Abstract
Purpose :
Successful signal propagation along axons is important for the survival of neural networks. In the optic nerve, oligodendrocytes produce the myelin sheath which provides insulation for retinal ganglion cell (RGC) axons. Optic neuritis (the demyelination and inflammation of the optic nerve) results in visual deficits and thinning of retinal nerve fiber and ganglion cell layers. Demyelination can induce functional deficits; however, the impact of demyelination on upstream network connectivity is not well understood. This study evaluates the effect of local optic nerve demyelination on axonal conduction and retinal connectivity.
Methods :
In the MBP-iCP9 transgenic mouse, the myelin basic protein (MBP) promoter drives expression of an inducible caspase 9 (iCP9) sequence and a DsRed reporter. Eye injection of the chemical inducer of dimerization (CID) results in the selective apoptosis of MBP+ oligodendrocytes and subsequent demyelination. MBP-iCP9 mice were injected at P14 with either CID or vehicle for three consecutive days. After two weeks, immunohistochemistry was used to quantify oligodendrocyte loss and retinal layer density and identify RGC subtypes. Optic nerve compound action potentials (CAPs) were measured using extracellular recordings. Axon populations were identified as axons with similar conduction speeds. Within-nerve controls used tetrodotoxin to isolate and remove the stimulus artifact during analysis.
Results :
CID-treated nerves had a significant loss of DsRed+ CC1+ oligodendrocytes (n>5, p=0.04). Average CAP area (a measure of functional axons) was reduced in CID-treated mice (n<5). CID-treated nerves retained the fastest-conducting axon populations but lost slow-conducting axons. Average ganglion cell layer density was significantly lower in CID-treated retinas (n>4, p=0.03), while inner and outer nuclear layer density were unaffected (p>0.05). Analysis of the ganglion cell layer found a significant loss of RBPMS+ (p=0.03) and Brn3a+ (p=0.02) cells in CID-treated retinas and a non-significant reduction of ChAT+ amacrine cells (p=0.06).
Conclusions :
These results suggest that partial demyelination of the optic nerve can induce a significant conduction deficit and disrupt retinal connectivity through the loss of alpha-RGCs. This provides valuable insight into the regulatory role of myelination in the maintenance of upstream neural networks.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.