5 Teleost Gonadotropins: Isolation, Biochemistry, and Function

Fish reproductive biology - Reflecting on five decades of fundamental and translational research

Driven by the broad diversity of species and physiologies and by reproduction-related bottlenecks in aquaculture, the field of fish reproductive biology has rapidly grown over the last five decades. This review provides my perspective on the field during this period, integrating fundamental and applied developments and milestones. Our basic understanding of the brain-pituitary-gonadal axis led to overcoming the failure of farmed fish to ovulate and spawn in captivity, allowing us to close the fish life cycle and establish a predictable, year-round production of eggs. Dissecting the molecular and hormonal mechanisms associated with sex determination and differentiation drove technologies for producing better performing mono-sex and reproductively-sterile fish. The growing contingent of passionate fish biologists, together with the availability of innovative platforms such as transgenesis and gene editing, as well as new models such as the zebrafish and medaka, have generated many discoveries, also leading to new insights of reproductive biology in higher vertebrates including humans. Consequently, fish have now been widely accepted as vertebrate reproductive models. Perhaps the best testament of the progress in our discipline is demonstrated at the International Symposia on Reproductive Physiology of Fish (ISRPF), at which our scientific family has convened every four years since the grandfather of the field, the late Ronald Billard, organized the inaugural 1977 meeting in Paimpont, France. As the one person who has been fortunate enough to attend all of these meetings since their inception, I have witnessed first-hand the astounding evolution of our field as we capitalized on the molecular and biotechnological revolutions in the life sciences, which enabled us to provide a higher resolution of fish reproductive and endocrine processes, answer more questions, and dive into deeper comprehension. Undoubtedly, the next (five) decades will be similarly exciting as we continue to integrate physiology with genomics, basic and translational research, and the small fish models with the aquacultured species.

Seasonal physiological dynamics of maturing female southern flounder (Paralichthys lethostigma)

Physiological information is rarely used in descriptions of maturity for managed, wild fish species; however, the use of physiological data holds great promise to provide important detail on the complexities of oocyte development and maturity. Investigating southern flounder (Paralichthys lethostigma)-an overfished commercial and recreational fishery resource-we examined pre-spawn physiological changes in females to provide further detail of the maturation process. Given that adults of this species complete maturation and spawn in unknown offshore locations, information on pre-spawn physiological changes is particularly informative for both size- and age-based patterns of maturity. We evaluated seasonal and ontogenetic changes in hormone concentrations in blood plasma that are commonly associated with sexual maturation, in addition to quantifying and classifying lipid stored in liver tissue. We found a strong positive relationship between body weight and lipid content during all months, as well as evidence for mobilization of lipids among larger females in September and October, presumably for gonadal development. Throughout the sampling period, the lipid content of smaller individuals was dominated by structural lipids (as opposed to storage lipids). In contrast, larger individuals possessed greater amounts of storage lipids. This suggests that larger, putatively maturing individuals were accumulating storage lipids for later production of vitellogenin. Females sampled for blood sex steroids and ovarian histology showed different testosterone and estradiol concentrations between putatively maturing and immature fish, and temporal variation with peaks in October and November. Overall, emerging patterns of liver lipid content and composition and blood steroid concentrations describe a multi-month maturation process that is often managed one dimensionally over short time periods. Insights from this work will improve our understanding of the life history of southern flounder, with the potential for better understanding of the dynamics of offshore spawning migration and informing subsequent species management.

The brain–pituitary–gonad axis in male teleosts, with special emphasis on flatfish (Pleuronectiformes)

The key component regulating vertebrate puberty and sexual maturation is the endocrine system primarily effectuated along the brain-pituitary-gonad (BPG) axis. By far most investigations on the teleost BPG axis have been performed on salmonids, carps, catfish and eels. Accordingly, earlier reviews on the BPG axis in teleosts have focused on these species, and mainly on females (e.g. 'Fish Physiology, vol. IXA. Reproduction (1983) pp. 97'; 'Proceedings of the Fourth International Symposium on the Reproductive Physiology of Fish. FishSymp91, Sheffield, UK, 1991, pp. 2'; 'Curr. Top. Dev. Biol. 30 (1995) pp. 103'; 'Rev. Fish Biol. Fish. 7 (1997) pp. 173'; 'Proceedings of the Sixth International Symposium on the Reproductive Physiology of Fish. John Grieg A/S, Bergen, Norway, 2000, pp. 211'). However, in recent years new data have emerged on the BPG axis in flatfish, especially at the level of the brain and pituitary. The evolutionarily advanced flatfishes are important model species both from an evolutionary point of view and also because many are candidates for aquaculture. The scope of this paper is to review the present status on the male teleost BPG axis, with an emphasis on flatfish. In doing so, we will first discuss the present understanding of the individual constituents of the axis in the best studied teleost models, and thereafter discuss available data on flatfish. Of the three constituents of the BPG axis, we will focus especially on the pituitary and gonadotropins. In addition to reviewing recent information on flatfish, we present some entirely new information on the phylogeny and molecular structure of teleost gonadotropins.

Possible gonadotropin cells in the lamprey pituitary: colocalization of mammalian LH-like immunoreactivity and glycoconjugate in adult sea lampreys (Petromyzon marinus)

In lampreys, although gonadotropin (GTH) has not yet been isolated from the pituitary gland, the presence of GTH has been strongly suggested. To detect possible GTH in the sea lamprey (Petromyzon marinus) pituitary, two different cytochemical probes were tested: One was the use of antibodies to GTHs, and the other was the use of lectin-screening kits for demonstration of glycoconjugate in hormonal molecules. GTH-like immunoreactivity was found in cells distributed in the ventral half of the proximal pars distalis. These cells were stained intensely by all four lots of anti-ovine LH including LHbeta, and were stained moderately or weakly by several other antibodies to LH-related GTHs, such as human LHbeta, hCGbeta, amphibian LH, and sturgeon GTH IIbeta. On the other hand, there were no positive reactions in the sea lamprey pituitary using the antibodies to FSH-related GTHs, thyrotropin (TSH), or pituitary glycoprotein hormones of teleost origin. Thus, GTH-like material in the sea lamprey pituitary seems to be more closely related to mammalian-like LH, rather than to FSH or TSH, as far as immunocytochemical determinants. A total of 21 kinds of lectins was tested. Among those, GTH-positive cells were also stained positively by concanavalin A and Vicia villosa agglutinin. Thus, the present study demonstrates colocalization of LH-like immunoreactivity and glycoconjugate in cells in the ventral half of the proximal pars distalis of the sea lamprey pituitary. It is suggested that those cells are most likely to be GTH cells in the sea lamprey pituitary.

Purification of gonadotropin II from a teleost fish, the hybrid striped bass, and development of a specific enzyme-linked immunosorbent assay

A highly purified gonadotropin II (GtH II), referred to as striped bass GtH II (stbGtH II), and its alpha and beta subunits were prepared from pituitaries of sexually mature hybrid striped bass. Pituitary glycoproteins were extracted with ethanol and intact stbGtH II purified by gel-filtration chromatography on Sephadex G-100, ion-exchange chromatography (IEC) on DE-52, and fast-performance liquid chromatography (FPLC) on Superdex 75. The presence of GtHs during the purification procedure was monitored by characteristic elution on reversed-phase high-performance liquid chromatography (rpHPLC) and in vitro steroidogenic activity. The stbGtH II alpha and beta subunits were purified from the pituitary ethanol extract by gel-filtration, IEC, and rpHPLC, and their identities assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), rpHPLC, and N-terminal amino acid sequencing. Molecular weights of intact stbGtH II and its alpha and beta subunits, determined by SDS-PAGE, were 34.5, 14.8, and 20.4 kDa, respectively. The stbGtH II beta subunit was used to produce specific antibodies, and a competitive enzyme-linked immunosorbent assay was developed using intact stbGtH II for the standard curve. The sensitivity of the assay was 156 pg/ml (15.6 pg/well) and the intra- and interassay coefficients of variation (at 50% binding) were 7.7% (n = 16) and 8.7% (n = 10), respectively. Physiological validation of the assay was performed by measuring changes of plasma GtH II levels in mature striped bass females, after injection of GnRHa ([d-Ala6,Pro9-NEt]-mGnRH, 100 microg/kg BW). A maximum surge of GtH II in plasma was observed at 12 hr postinjection (22.5 +/- 3. 01 ng/ml), whereas GtH II levels in control fish (around 4 ng/ml) remained unchanged. Displacement curves obtained with serial dilutions of plasma and pituitaries from a number of perciform species were parallel to the standard curve, indicating that this assay can be used for GtH II measurements in a variety of fish species.

Human chorionic gonadotropin (hCG) induces gonad reversal in a protogynous fish, the bluehead wrasse, Thalassoma bifasciatum (Teleostei, Labridae)

Female to male successive hermaphroditism (protogyny) is common in several groups of marine fish. Thalassoma bifasciatum, the bluehead wrasse (Labridae), found in the reefs of the Caribbean normally undergoes sex reversal after receiving behavioral cues. This report deals with the successful use of human chorionic gonadotropin (hCG) in inducing gonad reversal in this species. Eighty percent (n = 40) of the treated fish showed signs of reversal in 1-6 weeks; only 11% (n = 54) of the control (nontreated) group showed signs of reversal during the same period. The number of fish undergoing reversal increased with the length of the treatment period, 55% after 1 week, 100% after 6 weeks. A bluehead color pattern, typical of the terminal male phase, also appeared more frequently in the longer treated groups. To determine the efficacy of hCG in inducing gonad reversal, morphological criteria for reversal had to be established. For the majority of treated fish, the presence of both spermatogenic cysts and degenerating oocytes was sufficient to label these gonads unambiguously as undergoing reversal. However, at the extremes of this process, i.e., onset (early) and endpoint (late) stages, ambiguity could arise in identifying a gonad as undergoing reversal, and, therefore, key criteria were established for these stages. In our hands, the most consistent and reliable criterion for the early stage reversal was appearance of male germ line cells, clusters of "spermatogonial-like" cells. For the late stage, recognizable remnants of late-state oocytes had to be present. Some details of the histological changes that occur during early, middle, and late stages of reversal are also described. It is suggested that these results with hCG shed new light on the endocrine control of gonad reversal in T. bifasciatum.

Gonadotropins I and II in juvenile coho salmon

The present study was designed to obtain basic endocrine information on GTH I and GTH II in previtellogenic and prespermatogenic coho salmon (immature). Levels of GTH II in pituitary extracts were 6.5 ± 2.0 and 6.7 ± 2.0 pg/μg pituitary protein in male and female fish, respectively. In contrast, the pituitary content of GTH I was approximately 100-fold higher than GTH II (1.302 ± .22 and 1.173 ± .21 ng/μg pituitary protein in male and female fish, respectively). Plasma levels of GTH II in immature salmon were not detectable by RIA whereas plasma GTH I levels were approximately 0.62 ± 0.12 and 0.78 ± 0.13 ng/ml in male and female fish, respectively. Highly purified coho salmon GTH I and GTH II stimulated testicular testosterone production and ovarian estradiol productionin vitro in a similar manner, though GTH II appeared more potent than GTH I. Therefore, it appears that although the salmon pituitary contains predominantly GTH I prior to puberty, the gonad can respond to both GTH I and GTH II.

Short-term profiles of plasma gonadotropin and estradiol-17 beta levels in the female rainbow trout, from early ovarian recrudescence and throughout vitellogenesis

At various stages throughout the annual reproductive cycle, female rainbow trout (Salmo gairdneri) were fitted with a catheter in the dorsal aorta. They were bled via the catheter at frequencies of once every 30, 60, or 240 min over periods of 5 to 24 hr. Gonadotropin (GtH), estradiol-17 beta (E(2)17 beta). and estrone levels were measured in the plasma samples. At early ovarian recrudescence (March), short-term (1-2 hr), high-amplitude (delta GtH = up to 100 ng/ml), episodic pulses of GtH were recorded in the plasma of 12 of the 26 studied fish. In the others, GtH levels remained low and constant. No synchronization was found among the individual GtH profiles. E(2)17 beta levels in the same fish were low and constant while estrone was not detectable. At early stages of exogenous vitellogenesis (June), plasma GtH (1-3 ng/ml) and E(2)17 beta (0.5-1.5 ng/ml) levels were low and constant. At advanced stages of exogenous vitellogenesis (September-October), fluctuating GtH levels were found again in most of the females; basal GtH concentrations were only slightly higher than those recorded in June. The fluctuations consisted of short-term (1-3 hr) random GtH pulses of moderate amplitude (delta GtH = up to 5 ng/ml), occurring at a relatively high frequency (up to 5 per 12 hr). Although no regular synchronous daily pattern of GtH was noted, most of the GtH pulses were observed during the photophase and early scotophase. The appearance of GtH pulsatility during exogenous vitellogenesis was accompanied by a large increase in plasma E(2)17 beta up to levels ranging from 6 to 30 ng/ml. In contrast to the GtH profiles, the E(2)17 beta profiles showed continuous and progressive variations, superimposed the abrupt GtH pulses, and a high degree of synchronization. E(2)17 beta levels increased during the photophase and reached maxima toward and during early scotophase.

Molecular cloning and sequencing of salmon gonadotropin beta subunit

Gonadotropin (GTH) was purified from the pituitaries of the Pacific chinook salmon using a combination of stepwise ethanol precipitation and concanavalin-A affinity chromatography. The alpha and beta subunits were dissociated and fractionated by C-18 reverse-phase high-performance liquid chromatography with a 0.1% trifluoroacetic acid/acetonitrile gradient. An enriched cDNA library was screened for the beta-GTH gene(s) using two synthetic oligonucleotides based on partial protein data. A positive, full-size clone (E3) was identified and sequenced. It contains 657 base pairs and codes for a 142-amino-acid precursor protein. The mature salmon beta-GTH (119 amino acids) is structurally homologous to human luteinizing hormone and chorionic gonadotropin. The effect of testosterone implantation on pituitary GTH and beta-GTH mRNA was examined with radioimmunoassay and Northern blot analysis. There was a corresponding increase in both the pituitary GTH and mRNA levels.