Leptin stimulates gonadotropin release and ovarian development in marine teleost chub mackerel.

Leptin transmits information about energy stored in the periphery to the reproductive axis and is an essential signal for puberty initiation in mammals; however, to date, few studies have focused on the direct effects of leptin stimulation on reproductive factors in fish. This study demonstrated the effect of leptin stimulation on important reproductive factors and ovarian development in the marine teleost chub mackerel (Scomber japonicus). We prepared recombinant leptin and conducted functional analyses through in vitro bioassays using primary pituitary cells, long-term leptin treatment administered to pre-pubertal females, and intracerebroventricular (ICV) administration. The results showed that leptin stimulation strongly induced gonadotropin (follicle-stimulating hormone: FSH and luteinizing hormone: LH) secretion from pituitary cells collected from pre-pubertal females, and that long-term leptin treatment significantly promoted ovarian development and triggered pubertal onset. Furthermore, ICV administration of leptin did not affect kisspeptin gene expression but significantly upregulated gonadotropin-releasing hormone 1 (gnrh1), fshb and lhb gene expression in sexually immature females. These results strongly suggest leptin as an important signal for reproductive-axis activation in chub mackerel.

Possible role of the leptin system in controlling puberty in the male chub mackerel, Scomber japonicus.

Leptin directly regulates kisspeptin neurons in the hypothalamus and gonadotropin secretion from the pituitary, making it a central player in the onset of mammalian puberty. Recently, we identified two leptin genes (lepa and lepb) and a single leptin receptor (lepr) in the marine perciform fish chub mackerel; however, the expression of these genes did not correlate with the expression of important reproductive genes or ovarian stage during female puberty. Here, we expand upon these initial observations by evaluating the expression of lepa, lepb, and lepr during pubertal transition and under differential feeding conditions in the male chub mackerel. Reverse transcription-polymerase chain reaction (RT-PCR) showed that lepa was primarily expressed in the liver of pubertal and gonadal recrudescence adults, as well as in the brain of adult fish; lepb was primarily expressed in the brain of all fish tested; and lepr was widely expressed in a variety of tissues. qRT-PCR analyses revealed significant increases in the hepatic expression of lepa in accordance with testicular stage, whereas pituitary follicle-stimulating hormone (fshß) expression increased in unison with hepatic lepa. In contrast, expression of both brain lepa and lepb dramatically decreased during pubertal transition, with brain kisspeptin 1 (kiss1) expression strongly correlating with leptin expression patterns. In pre-pubertal males, lepa, lepb, and lper gene expression in the brain, pituitary gland, and liver decreased in fish given a high feed diet, relative to the controlled feeding group. Taken together, these results indicate high sexual specificity of leptin expression, suggesting a possible role for leptin signaling in endocrine and neuroendocrine functions during spermatogenesis in the pubertal male chub mackerel.

Two leptin genes and a leptin receptor gene of female chub mackerel (Scomber japonicus): Molecular cloning, tissue distribution and expression in different obesity indices and pubertal stages.

Leptin is a hormone produced by fat cells that regulates the amount of fat stored in the body and conveys nutritional status to the reproductive axis in mammals. In the present study we identified two subtypes of leptin genes (lepa and lepb) and a leptin receptor gene (lepr) from chub mackerel (Scomber japonicus) and there gene expression under different feeding conditions (control and high-feed) and pubertal development stages was analyzed using quantitative real-time PCR. The protein lengths of LepA, LepB and LepR were 161 amino acids (aa), 163 aa and 1149 aa, respectively and both leptin subtypes shared only 15% similarity in aa sequences. In pubertal females, lepa was expressed in the brain, pituitary gland, liver, adipose tissue and ovary; however, in adult (gonadal maturation after the second in the life) females, lepa was expressed only in the liver. lepb was expressed primarily in the brain of all fish tested and was expressed strongly in the adipose tissue of adults. lepr was characterized by expression in the pituitary. The high-feed group showed a high conditioning factor level; unexpectedly, hepatic lepa and brain lepr were significantly more weakly expressed compared with the control-feed group. Furthermore, the expression levels of lepa, lepb and lepr genes showed no significant differences between pre-pubertal and post-pubertal fish. On the other hand, pituitary fshß and lhß showed no significant differences between different feeding groups of pre-pubertal fish. In contrast, fshß and lhß expressed abundantly in the post-pubertal fish of control feed group. Based on these results, whether leptin plays an important role in the nutritional status and pubertal onset of chub mackerel remains unknown.

mRNA levels of kisspeptins, kisspeptin receptors, and GnRH1 in the brain of chub mackerel during puberty.

Kisspeptin (Kiss) and its cognate receptor (Kiss1R), implicated in the neuroendocrine control of GnRH secretion in mammals, have been proposed to be the key factors in regulating puberty. However, the mechanisms underlying the initiation of puberty in fish are poorly understood. The chub mackerel Scomber japonicus expresses two forms of Kiss (kiss1 and kiss2) and two Kiss receptor (kissr1 and kissr2) genes in the brain, which exhibit sexually dimorphic changes during the seasonal reproductive cycle. This indicates that the kisspeptin system plays an important role in gonadal recrudescence of chub mackerel; however, the involvement of the kisspeptin system in the pubertal process has not been identified. In the present study, we examined the mRNA expression of kiss1, kiss2, kissr1, kissr2, and gnrh1 (hypophysiotropic form) in the brain of a chub mackerel during puberty. In male fish, kiss2, kissr1 and kissr2 levels increased significantly at 14weeks post-hatch (wph), synchronously with an increase in type A spermatogonial populations in the testis; kiss2 and gnrh1 levels significantly increased at 22wph, just before the onset of meiosis in the testes. In female fish, kiss2 increased significantly at 14wph, synchronously with an increase in the number of perinucleolar oocytes in the ovary; kiss1 and kiss2 levels significantly increased concomitantly with an increase in the kissr1, kissr2, and gnrh1 levels at 24wph, just before the onset of vitellogenesis in oocytes. The present results suggest positive involvement of the kisspeptin-GnRH system in the pubertal process in the captive reared chub mackerel.