The processing of glycoprotein APP results in the formation of Abeta peptides, which aggregate to form amyloid deposition in the brains of patients with Alzheimer's disease (AD). APP was localized to the endoplasmic reticulum (ER), Golgi, plasma membrane, and was shown to undergo a number of posttranslational modifications, including formation of S-S bonds, glycosylation, phosphorylation, proteolytic processing, and secretion. AD-associated mutations lead to an increased production of Abeta42, a highly amyloidogenic peptide which is found in the ER and in the extracellular space. The functions of proteins critically depends on their proper folding. In the past few years, our understanding of the roles of ER resident chaperones in coordinating molecular events leading to the proper folding of glycoproteins has advanced substantially. Only properly folded species are allowed to transit to the Golgi, whereas, misfolded, aberrantly modified, and/or mutated glycoproteins are recognized, retranslocated from the ER into cytosol, and degraded by the ubiquitin-proteasome degradation pathway. This proposal extends this very recent line of research by defining the processes by which APP is folded, and determining the relationship of misfolded APP and the formation of Abeta42. Based on our preliminary data, we hypothesize that (a) key ER molecular chaperones, calnexin, calreticulin, and BiP are critically important for the proper folding, maturation, and secretion of APP; (b) misfolded or mutated APP are retranslocated into cytosol and degraded by the ubiquitin-proteasome degradation pathway; and (c) Abeta42 is generated from the misfolded and retranslocated APP. To address these hypothesis the following specific aims are proposed to define: (1) the contribution of the ER chaperones to the folding, maturation, and secretion of APP in transfected COS and HEK-293 cells; (2) the contribution of the ER chaperones to the folding of APP in neuronal NT2N, NIE 115, and B103 cells;(3) the contribution of disulfide bond formation to conformational maturation of APP; (4) the effect of AD-associated mutations on folding and degradation of APP; (5) the effect of interactions of ER chaperones with APP on the generation of Abeta42. Thus, by elucidating the role of ER chaperones in the folding of nascent APP, and demonstrating the effect of the ER quality control mechanism on AD-associated APP mutations, the proposed research will advance our understanding of the etiology of AD.
