Agonist-induced internalization of the G protein G11alpha and thyrotropin-releasing hormone receptors proceed on different time scales

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.

Identification and subcellular localization of molecular complexes of Gq/11α protein in HEK293 cells

Heterotrimeric G-proteins localized in the plasma membrane convey the signals from G-protein-coupled receptors (GPCRs) to different effectors. At least some types of G-protein α subunits have been shown to be partly released from plasma membranes and to move into the cytosol after receptor activation by the agonists. However, the mechanism underlying subcellular redistribution of trimeric G-proteins is not well understood and no definitive conclusions have been reached regarding the translocation of Gα subunits between membranes and cytosol. Here we used subcellular fractionation and clear-native polyacrylamide gel electrophoresis to identify molecular complexes of G(q/11)α protein and to determine their localization in isolated fractions and stability in naïve and thyrotropin-releasing hormone (TRH)-treated HEK293 cells expressing high levels of TRH receptor and G(11)α protein. We identified two high-molecular-weight complexes of 300 and 140 kDa in size comprising the G(q/11) protein, which were found to be membrane-bound. Both of these complexes dissociated after prolonged treatment with TRH. Still other G(q/11)α protein complexes of lower molecular weight were determined in the cytosol. These 70 kDa protein complexes were barely detectable under control conditions but their levels markedly increased after prolonged (4-16 h) hormone treatment. These results support the notion that a portion of G(q/11)α can undergo translocation from the membrane fraction into soluble fraction after a long-term activation of TRH receptor. At the same time, these findings indicate that the redistribution of G(q/11)α is brought about by the dissociation of high-molecular-weight complexes and concomitant formation of low-molecular-weight complexes containing the G(q/11)α protein.

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.

Protein alterations induced by long-term agonist treatment of HEK293 cells expressing thyrotropin-releasing hormone receptor and G11alpha protein

This study aimed to determine whether sustained stimulation with thyrotropin-releasing hormone (TRH), a peptide with important physiological functions, can possibly affect expression of plasma membrane proteins in HEK293 cells expressing high levels of TRH receptor and G(11)alpha protein. Our previous experiments using silver-stained two-dimensional polyacrylamide gel electrophoretograms did not reveal any significant changes in an overall composition of membrane microdomain proteins after long-term treatment with TRH of these cells (Matousek et al. 2005 Cell Biochem Biophys 42: 21-40). Here we used a purified plasma membrane fraction prepared by Percoll gradient centrifugation and proteins resolved by 2D electrophoresis were stained with SYPRO Ruby gel stain. The high enrichment in plasma membrane proteins of this preparation was confirmed by a multifold increase in the number of TRH receptors and agonist stimulated G-protein activity, compared to postnuclear supernatant. By a combination of these approaches we were able to determine a number of clearly discernible protein changes in the plasma membrane-enriched fraction isolated from cells treated with TRH (1 x 10(-5) M, 16 h): 4 proteins disappeared, the level of 18 proteins decreased and the level of 39 proteins increased. Our concomitant immunochemical determinations also indicated a clear down-regulation of G(q/11)alpha proteins in preparations from hormone-treated cells. In parallel, we observed decrease in caspase 3 and alterations in some other apoptotic marker proteins, which were in line with the presumed antiapoptotic effect of TRH.

Subcellular trafficking of the TRH receptor: effect of phosphorylation

Activation of the G protein-coupled TRH receptor leads to its phosphorylation and internalization. These studies addressed the fundamental question of whether phosphorylation regulates receptor trafficking or endosomal localization regulates the phosphorylation state of the receptor. Trafficking of phosphorylated and dephosphorylated TRH receptors was characterized using phosphosite-specific antibody after labeling surface receptors with antibody to an extracellular epitope tag. Rab5 and phosphoreceptor did not colocalize at the plasma membrane immediately after TRH addition but overlapped extensively by 15 min. Dominant-negative Rab5-S34N inhibited receptor internalization. Later, phosphoreceptor was in endosomes containing Rab5 and Rab4. Dephosphorylated receptor colocalized with Rab4 but not with Rab5. Dominant-negative Rab4, -5, or -11 did not affect receptor phosphorylation or dephosphorylation, showing that phosphorylation determines localization in Rab4(+)/Rab5(-) vesicles and not vice versa. No receptor colocalized with Rab7; a small amount of phosphoreceptor colocalized with Rab11. To characterize recycling, surface receptors were tagged with antibody, or surface receptors containing an N-terminal biotin ligase acceptor sequence were labeled with biotin. Most recycling receptors did not return to the plasma membrane for more than 2 h after TRH was removed, whereas the total cell surface receptor density was largely restored in less than 1 h, indicating that recruited receptors contribute heavily to early repopulation of the plasma membrane.

Rapid changes in shear stress induce dissociation of a G alpha(q/11)-platelet endothelial cell adhesion molecule-1 complex

It has been recently shown that endothelial platelet endothelial cell adhesion molecule-1 (PECAM-1) expression is pro-atherogenic. PECAM-1 is involved in sensing rapid changes in fluid shear stress but the mechanisms for activating signalling complexes at the endothelial cell junction have yet to be elucidated. Additional studies suggest the activation of membrane-bound G proteins G alpha(q/11) also mediate flow-induced responses. Here, we investigated whether PECAM-1 and G alpha(q/11) could act in unison to rapidly respond to fluid shear stress. With immunohistochemistry, we observed a co-localization of G alpha(q/11) and PECAM-1 at the cell-cell junction in the atheroprotected section of mouse aortae. In contrast, G alpha(q/11) was absent from junctions in atheroprone areas as well as in all arterial sections of PECAM-1 knockout mice. In primary human endothelial cells, temporal gradients in shear stress led to a rapid dissociation of the G alpha(q/11)-PECAM-1 complex within 30 s and a partial relocalization of the G alpha(q/11) staining to perinuclear areas within 150 min, whereas transitioning fluid flow devoid of temporal gradients did not disrupt the complex. Inhibition of G protein activation eliminated temporal gradient flow-induced G alpha(q/11)-PECAM-1 dissociation. These results allow us to conclude that G alpha(q/11)-PECAM-1 forms a mechanosensitive complex and its localization suggests the G alpha(q/11)-PECAM-1 complex is a critical mediator of vascular diseases.

The Importance of N-terminal Polycysteine and Polybasic Sequences for G14α and G16α Palmitoylation, Plasma Membrane Localization, and Signaling Function

Plasma membrane targeting of G protein alpha (Galpha) subunits is essential for competent receptor-to-G protein signaling. Many Galpha are tethered to the plasma membrane by covalent lipid modifications at their N terminus. Additionally, it is hypothesized that Gq family members (Gqalpha,G11alpha,G14alpha, and G16alpha) in particular utilize a polybasic sequence of amino acids in their N terminus to promote membrane attachment and protein palmitoylation. However, this hypothesis has not been tested, and nothing is known about other mechanisms that control subcellular localization and signaling properties of G14alpha and G16alpha. Here we report critical biochemical factors that mediate membrane attachment and signaling function of G14alpha and G16alpha. We find that G14alpha and G16alpha are palmitoylated at distinct polycysteine sequences in their N termini and that the polycysteine sequence along with the adjacent polybasic region are both important for G16alpha-mediated signaling at the plasma membrane. Surprisingly, the isolated N termini of G14alpha and G16alpha expressed as peptides fused to enhanced green fluorescent protein each exhibit differential requirements for palmitoylation and membrane targeting; individual cysteine residues, but not the polybasic regions, determine lipid modification and subcellular localization. However, full-length G16alpha, more so than G14alpha, displays a functional dependence on single cysteines for membrane localization and activity, and its full signaling potential depends on the integrity of the polybasic sequence. Together, these findings indicate that G14alpha and G16alpha are palmitoylated at distinct polycysteine sequences, and that the adjacent polybasic domain is not required for Galpha palmitoylation but is important for localization and functional activity of heterotrimeric G proteins.

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.

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.

High-affinity interactions between human alpha1A-adrenoceptor C-terminal splice variants produce homo- and heterodimers but do not generate the alpha1L-adrenoceptor

Using combinations of bioluminescence resonance energy transfer, time-resolved fluorescence resonance energy transfer and the functional complementation of pairs of inactive receptor-G protein fusion proteins, the human alpha(1A-1)-adrenoceptor was shown to form homodimeric/oligomeric complexes when expressed in human embryonic kidney (HEK) 293 cells. Saturation bioluminescence resonance energy transfer studies indicated the alpha(1A-1)-adrenoceptor homodimer interactions to be high affinity and some 75 times greater than interactions between the alpha(1A-1)-adrenoceptor and the delta opioid peptide receptor. Only a fraction of the alpha(1A-1)-adrenoceptors was at the plasma membrane of HEK293 cells at steady state. However, dimers of alpha(1A-1)-adrenoceptors were also present in intracellular membranes, and the dimer status of those delivered to the cell surface was unaffected by the presence of agonist. Splice variation can generate at least three forms of the human alpha(1A-1)-adrenoceptor with differences limited to the C-terminal tail. Each of the alpha(1A-1), alpha(1A-2a), and alpha(1A-3a)-adrenoceptor splice variants formed homodimers/oligomers, and all combinations of these splice variants were able to generate heterodimeric/oligomeric interactions. Despite the coexpression of these splice variants in human tissues that possess the pharmacologically defined alpha(1L)-adrenoceptor binding site, coexpression of any pair in HEK293 cells failed to generate ligand binding characteristic of the alpha(1L)-adrenoceptor.

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.

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.

Subcellular shifts of trimeric G-proteins following activation of baker's yeast by glucose

Addition of glucose to a resting cell suspension of the yeast Saccharomyces cerevisiae was accompanied by marked shifts of the G alpha-protein subunits from the plasma membrane to the cell interior. This process was rapid with half-times between < 10 and 20 s. The decrease of the plasma membrane pool of the Gi alpha/Go alpha- and Gq alpha/Gl 1 alpha-protein subunits correlated with an increase in acid-sensitive forms of these proteins which was recovered in the mitochondrial and/or lysosomal membrane fraction. In contrast to cells from higher organisms glucose-stimulated yeast exhibits an extremely rapid type of the redistribution (internalization). The question remains open as to the functional significance of the internalized forms of the G-proteins as these remain sequestered from the plasma membrane well after glucose has been consumed.

Membrane-Bound and cytosolic forms of heterotrimeric G proteins in young and adult rat myocardium: influence of neonatal hypo- and hyperthyroidism

Membrane and cytosolic fractions prepared from ventricular myocardium of young (21-day-old) hypo- or hyperthyroid rats and adult (84-day-old) previously hypo- or hyperthyroid rats were analyzed by immunoblotting with specific anti-G-protein antibodies for the relative content of Gs alpha, Gi alpha/Go alpha, Gq alpha/G11 alpha, and G beta. All tested G protein subunits were present not only in myocardial membranes but were at least partially distributed in the cytosol, except for Go alpha2, and G11 alpha. Cytosolic forms of the individual G proteins represented about 5-60% of total cellular amounts of these proteins. The long (Gs alpha-L) isoform of Gs alpha prevailed over the short (Gs alpha-S) isoform in both crude myocardial membranes and cytosol. The Gs alpha-L/Gs alpha-S ratio in membranes as well as in cytosol increased during maturation due to a substantial increase in Gs alpha-L. Interestingly, whereas the amount of membrane-bound Gi alpha/Go alpha and Gq alpha/G11 alpha proteins tend to lower during postnatal development, cytosolic forms of these G proteins mostly rise. Neonatal hypothyroidism reduced the amount of myocardial Gs alpha and increased that of Gi alpha/Go alpha proteins. By contrast, neonatal hyperthyroidism increased expression of Gs alpha and decreased that of Gi alpha and G11 alpha in young myocardium. Changes in G protein content induced by neonatal hypo- and hyperthyroidism in young rat myocardium were restored in adulthood. Alterations in the membrane-cytosol balance of G protein subunits associated with maturation or induced by altered thyroid status indicate physiological importance of cytosolic forms of these proteins in the rat myocardium.

Quantitative analysis of agonist-dependent parathyroid hormone receptor trafficking in whole cells using a functional green fluorescent protein conjugate

Many G-protein coupled receptors (GPCRs) undergo ligand-dependent internalization upon activation. The parathyroid hormone (PTH) receptor undergoes endocytosis following prolonged exposure to ligand although the ultimate fate of the receptor following internalization is largely unknown. To investigate compartmentalization of the PTH receptor, we have established a stable cell line expressing a PTH receptor-green fluorescent protein (PTHR-GFP) conjugate and an algorithm to quantify PTH receptor internalization. HEK 293 cells expressing the PTHR-GFP were compared with cells expressing the wild-type PTH receptor in whole-cell binding and functional assays. 125I-PTH binding studies revealed similar Bmax and kD values in cells expressing either the PTHR-GFP or the wild-type PTH receptor. PTH-induced cAMP accumulation was similar in both cell lines suggesting that addition of the GFP to the cytoplasmic tail of the PTH receptor does not alter the ligand binding or G-protein coupling properties of the receptor. Using confocal fluorescence microscopy, we demonstrated that PTH treatment of cells expressing the PTHR-GFP conjugate produced a time-dependent redistribution of the receptor to the endosomal compartment which was blocked by pretreatment with PTH antagonist peptides. Treatment with hypertonic sucrose prevented PTH-induced receptor internalization, suggesting that the PTH receptor internalizes via a clathrin-dependent mechanism. Moreover, co-localization with internalized transferrin showed that PTHR-GFP trafficking utilized the endocytic recycling compartment. Experiments using cycloheximide to inhibit protein synthesis demonstrated that recycling of the PTHR-GFP back to the plasma membrane was complete within 1-2 h of ligand removal and was partially blocked by pretreatment with cytochalasin D, but not nocodazole. We also demonstrated that the PTH receptor, upon recycling to the plasma membrane, is capable of undergoing a second round of internalization, a finding consistent with a role for receptor recycling in functional resensitization.

Visualization of a functional Galpha q-green fluorescent protein fusion in living cells. Association with the plasma membrane is disrupted by mutational activation and by elimination of palmitoylation sites, but not be activation mediated by receptors or AlF4-

To investigate how G protein alpha subunit localization is regulated under basal and activated conditions, we inserted green fluorescent protein (GFP) into an internal loop of Galpha(q). alpha(q)-GFP stimulates phospholipase C in response to activated receptors and inhibits betagamma-dependent activation of basal G protein-gated inwardly rectifying K(+) currents as effectively as alpha(q) does. Association of alpha(q)-GFP with the plasma membrane is reduced by mutational activation and eliminated by mutation of the alpha(q) palmitoylation sites, suggesting that alpha(q) must be in the inactive, palmitoylated state to be targeted to this location. We tested the effects of activation by receptors and by AlF(4)(-) on the localization of alpha(q)-GFP in cells expressing both alpha(q)-GFP and a protein kinase Cgamma-red fluorescent protein fusion that translocates to the plasma membrane in response to activation of G(q). In cells that clearly exhibit protein kinase Cgamma-red fluorescent protein translocation responses, relocalization of alpha(q)-GFP is not observed. Thus, under conditions associated with palmitate turnover and betagamma dissociation, alpha(q)-GFP remains associated with the plasma membrane. These results suggest that upon reaching the plasma membrane alpha(q) receives an anchoring signal in addition to palmitoylation and association with betagamma, or that during activation, one or both of these factors continues to retain alpha(q) in this location.

Molecular diversity of cyclic AMP signalling

Several neuroendocrine control systems are prominently controlled by G-protein coupled receptors that activate the cAMP signal transduction pathway. The discovery of multiple genes that encode the molecular machinery of cAMP metabolism has revolutionized our knowledge of cAMP mediated processes. This perhaps all too familiar second messenger can be generated by nine different membrane enzymes in the context of varied levels of activation of G proteins as well as Ca(2+)- and protein kinase C-dependent processes. The amplitude, length and subcellular distribution of the cAMP signal are further modulated by over twenty functionally distinct isotypes of cAMP-degrading phosphodiesterases in a cell- and stimulus-specific manner. The present review summarizes the key properties of the molecular machinery that generates the cAMP signal and highlights how it is deployed in neuroendocrine systems.

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

The role of detergent-insensitive membrane domains (DIMs) in desensitisation of the G protein-coupled receptor-mediated hormone response was studied in clone E2M11 of HEK293 cells which stably express high levels of both thyrotropin-releasing hormone (TRH) receptors and G(11)alpha G protein. DIMs were prepared by flotation in equilibrium sucrose density gradients and characterised by a panel of membrane markers representing peripheral, glycosylphosphatidylinositol-bound as well as integral membrane proteins (caveolin, CD29, CD55, CD59, CD147, the alpha subunit of Na, K-ATPase) and enzyme activities (alkaline phosphatase, adenylyl cyclase). Caveolin-containing DIMs represented only a small fraction of the overall pool of G(q)alpha/G(11)alpha-rich domains. Prolonged stimulation of E2M11 cells with TRH resulted in dramatic depletion of G(q)alpha/G(11)alpha from all DIMs, which was paralleled by a concomitant G(q)alpha/G(11)alpha increase in the high-density gradient fractions containing the bulk-phase membrane constituents soluble in 1% Triton X-100. Distribution of membrane markers was unchanged under these conditions. Membrane domains thus represent a substantial structural determinant of the G protein pool relevant to desensitisation of hormone action.

Activation of the beta(2)-adrenergic receptor-Galpha(s) complex leads to rapid depalmitoylation and inhibition of repalmitoylation of both the receptor and Galpha(s)

Palmitoylation is unique among lipid modifications in that it is reversible. In recent years, dynamic palmitoylation of G protein alpha subunits and of their cognate receptors has attracted considerable attention. However, very little is known concerning the acylation/deacylation cycle of the proteins in relation to their activity status. In particular, the relative contribution of the activation and desensitization of the signaling unit to the regulation of the receptors and G proteins palmitoylation state is unknown. To address this issue, we took advantage of the fact that a fusion protein composed of the stimulatory alpha subunit of trimeric G protein (Galpha(s)) covalently attached to the beta(2)-adrenergic receptor (beta(2)AR) as a carboxyl-terminal extension (beta(2)AR-Galpha(s)) can be stimulated by agonists but does not undergo rapid inactivation, desensitization, or internalization. When expressed in Sf9 cells, both the receptor and the Galpha(s) moieties of the fusion protein were found to be palmitoylated via thioester linkage. Stimulation with the beta-adrenergic agonist isoproterenol led to a rapid depalmitoylation of both the beta(2)AR and Galpha(s) and inhibited repalmitoylation. The extent of depalmitoylation induced by a series of agonists was correlated (0.99) with their intrinsic efficacy to stimulate the adenylyl cyclase activity. However, forskolin-stimulated cAMP production did not affect the palmitoylation state of beta(2)AR-Galpha(s), indicating that the agonist-promoted depalmitoylation is linked to conformational changes and not to second messenger generation. Given that, upon activation, the fusion protein mimics the activated receptor-G protein complex but cannot undergo desensitization, the data demonstrate that early steps in the activation process lead to the depalmitoylation of both receptor and G protein and that repalmitoylation requires later events that cannot be accommodated by the activated fusion protein.

Overexpression of the G protein G11alpha prevents desensitization of Ca2+ response to thyrotropin-releasing hormone

Doubly transfected human embryonal kidney cells (clone E2M11 of the HEK 293 cell line) expressing both thyrotropin-releasing hormone (TRH) receptors and G11alpha protein in high amounts were used to analyze the desensitization phenomenon of the Ca2+-mobilizing pathway. Quite unexpectedly, we did not observe any significant desensitization of the [Ca2+]i response to TRH in these cells after repeated or prolonged incubation with the hormone (up to 5 h). Under the same conditions, the TRH-induced [Ca2+]i response was completely desensitized in the parent cell line (293-E2 cels) expressing TRH receptors alone. In both cell lines, inositol phosphate response was desensitized after TRH exposure, although basal levels of inositol phospates in TRH-pretreated cells were much higher than in "naive" TRH-unexposed cells. These data suggest a significant role of the G protein G11alpha in desensitization of the Ca2+-mobilizing pathway occuring after repeated or long-term exposure of target cells to TRH-receptor agonists.

Signal transduction and hormone-dependent internalization of the thyrotropin-releasing hormone receptor in cells lacking Gq and G11

The thyrotropin-releasing hormone (TRH) receptor was expressed in embryonic fibroblasts from mice lacking the alpha subunits of Gq and G11 (Fq/11 cells) to determine whether G protein coupling is necessary for agonist-dependent receptor internalization. Neither TRH nor agonists acting on endogenous receptors increased intracellular calcium unless the cells were co-transfected with the alpha subunit of Gq. In contrast, temperature-dependent internalization of [3H]MeTRH in Fq/11 cells was the same whether Gqalpha was expressed or not. A rhodamine-labeled TRH analog and fluorescein-labeled transferrin co-localized in endocytic vesicles in Fq/11 cells, indicating that endocytosis took place via the normal clathrin pathway. Cotransfection with beta-arrestin or V53D beta-arrestin increased TRH-dependent receptor sequestration. Fq/11 cells were co-transfected with the TRH receptor and a green fluorescent protein (GFP)-beta-arrestin conjugate. GFP-beta-arrestin was uniformly distributed in the cytoplasm of untreated cells and quickly translocated to the periphery of the cells when TRH was added. A truncated TRH receptor that lacks potential phosphorylation sites in the cytoplasmic carboxyl terminus signaled but did not internalize or cause membrane localization of GFP-beta-arrestin. These results prove that calcium signaling by the TRH receptor requires coupling to a G protein in the Gq family, but TRH-dependent binding of beta-arrestin and sequestration do not.

Distinct mechanisms for the endocytosis of muscarinic receptors and Gq/11

To determine whether the agonist-mediated endocytosis of muscarinic receptors and Gq/11 are mechanistically related events, the internalization of Gq/11 was monitored under conditions established to prevent muscarinic receptor endocytosis. Incubation of SH-SY5Y neuroblastoma cells with oxotremorine-M resulted in the translocation of both muscarinic receptors and Gq/11 into a 'light vesicle' membrane fraction. Although muscarinic receptor translocation was prevented by either the depletion of phosphoinositides or by disruption of clathrin assembly, the endocytosis of Gq/11 was unaffected. These results demonstrate that the agonist-induced internalization of muscarinic receptors and Gq/11 proceed via distinct mechanisms.

Real time visualization of agonist-mediated redistribution and internalization of a green fluorescent protein-tagged form of the thyrotropin-releasing hormone receptor

The long isoform of the rat thyrotropin-releasing hormone receptor (TRHR) was modified by the addition of a vesicular stomatitis virus (VSV) epitope tag and green fluorescent protein (GFP). VSV-TRHR-GFP bound TRH with affinity similar to that of the unmodified receptor and stimulated [3H]inositol phosphate production. A clone stably expressing VSV-TRHR-GFP at some 120,000 copies/cell was selected to visualize this receptor during cellular exposure to TRH. Internalization was detected within 3-5 min after treatment with 1 x 10(-7) M TRH, with dramatic reductions in plasma membrane localization achieved within 10-15 min. The TRHR antagonist/inverse agonist chlordiazepoxide competitively inhibited internalization. Hyperosmotic sucrose inhibited internalization of VSV-TRHR-GFP, measured both by intact cell [3H]TRH binding studies and by confocal microscopy. Now TRH caused a redistribution of VSV-TRHR-GFP to highly punctate but plasma membrane-delineated foci. Pretreatment with the microtubule-disrupting agent nocodazole allowed internalization of the VSV-TRHR-GFP construct but only into vesicles that remained in close apposition to the plasma membrane. Covisualization of VSV-TRHR-GFP and Texas Red transferrin initially indicated entirely separate localizations. After exposure to TRH substantial amounts of VSV-TRHR-GFP were present in vesicles overlapping those containing Texas Red transferrin. Such results demonstrate the G protein-coupling capacity and provide real time visualization of the processes of internalization of a TRH-receptor-GFP construct in response to agonist.