INCREASED PRODUCTION OF AMYLOID PRECURSOR PROTEIN PROVIDES A SUBSTRATE FOR CASPASE-3 IN DYING MOTONEURONS

Biochemical and molecular mechanisms of neuronal cell death are curren tly an area of intense research. It is well documented that the lumbar spinal motoneurons of the chick embryo undergo a period of naturally occurring programmed cell death (PCD) requiring new gene expression an d activation of caspases. To identify genes that exhibit changed expre ssion levels in dying motoneurons, we used a PCR-based subtractive hyb ridization protocol to identify messages uniquely expressed in motoneu rons deprived of trophic support as compared with their healthy counte rparts. We report that one upregulated message in developing motoneuro ns undergoing cell death is the mRNA for amyloid precursor protein (AP P). Increased levels of APP and beta-amyloid protein are also detected within dying motoneurons. The predicted peptide sequence of APP indic ates two potential cleavage sites for caspase-3 (CPP-32), a caspase ac tivated in dying motoneurons. When peptide inhibitors of caspase-3 are administered to motoneurons destined to undergo PCD, decreased levels of APP protein and greatly reduced p-amyloid production are observed. Furthermore, we show that APP is cleaved by caspase-3. Our results su ggest that differential gene expression results in increased levels of APP, providing a potential substrate for one of the cell death-activa ted caspases that may ultimately cause the demise of the cell. These r esults, combined with information on the toxic role of APP and its pro teolytic by-product beta-amyloid, in the neurodegenerative disease Alz heimer's, suggest that events of developmental PCD may be reactivated in early stages of pathological neurodegeneration.