Lens Cell Proliferation, Differentiation and Development Require K6 on Ubiquitin

Abstract

Purpose: : Many cellular processes, such as cell cycle control, differentiation, signal transduction, transcription and removal of obsolete proteins are regulated by ubiquitin proteasome pathway (UPP). Surprisingly little is known about the ubiquitin molecule itself. There are 7 lysines on ubiquitin (Ub). The least understood is K6. For degradation of a substrate to precede, multiple Ubs, often linked to each other in isopeptide bonds which utilize K48 of one ubiquitin and the carboxyl terminus of another, are attached to the substrate.

Methods: : In previous studies, in order to investigate the role of Ub in lens proliferation we expressed K6W-Ub (a mutant that precludes lysine 6 linkages on ubiquitin and acts as dominant-negative inhibitor of the UPP) in human lens epithelial cells under control of an α-crytallin promoter.

Results: : We found that cell proliferation is inhibited, mainly due to the delay of the cell cycle at the G₂/M phase and impaired UPP-dependent degradation. Mice which express higher levels of K6W-Ub, even against endogenous wt ubiquitin, show severe cataracts with accumulation of high mass K6W-Ub containing conjugates, protein aggregation and decreased protein degradation. Newer data indicate that K6W-Ub expression inhibits lens cell proliferation and lens fiber differentiation, specifically delayed gamma-crystallin expression. This delays dilution of p27, phosphorylation of lamin, disassembly of the nuclear membrane and entry of DNase IIβ into the nucleus. Thus, fibers are abnormal and denucleation is impaired.

Conclusions: : This data establishes an unknown role for a UPP, specifically K6 on Ub, in lens differentiation (denucleation), development and homeostasis.

Keywords: differentiation • cataract • proteolysis