Background MRP8 and MRP14 are phagocyte-derived Damage Associated Molecular Pattern (DAMP) proteins and can be used as biomarkers in inflammatory diseases e.g. juvenile idiopathic arthritis. We found that the MRP8/14 complex (calprotectin) is highly elevated in the serum of patients with pyogenic arthritis, pyoderma gangrenosum, and acne (PAPA) syndrome and is even significantly higher in hypercalprotectinaemia and hyperzincaemia (Hz/Hc) . For PAPA, mutations in the proline serine threonine phosphatase-interacting protein 1 (PSTPIP1) gene are described . We have recently identified novel autosomal dominant mutations in PSTPIP1 Hz/Hc patients. All are heterozygous carrier of an E250K or E257K substitution encoded by exon 11 of the PSTPIP1 gene. These patients show an excessively high serum concentration (0.9-12.0 g/l, normal range <0.001 g/l) of MRP8/14 not present in any other inflammatory condition. The mechanism of secretion of MRP8/14 is not clear but depends on a tubulin-mediated alternative secretory pathway. MRP8/14 induces pro-inflammatory effects through interaction with Toll-like receptor 4 dependent signalling pathways .
Objectives The molecular link between PSTPIP1 mutations and elevated MRP8/14 concentrations is currently unknown. Therefore we investigated the role of PSTPIP1 during release of MRP8/14.
Methods MRP8/14 serum concentrations of patients were determined by ELISA. Monocytes from patients were isolated and MRP8/14 levels were measured in culture supernatants prior and after activation. Intracellular distribution of E250K and wildtype PSTPIP1 was analysed in transfected cells by immunofluorescence and interactions between PSTPIP1, MRP8/14 and microtubules were characterized via immunoprecipitations or microtubule binding assays. Protein interactions were further quantified in vitro by using a modified MRP8/14-ELISA with different constructs of PSTPIP1.
Results Monocytes from patients with PSTPIP1 mutations release significantly higher amounts of MRP8/14 than control cells. A co-localization of PSTPIP1 and MRP8/14 could be shown in monocytes and in vitro studies confirm this interaction as a calcium dependent and direct binding. By using deletion constructs of PSTPIP1 we could demonstrate that the MRP8/14 binding motif is mutated in PAPA and Hz/Hc. A mutual interference of PSTPIP1 and MRP8/14 on their interaction with microtubules could be shown which is altered by using the E250K mutated PSTPIP1.
Conclusions Phagocytes seem to be the responsible cell type for the high serum concentration of MRP8/14 in Hz/Hc and PAPA. The alarmin MRP8/14 interacts directly with PSTPIP1 in a calcium-dependent manner and mutations found in all patients are apparently located inside the PSTPIP1-MRP8/14 binding region. Interaction of these proteins seem to have a regulatory function on their tubulin binding capability and could be of important relevance for the tubulin-dependent secretion of MRP8/14 which may be a pathogenetic mechanism of MRP-driven inflammation in PAMI.
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Disclosure of Interest None declared