Neurotrophins are a family of soluble polypeptide growth factors widely recognized for their roles in the mammalian nervous system. One such neurotrophin, brain-derived neurotrophic factor (BDNF) was originally described in the nervous system but has now been shown to be expressed in a variety of nonneuronal tissues including endocrine tissues. We examined the human ovarian follicle for its possible secretion of BDNF and further studied mouse oocytes to determine BDNF's possible influence upon oocyte maturation. In a series of experiments derived from human specimens from in vitro fertilization following oocyte retrieval, BDNF was detected in human follicular fluid. To define the source of BDNF, cumulus granulosa cells (the cells that immediately surround the developing oocyte) were grown in cell culture for 1-2 d. BDNF protein increased over 24 h in the culture medium. Moreover, the release of BDNF was enhanced upon stimulation with cAMP or forskolin, an activator of cAMP. In contrast, mural granulosa (cells lining the follicle), oocytes, and embryos did not release appreciable quantities of BDNF. To examine possible targets of BDNF, mouse studies were used to localize the BDNF receptor, Trk B, immunocytochemically. The receptor was present on the surface of isolated oocytes. Moreover, BDNF promoted mouse oocyte maturation in culture. These experiments demonstrate for the first time the presence and secretion of BDNF from follicular cells in the human ovary and suggest a possible role for BDNF in the regulation and modulation of oocyte maturation.
Neurotrophins are a family of soluble polypeptide growth factors widely recognized for their role in the mammalian nervous system. We first reported the unique presence of one neurotrophin, brain-derived neurotrophic factor (BDNF), in the follicular fluid of the human ovarian follicle. The BDNF receptor, Trk B, was identified in mouse oocytes, and BDNF accelerated first polar body extrusion in vitro. In the present study, we examined human follicular fluid and mouse immature oocytes to determine whether another Trk B ligand, neurotrophin-4/5 (NT-4/5), is present within the ovarian follicle and if so, whether it demonstrates activity similar to that of BDNF. We also examined whether a non-Trk B neurotrophin ligand, neurotrophin-3 (NT-3), is present within the follicle and has a possible role in oocyte maturation. NT-4/5 and NT-3 were noted to be present in all human follicular fluid samples aspirated from follicles of women undergoing in vitro fertilization. NT-4/5, but not NT-3, significantly promoted mouse oocyte polar body extrusion. Trk C receptors were not noted to be present in mouse oocytes. This study demonstrates for the first time that NT-4/5 and NT-3 are present in the follicular fluid of the human ovary. These data suggest that NT-4/5, like BDNF, promotes oocyte nuclear maturation. In contrast, NT-3 does not promote oocyte maturation but may contribute to follicle-oocyte maturation by mechanisms not yet identified.
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