Pro-opiomelanocortin gene is expressed in post-implantation mouse embryos and enhances growth potential of myogenic cells

Impaired neonatal survival of pro-opiomelanocortin null mutants

Intercrosses of heterozygous pro-opiomelanocortin (POMC) mice result in homozygous null progeny at lower frequencies than expected. Genotyping offspring at pre-, peri-, and postnatal stages revealed that over half of homozygous null mutants die in the early postnatal stages. To investigate the reasons for this early postnatal lethality, we analyzed in detail different parameters in the initial hours after birth. POMC null mutants born to heterozygous dams presented at birth with corticosterone levels no different from wildtype littermates, were euglycemic, and had normal liver glycogen stores. However, already 30 min after birth corticosterone levels dropped by 80% and were undetectable thereafter, while corticosterone levels in wildtype animals increased during postnatal hours. Circulating adrenaline was almost below detection 1h after birth. Blood glucose levels fell sharply in all genotypes within 30 min after birth; however, wildtype and heterozygous pups overcame hypoglycemia within an hour, while mutant pups stayed hypoglycemic. The depletion of liver glycogen stores in mutant pups was significantly less efficient compared to their littermates in the hours after birth. POMC null mutant mice born to POMC null mutant dams completely lack corticosterone and die of the expected respiratory dysfunction. In contrast, POMC null mutant mice born to heterozygous dams do not die of respiratory problems, but rather due to hypoglycemia. Our studies confirm an essential involvement of POMC peptides and of adrenal glucocorticoids and catecholamines on glucose homeostasis critical for early postnatal survival.

Expression patterns of hormones, signaling molecules, and transcription factors during adenohypophysis development in the chick embryo

The chick embryo is an ideal model to study pituitary cell-type differentiation. Previous studies describing the temporal appearance of differentiated pituitary cell types in the chick embryo are contradictory. To resolve these controversies, we used RT-PCR to define the temporal onset and in situ hybridization and immunohistochemistry to define the spatial localization of hormone expression within the pituitary. RT-PCR detected low levels of Fshbeta (gonadotropes) and Pomc (corticotropes, melanotropes) mRNA at E4 and Gh (somatotropes), Prl (lactotropes), and Tshbeta (thyrotropes) mRNA at E8. For all hormones, sufficient accumulation of mRNA and/or protein to permit detection by in situ hybridization or immunohistochemistry was observed approximately 3 days later and in all cases corresponded to a notable increase in RT-PCR product. We also describe the expression patterns of signaling (Bmp2, Bmp4, Fgf8, Fgf10, Shh) and transcription factors (Pitx1, Pitx2, cLim3) known to be important for pituitary organogenesis in other model organisms.

Different developmental patterns of melanocortin MC3 and MC4 receptor mRNA: predominance of Mc4 in fetal rat nervous system

Melanocortins are thought to be involved in neuronal development and regeneration. Pro-opiomelanocortin (POMC), the precursor of alpha-melanocyte stimulating hormone (alpha-MSH), gamma-MSH, ACTH, and beta-endorphin, becomes detectable in rat hypothalamic neurons from gestational day (E) 12.5. We recently described stage- and region-specific ontogenetic patterns of binding sites for the alpha-MSH analogue [125I]-Nle4,D-Phe7-alpha-MSH ([125I]-NDP), with the first localizations in epithalamus and sympathetic chain at E13. [125I]-NDP binds to all known melanocortin receptors, including MC3-R and MC4-R, the predominant melanocortin receptors in nervous system. To identify the receptor type expressed during ontogeny, the developmental pattern of MC3-R and MC4-R mRNA was investigated by in situ hybridization in fetuses and offspring of time-pregnant Long Evans rats between E14 and postnatal day (P) 27. MC4-R mRNA was found to be the predominant species during the entire fetal period. It was localized in all fetal areas exhibiting distinct [125I]-NDP binding, starting with sympathetic ganglia and epithalamus (E14), and including sensory trigeminal nuclei (E16), dorsal motor nucleus of vagus (E16) and cranial nerve ganglia, inferior olive (E18) and cerebellum (E18), striatal regions (E16), and entorhinal cortex (E22). In contrast, MC3-R mRNA was detectable only in the postnatal period, with a fast increase in expression in the ventromedial and arcuate nuclei. The early presence of MC4-R mRNA in central and peripheral nervous system and transient regional peaks of mRNA expression, often concomitant with periods of neural network formation, suggest a role of this receptor type in early ontogeny. The MC3 receptor may be involved in analogous processes during postnatal development.

Paracrine communication in the anterior pituitary as studied in reaggregate cell cultures

The classical image of the endocrine system is that secretory function of a gland is regulated from outside that gland by other organs. Focused on the pituitary gland, hormone secretion by the anterior lobe is under control of peptides and biogenic amines produced by the hypothalamus. About a decade ago, our group launched the new idea that functioning of the anterior pituitary (AP) is also regulated from within, i.e., that the constituent cell types inter-communicate to control hormone secretion. Extensive in vitro research has now provided a body of evidence that paracrine communication plays an important role, not only in regulation of hormone secretion but also in development, growth, and differentiation of the AP [reviewed in Denef (1994) The Pituitary Gland, pp. 351-378]. It further revealed that crosstalk between the cells is mediated by local, paracrine, factors. The main objective of our research is to identify those factors, their actions and the producing and target cell type(s) in order to unravel the paracrine communication network that is functional in the AP. Equally important, we set the step towards in vivo examination of the results obtained in vitro using transgenic mice. In the present article, we will review the technology used, three examples of AP cell-to-cell interactions studied, and we will discuss the value of transgenic animal models in the study of AP paracrine communication.

Region- and stage-specific patterns of melanocortin receptor ontogeny in rat central nervous system, cranial nerve ganglia and sympathetic ganglia

Observations on developmental actions of melanotropic peptides in nervous system have been difficult to interpret in the absence of data on receptor ontogeny. We investigated binding of [125I]Nle4,D-Phe7-alpha-MSH ([125I]NDP) in developing Long Evans rats from gestational day (E) 13 by quantitative autoradiography. Regional [125I]NDP binding characteristics were assessed by competition experiments in early postnatal brain. The study revealed region- and stage-specific, often transient ontogenetic patterns. Sympathetic ganglia exhibit high [125I]NDP binding from E13, with a peak in superior cervical ganglion at E16-E18. The first central [125I]NDP binding sites transiently appear in parts of thalamus between E13 and E15. The early fetal period is characterized by prominent peaks of receptor density in somatosensory and viscerosensory nuclei (trigeminal sensory nuclei, solitary tract nucleus), paralleled by receptor expression in 5th, 7th, 9th and 10th cranial nerve ganglia. During late fetal life, receptor density peaks in dorsal motor nucleus of vagus and inferior olive; binding sites transiently appear in cerebellum. Caudate-putamen, nucleus accumbens, olfactory tubercle and septohippocampal nucleus show a high perinatal maximum. Starting with late fetal piriform cortex, [125I]NDP binding peaks sequentially in cerebral cortical areas, with highest levels in entorhinal cortex. Preoptic, septal, hypothalamic and amygdaloid areas known for elevated receptor densities in adulthood, exhibit a slow, peri- and postnatal receptor ontogeny. Temporal relations to regional developmental processes support the idea of a role of melanocortins during ontogeny.