Ras small GTPases are critical molecular switches
Ras small GTPases are critical molecular switches that connect extracellular signals to intracellular signaling pathways initiated by membrane receptors, such as growth factor receptors and G protein-coupled receptors (Simanshu et al., 2017). When shifting between active GTP-bound and inactive GDP-bound form, Ras-small GTPases coordinate several cellular functions as proliferation, cell differentiation, apoptosis, migration, adhesion, and gene Hyperoside (Sandí et al., 2017; Wen et al., 2018; Stankiewicz et al., 2015; Muñoz-Félix et al., 2016; Xu et al., 2016)
The subfamily of Ras-like small GTPases contains numerous members including classical Ras (H-, K-and N-Ras), R-Ras, TC21/R-Ras2, MRas/R-Ras3, DexRas1/RasD1, RalA/B, Rheb, Rit, Rin, Rap1 and Rap2, and atypical κB-Ras1 and κB-Ras2 (Goitre et al., 2014; Fenwick et al., 2000). Despite considerable structural, sequence and biochemical similarities, these proteins are found differentially expressed in tissues and play multiple and divergent roles, including regulation of immunity and inflammation (Johnson e Chen, 2012).
Macrophages in innate immune system have implicated distinct requirements for Ras-like small GTPases, including Rap1 (Katagiri et al., 2000; Caron et al., 2000; Tang et al., 2014). Rap1 is capable of regulating the activation of the integrin αMβ2 in response to LPS and other inflammatory mediators in macrophages (Caron et al., 2000). Expression of B-cell activating factor (BAFF) in macrophages stimulated with LPS is regulated, in part, through Epac1-mediated Rap1 activation (Moon et al., 2011). Pathogen associated molecular patterns (PAMPs) recognition by TLR2 and TLR5 in monocytes triggers inflammatory cell infiltration to the site of pathogen invasion or sterile tissue injury via Rap1 GTPase-dependent β2-integrin activation (Chung et al., 2014). In addition, through RasGRP3-dependent activation, Rap1 is implicated in Toll-like receptor (TLR)-triggered inflammatory response by limiting IL-6 cytokine production in macrophages (Tang et al., 2014).
Rap2, the closest relative of Rap1 and sharing ˜60% identity, had been long thought to be functionally analogous to Rap1. Contrary to Rap1 function, however, Rap2 was initially shown unable to reverse Ras-induced transformation of fibroblasts (Jimenez et al., 1991). To date, specific roles for Rap2 in a number of cellular processes have been reported (Bruurs and Bos, 2014; Wang et al., 2017; Kong et al., 2007; Meng et al., 2018). Biological functions of Rap2 in macrophages and inflammation, however, have remained largely unexplored. In addition, despite a number of guanine exchange factors implicated in the activity for Rap2 (De Rooij et al., 1999, 2000; Yaman et al., 2009; Gao et al., 2001), a few have been characterized in inflammatory responses (Scott et al., 2016; Silva et al., 2018 - submitted). Among reports associating Rap2 proteins with immunity, it has recently been demonstrated that microtubule BCR receptor polarization is impaired in Rap2c depleted B lymphocytes (Wang et al., 2018). Furthermore, the involvement of Rap2 proteins in neutrophil migration and adhesion has been described (Gera et al., 2017; Jenei et al., 2006).
Material and methods
Discussion It is not unusual to observe similar phenotypes for cells where proteins whose levels are both increased or silenced (Manzini et al., 2014; Wang et al., 2014). It is possible that overexpression of Rap2a may result into formation of unbalanced protein complexes, by sequestering or trapping critical proteins associated with intracellular events to NF-κB activation and inhibiting in turn its normal function. The notion that LPS (and other PAMPs) activate members of the Ras superfamily is not fully clarified. Ras can be activated by CpG in macrophages, and it is implicated in signaling initiated by the TLR ligand as an early event by associating with TLR9 and promoting IRAK1/TRAF6 complex formation, and MAPK and NF-κB activation (Xu et al., 2003). Rap1 has been shown to be a target of RasGRP3 in TLRs signalling in macrophages. Besides, the GEF may regulate IL-6 production via activating Rap1a (Tang et al., 2014).