In vitro evaluation of extracts from Mentha species for potential treatment of Alzheimer’s disease

With an increase in elderly population, there is an increase in pathological conditions that accompany ageing, e.g. Alzheimer’s disease (AD). AD develops via several causes such as oxidative stress, cholinergic neurons apoptotic death and accumulation of abnormal proteins in the brain. Current medications are only palliative, so there is a need for a drug for this disorder. Plants from the Lamiaceae have been used traditionally to improve memory, suggesting neuroprotective/curative potential. Hence, the present study focussed on evaluation of extracts from Mentha taxa (Lamiaceae) against a number of factors implicated in development of AD.28 mint samples, representing nineteen Mentha taxa, were purchased (with nine taxa purchased twice from two nurseries). Plants were grown under identical conditions for two months, then leaves were collected, dried and extracted. Air-dried herbarium specimens were also prepared. Micromorphological analysis of leaves of the 19 taxa was undertaken to characterise the taxa. Dimensions of lamina and petiole were measured. Stomatal distribution and density and trichome density were determined using stereo and scanning electron microscopy. Taxa showed variations in leaf macromorphology, i.e. different lamina and petiole lengths. Species in section Pulegium were distinguishable by small lamina, long petiole percentage and high adaxial stomatal density. Non-glandulartrichomes were diagnostic for some taxa.
The 19 taxa were screened for their phenolic composition and in vitro bioactivities relevant to AD (antioxidant, acetylcholinesterase, butyrylcholinesterase and histone deacetylase inhibition). Mint extracts showed strong antioxidant activity, moderate
acetylcholinesterase and butyrylcholinesterase inhibition, and good histone deacetylase inhibition, which strongly correlated to biophenol content. Apigenin-7-O-β-D-diglucuronide, chicoric acid and isosakuranetin were tentatively identified as new compounds in Mentha. Principal component analysis (PCA) was carried out to select six Mentha taxa for further screening, which covered the broad spectrum of bioactivities exhibited by mints: Moroccan and Corsican mints, with the highest phenolic content, antioxidant capacity and histone deacetylase inhibition; white peppermint with the
highest acetylcholinesterase inhibition, but lowest in other activities; the Australian mints, M. australis and M. diemenica, as they represented most of the investigated taxa having moderate activities; and peppermint, of dietary significance.
The six taxa were investigated for their β-secretase and amyloid β-aggregation inhibition, and their protective effect on H₂O₂-induced damage in SH-SY5Y cells through caspase activity. All six exhibited strong β-secretase inhibition. M. requienii showed excellent inhibition of amyloid β-aggregation, indicating that extracts have the potential to prevent the formation of amyloid β and also could prevent protein aggregation if already formed. M. diemenica and M. requienii lowered caspase activity in H₂O₂-induced oxidative stress and apoptosis indicating they could suppress apoptosis at cellular level.
Real-time PCR and Western blot analysis were carried out for the two most promising extracts to determine their effects on signalling pathways in SH-SY5Y cells. M. diemenica lowered the expression of pro-apoptotic protein, Bax, and increased that of anti-apoptotic protein, Bcl-xL, which could be mediated by a down-regulation of ASK1-JNK pathway.
This study suggests that mints have potential neuroprotective effects. Further studies are recommended to address their possibilities for use in prophylaxis from AD or as treatment.