β-arrestin 2 regulates Aβ generation and γ-secretase activity in Alzheimer's disease

Mice lacking GPR3 receptors display late-onset obese phenotype due to impaired thermogenic function in brown adipose tissue

We report an unexpected link between aging, thermogenesis and weight gain via the orphan G protein-coupled receptor GPR3. Mice lacking GPR3 and maintained on normal chow had similar body weights during their first 5 months of life, but gained considerably more weight thereafter and displayed reduced total energy expenditure and lower core body temperature. By the age of 5 months GPR3 KO mice already had lower thermogenic gene expression and uncoupling protein 1 protein level and showed impaired glucose uptake into interscapular brown adipose tissue (iBAT) relative to WT littermates. These molecular deviations in iBAT of GPR3 KO mice preceded measurable differences in body weight and core body temperature at ambient conditions, but were coupled to a failure to maintain thermal homeostasis during acute cold challenge. At the same time, the same cold challenge caused a 17-fold increase in Gpr3 expression in iBAT of WT mice. Thus, GPR3 appears to have a key role in the thermogenic response of iBAT and may represent a new therapeutic target in age-related obesity.

Angiotensin type 1a receptor deficiency decreases amyloid β-protein generation and ameliorates brain amyloid pathology

Alzheimer's disease is characterized by neuronal loss and cerebral accumulation of amyloid-β protein (Aβ) and lowering the generation of Aβ is a pivotal approach in the strategy of Alzheimer's disease treatment. Midlife hypertension is a major risk factor for the future onset of sporadic Alzheimer's disease and the use of some antihypertensive drugs may decrease the incidence of Alzheimer's disease. However, it is largely unknown how the blood pressure regulation system is associated with the pathogenesis of Alzheimer's disease. Here we found that the deficiency of angiotensin type 1a receptor (AT1a), a key receptor for regulating blood pressure, significantly decreased Aβ generation and amyloid plaque formation in a mouse model of Alzheimer's disease. The lack of AT1a inhibited the endocleavage of presenilin-1 (PS1), which is essential for γ-secretase complex formation and Aβ generation. Notably, the ligand of AT1a, angiotensin II, enhanced Aβ generation, PS1 endocleavage and γ-secretase complex formation. Our results suggest that AT1a activation is closely associated with Aβ generation and brain amyloid accumulation by regulating γ-secretase complex formation. Thus, removal of life style factors or stresses that stimulate AT1a to elevate blood pressure may decrease Aβ generation and brain amyloid accumulation, thereby preventing the pathogenesis of Alzheimer's disease.

The stress response neuropeptide CRF increases amyloid-β production by regulating γ-secretase activity

The biological underpinnings linking stress to Alzheimer's disease (AD) risk are poorly understood. We investigated how corticotrophin releasing factor (CRF), a critical stress response mediator, influences amyloid-β (Aβ) production. In cells, CRF treatment increases Aβ production and triggers CRF receptor 1 (CRFR1) and γ-secretase internalization. Co-immunoprecipitation studies establish that γ-secretase associates with CRFR1; this is mediated by β-arrestin binding motifs. Additionally, CRFR1 and γ-secretase co-localize in lipid raft fractions, with increased γ-secretase accumulation upon CRF treatment. CRF treatment also increases γ-secretase activity in vitro, revealing a second, receptor-independent mechanism of action. CRF is the first endogenous neuropeptide that can be shown to directly modulate γ-secretase activity. Unexpectedly, CRFR1 antagonists also increased Aβ. These data collectively link CRF to increased Aβ through γ-secretase and provide mechanistic insight into how stress may increase AD risk. They also suggest that direct targeting of CRF might be necessary to effectively modulate this pathway for therapeutic benefit in AD, as CRFR1 antagonists increase Aβ and in some cases preferentially increase Aβ42 via complex effects on γ-secretase.

β-Arrestin2 encourages inflammation-induced epithelial apoptosis through ER stress/PUMA in colitis

β-Arrestins (β-arrs) are regulators and mediators of G protein-coupled receptor signaling, and accumulating evidence suggests that they are functionally involved in inflammation and autoimmune diseases. However, the effect of β-arrs is unclear in inflammatory bowel disease (IBD), and the role of β-arr2 is unknown in ulcerative colitis (UC) and Crohn's disease (CD). The aim of this study is to investigate whether β-arr2 encourages inflammation-induced epithelial apoptosis through endoplasmic reticulum (ER) stress/p53-upregulated modulator of apoptosis (PUMA) in colitis. In the present study, the results showed that β-arr2 was increased in specimens from patients with UC or CD. Furthermore, a β-arr2 deficiency significantly repressed intestinal inflammation, ameliorated colitis, and alleviated mucosal apoptosis in mice. In addition, the targeted deletion of β-arr2 depressed ER stress, inhibited PUMA, and downregulated PUMA-mediated mitochondrial apoptotic signaling in colitis. β-Arr2, an important modulator of G protein-coupled receptor function, binds eIF2α to activate ER stress signaling. Furthermore, the knockdown of PUMA dramatically prevented β-arr2-induced apoptosis via alleviating ER stress in vitro. The results suggest that β-arr2 encourages inflammation-induced epithelial apoptosis through ER stress/PUMA in colitis and that β-arr2 is a potential therapeutic target for colitis.

Characterization of Pterocarpus erinaceus kino extract and its gamma-secretase inhibitory properties

ETHNOPHARMACOLOGICAL RELEVANCE

The aqueous decoction of Pterocarpus erinaceus has been traditionally used in Benin against memory troubles.

AIM OF THE STUDY

New strategies are needed against Alzheimer׳s disease (AD), for, to date, AD treatment is symptomatic and consists in drugs treating the cognitive decline. An interesting target is the β-amyloid peptide (Aβ), whose accumulation and progressive deposition into amyloid plaques are key events in AD aetiology. Identifying new and more selective γ-secretase inhibitors or modulators (none of the existing has proven so far to be selective or fully efficient) appears in this respect of particular interest. We studied the activity and mechanisms of action of Pterocarpus erinaceus kino aqueous extract, after the removal of catechic tannins (KAST).

METHODS AND RESULTS

We tested KAST at non-toxic concentrations on cells expressing the human Amyloid Precursor Protein (APP695), as well as on primary neurons. Pterocarpus erinaceus extract was found to inhibit Aβ release in both models. We further showed that KAST inhibited γ-secretase activity in cell-free and in vitro assays, strongly suggesting that KAST is a natural γ-secretase inhibitor. Importantly, this extract did not inhibit the cleavage of Notch, another γ-secretase substrate responsible for major detrimental side effects observed with γ-secretase inhibitors. Epicatechin was further identified in KAST by HPLC-MS.

CONCLUSION

Pterocarpus erinaceus kino extract appears therefore as a new γ-secretase inhibitor selective towards APP processing.

Complex regulation of γ-secretase: from obligatory to modulatory subunits

γ-Secretase is a four subunit, 19-pass transmembrane enzyme that cleaves amyloid precursor protein (APP), catalyzing the formation of amyloid beta (Aβ) peptides that form amyloid plaques, which contribute to Alzheimer’s disease (AD) pathogenesis. γ-Secretase also cleaves Notch, among many other type I transmembrane substrates. Despite its seemingly promiscuous enzymatic capacity, γ-secretase activity is tightly regulated. This regulation is a function of many cellular entities, including but not limited to the essential γ-secretase subunits, nonessential (modulatory) subunits, and γ-secretase substrates. Regulation is also accomplished by an array of cellular events, such as presenilin (active subunit of γ-secretase) endoproteolysis and hypoxia. In this review we discuss how γ-secretase is regulated with the hope that an advanced understanding of these mechanisms will aid in the development of effective therapeutics for γ-secretase-associated diseases like AD and Notch-addicted cancer.

Locus Coeruleus, norepinephrine and Aβ peptides in Alzheimer's disease

Monoaminergic brainstem systems have widespread projections that participate in many central processes and, when dysregulated, contribute to a plethora of neuropsychiatric and neurodegenerative disorders. Synapses are the foundation of these neuronal circuits, and their local dysfunction results in global aberrations leading to pathophysiological disease states. This review focuses on the locus coeruleus (LC) norepinephrine (NE) brainstem system and its underappreciated role in Alzheimer's disease (AD). Amyloid beta (Aβ), a peptide that accumulates aberrantly in AD has recently been implicated as a modulator of neuronal excitability at the synapse. Evidence is presented showing that disruption of the LC-NE system at a synaptic and circuit level during early stages of AD, due to conditions such as chronic stress, can potentially lead to amyloid accumulation and contribute to the progression of this neurodegenerative disorder. Additional factors that impact neurodegeneration include neuroinflammation, and network de-synchronization. Consequently, targeting the LC-NE system may have significant therapeutic potential for AD, as it may facilitate modulation of Aβ production, curtail neuroinflammation, and prevent sleep and behavioral disturbances that often lead to negative patient outcomes.

Hunting for the function of orphan GPCRs - beyond the search for the endogenous ligand

Seven transmembrane-spanning proteins (7TM), also called GPCRs, are among the most versatile and evolutionary successful protein families. Out of the 400 non-odourant members identified in the human genome, approximately 100 remain orphans that have not been matched with an endogenous ligand. Apart from the classical deorphanization strategies, several alternative strategies provided recent new insights into the function of these proteins, which hold promise for high therapeutic potential. These alternative strategies consist of the phenotypical characterization of organisms silenced or overexpressing orphan 7TM proteins, the search for constitutive receptor activity and formation of protein complexes including 7TM proteins as well as the development of synthetic, surrogate ligands. Taken together, a variety of ligand-independent functions can be attributed to orphan 7TM proteins that range from constitutive activity to complex formation with other proteins and include 'true' orphans for which no ligand exist and 'conditional' orphans that behave like orphans in the absence of ligand and as non-orphans in the presence of ligand.

Epigenetic dysregulation of hairy and enhancer of split 4 (HES4) is associated with striatal degeneration in postmortem Huntington brains

To investigate epigenetic contributions to Huntington's disease (HD) pathogenesis, we carried out genome-wide mapping of the transcriptional mark, trimethyl-histone H3-lysine 4 (H3K4me3) in neuronal nuclei extracted from prefrontal cortex of HD cases and controls using chromatin immunoprecipitation followed by deep-sequencing. Neuron-specific mapping of the genome-wide distribution of H3K4me3 revealed 136 differentially enriched loci associated with genes implicated in neuronal development and neurodegeneration, including GPR3, TMEM106B, PDIA6 and the Notch signaling genes hairy and enhancer of split 4 (HES4) and JAGGED2, supporting the view that the neuronal epigenome is affected in HD. Importantly, loss of H3K4me3 at CpG-rich sequences on the HES4 promoter was associated with excessive DNA methylation, reduced binding of nuclear proteins to the methylated region and altered expression of HES4 and HES4 targeted genes MASH1 and P21 involved in striatal development. Moreover, hypermethylation of HES4 promoter sequences was strikingly correlated with measures of striatal degeneration and age-of-onset in a cohort of 25 HD brains (r = 0.56, P = 0.006). Lastly, shRNA knockdown of HES4 in human neuroblastoma cells altered MASH1 and P21 mRNA expression and markedly increased mutated HTT-induced aggregates and cell death. These findings, taken together, suggest that epigenetic dysregulation of HES4 could play a critical role in modifying HD disease pathogenesis and severity.

Effects of corticotrophin-releasing factor receptor 1 antagonists on amyloid-β and behavior in Tg2576 mice

RATIONALE

Previous studies indicate that psychosocial stressors could accelerate amyloid-β (Aβ) levels and accelerate plaque deposition in mouse models of Alzheimer disease (AD). Stressors enhanced the release of corticotrophin-releasing factor (CRF), and exogenous CRF administration mimicked the effects of stress on Aβ levels in mouse models of AD. However, whether CRF receptor 1 (CRF1) antagonists could influence the stress-induced acceleration of an AD-like process in mouse models has not been well studied.

OBJECTIVE

We sought to examine whether CRF1 antagonists inhibit the effects of isolation stress on tissue Aβ levels, Aβ plaque deposition, and behaviors related to anxiety and memory in Tg2576 mice, and to investigate the molecular mechanism underlying such effects.

METHODS

Cohorts of Tg2576 mouse pups were isolated or group-housed at 21 days of age, and then the subgroups of these cohorts received daily intraperitoneal injections of the CRF1 antagonists, antalarmin or R121919 (5, 10, and 20 mg/kg), or vehicle for 1 week. Other cohorts of Tg2576 mouse pups were isolated or group-housed at 21 days of age, and then at 4 months of age, subgroups of these mice were administered antalarmin (20 mg/kg) or vehicle in their drinking water for 6 months. Finally, cultured primary hippocampal neurons from regular Tg2576 pups (P0) were incubated with CRF (0.1, 1, and 10 nM), antalarmin (100 nM) or H-89 (1 μM) for 48 h. Brain tissues or cultured neurons were collected for histological and biochemical analyses, and behavioral measures were collected in the cohorts of mice that were chronically stressed.

RESULTS

Administration of antalarmin at 20 mg/kg dose for 1 week significantly reduced Aβ1-42 levels in isolation stressed mice. Administration of antalarmin for 6 months significantly decreased plasma corticosterone levels, tissue Aβ1-42 levels, and Aβ plaque deposition in the brain and blocked the effects of isolation stress on behaviors related to anxiety and memory. Finally, incubation of neurons with 100 nM antalarmin inhibited the ability of 10 nM CRF to increase Aβ1-42 levels and protein kinase A IIβ expression. The effect of CRF1 on Aβ1-42 levels was also diminished by treatment with H-89, a c-AMP/PKA inhibitor.

CONCLUSIONS

These results suggest that CRF1 antagonists can slow an AD-like process in Tg2576 mice and that the c-AMP/PKA signaling pathway may be involved in this effect.

Role for endocytosis of a constitutively active GPCR (GPR185) in releasing vertebrate oocyte meiotic arrest

Vertebrate oocytes are naturally arrested at prophase of meiosis I for sustained periods of time before resuming meiosis in a process called oocyte maturation that prepares the egg for fertilization. Members of the constitutively active GPR3/6/12 family of G-protein coupled receptors represent important mediators of meiotic arrest. In the frog oocyte the GPR3/12 homolog GPRx (renamed GPR185) has been shown to sustain meiotic arrest by increasing intracellular cAMP levels through GαSβγ. Here we show that GPRx is enriched at the cell membrane (~80%), recycles through an endosomal compartment at steady state, and loses its ability to signal once trapped intracellularly. Progesterone-mediated oocyte maturation is associated with significant internalization of both endogenous and overexpressed GPRx. Furthermore, a GPRx mutant that does not internalize in response to progesterone is significantly more efficient than wild-type GPRx at blocking oocyte maturation. Collectively our results argue that internalization of the constitutively active GPRx is important to release oocyte meiotic arrest.

Sorting nexin 27 regulates Aβ production through modulating γ-secretase activity

Patients with Down syndrome (DS) invariably develop Alzheimer's disease (AD) pathology in their 40s. We have recently found that overexpression of a chromosome 21-encoded microRNA-155 results in decreased levels of the membrane trafficking component, SNX27, diminishing glutamate receptor recycling and thereby impairing synaptic functions in DS. Here, we report a function of SNX27 in regulating β-amyloid (Aβ) generation by modulating γ-secretase activity. Downregulation of SNX27 using RNAi increased Aβ production, whereas overexpression of full-length SNX27, but not SNX27ΔPDZ, reversed the RNAi-mediated Aβ elevation. Moreover, genetic deletion of Snx27 promoted Aβ production and neuronal loss, whereas overexpression of SNX27 using an adeno-associated viral (AAV) vector reduced hippocampal Aβ levels in a transgenic AD mouse model. SNX27 associates with the γ-secretase complex subunit presenilin 1; this interaction dissociates the γ-secretase complex, thus decreasing its proteolytic activity. Our study establishes a molecular mechanism for Aβ-dependent pathogenesis in both DS and AD.

Construction and analysis of the protein-protein interaction networks based on gene expression profiles of Parkinson's disease

Background

Parkinson's Disease (PD) is one of the most prevailing neurodegenerative diseases. Improving diagnoses and treatments of this disease is essential, as currently there exists no cure for this disease. Microarray and proteomics data have revealed abnormal expression of several genes and proteins responsible for PD. Nevertheless, few studies have been reported involving PD-specific protein-protein interactions.

Results

Microarray based gene expression data and protein-protein interaction (PPI) databases were combined to construct the PPI networks of differentially expressed (DE) genes in post mortem brain tissue samples of patients with Parkinson's disease. Samples were collected from the substantia nigra and the frontal cerebral cortex. From the microarray data, two sets of DE genes were selected by 2-tailed t-tests and Significance Analysis of Microarrays (SAM), run separately to construct two Query-Query PPI (QQPPI) networks. Several topological properties of these networks were studied. Nodes with High Connectivity (hubs) and High Betweenness Low Connectivity (bottlenecks) were identified to be the most significant nodes of the networks. Three and four-cliques were identified in the QQPPI networks. These cliques contain most of the topologically significant nodes of the networks which form core functional modules consisting of tightly knitted sub-networks. Hitherto unreported 37 PD disease markers were identified based on their topological significance in the networks. Of these 37 markers, eight were significantly involved in the core functional modules and showed significant change in co-expression levels. Four (ARRB2, STX1A, TFRC and MARCKS) out of the 37 markers were found to be associated with several neurotransmitters including dopamine.

Conclusion

This study represents a novel investigation of the PPI networks for PD, a complex disease. 37 proteins identified in our study can be considered as PD network biomarkers. These network biomarkers may provide as potential therapeutic targets for PD applications development.

Prediction of miRNA-mRNA associations in Alzheimer's disease mice using network topology

BACKGROUND

Little is known about the relationship between miRNA and mRNA expression in Alzheimer's disease (AD) at early- or late-symptomatic stages. Sequence-based target prediction algorithms and anti-correlation profiles have been applied to predict miRNA targets using omics data, but this approach often leads to false positive predictions. Here, we applied the joint profiling analysis of mRNA and miRNA expression levels to Tg6799 AD model mice at 4 and 8 months of age using a network topology-based method. We constructed gene regulatory networks and used the PageRank algorithm to predict significant interactions between miRNA and mRNA.

RESULTS

In total, 8 cluster modules were predicted by the transcriptome data for co-expression networks of AD pathology. In total, 54 miRNAs were identified as being differentially expressed in AD. Among these, 50 significant miRNA-mRNA interactions were predicted by integrating sequence target prediction, expression analysis, and the PageRank algorithm. We identified a set of miRNA-mRNA interactions that were changed in the hippocampus of Tg6799 AD model mice. We determined the expression levels of several candidate genes and miRNA. For functional validation in primary cultured neurons from Tg6799 mice (MT) and littermate (LM) controls, the overexpression of ARRDC3 enhanced PPP1R3C expression. ARRDC3 overexpression showed the tendency to decrease the expression of miR139-5p and miR3470a in both LM and MT primary cells. Pathological environment created by Aβ treatment increased the gene expression of PPP1R3C and Sfpq but did not significantly alter the expression of miR139-5p or miR3470a. Aβ treatment increased the promoter activity of ARRDC3 gene in LM primary cells but not in MT primary cells.

CONCLUSIONS

Our results demonstrate AD-specific changes in the miRNA regulatory system as well as the relationship between the expression levels of miRNAs and their targets in the hippocampus of Tg6799 mice. These data help further our understanding of the function and mechanism of various miRNAs and their target genes in the molecular pathology of AD.

Glutathione and mitochondria

Glutathione (GSH) is the main non-protein thiol in cells whose functions are dependent on the redox-active thiol of its cysteine moiety that serves as a cofactor for a number of antioxidant and detoxifying enzymes. While synthesized exclusively in the cytosol from its constituent amino acids, GSH is distributed in different compartments, including mitochondria where its concentration in the matrix equals that of the cytosol. This feature and its negative charge at physiological pH imply the existence of specific carriers to import GSH from the cytosol to the mitochondrial matrix, where it plays a key role in defense against respiration-induced reactive oxygen species and in the detoxification of lipid hydroperoxides and electrophiles. Moreover, as mitochondria play a central strategic role in the activation and mode of cell death, mitochondrial GSH has been shown to critically regulate the level of sensitization to secondary hits that induce mitochondrial membrane permeabilization and release of proteins confined in the intermembrane space that once in the cytosol engage the molecular machinery of cell death. In this review, we summarize recent data on the regulation of mitochondrial GSH and its role in cell death and prevalent human diseases, such as cancer, fatty liver disease, and Alzheimer’s disease.

OCIAD2 activates γ-secretase to enhance amyloid β production by interacting with nicastrin

The gamma (γ)-secretase holoenzyme is composed of four core proteins and cleaves APP to generate amyloid beta (Aβ), a key molecule that causes major neurotoxicity during the early stage of Alzheimer's disease (AD). However, despite its important role in Aβ production, little is known about the regulation of γ-secretase. OCIAD2, a novel modulator of γ-secretase that stimulates Aβ production, and which was isolated from a genome-wide functional screen using cell-based assays and a cDNA library comprising 6,178 genes. Ectopic expression of OCIAD2 enhanced Aβ production, while reduction of OCIAD2 expression suppressed it. OCIAD2 expression facilitated the formation of an active γ-secretase complex and enhanced subcellular localization of the enzyme components to lipid rafts. OCIAD2 interacted with nicastrin to stimulate γ-secretase activity. OCIAD2 also increased the interaction of nicastrin with C99 and stimulated APP processing via γ-secretase activation, but did not affect Notch processing. In addition, a cell-permeable Tat-OCIAD2 peptide that interfered with the interaction of OCIAD2 with nicastrin interrupted the γ-secretase-mediated AICD production. Finally, OCIAD2 expression was significantly elevated in the brain of AD patients and PDAPP mice. This study identifies OCIAD2 as a selective activator of γ-secretase to increase Aβ generation.

ARRB1/β-arrestin-1 mediates neuroprotection through coordination of BECN1-dependent autophagy in cerebral ischemia

Autophagy, a highly conserved process conferring cytoprotection against stress, contributes to the progression of cerebral ischemia. β-arrestins are multifunctional proteins that mediate receptor desensitization and serve as important signaling scaffolds involved in numerous physiopathological processes. Here, we show that both ARRB1 (arrestin, β 1) and ARRB2 (arrestin, β 2) were upregulated by cerebral ischemic stress. Knockout of Arrb1, but not Arrb2, aggravated the mortality, brain infarction, and neurological deficit in a mouse model of cerebral ischemia. Accordingly, Arrb1-deficient neurons exhibited enhanced cell injury upon oxygen-glucose deprivation (OGD), an in vitro model of ischemia. Deletion of Arrb1 did not affect the cerebral ischemia-induced inflammation, oxidative stress, and nicotinamide phosphoribosyltransferase upregulation, but markedly suppressed autophagy and induced neuronal apoptosis/necrosis in vivo and in vitro. Additionally, we found that ARRB1 interacted with BECN1/Beclin 1 and PIK3C3/Vps34, 2 major components of the BECN1 autophagic core complex, under the OGD condition but not normal conditions in neurons. Finally, deletion of Arrb1 impaired the interaction between BECN1 and PIK3C3, which is a critical event for autophagosome formation upon ischemic stress, and markedly reduced the kinase activity of PIK3C3. These findings reveal a neuroprotective role for ARRB1, in the context of cerebral ischemia, centered on the regulation of BECN1-dependent autophagosome formation.

Successful therapies for Alzheimer's disease: why so many in animal models and none in humans?

Peering into the field of Alzheimer’s disease (AD), the outsider realizes that many of the therapeutic strategies tested (in animal models) have been successful. One also may notice that there is a deficit in translational research, i.e., to take a successful drug in mice and translate it to the patient. Efforts are still focused on novel projects to expand the therapeutic arsenal to “cure mice.” Scientific reasons behind so many successful strategies are not obvious. This article aims to review the current approaches to combat AD and to open a debate on common mechanisms of cognitive enhancement and neuroprotection. In short, either the rodent models are not good and should be discontinued, or we should extract the most useful information from those models. An example of a question that may be debated for the advancement in AD therapy is: In addition to reducing amyloid and tau pathologies, would it be necessary to boost synaptic strength and cognition? The debate could provide clues to turn around the current negative output in generating effective drugs for patients. Furthermore, discovery of biomarkers in human body fluids, and a clear distinction between cognitive enhancers and disease modifying strategies, should be instrumental for advancing in anti-AD drug discovery.

Metabotropic glutamate receptor 5 knockout reduces cognitive impairment and pathogenesis in a mouse model of Alzheimer's disease

Background

Alzheimer’s disease (AD) pathology occurs in part as the result of excessive production of β-amyloid (Aβ). Metabotropic glutamate receptor 5 (mGluR5) is now considered a receptor for Aβ and consequently contributes to pathogenic Aβ signaling in AD.

Results

Genetic deletion of mGluR5 rescues the spatial learning deficits observed in APPswe/PS1ΔE9 AD mice. Moreover, both Aβ oligomer formation and Aβ plaque number are reduced in APPswe/PS1ΔE9 mice lacking mGluR5 expression. In addition to the observed increase in Aβ oligomers and plaques in APPswe/PS1ΔE9 mice, we found that both mTOR phosphorylation and fragile X mental retardation protein (FMRP) expression were increased in these mice. Genetic deletion of mGluR5 reduced Aβ oligomers, plaques, mTOR phosphorylation and FMRP expression in APPswe/PS1ΔE9 mice.

Conclusions

Thus, we propose that Aβ activation of mGluR5 appears to initiate a positive feedback loop resulting in increased Aβ formation and AD pathology in APPswe/PS1ΔE9 mice via mechanism that is regulated by FMRP.

Developmental expression of GPR3 in rodent cerebellar granule neurons is associated with cell survival and protects neurons from various apoptotic stimuli

G-protein coupled receptor 3 (GPR3), GPR6, and GPR12 belong to a family of constitutively active Gs-coupled receptors that activate 3'-5'-cyclic adenosine monophosphate (cAMP) and are highly expressed in the brain. Among these receptors, the endogenous expression of GPR3 in cerebellar granule neurons (CGNs) is increased following development. GPR3 is important for neurite outgrowth and neural maturation; however, the physiological functions of GPR3 remain to be fully elucidated. Here, we investigated the survival and antiapoptotic functions of GPR3 under normal and apoptosis-inducing culture conditions. Under normal culture conditions, CGNs from GPR3-knockout mice demonstrated lower survival than did CGNs from wild-type or GPR3-heterozygous mice. Cerebellar sections from GPR3-/- mice at P7, P14, and P21 revealed more caspase-3-positive neurons in the internal granular layer than in cerebellar sections from wild-type mice. Conversely, in a potassium-deprivation model of apoptosis, increased expression of these three receptors promoted neuronal survival. The antiapoptotic effect of GPR3 was also observed under hypoxic (1% O2/5% CO2) and reactive oxygen species (ROS)-induced apoptotic conditions. We further investigated the signaling pathways involved in the GPR3-mediated antiapoptotic effect. The addition of the PKA inhibitor KT5720, the MAP kinase inhibitor U0126, and the PI3 kinase inhibitor LY294002 abrogated the GPR3-mediated antiapoptotic effect in a potassium-deprivation model of apoptosis, whereas the PKC inhibitor Gö6976 did not affect the antiapoptotic function of GPR3. Furthermore, downregulation of endogenous GPR3 expression in CGNs resulted in a marked reduction in the basal levels of ERK and Akt phosphorylation under normal culture conditions. Finally, we used a transient middle cerebral artery occlusion (tMCAO) model in wild-type and GPR3-knockout mice to determine whether GPR3 expression modulates neuronal survival after brain ischemia. After tMCAO, GPR3-knockout mice exhibited a significantly larger infarct area than did wild-type mice. Collectively, these in vitro and in vivo results suggest that the developmental expression of constitutively active Gs-coupled GPR3 activates the ERK and Akt signaling pathways at the basal level, thereby protecting neurons from apoptosis that is induced by various stimuli.

Lipid rafts are required for signal transduction by angiotensin II receptor type 1 in neonatal glomerular mesangial cells

Angiotensin II (ANG-II) receptors (AGTRs) contribute to renal physiology and pathophysiology, but the underlying mechanisms that regulate AGTR function in glomerular mesangium are poorly understood. Here, we show that AGTR1 is the functional AGTR subtype expressed in neonatal pig glomerular mesangial cells (GMCs). Cyclodextrin (CDX)-mediated cholesterol depletion attenuated cell surface AGTR1 protein expression and ANG-II-induced intracellular Ca(2+) ([Ca(2+)]i) elevation in the cells. The COOH-terminus of porcine AGTR1 contains a caveolin (CAV)-binding motif. However, neonatal GMCs express CAV-1, but not CAV-2 and CAV-3. Colocalization and in situ proximity ligation assay detected an association between endogenous AGTR1 and CAV-1 in the cells. A synthetic peptide corresponding to the CAV-1 scaffolding domain (CSD) sequence also reduced ANG-II-induced [Ca(2+)]i elevation in the cells. Real-time imaging of cell growth revealed that ANG-II stimulates neonatal GMC proliferation. ANG-II-induced GMC growth was attenuated by EMD 66684, an AGTR1 antagonist; BAPTA, a [Ca(2+)]i chelator; KN-93, a Ca(2+)/calmodulin-dependent protein kinase II inhibitor; CDX; and a CSD peptide, but not PD 123319, a selective AGTR2 antagonist. Collectively, our data demonstrate [Ca(2+)]i-dependent proliferative effect of ANG-II and highlight a critical role for lipid raft microdomains in AGTR1-mediated signal transduction in neonatal GMCs.

Identification of a novel small-molecule agonist for human G protein-coupled receptor 3

G protein-coupled receptor 3 (GPR3) is an orphan G protein-coupled receptor (GPCR) predominantly expressed in mammalian brain and oocytes. GPR3 plays important roles in these two organs and is known as a Gαs-coupled receptor-activated constitutively in cells. However, the signal transduction pathway and pharmacological function of GPR3 remain unclear because of the lack of a specific ligand. By use of a human embryonic kidney 293 cell line stably expressing FLAG-GPR3-green fluorescent protein, a chemical screening for GPR3 ligands was performed using homogeneous time-resolved fluorescence cAMP assay. Diphenyleneiodonium chloride (DPI) was identified as a novel agonist of GPR3 with weak or no cross-reactivity with other GPCRs. DPI was further characterized to activate several GPR3-mediated signal transduction pathways, including Ca(2+) mobilization, cAMP accumulation, membrane recruitment of β-arrestin2, and receptor desensitization. Parallel studies revealed that the activity of DPI is much more pronounced than sphingosine 1-phosphate, a previously reported GPR3 agonist. Our study identified a novel and specific agonist of GPR3, which provides a useful tool for further study of this orphan GPCR.

Regulation of amyloid precursor protein processing by serotonin signaling

Proteolytic processing of the amyloid precursor protein (APP) by the β- and γ-secretases releases the amyloid-β peptide (Aβ), which deposits in senile plaques and contributes to the etiology of Alzheimer's disease (AD). The α-secretase cleaves APP in the Aβ peptide sequence to generate soluble APPα (sAPPα). Upregulation of α-secretase activity through the 5-hydroxytryptamine 4 (5-HT₄) receptor has been shown to reduce Aβ production, amyloid plaque load and to improve cognitive impairment in transgenic mouse models of AD. Consequently, activation of 5-HT₄ receptors following agonist stimulation is considered to be a therapeutic strategy for AD treatment; however, the signaling cascade involved in 5-HT₄ receptor-stimulated proteolysis of APP remains to be determined. Here we used chemical and siRNA inhibition to identify the proteins which mediate 5-HT_4d receptor-stimulated α-secretase activity in the SH-SY5Y human neuronal cell line. We show that G protein and Src dependent activation of phospholipase C are required for α-secretase activity, while, unexpectedly, adenylyl cyclase and cAMP are not involved. Further elucidation of the signaling pathway indicates that inositol triphosphate phosphorylation and casein kinase 2 activation is also a prerequisite for α-secretase activity. Our findings provide a novel route to explore the treatment of AD through 5-HT₄ receptor-induced α-secretase activation.

Trafficking regulation of proteins in Alzheimer's disease

The β-amyloid (Aβ) peptide has been postulated to be a key determinant in the pathogenesis of Alzheimer’s disease (AD). Aβ is produced through sequential cleavage of the β-amyloid precursor protein (APP) by β- and γ-secretases. APP and relevant secretases are transmembrane proteins and traffic through the secretory pathway in a highly regulated fashion. Perturbation of their intracellular trafficking may affect dynamic interactions among these proteins, thus altering Aβ generation and accelerating disease pathogenesis. Herein, we review recent progress elucidating the regulation of intracellular trafficking of these essential protein components in AD.

Allosteric and biased g protein-coupled receptor signaling regulation: potentials for new therapeutics

G protein-coupled receptors (GPCRs) are seven-transmembrane proteins that participate in many aspects of the endocrine function and are important targets for drug development. They transduce signals mainly, but not exclusively, via hetero-trimeric G proteins, leading to a diversity of intracellular signaling cascades. Ligands binding at the hormone orthosteric sites of receptors have been classified as agonists, antagonists, and/or inverse agonists based on their ability to mainly modulate G protein signaling. Accumulating evidence also indicates that such ligands, alone or in combination with other ones such as those acting outside the orthosteric hormone binding sites (e.g., allosteric modulators), have the ability to selectively engage subsets of signaling responses as compared to the natural endogenous ligands. Such modes of functioning have been variously referred to as "functional selectivity" or "ligand-biased signaling." In this review, we provide an overview of the current knowledge regarding GPCR-biased signaling and their functional regulation with a focus on the evolving concept that receptor domains can also be targeted to allosterically bias signaling, and discuss the usefulness of such modes of regulation for the design of more efficient therapeutics.

Arrestins as regulatory hubs in cancer signalling pathways

Non-visual arrestins were initially appreciated for the roles they play in the negative regulation of G protein-coupled receptors through the processes of desensitisation and endocytosis. The arrestins are also now known as protein scaffolding platforms that act downstream of multiple types of receptors, ensuring relevant transmission of information for an appropriate cellular response. They function as regulatory hubs in several important signalling pathways that are often dysregulated in human cancers. Interestingly, several recent studies have documented changes in expression and localisation of arrestins that occur during cancer progression and that correlate with clinical outcome. Here, we discuss these advances and how changes in expression/localisation may affect functional outputs of arrestins in cancer biology.

Quantifying biased β-arrestin signaling

It is now established that agonists do not uniformly activate pleiotropic signaling mechanisms initiated by receptors but rather can bias signals according to the unique receptor conformations they stabilize. One of the important emerging signaling systems where this can occur is through β-arrestin. This chapter discusses biased signaling where emphasis or de-emphasis of β-arrestin signaling is postulated (or been shown) to be beneficial. The chapter specifically focuses on methods to quantify biased effects; these methods furnish scales that can be used in the process of optimizing biased agonism (and antagonism) for therapeutic benefit. Specifically, methods to derive ΔΔLog(τ/K A) or ΔΔLog(Relative Activity) values are described to do this.

Role of β-arrestins and arrestin domain-containing proteins in G protein-coupled receptor trafficking

The arrestin clan can now be broadly divided into three structurally similar subgroups: the originally identified arrestins (visual and β-arrestins), the α-arrestins and a group of Vps26-related proteins. The visual and β-arrestins selectively bind to agonist-occupied phosphorylated G protein-coupled receptors (GPCRs) and inhibit GPCR coupling to heterotrimeric G proteins while the β-arrestins also function as adaptor proteins to regulate GPCR trafficking and G protein-independent signaling. The α-arrestins have also recently been implicated in regulating GPCR trafficking while Vps26 regulates retrograde trafficking. In this review, we provide an overview of the α-arrestins and β-arrestins with a focus on our current understanding of how these adaptor proteins regulate GPCR trafficking.

A fast growing spectrum of biological functions of γ-secretase in development and disease

γ-secretase, which assembles as a tetrameric complex, is an aspartyl protease that proteolytically cleaves substrate proteins within their membrane-spanning domain; a process also known as regulated intramembrane proteolysis (RIP). RIP regulates signaling pathways by abrogating or releasing signaling molecules. Since the discovery, already >15 years ago, of its catalytic component, presenilin, and even much earlier with the identification of amyloid precursor protein as its first substrate, γ-secretase has been commonly associated with Alzheimer's disease. However, starting with Notch and thereafter a continuously increasing number of novel substrates, γ-secretase is becoming linked to an equally broader range of biological processes. This review presents an updated overview of the current knowledge on the diverse molecular mechanisms and signaling pathways controlled by γ-secretase, with a focus on organ development, homeostasis and dysfunction. This article is part of a Special Issue entitled: Intramembrane Proteases.

Role of synaptic activity in the regulation of amyloid beta levels in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia. Accumulation of amyloid-beta (Aβ) peptides is regarded as the critical component associated with AD pathogenesis, which is derived from the amyloid precursor protein (APP) cleavage. Recent studies suggest that synaptic activity is one of the most important factors that regulate Aβ levels. It has been found that synaptic activity facilitates APP internalization and influences APP cleavage. Glutamatergic, cholinergic, serotonergic, leptin, adrenergic, orexin, and gamma-amino butyric acid receptors, as well as the activity-regulated cytoskeleton-associated protein (Arc) are all involved in these processes. The present review summarizes the evidence for synaptic activity-modulated Aβ levels and the mechanisms underlying this regulation. Interestingly, the immediate early gene product Arc may also be the downstream signaling molecule of several receptors in the synaptic activity-modulated Aβ levels. Elucidating how Aβ levels are regulated by synaptic activity may provide new insights in both the understanding of the pathogenesis of AD and in the development of therapies to slow down the progression of AD.

Gpr3 stimulates Aβ production via interactions with APP and β-arrestin2

The orphan G protein-coupled receptor (GPCR) GPR3 enhances the processing of Amyloid Precursor Protein (APP) to the neurotoxic beta-amyloid (Aβ) peptide via incompletely understood mechanisms. Through overexpression and shRNA knockdown experiments in HEK293 cells, we show that β-arrestin2 (βarr2), a GPCR-interacting scaffold protein reported to bind γ-secretase, is an essential factor for GPR3-stimulated Aβ production. For a panel of GPR3 receptor mutants, the degree of stimulation of Aβ production correlates with receptor-β-arrestin binding and receptor trafficking to endocytic vesicles. However, GPR3’s recruitment of βarr2 cannot be the sole explanation, because interaction with βarr2 is common to most GPCRs, whereas GPR3 is relatively unique among GPCRs in enhancing Aβ production. In addition to β-arrestin, APP is present in a complex with GPR3 and stimulation of Aβ production by GPR3 mutants correlates with their level of APP binding. Importantly, among a broader selection of GPCRs, only GPR3 and prostaglandin E receptor 2 subtype EP2 (PTGER2; another GPCR that increases Aβ production) interact with APP, and PTGER2 does so in an agonist-stimulated manner. These data indicate that a subset of GPCRs, including GPR3 and PTGER2, can associate with APP when internalized via βarr2, and thereby promote the cleavage of APP to generate Aβ.

Interaction of membrane/lipid rafts with the cytoskeleton: impact on signaling and function: membrane/lipid rafts, mediators of cytoskeletal arrangement and cell signaling

The plasma membrane in eukaryotic cells contains microdomains that are enriched in certain glycosphingolipids, gangliosides, and sterols (such as cholesterol) to form membrane/lipid rafts (MLR). These regions exist as caveolae, morphologically observable flask-like invaginations, or as a less easily detectable planar form. MLR are scaffolds for many molecular entities, including signaling receptors and ion channels that communicate extracellular stimuli to the intracellular milieu. Much evidence indicates that this organization and/or the clustering of MLR into more active signaling platforms depends upon interactions with and dynamic rearrangement of the cytoskeleton. Several cytoskeletal components and binding partners, as well as enzymes that regulate the cytoskeleton, localize to MLR and help regulate lateral diffusion of membrane proteins and lipids in response to extracellular events (e.g., receptor activation, shear stress, electrical conductance, and nutrient demand). MLR regulate cellular polarity, adherence to the extracellular matrix, signaling events (including ones that affect growth and migration), and are sites of cellular entry of certain pathogens, toxins and nanoparticles. The dynamic interaction between MLR and the underlying cytoskeleton thus regulates many facets of the function of eukaryotic cells and their adaptation to changing environments. Here, we review general features of MLR and caveolae and their role in several aspects of cellular function, including polarity of endothelial and epithelial cells, cell migration, mechanotransduction, lymphocyte activation, neuronal growth and signaling, and a variety of disease settings. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Regulation of Constitutive GPR3 Signaling and Surface Localization by GRK2 and β-arrestin-2 Overexpression in HEK293 Cells

G protein-coupled receptor 3 (GPR3) is a constitutively active receptor that maintains high 3′-5′-cyclic adenosine monophosphate (cAMP) levels required for meiotic arrest in oocytes and CNS function. Ligand-activated G protein-coupled receptors (GPCRs) signal at the cell surface and are silenced by phosphorylation and β-arrestin recruitment upon endocytosis. Some GPCRs can also signal from endosomes following internalization. Little is known about the localization, signaling, and regulation of constitutively active GPCRs. We demonstrate herein that exogenously-expressed GPR3 localizes to the cell membrane and undergoes internalization in HEK293 cells. Inhibition of endocytosis increased cell surface-localized GPR3 and cAMP levels while overexpression of GPCR-Kinase 2 (GRK2) and β-arrestin-2 decreased cell surface-localized GPR3 and cAMP levels. GRK2 by itself is sufficient to decrease cAMP production but both GRK2 and β-arrestin-2 are required to decrease cell surface GPR3. GRK2 regulates GPR3 independently of its kinase activity since a kinase inactive GRK2-K220R mutant significantly decreased cAMP levels. However, GRK2-K220R and β-arrestin-2 do not diminish cell surface GPR3, suggesting that phosphorylation is required to induce GPR3 internalization. To understand which residues are targeted for desensitization, we mutated potential phosphorylation sites in the third intracellular loop and C-terminus and examined the effect on cAMP and receptor surface localization. Mutation of residues in the third intracellular loop dramatically increased cAMP levels whereas mutation of residues in the C-terminus produced cAMP levels comparable to GPR3 wild type. Interestingly, both mutations significantly reduced cell surface expression of GPR3. These results demonstrate that GPR3 signals at the plasma membrane and can be silenced by GRK2/β-arrestin overexpression. These results also strongly implicate the serine and/or threonine residues in the third intracellular loop in the regulation of GPR3 activity.

β-Arrestins as potential therapeutic targets for Alzheimer's disease

β-arrestins represent a small family of G protein-coupled receptors (GPCRs) regulators, which provide modulating effects by facilitating desensitization and internalization of GPCRs as well as initiating their own signalings. Recent reports have demonstrated that β-arrestins levels were correlated with amyloid-β peptide (Aβ) pathology in brains of Alzheimer's disease (AD) patients and animal models. β-arrestins could enhance the activity of γ-secretase via interacting with anterior pharynx defective 1 subunit, which increased Aβ production and contributed to the pathogenesis of AD. In addition, Aβ-induced internalization of β2-adrenergic receptor internalization and loss of dendritic spine in neurons were proven to be mediated by β-arrestins, further establishing their pathogenic role in AD. More importantly, deletion of β-arrestins markedly attenuated AD pathology, without causing any gross abnormality. Here, we review the evidence about the roles of β-arrestins in the progression of AD. In addition, the established and postulated mechanisms by which β-arrestins mediated in AD pathogenesis are also discussed. Based on the role of β-arrestins in AD pathogenesis, genetically or pharmacologically targeting β-arrestins might provide new opportunities for AD treatment.

FTY720/fingolimod, a sphingosine analogue, reduces amyloid-β production in neurons

Sphingosine-1-phosphate (S1P) is a pluripotent lipophilic mediator working as a ligand for G-protein coupled S1P receptors (S1PR), which is currently highlighted as a therapeutic target for autoimmune diseases including relapsing forms of multiple sclerosis. Sphingosine related compounds, FTY720 and KRP203 known as S1PR modulators, are phosphorylated by sphingosine kinase 2 (SphK2) to yield the active metabolites FTY720-P and KRP203-P, which work as functional antagonists for S1PRs. Here we report that FTY720 and KRP203 decreased production of Amyloid-β peptide (Aβ), a pathogenic proteins causative for Alzheimer disease (AD), in cultured neuronal cells. Pharmacological analyses suggested that the mechanism of FTY720-mediated Aβ decrease in cells was independent of known downstream signaling pathways of S1PRs. Unexpectedly, 6-days treatment of APP transgenic mice with FTY720 resulted in a decrease in Aβ40, but an increase in Aβ42 levels in brains. These results suggest that S1PR modulators are novel type of regulators for Aβ metabolisms that are active in vitro and in vivo.

Development and mechanism of γ-secretase modulators for Alzheimer's disease

γ-Secretase is an aspartyl intramembranal protease composed of presenilin, Nicastrin, Aph1, and Pen2 with 19 transmembrane domains. γ-Secretase cleaves the amyloid precursor proteins (APP) to release Aβ peptides that likely play a causative role in the pathogenesis of Alzheimer's disease (AD). In addition, γ-secretase cleaves Notch and other type I membrane proteins. γ-Secretase inhibitors (GSIs) have been developed and used for clinical studies. However, clinical trials have shown adverse effects of GSIs that are potentially linked with nondiscriminatory inhibition of Notch signaling, overall APP processing, and other substrate cleavages. Therefore, these findings call for the development of disease-modifying agents that target γ-secretase activity to lower levels of Aβ42 production without blocking the overall processing of γ-secretase substrates. γ-Secretase modulators (GSMs) originally derived from nonsteroidal anti-inflammatory drugs (NSAIDs) display such characteristics and are the focus of this review. However, first-generation GSMs have limited potential because of the low potency and undesired neuropharmacokinetic properties. This generation of GSMs has been suggested to interact with the APP substrate, γ-secretase, or both. To improve the potency and brain availability, second-generation GSMs, including NSAID-derived carboxylic acid and non-NSAID-derived heterocyclic chemotypes, as well as natural product-derived GSMs have been developed. Animal studies of this generation of GSMs have shown encouraging preclinical profiles. Moreover, using potent GSM photoaffinity probes, multiple studies unambiguously have showed that both carboxylic acid and heterocyclic GSMs specifically target presenilin, the catalytic subunit of γ-secretase. In addition, two types of GSMs have distinct binding sites within the γ-secretase complex and exhibit different Aβ profiles. GSMs induce a conformational change of γ-secretase to achieve modulation. Various models are proposed and discussed. Despite the progress of GSM research, many outstanding issues remain to be investigated to achieve the ultimate goal of developing GSMs as effective AD therapies.

Targeting ubiquilin-1 in Alzheimer's disease

INTRODUCTION

Alzheimer's disease (AD) is a common neurodegenerative disorder affecting an increasing number of people worldwide as the population ages. Currently, there are no drugs available that could prevent AD pathogenesis or slow down its progression. Increasing evidence links ubiquilin-1, an ubiquitin-like protein, into the pathogenic mechanisms of AD and other neurodegenerative diseases. Ubiquilin-1 has been shown to play a key role in the regulation of the levels, subcellular targeting, aggregation and degradation of various neurodegenerative disease-associated proteins. These include the amyloid precursor protein and presenilins that are intimately involved in the mechanisms of AD.

AREAS COVERED

Here, the properties and diverse functions of ubiquilin-1 protein in the context of the pathogenesis of AD and other neurodegenerative disorders are discussed. This review recapitulates the available knowledge on the involvement of ubiquilin-1 in the genetic and molecular mechanisms in AD. Furthermore, the association of ubiquilin-1 with specific proteins and mechanisms involved in the pathogenesis of neurodegenerative diseases is described and the known ubiquilin-1-interacting proteins summarized.

EXPERT OPINION

The variety of ubiquilin-1-interacting proteins and its central role in the regulation of protein levels and degradation provides a number of novel candidates and approaches for future research and drug discovery.

Structural biology of presenilins and signal peptide peptidases

Presenilin and signal peptide peptidase are multispanning intramembrane-cleaving proteases with a conserved catalytic GxGD motif. Presenilin comprises the catalytic subunit of γ-secretase, a protease responsible for the generation of amyloid-β peptides causative of Alzheimer disease. Signal peptide peptidase proteins are implicated in the regulation of the immune system. Both protease family proteins have been recognized as druggable targets for several human diseases, but their detailed structure still remains unknown. Recently, the x-ray structures of some archaeal GxGD proteases have been determined. We review the recent progress in biochemical and biophysical probing of the structure of these atypical proteases.

Analyzing the role of receptor internalization in the regulation of melanin-concentrating hormone signaling

The regulation of appetite is complex, though our understanding of the process is improving. The potential role for the melanin-concentrating hormone (MCH) signaling pathway in the treatment of obesity is being explored by many. It was hypothesized that internalization of MCH receptors would act to potently desensitize cells to MCH. Despite potent desensitization of ERK signaling by MCH in BHK-570 cells, we were unable to observe MCH-mediated internalization of MCH receptor 1 (MCHR1) by fluorescence microscopy. A more quantitative approach using a cell-based ELISA indicated only 15% of receptors internalized, which is much lower than that reported in the literature. When β-arrestins were overexpressed in our system, removal of receptors from the cell surface was facilitated and signaling to a leptin promoter was diminished, suggesting that internalization of MCHR1 is sensitive to cellular β-arrestin levels. A dominant-negative GRK construct completely inhibited loss of receptors from the cell surface in response to MCH, suggesting that the internalization observed is phosphorylation-dependent. Since desensitization of MCH-mediated ERK signaling did not correlate with significant loss of MCHR1 from the cell surface, we hypothesize that in this model system regulation of MCH signaling may be the result of segregation of receptors from signaling components at the plasma membrane.