Monomeric SDF1 promotes axon regeneration and protects RGCs after optic nerve injury

Abstract

Purpose : Inflammation plays a key role in peripheral nerve regeneration and can also be recruited to promote regeneration in the CNS. In adult mammals, the inability of retinal ganglion cells (RGCs) to regrow injured axons through the optic nerve is partially reversed by inducing intraocular inflammation. One important mediator of this phenomenon is Oncomodulin (Ocm), a small Ca²⁺-binding protein that is expressed by infiltrative neutrophils and macrophages and that binds to a high-affinity receptor on RGCs in a cAMP-dependent manner. However, Ocm does not account for the full effects of intraocular inflammation, motivating us to identify other important mediators of inflammation-induced optic nerve repair.

Methods : We carried out immunostaining and qPCR to investigate the expression of stromal cell-derived factor 1 (SDF1, CXCL12) in the retina and in infiltrative cells in the vitreous. The effects of SDF1 on RGC survival and axon regeneration were investigated in cell culture and in vivo following optic nerve injury.

Results : We discovered that, along with Ocm, SDF1 is an important contributor to inflammation-induced optic nerve regeneration and RGC survival. SDF1 is expressed and secreted by invasive macrophages and binds to its receptor CXCR4 on RGCs. Monomeric SDF1, the active form of the protein, strongly enhanced RGC survival and promoted axon growth after optic nerve injury, and strongly enhanced the effects of Ocm. Deletion of SDF1 in macrophages (using cxcl12^flx/flxLysM^Cre/+ mice) or deletion of its receptor (CXCR4) in RGCs (cxcr4^flx/flx mice injected intraocularly with AAV2-Cre virus) reduced inflammation-induced optic nerve regeneration 2 weeks after nerve injury. In RGC cultures, SDF1, uniquely among many growth factors tested, strongly enhanced the effects of Ocm/cAMP. As expected, SDF1 elevated intraocular cAMP levels and enhanced PI3K signaling and S6 phosphorylation. In vivo, AAV2 expressing monomeric SDF1 augmented levels of regeneration induced by Ocm or intraocular inflammation, and when combined with Ocm/cAMP and pten deletion, SDF1 increased the number of axons that crossed the chiasm and entered the brain.

Conclusions : SDF1 is a major contributor to inflammation-induced optic nerve regeneration. Monomeric SDF1 strongly enhances axon regeneration when combined with other identified molecules, and is of potential clinical value to augment regeneration after optic nerve injury.

This is a 2020 ARVO Annual Meeting abstract.