Small RhoGTPases are single domain nucleotide dependent bina

Small RhoGTPases are single-domain nucleotide-dependent binary switches that act as highly-tuned regulators in signal transduction [1]. The cycling between active GTP-bound and inactive GDP-bound forms allows RhoGTPases to bind to or to dissociate from downstream effectors, respectively [2]. Guanine nucleotide exchange factors (GEFs) that catalyze the exchange of GDP for GTP, and GTPase-activating proteins (GAPs) that increase intrinsic GTP hydrolysis are respectively responsible for RhoGTPases switching between their active and inactive form [3]. Furthermore, switching between GDP and GTP may involve cytosol-membrane translocation, as farnesyl or geranylgeranyl-carrying RhoGTPases can form soluble complexes with guanine dissociation inhibitors (GDIs), thus preventing RhoGTPases from membrane-targeting and GEFs-mediated activation [4]. A remarkable feature of RhoGTPases-based signaling networks is that specific interacting patterns between GEFs and RhoGTPases, coupled with post-translational modifications and scaffolding molecules lead to spatiotemporal promotion of determined outcomes [5]. Ras-related C3 botulinum toxin substrate 1 (Rac1), a member of the RhoGTPases family, has a pivotal role in the regulation of apoptosis inducer polymerization during cytoskeletal rearrangement events [6]. Rac1-mediated actin regulation takes place through binding of Rac1 to the scaffolding molecule known as insulin receptor tyrosine kinase substrate p53 (IRSp53), thus leading to Rac1 binding to WASP-family verprolin-homologous (WAVE) proteins [7]. As a result, WAVEs bind to and activate the actin-nucleating protein, actin-related protein 2/3 (Arp2/3) complex, which initiates growth of new branched filaments [8]. Another way in which Rac1 can regulate the actin cytoskeleton is by binding to p21-activated kinases (PAKs) which in turn conducts cytoskeletal rearrangement via phosphorylating Lim kinases (LIMKs) [9]. LIMKs phosphorylate cofilin subsequently, thereby releasing it from actin filaments and thus suppressing actin-severing activity [10]. In this way, the Rac1/Pak1/LIMK1/cofilin axis may modulate the turnover of actin filaments at the lamellipodium [11]. Rac1-mediated actin regulation has important roles in cell-cell adhesion [12], cell-extracellular matrix (ECM) early interaction [13], cell polarization [14] and cell mobility [15]. These events are widely regarded as actin-related outcomes of the Rac1 signaling axis. Additionally, Rac1 is indispensable for the assembly of the membrane-located superoxide-producing NADPH oxidase (NOX) complexes, where it is required for the electron transfer from NADPH to oxygen [16]. NOX isoforms I and II (NOX1 and NOX2) are activated via Rac1 having relevant roles in physiology and in several human diseases including neurodegenerative pathologies [17]. Besides Rac1, the assembly of the NOX complex requires the binding of at least two cytoplasmic subunits (an activator and an organizer) to the membrane-located catalytic subunit, which in turn must be bound to a membrane-located anchorage subunit [18]. The cytoplasmic activators p67 and NOXA1 are the NOX2 and NOX1 components serving as targets for Rac1, respectively [19]. In the cytosol, prenylated Rac1 is inactive and bound to RhoGDIs [20]. Through various receptor-mediated signaling cascades that involve Rac1 GEFs, the Rac1-RhoGDI complex is translocated to the membrane [21], [22]. NOXs enzymes play significant roles in endothelial functions [23], cellular proliferation [24], cancer [25], establishment of neuronal polarity [26] and neurodegeneration [27]. Taken together, it is evident that Rac1 presents two different downstream outcomes; actin- and NOX-related events (Fig. 1). How Rac1 can promote each outcome in a coordinated manner is intriguing. An example of Rac1-mediated signaling bifurcation is seen in the context of MAP kinases function. In this way, Wu et al. [28] showed that downstream from Tat (Human immunodeficiency virus type 1 transactivator of transcription) signaling, two independent Rac1-mediated outcomes take place; activation of RhoA-Nox4-dependent Ras/ERK which favors proliferation, and activation of PAK1-Nox2-dependent JNK that promotes cytoskeletal rearrangement It has been suggested that RhoA may favor Nox4 activity via up-regulating its expression levels during fibroblast differentiation [29]. However, the correlation between RhoA and Nox4 is not resolved and appears to be context-dependent as recent evident shows that loss of Nox4 increases levels of RhoA in Huh7 and PLC/PRF/5 cells, while overexpression of Nox4 in SNU449 cells increased RhoA levels [30]. In the present review, we are interested in Rac1-mediated signaling bifurcation regarding ROS production and actin regulation outcomes, which have been studied mostly as separate events. Here, we bring together evidence of co-occurrence and crosstalk between both functions. One layer of modulation for such outcomes is provided by Rac1 regulators, namely GEFs, GAPs and GDIs. Then Rac1 regulators driving both actin- and NOX-related outcomes are discussed. Moreover, crosstalk events between Rac1 axes involving redox signaling are also addressed along with their physiological and pathological roles highlighting possible roles in neuronal functions.