Immunohistochemical characterization of γ-secretase activating protein expression in Alzheimer's disease brains

A recent study showed that γ-secretase activating protein (GSAP), derived from a C-terminal fragment of pigeon homolog (PION), increases amyloid-β (Aβ) production by interacting with presenilin-1 (PS1) and the β-secretase-cleaved C-terminal fragment of amyloid precursor protein (APP-CTF). In the study, knockdown of GSAP reduces production of Aβ and plaque formation in the brain of APPswe and PS1ΔE9 double transgenic mice without affecting the Notch-dependent pathway. Therefore, GSAP is an ideal target for designing γ-secretase modulators with least side effects in Alzheimer's disease (AD). However, at present, the precise distribution of GSAP in AD brains remains to be characterized.

By immunohistochemistry, we studied GSAP expression in the frontal cortex and the hippocampus of 11 aged AD and 17 age-matched control cases.

GSAP immunoreactivity exhibited distinct morphological features, such as fine granular cytoplasmic deposits, dense nodular and patchy deposits, beads and string-like deposits, and diffuse dot-like deposits. In both AD and control brains, a fairly small subset of cerebral cortical and hippocampal neurones expressed fine granular cytoplasmic deposits, while diffuse dot-like deposits were more frequently found in the neuropil and neuronal processes, particularly enriched in the hippocampal CA2 and CA3 regions. Among GSAP-immunoreactive deposits, dense nodular and patchy deposits, located in the neuropil and closely associated with PS1 expression and Aβ deposition, indicated the most distinguishing features of AD pathology.

Aberrant regulation of GSAP expression plays a key role in acceleration of γ-cleavage of APP-CTF and accumulation of Aβ in AD brains.