Novel approaches to glaucomatous neurodegeneration, based on the integrated stress response.

Glaucoma is a progressive neurodegenerative disease characterized by the gradual loss of retinal ganglion cells (RGCs), leading to visual impairment and potential blindness.1Casson R.J. Chidlow G. Wood J.P.M. Crowston J.G. Goldberg I. Definition of glaucoma: clinical and experimental concepts.Clin. Exp. Ophthalmol. 2012; 40: 341-349Crossref PubMed Scopus (307) Google Scholar The degeneration of RGCs is often associated with increased intraocular pressure (IOP), resulting from an imbalance in fluid dynamics within the eye. Elevated IOP leads to mechanical stress and compromises blood flow to the optic nerve, thereby triggering a cascade of neurodegenerative events. The various types of glaucoma are the major cause of irreversible vision loss worldwide.2Allison K. Patel D. Alabi O. Epidemiology of Glaucoma: The Past, Present, and Predictions for the Future.Cureus. 2020; 12: e11686PubMed Google Scholar Among a number of cellular mechanisms of importance to a variety of illnesses, endoplasmic reticulum (ER) stress has been studied in the context of neurodegenerative diseases, including glaucoma. Cells experience ER stress when there is an imbalance between the protein-folding capacity of the ER and the demand for proper protein folding, which activates the unfolded protein response (UPR). This adaptive response, composed by the PERK, ATF6, and IRE1 pathways, alleviates ER stress and restores protein homeostasis. However, non-resolved ER stress induces cell death.3Hetz C. Zhang K. Kaufman R.J. Mechanisms, regulation and functions of the unfolded protein response.Nat. Rev. Mol. Cell Biol. 2020; 21: 421-438Crossref PubMed Scopus (756) Google Scholar CHOP, one of the genes transcriptionally regulated by ATF4, is related to ER stress-induced cell death. In response to ER stress, PERK is activated and phosphorylates eIF2alpha, inhibiting eIF2B-dependent protein synthesis, which, in turn, upregulates the translation of ATF4. Among studies of neurodegeneration, a growing number of publications highlight the critical importance of ER stress in axonopathies and identify ATF4 and CHOP as potential targets for neuroprotection.4Hughes D. Mallucci G.R. The unfolded protein response in neurodegenerative disorders–therapeutic modulation of the PERK pathway.FEBS J. 2019; 286: 342-355Crossref PubMed Scopus (111) Google Scholar,5Kasetti R.B. Patel P.D. Maddineni P. Patil S. Kiehlbauch C. Millar J.C. Searby C.C. Raghunathan V. Sheffield V.C. Zode G.S. ATF4 leads to glaucoma by promoting protein synthesis and ER client protein load.Nat. Commun. 2020; 11: 5594Crossref PubMed Scopus (31) Google Scholar In a study published in Molecular Therapy – Nucleic Acids, Fang et al.6Fang F. Liu P. Huang H. Feng X. Li L. Sun Y. Kaufman R.J. Hu Y. RGC-Specific ATF4 and/or CHOP Deletion Rescues Glaucomatous Neurodegeneration and Visual Function.Mol. Ther. Nucleic Acids. 2023; 33: 286-295Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar investigated the role of the the ATF4-CHOP axis of the UPR in RGCs subject to models of glaucoma. The group had previously shown that either germline deletion of CHOP or systemic administration of inhibitors of ATF4/CHOP protected RGCs and their axons in a variety of mouse models of optic neuropathy. In the present work, they took a step further and tested the autonomous effects of RGC-specific deletion of ATF4 and CHOP upon both traumatic optic nerve injury and models of glaucoma. This new study contains a series of genetically modified mouse models, small molecule inhibitors, and CRISPR-mediated knockdown, with the aim to elucidate the potential neuroprotective effects of targeting the ATF4-CHOP axis. The researchers used genetically modified mice with floxed ATF4 and CHOP genes and specifically deleted those molecules in RGCs, which promoted the survival of the soma and axon of the RGCs, both after optic nerve crush and in a mouse model of glaucoma induced by ocular hypertension. Interestingly, the combined deletion of ATF4 and CHOP yielded better results than single deletions, suggesting a synergistic effect of neuroprotection. The latter finding highlights the significance of targeting both molecules simultaneously to enhance therapeutic outcomes. Overall, the study of Fang et al.6Fang F. Liu P. Huang H. Feng X. Li L. Sun Y. Kaufman R.J. Hu Y. RGC-Specific ATF4 and/or CHOP Deletion Rescues Glaucomatous Neurodegeneration and Visual Function.Mol. Ther. Nucleic Acids. 2023; 33: 286-295Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar provides valuable insights into the potential neuroprotective effects of targeting the ATF4-CHOP axis in glaucoma. In addition, the authors tested the effects of ISRIB, an inhibitor of the integrated stress response (ISR) that binds eIF2B and increases its activity.7Sidrauski C. McGeachy A.M. Ingolia N.T. Walter P. The small molecule ISRIB reverses the effects of eIF2α phosphorylation on translation and stress granule assembly.Elife. 2015; 4e05033Crossref Scopus (350) Google Scholar,8Zyryanova A.F. Weis F. Faille A. Alard A.A. Crespillo-Casado A. Sekine Y. Harding H.P. Allen F. Parts L. Fromont C. et al.Binding of ISRIB reveals a regulatory site in the nucleotide exchange factor eIF2B.Science. 2018; 359: 1533-1536Crossref PubMed Scopus (107) Google Scholar The central core of ISR is the phosphorylation of eIF2alpha, which inhibits eIF2B activity, therefore blocking protein synthesis and upregulating ATF4 mRNA translation. In addition to PERK, which is activated by ER stress, the ISR is also composed of the eIF2alpha kinases PKR, HRI, and GCN2, which are activated by distinct stress inputs.9Pakos-Zebrucka K. Koryga I. Mnich K. Ljujic M. Samali A. Gorman A.M. The integrated stress response.EMBO Rep. 2016; 17: 1374-1395Crossref PubMed Scopus (1193) Google Scholar Fang and co-workers showed that topical application of ISRIB significantly preserved visual function, RGC soma, and axons in the glaucoma model.6Fang F. Liu P. Huang H. Feng X. Li L. Sun Y. Kaufman R.J. Hu Y. RGC-Specific ATF4 and/or CHOP Deletion Rescues Glaucomatous Neurodegeneration and Visual Function.Mol. Ther. Nucleic Acids. 2023; 33: 286-295Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar In addition, the authors examined the GADD45a gene, which is also transcriptionally regulated by ATF4 in response to diverse stress.10Jiang H.Y. Jiang L. Wek R.C. The eukaryotic initiation factor-2 kinase pathway facilitates differential GADD45a expression in response to environmental stress.J. Biol. Chem. 2007; 282: 3755-3765Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar Here again, Fang and colleagues showed that CRISPR-mediated knockdown of Gadd45a in the glaucoma model protected RGCs and preserved visual function.6Fang F. Liu P. Huang H. Feng X. Li L. Sun Y. Kaufman R.J. Hu Y. RGC-Specific ATF4 and/or CHOP Deletion Rescues Glaucomatous Neurodegeneration and Visual Function.Mol. Ther. Nucleic Acids. 2023; 33: 286-295Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar The identification, in this study, of potential therapeutic targets of significant clinical importance paves the way for future preclinical and clinical investigations. By using a combination of genetically modified mouse models, small molecule inhibitors, and CRISPR-mediated knockdown, the group demonstrated the promise of these strategies for treating optic neuropathies and possibly other neurodegenerative diseases. The current findings add to a growing body of literature that supports the modulation of both ER stress response and ISR and encourages further research and development of novel treatments targeting these stress response pathways. RGCs in glaucoma may benefit from future studies of both ISR-eIF2alpha kinases and ISR-induced stress granules,7Sidrauski C. McGeachy A.M. Ingolia N.T. Walter P. The small molecule ISRIB reverses the effects of eIF2α phosphorylation on translation and stress granule assembly.Elife. 2015; 4e05033Crossref Scopus (350) Google Scholar as well as of the role of ATF6 and IRE1, two other branches of the UPR. Analysis of gene expression pattern after blocking CHOP and ATF4 may also be of interest to identify pathways involved with neuroprotection and neurodegeneration. Finally, one should also be aware of the high proportion of comorbidities associated with glaucoma, such as cataract,11Wang X. Yao S. Wang M. Cao G. Chen Z. Huang Z. Wu Y. Han L. Xu B. Hu Y. Multimorbidity among Two Million Adults in China.Int. J. Environ. Res. Publ. Health. 2020; 17: 3395Crossref PubMed Scopus (30) Google Scholar diabetes mellitus,12Zhou M. Wang W. Huang W. Zhang X. Diabetes Mellitus as a Risk Factor for Open-Angle Glaucoma: A Systematic Review and Meta-Analysis.PLoS One. 2014; 9e102972Google Scholar retinal vasculature dysfunction,13Wareham L.K. Calkins D.J. The Neurovascular Unit in Glaucomatous Neurodegeneration.Front. Cell Dev. Biol. 2020; 8: 452Crossref PubMed Scopus (48) Google Scholar,14Dascalu A.M. Stana D. Nicolae V.A. Cirstoveanu C. Vancea G. Serban D. Socea B. Association between vascular comorbidity and glaucoma progression: A four-year observational study.Exp. Ther. Med. 2021; 21: 283Crossref PubMed Google Scholar,15Haider A.A. Rex T.S. Wareham L.K. cGMP Signaling in the Neurovascular Unit-Implications for Retinal Ganglion Cell Survival in Glaucoma.Biomolecules. 2022; 12: 1671Crossref PubMed Scopus (2) Google Scholar obesity, asthma,16Garcia-Villanueva C. Milla E. Bolarin J.M. García-Medina J.J. Cruz-Espinosa J. Benítez-Del-Castillo J. Salgado-Borges J. Hernández-Martínez F.J. Bendala-Tufanisco E. Andrés-Blasco I. et al.Impact of Systemic Comorbidities on Ocular Hypertension and Open-Angle Glaucoma, in a Population from Spain and Portugal.J. Clin. Med. 2022; 11: 5649Crossref PubMed Scopus (4) Google Scholar as well as the active role of Müller cells and astrocytes in circumstances of comorbidity with glaucoma.17Yao X. Yang H. Han H. Kou X. Jiang Y. Luo M. Zhou Y. Wang J. Fan X. Wang X. et al.Genome-wide analysis of genetic pleiotropy and causal genes across three age-related ocular disorders.Hum. Genet. 2023; 142: 507-522Crossref PubMed Scopus (0) Google Scholar Importantly, both the ISR and the UPR have also been identified in the pathogenesis of multiple diseases inclusive of diabetes mellitus, vascular dysfunction, obesity, and metabolic dysfunction.18Costa-Mattioli M. Walter P. The integrated stress response: From mechanism to disease.Science. 2020; 368: 5314Crossref Scopus (166) Google Scholar Such complexities of comorbidities associated with glaucoma are likely to need further efforts besides RGC-specific targeted approaches. Funding for all authors is provided by CNPq, CAPES, FINEP, and UFRJ. L.B.C., H.P.-S., and R.L. conceived and wrote this commentary. The authors have no conflict of interest to declare.