Fig. 6.

Neuron-NG2 + cell RA synthesis and transfer in association with exosomes that is induced by neuronal activation of RARβ agonist. Schematic representation of a novel signalling pathway in axonal/neurite outgrowth. Treatment with an RARβ agonist induces upregulation of the receptor at the axonal tips and this promotes the endogenous synthesis of RA, via upregulation of ADH7 in the neuron. This increases the synthesis of retinal, which is the substrate for Raldh2. Raldh2 is not upregulated in the neurons, but rather in the NG2 + cells that invade the lesioned area after SCI. RA is metabolized from the retinal and secreted in association with exosomes. These are taken up by adjacent axons and serve as an important axonal/neurite growth cue.