Varadaraj K, Kushmerick C, Baldo G J, Bassnett S, Shiels A, Mathias R T
Department of Physiology and Biophysics, SUNY at Stony Brook, Stony Brook, NY 11794-8661, USA.
J Membr Biol. 1999 Aug 1;170(3):191-203. doi: 10.1007/s002329900549.
MIP 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 CatFr 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 (pH2O) on the order of 1 micron/sec whereas normal fiber cell membrane pH2O was 17 micron/sec frog, 32 micron/sec rabbit and 43 micron/sec mouse. CatFr mouse lens fiber cell pH2O was reduced by 13 micron/sec for heterozygous and 30 micron/sec for homozygous mutants when compared to wild type. Lastly, when expressed in oocytes, the pH2O conferred by MIP is not sensitive to Hg2+ whereas that of CHIP28 (AQP1) is blocked by Hg2+. The fiber cell membrane pH2O was also not sensitive to Hg2+ whereas lens epithelial cell pH2O (136 micron/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 CatFr mouse lenses. There was no detectable difference in either coupling or conductance between the wild-type and the mutant lenses.
MIP被推测为一种间隙连接蛋白、一种膜离子通道、一种膜水通道以及甘油运输和代谢的促进剂。这些可能的作用已通过MIP在晶状体间隙连接斑中的定位以及MIP在重构到人工膜中或在卵母细胞中外源表达时的特性间接表明。我们已经检查了晶状体纤维细胞，以确定这些功能是否存在以及它们是否受到在CatFr小鼠晶状体中发现的MIP突变的影响。在这五种推测的功能中，只有一种，即水通道的作用，似乎在原位纤维细胞中是真实存在的。根据置于高渗溶液中的囊泡体积变化速率，纤维细胞膜脂质的水渗透率（pH2O）较低，约为1微米/秒，而正常纤维细胞膜pH2O在青蛙中为17微米/秒，在兔子中为32微米/秒，在小鼠中为43微米/秒。与野生型相比，CatFr小鼠晶状体纤维细胞的pH2O对于杂合突变体降低了13微米/秒，对于纯合突变体降低了30微米/秒。最后，当在卵母细胞中表达时，MIP赋予的pH2O对Hg2+不敏感，而CHIP28（AQP1）的pH2O被Hg2+阻断。纤维细胞膜pH2O对Hg2+也不敏感，而晶状体上皮细胞pH2O（兔子中为136微米/秒）被Hg2+阻断。关于其他推测的作用，纤维细胞膜或脂质囊泡的甘油渗透率约为1纳米/秒，比MIP在卵母细胞中表达时赋予的甘油渗透率低一个数量级。采用阻抗研究来确定野生型和杂合CatFr小鼠晶状体中的间隙连接偶联和纤维细胞膜电导。野生型和突变型晶状体在偶联或电导方面均未检测到差异。
