Potential Effects of the Rosmarinus officinalis Extract and its Active Compounds on Synaptic Regulation and Adult Hippocampal Neurogenesis

Abstract

Adult hippocampal neurogenesis (AHN) is imeportant mechanism responsible for the maintenance and regulation of hippocampal functioning and synaptic plasticity. Impaired neurogenesis is correlated with the pathological hallmarks of debilitating neurodegenerative disorders (NDDs) like Alzheimer’s disease (AD), which remains incurable, despite the availability of symptomatic treatment. This necessitates the development of therapeutic options that favour neurogenesis and alleviate the associated cognitive and synaptic deficits. Rosmarinus officinalis (R. officinalis), also referred to as rosemary, is a medicinal herb known to exhibit significant pharmacological effects for e.g., anti-diabetic, antimicrobial, anti-inflammatory, antioxidant and antidepressant activities. The current study evaluates the ameliorative and medicinal effects of key phytochemicals of R. officinalis. High-resolution 1H-NMR analysis of R. officinalis crude extract was performed that revealed, RA, CA and UA, and as major bioactive constituents. The proneurogenic effects of the screened molecules were investigated in a β-amyloid 1-42 (Aβ1- 42)-induced mouse model of AD and comparisons were drawn with the standard drug donepezil. An in-vivo study was carried out on BALB/c mice which were segregated into ten groups. The disease groups were generated by the injection of Aβ1-42 in the hippocampus of the mice, through stereotaxic surgery. Phosphate-buffered saline (PBS) was injected to the remaining animals (control). Each set of five groups were administered orally with vehicle, an ethanolic extract of R. officinalis, UA, RA or donepezil. Behavior analysis was performed which was comprised of the Morris water maze task to evaluate the spatial memory, the novel object recognition task for asseeement of object recognition memory, elevated plus maze to measure anxiety-like behavior and social interaction test xx for determination of social affiliation and novelty preference. The mice were then sacrificed followed by the processing of the hippocampal tissue for histological analysis, immunohistochemistry and gene expression analysis. The results demonstrate a protective effect by UA and RA in the restoration of the deficits in spatial and recognition memory as well as alterations in anxiety and social interaction induced by Aβ1-42. Furthermore, a substantial reduction was observed in Aβ plaques in comparison to the diseased group and Aβ1- 42 + donepezil-treated group. An improvement in the neuronal density was also observed as evident via neuronal nuclei (Neu N), doublecortin (DCX) and Ki-67 immunoreactivity in the diseased groups treated with RA and UA in comparison to the Aβ1-42 and Aβ1–42+donepezil-treated groups. Treatment with UA and RA also normalized the expression levels of synaptic (Synaptophysin, PSD-95 and Syn I, II, III) and neurogenic (Ki67, Neu N and DCX) markers. Furthermore, in-silico analysis was performed comprising of molecular docking studies using AutoDock vina and Amber 20 for molecular dynamic (MD) simulations. The binding energy values of the interaction of bioactive compounds of R. officinalis with the target proteins were analysed to estimate the interaction potential which was further validated through MD simulation studies based on RMSF, RMSD and MM/GBSA. Prediction of the drug-like characteristics and pharmacokinetic properties of the compounds was obtained through Lipinski filter and admetSAR analysis. The findings indicate a strong interaction potential of the selected compounds with BACE1, AChE, and synapsin I, II and III. Moreover, RA and UA exhibited promising potential as inhibitors of BACE1 and AChE evidenced by comparable binding energy values to those of the positive control, donepezil. Analysis of pharmacokinetic properties and drug likeness revealed the drug-relevant xxi features of the compounds affirming their promising potential as therapeutic candidates against Aβ1-42-induced neurotoxicity in AD.