Effects of mechanical stretching on trabecular matrix metalloproteinases

PURPOSE. The homeostatic mechanisms responsible for intraocular pressure (I OP) regulation are not understood. Studies were conducted to evaluate the h ypothesis that trabecular mesh work (TM) cells sense increases in IOP as st retching or distortion of their extracellular matrix (ECM) and respond by i ncreasing ECM turnover enzymes. METHODS. Flow rates were increased in perfused human anterior segment organ cultures and the matrix metalloproteinase (MMP) levels and IOP were evalua ted. Human TMs in stationary anterior segment organ culture were mechanical ly stretched, and MMP levels were analyzed. TM cells were grown on membrane s, which were then stretched, and MMP levels were evaluated. Western immuno blots, zymography, and confocal immunohistochemistry were used to evaluate changes in MMPs and their tissue inhibitors, the TIMPs. RESULTS. Doubling the flow rate in perfused human organ cultures increased gelatinase A levels in the perfusate by 30% to 30% without affecting gelati nase B or stromelysin levels. Immediately after doubling the flow rate, the measured IOP doubled. However, over the next few days the IOP gradually re turned to the initial level, although the flow rate was maintained at doubl e the initial value. Stretching stationary organ cultures or stretching TM cells grown on membranes resulted in similar increases in gelatinase A with out changes in gelatinase B or stromelysin levels. The gelatinase A increas es occurred between 24 and 72 hours and were approximately proportional to the degree of stretching. Although coating the membranes with different ECM molecule affected the gelatinase A response, the optimum response occurred when the cells had been grown long enough to produce their own ECM. By Wes tern immunoblot and confocal immunohistochemistry, the stretch-induced incr eases in gelatinase A were accompanied by strong decreases in TIMP-2 levels and moderate increases in one membrane type MMP, MT1-MMP. After mechanical stretching of the membrane, gelatinase A, MT1-MMP and TIMP-2 all exhibited a similar punctate immunostaining pattern over the TM. cell surface. CONCLUSIONS. These results are compatible with the hypothesis that elevatio ns in IOP are sensed by TM cells as ECM stretch/distortion. TM cells respon d by increasing gelatinase A and MT1-MMP, while decreasing TIMP-2 levers. T his will increase ECM turnover rates, reduce the trabecular resistance to a queous humor outflow, and restore normal IOP levels. This hypothesis provid es a regulatory feedback mechanism for IOP homeostasis.