EDITORIAL FOCUS
What molecular events underlie heterologous desensitization? Focus on "Receptor phosphorylation does not mediate cross talk between muscarinic M3 and bradykinin B2 receptors"

M. Marlene Hosey

Department of Molecular Pharmacology and Biological Chemistry, Northwestern University Medical School, Chicago, Illinois 60611


    ARTICLE
TOP
ARTICLE
REFERENCES

G PROTEIN-COUPLED RECEPTORS (GPCRs) comprise one of the largest families of signaling molecules and play physiologically important roles in every cell type. Thousands of GPCRs are predicted to exist. Indeed, even in the tiny worm Caenorhabditis elegans, there are predicted to be ~650 different GPCRs (10). The GPCRs are activated by a diverse array of molecules, including hormones, neurotransmitters, paracrine factors, sensory molecules, ions, cyclic nucleotides, and proteases, among others. GPCRs signal in cells by activating heterotrimeric G proteins, which consist of alpha beta gamma -subunits. Because there are multiple genes encoding alpha -, beta -, and gamma -subunits, a large number of different heterotrimers can theoretically assemble. Once activated, the alpha - and beta gamma -subunits of G proteins dissociate, and each can modulate the activity of a growing list of effectors, including enzymes such as adenylyl cyclases and phospholipases, as well as ion channels (14). In contrast to the large number of GPCRs, the number of identified effectors is considerably smaller. Because many cells have multiple GPCRs that signal through common effectors, it is not surprising that cross-regulation can occur in the signaling pathways from one GPCR to another. This "cross talk" is commonly seen as an inhibition (desensitization) induced by one GPCR onto another GPCR, although stimulation (sensitization) of signaling of one GPCR by another has also been frequently observed.

Desensitization of GPCRs, or an attenuation of signaling that occurs despite the continued presence of a ligand, is a physiologically important and complex process that participates in the turning off of the GPCRs (3, 6, 12, 17, 18). Traditionally, desensitization has been divided into two forms. Homologous desensitization is a process whereby only the activated GPCRs are "turned off" or desensitized, whereas heterologous desensitization refers to processes whereby the activation of one GPCR can result in the inhibition of another, heterologous GPCR to signal. It is not clear why many GPCRs exhibit desensitization while others do not or why some GPCRs may exhibit either homologous or heterologous desensitization or both. For example, in the current article in focus, Willars et al. (Ref. 24, see page C859 in this issue) have observed heterologous desensitization of bradykinin B2 receptors after activation of M3 muscarinic receptors while the converse did not occur, i.e., the M3 receptors were not desensitized by the B2 receptors. Different cellular backgrounds can also influence the type and degree of desensitization that is observed, suggesting that multiple events are required for the different processes.

What are the molecular events underlying desensitization of GPCRs? Homologous desensitization has been extensively studied, and an important hallmark of this process is that it is exquisitely dependent on agonist occupancy of the GPCRs. Agonist-induced phosphorylation of GPCRs is an early event that is important in the process of homologous desensitization (3, 5, 17, 18). The processes underlying heterologous or cross-desensitization are less well understood, and it appears that there will be multiple mechanisms contributing to this type of inhibition. GPCRs that undergo heterologous desensitization do not need to be occupied by an agonist. Rather, the activation of one GPCR generates a signal that causes inhibition of signaling by a second, heterologous GPCR. This type of desensitization is due to cross talk in signaling pathways that involves modifications of the activities of GPCRs, G proteins, or effectors (8, 9, 12).

Because phosphorylation of the GPCRs plays a central role in homologous desensitization, many have asked whether receptor phosphorylation might play a role in heterologous desensitization. Activation of second messenger-dependent protein kinases, such as protein kinase A and protein kinase C (PKC), by one GPCR can result in the phosphorylation of other GPCRs in the same cell and result in heterologous desensitization (9, 12). Indeed, many GPCRs are phosphorylated by PKC, and an illustration of the role of PKC in cross-desensitization is provided from studies of chemoattractant receptors (1). Notably, PKC-mediated phosphorylation of platelet-activating factor (PAF) receptors is induced by activation of other chemoattractant receptors and inhibits both coupling of the PAF receptors to G proteins and mobilization of intracellular calcium stores (20). Interestingly, the cross-regulation is unidirectional, because activation of wild-type PAF receptors does not cross-regulate the chemoattractant receptors that induced the cross-regulation of the PAF receptors (20).

A unidirectional regulation of GPCRs was also found in the study by Willars et al. (24); however, the consequence of phosphorylation by PKC was markedly different. Willars et al. demonstrate that activation of bradykinin B2 receptors induced a PKC-dependent phosphorylation of the M3 muscarinic receptors. However, this phosphorylation did not appear to alter the ability of the M3 receptors to increase phosphoinositide or calcium signaling. On the other hand, activation of the M3 receptors did not induce phosphorylation of the bradykinin B2 receptors but did induce cross-inhibition of phosphoinositide and calcium signaling. Thus these data provide an example where the cross-regulation of receptor signaling did not appear to be linked to receptor cross-phosphorylation, whereas, in other cases, such as that cited above for the PAF receptors (1, 20), receptor phosphorylation plays an obligatory role in cross-desensitization. It is likely that multiple events participate in heterologous regulation of GPCRs. It is becoming increasingly clear that receptor phosphorylation is not the exclusive mediator of heterologous desensitization (1, 8) and that events downstream are clearly involved. Although the exact nature of these downstream events is not known in detail, many have suggested the involvement of, and/or modification of, G proteins, downstream effectors, and/or stores of small signaling molecules (1, 8, 9, 12).

What else might be contributing to cross-desensitization of GPCRs? In recent years, the identification and characterization of the growing family of regulators of G protein signaling (RGS) proteins has added another player to desensitization pathways (2, 11). Because RGS proteins have been demonstrated to be GTPase-activating proteins (GAPs) that accelerate the turnoff of activated G proteins, it is likely that RGS proteins contribute to desensitization. The role of RGS proteins in the regulation of GPCRs is under intensive study, and we are certain to learn more about how they participate in desensitization. Of interest, there are indications that the inhibition of activated Galpha -subunits by RGS proteins might allow for enhanced expression of Gbeta gamma -dependent pathways (4, 8, 16). Interestingly, certain effectors such as phospholipase C-beta 1 possess GAP activity, and this can be blocked by Gbeta gamma -subunits (7). It remains to be determined how GAPs contribute to cross-regulation.

In addition, previously unappreciated inhibitory signaling mechanisms are becoming more obvious contributors to heterologous desensitization. For example, it is possible that cross-desensitization might result when two GPCRs regulate a common effector by different signaling pathways. Recent studies of the regulation of voltage-dependent calcium channels by GPCRs have suggested this possibility (19). Potentially, the ability of one GPCR to regulate one effector may be more dominant and preclude the ability of another GPCR to exert its effect. Another area of developing interest is the role of lipids or lipid-derived signaling molecules. Arachidonic acid production has been linked to the ability of endothelin A receptors to heterologously inhibit the ability of µ-opiate receptors to stimulate G protein-activated inwardly rectifying potassium channels (GIRKs) (21). This is interesting because the endothelin receptors by themselves do not appear to exert effects on the GIRK channels (21).

Other mechanisms of GPCR regulation also may involve lipid-dependent regulation. The findings that arrestin-dependent internalization of GPCRs may possess a requirement for phosphoinositides (13), and that agonist-dependent internalization of GPCRs may be modulated by depletion of phosphoinositides (22), both suggest that the status of lipid stores may influence the desensitization of GPCRs. In addition, there appears to be a requirement for phosphoinositides in the GPCR-dependent regulation of GIRK channels (15, 23). Consequently, depletion of phosphatidylinositol 4,5-bisphosphate stores by GPCR-mediated activation of phospholipases could contribute to heterologous desensitization of GIRKs. Future studies should reveal other previously unrecognized signaling events that have roles in the desensitization of responses elicited by GPCRs. From what we know so far, it is likely that the molecular events underlying cross-regulation of GPCRs will be complex and that a universal mechanism will not be found responsible.


    FOOTNOTES

Address for reprint requests and other correspondence: M. M. Hosey, Dept. of Molecular Pharmacology and Biological Chemistry, Northwestern Univ. Medical School, 303 E. Chicago Ave.-S215, Chicago, IL 60611 (E-mail: mhosey{at}nwu.edu).


    REFERENCES
TOP
ARTICLE
REFERENCES

1.   Ali, H., R. M. Richardson, B. Haribabu, and R. Snyderman. Chemoattractant receptor cross-desensitization. J. Biol. Chem. 274: 6027-6030, 1999[Free Full Text].

2.   Berman, D. M., and A. G. Gilman. Mammalian RGS proteins: barbarians at the gate. J. Biol. Chem. 273: 1269-1272, 1998[Free Full Text].

3.   Bünemann, M., and M. M. Hosey. G-protein coupled receptor kinases as modulators of G-protein signalling. J. Physiol. (Lond.) 517: 5-23, 1999[Free Full Text].

4.   Bünemann, M., and M. M. Hosey. Regulators of G protein signaling (RGS) proteins constitutively activate Gbeta gamma -gated potassium channels. J. Biol. Chem. 273: 31186-31190, 1998[Abstract/Free Full Text].

5.   Bünemann, M., K. B. Lee, R. Pals-Rylaarsdam, A. G. Roseberry, and M. M. Hosey. Desensitization of G-protein-coupled receptors in the cardiovascular system. Annu. Rev. Physiol. 61: 169-192, 1999[Medline].

6.   Burchett, S. A., M. J. Bannon, and J. G. Granneman. RGS mRNA expression in rat striatum: modulation by dopamine receptors and effects of repeated amphetamine administration. J. Neurochem. 72: 1529-1533, 1999[Medline].

7.   Chidiac, P., and E. M. Ross. Phospholipase C-beta 1 directly accelerates GTP hydrolysis by Galpha q and acceleration is inhibited by Gbeta gamma subunits. J. Biol. Chem. 274: 19639-19643, 1999[Abstract/Free Full Text].

8.   Chuang, H. H., M. Yu, Y. N. Jan, and L. Y. Jan. Evidence that the nucleotide exchange and hydrolysis cycle of G proteins causes acute desensitization of G-protein gated inward rectifier K+ channels. Proc. Natl. Acad. Sci. USA 95: 11727-11732, 1998[Abstract/Free Full Text].

9.   Chuang, T. T., L. Iacovelli, M. Sallese, and A. De Blasi. G protein-coupled receptors: heterologous regulation of homologous desensitization and its implications. Trends Pharmacol. Sci. 17: 416-421, 1996[Medline].

10.   C. elegans Sequencing Consortium. Genome sequence of the nematode C. elegans: a platform for investigating biology. Science 282: 2012-2018, 1998[Abstract/Free Full Text].

11.   De Vries, L., and M. Gist Farquhar. RGS proteins: more than just GAPs for heterotrimeric G proteins. Trends Cell Biol. 9: 138-144, 1999.[Medline]

12.   Freedman, N. J., and R. J. Lefkowitz. Desensitization of G protein-coupled receptors. Recent Prog. Horm. Res. 51: 319-351, 1996[Medline].

13.   Gaidarov, I., J. G. Krupnick, J. R. Falck, J. L. Benovic, and J. H. Keen. Arrestin function in G protein-coupled receptor endocytosis requires phosphoinositide binding. EMBO J. 18: 871-881, 1999[Abstract/Free Full Text].

14.   Hamm, H. E. The many faces of G protein signaling. J. Biol. Chem. 273: 669-672, 1998[Free Full Text].

15.   Huang, C. L., S. Feng, and D. W. Hilgemann. Direct activation of inward rectifier potassium channels by PIP2 and its stabilization by Gbeta gamma . Nature 391: 803-806, 1998[Medline].

16.   Kammermeier, P. J., and S. R. Ikeda. Expression of RGS2 alters the coupling of metabotropic glutamate receptor 1a to M-type K+ and N-type Ca2+ channels. Neuron 22: 819-829, 1999[Medline].

17.   Krupnick, J. G., and J. L. Benovic. The role of receptor kinases and arrestins in G protein-coupled receptor regulation. Annu. Rev. Pharmacol. Toxicol. 38: 289-319, 1998[Medline].

18.   Pitcher, J. A., N. J. Freedman, and R. J. Lefkowitz. G protein-coupled receptor kinases. Annu. Rev. Biochem. 67: 653-692, 1998[Medline].

19.   Polo-Parada, L., and G. Pilar. Kappa- and mu-opioids reverse the somatostatin inhibition of Ca2+ currents in ciliary and dorsal root ganglion neurons. J. Neurosci. 19: 5213-5227, 1999[Abstract/Free Full Text].

20.   Richardson, R. M., B. Haribabu, H. Ali, and R. Snyderman. Cross-desensitization among receptors for platelet activating factor and peptide chemoattractants. Evidence for independent regulatory pathways. J. Biol. Chem. 271: 28717-28724, 1996[Abstract/Free Full Text].

21.   Rogalski, S. L., C. Cyr, and C. Chavkin. Activation of the endothelin receptor inhibits the G protein-coupled inwardly rectifying potassium channel by a phospholipase A2-mediated mechanism. J. Neurochem. 72: 1409-1416, 1999[Medline].

22.   Sorensen, S. D., D. A. Linseman, E. L. McEwen, A. M. Heacock, and S. K. Fisher. Inhibition of beta (2)-adrenergic and muscarinic cholinergic receptor endocytosis after depletion of phosphatidylinositol bisphosphate. J. Pharmacol. Exp. Ther. 290: 603-610, 1999[Abstract/Free Full Text].

23.   Sui, J. L., J. Petit-Jacques, and D. E. Logothetis. Activation of the atrial KACh channel by the beta gamma subunits of G proteins or intracellular Na+ ions depends on the presence of phosphatidylinositol phosphates. Proc. Natl. Acad. Sci. USA 95: 1307-1312, 1998[Abstract/Free Full Text].

24.   Willars, G. B., W. Müller-Esterl, and S. R. Nahorski. Receptor phosphorylation does not mediate cross talk between muscarinic M3 and bradykinin B2 receptors. Am. J. Physiol. 277 (Cell Physiol. 46): C859-C869, 1999[Abstract/Free Full Text].


Am J Physiol Cell Physiol 277(5):C856-C858
0002-9513/99 $5.00 Copyright © 1999 the American Physiological Society




This Article
Full Text (PDF)
Alert me when this article is cited
Alert me if a correction is posted
Citation Map
Services
Email this article to a friend
Similar articles in this journal
Similar articles in PubMed
Alert me to new issues of the journal
Download to citation manager
Google Scholar
Articles by Hosey, M. M.
Articles citing this Article
PubMed
PubMed Citation
Articles by Hosey, M. M.


HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online