02.28 The role of arginase I in osteoclasts

Background Osteoclasts are multi-nucleated, bone resorbing cells that derive from the monocyte-macrophage linage. They are further known as the main effector cells for development of age-related osteoporosis and are critically involved in the pathogenesis of rheumatic diseases. Arginase I is classically known as a marker for anti-inflammatory macrophage polarisation, but its importance for osteoclasts and their development has not been characterised. Our aim was therefore to investigate the importance of the enzyme in the context of osteoclastogenesis and osteoclast metabolism. Materials and methods We analysed osteoclastogenesis of C57BL/6J wildtype bone marrow cells in vitro in the presence and absence of recombinant Arginase 1 (recARG1). This approach was complemented via qPCR analysis of relevant osteoclast marker genes. We further investigated the effect of recARG1 regarding an in vivo model called serum transfer arthritis, where we treated C57BL/6J wildtype mice with the recombinant enzyme. Disease severity was then assessed using clinical scores and paw histology. Results We observed that Arginase 1 was downregulated from the progression of a precursor to a mature osteoclast. When we incubated osteoclast precursors stimulated with RANKL with recARG1, we completely inhibited the generation of osteoclasts. In line, recARG1 could also inhibit the mRNA expression of RANKL-induced osteoclastogenic marker genes such as NFATc1, Cathepsin K and others. Of note, recARG1 did not affect M-CSF-dependent generation of osteoclast precursors. In an in vivo model known as serum transfer arthritis, mice with recARG1 showed reduced signs of disease, assessed by clinical scores (mouse weight, paw swelling and grip strength). This finding was complemented by scoring of paw histological slides. Although synovial inflammation was only numerically reduced, treatment with recARG1 lead to a significant decrease in the numbers of osteoclasts in the inflamed paws. Conclusions We propose that the activity of the enzyme Arginase I is critical for the development of osteoclasts from myeloid precursors and hypothesise that administration or recombinant Arginase I might influence development and severity of osteoclast driven diseases.