In vitro effects of medicinal plant extracts and phytochemicals on factors implicated in Alzheimer's disease

BACKGROUND: Alzheimer's disease (AD) is a complex, multifactorial neurological disorder characterized by an insidious onset with progressive symptoms of memory impairment, language deficit, behavioural problems including agitation, mood disturbances and poor judgment. AD involves multiple pathogenetic factors such as A~ production and aggregation, oxidative stress, tau protein aggregation, metal ions (e.g. Cu2+, Zn2+, Fe2+) and reduced levels of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) enzymes. All these factors play important roles in neurodegeneration associated with AD. These provide diverse multiple targets for examining AD-modifying drugs. In view of this, the study was focussed on finding natural plant extracts used traditionally for centuries to treat memory and cognition related disorders specifically AD. Attention was also focussed on certain phytochemicals to find their therapeutic values in AD. AIMS: To find the neuroprotective, anti-cholinesterase and antioxidant activities of an aqueous extract of Withania somnifera, Bacopa monniera, Salvia patens, Salvia elegans, Capsicum annum, Uncaria tomentosa, Melissa officinalis and Centella asiatica plants. Four phytochemicals namely allicin, ajoene, capsaicin and asiatic acid were also examined for these activities. The targets chosen in this study were AChE and BuChE, neurotoxic A~ fibrils and oxidative stress pathway using in vitro models. METHDOLOGY: The aqueous extracts were obtained by boiling dried powdered plant materials in deionized water for 25 min. Enzyme assay were performed to determine anti-ChE activity by Ellman's method, while kinetics (Km and Vmax) were analyzed using Lineweaver-Burk plot method. Antioxidant capacities of these plant extracts and phytochemicals were determined using FRAP and TEAC assays. The neuroprotective activity of plant extracts and phytochemicals were determined under in vitro condition in differentiated pheochromocytoma (PC12) cell culture against H202 and A~ induced toxicities. X111 The anti-aggregation property was determined by transmission electron microscopy (TEM) and the Thioflavin T (ThT) fluorescence assay. RESUL TS: An aqueous extract of W somnifera showed dual inhibitory activities against both AChE and BuChE. The ICso values for AChE and BuChE were O.l5±0.007 and 0.80±0.005mg/mL, respectively. The potency of inhibition was greater for AChE compared with BuChE. The mode of inhibition was non- competitive mixed inhibition. Other plant extracts displayed either weak or negligible enzyme inhibition activity. In contrast, allicin demonstrated potent dual enzyme inhibitory activity. The ICso values for AChE and BuChE inhibition were 0.01±0.009 and 0.05±0.018mg/mL, respectively, while ajoene demonstrated weak enzyme inhibitory activity compared with allicin. The ICso values for AChE and BuChE inhibition of ajoene were 0.55±0.012 and 0.07±0.015mg/mL, respectively. Most of the plant extracts possessed high antioxidant capacities; FRAP values: 4242±112 to 1364644±130J,!mol Fe2+E/g dried weight; TEAC values: 1.89±0.12 to 26.20±0.05 mmol Trolox E/g dried weight. In contrast, allicin, ajoene and capsaicin showed weak antioxidant capacities; FRAP values: 1615±90, 934±35 and 1423±75 umol Fe2+E/g dried weight, respectively, whereas asiatic acid showed no apparent antioxidant capacity. These results led to the second part of the study in which these aqueous extracts and phytochemicals were examined for their neuroprotective properties under in vitro conditions in differentiated PC12 cells against H202 and A~ induced toxicity. The neuroprotective effect varied from plant to plant. An aqueous extract of W somnifera, B. monniera, S. elegans, U tomentosa, C. asiatica and pure compounds asiatic acid and capsaicin showed up to 70-90% protection of PC 12 cells against H202 and A~ induced toxicity whereas, allicin and M ofjicinalis extract showed no protective effects. Anti-aggregation measured by TEM and ThT fluorescence assay showed that an aqueous extract of W somnifera and allicin strongly inhibited fibril formation compared with control samples. These results suggest that the aqueous extract of W somnifera root and allicin have an ability to inhibit formation of mature fibrils which leads to plaque formation. XIV CONCLUSION: These finding demonstrate that that an aqueous extract prepared from these medicinal plants and phytochemicals have significant neuroprotective activities against different targets including AChE, BuChE, ROS and A~ implicated in AD. Hence, these results provide evidence of the usefulness of these medicinal plants and phytochemicals which may be used in the future to develop new therapeutic strategies for the prevention of and treatment of AD.EThOS - Electronic Theses Online ServiceGBUnited Kingdo