DLK orchestrates a modular transcriptional response to axon injury with separate roles for Fos and Jun

Axon injury initiates transcriptional reprogramming that in competent cells leads to regeneration. In vertebrate neurons, DLK acts upstream of Jun, STAT and Atf3, core transcription factors that mediate regeneration. It is unclear whether these three proteins are activated independently, or whether they function in a linear cascade. To investigate relationships between these transcription factors we wished to use Drosophila as a model system as it has one ortholog of each. However, the only transcription factor linked to DLK-mediated axon regeneration (AR) in flies was Fos. Using loss of function approaches we demonstrate that Jun, STAT and Atf3 are required for Drosophila sensory axon regeneration, indicating transcriptional control of axon regeneration is broadly conserved. We next investigated temporal roles for Fos, Jun, STAT and Atf3. Only Fos is required for the early transcriptional response, which coincides with neuroprotection, and its nuclear entry and homodimerization coincide with this phase. Reduction of Jun homodimerization occurs after axon injury downstream of DLK/JNK, but independently from Fos, at a later stage associated with axon regrowth. STAT nuclear entry occurs downstream of Jun as part of this stage, is inhibited by Fos, and does not require JAK, which is dispensable for axon regeneration. Atf3 nuclear exit is in turn downstream of Fos, Jun, and STAT. Our results suggest that DLK/JNK separately activates Fos and Jun, and that Jun initiates a transcriptional cascade that includes STAT and Atf3. These two transcriptional modules control separate steps of the injury response that culminates in axon regeneration.