Increased Expression of β‐Amyloid Protein Precursor and Microtubule‐Associated Protein τ During the Differentiation of Murine Embryonal Carcinoma Cells

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Abstract
Expression of the genes encoding the β/A4 amyloid protein precursor (APP) and microtubule‐associated protein τ was studied in an embryonal carcinoma cell line (P19) that differentiates in vitro into cholinergic neurons after treatment with retinoic acid. Expression of APP increased 34‐ (mRNA) and 50‐fold (protein) during neuronal differentiation; APP‐695 accounted for most of this increase. These remarkable increases in APP expression coincided with a proliferation of neuronal processes and with an increase in content of τ mRNA. Moreover, subsequent decreases in the levels of APP and τ mRNA coincided with the onset of the degeneration of the neuronal processes. Immunocytochemical staining suggested that >85% of the P19‐derived neurons are cholinergic and that APP is present in the neuronal processes and cell bodies. These results suggest that APP may play an important role in construction of neuronal networks and neuronal differentiation and also indicate that this embryonal carcinoma cell line provides an ideal model system to investigate biological functions of APP and the roles of APP and τ protein in development of Alzheimer's disease in cholinergic neurons.