Search

The relationship between depression, its etiology and therapy, and the cAMP signaling system have been studies for decades. This review will focus on cAMP, G proteins and adenylyl cyclase and depression or antidepressant action. Both human and animal studies are compared and contrasted. It is concluded that there is some synteny in the findings that cAMP signaling is attenuated in depression and that this is reversed by successful antidepressant therapy. The G protein that activates adenylyl cyclase, Gα s , appears to have diminished access to adenylyl cyclase in depression, and this is rectified by successful antidepressant treatment. Unfortunately, attempts to link specific isoforms of adenylyl cyclase to depression or antidepressant action suffer from discontinuity between human and animal studies., Competing Interests: MMR is a founder of Pax Neuroscience, Inc. and has a financial interest in that concern. The remaining author, JMS, declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Schappi and Rasenick.)

In contrast to healthy controls, the heterotrimeric G protein, Gsalpha (Gsα) is ensconced predominantly in lipid rafts in subjects with major depressive disorder (MDD) resulting in impaired stimulation of adenylyl cyclase. In this small proof-of-concept study, we examined the hypothesis that translocation of Gsα from lipid rafts toward a more facile activation of adenylyl cyclase is a biomarker for clinical response to antidepressants. There were 49 subjects with MDD (HamD 17 score ≥15) and 59 healthy controls at the screen visit. The AlphaScreen (PerkinElmer) assay measured both basal activity and prostaglandin E1 (PGE1) stimulation of Gsα-adenylyl cyclase to assess the extent of coupling of Gsα with adenylyl cyclase. At screen, platelet samples obtained from MDD subjects revealed significantly lower PGE1 activation of adenylyl cyclase activity than controls (p = 0.02). Subsequently, 19 consenting MDD subjects completed a 6-week open label antidepressant treatment trial. The 11 antidepressant responders (HamD 17 improvement ≥50% from screen) revealed significant increase in PGE1-stimulated adenylyl cyclase compared to non-responders (p = 0.05) with an effect size of 0.83 for the PGE1/Gsα lipid-raft biomarker. PGE1 stimulation increased by ≥30% from screen assessment in eight responders (72.7%) and two non-responders (25.0%) [Fisher exact = 0.07] with a positive predictive value for response of 80.0%. In this small, pilot study, increased PGE1 stimulated adenylyl cyclase was associated with antidepressant response in MDD subjects. These data suggest that a simple, high-throughput-capable assay for depression and antidepressant response can be developed. Future studies are needed to evaluate the utility of this biomarker for the treatment of MDD., (© 2021. This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply.)

Evidence from epidemiological and laboratory studies, as well as randomized placebo-controlled trials, suggests supplementation with n-3 polyunsaturated fatty acids (PUFAs) may be efficacious for treatment of major depressive disorder (MDD). The mechanisms underlying n-3 PUFAs potential therapeutic properties remain unknown. There are suggestions in the literature that glial hypofunction is associated with depressive symptoms and that antidepressants may normalize glial function. In this study, induced pluripotent stem cells (iPSC)-derived neuronal stem cell lines were generated from individuals with MDD. Astrocytes differentiated from patient-derived neuronal stem cells (iNSCs) were verified by GFAP. Cells were treated with eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) or stearic acid (SA). During astrocyte differentiation, we found that n-3 PUFAs increased GFAP expression and GFAP positive cell formation. BDNF and GDNF production were increased in the astrocytes derived from patients subsequent to n-3 PUFA treatment. Stearic Acid (SA) treatment did not have this effect. CREB activity (phosphorylated CREB) was also increased by DHA and EPA but not by SA. Furthermore, when these astrocytes were treated with n-3 PUFAs, the cAMP antagonist, RP-cAMPs did not block n-3 PUFA CREB activation. However, the CREB specific inhibitor (666-15) diminished BDNF and GDNF production induced by n-3 PUFA, suggesting CREB dependence. Together, these results suggested that n-3 PUFAs facilitate astrocyte differentiation, and may mimic effects of some antidepressants by increasing production of neurotrophic factors. The CREB-dependence and cAMP independence of this process suggests a manner in which n-3 PUFA could augment antidepressant effects. These data also suggest a role for astrocytes in both MDD and antidepressant action., (© 2020. The Author(s), under exclusive licence to Springer Nature Limited.)

Termination of antidepressant therapy often has negative consequences. Although symptoms of antidepressant withdrawal are widely recognized, the molecular processes that underlie them are not well characterized. We show that certain aspects of G α s signaling remain suppressed after antidepressant withdrawal, even after others have reverted to baseline. Antidepressant treatment causes translocation of G α s protein from lipid rafts to nonraft membrane regions. This results in augmented G α s signaling, including facilitated activation of adenylyl cyclase and increased cAMP accumulation. Using CC6 or SK-N-SH cells and a lipid raft-localized cAMP sensor, we show that G α s signaling is reduced in lipid rafts, even while signaling is enhanced elsewhere in the cell. These signaling changes mirror the changes in G α s localization observed after antidepressant treatment. Furthermore, we show that suppression of G α s signaling in lipid rafts persists at least 24 hours after cessation of antidepressant treatment. G α s localization was quantified after membrane isolation and sequential detergent extraction. We show that suppression of lipid raft G α s signaling persists for an extended time period after antidepressant withdrawal, whereas increased nonraft membrane G α s signaling reverts partially or fully upon cessation of antidepressant treatment. Translocation of G α s out of lipid rafts is also persistent. These events may reflect cellular adaptations to antidepressant treatment that contribute to antidepressant discontinuation syndromes and may aid in the discovery of new treatments and strategies to mitigate the symptoms of depression and antidepressant withdrawal. SIGNIFICANCE STATEMENT: This work explores, for the first time, the effects of antidepressants on G α s signaling after drug withdrawal. This provides novel insight into the cellular and molecular processes affected by antidepressant drugs and their persistence after discontinuation of treatment., (U.S. Government work not protected by U.S. copyright.)

While several therapeutic strategies exist for depression, most antidepressant drugs require several weeks before reaching full biochemical efficacy and remission is not achieved in many patients. Therefore, biomarkers for depression and drug-response would help tailor treatment strategies. This study made use of banked human lymphoblast cell lines (LCLs) from normal and depressed subjects; the latter divided into remitters and non-remitters. Due to the fact that previous studies have shown effects on growth factors, cytokines, and elements of the cAMP-generating system as potential biomarkers for depression and antidepressant action, these were examined in LCLs. Initial gene and protein expression profiles for signaling cascades related to neuroendocrine and inflammatory functions differ among the three groups. Growth factor genes, including VEGFA and BDNF were significantly down-regulated in cells from depressed subjects. In addition, omega-3 polyunsaturated fatty acids (n-3 PUFAs) have been reported to act as both antidepressants and anti-inflammatories, but the mechanisms for these effects are not established. Here we showed that n-3 PUFAs and escitalopram (selective serotonin reuptake inhibitors, SSRIs) treatment increased adenylyl cyclase (AC) and BDNF gene expression in LCLs. These data are consistent with clinical observations showing that n-3 PUFA and SSRI have antidepressant affects, which may be additive. Contrary to observations made in neuronal and glial cells, n-3 PUFA treatment attenuated cAMP accumulation in LCLs. However, while lymphoblasts show paradoxical responses to neurons and glia, patient-derived lymphoblasts appear to carry potential depression biomarkers making them an important tool for studying precision medicine in depressive patients. Furthermore, these data validate usefulness of n-3 PUFAs in treatment for depression., (© 2020. The Author(s), under exclusive licence to Springer Nature Limited.)

Migraine is the sixth most prevalent disease worldwide but the mechanisms that underlie migraine chronicity are poorly understood. Cytoskeletal flexibility is fundamental to neuronal-plasticity and is dependent on dynamic microtubules. Histone-deacetylase-6 (HDAC6) decreases microtubule dynamics by deacetylating its primary substrate, α-tubulin. We use validated mouse models of migraine to show that HDAC6-inhibition is a promising migraine treatment and reveal an undiscovered cytoarchitectural basis for migraine chronicity. The human migraine trigger, nitroglycerin, produced chronic migraine-associated pain and decreased neurite growth in headache-processing regions, which were reversed by HDAC6 inhibition. Cortical spreading depression (CSD), a physiological correlate of migraine aura, also decreased cortical neurite growth, while HDAC6-inhibitor restored neuronal complexity and decreased CSD. Importantly, a calcitonin gene-related peptide receptor antagonist also restored blunted neuronal complexity induced by nitroglycerin. Our results demonstrate that disruptions in neuronal cytoarchitecture are a feature of chronic migraine, and effective migraine therapies might include agents that restore microtubule/neuronal plasticity., Competing Interests: ZB, HS, ID, KS, AT, WW, ZS, PS, CC, EM, MA, VP, BK, PP, SB, MR, AP No competing interests declared, (© 2021, Bertels et al.)

Cytoskeletal proteins and post-translational modifications play a role in mood disorders. Post-translational modifications of tubulin also alter microtubule dynamics. Furthermore, tubulin interacts closely with Gα s , the G-protein responsible for activation of adenylyl cyclase. Postmortem tissue derived from depressed suicide brain showed increased Gα s in lipid-raft domains compared with normal subjects. Gα s , when ensconced in lipid rafts, couples less effectively with adenylyl cyclase to produce cAMP, and this is reversed by antidepressant treatment. A recent in vitro study demonstrated that tubulin anchors Gα s to lipid rafts and that increased tubulin acetylation (due to HDAC6 inhibition) and antidepressant treatment decreased the proportion of Gα s complexed with tubulin. This suggested that deacetylated-tubulin might be more prevalent in depression. This study examined tubulin acetylation in whole-tissue homogenate, plasma membrane, and lipid-raft membrane domains in tissue from normal control subjects, depressed suicides, and depressed nonsuicides (human males/females). While tissue homogenate showed no changes in tubulin acetylation between control, depressed suicides, and depressed nonsuicides, plasma membrane-associated tubulin showed significant decreases in acetylation from depressed suicides and depressed nonsuicides compared with controls. No change was seen in expression of the enzymes responsible for tubulin acetylation or deacetylation. These data suggest that, during depression, membrane-localized tubulin maintains a lower acetylation state, permitting increased sequestration of Gα s in lipid-raft domains, where it is less likely to couple to adenylyl cyclase for cAMP production. Thus, membrane tubulin may play a role in mood disorders, which could be exploited for diagnosis and treatment. SIGNIFICANCE STATEMENT There is little understanding about the molecular mechanisms involved in the development of depression and, in severe cases, suicide. Evidence for the role of microtubule modifications in progression of depressive disorders is emerging. These postmortem data provide strong evidence for membrane tubulin modification leading to reduced efficacy of the G protein, Gα s , in depression. This study reveals a direct link between decreased tubulin acetylation in human depression and the increased localization of Gα s in lipid-raft domains responsible for attenuated cAMP signaling. The evidence presented here suggest a novel diagnostic and therapeutic locus for depression., (Copyright © 2020 the authors.)

Background: Brain-derived neurotrophic factor (BDNF) has an essential role in synaptic plasticity and neurogenesis. BDNF mediates amygdala-dependent learning for both aversive and appetitive emotional memories. The expression of BDNF in limbic regions is posited to contribute the development of depression, and amygdala responsivity is a potential marker of depressive state., Methods: The present study examined the relationship between platelet BDNF levels and amygdala volume and function in major depressive disorder (MDD). Participants were 23 MDD (mean age 38.9 years) and 23 healthy controls (mean age 38.8 years). All participants were recruited from the community. MDD participants were in a current depressive episode of moderate severity and medication-free. Amygdala responses were acquired during a functional MRI task of implicit emotional processing with sad facial expressions., Results: Significant correlation was observed between platelet BDNF levels and left amygdala responses, but no significant correlations were found with right amygdala responses or with amygdala volumes., Limitations: Interactions with neuroprotective as well as neurotoxic metabolites in the kyneurenine pathway were not examined., Conclusions: Relationship between BDNF levels and amygdala responsivity to emotionally salient stimuli in MDD could reflect the importance of BDNF in amygdala-dependent learning with clinical implications for potential pathways for treatment., Competing Interests: Declaration of Competing Interest The study had been funded in part by Eli Lilly and Company. CHYF had held recent research grants from Eli Lilly and Company and GlaxoSmithKline. MMR had held recent research grants from Eli Lilly and Company and had received consulting fees from Eli Lilly, Roche, Sunnovion, Stroke-Med. MMR report ownership interest in Pax Neuroscience, and MMR reports partial salary support via a Merit Award from the Veterans Administration. LBM was a former employee and stockholder of Eli Lilly and Company., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Ketamine produces rapid and robust antidepressant effects in depressed patients within hours of administration, often when traditional antidepressant compounds have failed to alleviate symptoms. We hypothesized that ketamine would translocate Gα s from lipid rafts to non-raft microdomains, similarly to other antidepressants but with a distinct, abbreviated treatment duration. C6 glioma cells were treated with 10 µM ketamine for 15 min, which translocated Gα s from lipid raft domains to non-raft domains. Other NMDA antagonist did not translocate Gα s from lipid raft to non-raft domains. The ketamine-induced Gα s plasma membrane redistribution allows increased functional coupling of Gα s and adenylyl cyclase to increase intracellular cyclic adenosine monophosphate (cAMP). Moreover, increased intracellular cAMP increased phosphorylation of cAMP response element-binding protein (CREB), which, in turn, increased BDNF expression. The ketamine-induced increase in intracellular cAMP persisted after knocking out the NMDA receptor indicating an NMDA receptor-independent effect. Furthermore, 10 µM of the ketamine metabolite (2R,6R)-hydroxynorketamine (HNK) also induced Gα s redistribution and increased cAMP. These results reveal a novel antidepressant mechanism mediated by acute ketamine treatment that may contribute to ketamine's powerful antidepressant effect. They also suggest that the translocation of Gα s from lipid rafts is a reliable hallmark of antidepressant action that might be exploited for diagnosis or drug development.

Glycosphingolipid (GSL) accumulation is implicated in the neuropathology of several lysosomal conditions, such as Krabbe disease, and may also contribute to neuronal and glial dysfunction in adult-onset conditions such as Parkinson's disease, Alzheimer's disease and multiple sclerosis. GSLs accumulate in cellular membranes and disrupt their structure; however, how membrane disruption leads to cellular dysfunction remains unknown. Using authentic cellular and animal models for Krabbe disease, we provide a mechanism explaining the inactivation of lipid raft (LR)-associated IGF-1-PI3K-Akt-mTORC2, a pathway of crucial importance for neuronal function and survival. We show that psychosine, the GSL that accumulates in Krabbe disease, leads to a dose-dependent LR-mediated inhibition of this pathway by uncoupling IGF-1 receptor phosphorylation from downstream Akt activation. This occurs by interfering with the recruitment of PI3K and mTORC2 to LRs. Akt inhibition can be reversed by sustained IGF-1 stimulation, but only during a time window before psychosine accumulation reaches a threshold level. Our study shows a previously unknown connection between LR-dependent regulation of mTORC2 activity at the cell surface and a genetic neurodegenerative disease. Our results show that LR disruption by psychosine desensitizes cells to extracellular growth factors by inhibiting signal transmission from the plasma membrane to intracellular compartments. This mechanism serves also as a mechanistic model to understand how alterations of the membrane architecture by the progressive accumulation of lipids undermines cell function, with potential implications in other genetic sphingolipidoses and adult neurodegenerative conditions. This article has an associated First Person interview with the first author of the paper., Competing Interests: Competing interestsThe authors declare no competing interests, except for E.R.B., who is a consultant for Lysosomal Therapeutics, Inc. Lysosomal Therapeutics Inc. did not play a role in the study design, data collection and analysis, decision to publish, preparation of the manuscript, or financial support in the form of authors' salaries and/or research materials., (© 2019. Published by The Company of Biologists Ltd.)

The originally published version of this article contained an error in Fig. 1e (imipramine), which was a duplicate of Fig. 1a control. The correct figure appears in the correction article. This error did not affect numeric results, as quantitation shown in the paper was carried out with three correct blots, including the one shown below.

Lipid microenvironments in the plasma membrane are known to influence many signal transduction pathways. Several of those pathways are critical for both the etiology and treatment of depression. Further, several signaling proteins are modified, covalently, by lipids, a process that alters their interface with the microenvironments mentioned above. This review presents a brief discussion of the interface of the above elements as well as a discussion about the participation of lipids and lipid moieties in the action of antidepressants., (© 2019 Elsevier Inc. All rights reserved.)

Progress toward new antidepressant therapies has been relatively slow over the past few decades, with the result that individuals suffering from depression often struggle to find an effective treatment - a process often requiring months. Furthermore, the neural factors that contribute to depression remain poorly understood, and there are many open questions regarding the mechanism of action of existing antidepressants. A better understanding of the molecular processes that underlie depression and contribute to antidepressant efficacy is therefore badly needed. In this review we highlight research investigating the role of G-proteins and the regulators of G-protein signaling (RGS) proteins, two protein families that are intimately involved in both the genesis of depressive states and the action of antidepressant drugs. Many antidepressants are known to indirectly affect the function of these proteins. Conversely, dysfunction of the G-protein and RGS systems can affect antidepressant efficacy. However, a great deal remains unknown about how these proteins interact with antidepressants. Findings pertinent to each individual G-protein and RGS protein are summarized from in vitro , in vivo , and clinical studies.

Current antidepressant therapies meet with variable therapeutic success and there is increasing interest in therapeutic approaches not based on monoamine signaling. Histone deacetylase 6 (HDAC6), which also deacetylates α-tubulin shows altered expression in mood disorders and HDAC6 knockout mice mimic traditional antidepressant treatments. Nonetheless, a mechanistic understanding for HDAC6 inhibitors in the treatment of depression remains elusive. Previously, we have shown that sustained treatment of rats or glioma cells with several antidepressants translocates Gα s from lipid rafts toward increased association with adenylyl cyclase (AC). Concomitant with this is a sustained increase in cAMP production. While Gα s modifies microtubule dynamics, tubulin also acts as an anchor for Gα s in lipid-rafts. Since HDAC-6 inhibitors potentiate α-tubulin acetylation, we hypothesize that acetylation of α-tubulin disrupts tubulin-Gα s raft-anchoring, rendering Gα s free to activate AC. To test this, C6 Glioma (C6) cells were treated with the HDAC-6 inhibitor, tubastatin-A. Chronic treatment with tubastatin-A not only increased α-tubulin acetylation but also translocated Gα s from lipid-rafts, without changing total Gα s . Reciprocally, depletion of α-tubulin acetyl-transferase-1 ablated this phenomenon. While escitalopram and imipramine also disrupt Gα s /tubulin complexes and translocate Gα s from rafts, they evoke no change in tubulin acetylation. Finally, two indicators of downstream cAMP signaling, cAMP response element binding protein phosphorylation (pCREB) and expression of brain-derived-neurotrophic-factor (BDNF) were both elevated by tubastatin-A. These findings suggest HDAC6 inhibitors show a cellular profile resembling traditional antidepressants, but have a distinct mode of action. They also reinforce the validity of antidepressant-induced Gα s translocation from lipid-rafts as a biosignature for antidepressant response that may be useful in the development of new antidepressant compounds.

We previously reported that 3T3-L1 cells express a functional sweet taste receptor possibly as a T1R3 homomer that is coupled to Gs and negatively regulates adipogenesis by a Gαs-mediated but cAMP-independent mechanism. Here, we show that stimulation of this receptor with sucralose or saccharin induced disassembly of the microtubules in 3T3-L1 preadipocytes, which was attenuated by overexpression of the dominant-negative mutant of Gαs (Gαs-G226A). In contrast, overexpression of the constitutively active mutant of Gαs (Gαs-Q227L) as well as treatment with cholera toxin or isoproterenol but not with forskolin caused disassembly of the microtubules. Sweetener-induced microtubule disassembly was accompanied by activation of RhoA and Rho-associated kinase (ROCK). This was attenuated with by knockdown of GEF-H1, a microtubule-localized guanine nucleotide exchange factor for Rho GTPase. Furthermore, overexpression of the dominant-negative mutant of RhoA (RhoA-T19N) blocked sweetener-induced dephosphorylation of Akt and repression of PPARγ and C/EBPα in the early phase of adipogenic differentiation. These results suggest that the T1R3 homomeric sweet taste receptor negatively regulates adipogenesis through Gαs-mediated microtubule disassembly and consequent activation of the Rho/ROCK pathway.

Depression is the leading cause of disability worldwide, and even though many forms of therapy exist, about one third of patients treated with conventional antidepressants do not experience a response. For these reasons, new approaches to treat depression, including fish oil, are being investigated. Fish oil is known to have many beneficial side effects, and clinical trials demonstrate that supplementation with fish oil is beneficial in the management of depression. Fish oil contains omega-3 polyunsaturated fatty acids (PUFA), and there are several mechanisms by which PUFAs are thought to induce an antidepressant effect, including anti-inflammatory action and direct effects on membrane properties. This review will analyze and evaluate the clinical trials surrounding fish oil use in the treatment of depression, and will also review the likely sites of action of PUFAs at the cell membrane with special attention being placed on lipid rafts and G-proteins.

Depression is a significant public health problem for which currently available medications, if effective, require weeks to months of treatment before patients respond. Previous studies have shown that the G protein responsible for increasing cAMP (Gαs) is increasingly localized to lipid rafts in depressed subjects and that chronic antidepressant treatment translocates Gαs from lipid rafts. Translocation of Gαs, which shows delayed onset after chronic antidepressant treatment of rats or of C6 glioma cells, tracks with the delayed onset of therapeutic action of antidepressants. Because antidepressants appear to specifically modify Gαs localized to lipid rafts, we sought to determine whether structurally diverse antidepressants accumulate in lipid rafts. Sustained treatment of C6 glioma cells, which lack 5-hydroxytryptamine transporters, showed marked concentration of several antidepressants in raft fractions, as revealed by increased absorbance and by mass fingerprint. Closely related molecules without antidepressant activity did not concentrate in raft fractions. Thus, at least two classes of antidepressants accumulate in lipid rafts and effect translocation of Gαs to the non-raft membrane fraction, where it activates the cAMP-signaling cascade. Analysis of the structural determinants of raft localization may both help to explain the hysteresis of antidepressant action and lead to design and development of novel substrates for depression therapeutics., (© 2016 by The American Society for Biochemistry and Molecular Biology, Inc.)

Background: Plasma membrane localization can play a significant role in the ultimate function of certain proteins. Specific membrane domains like lipid rafts have been shown to be inhibitory domains to a number of signaling proteins, including Gsα, and chronic antidepressant treatment facilitates Gs signaling by removing Gsα form lipid rafts. The intent of this study is to compare the effects of the selective serotnin reuptake inhibitor, escitalopram, with that of the mood stabilizing drug, lithium., Results: There are a number of mechanisms of action proposed for lithium as a mood stabilizing agent, but the interactions between G proteins (particularly Gs) and mood stabilizing drugs are not well explored. Of particular interest was the possibility that there was some effect of mood stabilizers on the association between Gsα and cholesterol-rich membrane microdomains (lipid rafts), similar to that seen with long-term antidepressant treatment. This was examined by biochemical and imaging (fluorescence recovery after photobleaching: FRAP) approaches. Results indicate that escitalopram was effective at liberating Gsα from lipid rafts while lithium was not., Conclusions: There are a number of drug treatments for mood disorders and yet there is no unifying hypothesis for a cellular or molecular basis of action. It is evident that there may in fact not be a single mechanism, but rather a number of different mechanisms that converge at a common point. The results of this study indicate that the mood stabilizing agent, lithium, and the selective serotonin reuptake inhibitor, escitalopram, act on their cellular targets through mutually exclusive pathways. These results also validate the hypothesis that translocation of Gsα from lipid rafts could serve as a biosignature for antidepressant action.

The 5-hydroxytryptamine (5-HT, also known as serotonin) subtype 6 receptor (5-HT6R, also known as HTR6) plays roles in cognition, anxiety and learning and memory disorders, yet new details concerning its regulation remain poorly understood. In this study, we found that 5-HT6R directly interacted with SNX14 and that this interaction dramatically increased internalization and degradation of 5-HT6R. Knockdown of endogenous SNX14 had the opposite effect. SNX14 is highly expressed in the brain and contains a putative regulator of G-protein signaling (RGS) domain. Although its RGS domain was found to be non-functional as a GTPase activator for Gαs, we found that it specifically bound to and sequestered Gαs, thus inhibiting downstream cAMP production. We further found that protein kinase A (PKA)-mediated phosphorylation of SNX14 inhibited its binding to Gαs and diverted SNX14 from Gαs binding to 5-HT6R binding, thus facilitating the endocytic degradation of the receptor. Therefore, our results suggest that SNX14 is a dual endogenous negative regulator in 5-HT6R-mediated signaling pathway, modulating both signaling and trafficking of 5-HT6R., (© 2015. Published by The Company of Biologists Ltd.)

Signals that activate the G protein Gαs and promote neuronal differentiation evoke Gαs internalization in rat pheochromocytoma (PC12) cells. These agents also significantly increase Gαs association with microtubules, resulting in an increase in microtubule dynamics because of the activation of tubulin GTPase by Gαs. To determine the function of Gαs/microtubule association in neuronal development, we used real-time trafficking of a GFP-Gαs fusion protein. GFP-Gαs concentrates at the distal end of the neurites in differentiated living PC12 cells as well as in cultured hippocampal neurons. Gαs translocates to specialized membrane compartments at tips of growing neurites. A dominant-negative Gα chimera that interferes with Gαs binding to tubulin and activation of tubulin GTPase attenuates neurite elongation and neurite number both in PC12 cells and primary hippocampal neurons. This effect is greatest on differentiation induced by activated Gαs. Together, these data suggest that activated Gαs translocates from the plasma membrane and, through interaction with tubulin/microtubules in the cytosol, is important for neurite formation, development, and outgrowth. Characterization of neuronal G protein dynamics and their contribution to microtubule dynamics is important for understanding the molecular mechanisms by which G protein-coupled receptor signaling orchestrates neuronal growth and differentiation., (© 2015 by The American Society for Biochemistry and Molecular Biology, Inc.)

Background: Longitudinal neuroimaging studies of major depressive disorder (MDD) have most commonly assessed the effects of antidepressants from the serotonin reuptake inhibitor class and usually reporting a single measure. Multimodal neuroimaging assessments were acquired from MDD patients during an acute depressive episode with serial measures during a 12-week treatment with the serotonin-norepinephrine reuptake inhibitor (SNRI) duloxetine., Methods: Participants were medication-free MDD patients (n = 32; mean age 40.2 years) in an acute depressive episode and healthy controls matched for age, gender, and IQ (n = 25; mean age 38.8 years). MDD patients received treatment with duloxetine 60 mg daily for 12 weeks with an optional dose increase to 120 mg daily after 8 weeks. All participants had serial imaging at weeks 0, 1, 8, and 12 on a 3 Tesla magnetic resonance imaging (MRI) scanner. Neuroimaging tasks included emotional facial processing, negative attentional bias (emotional Stroop), resting state functional MRI and structural MRI., Results: A significant group by time interaction was identified in the anterior default mode network in which MDD patients showed increased connectivity with treatment, while there were no significant changes in healthy participants. In the emotional Stroop task, increased posterior cingulate activation in MDD patients normalized following treatment. No significant group by time effects were observed for happy or sad facial processing, including in amygdala responsiveness, or in regional cerebral volumes. Reduced baseline resting state connectivity within the orbitofrontal component of the default mode network was predictive of clinical response. An early increase in hippocampal volume was predictive of clinical response., Conclusions: Baseline resting state functional connectivity was predictive of subsequent clinical response. Complementary effects of treatment were observed from the functional neuroimaging correlates of affective facial expressions, negative attentional bias, and resting state. No significant effects were observed in affective facial processing, while the interaction effect in negative attentional bias and individual group effects in resting state connectivity could be related to the SNRI class of antidepressant medication. The specificity of the observed effects to SNRI pharmacological treatments requires further investigation., Trial Registration: Registered at clinicaltrials.gov ( NCT01051466 ).

GPCR signaling is modified both in major depressive disorder and by chronic antidepressant treatment. Endogenous Gαs redistributes from raft- to nonraft-membrane fractions after chronic antidepressant treatment. Modification of G protein anchoring may participate in this process. Regulation of Gαs signaling by antidepressants was studied using fluorescence recovery after photobleaching (FRAP) of GFP-Gαs. Here we find that extended antidepressant treatment both increases the half-time of maximum recovery of GFP-Gαs and decreases the extent of recovery. Furthermore, this effect parallels the movement of Gαs out of lipid rafts as determined by cold detergent membrane extraction with respect to both dose and duration of drug treatment. This effect was observed for several classes of compounds with antidepressant activity, whereas closely related molecules lacking antidepressant activity (eg, R-citalopram) did not produce the effect. These results are consistent with previously observed antidepressant-induced translocation of Gαs, but also suggest an alternate membrane attachment site for this G protein. Furthermore, FRAP analysis provides the possibility of a relatively high-throughput screening tool for compounds with putative antidepressant activity.

G proteins mediate signals from membrane G protein coupled receptors to the cell interior, evoking significant regulation of cell physiology. The cytoskeleton contributes to cell morphology, motility, division, and transport functions. This review will discuss the interplay between heterotrimeric G protein signaling and elements of the cytoskeleton. Also described and discussed will be the interplay between tubulin and G proteins that results in atypical modulation of signaling pathways and cytoskeletal dynamics. This will be extended to describe how tubulin and G proteins act in concert to influence various aspects of cellular behavior. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters.This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé., (© 2013.)

Dietary fish oil, a source of polyunsaturated fatty acids (n-3 PUFA), has become increasingly popular for antidepressant therapy, in part because about half of patients treated with conventional antidepressants either fail to remit or discontinue therapy due to side effects. The inception of n-3 PUFA as a putative depression therapeutic may have stemmed from reports suggesting that dietary n-3 PUFA deficiency is linked to both altered membrane PUFA content as well as clinical depression. Several studies have examined n-3 PUFA treatment in depression, either singly or in combination with conventional antidepressant drugs. While results have been encouraging, fish oil treatment remains controversial. At least some of the reason for this is the lack of a defined site of action for n-3 PUFA that would be consistent with an antidepressant effect. This review will address this issue. While it is possible, even likely, that n-3 PUFA have multiple sites of action, this chapter will focus on sites at which n-3 PUFA modify G protein signaling and how those sites relate to both depression and antidepressant action. Much of the focus herein will be on specialized membrane domains (lipid rafts) and the effects that agents modifying those rafts have on elements of G protein signaling cascades. The relevance of specific alterations of G protein signaling for both depression and antidepressant action will be discussed, as will the ability for n-3 PUFA to act either as an antidepressant or in concert with conventional antidepressants.

Microtubules, major components of the cytoskeleton, play important roles in a variety of cellular functions including mitosis, intracellular transport, and the modulation of cell morphology. Several studies have demonstrated that specific G-protein alpha subunits bind to tubulin with a high affinity (~130 nM) and elicit various functional effects on tubulin and microtubules. In this chapter, we present a description of the protocols for several methods that are used to determine the interaction between heterotrimeric G proteins and tubulin, as well as functional consequences of the interactions including protocols for protein purification, binding assays, tubulin GTPase assays, microtubule dynamics assays, and assays for cytoskeletal consequences of G-protein-coupled receptor signaling., (Copyright © 2013 Elsevier Inc. All rights reserved.)

This volume describes a series of psychiatric and neuropsychiatric disorders, connects some aspects of somatic and psychiatric medicine, and describes various current and emerging therapies. The purpose of this chapter is to set the stage for the volume by developing the theoretical basis of synaptic transmission and introducing the various neurotransmitters and their receptors involved in the process. The intent is to provide not only a historical context through which to understand neurotransmitters, but a current contextual basis for understanding neuronal signal transduction and applying this knowledge to facilitate treatment of maladies of the brain and mind., (Copyright © 2012 Elsevier B.V. All rights reserved.)

Although evidence has shown the regulating effect of n-3 poly-unsaturated fatty acid (n-3 PUFA) on cell signaling transduction, it remains unknown whether n-3 PUFA treatment modulates estrogen signaling. The current study showed that docosahexaenoic acid (DHA, C22:6), eicosapentaenoic acid (EPA, C20:5) shifted the pro-survival and proliferative effect of estrogen to a pro-apoptotic effect in human breast cancer (BCa) MCF-7 and T47D cells. 17 β-estradiol (E2) enhanced the inhibitory effect of n-3 PUFAs on BCa cell growth. The IC50 of DHA or EPA in MCF-7 cells decreased when combined with E2 (10 nM) treatment (from 173 µM for DHA only to 113 µM for DHA+E2, and from 187 µm for EPA only to 130 µm for EPA+E2). E2 also augmented apoptosis in n-3 PUFA-treated BCa cells. In contrast, in cells treated with stearic acid (SA, C18:0) as well as cells not treated with fatty acid, E2 promoted breast cancer cell growth. Classical (nuclear) estrogen receptors may not be involved in the pro-apoptotic effects of E2 on the n-3 PUFA-treated BCa cells because ERα agonist failed to elicit, and ERα knockdown failed to block E2 pro-apoptotic effects. Subsequent studies reveal that G protein coupled estrogen receptor 1 (GPER1) may mediate the pro-apoptotic effect of estrogen. N-3 PUFA treatment initiated the pro-apoptotic signaling of estrogen by increasing GPER1-cAMP-PKA signaling response, and blunting EGFR, Erk 1/2, and AKT activity. These findings may not only provide the evidence to link n-3 PUFAs biologic effects and the pro-apoptotic signaling of estrogen in breast cancer cells, but also shed new insight into the potential application of n-3 PUFAs in BCa treatment.

The heterotrimeric, G protein-coupled receptor-associated G protein, Gα(s), binds tubulin with nanomolar affinity and disrupts microtubules in cells and in vitro. Here we determine that the activated form of Gα(s) binds tubulin with a K(D) of 100 nm, stimulates tubulin GTPase, and promotes microtubule dynamic instability. Moreover, the data reveal that the α3-β5 region of Gα(s) is a functionally important motif in the Gα(s)-mediated microtubule destabilization. Indeed, peptides corresponding to that region of Gα(s) mimic Gα(s) protein in activating tubulin GTPase and increase microtubule dynamic instability. We have identified specific mutations in peptides or proteins that interfere with this process. The data allow for a model of the Gα(s)/tubulin interface in which Gα(s) binds to the microtubule plus-end and activates the intrinsic tubulin GTPase. This model illuminates both the role of tubulin as an "effector" (e.g. adenylyl cyclase) for Gα(s) and the role of Gα(s) as a GTPase activator for tubulin. Given the ability of Gα(s) to translocate intracellularly in response to agonist activation, Gα(s) may play a role in hormone- or neurotransmitter-induced regulation of cellular morphology.

Guanylyl cyclases (GCs), a ubiquitous family of enzymes that metabolize GTP to cyclic GMP (cGMP), are traditionally divided into membrane-bound forms (GC-A-G) that are activated by peptides and cytosolic forms that are activated by nitric oxide (NO) and carbon monoxide. However, recent data has shown that NO activated GC's (NOGC) also may be associated with membranes. In the present study, interactions of guanylyl cyclase A (GC-A), a caveolae-associated, membrane-bound, homodimer activated by atrial natriuretic peptide (ANP), with NOGC, a heme-containing heterodimer (alpha/beta) beta1 isoform of the beta subunit of NOGC (NOGCbeta1) was specifically focused. NOGCbeta1 co-localized with GC-A and caveolin on the membrane in human kidney (HK-2) cells. Interaction of GC-A with NOGCbeta1 was found using immunoprecipitations. In a second set of experiments, the possibility that NOGCbeta1 regulates signaling by GC-A in HK-2 cells was explored. ANP-stimulated membrane guanylyl cyclase activity (0.05 +/- 0.006 pmol/mg protein/5 min; P < 0.01) and intra cellular GMP (18.1 +/- 3.4 vs. 1.2 +/- 0.5 pmol/mg protein; P < 0.01) were reduced in cells in which NOGCbeta1 abundance was reduced using specific siRNA to NOGCbeta1. On the other hand, ANP-stimulated cGMP formation was increased in cells transiently transfected with NOGCbeta1 (530.2 +/- 141.4 vs. 26.1 +/- 13.6 pmol/mg protein; P < 0.01). siRNA to NOGCbeta1 attenuated inhibition of basolateral Na/K ATPase activity by ANP (192 +/- 22 vs. 92 +/- 9 nmol phosphate/mg protein/min; P < 0.05). In summary, the results show that NOGCbeta1 and GC-A interact and that NOGCbeta1 regulates ANP signaling in HK-2 cells. The results raise the novel possibility of cross-talk between NOGC and GC-A signaling pathways in membrane caveolae.

Chronic antidepressant treatment has been shown to increase adenylyl cyclase activity, in part, due to translocation of Galpha(s) from lipid rafts to a nonraft fraction of the plasma membrane where they engage in a more facile stimulation of adenylyl cyclase. This effect holds for multiple classes of antidepressants, and for serotonin uptake inhibitors, it occurs in the absence of the serotonin transporter. In the present study, we examined the change in the amount of Galpha(s) in lipid raft and whole cell lysate after exposing C6 cells to escitalopram. The results showed that chronic (but not acute) escitalopram decreased the content of Galpha(s) in lipid rafts, whereas there was no change in overall Galpha(s) content. These effects were drug dose- and exposure time-dependent. Although R-citalopram has been reported to antagonize some effects of escitalopram, this compound was without effect on Galpha(s) localization in lipid rafts, and R-citalopram did not inhibit these actions of escitalopram. Escitalopram treatment increased cAMP accumulation, and this seemed due to increased coupling between Galpha(s) and adenylyl cyclase. Thus, escitalopram is potent, rapid and efficacious in translocating Galpha(s) from lipid rafts, and this effect seems to occur independently of 5-hydroxytryptamine transporters. Our results suggest that, although antidepressants display distinct affinities for well identified targets (e.g., monoamine transporters), several presynaptic and postsynaptic molecules are probably modified during chronic antidepressant treatment, and these additional targets may be required for clinical efficacy of these drugs.

Lipid rafts and caveolae are specialized membrane microdomains implicated in regulating G protein-coupled receptor signaling cascades. Previous studies have suggested that rafts/caveolae may regulate beta-adrenergic receptor/Galpha(s) signaling, but underlying molecular mechanisms are largely undefined. Using a simplified model system in C6 glioma cells, this study disrupts rafts/caveolae using both pharmacological and genetic approaches to test whether caveolin-1 and lipid microdomains regulate G(s) trafficking and signaling. Lipid rafts/caveolae were disrupted in C6 cells by either short-term cholesterol chelation using methyl-beta-cyclodextrin or by stable knockdown of caveolin-1 and -2 by RNA interference. In imaging studies examining Galpha(s)-GFP during signaling, stimulation with the betaAR agonist isoproterenol resulted in internalization of Galpha(s)-GFP; however, this trafficking was blocked by methyl-beta-cyclodextrin or by caveolin knockdown. Caveolin knockdown significantly decreased Galpha(s) localization in detergent insoluble lipid raft/caveolae membrane fractions, suggesting that caveolin localizes a portion of Galpha(s) to these membrane microdomains. Methyl-beta-cyclodextrin or caveolin knockdown significantly increased isoproterenol or thyrotropin-stimulated cAMP accumulation. Furthermore, forskolin- and aluminum tetrafluoride-stimulated adenylyl cyclase activity was significantly increased by caveolin knockdown in cells or in brain membranes obtained from caveolin-1 knockout mice, indicating that caveolin attenuates signaling at the level of Galpha(s)/adenylyl cyclase and distal to GPCRs. Taken together, these results demonstrate that caveolin-1 and lipid microdomains exert a major effect on Galpha(s) trafficking and signaling. It is suggested that lipid rafts/caveolae are sites that remove Galpha(s) from membrane signaling cascades and caveolins might dampen globally Galpha(s)/adenylyl cyclase/cAMP signaling.

It is now evident that Galpha(s) traffics into cytosol following G protein-coupled receptor activation, and alpha subunits of some heterotrimeric G-proteins, including Galpha(s) bind to tubulin in vitro. Nevertheless, many features of G-protein-microtubule interaction and possible intracellular effects of G protein alpha subunits remain unclear. In this study, several biochemical approaches demonstrated that activated Galpha(s) directly bound to tubulin and cellular microtubules, and fluorescence microscopy showed that cholera toxin-activated Galpha(s) colocalized with microtubules. The activated, GTP-bound, Galpha(s) mimicked tubulin in serving as a GTPase activator for beta-tubulin. As a result, activated Galpha(s) made microtubules more dynamic, both in vitro and in cells, decreasing the pool of insoluble microtubules without changing total cellular tubulin content. The amount of acetylated tubulin (an indicator of microtubule stability) was reduced in the presence of Galpha(s) activated by mutation. Previous studies showed that cholera toxin and cAMP analogs may stimulate neurite outgrowth in PC12 cells. However, in this study, overexpression of a constitutively activated Galpha(s) or activation of Galpha(s) with cholera toxin in protein kinase A-deficient PC12 cells promoted neurite outgrowth in a cAMP-independent manner. Thus, it is suggested that activated Galpha(s) acts as an intracellular messenger to regulate directly microtubule dynamics and promote neurite outgrowth. These data serve to link G-protein signaling with modulation of the cytoskeleton and cell morphology.

A large percentage of current drugs target G-protein-coupled receptors, which couple to well-known signaling pathways involving cAMP or calcium. G-proteins themselves may subserve a second messenger function. Here, we review the role of tubulin and microtubules in directly mediating effects of heterotrimeric G-proteins on neuronal outgrowth, shape and differentiation. G-protein-tubulin interactions appear to be regulated by neurotransmitter activity, and, in turn, regulate the location of Galpha in membrane microdomains (such as lipid rafts) or the cytosol. Tubulin binds with nanomolar affinity to Gsalpha, Gialpha1 and Gqalpha (but not other Galpha subunits) as well as Gbeta(1)gamma(2) subunits. Galpha subunits destabilize microtubules by stimulating tubulin's GTPase, while Gbetagamma subunits promote microtubule stability. The same region on Gsalpha that binds adenylyl cyclase and Gbetagamma also interacts with tubulin, suggesting that cytoskeletal proteins are novel Galpha effectors. Additionally, intracellular Gialpha-GDP, in concert with other GTPase proteins and Gbetagamma, regulates the position of the mitotic spindle in mitosis. Thus, G-protein activation modulates cell growth and differentiation by directly altering microtubule stability. Further studies are needed to fully establish a structural mechanism of this interaction and its role in synaptic plasticity., (Copyright 2009 S. Karger AG, Basel.)

Heterotrimeric Gproteins participate in signal transduction by transferring signals from cell surface receptors to intracellular effector molecules. Gproteins also interact with microtubules and participate in microtubule-dependent centrosome/chromosome movement during cell division, as well as neuronal differentiation. In recent years, significant progress has been made in our understanding of the biochemical/functional interactions between Gprotein subunits (alpha and betagamma) and microtubules, and the molecular details emerging from these studies suggest that alpha and betagamma subunits of Gproteins interact with tubulin/microtubules to regulate the assembly/dynamics of microtubules, providing a novel mechanism for hormone- or neurotransmitter-induced rapid remodeling of cytoskeleton, regulation of the mitotic spindle for centrosome/chromosome movements in cell division, and neuronal differentiation in which structural plasticity mediated by microtubules is important for appropriate synaptic connections and signal transmission.

A number of studies have demonstrated interplay between the cytoskeleton and G protein signaling. Many of these studies have determined a specific interaction between tubulin, the building block of microtubules, and G proteins. The alpha subunits of some heterotrimeric G proteins, including Gsalpha, have been shown to interact strongly with tubulin. Binding of Galpha to tubulin results in increased dynamicity of microtubules due to activation of GTPase of tubulin. Tubulin also activates Gsalpha via a direct transfer of GTP between these molecules. Structural insight into the interaction between tubulin and Gsalpha was required, and was determined, in this report, through biochemical and molecular docking techniques. Solid phase peptide arrays suggested that a portion of the amino terminus, alpha2-beta4 (the region between switch II and switch III) and alpha3-beta5 (just distal to the switch III region) domains of Gsalpha are important for interaction with tubulin. Molecular docking studies revealed the best-fit models based on the biochemical data, showing an interface between the two molecules that includes the adenylyl cyclase/Gbetagamma interaction regions of Gsalpha and the exchangeable nucleotide-binding site of tubulin. These structural models explain the ability of tubulin to facilitate GTP exchange on Galpha and the ability of Galpha to activate tubulin GTPase.

Recent in vivo and in vitro studies have demonstrated that Gs alpha migrates from a Triton X-100 (TX-100)-insoluble membrane domain (lipid raft) to a TX-100-soluble nonraft membrane domain in response to chronic, but not acute, treatment with tricyclic or selective serotonin reuptake inhibitor antidepressants. This migration resulted in a more facile association with adenylyl cyclase. Our hypothesis is that Gs alpha may be ensconced, to a greater extent, in lipid rafts during depression, and that one action of chronic antidepressant treatment is to reverse this. In this postmortem study, we examined Gs alpha membrane localization in the cerebellum and prefrontal cortex of brains from nonpsychiatric control subjects and suicide cases with confirmed unipolar depression. Sequential TX-100 and TX-114 detergent extractions were performed on the brain tissue. In the cerebellum, the ratio of TX-100/TX-114-soluble Gs alpha is approximately 2:1 for control versus depressed suicides. Results with prefrontal cortex samples from each group demonstrate a similar trend. These data suggest that depression localizes Gs alpha to a membrane domain (lipid rafts) where it is less likely to couple to adenylyl cyclase and that antidepressants may upregulate Gs alpha signaling via disruption of membrane microenvironments. Raft localization of Gs alpha in human peripheral tissue may thus serve as a biomarker for depression and as a harbinger of antidepressant responsiveness.

The betagamma subunit of G proteins (Gbetagamma) is known to transfer signals from cell surface receptors to intracellular effector molecules. Recent results suggest that Gbetagamma also interacts with microtubules and is involved in the regulation of the mitotic spindle. In the current study, the anti-microtubular drug nocodazole was employed to investigate the mechanism by which Gbetagamma interacts with tubulin and its possible implications in microtubule assembly in cultured PC12 cells. Nocodazole-induced depolymerization of microtubules drastically inhibited the interaction between Gbetagamma and tubulin. Gbetagamma was preferentially bound to microtubules and treatment with nocodazole suggested that the dissociation of Gbetagamma from microtubules is an early step in the depolymerization process. When microtubules were allowed to recover after removal of nocodazole, the tubulin-Gbetagamma interaction was restored. Unlike Gbetagamma, however, the interaction between tubulin and the alpha subunit of the Gs protein (Gsalpha) was not inhibited by nocodazole, indicating that the inhibition of tubulin-Gbetagamma interactions during microtubule depolymerization is selective. We found that Gbetagamma also interacts with gamma-tubulin, colocalizes with gamma-tubulin in centrosomes, and co-sediments in centrosomal fractions. The interaction between Gbetagamma and gamma-tubulin was unaffected by nocodazole, suggesting that the Gbetagamma-gamma-tubulin interaction is not dependent on assembled microtubules. Taken together, our results suggest that Gbetagamma may play an important and definitive role in microtubule assembly and/or stability. We propose that betagamma-microtubule interaction is an important step for G protein-mediated cell activation. These results may also provide new insights into the mechanism of action of anti-microtubule drugs.

The cellular site of formation, Galpha-coupling preference, and agonist regulation of mu-delta opioid receptor (OR) heterooligomers were studied. Bioluminescence resonance energy transfer (BRET) showed that mu-deltaOR heterooligomers, composed of preformed mu and delta homooligomers, interacted constitutively in the endoplasmic reticulum (ER) with Galpha-proteins forming heteromeric signaling complexes before being targeted to the plasma membrane. Compared to muOR homooligomers, the mu-delta heterooligomers showed higher affinity and efficiency of interaction for Gz over Gi, indicating a switch in G-protein preference. Treatment with DAMGO or deltorphin II led to coregulated internalization of both receptors, whereas DPDPE and DSLET had no effect on mu-delta internalization. Staggered expression resulted in non-interacting mu and delta receptors, even though both receptors were colocalized at the cell surface. Agonists failed to induce BRET between staggered receptors, and resulted in internalization solely of the receptor targeted by agonist. Thus, mu-deltaOR heterooligomers form and preferentially associate with Gz to generate a signaling complex in the ER, and have a distinct agonist-internalization profile compared to either mu or delta homooligomers.

Mastoparan, a wasp venom toxin, has various pharmacological activities, the mechanisms of which are still unknown. To clarify the action of mastoparan on G protein-coupled receptor-mediated signaling, we previously examined the effect of mastoparan on G(q)-mediated signaling and demonstrated that mastoparan binds to gangliosides causing a decrease in Galpha(q/11) content in lipid rafts, and resulting in the inhibition of G(q)-mediated phosphoinositide hydrolysis (Sugama et al., Mol. Pharmacol., 68, 1466, 2005). In the present study, we examined the effect of mastoparan on beta-adrenoceptor-G(s) signaling in 1321N1 human astrocytoma cells. Mastoparan inhibited isoproterenol-induced elevation of cyclic AMP in a concentration-dependent manner. Although mastoparan is known to be an activator of G(i), pertussis toxin only slightly attenuated mastoparan-induced inhibition of cyclic AMP elevation, suggesting that a major part of the inhibition of cyclic AMP elevation induced by mastoparan is not mediated by Galpha(i). By contrast, mastoparan-induced inhibition of cyclic AMP elevation was clearly attenuated by preincubation of the cells with ganglioside mixtures. Moreover, mastoparan changed the localization of Galpha(s) in lipid rafts without disrupting the structure of lipid rafts. Fluorescent staining analysis showed that mastoparan released GFP-Galpha(s) from plasma membranes into the cytosol. These results suggest that the mastoparan-induced suppression of cyclic AMP elevation is mainly caused by changing the localization of Galpha(s) in lipid rafts into a compartment in the cellular interior where it is not available to activate adenylyl cyclase.

Lipid rafts are specialized structures on the plasma membrane that have an altered lipid composition as well as links to the cytoskeleton. It has been proposed that these structures are membrane domains in which neurotransmitter signalling might occur through a clustering of receptors and components of receptor-activated signalling cascades. The localization of these proteins in lipid rafts, which is affected by the cytoskeleton, also influences the potency and efficacy of neurotransmitter receptors and transporters. The effect of lipid rafts on neurotransmitter signalling has also been implicated in neurological and psychiatric diseases.

Chronic melatonin exposure produces microtubule rearrangements in Chinese hamster ovary (CHO) cells expressing the human MT1 melatonin receptor while at the same time desensitizing MT1 receptors. Because microtubule rearrangements parallel MT1 receptor desensitization, we tested whether microtubules modulate receptor responsiveness. We determined whether depolymerization of microtubules by Colcemid, which prevents melatonin-induced outgrowths in MT1-expressing CHO cells, also prevents MT1 receptor desensitization by affecting G(alpha)-GTP exchange on G-proteins. In this study, we found that depolymerization of microtubules in MT1 receptor expressing cells, prevented melatonin-induced receptor desensitization reflected by an increase in the number of high potency sites when compared with melatonin-treated cells. Further examination of the mechanism(s) underlying this desensitization suggested that these effects occurred at the level of G-proteins. Depolymerization of microtubules during melatonin-induced desensitization, attenuated forskolin-induced cAMP accumulation, the opposite of which usually occurs following melatonin exposure alone. Concomitant to this attenuation in the forskolin response was a reduction in the amount of G(i alpha) protein coupled to MT1 receptors and an increase in [32P] azidoanilido GTP incorporation into G(i) proteins. These data are consistent with the findings that microtubule depolymerization did not affect MT1/G(q) coupling nor did it affect melatonin-induced phosphoinositide hydrolysis following melatonin exposure. However, interestingly, microtubule depolymerization enhanced melatonin-induced protein kinase C activation that was blocked in the presence of pertussis toxin. These data demonstrate that microtubule dynamics can modulate melatonin receptor function through their actions on G(i) proteins and impact on downstream signaling cascades.

The microtubule-associated protein tau may be involved in cell morphogenesis and axonal maintenance. In addition to microtubules, tau has been shown to interact with actin in vitro. In the present study interaction of tau and actin was investigated in PC12 cells. No interaction between tau and actin was observed without NGF treatment. Under NGF stimulation, tau distributed at ends of cellular extensions, where it associated with actin in a microtubule-independent manner. F-actin disruption revealed that relocalization and assembly of F-actin at the ends of cellular extensions were necessary for NGF-induced tau reorganization and association with actin. A truncated tau-GFP (tau(1-186)-GFP, N-terminal of tau) did not associate with actin. However, tau23(174-352)-GFP (carboxyl-terminal of Tau23) did associate with actin and the requirement for NGF was lost. Nevertheless, NGF boosted tau23(174-352)-GFP interaction with actin and promoted colocalization at the ends of cellular extensions. This suggests that the C-terminal of tau is required for associating with actin and the tau N-terminal may play a regulatory role in this process. A possible role for tau-actin interaction in neurite outgrowth is postulated.

Heterotrimeric G proteins participate in signal transduction by transferring signals from cell surface receptors to intracellular effector molecules. Interestingly, recent results suggest that G proteins also interact with microtubules and participate in cell division and differentiation. It has been shown earlier that both alpha and betagamma subunits of G proteins modulate microtubule assembly in vitro. Since G protein activation and subsequent dissociation of alpha and betagamma subunits are necessary for G proteins to participate in signaling processes, here we asked if similar activation is required for modulation of microtubule assembly by G proteins. We reconstituted Galphabetagamma heterotrimer from myristoylated-Galpha and prenylated-Gbetagamma, and found that the heterotrimer blocks Gi1alpha activation of tubulin GTPase and inhibits the ability of Gbeta1gamma2 to promote in vitro microtubule assembly. Results suggest that G protein activation is required for functional coupling between Galpha/Gbetagamma and tubulin/microtubules, and supports the notion that regulation of microtubules is an integral component of G protein mediated signaling.

The arrest of meiotic prophase in mouse oocytes within antral follicles requires the G protein G(s) and an orphan member of the G protein-coupled receptor family, GPR3. To determine whether GPR3 activates G(s), the localization of Galpha(s) in follicle-enclosed oocytes from Gpr3(+/+) and Gpr3(-/-) mice was compared by using immunofluorescence and Galpha(s)GFP. GPR3 decreased the ratio of Galpha(s) in the oocyte plasma membrane versus the cytoplasm and also decreased the amount of Galpha(s) in the oocyte. Both of these properties indicate that GPR3 activates G(s). The follicle cells around the oocyte are also necessary to keep the oocyte in prophase, suggesting that they might activate GPR3. However, GPR3-dependent G(s) activity was similar in follicle-enclosed and follicle-free oocytes. Thus, the maintenance of prophase arrest depends on the constitutive activity of GPR3 in the oocyte, and the follicle cell signal acts by a means other than increasing GPR3 activity.

Previous studies demonstrated that Gsalpha migrates from a Triton X-100 (TTX-100) insoluble membrane domain to a TTX-100 soluble membrane domain in response to chronic treatment with the antidepressants desipramine and fluoxetine. Antidepressant treatment also causes a Gsalpha redistribution in cells as seen by confocal microscopy. The current studies have focused on examining the possibility that the association between Gsalpha and the plasma membrane and/or cytoskeleton is altered in response to antidepressant treatment, and that this is relevant to both Gsalpha redistribution and the increased coupling between Gsalpha and adenylyl cyclase seen after chronic antidepressant treatment. Chronic treatment of C6 cells with two fuctionally and structurally distinct antidepressants, desipramine and fluoxetine, decreased the Gsalpha content of TTX-100 insoluble membrane domains by as much as 60%, while the inactive fluoxetine analog LY368514 had no effect. Disruption of these membrane domains with the cholesterol chelator methyl-beta-cyclodextrin altered the localization of many proteins involved in the cAMP signaling cascade, but only Gsalpha localization was altered by antidepressant treatment. In addition, microtubule disruption with colchicine elicited the movement of Gsalpha out of detergent-resistant membrane domains in a manner identical to that seen with antidepressant treatment. The data presented here further substantiate the role of Gsalpha as a major player in antidepressant-induced modification of neuronal signaling and also raise the possibility that an interaction between Gsalpha and the cytoskeleton is involved in this process.