Insulin Receptor signaling potentiation by proinsulin preserves photoreceptors and rod synapses in the rd10 mouse model of retinitis pigmentosa

Resumen

Purpose : Retinitis pigmentosa is a group of inherited retinal dystrophies, that courses with death of photoreceptors, leading to blindness. We are focused in developing neuroprotective strategies aimed to extend the visual function. In the present study we characterize the neuroprotective effect of potentiating insulin receptor (IR) signaling by a treatment with proinsulin (Pi), the insulin precursor.Methods : IR isoforms were determined by RT-PCR. IR and pS6240/244(pS6) distribution were analyzed by immunostaining in cryosections. Analysis of retinal synapses was carried out by immunostaining and electron microscopy. Adeno-associated viral vectors carrying the human Pi cDNA (AAV-hPi) were administrated to rd10 mice at P10-12 by intramuscular injection, and serum, retinal and eye hPi levels were measured by ELISA. Metabolic effects of hPi were analyzed by body weight and the glucose level measurements. Visual function was assessed by electroretinography and optomotor test.Results : The IRA isoform was predominant in the retina and brain. IR was widely expressed in the retina but highly enriched in horizontal cell (HC) and ganglion cell axons. Concomitant to RP progression, the levels of IR in the HC axons and those of pS6 puncta staining at the HC terminal tips were selectively downregulated in the rd10 retina. Ultrastructural analysis of the rd10 retina at early stages of degeneration revealed decreased proportion of rod triad synapses and increased rate of disconnected rod terminals as well as that of degenerative rod spherules. Single AAV-hPi injection produced sustained serum hPi levels which did not have metabolic consequences and reached the eye and retina. hPi-treatment of rd10 mice restored pS6 levels in HC tips and exerted a neuroprotective effect on photoreceptor survival and rod synaptic connectivity. Moreover, hPi-treated rd10 mice showed extended visual function.Conclusions : IR expression and signaling is down regulated in rd10 retina. As part the RP degenerative rods undergo a process of synaptic disconnection. hPi treatment restored IR signaling by means of increasing pS6 levels, preserved photoreceptor cells and synaptic connectivity, and extended visual function in the rd10 mouse. All together, these results suggest that proinsulin is a multi-facet neuroprotective factor and a potential therapy for RP treatment.