Analysis of the amyloid precursor protein role in neuritogenesis reveals a biphasic SH-SY5Y neuronal cell differentiation model

The existence of an intrinsic programme controlling neuritogenesis and activated during early neuronal differentiation and regeneration stages is well established. However, the identity and role of each molecular player and event, as well as how such a programme is modified by environmental signals, remain a focus of research. The amyloid precursor protein (APP) is a neuromodulator of the developing and mature nervous system, although in a highly complex manner which is far from clear. To study APP-induced neuritogenesis, the retinoic acid (RA)-induced SH-SY5Y cell differentiation model was first minutely characterized in terms of RA dose, morphological outputs and relevant biochemical markers. The findings reported here unveiled two differentiation phases for the 10M RA dose: 1-4 (4days excluded) and 4-8days, clearly defined by fold increases in the ratio between APP and acetylated Tubulin. Moreover, we describe, for the first time, a unique peak of secreted APP (sAPP)/APP ratio in the first phase. Subsequent APP and sAPP modulations confirmed that a high sAPP/APP ratio potentiates the elongation of smaller processes at the earlier neuritogenic phase. This sAPP/APP ratio drops in the second phase, as holoAPP levels increase to assist the maintenance of the longer neurites, potentially via their stabilization.