In this report we focus on the characterization of
appican, the
chondroitin sulfate proteoglycan form of
amyloid precursor
protein (APP), and the role that it and other
proteoglycans may play in AD.
Appican is expressed by certain transformed cell lines of neural origin, namely C6 cells and N2a
neuroblastomas. It is detected in both human and rat brain and in primary cultures is expressed by astrocytes, but not neurons. The core
protein of
appican has been shown to be an alternatively spliced
isoform of APP, lacking exon 15 of the APP gene, originally identified in leukocytes (L-APP). Splicing out of exon 15 results in the joining of exons 14 and 16, and formation of an Asp-Xaa-Ser-Gly consensus sequence for
chondroitin sulfate chain attachment to
serine 619 of L-APP, which lies 16
amino acids upstream of the A beta
peptide sequence. Mutation of this
serine residue to an
alanine prevented
chondroitin sulfate chain addition to the core
protein. Levels of
appican expression could be regulated by growth conditions independently of APP, suggesting that these molecules may serve distinct physiological roles within the cell. Morphological changes were also observed in both astrocytic and transformed cell cultures, that appeared to reflect changes in levels of
appican expression. Preliminary data suggest that
appican may be a strong
cell adhesion molecule. Transfected C6
glioma cells overexpressing
appican remained attached to tissue culture dishes markedly better than either C6 cells over-expressing exon-15 containing APP or WT C6 cells.
Appican-enriched extracellular matrix (ECM) was also observed to serve as a much better substrate for attachment of N2a
neuroblastomas, pheocromocytoma PC12 cells and primary astrocytes compared to APP enriched ECM.