The role of MIP in lens fiber cell membrane transport

MTP has been hypothesized to be a gap junction protein, a membrane ion channel, a membrane water channel and a facilitator of glycerol transport and metabolism. These possible roles have been indirectly suggested by the localization of MIP in lens gap junctional plaques and the properties of MIP when reconstituted into artificial membranes or exogenously expressed in oocytes. We have examined lens fiber cells to see if these functions are present and whether they are affected by a mutation of MIP found in Cat(Fr) mouse lens. Of these five hypothesized functions, only one, the role of water channel, appears to be true of fiber cells in situ. Based on the rate of volume change of vesicles placed in a hypertonic solution, fiber cell membrane lipids have a low water permeability (p(H2O)) on the order of 1 mu m/sec whereas normal fiber cell membrane p(H2O) was 17 mu m/sec frog, 32 mu m/sec rabbit and 43 mu m/sec mouse. Cat(Fr) mouse lens fiber cell p(H2O) was reduced by 13 mu m/sec for heterozygous and 30 mu m/sec for homozygous mutants when compared to wild type, Lastly, when expressed in oocytes, the p(H2O) conferred by MIP is not sensitive to Hg2+ whereas that of CHIP28 (AQP1) is blocked by Hg2+. The fiber cell membrane p(H2O) was also not sensitive to Hg2+ whereas lens epithelial cell p(H2O) (136 mu m/sec in rabbit) was blocked by Hg2+. With regard to the other hypothesized roles, fiber cell membrane or lipid vesicles had a glycerol permeability on the order of 1 nm/sec, an order of magnitude less than that conferred by MIP when expressed in oocytes. Impedance studies were employed to determine gap junctional coupling and fiber cell membrane conductance in wild-type and heterozygous Cat(Fr) mouse lenses. There was no detectable difference in either coupling or conductance between the wild-type and the mutant lenses.