The zeta isoform of 14-3-3 proteins interacts with the third intracellular loop of different alpha2-adrenergic receptor subtypes

Modulating GPCR and 14-3-3 protein interactions: Prospects for CNS drug discovery

The activation of G-protein-coupled receptors (GPCRs) triggers a series of protein-protein interaction events that subsequently induce a chain of reactions, including alteration of receptor structures, phosphorylation, recruitment of associated proteins, protein trafficking and gene expression. Multiple GPCR signaling transduction pathways are evident - two well-studied pathways are the GPCR-mediated G-protein and β-arrestin pathways. Recently, ligand-induced interactions between GPCRs and 14-3-3 proteins have been demonstrated. This linking of GPCRs to 14-3-3 protein signal hubs opens up a whole new realm of signal transduction possibilities. 14-3-3 proteins play a key part in GPCR trafficking and signal transduction. GPCR-mediated 14-3-3 protein signaling can be harnessed for the study of GPCR function and therapeutics.

Dexmedetomidine and acute kidney injury following cardiac surgery in pediatric patients—An updated systematic review and meta-analysis

Background

Acute kidney injury (AKI) is a common postoperative complication in pediatric patients undergoing cardiac surgery and associated with poor outcomes. Dexmedetomidine has the pharmacological features of organ protection in cardiac surgery patients. The aim of this meta-analysis is to investigate the effect of dexmedetomidine infusion on the incidence of AKI after cardiac surgery in pediatric patients.

Methods

The databases of Pubmed, Embase, and Cochrane Library were searched until April 24, 2022 following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. RevMan 5.3 was used to perform statistical analyses.

Results

Five relevant trials with a total of 630 patients were included. The pooled result using fixed-effects model with OR demonstrated significant difference in the incidence of AKI between patients with dexmedetomidine and placebo (OR = 0.49, 95% CI: [0.33, 0.73], I² = 0%, p for effect = 0.0004). Subgroup analyses were performed based on congenital heart disease (CHD) types and dexmedetomidine intervention time. Pooled results did not demonstrate considerable difference in the incidence of AKI in pediatric patients receiving intraoperative (OR = 0.53, 95% CI: [0.29, 0.99], I² = 0%, p for effect = 0.05) or postoperative dexmedetomidine infusion (OR = 0.56, 95% CI: [0.31, 1.04], p for effect = 0.07), but a significant difference in patients receiving combination of intra- and postoperative dexmedetomidine infusion (OR = 0.27, 95% CI: [0.09, 0.77], p for effect = 0.01). Besides, there was no significant difference in duration of mechanical ventilation (SMD: –0.19, 95% CI: –0.46 to 0.08, p for effect = 0.16; SMD: –0.16, 95% CI: –0.37 to 0.06, p for effect = 0.15), length of ICU (SMD: 0.02, 95% CI: –0.41 to 0.44, p for effect = 0.93) and hospital stay (SMD: 0.2, 95% CI: –0.13 to 0.54, p for effect = 0.23), and in-hospital mortality (OR = 1.26, 95% CI: 0.33–4.84, p for effect = 0.73) after surgery according to the pooled results of the secondary outcomes.

Conclusion

Compared to placebo, dexmedetomidine could significantly reduce the postoperative incidence of AKI in pediatric patients undergoing cardiac surgery with cardiopulmonary bypass (CPB), but the considerable difference was reflected in the pediatric patients receiving combination of intra- and postoperative dexmedetomidine infusion. Besides, there was no significant difference in duration of mechanical ventilation, length of ICU and hospital stay, or in-hospital mortality after surgery.

Pushing Forward the Future Tense: Perspectives of a Scientist

This review is a somewhat chronological tale of my scientific life, emphasizing the why of the questions we asked in the lab and lessons learned that may be of value to nascent scientists. The reader will come to realize that the flow of my life has been driven by a combined life of the mind and life of the soul, intertwining like the strands of DNA. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The role of 14-3-3 proteins in cell signalling pathways and virus infection

14-3-3 proteins are highly conserved in species ranging from yeast to mammals and regulate numerous signalling pathways via direct interactions with proteins carrying phosphorylated 14-3-3-binding motifs. Recent studies have shown that 14-3-3 proteins can also play a role in viral infections. This review summarizes the biological functions of 14-3-3 proteins in protein trafficking, cell-cycle control, apoptosis, autophagy and other cell signal transduction pathways, as well as the associated mechanisms. Recent findings regarding the role of 14-3-3 proteins in viral infection and innate immunity are also reviewed.

14-3-3ζ mediates an alternative, non-thermogenic mechanism in male mice to reduce heat loss and improve cold tolerance

Objective

Adaptive thermogenesis, which is partly mediated by sympathetic input on brown adipose tissue (BAT), is a mechanism of heat production that confers protection against prolonged cold exposure. Various endogenous stimuli, for example, norepinephrine and FGF-21, can also promote the conversion of inguinal white adipocytes to beige adipocytes, which may represent a secondary cell type that contributes to adaptive thermogenesis. We previously identified an essential role of the molecular scaffold 14-3-3ζ in adipogenesis, but one of the earliest, identified functions of 14-3-3ζ is its regulatory effects on the activity of tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of norepinephrine. Herein, we examined whether 14-3-3ζ could influence adaptive thermogenesis via actions on BAT activation or the beiging of white adipocytes.

Methods

Transgenic mice over-expressing a TAP-tagged human 14-3-3ζ molecule or heterozygous mice without one allele of Ywhaz, the gene encoding 14-3-3ζ, were used to explore the contribution of 14-3-3ζ to acute (3 h) and prolonged (3 days) cold (4 °C) exposure. Metabolic caging experiments, PET-CT imaging, and laser Doppler imaging were used to determine the effect of 14-3-3ζ over-expression on thermogenic and vasoconstrictive mechanisms in response to cold.

Results

Transgenic over-expression of 14-3-3ζ (TAP) in male mice significantly improved tolerance to acute and prolonged cold. In response to cold, body temperatures in TAP mice did not decrease to the same extent when compared to wildtype (WT) mice, and this was associated with increased UCP1 expression in beige inguinal white tissue (iWAT) and BAT. Of note was the paradoxical finding that cold-induced changes in body temperatures of TAP mice were associated with significantly decreased energy expenditure. The marked improvements in tolerance to prolonged cold were not due to changes in sensitivity to β-adrenergic stimulation or BAT or iWAT oxidative metabolism; instead, over-expression of 14-3-3ζ significantly decreased thermal conductance and heat loss in mice via increased peripheral vasoconstriction.

Conclusions

Despite being associated with elevations in cold-induced UCP1 expression in brown or beige adipocytes, these findings suggest that 14-3-3ζ regulates an alternative, non-thermogenic mechanism via vasoconstriction to minimize heat loss during cold exposure.

•

14-3-3ζ over-expression in male mice improves tolerance to acute and prolonged cold.

•

Increasing 14-3-3ζ expression promotes beiging of inguinal white adipose tissue.

•

Cold-induced changes in body temperature can be dissociated from energy expenditure.

•

14-3-3ζ-dependent decreases in heat loss are associated with vasoconstriction.

Regulation of α2B-Adrenergic Receptor Cell Surface Transport by GGA1 and GGA2

The molecular mechanisms that control the targeting of newly synthesized G protein-coupled receptors (GPCRs) to the functional destinations remain poorly elucidated. Here, we have determined the role of Golgi-localized, γ-adaptin ear domain homology, ADP ribosylation factor-binding proteins 1 and 2 (GGA1 and GGA2) in the cell surface transport of α_2B-adrenergic receptor (α_2B-AR), a prototypic GPCR, and studied the underlying mechanisms. We demonstrated that knockdown of GGA1 and GGA2 by shRNA and siRNA significantly reduced the cell surface expression of inducibly expressed α_2B-AR and arrested the receptor in the perinuclear region. Knockdown of each GGA markedly inhibited the dendritic expression of α_2B-AR in primary cortical neurons. Consistently, depleting GGA1 and GGA2 attenuated receptor-mediated signal transduction measured as ERK1/2 activation and cAMP inhibition. Although full length α_2B-AR associated with GGA2 but not GGA1, its third intracellular loop was found to directly interact with both GGA1 and GGA2. More interestingly, further mapping of interaction domains showed that the GGA1 hinge region and the GGA2 GAE domain bound to multiple subdomains of the loop. These studies have identified an important function and revealed novel mechanisms of the GGA family proteins in the forward trafficking of a cell surface GPCR.

G-protein-coupled receptors mediate 14-3-3 signal transduction

G-protein-coupled receptor (GPCR)-interacting proteins likely participate in regulating GPCR signaling by eliciting specific signal transduction cascades, inducing cross-talk with other pathways, and fine tuning the signal. However, except for G-proteins and β-arrestins, other GPCR-interacting proteins are poorly characterized. 14-3-3 proteins are signal adaptors, and their participation in GPCR signaling is not well understood or recognized. Here we demonstrate that GPCR-mediated 14-3-3 signaling is ligand-regulated and is likely to be a more general phenomenon than suggested by the previous reports of 14-3-3 involvement with a few GPCRs. For the first time, we can pharmacologically characterize GPCR/14-3-3 signaling. We have shown that GPCR-mediated 14-3-3 signaling is phosphorylation-dependent, and that the GPCR/14-3-3 interaction likely occurs later than receptor desensitization and internalization. GPCR-mediated 14-3-3 signaling can be β-arrestin-independent, and individual agonists can have different potencies on 14-3-3 and β-arrestin signaling. GPCRs can also mediate the interaction between 14-3-3 and Raf-1. Our work opens up a new broad realm of previously unappreciated GPCR signal transduction. Linking GPCRs to 14-3-3 signal transduction creates the potential for the development of new research directions and provides a new signaling pathway for drug discovery.

X-exome sequencing of 405 unresolved families identifies seven novel intellectual disability genes

X-linked intellectual disability (XLID) is a clinically and genetically heterogeneous disorder. During the past two decades in excess of 100 X-chromosome ID genes have been identified. Yet, a large number of families mapping to the X-chromosome remained unresolved suggesting that more XLID genes or loci are yet to be identified. Here, we have investigated 405 unresolved families with XLID. We employed massively parallel sequencing of all X-chromosome exons in the index males. The majority of these males were previously tested negative for copy number variations and for mutations in a subset of known XLID genes by Sanger sequencing. In total, 745 X-chromosomal genes were screened. After stringent filtering, a total of 1297 non-recurrent exonic variants remained for prioritization. Co-segregation analysis of potential clinically relevant changes revealed that 80 families (20%) carried pathogenic variants in established XLID genes. In 19 families, we detected likely causative protein truncating and missense variants in 7 novel and validated XLID genes (CLCN4, CNKSR2, FRMPD4, KLHL15, LAS1L, RLIM and USP27X) and potentially deleterious variants in 2 novel candidate XLID genes (CDK16 and TAF1). We show that the CLCN4 and CNKSR2 variants impair protein functions as indicated by electrophysiological studies and altered differentiation of cultured primary neurons from Clcn4(-/-) mice or after mRNA knock-down. The newly identified and candidate XLID proteins belong to pathways and networks with established roles in cognitive function and intellectual disability in particular. We suggest that systematic sequencing of all X-chromosomal genes in a cohort of patients with genetic evidence for X-chromosome locus involvement may resolve up to 58% of Fragile X-negative cases.

Downregulation of urea transporter UT-A1 activity by 14-3-3 protein

Urea transporter (UT)-A1 in the kidney inner medulla plays a critical role in the urinary concentrating mechanism and thereby in the regulation of water balance. The 14-3-3 proteins are a family of seven isoforms. They are multifunctional regulatory proteins that mainly bind to phosphorylated serine/threonine residues in target proteins. In the present study, we found that all seven 14-3-3 isoforms were detected in the kidney inner medulla. However, only the 14-3-3 γ-isoform was specifically and highly associated with UT-A1, as demonstrated by a glutathione-S-transferase-14-3-3 pulldown assay. The cAMP/adenylyl cyclase stimulator forskolin significantly enhanced their binding. Coinjection of 14-3-3γ cRNA into oocytes resulted in a decrease of UT-A1 function. In addition, 14-3-3γ increased UT-A1 ubiquitination and protein degradation. 14-3-3γ can interact with both UT-A1 and mouse double minute 2, the E3 ubiquitin ligase for UT-A1. Thus, activation of cAMP/PKA increases 14-3-3γ interactions with UT-A1 and stimulates mouse double minute 2-mediated UT-A1 ubiquitination and degradation, thereby forming a novel regulatory mechanism of urea transport activity.

Minireview: Role of intracellular scaffolding proteins in the regulation of endocrine G protein-coupled receptor signaling

The majority of hormones stimulates and mediates their signal transduction via G protein-coupled receptors (GPCRs). The signal is transmitted into the cell due to the association of the GPCRs with heterotrimeric G proteins, which in turn activates an extensive array of signaling pathways to regulate cell physiology. However, GPCRs also function as scaffolds for the recruitment of a variety of cytoplasmic protein-interacting proteins that bind to both the intracellular face and protein interaction motifs encoded by GPCRs. The structural scaffolding of these proteins allows GPCRs to recruit large functional complexes that serve to modulate both G protein-dependent and -independent cellular signaling pathways and modulate GPCR intracellular trafficking. This review focuses on GPCR interacting PSD95-disc large-zona occludens domain containing scaffolds in the regulation of endocrine receptor signaling as well as their potential role as therapeutic targets for the treatment of endocrinopathies.

Up-regulated 14-3-3β and 14-3-3ζ proteins in prefrontal cortex of subjects with schizophrenia: effect of psychotropic treatment

14-3-3 is a family of conserved regulatory proteins that bind to a multitude of functionally diverse signalling proteins. Various genetic studies and gene expression and proteomic analyses have involved 14-3-3 proteins in schizophrenia (SZ). On the other hand, studies about the status of these proteins in major depressive disorder (MD) are still missing. Immunoreactivity values of cytosolic 14-3-3β and 14-3-3ζ proteins were evaluated by Western blot in prefrontal cortex (PFC) of subjects with schizophrenia (SZ; n=22), subjects with major depressive disorder (MD; n=21) and age-, gender- and postmortem delay-matched control subjects (n=52). The modulation of 14-3-3β and 14-3-3ζ proteins by psychotropic medication was also assessed. The analysis of both proteins in SZ subjects with respect to matched control subjects showed increased 14-3-3β (Δ=33±10%, p<0.05) and 14-3-3ζ (Δ=29±6%, p<0.05) immunoreactivity in antipsychotic-free but not in antipsychotic-treated SZ subjects. Immunoreactivity values of 14-3-3β and 14-3-3ζ were not altered in MD subjects. These results show the specific up-regulation of 14-3-3β and 14-3-3ζ proteins in PFC of SZ subjects and suggest a possible down-regulation of both proteins by antipsychotic treatment.

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

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

Regulation of G protein signaling by the 70kDa heat shock protein

G protein-coupled receptors (GPCRs) transduce extracellular signals to the interior of the cell by activating membrane-bound guanine nucleotide-binding regulatory proteins (G proteins). An increasing number of proteins have been reported to bind to and regulate GPCRs. We report a novel regulation of the alpha(2A) adrenergic receptor (α(2A)-R) by the ubiquitous stress-inducible 70kDa heat shock protein, hsp70. Hsp70, but not hsp90, attenuated G protein-dependent high affinity agonist binding to the α(2A)-R in Sf9 membranes. Antagonist binding was unchanged, suggesting that hsp70 uncouples G proteins from the receptor. As hsp70 did not bind G proteins but complexed with the α(2A)-R in intact cells, a direct interaction with the receptor seems likely. In the presence of hsp70, α(2A)-R-catalyzed [(35)S]GTPγS binding was reduced by approximately 70%. In contrast, approximately 50-fold higher concentrations of hsp70 were required to reduce agonist binding to the stress-inducible 5-hydroxytryptamine(1A) receptor (5-HT(1A)-R). In heat-stressed CHO cells, the α(2A)-R was significantly uncoupled from G proteins, coincident with an increased localization of hsp70 at the membrane. The contrasting effect of hsp70 on the α(2A)-R compared to the 5-HT(1A)-R suggests that during stress, upregulation of hsp70 may attenuate signaling from specific GPCRs as part of the stress response to foster survival.

Protein-protein interactions at the adrenergic receptors

The adrenergic receptors are among the best characterized G protein-coupled receptors (GPCRs) and knowledge on this receptor family has provided several important paradigms about GPCR function and regulation. One of the most recent paradigms initially supported by studies on adrenergic receptors is that both βarrestins and G protein-coupled receptors themselves can act as scaffolds binding a variety of proteins and this can result in growing complexity of the receptor-mediated cellular effects. In this review we will briefly summarize the main features of βarrestin binding to the adrenergic receptor subtypes and we will review more in detail the main proteins found to selectively interact with distinct AR subtype. At the end, we will review the main findings on oligomerization of the AR subtypes.

14-3-3ζ Protein regulates anterograde transport of the human κ-opioid receptor (hKOPR)

By proteomic analysis, we found that 14-3-3ζ was one of the proteins co-immunoprecipitated with human κ-opioid receptor (hKOPR) from extracts of solubilized Neuro2A cells stably expressing FLAG-hKOPR (N2A-FLAG-hKOPR cells). 14-3-3 proteins are a family of conserved regulatory molecules in eukaryotic cells, where they participate in signal transduction, metabolism, and membrane protein transport. 14-3-3ζ co-localized with the hKOPR in N2A cells. The hKOPR C-tail interacted with 14-3-3ζ in rat brain extracts and bound directly to purified 14-3-3ζ as demonstrated by pulldown techniques. 14-3-3ζ siRNA decreased expression of the hKOPR in N2A-FLAG-hKOPR cells and cultured primary cortical neurons of E19 rats by ~25% as determined by immunoblotting, ligand binding, and flow cytometry. The effect of 14-3-3ζ siRNA was reversed by overexpression of 14-3-3ζ. Expression of the 14-3-3 scavenger protein pGpLI-R18 also decreased hKOPR expression. 14-3-3ζ siRNA did not change expressions of the hDOPR and rMOPR in N2A cells. Pulse-chase study showed that 14-3-3ζ siRNA decreased the amount of mature hKOPR but did not change the rate of maturation or stability of hKOPR protein. Mutations of R354A/S358A in the putative 14-3-3 interaction motif (354)RQSTS(358) in the hKOPR C-tail reduced interaction of the hKOPR with 14-3-3ζ and abolished the effect of 14-3-3ζ knockdown on hKOPR expression. Mutation of the endoplasmic reticulum retention motif (359)RVR adjacent to the 14-3-3 interaction motif in the hKOPR C-tail decreased interaction of coatomer protein I (COPI) with the hKOPR and abolished 14-3-3ζ-mediated regulation of hKOPR expression. 14-3-3ζ knockdown increased association of COPI with the hKOPR. These results suggest that 14-3-3ζ promotes expression of the hKOPR by inhibiting COPI and RVR motif-mediated endoplasmic reticulum localization machinery.

Impairment of GABAB receptor dimer by endogenous 14-3-3ζ in chronic pain conditions

In the central nervous system, the inhibitory GABAB receptor is the archetype of heterodimeric G protein-coupled receptors (GPCRs). However, the regulation of GABAB dimerization, and more generally of GPCR oligomerization, remains largely unknown. We propose a novel mechanism for inhibition of GPCR activity through de-dimerization in pathological conditions. We show here that 14-3-3ζ, a GABAB1-binding protein, dissociates the GABAB heterodimer, resulting in the impairment of GABAB signalling in spinal neurons. In the dorsal spinal cord of neuropathic rats, 14-3-3ζ is overexpressed and weakens GABAB inhibition. Using anti-14-3-3ζ siRNA or competing peptides disrupts 14-3-3ζ/GABAB1 interaction and restores functional GABAB heterodimers in the dorsal horn. Importantly, both strategies greatly enhance the anti-nociceptive effect of intrathecal Baclofen in neuropathic rats. Taken together, our data provide the first example of endogenous regulation of a GPCR oligomeric state and demonstrate its functional impact on the pathophysiological process of neuropathic pain sensitization.

Influence of the accessory protein SET on M3 muscarinic receptor phosphorylation and G protein coupling

The proto-oncogene and inhibitor of protein phosphatase 2A (PP2A), SET, interacts with the third intracellular loop of the M3 muscarinic receptor (M3-MR), and SET knockdown with small interfering RNA (siRNA) in Chinese hamster ovary (CHO) cells augments M3-MR signaling. However, the mechanism of this action of SET on receptor signaling has not been defined, and we initiated studies to address this question. Knockdown of SET by siRNA in CHO cells stably expressing the M3-MR did not alter agonist-induced receptor phosphorylation or receptor internalization. Instead, it increased the extent of receptor dephosphorylation after agonist removal by ∼60%. In competition binding assays, SET knockdown increased high-affinity binding of agonist in intact cells and membrane preparations. Glutathione transferase pull-down assays and site-directed mutagenesis revealed a SET binding site adjacent to and perhaps overlapping the G protein-binding site within the third intracellular loop of the receptor. Mutation of this region in the M3-MR altered receptor coupling to G protein. These data indicate that SET decreases M3-MR dephosphorylation and regulates receptor engagement with G protein, both of which may contribute to the inhibitory action of SET on M3-MR signaling.

Brainstem deficiency of the 14-3-3 regulator of serotonin synthesis: a proteomics analysis in the sudden infant death syndrome

Impaired brainstem responses to homeostatic challenges during sleep may result in the sudden infant death syndrome (SIDS). Previously we reported a deficiency of serotonin (5-HT) and its key biosynthetic enzyme, tryptophan hydroxylase (TPH2), in SIDS infants in the medullary 5-HT system that modulates homeostatic responses during sleep. Yet, the underlying basis of the TPH2 and 5-HT deficiency is unknown. In this study, we tested the hypothesis that proteomics would uncover previously unrecognized abnormal levels of proteins related to TPH2 and 5-HT regulation in SIDS cases compared with controls, which could provide novel insight into the basis of their deficiency. We first performed a discovery proteomic analysis of the gigantocellularis of the medullary 5-HT system in the same data set with deficiencies of TPH2 and 5-HT levels. Analysis in 6 SIDS cases and 4 controls revealed a 42-75% reduction in abundance in 5 of the 6 isoforms identified of the 14-3-3 signal transduction family, which is known to influence TPH2 activity (p < 0.07). These findings were corroborated in an additional SIDS and control sample using an orthogonal MS(E)-based quantitative proteomic strategy. To confirm these proteomics results in a larger data set (38 SIDS, 11 controls), we applied Western blot analysis in the gigantocellularis and found that 4/7 14-3-3 isoforms identified were significantly reduced in SIDS cases (p ≤ 0.02), with a 43% reduction in all 14-3-3 isoforms combined (p < 0.001). Abnormalities in 5-HT and TPH2 levels and 5-HT(1A) receptor binding were associated with the 14-3-3 deficits in the same SIDS cases. These data suggest a potential molecular defect in SIDS related to TPH2 regulation, as 14-3-3 is critical in this process.

Study of GPCR-protein interactions using gel overlay assays and glutathione-S-transferase-fusion protein pull-downs

Numerous recent studies have suggested that the predicted cytosolic domains of G protein-coupled receptors represent a surface for association with proteins that may serve multiple roles in receptor localization, turnover, and signaling beyond the well-characterized interactions of these receptors with heterotrimeric G proteins. This Chapter describes two in vitro methods for ascertaining interactions between G protein-coupled receptors and various binding partners: gel overlay strategies and GST-fusion protein pull-downs.

Translational control by RGS2

The regulator of G protein signaling (RGS) proteins are a family of guanosine triphosphatase (GTPase)-accelerating proteins. We have discovered a novel function for RGS2 in the control of protein synthesis. RGS2 was found to bind to eIF2Bepsilon (eukaryotic initiation factor 2B epsilon subunit) and inhibit the translation of messenger RNA (mRNA) into new protein. This effect was not observed for other RGS proteins tested. This novel function of RGS2 is distinct from its ability to regulate G protein-mediated signals and maps to a stretch of 37 amino acid residues within its conserved RGS domain. Moreover, RGS2 was capable of interfering with the eIF2-eIF2B GTPase cycle, which is a requisite step for the initiation of mRNA translation. Collectively, this study has identified a novel role for RGS2 in the control of protein synthesis that is independent of its established RGS domain function.

Regulation of G protein-coupled receptor signalling: focus on the cardiovascular system and regulator of G protein signalling proteins

G protein-coupled receptors (GPCRs) are involved in many biological processes. Therefore, GPCR function is tightly controlled both at receptor level and at the level of signalling components. Well-known mechanisms by which GPCR function can be regulated comprise desensitization/resensitization processes and GPCR up- and downregulation. GPCR function can also be regulated by several proteins that directly interact with the receptor and thereby modulate receptor activity. An additional mechanism by which receptor signalling is regulated involves an emerging class of proteins, the so-called regulators of G protein signalling (RGS). In this review we will describe some of these control mechanisms in more detail with some specific examples in the cardiovascular system. In addition, we will provide an overview on RGS proteins and the involvement of RGS proteins in cardiovascular function.

Arrestins and spinophilin competitively regulate Na+,K+-ATPase trafficking through association with a large cytoplasmic loop of the Na+,K+-ATPase

The activity and trafficking of the Na(+),K(+)-ATPase are regulated by several hormones, including dopamine, vasopressin, and adrenergic hormones through the action of G-protein-coupled receptors (GPCRs). Arrestins, GPCR kinases (GRKs), 14-3-3 proteins, and spinophilin interact with GPCRs and modulate the duration and magnitude of receptor signaling. We have found that arrestin 2 and 3, GRK 2 and 3, 14-3-3 epsilon, and spinophilin directly associate with the Na(+),K(+)-ATPase and that the associations with arrestins, GRKs, or 14-3-3 epsilon are blocked in the presence of spinophilin. In COS cells that overexpressed arrestin, the Na(+),K(+)-ATPase was redistributed to intracellular compartments. This effect was not seen in mock-transfected cells or in cells expressing spinophilin. Furthermore, expression of spinophilin appeared to slow, whereas overexpression of beta-arrestins accelerated internalization of the Na(+),K(+)-ATPase endocytosis. We also find that GRKs phosphorylate the Na(+),K(+)-ATPase in vitro on its large cytoplasmic loop. Taken together, it appears that association with arrestins, GRKs, 14-3-3 epsilon, and spinophilin may be important modulators of Na(+),K(+)-ATPase trafficking.

Stop that cell! Beta-arrestin-dependent chemotaxis: a tale of localized actin assembly and receptor desensitization

Beta-arrestins have recently emerged as key regulators of directed cell migration or chemotaxis. Given their traditional role as mediators of receptor desensitization, one theory is that beta-arrestins contribute to cell polarity during chemotaxis by quenching the signal at the trailing edge of the cell. A second theory is that they scaffold signaling molecules involved in cytoskeletal reorganization to promote localized actin assembly events leading to the formation of a leading edge. This review addresses both models. It discusses studies demonstrating the involvement of beta-arrestins in chemotaxis both in vivo and in vitro as well as recent evidence that beta-arrestins directly bind and regulate proteins involved in actin reorganization.

Regulation of alpha2AR trafficking and signaling by interacting proteins

The continuing discovery of new G protein-coupled receptor (GPCR) interacting proteins and clarification of the functional consequences of these interactions has revealed multiple roles for these events. Some of these interactions serve to scaffold GPCRs to particular cellular micro-compartments or to tether them to defined signaling molecules, while other GPCR-protein interactions control GPCR trafficking and the kinetics of GPCR-mediated signaling transduction. This review provides a general overview of the variety of GPCR-protein interactions reported to date, and then focuses on one prototypical GPCR, the alpha(2)AR, and the in vitro and in vivo significance of its reciprocal interactions with arrestin and spinophilin. It seems appropriate to recognize the life and career of Arthur Hancock with a summary of studies that both affirm and surprise our preconceived notions of how nature is designed, as his career-long efforts similarly affirmed the complexity of human biology and attempted to surprise pathological changes in that biology with novel, discovery-based therapeutic interventions. Dr. Hancock's love of life, of family, and of commitment to making the world a better place are a model of the life well lived, and truly missed by those who were privileged to know, and thus love, him.

Association of 14-3-3 proteins to beta1-adrenergic receptors modulates Kv11.1 K+ channel activity in recombinant systems

We identify a new mechanism for the beta(1)-adrenergic receptor (beta(1)AR)-mediated regulation of human ether-a-go-go-related gene (HERG) potassium channel (Kv11.1). We find that the previously reported modulatory interaction between Kv11.1 channels and 14-3-3epsilon proteins is competed by wild type beta(1)AR by means of a novel interaction between this receptor and 14-3-3epsilon. The association between beta(1)AR and 14-3-3epsilon is increased by agonist stimulation in both transfected cells and heart tissue and requires cAMP-dependent protein kinase (PKA) activity. The beta(1)AR/14-3-3epsilon association is direct, since it can be recapitulated using purified 14-3-3epsilon and beta(1)AR fusion proteins and is abolished in cells expressing beta(1)AR phosphorylation-deficient mutants. Biochemical and electrophysiological studies of the effects of isoproterenol on Kv11.1 currents recorded using the whole-cell patch clamp demonstrated that beta(1)AR phosphorylation-deficient mutants do not recruit 14-3-3epsilon away from Kv11.1 and display a markedly altered agonist-mediated modulation of Kv11.1 currents compared with wild-type beta(1)AR, increasing instead of inhibiting current amplitudes. Interestingly, such differential modulation is not observed in the presence of 14-3-3 inhibitors. Our results suggest that the dynamic association of 14-3-3 proteins to both beta(1)AR and Kv11.1 channels is involved in the adrenergic modulation of this critical regulator of cardiac repolarization and refractoriness.

Differential targeting and function of alpha2A and alpha2C adrenergic receptor subtypes in cultured sympathetic neurons

Previous research suggested that alpha2A and alpha2C adrenergic receptor (AR) subtypes have overlapping but unique physiological roles in neuronal signaling; however, the basis for these dissimilarities is not completely known. To better understand the observed functional differences between these autoreceptors, we investigated targeting and signaling of endogenously expressed alpha2A and alpha2CARs in cultured sympathetic ganglion neurons (SGN). At Days 1 and 4, alpha2A and alpha2CARs could be readily detected in SGN from wild-type mice. By Day 8, alpha2A ARs were targeted to cell body, as well as axonal and dendritic sites, whereas alpha2C ARs were primarily localized to an intracellular vesicular pool within the cell body and proximal dendritic projections. Expression of synaptic vesicle marker protein SV2 did not differ at Day 8 nor co-localize with either subtype. By Day 16, however, alpha2C ARs had relocated to somatodendritic and axonal sites and, unlike alpha2A ARs, co-localized with SV2 at synaptic contact sites. Consistent with a functional role for alpha2 ARs, we also observed that dexmedetomidine stimulation of cultured SGN more efficiently inhibited depolarization-induced calcium entry into older, compared to younger, cultures. These results provide direct evidence of distinct developmental patterns of endogenous alpha2A and alpha2C AR targeting and function in a native cell system and that maturation of SGN in culture leads to alterations in neuronal properties required for proper targeting. More importantly, the co-localization at Day 16 of alpha2C ARs at sites of synaptic contact may partially explain the differential modulation of neurotransmitter release and responsiveness to action potential frequency observed between alpha2A and alpha2C ARs in SGN.

Modulation of subfamily B/R4 RGS protein function by 14-3-3 proteins

Regulator of G protein signalling (RGS) proteins are primarily known for their ability to act as GTPase activating proteins (GAPs) and thus attenuate G protein function within G protein-coupled receptor (GPCR) signalling pathways. However, RGS proteins have been found to interact with additional binding partners, and this has introduced more complexity to our understanding of their potential role in vivo. Here, we identify a novel interaction between RGS proteins (RGS4, RGS5, RGS16) and the multifunctional protein 14-3-3. Two isoforms, 14-3-3beta and 14-3-3epsilon, directly interact with all three purified RGS proteins and data from in vitro steady state GTP hydrolysis assays show that 14-3-3 inhibits the GTPase activity of RGS4 and RGS16, but has limited effects on RGS5 under comparable conditions. Moreover in a competitive pull-down experiment, 14-3-3epsilon competes with Galphao for RGS4, but not for RGS5. This mechanism is further reinforced in living cells, where 14-3-3epsilon sequesters RGS4 in the cytoplasm and impedes its recruitment to the plasma membrane by Galpha protein. Thus, 14-3-3 might act as a molecular chelator, preventing RGS proteins from interacting with Galpha, and ultimately prolonging the signal transduction pathway. In conclusion, our findings suggest that 14-3-3 proteins may indirectly promote GPCR signalling via their inhibitory effects on RGS GAP function.

G-protein-coupled receptor domain overexpression in Halobacterium salinarum: long-range transmembrane interactions in heptahelical membrane proteins

The aminergic alpha(2b)-adrenergic receptor (alpha(2b)-AR) third intracellular loop (alpha(2b)-AR 3i) mediates receptor subcellular compartmentalization and signal transduction processes via ligand-dependent interaction with G(i)- and G(o)- proteins. To understand the structural origins of these processes we engineered several lengths of alpha(2b)-AR 3i into the third intracellular loop of the proton pump bacteriorhodopsin (bR) and produced the fusion proteins in quantities suitable for physical studies. The fusion proteins were expressed in the Archaeon Halobacterium salinarum and purified. A highly expressed fusion protein was crystallized from bicelles and diffracted to low resolution on an in-house diffractometer. The bR-alpha(2b)-AR 3i(203-292) protein possessed a photocycle slightly perturbed from that of the wild-type bR. The first half of the fusion protein photocycle, correlated with proton release, is accelerated by a factor of 3, whereas the second half, correlated with proton uptake, is slightly slower than wild-type bR. In addition, there is a large decrease in the pK(a), (from 9.6 to 8.3) of the terminal proton release group in the unphotolyzed state of bR-alpha(2b)-AR 3i as deduced from the pH-dependence of the M-formation. Perturbation of a cytoplasmic loop has thus resulted in the perturbation of proton release at the extracellular surface. The current work indicates that long-range and highly coupled intramolecular interactions exist that are capable of "transducing" structural perturbations (e.g., signals) across the cellular membrane. This gene fusion approach may have general applicability for physical studies of G-protein-coupled receptor domains in the context of the bR structural scaffold.

14-3-3 proteins: a historic overview

This chapter includes a historic overview of 14-3-3 proteins with an emphasis on the differences between potentially cancer-relevant isoforms on the genomic, protein and functional level. The focus will therefore be on mammalian 14-3-3s although many important developments in the field have involved Drosophila 14-3-3 proteins for example and the cross-fertilisation from parallel studies on plant 14-3-3 should not be underestimated. In the major part of this review I will attempt to focus on some novel data and aspects of 14-3-3 structure and function, in particular regulation of 14-3-3 isoforms by oncogene-related protein kinase phosphorylation and aspects of 14-3-3 research with which newcomers to the field may be less familiar.

The structural basis of arrestin-mediated regulation of G-protein-coupled receptors

The 4 mammalian arrestins serve as almost universal regulators of the largest known family of signaling proteins, G-protein-coupled receptors (GPCRs). Arrestins terminate receptor interactions with G proteins, redirect the signaling to a variety of alternative pathways, and orchestrate receptor internalization and subsequent intracellular trafficking. The elucidation of the structural basis and fine molecular mechanisms of the arrestin-receptor interaction paved the way to the targeted manipulation of this interaction from both sides to produce very stable or extremely transient complexes that helped to understand the regulation of many biologically important processes initiated by active GPCRs. The elucidation of the structural basis of arrestin interactions with numerous non-receptor-binding partners is long overdue. It will allow the construction of fully functional arrestins in which the ability to interact with individual partners is specifically disrupted or enhanced by targeted mutagenesis. These "custom-designed" arrestin mutants will be valuable tools in defining the role of various interactions in the intricate interplay of multiple signaling pathways in the living cell. The identification of arrestin-binding sites for various signaling molecules will also set the stage for designing molecular tools for therapeutic intervention that may prove useful in numerous disorders associated with congenital or acquired disregulation of GPCR signaling.

14-3-3zeta interacts with human thromboxane receptors and is involved in the agonist-induced activation of the extracellular-signal-regulated kinase

Thromboxane receptor (TP) signaling results in a broad range of cellular responses including kinase activation and subsequent nuclear signaling events involved in cell transformation, proliferation, and cell survival. Proteins that may participate in the early signaling following receptor activation remain to be identified. We found that 14-3-3zeta is a novel protein interacting with TP intracellular loop 3 (i3) by yeast two-hybrid system. This interaction was further confirmed by GST pull-down and co-immunoprecipitation methods. Site-directed mutagenesis studies indicated that Pro-236 of the TP-i3 was involved in the binding to the 14-3-3zeta. Co-immunoprecipitation studies in the same cell lysate by TP antibody showed that TP binds not only with the 14-3-3zeta but also with the Raf-1. Our data also demonstrated that TP receptor activation induced by agonist rapidly recruited 14-3-3zeta and Raf-1 to form a complex with the TP on the plasma membrane. The significance of assembling this protein complex was examined by TP agonist-induced extracellular-signal-regulated kinase (ERK) phosphorylation in intact cells. TP agonist, I-BOP, induced ERK phosphorylation in HEK 293 cells expressing wild type TPalpha but significantly lower in those expressing TPalpha-P236V mutant. Attenuation of the expression of 14-3-3zeta by 14-3-3zeta siRNA decreased I-BOP-induced ERK phosphorylation indicating the involvement of the 14-3-3zeta in the signal transduction process. These results suggest that 14-3-3zeta may serve as a scaffold protein to form a protein complex consisting of TP, 14-3-3zeta, and Raf-1, and that this protein complex may be involved in the activation of ERK pathway following TP receptor activation.

14-3-3 proteins in membrane protein transport

14-3-3 proteins affect the cell surface expression of several unrelated cargo membrane proteins, e.g., MHC II invariant chain, the two-pore potassium channels KCNK3 and KCNK9, and a number of different reporter proteins exposing Arg-based endoplasmic reticulum localization signals in mammalian and yeast cells. These multimeric membrane proteins have a common feature in that they all expose coatomer protein complex I (COPI)- and 14-3-3-binding motifs. 14-3-3 binding depends on phosphorylation of the membrane protein in some and on multimerization of the membrane protein in other cases. Evidence from mutant proteins that are unable to interact with either COPI or 14-3-3 and from yeast cells with an altered 14-3-3 content suggests that 14-3-3 proteins affect forward transport in the secretory pathway. Mechanistically, this could be explained by clamping, masking, or scaffolding. In the clamping mechanism, 14-3-3 binding alters the conformation of the signal-exposing tail of the membrane protein, whereas masking or scaffolding would abolish or allow the interaction of the membrane protein with other proteins or complexes. Interaction partners identified as putative 14-3-3 binding partners in affinity purification approaches constitute a pool of candidate proteins for downstream effectors, such as coat components, coat recruitment GTPases, Rab GTPases, GTPase-activating proteins (GAPs), guanine-nucleotide exchange factors (GEFs) and motor proteins.

Proteomic analysis of human norepinephrine transporter complexes reveals associations with protein phosphatase 2A anchoring subunit and 14-3-3 proteins

The norepinephrine transporter (NET) terminates noradrenergic signals by clearing released NE at synapses. NET regulation by receptors and intracellular signaling pathways is supported by a growing list of associated proteins including syntaxin1A, protein phosphatase 2A (PP2A) catalytic subunit (PP2A-C), PICK1, and Hic-5. In the present study, we sought evidence for additional partnerships by mass spectrometry-based analysis of proteins co-immunoprecipitated with human NET (hNET) stably expressed in a mouse noradrenergic neuroblastoma cell line. Our initial proteomic analyses reveal multiple peptides derived from hNET, peptides arising from the mouse PP2A anchoring subunit (PP2A-Ar) and peptides derived from 14-3-3 proteins. We verified physical association of NET with PP2A-Ar via co-immunoprecipitation studies using mouse vas deferens extracts and with 14-3-3 via a fusion pull-down approach, implicating specifically the hNET NH2-terminus for interactions. The transporter complexes described likely support mechanisms regulating transporter activity, localization, and trafficking.

G protein-coupled receptors show unusual patterns of intrinsic unfolding

Intrinsically unstructured proteins (IUPs) or IUP-like regions often play key roles in controlling processes ranging from transcription to the cell cycle. In silico such proteins can be identified by their sequence properties; they have low hydrophobicity and high net charge. In this study, we applied the FoldIndex (http://bioportal.weizmann.ac.il/fldbin/findex) program to analyze human G protein-coupled receptors and compared them with membrane proteins of known structure and with IUPs. We show that human G protein-coupled receptor (GPCR) extramembranous domains include long (>50 residues) disordered segments, unlike membrane proteins of known structure. The predicted disorder occurred primarily in the N-terminal, C-terminal and third intracellular domain regions: 55, 69 and 56% of the human GPCRs were disordered in these regions, respectively. This increased flexibility may therefore be critical for GPCR function. Surprisingly, however, the kinds of residues used in GPCR unstructured regions were different than in hitherto-identified IUPs. The GPCR third intracellular loop domains contain very high percentages of Arg, Lys and His residues, especially Arg, but the percentage of Glu, Asp and Pro is no higher than in folded proteins. We propose that this has structural and functional consequences.

Emerging role of homo- and heterodimerization in G-protein-coupled receptor biosynthesis and maturation

The idea that G-protein-coupled receptors (GPCRs) can function as dimers is now generally accepted. Although an increasing amount of data suggests that dimers represent the basic signaling unit for most, if not all, members of this receptor family, GPCR dimerization might also be necessary to pass quality-control checkpoints of the biosynthetic pathway of GPCRs. To date, this hypothesis has been demonstrated unambiguously only for a small number of receptors that must form heterodimers to be exported properly to the plasma membrane (referred to as obligatory heterodimers). However, increasing evidence suggests that homodimerization might have a similar role in the receptor maturation process for many GPCRs.

GPCR interacting proteins (GIP)

G protein-coupled receptors (GPCR) interact not only with heterotrimeric G proteins but also with accessory proteins called GPCR interacting proteins (GIP). These proteins have important functions. They are implicated in GPCR targeting to specific cellular compartments, in their assembling into large functional complexes called "receptosomes," in their trafficking to and from the plasma membrane, and in the fine-tuning of their signaling properties. There are several types of GIPs. Some are transmembrane proteins such as another GPCR (homodimerization and heterodimerization), ionic channels, ionotropic receptors, and single transmembrane proteins. The latter is implicated in the fine-tuning of receptor pharmacology or signaling. Other GIPs are soluble proteins interacting mainly with the "magic" C-terminal tail. Among them, PDZ domain-containing proteins are the most abundant. They generally, but not always, interact with the extreme C-terminal domain of GPCRs. Some GIPs interact with specific sequences of the C-terminal such as the Homer binding sequence (-PPxxFR-), the dopamine receptor interacting protein (DRIP) binding sequence (-FxxxFxxxF-), etc. Finally, only few GIPs have been found thus far to interact with the third intracellular loop of GPCRs. The future will tell us if this situation is only due to technical reasons.

Structure/function analysis of alpha2A-adrenergic receptor interaction with G protein-coupled receptor kinase 2

G protein-coupled receptors (GPCRs) mediate the ability of a diverse array of extracellular stimuli to control intracellular signaling. Many GPCRs are phosphorylated by G protein-coupled receptor kinases (GRKs), a process that mediates agonist-specific desensitization in many cells. Although GRK binding to activated GPCRs results in kinase activation and receptor phosphorylation, relatively little is known about the mechanism of GRK/GPCR interaction or how this interaction results in kinase activation. Here, we used the alpha2A-adrenergic receptor (alpha(2A)AR) as a model to study GRK/receptor interaction because GRK2 phosphorylation of four adjacent serines within the large third intracellular loop of this receptor is known to mediate desensitization. Various domains of the alpha(2A)AR were expressed as glutathione S-transferase fusion proteins and tested for the ability to bind purified GRK2. The second and third intracellular loops of the alpha(2A)AR directly interacted with GRK2, whereas the first intracellular loop and C-terminal domain did not. Truncation mutagenesis identified three discrete regions within the third loop that contributed to GRK2 binding, the membrane proximal N- and C-terminal regions as well as a central region adjacent to the phosphorylation sites. Site-directed mutagenesis revealed a critical role for specific basic residues within these regions in mediating GRK2 interaction with the alpha(2A)AR. Mutation of these residues within the holo-alpha(2A)AR diminished GRK2-promoted phosphorylation of the receptor as well as the ability of the kinase to be activated by receptor binding. These studies provide new insight into the mechanism of interaction and activation of GRK2 by GPCRs and suggest that GRK2 binding is critical not only for receptor phosphorylation but also for full activity of the kinase.

Molecular structure and physiological functions of GABA(B) receptors

GABA(B) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. The cloning of the first GABA(B) receptor cDNAs in 1997 revived interest in these receptors and their potential as therapeutic targets. With the availability of molecular tools, rapid progress was made in our understanding of the GABA(B) system. This led to the surprising discovery that GABA(B) receptors need to assemble from distinct subunits to function and provided exciting new insights into the structure of G protein-coupled receptors (GPCRs) in general. As a consequence of this discovery, it is now widely accepted that GPCRs can exist as heterodimers. The cloning of GABA(B) receptors allowed some important questions in the field to be answered. It is now clear that molecular studies do not support the existence of pharmacologically distinct GABA(B) receptors, as predicted by work on native receptors. Advances were also made in clarifying the relationship between GABA(B) receptors and the receptors for gamma-hydroxybutyrate, an emerging drug of abuse. There are now the first indications linking GABA(B) receptor polymorphisms to epilepsy. Significantly, the cloning of GABA(B) receptors enabled identification of the first allosteric GABA(B) receptor compounds, which is expected to broaden the spectrum of therapeutic applications. Here we review current concepts on the molecular composition and function of GABA(B) receptors and discuss ongoing drug-discovery efforts.

Human follitropin receptor (FSHR) interacts with the adapter protein 14-3-3tau

The human follitropin (follicle stimulating hormone, FSH) receptor (FSHR) is a G protein-coupled receptor (GPCR). To identify cytoplasmic proteins that may regulate FSHR function, a yeast-based interaction trap was performed. A linked construct of the first and second intracellular loops (iL1-iL2 bait) of FSHR was used as bait and a human ovarian cDNA library was used as prey. Among the proteins identified that interacted with the bait was 14-3-3tau, a member of a family of homodimeric cytoplasmic adapter proteins. Human granulosa cells, the site of FSHR expression in the ovary, were found to contain 14-3-3tau. Importantly, 14-3-3tau co-immunoprecipitated with FSHR stably expressed in HEK 293 cells. Its association with FSHR was follitropin-dependent. Over-expression of 14-3-3tau resulted in a modest decrease of follitropin-induced cAMP accumulation. Collectively, these data support a role for 14-3-3tau in follitropin action. The finding that 14-3-3tau interacts with FSHR is novel and should lead to new insights into the regulation of GPCR in general and FSHR specifically.

Membrane trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) modulate diverse physiological and behavioral signaling pathways by virtue of changes in receptor activation and inactivation states. Functional changes in receptor properties include dynamic interactions with regulatory molecules and trafficking to various cellular compartments at various stages of the life cycle of a GPCR. This review focuses on trafficking of GPCRs to the cell surface, stabilization there, and agonist-regulated turnover. GPCR interactions with a variety of newly revealed partners also are reviewed with the intention of provoking further analysis of the relevance of these interactions in GPCR trafficking, signaling, or both. The disease consequences of mislocalization of GPCRs also are described.

Calmodulin Interacts with the Third Intracellular Loop of the Serotonin 5-Hydroxytryptamine1A Receptor at Two Distinct Sites

The serotonin 5-HT(1A) receptor couples to heterotrimeric G proteins and intracellular second messengers, yet no studies have investigated the possible role of additional receptor-interacting proteins in 5-HT(1A) receptor signaling. We have found that the ubiquitous Ca(2+)-sensor calmodulin (CaM) co-immunoprecipitates with the 5-HT(1A) receptor in Chinese hamster ovary fibroblasts. The human 5-HT(1A) receptor contains two putative CaM binding motifs, located in the N- and C-terminal juxtamembrane regions of the third intracellular loop of the receptor. Peptides encompassing both the N-terminal (i3N) and C-terminal (i3C) CaM-binding domains were tested for CaM binding. Using in vitro binding assays in combination with gel shift analysis, we demonstrated Ca(2+)-dependent formation of complexes between CaM and both peptides. We determined kinetic data using a combination of BIAcore surface plasmon resonance (SPR) and dansyl-CaM fluorescence. SPR analysis gave an apparent K(D) of approximately 110 nm for the i3N peptide and approximately 700 nm for the i3C peptide. Both peptides also caused characteristic shifts in the fluorescence emission spectrum of dansyl-CaM, with apparent affinities of 87 +/- 23 nm and 1.70 +/- 0.16 microm. We used bioluminescence resonance energy transfer to show that CaM interacts with the 5-HT(1A) receptor in living cells, representing the first in vivo evidence of a G protein-coupled receptor interacting with CaM. Finally, we showed that CaM binding and phosphorylation of the 5-HT(1A) receptor i3 loop peptides by protein kinase C are antagonistic in vitro, suggesting a possible role for CaM in the regulation of 5-HT(1A) receptor phosphorylation and desensitization. These data suggest that the 5-HT(1A) receptor contains high and moderate affinity CaM binding regions that may play important roles in receptor signaling and function.

The phosphoserine-585-dependent pathway of the GM-CSF/IL-3/IL-5 receptors mediates hematopoietic cell survival through activation of NF-kappaB and induction of bcl-2

We have recently identified a novel mechanism of hematopoietic cell survival that involves site-specific serine phosphorylation of the common beta subunit (beta(c)) of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors. However, the downstream components of this pathway are not known, nor is its relationship to survival signals triggered by tyrosine phosphorylation of the receptor clear. We have now found that phosphorylation of Ser585 of beta(c) in response to GM-CSF recruited 14-3-3 and phosphatidyl inositol 3-OH kinase (PI 3-kinase) to the receptor, while phosphorylation of the neighboring Tyr577 within this "viability domain" promoted the activation of both Src homology and collagen (Shc) and Ras. These are independent processes as demonstrated by the intact reactivity of phosphospecific anti-Ser585 and anti-Tyr577 antibodies on the cytotoxic T-lymphocyte-ecotrophic retroviral receptor neomycin (CTL-EN) mutants beta(c)Tyr577Phe and beta(c)Ser585Gly, respectively. Importantly, while mutants in which either Ser585 (beta(c)Ser585Gly) or all tyrosines (beta(c)F8) were substituted showed a defect in Akt phosphorylation, nuclear factor kappaB (NF-kappaB) activation, bcl-2 induction, and cell survival, the mutant beta(c)Tyr577Phe was defective in Shc, Ras, and extracellular signal-related kinase (ERK) activation, but supported CTL-EN cell survival in response to GM-CSF. These results demonstrate that both serine and tyrosine phosphorylation pathways play a role in hematopoietic cell survival, are initially independent of each other, and converge on NF-kappaB to promote bcl-2 expression.

Spinophilin stabilizes cell surface expression of alpha 2B-adrenergic receptors

The third intracellular (3i) loops of the alpha 2A- and alpha 2B-adrenergic receptor (AR) subtypes are critical for retention of these receptors at the basolateral surface of polarized Madin-Darby canine kidney (MDCKII) cells at steady state. The third intracellular loops of the alpha 2A, alpha 2B, and alpha 2C-AR subtypes interact with spinophilin, a multidomain protein that, like the three alpha 2-AR subtypes, is enriched at the basolateral surface of MDCKII cells. The present studies provide evidence that alpha 2-AR interaction with spinophilin contributes to cell surface stabilization of the receptor. We exploited the unique targeting profile of the alpha 2B-AR subtype in MDCKII cells: random delivery to apical and basolateral surfaces with rapid (t(1/2) < or = 60 min) apical versus slower (t(1/2) = 10-12 h) basolateral turnover. Apical delivery of a spinophilin subdomain containing the alpha 2-AR-interacting region (Sp151-483) by fusion with apically targeted p75NTR extended the half-life of alpha 2B-AR at the apical surface to approximately 3.6 h and eliminated the rapid phase (0-60 min) of alpha 2B-AR turnover on that surface. Furthermore, we examined alpha 2B-AR turnover at the surface of mouse embryo fibroblasts derived from wild type (Sp+/+) or spinophilin knock-out (Sp-/-) mice. Two independent experimental approaches demonstrated that agonist-evoked internalization of HA-alpha 2B-AR was accelerated in mouse embryo fibroblasts derived from Sp-/- mice. These findings are consistent with the interpretation that endogenous spinophilin contributes to the stabilization of alpha 2B-AR and presumably all three alpha2-AR subtypes at the surface of target cells and may act as a scaffold that could link alpha 2-ARs to proteins interacting with spinophilin via other domains.

Monomeric 14-3-3 protein is sufficient to modulate the activity of the Drosophila slowpoke calcium-dependent potassium channel

Drosophila 14-3-3zeta (D14-3-3zeta) modulates the activity of the Slowpoke calcium-dependent potassium channel (dSlo) by interacting with the dSlo binding protein, Slob. We show here that D14-3-3zeta forms dimers in vitro. Site-directed mutations in its putative dimerization interface result in a dimerization-deficient form of D14-3-3zeta. Both the wild-type and dimerization-deficient forms of D14-3-3zeta bind to Slob with similar affinity and form complexes with dSlo. When dSlo and Slob are expressed in mammalian cells, the dSlo channel activity is similarly modulated by co-expression of either the wild-type or the dimerization-deficient form of D14-3-3zeta. In addition, dSlo is still modulated by wild-type D14-3-3zeta in the presence of a 14-3-3 mutant, which does not itself bind to Slob but forms heterodimers with the wild-type 14-3-3. These data, taken together, suggest that monomeric D14-3-3zeta is capable of modulating dSlo channel activity in this regulatory complex.

Heterodimerization of alpha 2A- and beta 1-adrenergic receptors

beta- and alpha(2)-adrenergic receptors are known to exhibit substantial cross-talk and mutual regulation in tissues where they are expressed together. We have found that the beta(1)-adrenergic receptor (beta(1)AR) and alpha(2A)-adrenergic receptor (alpha(2A)AR) heterodimerize when coexpressed in cells. Immunoprecipitation studies with differentially tagged beta(1)AR and alpha(2A)AR expressed in HEK-293 cells revealed robust co-immunoprecipitation of the two receptors. Moreover, agonist stimulation of alpha(2A)AR was found to induce substantial internalization of coexpressed beta(1)AR, providing further evidence for a physical association between the two receptors in a cellular environment. Ligand binding assays examining displacement of [(3)H]dihydroalprenolol binding to the beta(1)AR by various ligands revealed that beta(1)AR pharmacological properties were significantly altered when the receptor was coexpressed with alpha(2A)AR. Finally, beta(1)AR/alpha(2A)AR heterodimerization was found to be markedly enhanced by a beta(1)AR point mutation (N15A) that blocks N-linked glycosylation of the beta(1)AR as well as by point mutations (N10A/N14A) that block N-linked glycosylation of the alpha(2A)AR. These data reveal an interaction between beta(1)AR and alpha(2A)AR that is regulated by glycosylation and that may play a key role in cross-talk and mutual regulation between these receptors.

Interaction of the parathyroid hormone receptor with the 14-3-3 protein

The receptor for parathyroid hormone (PTH) and PTH-related protein (PTHrP) regulates calcium homeostasis, bone remodeling and skeletal development. 14-3-3 proteins bind to signaling proteins and act as molecular scaffolds and regulators of subcellular localization. We show that the parathyroid hormone receptor (PTHR) interacts with 14-3-3 and the proteins colocalize within the cell. 14-3-3 interacts with the C-terminal tail of the receptor containing a consensus 14-3-3 binding motif, but additional binding sites are also used. Protein kinase-A treatment of the receptor and especially the C-terminal tail reduces 14-3-3 binding. The expressed C-terminal tail is primarily localized in the nucleus, supporting the function of a putative nuclear localization signal that could be involved in the previously described nuclear localization of PTHR. The observed interaction between PTHR and the 14-3-3 protein implies that 14-3-3 could contribute to regulation of PTHR signaling.

Characterization of zetin 1/rBSPRY, a novel binding partner of 14-3-3 proteins

14-3-3 proteins are ubiquitously expressed proteins which serve as central adaptors in different signal transduction cascades. In this study, yeast two-hybrid screening of a rat brain cDNA library identified a novel gene product termed zetin 1/rBSPRY that interacts with 14-3-3 zeta. The zetin 1/rBSPRY gene is ubiquitously expressed in a variety of rat tissues, with highest expression being found in testis. In adult brain, high levels of zetin 1/rBSPRY mRNA were observed in the hippocampus, cerebral cortex, and piriform cortex. Biochemical studies confirmed zetin 1/rBSPRY to interact with 14-3-3 zeta. Transient co-transfection in COS 7 cells caused a partial redistribution of zetin 1/rBSPRY into 14-3-3 zeta enriched submembranous foci at leading edges. Our results suggest a role for zetin 1/rBSPRY-14-3-3 interactions at specialized submembrane domains.