Baicalein reduces β-amyloid and promotes nonamyloidogenic amyloid precursor protein processing in an Alzheimer’s disease transgenic mouse model

Alzheimer’s disease (AD) is a neurodegenerative disease characterized pathologically by senile plaques (SPs) and neurofibrillary tangles (NFTs) and clinically by progressive cognitive deterioration. Over the last 2 decades, the β-amyloid (Aβ) hypothesis has been the dominant theory for the mechanism of AD pathogenesis (Hardy and Higgins, 1992; Hardy et al., 1998; Hardy and Selkoe, 2002). Amyloid precursor protein (APP) proteolysis is the fundamental process for the production of Aβ peptides implicated in AD pathology. Aβ peptides are produced by the initial action of β-secretase (BACE) cleavage, which creates an Aβ-containing C-terminal fragment (CTF) known as β-CTF and an N-terminal, soluble APP-β (sAPPβ) fragment, which is released extracellularly. Intracellularly, β-CTF is then cleaved by a multiprotein γ-secretase complex that results in generation of the Aβ peptide and a smaller γ-CTF. Under physiological conditions, Aβ is constitutively generated at relatively low levels and, hence, most APP is processed by the nonamyloidogenic pathway involving α-secretase. In the nonamyloidogenic pathway, APP is first cleaved at the α-secretase site, which results in the release of N-terminal sAPPα and the generation of α-CTF. Because of the limiting amount of APP in the cell and the failure to saturate the BACE pathway during APP overexpression, it is believed that the above-mentioned amyloidogenic and nonamyloidogenic pathways compete for substrate in the process of APP proteolysis (Hardy and Selkoe, 2002). Therefore, it is often inferred that extracellular elevation of sAPPα can be taken as indirect evidence of activation of α-secretase or inhibition of BACE and the associated amyloidogenic pathway (Rezai-Zadeh et al., 2005). Over the past decade, intense focus has been given to investigating the processes of APP proteolysis and Aβ metabolism as possible targets for AD therapy. One naturally occurring compound is baicalein (5,6,7-trihydroxyflavone), extracted from the root of the Chinese medicinal herb Scutellaria baicalensis Georgi (Jianjun and Huiru, 2008). Previous studies have demonstrated the usefulness of baicalein in reducing cognitive deficits in rats induced by chronic cerebral hypoperfusion and improving memory in ibotenic acid-induced rat models (Liu et al., 2007; Heo et al., 2009). Further, Wang et al. (2004) have shown baicalein can reduce β-amyloid peptide-(25–35)-induced amnesia in mice. Heo et al. (2004) reported that baicalein potently reduces Aβ-induced neurotoxicity in PC12 cells, possibly by a reduction of oxidative stress. Likewise, Zhu et al. (2004) observed that baicalein can inhibit fibrillation of α-synuclein and disaggregate existing fibrils, which raises the possibility of baicalein as a potential treatment for neurodegenerative diseases, such as Parkinson’s disease. However, whether baicalein can directly inhibit Aβ production and ameliorate the subsequent cognitive impairments observed in an animal model of AD has not been determined. The present study shows that baicalein significantly decreased Aβ production both in Chinese hamster ovary cells expressing a human wild-type APP gene (CHO/APPwt) and in primary neurons from transgenic Tg2576 mice. In concert with these observations, we found that baicalein promotes cleavage of α-CTF and elevates sAPPα. These cleavage events are attenuated with a specific GABA receptor antagonist, bicuculline. As a validation of these findings in vivo, we treated Aβ-overproducing Tg2576 transgenic mice with baicalein for 8 weeks and found decreased Aβ levels in the brain associated with promotion of the nonamyloidogenic α-secretase proteolytic pathway. Furthermore, we found the cognitive performance of these mice to be significantly improved compared with untreated mice, suggesting that this natural compound should be tested further for translation into humans.