A nerve is a complex cell community. We therefore used microfluidic chambers containing neurons and purified Schwann cells to test whether the poor axon growth in mutants was caused by disturbance of direct axon-Schwann cell interactions or whether the effect depended on other cells. Axon regeneration by axotomized, adult WT DRG neurons was strongly stimulated by control Schwann cells relative to laminin substrate alone,

as expected (Figures 5I–5K). The c-Jun mutant cells, however, were ineffective, the number and area of axons extending on their surface falling to only 40%–50% of that seen on WT cells. Importantly, reactivation of Trametinib c-Jun in mutant cells by adenoviral gene transfer, fully restored

axon number and length to WT levels. These experiments show that injury-activated Schwann cell c-Jun controls direct communication between Schwann cells and growing neurites. We have shown that c-Jun controls three important functions of denervated Schwann cells, formation of regeneration tracks, support of neuronal survival, and promotion Trichostatin A solubility dmso of axon regrowth. A fourth major role classically ascribed to these cells is removal of myelin and associated growth inhibitors, a task they accomplish by breaking down myelin early after injury and indirectly by instructing macrophages to complete myelin clearance (Hirata and Kawabuchi, 2002). We found that myelin clearance was substantially delayed

in mutants. Four weeks after sciatic nerve transection (without regeneration), the distal stump of WT nerves was translucent, while mutant nerves remained gray/white (Figure 6A). Osmium stained lipid debris occupied about 10-fold larger area in the mutant than WT nerves (Figure 6B). Electron microscopy revealed that although transected mutant nerves did not contain intact myelin, many Schwann cells contained lipid droplets, a late product of myelin breakdown (Figure 6C). This was not seen in 4 week transected WT controls. We therefore tested whether myelin breakdown was impaired in mutant Schwann cells. First, in cut adult nerves, the loss of myelin sheaths was delayed in the mutants (Figure 6D). This was not due to infiltrating macrophages, ALOX15 because the difference between WT and mutants was fully maintained when the cut nerves were maintained in vitro (Figure 6E). Second, this delay was confirmed by slower breakdown of myelin basic protein (MBP) in vivo (Figures S5A and S5B). Third, when myelinating cells from postnatal day 8 nerves were cultured, myelin proteins were broken down slowly by c-Jun mutant cells compared to WT, and mutant cultures contained many Schwann cells bloated with myelin debris (Figures 6F–6H). Both types of culture contained similar numbers of F4/80+ macrophages (5.6+/−1.8% and 5.8+/−1.