Regulation of the cell proliferation and migration as extra-pituitary functions of GnRH.

GnRH was originally identified as a hypothalamic factor which promotes gonadotropin release from the pituitary and was named gonadotropin-releasing hormone (GnRH). However, broad tissue distributions of GnRH and the GnRH receptor in various extrapituitary tissues and organs have been revealed and it has been suggested that GnRH has extrapituitary effects such as neuromodulation, immunomodulation, and regulation of follicular atresia and ovulation. Although a number of studies have been performed on these effects, little is known about the molecular mechanisms and physiological settings in which GnRH exerts its activities in extrapituitary organs or tissues. Our recent studies had demonstrated that GnRH is able to regulate both cell proliferation and cell migration at much lower concentration than that in the peripheral circulation by using human carcinoma cell lines. Moreover, stimulating activity of GnRH on the developing chick embryonic GnRH neurons was also demonstrated and strongly suggests possible involvement of GnRH in some of extrapituitary functions. This mini-review intends to provide solid evidence of GnRH activity in the regulation of cell proliferation and migration and its physiological relevance in extra-pituitary functions. Recent other research, including that in various invertebrates, provides new insight into the evolutionary scenarios of GnRH signaling systems, and GnRH functions. Both proliferating and migrating activities are important fundamental cellular activities and could provide an important clue into understanding what the driving force behind the evolution of the GnRH signaling system was.

Neurotrophic effect of gonadotropin-releasing hormone on neurite extension and neuronal migration of embryonic gonadotropin-releasing hormone neurons in chick olfactory nerve bundle culture.

Hypothalamic gonadotropin-releasing hormone (GnRH) neurons play a pivotal role in regulating the reproductive function of vertebrates. These neurons are known to originate in the olfactory placode and migrate along olfactory-related axons to reach the forebrain during embryonic development. Although GnRH is suggested to be secreted during such migration, its physiological significance is unknown. This point is difficult to explore in vivo because recent studies suggest that GnRH is an important factor for normal brain development and that modification of the embryonic GnRH system by exogenous GnRH analogue or genetic methods would result in dysgenesis of the brain. Therefore, to study the role of GnRH in the migratory process of GnRH neurons, we established an in vitro chick embryonic olfactory nerve bundle explant model. Embryonic day 7.5-8 olfactory nerve bundles were cultured in a mixture of Matrigel and collagen gel. At day 3 of culture, GnRH neurons extended their unbranched neurites and migrated out from both edges of the explant. The nature of neurite extension and migratory behavior of GnRH neurons was well maintained in the gel containing 25% Matrigel and 50% collagen. With this culture system, we examined the effect of GnRH on the migrating GnRH neurons. Cetrorelix, a GnRH antagonist, was found to inhibit significantly neurite growth and neuronal migration of GnRH neurons, the effects of which were repressed by the addition of chicken GnRH-I. These results suggest that GnRH functions as one of the regulating factors of GnRH neuronal development by promoting neurite extension and neuronal migration.

Expression of sex steroid hormone-related genes in the embryo of the leopard gecko.

Sex steroid hormones are known to play a central role in vertebrate sex determination and differentiation. However, the tissues in which they are produced or received during development, especially around the period of sex determination of the gonads, have rarely been investigated. In this study, we identified the cDNA sequence, including the full-length of the coding region of cholesterol side-chain cleavage enzyme (P450scc), from the leopard gecko; a lizard with temperature-dependent sex determination. Embryonic expression analysis of two steroidogenic enzymes, P450scc and P450 aromatase (P450arom), and four sex steroid hormone receptors, androgen receptor, estrogen receptor alpha and beta, and progesterone receptor, was subsequently conducted. mRNA expression of both steroidogenic enzymes was observed in the brain and gonads prior to the temperature-sensitive period of sex determination. The mRNAs of the four sex steroid hormone receptors were also detected in the brain and gonads at all stages examined. These results suggest the existence of a gonad-independent sex steroid hormone signaling system in the developing leopard gecko brain.

Molecular characterization of thyroid hormone receptors from the leopard gecko, and their differential expression in the skin.

Thyroid hormones (THs) play crucial roles in various developmental and physiological processes in vertebrates, including squamate reptiles. The effect of THs on shedding frequency is interesting in Squamata, since the effects on lizards are quite the reverse of those in snakes: injection of thyroxine increases shedding frequency in lizards, but decreases it in snakes. However, the mechanism underlying this differential effect remains unclear. To facilitate the investigation of the molecular mechanism of the physiological functions of THs in Squamata, their two specific receptor (TRalpha and beta) cDNAs, which are members of the nuclear hormone receptor superfamily, were cloned from a lizard, the leopard gecko, Eublepharis macularius. This is the first molecular cloning of thyroid hormone receptors (TRs) from reptiles. The deduced amino acid sequences showed high identity with those of other species, especially in the C and E/F domains, which are characteristic domains in nuclear hormone receptors. Expression analysis revealed that TRs were widely expressed in many tissues and organs, as in other animals. To analyze their role in the skin, temporal expression analysis was performed by RT-PCR, revealing that the two TRs had opposing expression patterns: TRalpha was expressed more strongly after than before skin shedding, whereas TRbeta was expressed more strongly before than after skin shedding. This provides good evidence that THs play important roles in the skin, and that the roles of their two receptor isoforms are distinct from each other.