Abstract
Purpose:
The extreme longevity of cells located on the optic axis of the lens implies the presence of mechanisms that effectively suppress apoptosis in this cell population. In this study, we used an integrated approach to examine the expression and function of Birc7, a member of the Inhibitor of Apoptosis Protein (IAP) family, in the mouse lens.
Methods:
DNA microarray, Q-PCR, in situ hybridization, immunofluorescence, and Western blotting were used to analyze Birc7 distribution in lens cells. Conditional knockout of Birc7 was achieved by crossing Birc7flox/flox mice with Le-Cre mice. Caspase-3 activity and DNA degradation were determined using Image-It Green Caspase 3/7 kit and TUNEL, respectively.
Results:
Birc7 was undetectable at E12.5 but strongly expressed in the lens by E16.5. Q-PCR analysis indicated that Birc7 was essentially a fiber-specific transcript. In situ hybridization further revealed that Birc7 was expressed by deep cortical fiber cells only, a finding confirmed subsequently by immunofluorescence analysis and western blotting of fractionated lenses. Lenses from Birc7-conditional knockout animals were small and cataractous. Histological analysis indicated that Birc7-null fibers were highly vacuolated with displaced nuclei. High levels of caspase-3 activity and the presence of TUNEL-positive nuclei indicated that the absence of Birc7 was associated with elevated levels of apoptotic cell death.
Conclusions:
This is the first study to show significant expression of Birc7 in a non-transformed cell type. We hypothesize that expression of Birc7 by the inner lens fiber cells contributes to the extreme longevity of these cells.
Keywords: 426 apoptosis/cell death •
449 cell survival •
445 cataract