Characterization and localization of mouse hypothalamic growth hormone-releasing factor and effect of gold thioglucose-induced hypothalamic lesions

Deficiency in prohormone convertase PC1 impairs prohormone processing in Prader-Willi syndrome

Prader-Willi syndrome (PWS) is caused by a loss of paternally expressed genes in an imprinted region of chromosome 15q. Among the canonical PWS phenotypes are hyperphagic obesity, central hypogonadism, and low growth hormone (GH). Rare microdeletions in PWS patients define a 91-kb minimum critical deletion region encompassing 3 genes, including the noncoding RNA gene SNORD116. Here, we found that protein and transcript levels of nescient helix loop helix 2 (NHLH2) and the prohormone convertase PC1 (encoded by PCSK1) were reduced in PWS patient induced pluripotent stem cell-derived (iPSC-derived) neurons. Moreover, Nhlh2 and Pcsk1 expression were reduced in hypothalami of fasted Snord116 paternal knockout (Snord116p-/m+) mice. Hypothalamic Agrp and Npy remained elevated following refeeding in association with relative hyperphagia in Snord116p-/m+ mice. Nhlh2-deficient mice display growth deficiencies as adolescents and hypogonadism, hyperphagia, and obesity as adults. Nhlh2 has also been shown to promote Pcsk1 expression. Humans and mice deficient in PC1 display hyperphagic obesity, hypogonadism, decreased GH, and hypoinsulinemic diabetes due to impaired prohormone processing. Here, we found that Snord116p-/m+ mice displayed in vivo functional defects in prohormone processing of proinsulin, pro-GH-releasing hormone, and proghrelin in association with reductions in islet, hypothalamic, and stomach PC1 content. Our findings suggest that the major neuroendocrine features of PWS are due to PC1 deficiency.

Differential response of arcuate proopiomelanocortin- and neuropeptide Y-containing neurons to the lesion produced by gold thioglucose administration

The effect of gold thioglucose (GTG) administration on neurons containing feeding-related peptides in the hypothalamic arcuate nucleus was examined in mice. Intraperitoneal GTG injection increased the body weight and produced a hypothalamic lesion that extended from the ventral part of the ventromedial nucleus to the dorsal part of the arcuate nucleus. Neurons containing proopiomelanocortin (POMC) and neuropeptide Y (NPY) present in the dorsal part of the arcuate nucleus were destroyed by GTG. In addition, the peptide-containing fibers that extended from the remaining arcuate neurons were degenerated at the lesion site. The number of POMC-containing fibers in the paraventricular nucleus, dorsomedial nucleus, and lateral hypothalamus was found to have decreased significantly when examined at 2 days and 2 weeks after the GTG treatment. In contrast, the number of NPY-containing fibers in the lateral hypothalamus remained unchanged after the GTG treatment, probably because of the presence of an unaffected NPY-containing fiber pathway passing through the tuberal region and projecting onto the lateral hypothalamus. The number of NPY-immunoreactive fibers in the paraventricular and dorsomedial nuclei showed a moderate but significant decrease at 2 days after the GTG treatment, but it recovered to the normal levels 2 weeks later. The NPY-containing fibers were found to have regenerated across the lesion site 2 weeks later, and this might contribute to the recovery of the NPY-immunoreactive fibers in these regions. The present results first demonstrate that POMC- and NPY-containing neurons in the arcuate nucleus respond differently to the lesion produced by the GTG treatment.

Hypothalamic 3',5'-cyclic adenosine monophosphate response element-binding protein loss causes anterior pituitary hypoplasia and dwarfism in mice

The principal regulation of body growth is via a cascade of hormone signals emanating from the hypothalamus, by release of GHRH, which then directs the somatotroph cells of the pituitary to release GH into the blood stream. This in turn leads to activation of signal transducer and activator of transcription 5-dependent expression of genes such as IGF-I in hepatocytes, acid labile substance, and serine protease inhibitor 2.1, resulting in body growth. Here, using conditional cAMP response element binding protein (CREB) mutant mice, we show that loss of the CREB transcription factor in the brain, but not the pituitary, results in reduced postnatal growth consistent with dwarfism caused by GH deficiency. We demonstrate that although there appears to be no significant impact upon the expression of GHRH mRNA in CREB mutant mice, the amount of GHRH peptide is reduced. These findings show that CREB is required for the efficient production of GHRH in hypothalamus, in addition to its previously reported role in pituitary GH production and somatotroph expansion.

Age-related changes in growth hormone (GH) cells in the pituitary gland of male mice are mediated by GH-releasing hormone but not by somatostatin in the hypothalamus

Using immunocytochemical and morphometric methods, we examine changes with age of growth hormone-releasing hormone (GHRH) in the arcuate nucleus (ARC), changes of somatostatin (SS) in the periventricular nucleus (PeN) of the hypothalamus, and changes of growth hormone (GH) cells in the anterior pituitary in male C57BL/6J mice at 2 months old (2 M), 4 M, 12 M and 24 M. The number of GHRH-ir neurons decreased significantly with age. The number of SS-ir neurons did not differ significantly between these all age groups. The volume of the anterior pituitary and the number of adenohypophysial parenchymal cells fell dramatically from 4 to 12 M. The proportion of GH-ir cells decreased significantly with age, and in absolute number from 4 to 12 M and in size from 2 to 4 M and from 4 to 12 M. These results suggest that the reduction in GH-ir cells in male mice is modulated by the reduction in GHRH-ir neurons, but not by SS-ir neurons.

Furin and prohormone convertase 1/3 are major convertases in the processing of mouse pro-growth hormone-releasing hormone

We investigated the proteolytic processing of mouse pro-GHRH [84 amino acids (aa)] by furin, PC1/3, PC2, and PC5/6A. We created six point mutations in the N- and C-terminal cleavage sites, RXXR decreased and RXRXXR decreased, respectively. The following results were obtained after transient transfection/cotransfection and metabolic pulse-chase labeling studies in several neuroendocrine cells. 1) Furin was the most efficient convertase in cleaving the N-terminal RXXR/RXRR site to generate intermediate I, 12-84aa, whereas PC1/3 was the most potent in processing the C-terminal RXRXXR site to yield mature GHRH, 12-53aa. 2) Both PC1/3 and PC5/6A also processed the N-terminal site but less efficiently than furin. 3) PC2 was much weaker in cleaving the C-terminal site relative to PC1/3 to generate mature GHRH. 4) The Q10R mutant was significantly more susceptible to furin cleavage at the N-terminal site than the wild-type pro-GHRH. And 5) the N- and C-terminal P1 Arg residues, R11 and R54, respectively, were essential for mature GHRH production. We also showed localization of the GHRH immunoreactive peptides in Golgi and secretory granules in neuroendocrine cells by an immunofluorescence assay. We conclude that the efficient production of mature GHRH from pro-GHRH is a stepwise process mediated predominantly by furin at the N-terminal cleavage site followed by PC1/3 at the C terminus.

Disruption of PC1/3 expression in mice causes dwarfism and multiple neuroendocrine peptide processing defects

The subtilisin-like proprotein convertases PC1/3 (SPC3) and PC2 (SPC2) are believed to be the major endoproteolytic processing enzymes of the regulated secretory pathway. They are expressed together or separately in neuroendocrine cells throughout the brain and dispersed endocrine system in both vertebrates and invertebrates. Disruption of the gene-encoding mouse PC1/3 has now been accomplished and results in a syndrome of severe postnatal growth impairment and multiple defects in processing many hormone precursors, including hypothalamic growth hormone-releasing hormone (GHRH), pituitary proopiomelanocortin to adrenocorticotropic hormone, islet proinsulin to insulin and intestinal proglucagon to glucagon-like peptide-1 and -2. Mice lacking PC1/3 are normal at birth, but fail to grow normally and are about 60% of normal size at 10 weeks. They lack mature GHRH, have low pituitary growth hormone (GH) and hepatic insulin-like growth factor-1 mRNA levels and resemble phenotypically the "little" mouse (Gaylinn, B. D., Dealmeida, V. I., Lyons, C. E., Jr., Wu, K. C., Mayo, K. E. & Thorner, M. O. (1999) Endocrinology 140, 5066-5074) that has a mutant GHRH receptor. Despite a severe defect in pituitary proopiomelanocortin processing to mature adrenocorticotropic hormone, blood corticosterone levels are essentially normal. There is marked hyperproinsulinemia but without impairment of glucose tolerance. In contrast, PC2-null mice lack mature glucagon and are chronically hypoglycemic (Furuta, M., Yano, H., Zhou, A., Rouille, Y., Holst, J., Carroll, R., Ravazzola, M., Orci, L., Furuta, H. & Steiner, D. (1997) Proc. Natl. Acad. Sci. USA 94, 6646-6651). The PC1/3-null mice differ from a human subject reported with compound heterozygosity for defects in this gene, who was of normal stature but markedly obese from early life. The PC1/3-null mice are not obese. The basis for these phenotypic differences is an interesting topic for further study. These findings prove the importance of PC1/3 as a key neuroendocrine convertase.

Distribution of galaninergic immunoreactivity in the brain of the mouse

The distribution of galaninergic immunoreactive (-ir) profiles was studied in the brain of colchicine-pretreated and non-pretreated mice. Galanin (GAL)-ir neurons and fibers were observed throughout all encephalic vesicles. Telencephalic GAL-ir neurons were found in the olfactory bulb, cerebral cortex, lateral and medial septum, diagonal band of Broca, nucleus basalis of Meynert, bed nucleus of stria terminalis, amygdala, and hippocampus. The thalamus displayed GAL-ir neurons within the anterodorsal, paraventricular, central lateral, paracentral, and central medial nuclei. GAL-ir neurons were found in several regions of the hypothalamus. In the midbrain, GAL-ir neurons appeared in the pretectal olivary nucleus, oculomotor nucleus, the medial and lateral lemniscus, periaqueductal gray, and the interpeduncular nucleus. The pons contained GAL-ir neurons within the dorsal subcoeruleus, locus coeruleus, and dorsal raphe. In the medulla oblongata, GAL-ir neurons appear in the anterodorsal and dorsal cochlear nuclei, salivatory nucleus, A5 noradrenergic cells, gigantocellular nucleus, inferior olive, solitary tract nucleus, dorsal vagal motor and hypoglossal nuclei. Only GAL-ir fibers were seen in the lateral habenula nucleus, substantia nigra, parabrachial complex, cerebellum, spinal trigeminal tract, as well as the motor root of the trigeminal and facial nerves. GAL-ir was also observed in several circumventricular organs. The widespread distribution of galanin in the mouse brain suggests that this neuropeptide plays a role in the regulation of cognitive and homeostatic functions.

Role of the gonads in sex differentiation of growth hormone-releasing hormone and somatostatin neurons in the mouse hypothalamus during postnatal development

We clarify the mechanism of sexual dimorphism of growth hormone releasing hormone (GHRH) neurons in the arcuate nucleus (ARC) and somatostatin (SS) neurons in periventricular nucleus (PeN), by studying the role of the gonads during the neonatal period and after puberty using immunohistochemical and morphometric methods. As in our previous works the numbers of ARC GHRH-ir and PeN SS-ir neurons were significantly greater in adult normal male (NM) mice than in adult normal female (NF) mice. Adult female mice that were ovariectomized neonatally (NOF) increased the expression of GHRH-ir neurons to the male pattern, but adult female mice ovariectomized after puberty (APO) did not change. Adult male mice castrated neonatally and after puberty (NCM and APC, respectively) were not significantly different from NM mice. However, NCT male mice, which were castrated neonatally and transplanted with ovary just before puberty, showed a significantly reduced number of GHRH-ir neurons compared with NCM mice, but no significant difference from NM and NF mice. On the other hand, the PeN SS-ir neuron expression in NCM mice and APC mice showed a significant reduction compared with NM mice, but no significant difference from NF mice. The number of PeN SS-ir neurons in NOF increased to match that of NM mice. Our results suggest that the presence of the ovary during postnatal life inhibits the development of ARC GHRH-ir neurons. The presence of the testis during postnatal life may stimulate the development of PeN SS-ir neurons, while the presence of the ovary during neonatal period may inhibit the development of PeN SS-ir neurons; the presence of ovary after puberty does not inhibit.

Sex differentiation of growth hormone-releasing hormone and somatostatin neurons in the mouse hypothalamus: an immunohistochemical and morphological study

We examine sexual dimorphism in growth hormone-releasing hormone (GHRH) in the arcuate nucleus (ARC), and somatostatin (SS) in the periventricular nucleus (PeN) of the hypothalamus, and investigate when it becomes evident. Using immunohistochemical staining and morphometry, we observed ARC GHRH-immunoreactive (ir) neurons, ARC SS-ir neurons and PeN SS-ir neurons in male and female mice at 5, 20, 30, 40 and 60 days old. The number of ARC GHRH-ir neurons was significantly higher in males than females, after 20 days old. ARC SS-ir neurons showed no significant differences between sexes. On the other hand, PeN SS-ir neurons were significantly more numerous in males at 30, 40 and 60 days than in females. During postnatal development, these GHRH- and SS-ir neurons changed in different patterns from ages 20 to 60 days. The number of ARC GHRH-ir neurons in both sexes decreased from 5 to 20 days, increased until day 40, and then decreased at day 60, while ARC SS-ir neurons in both sexes increased from day 5 to day 60. PeN SS-ir neurons in both sexes increased from days 5 to 20 to 116% in males and 189% in females. Furthermore, in male mice, the increase continued until 40 days of age, while in females, there was no significant difference from days 20 to 60. There were no apoptotic cells; a few proliferating cell nuclear antigen (PCNA) stained cells were found in the ARC and PeN. Our results suggest that the sex difference of ARC GHRH neurons and PeN SS neurons appears by stimulation with testosterone during the development life. The developmental fluctuation in the number of ARC GHRH-ir neurons may not be modulated by testosterone, but by ARC SS neurons.

Expression of a fusion gene consisting of the mouse growth hormone-releasing hormone gene promoter linked to the SV40 T-antigen gene in transgenic mice

Limited information is available concerning the regulation of growth hormone-releasing hormone (GHRH) gene expression in the hypothalamus, largely because of the lack of a suitable cellular model. In an attempt to immortalize hypothalamic GHRH-producing neurons, we have generated a transgenic mouse model which expresses the simian virus 40 (SV40) T-antigen gene (Tag) under the control of the GHRH gene promoter. The transgene contains approximately 5 kb of mouse GHRH gene sequences, including 3.5 kb of the 5'-flanking region, the entire hypothalamic exon 1 and 1.5 kb of intron 1, fused to the SV40 Tag gene. This construct was microinjected into fertilized oocytes. Fourteen of 96 mice born had integrated the transgene. These mice were fertile and showed no signs of central or peripheral tumors. The pattern of expression of the SV40 Tag gene was analyzed in four different transgenic lines by RT-PCR. The tissues tested include: hypothalamus, pituitary, cortex, cerebellum, spinal cord, adrenal, testis, spleen and lung. Transgene expression was consistently detected in the hypothalamus of all lines. In addition, SV40 Tag expression was also detected in the hypothalamus by Northern blot analysis in two of the transgenic lines. SV40 Tag expression was also detected in the testis of all transgenic lines by RT-PCR. This result was not expected since the GHRH gene sequences present in the transgene do not include the testis-specific transcription initiation site previously described. This suggests that GHRH gene expression in the mouse testis can be directed by regulatory sequences located downstream of the testis specific transcription start site. We conclude that the promoter region of the GHRH gene included in this construct contains the regulatory elements necessary to drive hypothalamic and testis expression in vivo. In addition, all mice from one of the transgenic lines developed cataracts in both eyes. SV40 Tag expression was detected not only in eyes with cataracts, but also, to a lesser extent, in eyes from other transgenic lines. Furthermore, the endogenous GHRH gene was found to be expressed in the eyes of normal mice.

Immunohistochemical evidence of neurons with GHRH or LHRH in the arcuate nucleus of male mice and their possible role in the postnatal development of adenohypophysial cells

BACKGROUND

The neonatal administration of monosodium L-glutamate (MSG) has been used in investigations of the possible role of the arcuate nucleus in neuroendocrine regulation during postnatal development. We used this method to examine whether the mouse arcuate contained cell bodies immunoreactive with antisera to growth hormone releasing hormone (GHRH) or luteinizing hormone releasing hormone (LHRH), and whether these hypothalamic peptides affect synthesis and secretion of growth hormone and gonadotropin and the testis.

METHODS

The hypothalamus, pituitary, and testes of adult male mice treated with MSG during the neonatal period were fixed in Bouin's fluid or 10% neutral formalin. The hypothalamus was used in immune staining, the pituitary was used in both morphometry and immune staining, and the testis was stained with hematoxylin and eosin.

RESULTS

Body weights in control and treated mice were not different. The treated mice had more subcutaneous adipose tissue and a shorter body than the control mice. The testes were heavier in the controls. Many perikarya immunoreactive with antisera to GHRH or LHRH were found in the arcuate nucleus in control mice, but few such perikarya were found in this nucleus in treated mice. The size of the anterior lobe and the number and size of GH cells, follicle stimulating hormone (FSH) cells, and prolactin (PRL) cells in treated mice were less than those of control mice.

CONCLUSIONS

GHRH and LHRH neurons in the arcuate nucleus in male mice may cause body and testis weight to increase via GH and LH cells, respectively, in the adenohypophysis during postnatal development. There are some differences in the hypothalamo-pituitary-testis axis of mice and rats.

Molecular cloning of cDNA encoding the precursor for hamster hypothalamic growth hormone-releasing factor

The structure of rat and mouse growth hormone-releasing factor (GRF) peptide and precursor shows considerable divergence from that of the human counterpart and also within rodents themselves. To study such structural divergence in another rodent, we cloned a cDNA encoding the GRF precursor from golden hamster. The hamster GRF (haGRF) cDNA clone had an open-reading frame that predicts a haGRF precursor protein with 107 amino acids. The haGRF precursor bore greater overall homology (82%) to the human than the same rodent homologue (58-64%) and contained two processing sites identical to the human sequence that would generate mature haGRF peptide. Furthermore, the haGRF peptide, like human but unlike rat or mouse GRF, consisted of 44 amino acids and also had greater homology to the human (89%) than the rodent sequence (64-75%), conserving a Tyr residue at the N-terminus and an amidated Leu residue at the C-terminus. Thus, both haGRF precursor and peptide are structurally more related to those of human than of other rodents, suggesting that rodent GRF precursor diverged from the human sequence at differential rates within the species.