Epac- and Rap- independent ERK1/2 phosphorylation induced by Gs-coupled receptor stimulation in HEK293 cells

The RasGrf family of mammalian guanine nucleotide exchange factors

RasGrf1 and RasGrf2 are highly homologous mammalian guanine nucleotide exchange factors which are able to activate specific Ras or Rho GTPases. The RasGrf genes are preferentially expressed in the central nervous system, although specific expression of either locus may also occur elsewhere. RasGrf1 is a paternally-expressed, imprinted gene that is expressed only after birth. In contrast, RasGrf2 is not imprinted and shows a wider expression pattern. A variety of isoforms for both genes are also detectable in different cellular contexts. The RasGrf proteins exhibit modular structures composed by multiple domains including CDC25H and DHPH motifs responsible for promoting GDP/GTP exchange, respectively, on Ras or Rho GTPase targets. The various domains are essential to define their intrinsic exchanger activity and to modulate the specificity of their functional activity so as to connect different upstream signals to various downstream targets and cellular responses. Despite their homology, RasGrf1 and RasGrf2 display differing target specificities and non overlapping functional roles in a variety of signaling contexts related to cell growth and differentiation as well as neuronal excitability and response or synaptic plasticity. Whereas both RasGrfs are activatable by glutamate receptors, G-protein-coupled receptors or changes in intracellular calcium concentration, only RasGrf1 is reported to be activated by LPA, cAMP, or agonist-activated Trk and cannabinoid receptors. Analysis of various knockout mice strains has uncovered a specific functional contribution of RasGrf1 in processes of memory and learning, photoreception, control of post-natal growth and body size and pancreatic β-cell function and glucose homeostasis. For RasGrf2, specific roles in lymphocyte proliferation, T-cell signaling responses and lymphomagenesis have been described.

G protein regulation of MAPK networks

G proteins provide signal-coupling mechanisms to heptahelical cell surface receptors and are critically involved in the regulation of different mitogen-activated protein kinase (MAPK) networks. The four classes of G proteins, defined by the G(s), G(i), G(q) and G(12) families, regulate ERK1/2, JNK, p38MAPK, ERK5 and ERK6 modules by different mechanisms. The alpha- as well as betagamma-subunits are involved in the regulation of these MAPK modules in a context-specific manner. While the alpha- and betagamma-subunits primarily regulate the MAPK pathways via their respective effector-mediated signaling pathways, recent studies have unraveled several novel signaling intermediates including receptor tyrosine kinases and small GTPases through which these G-protein subunits positively as well as negatively regulate specific MAPK modules. Multiple mechanisms together with specific scaffold proteins that can link G-protein-coupled receptors or G proteins to distinct MAPK modules contribute to the context-specific and spatio-temporal regulation of mitogen-activated protein signaling networks by G proteins.

Novel aspects of G-protein-coupled receptor signalling--different ways to achieve specificity

Our understanding of signal transduction via G-protein-coupled receptors (GPCR) has developed dramatically during the last decades. The initial idea of linear signalling pathways transferring information from the cell membrane to the nucleus has evolved into a complicated network of signalling pathways offering the possibility of crosstalk, fine tuning and specific regulation at multiple levels. During the recent meeting on GPCRs at the Karolinska Institutet, Stockholm novel aspects of GPCR signalling were presented and discussed. Here, we will discuss several possibilities for GPCRs to achieve specificity in signal transduction, such as the phenomenon of biased agonism, receptor multimerization, the role of co-receptors, the regulation of heterotrimeric G proteins as well as multiple G(s)-dependent pathways to extracellular single-regulated protein kinases.