Platelet C3G: a key player in vesicle exocytosis, spreading and clot retraction

C3G is a Rap1 GEF that plays a pivotal role in platelet-mediated processes such as angiogenesis, tumor growth, and metastasis by modulating the platelet secretome. Here, we explore the mechanisms through which C3G governs platelet secretion. For this, we utilized animal models featuring either overexpression or deletion of C3G in platelets, as well as PC12 cell clones expressing C3G mutants. We found that C3G specifically regulates α-granule secretion via PKCδ, but it does not affect δ-granules or lysosomes. C3G activated RalA through a GEF-dependent mechanism, facilitating vesicle docking, while interfering with the formation of the trans-SNARE complex, thereby restricting vesicle fusion. Furthermore, C3G promotes the formation of lamellipodia during platelet spreading on specific substrates by enhancing actin polymerization via Src and Rac1-Arp2/3 pathways, but not Rap1. Consequently, C3G deletion in platelets favored kiss-and-run exocytosis. C3G also controlled granule secretion in PC12 cells, including pore formation. Additionally, C3G-deficient platelets exhibited reduced phosphatidylserine exposure, resulting in decreased thrombin generation, which along with defective actin polymerization and spreading, led to impaired clot retraction. In summary, platelet C3G plays a dual role by facilitating platelet spreading and clot retraction through the promotion of outside-in signaling while concurrently downregulating α-granule secretion by restricting granule fusion.

Crk proteins activate the Rap1 guanine nucleotide exchange factor C3G by segregated adaptor-dependent and -independent mechanisms

BACKGROUND

C3G is a guanine nucleotide exchange factor (GEF) that activates Rap1 to promote cell adhesion. Resting C3G is autoinhibited and the GEF activity is released by stimuli that signal through tyrosine kinases. C3G is activated by tyrosine phosphorylation and interaction with Crk adaptor proteins, whose expression is elevated in multiple human cancers. However, the molecular details of C3G activation and the interplay between phosphorylation and Crk interaction are poorly understood.

METHODS

We combined biochemical, biophysical, and cell biology approaches to elucidate the mechanisms of C3G activation. Binding of Crk adaptor proteins to four proline-rich motifs (P1 to P4) in C3G was characterized in vitro using isothermal titration calorimetry and sedimentation velocity, and in Jurkat and HEK293T cells by affinity pull-down assays. The nucleotide exchange activity of C3G over Rap1 was measured using nucleotide-dissociation kinetic assays. Jurkat cells were also used to analyze C3G translocation to the plasma membrane and the C3G-dependent activation of Rap1 upon ligation of T cell receptors.

RESULTS

CrkL interacts through its SH3N domain with sites P1 and P2 of inactive C3G in vitro and in Jurkat and HEK293T cells, and these sites are necessary to recruit C3G to the plasma membrane. However, direct stimulation of the GEF activity requires binding of Crk proteins to the P3 and P4 sites. P3 is occluded in resting C3G and is essential for activation, while P4 contributes secondarily towards complete stimulation. Tyrosine phosphorylation of C3G alone causes marginal activation. Instead, phosphorylation primes C3G lowering the concentration of Crk proteins required for activation and increasing the maximum activity. Unexpectedly, optimal activation also requires the interaction of CrkL-SH2 domain with phosphorylated C3G.

CONCLUSION

Our study revealed that phosphorylation of C3G by Src and Crk-binding form a two-factor mechanism that ensures tight control of C3G activation. Additionally, the simultaneous SH2 and SH3N interaction of CrkL with C3G, required for the activation, reveals a novel adaptor-independent function of Crk proteins relevant to understanding their role in physiological signaling and their deregulation in diseases. Video abstract.

Adapter Protein RapGEF1 Is Required for ERK1/2 Signaling in Response to Elevated Phosphate in Vascular Smooth Muscle Cells

The sodium-dependent phosphate transporter, SLC20A1, is required for elevated inorganic phosphate (Pi) induced vascular smooth muscle cell (VSMC) matrix mineralization and phenotype transdifferentiation. Recently, elevated Pi was shown to induce ERK1/2 phosphorylation through SLC20A1 by Pi uptake-independent functions in VSMCs, suggesting a cell signaling response to elevated Pi. Previous studies identified Rap1 guanine nucleotide exchange factor (RapGEF1) as an SLC20A1-interacting protein and RapGEF1 promotes ERK1/2 phosphorylation through Rap1 activation. In this study, we tested the hypothesis that RapGEF1 is a critical component of the SLC20A1-mediated Pi-induced ERK1/2 phosphorylation pathway. Co-localization of SLC20A1 and RapGEF1, knockdown of RapGEF1 with siRNA, and small molecule inhibitors of Rap1, B-Raf, and Mek1/2 were investigated. SLC20A1 and RapGEF1 were co-localized in peri-membranous structures in VSMCs. Knockdown of RapGEF1 and small molecule inhibitors against Rap1, B-Raf, and Mek1/2 eliminated elevated Pi-induced ERK1/2 phosphorylation. Knockdown of RapGEF1 inhibited SM22α mRNA expression and blocked elevated Pi-induced downregulation of SM22α mRNA. Together, these data suggest that RapGEF1 is required for SLC20A1-mediated elevated Pi signaling through a Rap1/B-Raf/Mek1/2 cell signaling pathway, thereby promoting ERK1/2 phosphorylation and inhibiting SM22α gene expression in VSMCs.

Complex formation and reciprocal regulation between GSK3β and C3G

GSK3β, a ubiquitously expressed Ser/Thr kinase, regulates cell metabolism, proliferation and differentiation. Its activity is spatially and temporally regulated dependent on external stimuli and interacting partners, and its deregulation is associated with various human disorders. In this study, we identify C3G (RapGEF1), a protein essential for mammalian embryonic development as an interacting partner and substrate of GSK3β. In vivo and in vitro interaction assays demonstrated that GSK3β and Akt are present in complex with C3G. Molecular modelling and mutational analysis identified a domain in C3G that aids interaction with GSK3β, and overlaps with its nuclear export sequence. GSK3β phosphorylates C3G on primed as well as unprimed sites, and regulates its subcellular localization. Over-expression of C3G resulted in activation of Akt and inactivation of GSK3β. Huntingtin aggregate formation, dependent on GSK3β inhibition, was enhanced upon C3G overexpression. Stable clones of C2C12 cells generated by CRISPR/Cas9 mediated knockdown of C3G, that cannot differentiate, show reduced Akt activity and S9-GSK3β phosphorylation compared to wild type cells. Co-expression of catalytically active GSK3β inhibited C3G induced myocyte differentiation. C3G mutant defective for GSK3β phosphorylation, does not alter S9-GSK3β phosphorylation and, is compromised for inducing myocyte differentiation. Our results show complex formation and reciprocal regulation between GSK3β and C3G. We have identified a novel function of C3G as a negative regulator of GSK3β, a property important for its ability to induce myogenic differentiation.