Abstract
Purpose :
The retraction of retinal ganglion cell (RGC) dendrites and synapse loss are proposed to be an early pathological alteration in glaucoma. Aberrant insulin signaling has been associated with neurodegenerative diseases characterized by dendritic pathology, however, its role in the response of vulnerable neurons is poorly understood. We investigated the effect of insulin on dendrite and synapse regeneration and asked whether insulin resistance alters the regenerative ability of RGC
Methods :
Transgenic mice expressing YFP protein in RGC were subjected to optic nerve axotomy or microbead-induced ocular hypertension (OHT). Insulin was administered daily (i.p. or drops) starting when RGC dendrites had already retracted, dendritic trees and synapses were analyzed 4 to 7days later. Excitatory post-synaptic sites were visualized after biolistic transfection of a CMV:PSD95-YFP plasmid. RGC survival was assessed by quantification of RBPMS-positive cells and their light response measured by photopic negative response (PhNR) recordings. A group received a high fat diet to induce pre-diabetic insulin resistance
Results :
Insulin, independently of delivery route, promoted robust RGC dendrite regeneration, restoring process length, arbor area and complexity after axotomy or OHT. Quantitative analysis of dendritic parameters demonstrated a 36% increase in process length and 43% larger arbor area with insulin relative to vehicle-treated neurons (Vehicle: length= 2961 ± 180 µm, area= 77 ± 6 x 103 µm2, Insulin: length= 4677 ± 136 µm, area= 142 ± 7 103 µm2; N= 5-6 mice/group, n= 40-50 RGCs/group). Insulin restored excitatory postsynaptic sites on RGC dendrites independently of subtype, increasing PSD95 puncta density by 76%, 93%, and 93% in ON-S, OFF-S, and OFF-T RGCs, respectively (N=5-6/group). Insulin rescued the PhNR amplitude and promoted robust RGC neuroprotection. High fat diet intake, leading to insulin resistance, blocked the ability of insulin to promote RGC dendrite regeneration and survival
Conclusions :
Our study identifies insulin as a critical factor for RGC dendrite regeneration and functional recovery, while high fat diet leading to insulin resistance impaired this pro-regenerative response. A better understanding of RGC insulin signaling in glaucoma, including pre- and diabetic conditions, might have implications to prevent synaptic loss and visual deficits
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.