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M.J. Marian, H. Li, C.A. Paterson, D. Borchman; Plasma Membrane Ca+2-ATPase Expression in the Human Lens . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4476.
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Purpose: Plasma membrane Ca+2-ATPase (PMCA) plays a pivotal role in maintaining a low calcium concentration in the cell. A low intracellular calcium concentration is essential for lens clarity since a correlation between elevated calcium levels and lens opacity has been established. Four different genes code for four PMCA isoforms and alternative splicing of the transcript generates additional isoforms. The tissue distribution of PMCA isoforms varies significantly. PMCA1 and PMCA4 are "housekeeping isoforms" because of their expression in a wide range of tissues. PMCA2 is expressed mostly in brain and heart, while PMCA3 expression is restricted in brain and skeletal muscle. In previous studies, we have shown that all four PMCA isoforms were expressed in bovine lens epithelium. However, PMCA isoform expression in human lens epithelium remains to be explored and is the focus of this study. Methods: Human lenses were obtained from The Kentucky Lions Eye Banks in Lexington and Louisville Kentucky. Lens epithelium, cortex, and nucleus were separated, prepared, and used for western blot analysis to identify different PMCA isoforms. We used isoform- specific poly and monoclonal antibodies to detect different PMCAs in different parts of human lens. Results: In western blot experiments, a band of approximately 120 kDa representing PMCA1 was detected, however no band was identified for PMCA1 in cortex or nucleus. Similarly, a band of approximately125 kDa was recognized for PMCA2 only in epithelium, not in cortex or nucleus. We did not identify any band for PMCA3 in any part of the lens. The monoclonal PMCA4 antibody labeled a band of approximately 135 kDa in epithelium. No band for PMCA4 was identified in cortex or nucleus. Conclusion: We have shown, unequivocally, that PMCA1, 2, and 4 are expressed in human lens epithelial cells but, not in the cortical or nuclear region of the lens. The fiber membrane PMCA pumps are either degraded or undetectable by our antibodies. Of the numerous bands shown for PMCA2 in epithelial cells, the lower molecular weight bands probably represent proteolytic break down products of the PMCA2 protein. We did not detect PMCA3 in any part of the lens. The findings of this study provide a basis for further studies to examine the expression pattern of PMCA isoforms with respect to age, region and cataractogenesis.
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