Abstract
Purpose :
Glaucomatous optic neuropathies are, besides axonal degeneration and ultimate retinal ganglion cell (RGC) death, characterized by early retinal synaptic losses and RGC dendritic shrinkage. Functional neuronal network repair and consequent recovery of sight after optic nerve damage will therefore not only depend on axonal regeneration, but also on the proper restoration of dendritic morphology and connectivity. Dendrites, however, have been overlooked for decades in regenerative research.
Methods :
To unravel the inherent dendritic response of vertebrate neurons undergoing successful axonal regeneration, regeneration-competent adult zebrafish were subjected to an optic nerve crush (ONC) model. Retrograde biocytin labeling was used to assess brain reinnervation by regenerating RGC axons. RGC dendritic remodeling was studied via morphometric and spatiotemporal expression analyses of dendritic (Map2) and synaptic (Sv2, PSD-95, Znp-1) markers during different phases of RGC axonal regeneration. Primary functional recovery was determined using the dorsal light reflex test. To investigate a potential antagonistic relationship between RGC axonal regeneration and dendritic remodeling, we pharmacologically manipulated pathways known to affect dendritic stability, via repeated intravitreal injections of rapamycin, a validated inhibitor of mechanistic target of rapamycin (mTOR), or a broad-spectrum matrix metalloproteinase (MMP) inhibitor.
Results :
A longitudinal study in which RGC axonal regrowth and dendritic remodeling were assessed side-by-side after ONC, revealed that – as during development - RGC axogenesis precedes dendritogenesis during central nervous system (CNS) repair. Moreover, dendrites majorly shrank before the start of axonal regrowth and were only triggered to regrow upon RGC target contact initiation, altogether suggestive for a counteractive interplay between axons and dendrites after neuronal injury. Strikingly, both retinal mTOR and MMP inhibition after ONC, consecutively inhibited synapto-dendritic deterioration and RGC axonal regrowth, thus invigorating an antagonistic interplay wherein mature dendrites restrain axonal regrowth.
Conclusions :
Overall, our data provide pioneering indications that the orderly developmental program of neurite outgrowth repeats during CNS repair and suggest dendritic shrinkage as an important incentive for axonal regrowth and associated neuronal recovery.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.