A Greek Tragedy: The Growing Complexity of Alzheimer Amyloid Precursor Protein Proteolysis

Abstract

Proteolysis of the amyloid precursor protein (APP) liberates various fragments including the proposed initiator of Alzheimer disease-associated dysfunctions, amyloid-␤. However, recent evidence suggests that the accepted view of APP proteolysis by the canonical ␣-, ␤-, and ␥-secretases is simplistic, with the discovery of a number of novel APP secretases (including ␦-and -secretases, alternative ␤-secretases) and additional metabolites, some of which may also cause synaptic dysfunction. Furthermore, various proteins have been identified that interact with APP and modulate its cleavage by the secretases. Here, we give an overview of the increasingly complex picture of APP proteolysis.Currently over 46 million people worldwide are living with dementia (see the Alzheimer's Disease International website) with Alzheimer disease (AD) 3 representing the most common form of dementia. In AD, the amyloid cascade hypothesis posits that amyloid-␤ (A␤), produced through the sequential proteolytic cleavage of the amyloid precursor protein (APP) by the ␤-and ␥-secretases, is a key molecule in initiating and propagating disease pathology including neurofibrillary tangle formation, neuronal cell loss, aberrant synaptic activity, and brain atrophy that lead to the clinically recognized symptoms of dementia (1). However, identification of the A␤ peptide 25 years ago has not yet led to the advent of a viable therapeutic strategy that can slow or halt the progression of AD. Recent studies have revealed new complexities in the proteolytic processing of APP, including the identification of novel secretases which generate APP metabolites that accumulate in the brains of AD patients and may contribute to the synaptic dysfunction observed in the disease. In addition, numerous proteins are being identified that interact with APP, modulating its proteolysis and A␤ production. These new APP secretases and metabolites, along with the APP interactors, may present novel therapeutic targets that are independent of direct modulation of the canonical secretases and that will need to be considered when evaluating the results from current A␤-directed therapies. In this Minireview, we summarize the recent developments in APP proteolysis focusing on the novel secretases, APP interactors, and APP metabolites that are impacting on our understanding of both APP biology and the neurodegenerative disease process. The Canonical ␣-, ␤-, and ␥-Secretases and APP FragmentsThe generally accepted model of APP proteolysis is that APP is processed by one of two distinct proteolytic pathways (Fig. 1A). In the amyloidogenic pathway, ␤-secretase, the ␤-site APP-cleaving enzyme 1 (BACE1), cleaves APP within the ectodomain and liberates a soluble proteolytic fragment, termed soluble APP␤ (sAPP␤), primarily in the endosomal system from the transmembrane APP holoprotein (2). The remaining C-terminal membrane-bound APP fragment, CTF␤, is subsequently cleaved by the presenilin (PS)-containing ␥-secretase multisubunit complex to liberate the A␤ peptide and the APP intrace...

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