Disruption of the RIIbeta subunit of PKA reverses the obesity syndrome of Agouti lethal yellow mice

A bioluminescent and homogeneous assay for monitoring GPCR-mediated cAMP modulation and PDE activity

3',5'-Cyclic adenosine monophosphate (cAMP), the first identified second messenger, is implicated in diverse cellular processes involving cellular metabolism, cell proliferation and differentiation, apoptosis, and gene expression. cAMP is synthesized by adenylyl cyclase (AC), which converts ATP to cAMP upon activation of Gαs-protein coupled receptors (GPCRs) in most cases and hydrolyzed by cyclic nucleotide phosphodiesterases (PDEs) to 5'-AMP. Dysregulation of cAMP signaling is implicated in a wide range of pathophysiological conditions such as cardiovascular diseases, neurodegenerative and behavioral disorders, cancers, diabetes, obesity, cataracts, and others. Therefore, cAMP targeted therapies have been and are still undergoing intense investigation for the treatment of these and other diseases. This highlights the need for developing assays to detect and monitor cAMP levels. In this study, we show cAMP Lumit assay as a highly specific homogeneous bioluminescent assay suitable for high throughput screenings with a large assay window and a wide dynamic range for cAMP detection. We believe that this assay will aid and simplify drug discovery screening efforts for cAMP signaling targeted therapies.

RIIβ-PKA in GABAergic Neurons of Dorsal Median Hypothalamus Governs White Adipose Browning

The RIIβ subunit of  cAMP-dependent protein kinase A (PKA) is expressed in the brain and adipose tissue. RIIβ-knockout mice show leanness and increased UCP1 in brown adipose tissue. The authors have previously reported that RIIβ reexpression in hypothalamic GABAergic neurons rescues the leanness. However, whether white adipose tissue (WAT) browning contributes to the leanness and whether RIIβ-PKA in these neurons governs WAT browning are unknown. Here, this work reports that RIIβ-KO mice exhibit a robust WAT browning. RIIβ reexpression in dorsal median hypothalamic GABAergic neurons (DMH GABAergic neurons) abrogates WAT browning. Single-cell sequencing, transcriptome sequencing, and electrophysiological studies show increased GABAergic activity in DMH GABAergic neurons of RIIβ-KO mice. Activation of DMH GABAergic neurons or inhibition of PKA in these neurons elicits WAT browning and thus lowers body weight. These findings reveal that RIIβ-PKA in DMH GABAergic neurons regulates WAT browning. Targeting RIIβ-PKA in DMH GABAergic neurons may offer a clinically useful way to promote WAT browning for treating obesity and other metabolic disorders.

The regulation of PKA signaling in obesity and in the maintenance of metabolic health

The cAMP-dependent protein kinase (PKA) system represents a primary cell-signaling pathway throughout systems and across species. PKA facilitates the actions of hormones, neurotransmitters and other signaling molecules that bind G-protein coupled receptors (GPCR) to modulate cAMP levels. Through its control of synaptic events, exocytosis, transcriptional regulation, and more, PKA signaling regulates cellular metabolism and emotional and stress responses making it integral in the maintenance and dysregulation of energy homeostasis. Neural PKA signaling is regulated by afferent and peripheral efferent signals that link specific neural cell populations to the regulation of metabolic processes in adipose tissue, liver, pancreas, adrenal, skeletal muscle, and gut. Mouse models have provided invaluable information on the roles for PKA subunits in brain and key metabolic organs. While limited, human studies infer differential regulation of the PKA system in obese compared to lean individuals. Variants identified in PKA subunit genes cause Cushing syndrome that is characterized by metabolic dysregulation associated with endogenous glucocorticoid excess. Under healthy physiologic conditions, the PKA system is exquisitely regulated by stimuli that activate GPCRs to alter intracellular cAMP concentrations, and by PKA cellular localization and holoenzyme stability. Adenylate cyclase activity generates cAMP while phosphodiesterase-mediated cAMP degradation to AMP decreases cAMP levels downstream of GPCRs. Chronic perturbations in PKA signaling appear to be capable of resetting PKA regulation at several levels; in addition, sex differences in PKA signaling regulation, while not well understood, impact the physiologic consequences of metabolic dysregulation and obesity. This review explores the roles for PKA signaling in the pathogenesis of metabolic diseases including obesity, type 2 diabetes mellitus and associated co-morbidities through neural-peripheral crosstalk and cAMP/PKA signaling pathway targets that hold therapeutic potential.

The Role of Neuropeptide Y in the Nucleus Accumbens

Neuropeptide Y (NPY), an abundant peptide in the central nervous system, is expressed in neurons of various regions throughout the brain. The physiological and behavioral effects of NPY are mainly mediated through Y1, Y2, and Y5 receptor subtypes, which are expressed in regions regulating food intake, fear and anxiety, learning and memory, depression, and posttraumatic stress. In particular, the nucleus accumbens (NAc) has one of the highest NPY concentrations in the brain. In this review, we summarize the role of NPY in the NAc. NPY is expressed principally in medium-sized aspiny neurons, and numerous NPY immunoreactive fibers are observed in the NAc. Alterations in NPY expression under certain conditions through intra-NAc injections of NPY or receptor agonists/antagonists revealed NPY to be involved in the characteristic functions of the NAc, such as alcohol intake and drug addiction. In addition, control of mesolimbic dopaminergic release via NPY receptors may take part in these functions. NPY in the NAc also participates in fat intake and emotional behavior. Accumbal NPY neurons and fibers may exert physiological and pathophysiological actions partly through neuroendocrine mechanisms and the autonomic nervous system.

PKA functions in metabolism and resistance to obesity: lessons from mouse and human studies

Both direct and indirect evidence demonstrate a central role for the cAMP-dependent protein kinase (PKA) signaling pathway in the regulation of energy balance and metabolism across multiple systems. However, the ubiquitous pattern of PKA expression across cell types poses a challenge in pinpointing its tissue-specific regulatory functions and further characterizing its many downstream effects in certain organs or cells. Mouse models of PKA deficiency and over-expression and studies in living cells have helped clarify PKA function in adipose tissue (AT), liver, adrenal, pancreas, and specific brain nuclei, as they pertain to energy balance and metabolic dysregulation. Limited studies in humans suggest differential regulation of PKA in AT of obese compared to lean individuals and an overall dysregulation of PKA signaling in obesity. Despite its complexity, under normal physiologic conditions, the PKA system is tightly regulated by changes in cAMP concentrations upstream via adenylate cyclase and downstream by phosphodiesterase-mediated cAMP degradation to AMP and by changes in PKA holoenzyme stability. Adjustments in the PKA system appear to be important to the development and maintenance of the obese state and its associated metabolic perturbations. In this review we discuss the important role of PKA in obesity and its involvement in resistance to obesity, through studies in humans and in mouse models, with a focus on the regulation of PKA in energy expenditure, intake behavior, and lipid and glucose metabolism.

Neuronal cAMP/PKA Signaling and Energy Homeostasis

The brain plays a key role in the regulation of body weight and glucose metabolism. Peripheral signals including hormones, metabolites, and neural afferent signals are received and processed by the brain which in turn elicits proper behavioral and metabolic responses for maintaining energy and glucose homeostasis. The cAMP/protein kinase A (PKA) pathway acts downstream G-protein-coupled receptors (GPCR) to mediate the physiological effects of many hormones and neurotransmitters. Activated PKA phosphorylates various proteins including ion channels, enzymes, and transcription factors and regulates their activity. Recent studies have shown that neuronal cAMP/PKA activity in multiple brain regions are involved in the regulation of feeding, energy expenditure, and glucose homeostasis. In this chapter I summarize recent genetic and pharmacological studies concerning the regulation of body weight and glucose homeostasis by cAMP/PKA signaling in the brain.

Whole-genome resequencing reveals signatures of selection and timing of duck domestication

Background

The genetic basis of animal domestication remains poorly understood, and systems with substantial phenotypic differences between wild and domestic populations are useful for elucidating the genetic basis of adaptation to new environments as well as the genetic basis of rapid phenotypic change. Here, we sequenced the whole genome of 78 individual ducks, from two wild and seven domesticated populations, with an average sequencing depth of 6.42X per individual.

Results

Our population and demographic analyses indicate a complex history of domestication, with early selection for separate meat and egg lineages. Genomic comparison of wild to domesticated populations suggests that genes that affect brain and neuronal development have undergone strong positive selection during domestication. Our F_ST analysis also indicates that the duck white plumage is the result of selection at the melanogenesis-associated transcription factor locus.

Conclusions

Our results advance the understanding of animal domestication and selection for complex phenotypic traits.

Molecular characterization of feline melanocortin 4 receptor and melanocortin 2 receptor accessory protein 2

Melanocortin 4 receptor (MC4R), which is a member of the G protein-coupled receptor (GPCR) family, mediates regulation of energy homeostasis upon the binding of α-melanocyte-stimulating hormone (α-MSH) in the central nervous system (CNS). Melanocortin 2 receptor accessory protein 2 (MRAP2) modulates the function of MC4R. We performed cDNA cloning of cat MC4R and MRAP2 and characterized their amino acid sequences, mRNA expression patterns in cat tissues, protein-protein interactions, and functions. We found high sequence homology (>88%) with other mammalian MC4R and MRAP2 encoding 332 and 206 amino acid residues, respectively. Reverse transcription-polymerase chain reaction analysis revealed that cat MC4R and MRAP2 mRNA were expressed highly in the CNS. In CHO-K1 cells transfected with cat MC4R, stimulation with α-MSH increased intracellular cyclic adenosine monophosphate (cAMP) concentration in a dose-dependent manner. Furthermore, the presence of MRAP2 enhanced the cat MC4R-mediated cAMP production. These results suggested that cat MC4R acts as a neuronal mediator in the CNS and that its function is modulated by MRAP2. In addition, our NanoBiT study showed the dynamics of their interactions in living cells; stimulation with α-MSH slightly affected the interaction between MC4R and MRAP2, and did not affect MC4R homodimerization, suggesting that they interact in the basal state and that structural change of MC4R by activation may affect the interaction between MC4R and MRAP2.

Obesity-associated sympathetic overactivity in children and adolescents: the role of catecholamine resistance in lipid metabolism

BACKGROUND

Obesity in children and adolescents is characterized by chronic sympathetic overdrive and reduced epinephrine-stimulated lipolysis. This resistance to catecholamines occurs during the dynamic phase of fat accumulation. This review will focus on the relationship between sympathetic-adrenal activity and lipid metabolism, thereby highlighting the role of catecholamine resistance in the development of childhood obesity.

RESULTS AND CONCLUSIONS

Catecholamine resistance causes lipid accumulation in adipose tissue by reducing lipolysis, increasing lipogenesis and impeding free fatty acid (FFA) transportation. Exercise improves catecholamine resistance, as evidenced by attenuated systemic sympathetic activity, reduced circulating catecholamine levels and enhanced β-adrenergic receptor signaling. Insulin resistance is mostly a casual result rather than a cause of childhood obesity. Therefore, catecholamine resistance in childhood obesity may promote insulin signaling in adipose tissue, thereby increasing lipogenesis. This review outlines a series of evidence for the role of catecholamine resistance as an upstream mechanism leading to childhood obesity.

Hypothalamic PKA regulates leptin sensitivity and adiposity

Mice lacking the RIIβ regulatory subunit of cyclic AMP-dependent protein kinase A (PKA) display reduced adiposity and resistance to diet-induced obesity. Here we show that RIIβ knockout (KO) mice have enhanced sensitivity to leptin's effects on both feeding and energy metabolism. After administration of a low dose of leptin, the duration of hypothalamic JAK/STAT3 signalling is increased, resulting in enhanced POMC mRNA induction. Consistent with the extended JAK/STAT3 activation, we find that the negative feedback regulator of leptin receptor signalling, Socs3, is inhibited in the hypothalamus of RIIβ KO mice. During fasting, RIIβ–PKA is activated and this correlates with an increase in CREB phosphorylation. The increase in CREB phosphorylation is absent in the fasted RIIβ KO hypothalamus. Selective inhibition of PKA activity in AgRP neurons partially recapitulates the leanness and resistance to diet-induced obesity of RIIβ KO mice. Our findings suggest that RIIβ–PKA modulates the duration of leptin receptor signalling and therefore the magnitude of the catabolic response to leptin.

Mice lacking RIIβ, a regulatory subunit of protein kinase A, are lean and resistant to diet-induced obesity. Here, the authors show that RIIβ regulates leptin sensitivity, acting as a physiological brake on leptin responsiveness and the duration of leptin signalling in the hypothalamus.

Melanocortin 4 receptor activates ERK-cFos pathway to increase brain-derived neurotrophic factor expression in rat astrocytes and hypothalamus

Melanocortins are neuropeptides with well recognized anti-inflammatory and anti-apoptotic effects in the brain. Of the five melanocortin receptors (MCR), MC4R is abundantly expressed in the brain and is the only MCR present in astrocytes. We have previously shown that MC4R activation by the α-melanocyte stimulating hormone (α-MSH) analog, NDP-MSH, increased brain-derived neurotrophic factor (BDNF) expression through the classic cAMP-Protein kinase A-cAMP responsive element binding protein pathway in rat astrocytes. Now, we examined the participation of the mitogen activated protein kinases pathway in MC4R signaling. Rat cultured astrocytes treated with NDP-MSH 1 µM for 1 h showed increased BDNF expression. Inhibition of extracellular signal-regulated kinase (ERK) and ribosomal p90 S6 kinase (RSK), an ERK substrate, but not of p38 or JNK, prevented the increase in BDNF expression induced by NDP-MSH. Activation of MC4R increased cFos expression, a target of both ERK and RSK. ERK activation by MC4R involves cAMP, phosphoinositide-3 kinase (PI3K) and the non receptor tyrosine kinase, Src. Both PI3K and Src inhibition abolished NDP-MSH-induced BDNF expression. Moreover, we found that intraperitoneal injection of α-MSH induces BDNF and MC4R expression and activates ERK and cFos in male rat hypothalamus. Our results show for the first time that MC4R-induced BDNF expression in astrocytes involves ERK-RSK-cFos pathway which is dependent on PI3K and Src, and that melanocortins induce BDNF expression and ERK-cFos activation in rat hypothalamus.

Single Turnover Autophosphorylation Cycle of the PKA RIIβ Holoenzyme

To provide tight spatiotemporal signaling control, the cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) holoenzyme typically nucleates a macromolecular complex or a "PKA signalosome." Using the RIIβ holoenzyme as a prototype, we show how autophosphorylation/dephosphorylation of the RIIβ subunit, as well as cAMP and metal ions, contribute to the dynamics of PKA signaling. While we showed previously that the RIIβ holoenzyme could undergo a single turnover autophosphorylation with adenosine triphosphate and magnesium (MgATP) and trap both products in the crystal lattice, we asked here whether calcium could trap an ATP:RIIβ holoenzyme since the RIIβ holoenzyme is located close to ion channels. The 2.8Å structure of an RIIβp2:C2:(Ca2ADP)2 holoenzyme, supported by biochemical and biophysical data, reveals a trapped single phosphorylation event similar to MgATP. Thus, calcium can mediate a single turnover event with either ATP or adenosine-5'-(β,γ-imido)triphosphate (AMP-PNP), even though it cannot support steady-state catalysis efficiently. The holoenzyme serves as a "product trap" because of the slow off-rate of the pRIIβ subunit, which is controlled by cAMP, not by phosphorylation of the inhibitor site. By quantitatively defining the RIIβ signaling cycle, we show that release of pRIIβ in the presence of cAMP is reduced by calcium, whereas autophosphorylation at the phosphorylation site (P-site) inhibits holoenzyme reassociation with the catalytic subunit. Adding a single phosphoryl group to the preformed RIIβ holoenzyme thus creates a signaling cycle in which phosphatases become an essential partner. This previously unappreciated molecular mechanism is an integral part of PKA signaling for type II holoenzymes.

Differentially regulated protein kinase A (PKA) activity in adipose tissue and liver is associated with resistance to diet-induced obesity and glucose intolerance in mice that lack PKA regulatory subunit type IIα

The cAMP-dependent protein kinase A (PKA) signaling system is widely expressed and has a central role in regulating cellular metabolism in all organ systems affected by obesity. PKA has four regulatory (RIα, RIIα, RIβ, RIIβ) and four catalytic (Cα, Cβ, Cγ, Prkx) subunit isoforms that have tissue-specific expression profiles. In mice, knockout (KO) of RIIβ, the primary PKA regulatory subunit in adipose tissue or knockout of the catalytic subunit Cβ resulted in a lean phenotype that resists diet-induced obesity and associated metabolic complications. Here we report that the disruption of the ubiquitously expressed PKA RIIα subunit in mice (RIIαKO) confers resistance to diet-induced obesity, glucose intolerance, and hepatic steatosis. After 2-week high-fat diet exposure, RIIαKO mice weighed less than wild-type littermates. Over time this effect was more pronounced in female mice that were also leaner than their wild-type counterparts, regardless of the diet. Decreased intake of a high-fat diet contributed to the attenuated weight gain in RIIαKO mice. Additionally, RIIα deficiency caused differential regulation of PKA in key metabolic organs: cAMP-stimulated PKA activity was decreased in liver and increased in gonadal adipose tissue. We conclude that RIIα represents a potential target for therapeutic interventions in obesity, glucose intolerance, and nonalcoholic fatty liver disease.

Local cAMP signaling in disease at a glance

The second messenger cyclic AMP (cAMP) operates in discrete subcellular regions within which proteins that synthesize, break down or respond to the second messenger are precisely organized. A burgeoning knowledge of compartmentalized cAMP signaling is revealing how the local control of signaling enzyme activity impacts upon disease. The aim of this Cell Science at a Glance article and the accompanying poster is to highlight how misregulation of local cyclic AMP signaling can have pathophysiological consequences. We first introduce the core molecular machinery for cAMP signaling, which includes the cAMP-dependent protein kinase (PKA), and then consider the role of A-kinase anchoring proteins (AKAPs) in coordinating different cAMP-responsive proteins. The latter sections illustrate the emerging role of local cAMP signaling in four disease areas: cataracts, cancer, diabetes and cardiovascular diseases.

Protein kinase cAMP-dependent regulatory type II beta (PRKAR2B) gene variants in antipsychotic-induced weight gain

OBJECTIVE

Antipsychotics are effective in treating schizophrenia symptoms. However, the use of clozapine and olanzapine in particular are associated with significant weight gain. Mouse and human studies suggest that the protein kinase cAMP-dependent regulatory type II beta (PRKAR2B) gene may be involved in energy metabolism, and there is evidence that it is associated with clozapine's effects on triglyceride levels. We aimed at assessing PRKAR2B's role in antipsychotic-induced weight gain in schizophrenia patients.

METHODS

DNA samples from adult schizophrenia or schizoaffective disorder patients of mixed ancestry were genotyped, and weight gain was assessed. We analyzed 16 tag single-nucleotide polymorphisms across the PRKAR2B gene in a Caucasian subset treated either with clozapine or olanzapine (N = 99). Linear regression based on an additive model was performed with the inclusion of relevant covariates.

RESULTS

Normalized per cent weight change was analyzed, revealing that patients with the minor allele at rs9656135 had a mean weight increase of 4.1%, whereas patients without this allele had an increase of 3.4%. This association is not significant after correcting for multiple testing.

CONCLUSIONS

Because of limited power, PRKAR2B's role in antipsychotic-induced weight gain is unclear, but biological evidence suggests that PRKAR2B may be involved. Further research in larger sample sizes is warranted.

cAMP/PKA signaling defects in tumors: genetics and tissue-specific pluripotential cell-derived lesions in human and mouse

In the last few years, bench and clinical studies led to significant new insight into how cyclic adenosine monophosphate (cAMP) signaling, the molecular pathway that had been identified in the early 2000s as the one involved in most benign cortisol-producing adrenal hyperplasias, affects adrenocortical growth and development, as well as tumor formation. A major discovery was the identification of tissue-specific pluripotential cells (TSPCs) as the culprit behind tumor formation not only in the adrenal, but also in bone. Discoveries in animal studies complemented a number of clinical observations in patients. Gene identification continued in parallel with mouse and other studies on the cAMP signaling and other pathways.

Deficiency of the RIIβ subunit of PKA affects locomotor activity and energy homeostasis in distinct neuronal populations

Targeted disruption of RIIβ-protein kinase A (PKA) in mice leads to a lean phenotype, increased nocturnal locomotor activity, and activation of brown adipose tissue. Because RIIβ is abundantly expressed in both white and brown adipose tissue as well as the brain, the contribution of neuronal vs. peripheral PKA to these phenotypes was investigated. We used a Cre-Lox strategy to reexpress RIIβ in a tissue-specific manner in either adipocytes or neurons. Mice with adipocyte-specific RIIβ reexpression remained hyperactive and lean, but pan-neuronal RIIβ reexpression reversed both phenotypes. Selective RIIβ reexpression in all striatal medium spiny neurons with Darpp32-Cre corrected the hyperlocomotor phenotype, but the mice remained lean. Further analysis revealed that RIIβ reexpression in D2 dopamine receptor-expressing medium spiny neurons corrected the hyperlocomotor phenotype, which demonstrated that the lean phenotype in RIIβ-PKA-deficient mice does not develop because of increased locomotor activity. To identify the neurons responsible for the lean phenotype, we used specific Cre-driver mice to reexpress RIIβ in agouti-related peptide (AgRP)-, proopiomelanocortin (POMC)-, single-minded 1 (Sim1)-, or steroidogenic factor 1 (SF1)-expressing neurons in the hypothalamus, but observed no rescue of the lean phenotype. However, when RIIβ was reexpressed in multiple regions of the hypothalamus and striatum driven by Rip2-Cre, or specifically in GABAergic neurons driven by Vgat-ires-Cre, both the hyperactive and lean phenotypes were completely corrected. Bilateral injection of adeno-associated virus1 (AAV1)-Cre directly into the hypothalamus caused reexpression of RIIβ and partially reversed the lean phenotype. These data demonstrate that RIIβ-PKA deficiency in a subset of hypothalamic GABAergic neurons leads to the lean phenotype.

Pleiotropic functions of the transmembrane domain 6 of human melanocortin-4 receptor

The melanocortin-4 receptor (MC4R) is a critical regulator of energy homeostasis and has emerged as a premier target for obesity treatment. Numerous mutations in transmembrane domain 6 (TM6) of MC4R resulting in functional alterations have been identified in obese patients. Several mutagenesis studies also provided some data suggesting the importance of this domain in receptor function. To gain a better understanding of the structure-function relationship of the receptor, we performed alanine-scanning mutagenesis in TM6 to determine the functions of side chains. Of the 31 residues, two were important for cell surface expression, five were indispensable for α-melanocyte-stimulating hormone (α-MSH) and β-MSH binding, and six were important for signaling in the Gs-cAMP-PKA pathway. H264A, targeted normally to the plasma membrane, was undetectable by competitive binding assay and severely defective in basal and stimulated cAMP production and ERK1/2 phosphorylation. Nine mutants had decreased basal cAMP signaling. Seven mutants were constitutively active in cAMP signaling and their basal activities could be inhibited by two MC4R inverse agonists, Ipsen 5i and ML00253764. Five mutants were also constitutively active in the MAPK pathway with enhanced basal ERK1/2 phosphorylation. In summary, our study provided comprehensive data on the structure-function relationship of the TM6 of MC4R. We identified residues that are important for cell surface expression, ligand binding, cAMP generation, and residues for maintaining the WT receptor in active conformation. We also reported constitutive activation of the MAPK pathway and biased signaling. These data will be useful for rationally designing MC4R agonists and antagonists for treatment of eating disorders.

Assembly of allosteric macromolecular switches: lessons from PKA

Protein kinases are dynamic molecular switches that have evolved to be only transiently activated. Kinase activity is embedded within a conserved kinase core, which is typically regulated by associated domains, linkers and interacting proteins. Moreover, protein kinases are often tethered to large macromolecular complexes to provide tighter spatiotemporal control. Thus, structural characterization of kinase domains alone is insufficient to explain protein kinase function and regulation in vivo. Recent progress in structural characterization of cyclic AMP-dependent protein kinase (PKA) exemplifies how our knowledge of kinase signalling has evolved by shifting the focus of structural studies from single kinase subunits to macromolecular complexes.

Neuropeptide Y signaling modulates the expression of ethanol-induced behavioral sensitization in mice

Neuropeptide Y (NPY) and protein kinase A (PKA) have been implicated in neurobiological responses to ethanol. We have previously reported that mutant mice lacking normal production of the RIIβ subunit of PKA (RIIβ-/- mice) show enhanced sensitivity to the locomotor stimulant effects of ethanol and increased behavioral sensitization relative to littermate wild-type RIIβ+/+ mice. We now report that RIIβ-/- mice also show increased NPY immunoreactivity in the nucleus accumbens (NAc) core and the ventral striatum relative to RIIβ+/+ mice. These observations suggest that elevated NPY signaling in the NAc and/or striatum may contribute to the increased sensitivity to ethanol-induced behavioral sensitization that is a characteristic of RIIβ-/- mice. Consistently, NPY-/- mice failed to display ethanol-induced behavioral sensitization that was evident in littermate NPY+/+ mice. To examine more directly the role of NPY in the locomotor stimulant effects of ethanol, we infused a recombinant adeno-associated virus (rAAV) into the region of the NAc core of DBA/2J mice. The rAAV-fibronectin (FIB)-NPY(13-36) vector expresses and constitutively secretes the NPY fragment NPY(13-36) (a selective Y(2) receptor agonist) from infected cells in vivo. Mice treated with the rAAV-FIB-NPY(13-36) vector exhibited reduced expression of ethanol-induced behavioral sensitization compared with mice treated with a control vector. Taken together, the current data provide the first evidence that NPY signaling in the NAc core and the Y(2) receptor modulate ethanol-induced behavioral sensitization.

Melanocortin-induced PKA activation inhibits AMPK activity via ERK-1/2 and LKB-1 in hypothalamic GT1-7 cells

α-Melanocyte-stimulating hormone (α-MSH)-induced activation of the melanocortin-4 receptor in hypothalamic neurons increases energy expenditure and inhibits food intake. Active hypothalamic AMP-activated protein kinase (AMPK) has recently been reported to enhance food intake, and in vivo experiments suggested that intrahypothalamic injection of melanocortins decreased food intake due to the inhibition of AMPK activity. However, it is not clear whether α-MSH affects AMPK via direct intracellular signaling cascades or if the release of paracrine factors is involved. Here, we used a murine, hypothalamic cell line (GT1-7 cells) and monitored AMPK phosphorylation at Thr(172), which has been suggested to increase AMPK activity. We found that α-MSH dephosphorylated AMPK at Thr(172) and consequently decreased phosphorylation of the established AMPK substrate acetyl-coenzyme A-carboxylase at Ser(79). Inhibitory effects of α-MSH on AMPK were blocked by specific inhibitors of protein kinase A (PKA) or ERK-1/2, pointing to an important role of both kinases in this process. Because α-MSH-induced activation of ERK-1/2 was blunted by PKA inhibitors, we propose that ERK-1/2 serves as a link between PKA and AMPK in GT1-7 cells. Furthermore, down-regulation of liver kinase B-1, but not inhibition of calcium-calmodulin-dependent kinase kinase-β or TGFβ-activated kinase-1 decreased basal phosphorylation of AMPK and its dephosphorylation induced by α-MSH. Thus, we propose that α-MSH inhibits AMPK activity via a linear pathway, including PKA, ERK-1/2, and liver kinase B-1 in GT1-7 cells. Given the importance of the melanocortin system in the formation of adipositas, detailed knowledge about this pathway might help to develop drugs targeting obesity.

Structure and allostery of the PKA RIIβ tetrameric holoenzyme

In its physiological state, cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) is a tetramer that contains a regulatory (R) subunit dimer and two catalytic (C) subunits. We describe here the 2.3 angstrom structure of full-length tetrameric RIIβ(2):C(2) holoenzyme. This structure showing a dimer of dimers provides a mechanistic understanding of allosteric activation by cAMP. The heterodimers are anchored together by an interface created by the β4-β5 loop in the RIIβ subunit, which docks onto the carboxyl-terminal tail of the adjacent C subunit, thereby forcing the C subunit into a fully closed conformation in the absence of nucleotide. Diffusion of magnesium adenosine triphosphate (ATP) into these crystals trapped not ATP, but the reaction products, adenosine diphosphate and the phosphorylated RIIβ subunit. This complex has implications for the dissociation-reassociation cycling of PKA. The quaternary structure of the RIIβ tetramer differs appreciably from our model of the RIα tetramer, confirming the small-angle x-ray scattering prediction that the structures of each PKA tetramer are different.

Melanocortin 4 receptor activation induces brain-derived neurotrophic factor expression in rat astrocytes through cyclic AMP-protein kinase A pathway

Melanocortin 4 receptors (MC4R) are mainly expressed in the brain. We previously showed that the anti-inflammatory action of α-melanocyte-stimulating hormone (α-MSH) in rat hypothalamus and in cultured astrocytes involved MC4R activation. However, MC4R mechanisms of action remain undetermined. Since brain-derived neurotrophic factor (BDNF) may be mediating MC4R hypothalamic anorexigenic actions, we determined melanocortin effects on BDNF expression in rat cultured astrocytes and certain mechanisms involved in MC4R signaling. α-MSH and its analogue NDP-MSH, induced production of cAMP in astrocytes. This effect was completely blocked by the MC4R antagonist, HS024. We found that NDP-MSH increased BDNF mRNA and protein levels in astrocytes. The effect of NDP-MSH on BDNF expression was abolished by the adenylate cyclase inhibitor SQ22536, and decreased by the PKA inhibitor Rp-cAMP. Since melanocortins are immunomodulators, we investigated their actions with bacterial lipopolysaccharide (LPS) and interferon-γ (IFN-γ) stimulus. Although both α-MSH and LPS+IFN-γ increased cAMP responding element binding protein (CREB) activation, LPS+IFN-γ did not modify BDNF expression. On the other hand, α-MSH did not modify basal or LPS+IFN-γ-induced nuclear factor-κB activation. Our results show for the first time that MC4R activation in astrocytes induces BDNF expression through cAMP-PKA-CREB pathway without involving NF-κB.

Adiponectin inhibits lipolysis in mouse adipocytes

OBJECTIVE

Adiponectin is an adipocyte-derived hormone that sensitizes insulin and improves energy metabolism in tissues. This study was designed to investigate the direct regulatory effects of adiponectin on lipid metabolism in adipocytes.

RESEARCH DESIGN AND METHODS

Basal and hormone-stimulated lipolysis were comparatively analyzed using white adipose tissues or primary adipocytes from adiponectin gene knockout and control mice. To further study the underlying mechanisms through which adiponectin suppresses lipolysis, cultured 3T3-L1 adipocytes and adenovirus-mediated gene transduction were used.

RESULTS

Significantly increased lipolysis was observed in both adiponectin gene knockout mice and primary adipocytes from these mice. Hormone-stimulated glycerol release was inhibited in adiponectin-treated adipocytes. Adiponectin suppressed hormone-sensitive lipase activation without altering adipose triglyceride lipase and CGI-58 expression in adipocytes. Moreover, adiponectin reduced protein levels of the type 2 regulatory subunit RIIα of protein kinase A by reducing its protein stability. Ectopic expression of RIIα abolished the inhibitory effects of adiponectin on lipolysis in adipocytes.

CONCLUSIONS

This study demonstrates that adiponectin inhibits lipolysis in adipocytes and reveals a novel function of adiponectin in lipid metabolism in adipocytes.

Genomewide pharmacogenomic study of metabolic side effects to antipsychotic drugs

Understanding individual differences in the susceptibility to metabolic side effects as a response to antipsychotic therapy is essential to optimize the treatment of schizophrenia. Here, we perform genomewide association studies (GWAS) to search for genetic variation affecting the susceptibility to metabolic side effects. The analysis sample consisted of 738 schizophrenia patients, successfully genotyped for 492K single nucleotide polymorphisms (SNPs), from the genomic subsample of the Clinical Antipsychotic Trial of Intervention Effectiveness study. Outcomes included 12 indicators of metabolic side effects, quantifying antipsychotic-induced change in weight, blood lipids, glucose and hemoglobin A1c, blood pressure and heart rate. Our criterion for genomewide significance was a pre-specified threshold that ensures, on average, only 10% of the significant findings are false discoveries. A total of 21 SNPs satisfied this criterion. The top finding indicated that a SNP in Meis homeobox 2 (MEIS2) mediated the effects of risperidone on hip circumference (q=0.004). The same SNP was also found to mediate risperidone's effect on waist circumference (q=0.055). Genomewide significant finding were also found for SNPs in PRKAR2B, GPR98, FHOD3, RNF144A, ASTN2, SOX5 and ATF7IP2, as well as in several intergenic markers. PRKAR2B and MEIS2 both have previous research indicating metabolic involvement, and PRKAR2B has previously been shown to mediate antipsychotic response. Although our findings require replication and functional validation, this study shows the potential of GWAS to discover genes and pathways that potentially mediate adverse effects of antipsychotic medication.

Alternative G protein coupling and biased agonism: new insights into melanocortin-4 receptor signalling

The melanocortin-4 receptor (MC4R) is a prototypical G protein-coupled receptor (GPCR) that plays a considerable role in controlling appetite and energy homeostasis. Signalling initiated by MC4R is orchestrated by multiple agonists, inverse agonism and by interactions with accessory proteins. The exact molecular events translating MC4R signalling into its physiological role, however, are not fully understood. This review is an attempt to summarize new aspects of MC4R signalling in the context of its recently discovered alternative G protein coupling, and to give a perspective on how future research could improve our knowledge about the intertwining molecular mechanisms that are responsible for the regulation of energy homeostasis by the melanocortin system.

Lack of cAMP-response element-binding protein 1 in the hypothalamus causes obesity

The melanocortin system in the hypothalamus controls food intake and energy expenditure. Its disruption causes severe obesity in mice and humans. cAMP-response element-binding protein 1 (CREB1) has been postulated to play an important role downstream of the melanocortin-4 receptor (MC4R), but this hypothesis has never been confirmed in vivo. To test this, we generated mice that lack CREB1 in SIM1-expressing neurons, of the paraventricular nucleus (PVN), which are known to be MC4R-positive. Interestingly, CREB1(ΔSIM1) mice developed obesity as a result of decreased energy expenditure and impairment in maintaining their core body temperature and not because of hyperphagia, defining a new role for CREB1 in the PVN. In addition, the lack of CREB1 in the PVN caused a reduction in vasopressin expression but did not affect adrenal or thyroid function. Surprisingly, MC4R function tested pharmacologically was normal in CREB1(ΔSIM1) mice, suggesting that CREB1 is not required for intact MC4R signaling. Thus CREB1 may affect other pathways that are implicated in the regulation of body weight.

Protein kinase A is a target for aging and the aging heart

PKA is an important mediator of signal transduction downstream of G-protein-coupled receptors and plays a key role in the regulation of metabolism and triglyceride storage. It is a ubiquitous cellular kinase that phosphorylates serine and threonine residues in response to cAMP. PKA consists of two regulatory subunits, RI and RII, that are activated by cAMP to release two catalytic subunits, Calpha and Cbeta. We have shown that C57/BL6J male mice lacking the regulatory RIIbeta subunit have extended lifespan and are resistant to age-related conditions including cardiac decline. In addition to being protected from diet-induced pathologies, PKA Cbeta null mutant mice are protected from age-related problems such as weight gain and enlarged livers, as well as cardiac dysfunction and hypertrophy. Several possible mechanisms for the age sparing effects of PKA inhibition are discussed including A kinase anchoring protein signaling, alterations in the beta-adrenergic pathway, and activation of AMPK. Since PKA is a major metabolic regulator of gene signaling, the human gene homologs are potential pharmacological targets for age-related conditions including heart disease associated with declining cardiac performance.

Activation of the melanocortin-4 receptor causes enhanced excitation in presympathetic paraventricular neurons in obese Zucker rats

Sympathetic nerve activity is increased in obesity-related hypertension. However, the central mechanisms involved in the increased sympathetic outflow remain unclear. The hypothalamic melanocortin system is important for regulating energy balance and sympathetic outflow. To understand the mechanisms by which the melanocortin systems regulates sympathetic outflow, we investigated the role of melanocortin 4 receptors (MC4R) in regulating presympathetic paraventricular nucleus (PVN) neurons. We performed whole-cell patch-clamp recordings on retrogradely labeled PVN neurons projecting to the rostral ventrolateral medulla in brain slices from obese zucker rats (OZRs) and lean zucker rats (LZRs). The MC4R agonists melanotan II (MTII) and α-melanocyte-stimulating hormone (α-MSH) increased the firing activity and depolarized the labeled PVN neurons from both LZRs and OZRs in a concentration-dependent manner. MTII produced significant greater increase in the firing activity in OZRs than in LZRs. Blocking MC4R with the specific antagonist SHU9119 had no effect on the basal firing rate but abolished the MTII-induced increase in the firing rate in both OZRs and LZRs. Furthermore, intracellular dialysis of guanosine 5'-O-(2-thodiphosphate), but not bath application of kynurenic acid and bicuculline, eliminated the MTII-induced increase in firing activity. In addition, MTII had no effect on the frequency and amplitude of glutamatergic excitatory postsynaptic currents and GABAergic inhibitory postsynaptic currents in labeled PVN neurons. Collectively, our findings suggest that MC4R contributes to the elevated excitability of PVN presympathetic neurons, which may be involved in obesity-related hypertension.

3.2 Mb microdeletion in chromosome 7 bands q22.2-q22.3 associated with overgrowth and delayed bone age

We report a patient with mental retardation, epilepsy, overgrowth, delayed bone age, peculiar facial features, corpus callosum hypoplasia, enlarged cisterna magna and right cerebellar hypoplasia. Array-CGH analysis revealed the presence of a de novo 3.2 Mb interstitial deletion of the long arm of chromosome 7 involving bands q22.2-q22.3. The rearrangement includes 15 genes and encompasses a genomic region that represents a site of frequent loss of heterozygosity in myeloid malignancies. Four genes are implicated in the control of cell cycle: SRPK2, MLL5, RINT1 and LHFPL3. Haploinsufficiency of these genes might therefore be associated with overgrowth and could confer susceptibility to cancers or other tumours, so that attention to this possibility would be appropriate during regular medical review. In conclusion, array-CGH analysis should be performed in patients with overgrowth where the known causes have already been excluded, because some still unclassified overgrowth syndromes may be caused by subtle genomic imbalances.

Attenuation of age-related metabolic dysfunction in mice with a targeted disruption of the Cbeta subunit of protein kinase A

The cyclic adenosine monophosphate-dependent protein kinase A (PKA) pathway helps regulate both cell growth and division, and triglyceride storage and metabolism in response to nutrient status. Studies in yeast show that disruption of this pathway promotes longevity in a manner similar to caloric restriction. Because PKA is highly conserved, it can be studied in mammalian systems. This report describes the metabolic phenotype of mice lacking the PKA catalytic subunit Cbeta. We confirmed that Cbeta has high levels of expression in the brain but also showed moderate levels in liver. Cbeta-null animals had reduced basal PKA activity while appearing overtly normal when fed standard rodent chow. However, the absence of Cbeta protected mice from diet-induced obesity, steatosis, dyslipoproteinemia, and insulin resistance, without any differences in caloric intake or locomotor activity. These findings have relevant pharmacological implications because aging in mammals is characterized by metabolic decline associated with obesity, altered body fat distribution, and insulin resistance.

Novel isoform-specific interfaces revealed by PKA RIIbeta holoenzyme structures

The cAMP-dependent protein kinase catalytic (C) subunit is inhibited by two classes of functionally nonredundant regulatory (R) subunits, RI and RII. Unlike RI subunits, RII subunits are both substrates and inhibitors. Because RIIbeta knockout mice have important disease phenotypes, the RIIbeta holoenzyme is a target for developing isoform-specific agonists and/or antagonists. We also know little about the linker region that connects the inhibitor site to the N-terminal dimerization domain, although this linker determines the unique globular architecture of the RIIbeta holoenzyme. To understand how RIIbeta functions as both an inhibitor and a substrate and to elucidate the structural role of the linker, we engineered different RIIbeta constructs. In the absence of nucleotide, RIIbeta(108-268), which contains a single cyclic nucleotide binding domain, bound C subunit poorly, whereas with AMP-PNP, a non-hydrolyzable ATP analog, the affinity was 11 nM. The RIIbeta(108-268) holoenzyme structure (1.62 A) with AMP-PNP/Mn(2+) showed that we trapped the RIIbeta subunit in an enzyme:substrate complex with the C subunit in a closed conformation. The enhanced affinity afforded by AMP-PNP/Mn(2+) may be a useful strategy for increasing affinity and trapping other protein substrates with their cognate protein kinase. Because mutagenesis predicted that the region N-terminal to the inhibitor site might dock differently to RI and RII, we also engineered RIIbeta(102-265), which contained six additional linker residues. The additional linker residues in RIIbeta(102-265) increased the affinity to 1.6 nM, suggesting that docking to this surface may also enhance catalytic efficiency. In the corresponding holoenzyme structure, this linker docks as an extended strand onto the surface of the large lobe. This hydrophobic pocket, formed by the alphaF-alphaG loop and conserved in many protein kinases, also provides a docking site for the amphipathic helix of PKI. This novel orientation of the linker peptide provides the first clues as to how this region contributes to the unique organization of the RIIbeta holoenzyme.

Pertussis toxin-sensitive signaling of melanocortin-4 receptors in hypothalamic GT1-7 cells defines agouti-related protein as a biased agonist

Melanocortin-4 receptor (MC4R)-induced anorexigenic signaling in the hypothalamus controls body weight and energy homeostasis. So far, MC4R-induced signaling has been exclusively attributed to its coupling to G(s) proteins. In line with this monogamous G protein coupling profile, most MC4R mutants isolated from obese individuals showed a reduced ability to activate G(s). However, some mutants displayed enhanced G(s) coupling, suggesting that signaling pathways independent of G(s) may be involved in MC4R-mediated anorexigenic signaling. Here we report that the G(s) signaling-deficient MC4R-D90N mutant activates G proteins in a pertussis toxin-sensitive manner, indicating that this mutant is able to selectively interact with G(i/o) proteins. Analyzing a hypothalamic cell line (GT1-7 cells), we observed activation of pertussis toxin-sensitive G proteins by the wild-type MC4R as well, reflecting multiple coupling of the MC4R to G(s) and G(i/o) proteins in an endogenous cell system. Surprisingly, the agouti-related protein, which has been classified as a MC4R antagonist, selectively activates G(i/o) signaling in GT1-7 cells. Thus, the agouti-related protein antagonizes melanocortin-dependent G(s) activation not only by competitive antagonism but additionally by initiating G(i/o) protein-induced signaling as a biased agonist.

Protein kinase A regulatory subunits in human adipose tissue: decreased R2B expression and activity in adipocytes from obese subjects

OBJECTIVE

In human adipocytes, the cAMP-dependent pathway mediates signals originating from beta-adrenergic activation, thus playing a key role in the regulation of important metabolic processes, i.e., lipolysis and thermogenesis. Cyclic AMP effects are mainly mediated by protein kinase A (PKA), whose R2B regulatory isoform is the most expressed in mouse adipose tissue, where it protects against diet-induced obesity and fatty liver development. The aim of the study was to investigate possible differences in R2B expression, PKA activity, and lipolysis in adipose tissues from obese and nonobese subjects.

RESEARCH DESIGN AND METHODS

The expression of the different PKA regulatory subunits was evaluated by immunohistochemistry, Western blot, and real-time PCR in subcutaneous and visceral adipose tissue samples from 20 nonobese and 67 obese patients. PKA activity and glycerol release were evaluated in total protein extract and adipocytes isolated from fresh tissue samples, respectively.

RESULTS

Expression techniques showed that R2B was the most abundant regulatory protein, both at mRNA and protein level. Interestingly, R2B mRNA levels were significantly lower in both subcutaneous and visceral adipose tissues from obese than nonobese patients and negatively correlated with BMI, waist circumference, insulin levels, and homeostasis model assessment of insulin resistance. Moreover, both basal and stimulated PKA activity and glycerol release were significantly lower in visceral adipose tissue from obese patients then nonobese subjects.

CONCLUSIONS

Our results first indicate that, in human adipose tissue, there are important BMI-related differences in R2B expression and PKA activation, which might be included among the multiple determinants involved in the different lipolytic response to beta-adrenergic activation in obesity.

Phosphodiesterase inhibitor-dependent inverse agonism of agouti-related protein on melanocortin 4 receptor in sea bass (Dicentrarchus labrax)

The melanocortin 4 receptor (MC4R) is a G protein-coupled receptor mainly expressed in the central nervous system of vertebrates. Activation of the MC4R leads to a decrease in food intake, whereas inactivating mutations are a genetic cause of obesity. The binding of agouti-related protein (AGRP) reduces not only agonist-stimulated cAMP production (competitive antagonist) but also the basal activity of the receptor, as an inverse agonist. Transgenic zebrafish overexpressing AGRP display increased food intake and linear growth, indicative of a physiological role for the melanocortin system in the control of the energy balance in fish. We report on the cloning, pharmacological characterization, tissue distribution, and detailed brain mapping of a sea bass (Dicentrarchus labrax) MC4R ortholog. Sea bass MC4R is profusely expressed within food intake-controlling pathways of the fish brain. However, the activity of the melanocortin system during progressive fasting does not depend on the hypothalamic/pituitary proopiomelanocortin (POMC) and MC4R expression, which suggests that sea bass MC4R is constitutively activated and regulated by AGRP binding. We demonstrate that AGRP acts as competitive antagonist and reduces MTII-induced cAMP production. AGRP also decreases the basal activity of the receptor as an inverse agonist. This observation suggests that MC4R is constitutively active and supports the evolutionary conservation of the AGRP/MC4R interactions. The inverse agonism, but not the competitive antagonism, depends on the presence of a phosphodiesterase inhibitor (IBMX). This suggests that inverse agonism and competitive antagonism operate through different intracellular signaling pathways, a view that opens up new targets for the treatment of melanocortin-induced metabolic syndrome.

Microarray analysis sheds light on the dedifferentiating role of agouti signal protein in murine melanocytes via the Mc1r

The melanocortin-1 receptor (MC1R) is a key regulator of pigmentation in mammals and is tightly linked to an increased risk of skin cancers, including melanoma, in humans. Physiologically activated by alpha-melanocyte stimulating hormone (alphaMSH), MC1R function can be antagonized by a secreted factor, agouti signal protein (ASP), which is responsible for the lighter phenotypes in mammals (including humans), and is also associated with increased risk of skin cancer. It is therefore of great interest to characterize the molecular effects elicited by those MC1R ligands. In this study, we determined the gene expression profiles of murine melan-a melanocytes treated with ASP or alphaMSH over a 4-day time course using genome-wide oligonucleotide microarrays. As expected, there were significant reductions in expression of numerous melanogenic proteins elicited by ASP, which correlates with its inhibition of pigmentation. ASP also unexpectedly modulated the expression of genes involved in various other cellular pathways, including glutathione synthesis and redox metabolism. Many genes up-regulated by ASP are involved in morphogenesis (especially in nervous system development), cell adhesion, and extracellular matrix-receptor interactions. Concomitantly, ASP enhanced the migratory potential and the invasiveness of melanocytic cells in vitro. These results demonstrate the role of ASP in the dedifferentiation of melanocytes, identify pigment-related genes targeted by ASP and by alphaMSH, and provide insights into the pleiotropic molecular effects of MC1R signaling that may function during development and may affect skin cancer risk.