Understanding the mechanisms underlying the neuroprotective role of calorie restriction in Parkinson’s Disease

Parkinson’s Disease (PD) is a debilitating neurological condition classified by a reduction of dopamine in the nigrostriatal region of the brain, resulting in movement disorders. Calorie restriction (CR) has shown to be neuroprotective during PD however, adhering to CR is difficult. In this thesis we attempted to create an alternative option that can mimic CR without having to reduce the amount of calories we consume. Initially we focused on the “hunger hormone” ghrelin, as it is elevated in the plasma during CR and is known to be protective in PD. Ghrelin exists in two distinctive isoforms, each with its own metabolic profile. In PD acyl ghrelin administration is neuroprotective, however, the role of des-acylated ghrelin is unknown. We wanted to identify the relative contributions each isoform plays using the MPTP model of PD. Chronic administration of acyl ghrelin in mice lacking both isoforms of ghrelin (Ghrelin KO) attenuated the MPTP-induced loss on Tyrosine Hydroxylase (TH; marker for dopamine) neuronal number and volume and TH protein concentration in the nigrostriatal pathway. However, injection of acyl ghrelin also elevated plasma des-acylated ghrelin, indicating in vivo deacetylation. Next, we chronically administered des-acylated ghrelin to Ghrelin KO mice and observed no neuroprotective effects. The lack of a protective effect was mirrored in Ghrelin-O-Acyltransferase (GOAT) KO mice, which lacks the ability to acylate ghrelin and consequently chronically increases plasma des-acyl ghrelin. Using this information we wanted to determine if acyl ghrelin was responsible for the neuroprotective actions of CR. CR attenuated the MPTP-induced loss of substantia nigra (SN) dopamine neurons and striatal dopamine turnover in Ghrelin WT but not KO mice, demonstrating that ghrelin mediates CR’s neuroprotective effect. CR elevated phosphorylated AMPK-activated kinase (AMPK) levels in the SN of WT but not KO mice suggesting that AMPK is a target for ghrelin-induced neuroprotection. Indeed, exogenous acyl ghrelin significantly increased pAMPK in the SN. Genetic deletion of AMPKβ1 and 2 subunits only in dopamine neurons (AMPK KO) prevented ghrelin-induced AMPK phosphorylation and neuroprotection. Hence, ghrelin signaling through AMPK in SN dopamine neurons mediates CR’s neuroprotective effects. Next we wanted to recreate the neuroprotective actions of CR with an already safe therapeutic. Metformin is the most commonly used drug to treat type 2 diabetes. It acts via AMPK activation in the periphery to ultimately lower blood glucose levels. Recently Metformin has been shown to be neuroprotective in PD. We wanted to determine if this was due to a direct effect on AMPK activity in dopaminergic neurons. We show that Metformin is neuroprotective in a mouse model of PD by attenuating dopaminergic cell loss and gliosis. This effect was present in both AMPK WT and KO mice indicating that Metformin’s neuroprotective actions are not due to AMPK activation in the SN dopaminergic neurons. Overall, these studies suggest a pathway linking CR with elevated acyl ghrelin which in turn phosphorylates AMPK in dopaminergic neurons to elicit a neuroprotective effect. CR mimetics should focus on AMPK activation in dopaminergic neurons as one potential target for the treatment of PD.