br Introduction br Rac dependent

Introduction
Rac1-dependent ROS and Gemcitabine regulation in neuronal functions NOX-mediated ROS have an important role as physiological messengers. One remarkable example regarding such a function is during axonal formation. In this line, increased p40phox/NOX2 levels and co-localization at growth cone contact sites with apCAM beads and interacting growth cones have been observed. Thus apCAM-clustering promotes actin rearrangement and NADPH oxidase activation during neurite outgrowth [62]. Based on the latter, Munnamalai et al. [62] proposed that cytosolic NADPH oxidase subunits such as p40 are associated with actin structures in unstimulated growth cones. Here, NOX2 subunits p47phox, p67phox, p40phox, and Rac1 translocate to the plasma membrane (with or without F-actin) and activate NOX2 upon growth cone stimulation by external cues. In addition, it has been shown that in order to sustain axonal development, Rac1 is activated via RyR-mediated Ca2+ release from the ER [63]. In this mechanism, ER Ca2+ release promotes Rac1-activation, which in turn activates NOX2 leading to ROS production. Since RyR activity is promoted by ROS and Ca2+[64], a feed-forward loop in which activated Rac1 maintains axonal growth and NOX-mediated ROS production was established [63]. Notably, this loop could be abrogated by applying NSC23766, which blocks the interaction of Rac1 with its GEFs Tiam1 and TRIO. Thus ROS production would be sustained in neurons via RyR-mediated Ca2+ release/GEFs/Rac1/NOX pathway.
Involvement of Rac1 regulators in redox and actin events
Crosstalk between actin and ROS-related events mediated by Rac1
Crosstalk between redox and scaffolding events in Rac1-dependent actin regulation
Deregulated Rac1 activation in neurodegeneration It has been reported that fibrillar amyloid-beta peptide, which is observed in AD, promotes increased Rac1 activity via Tiam1 activation by a Ca2+-dependent mechanism, this phenomenon also involves enhanced actin polymerization [216]. Here, amyloid beta mediates Rac1 activation through phosphorylation and translocation of Tiam1; in fact, Fibrillar A1-42 induced a significant increase (1.5-fold) in the level of Thr phosphorylation of Tiam1. Calcium-dependent PKC activity was responsible for Tiam1 phosphorylation. Therefore, upon amyloid beta exposure, Rac1 is over-activated by Ca2+ signaling (conventional PKC activity) that promotes Tiam1 activation and its translocation to the membrane [217]. A link between actin dynamics and ROS production upon amyloid beta exposure has been proposed by Tsoy et al. [218], whom by using immortalized cerebral endothelial cells (bEnd3) observed that Aβ42 promoted ROS production by up to 83% after 60min of treatment. They also demonstrated that Aβ42 favored actin polymerization, while pretreatment with the antioxidant N-acetylcysteine (NAC) suppressed Aβ-induced actin polymerization and cytoskeletal rearrangement [218]. Manterola et al. [219] also found a positive correlation between Aβ42 exposure and Rac1 activation in SN4741 cells (a line originated from substantia nigra dopaminergic cells derived from transgenic mouse embryos), primary embryonic cortical neurons from rats and in neuronal organotypic cultures of the hippocampus and the entorhinal cortex. They observed that Aβ1–42 peptide stimulates the Rac1 pathway through Tiam-1 phosphorylation by novel PKCs. These kinases are not calcium dependent. Similarly, in a mice model of the Fragile X syndrome, characterized by thin, long and immature high density dendritic spines, Bongmba et al. [220] found that Rac1 is over-activated in the mouse brain. It has been observed that NOX2-derived ROS is locally produced at synapses and that NOX2 has a post-synaptic localization [214], [221], [222]. Remarkably, Abdel-Rahman et al. [223] showed that NOX2 rather than the mitochondria is the major source of synaptic ROS in forebrain synaptosomes from mice. Therefore, it might be possible that neurodegenerative hyper-activation of Rac1 could be inducing deregulated NOX activation.