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 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 from lipid raft domains to non-raft domains. Other NMDA antagonist did not translocate Gα from lipid raft to non-raft domains. The ketamine-induced Gα plasma membrane redistribution allows increased functional coupling of Gα 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 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 from lipid rafts is a reliable hallmark of antidepressant action that might be exploited for diagnosis or drug development.

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.

GPCR signaling is modified both in major depressive disorder and by chronic antidepressant treatment. Endogenous Ga s redistributes from raft-to nonraft-membrane fractions after chronic antidepressant treatment. Modification of G protein anchoring may participate in this process. Regulation of Ga s signaling by antidepressants was studied using fluorescence recovery after photobleaching (FRAP) of GFP-Ga s . Here we find that extended antidepressant treatment both increases the half-time of maximum recovery of GFP-Ga s and decreases the extent of recovery. Furthermore, this effect parallels the movement of Ga s 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 Ga s , 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.

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 (G␣ s ) is increasingly localized to lipid rafts in depressed subjects and that chronic antidepressant treatment translocates G␣ s from lipid rafts. Translocation of G␣ s , 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 G␣ s 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 G␣ s 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.Depression is the leading cause of long term disability in the industrialized world (1). Although depression is a significant health problem in the United States and antidepressants are heavily prescribed (2), the mechanism of action for these drugs is not understood. Further, nearly a third of those treated with these drugs do not achieve remission of their depression (3). Although most of these drugs do interfere with monoamine uptake or catabolism, they exert this effect within hours, even though most of the compounds require weeks before alleviation of symptoms is observed (4). Thus, other targets for antidepressant drugs may exist (4).Chronic antidepressant treatment engages signaling pathways apart from increasing monoamine density in the synaptic cleft. One of these is an increased accumulation of cellular cAMP and sequelae thereof, such as increased cAMP-response element-binding protein (CREB) phosphorylation and increased transcription of cAMP-regulated genes (e.g. BDNF) (5). Moreover, positron emission tomography (PET) evidence suggests that cAMP is diminished throughout the brain of depressed human subjects (6). Thus, it is possible that some antidepressant effects are mediated through induction of the cAMP-generating system, including G␣ s and adenylyl cyclase.Previous studies demonstrated that chronic antidepressant treatment translocates G␣ s from lipid rafts, whereupon it engages in a more facile activation of adenylyl cyclase (7,8). Lipid rafts are regions of the plasma membrane rich in caveolin, cholesterol, sphingolipids, and cytoskeletal an...

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 from lipid rafts toward increased association with adenylyl cyclase (AC). Concomitant with this is a sustained increase in cAMP production. While Gα modifies microtubule dynamics, tubulin also acts as an anchor for Gα in lipid-rafts. Since HDAC-6 inhibitors potentiate α-tubulin acetylation, we hypothesize that acetylation of α-tubulin disrupts tubulin-Gα raft-anchoring, rendering Gα 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 from lipid-rafts, without changing total Gα. Reciprocally, depletion of α-tubulin acetyl-transferase-1 ablated this phenomenon. While escitalopram and imipramine also disrupt Gα/tubulin complexes and translocate Gα 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 translocation from lipid-rafts as a biosignature for antidepressant response that may be useful in the development of new antidepressant compounds.