The Human Rho-GEF Trio and Its Target GTPase RhoG Are Involved in the NGF Pathway, Leading to Neurite Outgrowth

Rho-GTPases control a wide range of physiological processes by regulating actin cytoskeleton dynamics. Numerous studies on neuronal cell lines have established that Rac, Cdc42, and RhoG activate neurite extension, while RhoA mediates neurite retraction. Guanine nucleotide exchange factors (GEFs) activate Rho-GTPases by accelerating GDP/GTP exchange. Trio displays two Rho-GEF domains, GEFD1, activating the Rac pathway via RhoG, and GEFD2, acting on RhoA, and contains numerous signaling motifs whose contribution to Trio function has not yet been investigated. Genetic analyses in Drosophila and in Caenorhabditis elegans indicate that Trio is involved in axon guidance and cell motility via a GEFD1-dependent process, suggesting that the activity of its Rho-GEFs is strictly regulated. Here, we show that human Trio induces neurite outgrowth in PC12 cells in a GEFD1-dependent manner. Interestingly, the spectrin repeats and the SH3-1 domain of Trio are essential for GEFD1-mediated neurite outgrowth, revealing an unexpected role for these motifs in Trio function. Moreover, we demonstrate that Trio-induced neurite outgrowth is mediated by the GEFD1-dependent activation of RhoG, previously shown to be part of the NGF (nerve growth factor) pathway. The expression of different Trio mutants interferes with NGF-induced neurite outgrowth, suggesting that Trio may be an upstream regulator of RhoG in this pathway. In addition, we show that Trio protein accumulates under NGF stimulation. Thus, Trio is the first identified Rho-GEF involved in the NGF-differentiation signaling.