Different expression of calreticulin and immunoglobulin binding protein in Alzheimer's disease brain

Both calreticulin (CRT) and immunoglobulin binding protein (Bip) have a role in the folding and assembly of oligomeric membrane proteins in the endoplasmic reticulum (ER). Recent studies have demonstrated the generation of beta-amyloid protein (Abeta) 1-42, a key peptide for amyloid deposits, in the ER. We, therefore, examined the localization and expression of CRT, Bip and their mRNA by immunohistochemistry, Western blot, in situ hybridization and semiquantitative reverse transcription polymerase chain reaction (RT-PCR) in both neurologically normal and Alzheimer's disease (AD) brains. Two polyclonal anti-CRT antibodies gave similar positive staining of CRT in neurons and glia. In neuronal cells, the cytoplasm, nucleoli and their processes were positive for CRT. In glial cells, perinuclear staining was frequently seen and the processes of some glial cells were also stained. In AD, these antibodies stained clearly damaged neurons but the number and the intensity of positive cells were decreased compared to controls. Processes of microglial cells were markedly positive in the AD white matter. Western blots using an anti-CRT antibody showed significantly lower immunoreactive bands in AD than control brains. By in situ hybridization, the number of neurons which express the CRT mRNA was less in AD than in controls. Using RT-PCR, the relative levels of the CRT mRNA in AD brains were also found to be significantly lower than those in controls. On the other hand, the number of Bip-positive cell, the production of Bip and the expression of mRNA for Bip did not differ between control and AD brains. These results suggest that CRT may be a multifunctional protein in human brain, and that the weak expression of CRT and the positive staining of microglial processes in AD brain may be part of the pathological processes in AD.