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 μM protease substrate, (t-butoxycarbonyl-leu-met-7-amino-4-chloromethylcoumarin: BOC-Leu-Met-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 × 10^-3 M, 10^-6 M, and 10^-8 M [Ca²⁺]_o were: 24.7 ± 1.3, 53.0 ± 2.8, and more than 120 minutes, respectively. A significant increase in T_g (∼ 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 [Ca²⁺]_i, or the protease substrate to competitively inhibit degradation of cellular proteins. In the presence of Ringer's solution containing 2 × 10^-3 M [Ca²⁺]_o and 0.5 mM of the cysteine protease inhibitor, leupeptin T_g increased to 100 minutes, without affecting [Ca²⁺]_i. The proteolytic activity of fiber cells in Ringer's solution containing 10^-6 M and 2 × 10^-3 M [Ca²⁺]_o increased by approximately 7- and 12-fold, respectively compared with sucrose-EDTA solution or Ringer's solution containing 10^-8 M [Ca²⁺]_o. This increase in proteolytic activity was inhibited by leupeptin. CONCLUSIONS. Elevation of calcium in the medium results in a proportionate increase in [Ca²⁺]_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 [Ca²⁺]_i. These results suggest that globulization of isolated fiber cells in physiological salt solutions is mediated by Ca²⁺-activated protease(s).