Neurotrophic factors in peripheral nervous system development : Function and specificity

Sammanfattning: Neurotrophic factors control a range of activities in developing neurons, including survival, proliferation, migration and neuronal differentiation. Two classes of neurotrophic factors that are strongly linked to neuronal development of the peripheral nervous system are the neurotrophins and GDNF family ligands. Like other trophic factors they act by triggering signalling through receptor tyrosine kinases. This thesis addresses novel functions for the receptor of GDNF family ligands in peripheral nervous system development and how specificity is achieved at different levels of the trophic factor signalling pathway. In paper I, NT3, which has been considered to be a promiscuous neurotrophin, is demonstrated to only signal through the TrkC receptor, despite the presence of TrkB. In genetically modified mice, NT3/TrkC, but not NT3/TrkB, could promote survival in the majority of cochleovestibular neurons. Furthermore, NT3/TrkC could not substitute for BDNF/TrkB in final target innervation of the vestibular system of the inner ear. In paper II and III, Ret, the receptor for GDNF family ligands, is shown to require the binding of the adaptor protein Frs2 to its phosphorylated tyrosine 1062 for migration. In addition, phosphorylated tyrosine 981 was also necessary for a full migratory response, showing cooperation between these tyrosines. Moreover, the subcellular localization of Ret was dependent on which adaptor that binds to tyrosine 1062, highlighting a new function for adaptor proteins. In paper IV we have ablated Ret in a subpopulation of nociceptive neurons in vivo. We show that Ret was important for the proper expression of ion channels and G protein-coupled receptors. The loss of Ret also led to behavioural changes when mechanical and hot stimuli were applied to the genetically modified mice. This demonstrates the necessity of Ret for normal development of mechano- and thermosensation. In conclusion this thesis contributes to the elucidation of how specificity in neurotrophic factor signalling is achieved and the definition of new roles for neurotrophic factors in development.