Disruption of E3 ligase NEDD4 in peripheral neurons interrupts axon outgrowth: Linkage to PTEN.

Exploiting molecules and pathways important in developmental axon behaviour may offer new insights into regenerative behaviour of adult peripheral neurons after injury. In previous work, we have provided evidence that inhibition or knockdown of PTEN (phosphatase and tensin homolog deleted on chromosome ten) dramatically increases adult peripheral axon outgrowth, especially in preconditioned neurons (Christie et al., 2010). PTEN appears to operate as an endogenous brake to regeneration. Recent reports from Drinjakovic et al. (2010) have highlighted a role for the ubiquitin proteasome system (UPS) during neurite outgrowth in developing Xenopus retinal ganglion cells. Specifically, disruption of the UPS E3 ligase Nedd4 (neural precursor cell-expressed developmentally down-regulated protein 4) inhibited neurite branching through up-regulation of PTEN. We explored the potential role of Nedd4 in the peripheral neurons of adult rat dorsal root ganglia (DRG), particularly its impact on regenerative behaviour. Global inhibition of the UPS in vitro was associated with a severe decrease in neurite branching, both in preconditioned (injured) and control DRG sensory neurons. These involved neurons however maintained or qualitatively increased their PTEN expression, suggesting ongoing PTEN activity during UPS inhibition. Considering component's of UPS more specifically, Nedd4 co-localized with PTEN within sensory neurons in vivo and in vitro. Nedd4 also co-localized with PTEN and NF200 labelled regenerating axons at the injury site in the periphery following a 3 day sciatic nerve cut. A significant role for this unique co-expression was observed with fluorescently tagged siRNA inhibition of Nedd4, which decreased neurite outgrowth, an impact associated with greater expression of PTEN and that was completely reversed with application of a PTEN inhibitor. Overall, our results suggest an important role for Nedd4 regulation of PTEN in the response of peripheral neurons to injury. By degrading PTEN among other potential actions, Nedd4 supports axonal outgrowth whereas its inhibition facilitates PTEN inhibition of regeneration.