Evaluation of Neuroprotective Effects o a DNA Polymerase Inhibitor and Selective Nutraceuticals in Alzheimer's Disease Models

Abstract

Alzheimer's disease (AD) is a neurodegenerative disease of the brain that causes progressive impairment in memory, intelligence, behaviour, and ability to undertake activities of daily living. About 60% of dementia cases are caused by AD, which currently affects more than 35 million people worldwide and the number of people with AD is expected to increase by more than 66 million by 2030. The disease is identified by the presence of critical pathological hallmarks such as senile plaque, neurofibrillary tangles and neuroinflammation. AD is categorized into two types i.e., Early onset AD (EOAD) and late-onset AD (LOAD). EOAD is caused due to genetic modification of genes such as PSN I, PSN II, APP etc. Whereas LOAD is caused due to multiple factors such as age, ApoE, acquired conditions due to environmental or metabolic changes. Research in AD is guided by major hypotheses such as cholinergic hypothesis, amyloid cascade hypothesis, tau hypothesis, neuroinflammation and oxidative stress. Current approved treatments are classified anti-cholinergic and are approved only for symptomatic management of the disease. However, current research strategies are focused at developing molecules that are targeting critical parameters like Aβ, p-tau, inflammatory markers and other mode of action and thereby are expected to prevent or cure the disease instead of imparting just symptomatic management like current approved medications. According to the amyloid cascade hypothesis, Aβ is the "driver" of a series of downstream events that lead to neuronal death and AD symptoms. The hypothesis also underlines correlation between up regulation of Aβ production and down regulation of its clearance by enzymatic pathway performed by amyloid degrading enzymes ADE and non-enzymatic pathway performed by localized transporters. It is also reported that amyloid deposition can trigger abnormal phosphorylation and aggregation of tau followed by neuroinflammation and oxidative stress. Taking the basis of the hypothesis, current research work was undertaken to evaluate Vitamin D3 (2.5 µg/kg and 5.0 µg/kg), Valacyclovir (100 mg/kg and 150 mg/kg), Berberine (50 mg/kg and 100 mg/kg). Donepezil (5 mg/kg) was taken as standard. Vitamin D3 was selected due to its strong correlation with incidence of dementia in Vitamin D deficient population, Berberine was selected based on reported evidence of this natural alkaloid for its ability to improve memory and other neurological conditions. Valacyclovir was being chosen due to reported clinical benefits of this antiviral in AD patients and evaluate the possibility of repurposing the drug for treatment of AD. The purpose of our research was to evaluate in-depth neuroprotective benefits of these drugs and understand their impact on AD specific parameters and mechanism of action. For valacyclovir, this is the first ever reported pre-clinical studies that studies its neuroprotective role in AD. Primarily, In-silico studies were performed for the selected treatments followed by in-vivo studies were performed using Scopolamine induced amnesia rat model, Streptozotocin ICV AD rat model and Aβ1-42 induced AD rat model. In-silico docking studies evaluated binding efficiency of selected treatments with Acetylcholinesterase (AChE), Butyrylcholinesterase (BuChE) and Beta-secretase1 enzyme (BACE-1). The study outcome indicated significant binding efficiency with these enzymes indicating potential biological action. In-vivo models evaluated effect of these treatments on AD specific parameters like Aβ1-40, Aβ1-42, p-tau, BACE-1, AChE and were found to be consistently reduced with all treatment groups as compared to standard Donepezil whereas the concentration of NEP and IDE was increased significantly. Similarly inflammatory markers such as TNF-α, IFN-γ, IL-6, IL-1β, NF-κβ and oxidative parameters were also significantly altered by all selected treatments as compared to standard Donepezil. The results were found to be comparable and consistent in all three animal models. Further, results also finds that the selected treatments were able to downregulate amyloid proteins, (Aβ1-40 and Aβ1-42), p-Tau and BACE-1 and showed parallel upregulation of ADE enzymes i.e NEP and IDE. Therefore, as per the hypothesis, it could be possible that amyloid plaque can be reduced by increasing its clearance and at the same time control its abnormal production to have significant impact on progression of the disease. Hence, an ideal treatment for AD should have two-way approach while targeting its key pathological hallmarks. Based on the findings, the neuroprotective effects of Vitamin D3, Valacyclovir and Berberine supplementation can be concluded. They may also contribute to delaying the development of cognitive deficits and neurodegeneration and thereby support improvement in learning and memory. The clinical benefits of treatments are found to be comparable with Donepezil. Results are suggestive of efficiency of selected treatments in delaying the progression of AD by slowing down production of Aβ proteins, increase in clearance of Aβ proteins, reduction in p-Tau, BACE-1 markers and reduction in inflammatory markers and oxidative stress. The mechanistic approach of treatments appears to be multifaceted. The findings are consistent with outcomes of histopathological examination of brain tissue. However, more studies are warranted to evaluate clinical significance and to conclude significant therapeutic benefits of Vitamin D3, Valacyclovir and Berberine in Alzheimer's Disease.