Neuronal regeneration in the injured central nervous system is hampered by multiple extracellular proteins. These proteins exert their inhibitory action through interactions with receptors that are located in cholesterol rich compartments of the membrane termed lipid rafts. Here we show that cholesterol-synthesis inhibition prevents the association of the Neogenin receptor with lipid rafts. Furthermore, we show that cholesterol-synthesis inhibition enhances axonal growth both on inhibitory -myelin and -RGMa substrates. Following optic nerve injury, lowering cholesterol synthesis with both drugs and siRNA-strategies allows for robust axonal regeneration and promotes neuronal survival. Cholesterol inhibition also enhanced photoreceptor survival in a model of Retinitis Pigmentosa. Our data reveal that Lovastatin leads to several opposing effects on regenerating axons: cholesterol synthesis inhibition promotes regeneration whereas altered prenylation impairs regeneration. We also show that the lactone prodrug form of lovastatin has differing effects on regeneration when compared to the ring-open hydroxy-acid form. Thus the association of cell surface receptors with lipid rafts contributes to axonal regeneration inhibition, and blocking cholesterol synthesis provides a potential therapeutic approach to promote neuronal regeneration and survival in the diseased Central Nervous System. SIGNIFICANCE STATEMENT: Statins have been intensively used to treat high levels of cholesterol in humans. However, the effect of cholesterol inhibition in both the healthy and the diseased brain remains controversial. In particular, it is unclear whether cholesterol inhibition with statins can promote regeneration and survival following injuries. Here we show that late stage cholesterol inhibition promotes robust axonal regeneration following optic nerve injury. We identified distinct mechanisms of action for activated vs non-activated Lovastatin that may account for discrepancies found in the literature. We show that late stage cholesterol synthesis inhibition alters Neogenin association with lipid rafts, thereby i) neutralizing the inhibitory function of its ligand and ii) offering a novel opportunity to promote CNS regeneration and survival following injuries.

Abortive cell cycle (ACC) re-entry of apoptotic neurons is a recently characterized phenomenon that occurs after central nervous system (CNS) injury or over the course of CNS disease. Consequently, inhibiting cell cycle progression is neuroprotective in numerous CNS pathology models. Primary cilia are ubiquitous, centriole-based cellular organelles that prevent cell cycling, but their ability to modulate abortive cell cycle has not been described. Here, we show that neuronal cilia are ablated in-vitro and in-vivo following injury by hypoxia or optic nerve transection (ONT), respectively. Furthermore, forced cilia resorption sensitized neurons to these injuries and enhanced cell death. In contrast, pharmacological inhibition or shRNA knockdown of the proteins that disassemble the cilia increased neuron survival and decreased the phosphorylation of retinoblastoma (Rb), a master switch for cell cycle re-entry. Our findings show that the stabilization of neuronal primary cilia inhibits, at least transiently, apoptotic cell cycling, which has implications for future therapeutic strategies that halt or slow the progression of neurodegenerative diseases and acute CNS injuries.