Serpin=serine protease-like complexes within neurofilament conglomerates of motoneurons in amyotrophic lateral sclerosis

Abstract

Neurofilamentous conglomerates (NfCg), as axonal spheroids or conglomerates in motoneurons, are the histopathologic hallmarks for early stages of amyotrophic lateral sclerosis (ALS). We hypothesize that NfCg may be formed by post-translational modifications of altered Nf proteins that include: (1) hyperphosphorylation, (2) glycosylation (or glycoxidation), (3) nitration, (4) ubiquitination and/or (5) crosslinking by the Ca++-dependent transglutaminase (TGase). These, as well as other changes, are predicted to be initiated or accentuated by oxidative damage. The damaged Nf proteins then activate cascades of intracellular protein degradation which include ATP-dependent ubiquitin/proteasome proteolysis. Other proteolytic systems, either Ca++-dependent or independent, may also be activated, such as serine and cysteine protease systems. These enzymes, either lysosomal or non-lysosomal may also participate in the degradation of damaged Nf proteins being balanced by their cognate inhibitors. Protein complexes formed by these protease=inhibitor systems, along with damaged Nf proteins, may accumulate within the cell bodies as neuronal inclusions, since a number of intracellular inclusions are found in motor neurons in ALS. In the current study, we investigated the involvement of serine proteases and their serpins in NfCg formation. Pairs of three serine proteases (trypsin, chymotrypsin and thrombin) and their cognate serpins (alpha1-anti-trypsin, alpha1-anti-chymotrypsin, and protease nexin I) were probed in motoneurons with their antibodies for both NfCg and inclusions. Positive immunoreactivities for all serine proteases and their cognate serpins support the contention that the imbalance of serine proteases and internalized serpins may have a role in formation of NfCg and inclusions, and hence, the pathogenesis of ALS.