Genistein inhibits aggregation of exogenous amyloid-beta_1–40 and alleviates astrogliosis in the hippocampus of rats

Abstract

We addressed the question of whether injection of Amyloid beta (Aβ)_1–40 in the rat brain is associated with pathology in the hippocampus, and if genistein has any protective effect against the neuronal damage caused by Aβ_1–40. Genistein is a plant-derived compound with a structure similar to that of the female sex hormone estrogen and it was recently shown that pretreatment with a single dose of genistein ameliorated learning and memory deficits in an (Aβ)_1–40 rat model of Alzheimer's disease. Here, we report that injection of the amyloid peptide into the hippocampus of rats led to formation of Aβ_1–40 positive aggregates close to the lateral blade of the dentate gyrus (DGlb). We also observed the following in the hippocampus: extensive cell death in the DGlb (P < 0.0001), CA1 (P = 0.03), and CA3 (P = 0.002); an increased number of iNOS-expressing cells (P = 0.01) and gliosis. Genistein given to rats by gavage 1 h before injection of Aβ_1–40 inhibited the formation of Aβ_1–40 positive aggregates in the brain tissue and led to increased number of nNOS⁺ (P = 0.0001) cells in the hippocampus compared to sham-operated genistein-treated controls. Treatment with genistein also alleviated the extensive astrogliosis that occurred in Aβ_1–40-injected hippocampus to a level similar to that observed in sham-operated rats. We conclude that the neurons in the DGlb are most sensitive to Aβ_1–40, and a single dose of genistein can ameliorate Aβ_1–40 induced pathology.

Introduction

In patients with Alzheimer's disease (AD), the brain shows extracellular β-amyloid (Aβ) deposition as well as intracellular neurofibrillary tangles. Dystrophic neuritis, synaptic loss, and neuronal death are additional pathological hallmarks of AD. Much of the research on the pathogenesis of this disease has focused on the role of abnormally high amyloid secretion, which is believed to be the central event in neuronal degeneration. In vitro, the presence of Aβ is associated with degeneration of neurites (Horiuchi et al., 2010), and overexpression of Aβ in transgenic mice induces neuronal degeneration consistent with that observed in Alzheimer's patients (Games et al., 1995). Also injection of Aβ into the rat brain causes neuronal damage (Miguel-Hidalgo et al., 1998, Miguel-Hidalgo et al., 2002). Even though much progress has been made in elucidating the biological mechanism of development of this disease, there is still no cure. The increasing number of patients suffering from AD throughout the world (Fratiglioni et al., 2010) indicates an urgent need for preventive measures and effective therapy.

Soy protein contains a large amount of isoflavones which are associated with a wide variety of beneficial health effects. One of the best-known isoflavones i.e. genistein is absorbed in the small intestine (Picherit et al., 2000), can be detected in plasma and serum after oral administration (Rowland et al., 2003), and crosses the blood–brain barrier (Tsai, 2005). Genistein has a structure similar to 17β-oestradiol, and it can bind to the estrogen receptors (Lephart et al., 2004). Estrogen offers some protection against Aβ-induced cell death, but it also has serious oncogenic effects on non-neuronal cells (Bang et al., 2004), and thus renders this hormone of limited use for treatment purposes. Genistein may have a beneficial influence similar to that of estrogen but without the negative side effects (Bang et al., 2004). Genistein acts via estrogen receptors to stimulate MAP kinases; these proteins activate the NFĸB signaling pathway and thereby induce overexpression of manganese superoxide dismutase (MnSOD), which serves as an antioxidant in the cell (Akiyama et al., 1987, Borras et al., 2006). Recently, Huang and Zhang (Huang and Zhang, 2010) observed that chronic ingestion of genistein reduced neuronal apoptosis in the brain of ovariectomized rats. Furthermore, Valles et al. (2008) found that pretreatment with genistein attenuated Aβ-induced death of cortical neurons in vitro by lowering oxidative stress. It is plausible that genistein provides a protective effect via its anti-inflammatory influence, or via inhibition of the endoplasmic reticulum stress that arises due to accumulation of unfolded proteins (Park et al., 2010). Overall, it appears that genistein has a positive impact on various cellular mechanisms that are assumed to underlie the development of AD. Therefore, genistein may be a good candidate in the search for compounds that can be used to prevent or treat AD in the future. We have previously observed that genistein ameliorated impairment of short-term spatial memory induced by intrahippocampal injection of Aβ_1–40 in rats (Bagheri et al., 2011). In the current study, we addressed the question of whether injection of Aβ_1–40 in the rat brain is associated with pathology in the hippocampus, and if genistein has any protective effect against the neuronal damage caused by Aβ_1–40.