Insulin receptor activation by proinsulin preserves synapses and vision in retinitis pigmentosa

Abstract

Background Synaptic loss, neuronal cell death, and circuit remodeling are common features of central nervous system neurodegenerative disorders. Retinitis pigmentosa, the leading cause of inherited blindness, is a group of retinal dystrophies characterized by photoreceptor cell dysfunction and death. The insulin receptor, a key controller of metabolism, also regulates neuronal survival and synaptic formation, maintenance, and activity. Indeed, deficient insulin receptor signaling has been implicated in several brain neurodegenerative pathologies. Methods We employed a gene therapy strategy to enhance insulin receptor signaling in the Pde6b rd10/rd10 mouse model of RP by using recombinant AAV serotype 2/1 viral vectors bearing cDNA from the human proinsulin gene. Insulin receptor expression and signaling were analyzed by PCR and immunostaining. Synaptic connectivity was evaluated by electron microscopy and immunostaining. Proinsulin levels were measured by ELISA. Photoreceptor preservation was assayed by histological analysis and visual function was assessed by electroretinography and optomotor test. Results We present evidence linking impaired insulin receptor signaling with retinitis pigmentosa. We found a selective decrease in the levels of the insulin receptor and its downstream effector phospho-S6 in retinal horizontal cell axons in the rd10 mouse model of retinitis pigmentosa, as well as aberrant synapses between rod photoreceptors and the postsynaptic terminals of horizontal and bipolar cells. A gene therapy strategy to induce sustained proinsulin production restored retinal insulin receptor signaling, by increasing S6 phosphorylation, without peripheral metabolic consequences. Moreover, proinsulin preserved photoreceptor synaptic connectivity and prolonged visual function as determined by electroretinography and optomotor tests. Conclusion These findings point to a disease-modifying role of insulin receptor and support the therapeutic potential of proinsulin in retinitis pigmentosa.