Role of calcium-dependent protease(s) in globulization of isolated rat lens cortical fiber cells.

PURPOSE To investigate the role of calcium-activated proteases in calcium-dependent disintegrative globulization of isolated rat lens cortex fiber cells. METHODS Rat lens fiber cells were isolated and plated on coverslips at the bottom of a temperature-controlled chamber. The fiber cells were incubated with 10 microM protease substrate, (t-butoxycarbonyl-leu-met-7-amino-4-chloromethylcoumarin:BOC-Leu-M et- CMAC) and the proteolytic activity in the fiber cells was determined by observing the increase in fluorescence, using an excitation wavelength of 360 nm, and measuring emission at 410 nm. Free intracellular calcium was measured using the cell-permeable calcium indicator Fluo-3-AM, and the globulization time (T(g)) was determined using image analysis. RESULTS T:(g) of fiber cells superfused with Ringer's solution containing 2 x 10(-)(3) M, 10(-)(6) M, and 10(-)(8) M [Ca(2+)](o) were: 24.7 +/- 1.3, 53.0 +/- 2.8, and more than 120 minutes, respectively. A significant increase in T:(g) ( approximately 95 minutes) was observed when the fibers were preincubated with acetoxymethyl ester of 1,2-bis (2-amino-phenoxy) ethane N:, N:, N:, N:-tetra-acetic acid (BAPTA-AM) to buffer changes in [Ca(2+)](i), or the protease substrate to competitively inhibit degradation of cellular proteins. In the presence of Ringer's solution containing 2 x 10(-)(3) M [Ca(2+)](o) and 0.5 mM of the cysteine protease inhibitor, leupeptin, T:(g) increased to 100 minutes, without affecting [Ca(2+)](i). The proteolytic activity of fiber cells in Ringer's solution containing 10(-)(6) M and 2 x 10(-)(3) M [Ca(2+)](o) increased by approximately 7- and 12-fold, respectively, compared with sucrose-EDTA solution or Ringer's solution containing 10(-)(8) M [Ca(2+)](o). This increase in proteolytic activity was inhibited by leupeptin. CONCLUSIONS Elevation of calcium in the medium results in a proportionate increase in [Ca(2+)](i) and the proteolytic activity in isolated lens fiber cells. The increase in the proteolytic activity is accompanied by an increase in the rate of globulization of the fiber cells. Inhibition of the proteolytic activity by leupeptin increases T:(g) without affecting the gain in [Ca(2+)](i). These results suggest that globulization of isolated fiber cells in physiological salt solutions is mediated by Ca(2+)-activated protease(s).