46PThe endothelial DLL4-muscular NOTCH2 axis drives muscle atrophy in dermatomyositis: insights from single-nucleus transcriptomics

Muscle atrophy, a debilitating feature of neuromuscular disorders, remains mechanistically unresolved. Recent breakthroughs in murine models identified the endothelial-derived NOTCH ligand DLL4 as a critical activator of muscular NOTCH2, triggering atrophy in disuse and diabetes. However, its role in human muscle diseases is unexplored. This study investigates the DLL4–NOTCH2 axis in autoimmune and hereditary muscle diseases, focusing on its therapeutic potential. Single-nucleus RNA sequencing (snRNA-seq) was performed on frozen muscle biopsies from 34 patients: autoimmune myositis (inclusion body myositis (IBM: n=2), immune-mediated necrotizing myopathy (IMNM: n=4), anti-synthetase syndrome (ASS: n=4), and dermatomyositis (DM: n=6), hereditary muscle diseases (n=16), ALS (n=2), and non-disease controls (n=4). DLL4 and NOTCH2 expression was validated via immunofluorescence and immunoelectron microscopy. Proximity ligation assay (PLA) assessed direct interactions. snRNA-seq revealed that DLL4 was exclusively expressed in endothelial cells (ECs), with marked upregulation in DM versus other groups. NOTCH2 was highly expressed in the affected myonuclei in several muscle diseases, including DM. NOTCH2 signal was localized on the sarcolemma, and DLL4 positive ECs densely surrounded small-diameter fibers in DM. In situ PLA revealed that direct DLL4-NOTCH2 interactions occur in DM. Our results indicate that the endothelial DLL4–muscular Notch2 axis is a potential driver of muscle atrophy in DM. Targeting this pathway could mitigate muscle wasting, offering a novel therapeutic strategy for myositis and related disorders.