Genetic ablation of formin protein accelerates axon regeneration via modulation of microtubule dynamics

Peripheral nerve injury (PNI) reactivates the intrinsic growth machinery necessary for axonal outgrowth, which enable axon regeneration after injury. Proximal PNI requires axon regeneration over a long-distance for target reinnervation which usually take months or years to fully extend to distal muscle targets. By the time when axons finally reach their original targets, they failed to re-establish connections and form functional synapses after chronic denervation. This accounts for the poor functional recovery in patients with proximal PNI. We demonstrated that the damaged axons must extend to the distal muscle within a critical period of 35 days in adult mice for complete restoration of motor functions, and by accelerating axonal regrowth it is plausible to promote functional recovery after PNI. Formin proteins are protein superfamily which share highly conserved FH1 and FH2 domains. Ubiquitous expression of formin proteins is detected in both central and peripheral nervous system; however, its functions, especially in regulation of axon regeneration, remain largely unknown. Compelling evidence suggested that both FH1 and FH2 domains in formin proteins can bind to microtubule and modulate its dynamics and stability, which is a crucial determinant for successful axon regeneration. Our pilot study has demonstrated that PNI induced down-regulation of formin protein. In vivo silencing of formin protein using target-specific formin-short interfering RNA (formin-siRNA) markedly accelerated axonal regrowth, and promoted sensory and motor functional recovery after sciatic nerve crush injury in adult mice. In the current study, we further tested if complete ablation of formin protein could accelerate axon regeneration after PNI using formin complete knockout mice (formin-KO). Cultured dorsal root ganglion (DRG) neurons prepared from formin-KO mice exhibited significantly longer neurites with more axonal branching compared with their wild-type littermates. We then performed sciatic nerve crush injury on formin-KO mice and assessed the distal extent of axon regeneration using sciatic nerve pinch test. In line with our in vitro results, complete ablation of formin protein markedly accelerated axonal regrowth 3 days after crush injury. Sensory and motor functional recovery also markedly improved in formin-KO mice as assessed by an exhaustive list of neurobehavioral and electrophysiological studies after crush injury. Further investigation of molecular mechanisms underlying the formin-mediated microtubule dynamics shed new light in developing novel therapeutic approaches to promote functional restoration in patients with proximal PNI.