When 7 transmembrane receptors are not G protein–coupled receptors Keshava Rajagopal, … , Robert J. Lefkowitz, Howard A. Rockman J Clin Invest. 2005;115(11):2971-2974. https://doi.org/10.1172/JCI26950. Commentary Classically, 7 transmembrane receptors transduce extracellular signals by coupling to heterotrimeric G proteins, although recent in vitro studies have clearly demonstrated that they can also signal via G protein–independent mechanisms. However, the physiologic consequences of this unconventional signaling, particularly in vivo, have not been explored. In this issue of the JCI, Zhai et al. demonstrate in vivo effects of G protein–independent signaling by the angiotensin II type 1 receptor (AT1R). In studies of the mouse heart, they compare the physiologic and biochemical consequences of transgenic cardiac-specific overexpression of a mutant AT1R incapable of G protein coupling with those of a wild-type receptor. Their results not only provide the first glimpse of the physiologic effects of this newly appreciated mode of signaling but also provide important and previously unappreciated clues as to the underlying molecular mechanisms. Find the latest version: https://jci.me/26950/pdf commentaries gene, spastin, regulates microtubule stability to 16. Wittmann, C.W., et al. 2001. Tauopathy in Dro- 21. Chan, Y.B., et al. 2003. Neuromuscular defects in modulate synaptic structure and function. Curr. sophila: neurodegeneration without neurofibril- a Drosophila survival motor neuron gene mutant. Biol. 14:1135–1147. lary tangles. Science. 293:711–714. Hum. Mol. Genet. 12:1367–1376. 12. Sherwood, N.T., Sun, Q., Xue, M., Zhang, B., and 17. Shapiro, W.R., and Young, D.F. 1984. Neurological 22. Warrick, J.M., et al. 1998. Expanded polyglutamine Zinn, K. 2004. Drosophila spastin regulates synap- complications of antineoplastic therapy. Acta Neurol. protein forms nuclear inclusions and causes neural tic microtubule networks and is required for nor- Scand. Suppl. 100:125–132. degeneration in Drosophila. Cell. 93:939–949. mal motor function. PLoS Biol. 2:e429. 18. Bonini, N.M., and Fortini, M.E. 2003. Human neu- 23. Jackson, G.R., et al. 1998. Polyglutamine-expanded 13. Marques, G. 2005. Morphogens and synaptogen- rodegenerative disease modeling using Drosophila. human huntingtin transgenes induce degenera- esis in Drosophila. J. Neurobiol. 64:417–434. Annu. Rev. Neurosci. 26:627–656. tion of Drosophila photoreceptor neurons. Neuron. 14. Jackson, A.L., et al. 2003. Expression profiling 19. Min, K.T., and Benzer, S. 1999. Preventing neurode- 21:633–642. reveals off-target gene regulation by RNAi. Nat. generation in the Drosophila mutant bubblegum. 24. Feany, M.B., and Bender, W.W. 2000. A Drosophila Biotechnol. 21:635–637. Science. 284:1985–1988. model of Parkinson’s disease. Nature. 404:394–398. 15. Sledz, C.A., Holko, M., de Veer, M.J., Silverman, 20. Ye, Y., and Fortini, M.E. 1999. Apoptotic activi- 25. Fossgreen, A., et al. 1998. Transgenic Drosophila R.H., and Williams, B.R. 2003. Activation of the ties of wild-type and Alzheimer’s disease-related expressing human amyloid precursor protein show interferon system by short-interfering RNAs. Nat. mutant presenilins in Drosophila melanogaster. gamma-secretase activity and a blistered-wing phe- Cell Biol. 5:834–839. J. Cell Biol. 146:1351–1364. notype. Proc. Natl. Acad. Sci. U. S. A. 95:13703–13708. When 7 transmembrane receptors are not G protein–coupled receptors Keshava Rajagopal,1 Robert J. Lefkowitz,2,3,4 and Howard A. Rockman2,5,6 1Department of Surgery, 2Department of Medicine, 3Department of Biochemistry, 4Howard Hughes Medical Institute, 5Department of Cell Biology, and 6Department of Molecular Genetics, Duke University Medical Center, Durham, North Carolina, USA. Classically, 7 transmembrane receptors transduce extracellular signals by pression on cardiac morphology, physiol- coupling to heterotrimeric G proteins, although recent in vitro studies have ogy, and signal transduction were assessed clearly demonstrated that they can also signal via G protein–independent and compared with those of AT1-WT. mechanisms. However, the physiologic consequences of this unconventional signaling, particularly in vivo, have not been explored. In this issue of the G protein–independent signal JCI, Zhai et al. demonstrate in vivo effects of G protein–independent sig- transduction via the AT1R naling by the angiotensin II type 1 receptor (AT1R) (see the related article results in ventricular hypertrophy beginning on page 3045). In studies of the mouse heart, they compare the with diminished apoptosis physiologic and biochemical consequences of transgenic cardiac-specific and a unique signaling profile overexpression of a mutant AT1R incapable of G protein coupling with those The Tg-i2m mice display a pronounced of a wild-type receptor. Their results not only provide the first glimpse of cardiac phenotype, which is distinct from the physiologic effects of this newly appreciated mode of signaling but also that of the Tg-WT mice (3). Marked ven- provide important and previously unappreciated clues as to the underlying tricular dilatation and eccentric hyper- molecular mechanisms. trophy are present in the Tg-i2m hearts to a greater extent than are observed in All vital physiologic functions of higher- of 7TMR-mediated signaling apparently Tg-WT hearts. However, less cardiomyo- order animals are critically regulated by occur independently of G protein acti- cyte apoptosis is observed in the Tg-i2m signal transduction through 7 trans- vation (2). Yet, the physiologic and/or hearts than in the Tg-WT hearts. These membrane receptors (7TMRs), which in pathophysiologic roles that such novel data suggest that G protein–independent the in vivo context has traditionally been mechanisms of signal transduction play signal transduction via AT1-i2m results in understood to be mediated via heterotri- in vivo are unknown. induction of cytoprotective pathways in meric G proteins and downstream second- Zhai et al. now provide compelling the heart, but that in spite of this, adverse messenger molecules (1). However, for a evidence for distinct physiologic conse- ventricular remodeling occurs. number of years, biochemical and cellular quences of G protein–independent signal The Tg-i2m mice also display unique studies have suggested that some aspects transduction via the angiotensin II type electrophysiologic disturbances, as they 1 receptor (AT1R) in the heart based on have third-degree atrioventricular (AV) Nonstandard abbreviations used: AT1R, angiotensin their generation and study of transgenic block due to impaired development of II type 1 receptor; AT1-i2m, AT1R second intracellular mice with cardiac-specific overexpression the AV node (3). Isolated cardiomyocytes loop mutant; AT1-WT, WT AT1R; GRK, G protein–cou- of a WT AT1R (AT1-WT; Tg-WT mice) or from Tg-i2m mice display diminished pled receptor kinase; Tg-i2m mice, transgenic 2+ mice with cardiac-specific overexpression of an AT1R second intracellular loop mutant L-type Ca channel currents whereas these AT1-i2m; Tg-WT mice, transgenic mice with (AT1-i2m; Tg-i2m mice) (3). AT1-i2m has are unimpaired in Tg-WT cells. These data cardiac-specific overexpression of AT1-WT; 7TMR, been shown in previous in vitro studies to point to a possible role for AT1R-medi- 7 transmembrane receptor. be completely incapable of activating G q ated, G protein–independent signaling in Conflict of interest: The authors have declared that no α conflict of interest exists. and Gαi, while retaining the ability to acti- the regulation of cardiac ion channels. The Citation for this article: J. Clin. Invest. 115:2971–2974 vate molecular effectors such as Src and authors also performed invasive hemody- (2005). doi:10.1172/JCI26950. ERK (4). The effects of AT1-i2m overex- namic studies that suggest a possible great- The Journal of Clinical Investigation http://www.jci.org Volume 115 Number 11 November 2005 2971 commentaries Table 1 therein) as well as for other types of recep- Comparison between the in vivo signaling properties of the AT1-i2m and known data tors. Thus, the GRK/β-arrestin system is regarding GRK/β-arrestin–mediated signaling actually bifunctional, as it initiates new G protein–independent signaling pathways Biochemical parameter AT1-i2m GRK/β-arrestin– even as it inactivates G protein–mediated mediated signals signaling. These G protein–independent Gαq activation Absent Absent signaling functions may in fact be evolu- Src activation Yes Yes tionarily conserved, as evidenced by recent PKC activation/translocation Absent Absent data demonstrating that β-arrestin–like Cytoplasmic phospho-ERK Elevated Elevated proteins transduce signals from a primi- Nuclear phospho-ERK Unaffected Unaffected tive 7TM pH sensor in fungi (8). A JNK activation Absent β-arrestin–mediated angiotensin II– B p38 activation Absent Absent stimulated activation of ERK1/2 has been Apoptosis Decreased relative to AT1-WT Decreased; PI3K, Akt, extensively studied in an in vitro human prosurvival ERK activated embryonic kidney 293 cell system (reviewed The signaling profile of AT1-i2m (middle column) and known effects of GRK/β-arrestin–mediated in ref. 7). The signal appears to be carried signal transduction (right column) are compared for each biochemical parameter listed. AGRK/ by β-arrestin2, with β-arrestin1 actually -arrestin–mediated signaling results in activation of JNK3 specifically (21); evidence also demon- β being inhibitory. As with other -arrestin– strates that β-arrestin recruits a phosphatase