Myr+-Gi2 alpha and Go alpha subunits restore the efficacy of opioids, clonidine and neurotensin giving rise to antinociception in G-protein knock-down mice

Over-expression of GTP-binding proteins and GTPase activity in mouse astrocyte membranes in response to Theiler's murine encephalomyelitis virus infection

Intracerebral infection with Theiler's murine encephalomyelitis virus (TMEV) induces a demyelinating disease that resembles human multiple sclerosis. In order to delineate the early events in this virus-induced neuroinflammatory disease, we have analyzed global GTPases gene activation following TMEV infection of murine brain astrocytes. DNA hybridization microchip analysis demonstrated that 10 sequences described as GTPbinding proteins and GTPases in different protein databases were over-expressed, in response to this infectious agent in astroglial cells. We have first characterized both the GTP-binding and GTPase activities in uninfected astrocyte membranes from a biochemical point of view. The increase in such activities was further validated in TMEV-infected astrocytes, peaking 2-4 h after infection. Over-expression is also induced by the inflammation-related chemokines interleukin-6 and interferon-gamma but not by interleukin-1alpha or tumor necrosis factor-alpha. From the many GTPases that could be over-expressed we have studied two, because of its biological significance; Ras p21 and the subunit alphai2 of G proteins. Western blots revealed increases in both proteins after infection with TMEV, in accordance with the previous enzymologic results. An increase in the active form of Ras (the GTP bound form) in cell lysates was also confirmed by affinity binding to a glutathione-S-transferase-fusion protein, following TMEV infection. A final demonstration of physiological up-regulation is provided by UV cross-linking of membrane proteins with the hydrolysis-resistant GTP agonist GTP [gamma-(35)S]. This technique allow us to detect, after SDS-PAGE, the increase of two further majoritary GTPbinding proteins with MW of 62 and 49 KDa. A quantitative analysis of four selected genes coding for p21 ras, Galphai2 subunit of protein G, Munc-18 and protein interacting with C kinase 1, was performed by real-time RT-PCR to verify the microarray results. The study of GTPase activity and of the above genes by RT-PCR in brains of sick mice, demonstrated a significative increase in mRNA coding for p21ras and protein interacting with C kinase 1 in vivo. Here we demonstrate that one of the mechanisms triggered by TMEV infection of astrocytes is the up-regulation of proteins related to GTP metabolism, one important signal transduction system in mammalian cells.

RGS-Rz and RGS9-2 proteins control mu-opioid receptor desensitisation in CNS: the role of activated Galphaz subunits

Two consecutive i.c.v. administrations of analgesic doses of mu-opioid receptor agonists lead to a profound desensitisation of the latter receptors; a third dose produced less than 20% of the effect obtained with the first administration. Desensitisation was still effective 24h later. Impairing the activity of Galphaz but not Galphai2 subunits prevented tolerance developing after the administration of three consecutive doses of morphine. Further, the i.c.v. injection of Galphai2 subunits potentiated morphine analgesia and abolished acute tolerance, whereas i.c.v.-administered Galphaz subunits produced a rapid and robust loss of the response to morphine. The RGSZ1 and RGSZ2 proteins selectively deactivate GalphazGTP subunits, and their knockdown increased the effects produced by the first dose of morphine. However, impairing their activity also accelerated tachyphylaxis following successive doses of morphine, and facilitated the development of acute morphine tolerance. In contrast, inhibiting the RGS9-2 proteins, which bind to GalphaoGTP and GalphaiGTP but only weakly deactivates them, preserved the effects of consecutive morphine doses and abolished the generation of acute tolerance. Therefore, desensitisation of mu-opioid receptors can be achieved by reducing the responsiveness of post-receptor elements (via the possible action of activated Galphaz subunits) and/or by depleting the pool of receptor-regulated G proteins that agonists need to propagate their effects, e.g., through the activity of RGS9-2 proteins.

Inhibitors of serine/threonine protein phosphatases antagonize the antinociception induced by agonists of alpha 2 adrenoceptors and GABAB but not kappa-opioid receptors in the tail flick test in mice

We previously reported that serine/threonine protein phosphatases (PPs) play a role in the antinociception induced by the mu-opioid receptor agonist morphine. In this study we evaluated the possible involvement of PPs on the antinociception induced by agonists of others G protein-coupled receptors in the tail flick test in mice. The subcutaneous administration of clonidine (0.25-4 mg/kg), baclofen (2-32 mg/kg) or U50,488H (2-16 mg/kg) (agonists of alpha(2) adrenoceptors, GABA(B) and kappa-opioid receptors, respectively) produced dose-dependent antinociception. The antinociceptive effects of clonidine and baclofen were antagonized in a dose-dependent way by the protein phosphatase inhibitors okadaic acid (0.001-10 pg/mouse, i.c.v.) and cantharidin (0.001-10 ng/mouse, i.c.v.), and okadaic acid was 1000 times more potent than cantharidin in producing this effect. The effects of these drugs appear to be specifically due to the blockade of PPs, since L-norokadaone (an analogue of okadaic acid that has no effect on PPs) did not modify clonidine- or baclofen-induced antinociception over the wide range of doses used (0.001-1000 pg/mouse, i.c.v.). On the other hand, the antinociception induced by activation of kappa-opioid receptors with U50,488H was not modified by okadaic acid or cantharidin. In conclusion, our data support the idea that serine/threonine PPs are differentially involved in the antinociceptive effects of several agonists of G protein-coupled receptors in mice.

Functional analysis of MOR-1 splice variants of the mouse mu opioid receptor gene Oprm

A series of mu opioid receptor gene Oprm splice variants have been reported that differ only at their C-terminus. These variants all contain exons 1, 2, and 3 of the gene, the exons responsible for coding all seven transmembrane domains. Whereas MOR-1 also has exon 4 that encodes for an additional 12 amino acids at the tip of the C-terminus, the other MOR-1 variants have unique amino acid sequences distinct from those in MOR-1 due to alternative splicing. All these variants are mu-selective in binding assays. The current study explored the ability of these variants to stimulate [35S]GTPgammaS binding to assess them functionally. Only mu opioids stimulated [35S]GTPgammaS binding. Among the mu opioids we noted marked differences in their maximal stimulation among the clones. This was most prominent with beta-endorphin, which stimulated [35S]GTPgammaS binding in the MOR-1E expressing cells to a greater degree than [D-Ala2,MePhe4,Gly(ol)5]enkephalin (DAMGO; 130%) and was far less effective than DAMGO in MOR-1C cells (44%). The rank order of maximal stimulation of the drugs varied among the clones as well. Dynorphin A, beta-endorphin and morphine were most effective in stimulating [35S]GTPgammaS binding in MOR-1E, while M6G and fentanyl were most effective in MOR-1 expressing cells. The potency (EC50) of some of the drugs also varied extensively among the clones, with a poor correlation between the potency of the drugs to stimulate [35S]GTPgammaS binding and their binding affinity. Together, these findings reveal marked functional differences among the variants that only can be explained by their structural differences at the tip of their C-terminus.

α-2 agonists induce amnesia through activation of the Gi-protein signalling pathway

The post-receptorial mechanism of the amnesic action of the alpha2-agonists clonidine and guanabenz was investigated in the mouse passive avoidance test. Animals were i.c.v. injected with pertussis toxin (PTX) or with antisense oligonucleotides, complementary to the sequence of the alpha-subunit mRNA of Gi1, Gi2, Gi3, Go1 and Go2 proteins. The administration of PTX (0.25 microg per mouse i.c.v.) reversed the amnesia induced by both alpha2-agonists. Similarly, anti-Gialpha1 (6.25-12.5 microg per mouse i.c.v.), anti-Gialpha3 (3.12-12.5 microg per mouse i.c.v.), anti-Goalpha1 (12.5-25 microg per mouse i.c.v.) antagonised the detrimental effect induced by clonidine and guanabenz. By contrast, pretreatment with anti-Gialpha2 (3.12-25 microg per mouse i.c.v.) and anti-Goalpha2 (12.5-25 microg per mouse i.c.v.) never modified the impairment of memory processes induced by the alpha2-agonists. At the highest effective doses, none of the compounds used impaired motor coordination (rota rod test), nor modified spontaneous motility and inspection activity, (hole board test). These results indicate the involvement of Gi1, Gi3, and Go1, but not Gi2 and Go2, protein subtypes in the transduction mechanism responsible for the induction of amnesia by clonidine and guanabenz.

The R7 subfamily of RGS proteins assists tachyphylaxis and acute tolerance at mu-opioid receptors

Members of the R7 subfamily of regulators of G-protein signaling (RGS) proteins (RGS6, RGS7, RGS9-2, and RGS11) are found in the mouse CNS. The expression of these proteins was effectively reduced in different neural structures by blocking their mRNA with antisense oligodeoxynucleotides (ODNs). This was achieved without noticeable changes in the binding characteristics of labeled beta-endorphin to opioid receptors. Knockdown of R7 proteins enhanced the potency of antinociception promoted by morphine and [D-Ala(2), N-MePhe(4), Gly-ol(5)]-enkephalin (DAMGO)-both agonists at mu-opioid receptors. The duration of morphine analgesia was greatly increased in RGS9-2 and in RGS11 knockdown mice. The impairment of R7 proteins brought about different changes in the analgesic activity of selective delta agonists. Knockdown of RGS11 reduced [D-Ala(2)]deltorphin II analgesic effects. Those of RGS6 and RGS9-2 proteins caused [D-Ala(2)]deltorphin II to produce a smoothened time-course curve-the peak effect blunted and analgesia extended during the declining phase. RGS9-2 impairment also promoted a similar pattern of change for [D-Pen(2,5)]-enkephalin (DPDPE). RGS7-deficient mice showed an increased response to both [D-Ala(2)]deltorphin II and DPDPE analgesic effects. A single intracerebroventricular (i.c.v.) ED(80) analgesic dose of morphine gave rise to acute tolerance in control mice, but did not promote tolerance in RGS6, RGS7, RGS9-2, or RGS11 knockdown animals. Thus, R7 proteins play a critical role in agonist tachyphylaxis and acute tolerance at mu-opioid receptors, and show differences in their modulation of delta-opioid receptors.

Biochemical and pharmacological control of the multiplicity of coupling at G-protein-coupled receptors

For decades, it has been generally proposed that a given receptor always interacts with a particular GTP-binding protein (G-protein) or with multiple G-proteins within one family. However, for several G-protein-coupled receptors (GPCR), it now becomes generally accepted that simultaneous functional coupling with distinct unrelated G-proteins can be observed, leading to the activation of multiple intracellular effectors with distinct efficacies and/or potencies. Multiplicity in G-protein coupling is frequently observed in artificial expression systems where high densities of receptors are obtained, raising the question of whether such complex signalling reveals artefactual promiscuous coupling or is a genuine property of GPCRs. Multiple biochemical and pharmacological evidence in favour of an intrinsic property of GPCRs were obtained in recent studies. Thus, there are now many examples showing that the coupling to multiple signalling pathways is dependent on the agonist used (agonist trafficking of receptor signals). In addition, the different couplings were demonstrated to involve distinct molecular determinants of the receptor and to show distinct desensitisation kinetics. Such multiplicity of signalling at the level of G-protein coupling leads to a further complexity in the functional response to agonist stimulation of one of the most elaborate cellular transmission systems. Indeed, the physiological relevance of such versatility in signalling associated with a single receptor requires the existence of critical mechanisms of dynamic regulation of the expression, the compartmentalisation, and the activity of the signalling partners. This review aims at summarising the different studies that support the concept of multiplicity of G-protein coupling. The physiological and pharmacological relevance of this coupling promiscuity will be discussed.

Exogenous myristoylated-G(i2)alpha subunits of GTP-binding proteins are mitogens following their internalization by astrocytes in culture

Heterotrimeric GTP-binding proteins (G proteins) are involved in the coupling of a variety of cell surface receptors to different intracellular signalling pathways, some of which take part in the regulation of growth by affecting cell proliferation and/or differentiation. In cultured astrocytes, many receptors of neuropeptides and hormones are coupled to the heterotrimeric G(i) proteins which regulate the mitogen-activated protein kinase (MAPK/ERK) cascade through both the Galpha and Gbetagamma subunits. We have previously reported that functionally active recombinant myr-G(i2)alpha subunits added to such cultures are internalised and distributed within the plasma membrane and cytosol as well as in the nuclei of dividing astrocytes. Here we show that astrocytes proliferate dose-dependently in response to exogenous myr-G(i2)alpha subunits. Concentrations of 100 pM-30 nM myr-G(i2)alpha caused more than 2.5-fold increase of [3H]thymidine incorporation over basal levels. Other classes of myr-Galpha subunits, such as G(i3)alpha or G(o)alpha, induced a much lower proliferative effect. The addition of G(i1)alpha subunits to the cultures produced no change, indicating the selectivity of this effect. Even though myr-G(i2)alpha subunits are internalised by the cells regardless of their guanine nucleotide-bound state, much less [3H]thymidine incorporation was observed in the presence of GDPbetaS-myr-G(i2)alpha or GTPgammaS-myr-G(i2)alpha. Further, the fluorescent labelling was dissimilarly distributed, the signal being concentrated in the nucleus and perinuclear regions of the astrocytes. Selective disassembly of caveolae impaired both myr-G(i2)alpha internalisation and DNA induction. Together, these data reveal a proliferative effect of myr-G(i2)alpha subunits in astrocytes, and provide evidence for the incorporation of exogenous myr-G(i2)alpha subunits into the mitogen cascade activated by neurotransmitters or growth factors. The fact that Galpha proteins can enter cells is particularly interesting because options for delivering functional proteins into cells are limited. Thus, these proteins may have clinical applications for compensating deficits in the transduction mechanisms associated with several neurological diseases, or as a non-invasive membrane traversing carriers.

Glycosylated phosducin-like protein long regulates opioid receptor function in mouse brain

Phosducin (Phd), a protein that in retina regulates rhodopsin desensitization by controlling the activity of Gt beta gamma-dependent G-protein-coupled receptor kinases (GRKs), is present in very low levels in the CNS of mammals. However, this tissue contains proteins of related sequence and function. This paper reports the presence of N-glycosylated phosducin-like protein long (PhLP(L)) in all structures of mouse CNS, mainly in synaptic plasma membranes and associated with G beta subunits and 14-3-3 proteins. To analyze the role PhLP(L) in opioid receptor desensitization, its expression was reduced by the use of antisense oligodeoxynucleotides (ODNs). The antinociception induced by morphine, [D-Ala(2), N-MePhe(4),Gly-ol(5)]-enkephalin (DAMGO), beta-endorphin, [D-Ala(2)]deltorphin II, [D-Pen(2,5)]-enkephalin (DPDPE) or clonidine in the tail-flick test was reduced in PhLP(L)-knock-down mice. A single intracerebroventricular (icv)-ED(80) analgesic dose of morphine gave rise to acute tolerance that lasted for 4 days, but which was prevented or reversed by icv-injection of myristoylated (myr(+)) G(i2)alpha subunits. PhLP(L) knock-down brought about a myr(+)-G(i2)alpha subunit-insensitive acute tolerance to morphine that was still present after 8 days. It also diminished the specific binding of (125)I-Tyr(27)-beta-endorphin-(1-31) (human) to mouse periaqueductal gray matter membranes. After being exposed to chronic morphine treatment, post-dependent mice required about 10 days for complete recovery of morphine antinociception. The impairment of PhLP(L) extended this period beyond 17 days. It is concluded that PhLP(L) knock-down facilitates desensitization and uncoupling of opioid receptors.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Administration of myr(+)-G(i2)alpha subunits prevents acute tolerance (tachyphylaxis) to mu-opioid effects in mice

The administration of efficacious doses of morphine or beta-endorphin causes acute tolerance (tachyphylaxis) to the effects of additional administrations of these opioids. Mice intracerebroventricularly (icv)-injected with biologically active myristoylated (myr(+))-G(i2)alpha subunits developed no tachyphylaxis to morphine antinociception in the tail-flick test. This treatment increased the potency of opioid-induced analgesia during the declining phase. Moreover, animals showing tachyphylaxis to opioid effects exhibited normal responses to the agonists after icv-administration of myr(+)-G(i2)alpha subunits. In morphine tolerant/dependent mice, an icv dose of 12 pmol/mouse myr(+)-G(i2)alpha subunits facilitated complete restoration of morphine antinociception in only 4 or 5 days instead of the 10 to 11 days required for post-dependent mice. This was observed when myr(+)-G alpha subunits were injected within the first 24 h of chronic morphine administration -- but not later when long-term tolerance takes place. These results suggest that during the course of an opioid effect a progressive reduction of receptor-regulated G-proteins occurs, and hence tachyphylaxis develops. Exogenous administration of myr(+)-G alpha subunits may be of therapeutic potential in improving agonist activity and accelerating the recovery of post-dependent receptors.

Evidence for the dual coupling of the rat neurotensin receptor with pertussis toxin-sensitive and insensitive G-proteins

We previously demonstrated the functional coupling of the rat neurotensin receptor NTS1 with G-proteins on transfected CHO cell homogenates by showing modulation of agonist affinity by guanylyl nucleotides and agonist-mediated stimulation of [(35)S]GTP gamma S binding. In the present study, we observed that G(i/o)-type G-protein inactivation by pertussis toxin (PTx) resulted in a dramatic reduction of the NT-induced [(35)S]GTP gamma S binding whereas the effect of guanylyl nucleotide was almost not affected. As expected, NT-mediated phosphoinositide hydrolysis and intracellular calcium mobilization were not altered after PTx treatment. This suggests the existence of multiple signaling cascades activated by NT. Accordingly, using PTx and the PLC inhibitor U-73122, we showed that both signaling pathways contribute to the NT-mediated production of arachidonic acid. These results support evidence for a dual coupling of the NTS1 with PTx-sensitive and insensitive G-proteins.

Nuclear and cytoskeletal translocation and localization of heterotrimeric G-proteins

Heterotrimeric GTP-binding proteins (G-proteins) are involved in a diverse array of signalling pathways. They are generally thought to be membrane-bound proteins, which disassociate on receptor activation and binding of GTP. A model to explain this has been proposed, which is often described as 'the G-protein cycle'. The 'G-protein cycle' is discussed in the present paper in relation to evidence that now exists regarding the non- membranous localization of G-proteins. Specifically, the experimental evidence demonstrating association of G-proteins with the cytoskeleton and the nucleus, and the mechanisms by which G-proteins translocate to these sites are reviewed. Furthermore, the possible effector pathways and the physiological function of G-proteins at these sites are discussed.