Title: Investigation of the biochemistry and function of neutrophil serine protease 4 (NSP4)
Abstract: Serine proteases in cytoplasmic granules of neutrophils (NSPs), namely neutrophil elastase, cathepsin G (CG) and proteinase 3, have been under intense investigation for several decades. They are mainly known for their role in intracellular killing of pathogens and are also increasingly recognized as key regulators of innate immune responses. In 2009, I identified a fourth serine protease in neutrophils that has been completely overlooked and neglected so far. The aim of this thesis was an in-depth biochemical and functional characterization of this novel serine protease 4 (NSP4) of human neutrophils.
Using monoclonal antibodies to NSP4, the distribution of NSP4 in normal human tissues was studied. NSP4 was observed only in neutrophils and neutrophil precursors of the bone marrow. The content of NSP4 in neutrophil lysates was about 20-fold lower compared to CG. Nevertheless, NSP4 was found to be released into the supernatant upon neutrophil activation. NSP4 could be further identified as a novel azurophil granule protein of neutrophils by Western blot analyses of subcellular fractions. For the functional analysis, the production and yield of recombinant NSP4 was clearly improved using different expression systems and DNA construct modifications. The proteolytic specificity was analyzed using E. coli peptide libraries, mass spectrometry and several synthetic peptide libraries. All these analyses clearly revealed an arginine specificity for NSP4. Consistent with this, NSP4 was strongly inhibited by heparin-accelerated antithrombin and C1 inhibitor and, with lower efficacy, by α1-proteinase inhibitor (α1PI). The data allowed me to generate an NSP4-specific α1PI variant that was shown to form covalent complexes with all NSP4 of neutrophil lysates and supernatants of activated neutrophils. This finding strongly indicated that NSP4 is fully processed and stored as an already activated enzyme in azurophil granules. In addition, dipeptidyl peptidase I (DPPI) was identified as the activator of NSP4 in vivo, as DPPI deficiency resulted in complete absence of NSP4 in a Papillon-Lefevre patient. Analysis of cell-based calcium assays revealed that proteinase-activated receptor-2 may represent a potential natural substrate of NSP4. So far, NSP4-deficient mice did not show an abnormal phenotype under clean housing conditions. Activation of isolated neutrophils by phorbol esters or immune complexes was also not impaired. This study establishes NSP4 as the only arginine-specific pre-activated serine protease stored in azurophil granules of neutrophils that may fullfil a quite distinct, supportive role in neutrophil responses to tissue damage and bacterial infections.
Publication Year: 2014
Publication Date: 2014-01-31
Language: en
Type: article
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