Effect of major triterpenoids of centella asiatica on neurite outgrowth in neuro-2a cells

Abstract

Centella asiatica has been reported to demonstrate neuroprotection and memory improvement in various animal models. Triterpenoid glycosides, madecassoside (MS), asiaticoside (AS), and their aglycones, madecassic acid (MA) and asiatic acid (AA) are considered as major neuroactive constituents in C. asiatica. In this study we aimed to compared MS, AS, MA and AA for their neurite outgrowth activities and mechanisms in Neuro-2a cells. Immunofluorescent staining against ßIII-tubulin showed that MS and AS significantly increased the percentage of neurite-bearing cells and neurite length with higher potency than MA and AA. Our mechanistic studies revealed the differences in signal activation between glycosides and aglycones. Neurite outgrowth activities of MS and AS were found to be regulated by activation of TrkA receptor signaling, resulting in sustained ERK1/2 activation coupled with cAMP response element binding (CREB) phosphorylation which involved with neuronal differentiation. Activation on TrkA receptor also triggered Akt phosphorylation, resulting in inhibitory effects on glycogen synthase kinase (GSK) 3ß and RhoA, which controlled cytoskeleton dynamic, resulting in allowing neurite extension. Translocation of nuclear factor erythroid 2–related factor 2 (Nrf2) was also regulated by Akt signaling, which might be associated with neurite outgrowth. On the contrary, MA and AA induced neurite outgrowth with different mechanisms by ERK1/2-independent activation of CREB and Akt-independent inhibition on GSK3ß, while Nrf2 translocation and RhoA inhibition were also mediated by Akt signaling. These differences between glycosides and aglycones might be associated with neurite outgrowth potency that glycosides exhibited higher potency than aglycones. Taken together, our findings have provided the evidence to support the neuroprotection and memory improvement activity of active constituents in C. asiatica and might be useful for drug design.