HSP90ß prevents aging-related cataract formation through regulation of the charged multivesicular body protein (CHMP4B) and p53.

Cataract is a leading ocular disease causing global blindness. The mechanism of cataractogenesis has not been well defined. Here, we demonstrate that the heat shock protein 90ß (HSP90ß) plays a fundamental role in suppressing cataractogenesis. HSP90ß is the most dominant HSP in normal lens, and its constitutive high level of expression is largely derived from regulation by Sp1 family transcription factors. More importantly, HSP90ß is significantly down-regulated in human cataract patients and in aging mouse lenses, whereas HSP90ß silencing in zebrafish causes cataractogenesis, which can only be rescued by itself but not other HSP90 genes. Mechanistically, HSP90ß can directly interact with CHMP4B, a newly-found client protein involved in control of cytokinesis. HSP90ß silencing causes upregulation of CHMP4B and another client protein, the tumor suppressor p53. CHMP4B upregulation or overexpression induces excessive division of lens epithelial cells without proper differentiation. As a result, these cells were triggered to undergo apoptosis due to activation of the p53/Bak-Bim pathway, leading to cataractogenesis and microphthalmia. Silence of both HSP90ß and CHMP4B restored normal phenotype of zebrafish eye. Together, our results reveal that HSP90ß is a critical inhibitor of cataractogenesis through negative regulation of CHMP4B and the p53-Bak/Bim pathway.

MAB21L1 promotes survival of lens epithelial cells through control of αB-crystallin and ATR/CHK1/p53 pathway.

The male abnormal gene family 21 (mab21), was initially identified in C. elegans. Since its identification, studies from different groups have shown that it regulates development of ocular tissues, brain, heart and liver. However, its functional mechanism remains largely unknown. Here, we demonstrate that Mab21L1 promotes survival of lens epithelial cells. Mechanistically, Mab21L1 upregulates expression of αB-crystallin. Moreover, our results show that αB-crystallin prevents stress-induced phosphorylation of p53 at S-20 and S-37 through abrogating the activation of the upstream kinases, ATR and CHK1. As a result of suppressing p53 activity by αB-crystallin, Mab21L1 downregulates expression of Bak but upregulates Mcl-1 during stress insult. Taken together, our results demonstrate that Mab21L1 promotes survival of lens epithelial cells through upregulation of αB-crystallin to suppress ATR/CHK1/p53 pathway.

MYPT1/PP1-Mediated EZH2 Dephosphorylation at S21 Promotes Epithelial-Mesenchymal Transition in Fibrosis through Control of Multiple Families of Genes.

The methyltransferase EZH2 plays an important role in regulating chromatin conformation and gene transcription. Phosphorylation of EZH2 at S21 by AKT kinase suppresses its function. However, protein phosphatases responsible for the dephosphorylation of EZH2-S21 remain elusive. Here, it is demonstrated that EZH2 is highly expressed in the ocular lens, and AKT-EZH2 axis is important in TGFß-induced epithelial-mesenchymal transition (EMT). More importantly, it is identified that MYPT1/PP1 dephosphorylates EZH2-S21 and thus modulates its functions. MYPT1 knockout accelerates EMT, but expression of the EZH2-S21A mutant suppresses EMT through control of multiple families of genes. Furthermore, the phosphorylation status and gene expression modulation of EZH2 are implicated in control of anterior subcapsular cataracts (ASC) in human and mouse eyes. Together, the results identify the specific phosphatase for EZH2-S21 and reveal EZH2 dephosphorylation control of several families of genes implicated in lens EMT and ASC pathogenesis. These results provide important novel information in EZH2 function and regulation.

The E3 Ligase PIAS1 Regulates p53 Sumoylation to Control Stress-Induced Apoptosis of Lens Epithelial Cells Through the Proapoptotic Regulator Bax.

Protein sumoylation is one of the most important post-translational modifications regulating many biological processes (Flotho A & Melchior F. 2013. Ann Rev. Biochem. 82:357-85). Our previous studies have shown that sumoylation plays a fundamental role in regulating lens differentiation (Yan et al., 2010. PNAS, 107(49):21034-9.; Gong et al., 2014. PNAS. 111(15):5574-9). Whether sumoylation is implicated in lens pathogenesis remains elusive. Here, we present evidence to show that the protein inhibitor of activated STAT-1 (PIAS1), a E3 ligase for sumoylation, is implicated in regulating stress-induced lens pathogenesis. During oxidative stress-induced cataractogenesis, expression of PIAS1 is significantly altered at both mRNA and protein levels. Upregulation and overexpression of exogenous PIAS1 significantly enhances stress-induced apoptosis. In contrast, silence of PIAS1 with CRISPR/Cas9 technology attenuates stress-induced apoptosis. Mechanistically, different from other cells, PIAS1 has little effect to activate JNK but upregulates Bax, a major proapoptotic regulator. Moreover, Bax upregulation is derived from the enhanced transcription activity of the upstream transcription factor, p53. As revealed previously in other cells by different laboratories, our data also demonstrate that PIAS1 promotes SUMO1 conjugation of p53 at K386 residue in lens epithelial cells and thus enhances p53 transcription activity to promote Bax upregulation. Silence of Bax expression largely abrogates PIAS1-mediated enhancement of stress-induced apoptosis. Thus, our results demonstrated that PIAS1 promotes oxidative stress-induced apoptosis through positive control of p53, which specifically upregulates expression of the downstream proapoptotic regulator Bax. As a result, PIAS1-promoted apoptosis induced by oxidative stress is implicated in lens pathogenesis.

Aging-dependent loss of GAP junction proteins Cx46 and Cx50 in the fiber cells of human and mouse lenses accounts for the diminished coupling conductance.

The homeostasis of the ocular lens is maintained by a microcirculation system propagated through gap junction channels. It is well established that the intercellular communications of the lens become deteriorative during aging. However, the molecular basis for this change in human lenses has not been well defined. Here, we present evidence to show that over 90% of Cx46 and Cx50 are lost in the fiber cells of normal human lenses aged 50 and above. From transparent to cataractous lenses, while Cx43 was upregulated, both Cx46 and Cx50 were significantly down-regulated in the lens epithelia. During aging of mouse lenses, Cx43 remained unchanged, but both Cx46 and Cx50 were significantly downregulated. Under oxidative stress treatment, mouse lenses develop in vitro cataractogenesis. Associated with this process, Cx43 was significantly upregulated, in contrast, Cx46 and Cx50 were sharply downregulated. Together, our results for the first time reveal that downregulation in Cx46 and Cx50 levels appears to be the major reason for the diminished coupling conductance, and the aging-dependent loss of Cx46 and Cx50 promotes senile cataractogenesis.