Identification of growth hormone molecular variants in chicken serum

Expression, cellular distribution, and heterogeneity of growth hormone in the chicken cerebellum during development

Although growth hormone (GH) is mainly synthesized and secreted by pituitary somatotrophs, it is now well established that the GH gene can be expressed in many extrapituitary tissues, including the central nervous system (CNS). Here we studied the expression of GH in the chicken cerebellum. Cerebellar GH expression was analyzed by in situ hybridization and cDNA sequencing, as well as by immunohistochemistry and confocal microscopy. GH heterogeneity was studied by Western blotting. We demonstrated that the GH gene was expressed in the chicken cerebellum and that its nucleotide sequence is closely homologous to pituitary GH cDNA. Within the cerebellum, GH mRNA is mainly expressed in Purkinje cells and in cells of the granular layer. GH-immunoreactivity (IR) is also widespread in the cerebellum and is similarly most abundant in the Purkinje and granular cells as identified by specific neuronal markers and histochemical techniques. The GH concentration in the cerebellum is age-related and higher in adult birds than in embryos and juveniles. Cerebellar GH-IR, as determined by Western blot under reducing conditions, is associated with several size variants (of 15, 23, 26, 29, 35, 45, 50, 55, 80 kDa), of which the 15 kDa isoform predominates (>30% among all developmental stages). GH receptor (GHR) mRNA and protein are also present in the cerebellum and are similarly mainly present in Purkinje and granular cells. Together, these data suggest that GH and GHR are locally expressed within the cerebellum and that this hormone may act as a local autocrine/paracrine factor during development of this neural tissue.

O-Glycosylated 24 kDa human growth hormone has a mucin-like biantennary disialylated tetrasaccharide attached at Thr-60

MS was used to characterize the 24 kDa human growth hormone (hGH) glycoprotein isoform and determine the locus of O-linked oligosaccharide attachment, the oligosaccharide branching topology, and the monosaccharide sequence. MALDI-TOF/MS and ESI-MS/MS analyses of glycosylated 24 kDa hGH tryptic peptides showed that this hGH isoform is a product of the hGH normal gene. Analysis of the glycoprotein hydrolysate by high-performance anion-exchange chromatography with pulsed amperometric detection and HPLC with fluorescent detection for N-acetyl neuraminic acid (NeuAc) yielded the oligosaccharide composition (NeuAc(2), N-acetyl galactosamine(1), Gal(1)). After beta-elimination to release the oligosaccharide from glycosylated 24 kDa hGH, collision-induced dissociation of tryptic glycopeptide T6 indicated that there had been an O-linked oligosaccharide attached to Thr-60. The sequence and branching structure of the oligosaccharide were determined by ESI-MS/MS analysis of tryptic glycopeptide T6. The mucin-like O-oligosaccharide sequence linked to Thr-60 begins with N-acetyl galactosamine and branches in a bifurcated topology with one appendage consisting of galactose followed by NeuAc and the other consisting of a single NeuAc. The oligosaccharide moiety lies in the high-affinity binding site 1 structural epitope of hGH that interfaces with both the growth hormone and the prolactin receptors and is predicted to sterically affect receptor interactions and alter the biological actions of hGH.

Chicken growth hormone: further characterization and ontogenic changes of an N-glycosylated isoform in the anterior pituitary gland

Glycosylation is one of the post-translational modifications that growth hormone (GH) can undergo. This has been reported for human, rat, mouse, pig, chicken and buffalo GH. The nature and significance of GH glycosylation remains to be elucidated. This present study further characterizes glycosylated chicken GH (G-cGH) and examines changes in the pituitary concentration of G-cGH during embryonic development and post hatching growth. G-cGH was purified from chicken pituitaries by affinity chromatography (Concanavalin A-Sepharose and monoclonal antibody bound to Sepharose). Immunoreactive G-cGH has a MW of 26 kDa or 29 kDa as determined by SDS-PAGE, respectively, under non-reducing and reducing conditions. Evidence that it is N-glycosylated comes from its susceptibility to peptide N-glycosidase F, and its resistance to O-glycosidase. Based on the ability of G-cGH to bind Concanavalin A or wheat germ agglutinin but not other lectins and its susceptibility to peptide N-glycosidase F, a hybrid or biantennary type glycopeptide (GlcNac2, Man) structure is proposed. Some G-cGH can be observed in the pituitary at most ages examined (from 15-day embryo to adult). Moreover, electron microscopy revealed the presence of both immuno-reactive GH and Concanavalin A-reactive sites in the same secretory granules in the somatotrope. There were marked changes in the level and relative proportion of G-cGH in the pituitary gland during development and growth, the proportion of G-cGH rising during late embryonic development (e.g., between 15 and 18 days of development) and with further increases between 9 weeks and 15 weeks old. G-cGH was able to bind to chicken liver membrane preparations with less affinity than non-glycosylated monomer; on the other hand, however, G-cGH stimulated cell proliferation on Nb2 lymphoma bioassay whereas the non-glycosylated monomer was uncapable to do it.

The chicken pituitary expresses an ovoinhibitor-like protein in subpopulations of some, but not all, hormone-producing cell types

Ovoinhibitor is a serine protease-inhibiting protein that was originally purified from egg whites. It is secreted by the oviduct under the control of estrogen and progesterone and it specifically inhibits serine proteinases such as trypsin and chymotrypsin. During recent attempts to raise monoclonal antibodies (Mabs) against chicken bursa of Fabricius proteins, one Mab was produced that specifically recognized chicken ovoinhibitor. This was the first demonstration of ovoinhibitor in an avian immune organ. We presently report on the expression of an ovoinhibitor-like molecule by the pituitary of the chicken as revealed by immunocytochemistry and RT-PCR. Immunofluorescent dual staining experiments using the mouse anti-ovoinhibitor Mab in conjunction with polyclonal antibodies against various hypophysial hormones revealed partial co-localization of an ovoinhibitor-like molecule with growth hormone (GH), luteinizing hormone (LH), and pro-opiomelanocortin (POMC), in a subset of the respective hormone producing cells. By contrast, no co-localization with prolactin (PRL) could be reliably demonstrated. RT-PCR of hypophysial mRNA using ovoinhibitor gene-specific primers yielded an amplicon that was 20% shorter than predicted on the basis of the published ovoinhibitor sequence. Sequencing revealed that of the represented exons only the central portion was expressed in the pituitary and that both 5' and 3' ends of each exon had been truncated. While expression of ovalbumin-like serine protease inhibitors (serpins) has been previously reported in the rat pituitary, to our knowledge, this is the first report of a Kazal-type serine protease inhibitor in the vertebrate neuroendocrine system.

Differential secretion of chicken growth hormone variants after growth hormone-releasing hormone stimulation in vitro

Variants of growth hormone (GH) are present in most vertebrates. Chicken GH (cGH) undergoes posttranslational modifications that contribute to its structural diversity. Although the 22-kDa form of GH is the most abundant, some other variants have discrete bioactivities that may not be shared by others. The proportion of cGH variants changes during ontogeny, suggesting that they are regulated differentially. The effect of growth hormone-releasing hormone (GHRH) on the release of cGH variants was studied in both pituitary gland and primary cell cultures, employing sodium dodecyl sulfate polyacrylamide gel electrophoresis, Western blotting, and densitometry. GHRH (2 nM, 2 h) stimulated the secretion of most of the size variants of cGH although the amplitude of increase was not equal for each one. A differential effect on the secretion of GH size variants, particularly on the 22- (monomer) and 26-kDa (putatively glycosylated) cGH isoforms was found in both systems. In the whole pituitary culture, the proportion of the 26-kDa immunoreactive cGH increased 35% while the 22 kDa decreased 31% after GHRH treatment in comparison with the controls. In the primary cell culture system, the proportion of the glycosylated variant increased 43% whereas the monomer and the dimer decreased 22.26 and 29%, respectively, after GHRH stimulation. Activators of intracellular signals such as 1 mM 8-bromo-cAMP and 1 microM phorbol myristate acetate had a similar effect to that obtained with GHRH. The data support the hypothesis that GH variants may be under differential control and that GHRH promotes the release of a glycosylated cGH variant that has an extended half-life in circulation.

Growth hormone size variants: changes in the pituitary during development of the chicken

There is considerable evidence for the existence of structural variants of growth hormone (GH). The chicken is a useful model for investigating GH heterogeneity as both size and charge immunoreactive-(ir) variants have been observed in the pituitary and plasma. The present study examined the size distribution of ir-GH in the pituitary gland of chicken, from late embryogenesis through adulthood. Pituitaries were homogenized in the presence of protease inhibitor, and the GH size variants were separated by SDS-PAGE, transferred by Western blotting, immunostained with a specific antiserum to chicken GH, and quantitated by chemiluminescence followed by laser densitometry (chemiluminescent assay). Under nonreducing conditions ir-GH bands of 15, 22, 25, 44, 50, 66, 80, 98, 105 and >110 kDa were observed. Both the relative proportion of the GH size variants and the total pituitary content varied with developmental stage and age. The proportion of the 15-kDa fragment was greatest in the embryonic stage, and then it decreased. The proportion of the monomeric 22-kDa form was lowest at 18 days of embryogenesis (dE) and highest at 20 dE. In contrast, the high MW forms (>/=66 kDa) were lowest in embryos, and they increased (P < 0.05) after hatching. The 22-, 44-, 66-, and 80-kDa forms were assayed for activity by radioreceptor assay following isolation by semipreparative SDS-PAGE. Only the 22-kDa GH variant showed radioreceptor activity. Under reducing conditions for SDS-PAGE, ir-GH bands of 13, 15, 18, 23, 26, 36, 39, 44, 48, 59 and 72 kDa were oberved, but most of the high MW form disappeared. There was a concomitant increase in the proportion of the monomeric band and of several submonomeric forms. The present data indicate that the expression, processing, and/or release of some if not all size variants are under some differential control during growth and development of the chicken.

Baculovirus-mediated expression of chicken growth hormone

A full-length chicken growth hormone (cGH) cDNA was placed downstream from the Autograph californica nuclear polyhedron virus, AcNPV, polyhedron gene promoter and expressed in Sf9 insect cells. Secreted recombinant cGH levels averaged 2-10 micrograms/ml from day 5-10 postinfection. The recombinant cGH analyzed by SDS-PAGE gels and Western blotting consisted of a doublet with M(r) of 26.5 and 23.5 kDa. Analysis by 2-D electrophoresis of partially-purified recombinant cGH and purified native cGH revealed similar immunoreactive charge isoforms and M(r) variants. The recombinant hormone was biologically active in a homologous radioreceptor assay. The results show that cGH expressed in insect cells is biologically and immunologically active, and that a variety of isoforms are secreted which exhibit size and charge properties similar to those of pituitary-derived cGH.

Ontogeny of pituitary and serum growth hormone in growing turkeys as measured by radioimmunoassay and radioreceptor assay

Some mammalian studies have revealed a wide discrepancy in pituitary and circulating growth hormone (GH) measurements determined by immunological and biological assay methods. Recent studies demonstrating that avian GH exists in numerous isoforms raise concerns that immunological measurement of GH may not accurately reflect the amount of biologically active hormone present. We sampled eight different male turkeys of a commercial strain weekly until 6 wk of age, and then biweekly until 20 wk. Total pituitary GH content and serum GH concentration were measured by avian GH RIA and radioreceptor assay (RRA). The highest mean serum GH concentration occurred at 3 wk, and the ontogeny of serum GH content from 1 to 8 wk was similar whether measured by RIA or RRA. Pituitary GH content was highest at 6 wk, but RIA and RRA estimates differed markedly throughout the study. Pituitary content of biologically active GH, as estimated by RRA, exceeded that of immunologically active GH from 2 to 10 wk, whereas the reverse was true at 14, 18, and 20 wk. We conclude that this avian GH RIA accurately measures bioactive circulating turkey GH, but that the pituitary of the young turkey may contain bioactive GH isoforms that have poor immunological activity in our RIA.