Immunohistochemical localization of the proteinase inhibitor region of amyloid precursor proteins in the neocortex of Alzheimer's disease and aged controls

The immunohistochemical localization of the proteinase inhibitor region of amyloid protein precursors (APPI) in the postmortem human neocortex was studied using a polyclonal antibody raised against a purified recombinant human APPI derivative produced by COS-1 cells. APPI-like immunoreactivity (APPI-LI) was found diffusely in the human neocortex. APPI-LI appeared as irregularly shaped granular structures. The size of the APPI-LI structures was 1-4 μm in diameter. APPI-LI usually formed a cluster of 10- to 20-μm diameter in the cortical gray matter and 20- to 40-μm diameter in the subcortical white matter. Double staining for APPI and glial fibrillary acidic protein indicated that APPI-LI in the white matter and molecular layer was localized exclusively in the fibrillary astrocytes. In contrast, APPI-LI was found in neurons as well as in the fibrillary astrocytes in layers II through to VI. Under fluorescence microscopy, APPI-LI in both neurons and fibrillary astrocytes were found in close association with lipofuscin. The present observations indicate that APPI is localized in neurons and astrocytes in the human neocortex and that APPI may be associated with lipofuscin or lysosome in the human neocortex.