Skeletal muscle is paradigmatic of a regenerative tissue that repairs itself via the activation of a resident stem cell 1 . Termed the satellite cell, these normally quiescent cells are induced to proliferate by ill-defined wound-derived signals 2 . Identifying the source and nature of these proregenerative cues has been hampered by an inability to visualise the complex cellular interactions that occur within the wound environment. We therefore developed a zebrafish muscle injury model to systematically capture satellite cell interactions within the injury site, in real time, throughout the repair process. This analysis identified that a specific subset of macrophages 'dwells' within the injury, establishing a transient but obligate stem cell niche required for stem cell proliferation. Single cell profiling identified specific signals secreted from dwelling macrophages that include the cytokine, Nicotinamide phosphoribosyltransferase (NAMPT/Visfatin/PBEF). Here we show that NAMPT secretion from the macrophage niche is required for muscle regeneration, acting through the C-C motif chemokine receptor type 5 (CCR5) expressed on muscle stem cells. This analysis reveals that along with their well-described ability to modulate the pro-inflammatory and antiinflammatory phases of wound repair, specific macrophage populations also provide a transient stem cell-activating niche, directly supplying pro-proliferative cues that govern the timing and rate of muscle stem cell-mediated repair processes.
MainLive imaging of the collective cellular response to tissue injury remains a long-term goal of the regenerative medicine field. In an attempt to attain this goal we have developed zebrafish models of tissue injury where the optical accessibility of the larvae allows the application of noninvasive techniques to assay repair in real time. Here we apply this approach to regenerating skeletal muscle in order to determine the cellular and molecular events that define regeneration in vivo. Transgenic zebrafish reporter lines fluorescently tagging wound-present cellular components were subject to acute injury, enabling the location and response dynamics of individual woundoccupying cells to be correlated to the stem cell compartment during muscle repair. We developed two injury paradigms: a focal laser ablation model that severs 2-6 fibres per injury and a needle stab
The mechanisms of coordinated changes in proteome composition and their relevance for the differentiation of neutrophil granulocytes are not well studied. Here, we discover two novel human genetic defects in SRPRA and SRP19, constituents of the mammalian co-translational targeting machinery and characterize their role in neutrophil granulocyte differentiation. We systematically study the proteome of neutrophil granulocytes from patients with variants in the signal recognition particle (SRP) genes, HAX1, and ELANE and identify global as well as specific proteome aberrations. Using in vitro differentiation of human induced pluripotent stem cells and in vivo zebrafish models, we study the effects of SRP-deficiency on neutrophil granulocyte development. In a heterologous cell-based inducible protein expression system, we validate the effects conferred by SRP dysfunction for selected proteins that we identified in our proteome screen. Thus, SRP-dependent protein processing, intracellular trafficking and homeostasis are critically important for the differentiation of neutrophil granulocytes.
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