-1 for the lifespan extension of sgk-1 mutants and the further extension conferred by PHB depletion. Moreover, although the mitochondrial unfolded protein response (UPRmt ) and autophagy are induced in sgk-1 mutants and upon PHB depletion, they are dispensable for lifespan. However, the enhanced longevity caused by PHB depletion in sgk-1 mutants requires both, the UPRmt and autophagy, but not mitophagy. We hypothesize that UPRmt induction upon PHB depletion extends lifespan of sgk-1 mutants through autophagy and probably modulation of lipid metabolism.
    Keywords:  SGK-1; UPRmt; autophagy; lipogenesis; mitochondria; prohibitin
    DOI:  https://doi.org/10.1111/acel.13359
  11. Cells. 2021 Apr 30. pii: 1067. [Epub ahead of print]10(5):
    Metformin Dysregulates the Unfolded Protein Response and the WNT/β-Catenin Pathway in Endometrial Cancer Cells through an AMPK-Independent Mechanism.
    Domenico Conza, Paola Mirra, Gaetano Calì, Luigi Insabato, Francesca Fiory, Francesco Beguinot, Luca Ulianich.
      Multiple lines of evidence suggest that metformin, an antidiabetic drug, exerts anti-tumorigenic effects in different types of cancer. Metformin has been reported to affect cancer cells' metabolism and proliferation mainly through the activation of AMP-activated protein kinase (AMPK). Here, we show that metformin inhibits, indeed, endometrial cancer cells' growth and induces apoptosis. More importantly, we report that metformin affects two important pro-survival pathways, such as the Unfolded Protein Response (UPR), following endoplasmic reticulum stress, and the WNT/β-catenin pathway. GRP78, a key protein in the pro-survival arm of the UPR, was indeed downregulated, while GADD153/CHOP, a transcription factor that mediates the pro-apoptotic response of the UPR, was upregulated at both the mRNA and protein level. Furthermore, metformin dramatically inhibited β-catenin mRNA and protein expression. This was paralleled by a reduction in β-catenin transcriptional activity, since metformin inhibited the activity of a TCF/LEF-luciferase promoter. Intriguingly, compound C, a well-known inhibitor of AMPK, was unable to prevent all these effects, suggesting that metformin might inhibit endometrial cancer cells' growth and survival through the modulation of specific branches of the UPR and the inhibition of the Wnt/β-catenin pathway in an AMPK-independent manner. Our findings may provide new insights on the mechanisms of action of metformin and refine the use of this drug in the treatment of endometrial cancer.
    Keywords:  AMPK; UPR; Wnt/β-catenin; endometrial cancer; metformin
    DOI:  https://doi.org/10.3390/cells10051067
  12. Genetics. 2020 Feb 01. 214(2): 409-418
    FSHR-1/GPCR Regulates the Mitochondrial Unfolded Protein Response in Caenorhabditis elegans.
    Sungjin Kim, Derek Sieburth.
      The mitochondrial unfolded protein response (UPRmt) is an evolutionarily conserved adaptive response that functions to maintain mitochondrial homeostasis following mitochondrial damage. In Caenorhabditis elegans, the nervous system plays a central role in responding to mitochondrial stress by releasing endocrine signals that act upon distal tissues to activate the UPRmt. The mechanisms by which mitochondrial stress is sensed by neurons and transmitted to distal tissues are not fully understood. Here, we identify a role for the conserved follicle-stimulating hormone G protein-coupled receptor, FSHR-1, in promoting UPRmt activation. Genetic deficiency of fshr-1 severely attenuates UPRmt activation and organism-wide survival in response to mitochondrial stress. FSHR-1 functions in a common genetic pathway with SPHK-1/sphingosine kinase to promote UPRmt activation, and FSHR-1 regulates the mitochondrial association of SPHK-1 in the intestine. Through tissue-specific rescue assays, we show that FSHR-1 functions in neurons to activate the UPRmt, to promote mitochondrial association of SPHK-1 in the intestine, and to promote organism-wide survival in response to mitochondrial stress. We propose that FSHR-1 functions cell nonautonomously in neurons to activate UPRmt upstream of SPHK-1 signaling in the intestine.
    Keywords:  FSHR-1; UPRmt; paraquat; sphingosine kinase
    DOI:  https://doi.org/10.1534/genetics.119.302947
  13. Front Mol Biosci. 2021 ;8 648752
    The Prognostic Value of PERK in Cancer and Its Relationship With Immune Cell Infiltration.
    Peng Wang, Liying Han, Moxin Yu, Zhengyu Cao, Xiaoning Li, Yunxia Shao, Guoping Zhu.
      Background: Protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) is a type I transmembrane protein that functions as an endoplasmic reticulum (ER) stress sensor to regulate global protein synthesis. Recent research studies suggest that PERK, as an important receptor protein of unfolded protein response, is involved in the pathogenesis of many cancers. This study aimed to investigate PERK expression and its relationship with prognosis in pan-cancer and attempted to explore the relevant mechanism of PERK involved in the regulation of cancer pathogenesis. Methods: The Oncomine and TIMER databases were used to analyze the expression of PERK between pan-cancer samples and normal samples. Survival analysis was performed using the PrognoScan, Kaplan-Meier (K-M) plotter, and UALCAN databases. Gene set enrichment analysis (GSEA) was used to perform the functional enrichment analysis of the PERK gene in breast invasive carcinoma (BRCA), head and neck squamous cell carcinoma (HNSC), and thyroid carcinoma (THCA). The TIMER database was used to investigate the correlation between PERK expression and tumor-infiltrating immune cells and analyze the relationship of PERK with marker genes of immune cells which were downloaded from the CellMarker database in BRCA, HNSC, and THCA. Results: PERK was differentially expressed in various cancers, such as breast cancer, liver cancer, lung cancer, gastric carcinoma, lymphoma, thyroid cancer, leukemia, and head and neck squamous cell carcinomas. The high expression of PERK was associated with a poor prognosis in KIRP, LGG, BRCA, and THCA and with a favorable prognosis in HNSC. The results of GSEA indicated that PERK was mainly enriched in immune-related signaling pathways in BRCA, HNSC, and THCA. Moreover, PERK expression was significant positively correlated with infiltrating levels of macrophages and dendritic cells and was strongly associated with a variety of immune markers, especially macrophage mannose receptor 1 (MRC1, also called CD206) and T-helper cells (Th). Conclusion: The high expression of PERK could promote the infiltration of multiple immune cells in the tumor microenvironment and could deteriorate the outcomes of patients with breast and thyroid cancers, suggesting that PERK as well as tumor-infiltrating immune cells could be taken as potential biomarkers of prognosis.
    Keywords:  PERK; biomarker; cancer; immune cell infiltration; prognosis
    DOI:  https://doi.org/10.3389/fmolb.2021.648752
  14. FASEB J. 2021 Jun;35(6): e21579
    Preservation of endoplasmic reticulum (ER) Ca2+ stores by deletion of inositol-1,4,5-trisphosphate receptor type 1 promotes ER retrotranslocation, proteostasis, and protein outer segment localization in cyclic nucleotide-gated channel-deficient cone photoreceptors.
    Fan Yang, Hongwei Ma, Michael R Butler, Xi-Qin Ding.
      Endoplasmic reticulum (ER) Ca2+ homeostasis relies on an appropriate balance between efflux- and influx-channel activity responding to dynamic changes of intracellular Ca2+ levels. Dysregulation of this complex signaling network has been shown to contribute to neuronal and photoreceptor death in neuro- and retinal degenerative diseases, respectively. In mice with cone cyclic nucleotide-gated (CNG) channel deficiency, a model of achromatopsia/cone dystrophy, cones display early-onset ER stress-associated apoptosis and protein mislocalization. Cones in these mice also show reduced cytosolic Ca2+ level and subsequent elevation in the ER Ca2+ -efflux-channel activity, specifically the inositol-1,4,5-trisphosphate receptor type 1 (IP3 R1), and deletion of IP3 R1 results in preservation of cones. This work investigated how preservation of ER Ca2+ stores leads to cone protection. We examined the effects of cone specific deletion of IP3 R1 on ER stress responses/cone death, protein localization, and ER proteostasis/ER-associated degradation. We demonstrated that deletion of IP3 R1 improves trafficking of cone-specific proteins M-/S-opsin and phosphodiesterase 6C to cone outer segments and reduces localization to cone inner segments. Consistent with the improved protein localization, deletion of IP3 R1 results in increased ER retrotranslocation protein expression, reduced proteasome subunit expression, reduced ER stress/cone death, and reduced retinal remodeling. We also observed the enhanced ER retrotranslocation in mice that have been treated with a chemical chaperone, supporting the connection between improved ER retrotranslocation/proteostasis and alleviation of ER stress. Findings from this work demonstrate the importance of ER Ca2+ stores in ER proteostasis and protein trafficking/localization in photoreceptors, strengthen the link between dysregulation of ER Ca2+ homeostasis and ER stress/cone degeneration, and support an involvement of improved ER proteostasis in ER Ca2+ preservation-induced cone protection; thereby identifying IP3 R1 as a critical mediator of ER stress and protein mislocalization and as a potential target to preserve cones in CNG channel deficiency.
    Keywords:  CNG channel; ER Ca2+ stores; ER stress; cone photoreceptors; inositol-1,4,5-trisphosphate receptor; retinal degeneration
    DOI:  https://doi.org/10.1096/fj.202002711R
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