It also includes the morphogenetic check details processes that change myelinating cells to process bearing cells forming regeneration tracks,

and the conversion of Schwann cells to cells equipped to rapidly clear myelin from injured nerves (Stoll et al., 2002; Chen et al., 2007; Vargas and Barres, 2007; Wang et al., 2008; Gordon et al., 2009; Höke and Brushart, 2010; Angeloni et al., 2011). The exceptional repair potential of peripheral nerves is likely due to the coordinated functions of the repair program. Yet individual factors can also be presumed to play a prominent role, as exemplified by the enhanced regeneration seen when GDNF and artemin levels are increased in c-Jun mutant facial nerves (Fontana et al., 2012). c-Jun is absent from Schwann cell precursors, expressed in immature cells in vivo and in cultured Schwann cells, suppressed by Krox-20 on myelination, but rapidly re-expressed at high levels in Schwann cells of injured nerves (Parkinson et al., 2004, 2008; D.K.W., unpublished). Among potential intracellular activators of c-Jun is the AP-1 transcription complex, of which c-Jun is a key component. AP-1 activity, in turn, is controlled by numerous signals, including the major MAPK pathways Erk1/2, JNK, and p38. These are all activated in injured nerves and therefore potential upstream regulators of c-Jun (Sheu et al.,

2000; Myers et al., 2003; Harrisingh et al., FG-4592 chemical structure 2004; Jessen and Mirsky, 2008: Parkinson et al., 2008; Napoli et al., 2012; Yang et al., 2012). Genetically, the transcription factor Sox2 is not downstream of c-Jun, since

Sox2 remains normally upregulated in injured c-Jun mutant nerves (Figure S4). We described previously that c-Jun shows cross-inhibitory interactions with the pro-myelin transcription factor Krox20 (Parkinson et al., 2008). Mirroring the function of c-Jun in denervated cells, Krox20 is involved in the regulation of 100–200 genes in myelinating Schwann cells (P. Topilko, personal communication) and is required for the normal activation of the myelin program. We therefore suggest PAK6 that Krox20/c-Jun are central components of a cross-inhibitory switch that regulates cell fate in injured and regenerating nerves. The long term persistence of Schwann cell lipid droplets and large multivacuolated (foamy) macrophages in transected mutant nerves suggests problems with lipid clearance and macrophage activation and exit. Recent evidence indicates that failure of lipid breakdown may delay regeneration (Winzeler et al., 2011). The reduced macrophage numbers in the mutant early after injury is unlikely to contribute substantially to the regeneration problems, a conclusion supported by the microfluidic chamber experiments, where axon growth fails in the presence of mutant Schwann cells, even in the absence of macrophages.