Differential effects of BDNF and neurotrophin 4 (NT4) on endocytic sorting of TrkB receptors

Neurotrophins are a family of growth factors playing key roles in the survival, development, and function of neurons. The neurotrophins brain-derived neurotrophic factor (BDNF) and NT4 both bind to and activate TrkB receptors, however, they mediate distinct neuronal functions. The molecular mechanism of how TrkB activation by BDNF and NT4 leads to diverse outcomes is unknown. Here, we report that BDNF and NT4 lead to differential endocytic sorting of TrkB receptors resulting in diverse biological functions in cultured cortical neurons. Fluorescent microscopy and surface biotinylation experiments showed that both neurotrophins stimulate internalization of TrkB with similar kinetics. Exposure to BDNF for 2-3h reduced the surface pool of TrkB receptors to half, whereas a longer treatment (4-5h) with NT4 was necessary to achieve a similar level of down-regulation. Although BDNF and NT4 induced TrkB phosphorylation with similar intensities, BDNF induced more rapid ubiquitination and degradation of TrkB than NT4. Interestingly, TrkB receptor ubiquitination by these ligands have substantially different pH sensitivities, resulting in varying degrees of receptor ubiquitination at lower pH levels. Consequently, NT4 was capable of maintaining longer sustained downstream signaling activation that correlated with reduced TrkB ubiquitination at endosomal pH. Thus, by leading to altered endocytic trafficking itineraries for TrkB receptors, BDNF and NT4 elicit differential TrkB signaling in terms of duration, intensity, and specificity, which may contribute to their functional differences invivo.