Abstract:
Present study reports differential expression of the two insulin receptor (IR) subtypes in zebrafish ovary
at various stages of follicular growth and potential involvement of IR in insulin-induced oocyte maturation.
The results showed that mRNA expression for IR subtypes, insra and insrb, exhibited higher levels in
mid-vitellogenic (MV) and full-grown (FG) rather than pre-vitellogenic (PV) oocytes. Interestingly, compared
to the levels in denuded oocytes, mRNAs for both insra and insrb were expressed at much higher
level in the follicle layer harvested from FG oocytes. Immunoprecipitation using IRb antibody could
detect a protein band of desired size ( 95 kDa) in FG oocyte lysates. Further, IRb immunoreactivity
was detected in ovarian tissue sections, especially at the follicle layer and oocyte membrane of MV
and FG, but not PV stage oocytes. While hCG (10 IU/ml) stimulation was without effect, priming with
insulin (5 lM) could promote oocyte maturation of MV oocytes in a manner sensitive to de novo protein
and steroid biosynthesis. Compared to hCG, in insulin pre-incubated MV oocytes, stimulation with maturation
inducing steroid (MIS), 17a,20b-dihydroxy-4-pregnen-3-one (DHP) elicited higher maturational
response. Potential involvement of insulin-mediated action on acquisition of maturational competence
and regulation of oocyte maturation was further manifested through up regulation of 20bhydroxysteroid
dehydrogenase (20b-hsd), MIS receptor (mPRa), insulin-like growth factor 3 (igf3) and
IGF1 receptor (igf1rb), but not cyp19a expression in MV oocytes. Moreover, priming with anti-IRb attenuated
insulin action on meiotic G2-M1 transition indicating the specificity of insulin action and physiological
relevance of IR in zebrafish ovary.
