Beta-arrestin scaffolding of phosphatidylinositol 4-phosphate 5-kinase Ialpha promotes agonist-stimulated sequestration of the beta2-adrenergic receptor

Seasonal dynamics and driving mechanisms of microbial biogenic elements cycling function, assembly process, and co-occurrence network in plateau lake sediments

Microbial community diversity significantly varies with seasonality. However, little is known about seasonal variation of microbial community functions in lake sediments and their associated environmental influences. In this study, metagenomic sequencing of sediments collected from winter, summer, and autumn from Caohai Lake, Guizhou Plateau, were used to evaluate the composition and function of sediment microbial communities, the potential interactions of functional genes, key genes associated with seasons, and community assembly mechanisms. The average concentrations of nitrogen (TN) and phosphorus (TP) in lake sediments were higher, which were 6.136 and 0.501 g/kg, respectively. TN and organic matter (OM) were the primary factors associated with sediment community composition and functional profiles. The diversity and structure of the microbial communities varied with seasons, and Proteobacteria relative abundances were significantly lower in summer than in other seasons (58.43-44.12 %). Seasons were also associated with the relative abundances of functional genes, and in particular korA, metF, narC, nrfA, pstC/S, and soxB genes. Network complexity was highest in the summer and key genes in the network also varied across seasons. Neutral community model analysis revealed that the assembly mechanisms related to carbon (C), nitrogen (N), phosphorus (P), and sulfur (S) cycle-related genes were primarily associated with random processes. In summary, diverse functional genes were identified in lake sediments and exhibited evidence for synergistic interactions (Positive proportion: 74.91-99.82 %), while seasonal factors influenced their distribution. The results of this study provide new insights into seasonal impacts on microbial-driven biogeochemical cycling in shallow lakes.

Phycospheric bacteria limits the effect of nitrogen and phosphorus imbalance on diatom bloom

Human activities have caused an imbalance in the input nitrogen and phosphorus (N/P) in the biosphere. The imbalance of N/P is one of the characteristics of water eutrophication, which is the fundamental factor responsible for the blooms. The effects of the N/P imbalance on diatom and phycospheric bacteria in blooms are poorly understood. In this study, the N/P molar ratio in real water (14:1) and the predicted N/P molar ratio in future water (65:1) were simulated to analyze the response of Cyclotella sp. and phycospheric bacteria to the N/P imbalance. The results showed that the N/P imbalance inhibited the growth of Cyclotella sp., but prolonged diatom bloom duration. The resistance of Cyclotella sp. to the N/P imbalance is related to phycospheric bacteria, and there are dynamic regulatory mechanisms within the phycospheric bacteria community to resist the N/P imbalance: (1) the increase of HNA bacterial density, the decrease of LNA bacterial density, (2) the increase of phycospheric bacterial diversity and eutrophic bacteria abundance, and the change of denitrifying bacteria abundance, (3) the activity of nitrogen and phosphorus metabolism of HNA bacteria enhanced, while that of LNA bacteria decreased. And the gene hosts of nitrogen and phosphorus metabolism were most enriched in Proteobacteria, indicating that Proteobacteria played an important role in maintaining the stability of phycospheric bacteria and was the dominant phylum resistant to the N/P imbalance. This study clarified that the algal-bacteria system was resistant to the N/P imbalance and implied that the N/P imbalance had little effect on the occurrence of diatom bloom events due to the presence of phycospheric bacteria.

G protein-coupled receptor endocytosis generates spatiotemporal bias in β-arrestin signaling

The stabilization of different active conformations of G protein-coupled receptors is thought to underlie the varying efficacies of biased and balanced agonists. Here, profiling the activation of signal transducers by angiotensin II type 1 receptor (AT1R) agonists revealed that the extent and kinetics of β-arrestin binding exhibited substantial ligand-dependent differences, which were lost when receptor internalization was inhibited. When AT1R endocytosis was prevented, even weak partial agonists of the β-arrestin pathway acted as full or near-full agonists, suggesting that receptor conformation did not exclusively determine β-arrestin recruitment. The ligand-dependent variance in β-arrestin translocation was much larger at endosomes than at the plasma membrane, showing that ligand efficacy in the β-arrestin pathway was spatiotemporally determined. Experimental investigations and mathematical modeling demonstrated how multiple factors concurrently shaped the effects of agonists on endosomal receptor-β-arrestin binding and thus determined the extent of functional selectivity. Ligand dissociation rate and G protein activity had particularly strong, internalization-dependent effects on the receptor-β-arrestin interaction. We also showed that endocytosis regulated the agonist efficacies of two other receptors with sustained β-arrestin binding: the V2 vasopressin receptor and a mutant β2-adrenergic receptor. In the absence of endocytosis, the agonist-dependent variance in β-arrestin2 binding was markedly diminished. Our results suggest that endocytosis determines the spatiotemporal bias in GPCR signaling and can aid in the development of more efficacious, functionally selective compounds.

Interaction of Angiotensin II AT1 Receptors with Purinergic P2X Receptors in Regulating Renal Afferent Arterioles in Angiotensin II-Dependent Hypertension

In angiotensin II (Ang II)-dependent hypertension, Ang II activates angiotensin II type 1 receptors (AT1R) on renal vascular smooth muscle cells, leading to renal vasoconstriction with eventual glomerular and tubular injury and interstitial inflammation. While afferent arteriolar vasoconstriction is initiated by the increased intrarenal levels of Ang II activating AT1R, the progressive increases in arterial pressure stimulate the paracrine secretion of adenosine triphosphate (ATP), leading to the purinergic P2X receptor (P2XR)-mediated constriction of afferent arterioles. Thus, the afferent arteriolar tone is maintained by two powerful systems eliciting the co-existing activation of P2XR and AT1R. This raises the conundrum of how the AT1R and P2XR can both be responsible for most of the increased renal afferent vascular resistance existing in angiotensin-dependent hypertension. Its resolution implies that AT1R and P2XR share common receptor or post receptor signaling mechanisms which converge to maintain renal vasoconstriction in Ang II-dependent hypertension. In this review, we briefly discuss (1) the regulation of renal afferent arterioles in Ang II-dependent hypertension, (2) the interaction of AT1R and P2XR activation in regulating renal afferent arterioles in a setting of hypertension, (3) mechanisms regulating ATP release and effect of angiotensin II on ATP release, and (4) the possible intracellular pathways involved in AT1R and P2XR interactions. Emerging evidence supports the hypothesis that P2X1R, P2X7R, and AT1R actions converge at receptor or post-receptor signaling pathways but that P2XR exerts a dominant influence abrogating the actions of AT1R on renal afferent arterioles in Ang II-dependent hypertension. This finding raises clinical implications for the design of therapeutic interventions that will prevent the impairment of kidney function and subsequent tissue injury.

Biophysical physiology of phosphoinositide rapid dynamics and regulation in living cells

Phosphoinositide membrane lipids are ubiquitous low-abundance signaling molecules. They direct many physiological processes that involve ion channels, membrane identification, fusion of membrane vesicles, and vesicular endocytosis. Pools of these lipids are continually broken down and refilled in living cells, and the rates of some of these reactions are strongly accelerated by physiological stimuli. Recent biophysical experiments described here measure and model the kinetics and regulation of these lipid signals in intact cells. Rapid on-line monitoring of phosphoinositide metabolism is made possible by optical tools and electrophysiology. The experiments reviewed here reveal that as for other cellular second messengers, the dynamic turnover and lifetimes of membrane phosphoinositides are measured in seconds, controlling and timing rapid physiological responses, and the signaling is under strong metabolic regulation. The underlying mechanisms of this metabolic regulation remain questions for the future.

Propranolol and low-dose isoproterenol ameliorate insulin resistance, enhance β-arrestin2 signaling, and reduce cardiac remodeling in high-fructose, high-fat diet-fed mice: Comparative study with metformin

AIMS

β-Arrestin2 signaling has emerged as a promising therapeutic target for the management of insulin resistance and related complications. Moreover, recent studies have shown that certain G protein-coupled receptor (GPCR) ligands can modulate β-arrestin2 signaling. The current study examined the effects of the β-blocker propranolol and a low dose of the agonist isoproterenol (L-D-ISOPROT) on β-arrestin2 signaling, insulin resistance, and cardiac remodeling in high-fructose, high-fat diet (HFrHFD)-fed mice. In addition, the effects of these agents were compared to those of the clinical antidiabetic agent, metformin.

MATERIALS AND METHODS

Insulin resistance was induced by HFrHFD feeding for 16 weeks. Mice were then randomly allocated to groups receiving propranolol, L-D-ISOPROT, metformin, or vehicle (control) for 4 weeks starting on week 13 of HFrHFD feeding. Survival rate, body weight, visceral fat weight, blood glucose, serum insulin, insulin resistance index, hepatic β-arrestin2 signaling, heart weight, left and right ventricular thicknesses, cardiac fibrosis severity, serum endothelin-1, cardiac cardiotrophin-1, and cardiac β-arrestin2 signaling were then compared among groups.

KEY FINDINGS

HFrHFD for 16 weeks significantly increased insulin resistance index, cardiac fibrosis area, and serum endothelin-1, and reduced hepatic β-arrestin2 signaling, cardiac cardiotrophin-1, and cardiac β-arrestin2 signaling without significant changes in survival rate, body weight, visceral fat weight, heart weight, or left and right ventricular thicknesses. All three drugs reduced insulin resistance and cardiac remodeling parameters and enhanced β-arrestin2 signaling with variable efficacies.

SIGNIFICANCE

Propranolol and L-D-ISOPROT, like metformin, can reduce insulin-resistance and cardiac remodeling in HFrHFD-fed mice, possibly by upregulating β-arrestin2 signaling activity. Therefore, β-arrestin2-signaling modulation might be a promising strategy for insulin-resistance treatment.

Acute and chronic metabolic effects of carvedilol in high-fructose, high-fat diet-fed mice: implication of β-arrestin2 pathway

We aimed to investigate the acute and chronic effects of carvedilol on insulin resistance in high-fructose, high-fat diet (HFrHFD) - fed mice and the implication of the β-arrestin2 pathway. The acute effect of carvedilol (10 mg/kg, i.p.) on glucose tolerance and hepatic lipid signaling in normal and insulin resistant mice was investigated. Then, the chronic effect of carvedilol on insulin resistance and dyslipidemia in HFrHFD-fed mice was examined. Changes in β-arrestin2 and its downstream signals in liver, skeletal muscle, and adipose tissue were measured. This involved measuring phosphatidylinositol 4,5-bisphosphate (PIP2) and diacylglycerol (DAG) levels and protein kinase B (AKT) activity. Carvedilol acutely reduced fasting blood glucose levels in both normal and insulin resistant mice without significantly affecting the glucose tolerance. These acute effects were associated with increased hepatic PIP2 but decreased hepatic DAG levels. Chronic administration of carvedilol significantly ameliorated insulin resistance and dyslipidemia in HFrHFD-fed mice. These chronic effects were associated with increased β-arrestin2, PIP2, and AKT activity levels but decreased DAG levels in the classical insulin target tissues. In conclusion, carvedilol acutely maintains glucose homeostasis and chronically ameliorates insulin resistance and dyslipidemia in HFrHFD-fed mice. The insulin sensitizing effects of carvedilol are highly correlated with the upregulation of β-arrestin2 pathway.

Pioglitazone Enhances β-Arrestin2 Signaling and Ameliorates Insulin Resistance in Classical Insulin Target Tissues

INTRODUCTION

Pioglitazone is a thiazolidinedione oral antidiabetic agent. This study aimed to investigate the effects of pioglitazone as insulin sensitizer on β-arrestin2 signaling in classical insulin target tissues.

METHODS

Experiments involved three groups of mice; the first one involved mice fed standard chow diet for 16 weeks; the second one involved mice fed high-fructose, high-fat diet (HFrHFD) for 16 weeks; and the third one involved mice fed HFrHFD for 16 weeks and received pioglitazone (30 mg/kg/day, orally) in the last four weeks of feeding HFrHFD.

RESULTS

The results showed significant improvement in the insulin sensitivity of pioglitazone-treated mice as manifested by significant reduction in the insulin resistance index. This improvement in insulin sensitivity was associated with significant increases in the β-arrestin2 levels in the adipose tissue, liver, and skeletal muscle. Moreover, pioglitazone significantly increased β-arrestin2 signaling in all the examined tissues as estimated from significant increases in phosphatidylinositol 4,5 bisphosphate and phosphorylation of Akt at serine 473 and significant decrease in diacylglycerol level.

CONCLUSION

To the best of our knowledge, our work reports a new mechanism of action for pioglitazone through which it can enhance the insulin sensitivity. Pioglitazone increases β-arrestin2 signaling in the adipose tissue, liver, and skeletal muscle of HFrHFD-fed mice.

β-arrestin-dependent PI(4,5)P2 synthesis boosts GPCR endocytosis

β-arrestins regulate many cellular functions including intracellular signaling and desensitization of G protein-coupled receptors (GPCRs). Previous studies show that β-arrestin signaling and receptor endocytosis are modulated by the plasma membrane phosphoinositide lipid phosphatidylinositol-(4, 5)-bisphosphate (PI(4,5)P2). We found that β-arrestin also helped promote synthesis of PI(4,5)P2 and up-regulated GPCR endocytosis. We studied these questions with the Gq-coupled protease-activated receptor 2 (PAR2), which activates phospholipase C, desensitizes quickly, and undergoes extensive endocytosis. Phosphoinositides were monitored and controlled in live cells using lipid-specific fluorescent probes and genetic tools. Applying PAR2 agonist initiated depletion of PI(4,5)P2, which then recovered during rapid receptor desensitization, giving way to endocytosis. This endocytosis could be reduced by various manipulations that depleted phosphoinositides again right after phosphoinositide recovery: PI(4)P, a precusor of PI(4,5)P2, could be depleted at either the Golgi or the plasma membrane (PM) using a recruitable lipid 4-phosphatase enzyme and PI(4,5)P2 could be depleted at the PM using a recruitable 5-phosphatase. Endocytosis required the phosphoinositides. Knock-down of β-arrestin revealed that endogenous β-arrestin normally doubles the rate of PIP5-kinase (PIP5K) after PAR2 desensitization, boosting PI(4,5)P2-dependent formation of clathrin-coated pits (CCPs) at the PM. Desensitized PAR2 receptors were swiftly immobilized when they encountered CCPs, showing a dwell time of ∼90 s, 100 times longer than for unactivated receptors. PAR2/β-arrestin complexes eventually accumulated around the edges or across the surface of CCPs promoting transient binding of PIP5K-Iγ. Taken together, β-arrestins can coordinate potentiation of PIP5K activity at CCPs to induce local PI(4,5)P2 generation that promotes recruitment of PI(4,5)P2-dependent endocytic machinery.

Hedgehog signaling and Tre1 regulate actin dynamics through PI(4,5)P2 to direct migration of Drosophila embryonic germ cells

The Tre1 G-protein coupled receptor (GPCR) was discovered to be required for Drosophila germ cell (GC) coalescence almost two decades ago, yet the molecular events both upstream and downstream of Tre1 activation remain poorly understood. To gain insight into these events, we describe a bona fide null allele and both untagged and tagged versions of Tre1. We find that the primary defect with complete Tre1 loss is the failure of GCs to properly navigate, with GC mis-migration occurring from early stages. We find that Tre1 localizes with F-actin at the migration front, along with PI(4,5)P₂; dPIP5K, an enzyme that generates PI(4,5)P₂; and dWIP, a protein that binds activated Wiskott-Aldrich syndrome protein (WASP), which stimulates F-actin polymerization. We show that Tre1 is required for polarized accumulation of F-actin, PI(4,5)P₂, and dPIP5K. Smoothened also localizes with F-actin at the migration front, and Hh, through Smo, increases levels of Tre1 at the plasma membrane and Tre1’s association with dPIP5K.

Kim et al. uncover molecular and cellular events upstream and downstream of the Tre1 G-protein coupled receptor (GPCR), which is required for germ cell navigation in Drosophila. Hedgehog signaling through Smoothened localizes Tre1 to activate F-actin assembly through dPIP5K, PI(4,5)P₂, and WASP.

Quercetin and lithium chloride potentiate the protective effects of carvedilol against renal ischemia-reperfusion injury in high-fructose, high-fat diet-fed Swiss albino mice independent of renal lipid signaling

Renal ischemia-reperfusion injury (R-IRI) is the main cause of acute renal failure. Carvedilol has been shown to protect against R-IRI. However, the underlying mechanisms are still not completely clarified. This study aimed to investigate the role of lipid signaling in mediating carvedilol protective effects against R-IRI in insulin-resistant mice by using two different lipid signaling modulators, quercetin and lithium chloride (LiCl). Mice were fed high-fructose, high-fat diet (HFrHFD) for 16 weeks to induce insulin resistance. At the end of feeding period, mice were randomly distributed into five groups; Sham, R-IRI, Carvedilol (20 mg/kg, i.p.), Carvedilol + Quercetin (10 mg/kg, i.p.), Carvedilol + LiCl (200 mg/kg, i.p.). R-IRI was performed by applying 30 min of unilateral renal ischemia followed by one hour of reperfusion. Quercetin and LiCl were administered 30 min before carvedilol administration and carvedilol was administered 30 min before ischemia. Changes in kidney function tests, histopathology, fibrosis area, lipid signaling, inflammatory, apoptosis and oxidative stress markers in the kidney were measured. Results showed that R-IRI decreased kidney function, impaired renal tissue integrity, modulated lipid signaling and increased renal inflammation, apoptosis and oxidative stress. Carvedilol treatment decreased the detrimental effects induced by R-IRI. In addition, pre-injection of both quercetin and LiCl potentiated the reno-protective effects of carvedilol against R-IRI independent of changes in lipid mediators like phosphatidyl inositol 4,5 bisphosphate (PIP2) and diacylglycerol (DAG). In conclusion, quercetin and LiCl potentiate the protective effects of carvedilol against R-IRI in HFrHFD-fed mice by reducing inflammation and oxidative stress independent of lipid signaling.

Metabolic Deregulation of the Blood-Outer Retinal Barrier in Retinitis Pigmentosa

Retinitis pigmentosa (RP) initiates with diminished rod photoreceptor function, causing peripheral and nighttime vision loss. However, subsequent loss of cone function and high-resolution daylight and color vision is most debilitating. Visual pigment-rich photoreceptor outer segments (OS) undergo phagocytosis by the retinal pigment epithelium (RPE), and the RPE also acts as a blood-outer retinal barrier transporting nutrients, including glucose, to photoreceptors. We provide evidence that contact between externalized phosphatidylserine (PS) on OS tips and apical RPE receptors activates Akt, linking phagocytosis with glucose transport to photoreceptors for new OS synthesis. As abundant mutant rod OS tips shorten in RP, Akt activation is lost, and onset of glucose metabolism in the RPE and diminished glucose transport combine to cause photoreceptor starvation and accompanying retinal metabolome changes. Subretinal injection of OS tip mimetics displaying PS restores Akt activation, glucose transport, and cone function in end-stage RP after rods are lost.

Wang et al. show that onset of glucose metabolism in the retinal pigment epithelium (RPE), which acts as the blood-outer retinal barrier, and inhibition of RPE glucose transport to photoreceptors combine to cause photoreceptor starvation and vision loss in retinitis pigmentosa.

Phosphatidylinositol-4-phosphate 5-kinase type 1α attenuates Aβ production by promoting non-amyloidogenic processing of amyloid precursor protein

Alzheimer's disease (AD) is characterized by a chronic decline in cognitive function and is pathologically typified by cerebral deposition of amyloid-β peptide (Aβ). The production of Aβ is mediated by sequential proteolysis of amyloid precursor protein (APP) by β- and γ-secretases, and has been implicated as the essential determinant of AD pathology. Previous studies have demonstrated that the level of phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] in the membrane may potentially modulate Aβ production. Given that PI(4,5)P2 is produced by type 1 phosphatidylinositol-4-phosphate 5-kinases (PIP5Ks), we sought to determine whether the level of PIP5K type Iα (PIP5K1A) can affect production of Aβ by modulating the lipid composition of the membrane. Using a HEK-derived cell line that constitutively expresses yellow fluorescent protein-tagged APP (APP-YFP), we demonstrated that overexpression of PIP5K1A results in significant enhancement of non-amyloidogenic APP processing and a concomitant suppression of the amyloidogenic pathway, leading to a marked decrease in secreted Aβ. Consistently, cells overexpressing PIP5K1A exhibited a significant redistribution of APP-YFP from endosomal compartments to the cell surface. Our findings suggest that PIP5K1A may play a critical role in governing Aβ production by modulating membrane distribution of APP, and as such, the pathway may be a valuable therapeutic target for AD.

Functional profiles of phycospheric microorganisms during a marine dinoflagellate bloom

Harmful algal blooms (HABs) are an ecological concern but relatively few studies have investigated the functional potential of bacterioplankton over a complete algal bloom cycle, which is critical for determining their contribution to the fate of algal blooms. To address this point, we carried out a time-series metagenomic analysis of the functional features of microbial communities at three different Gymnodinium catenatum bloom stages (pre-, peak-, and post-bloom). Different microbial composition were observed during the blooming stages. The environmental parameters and correlation networks co-contribute to microbial variability, and the former explained 38.4% of total variations of the bacterioplankton community composition. Functionally, a range of pathways involved in carbon, nitrogen, phosphorus and sulfur cycling were significantly different during the various HAB stages. Genes associated with carbohydrate-active enzymes, denitrification, and iron oxidation were enriched at the pre-bloom stage; genes involved in reductive citrate cycle for carbon fixation, carbon degradation, nitrification and phosphate transport were enhanced at the peak stage; and relative gene abundance related to sulfur oxidation, vitamin synthesis, and iron transport and storage was increased at the post-bloom stage. The ecological linkage analysis has shown that microbial functional potential especially the C/P/Fe metabolism were significantly linked to the fate of the algal blooms. Taken together, our results demonstrated that microorganisms displayed successional patterns not only at the community level, but also in the metabolic potential on HAB's progression. This work contributes to a growing understanding of microbial structural elasticity and functional plasticity and shed light on the potential mechanisms of microbial-mediated HAB trajectory.

Carvedilol Diminishes Cardiac Remodeling Induced by High-Fructose/High-Fat Diet in Mice via Enhancing Cardiac β-Arrestin2 Signaling

BACKGROUND

Insulin resistance (IR) is a well-known risk factor for cardiovascular complications. This study aimed to investigate the effect of a dietary model of IR in mice on cardiac remodeling, cardiac β-arrestin2 signaling, and the protective effects of carvedilol as a β-arrestin-biased agonist.

METHODS AND RESULTS

Insulin resistance was induced by feeding mice high-fructose/high-fat diet (HFrHFD) for 16 weeks. Carvedilol was adiministered for 4 weeks starting at week 13. At the end of the experiment, body weight, heart weight, left and right ventricular thickness, visceral fat weight, fasting blood glucose (FBG), serum insulin, IR index, and serum endothelin-1 were measured. In addition, cardiac tissue samples were histopathologically examined. Also, cardiac levels of cardiotrophin-1, β-arrestin2, phosphatidylinositol 4,5 bisphosphate (PIP2), diacylglycerol (DAG), and phosphoserine 473 Akt (pS473 Akt) were measured. Results showed significant increases in the FBG, serum insulin, IR index, serum endothelin-1, cardiac DAG, cardiac fibrosis, and degenerated cardiac myofibrils in HFrHFD-fed mice associated with a significant reduction in cardiac levels of cardiotrophin-1, β-arrestin2, PIP2, and pS473 Akt. On the other hand, carvedilol significantly reduced the heart weight, FBG, serum insulin, IR index, serum endothelin-1, cardiac DAG, left ventricular thickness, right ventricular fibrosis, and degeneration of cardiac myofibrils. In addition, carvedilol significantly increased cardiac levels of cardiotrophin-1, β-arrestin2, PIP2, and pS473 Akt.

CONCLUSION

Carvedilol enhances cardiac β-arrestin2 signaling and reduces cardiac remodeling in HFrHFD-fed mice.

Phospholipids: Pulling Back the Actin Curtain for Granule Delivery to the Immune Synapse

Phosphoinositides, together with the phospholipids phosphatidylserine and phosphatidic acid, are important components of the plasma membrane acting as second messengers that, with diacylglycerol, regulate a diverse range of signaling events converting extracellular changes into cellular responses. Local changes in their distribution and membrane charge on the inner leaflet of the plasma membrane play important roles in immune cell function. Here we discuss their distribution and regulators highlighting the importance of membrane changes across the immune synapse on the cytoskeleton and the impact on the function of cytotoxic T lymphocytes.

The Diverse Roles of Arrestin Scaffolds in G Protein-Coupled Receptor Signaling

The visual/β-arrestins, a small family of proteins originally described for their role in the desensitization and intracellular trafficking of G protein-coupled receptors (GPCRs), have emerged as key regulators of multiple signaling pathways. Evolutionarily related to a larger group of regulatory scaffolds that share a common arrestin fold, the visual/β-arrestins acquired the capacity to detect and bind activated GPCRs on the plasma membrane, which enables them to control GPCR desensitization, internalization, and intracellular trafficking. By acting as scaffolds that bind key pathway intermediates, visual/β-arrestins both influence the tonic level of pathway activity in cells and, in some cases, serve as ligand-regulated scaffolds for GPCR-mediated signaling. Growing evidence supports the physiologic and pathophysiologic roles of arrestins and underscores their potential as therapeutic targets. Circumventing arrestin-dependent GPCR desensitization may alleviate the problem of tachyphylaxis to drugs that target GPCRs, and find application in the management of chronic pain, asthma, and psychiatric illness. As signaling scaffolds, arrestins are also central regulators of pathways controlling cell growth, migration, and survival, suggesting that manipulating their scaffolding functions may be beneficial in inflammatory diseases, fibrosis, and cancer. In this review we examine the structure-function relationships that enable arrestins to perform their diverse roles, addressing arrestin structure at the molecular level, the relationship between arrestin conformation and function, and sites of interaction between arrestins, GPCRs, and nonreceptor-binding partners. We conclude with a discussion of arrestins as therapeutic targets and the settings in which manipulating arrestin function might be of clinical benefit.

Contributions of protein kinases and β-arrestin to termination of protease-activated receptor 2 signaling

Systematic imaging studies and modeling reveal new details of the regulation of the G_q-coupled GPCR, protease-activated receptor 2, by phosphorylation and β-arrestin.

Activated G_q protein–coupled receptors (G_qPCRs) can be desensitized by phosphorylation and β-arrestin binding. The kinetics and individual contributions of these two mechanisms to receptor desensitization have not been fully distinguished. Here, we describe the shut off of protease-activated receptor 2 (PAR2). PAR2 activates G_q and phospholipase C (PLC) to hydrolyze phosphatidylinositol 4,5-bisphosphate (PIP₂) into diacylglycerol and inositol trisphosphate (IP₃). We used fluorescent protein–tagged optical probes to monitor several consequences of PAR2 signaling, including PIP₂ depletion and β-arrestin translocation in real time. During continuous activation of PAR2, PIP₂ was depleted transiently and then restored within a few minutes, indicating fast receptor activation followed by desensitization. Knockdown of β-arrestin 1 and 2 using siRNA diminished the desensitization, slowing PIP₂ restoration significantly and even adding a delayed secondary phase of further PIP₂ depletion. These effects of β-arrestin knockdown on PIP₂ recovery were prevented when serine/threonine phosphatases that dephosphorylate GPCRs were inhibited. Thus, PAR2 may continuously regain its activity via dephosphorylation when there is insufficient β-arrestin to trap phosphorylated receptors. Similarly, blockers of protein kinase C (PKC) and G protein–coupled receptor kinase potentiated the PIP₂ depletion. In contrast, an activator of PKC inhibited receptor activation, presumably by augmenting phosphorylation of PAR2. Our interpretations were strengthened by modeling. Simulations supported the conclusions that phosphorylation of PAR2 by protein kinases initiates receptor desensitization and that recruited β-arrestin traps the phosphorylated state of the receptor, protecting it from phosphatases. Speculative thinking suggested a sequestration of phosphatidylinositol 4-phosphate 5 kinase (PIP5K) to the plasma membrane by β-arrestin to explain why knockdown of β-arrestin led to secondary depletion of PIP₂. Indeed, artificial recruitment of PIP5K removed the secondary loss of PIP₂ completely. Altogether, our experimental and theoretical approaches demonstrate roles and dynamics of the protein kinases, β-arrestin, and PIP5K in the desensitization of PAR2.

β-arrestins and G protein-coupled receptor trafficking

Nonvisual arrestins (β-arrestin-1 and β-arrestin-2) are adaptor proteins that function to regulate G protein-coupled receptor (GPCR) signaling and trafficking. β-arrestins are ubiquitously expressed and function to inhibit GPCR/G protein coupling, a process called desensitization, and promote GPCR trafficking and arrestin-mediated signaling. β-arrestin-mediated endocytosis of GPCRs requires the coordinated interaction of β-arrestins with clathrin, adaptor protein 2 (AP2), and phosphoinositides. These interactions are facilitated by a conformational change in β-arrestin that is thought to occur upon binding to a phosphorylated activated GPCR. In this review, we provide an overview of the key interactions involved in β-arrestin-mediated trafficking of GPCRs.

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.

β-arrestin promotes Wnt-induced low density lipoprotein receptor-related protein 6 (Lrp6) phosphorylation via increased membrane recruitment of Amer1 protein

β-Arrestin is a scaffold protein that regulates signal transduction by seven transmembrane-spanning receptors. Among other functions it is also critically required for Wnt/β-catenin signal transduction. In the present study we provide for the first time a mechanistic basis for the β-arrestin function in Wnt/β-catenin signaling. We demonstrate that β-arrestin is required for efficient Wnt3a-induced Lrp6 phosphorylation, a key event in downstream signaling. β-Arrestin regulates Lrp6 phosphorylation via a novel interaction with phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2)-binding protein Amer1/WTX/Fam123b. Amer1 has been shown very recently to bridge Wnt-induced and Dishevelled-associated PtdIns(4,5)P2 production to the phosphorylation of Lrp6. Using fluorescence recovery after photobleaching we show here that β-arrestin is required for the Wnt3a-induced Amer1 membrane dynamics and downstream signaling. Finally, we show that β-arrestin interacts with PtdIns kinases PI4KIIα and PIP5KIβ. Importantly, cells lacking β-arrestin showed higher steady-state levels of the relevant PtdInsP and were unable to increase levels of these PtdInsP in response to Wnt3a. In summary, our data show that β-arrestins regulate Wnt3a-induced Lrp6 phosphorylation by the regulation of the membrane dynamics of Amer1. We propose that β-arrestins via their scaffolding function facilitate Amer1 interaction with PtdIns(4,5)P2, which is produced locally upon Wnt3a stimulation by β-arrestin- and Dishevelled-associated kinases.

β-arrestin-selective G protein-coupled receptor agonists engender unique biological efficacy in vivo

Biased G protein-coupled receptor agonists are orthosteric ligands that possess pathway-selective efficacy, activating or inhibiting only a subset of the signaling repertoire of their cognate receptors. In vitro, D-Trp(12),Tyr(34)-bPTH(7-34) [bPTH(7-34)], a biased agonist for the type 1 PTH receptor, antagonizes receptor-G protein coupling but activates arrestin-dependent signaling. In vivo, both bPTH(7-34) and the conventional agonist hPTH(1-34) stimulate anabolic bone formation. To understand how two PTH receptor ligands with markedly different in vitro efficacy could elicit similar in vivo responses, we analyzed transcriptional profiles from calvarial bone of mice treated for 8 wk with vehicle, bPTH(7-34) or hPTH(1-34). Treatment of wild-type mice with bPTH(7-34) primarily affected pathways that promote expansion of the osteoblast pool, notably cell cycle regulation, cell survival, and migration. These responses were absent in β-arrestin2-null mice, identifying them as downstream targets of β-arrestin2-mediated signaling. In contrast, hPTH(1-34) primarily affected pathways classically associated with enhanced bone formation, including collagen synthesis and matrix mineralization. hPTH(1-34) actions were less dependent on β-arrestin2, as might be expected of a ligand capable of G protein activation. In vitro, bPTH(7-34) slowed the rate of preosteoblast proliferation, enhanced osteoblast survival when exposed to an apoptotic stimulus, and stimulated cell migration in wild-type, but not β-arrestin2-null, calvarial osteoblasts. These results suggest that bPTH(7-34) and hPTH(1-34) affect bone mass in vivo through predominantly separate genomic mechanisms created by largely distinct receptor-signaling networks and demonstrate that functional selectivity can be exploited to change the quality of G protein-coupled receptor efficacy.

Arrestins as regulators of kinases and phosphatases

The discovery that, in addition to mediating G protein-coupled receptor (GPCR) desensitization and endocytosis, arrestins bind to diverse catalytically active nonreceptor proteins and act as ligand-regulated signaling scaffolds led to a paradigm shift in the study of GPCR signal transduction. Research over the past decade has solidified the concept that arrestins confer novel GPCR-signaling capacity by recruiting protein and lipid kinase, phosphatase, phosphodiesterase, and ubiquitin ligase activity into receptor-based multiprotein "signalsome" complexes. Signalsomes regulate downstream pathways controlled by Src family nonreceptor tyrosine kinases, mitogen-activated protein kinases, protein kinase B (AKT), glycogen synthase kinase 3, protein phosphatase 2A, nuclear factor-κB, and several others, imposing spatial and temporal control on their function. While many arrestin-bound kinases and phosphatases are involved in the control of cytoskeletal rearrangement, vesicle endocytosis, exocytosis, and cell migration, other signals reach into the nucleus, affecting cell proliferation, apoptosis, and survival. Indeed, the kinase/phosphatase network regulated by arrestins may be fully as diverse as that regulated by heterotrimeric G proteins.

Arrestins: role in the desensitization, sequestration, and vesicular trafficking of G protein-coupled receptors

Over the years, β-arrestins have emerged as multifunctional molecular scaffolding proteins regulating almost every imaginable G protein-coupled receptor (GPCR) function. Originally discovered as GPCR-desensitizing molecules, they have been shown to also serve as important regulators of GPCR signaling, sequestration, and vesicular trafficking. This broad functional role implicates β-arrestins as key regulatory proteins for cellular function. Hence, this chapter summarizes the current understanding of the β-arrestin family's unique ability to control the kinetics as well as the extent of GPCR activity at the level of desensitization, sequestration, and subsequent intracellular trafficking.

The very large G protein coupled receptor (Vlgr1) in hair cells

The very large G protein coupled receptor (Vlgr1) is a member of adhesion receptors or large N-terminal family B-7 transmembrane helixes (LNB7TM) receptors within the seven trans-membrane receptor superfamily. Vlgr1 is the largest GPCR identified to date; its mRNA spans 19 kb and encodes 6,300 amino acids. Vlgr1 is a core component of ankle-link complex in inner ear hair cells. Knock-out and mutation mouse models show that loss of Vlgr1 function leads to abnormal stereociliary development and hearing loss, indicating crucial roles of Vlgr1 in hearing transduction or auditory system development. Over the past 10 or so years, human genetics data suggested that Vlgr1 mutations cause Usher syndromes and seizures. Although significant progresses have been made, the details of Vlgr1's function in hair cells, its signaling cascade, and the mechanisms underlying causative effects of Vlgr1 mutations in human diseases remain elusive and ask for further investigation.

Ubiquitin-mediated regulation of endocytosis by proteins of the arrestin family

In metazoans, proteins of the arrestin family are key players of G-protein-coupled receptors (GPCRS) signaling and trafficking. Following stimulation, activated receptors are phosphorylated, thus allowing the binding of arrestins and hence an “arrest” of receptor signaling. Arrestins act by uncoupling receptors from G proteins and contribute to the recruitment of endocytic proteins, such as clathrin, to direct receptor trafficking into the endocytic pathway. Arrestins also serve as adaptor proteins by promoting the recruitment of ubiquitin ligases and participate in the agonist-induced ubiquitylation of receptors, known to have impact on their subcellular localization and stability. Recently, the arrestin family has expanded following the discovery of arrestin-related proteins in other eukaryotes such as yeasts or fungi. Surprisingly, most of these proteins are also involved in the ubiquitylation and endocytosis of plasma membrane proteins, thus suggesting that the role of arrestins as ubiquitin ligase adaptors is at the core of these proteins' functions. Importantly, arrestins are themselves ubiquitylated, and this modification is crucial for their function. In this paper, we discuss recent data on the intricate connections between arrestins and the ubiquitin pathway in the control of endocytosis.

β2-adrenergic receptor and astrocyte glucose metabolism

Astrocyte glucose metabolism functions to maintain brain activity in both normal and stress conditions. Dysregulation of astrocyte glucose metabolism relates to development of neuronal disease, such as multiple sclerosis and Alzheimer's disease. In response to acute stress, beta2-adrenergic receptor is activated and initiates multiple signaling events mediated by Gs, Gi, arrestin, or other effectors depending on specific cellular contexts. In astrocytes, beta2-adrenergic receptor promotes glucose uptake through GLUT1 and accelerates glycogen degradation via coupling to Gs and second messenger cAMP-dependent pathway. Beta2-adrenergic receptor may regulate other steps in astrocyte glucose metabolism, such as lactate production or transduction. Inappropriate regulation of beta2-adrenergic receptor activity can disrupt normal glucose metabolism, and leads to accelerate neuronal disease development. It was demonstrated that the absence of beta2-adrenergic receptor in astrocytes occurred in multiple sclerosis patients, and the increased beta2-adrenergic receptor activity relates to Alzheimer's disease. A clear view of beta2-adrenergic receptor-mediated signaling pathways in regulating astrocyte glucose metabolism could help us to develop neuronal diseases treatment by targeting to the beta2-adrenergic receptor.

Phosphatidylinositol 4, 5 bisphosphate and the actin cytoskeleton

Dynamic changes in PM PIP(2) have been implicated in the regulation of many processes that are dependent on actin polymerization and remodeling. PIP(2) is synthesized primarily by the type I phosphatidylinositol 4 phosphate 5 kinases (PIP5Ks), and there are three major isoforms, called a, b and g. There is emerging evidence that these PIP5Ks have unique as well as overlapping functions. This review will focus on the isoform-specific roles of individual PIP5K as they relate to the regulation of the actin cytoskeleton. We will review recent advances that establish PIP(2) as a critical regulator of actin polymerization and cytoskeleton/membrane linkages, and show how binding of cytoskeletal proteins to membrane PIP(2) might alter lateral or transverse movement of lipids to affect raft formation or lipid asymmetry. The mechanisms for specifying localized increase in PIP(2) to regulate dynamic actin remodeling will also be discussed.

Role of phosphoinositides at the neuronal synapse

Synaptic transmission is amongst the most sophisticated and tightly controlled biological phenomena in higher eukaryotes. In the past few decades, tremendous progress has been made in our understanding of the molecular mechanisms underlying multiple facets of neurotransmission, both pre- and postsynaptically. Brought under the spotlight by pioneer studies in the areas of secretion and signal transduction, phosphoinositides and their metabolizing enzymes have been increasingly recognized as key protagonists in fundamental aspects of neurotransmission. Not surprisingly, dysregulation of phosphoinositide metabolism has also been implicated in synaptic malfunction associated with a variety of brain disorders. In the present chapter, we summarize current knowledge on the role of phosphoinositides at the neuronal synapse and highlight some of the outstanding questions in this research field.

Functional relevance of biased signaling at the angiotensin II type 1 receptor

Angiotensin II type 1 receptor antagonists (AT1R blockers, or ARBs) are used commonly in the treatment of cardiovascular disorders such as heart failure and hypertension. Their clinical success arises from their ability to prevent deleterious Gα(q) protein activation downstream of AT1R, which leads to a decrease in morbidity and mortality. Recent studies have identified AT1R ligands that concurrently inhibit Gα(q) protein-dependent signaling and activate Gα(q) protein-independent/β-arrestin-dependent signaling downstream of AT1R, events that may actually improve cardiovascular performance more than conventional ARBs. The ability of such ligands to induce intracellular signaling events in an AT1R-β-arrestin-dependent manner while preventing AT1R-Gα(q) protein activity defines them as biased AT1R ligands. This mini-review will highlight recent studies that have defined biased signaling at the AT1R and discuss the possible clinical relevance of β-arrestin-biased AT1R ligands in the cardiovascular system.

β-arrestin-dependent actin reorganization: bringing the right players together at the leading edge

First identified as mediators of G-protein-coupled receptor desensitization and internalization and later as signaling platforms, β-arrestins play a requisite role in chemotaxis and reorganization of the actin cytoskeleton, downstream of multiple receptors. However, the precise molecular mechanisms underlying their involvement have remained elusive. Initial interest in β-arrestins as facilitators of cell migration and actin reorganization stemmed from the known interplay between receptor endocytosis and actin filament formation, because disruption of the actin cytoskeleton inhibits these β-arrestin-dependent events. With growing interest in the mechanisms by which cells can sense a gradient of agonist during cell migration, investigators began to hypothesize that β-arrestins may contribute to directed migration by controlling chemotactic receptor turnover at the plasma membrane. Finally, increasing evidence emerged that β-arrestins are more than just clathrin adaptor proteins involved in turning off receptor signals; they are actually capable of generating their own signals by scaffolding signaling molecules and controlling the activity of multiple cellular enzymes. This new role of β-arrestins as signaling scaffolds has led to the hypothesis that they can facilitate cell migration by sequestering actin assembly activities and upstream regulators of actin assembly at the leading edge. This Minireview discusses recent advances in our understanding of how β-arrestin scaffolds contribute to cell migration, focusing on recently identified β-arrestin interacting proteins and phosphorylation targets that have known roles in actin reorganization.

β-Arrestin-mediated receptor trafficking and signal transduction

β-Arrestins function as endocytic adaptors and mediate trafficking of a variety of cell-surface receptors, including seven-transmembrane receptors (7TMRs). In the case of 7TMRs, β-arrestins carry out these tasks while simultaneously inhibiting upstream G-protein-dependent signaling and promoting alternate downstream signaling pathways. The mechanisms by which β-arrestins interact with a continuously expanding ensemble of protein partners and perform their multiple functions including trafficking and signaling are currently being uncovered. Molecular changes at the level of protein conformation as well as post-translational modifications of β-arrestins probably form the basis for their dynamic interactions during receptor trafficking and signaling. It is becoming increasingly evident that β-arrestins, originally discovered as 7TMR adaptor proteins, indeed have much broader and more versatile roles in maintaining cellular homeostasis. In this review paper, we assess the traditional and novel functions of β-arrestins and discuss the molecular attributes that might facilitate multiple interactions in regulating cell signaling and receptor trafficking.

Beyond desensitization: physiological relevance of arrestin-dependent signaling

Heptahelical G protein-coupled receptors are the most diverse and therapeutically important family of receptors in the human genome. Ligand binding activates heterotrimeric G proteins that transmit intracellular signals by regulating effector enzymes or ion channels. G protein signaling is terminated, in large part, by arrestin binding, which uncouples the receptor and G protein and targets the receptor for internalization. It is clear, however, that heptahelical receptor signaling does not end with desensitization. Arrestins bind a host of catalytically active proteins and serve as ligand-regulated scaffolds that recruit protein and lipid kinase, phosphatase, phosphodiesterase, and ubiquitin ligase activity into the receptor-arrestin complex. Although many of these arrestin-bound effectors serve to modulate G protein signaling, degrading second messengers and regulating endocytosis and trafficking, other signals seem to extend beyond the receptor-arrestin complex to regulate such processes as protein translation and gene transcription. Although these findings have led to a re-envisioning of heptahelical receptor signaling, little is known about the physiological roles of arrestin-dependent signaling. In vivo, the duality of arrestin function makes it difficult to dissociate the consequences of arrestin-dependent desensitization from those that might be ascribed to arrestin-mediated signaling. Nonetheless, recent evidence generated using arrestin knockouts, G protein-uncoupled receptor mutants, and arrestin pathway-selective "biased agonists" is beginning to reveal that arrestin signaling plays important roles in the retina, central nervous system, cardiovascular system, bone remodeling, immune system, and cancer. Understanding the signaling roles of arrestins may foster the development of pathway-selective drugs that exploit these pathways for therapeutic benefit.

Arrestin development: emerging roles for beta-arrestins in developmental signaling pathways

Arrestins were identified as mediators of G protein-coupled receptor (GPCR) desensitization and endocytosis. However, it is now clear that they scaffold many intracellular signaling networks to modulate the strength and duration of signaling by diverse types of receptors--including those relevant to the Hedgehog, Wnt, Notch, and TGFbeta pathways--and downstream kinases such as the MAPK and Akt/PI3K cascades. The involvement of arrestins in many discrete developmental signaling events suggests an indispensable role for these multifaceted molecular scaffolds.

Regulators of G protein signaling proteins as central components of G protein-coupled receptor signaling complexes

The regulators of G protein signaling (RGS) proteins bind directly to G protein alpha (Gα) subunits to regulate the signaling functions of Gα and their linked G protein-coupled receptors (GPCRs). Recent studies indicate that RGS proteins also interact with GPCRs, not just G proteins, to form preferred functional pairs. Interactions between GPCRs and RGS proteins may be direct or indirect (via a linker protein) and are dictated by the receptors, rather than the linked G proteins. Emerging models suggest that GPCRs serve as platforms for assembling an overlapping and distinct constellation of signaling proteins that perform receptor-specific signaling tasks. Compelling evidence now indicates that RGS proteins are central components of these GPCR signaling complexes. This review will outline recent discoveries of GPCR/RGS pairs as well as new data in support of the idea that GPCRs serve as platforms for the formation of multiprotein signaling complexes.

Mammalian phosphoinositide kinases and phosphatases

Phosphoinositides are lipids that are present in the cytoplasmic leaflet of a cell's plasma and internal membranes and play pivotal roles in the regulation of a wide variety of cellular processes. Phosphoinositides are molecularly diverse due to variable phosphorylation of the hydroxyl groups of their inositol rings. The rapid and reversible configuration of the seven known phosphoinositide species is controlled by a battery of phosphoinositide kinases and phosphoinositide phosphatases, which are thus critical for phosphoinositide isomer-specific localization and functions. Significantly, a given phosphoinositide generated by different isozymes of these phosphoinositide kinases and phosphatases can have different biological effects. In mammals, close to 50 genes encode the phosphoinositide kinases and phosphoinositide phosphatases that regulate phosphoinositide metabolism and thus allow cells to respond rapidly and effectively to ever-changing environmental cues. Understanding the distinct and overlapping functions of these phosphoinositide-metabolizing enzymes is important for our knowledge of both normal human physiology and the growing list of human diseases whose etiologies involve these proteins. This review summarizes the structural and biological properties of all the known mammalian phosphoinositide kinases and phosphoinositide phosphatases, as well as their associations with human disorders.

beta-Arrestin mediates beta1-adrenergic receptor-epidermal growth factor receptor interaction and downstream signaling

beta1-Adrenergic receptor (beta1AR) stimulation confers cardioprotection via beta-arrestin-de pend ent transactivation of epidermal growth factor receptors (EGFRs), however, the precise mechanism for this salutary process is unknown. We tested the hypothesis that the beta1AR and EGFR form a complex that differentially directs intracellular signaling pathways. beta1AR stimulation and EGF ligand can each induce equivalent EGFR phosphorylation, internalization, and downstream activation of ERK1/2, but only EGF ligand causes translocation of activated ERK to the nucleus, whereas beta1AR-stimulated/EGFR-transactivated ERK is restricted to the cytoplasm. beta1AR and EGFR are shown to interact as a receptor complex both in cell culture and endogenously in human heart, an interaction that is selective and undergoes dynamic regulation by ligand stimulation. Although catecholamine stimulation mediates the retention of beta1AR-EGFR interaction throughout receptor internalization, direct EGF ligand stimulation initiates the internalization of EGFR alone. Continued interaction of beta1AR with EGFR following activation is dependent upon C-terminal tail GRK phosphorylation sites of the beta1AR and recruitment of beta-arrestin. These data reveal a new signaling paradigm in which beta-arrestin is required for the maintenance of a beta1AR-EGFR interaction that can direct cytosolic targeting of ERK in response to catecholamine stimulation.

Clathrin regulates the association of PIPKIgamma661 with the AP-2 adaptor beta2 appendage

The AP-2 clathrin adaptor differs fundamentally from the related AP-1, AP-3, and AP-4 sorting complexes because membrane deposition does not depend directly on an Arf family GTPase. Instead phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) appears to act as the principal compartmental cue for AP-2 placement at the plasma membrane as well as for the docking of numerous other important clathrin coat components at the nascent bud site. This PtdIns(4,5)P(2) dependence makes type I phosphatidylinositol 4-phosphate 5-kinases (PIPKIs) lynchpin enzymes in the assembly of clathrin-coated structures at the cell surface. PIPKIgamma is the chief 5-kinase at nerve terminals, and here we show that the 26-amino acid, alternatively spliced C terminus of PIPKIgamma661 is an intrinsically unstructured polypeptide that binds directly to the sandwich subdomain of the AP-2 beta2 subunit appendage. An aromatic side chain-based, extended interaction motif that also includes the two bulky C-terminal residues of the short PIPKIgamma635 variant is necessary for beta2 appendage engagement. The clathrin heavy chain accesses the same contact surface on the AP-2 beta2 appendage, but because of additional clathrin binding sites located within the unstructured hinge segment of the beta2 subunit, clathrin binds the beta2 chain with a higher apparent affinity than PIPKIgamma661. A clathrin-regulated interaction with AP-2 could allow PIPKIgamma661 to be strategically positioned for regional PtdIns(4,5)P(2) generation during clathrin-coated vesicle assembly at the synapse.

Knockout of spinophilin, an endogenous antagonist of arrestin-dependent alpha2-adrenoceptor functions, enhances receptor-mediated antinociception yet does not eliminate sex-related differences

We have previously shown gonadal steroid-dependent, gender specific modulation of nociception by alpha(2)-adrenoceptors. Agonist activation of the receptor enhances its association with spinophilin that antagonizes arrestin functions both by diminishing receptor phosphorylation by G-protein-coupled receptor kinase 2 (GRK2) and by competing for receptor interactions with arrestin. Since spinophilin is highly enriched in dendritic spines, we investigated whether alpha(2)-adrenoceptor-induced antinociception as well as sex-related differences are modified in spinophilin knockout mice. We evaluated alpha(2)-adrenoceptor antinociception in a heat-evoked tail flick test in spinophilin wild type (Sp(+/+)) and knockout (Sp(-/-)) mice. Baseline tail flick latencies (TFLs) did not change between any groups. Interestingly, the alpha(2)-adrenoceptor agonist, clonidine, increased TFL in male and diestrous (low estrogen) Sp(-/-) as well as Sp(+/+) mice; in fact, this increase in TFL was significantly higher in Sp(-/-) male and diestrous groups than in their Sp(+/+) counterparts. This unexpected finding is consistent with enhanced alpha(2)-adrenoceptor-mediated sedation observed previously in Sp(-/-) mice, presumably due to accelerated endocytosis of desensitized receptors and recycling of refreshed receptors when arrestin is not competed for by spinophilin in Sp(-/-) mice. Despite modulation of alpha(2)-adrenoceptor effects in Sp(-/-) mice, sex-related differences were retained; thus, clonidine was ineffective in proestrous females (highest estrogen levels), in both Sp(-/-) and Sp(+/+) mice, reaffirming that estrogen suppresses alpha(2)-adrenoceptor-evoked antinociception. These findings show that elimination of spinophilin enhances alpha(2)-adrenoceptor-evoked antinociception in estrogen-deprived physiological settings, suggesting a role for spinophilin to suppress these effects, and yet this enhanced response cannot overcome the absence of antinociception with elevated estrogen levels.