POSTNATAL EXPRESSION OF NERVE GROWTH-FACTOR RECEPTORS IN THE RAT TESTIS

Because nerve growth factor beta (NGF beta) and its corresponding rece ptors have been implicated in the paracrine regulation of spermatogene sis, we examined the postnatal developmental expression of the low- an d high-affinity NGF receptors in the fat testis, and localized their e xpression to specific testicular cell types. The neurotropin receptors consist of a low-affinity p75 nerve growth factor receptor (LNGFR) an d a family of high-affinity tyrosine receptor kinases (trk). Both the p75 LNGFR gene product and the trk receptor gene product were detected in immature rat testes, with maximal expression in 10- and 20-day-old rats. Expression of the testicular p75 LNGFR and the trk receptor pro gressively declined in older animals so that they were barely detectab le in 90-day-old adult rats. The 75-kDa LNGFR was detected in membrane fractions of Sertoli cells, whereas the p75 LNGFR was not detected by Western blot in membrane fractions of round spermatids and primary sp ermatocytes. Interestingly, microsomal fractions of peritubular myoid cells were immunoreactive for a 65-kDa band on Western blots with the p75 LNGFR monoclonal antibody. Immunoblot analysis of the trk receptor in cell lysates of isolated cell types was inconclusive. Excess NGF b eta and round spermatid protein, which is known to contain a NGF-iike protein, were both capable of displacing the binding of I-125-NGF beta from the surface of Sertoli cells. Scatchard analysis of I-125-NGF be ta binding to Sertoli cells identified a low-affinity binding site (K- d = 6.3 +/- 2.8 x 10(-9) M), consistent with the characteristics of th e p75 LNGFR, and a high-affinity binding site (K-d = 3.2 +/- 1.2 x 10( -9) M), consistent with the characteristics of the trk receptor. These results characterize the postnatal developmental expression of a p75 LNGFR and a trk receptor localized to Sertoli cells consistent with th eir role in the paracrine regulation of spermatogenesis.