Urocortins: Emerging Metabolic and Energy Homeostasis Perspectives

Review Urocortins: emerging metabolic and energy homeostasis perspectives

Yael Kuperman and Alon Chen

Department of Neurobiology, Weizmann Institute of Science, Rehovot 76100, Israel

The effects of stress on energy balance and the through the expression of neuropeptide Y and agouti-related pep- involvement of the neuropeptide corticotropin releasing tide. The Arc is accessible to circulating signals of energy balance, factor in modulating the anorexia of stress and sympath- through the underlying median eminence, because this region of the etic nervous system tone are well recognized. Currently, brain is not protected by the blood–brain barrier. Thus, the Arc serves as an integrative center responsible for information processing and studies centered on the roles of the more recently coordinating appropriate output [58]. described members of this family of ligands, the urocor- Bed nucleus of the stria terminalis (BNST): the BNST is located within tins, and their preferred receptor, the corticotropin the basal forebrain and is considered to be the extended amygdala. It releasing factor type 2 receptor, suggest that they are seems to be involved in a number of complex functions, including important modulators of centrally controlled metabolic sexual behavior, autonomic function, anxiety and aversiveness of opiate withdrawal [58]. functions. In addition, urocortins also regulate fuel util- Dorsal raphe nucleus (DRN): the raphe nuclei are located in the ization in the periphery by acting locally within key midbrain and provide the major ascending serotonergic projection metabolic tissues through autocrine and/or paracrine to the forebrain. The raphe projects to the striatum, amygdala, mechanisms. Recent ﬁndings have demonstrated that caudate putamen, hippocampus, substantia nigra and locus coeru- urocortin 2 and urocortin 3, by acting through their leus. The dorsal and medial raphe nuclei receive substantial afferents from the parabrachial nucleus and the hypothalamic nuclei. The DRN speciﬁc receptor in peripheral tissues, are novel modu- contains the largest and densest 5-hydroxytryptamine (5-HT) aggre- lators of glucose homeostasis and metabolic functions. gate in the brain. In contrast to the median raphe nucleus, there is no age-related loss of 5-HT neurons within the DRN, although 5-HT- Introduction producing cell size and dendritic length decline with age [58]. The maintenance of energy homeostasis in the presence of Dorso-medial hypothalamic nucleus (DMH): the DMH receives afferents from the BNST, from many parts of the brain stem and from most physiologically or psychologically stressful stimuli requires parts of the hypothalamus. The projections of the DMH are mostly the activation of coordinated adaptive responses, with regu- intrahypothalamic. The DMH has been implicated in the regulation of latory and functional changes in both central and peripheral ingestive behavior, stress, reproduction, circadian rhythms and systems. Although the roles of the corticotropin releasing thermogenesis [58]. Lateral septum (LS): the LS nucleus is the largest nucleus of the septum (the medial interventricular wall of the telencephalon). Most of the septal region develops morphofunctional links with the hippo- Glossary campus and the amygdala. The septal region is rich in g-aminobutyric acid (GABA)-ergic neurons. The septal region does not form a func- Amygdala: the amygdala consists of a heterogeneous gray complex tional unit by itself, but it comprises complex and parallel circuits that which is divided to nuclear groups on the basis of cytoarchitectonic, might form loops with the hippocampal formation and the hypothala- histochemical, immunocytochemical and hodological studies. The mus [58]. amygdala has been shown to be involved in the modulation of Paraventicular nucleus (PVN): The PVN is located in the anterior part neuroendocrine function, visceral effector mechanisms and complex of the hypothalamic periventricular zone. The PVN contains magno- patterns of integrated behavior, such as defense, ingestion, aggres- cellular neurosecretory cells, which produce oxytocin and vasopressin sion, reproduction, memory and learning. Such a modulation is and whose axons extend into the posterior pituitary, and parvocellular exercised, at least in part, through a vast network of connections neurosecretory cells, which produce CRF, vasopressin and thyrotro- with other brain regions, such as the hypothalamus, the brain stem pin-releasing hormone. The parvocellular neurons project to the and spinal cord autonomic cell aggregates [58]. median eminence, and the secreted peptides are carried to the anterior Arcuate nucleus (Arc): the Arc is located in the tuberal part of the pituitary by the blood vessels of the hypothalamo–pituitary portal hypothalamic periventricular zone. The Arc receives strong input system. The PVN also contains neurons that project to regions contain- from hypothalamic structures such as the periventricular nuclei, ing preganglionic autonomic neurons. Accordingly, the PVN has a the PVN, the dorso-medial hypothalamic nucleus, the preoptic central role in mediating hypothalamic responses to stress, feeding nucleus and the premammillary nucleus. Extrahypothalamic input and drinking behavior, and participates in a variety of autonomic involves the bed nucleus of the stria terminalis, the amygdala, the responses [58]. lateral septal nucleus and the brain stem. Two primary populations of Ventro-medial hypothalamic nucleus (VMH): The VMH receives affer- Arc neurons exert opposing actions on energy balance. One neuronal ent projections from the amygdala and ventral subiculum, from many population located mainly in the ventro-lateral subdivision of the Arc hypothalamic nuclei and from the brain stem. The VMH projections are inhibits food intake through the expression of pro-opiomelanocortin- consistent with its proposed role in mediating somatomotor aspects of derived peptide, the a-melanocyte-stimulating hormone and complex motivated behavior. It shares connections with forebrain and cocaine- and amphetamine-regulated transcript. The other popu- brain stem regions that are involved in mediating reproductive beha- lation in the far ventro-medial part of the Arc stimulates food intake vior and with regions involved with appetite behaviors such as the PVN and DMH. The VMH sends massive projections to other parts of the hypothalamic medial zone, to the amygdala and the septum [58] (see Glossary Figure I). Corresponding author: Chen, A. ([email protected]).

Glossary Figure I. Neuroanatomy involved in energy homeostasis and the stress response. A schematic depicting a mammalian brain, highlighting various regions implicated in energy homeostasis and the stress response.

factor (CRF) peptide and its cognate type 1 CRF receptor (see Glossary) in the hypothalamus, represents the ﬁnal (CRF1) in the regulation of the hypothalamic–pituitary– common pathway for the integration of the neuroendocrine adrenal (HPA) axis and stress-related behavioral responses stress response in the brain [1]. CRF has an important and are well established, the physiological roles of the related well-established role in the regulation of the HPA axis urocortin peptides and their preferred receptor, the type 2 under basal and stress conditions [2,3]. In addition to this CRF receptor (CRF2), in responses to such challenges are hypophysiotropic action, CRF integrates the behavioral, less understood. autonomic and metabolic responses to stressors [4–6]. CRF To date, the mammalian CRF–urocortin family includes is involved in the control of arousal, anxiety, cognitive four structurally related peptides (CRF, and urocortin 1, -2 functions and appetite [7–13]. Dysregulation of the stress and -3). All are encoded by separate genes and show response can have severe psychological and physiological differential expression patterns within both central and consequences [14,15], and chronic hyperactivation of the peripheral tissues. The physiological effects of these pep- CRF system has been linked to stress-related emotional tides are mediated through two related seven transmem- disorders such as anxiety, anorexia nervosa and depres- brane domain receptors (CRF1 and CRF2), which are sion [7–15]. expressed as multiple isoforms arising from alternative In addition to CRF, the CRF–urocortin family of pep- splicing of the genes. In addition to the brain and pituitary tides includes the more recently described urocortins: uro- gland, the CRF–urocortin family of peptides and receptors cortin 1 [16], urocortin 2 (or stresscopin-related peptide are highly expressed in several peripheral tissues, and an encoded by the human ortholog) [17,18] and urocortin 3 (or increasing body of evidence suggests a role for these pep- stresscopin encoded by the human ortholog) [18,19] tides and receptors in regulating energy metabolism not (Figure 1). CRF and urocortin peptides mediate their only centrally, but also by acting locally within key meta- effects through activation of two membrane-bound G- bolic tissues, including skeletal muscle and the endocrine protein-coupled receptors, CRF1 [20–23] and CRF2 [24– pancreas. Here, we discuss the recent ﬁndings demonstrat- 27] (Figure 1). CRF1 mRNA is widely expressed in mam- ing the potential for urocortin 2 and urocortin 3, acting malian brain and pituitary, with high levels in the anterior through CRF2, to modulate glucose homeostasis and pituitary, cerebral cortex, arcuate nucleus (Arc), cerebel- metabolic functions. lum, amygdala, hippocampus and olfactory bulb [28]. CRF2 has three apparent membrane bound splice variants in Stress, CRF family members and energy homeostasis humans (a, b and c) and two in rats (a and b) that are Perception of physical or psychological stress by an organ- produced by the use of alternate 50 exons [24–27,29].In ism is followed by a series of events which result in changes rodents, CRF2a is predominantly expressed in the brain in in emotional and cognitive functions, modulation of auto- a discrete pattern, with highest densities in the lateral nomic activities and the secretion of glucocorticoids from septal nucleus, bed nucleus of the stria terminalis, ventro- the adrenal cortex. Both activation and termination of medial hypothalamic nucleus (VMH), olfactory bulb, the behavioral, autonomic and adrenocortical stress mesencephalic raphe nuclei and medial amygdala [28]. responses are crucial for adaptation and survival. The The CRF2b splice form is expressed primarily in peripheral neuropeptide CRF, expressed and secreted from the par- tissues, with the highest levels of expression in the skeletal vocellular neurons of the paraventricular nucleus (PVN) muscle and heart, the choroid plexus of the brain and the

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Figure 1. Schematic representation of the mammalian CRF–urocortin family of peptides, receptors and binding proteins. Colored arrows indicate the receptors and binding proteins with which each ligand interacts. Dotted arrow indicates relatively lower affinity, as compared with unbroken arrow. CRF has relatively lower affinity for CRF2 compared with its affinity for CRF1. Urocortin 1 has approximately equal affinity for both receptors, and urocortin 2 and urocortin 3 seem to be selective for CRF2. The signaling cascade also includes CRF-BP and the recently identified sCRF2a. Both CRF-BP and sCRF2a bind to CRF and urocortin 1 with high affinity. CRF stimulates the secretion of adrenocorticotropic hormone (ACTH), through CRF1 located on corticotropic cells in the anterior pituitary; this hormone exerts an effect on the adrenal cortex to produce and secrete glucocorticoids (corticosterone and cortisol) in response to stimulation by ACTH. Glucocorticoids, in turn, feed back on the hypothalamus and pituitary, to suppress CRF and ACTH production, in a negative feedback cycle. Each peptide is represented by different color: CRF in red; urocortin 1 in green, urocortin 2 in blue and urocortin 3 in purple. CRF2 has two apparent membrane-bound splice variants in rodents, resulting in two receptor proteins of 411 and 431 amino acids (CRF2a and CRFR2b,respectively).

gastrointestinal tract [24,27]. Interestingly, human CRF2a [28]. More recently, studies of the physiology of urocortin 2 is predominantly found in the periphery, indicating that and urocortin 3, which are highly expressed in skeletal receptor isoform distribution differs across species. muscle and the pancreas, respectively, have provided intri- Receptor binding and intracellular cAMP accumulation guing evidence that they are novel peripheral modulators studies in cells stably transfected with CRF receptors have of glucose homeostasis and metabolic functions. demonstrated that CRF1 and CRF2 differ pharmacologically. CRF has a relatively lower affinity for CRF2 compared Urocortin 2 and insulin sensitivity in skeletal muscle with its affinity for CRF1 [16], whereas urocortin 1 has equal Peptides encoded by the gene encoding urocortin 2 were affinity for both receptors, and urocortin 2 and urocortin 3 identified and described as new putative members of the are highly selective for CRF2 [16,17,19] (Figure 1). The CRF family in 2001 [17,19]. The predicted mature forms of existence of the CRF-binding protein (CRF-BP) [30] and human and mouse urocortin 2 peptides following processing the recently identified soluble splice variant of CRF2a are 76% identical, have a 38-amino acid sequence associated (sCRF2a) [31], both of which bind to CRF and urocortin 1 with high biological activity and are structurally more with high affinity, add a further level of complexity to the related to urocortin 3 than to urocortin 1 or CRF control of the action of these ligands (Figure 1). [19,35,36]. Whereas mouse urocortin 2 has a glycine and Maintenance of energy homeostasis and body weight is a pair of basic residues (R-R) at the C-terminus that are achieved by an intricate balance between energy intake presumed to be involved in amidation and cleavage from the (food consumption) and expenditure. The brain ultimately precursor, the human urocortin 2 ortholog lacks the stan- governs this energy homeostasis. Afferent signals indicat- dard consensus site found for proteolytic cleavage and C- ing the nutritional status of the animal and the state of its terminal amidation, a requisite for biological potency [36]. external environment, including the presence of physio- Transcripts encoding urocortin 2 are expressed in dis- logical and/or psychological stressors, are integrated cen- crete regions of the rodent central nervous system, including trally, and the efferent pathways controlling feeding the hypothalamic paraventricular, supraoptic and arcuate behavior and energy expenditure are modulated accord- nuclei and the locus coeruleus in the brainstem [17],and ingly [32]. The core site of these integrative processes is the urocortin 2 is thus well positioned to be a potential modu- hypothalamus, where an array of neurotransmitters, in- lator of neuroendocrine activity and stress-related behavior cluding many neuropeptides, modulates signals through [37,38]. Urocortin 2 mRNA is also widely expressed in a complex neural circuits [32]. variety of mouse peripheral tissues, including the adrenal Much evidence has accumulated over the years impli- gland, lung and the gastrointestinal tract, but is most highly cating CRF-related neuropeptides and receptors as players expressed in skeletal muscle and skin [39]. in this complex central network regulating energy balance The generation and study of urocortin 2 knockout mice [32–34]. The effects of stress on feeding behavior and has revealed an interesting metabolic phenotype and a energy homeostasis are well documented, and both potential role for urocortin 2 as a peripheral modulator of CRF1 and CRF2 are highly expressed in hypothalamic glucose utilization and insulin sensitivity in skeletal regions directly associated with control of feeding and muscle [40] (Figure 2). The initial observation that glucose energy balance, including the Arc and the VMH nuclei tolerance is significantly enhanced in urocortin 2-deficient

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Figure 2. Metabolic phenotype of urocortin 2-null mice. (a) Urocortin 2-null mice demonstrate enhanced glucose tolerance, as reflected by the ability of such mice and their WT littermates to handle a glucose load using a standard glucose tolerance test. Administration of synthetic urocortin 2 (Ucn 2) peptide to mutant mice before the glucose tolerance test restores blood glucose to WT levels (inset). (b) Administration of the CRF2-specific antagonist astressin 2B (Ast 2B) into WT mice mimics the urocortin 2 mutant mice glucose tolerance test profile. (c) The enhanced glucose tolerance in the urocortin 2-null mice is not due to increased insulin secretion measured following glucose injection. (d) Urocortin 2-null mice demonstrate increased insulin sensitivity in an insulin tolerance test, and administration of synthetic urocortin 2 peptide to mutant mice, before the insulin tolerance test, restores blood glucose to WT levels (inset). Reproduced, with permission, from Ref. [39]. mice when compared with their wild-type (WT) littermates That urocortin 2-null mice and their WT littermates was demonstrated by the improved ability of these mice to showed similar fasting and glucose-stimulated insulin metabolize a glucose load [40] (Figure 2). Systemic admin- levels suggested that the mechanism for enhanced glucose istration of synthetic urocortin 2 peptide to the mutant tolerance in urocortin 2-null mice was not due to increased mice, before the glucose tolerance test, restored the glucose insulin secretion into the bloodstream, but rather to an profile to one comparable with that of WT mice, whereas increase in insulin sensitivity of target tissues. This was administration of the CRF2-selective antagonist astressin demonstrated by measurement of whole-body glucose 2B to WT littermates resulted in a glucose tolerance profile homeostasis using the hyperinsulinemic euglycemic glucose mirroring that of urocortin 2-null mice [40] (Figure 2). The clamp technique. Higher glucose infusion rates were efficacy of peripherally administered peptide or antagonist required to maintain a set physiological blood glucose level in mediating these effects strongly suggests that the in urocortin 2-null mice subjected to constant hyperinsuli- observed phenotype is peripherally, rather than centrally, nemia than in WT littermates [40]. During these clamp mediated. studies, whole body glucose uptake, as measured using Urocortin 2-null mice also demonstrate increased radiolabeled glucose analogues (which measure peripheral insulin sensitivity when compared with their WT litter- insulin sensitivity), glycolysis and insulin-mediated sup- mates, as determined using an insulin tolerance test. pression of hepatic glucose production rates (i.e. hepatic Again, administration of synthetic urocortin 2 peptide insulin sensitivity) were all demonstrated to be significantly before the test restored the blood glucose response profile increased in urocortin 2-deficient mice [40]. Furthermore, in response to insulin to that of WT animals [40] (Figure 2). specific glucose uptake into the gastrocnemius muscle was

125 Review Trends in Endocrinology and Metabolism Vol.19 No.4 significantly higher in mutant mice. These effects were mRNA levels were found to be significantly higher in obese observed in mice maintained on standard chow or a high- leptin-deficient ob/ob mice and in rats fed a high-fat diet fat diet, suggesting that urocortin 2-null mice are protected [44], and pharmacological blockade of CRF2 with astressin against the deleterious effects of a high-fat diet on insulin 2B antagonist or immunoneutralization of urocortin 3 sensitivity and glucose tolerance. attenuated high, but not low, glucose-induced insulin Accordingly, although urocortin 2-knockout mice and secretion from isolated pancreatic islets in vitro [44]. These WT littermates placed on standard chow or a high-fat diet data indicated that endogenous urocortin 3 is released for 16 weeks showed similar weight gain and food con- under conditions of high glucose and then signals through sumption, significant increases in blood glucose and insu- islet CRF2 to facilitate insulin secretion. lin levels were observed only in the WT mice [40]. Body The recent generation of urocortin 3-knockout mice has composition measurements determined that urocortin 2- provided further insights into the endogenous role of uro- null mice placed on a high-fat diet showed decreases in cortin 3 in vivo [44]. Urocortin 3-null mice, when fed a adipose tissue and increases in lean tissue when compared standard chow diet, showed similar responses to their WT with WT littermates [40]. littermates in glucose and insulin tolerance tests. How- Further mechanistic insights were provided by in vitro ever, pancreatic islets isolated from urocortin 3-null mice demonstrations of urocortin 2 inhibition of insulin-induced secreted significantly less insulin in response to high, but phosphorylation of Akt and extracellular-signal-regulated not low or medium, glucose concentrations [44], further kinases 1 and -2 in cultured skeletal muscle cells and in a supporting the purported role of endogenous urocortin 3 in myotube cell line (C2C12), and of insulin-induced glucose induction of insulin secretion in response to high glucose uptake by C2C12 myotubes [40]. These results suggest conditions. Urocortin 3-null mice fed a high-fat diet for 16 that urocortin 2 functions to inhibit interactions between weeks showed significantly lower fasting blood glucose and insulin signaling pathway components, although further plasma insulin concentrations when compared with their studies are required to determine the precise molecular WT littermates [44]. Unlike WT mice, which developed mechanisms by which urocortin 2, acting through its impaired glucose tolerance and insulin resistance when on specific G-protein-coupled receptor CRF2b, inhibits insulin the high-fat diet, urocortin 3-null mice did not show a receptor signaling. decrease in glucose tolerance and were protected against dietary high fat-induced hyperinsulinemia, hyperglyce- Pancreatic urocortin 3, insulin secretion and energy mia, hepatic steatosis and hypertriglyceridemia [44]. homeostasis These results suggest that endogenous pancreatic urocor- The urocortin 3 gene encodes a predicted mature 38-amino tin 3, induced under excessive caloric conditions, functions acid peptide [18,19]. The synthetic amidated urocortin 3 locally to augment insulin production, which might con- peptide selectively binds to and activates CRF2 with high tribute in the long term to reduced insulin sensitivity and affinity, suggesting that urocortin 3, in addition to urocor- harmful metabolic consequences. tin 2, is an endogenous selective CRF2 agonist. mRNA transcripts encoding urocortin 3 are expressed in discrete Energy homeostasis and CRF2 regions of the rodent central nervous system, predomi- In addition to the peripheral tissues involved in the hand- nately within the hypothalamus and medial amygdala ling of fuel molecules discussed earlier, CRF2 is present in [41,42]. In the hypothalamus, urocortin 3-expressing brain regions intimately associated with the central regu- neurons are present in the median preoptic nucleus and lation of feeding, glucose homeostasis and energy balance. in the rostral perifornical area lateral to the PVN [19,41], The CRF2a splice variant is highly expressed in the VMH, with fibers distributed mainly to hypothalamic and limbic and to a lesser extent in the arcuate nucleus, dorso-medial structures. Several major urocortin 3 terminal fields, in- hypothalamus, lateral hypothalamus and PVN [27,28,45]. cluding the lateral septum and the ventro-medial hypo- Important insights into the role of central CRF2 in mod- thalamus, express high levels of CRF2 [41]. Besides the ulating energy homeostasis and metabolic function are brain, urocortin 3, similarly to urocortin 2, is also provided by studies conducted in CRF2-deficient mice expressed in peripheral tissues but with a distinct pattern [46–48]. These studies provide evidence that central of distribution, with high levels of expression in the pan- CRF2 is important in modulating not only metabolic rate, creas [43], adrenal gland and gastrointestinal tract of but also appetite and feeding behaviors. rodents. Using in situ hybridization and immunohisto- CRF2-null mice recovered completely and more rapidly chemical techniques, Li et al. [43] demonstrated that uro- from urocortin 1-induced hypophagia compared with their cortin 3 is highly expressed by the b cells of pancreatic WT littermates [47]. CRF2-null mice exhibited normal islets and also by the mouse b-cell line MIN6 [43]. High basal feeding behavior and weight gain [46–48] but glucose, high potassium and forskolin were demonstrated decreased their food intake following food deprivation. to stimulate urocortin 3 secretion from MIN6 cells [43], and There were no differences in body weight following food the dose-dependent effects of glucose on urocortin 3 deprivation or refeeding; this effect on food intake could secretion were found to be mediated by the KATP channel thus be interpreted as a direct adaptation to the effect of [43]. The insulin secretagogues exendin-4 (a glucagon-like CRF2 deficiency on metabolic rate. The observed decrease in peptide analogue) and carbachol (a muscarinic acetyl- food intake could also be an anxiety-related behavior elicited choline receptor agonist) significantly stimulated urocortin by the stress of food deprivation, given that the CRF2-null 3 release from MIN6 cells in the presence of high physio- mice have a recognized anxiogenic phenotype [46]. CRF2- logical (10 mM) glucose levels [44]. Pancreatic urocortin 3 null mice maintained on a high-fat diet have increased food

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intake when compared with their WT littermates and do not models, targeting specific CRF2-expressing brain nuclei or exhibit the weight gain or hypercholesterolemia induced in individual peripheral tissues. the WT animals [49]. They also demonstrate enhanced In addition to the effects on metabolism mediated glucose tolerance and increased insulin sensitivity, and through the autonomic nervous system, both central and protection from insulin resistance induced by a high-fat diet peripheral stimulation of CRF2 produces satiety. Central [49]. Examination of the interscapular brown adipose tissue administration of CRF1 agonists is well recognized to elicit (IBAT) revealed elevated levels of uncoupling protein-1 in rapid onset and short-term anorexia, independently of CRF2-null mice [49] and significantly elevated basal IBAT CRF2. However, the observed anorexia following urocortin thermogenesis with prolonged adrenergic responsivity of 2 or urocortin 3 administration is delayed and of greater IBAT in older mice. These data suggest that CRF2-null mice duration, suggesting differential roles for the CRF receptor might have increased sympathetic nervous system (SNS) types in modulating food intake. Moreover, intracerebro- outflow, and that, in older mice, the SNS pathway might ventricular administration of urocortin 2 does not elicit remain, unlike in WT mice, sensitive to adrenergic stimu- the malaise, behavioral arousal or anxiogenesis associated lation in the absence of CRF2 [50].CRF2-null mice, main- with CRF1 agonist administration [51,52]. It is also tained on a high-fat diet, have a reduced respiratory noteworthy that CRF1 stimulation could mediate exchange ratio, which indicates a reduction in carbohydrate negative effects on ingestive behavior that are secondary oxidation and an increase in fatty acid oxidation as a fraction to the anxiety-and fear-like behaviors triggered by non- of the total energy consumed by these mice. The reduced selective CRF receptor agonists. respiratory exchange ratio was reversible by CRF1 antagon- Additional analyses have shown that infusion of uro- ist treatment, suggesting a role for CRF2 in impeding cortin 3 to the VMH reduces meal frequency, prolongs CRF1-induced SNS activity [50]. Clearly, the metabolic postmeal intervals, slows the eating rate and reduces meal phenotype of the CRF2-null mice is complex, and the size [53]. These observations are similar to those reported reported findings are in some instances contradictory and for intracerebroventricular leptin administration and might be confounded by their behavioral phenotype. The might reflect a functional relationship between leptin role of this receptor in the control of energy processes might and CRF2 in the VMH [53]. A positive correlation between be further resolved by the study of more sophisticated mice leptin serum levels and CRF2 mRNA levels in the VMH has

Figure 3. Schematic representation summarizing the proposed roles of central and peripheral CRF and urocortin peptides and receptors in modulating glucose homeostasis. Following stressful stimuli, glucocorticoid exposure resulting from HPA axis activation by hypothalamic CRF and changes in autonomic activity will modulate skeletal muscle, and pancreatic and hepatic function. CRF and urocortins, functioning through both type 1 and type 2 CRF receptors in the brain, will modulate food intake and glucose homeostasis. Peripherally, urocortin 2 produced in skeletal muscle and acting locally at the CRF2 receptor will regulate glucose uptake in skeletal muscle by inhibiting insulin signaling. Urocortin 3, produced by the pancreatic b cells, regulates high glucose-induced insulin secretion. Potential regulation of pancreatic and skeletal muscle functions by CRF-related peptides and receptors requires further investigation. Although, both urocortin 2- and urocortin 3-deficient mice demonstrate alterations in metabolic liver functions, studies have failed to demonstrate hepatic expression of CRF-related peptides or receptors (denoted by ‘?’ in the liver). Therefore, these observed effects in the mutant mice would seem likely to be secondary to their altered energy homeostasis resulting from the direct effects on muscle or pancreatic physiology.

127 Review Trends in Endocrinology and Metabolism Vol.19 No.4 been shown [54,55]. Physiological perturbations and stres- Medical Research Institute, Edinburgh, UK, for their critical reading and sors, including adrenalectomy, starvation and repeated constructive comments. The authors would like to thank Ms Genia Brodsky and Mr Ziv Ariely from the Graphics Department of the cold stress, caused a decrease in both leptin and CRF2 Weizmann Institute of Science for their assistance in generating the mRNA levels in the VMH [45,54,55]. illustrations for this review. A further intriguing role for CRF receptors in the VMH in mediating counterregulatory responses to acute hypo- References glycemia has been suggested. The VMH contains neurons 1 Vale, W. et al. (1981) Characterization of a 41-residue ovine hypothalamic peptide that stimulates secretion of corticotropin and which react to changes in circulating glucose levels. beta-endorphin. 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