γ-Secretase mediated proteolytic processing of the triggering receptor expressed on myeloid cells-2

Abstract

Alzheimer's disease (AD) is a progressive neurodegenerative disorder, affecting millions of people worldwide. AD is histopathologically characterized by the appearance of neurofibrillary tangles, which are intraneuronal accumulations of hyperphosphorylated tau protein, and extracellular β-amyloid plaques. β-Amyloid plaques arise from progressive accumulation of Aβ, a small hydrophobic peptide, in the brain. Aβ derives from sequential proteolytic processing of the amyloid precursor protein (APP). The final processing step in the generation of Aβ is catalyzed by the γ-secretase complex, consisting of presenilin-1 (PS1), the active subunit, nicastrin, Aph-1 (anterior pharynx defective-1) and PEN-2 (presenilin enhancer element-2). Besides APP, the γ- secretase has several other substrates and is also involved in the endocytosis of membrane bound lipoprotein receptors.<br /> In addition to neurofibrillary tangles and amyloid plaques, activation of microglia and inflammatory processes are also fundamental characteristics in the brain of AD patients. Activated microglia appear to play a dual role in AD. On one hand they produce pro-inflammatory cytokines, reactive oxygen species and nitric oxide, augmenting inflammatory processes and oxidative stress which might promote neuronal damage. On the other hand microglia can phagocytose Aβ, thereby contributing to its clearance from the brain. However, the accumulation of Aβ in AD brains indicates an insufficient clearance of Aβ in AD pathogenesis which is not yet understood. Since γ-secretase was previously linked to endocytosis, it might also be implicated in phagocytic processes by microglia and clearance of Aβ.<br /> Here it is shown by biochemical experiments in cell culture models, that the triggering receptor expressed on myeloid cells-2 (TREM2) represents a novel substrate for γ-secretase in microglia. Pharmacological inhibition of γ-secretase resulted in accumulation of a TREM2 C-terminal fragment. This fragment also accumulated upon expression of a dominant negative variant of PS1. Immunofluorescence and biotinylation experiments further indicated that the processing of TREM2 occurs at the plasma membrane. In addition, cell biological experiments demonstrated shedding of the TREM2 ectodomain. Thus, TREM2 follows the canonical proteolytic processing pathway of γ-secretase substrates which consists of an initial cleavage within the ectodomain followed by intramembranous cleavage of the resulting membrane-tethered CTF by γ-secretase. The usage of selective protease inhibitors also indicated the involvement of a metalloprotease, likely of the ADAM family, in TREM2 ectodomain shedding. TREM2 dependent signaling required the interaction with its co-receptor DAP12. Interestingly, co-immunoprecipitations revealed impaired interaction of TREM2 and DAP12 upon γ-secretase inhibition Moreover, the impaired interaction resulted in decreased phosphorylation of DAP12. Expression of different PS1 FAD mutants, led to decreased phagocytosis of Aβ. Thus, a partial loss of γ-secretase activity might decrease the capacity of microglia to clear Aβ.<br /> Taken together, these results indicate a critical function of γ-secretase in microglia and might help to understand molecular mechanisms underlying impaired Aβ clearance in the pathogenesis of AD.