Review

Urocortins: emerging metabolic andenergy homeostasis perspectivesYael Kuperman and Alon Chen

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

through the expression of neuropeptide Y and agouti-related pep-

tide. The Arc is accessible to circulating signals of energy balance,

through the underlying median eminence, because this region of the

brain is not protected by the blood–brain barrier. Thus, the Arc serves

as an integrative center responsible for information processing and

coordinating appropriate output [58].

Bed nucleus of the stria terminalis (BNST): the BNST is located within

the basal forebrain and is considered to be the extended amygdala. It

seems to be involved in a number of complex functions, including

sexual behavior, autonomic function, anxiety and aversiveness of

opiate withdrawal [58].

Dorsal raphe nucleus (DRN): the raphe nuclei are located in the

midbrain and provide the major ascending serotonergic projection

to the forebrain. The raphe projects to the striatum, amygdala,

caudate putamen, hippocampus, substantia nigra and locus coeru-

leus. The dorsal and medial raphe nuclei receive substantial afferents

from the parabrachial nucleus and the hypothalamic nuclei. The DRN

contains the largest and densest 5-hydroxytryptamine (5-HT) aggre-

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-

producing cell size and dendritic length decline with age [58].

Dorso-medial hypothalamic nucleus (DMH): the DMH receives affer-

ents from the BNST, from many parts of the brain stem and from most

parts of the hypothalamus. The projections of the DMH are mostly

intrahypothalamic. The DMH has been implicated in the regulation of

ingestive behavior, stress, reproduction, circadian rhythms and

thermogenesis [58].

The effects of stress on energy balance and theinvolvement of the neuropeptide corticotropin releasingfactor in modulating the anorexia of stress and sympath-etic nervous system tone are well recognized. Currently,studies centered on the roles of the more recentlydescribed members of this family of ligands, the urocor-tins, and their preferred receptor, the corticotropinreleasing factor type 2 receptor, suggest that they areimportant modulators of centrally controlled metabolicfunctions. In addition, urocortins also regulate fuel util-ization in the periphery by acting locally within keymetabolic tissues through autocrine and/or paracrinemechanisms. Recent findings have demonstrated thaturocortin 2 and urocortin 3, by acting through theirspecific receptor in peripheral tissues, are novel modu-lators of glucose homeostasis and metabolic functions.

IntroductionThe maintenance of energy homeostasis in the presence ofphysiologically or psychologically stressful stimuli requiresthe activation of coordinated adaptive responses, with regu-latory and functional changes inboth central and peripheralsystems. Although the roles of the corticotropin releasing

Glossary

Amygdala: the amygdala consists of a heterogeneous gray complex

which is divided to nuclear groups on the basis of cytoarchitectonic,

histochemical, immunocytochemical and hodological studies. The

amygdala has been shown to be involved in the modulation of

neuroendocrine function, visceral effector mechanisms and complex

patterns of integrated behavior, such as defense, ingestion, aggres-

sion, reproduction, memory and learning. Such a modulation is

exercised, at least in part, through a vast network of connections

with other brain regions, such as the hypothalamus, the brain stem

and spinal cord autonomic cell aggregates [58].

Arcuate nucleus (Arc): the Arc is located in the tuberal part of the

hypothalamic periventricular zone. The Arc receives strong input

from hypothalamic structures such as the periventricular nuclei,

the PVN, the dorso-medial hypothalamic nucleus, the preoptic

nucleus and the premammillary nucleus. Extrahypothalamic input

involves the bed nucleus of the stria terminalis, the amygdala, the

lateral septal nucleus and the brain stem. Two primary populations of

Arc neurons exert opposing actions on energy balance. One neuronal

population located mainly in the ventro-lateral subdivision of the Arc

inhibits food intake through the expression of pro-opiomelanocortin-

derived peptide, the a-melanocyte-stimulating hormone and

cocaine- and amphetamine-regulated transcript. The other popu-

lation in the far ventro-medial part of the Arc stimulates food intake

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-

campus and the amygdala. The septal region is rich in g-aminobutyric

acid (GABA)-ergic neurons. The septal region does not form a func-

tional unit by itself, but it comprises complex and parallel circuits that

might form loops with the hippocampal formation and the hypothala-

mus [58].

Paraventicular nucleus (PVN): The PVN is located in the anterior part

of the hypothalamic periventricular zone. The PVN contains magno-

cellular neurosecretory cells, which produce oxytocin and vasopressin

and whose axons extend into the posterior pituitary, and parvocellular

neurosecretory cells, which produce CRF, vasopressin and thyrotro-

pin-releasing hormone. The parvocellular neurons project to the

median eminence, and the secreted peptides are carried to the anterior

pituitary by the blood vessels of the hypothalamo–pituitary portal

system. The PVN also contains neurons that project to regions contain-

ing preganglionic autonomic neurons. Accordingly, the PVN has a

central role in mediating hypothalamic responses to stress, feeding

and drinking behavior, and participates in a variety of autonomic

responses [58].

Ventro-medial hypothalamic nucleus (VMH): The VMH receives affer-

ent projections from the amygdala and ventral subiculum, from many

hypothalamic nuclei and from the brain stem. The VMH projections are

consistent with its proposed role in mediating somatomotor aspects of

complex motivated behavior. It shares connections with forebrain and

brain stem regions that are involved in mediating reproductive beha-

vior and with regions involved with appetite behaviors such as thePVN

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. (alon.chen@weizmann.ac.il).

122 1043-2760/$ – see front matter � 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tem.2007.12.002 Available online 11 March 2008

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.

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factor (CRF) peptide and its cognate type 1 CRF receptor(CRF1) in the regulation of the hypothalamic–pituitary–adrenal (HPA) axis and stress-related behavioral responsesare well established, the physiological roles of the relatedurocortin peptides and their preferred receptor, the type 2CRF receptor (CRF2), in responses to such challenges areless understood.

To date, themammalian CRF–urocortin family includesfour structurally related peptides (CRF, and urocortin 1, -2and -3). All are encoded by separate genes and showdifferential expression patterns within both central andperipheral tissues. The physiological effects of these pep-tides are mediated through two related seven transmem-brane domain receptors (CRF1 and CRF2), which areexpressed as multiple isoforms arising from alternativesplicing of the genes. In addition to the brain and pituitarygland, the CRF–urocortin family of peptides and receptorsare highly expressed in several peripheral tissues, and anincreasing body of evidence suggests a role for these pep-tides and receptors in regulating energy metabolism notonly centrally, but also by acting locally within key meta-bolic tissues, including skeletal muscle and the endocrinepancreas. Here, we discuss the recent findings demonstrat-ing the potential for urocortin 2 and urocortin 3, actingthrough CRF2, to modulate glucose homeostasis andmetabolic functions.

Stress, CRF family members and energy homeostasisPerception of physical or psychological stress by an organ-ism is followed by a series of events which result in changesin emotional and cognitive functions, modulation of auto-nomic activities and the secretion of glucocorticoids fromthe adrenal cortex. Both activation and termination ofthe behavioral, autonomic and adrenocortical stressresponses are crucial for adaptation and survival. Theneuropeptide CRF, expressed and secreted from the par-vocellular neurons of the paraventricular nucleus (PVN)

(see Glossary) in the hypothalamus, represents the finalcommon pathway for the integration of the neuroendocrinestress response in the brain [1]. CRF has an important andwell-established role in the regulation of the HPA axisunder basal and stress conditions [2,3]. In addition to thishypophysiotropic action, CRF integrates the behavioral,autonomic andmetabolic responses to stressors [4–6]. CRFis involved in the control of arousal, anxiety, cognitivefunctions and appetite [7–13]. Dysregulation of the stressresponse can have severe psychological and physiologicalconsequences [14,15], and chronic hyperactivation of theCRF system has been linked to stress-related emotionaldisorders such as anxiety, anorexia nervosa and depres-sion [7–15].

In addition to CRF, the CRF–urocortin family of pep-tides includes the more recently described urocortins: uro-cortin 1 [16], urocortin 2 (or stresscopin-related peptideencoded by the human ortholog) [17,18] and urocortin 3 (orstresscopin encoded by the human ortholog) [18,19](Figure 1). CRF and urocortin peptides mediate theireffects through activation of two membrane-bound G-protein-coupled receptors, CRF1 [20–23] and CRF2 [24–27] (Figure 1). CRF1 mRNA is widely expressed in mam-malian brain and pituitary, with high levels in the anteriorpituitary, cerebral cortex, arcuate nucleus (Arc), cerebel-lum, amygdala, hippocampus and olfactory bulb [28]. CRF2

has three apparent membrane bound splice variants inhumans (a, b and c) and two in rats (a and b) that areproduced by the use of alternate 50 exons [24–27,29]. Inrodents, CRF2a is predominantly expressed in the brain ina discrete pattern, with highest densities in the lateralseptal nucleus, bed nucleus of the stria terminalis, ventro-medial hypothalamic nucleus (VMH), olfactory bulb,mesencephalic raphe nuclei and medial amygdala [28].The CRF2b splice form is expressed primarily in peripheraltissues, with the highest levels of expression in the skeletalmuscle 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).

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gastrointestinal tract [24,27]. Interestingly, human CRF2a

is predominantly found in the periphery, indicating thatreceptor isoform distribution differs across species.

Receptor binding and intracellular cAMP accumulationstudies in cells stably transfected with CRF receptors havedemonstrated that CRF1 and CRF2 differ pharmacologi-cally. CRF has a relatively lower affinity for CRF2 comparedwith its affinity forCRF1 [16], whereasurocortin 1 has equalaffinity for both receptors, and urocortin 2 and urocortin 3are highly selective for CRF2 [16,17,19] (Figure 1). Theexistence of the CRF-binding protein (CRF-BP) [30] andthe recently identified soluble splice variant of CRF2a

(sCRF2a) [31], both of which bind to CRF and urocortin 1with high affinity, add a further level of complexity to thecontrol of the action of these ligands (Figure 1).

Maintenance of energy homeostasis and body weight isachieved by an intricate balance between energy intake(food consumption) and expenditure. The brain ultimatelygoverns this energy homeostasis. Afferent signals indicat-ing the nutritional status of the animal and the state of itsexternal environment, including the presence of physio-logical and/or psychological stressors, are integrated cen-trally, and the efferent pathways controlling feedingbehavior and energy expenditure are modulated accord-ingly [32]. The core site of these integrative processes is thehypothalamus, where an array of neurotransmitters, in-cluding many neuropeptides, modulates signals throughcomplex neural circuits [32].

Much evidence has accumulated over the years impli-cating CRF-related neuropeptides and receptors as playersin this complex central network regulating energy balance[32–34]. The effects of stress on feeding behavior andenergy homeostasis are well documented, and bothCRF1 and CRF2 are highly expressed in hypothalamicregions directly associated with control of feeding andenergy balance, including the Arc and the VMH nuclei

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[28]. More recently, studies of the physiology of urocortin 2and urocortin 3, which are highly expressed in skeletalmuscle and the pancreas, respectively, have provided intri-guing evidence that they are novel peripheral modulatorsof glucose homeostasis and metabolic functions.

Urocortin 2 and insulin sensitivity in skeletal musclePeptides encoded by the gene encoding urocortin 2 wereidentified and described as new putative members of theCRF family in 2001 [17,19]. The predicted mature forms ofhumanandmouse urocortin 2 peptides following processingare 76% identical, have a 38-aminoacid sequence associatedwith high biological activity and are structurally morerelated to urocortin 3 than to urocortin 1 or CRF[19,35,36]. Whereas mouse urocortin 2 has a glycine anda pair of basic residues (R-R) at the C-terminus that arepresumed to be involved in amidation and cleavage from theprecursor, the human urocortin 2 ortholog lacks the stan-dard consensus site found for proteolytic cleavage and C-terminal amidation, a requisite for biological potency [36].

Transcripts encoding urocortin 2 are expressed in dis-crete regions of the rodent central nervous system, includingthe hypothalamic paraventricular, supraoptic and arcuatenuclei and the locus coeruleus in the brainstem [17], andurocortin 2 is thus well positioned to be a potential modu-lator of neuroendocrine activity and stress-related behavior[37,38]. Urocortin 2 mRNA is also widely expressed in avariety of mouse peripheral tissues, including the adrenalgland, lungand the gastrointestinal tract, but ismosthighlyexpressed in skeletal muscle and skin [39].

The generation and study of urocortin 2 knockout micehas revealed an interesting metabolic phenotype and apotential role for urocortin 2 as a peripheral modulator ofglucose utilization and insulin sensitivity in skeletalmuscle [40] (Figure 2). The initial observation that glucosetolerance is significantly enhanced in urocortin 2-deficient

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].

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mice when compared with their wild-type (WT) littermateswas demonstrated by the improved ability of these mice tometabolize a glucose load [40] (Figure 2). Systemic admin-istration of synthetic urocortin 2 peptide to the mutantmice, before the glucose tolerance test, restored the glucoseprofile to one comparable with that of WT mice, whereasadministration of the CRF2-selective antagonist astressin2B toWT littermates resulted in a glucose tolerance profilemirroring that of urocortin 2-null mice [40] (Figure 2). Theefficacy of peripherally administered peptide or antagonistin mediating these effects strongly suggests that theobserved phenotype is peripherally, rather than centrally,mediated.

Urocortin 2-null mice also demonstrate increasedinsulin sensitivity when compared with their WT litter-mates, as determined using an insulin tolerance test.Again, administration of synthetic urocortin 2 peptidebefore the test restored the blood glucose response profilein response to insulin to that ofWT animals [40] (Figure 2).

That urocortin 2-null mice and their WT littermatesshowed similar fasting and glucose-stimulated insulinlevels suggested that the mechanism for enhanced glucosetolerance in urocortin 2-null mice was not due to increasedinsulin secretion into the bloodstream, but rather to anincrease in insulin sensitivity of target tissues. This wasdemonstrated by measurement of whole-body glucosehomeostasis using thehyperinsulinemic euglycemic glucoseclamp technique. Higher glucose infusion rates wererequired to maintain a set physiological blood glucose levelin urocortin 2-null mice subjected to constant hyperinsuli-nemia than in WT littermates [40]. During these clampstudies, whole body glucose uptake, as measured usingradiolabeled glucose analogues (which measure peripheralinsulin sensitivity), glycolysis and insulin-mediated sup-pression of hepatic glucose production rates (i.e. hepaticinsulin sensitivity)were all demonstrated to be significantlyincreased in urocortin 2-deficient mice [40]. Furthermore,specific glucose uptake into the gastrocnemius muscle was

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significantly higher in mutant mice. These effects wereobserved in mice maintained on standard chow or a high-fat diet, suggesting that urocortin 2-null mice are protectedagainst the deleterious effects of a high-fat diet on insulinsensitivity and glucose tolerance.

Accordingly, although urocortin 2-knockout mice andWT littermates placed on standard chow or a high-fat dietfor 16 weeks showed similar weight gain and food con-sumption, significant increases in blood glucose and insu-lin levels were observed only in the WT mice [40]. Bodycomposition measurements determined that urocortin 2-null mice placed on a high-fat diet showed decreases inadipose tissue and increases in lean tissue when comparedwith WT littermates [40].

Further mechanistic insights were provided by in vitrodemonstrations of urocortin 2 inhibition of insulin-inducedphosphorylation of Akt and extracellular-signal-regulatedkinases 1 and -2 in cultured skeletal muscle cells and in amyotube cell line (C2C12), and of insulin-induced glucoseuptake by C2C12 myotubes [40]. These results suggestthat urocortin 2 functions to inhibit interactions betweeninsulin signaling pathway components, although furtherstudies are required to determine the precise molecularmechanisms by which urocortin 2, acting through itsspecific G-protein-coupled receptor CRF2b, inhibits insulinreceptor signaling.

Pancreatic urocortin 3, insulin secretion and energyhomeostasisThe urocortin 3 gene encodes a predicted mature 38-aminoacid peptide [18,19]. The synthetic amidated urocortin 3peptide selectively binds to and activates CRF2 with highaffinity, suggesting that urocortin 3, in addition to urocor-tin 2, is an endogenous selective CRF2 agonist. mRNAtranscripts encoding urocortin 3 are expressed in discreteregions of the rodent central nervous system, predomi-nately within the hypothalamus and medial amygdala[41,42]. In the hypothalamus, urocortin 3-expressingneurons are present in the median preoptic nucleus andin the rostral perifornical area lateral to the PVN [19,41],with fibers distributed mainly to hypothalamic and limbicstructures. Several major urocortin 3 terminal fields, in-cluding the lateral septum and the ventro-medial hypo-thalamus, express high levels of CRF2 [41]. Besides thebrain, urocortin 3, similarly to urocortin 2, is alsoexpressed in peripheral tissues but with a distinct patternof distribution, with high levels of expression in the pan-creas [43], adrenal gland and gastrointestinal tract ofrodents. Using in situ hybridization and immunohisto-chemical techniques, Li et al. [43] demonstrated that uro-cortin 3 is highly expressed by the b cells of pancreaticislets and also by the mouse b-cell line MIN6 [43]. Highglucose, high potassium and forskolin were demonstratedto stimulate urocortin 3 secretion fromMIN6 cells [43], andthe dose-dependent effects of glucose on urocortin 3secretion were found to be mediated by the KATP channel[43]. The insulin secretagogues exendin-4 (a glucagon-likepeptide analogue) and carbachol (a muscarinic acetyl-choline receptor agonist) significantly stimulated urocortin3 release from MIN6 cells in the presence of high physio-logical (10 mM) glucose levels [44]. Pancreatic urocortin 3

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mRNA levels were found to be significantly higher in obeseleptin-deficient ob/ob mice and in rats fed a high-fat diet[44], and pharmacological blockade of CRF2 with astressin2B antagonist or immunoneutralization of urocortin 3attenuated high, but not low, glucose-induced insulinsecretion from isolated pancreatic islets in vitro [44]. Thesedata indicated that endogenous urocortin 3 is releasedunder conditions of high glucose and then signals throughislet CRF2 to facilitate insulin secretion.

The recent generation of urocortin 3-knockout mice hasprovided further insights into the endogenous role of uro-cortin 3 in vivo [44]. Urocortin 3-null mice, when fed astandard chow diet, showed similar responses to their WTlittermates in glucose and insulin tolerance tests. How-ever, pancreatic islets isolated from urocortin 3-null micesecreted significantly less insulin in response to high, butnot low or medium, glucose concentrations [44], furthersupporting the purported role of endogenous urocortin 3 ininduction of insulin secretion in response to high glucoseconditions. Urocortin 3-null mice fed a high-fat diet for 16weeks showed significantly lower fasting blood glucose andplasma insulin concentrations when compared with theirWT littermates [44]. Unlike WT mice, which developedimpaired glucose tolerance and insulin resistance when onthe high-fat diet, urocortin 3-null mice did not show adecrease in glucose tolerance and were protected againstdietary high fat-induced hyperinsulinemia, hyperglyce-mia, hepatic steatosis and hypertriglyceridemia [44].These results suggest that endogenous pancreatic urocor-tin 3, induced under excessive caloric conditions, functionslocally to augment insulin production, which might con-tribute in the long term to reduced insulin sensitivity andharmful metabolic consequences.

Energy homeostasis and CRF2

In addition to the peripheral tissues involved in the hand-ling of fuel molecules discussed earlier, CRF2 is present inbrain regions intimately associated with the central regu-lation of feeding, glucose homeostasis and energy balance.The CRF2a splice variant is highly expressed in the VMH,and to a lesser extent in the arcuate nucleus, dorso-medialhypothalamus, lateral hypothalamus and PVN [27,28,45].Important insights into the role of central CRF2 in mod-ulating energy homeostasis and metabolic function areprovided by studies conducted in CRF2-deficient mice[46–48]. These studies provide evidence that centralCRF2 is important in modulating not only metabolic rate,but also appetite and feeding behaviors.

CRF2-null mice recovered completely and more rapidlyfrom urocortin 1-induced hypophagia compared with theirWT littermates [47]. CRF2-null mice exhibited normalbasal feeding behavior and weight gain [46–48] butdecreased their food intake following food deprivation.There were no differences in body weight following fooddeprivation or refeeding; this effect on food intake couldthus be interpreted as a direct adaptation to the effect ofCRF2 deficiency onmetabolic rate. The observed decrease infood intake couldalsobeananxiety-relatedbehavior elicitedby the stress of food deprivation, given that the CRF2-nullmice have a recognized anxiogenic phenotype [46]. CRF2-null mice maintained on a high-fat diet have increased food

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intakewhen comparedwith theirWT littermates and donotexhibit the weight gain or hypercholesterolemia induced inthe WT animals [49]. They also demonstrate enhancedglucose tolerance and increased insulin sensitivity, andprotection from insulin resistance induced by a high-fat diet[49]. Examination of the interscapular brown adipose tissue(IBAT) revealed elevated levels of uncoupling protein-1 inCRF2-null mice [49] and significantly elevated basal IBATthermogenesis with prolonged adrenergic responsivity ofIBAT in oldermice. These data suggest thatCRF2-nullmicemight have increased sympathetic nervous system (SNS)outflow, and that, in older mice, the SNS pathway mightremain, unlike in WT mice, sensitive to adrenergic stimu-lation in the absence of CRF2 [50]. CRF2-null mice, main-tained on a high-fat diet, have a reduced respiratoryexchange ratio, which indicates a reduction in carbohydrateoxidationandan increase in fattyacidoxidationasa fractionof the total energy consumed by these mice. The reducedrespiratory exchange ratiowas reversible byCRF1 antagon-ist treatment, suggesting a role for CRF2 in impedingCRF1-induced SNS activity [50]. Clearly, the metabolicphenotype of the CRF2-null mice is complex, and thereported findings are in some instances contradictory andmight be confounded by their behavioral phenotype. Therole of this receptor in the control of energy processesmightbe further resolved by the study of more sophisticated mice

Figure 3. Schematic representation summarizing the proposed roles of central and

homeostasis. Following stressful stimuli, glucocorticoid exposure resulting from HPA ax

skeletal muscle, and pancreatic and hepatic function. CRF and urocortins, functioning th

and glucose homeostasis. Peripherally, urocortin 2 produced in skeletal muscle and ac

inhibiting insulin signaling. Urocortin 3, produced by the pancreatic b cells, regulates hig

muscle functions by CRF-related peptides and receptors requires further investigation. A

metabolic liver functions, studies have failed to demonstrate hepatic expression of CRF-

effects in the mutant mice would seem likely to be secondary to their altered energy h

models, targeting specific CRF2-expressing brain nuclei orindividual peripheral tissues.

In addition to the effects on metabolism mediatedthrough the autonomic nervous system, both central andperipheral stimulation of CRF2 produces satiety. Centraladministration of CRF1 agonists is well recognized to elicitrapid onset and short-term anorexia, independently ofCRF2. However, the observed anorexia following urocortin2 or urocortin 3 administration is delayed and of greaterduration, suggesting differential roles for the CRF receptortypes in modulating food intake. Moreover, intracerebro-ventricular administration of urocortin 2 does not elicitthe malaise, behavioral arousal or anxiogenesis associatedwith CRF1 agonist administration [51,52]. It is alsonoteworthy that CRF1 stimulation could mediatenegative effects on ingestive behavior that are secondaryto the anxiety-and fear-like behaviors triggered by non-selective CRF receptor agonists.

Additional analyses have shown that infusion of uro-cortin 3 to the VMH reduces meal frequency, prolongspostmeal intervals, slows the eating rate and reduces mealsize [53]. These observations are similar to those reportedfor intracerebroventricular leptin administration andmight reflect a functional relationship between leptinand CRF2 in the VMH [53]. A positive correlation betweenleptin serum levels andCRF2mRNA levels in theVMHhas

peripheral CRF and urocortin peptides and receptors in modulating glucose

is activation by hypothalamic CRF and changes in autonomic activity will modulate

rough both type 1 and type 2 CRF receptors in the brain, will modulate food intake

ting locally at the CRF2 receptor will regulate glucose uptake in skeletal muscle by

h glucose-induced insulin secretion. Potential regulation of pancreatic and skeletal

lthough, both urocortin 2- and urocortin 3-deficient mice demonstrate alterations in

related peptides or receptors (denoted by ‘?’ in the liver). Therefore, these observed

omeostasis resulting from the direct effects on muscle or pancreatic physiology.

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been shown [54,55]. Physiological perturbations and stres-sors, including adrenalectomy, starvation and repeatedcold stress, caused a decrease in both leptin and CRF2

mRNA levels in the VMH [45,54,55].A further intriguing role for CRF receptors in the VMH

in mediating counterregulatory responses to acute hypo-glycemia has been suggested. The VMH contains neuronswhich react to changes in circulating glucose levels.Changes in systemic glucose are translated into a changein neuronal firing rate through glucokinase and ATP-sen-sitive potassium channel activity [56]. It was recentlyshown that CRF2 activation within the VMH has a sup-pressive effect on counterregulatory responses to acutehypoglycemia, whereas CRF1 activation in the VMHamplifies these responses [56,57].

Concluding remarksMaintaining energy homeostasis in the presence of diversechallenges, such as starvation, exercise or high-fat diet,requires numerous adaptive responses in both central andperipheral tissues. The recent studies summarized herehave clearly demonstrated that urocortin 2 and urocortin3, functioning through the type 2 CRF receptor, can serveas autocrine and/or paracrine regulators of glucose homeo-stasis by modulating insulin sensitivity in skeletal muscleor by regulating glucose-induced insulin secretion in the b

cells of the pancreas, respectively (Figure 3). The anatom-ical distribution of urocortin 2, urocortin 3 and CRF2

within peripheral and central tissues key to the regulationof energy homeostasis, together with the robust metabolicphenotypes of mice deficient in these factors, leaves thempoised as major new players in this field (Figure 3).

The CRF–urocortin family of peptides and receptors arenot only structurally and pharmacologically related, butalso share additional commonalities, such as regulationby glucocorticoids [39,45], supporting the concept that theyare likely to operate in concert as a single functional system.Further insights into the detailed physiology of this systemwill be aided by the generation of tissue-specific urocortin 2,urocortin 3 and CRF2 transgenic animal models for manip-ulation of expression levels of the receptor or ligands. Studyof such models will facilitate our understanding of thespecific roles of central and peripheral CRF2 in modulatingmetabolic functions. Likewise, understanding of thedetailed regulation of central and peripheral CRF2 andurocortins under different physiological conditions (basalor challenged)will also contribute to elucidating the cellularand molecular mechanisms mediating their effects.

The novel functions for CRF2 and its ligands urocortin 2and urocortin-3 as local regulators of glucose uptake inmuscle, and of insulin secretion in the pancreas, highlightedhere, not only add to out current understanding of thephysiology of energy metabolism, but are also of potentialinterest as therapeutic targets for themanagement of type 2diabetes and other metabolic disorders.

AcknowledgementsThe authors would like to thank Dr Wylie Vale from the ClaytonFoundation Laboratories for Peptide Biology, The Salk Institute forBiological Studies, La Jolla, CA, USA, and Dr Pauline Jamieson,Endocrinology Unit, Centre for Cardiovascular Science, The Queen’s

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Medical Research Institute, Edinburgh, UK, for their critical reading andconstructive comments. The authors would like to thank Ms GeniaBrodsky and Mr Ziv Ariely from the Graphics Department of theWeizmann Institute of Science for their assistance in generating theillustrations for this review.

References1 Vale, W. et al. (1981) Characterization of a 41-residue ovine

hypothalamic peptide that stimulates secretion of corticotropin andbeta-endorphin. Science 213, 1394–1397

2 Rivier, C. and Vale, W. (1983) Modulation of stress-induced ACTHrelease by corticotropin releasing factor, catecholamines andvasopressin. Nature 305, 325–327

3 Muglia, L. et al. (1995) Corticotropin-releasing hormone deficiencyreveals major fetal but not adult glucocorticoid need. Nature 373,427–432

4 Sutton, R.E. et al. (1982) Corticotropin-releasing factor (CRF) producesbehavioral activation in rats. Nature 297, 331–333

5 Brown, M.R. et al. (1982) Corticotropin-releasing factor (CRF): actionson the sympathetic nervous system and metabolism. Endocrinology111, 928–931

6 Koob, G.F. and Heinrichs, S.C. (1999) A role for corticotropin releasingfactor and urocortin in behavioral responses to stressors. Brain Res.848, 141–152

7 Zorrilla, E.P. and Koob, G.F. (2004) The therapeutic potential of CRF1

antagonists for anxiety. Expert Opin. Investig. Drugs 13, 799–8288 Heinrichs, S.C. and Koob, G.F. (2004) Corticotropin-releasing factor in

brain: a role in activation, arousal, and affect regulation. J. Pharmacol.Exp. Ther. 311, 427–440

9 Holsboer, F. (1999) The rationale for corticotropin-releasing hormonereceptor (CRH-R) antagonists to treat depression and anxiety.J. Psychiatr. Res. 33, 181–214

10 Arborelius, L. et al. (1999) The role of corticotropin-releasing factor indepression and anxiety disorders. J. Endocrinol. 160, 1–12

11 Holmes, A. et al. (2003) Neuropeptide systems as novel therapeutictargets for depression and anxiety disorders. Trends Pharmacol. Sci.24, 580–588

12 Bale, T.L. (2005) Sensitivity to stress: dysregulation of CRF pathwaysand disease development. Horm. Behav. 48, 1–10

13 Chrousos, G.P. and Gold, P.W. (1992) The concepts of stress and stresssystem disorders. Overview of physical and behavioral homeostasis.JAMA 267, 1244–1252

14 de Kloet, E.R. et al. (2005) Stress and the brain: from adaptation todisease. Nat. Rev. Neurosci. 6, 463–475

15 McEwen, B.S. (2004) Protection and damage from acute and chronicstress: allostasis and allostatic overload and relevance to thepathophysiology of psychiatric disorders. Ann. N. Y. Acad. Sci. 1032,1–7

16 Vaughan, J. et al. (1995) Urocortin, amammalian neuropeptide relatedto fish urotensin I and to corticotropin-releasing factor. Nature 378,287–292

17 Reyes, T.M. et al. (2001) Urocortin II: a member of the corticotropin-releasing factor (CRF) neuropeptide family that is selectively bound bytype 2 CRF receptors. Proc. Natl. Acad. Sci. U. S. A. 98, 2843–2848

18 Hsu, S.Y. and Hsueh, A.J. (2001) Human stresscopin and stresscopin-related peptide are selective ligands for the type 2 corticotropin-releasing hormone receptor. Nat. Med. 7, 605–611

19 Lewis, K. et al. (2001) Identification of urocortin III, an additionalmember of the corticotropin-releasing factor (CRF) family with highaffinity for the CRF2 receptor. Proc. Natl. Acad. Sci. U. S. A. 98, 7570–7575

20 Chen, R. et al. (1993) Expression cloning of a human corticotropinreleasing factor (CRF) receptor.Proc. Natl. Acad. Sci. U. S. A. 90, 8967–8971

21 Perrin, M.H. et al. (1993) Cloning and functional expression of a ratbrain corticotropin releasing factor (CRF) receptor.Endocrinology 133,3058–3061

22 Vita, N. et al. (1993) Primary structure and functional expression ofmouse pituitary and human brain corticotrophin releasing factorreceptors. FEBS Lett. 335, 1–5

23 Chang, C-P. et al. (1993) Identification of a seven transmembrane helixreceptor for corticotropin-releasing factor and sauvagine inmammalian brain. Neuron 11, 1187–1195

Review Trends in Endocrinology and Metabolism Vol.19 No.4

24 Perrin,M. et al. (1995) Identification of a second corticotropin-releasingfactor receptor gene and characterization of a cDNA expressed in heart.Proc. Natl. Acad. Sci. U. S. A. 92, 2969–2973

25 Stenzel, P. et al. (1995) Identification of a novel murine receptor forcorticotropin-releasing hormone expressed in the heart. Mol.Endocrinol. 9, 637–645

26 Kishimoto, T. et al. (1995) A sauvagine/corticotropin-releasing factorreceptor expressed in heart and skeletal muscle. Proc. Natl. Acad. Sci.U. S. A. 92, 1108–1112

27 Lovenberg, T.W. et al. (1995) Cloning and characterization of afunctionally distinct corticotropin-releasing factor receptor subtypefrom rat brain. Proc. Natl. Acad. Sci. U. S. A. 92, 836–840

28 Van Pett, K. et al. (2000) Distribution of mRNAs encoding CRFreceptors in brain and pituitary of rat and mouse. J. Comp. Neurol.428, 191–212

29 Kostich, W.A. et al. (1998) Molecular identification and analysis of anovel human corticotropin-releasing factor (CRF) receptor: the CRF2g

receptor. Mol. Endocrinol. 12, 1077–108530 Potter, E. et al. (1991) Cloning and characterization of the cDNAs for

human and rat corticotropin releasing factor-binding proteins. Nature349, 423–426

31 Chen, A.M. et al. (2005) A soluble mouse brain splice variant of type 2a

corticotropin-releasing factor (CRF) receptor binds ligands andmodulates their activity. Proc. Natl. Acad. Sci. U. S. A. 102, 2620–2625

32 Dallman, M.F. et al. (2006) Glucocorticoids, chronic stress, and obesity.Prog. Brain Res. 153, 75–105

33 Zorrilla, E.P. et al. (2003) Nibbling at CRF receptor control of feedingand gastrocolonic motility. Trends Pharmacol. Sci. 24, 421–427

34 Richard, D. et al. (2002) The corticotropin-releasing factor family ofpeptides and CRF receptors: their roles in the regulation of energybalance. Eur. J. Pharmacol. 440, 189–197

35 Perrin, M.H. and Vale, W.W. (1999) Corticotropin releasingfactor receptors and their ligand family. Ann. N. Y. Acad. Sci. 885,312–328

36 Dautzenberg, F.M. and Hauger, R.L. (2002) The CRF peptide familyand their receptors: yet more partners discovered. Trends Pharmacol.Sci 23, 71–77

37 Chen, A. et al. (2006) Urocortin 2-deficient mice exhibit gender-specificalterations in circadian hypothalamus-pituitary-adrenal axis anddepressive-like behavior. J. Neurosci. 26, 5500–5510

38 Chen, A. et al. (2003) Glucocorticoids regulate the expression of themouse urocortin II gene: a putative connection between thecorticotropin-releasing factor receptor pathways. Mol. Endocrinol.17, 1622–1639

39 Chen, A. et al. (2004) Urocortin II gene is highly expressed in mouseskin and skeletal muscle tissues: localization, basal expression inCRFR1 and CRFR2-null mice and regulation by glucocorticoids.Endocrinology 145, 2445–2457

40 Chen, A. et al. (2006) Urocortin 2 modulates glucose utilization andinsulin sensitivity in skeletal muscle. Proc. Natl. Acad. Sci. U. S. A.103, 16580–16585

41 Li, C. et al. (2002) Urocortin III-immunoreactive projections in ratbrain: partial overlap with sites of type 2 corticotrophin-releasingfactor receptor expression. J. Neurosci. 22, 991–1001

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42 Jamieson, P.M. et al. (2006) Urocortin 3 modulates the neuroendocrinestress response and is regulated in rat amygdala and hypothalamus bystress and glucocorticoids. Endocrinology 147, 4578–4588

43 Li, C. et al. (2003) Urocortin III is expressed in pancreatic beta-cellsand stimulates insulin and glucagon secretion. Endocrinology 144,3216–3224

44 Li, C. et al. (2007) Urocortin 3 regulates glucose-stimulated insulinsecretion and energy homeostasis. Proc. Natl. Acad. Sci. U. S. A. 104,4206–4211

45 Chen, A. et al. (2005) Mouse corticotropin-releasing factor receptortype 2a gene: isolation, distribution, pharmacological characterizationand regulation by stress and glucocorticoids. Mol. Endocrinol. 19, 441–458

46 Bale, T.L. et al. (2000) Mice deficient for corticotropin-releasinghormone receptor-2 display anxiety-like behaviour and arehypersensitive to stress. Nat. Genet. 24, 410–414

47 Coste, S.C. et al. (2000) Abnormal adaptations to stress and impairedcardiovascular function inmice lacking corticotropin-releasing hormonereceptor-2. Nat. Genet. 24, 403–409

48 Kishimoto, T. et al. (2000) Deletion of crhr2 reveals an anxiolytic rolefor corticotropin-releasing hormone receptor-2.Nat. Genet. 24, 415–419

49 Bale, T.L. et al. (2003) Corticotropin-releasing factor receptor-2-deficient mice display abnormal homeostatic response to challengesof increased dietary fat and cold. Endocrinology 144, 2580–2587

50 Carlin, K.M. et al. (2006) Vital functions of corticotropin-releasingfactor (CRF) pathways in maintenance and regulation of energyhomeostasis. Proc. Natl. Acad. Sci. U. S. A. 103, 3462–3467

51 Zorrilla, E.P. et al. (2004) Human urocortin 2, a corticotropin-releasingfactor (CRF)2 agonist, and ovine CRF, a CRF1 agonist, differentiallyalter feeding and motor activity. J. Pharmacol. Exp. Ther. 310, 1027–1034

52 Inoue, K. et al. (2003) Human urocortin II, a selective agonist for thetype 2 corticotropin-releasing factor receptor, decreases feeding anddrinking in the rat. J. Pharmacol. Exp. Ther. 305, 385–393

53 Fekete, E.M. et al. (2007) Delayed satiety-like actions and alteredfeeding microstructure by a selective type 2 corticotropin-releasingfactor agonist in rats: intra-hypothalamic urocortin 3 administrationreduces food intake by prolonging the post-meal interval.Neuropsychopharmacology 32, 1052–1068

54 Makino, S. et al. (1998) Altered expression of type 2 CRH receptormRNA in the VMH by glucocorticoids and starvation. Am. J. Physiol.275, R1138–R1145

55 Makino, S. et al. (1999) Decreased type 2 corticotrophin-releasing hormone receptor mRNA expression in theventromedial hypothalamus during repeated immobilization stress.Neuroendocrinology 70, 160–167

56 McCrimmon, R.J. et al. (2006) Corticotrophin-releasing factor receptorswithin the ventromedial hypothalamus regulate hypoglycemia-inducedhormonal counterregulation. J. Clin. Invest. 116, 1723–1730

57 Cheng, H. et al. (2007) Type 1 corticotrophin-releasing factor receptors(CRFR1) in the ventromedial hypothalamus (VMH) promotehypoglycemia-induced hormonal counterregulation. Am. J. Physiol.Endocrinol. Metab. 293, E705–E712

58 Paxinos, G, ed. (2004) The rat Nervous System, Elsevier Academic Press

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