Retinoic acid synthesis by NG2 expressing cells promotes a permissive environment for axonal outgrowth

Stimulation of retinoic acid (RA) mediated signalling pathways following neural injury leads to regeneration in the adult nervous system and numerous studies have shown that the specific activation of the retinoic acid receptor β (RARβ) is required for this process. Here we identify a novel mechanism by which neuronal RARβ activation results in the endogenous synthesis of RA which is released in association with exosomes and acts as a positive cue to axonal/neurite outgrowth. Using an established rodent model of RARβ induced axonal regeneration, we show that neuronal RARβ activation upregulates the enzymes involved in RA synthesis in a cell specific manner; alcohol dehydrogenase7 (ADH7) in neurons and aldehyde dehydrogenase 2 (Raldh2) in NG2 expressing cells (NG2+ cells). These release RA in association with exosomes providing a permissive substrate to neurite outgrowth. Conversely, deletion of Raldh2 in the NG2+ cells in our in vivo regeneration model is sufficient to compromise axonal outgrowth. This hitherto unidentified RA paracrine signalling is required for axonal/neurite outgrowth and is initiated by the activation of neuronal RARβ signalling.