Peripheral administration of the N-terminal pro-opiomelanocortin fragment 1-28 to Pomc-/- mice reduces food intake and weight but does not affect adrenal growth or corticosterone production

Identification of Three POMCa Genotypes in Largemouth Bass (Micropterus salmoides) and Their Differential Physiological Responses to Feed Domestication

Diverse feeding habits in teleosts involve a wide range of appetite-regulating factors. As an appetite-suppressing gene, the polymorphisms of POMCa in largemouth bass (Micropterus salmoides) were validated via sequencing and high-resolution melting (HRM). The frequency distribution of different POMCa genotypes were analyzed in two populations, and physiological responses of different POMCa genotypes to feed domestication were investigated. The indel of an 18 bp AU-rich element (ARE) in the 3' UTR and four interlocked SNP loci in the ORF of 1828 bp of POMCa cDNA sequence were identified in largemouth bass and constituted three genotypes of POMC-A I, II, and III, respectively. POMC-A I and Allele I had increased frequencies in the selection population than in the non-selection population (p < 0.01), 63.55% vs. 43.33% and 0.7850 vs. 0.6778, respectively. POMC-A I possessed the lowest value of POMCa mRNA during fasting (p < 0.05) and exhibited growth and physiological advantages under food deprivation and refeeding according to the levels of body mass and four physiological indicators, i.e., cortisol (Cor), growth hormone (GH), insulin-like growth factor-1 (IGF-1), and glucose (Glu). The identification of three POMCa genotypes, alongside their varying physiological responses during feed domestication, suggests a selective advantage that could be leveraged in molecular marker-assisted breeding of largemouth bass that are adapted to feeding on formula diet.

Current insight into the transient X-zone in the adrenal gland cortex

Mouse models have been widely used in the study of adrenal gland development and diseases. The X-zone is a unique structure of the mouse adrenal gland and lineage-tracing studies show that the X-zone is a remnant of the fetal adrenal cortex. Although the X-zone is considered analogous to the fetal zone in the human adrenal cortex, the functional significance of the X-zone has remained comparatively more obscure. The X-zone forms during the early postnatal stages of adrenal development and regresses later in a remarkable sexually dimorphic fashion. The formation and regression of the X-zone can be different in mice with different genetic backgrounds. Mouse models with gene mutations, hormone/chemical treatments, and/or gonadectomy can also display an aberrant development of the X-zone or alternatively a dysregulated X-zone regression. These models have shed light on the molecular mechanisms regulating the development and regression of these unique adrenocortical cells. This review paper briefly describes the development of the adrenal gland including the formation and regression processes of the X-zone. It also summarizes and lists mouse models that demonstrate different X-zone phenotypes.

Adrenal cortex renewal in health and disease

Resident progenitor and/or stem cell populations in the adult adrenal cortex enable cortical cells to undergo homeostatic renewal and regeneration after injury. Renewal occurs predominantly in the outer layers of the adrenal gland but newly formed cells undergo centripetal migration, differentiation and lineage conversion in the process of forming the different functional steroidogenic zones. Over the past 10 years, advances in the genetic characterization of adrenal diseases and studies of mouse models with altered adrenal phenotypes have helped to elucidate the molecular pathways that regulate adrenal tissue renewal, several of which are fine-tuned via complex paracrine and endocrine influences. Moreover, the adrenal gland is a sexually dimorphic organ, and testicular androgens have inhibitory effects on cell proliferation and progenitor cell recruitment in the adrenal cortex. This Review integrates these advances, including the emerging role of sex hormones, into existing knowledge on adrenocortical cell renewal. An in-depth understanding of these mechanisms is expected to contribute to the development of novel therapies for severe endocrine diseases, for which current treatments are unsatisfactory.

ACTH signalling and adrenal development: lessons from mouse models

The melanocortin-2-receptor (MC2R), also known as the ACTH receptor, is a critical component of the hypothalamic-pituitary-adrenal axis. The importance of MC2R in adrenal physiology is exemplified by the condition familial glucocorticoid deficiency (FGD), a potentially fatal disease characterised by isolated cortisol deficiency. MC2R mutations cause ~25% of cases. The discovery of a MC2R accessory protein MRAP, mutations of which account for ~20% of FGD, has provided insight into MC2R trafficking and signalling. MRAP is a single transmembrane domain accessory protein highly expressed in the adrenal gland and essential for MC2R expression and function. Mouse models helped elucidate the action of ACTH. The Mc2r-knockout (Mc2r - / - ) mice was the first mouse model developed to have adrenal insufficiency with deficiencies in glucocorticoid, mineralocorticoid and catecholamines. We recently reported the generation of the Mrap - / - mice which better mimics the human FGD phenotype with isolated glucocorticoid deficiency alone. The adrenal glands of adult Mrap - / - mice were grossly dysmorphic with a thickened capsule, deranged zonation and deranged WNT4/beta-catenin and sonic hedgehog (SHH) pathway signalling. Collectively, these mouse models of FGD highlight the importance of ACTH and MRAP in adrenal progenitor cell regulation, cortex maintenance and zonation.

POMC: The Physiological Power of Hormone Processing

Pro-opiomelanocortin (POMC) is the archetypal polypeptide precursor of hormones and neuropeptides. In this review, we examine the variability in the individual peptides produced in different tissues and the impact of the simultaneous presence of their precursors or fragments. We also discuss the problems inherent in accurately measuring which of the precursors and their derived peptides are present in biological samples. We address how not being able to measure all the combinations of precursors and fragments quantitatively has affected our understanding of the pathophysiology associated with POMC processing. To understand how different ratios of peptides arise, we describe the role of the pro-hormone convertases (PCs) and their tissue specificities and consider the cellular processing pathways which enable regulated secretion of different peptides that play crucial roles in integrating a range of vital physiological functions. In the pituitary, correct processing of POMC peptides is essential to maintain the hypothalamic-pituitary-adrenal axis, and this processing can be disrupted in POMC-expressing tumors. In hypothalamic neurons expressing POMC, abnormalities in processing critically impact on the regulation of appetite, energy homeostasis, and body composition. More work is needed to understand whether expression of the POMC gene in a tissue equates to release of bioactive peptides. We suggest that this comprehensive view of POMC processing, with a focus on gaining a better understanding of the combination of peptides produced and their relative bioactivity, is a necessity for all involved in studying this fascinating physiological regulatory phenomenon.

Corticosteroids Are Essential for Maintaining Cardiovascular Function in Male Mice

Activation of the hypothalamic-pituitary-adrenal axis results in the release of hormones from the adrenal glands, including glucocorticoids and mineralocorticoids. The physiological association between corticosteroids and cardiac disease is becoming increasingly recognized; however, the mechanisms underlying this association are not well understood. To determine the biological effects of corticosteroids on the heart, we investigated the impact of adrenalectomy in C57BL/6 male mice. Animals were adrenalectomized (ADX) at 1 month of age and maintained for 3-6 months after surgery to evaluate the effects of long-term adrenalectomy on cardiac function. Morphological evaluation suggested that ADX mice showed significantly enlarged hearts compared with age-matched intact controls. These changes in morphology correlated with deficits in left ventricular (LV) function and electrocardiogram (ECG) abnormalities in ADX mice. Correlating with these functional defects, gene expression analysis of ADX hearts revealed aberrant expression of a large cohort of genes associated with cardiac hypertrophy and arrhythmia. Combined corticosterone and aldosterone replacement treatment prevented the emergence of cardiac abnormalities in ADX mice, whereas corticosterone replacement prevented the effects of adrenalectomy on LV function but did not block the emergence of ECG alterations. Aldosterone replacement did not preserve the LV function but prevented ECG abnormalities. Together, the data indicate that adrenal glucocorticoids and mineralocorticoids either directly or indirectly have selective effects in the heart and their signaling pathways are essential in maintaining normal cardiac function.

Functional Zonation of the Adult Mammalian Adrenal Cortex

The standard model of adrenocortical zonation holds that the three main zones, glomerulosa, fasciculata, and reticularis each have a distinct function, producing mineralocorticoids (in fact just aldosterone), glucocorticoids, and androgens respectively. Moreover, each zone has its specific mechanism of regulation, though ACTH has actions throughout. Finally, the cells of the cortex originate from a stem cell population in the outer cortex or capsule, and migrate centripetally, changing their phenotype as they progress through the zones. Recent progress in understanding the development of the gland and the distribution of steroidogenic enzymes, trophic hormone receptors, and other factors suggests that this model needs refinement. Firstly, proliferation can take place throughout the gland, and although the stem cells are certainly located in the periphery, zonal replenishment can take place within zones. Perhaps more importantly, neither the distribution of enzymes nor receptors suggest that the individual zones are necessarily autonomous in their production of steroid. This is particularly true of the glomerulosa, which does not seem to have the full suite of enzymes required for aldosterone biosynthesis. Nor, in the rat anyway, does it express MC2R to account for the response of aldosterone to ACTH. It is known that in development, recruitment of stem cells is stimulated by signals from within the glomerulosa. Furthermore, throughout the cortex local regulatory factors, including cytokines, catecholamines and the tissue renin-angiotensin system, modify and refine the effects of the systemic trophic factors. In these and other ways it more and more appears that the functions of the gland should be viewed as an integrated whole, greater than the sum of its component parts.

60 YEARS OF POMC: Purification and biological characterisation of melanotrophins and corticotrophins

The remarkable conservation of the primary structures and anatomical location of dogfish α-melanocyte-stimulating hormone (MSH), corticotrophin-like intermediate lobe peptide (CLIP) and adrenocorticotrophic hormone (ACTH) compared with mammals reinforced the tissue-specific processing hypothesis of ACTH peptides in the pituitary gland. The cloning of dogfish pro-opiomelanocortin (POMC) led to the identification of δ-MSH and simultaneously revealed the high conservation of the γ-MSH sequence during evolution. These studies have also shown that β-MSH is much less conserved during evolution and in some species is not even processed from β-LPH. Human pro-γ-MSH potentiates the corticosteroidogenic activity of ACTH and peptides generated from its N-terminal, in particular big-γ-MSH, appear to have adrenal mitogenic activity. Human big-γ-MSH (from the zona intermedia) may also cause the adrenache. The review finishes with a cautionary note with regard to the misdiagnosis of the ectopic ACTH syndrome in which partial processing of ACTH can result in large concentrations of α-MSH and CLIP, which can interfere in the performance of two-site immunoassays, and the problem of the correct disulphide bridge arrangement in synthetic N-POMC peptides is also discussed.

60 YEARS OF POMC: N-terminal POMC peptides and adrenal growth

The peptide hormones contained within the sequence of proopiomelanocortin (POMC) have diverse roles ranging from pigmentation to regulation of adrenal function to control of our appetite. It is generally acknowledged to be the archetypal hormone precursor, and as its biology has been unravelled, so too have many of the basic principles of hormone biosynthesis and processing. This short review focuses on one group of its peptide products, namely, those derived from the N-terminal of POMC and their role in the regulation of adrenal growth. From a historical and a personal perspective, it describes how their role in regulating proliferation of the adrenal cortex was identified and also highlights the key questions that remain to be answered.

60 YEARS OF POMC: The proopiomelanocortin gene: discovery, deletion and disease

The cloning of the bovine proopiomelanocortin (POMC) cDNA in 1978 by Nakanishi and colleagues was the result of a remarkable series of exacting and ingenious experiments. With this work, they instantly confirmed the single precursor hypothesis for adrenocorticotrophic hormone-β-lipotropin, as it was then known, and in so doing revealed the existence of additional, largely unpredicted, N-terminal peptides that together formed the POMC precursor peptide. This work paved the way for a host of additional studies into the physiology of these peptides and their regulation. Furthermore, the cloning of the murine Pomc gene was essential for subsequent studies, in which Pomc was intentionally deleted in the mouse illuminating its substantial role in body weight regulation and adrenal function. Contemporaneously with this work, naturally occurring mutations in human POMC came to light underlining the vital role of this gene in appetite regulation. This article reviews each of these aspects of POMC with the benefit of several decades of hindsight and informed by more recent genomic and transcriptomic data.

Comparative Effect of ACTH and Related Peptides on Proliferation and Growth of Rat Adrenal Gland

Pro-opiomelanocortin (POMC) is a polypeptide precursor known to yield biologically active peptides related to a range of functions. These active peptides include the adrenocorticotropic hormone (ACTH), which is essential for maintenance of adrenal growth and steroidogenesis, and the alpha-melanocyte stimulation hormone, which plays a key role in energy homeostasis. However, the role of the highly conserved N-terminal region of POMC peptide fragments has begun to be unraveled only recently. Here, we review the cascade of events involved in regulation of proliferation and growth of murine adrenal cortex triggered by ACTH and other POMC-derived peptides. Key findings regarding signaling pathways and modulation of genes and proteins required for the regulation of adrenal growth are summarized. We have outlined the known mechanisms as well as future challenges for research on the regulation of adrenal proliferation and growth triggered by these peptides.

The involvement of Nek2 and Notch in the proliferation of rat adrenal cortex triggered by POMC-derived peptides

The adrenal gland is a dynamic organ that undergoes constant cell turnover. This allows for rapid organ remodeling in response to the physiological demands of the HPA axis, which is controlled by proopiomelanocortin (POMC)-derived peptides, such as adrenocorticotropic hormone (ACTH) and N-Terminal peptides (N-POMC). In the rat adrenal cortex, POMC-derived peptides trigger a mitogenic effect, and this process increases cyclins D and E, while inhibiting p27Kip1. The goal of the present study was to further explore the mitogenic effect of ACTH and synthetic N-POMC_1–28 peptides by investigating the differences in the expression of key genes involved in the cell cycle of the rat adrenal cortex, following inhibition of the HPA axis. Moreover, we evaluated the differences between the inner and outer fractions of the adrenal cortex (ZF-fraction and ZG-fraction) in terms of their response patterns to different stimuli. In the current study, the inhibition of the HPA axis repressed the expression of Ccnb2, Camk2a, and Nek2 genes throughout the adrenal cortex, while treatments with POMC-derived peptides stimulated Nek2, gene and protein expression, and Notch2 gene expression. Furthermore, Notch1 protein expression was restricted to the subcapsular region of the cortex, an area of the adrenal cortex that is well-known for proliferation. We also showed that different regions of the adrenal cortex respond to HPA-axis inhibition and to induction with POMC-derived peptides at different times. These results suggest that cells in the ZG and ZF fractions could be at different phases of the cell cycle. Our results contribute to the understanding of the mechanisms involved in cell cycle regulation in adrenocortical cells triggered by N-POMC peptides and ACTH, and highlight the involvement of genes such as Nek2 and Notch.

11-Dehydrocorticosterone causes metabolic syndrome, which is prevented when 11β-HSD1 is knocked out in livers of male mice

Metabolic syndrome is growing in importance with the rising levels of obesity, type 2 diabetes, and insulin resistance. Metabolic syndrome shares many characteristics with Cushing's syndrome, which has led to investigation of the link between excess glucocorticoids and metabolic syndrome. Indeed, increased glucocorticoids from intracellular regeneration by 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) drives insulin resistance and increases adiposity, but these metabolic changes are assumed to be due to increased circulating glucocorticoids. We hypothesized that increasing the substrate for 11β-HSD1 (11-dehydrocorticosterone, 11-DHC) would adversely affect metabolic parameters. We found that chronic administration of 11-DHC to male C57BL/6J mice resulted in increased circulating glucocorticoids, and down-regulation of the hypothalamic-pituitary-adrenal axis. This elevated 11β-HSD1-derived corticosterone led to increased body weight gain and adiposity and produced marked insulin resistance. Surprisingly liver-specific 11β-HSD1 knockout (LKO) mice given 11-DHC did not show any of the adverse metabolic effects seen in wild-type mice. This occurred despite the 11-DHC administration resulting in elevated circulating corticosterone, presumably from adipose tissue. Mice with global deletion of 11β-HSD1 (global knockout) were unaffected by treatment with 11-DHC, having no increase in circulating corticosterone and exhibiting no signs of metabolic impairment. Taken together, these data show that in the absence of 11β-HSD1 in the liver, mice are protected from the metabolic effects of 11-DHC administration, even though circulating glucocorticoids are increased. This implies that liver-derived intratissue glucocorticoids, rather than circulating glucocorticoids, contribute significantly to the development of metabolic syndrome and suggest that local action within hepatic tissue mediates these effects.

Loss of agouti-related peptide does not significantly impact the phenotype of murine POMC deficiency

The hypothalamic melanocortin system is unique among neuropeptide systems controlling energy homeostasis, in that both anorexigenic proopiomelanocortin (POMC)-derived and orexigenic Agouti related-peptide (AgRP)-derived ligands act at the same receptors, namely melanocortin 3 and 4 receptors (MC3/4R). AgRP clearly acts as a competitive antagonist at MC3R and MC4R but may also have an inverse agonist action at these receptors. The physiological relevance of this remains uncertain. We generated a mouse lacking both POMC and AgRP [double knockout (DKO) mouse]. Phenotyping was performed in the absence and presence of glucocorticoids, and the response to central peptide administration was studied. The phenotype of DKO mice is indistinguishable from that of mice lacking Pomc alone, with both exhibiting highly similar degrees of hyperphagia and increased body length, fat, and lean mass compared with wild-type controls. After a 24-h fast, there was no difference in the refeeding response between Pomc(-/-) and DKO mice. Similarly, corticosterone supplementation caused an equivalent increase in food intake and body weight in both genotypes. Although the central administration of [Nle⁴, d-Phe⁷]-α-MSH to DKO mice caused a decrease in food intake and an increase in brown adipose tissue Ucp1 expression, both of which could be antagonized with the coadministration of AgRP, there was no effect of AgRP alone. These data suggest AgRP acts predominantly as a melanocortin antagonist. If AgRP has significant melanocortin-independent actions, these are of insufficient magnitude in vivo to impact any of the detailed phenotypes we have measured under a wide variety of conditions.

The proliferative effect of synthetic N-POMC(1-28) peptides in rat adrenal cortex: a possible role for cyclin E

Modified synthetic N-POMC(1-28) without disulfide bridges has been shown to act as an adrenal mitogen. Cyclins and their inhibitors are the major cell cycle controls, but in the adrenal cortex the effect of ACTH and N-POMC on the expression of these proteins remains unclear. In this work, we evaluate the effect of different synthetic N-POMC peptides on the S-phase of the cell cycle. In addition, we examine the cyclin E expression in rat adrenal cortex. Rats treated with dexamethasone were injected with ACTH and/or synthetic modified N-POMC and/or synthetic N-POMC with disulfide bridges. DNA synthesis was determined by BrdU incorporation and protein expression was analyzed by immunoblotting and immunohistochemistry. The results showed that similarly to modified N-POMC without disulfide bridges, administration of synthetic N-POMC with disulfide bridges and the combination of ACTH and N-POMC promoted an increase of BrdU-positive nuclei in adrenal cortex. However, the proliferative effect of N-POMC was comparable to that of ACTH only in the zona glomerulosa. An increase in cyclin E expression was observed 6 h after N-POMC treatment in the outer fraction of the adrenal cortex, in agreement with immunohistochemical findings in the zona glomerulosa. In summary, the effect of synthetic N-POMC with disulfide bridges was similar to modified synthetic N-POMC, increasing proliferation in the adrenal cortex, confirming previous evidence that disulfide bridges are not essential to the N-POMC mitogenic effect. Moreover, cyclin E appears to be involved in the N-POMC- and ACTH-stimulated proliferation in the zona glomerulosa of the adrenal cortex.

Modulatory role of serotonin on feeding behavior

The appearance, the odor, and the flavor of foods, all send messages to the encephalic area of the brain. The hypothalamus, in particular, plays a key role in the mechanisms that control the feeding behavior. These signals modulate the expression and the action of anorexigenic or orexigenic substances that influence feeding behavior. The serotonergic system of neurotransmission consists of neurons that produce and liberate serotonin as well as the serotonin-specific receptor. It has been proven that some serotonergic drugs are effective in modulating the mechanisms of control of feeding behavior. Obesity and its associated illnesses have become significant public health problems. Some drugs that manipulate the serotonergic systems have been demonstrated to be effective interventions in the treatment of obesity. The complex interplay between serotonin and its receptors, and the resultant effects on feeding behavior have become of great interest in the scientific community.

Effects of melanocortins on adrenal gland physiology

The melanocortin-2-receptor (MC(2) receptor), also known as the ACTH receptor, is a critical component of the hypothalamic-pituitary-adrenal axis. The importance of MC(2) receptor in adrenal physiology is exemplified by the condition familial glucocorticoid deficiency, a potentially fatal disease characterised by isolated cortisol deficiency. MC(2)receptor mutations cause ~25% of cases. The discovery of a MC(2) receptor accessory protein MRAP, mutations of which account for ~15%-20% of familial glucocorticoid deficiency, has provided insight into MC(2) receptor trafficking and signalling. MRAP is essential for the functional expression of MC(2) receptor. MRAP2, a novel homolog of MRAP, can also facilitate MC(2) receptor cell surface expression and function. Like MRAP, MRAP2 is a small transmembrane domain glycoprotein capable of homodimerising. In addition, MRAP/MRAP2 can heterodimerise. The presence of MRAP2 adrenal expression suggests a possible role for MRAP2 in adrenal physiology, which has yet to be elucidated. Importantly, new data shows that the MRAPs can interact with all the other melanocortin receptors (MC(1,3,4,5) receptor). In contrast to MC(2) receptor, this interaction results in reduced melanocortin receptor surface expression and signalling. MRAP2 is predominantly expressed in brain. Hypothalamic expression has been demonstrated for both MRAP and MRAP2. The ability of MRAPs to modulate different members of the melanocortin receptor family in a bidirectional manner is intriguing. Furthermore, central nervous system expression of MRAPs points to a role beyond MC(2) receptor mediated adrenal steroidogenesis.

Melanocortins and body weight regulation: glucocorticoids, Agouti-related protein and beyond

In the intervening three decades since Panksepp observed for the first time that centrally administered α-melanocyte stimulating hormone decreased food intake (Panksepp and Meeker, 1976), a wealth of data have accrued to firmly establish melanocortin signaling as a central regulator of food intake and fat mass. Advances in molecular biology have not only allowed detailed studies of spontaneously occurring obese mice with altered melanocortin signaling to be undertaken but also permitted the generation of a plethora of mouse models with precise perturbations at critical steps in the melanocortin system to finesse further the cellular and molecular architecture of relevant pathways. In this article we focus in upon a number of these mouse models which continue to help us tease apart the complexities of this critical system. Further, we review data on the important interaction between pro-opiomelanocortin derived peptides and the adrenal system and the relationship between agonist and antagonist peptides acting at central melanocortin receptors.

The genetics of familial glucocorticoid deficiency

Familial glucocorticoid deficiency is an autosomal recessive disorder resulting from defects in the action of adrenocorticotropic hormone (ACTH) to stimulate glucocorticoid synthesis in the adrenal. Production of mineralocorticoids by the adrenal is normal. Patients present in early life with low or undetectable cortisol and--because of the failure of the negative feedback loop to the pituitary and hypothalamus--grossly elevated ACTH levels. About half of all cases result from mutations in the ACTH receptor (melanocortin 2 receptor) or from mutations in the melanocortin 2 receptor accessory protein (MRAP), but other genetic causes of this potentially lethal disorder remain to be discovered.

In vivo evidence for inverse agonism of Agouti-related peptide in the central nervous system of proopiomelanocortin-deficient mice

OBJECTIVE

Melanocyte-stimulating hormone (MSH) peptides processed from proopiomelanocortin (POMC) regulate energy homeostasis by activating neuronal melanocortin receptor (MC-R) signaling. Agouti-related peptide (AgRP) is a naturally occurring MC-R antagonist but also displays inverse agonism at constitutively active melanocortin-4 receptor (MC4-R) expressed on transfected cells. We investigated whether AgRP functions similarly in vivo using mouse models that lack all neuronal MSH, thereby precluding competitive antagonism of MC-R by AgRP.

RESEARCH DESIGN AND METHODS

Feeding and metabolic effects of the MC-R agonist melanotan II (MTII), AgRP, and ghrelin were investigated after intracerebroventricular injection in neural-specific POMC-deficient (Pomc(-/-)Tg/+) and global POMC-deficient (Pomc(-/-)) mice. Gene expression was quantified by RT-PCR.

RESULTS

Hyperphagic POMC-deficient mice were more sensitive than wild-type mice to the anorectic effects of MTII. Hypothalamic melanocortin-3 (MC3)/4-R mRNAs in POMC-deficient mice were unchanged, suggesting increased receptor sensitivity as a possible mechanism for the heightened anorexia. AgRP reversed MTII-induced anorexia in both mutant strains, demonstrating its ability to antagonize MSH agonists at central MC3/4-R, but did not produce an acute orexigenic response by itself. The action of ghrelin was attenuated in Pomc(-/-)Tg/+ mice, suggesting decreased sensitivity to additional orexigenic signals. However, AgRP induced delayed and long-lasting modifications of energy balance in Pomc(-/-)Tg/+, but not glucocorticoid-deficient Pomc(-/-) mice, by decreasing oxygen consumption, increasing the respiratory exchange ratio, and increasing food intake.

CONCLUSIONS

These data demonstrate that AgRP can modulate energy balance via a mechanism independent of MSH and MC3/4-R competitive antagonism, consistent with either inverse agonist activity at MC-R or interaction with a distinct receptor.

Melanocortin 2 receptor is required for adrenal gland development, steroidogenesis, and neonatal gluconeogenesis

ACTH (i.e., corticotropin) is the principal regulator of the hypothalamus-pituitary-adrenal axis and stimulates steroidogenesis in the adrenal gland via the specific cell-surface melanocortin 2 receptor (MC2R). Here, we generated mice with an inactivation mutation of the MC2R gene to elucidate the roles of MC2R in adrenal development, steroidogenesis, and carbohydrate metabolism. These mice, the last of the knockout (KO) mice to be generated for melanocortin family receptors, provide the opportunity to compare the phenotype of proopiomelanocortin KO mice with that of MC1R-MC5R KO mice. We found that the MC2R KO mutation led to neonatal lethality in three-quarters of the mice, possibly as a result of hypoglycemia. Those surviving to adulthood exhibited macroscopically detectable adrenal glands with markedly atrophied zona fasciculata, whereas the zona glomerulosa and the medulla remained fairly intact. Mutations of MC2R have been reported to be responsible for 25% of familial glucocorticoid deficiency (FGD) cases. Adult MC2R KO mice resembled FGD patients in several aspects, such as undetectable levels of corticosterone despite high levels of ACTH, unresponsiveness to ACTH, and hypoglycemia after prolonged (36 h) fasting. However, MC2R KO mice differ from patients with MC2R-null mutations in several aspects, such as low aldosterone levels and unaltered body length. These results indicate that MC2R is required for postnatal adrenal development and adrenal steroidogenesis and that MC2R KO mice provide a useful animal model by which to study FGD.

A comparative study of the central effects of specific proopiomelancortin (POMC)-derived melanocortin peptides on food intake and body weight in pomc null mice

Functional disruption of either MC3R or MC4R results in obesity, implicating both in the control of energy homeostasis. The ligands for these receptors are derived from the prohormone proopiomelancortin (POMC), which is posttranslationally processed to produce a set of melanocortin peptides with a range of activities at the MC3R and MC4R. The relative importance of each of these peptides alpha-MSH, gamma3-MSH, gamma2-MSH, gamma-lipotropin (gamma-LPH) and, in man but not in rodents, beta-MSH] in the maintenance of energy homeostasis is, as yet, unclear. To investigate this further, equimolar amounts (2 nmol) of each peptide were centrally administered to freely feeding, corticosterone-supplemented, Pomc null (Pomc-/-) mice. After a single dose at the onset of the dark cycle, alpha-MSH had the most potent anorexigenic effect, reducing food intake to 35% of sham-treated animals. beta-MSH, gamma-LPH, and gamma3- and gamma2-MSH all reduced food intake but to a lesser degree. The effects of peptide administration over 3 d were also assessed. Only alpha-MSH significantly reduced body weight, affecting both fat and lean mass. Other peptides had no significant effect on body weight. Pair-feeding of sham-treated mice to those treated with alpha-MSH resulted in identical changes in total weight, fat and lean mass indicating that the effects of alpha-MSH were primarily due to reduced food intake rather than increased energy expenditure. Although other melanocortins can reduce food intake in the short-term, only alpha-MSH can reduce the excess fat and lean mass found in Pomc-/- mice, mediated largely through an effect on food intake.

Pituitary-interrenal interaction in zebrafish interrenal organ development

To further elucidate pituitary adrenal interactions during development, we studied the organogenesis of the interrenal organ, the teleost homolog of the mammalian adrenal gland, in zebrafish. To this end we compared wild-type zebrafish interrenal development with that of mutants lacking pituitary cell types including corticotrophs. In addition, we studied the effects of ACTH receptor (Mc2r) knockdown and dexamethasone (dex) on interrenal development and pituitary feedback. Until 2 d post fertilization (2 dpf) interrenal development assessed by transcripts of key steroidogenic genes (cyp11a1, mc2r, star) is independent of proopiomelanocortin (Pomc) as demonstrated in aal/eya1and lia/fgf3 mutants. However, at 5 dpf lack of pituitary cells leads to reduced expression of steroidogenic genes at both the transcriptional and the protein level. Pituitary control of interrenal development resides in corticotrophs, because pit1 mutants lacking pituitary cells except corticotrophs have a phenotype similar to that of wild-type controls. Furthermore, development in mc2r knockdown morphants does not differ from aal/eya1 and lia/fgf3 mutants. Inhibition of steroidogenesis by mc2r knockdown induces up-regulation of pomc expression in the anterior domain of pituitary corticotrophs. Accordingly, dex suppresses pomc in the anterior domain only, leading to impaired expression of steroidogenic genes commencing at 3 dpf and interrenal hypoplasia via reduced interrenal proliferation. In contrast, negative feedback on pituitary corticotrophs by dex is evident at 2 dpf and precedes effects of Pomc on the interrenal primordium. These data demonstrate a gradual transition from early pituitary-independent interrenal organogenesis to developmental control by the anterior domain of pituitary corticotrophs acting via Mc2 receptors.