SignificanceNeurotrophin receptors are a class of receptor tyrosine kinases that couple to signaling pathways critical for neuronal survival and growth. One member, TrkB, is particularly interesting because it plays a role in many severe degenerative neurological diseases. The TrkB natural ligand brain-derived neurotrophic factor (BDNF) is not suitable to be developed as a drug or therapy as proved by previous unsuccessful clinical trials. Here we report a selection method that produced potent full agonist antibodies that mimic BDNF function, yet with better biophysical properties. This study paves the road for the development of agonist antibodies for other receptor tyrosine kinases.

In the central nervous system, most neurons co‐express TrkB and TrkC, the tyrosine kinase receptors for brain‐derived neurotrophic factor (BDNF) and neurotrophin‐3 (NT3). As NT3 can also activate TrkB, it has been difficult to understand how NT3 and TrkC can exert unique roles in the assembly of neuronal circuits. Using neurons differentiated from human embryonic stem cells expressing both TrkB and TrkC, we compared Trk activation by BDNF and NT3. To avoid the complications resulting from TrkB activation by NT3, we also generated neurons from stem cells engineered to lack TrkB. We found that NT3 activates TrkC at concentrations lower than those of BDNF needed to activate TrkB. Downstream of Trk activation, the changes in gene expression caused by TrkC activation were found to be similar to those resulting from TrkB activation by BDNF, including a number of genes involved in synaptic plasticity. At high NT3 concentrations, receptor selectivity was lost as a result of TrkB activation. In addition, TrkC was down‐regulated, as was also the case with TrkB at high BDNF concentrations. By contrast, receptor selectivity as well as reactivation were preserved when neurons were exposed to low neurotrophin concentrations. These results indicate that the selectivity of NT3/TrkC signalling can be explained by the ability of NT3 to activate TrkC at concentrations lower than those needed to activate TrkB. They also suggest that in a therapeutic perspective, the dosage of Trk receptor agonists will need to be taken into account if prolonged receptor activation is to be achieved.

This review focuses on neurotrophins and their tyrosine kinase receptors, with an emphasis on their relevance to the function and dysfunction in the human nervous system. It also deals with measurements of BDNF levels and highlights recent findings from our laboratory on TrkB and TrkC signalling in human neurons. These include ligand selectivity and Trk activation by neurotrophins and non-neurotrophin ligands. The ligand-induced down-regulation and re-activation of Trk receptors is also discussed.