Requirements of Gαi1 and Gαi3 for Brain‐Derived Neurotrophic Factor (BDNF) and Nerve Growth Factor (NGF) induced signaling

The inhibitory alpha subunits of heterotrimeric guanine nucleotide–binding proteins (Gαi proteins) couple to heptahelical cell surface receptors to convey extracellular messages to their effectors. Our previous study has indicated an unexpected and critical role of Gαi proteins in mediating receptor tyrosine kinase signaling (Cao et al., Science Signaling , 2009). Here we discovered a novel function of Gαi proteins in Brain‐derived neurotrophic factor (BDNF)/Nerve growth factor (NGF) signaling. Gαi proteins are required for BDNF and NGF to transduce signals from Trk receptor to stimulate mitogen‐activated protein kinases (MAPKs, p38, JNK and ERK) and AKT/mTORC1 signaling. Knockout of Gαi1 and Gαi3 completely blocks BDNF and NGF induced AKT/mTORC1 and MAPK activation in MEFs. Both Gαi1 and Gαi3 associated with the Grb2‐associated binding protein 1 (Gab1), and were required for the interaction of Gab1 with SHP2 as well as the interactions of Grb2 with SHP2, Shc, MEKK1 and MEKK2, in response to BDNF and NGF. Loss of Gαi1 and Gαi3 impaired the phosphorylation of Gab1, SHP2 and Shc in response to BDNF and NGF. Importantly, siRNA knockdown of both Gαi1 and Gαi3 inhibits BDNF and NGF induced AKT/mTORC1 and MAPK activation in cerebellar granule neuron, and BDNF induced neuronal survival, neuronal migration and neurite outgrowth is also largely impaired in cerebellar neurons with Gαi1 and Gαi3 deficiency. Notably, in mice brain tissues (cortex and cerebellum); double knockout of Gαi1 and Gαi3 also totally abolishes BDNF induced signaling. Overall, these results show that Gαi1 and Gαi3 lie downstream of Trk receptor, but upstream of Gab1‐Shc‐Grb2‐Shp2, mediating MAPK and AKT/mTORC1 signaling as well neuronal survival and migration.