Overexpression of ATF3 or the combination of ATF3, c-Jun, STAT3 and Smad1 promotes regeneration of the central axon branch of sensory neurons but without synergistic effects

Peripheral nerve injury results in the activation of a number of transcription factors (TFs) in injured neurons, some of which may be key regulators of the regeneration-associated gene (RAG) programme. Among known RAG TFs, ATF3, Smad1, STAT3 and c-Jun have all been linked to successful axonal regeneration and have known functional and physical interactions. We hypothesised that TF expression would promote regeneration of the central axon branch of DRG neurons in the absence of a peripheral nerve lesion and that simultaneous overexpression of multiple RAG TFs would lead to greater effects than delivery of a single TF. Using adeno-associated viral vectors, we overexpressed either the combination of ATF3, Smad1, STAT3 and c-Jun with farnesylated GFP (fGFP), ATF3 only with fGFP, or fGFP only, in DRG neurons and assessed axonal regeneration after dorsal root transection or dorsal column injury and functional improvement after dorsal root injury. ATF3 alone and the combination of TFs promoted faster regeneration in the injured dorsal root. Surprisingly, however, the combination did not perform better than ATF3 alone. Neither treatment was able to induce functional improvement on sensory tests after dorsal root injury or promote regeneration in a dorsal column injury model. The lack of synergistic effects among these factors indicates that while they do increase the speed of axon growth, there may be functional redundancy between these TFs. Because axon growth is considerably less than that seen after a conditioning lesion, it appears these TFs do not induce the full regeneration programme.