p23 protein function and its role in APP family protein trafficking

Alzheimer's disease (AD) is the leading cause of worldwide dementia. Current data suggest that cerebral beta-amyloid (Abeta) plaque formation, generated form proteolytic processing of amyloid precursor protein (APP) and extracellular deposition of Abeta, initiates AD pathogenesis. Early secretory pathway regulation of APP trafficking is critical for its subsequent proteolytic processing in the trans-Golgi network and post-Golgi organelles; however, proteins participating in this regulation have not been well delineated.; p24 proteins are a family of type I membrane proteins that populate the early secretory pathway and are necessary for mammalian embryogenesis. Overexpression of an individual p24 protein (p23) in cultured mammalian cells does not replace the other p24 proteins. Meanwhile, p23 overexpression selectively increases stability of immature APP family polypeptides while reducing their complex maturation and proteolytic processing. Further biochemical characterization of p23 overexpression indicates that early secretory pathway retention of an APP family protein (APLPI) is possibly regulated by a direct interaction with the lumenal domain of p23. Interestingly, co-expression of APP C-terminal binding proteins (that promote maturation of APP family proteins) and p23 reveals that specific protein interactions with both the N- and C-terminal domains of APP family proteins are important for regulating their early secretory pathway trafficking.; p23 knockdown by siRNA reduces expression levels of all p24 proteins, providing a method for depletion of the p24 system in cultured cells. p24 protein family knockdown results in dispersal of cis-Golgi proteins as well as delayed Golgi tubulation and fusion with the endoplasmic reticulum following brefeldin-A treatment. p24 depleted cells exhibit a decreased level of beta-COP recruitment to membranes in in vitro binding assays; however, these cells also have more endogenous beta-COP bound to isolated membranes. Meanwhile, APP biochemical analyses in p24 protein depleted cells indicate that APP matures faster, is stabilized better, and is processed more efficiently to generate both secreted sAPP and Abeta following p24 depletion. These data collectively suggest that the p24 family is important for regulating Golgi to ER retrograde COP I vesicle budding and plays a significant role in the early stages of APP intracellular trafficking that impact on Abeta production.