Thyrotropin-releasing hormone-induced depletion of G(q)alpha/G(11)alpha proteins from detergent-insensitive membrane domains

Biochemical and physiological insights into TRH receptor-mediated signaling

Thyrotropin-releasing hormone (TRH) is an important endocrine agent that regulates the function of cells in the anterior pituitary and the central and peripheral nervous systems. By controlling the synthesis and release of thyroid hormones, TRH affects many physiological functions, including energy homeostasis. This hormone exerts its effects through G protein-coupled TRH receptors, which signal primarily through G_q/11 but may also utilize other G protein classes under certain conditions. Because of the potential therapeutic benefit, considerable attention has been devoted to the synthesis of new TRH analogs that may have some advantageous properties compared with TRH. In this context, it may be interesting to consider the phenomenon of biased agonism and signaling at the TRH receptor. This possibility is supported by some recent findings. Although knowledge about the mechanisms of TRH receptor-mediated signaling has increased steadily over the past decades, there are still many unanswered questions, particularly about the molecular details of post-receptor signaling. In this review, we summarize what has been learned to date about TRH receptor-mediated signaling, including some previously undiscussed information, and point to future directions in TRH research that may offer new insights into the molecular mechanisms of TRH receptor-triggered actions and possible ways to modulate TRH receptor-mediated signaling.

The Role of G-proteins and G-protein Regulating Proteins in Depressive Disorders

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.

Gαq signalling: the new and the old

In the last few years the interactome of Gαq has expanded considerably, contributing to improve our understanding of the cellular and physiological events controlled by this G alpha subunit. The availability of high-resolution crystal structures has led the identification of an effector-binding region within the surface of Gαq that is able to recognise a variety of effector proteins. Consequently, it has been possible to ascribe different Gαq functions to specific cellular players and to identify important processes that are triggered independently of the canonical activation of phospholipase Cβ (PLCβ), the first identified Gαq effector. Novel effectors include p63RhoGEF, that provides a link between G protein-coupled receptors and RhoA activation, phosphatidylinositol 3-kinase (PI3K), implicated in the regulation of the Akt pathway, or the cold-activated TRPM8 channel, which is directly inhibited upon Gαq binding. Recently, the activation of ERK5 MAPK by Gq-coupled receptors has also been described as a novel PLCβ-independent signalling axis that relies upon the interaction between this G protein and two novel effectors (PKCζ and MEK5). Additionally, the association of Gαq with different regulatory proteins can modulate its effector coupling ability and, therefore, its signalling potential. Regulators include accessory proteins that facilitate effector activation or, alternatively, inhibitory proteins that downregulate effector binding or promote signal termination. Moreover, Gαq is known to interact with several components of the cytoskeleton as well as with important organisers of membrane microdomains, which suggests that efficient signalling complexes might be confined to specific subcellular environments. Overall, the complex interaction network of Gαq underlies an ever-expanding functional diversity that puts forward this G alpha subunit as a major player in the control of physiological functions and in the development of different pathological situations.

G protein trafficking

The classical view of heterotrimeric G protein signaling places G -proteins at the cytoplasmic surface of the cell's plasma membrane where they are activated by an appropriate G protein-coupled receptor. Once activated, the GTP-bound Gα and the free Gβγ are able to regulate plasma membrane-localized effectors, such as adenylyl cyclase, phospholipase C-β, RhoGEFs and ion channels. Hydrolysis of GTP by the Gα subunit returns the G protein to the inactive Gαβγ heterotrimer. Although all of these events in the G protein cycle can be restricted to the cytoplasmic surface of the plasma membrane, G protein localization is dynamic. Thus, it has become increasingly clear that G proteins are able to move to diverse subcellular locations where they perform non-canonical signaling functions. This chapter will highlight our current understanding of trafficking pathways that target newly synthesized G proteins to the plasma membrane, activation-induced and reversible translocation of G proteins from the plasma membrane to intracellular locations, and constitutive trafficking of G proteins.

Disruption of the plasma membrane integrity by cholesterol depletion impairs effectiveness of TRH receptor-mediated signal transduction via G(q)/G(11)alpha proteins

We monitored the radioligand-binding characteristics of thyrotropin-releasing hormone (TRH) receptors, functional activity of G(q/11)alpha proteins, and functional status of the whole signaling cascade in HEK293 expressing high levels of TRH receptors and G(11)alpha. Our analyses indicated that disruption of plasma membrane microdomains by cholesterol depletion did not markedly influence the binding parameters of TRH receptors, but it altered efficacy of signal transduction. The functional coupling between TRH receptor and G(q/11)alpha was assessed by agonist-stimulated [(35)S]GTPgammaS binding, and results of these measurements pointed out to significantly lower potency of TRH to mediate G protein activation in the plasma membrane fraction isolated from cholesterol-depleted cells; there was a shift in sensitivity by one order of magnitude to the higher concentrations. A markedly lower sensitivity to stimulation with TRH was also observed in our experiments dealing with determination of hormone-induced Ca(2+) response. These data suggest that the intact structure of plasma membranes is an important optimum signal transduction initiated by TRH receptors and mediated by G(q/11)alpha proteins.

FasL, Fas, and death-inducing signaling complex (DISC) proteins are recruited to membrane rafts after spinal cord injury

The Fas/CD95 receptor-ligand system plays an essential role in apoptosis that contributes to secondary damage after spinal cord injury (SCI), but the mechanism regulating the efficiency of FasL/Fas signaling in the central nervous system (CNS) is unknown. Here, FasL/Fas signaling complexes in membrane rafts were investigated in the spinal cord of adult female Fischer rats subjected to moderate cervical SCI and sham operation controls. In sham-operated animals, a portion of FasL, but not Fas was present in membrane rafts. SCI resulted in FasL and Fas translocation into membrane raft microdomains where Fas associates with the adaptor proteins Fas-associated death domain (FADD), caspase-8, cellular FLIP long form (cFLIPL ), and caspase-3, forming a death-inducing signaling complex (DISC). Moreover, SCI induced expression of Fas in clusters around the nucleus in both neurons and astrocytes. The formation of the DISC signaling platform leads to rapid activation of initiator caspase-8 and effector caspase-3, and the modification of signaling intermediates such as FADD and cFLIP(L) . Thus, FasL/Fas-mediated signaling after SCI is similar to Fas expressing Type I cell apoptosis.

Assembly and trafficking of heterotrimeric G proteins

To be activated by cell surface G protein-coupled receptors, heterotrimeric G proteins must localize at the cytoplasmic surface of plasma membranes. Moreover, some G protein subunits are able to traffic reversibly from the plasma membrane to intracellular locations upon activation. This current topic will highlight new insights into how nascent G protein subunits are assembled and how they arrive at plasma membranes. In addition, recent reports have increased our knowledge of activation-induced trafficking of G proteins. Understanding G protein assembly and trafficking will lead to a greater understanding of novel ways that cells regulate G protein signaling.

Lipid raft microdomains and neurotransmitter signalling

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.

Dominant Portion of Thyrotropin-Releasing Hormone Receptor Is Excluded from Lipid Domains. Detergent-Resistant and Detergent-Sensitive Pools of TRH Receptor and Gqα/G11α Protein

Some G protein-coupled receptors might be spacially targetted to discrete domains within the plasma membrane. Here we assessed the localization in membrane domains of the epitope-tagged, fluorescent version of thyrotropin-releasing hormone receptor (VSV-TRH-R-GFP) expressed in HEK293 cells. Our comparison of three different methods of cell fractionation (detergent extraction, alkaline treatment/sonication and mechanical homogenization) indicated that the dominant portion of plasma membrane pool of the receptor was totally solubilized by Triton X-100 and its distribution was similar to that of transmembrane plasma membrane proteins (glycosylated and non-glycosylated forms of CD147, MHCI, CD29, CD44, transmembrane form of CD58, Tapa1 and Na,K-ATPase). As expected, caveolin and GPI-bound proteins CD55, CD59 and GPI-bound form of CD58 were preferentially localized in detergent-resistant membrane domains (DRMs). Trimeric G proteins G(q)alpha/G(11)alpha, G(i)alpha1/G(i)alpha2, G(s)alphaL/G(s)alphaS and Gbeta were distributed almost equally between detergent-resistant and detergent-solubilized pools. In contrast, VSV-TRH-R-GFP, Galpha, Gbeta and caveolin were localized massively only in low-density membrane fragments of plasma membranes, which were generated by alkaline treatment/sonication or by mechanical homogenization of cells. These data indicate that VSV-TRH-R-GFP as well as other transmembrane markers of plasma membranes are excluded from TX-100-resistant, caveolin-enriched membrane domains. Trimeric G protein G(q)alpha/G(11)alpha occurs in both DRMs and in the bulk of plasma membranes, which is totally solubilized by TX-100.

Agonist-induced tyrosine phosphorylation of Gq/G11α requires the intact structure of membrane domains

Stimulation of receptors coupled to G(q)/G(11) protein may induce phosphorylation on a tyrosine residue of the alpha subunit of this G protein, which is an essential event for G(q)/G(11) activation. Here we observed that in HEK293 cells stably expressing high levels of thyrotropin-releasing hormone (TRH) receptors and G(11)alpha protein the maximal tyrosine phosphorylation of G(q)/G(11)alpha was reached within 10 min of TRH stimulation and then it faded away at longer time periods of agonist exposure. The G(q)/G(11)alpha protein levels did not change during this treatment. Incubation of intact cells with beta-cyclodextrin (beta CD) for 40 min prior to hormone exposure significantly decreased the rapid transient tyrosine phosphorylation. Subsequent replenishment of cholesterol levels reversed the former negative effect of beta CD. Isolation of caveolin-enriched, detergent-resistant membrane domains indicated destruction of these structures in beta CD-treated cells. These data indicate that the preserved integrity of plasma membrane domains/caveolae is required for complete agonist-induced phosphorylation of G(q)/G(11)alpha.

Long-term agonist stimulation of IP prostanoid receptor depletes the cognate G(s)alpha protein in membrane domains but does not change the receptor level

Iloprost (IP) stimulation (1 microM, 2 h) of Flag-epitope-tagged human IP prostanoid receptor (FhIPR) expressed in HEK293 cells resulted in specific decrease of endogenous G(s)alpha protein in detergent-insensitive, caveolin-enriched, membrane domains (DIMs). Receptor protein FhIPR, caveolin, G(i)alpha and GPI-linked, domain markers CD55 and CD59 were unchanged. The same result was obtained in HEK293 cells expressing FhIPR-G(s)alpha fusion protein. The endogenous G(s)alpha decreased, but the level of Flag-hIPR-G(s)alpha protein did not change. The specific depletion of domain-bound pool of G(s)alpha as consequence of iloprost stimulation was also demonstrated in membrane domains prepared according to alkaline treatment plus sonication protocol (detergent-free procedure of Song et al.). Our data further indicated that in control, quiescent cells only a very small amount of IP prostanoid receptor was present in DIMs together with large amount of its cognate G(s)alpha protein. Expressed in quantitative terms, DIMs contained 30-40% of the total cellular amount of G proteins whereas the content of IP prostanoid receptors was 1-3%. The dominant portion (>95%) of FhIPR as well as FhIPR-G(s)alpha was localised in high-density area of the gradient containing detergent-solubilised proteins. FhIPR and FhIPR-G(s)alpha distribution was similar to that of transmembrane plasma membrane (PM) markers (CD147, MHCI, CD29, Tapa1, the alpha subunit of Na,K-ATPase, transmembrane form of CD58 and CD44). All these proteins are known to be fully solubilised by detergent and thus unable to float in density gradient. Our data indicate that (i) long-term agonist stimulation of IP prostanoid receptor is associated with preferential decrease of its cognate G protein G(s)alpha from membrane domains; receptor level is unchanged. (ii) Very small fraction (1-3%) of total cellular amount of receptors is recovered in DIMs together with roughly 40% of G proteins. These data suggest a "supra-stoichiometric" arrangement of G proteins and corresponding receptors in DIMs.

Different methods of membrane domains isolation result in similar 2-D distribution patterns of membrane domain proteins

Membrane domains are highly specialized parts of the cell plasma membrane, carrying on and augmenting the incoming signals. To study their structural and functional properties, it is crucial to find the least damaging mode of their isolation. Using two different cell lines, epithelial HEK cells (clone E2M11) and S49 lymphoma cells, three methods of membrane domain isolation (i.e., detergent extraction, alkaline treatment, and "drastic" homogenization) were tested for similarity and reproducibility by 2-D electrophoresis. Our data show that the protein composition of membrane domains obtained by different isolation methods is similar and that approximately 60% of the spots are present in all membrane domain preparations. Furthermore, the same degree of similarity of 2-D profiles of the most intensively silver stained spots found in membrane domains of the two cell lines derived from different tissues suggests that the composition of a large part of membrane domains proteins is conservative. We suggest that these proteins may either be involved in the organization of membrane domain structure or represent the conservative component of signal transduction machinery.

Differential recruitment of Kv1.4 and Kv4.2 to lipid rafts by PSD-95

The activity of voltage-gated potassium (Kv) channels, and consequently their influence on cellular functions, can be substantially altered by phosphorylation. Several protein kinases that modulate Kv channel activity are found in membrane subdomains known as lipid rafts, which are thought to organize signaling complexes in the cell. Thus, we asked whether Kv1.4 and Kv4.2, two channels with critical roles in excitable cells, are found in lipid rafts. Acylation can target proteins to raft regions; however, Kv channels are not acylated, and therefore, a different mechanism must exist to bring them into these membrane subdomains. Because both Kv1.4 and Kv4.2 interact with postsynaptic density protein 95 (PSD-95), which is acylated (specifically, palmitoylated), we examined whether PSD-95 can recruit these channels to lipid rafts. We found that a portion of Kv1.4 and Kv4.2 protein in rat brain membranes is raft-associated. Lipid raft patching and immunostaining confirmed that some Kv4.2 is in Thy-1-containing rafts in rat hippocampal neurons. Using a heterologous expression system, we determined that palmitoylation of PSD-95 was crucial to its localization to lipid rafts. We then assessed the contribution of PSD-95 to the raft association of these channels. Co-expression of PSD-95 increased the amount of Kv1.4, but not Kv4.2, in lipid rafts. Deleting the PSD-95 binding motif of Kv1.4 eliminated this recruitment, as did substituting a palmitoylation-deficient PSD-95 mutant. This work represents the first evidence that PSD-95 binding can recruit Kv channels into lipid rafts, a process that could facilitate interactions with the protein kinases that affect channel activity.

delta-Opioid receptors exhibit high efficiency when activating trimeric G proteins in membrane domains

Low-density membrane fragments (domains) were separated from the bulk of plasma membranes of human embryonic kidney (HEK)293 cells expressing a delta-opioid (DOP) receptor-Gi1alpha fusion protein by drastic homogenization and flotation on equilibrium sucrose density gradients. The functional activity of trimeric G proteins and capacity of the DOP receptor to stimulate both the fusion protein-linked Gi1alpha and endogenous pertussis-toxin sensitive G proteins was measured as d-Ala2, d-Leu5-enkephalin stimulated high-affinity GTPase or guanosine-5'-[gamma-35S]triphosphate ([35S]GTPgammaS) binding. The maximum d-Ala2-d-Leu5 enkephalin (DADLE)-stimulated GTPase was two times higher in low-density membrane fragments than in bulk of plasma membranes; 58 and 27 pmol/mg/min, respectively. The same difference was obtained for [35S]GTPgammaS binding. Contrarily, the low-density domains contained no more than half the DOP receptor binding sites (Bmax = 6.6 pmol/mg versus 13.6 pmol/mg). Thus, when corrected for expression levels of the receptor, low-density domains exhibited four times higher agonist-stimulated GTPase and [35S]GTPgammaS binding than the bulk plasma membranes. The regulator of G protein signaling RGS1, enhanced further the G protein functional activity but did not remove the difference between domain-bound and plasma membrane pools of G protein. The potency of the agonist in functional studies and the affinity of specific [3H]DADLE binding to the receptor were, however, the same in both types of membranes - EC50 = 4.5 +/- 0.1 x 10(-8) and 3.2 +/- 1.4 x 10(-8) m for GTPase; Kd = 1.2 +/- 0.1 and 1.3 +/- 0.1 nm for [3H]DADLE radioligand binding assay. Similar results were obtained when sodium bicarbonate was used for alkaline isolation of membrane domains. By contrast, detergent-insensitive membrane domains isolated following treatment of cells with Triton X100 exhibited no DADLE-stimulated GTPase or GTPgammaS binding. Functional coupling between the DOP receptor and cognate G proteins was also blocked by high-energy ultrasound and repeated freezing-thawing. Our data indicate, for the first time, that membrane domains isolated using 'detergent-free' procedures exhibit higher efficiency of coupling between a G protein-coupled receptor and its corresponding G protein(s) than bulk plasma membranes. Detergent-extraction diminishes these interactions, even when the receptor and G proteins are physically tethered together.

G(s) signaling is intact after disruption of lipid rafts

Membrane microdomains enriched in cholesterol and sphingolipids modulate a number of signal transduction pathways and provide a residence for heterotrimeric G proteins, their receptors, and their effectors. We investigated whether signaling through G(s) was dependent on these membrane domains, characterized by their resistance to detergents, by depleting cells of cholesterol and sphingolipids. For cholesterol depletion, rat salivary epithelial A5 cells were cultured under low-cholesterol conditions, and then treated with the cholesterol chelator methyl-beta-cyclodextrin. For sphingolipid depletion, LY-B cells, a mutant CHO cell line that is unable to synthesize sphingolipids, were incubated under low-sphingolipid conditions. Depletion of cholesterol or sphingolipid led to a loss or decrease, respectively, in the amount of Galpha(s) from the detergent-resistant membranes without any change in the cellular or membrane-bound amounts of Galpha(s). The cAMP accumulation in response to a receptor agonist was intact and the level slightly increased in cells depleted of cholesterol or sphingolipids compared to that in control cells. These results indicate that localization of Galpha(s) to detergent-resistant membranes was not required for G(s) signaling. Analysis of the role of lipid rafts on the kinetics of protein associations in the membrane suggests that compartmentalization in lipid rafts may be more effective in inhibiting protein interactions and, depending on the pathway, ultimately inhibit or promote signaling.