Aβ_1–42 modulation of Akt phosphorylation via α7 nAChR and NMDA receptors

Abstract

Elevated Aβ and its deposition as senile plaques are pathogenic features of Alzheimer's disease. Aβ has been shown to be toxic to neurons and to inhibit long-term potentiation yet, the intracellular signalling pathways underlying these actions are unknown. We report for the first time that acute exposure of primary mouse neurons to 400 nM Aβ_1–42 increased Akt phosphorylation in an α₇ nicotinic receptor and NMDA receptor dependant manner. However, prolonged incubation resulted in Akt phosphorylation returning to baseline consistent with the action of a physiological regulator. Analysis of an APP transgenic mouse (TAS10) revealed a significant deficit in hippocampal Akt phosphorylation at 13 months. This time point corresponds to the emergence of plaque formation and memory impairments in these mice. The present study suggests that Aβ_1–42 regulates Akt phosphorylation in a complex manner. Acutely, Aβ_1–42 stimulates Akt phosphorylation however, chronic exposure to Aβ in TAS10 mice resulted in a downregulation of Akt phosphorylation consistent with abnormalities in excitatory neurotransmission in these mice and with recent reports of Aβ_1–42 driven internalisation of NMDA receptors.

Introduction

The pathophysiological role of the beta-amyloid peptide, Aβ, in neuronal signalling has been the subject of intensive study as elevated cerebral levels of the peptide have been hypothesised to be a cause of neurodegeneration and cognitive impairment in Alzheimer's disease (AD). Central to this has been both in vitro and in vivo evidence that Aβ is toxic to neurons (Harkany et al., 2000, Kim et al., 2000, Selkoe, 1994, Selkoe, 2001, Small and McLean, 1999) and can inhibit long-term potentiation (LTP), a correlate of synaptic efficacy (Freir and Herron, 2003). From a neurochemical standpoint dysfunction of both glutamatergic and cholinergic neurotransmission are considered to underlie the cognitive impairment seen in AD (Francis et al., 1999).

Specific interactions between Aβ and acetylcholine receptors have been reported. In particular, Aβ binds with high affinity to the α₇ subtype of the nicotinic acetylcholine receptor α₇nAChR (Wang et al., 2000a, Wang et al., 2000b). Whether or not Aβ acts as an agonist or an antagonist at α₇nAChR remains controversial (Dineley et al., 2001, Pettit et al., 2001, Spencer, 2006). In relation to intracellular signalling, phosphatidylinositol 3-kinase (PI3K) mediation of α₇nAChR-dependant phosphorylation of extracellular signal-regulated kinase (ERK) and Akt has been reported (Dineley et al., 2001, Kihara et al., 2001, Shaw et al., 2002, Shimohama and Kihara, 2001). Phosphorylation of Akt has a well-established role in cell survival (Man et al., 2003, Mizuno et al., 2003, Qin et al., 2005, Sanna et al., 2002) and a link between deficits in Akt phosphorylation, intraneuronal Aβ accumulation and apoptosis has been proposed (Magrane et al., 2005). However, other studies suggest an additional potential role for Akt in the induction of LTP, following NMDA receptor activation of PI3K (Man et al., 2003, Mizuno et al., 2003, Qin et al., 2005, Sanna et al., 2002). Aβ may reduce glutamatergic transmission via α₇nAChR dependent NMDA receptor endocytosis (Snyder et al., 2005) which could result in inhibition of Akt phosphorylation (Cedazo-Minguez et al., 2003, Kubo et al., 2002). Finally, alterations in Akt phosphorylation in AD have been reported and to correlate with the staging and severity of the disease (Pei et al., 2003).

We wished to investigate early Aβ-driven signalling events in neurons which might account for observed changes in neurotransmission and cognitive performance in AD. To this end, the phosphorylation status of Akt was analysed in primary cortical neurons exposed to Aβ_1–42 and in TAS10 transgenic mice carrying the human Swedish familial mutation of the amyloid precursor protein, APP (Brown et al., 2005, Richardson et al., 2003).