Resource: National Key Laboratory of Ophthalmic Disease Prevention and Treatment
Proofread by: Jiawei Wang
Reviewed by: Lingyi Liang
Edited by: Xianjing Wei
Cataract is the leading cause of blindness over the world, and it is derived from genetic mutations, stress factors, and aging. Although the molecular mechanisms mediating cataractogenesis are complicated, the common molecular characteristics is the protein damage imposed by the above factors. The damaged proteins become aggregated and cause light scattering, thus cataracts occur. The alpha-crystallin proteins, being part of the lens structure proteins, belong to the small heat shock protein family, and bear certain aspects of the molecular chaperone function. By binding to the damaged proteins, they help to increase the solubility of the damaged proteins. However, alpha-crystallins lack the enzyme activity of ATP hydrolase and are unable to repair the bound damaged proteins. As more and more damaged proteins are bound to alpha-crystallins, they ultimately become exhausted, and the large molecular complexes become insoluble, leading to cataractogenesis. Therefore, maintaining lens transparency requires the function of a truly efficient and reparative molecular chaperone.
Professor David Wan-Cheng Li's research team discovered that the heat shock protein HSP90 beta (HSP90β) is the most important molecular chaperone in the lens and plays an indispensable role in maintaining the transparent optical properties of the ocular lens. These exciting results were recently published in the Proceedings of National Academy of Sciences of USA (CAS Zone 1 Journal with an impact factor of 11.1), entitled as "HSP90β prevents aging-related cataract formation through regulation of the charged multivesicular body protein (CHMP4B) and p53” (see figure below). This elegant work identified the key factors for maintaining the lens transparency and elucidated an important molecular mechanism guarding cataractogenesis. More importantly, the present study pointed out an important research direction for the prevention and treatment of cataracts.
In this study, the researchers utilized the WES technology from the Central Research Facilities in the Zhongshan Ophthalmic Center, Sun Yat-sen University, and examined the expression patterns of multiple HSP90 subtypes (HSP90a, HSP90β, TRAP-1 and GRP94) and also different HSPs (HSP90, HSP70, HSP60, and HSP40) in normal transparent human lenses and lens capsular epithelial samples of over 100 cataract patients, and the results showed that HSP90β is the heat shock protein with the highest level of expression. More importantly, such high level of HSP90β expression is significantly downregulated in cataract patients. In addition, the authors also examined the expression patterns of the 4 HSP90 subtypes and different HSPs in young and aging mouse lenses. It was found that HSP90β displayed the highest level of expression in young mouse lenses (1-month old) but drastically downregulated in old mouse lenses (8- and 14-month old). To understand why HSP90β displayed such high level of expression in normal human lenses, the core and proximal promoter of the HSP90β gene was examined, and multiple copies of cis-elements for Sp1 and Sp4 binding were identified. Gel mobility shifting assays, luciferase reporting gene activity analysis and ChIP assays were used to verify the strong positive control of the HSP90β gene expression by the two transcriptional factors, Sp1 and Sp4. In Zebrafish, silence of the HSP90β gene expression but not other subtypes of HSP90 lead to microphthalmia and cataract, and this defect can only be rescued by HSP90β gene itself but not by other HSP90 subtypes.
Subsequently, to further explore the potential mechanisms underlying the cataract formation caused by HSP90β reduction, the researchers used mass spectrometry analysis and identified a novel client protein (CHMP4B) of HSP90β. Chmp4b is a pathogenic gene and its anomaly causes congenital cataract formation. Silence HSP90β by morpholino oligos induced upregulation of CHMP4B in Zebrafish, which led to a significant increase in the level of lens epithelial cell proliferation. The excess numbers of lens epithelial cells derived from CHMP4B upregulation were induced to undergo apoptosis due to upregulation of p53, another client protein of HSP90β. The phenotype of microphthalmia and cataract due to HSP90β silence in zebrafish were rescued by double knockdown of HSP90β and CHMP4B. In summary, a series of experimental studies with patient samples, cultured cells and zebrafish models, Dr. Li’s team proved that HSP90β, through interaction with CHMP4B and p53, maintains normal proliferation of lens epithelial cells and suppresses cell apoptosis to protect normal development, and guard cataractogenesis (see figure below).
Jia-Ling Fu, Shu-Yu Zheng, Yan Wang, Xue-Bin Hu and Yuan Xiao are the co-first authors of the article, David Wan-Cheng Li is the corresponding author, and Zhongshan Ophthalmic Center, Sun Yat-sen University and the National Key Laboratory of Ophthalmic Disease Prevention and Treatment are the academic units hosting this research project. This study was partially supported by the National Natural Science Foundation of China (#81970787; #82271071; #81970784; and #82000876), the Key Project of Guangdong Province Natural Science Foundation and Guangzhou City Natural Science Foundation Joint Program (2019B1515120014), and the Fund from the National Key Laboratory of Ophthalmic Disease Prevention and Treatment (#3030901010111;# 3030901010110).
Original Article Online Link: https://www.pnas.org/eprint/WYJTWGSXBNVTQRFCUWXE/full
Online Link: https://www.pnas.org/eprint/WYJTWGSXBNVTQRFCUWXE/full
