Polycomb Group Protein Ezh2 Supports Mammalian Axon Regeneration in Peripheral and Central Nervous System

Abstract

In mammals, neurons in the central nervous system (CNS) lose their axon growth ability as they mature. On the contrary, mature neurons in the peripheral nervous system (PNS) still possess such an ability and can spontaneously regenerate axons upon axonal injury by initiating a regenerative response. Emerging evidence suggests that the transition from a non-regenerative state to a regenerative state of injured PNS neurons largely involves epigenetic regulation of chromatin accessibility. Here we investigated the role of Ezh2, a histone methyltransferase that adds methyl groups onto lysine 27 on histone H3 (H3K27) to suppress gene transcription, in mammalian axon regeneration. Western blot results showed that Ezh2 level was significantly upregulated in mouse lumbar 4 and 5 dorsal root ganglia (L4/5 DRGs) 3 days following sciatic nerve axotomy. Functionally, Ezh2 knockdown or knockout in cultured DRG neurons impaired axon growth in vitro. In consistence, knockdown or knockout of Ezh2 in L4/5 DRGs by in vivo electroporation or conditional knockout of Ezh2 in sensory neurons impaired sensory axon regeneration after sciatic nerve crush. More importantly, using optic nerve regeneration model, we found that overexpressing Ezh2 in retinal ganglion cells (RGCs) by intravitreal injection of AAV2-Ezh2 virus significantly promoted axon regeneration after optic nerve crush. RNA-seq analysis of RGCs enriched by fluorescence-activated cell sorting revealed that the transcription of a large number of genes involved in synaptic transmission were downregulated by Ezh2 overexpression, indicating that Ezh2 supports axon regeneration by rejuvenating adult neurons to a younger state. Using optic nerve regeneration model again, we showed that overexpression of Slc6a13 (encoding Gat2), a gene suppressed by Ezh2, partially blocked RGC axon regeneration induced by Ezh2 overexpression. Our study demonstrated that Ezh2 was required for the successful spontaneous axon regeneration in the PNS, and that Ezh2 gain-of-function could promote axon regeneration in the CNS by silencing synaptic transmission-associated genes.