Up-regulation of novel intermediate filament proteins in primary fiber cells: An indicator of all vertebrate lens fiber differentiation?

The early embryonic development and expression patterns of the eye lens spe cific cytoskeletal proteins, CP49 and CP95, were determined for the chick a nd were found to be similar in both human and mouse. These proteins, as wel l as their homologs in other species, are obligate polymerization partners which form unique filamentous structures termed "beaded filaments." CP49 an d CP95 appeared as protein products after 3 days of embryonic development i n the chick during the elongation of primary fiber cells. Although limited data were obtained for human embryos at these early developmental timepoint s, they were consistent with the interpretation that the up-regulation of t hese lens specific proteins began only after the initiation of lens vesicle closure. In situ hybridization with the mouse lens confirmed that message levels for beaded filament proteins were greatly elevated in differentiatin g primary fiber cells. Nuclease protection assays established that mRNA lev els for CP49 remained relatively constant while CP95 mRNA levels increased once the process of secondary fiber formation was under way. Although prese nt in relatively low abundance, the mRNA for a unique splice variant of CP4 9, CP49(INS), was also detected early in embryonic development and into adu lthood. Peptide-specific antibodies directed against unique predicted seque nces were able to confirm the protein expression of CP49(INS) in both embry onic and adult chick lens cells. These data present the first detailed stud y of the expression of CP49 and CP95 during early lens development. They su ggest that the up-regulated expression of CP49 and CP95 could serve as pan- specific markers for all vertebrate lens fiber development. Anat Rec 258:25 -33, 2000. (C) 2000 Wiley-Liss, Inc.