Genetically encoded nAChR upregulation is neuroprotective in female parkinsonian mice

Parkinson's disease is projected to rise to pandemic proportions by 2050, which has resulted in an urgent need for disease-modifying treatments. In this regard, we previously showed that in a mouse model of parkinsonism with unilateral 6-hydroxydopamine (6-OHDA) injection into the dorsolateral striatum (DLS), low doses of the neuronal nicotinic acetylcholine receptor (nAChR) partial agonist and smoking cessation drug, cytisine exerts sex-specific neuroprotection in substantia nigra pars compacta (SNc) dopaminergic (DA) neurons of only female mice by reducing apoptotic endoplasmic reticulum (ER) stress. Although these data suggest that neuroprotection might occur via cytisine-mediated upregulation of β2 subunit-containing (β2*) nAChRs in SNc DA neurons, there is no direct evidence to support this idea. Therefore, this study asks the critical question of whether upregulation of β2* nAChRs in SNc DA neurons alone is sufficient to reduce apoptotic ER stress and exert neuroprotection in a preclinical unilateral DLS mouse model of 6-OHDA-induced parkinsonism. To address this question, we generate and characterize a novel β2-upregulated transgenic mouse line. These transgenic mice possess mutations in the M3-M4 intracytoplasmic loop of β2 subunits that cause constitutive upregulation of β2* nAChRs without nicotinic ligands. Surprisingly, when compared to wild-type littermates, only female β2-upregulated transgenic mice demonstrate upregulation of β2* nAChRs in SNc DA neurons as assessed by significant increases in Sec24D-containing ER exit sites (Sec24D-ERES). Using the optogenetic calcium and dopamine sensors, GCaMP6f and GRABDA respectively, we found significant increases in dihydro-beta-erythroidine (DhβE)-sensitive β2* nAChR-mediated calcium influx in SNc DA neuron dendrites and DhβE-sensitive acetylcholine (ACh)-evoked dopamine release at SNc DA neuron terminals of the DLS in female transgenic mice. We then used four independent readouts to assess neuroprotection of SNc DA neurons following unilateral 6-OHDA injection into the DLS, viz ., contralateral apomorphine-induced rotations, preservation of SNc DA neurons, inhibition of a major proapoptotic ER stress protein, C/EBP homologous protein (CHOP) and glial fibrillary acid protein (GFAP) expression in SNc astrocytes. In all four readouts, female β2-upregulated transgenic mice showed significant neuroprotection. From a clinical perspective, this study shows that upregulation without nicotinic ligand-mediated activation of β2* nAChRs in SNc DA neurons can be a translationally viable disease-modifying strategy for Parkinson's disease. In addition, we envision that the novel transgenic β2-upregulated mice created in this study will provide a valuable tool for understanding the role of nAChR upregulation in major neurological disorders such as addiction, anxiety, depression and dementia.