Recovery of Axonal Transport after Partial Optic Nerve Damage Is Associated with Secondary Retinal Ganglion Cell Death In Vivo

Abstract

Traumatic injury of the optic nerve leads to retrograde cell death of retinal ganglion cells (RGCs) but usually a certain percentage of neurons survive. It has been suggested that recovery of axonal transport is beneficial for survival. The present study was therefore performed to provide a synopsis of the temporal pattern of axonal transport decline/recovery and the viability of RGCs after optic nerve crush (ONC). Fluorescent dyes were injected into the superior colliculus to retrogradely label RGCs. Axonal transport kinetics into the RGCs was visualized with in vivo confocal neuroimaging (ICON) in uninjured rats and in rats which had mild or moderate ONC. Red fluorescent beads were injected on day 2 post-ONC and green beads on day 7. At 2 to 4 days post-ONC significant axonal transport was detected, but within 1 week the transport of the fluorescent beads was decreased. Interestingly, during post-ONC week 3 the axon transport slowly recovered. However, despite this recovery, retrograde cell death rate continued and was even increased in a "second wave" of cell death in those neurons that displayed axon transport recovery. After damage many surviving RGCs lose their axon transport, but after approximately 3 weeks, this transport recovers again, a sign of intrinsic axon repair. Contrary to the prediction, axon transport recovery is not associated with better cell survival but rather with a second wave of cell death. Thus, the accelerated cell death associated with recovery of axon transport suggests the existence of a late retrograde cell death signal.