DIFFERENTIAL GENE-EXPRESSION WITHIN THE OVINE CORPUS-LUTEUM - IDENTIFICATION OF SECRETED PROTEIN ACIDIC AND RICH IN CYSTEINE AS A MAJOR SECRETORY PRODUCT OF SMALL BUT NOT LARGE LUTEAL CELLS

Limited information is available regarding secretory proteins of the c orpus luteum (CL), and the potential local role these proteins may pla y in control of luteal function. An ovine small luteal cell complement ary DNA library was immunologically screened with a polyclonal antiser um generated against small cell secretory proteins. A relatively abund ant complementary DNA (similar to 2.1 kb) encoding a calcium binding g lycoprotein Secreted Protein Acidic and Rich in Cysteine (SPARC) was i solated. Production of SPARC protein by ovine luteal cells was confirm ed by immunoprecipitating it from labeled culture medium. Expression o f SPARC messenger RNA (mRNA) was examined within CL collected on days 3, 7, 10, 13, and 16 post estrus (n = 4, 4, 4, 3, and 4 respectively), and within pools of purified small (n = 3) and large (n = 4) luteal c ells by Northern and dot blot analysis. Amounts of SPARC mRNA increase d during the early luteal phase, peaked by day 7 (P < 0.05) and subseq uently declined on days 10 and 13 (P < 0.05). SPARC mRNA content was s ignificantly higher in the small than in the large cells (P < 0.003). In situ hybridization showed that SPARC mRNA was localized to the thec al layer of Graafian follicles and to day 3 and day 10 CL. Within CL, immunohistochemistry indicated that SPARC protein was associated with small luteal cells (spindle shaped, avg = 17 mu M in diameter) but not with large cells. This specific localization to small cells was confi rmed by colocalization of SPARC with 3 beta-hydroxysteroid dehydrogena se. We conclude that SPARC is a major secretory product of small stero idogenic luteal cells of the ovine CL. As SPARC is known to modulate m any aspects of tissue reorganization, expression by small luteal cells may play a role in regulation of CL maturation.