Expression of activin receptor II and IIB mRNA isoforms in mouse reproductive organs and oocytes

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Abstract
Activins, the dimeric polypeptides of inhibin β‐subunits, exhibit paracrine effects on cell proliferation, differentiation, and various other cell functions. The complex biological response to activin appears to involve multiple receptors. In the present study, we examined the isoform mRNA expression of both activin receptor type II (ActR‐II) and type IIB (ActR‐IIB) genes in mouse reproductive organs, cumulus–oocyte complexes (COCs), and ovulated oocytes. Northern blot analyses of female and male reproductive organs with single‐stranded ActR‐II cDNA probes revealed that mouse ovaries expressed high levels of the 6.0 kilobase (kb) mRNA, whereas the 3.0 kb transcript was the major mRNA species found in the testis. Reverse transcriptase–polymerase chain reaction (RTPCR) showed that both COCs and oocytes contained ActR‐II mRNA. To examine the expression of ActR‐IIB gene, primer selection was made outside the two alternative splicing sites in order to amplify the cDNAs of all four distinct receptor isoforms. The results of RT‐PCR demonstrated that isoforms IIB2 and IIB4 were the major mRNA species expressed in both female and male gonads and extragonadal reproductive tissues. The ovary expressed all four mRNA isoforms, whereas the testes expressed only three isoforms. Furthermore, COCs and oocytes contained only the ActR‐IIB2 isoform. The differential expression of both activin receptor mRNA isoforms in the reproductive organs suggests that distinct alternative splicing mechanisms are involved in activin receptor gene expression in male and female gonads, and that each of the activin receptors may have its own biological function in reproduction. The expression of activin receptor genes in COCs and oocytes indicates that activin, produced by the granulosa cells of maturing follicles, may act locally to regulate follicular development and oocyte maturation.