Role of the 39-kDa receptor-associated protein (RAP) in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia that gradually worsens over time and leads to death. AD is characterized by an accumulation of ˜í‚â§-amyloid protein (A˜í‚â§) in the brain. Recently, the 39-kDa receptor-associated protein (RAP) has been implicated in the AD pathology. RAP is found mainly in endoplasmic reticulum (ER) and functions as a chaperone for the maturation and trafficking of the low density lipoprotein (LDL) receptor family. Polymorphisms in the RAP gene have been associated with an increased risk of AD. Two studies have shown that down-regulation of RAP expression exacerbates A˜í‚⧠pathology in transgenic mouse models of AD, suggesting that RAP may have an important role in A˜í‚⧠production and clearance. It has been also shown that RAP binds strongly to A˜í‚â§, leading to an inhibition of A˜í‚⧠aggregation and neurotoxicity. Furthermore, a recent study has shown that the level of RAP expression is significantly decreased in the AD brain compared with healthy controls. This study aimed to identify the region of RAP which binds to A˜í‚⧠and the effect of RAP over-expression and treatment on APP metabolism and A˜í‚⧠production. The possibility that A˜í‚⧠can bind to RAP in the human CSF was also examined. To determine the A˜í‚â§-binding region on RAP, an in-vitro assay was established and validated to study A˜í‚â§-self association, based on the binding of biotin labelled A˜í‚â§42 to synthetic A˜í‚â§42 seeded in the wells of microplates. The A˜í‚â§-binding region on RAP was identified by measuring A˜í‚â§-self association in the presence of different RAP fragments. The results indicated that the A˜í‚â§-binding site was located between amino acid residues 206 and 216 of the RAP sequence in loop region between domains D2 and D3. To examine the effect of RAP on APP metabolism, two strategies were used. The first strategy involved over-expression of RAP in sweAPP-CHO and sweAPP-SH-SY5Y cells, after which the effect of RAP on APP processing and A˜í‚⧠production was analysed. As a second strategy, the effect of RAP on APP metabolism was examined in primary cortical neurons derived from Tg2576 transgenic mice. The results showed that over-expression or treatment of RAP had no significant effect on the level of total APP. However, the levels of sAPP˜í‚â§, C99 and A˜í‚⧠were decreased and the production of C83 was significantly increased in RAP-transfected cells. The results suggest that RAP could decrease A˜í‚⧠production by decreasing ˜í‚â§-cleavage. Therefore, a decreased level of RAP in the brain could contribute to the pathogenesis of AD. Attempts to determine if A˜í‚⧠can bind with RAP in the human CSF were not successful; the preliminary results, however, suggested that Clusterin/CLU is the main A˜í‚â§-binding protein in the human CSF. Taken together, these data suggest that RAP is a protective factor against Alzheimer's disease.