Neuronal Function of the Amyloid Precursor Protein of Alzheimer's Disease

Abstract

The amyloid precursor protein (APP) is a type I transmembrane protein with several characterized biochemical domains. However, since APP -/- mice show a normal phenotype, the function of APP remains elusive. Neuronal function of APP was investigated by expression of human APP (hAPP) or inhibition of endogenous APP production in primary cultures of cortical neurons. Primary cultures of neurons were prepared from rat or mouse embryos. Mouse embryonic fibroblasts (MEFs) were maintained in culture as cell lines. hAPP or shRNA expression was performed using recombinant adeno- or lentiviruses. Calcium oscillations and calcium currents were measured using FURA2 and patch clamp, respectively. APP target genes, and HMGCoA reductase mRNA expressions were quantified by qRT-PCR, while the corresponding proteins were quantified by Western blotting. Focal expression of hAPP in some neurons of a network interrupted spontaneous synchronous calcium oscillations in the entire network. hAPP expressing neurons showed increased activity of L-type voltage-dependent calcium channels. This in turn activated calcium-activated K+ channels, responsible for an increased medium afterhyperpolarisation (mAHP) phase following action potentials, preventing propagation of calcium oscillations (1). Neuronal expression of hAPP decreased HMGCoA reductase expression and cholesterol synthesis. This inhibition resulted from the impairment of SREBP processing in the Golgi compartment, where a direct interaction between SREPB and APP was demonstrated by co-immunoprecipitation. Expression of aquaporin 1 (AQP1) was strongly decreased in MEFs lacking APP. AQP1 expression was restored in these cells by expression of hAPP. The transcriptional activity of AQP1 gene promoter was identical in APP+/+ and APP-/- MEFs, as was the stability of AQP1 mRNA. Regulation of AQP1 expression by APP was sensitive to histone acetylation, and HDAC activity co-immunoprecipitated with APP. Efficient expression of hAPP in primary cultures of cortical neurons, as well as inhibition of endogenous APP expression allowed to identify important neuronal functions of APP, including control of neuronal excitability, cholesterol homeostasis, and epigenetic control of gene expression. (1) Ferrao Santos et al. (2009) J. Neurosci. 29, 4708-4718.