Testis- and developmental stage-specific expression of hnRNP A2/B1 splicing isoforms, B0a/b

Heterogeneous nuclear ribonucleoproteins (hnRNPs) A2 and B1 are abundant nu clear proteins that bind to nascent RNAs synthesized by RNA polymerase II. Previously we had found that the splicing isoforms hnRNP B0a/b, from which the ninth exon of the A2/B1 gene Is excluded, are abundantly expressed in t estis. We postulated that B0a/b are testis-specific isoforms, and investiga ted the expression of A2/B1 and B0a/b in rat tissues and in postnatal devel opment of rat testes using RNase protection assay, immunoblotting, and immu nohistochemistry. We found that hnRNP B0a/b mRNAs are expressed in several tissues but that the testis alone expresses B0a/b proteins. A sequential st udy using neonatal rat testes demonstrated that B0a/b mRNAs are produced af ter 17 days of age, but not translated until 4 weeks of age when round sper matids appear in addition to spermatogonia and spermatocytes. Immunohistoch emically, hnRNP A2/B1 isoforms are expressed during spermatogenesis from sp ermatogonia through round spermatids, whereas the expression of A1 is restr icted to spermatogonia. This expression pattern in the rat testis is mainta ined from birth through adulthood. These results suggest that the expressio n of the hnRNP A2/B1 gene is partly regulated by a testis-specific post-tra nscriptional mechanism, and that the products of the A2/B1 gene, especially hnRNP B0a/b, are involved in spermatogenesis. (C) 2000 Elsevier Science B. V. All rights reserved.