Mutation in the TRKB Cholesterol Recognition Site that blocks Antidepressant Binding does not Influence the Basal or BDNF-Stimulated Activation of TRKB

Brain-derived neurotrophic factor (BDNF) acting upon its receptor Neurotrophic tyrosine kinase receptor 2 (NTRK2, TRKB) plays a central role in the development and maintenance of synaptic function and activity- or drug-induced plasticity. TRKB possesses an inverted cholesterol recognition and alignment consensus sequence (CARC), suggesting this receptor can act as a cholesterol sensor. We have recently shown that antidepressant drugs directly bind to the CARC domain of TRKB dimers, and that this binding as well as biochemical and behavioral responses to antidepressants are lost with a mutation in the TRKB CARC motif (Tyr433Phe). However, it is not clear if this mutation can also compromise the receptor function and lead to behavioral alterations. Here, we observed that Tyr433Phe mutation does not alter BDNF binding to TRKB, or BDNF-induced dimerization of TRKB. In this line, primary cultures from embryos of heterozygous Tyr433Phe mutant mice (hTRKB.Tyr433Phe) are responsive to BDNF-induced activation of TRKB, and samples from adult mice do not show any difference on TRKB activation compared to wild-type littermates (TRKB.wt). The behavioral phenotype of hTRKB.Tyr433Phe mice is indistinguishable from the wild-type mice in cued fear conditioning, contextual discrimination task, or the elevated plus maze, whereas mice heterozygous to BDNF null allele show a phenotype in context discrimination task. Taken together, our results indicate that Tyr433Phe mutation in the TRKB CARC motif does not show signs of loss-of-function of BDNF responses, while antidepressant binding to TRKB and responses to antidepressants are lost in Tyr433Phe mutants, making them an interesting mouse model for antidepressant research.