Intravital microscopy of satellite cell dynamics and their interaction with myeloid cells during skeletal muscle regeneration

Skeletal muscle regeneration requires the highly coordinated cooperation of muscle satellite cells (MuSCs) with other cellular components. However, due to technical limitations, it remains unclear how MuSCs dynamically interact with non-myogenic cells, especially myeloid cells, in live animals. In this work, we developed a dual-laser multimodal nonlinear optical microscope platform to serve as an effective tool for studying the real-time interaction between MuSCs and non-myogenic cells during the early phase of muscle regeneration. Increased cell volume and mitochondrial mass, cell density, and myotube formation are indicative of MuSCs activation/growth, proliferation, and differentiation, respectively. Using 3D time-lapse imaging on live reporter mice containing both red fluorescence protein (RFP)-labeled macrophages and yellow fluorescence protein (YFP)-labeled MuSCs, and taking advantages of the autofluorescence of reduced nicotinamide adenine dinucleotide (NADH), we monitored the real-time spatiotemporal interaction between RFP+ macrophages/RFP- non-myogenic cells and YFP+ muscle stem/progenitor cells during the activation and the proliferation stages of regeneration. Our results indicated that their cell-cell contact was transient in nature. By inhibiting macrophage infiltration, we further showed that direct cell-cell contact between macrophages and MuSCs was not required for early activation of MuSCs before the proliferation stage. However, decreased macrophage infiltration impeded the proliferation and differentiation of MuSCs and also led to intramuscular fibrosis. Besides, neutrophil depletion in the CCR2 deficient mice did not delay the initial growth of MuSCs. These findings provide a new perspective on myeloid cells’ role during muscle regeneration. ### Competing Interest Statement The authors have declared no competing interest.