Optogenetic Modulation of the GABAergic System in Alzheimer's Disease

Abstract

Alzheimer’s Disease (AD) is a neurodegenerative disorder accounting for majority of all dementia cases in the world. At present, a clear understanding of the mechanisms underlying AD pathology has not been achieved. Evidence points to a potential remodelling of the inhibitory neurotransmitter γ-Aminobutyric acid (GABA) system in AD. The implication of a role for the GABAergic system in AD pathogenesis has led to its exploration as a therapeutic target by several drug treatments but none so far have been able to target specific GABAergic neuronal populations with spatio-temporal accuracy and cell-type specificity. In this study, we aimed to circumvent these issues by using optogenetics to specifically modulate GABAergic inhibition in an in vivo AD mouse model and prevent cognitive deficits. C57BL6 mice were stereotactically injected bilaterally in the CA1 hippocampal region with a lentiviral vector, LV.GAD67.NpHR3.0.YFP, containing GAD67-specific promoter fused with enhanced inhibitory halorhodopsin (NpHR) and yellow fluorescent protein (YFP). 35 days after viral vector expression, the mice were stereotactically injected bilaterally in the CA1 hippocampal region with amyloid-beta (Aβ1-42) and implanted with the optogenetic device and optogenetically inhibited immediately after surgery for 30 minutes continuously at 40 Hz frequency. We compared the effects of the optogenetic inhibition stimulus to an Aβ1-42 -only (AB) group and we also included naive control, implant only control and an implant group inhibited without viral vector expression. Mice were tested for novel object alteration and novel object recognition tasks 7 days post stimulus (AB+V+S_7 D) and repeated at 30 days post stimulus (AB+V+S_30 D) for short-term and long-term assessment of the optogenetic inhibition, and were euthanised post-testing. Fluorescent immunohistochemistry was used to characterize the cellular and molecular changes in specific hippocampal-subregions. We show significant (p<0.05) improvement in cellular, molecular, and behavioural changes in optogenetically modulated Aβ1-42-injected mice tested 7 days post stimulus (AB+V+S_7 D) compared to the Aβ1-42-only injected mice. The optogenetic inhibition was able to prevent longterm spatial memory impairment and pyramidal cell loss in the AB+V+S_7 D mice. Aβ1-42- induced microglial overactivation was significantly reduced in the dentate gyrus (DG) region of the hippocampus in AB+V+S_7 D mice indicated by lowered expression levels of microglial marker Iba-1 compared to Aβ1-42-injected mice. The optogenetic inhibition was able to downregulate α5-GABAAR subunit expression in the DG of AB+V+S_7 D mice compared to Aβ1-42-injected mice. These findings demonstrate that our optogenetic approach could prevent cognitive deficits in an Aβ1-42-induced in vivo AD mouse model by inhibiting the GAD67-positive neurons. This necessitates the need for further investigation in the contribution of the GABAergic system in AD pathogenesis, and highlights the potential of optogenetic inhibition as a specific and effective alternate treatment against AD.