Interrelation of inflammation and APP in sIBM: IL-1β induces accumulation of β-amyloid in skeletal muscle

Distinct interrelationships between inflammation and β-amyloid-associated degeneration, the two major hallmarks of the skeletal muscle pathology in sporadic inclusion body myositis (sIBM), have remained elusive. Expression of markers relevant for these pathomechanisms were analysed in biopsies of sIBM, polymyositis (PM), dermatomyositis (DM), dystrophic and non-myopathic muscle as controls, and cultured human myotubes. By quantitative PCR, a higher upregulation was noted for the mRNA-expression of CXCL-9, CCL-3, CCL-4, IFN-γ, TNF-α and IL-1β in sIBM muscle compared to PM, DM and controls. All inflammatory myopathies displayed overexpression of degeneration-associated markers, yet only in sIBM, expression of the mRNA of amyloid precursor protein (APP) significantly and consistently correlated with inflammation in the muscle and mRNA-levels of chemokines and IFN-γ. Only in sIBM, immunohistochemical analysis revealed that inflammatory mediators including IL-1β co-localized to β-amyloid depositions within myofibres. In human myotubes, exposure to IL-1β caused upregulation of APP with subsequent intracellular aggregation of β-amyloid. Our data suggest that, in sIBM muscle, production of high amounts of pro-inflammatory mediators specifically induces β-amyloid-associated degeneration. The observations may help to design targeted treatment strategies for chronic inflammatory disorders of the skeletal muscle.