Identification of mechanisms by which cytisine and estrogen inhibit ER stress and exert neuroprotection in Parkinson's disease

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Parkinson’s disease (PD) is the second most common neurodegenerative disorder, predicted to affect over 12 million people worldwide by 2040. There is no cure for PD and current treatments are merely symptomatic, which underscores an urgent and unmet need for neuroprotective drugs. One major barrier for the inability to develop effective and clinically translatable neuroprotective drugs is that no single animal model recapitulates the complex process that leads to dopaminergic (DA) neuron loss in PD. Therefore, most drugs that succeed in preclinical animal models fail to work in PD patients. The current proposal addresses this barrier by focusing on hyperactivated endoplasmic reticulum (ER) stress, which is an established convergent mechanism downstream of multiple PD-related toxicities. ER stress consists of three signaling arms that culminate in the translocation of three key proteins – ATF6, XBP1, and CHOP into the nucleus of DA neurons. Moderate activation of ATF6 and XBP1 is neuroprotective, while chronic, uncontrolled activation of these proteins leads to DA cell death. By contrast, activation of CHOP invariably causes cell death. Experiments showed that the nicotinic compound cytisine causes an upregulation of specialized structures called ER exit sites (ERES), and this is associated with an inhibition of ATF6 and XBP1. Surprisingly, experiments in a 6-hydroxydopamine (6-OHDA)-induced preclinical toxin model of PD showed that cytisine is neuroprotective only in female mice. Additional preliminary experiments showed that 17-β-estradiol inhibits CHOP expression in DA neurons, which fits with strong sex-differences observed in the incidence and phenotype of PD, with men being more prone to developing PD than women. Based on these data, this proposal hypothesizes that cytisine and estrogen exert neuroprotection in female mice by synergistically inhibiting all three arms (ATF6, XBP1 and CHOP) of hyperactivated ER stress in DA neurons. Experiments are proposed to determine the extent to which cytisine requires ERES for exerting neuroprotection by specifically deleting ERES in DA neurons or by using nicotinic compounds that cannot upregulate ERES. Female mice will be ovariectomized to determine mechanisms by which specific subtypes of estrogen receptors exert neuroprotection in female mice. On completing experiments in this proposal, key signaling targets through which cytisine and estrogen mediate neuroprotective effects will be identified. Success in this proposal will lay groundwork for the long-term goal of discovering clinically translatable neuroprotective PD drugs that work in both men and women.