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The role of MIP in lens fiber cell membrane transport
K Varadaraj1, C Kushmerick, G J Baldo
1Department of Physiology and Biophysics, SUNY at Stony Brook, Stony Brook, NY 11794-8661, USA.
Abstract:
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.
Insights
The major intrinsic protein (MIP) functions as a water channel in lens fiber cells, but not as a glycerol transporter or gap junction protein. A mutation in MIP affects water permeability in the CatFr mouse lens.
Area of Science:
- Membrane biophysics
- Ocular physiology
- Molecular biology
Background:
- The major intrinsic protein (MIP) is hypothesized to function as a gap junction protein, ion channel, water channel, and facilitator of glycerol transport.
- Previous studies suggested these roles based on MIP's localization and properties in artificial systems.
Purpose of the Study:
- To investigate the in situ functions of MIP in lens fiber cells.
- To determine if a mutation in MIP, observed in the CatFr mouse, affects these hypothesized functions.
Main Methods:
- Examined lens fiber cells for water and glycerol permeability.
- Utilized hypertonic solutions to measure vesicle volume changes.
- Assessed the effect of mercury ions (Hg2+) on water permeability.
- Employed impedance studies to evaluate gap junctional coupling and membrane conductance.
Main Results:
- MIP functions as a water channel in lens fiber cells in situ.
- MIP's role in glycerol transport and gap junction function was not supported in fiber cells.
- The CatFr mouse mutation significantly reduced water permeability in lens fiber cells.
- MIP-mediated water transport was insensitive to Hg2+, unlike CHIP28 (AQP1).
Conclusions:
- MIP's primary function in lens fiber cells is water channel activity.
- The CatFr mutation impairs MIP's water channel function, impacting lens water permeability.
- MIP does not appear to function as a glycerol transporter or gap junction protein in lens fiber cells.