Analysis of Brain-derived Neurotrophic Factor Signaling in Spinal Muscular Atrophy

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Degeneration of anterior horn cells and denervation of muscles are the main clinical characteristics of spinal muscular atrophy (SMA) and well represented in SMA mouse models. Primary motoneurons from SMA type I mouse models exhibit altered axon elongation combined with reduced actin mRNA and protein levels in smaller growth cones. The differentiation defects correspond to impaired excitability at the nerve terminals due to reduced accumulation of N-type specific voltage-gated calcium channels (Cav2.2). These data raise the question whether the localization of e.g. growth factor receptors and their corresponding signaling pathways, which are important for differentiation and maturation of motoneurons, is affected in Smn-deficient motoneurons.In a first approach we could clearly show that the level of the Brain-derived Neurotrophic Factor (BDNF) receptor termed Tropomyosin-receptor-kinase B (TrkB) is reduced in growth cones of Smn-deficient motoneurons. Based on this observation we would like to investigate in detail how BDNF/TrkB signaling corresponds to affected actin cytoskeleton, excitability and neurotransmission in Smn-deficient motoneurons. The project shall be structured in three parts. In the first part we will focus on the time course of TrkB translocation at the growth cone in control and Smn-deficient motoneurons to understand how actin dynamics interfere with shifting TrkB to the cell surface in the presynaptic compartment of the neuromuscular endplate. As we know from our preliminary studies that the level of phospho-TrkB is significantly reduced in Smn-deficient growth cones, in the second part of our project we will try to figure out which signaling pathways downstream of TrkB (MAK kinase or Akt/mTOR pathway) support proper differentiation and maturation of presynapses in control motoneurons and which are affected in Smn-deficient motoneurons. It is in general an open question whether these pathways directly lead to altered release probability of synaptic vesicles or if newly synthesized proteins or rapid changes in the actin cytoskeleton are needed. Finally, the detailed analysis of affected BDNF/TrkB signaling in Smn-deficient motoneurons will be completed by experiments that focus on the intracellular rescue of actin transcript and proteins levels to understand whether defective BDNF/TrkB signaling, excitability disturbances and differentiation defects in Smn-deficient motoneurons can be compensated by proper formation of the actin cytoskeleton. Our expected results are thus important for a better understanding of the pathophysiology of SMA, in which presynaptic excitability, actin dynamics and neurotransmission are affected and thought to cause early events in the etiopathology.