Viral Vector-based Improvement of Optic Nerve Regeneration: Characterization of Individual Axons' Growth Patterns and Synaptogenesis in a Visual Target

Lack of axon growth ability in the central nervous system poses a major barrier to achieving functional connectivity after injury. Thus, a non-transgenic regenerative approach to reinnervating targets has important implications in clinical and research settings. Previous studies using knockout (KO) mice have demonstrated long-distance axon regeneration. Using an optic nerve injury model, here we evaluate the efficacy of viral, RNA interference (RNAi) and pharmacological approaches that target the phosphatase and tensin homolog (PTEN) and signal transducer and activator of transcription-3 pathways to improve long-distance axon regeneration in wild-type mice. Our data show that adeno-associated virus (AAV) expressing short hairpin RNA (shRNA) against PTEN (shPTEN) enhances retinal ganglion cell axon regeneration after crush injury. However, compared with the previous data in PTEN KO mice, AAV-shRNA results in a lesser degree of regeneration, likely due to incomplete gene silencing inherent to RNAi. In comparison, an extensive enhancement in regeneration is seen when AAV-shPTEN is coupled to AAV encoding ciliary neurotrophic factor (CNTF) and to a cyclic adenosine monophosphate (cAMP) analog, allowing axons to travel long distances and reach their target. We apply whole-tissue imaging that facilitates three-dimensional visualization of single regenerating axons and document heterogeneous terminal patterns in the targets. This shows that some axonal populations generate extensive arbors and make synapses with the target neurons. Collectively, we show a combinatorial viral RNAi and pharmacological strategy that improves long-distance regeneration in wild-type animals and provide single fiber projection data that indicates a degree of preservation of target recognition.