Muscle paralysis in Amyotrophic Lateral Sclerosis (ALS) occurs when motoneuron axons disconnect from the muscle fibers at the neuromuscular junction (NMJ). Even though it remains outstanding whether degeneration of the motoneuron or, instead, denervation of the NMJ are the primary cause of paralysis in ALS, preserving NMJ innervation is an attractive strategy to delay muscle weakness in ALS. An important compensatory mechanism for muscle denervation involves the sprouting of terminal axons, resulting in re-innervation of the muscle and enlargement of motor units.
Neurotrophic factors expressed by the muscle play an important role in the development and maintenance of NMJs. We recently provided genetic evidence in mice and humans implicating reduced levels of vascular endothelial growth factor (VEGF) in ALS and demonstrated that VEGF gene or protein treatment prolongs the survival of SOD1G93A rodent models of ALS, in part via direct neuroprotective effects of VEGF on motoneuron cell bodies. In my study, we explored whether VEGF directly affects NMJ innervation and stimulates terminal axon sprouting.
Therefore, recombinant VEGF was administered subcutaneously to mice from birth onwards for up to 30 days. The effect of VEGF on the branching pattern of individual motor axons was determined by reconstructing labeled axonal arbors from transgenic mice expressing a fluorescent protein in subsets of motoneurons. Our results show that individual axons in control animals branched sporadically to innervate NMJs within circumscribed regions in the muscle, whereas motor units from VEGF injected animals developed more numerous axon branches and exhibited a higher degree of localized axon arborization such that adjacent muscle fibers were often innervated by the same axon. Administration of VEGF from P0 till P30 also seemed to prolong innervation in a dose dependent manner. We are currently further exploring the effect, mechanism and therapeutic potential of VEGF on axon branching and NMJ innervation.