G protein-coupled receptor kinases, beta-arrestin-2 and associated regulatory proteins in the human brain: postmortem changes, effect of age and subcellular distribution

The multifaceted functions of β-arrestins and their therapeutic potential in neurodegenerative diseases

Arrestins are multifunctional proteins that regulate G-protein-coupled receptor (GPCR) desensitization, signaling, and internalization. The arrestin family consists of four subtypes: visual arrestin1, β-arrestin1, β-arrestin2, and visual arrestin-4. Recent studies have revealed the multifunctional roles of β-arrestins beyond GPCR signaling, including scaffolding and adapter functions, and physically interacting with non-GPCR receptors. Increasing evidence suggests that β-arrestins are involved in the pathogenesis of a variety of neurodegenerative diseases, including Alzheimer's disease (AD), frontotemporal dementia (FTD), and Parkinson's disease (PD). β-arrestins physically interact with γ-secretase, leading to increased production and accumulation of amyloid-beta in AD. Furthermore, β-arrestin oligomers inhibit the autophagy cargo receptor p62/SQSTM1, resulting in tau accumulation and aggregation in FTD. In PD, β-arrestins are upregulated in postmortem brain tissue and an MPTP model, and the β2AR regulates SNCA gene expression. In this review, we aim to provide an overview of β-arrestin1 and β-arrestin2, and describe their physiological functions and roles in neurodegenerative diseases. The multifaceted roles of β-arrestins and their involvement in neurodegenerative diseases suggest that they may serve as promising therapeutic targets.

G protein-coupled receptor kinases are associated with Alzheimer's disease pathology

AIM

Alzheimer's disease (AD) is characterised by extracellular deposition of amyloid-β (Aβ) in amyloid plaques and intracellular aggregation and accumulation of hyperphosphorylated tau in neurofibrillary tangles (NFTs). Although several kinases have been identified to contribute to the pathological phosphorylation of tau, kinase-targeted therapies for AD have not been successful in clinical trials. Critically, the kinases responsible for numerous identified tau phosphorylation sites remain unknown. G protein-coupled receptor (GPCR) kinases (GRKs) have recently been implicated in phosphorylation of non-GPCR substrates, for example, tubulin and α-synuclein, and in neurological disorders, including schizophrenia and Parkinson's disease. Accordingly, we investigated the involvement of GRKs in the pathophysiology of AD.

METHODS

We performed a comprehensive immunohistochemical and biochemical analysis of the ubiquitously expressed GRKs, namely, GRK2, 3, 5 and 6, in postmortem human brain tissue of control subjects and AD patients.

RESULTS

GRKs display unique cell-type-specific expression patterns in neurons, astrocytes and microglia. Levels of GRKs 2, 5 and 6 are specifically decreased in the CA1 region of the AD hippocampus. Biochemical evidence indicates that the GRKs differentially associate with total, soluble and insoluble pools of tau in the AD brain. Complementary immunohistochemical studies indicate that the GRKs differentially colocalise with total tau, phosphorylated tau and NFTs. Notably, GRKs 3 and 5 also colocalise with amyloid plaques.

CONCLUSION

These studies establish a link between GRKs and the pathological phosphorylation and accumulation of tau and amyloid pathology in AD brains and suggest a novel role for these kinases in regulation of the pathological hallmarks of AD.

β-Arrestin2 is a critical component of the GPCR-eNOS signalosome

Endothelial cell nitric oxide (NO) synthase (eNOS), the enzyme responsible for synthesis of NO in endothelial cells, is regulated by complex posttranslational mechanisms. Sinusoidal portal hypertension, a disorder characterized by liver sinusoidal endothelial cell (SEC) injury with resultant reduced eNOS activity and NO production within the liver, has been associated with defects in eNOS protein-protein interactions and posttranslational modifications. We and others have previously identified novel eNOS interactors, including G protein-coupled receptor (GPCR) kinase interactor 1 (GIT1), which we found to play an unexpected stimulatory role in GPCR-mediated eNOS signaling. Here we report that β-arrestin 2 (β-Arr2), a canonical GPCR signaling partner, localizes in SECs with eNOS in a GIT1/eNOS/NO signaling module. Most importantly, we show that β-Arr2 stimulates eNOS activity, and that β-Arr2 expression is reduced and formation of the GIT1/eNOS/NO signaling module is interrupted during liver injury. In β-Arr2-deficient mice, bile duct ligation injury (BDL) led to significantly reduced eNOS activity and to a dramatic increase in portal hypertension compared to BDL in wild-type mice. Overexpression of β-Arr2 in injured or β-Arr2-deficient SECs rescued eNOS function by increasing eNOS complex formation and NO production. We also found that β-Arr2-mediated GIT1/eNOS complex formation is dependent on Erk1/2 and Src, two kinases known to interact with and be activated by β-Arr2 in response to GCPR activation. Our data emphasize that β-Arr2 is an integral component of the GIT1/eNOS/NO signaling pathway and have implications for the pathogenesis of sinusoidal portal hypertension.

β-Arrestin Based Receptor Signaling Paradigms: Potential Therapeutic Targets for Complex Age-Related Disorders

G protein coupled receptors (GPCRs) were first characterized as signal transducers that elicit downstream effects through modulation of guanine (G) nucleotide-binding proteins. The pharmacotherapeutic exploitation of this signaling paradigm has created a drug-based field covering nearly 50% of the current pharmacopeia. Since the groundbreaking discoveries of the late 1990s to the present day, it is now clear however that GPCRs can also generate productive signaling cascades through the modulation of β-arrestin functionality. β-Arrestins were first thought to only regulate receptor desensitization and internalization - exemplified by the action of visual arrestin with respect to rhodopsin desensitization. Nearly 20 years ago, it was found that rather than controlling GPCR signal termination, productive β-arrestin dependent GPCR signaling paradigms were highly dependent on multi-protein complex formation and generated long-lasting cellular effects, in contrast to G protein signaling which is transient and functions through soluble second messenger systems. β-Arrestin signaling was then first shown to activate mitogen activated protein kinase signaling in a G protein-independent manner and eventually initiate protein transcription - thus controlling expression patterns of downstream proteins. While the possibility of developing β-arrestin biased or functionally selective ligands is now being investigated, no additional research has been performed on its possible contextual specificity in treating age-related disorders. The ability of β-arrestin-dependent signaling to control complex and multidimensional protein expression patterns makes this therapeutic strategy feasible, as treating complex age-related disorders will likely require therapeutics that can exert network-level efficacy profiles. It is our understanding that therapeutically targeting G protein-independent effectors such as β-arrestin will aid in the development of precision medicines with tailored efficacy profiles for disease/age-specific contextualities.

Effect of the 5-HT4 receptor agonist tegaserod on the expression of GRK2 and GRK6 in the rat gastrointestinal tract

Objective

Tegaserod is a 5-hydroxytryptamine type 4 (5-HT₄) receptor agonist, formerly used in treating constipation predominant irritable bowel syndrome, which desensitizes 5-HT₄ receptors in rat oesophagus and colon in vitro. Desensitization of 5-HT₄ receptors is regulated by G-protein coupled receptor kinases. This study was designed to assess the effect of 5-HT₄ receptor activation on the expression of GRK2 and GRK6 in the rat oesophagus and distal colon by acute administration of tegaserod.

Results

Rats were treated with a single dose of tegaserod (5 mg/kg) and tissue samples of the oesophagus and distal colon were prepared and level of GRK2 and GRK6 protein expression was determined using western blotting. The immunodensity of GRK2 and GRK6 was normalized against the loading control β-actin and compared with control animals. Acute administration of tegaserod for 1, 2, 3, 4, 6, and 8 h did not change significantly the immunodensity of GRK2 or GRK6 in the oesophagus or GRK2 in the distal colon when compared with control animals. This may indicate that the basal level of GRK2 and GRK6 expression is sufficient to regulate the desensitization of 5-HT₄ receptors in acute drug treatment.

Age-dependent antinociception and behavioral inhibition by morphine

In current clinical practice, morphine is dosed in older patients based on patient-weight, with different calculations for adjustment. However, at present, neither clinical experience nor the literature offers a clear evidence base for the relationship between antinociception, behavioral effects and morphine administration in older patients. In this study, we compared the nociceptive response of 8 and 24 week old rats after subcutaneous administration of morphine per body weight and analyzed their behavior using an advanced multi-conditioning system. Residual morphine in all major tissues was determined. We observed prolonged morphine-induced antinociception in older rats compared to younger rats. Moreover, morphine significantly stimulated locomotor and rearing behavior 180 min after injection, which was significantly higher in the 8 week compared to 24 week old rats. Tissue analysis from animals extracted 240 min post-injection revealed a significantly higher concentration of residual morphine in the brains of older versus younger animals when standardized on tissue weight. However, this effect was not observed when residual morphine was standardized on protein content. Collectively, our data suggest that in older rats morphine exhibits higher antinociception and increased behavioral inhibition compared to younger animals. This effect is likely due to a significantly higher accumulation of morphine in the brain of older animals.

Sarcolemmal dependence of cardiac protection and stress-resistance: roles in aged or diseased hearts

Disruption of the sarcolemmal membrane is a defining feature of oncotic death in cardiac ischaemia-reperfusion (I-R), and its molecular makeup not only fundamentally governs this process but also affects multiple determinants of both myocardial I-R injury and responsiveness to cardioprotective stimuli. Beyond the influences of membrane lipids on the cytoprotective (and death) receptors intimately embedded within this bilayer, myocardial ionic homeostasis, substrate metabolism, intercellular communication and electrical conduction are all sensitive to sarcolemmal makeup, and critical to outcomes from I-R. As will be outlined in this review, these crucial sarcolemmal dependencies may underlie not only the negative effects of age and common co-morbidities on myocardial ischaemic tolerance but also the on-going challenge of implementing efficacious cardioprotection in patients suffering accidental or surgically induced I-R. We review evidence for the involvement of sarcolemmal makeup changes in the impairment of stress-resistance and cardioprotection observed with ageing and highly prevalent co-morbid conditions including diabetes and hypercholesterolaemia. A greater understanding of membrane changes with age/disease, and the inter-dependences of ischaemic tolerance and cardioprotection on sarcolemmal makeup, can facilitate the development of strategies to preserve membrane integrity and cell viability, and advance the challenging goal of implementing efficacious 'cardioprotection' in clinically relevant patient cohorts. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

Revisiting the Serotonin Hypothesis: Implications for Major Depressive Disorders

Major depressive disorder (MDD) is a heritable neuropsychiatric disease associated with severe changes at cellular and molecular levels. Its diagnosis mainly relies on the characterization of a wide range of symptoms including changes in mood and behavior. Despite the availability of antidepressant drugs, 10 to 30 % of patients fail to respond after a single or multiple treatments, and the recurrence of depression among responsive patients is very high. Evidence from the past decades suggests that the brain neurotransmitter serotonin (5-HT) is incriminated in MDD, and that a dysfunction of 5-HT receptors may play a role in the genesis of this disease. The 5-HT membrane transporter protein (SERT), which helps regulate the serotonergic transmission, is also implicated in MDD and is one of the main targets of antidepressant therapy. Although a number of behavioral tests and animal models have been developed to study depression, little is known about the neurobiological bases of MDD. Understanding the role of the serotonergic pathway will significantly help improve our knowledge of the pathophysiology of depression and may open up avenues for the development of new antidepressant drugs. The overarching goal of this review is to present recent findings from studies examining the serotonergic pathway in MDD, with a focus on SERT and the serotonin 1A (5-HT1A), serotonin 1B (5-HT1B), and serotonin 2A (5-HT2A) receptors. This paper also describes some of the main molecules involved in the internalization of 5-HT receptors and illustrates the changes in 5-HT neurotransmission in knockout mice and animal model of depression.

Molecular regulation of hypothalamus-pituitary-gonads axis in males

The hypothalamic-pituitary-gonadal axis (HPG) plays vital roles in reproduction and steroid hormone production in both sexes. The focus of this review is upon gene structures, receptor structures and the signaling pathways of gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The hormones' functions in reproduction as well as consequences resulting from mutations are also summarized. Specific characteristics of hormones such as the pulsatile secretions of GnRH are also covered. The different regulators of the HPG axis are introduced including kisspeptin, activin, inhibin, follistatin, androgens and estrogen. This review includes not only their basic information, but also their unique function in the HPG axis. Here we view the HPG axis as a whole, so relations between ligands and receptors are well described crossing different levels of the HPG axis. Hormone interactions and transformations are also considered. The major information of this article is depicted in three figures summarizing the current discoveries on the HPG axis. This article systematically introduces the basic knowledge of the HPG axis and provides information of the current advances relating to reproductive hormones.

Balancing GRK2 and EPAC1 levels prevents and relieves chronic pain

Chronic pain is a major clinical problem, yet the mechanisms underlying the transition from acute to chronic pain remain poorly understood. In mice, reduced expression of GPCR kinase 2 (GRK2) in nociceptors promotes cAMP signaling to the guanine nucleotide exchange factor EPAC1 and prolongs the PGE2-induced increase in pain sensitivity (hyperalgesia). Here we hypothesized that reduction of GRK2 or increased EPAC1 in dorsal root ganglion (DRG) neurons would promote the transition to chronic pain. We used 2 mouse models of hyperalgesic priming in which the transition from acute to chronic PGE2-induced hyperalgesia occurs. Hyperalgesic priming with carrageenan induced a sustained decrease in nociceptor GRK2, whereas priming with the PKCε agonist ΨεRACK increased DRG EPAC1. When either GRK2 was increased in vivo by viral-based gene transfer or EPAC1 was decreased in vivo, as was the case for mice heterozygous for Epac1 or mice treated with Epac1 antisense oligodeoxynucleotides, chronic PGE2-induced hyperalgesia development was prevented in the 2 priming models. Using the CFA model of chronic inflammatory pain, we found that increasing GRK2 or decreasing EPAC1 inhibited chronic hyperalgesia. Our data suggest that therapies targeted at balancing nociceptor GRK2 and EPAC1 levels have promise for the prevention and treatment of chronic pain.

Reduced neutrophil chemotaxis and infiltration contributes to delayed resolution of cutaneous wound infection with advanced age

Advanced age is associated with alterations in innate and adaptive immune responses, which contribute to an increased risk of infection in elderly patients. Coupled with this immune dysfunction, elderly patients demonstrate impaired wound healing with elevated rates of wound dehiscence and chronic wounds. To evaluate how advanced age alters the host immune response to cutaneous wound infection, we developed a murine model of cutaneous Staphylococcus aureus wound infection in young (3-4 mo) and aged (18-20 mo) BALB/c mice. Aged mice exhibit increased bacterial colonization and delayed wound closure over time compared with young mice. These differences were not attributed to alterations in wound neutrophil or macrophage TLR2 or FcγRIII expression, or age-related changes in phagocytic potential and bactericidal activity. To evaluate the role of chemotaxis in our model, we first examined in vivo chemotaxis in the absence of wound injury to KC, a neutrophil chemokine. In response to a s.c. injection of KC, aged mice recruited fewer neutrophils at increasing doses of KC compared with young mice. This paralleled our model of wound infection, where diminished neutrophil and macrophage recruitment was observed in aged mice relative to young mice despite equivalent levels of KC, MIP-2, and MCP-1 chemokine levels at the wound site. This reduced leukocyte accumulation was also associated with lower levels of ICAM-1 in wounds from aged mice at early time points. These age-mediated defects in early neutrophil recruitment may alter the dynamics of the inflammatory phase of wound healing, impacting macrophage recruitment, bacterial clearance, and wound closure.

Regulation of munc18-1 and syntaxin-1A interactive partners in schizophrenia prefrontal cortex: down-regulation of munc18-1a isoform and 75 kDa SNARE complex after antipsychotic treatment

Munc18-1 and syntaxin-1 are crucial interacting molecules for synaptic membrane fusion and neurotransmitter release. Contrasting abnormalities of several proteins of the exocytotic machinery, including the formation of SNARE (synaptobrevin, SNAP-25 and syntaxin-1) complexes, have been reported in schizophrenia. This study quantified in the dorsolateral prefrontal cortex (PFC, Brodmann area 9) the immunocontent of munc18-1a/b isoforms, syntaxin-1A, other presynaptic proteins (synaptotagmin, synaptophysin), and SNARE complexes, as well as the effects of psychoactive drug exposure, in schizophrenia (SZ, n=24), non-schizophrenia suicide (SD, n=13) and major depression (MD, n=15) subjects compared to matched controls (n=39). SZ was associated with normal expression of munc18-1a/b and increased syntaxin-1A (+44%). The presence of antipsychotic drugs reduced the basal content of munc18-1a isoform (-23%) and synaptobrevin (-32%), and modestly reduced that of up-regulated syntaxin-1A (-16%). Munc18-1a and syntaxin-1A protein expression correlated positively in controls but showed a markedly opposite pattern in SZ, regardless of antipsychotic treatment. Thus, the ratio of syntaxin-1A to munc18-1a showed a net increase in SZ (+53/114%). The SNARE complex (75 kDa) was found unaltered in antipsychotic-free and reduced (-28%) in antipsychotic-treated SZ subjects. None of these abnormalities were observed in SD and MD subjects, unexposed or exposed to psychoactive drugs. The results reveal some exocytotic dysfunctions in SZ that are probably related to an imbalance of the interaction between munc18-1a and SNARE (mainly syntaxin-1A) complex. Moreover, antipsychotic drug treatment is associated with lower content of key proteins of the exocytotic machinery, which could result in a destabilization/impairment of neurosecretion.

Reduced platelet G protein-coupled receptor kinase 2 in major depressive disorder: antidepressant treatment-induced upregulation of GRK2 protein discriminates between responder and non-responder patients

The homologous regulation of neurotransmitter receptors by G protein-coupled receptor kinases (GRKs) is important in the pathogenesis and treatment of major depressive disorder (MDD). Previous studies have reported that the basal status of GRK2 is different in brains (upregulation) and platelets (downregulation) of subjects with MDD. The principal aim of this study was to re-examine the status of platelet membrane GRK2 protein in patients with MDD, along with GRK3 (a close kinase homolog) and GRK5 (a kinase with different properties), before and after treatment with serotonin-selective reuptake inhibitor (SSRI) or serotonin noradrenaline reuptake inhibitor (SNRI) antidepressants. The main findings indicated that platelet GRK2 and p-Ser670 GRK2 were reduced (36-41%) in unmedicated MDD subjects, and that GRK2 content correlated inversely with the severity of depression (r=-0.51). Effective antidepressant treatments normalized platelet GRK2, and, notably, GRK2 upregulation discriminated between responder and non-responder patients. Other findings revealed a modest reduction of platelet GRK3 (23%) and no alteration of platelet GRK5 content. In untreated subjects with MDD, lymphocyte GRK2 and GRK5 mRNAs were unaltered but antidepressant treatment-induced upregulation of GRK2 mRNA expression. The reduced content of platelet GRK2 protein is a relevant target in MDD. Although this peripheral GRK2 defect does not mirror the canonical regulation of brain GRK2 in depressed suicides, it could well represent a disease state marker as well as a surrogate of response to effective antidepressant treatment.

Tissue dependent differences in G-protein coupled receptor kinases associated with 5-HT4 receptor desensitization in the rat gastro-intestinal tract

Desensitization of 5-HT(4) receptors is regulated by G-protein coupled receptor kinases (GRKs). However, the specific GRK(s) that regulates the desensitization of 5-HT(4) receptors in the in vivo setting is unknown. We investigated the in situ expression of 5-HT(4) receptors and the GRKs in the rat gastrointestinal tract using immunohistochemistry and their interaction using coimmunoprecipitation. 5-HT(4) receptors were expressed in the tunica muscularis mucosae of the oesophagus, longitudinal muscle, myenteric plexus, circular muscle, submucosal plexus and muscularis mucosae of both the proximal and distal colon. GRK2 was expressed in longitudinal muscle and occasionally in myenteric plexus whilst GRK5 showed limited expression in the nerve endings of the myenteric plexus and submucosal plexus of the colon. GRK3 was expressed in the tunica muscularis mucosae of the oesophagus, circular muscle, submucosal plexus and muscularis mucosae of the colon. GRK6 was expressed in the tunica muscularis mucosae of the oesophagus, longitudinal muscle, circular muscle, and muscularis mucosae of the colon. Stimulation of tunica muscularis mucosae of the oesophagus and distal colon using the 5-HT(4) receptor agonist, tegaserod, followed by analysis of the 5-HT(4) receptor antibody immunoprecipitate, revealed the coimmunoprecipitation of GRK6 with 5-HT(4) receptors in the tunica muscularis mucosae of oesophagus while GRK2 and GRK6 were coimmunoprecipitated with 5-HT(4) receptors in the distal colon. This study indicates that GRK6 may be involved in the regulation of the desensitization of 5-HT(4) receptors in the rat oesophagus whilst GRK2 and GRK6 may be involved in regulation of the desensitization of 5-HT(4) receptors in the distal colon.

Sample preparation and fractionation for proteome analysis and cancer biomarker discovery by mass spectrometry

Sample preparation and fractionation technologies are one of the most crucial processes in proteomic analysis and biomarker discovery in solubilized samples. Chromatographic or electrophoretic proteomic technologies are also available for separation of cellular protein components. There are, however, considerable limitations in currently available proteomic technologies as none of them allows for the analysis of the entire proteome in a simple step because of the large number of peptides, and because of the wide concentration dynamic range of the proteome in clinical blood samples. The results of any undertaken experiment depend on the condition of the starting material. Therefore, proper experimental design and pertinent sample preparation is essential to obtain meaningful results, particularly in comparative clinical proteomics in which one is looking for minor differences between experimental (diseased) and control (nondiseased) samples. This review discusses problems associated with general and specialized strategies of sample preparation and fractionation, dealing with samples that are solution or suspension, in a frozen tissue state, or formalin-preserved tissue archival samples, and illustrates how sample processing might influence detection with mass spectrometric techniques. Strategies that dramatically improve the potential for cancer biomarker discovery in minimally invasive, blood-collected human samples are also presented.

Low endogenous G-protein-coupled receptor kinase 2 sensitizes the immature brain to hypoxia-ischemia-induced gray and white matter damage

Hypoxic-ischemic brain injury is regulated in part by neurotransmitter and chemokine signaling via G-protein-coupled receptors (GPCRs). GPCR-kinase 2 (GRK2) protects these receptors against overstimulation by inducing desensitization. Neonatal hypoxic-ischemic brain damage is preceded by a reduction in cerebral GRK2 expression. We determined the functional importance of GRK2 in hypoxic-ischemic brain damage. Nine-day-old wild-type and GRK2(+/-) mice with a approximately 50% reduction in GRK2 protein were exposed to unilateral carotid artery occlusion and hypoxia. In GRK2(+/-) animals, gray and white matter damage was aggravated at 3 weeks after hypoxia-ischemia. In addition, cerebral neutrophil infiltration was increased in GRK2(+/-) animals. Neutrophil depletion reduced brain damage, but neuronal loss was still more pronounced in GRK2(+/-) animals. Onset of neuronal loss was advanced in GRK2(+/-) animals regardless of neutrophil depletion. White matter injury was advanced in GRK2(+/-) animals and was not affected by neutrophil depletion. Activation/infiltration of microglia/macrophages was stronger in GRK2(+/-) brains but only occurred 24 h after hypoxia-ischemia and is therefore not the primary cause of increased damage. During hypoxia, cerebral blood flow was reduced to the same extent in both genotypes. In vitro, GRK2(+/-) hippocampal slices and cerebellar granular neurons were more sensitive to glutamate-induced death. We propose the novel concept that the kinase GRK2 regulates onset and magnitude of hypoxic-ischemic brain damage. Increased gray and white matter damage in GRK2(+/-) animals was not dependent on infiltrating neutrophils and occurred before microglia/macrophage activation was detected. Collectively, our data suggest that cerebral GRK2 has an important endogenous neuroprotective role in ischemic cerebral damage.

Altered expression and subcellular distribution of GRK subtypes in the dopamine-depleted rat basal ganglia is not normalized by l-DOPA treatment

Dysregulation of dopamine (DA) receptors is believed to underlie Parkinson's disease pathology and l-DOPA-induced motor complications. DA receptors are subject to regulation by G protein-coupled receptor kinases (GRKs) and arrestins. DA lesion with 6-hydroxydopamine caused multiple protein- and brain region-specific changes in the expression of GRKs. In the globus pallidus, all four GRK isoforms (GRK2, 3, 5, 6) were reduced in the lesioned hemisphere. In the caudal caudate-putamen (cCPu) three GRK isoforms (GRK2, 3, 6) were decreased by DA depletion. The decrease in GRK proteins in globus pallidus, but not cCPu, was mirrored by reduction in mRNA. GRK3 protein was reduced in the rostral caudate-putamen (rCPu), whereas other isoforms were either unchanged or up-regulated. GRK6 protein and mRNA were up-regulated in rCPu and nucleus accumbens. l-DOPA (25 mg/kg, twice daily for 10 days) failed to reverse changes caused by DA depletion, whereas D(2)/D(3) agonist pergolide (0.25 mg/kg daily for 10 days) restored normal levels of expression of GRK5 and 6. In rCPu, GRK2 protein was increased in most subcellular fractions by l-DOPA but not by DA depletion alone. Similarly, l-DOPA up-regulated arrestin3 in membrane fractions in both regions. GRK5 was down-regulated by l-DOPA in cCPu in the light membrane fraction, where this isoform is the most abundant. The data suggest that alterations in the expression and subcellular distribution of arrestins and GRKs contribute to pathophysiology of Parkinson's disease. Thus, these proteins may be targets for antiparkinsonian therapy.

Methods for samples preparation in proteomic research

Sample preparation is one of the most crucial processes in proteomics research. The results of the experiment depend on the condition of the starting material. Therefore, the proper experimental model and careful sample preparation is vital to obtain significant and trustworthy results, particularly in comparative proteomics, where we are usually looking for minor differences between experimental-, and control samples. In this review we discuss problems associated with general strategies of samples preparation, and experimental demands for these processes.

Identification of G-protein coupled receptor kinase 2 in paired helical filaments and neurofibrillary tangles

G-protein coupled receptor kinases (GRKs) constitute a serine/threonine kinase family playing a major role in agonist-induced phosphorylation and desensitization of G-protein coupled receptors. Recently, GRK2 and GRK5 have been demonstrated to phosphorylate alpha-synuclein (Ser129) and other synuclein isoforms. We studied colocalization of GRK2, GRK5, alpha-synuclein, and tau in neurodegenerative disorders characterized by fibrillary tau inclusions and/or alpha-synuclein-enriched Lewy bodies. We found that Lewy bodies were negative for both GRK2 and GRK5 in Lewy body disease (LBD) and LBD mixed with Alzheimer disease (AD + LBD). Instead, GRK2 but not GRK5 colocalized with 40% to 50% of neurofibrillary tangles in AD + LBD and AD brains. In disorders with less prominent alpha-synucleinopathy, neuronal and glial fibrillary tau deposits known to contain distinct subsets of tau isoforms were also positive for GRK2. These deposits included tufted astrocytes and coiled bodies in progressive supranuclear palsy, astrocytic plaques in corticobasal degeneration, and Pick bodies in Pick disease. In addition, paired helical filaments isolated from AD and AD + LBD brains were found to immunogold-label for GRK2, suggesting that GRK2 could be a potential tau kinase associated with fibrillary tau. Our studies indicate that GRK2 is a novel component of neuronal and glial fibrillary tau deposits with no preference in tau isoform binding. GRK2 may play a role in hyperphosphorylation of tau in tauopathies.

Reduced alpha adrenergic mediated contraction of renal preglomerular blood vessels as a function of gender and aging

As human males age, a decline in baroreflex-mediated elevation of blood pressure occurs due, at least in part, to a reduction in alpha-1 adrenergic vasoconstrictor function. Alpha adrenergic constriction is mediated by guanosine triphosphate binding Protein (G Protein) coupled signaling pathways. Alpha-1 A/C, B, and D adrenergic receptor expressions, measured by GeneChip array, are not reduced during aging in renal blood vessels of male or female rats. Alpha-1 A GeneChip expression is greater, at all ages studied, in females than in males. Prazosin binding by alpha-1 adrenergic receptors is greater in young adult female rats than in young adult male rats; however, it is reduced with aging in both male and female rats. G alpha q GeneChip expression declines while expression of adrenergic receptor kinase (GRK2) and tyrosine phosphatases (TyrP) increase with aging in male rats. The declines in alpha-1 adrenergic receptor binding and G alpha q expression and also the increases in GRK2 and TyrP expression likely relate to the age-related decline of vasoconstriction in male rats. The information that the expression of alpha-1 A adrenergic receptors is greater in female rats and (GRK2) expression does not increase during aging could relate to the gender differences in vasoconstrictor function with aging. Gene therapy to ameliorate the age-related decline in renal function could possibly reduce the need for renal dialysis. Signaling pathways such as those reviewed herein may provide an outline of the molecular pathways needed to move toward successful renal gene therapy for aging individuals.

Imaging of muscarinic acetylcholine receptor signaling in hippocampal neurons: evidence for phosphorylation-dependent and -independent regulation by G-protein-coupled receptor kinases

We used the inositol 1,4,5-trisphosphate (IP3) biosensor, the pleckstrin homology (PH) domain of PLCdelta1 (phospholipase C) tagged with enhanced green fluorescent protein (eGFP-PH(PLCdelta)), to examine muscarinic acetylcholine (mACh) receptor regulation of phospholipase C/IP3 signaling in intact single hippocampal neurons in "real time." Initial experiments produced a pharmacological profile consistent with the presence of a predominant M1 mACh receptor population coupled to the IP3 response. To investigate M1 mACh receptor regulation, neurons were stimulated with approximate EC50 concentrations of the mACh receptor agonist methacholine before (R1) and after (R2) a short (60 sec) exposure to a high concentration of agonist. This resulted in a marked attenuation in the R2 relative to R1 response. Inhibition of endogenous GRK6 (G-protein-coupled receptor kinase) activity, by the introduction of catalytically inactive (K215R)GRK6, partially reversed the attenuation of agonist-induced responsiveness, whereas overexpression of wild-type GRK6 increased receptor desensitization. Manipulation of endogenous GRK2 activity through introduction of either wild-type or catalytically inactive GRK2 ((K220R)GRK2) almost completely inhibited agonist-stimulated IP3 production, implying a phosphorylation-independent regulation of M1 mACh receptor signaling, most probably mediated by a GRK2 N-terminal RGS-like (regulator of G-protein signaling) domain interaction with GTP-bound Galpha(q/11). Together, our data suggest a role for both phosphorylation-dependent and -independent regulation of M1 mACh receptors in hippocampal neurons.

Decreased immunodensities of micro-opioid receptors, receptor kinases GRK 2/6 and beta-arrestin-2 in postmortem brains of opiate addicts

The homologous regulation of opioid receptors, through G protein-coupled receptor kinases (GRKs) and beta-arrestins, is an initial step in the complex molecular mechanisms leading to opiate tolerance and dependence. This study was designed to evaluate in parallel the contents of immunolabeled micro-opioid receptors (glycosylated proteins), two representative GRKs (GRK 2 and GRK 6) and beta-arrestin-2 in brains of opiate addicts who had died of an opiate overdose (heroin or methadone). The immunodensities of micro-opioid receptors were decreased (66 kDa protein: 24%, n=24, P<0.0001; 85 kDa protein: 16%, n=24, P<0.05) in the prefrontal cortex of opiate addicts compared with sex-, age-, and PMD-matched controls. This down-regulation of brain micro-opioid receptors was more pronounced in opiate addicts dying of a heroin overdose (27-30%, n=13) than in those who died of a methadone overdose (5-16%, n=11). In the same brains, significant decreases in the immunodensities of GRK 2 (19%, n=24, P<0.05), GRK 6 (25%, n=24, P<0.002) and beta-arrestin-2 (22%, n=24, P< 0.0005) were also quantitated. In contrast, the content of alpha-internexin (a neuronal marker used as a negative control) was not changed in brains of opiate addicts. In these subjects, there was a significant correlation between the densities of GRK 6 and beta-arrestin-2 (r=0.63, n=24, P=0.001), suggesting that both proteins are regulated in a coordinated manner by opiate drugs in the brain. The results indicate that opiate addiction in humans (tolerant state) is associated with down-regulation of brain micro-opioid receptors and regulatory GRK 2/6 and beta-arrestin-2 proteins. These molecular adaptations may be relevant mechanisms for the induction of opiate tolerance in brains of opiate addicts.

Regulation of platelet alpha 2A-adrenoceptors, Gi proteins and receptor kinases in major depression: effects of mirtazapine treatment

Major depression is associated with the upregulation of alpha(2A)-adrenoceptors in brain tissue and blood platelets. The homologous regulation of these receptors by G-protein-coupled receptor kinases (GRKs) might play a relevant role in the pathogenesis and treatment of depression. This study was designed to assess the status of the complex alpha(2A)-adrenoceptor/Galphai/GRK 2 in the platelets of depressed patients (n=22) before and after treatment with the antidepressant mirtazapine, an antagonist at alpha(2A)-adrenoceptors (30-45 mg/day for up to 6 months). A second series of depressed suicide attempters (n=32) were also investigated to further assess the status of platelet GRK 2 and GRK 6. Platelet alpha(2A)-adrenoceptors and Galphai protein immunoreactivities were increased in depressed patients (49 and 35%) compared with matched controls. In contrast, GRK 2 content was decreased in the two series of depressed patients (27 and 28%). GRK 6 (a GRK with different properties) was found unchanged. In drug-free depressed patients, the severity of depression (behavioral ratings with two different instruments) correlated inversely with the content of platelet GRK 2 (r=-0.46, n=22, p=0.032, and r=-0.55, n=22, p=0.009). After 4-24 weeks of treatment, mirtazapine induced downregulation of platelet alpha(2A)-adrenoceptors (up to 34%) and Galphai proteins (up to 28%), and the upregulation of GRK 2 (up to 30%). The results indicate that major depression is associated with reduced platelet GRK 2, suggesting that a defect of this kinase may contribute to the observed upregulation of alpha(2A)-adrenoceptors. Moreover, treatment with mirtazapine reversed this abnormality and induced downregulation of alpha(2A)-adrenoceptor/Galphai complex. The results support a role of supersensitive alpha(2A)-adrenoceptors in the pathogenesis and treatment of major depression.