It is well established that GSIS from pancreatic β-cells requires the generation of soluble second messengers including cyclic nucleotides (e.g., cAMP), biologically active hydrolytic products of phospholipases (diacyl glycerol and inositol triphosphate), adenine nucleotides (e.g., ATP) and guanine nucleotides (e.g., GTP). Intracellular generation of inositol triphosphates leads to mobilization of calcium from the endoplasmic reticulum to the soluble compartment, which is essential for the transport of insulin-laden secretory granules to the plasma membrane for fusion and exocytotic secretion of insulin into circulation (MacDonald 1990; Newgard and McGarry 1995; Prentki et al., 2013). Extant studies have demonstrated novel roles for small GTP-binding proteins (G proteins) belonging to the Rho, Ras, Rab and Arf subfamilies in insulin secretion, primarily via modulation of cytoskeletal remodeling and vesicle transport and fusion with the plasma membrane culminating in insulin secretion. Along these lines, previous studies have demonstrated roles for a number of GTP/GDP exchange factors (GEFs), including Tiam1, β-PIX and ARNO, in G protein-mediated insulin secretion (Kowluru 2020; Veluthakal and Thurmond 2021). Moreover, using a variety of pharmacological and molecular biological approaches, previous studies have demonstrated that post-translational modifications (e.g., prenylation) of Rho and Rab G proteins are necessary for insulin secretion to occur (Kowluru and Kowluru 2015; Kowluru 2020; Veluthakal and Thurmond 2021).

In the context of functional regulation of small G proteins, emerging evidence implicates novel roles for small G protein GDP-dissociation stimulator (smgGDS) in the functional activation of a variety of small G proteins. Original studies by Yamamoto and coworkers have reported functionally active smgGDS (∼61 kDa) in bovine brain cytosol (Yamamoto et al., 1990), which was later cloned by Kaibuchi et al. (Kaibuchi et al., 1991). Subsequent investigations have demonstrated GEF-like properties for smgGDS toward Rho subfamily of G-proteins, including Rac1, RhoA and Rap1B (Hiraoka et al., 1992; Williams 2003). Investigations by Chuang et al. have revealed that smgGDS stimulates GTP/GDP exchange via stabilization of nucleotide-bound and -free forms of Rac1 (Chuang et al., 1994). Subsequent studies by Hamel and coworkers have reported specific activation of RhoA and RhoC by smgGDS (Hamel et al., 2011). More recent investigations by Bergom and associates have revealed binding of DiRas1 to smgGDS, thereby antagonizing the interaction of smgGDS to other oncogenic small G proteins (Bergom et al., 2016). Together, these studies provided compelling evidence for regulatory control of G protein activation by smgGDS.

In addition to its GEF-like properties for smgGDS, several recent investigations have implicated novel roles for smgGDS in the regulatory control of newly synthesized small G protein prenylation signaling modules (Brandt et al., 2021). Two splice variants of smgGDS have been reported, each with specific and individual properties differentiating their intrinsic functionality. smgGDS-607 binds unprenylated small G proteins while smgGDS-558, which lacks one of the thirteen armadillo domains, binds prenylated small G proteins (Brandt et al., 2021). These data display a prominent role in the prenylation pathway of small G proteins in both transport to the prenylation machinery (smgGDS-607) and from the site of prenylation to subsequent downstream localization (smgGDS-558). It is noteworthy that recent studies by Garcia-Torres and Fierke have defined dual roles for smgGDS-607 in the activation and inhibition of farnesylation of H-Ras and DiRas1, respectively, suggesting multiple modes of substrate recognition by smgGDS (García-Torres and Fierke 2019).

Given the critical regulatory roles for post-translational prenylation and GEF-mediated activation of small G proteins in insulin secretion (as above) and based on emerging roles of smgGDS as a mediator of these two signaling pathways/steps, we undertook the current investigation to decipher roles for smgGDS in insulin secretion in INS-1 832/13 cells. Specifically, we assessed the roles of smgGDS in glucose-, KCl (calcium-induced), and forskolin/isobutyl-methylxanthine (cAMP-mediated) insulin secretion. We present the first evidence of expression of smgGDS in human islets, rodent islets, and clonal β-cells. We also report novel roles of these proteins in insulin secretion derived from glucose metabolic events, including calcium- and cAMP-dependent signaling steps.