Spontaneous Isomerization of Long-Lived Proteins Providesa Molecular Mechanism for the Lysosomal Failure Observed in Alzheimer’sDisease

摘要

Proteinaceous aggregation is a well-known observable in Alzheimer’s disease (AD), but failure and storage of lysosomal bodies within neurons is equally ubiquitous and actually precedes bulk accumulation of extracellular amyloid plaque. In fact, AD shares many similarities with certain lysosomal storage disorders though establishing a biochemical connection has proven difficult. Herein, we demonstrate that isomerization and epimerization, which are spontaneous chemical modifications that occur in long-lived proteins, prevent digestion by the proteases in the lysosome (namely, the cathepsins). For example, isomerization of aspartic acid into l-isoAsp prevents digestion of the N-terminal portion of Aβ by cathepsin L, one of the most aggressive lysosomal proteases. Similar results were obtained after examination of various target peptides with a full series of cathepsins, including endo-, amino-, and carboxy-peptidases. In all cases peptide fragments too long for transporter recognition or release from the lysosome persisted after treatment, providing a mechanism for eventual lysosomal storage andbridging the gap between AD and lysosomal storage disorders. Additionalexperiments with microglial cells confirmed that isomerization disruptsproteolysis in active lysosomes. These results are easily rationalizedin terms of protease active sites, which are engineered to preciselyorient the peptide backbone and cannot accommodate the backbone shiftcaused by isoaspartic acid or side chain dislocation resulting fromepimerization. Although Aβ is known to be isomerized and epimerizedin plaques present in AD brains, we further establish that the ratesof modification for aspartic acid in positions 1 and 7 are fast andcould accrue prior to plaque formation. Spontaneous chemistry cantherefore provide modified substrates capable of inducing graduallysosomal failure, which may play an important role in the cascadeof events leading to the disrupted proteostasis, amyloid formation,and tauopathies associated with AD.