Functional divergence of thyrotropin beta-subunit paralogs gives new insights into salmon smoltification metamorphosis

Smoltification is a metamorphic event in salmon life history, which initiates downstream migration and pre-adapts juvenile salmon for seawater entry. While a number of reports concern thyroid hormones and smoltification, few and inconclusive studies have addressed the potential role of thyrotropin (TSH). TSH is composed of a α-subunit common to gonadotropins, and a β-subunit conferring hormone specificity. We report the presence and functional divergence of duplicated TSH β-subunit paralogs (tshβa and tshβb) in Atlantic salmon. Phylogeny and synteny analyses allowed us to infer that they originated from teleost-specific whole genome duplication. Expression profiles of both paralogs in the pituitary were measured by qPCR throughout smoltification in Atlantic salmon from the endangered Loire-Allier population raised in a conservation hatchery. This revealed a striking peak of tshβb expression in April, concomitant with downstream migration initiation, while tshβa expression remained relatively constant. In situ hybridization showed two distinct pituitary cell populations, tshβa cells in the anterior adenohypophysis, and tshβb cells near to the pituitary stalk, a location comparable to the pars tuberalis TSH cells involved in seasonal physiology and behaviour in birds and mammals. Functional divergence of tshβ paralogs in Atlantic salmon supports a specific role of tshβb in smoltification.

Identification and localization of eight distinct hormone-producing cell types in the pituitary of male Atlantic halibut (Hippoglossus hippoglossus L.)

The eight distinct hormone-producing cell types in the adenohypophysis of male Atlantic halibut (Hippoglossus hippoglossus L.) were identified and localized using immunohistochemistry and in situ hybridization. Lactotropes either occupied most of the rostral pars distalis (RPD) or they were arranged in follicular structures located along the periphery of the RPD. Corticotropes were confined to a thin layer of RPD cells bordering the pars nervosa (PN). The somatotropes were arranged in multicellular layers bordering the highly convoluted PN penetrating the proximal pars distalis (PPD), while thyrotropes, scattered in small islets in between the somatotropes, were located in the centro-dorsal part of the PPD. Gonadotropes were found throughout the PPD. Immunoreactivity to glycoprotein-alpha and luteinizing hormone beta-subunit was also observed along the periphery of the pars intermedia (PI), indicating that a thin extension of the PPD surrounded the PI. In situ hybridization showed that follicle-stimulating hormone and luteinizing hormone were produced in distinct cells of the PPD. PI contained somatolactotropes bordering the highly convoluted PN, and melanotropes that showed positive immunostaining against both anti-alpha-melanocyte-stimulating hormone and anti-beta-endorphin. The general cellular organization was similar to that of other teleost fish. These results lay the basis for future investigations on Atlantic halibut pituitary physiology.

The adenohypophysis of Mediterranean yellowtail, Seriola dumerilii (Risso, 1810): an immunocytochemical study

The adenohypophysis (ADH) of the Mediterranean yellowtail was studied using the peroxidase-antiperoxidase technique. Human corticotropin (ACTH) (1-24)-immunoreactive (ir) cells were found bordering the neurohypophysis (NH) and salmon prolactin (PRL)-ir cells were arranged in thick cords, both in the rostral pars distalis (RPD). Gonadotropin (GTH)-, thyrotropin (TSH)- and growth hormone (GH)-ir cells were observed in the proximal pars distalis (PPD). Anti-chum salmon GTH I and anti-chum salmon GTH II immunostained the same cells in the outermost part of the ADH at the level of the PPD and the PI. In addition to these cells, some cells grouped in the inner areas of the posterior PPD were revealed by catfish alpha, beta-GTH antiserum. Human beta-TSH-ir cells formed small groups and discontinuous strands in the PPD often in contact with the NH. Tilapia GH-ir cells formed cords mainly surrounding the NH in the central PPD, while cod somatolactin- and alpha MSH-ir cells mainly surrounded the NH branches in the PI.

Salmonid pituitary gonadotrophs. I. Distinct cellular distributions of two gonadotropins, GTH I and GTH II

Using antisera specific for the beta subunits of two distinct coho salmon gonadotropins, GTH I and GTH II, an immunocytochemical study of rainbow trout and Atlantic salmon pituitaries was done. Cells which immunostained with anti-GTH I beta were distributed in the periphery of the glandular cords of the proximal pars distalis (PPD), in close association with somatotrophs. On the other hand, cells immunostained with anti-GTH II beta were located in the central parts of the glandular cords of the PPD. Neither the GTH I-producing nor the GTH II-producing cells stained with antisera against chum salmon growth hormone or the beta subunit of human thyroid-stimulating hormone. Moreover, GTH I and GTH II were localized in distinctly different cells. In no case was colocalization of these GTHs in the same cell observed. Finally, it was concluded that classification of GTH cells as globular and vesicular forms does not reflect the type of hormone produced by the cell, but may reflect differences in the physiological conditions of the cells.

Immunocytochemistry of gonadotropic cells in the pituitary of some teleost species

An antiserum (anti-sGTH) raised against salmon gonadotropin (SG-G100) was tested on pituitary sections of Sarotherodon mossambicus, Carassius auratus, Anguilla anguilla (intact and estradiol-treated to induce the development of the gonadotropic (GTH) cells), Salmo trutta fario, and a protandric marine teleost (Sarpa salpa, Sparidae). Using an immunoenzymologic technique, anti-sGTH stained GTH and thyrotropic (TSH) cells in the species studied. Anti-sGTH stained the granulated GTH cells in S. trutta fario and S. salpa; the vacuolated cells remained faintly stained or unlabeled in most cases. The results are discussed in the light of cytological, ultrastructural, and biochemical data which suggest that two forms of GTH cells may be present in some teleost pituitaries.