The Rab5 Activator ALS2/alsin Acts as a Novel Rac1 Effector through Rac1-activated Endocytosis

Abstract

Mutations in the ALS2 gene cause a number of recessive motor neuron diseases, indicating that the ALS2 protein (ALS2/ alsin) is vital for motor neurons. ALS2 acts as a guanine nucleotide exchange factor (GEF) for Rab5 (Rab5GEF) and is involved in endosome dynamics. However, the spatiotemporal regulation of the ALS2-mediated Rab5 activation is unclear. Here we identified an upstream activator for ALS2 and showed a functional significance of the ALS2 activation in endosome dynamics. ALS2 preferentially interacts with activated Rac1. In the cells activated Rac1 recruits cytoplasmic ALS2 to membrane ruffles and subsequently to nascent macropinosomes via Rac1-activated macropinocytosis. At later endocytic stages macropinosomal ALS2 augments fusion of the ALS2-localized macropinosomes with the transferrin-positive endosomes, depending on the ALS2-associated Rab5GEF activity. These results indicate that Rac1 promotes the ALS2 membranous localization, thereby rendering ALS2 active via Rac1-activated endocytosis. Thus, ALS2 is a novel Rac1 effector and is involved in Rac1-activated macropinocytosis. All together, loss of ALS2 may perturb macropinocytosis and/or the following membrane trafficking, which gives rise to neuronal dysfunction in the ALS2-linked motor neuron diseases.

Redox Modifier Genes and Pathways in Amyotrophic Lateral Sclerosis

“…Accordingly, ligand-triggered redoxosome formation mobilizes Nox to the early endosomal (EE) compartment, where Rac1=SOD1 complex formation leads to Nox activation and ROS production (55, 82). Macropinosomes (MP) form through the previously described process involving Alsin, Rac1, and Rab5 (76), and this compartment may be important for directing redoxosomes to the recycling endosomal (RE) compartment. This Alsin=Rab5-facilitated fusion event could lead to the processing of redoxosomal components and inactivation of the Nox complex.…”

“…Alsin was originally thought to serve as a GEF for both Rab5 and Rac1 GTPases, and hence is considered a candidate activator of each of these proteins (106,131). Although the GEF activity of Alsin with respect to Rab5 has been reproduced by several research laboratories, a relatively recent study suggests that Alsin functions as an effector, rather than a GEF, of Rac1 (76). This finding is of considerable interest since SOD1 also acts as an effector for Rac1 (rather than as a traditional GEF or GTPaseactivating protein (GAP)) (Fig.…”

“…Both the Rab5-GEF and Rac1-effector functions of Alsin appear to influence endocytic mechanisms and endosomal dynamics; activated Rac1 appears to recruit Alsin to the plasma membrane at sites of Rab5-dependent macropinocytosis, and Alsin appears to also promote the fusion of macropinosomes with EEA1=Rab5-positive early endosomes (Fig. 5A) (76). These functions of Alsin in endosome formation and trafficking are intriguing when compared to the biology of receptor-mediated redoxosome formation in the context of IL-1b and TNFa signaling pathways.…”

“…3). However, in contrast to Rac1=Alsin-dependent macropinocytosis, which is most likely dynamin-independent (14,31,76), endocytosis of ligand-activated IL-1R1 is dependent on dynamin rather than on Rac1 (82). Interestingly, Alsin has been shown to selectively bind ALS mutants of SOD1 via its DH=PH domain (69), a finding reminiscent of enhanced binding of ALS SOD1 mutants to Rac1 (55).…”

“…5C). Hence, Rac1=SOD1=Alsin=Rab5 signaling networks may act in concert to regulate cell signaling at the endosomal level, and dysfunction of these pathways in ALS Alsin plays an important role in macropinocytosis following its recruitment by Rac1 to plasma membrane ruffles at sites of organized actin filaments (76). In this context, Alsin may stabilize Rac1-GTP and hence act as a Rac1-effector.…”