Targeted deletion of PAC1 receptors in retinal neurons enhances neuron loss and axonopathy in a model of multiple sclerosis and optic neuritis.

Chronic inflammation drives synaptic loss in multiple sclerosis (MS) and is also commonly observed in other neurodegenerative diseases. Clinically approved treatments for MS provide symptomatic relief but fail to halt neurodegeneration and neurological decline. Studies in animal disease models have demonstrated that the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP, ADCYAP1) exhibits anti-inflammatory, neuroprotective and regenerative properties. Anti-inflammatory actions appear to be mediated primarily by two receptors, VPAC1 and VPAC2, which also bind vasoactive intestinal peptide (VIP). Pharmacological experiments indicate that another receptor, PAC1 (ADCYAP1R1), which is highly selective for PACAP, provides protection to neurons, although genetic evidence and other mechanistic information is lacking. To determine if PAC1 receptors protect neurons in a cell-autonomous manner, we used adeno-associated virus (AAV2) to deliver Cre recombinase to the retina of mice harboring floxed PAC1 alleles. Mice were then subjected to chronic experimental autoimmune encephalomyelitis (EAE), a disease model that recapitulates major clinical and pathological features of MS and associated optic neuritis. Unexpectedly, deletion of PAC1 in naïve mice resulted in a deficit of retinal ganglionic neurons (RGNs) and their dendrites, suggesting a homeostatic role of PAC1. Moreover, deletion of PAC1 resulted in increased EAE-induced loss of a subpopulation of RGNs purported to be vulnerable in animal models of glaucoma. Increased axonal pathology and increased secondary presence of microglia/macrophages was also prominently seen in the optic nerve. These findings demonstrate that neuronal PAC1 receptors play a homeostatic role in protecting RGNs and directly protects neurons and their axons against neuroinflammatory challenge. SIGNIFICANCE STATEMENT: Chronic inflammation is a major component of neurodegenerative diseases and plays a central role in multiple sclerosis (MS). Current treatments for MS do not prevent neurodegeneration and/or neurological decline. The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) has been shown to have anti-inflammatory, neuroprotective and regenerative properties but the cell type- and receptor-specific mechanisms are not clear. To test whether the protective effects of PACAP are direct on the PAC1 receptor subtype on neurons, we delete PAC1 receptors from neurons and investigate neuropathologigical changes in an animal model of MS. The findings demonstrate that PAC1 receptors on neurons play a homeostatic role in maintaining neuron health and can directly protect neurons and their axons during neuroinflammatory disease.