TrkB Regulates N-Methyl-D-Aspartate Receptor Signaling by Uncoupling and Recruiting the Brain-Specific Guanine Nucleotide Exchange Factor, RasGrf1.

Brain-derived neurotrophic factor (BDNF) facilitates multiple aspects of neuronal differentiation and cellular physiology by activating the high-affinity receptor tyrosine kinase, TrkB. While it is known that both BDNF and TrkB modulate cellular processes involved in learning and memory, exactly how TrkB cross-talks and modulates signaling downstream of excitatory ionotropic receptors, such as the NMDA receptor (NMDAR), are not well understood. A model that we have investigated involves the signaling molecule RasGrf1, a guanine nucleotide exchange factor for both Ras and Rac. We previously identified RasGrf1 as a novel Trk binding partner that facilitates neurite outgrowth in response to both nerve growth factor (NGF) (Robinson et al. in J Biol Chem 280:225-235, 2005) and BDNF (Talebian et al. in J Mol Neurosci 49:38-51, 2013); however, RasGrf1 can also bind the NR2B subunit of the NMDAR (Krapivinsky et al. in Neuron 40:775-784, 2003) and stimulate long-term depression (LTD) (Li et al. in J Neurosci 26:1721-1729, 2006). We have addressed a model that TrkB facilitates learning and memory via two processes. First, TrkB uncouples RasGrf1 from NR2B and facilitates a decrease in NMDA signaling associated with LTD (p38-MAPK). Second, the recruitment of RasGrf1 to TrkB enhances neurite outgrowth and pERK activation and signaling associated with learning and memory. We demonstrate that NMDA recruits RasGrf1 to NR2B; however, co-stimulation with BDNF uncouples this association and recruits RasGrf1 to TrkB. In addition, activation of TrkB stimulates the tyrosine phosphorylation of RasGrf1 which increases neurite outgrowth (Talebian et al. in J Mol Neurosci 49:38-51, 2013), and the tyrosine phosphorylation of NR2B (Tyr1472) (Nakazawa et al. in J Biol Chem 276:693-699, 2001) which facilitates NMDAR cell surface retention (Zhang et al. in J Neurosci 28:415-24, 2008). Collectively, these data demonstrate that TrkB alters NMDA signaling by a dual mechanism that uncouples LTD and, in turn, stimulates neuronal growth and the signaling pathways associated with learning and memory.

Ras guanine nucleotide releasing factor 1 (RasGrf1) enhancement of Trk receptor-mediated neurite outgrowth requires activation of both H-Ras and Rac.

We previously demonstrated that the guanine nucleotide exchange factor, RasGrf1, binds nerve growth factor (NGF)-activated TrkA and facilitates neurotrophin-induced neurite outgrowth in PC12 cells. RasGrf1 can activate both Ras and Rac, via intrinsic Cdc25 and DH domains, respectively, suggesting that the activation of both could contribute to this process. Previous studies have assayed constitutive neurite outgrowth following RasGrf1 over-expression in PC12 cells, in either the absence or presence of ectopic H-Ras, and have suggested an essential role for either Ras or Rac depending on the presence of H-Ras over-expression. In contrast, in this study, we have addressed the mechanism of how RasGrf1 facilitates neurite outgrowth in response to the neurotrophins, NGF and BDNF. Using Ras/Rac activation assays and site-directed RasGrf1 mutants, we find that both Ras and Rac are essential to neurotrophin-induced neurite outgrowth. Moreover, we find that H-Ras over-expression rescues the loss of neurite outgrowth observed with a Rac minus mutant and that RasGrf1 differentially stimulates NGF-dependent activation of Rac or Ras, depending on cell type. Collectively, these studies clarify the mechanism of how RasGrf1 expression facilitates neurotrophin-induced neurite outgrowth. Moreover, they suggest that H-Ras over-expression should be used with caution to measure phenotypic responses.