Inhibitory G protein

Apelin receptors activate several signalling pathways including coupling through inhibitory G-proteins (G ) and Ras-independent activation of extracellular-regulated kinases (ERKs) via protein kinase C (PKC). The apelin receptor is one of number of G-protein-coupled receptors that can act as an alternative coreceptor for entry into cells of HIV and simian immunodeficiency vims (SIV) strains in human U87 cells expressing CD4 in vitro. Apelin peptides blocks entry of HIV but display different potencies, with apelin-36 being more effective than shorter sequences [3]. 

Figure 6.1. Regulation of adenylate cyclase activity by G-proteins. Occupancy of receptors such as the /3-adrenergic receptor result in the activation (+) of adenylate cyclase via coupling through stimulatory G-proteins (Gs). Alternatively, occupancy of receptors such as the 2-adrenergic receptor inhibit (-) adenylate cyclase via coupling through inhibitory G-proteins (Gj).

Figure 6.3. Mechanism of action of heterotrimeric G-proteins. Upon receptor occupancy, the Ga-subunit binds GTP in exchange for GDP, and then moves in the membrane until it encounters its target enzyme, shown here as adenylate cyclase (alternatively, a phospholipase). The activated target enzyme then becomes functional. Inherent GTPase activity within the a-subunit then hydrolyses bound GTP to GDP, and the a-subunit dissociates from its target enzyme (which becomes inactive) and rebinds the / - and ysubunits. Upon continued receptor occupancy, further catalytic cycles of GTP exchange and target enzyme activation may occur. The scheme shown is for a stimulatory G-protein (Got,), but similar sequences of events occur with inhibitory G-proteins (Gcx,) except that the interaction of the a-subunit with adenylate cyclase will result in its inhibition. The sites of action of pertussis and cholera toxins are shown.

An inhibitory neurotransmitter activates an inhibitory G-protein thereby leading to a reduction in cAMP synthesis. 

G proteins are divided into several types, depending on their effects. Stimulatory G proteins (Cs) are widespread. They activate adenylate cyclases (see below) or influence ion channels. Inhibitory G proteins (Cj) inhibit adenylate cyclase. G proteins in the Gq family activate another effector enzyme—phospholipase c (see p. 386). 

One of the best-characterized effectors and second messenger systems is the cAMP cascade that can be either activated or inhibited by neurotransmit-ter/neuropeptide receptors, including those implicated in anxiety/stress such as CRE Receptors that activate cAMP synthesis couple with the stimulatory G protein, Gsa, and those that inhibit this second messenger couple with the inhibitory G protein, Gia, and these either stimulate or inhibit adenylyl cyclase, the effector enzyme responsible for synthesis of cAMP (Duman and Nestler 1999). There are at least nine different forms of adenylyl cyclase that have been identified by molecular cloning, each with a unique distribution in the brain. The different types of adenylyl cyclase are activated by Gsa as well as the diterpene forskolin, but are differentially regulated by Gia, the Py subunits, Ca, and by phosphorylation. This provides for fine control of adenylyl cyclase enzyme activity and regulation by other effector pathways. 

Garattini S, Mennini T Pharmacology of amineptine synthesis and updating. Chn Neuropharmacol 12 (suppl) S13-S 18, 1989 Garcia-Sainz JA, Gutierrez VG Activation of protein kinase C alters the interaction of alpha2 adrenoceptors and the inhibitory G protein (Gi) in human platelets. EEBS Lett 257 427-430, 1989... 

The first members of the Gi subfamily to be discovered displayed an inhibitory effect on adenylyl cyclase, thus the name Gi, for inhibitory G-proteins. Further members of the Gi subfamily have phospholipase C as the corresponding effector molecule. Signal transmission via phospholipase C flows into the inositol triphosphate and diacylglyce-rol pathways (see Chapter 6). 

As described in Chapter 31, opioids comprise a large family of endogenous and exogenous agonists at three G protein-coupled receptors the N-, K-, and 5-opioid receptors. Although all three receptors couple to inhibitory G proteins (ie, they all inhibit adenylyl cyclase), they have distinct. 

FIGURE 12-12 Transduction of the epinephrine signal the /J-adrenergic pathway. The seven steps of the mechanism that couples binding of epinephrine (E) to its receptor (Rec) with activation of adenylyl cyclase (AC) are discussed further in the text. The same adenylyl cyclase molecule in the plasma membrane may be regulated by a stimulatory G protein (Gs), as shown, or an inhibitory G protein (G, not shown). Gs and G, are under the influence of different hormones. Hormones that induce GTP binding to G, cause inhibition of adenylyl cyclase, resulting in lower cellular [cAMP]. 

G protein-coupled receptor kinases (GRKs) 441 scaffold proteins 441 inhibitory G protein (GO 441 calmodulin (CaM) 444 Ca2+/calmodulin-dependent protein kinases (CaM kinases I IV)) 444 two-component signaling systems 452 receptor His kinase 452 response regulator 452 receptorlike kinase (RLK) 455... 

The three-dimensional structure of the GDP complex of the intact transducin heterotrimer195 also shows a tight interaction between a and P subunits. The major interaction is probably disrupted by replacement of the bound GDP by GTP and the conformational change that occurs around the y- phospho group. This explains the dissociation of the a subunit from Py upon activation. An entirely similar picture has been obtained for the action of the inhibitory G protein, Gi2, for which structures of the a subunit and of the aPy heterotrimer (Fig. 11-7,B,C) have been determined.188 233 234 242 The structures resemble those of transducin, but differ in details. 

Phospholipase C, which initiates the release of phosphatidylinositol derivatives, also requires Ca2+ for activity. It is difficult to determine whether release of Ca2+ is a primary or secondary response. There are three isoenzyme types of phospholipase C-(3, y, and 8- and several sub forms of each with a variety of regulatory mechanisms.298 3"" For example, the y isoenzymes are activated by binding to the tyrosine kinase domain of receptors such as that for epidermal growth factor (see Fig. 11-13). In contrast, the (3 forms are often activated by inhibitory G proteins and also by G, which is specific for inositol phosphate release. 

Additional classification schemes will not be discussed here in any detail but include those with related gene and/or chromosome localizations and those with the same effector systems, (e.g., stimulatory or inhibitory G proteins or sodium, potassium, chloride, or calcium channels). These features of different receptors will be discussed as specific neurotransmitter receptors are mentioned throughout the rest of the book. 

Figure 10. Scheme illustrating the main locations and functional role of H3 receptors in the vessels. H3 receptors coupled with inhibitory G proteins (Gj) occur as prejunctional receptors in the adrenergic varicosities, where they negatively modulate noradrenaline (NA) release. Moreover, their activation in endothelial cells can induce muscle relaxation, by the release of inhibitory factors, such as nitric oxide (NO) and prostacyclin (PGI2). In some districts, excitatory H3 receptors were found in muscle cells and they mediate muscle contraction. MC = mast cell NOS = NO synthase COX = cyclooxygenase. 

El-Armouche A, Zolk O, Rau T, Eschenhagen T. Inhibitory G-proteins and their role in desensitization of the adenylyl cyclase pathway in heart failure. Cardio-vascRes. 2003 60 478-487. 

Raymond JR, Olsen CL, Gettys TW. Cell-specific physical and functional coupling of human 5-HT1A receptors to inhibitory G protein alpha-subunits and lack of coupling to Gs alpha. Biochemistry 1993 32 11,064-11,073. 

Gettys TW, Fields TA, Raymond JR. Selective activation of inhibitory G protein alpha-subunits by partial agonists of the human 5-HT1A receptor. Biochemistry 1994 33 4283-4290. 

Eschenhagen, T., Mende, U., Nose, M., Schmitz, W., Scholz, H., Haverich, A., Hirt, S., Doring, V., Kalmar, R, Hoppner, W., et al. 1992. Increased messenger RNA level of the inhibitory G protein ot subunit Gia-2 in human end-stage heart failure. Circ. Res. 70 688-696. 

Kawamoto, H., Ohyanagi, M., Nakamura, K., Yamamoto, J., and Iwasaki, T. 1994. Increased levels of inhibitory G protein in myocardium with heart failure. Jpn. Circ. J. 58 913-924. 

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