Biomedical subjects

Pei-San Tsai

Publications and source records attributed to Pei-San Tsai.

14 recordsLinked to original sources

PubMed2006

Characterization and steroidal regulation of gonadotropin beta subunits in the male leopard frog, Rana pipiens.

In ranid frogs, the secretion of gonadotropins (GtHs), luteinizing hormone (LH), and follicle-stimulating hormone (FSH), is potently regulated by gonadal steroids. To better understand the gonadal regulation of GtHs at the molecular level, we elucidated the full-length cDNA sequences of LH and FSH beta subunits from the leopard frog, Rana pipiens. The cDNAs for LHbeta and FSHbeta were 1084 and 667 bp in size excluding the poly (A) tail, and encoded proteins of 138 and 127 amino acids, respectively. Using reverse-transcription polymerase chain reaction (RT-PCR), the messages for LHbeta and FSHbeta were found in the pituitary, but not in the brain, heart, kidney, or the liver. Semi-quantitative RT-PCR revealed a significant elevation of FSHbeta, but not LHbeta, in mature male R. pipiens 21 days after gonadectomy (GDX). 17beta-estradiol implant for 21 days in GDX male frogs significantly suppressed the levels of both LHbeta and FSHbeta transcripts, whereas 5alpha-dihydrotestosterone implant suppressed only the latter. Together, these results laid the groundwork for investigating gonadal regulation of GtHbeta subunits in a ranid frog. Importantly, these data also revealed differential feedback effects of an androgen and an estrogen upon GtHbeta expression.

Amino Acid Sequence↗

PubMed2006

Mechanisms of disease: Insights into X-linked and autosomal-dominant Kallmann syndrome.

Kallmann syndrome (KS) is a disorder characterized by hypogonadotropic hypogonadism and anosmia. Although KS is genetically heterogeneous, only two causal genes have been identified to date. These include an X-linked gene that encodes anosmin 1 and an autosomal gene that encodes fibroblast growth factor receptor 1. Mutations in these two genes result in disorders that often include, but are not limited to, severe defects in olfactory and reproductive functions. In this respect, KS can be regarded as a 'human model' for understanding critical factors that regulate olfactory and reproductive development. Here we give an overview of the disorders that stem from mutations in these two genes, with special emphasis on the cellular mechanisms underlying olfactory and reproductive anomalies. Other, less well-known aspects of KS, such as the convergence of symptoms in patients with different genetic forms of KS and the unpredictable manifestation of KS symptoms, are also discussed.

Extracellular Matrix Proteins↗

PubMed2005

Expression of a dominant negative FGF receptor in developing GNRH1 neurons disrupts axon outgrowth and targeting to the median eminence.

During development, neurons that synthesize and release gonadotropin-releasing hormone (GNRH1) extend their axons to the median eminence (ME) to establish neurosecretory contacts necessary for hormone secretion. Signals that coordinate this process are not known, but could involve the activation of fibroblast growth factor receptors (FGFRs) expressed on developing GNRH1 neurons. Using both whole-animal and cell culture approaches, this study examines the direct role of FGFR signaling in the extension and guidance of GNRH1 axons to the ME. In vivo retrograde labeling with fluorogold (FG) first showed a significant reduction in the projections of GNRH1 axons to the circumventricular organs (including the ME) in transgenic mice expressing a dominant negative FGF receptor (dnFGFR) in GNRH1 neurons. Using a primary GNRH1 neuronal culture system, we examined if compromised axon extension and directional growth led to the reduced axon targeting efficiency seen in vivo. Primary cultures of GNRH1 neurons were established from Embryonic Day 15.5 embryos, an age when GNRH1 neurons are actively targeting the ME. Cultured GNRH1 neurons expressing dnFGFR (dnFGFR neurons) exhibited attenuated activation of signaling pathways and reduced neurite outgrowth in response to FGF2. Further, dnFGFR neurons failed to preferentially target neurites toward cocultured ME explant and FGF2-coated beads, suggesting a defect in axon pathfinding. Together, these findings describe a direct role of FGFR signaling in the elongation and guidance of GNRH1 axons to the ME.

Animals↗

PubMed2005

Gonadotropin-releasing hormone in invertebrates: structure, function, and evolution.

Gonadotropin-releasing hormone (GnRH) is central to the initiation and maintenance of reproduction in vertebrates. GnRH is found in all major groups of Phylum Chordata, including the protochordates. Studies on functional and structural evolution of GnRH have, in the past, focused exclusively on chordates. However, the recent structural elucidation of an octopus GnRH-like molecule and increasing evidence that GnRH-like substances are present in multiple invertebrate phyla suggest GnRH is an ancient peptide that arose prior to the divergence of protostomes and deuterostomes. The extraordinary conservation of GnRH structure and function raises interesting questions regarding the functional role assumed by GnRH over the course of evolution. This review will focus on the current understanding of GnRH structure and function in non-chordate invertebrates. Special emphasis will be placed upon the possible and speculated functions of GnRH in mollusks.

Amino Acid Sequence↗

PubMed2005

Steroid-induced changes in the morphology of GnRH neurons in the male leopard frog, Rana pipiens: correlation with plasma gonadotropin and gonadal size.

Previously, we reported that hypothalamic explants isolated from male leopard frogs (Rana pipiens) implanted with 17beta-estradiol (E2), but not 5alpha-dihydrotestosterone (DHT), released significantly higher levels of gonadotropin-releasing hormone (GnRH) in response to a veratridine challenge. In this study, we measured changes in GnRH soma size, circulating luteinizing hormone (LH), and gonadosomatic index (GSI) in response to these two steroid hormones to further assess the impact of these hormones on the hypothalamic-pituitary-gonadal axis. Sexually mature male R. pipiens were implanted with silastic capsules containing cholesterol (Ch; control), E2, or DHT for 20 days. GnRH immunocytochemistry (ICC) revealed that both E2 and DHT significantly enlarged GnRH soma size without affecting the total number of GnRH neurons in the forebrain. The effects of E2 and DHT were specific, since neither hormone altered the soma size of tyrosine hydroxylase neurons in the dorsomedial posterior tuberculum. Circulating LH levels were significantly reduced in animals treated with both steroid hormones, with E2 exerting the most potent inhibitory effect. A significant inverse correlation was observed between the GSI and GnRH soma size in Ch controls, suggesting animals with larger GnRH neurons tended to have smaller gonads. Overall, our results showed that both steroid hormones induced the accumulation of GnRH and ultimately the swelling of the GnRH soma. Further, larger GnRH neurons were associated with smaller gonads and lower circulating levels of LH, suggesting a link between enlarged GnRH neurons and an overall decrease in the reproductive activity of R. pipiens.

Animals↗

PubMed2005

Alteration of the hypothalamic-pituitary-gonadal axis in estrogen- and androgen-treated adult male leopard frog, Rana pipiens.

BACKGROUND: Gonadal steroids, in particular 5 alpha-dihydrotestosterone (DHT) and 17 beta-estradiol (E2), have been shown to feed back on the hypothalamic-pituitary-gonadal (HPG) axis of the ranid frog. However, questions still remain on how DHT and E2 impact two of the less-studied components of the ranid HPG axis, the hypothalamus and the gonad, and if the feedback effects are consistently negative. Thus, the goal of the study was to examine the effects of DHT and E2 upon the HPG axis of the gonadally-intact, sexually mature male leopard frogs, Rana pipiens. METHODS: R. pipiens were implanted with silastic capsules containing either cholesterol (Ch, a control), DHT, or E2 for 10 or 30 days. At each time point, steroid-induced changes in hypothalamic GnRH and pituitary LH concentrations, circulating luteinizing hormone (LH), and testicular histology were examined. RESULTS: Frogs implanted with DHT or E2 for 10 days did not show significant alterations in the HPG axis. In contrast, frogs implanted with hormones for 30 days had significantly lower circulating LH (for both DHT and E2), decreased pituitary LH concentration (for E2 only), and disrupted spermatogenesis (for both DHT and E2). The disruption of spermatogenesis was qualitatively similar between DHT and E2, although the effects of E2 were consistently more potent. In both DHT and E2-treated animals, a marked loss of all pre-meiotic germ cells was observed, although the loss of secondary spermatogonia appeared to be the primary cause of disrupted spermatogenesis. Unexpectedly, the presence of post-meiotic germ cells was either unaffected or enhanced by DHT or E2 treatment. CONCLUSIONS: Overall, these results showed that both DHT and E2 inhibited circulating LH and disrupted spermatogenesis progressively in a time-dependent manner, with the longer duration of treatment producing the more pronounced effects. Further, the feedback effects exerted by both steroid hormones upon the HPG axis were largely negative, although the possibility exists for a stimulatory effect upon the post-meiotic germ cells.

Animals↗

PubMed2004

Targeted expression of a dominant-negative fibroblast growth factor (FGF) receptor in gonadotropin-releasing hormone (GnRH) neurons reduces FGF responsiveness and the size of GnRH neuronal population.

Increasing evidence suggests that fibroblast growth factors (FGFs) are neurotrophic in GnRH neurons. However, the extent to which FGFs are involved in establishing a functional GnRH system in the whole organism has not been investigated. In this study, transgenic mice with the expression of a dominant-negative FGF receptor mutant (FGFRm) targeted to GnRH neurons were generated to examine the consequence of disrupted FGF signaling on the formation of the GnRH system. To first test the effectiveness of this strategy, GT1 cells, a GnRH neuronal cell line, were stably transfected with FGFRm. The transfected cells showed attenuated neurite outgrowth, diminished FGF-2 responsiveness in a cell survival assay, and blunted activation of the signaling pathway in response to FGF-2. Transgenic mice expressing FGFRm in a GnRH neuron-specific manner exhibited a 30% reduction in GnRH neuron number, but the anatomical distribution of GnRH neurons was unaltered. Although these mice were initially fertile, they displayed several reproductive defects, including delayed puberty, reduced litter size, and early reproductive senescence. Overall, our results are the first to show, at the level of the organism, that FGFs are one of the important components involved in the formation and maintenance of the GnRH system.

Animals↗

PubMed2004

Developmental regulation of gonadotropin-releasing hormone neurons by fibroblast growth factor signaling.

GnRH neurons are central to the initiation and maintenance of reproductive function in diverse vertebrates. The formation of a functional GnRH system during development is a highly complex event that likely requires extensive guidance by neurotrophic factors. In this study, we examined whether members of the fibroblast growth factor (FGF) family are critically involved in the development of endogenous GnRH neurons. Immunocytochemistry revealed the presence of FGF receptors (FGFRs) 1, 2, and 3, but not 4, in embryonic day (E) 10.5 medial nasal placode, an area and time consistent with the first appearance of GnRH neurons in mice. Dual immunocytochemistry confirmed the presence of FGFRs 1 and 3, but not 2 and 4, in a substantial fraction of E15.5 and postnatal day (P) 3 GnRH neurons. To examine whether FGF signaling was essential for the specification of GnRH neuronal fate, a nasal explant culture that supported the in vitro emergence of GnRH neurons from E10.5 noses was established. In this system, the addition of SU5402, a FGFR antagonist, suppressed the emergence of GnRH neurons. Lastly, we investigated whether FGF signaling altered the extension of neurites in cultures of dispersed GnRH neurons. The addition of FGF2 to E15.5 and P3 GnRH neurons expressing the green fluorescent protein significantly stimulated neurite outgrowth (E15.5 and P3) and branching (P3), suggesting a regulatory role of FGFs in GnRH axon targeting. Together these results demonstrated that FGF signaling critically regulates multiple phases of development in a neuroendocrine system essential for vertebrate reproduction.

Animals↗

PubMed2003

Steroidal modulation of in vitro gonadotropin secretion in prepubertal and adult male Siberian hamsters.

The effects of exogenous gonadal steroids, testosterone (T), and 17beta-estradiol (E(2)) upon the hypothalamo-pituitary-gonadal axis were reported to be different between prepubertal and adult Siberian hamsters. Utilizing an in vitro static culture system, we investigated if age-related differences in steroid responsiveness occurs at the pituitary. Prepubertal (20 days old) or adult (140 days old) male Siberian hamsters were implanted with 1 mm silastic capsules containing undiluted T, E(2) or cholesterol (Ch, control). After 15 days, pituitaries were removed, incubated in vitro, and subjected to the following treatments: two baseline measurements, one challenge with 10ng/ml of D-Lys(6)-gonadotropin-releasing hormone (GnRH), and three post-challenge washes. Fractions were collected every 30 minutes and measured for follicle-stimulating hormone (FSH) and luteinizing hormone (LH). T and E(2 )reduced basal secretion of LH and FSH in juveniles but not adults. In juveniles, E(2) increased GnRH-induced FSH and LH secretion, while T augmented GnRH-induced FSH secretion but attenuated GnRH-induced LH secretion. Steroid treatment had no effect on GnRH-stimulated LH or FSH release in adults. The only effect of steroid hormones upon adult pituitaries was the more rapid return of gonadotropin secretion to baseline levels following a GnRH challenge. These data suggest both basal and GnRH-induced gonadotropin secretion are more sensitive to steroid treatment in juvenile hamsters than adults. Further, differential steroidal regulation of FSH and LH at the level of the pituitary in juveniles might be a mechanism for the change in sensitivity to the negative effects of steroid hormones that occurs during the pubertal transition.

Aging↗

PubMed2003

Localization of gonadotropin-releasing hormone in the central nervous system and a peripheral chemosensory organ of Aplysia californica.

Gonadotropin-releasing hormone (GnRH) is a neurohormone crucial for the regulation of reproductive and neural functions in vertebrates. Recent discoveries of GnRH immunoreactivity (IR) in a number of invertebrates raised the possibility that GnRH may be an ancient molecule that had arisen before the emergence of Phylum Chordata. We previously demonstrated the presence of a GnRH IR similar to the mammalian (m) and tunicate I (tI) forms of GnRH in the hemolymph and ovotestis of an opisthobranch mollusk, Aplysia californica; however, the presence of GnRH in the central nervous system (CNS) of A. californica could not be detected with the available antisera against various forms of chordate GnRH. In the present study, we performed immunohistochemistry (IHC) to localize the presence of GnRH in the CNS and a peripheral chemosensory organ, the osphradium, of A. californica. A newly generated antiserum against tI-GnRH revealed the strong expression of GnRH IR in neurons of all CNS ganglia. A notable asymmetry in immunostaining was detected in the left and right abdominal hemiganglia. The CNS is rich in tI-GnRH immunoreactive neurons but lacks mGnRH IR, whereas the osphradium contains abundant mGnRH immunoreactive neurons but lacks tI-GnRH IR. The extract of CNS failed to stimulate the release of LH from mouse pituitary, demonstrating that the A. californica GnRH IR is structurally different from what is required to bind and activate mammalian GnRH receptor. Together, these results indicate the presence of at least two distinct GnRH systems in A. californica. The presence of GnRH in the osphradium is consistent with the long-standing anatomical relationship between GnRH and the chemosensory system observed in vertebrates.

Animals↗

PubMed2003

Development of a novel enzyme immunoassay for the measurement of in vitro GnRH release from rat and bullfrog hypothalamic explants.

Gonadotropin-releasing hormone (GnRH) is critical for the initiation and maintenance of reproduction in vertebrates. Information regarding GnRH release is abundant in mammals, but absent in poikilothermic tetrapods. In this study, we established a novel GnRH enzyme immunoassay (EIA) to measure GnRH release over time from hypothalamic explants isolated from mature field-caught and commercially-acquired male bullfrogs, Rana catesbeiana. Hypothalamic explants from rats were used as a positive control to test the sensitivity and accuracy of our EIA and to ensure our in vitro system could detect GnRH pulses. Prominent GnRH pulses were present in the majority (9/10) of rat hypothalamic explants, but absent in all (17/17) of the commercial bullfrogs and the majority (5/8) of field-caught bullfrogs. In three cases where GnRH pulses were observed in field-caught bullfrogs, there was only one pulse during the 2-h incubation period; high-frequency pulses similar to those observed in rats were not observed. Veratridine, which opens voltage-gated sodium channels, stimulated GnRH release in all explants cultured in the presence of Ca(2+), demonstrating explant viability. The levels of both spontaneous and veratridine-induced GnRH release were significantly higher in field-caught than commercial bullfrogs. This study demonstrated, for the first time, the temporal pattern of GnRH release in a poikilothermic tetrapod. Further, our results suggest the levels and patterns of GnRH output in bullfrogs are subject to the dynamic regulation by physiological and environmental cues.

Animals↗

PubMed2002

Disruption of pubertal onset by exogenous testosterone and estrogen in two species of rodents.

The administration of adult physiological levels of testosterone (T) and 17beta-estradiol (E(2)) to male Siberian hamsters was previously shown to delay the onset of puberty. To examine whether this is a response common to other rodents, we investigated whether exogenous steroids also alter the onset of puberty in Syrian hamsters and mice. Juvenile male Syrian hamsters and mice were implanted with Silastic capsules containing T, E(2), or cholesterol control. After 15 days, plasma, pituitaries, and testes were processed for histological analysis or measurements of gonadotropins and circulating steroid hormones. T and E(2) implants reduced testis mass and gonadotropin stores in both species and arrested spermatogenesis in Syrian hamsters. In contrast, spermatogenesis in mice was unaffected by T and only modestly affected by E(2). Although E(2) inhibited circulating follicle-stimulating hormone (FSH) in both species, T inhibited circulating FSH in mice only. Overall, our results demonstrate that the hypothalamic-pituitary-gonadal axis of each rodent species responds uniquely to T and E(2) during the pubertal transition. Despite the highly varied effects of T and E(2) in these two species, the ability of steroid hormones to disrupt the onset of puberty appears to be a feature common to many rodents.

Animals↗

PubMed2002

Differential alteration of the reproductive axis by testosterone and estrogen in peripubertal and adult male Siberian hamsters (Phodopus sungorus).

In male Siberian hamsters, administration of adult physiological levels of testosterone (T) and estrogen (E2) to juveniles inhibited pubertal onset by distinct pathways. It is presently unclear if T and E2 also exert an inhibitory effect on the reproductive function of sexually mature and sexually maturing hamsters. This study aims to determine if there is an age-dependent decline in the sensitivity of the hypothalamic-pituitary-gonadal (HPG) axis to these inhibitory steroids and if their actions remain distinct. Peripubertal and adult male Siberian hamsters were implanted with a silastic capsule containing T, E2, or cholesterol (Ch, control). Testosterone treatment significantly reduced testes mass and length and impaired spermatogenesis in both ages. In contrast, E2 treatment reduced testes mass only in peripubertal, but not adult, animals. In fact, E2 treatment significantly increased testes mass in adults without altering spermatogenesis. In addition, circulating E2 is very high immediately prior to pubertal onset and declines thereafter. Our results showed the inhibitory effects of T persist into adulthood whereas those of E2 subside as the animals become sexually mature. The decreased sensitivity of the HPG axis to the inhibitory effects of E2 in adult animals and the high level of circulating E2 immediately prior to pubertal onset suggest E2 may play an important role in the regulation of puberty in this species.

Aging↗

PubMed2002

Estrogen accelerates gonadal recrudescence in photo-regressed male siberian hamsters.

Seasonal gonadal recrudescence in the male Siberian hamster is accompanied by the initiation of spermatogenesis, weight gain, and darkening of coat color. The downstream endocrine regulators responsible for these changes have been definitively identified. We have previously shown that the administration of exogenous 17beta-estradiol (E(2)) to adult male Siberian hamsters kept under long-day photoperiod increased testicular mass without altering spermatogenesis. In this study, we examine if E(2) can initiate testicular growth in photo-regressed adult Siberian hamsters and if this testicular growth is accompanied by weight gain and pelage color change. Photo-regressed adult male Siberian hamsters were subcutaneously implanted with a 1 mm silastic capsule containing E(2) or cholesterol control. After 15 days, robust initiation of spermatogenesis was observed in E(2)-implanted animals in the absence of body weight and pelage color change. While circulating follicle-stimulating hormone (FSH) remained undetected in both control and E(2)-treated animals, E(2) significantly reduced pituitary gonadotropin stores. Overall, we showed E(2) stimulated gonadal recrudescence via a pathway that has diverged from body weight and pelage color change. Further, we demonstrated a novel role of E(2) in the initiation of spermatogenesis, possibly via a mechanism independent of FSH.

Animals↗