Monocytic elastase-mediated apolipoprotein-E degradation: Potential involvement of microglial elastase-like proteases in apolipoprotein-E proteolysis in brains with Alzheimers disease

• The key protease for ApoE degradation was studied in monocytes and microglia.
• High-salt extracts from monocytes and microglia exhibit protease activity to ApoE.
• Elastase-like protease was partially purified in high salt extracts of monocytes.
• Elastase inhibitor or anti-elastase moAb inhibits ApoE degradation by the extracts.
• Microglia surrounding Aβ plaque express elastase in Alzheimer's disease brain.

Impaired clearance of soluble Aβ (amyloid-β) promotes Aβ aggregation in brains with Alzheimer's disease (AD), while apolipoprotein-E (ApoE) in microglia mediates Aβ clearance. We studied the protease responsible for ApoE4 degradation in human peripheral monocyte extracts, which are from the same lineage as microglia. We detected the hydrolytic activity for ApoE4 in high-salt extracts with 2 M NaCl and found that the activity was inhibited by a serine protease inhibitor and an elastase-specific inhibitor, but not by other protease inhibitors. The extracts exhibited higher activity for the elastase substrate, and we followed the activity with ion-exchange and gel-filtration chromatography. Through silver staining, we partially purified a protein of 28 kDa, which was clarified as elastase by liquid chromatography–tandem mass spectrometry. These observations suggest that elastase is the key protease for ApoE4 degradation. We also detected ApoE4 hydrolytic activity in high-salt extracts in mouse microglial (BV-2) cell lysates, and showed that the ApoE4 fragments by the BV-2 extracts differed from the fragments by the monocyte extracts. Though the ApoE4 degradation by the extracts was not inhibited with elastase-specific inhibitors, it was inhibited by an elastase-specific monoclonal antibody, suggesting that elastase-like proteases in microglia differ from those of monocytes. Immunohistochemistry revealed that both elastase and ApoE were expressed in the senile plaques of brains with AD. In vitro studies also disclosed the localization of elastase in the microglial cell line, BV-2. Our results suggest that elastase-like proteases in the microglial cells surrounding Aβ plaques are responsible for ApoE degradation in the brain.