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Acute skeletal muscle injury: CCL2 expression by both monocytes and injured muscle is required for repair

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TLDR
It is concluded that CCL2 is a major ligand of CCR2 to recruit MOs/MPs into injured muscles to conduct phagocytosis and produce IGF‐1 for injury repair.
Abstract
CC chemokine ligand 2 (CCL2), a ligand of CC chemokine receptor 2 (CCR2), is essential to mount an adequate inflammatory response to repair acute skeletal muscle injury. We studied the mechanisms by which CCL2 regulates muscle inflammation and regeneration. Mobilization of monocytes/macrophages (MOs/MPs) but not lymphocytes or neutrophils was impaired from bone marrow to blood and from blood to injured muscles in Ccl2(-/-) mice. This was accompanied by poor phagocytosis, reduced up-regulation of insulin-like growth factor-1 (IGF-1), and impaired muscle regeneration. Bone marrow transfer from wild-type mice to irradiated Ccr2(-/-) but not Ccl2(-/-) mice restored muscle inflammation. Intravenously injected CCL2-deficient bone marrow monocytes could not enter wild-type injured muscles as well as wild-type bone marrow monocytes. Intravenously injected wild-type bone marrow monocytes could not enter CCL2-deficient injured muscles as well as wild-type injured muscles. CCL2 stimulated IGF-1 expression by wild-type but not CCR2-deficient intramuscular macrophages. A single intramuscular injection of IGF-1, but not PBS, markedly improved muscle regeneration in Ccl2(-/-) mice. We conclude that CCL2 is a major ligand of CCR2 to recruit MOs/MPs into injured muscles to conduct phagocytosis and produce IGF-1 for injury repair. CCL2 needs to be expressed by bone marrow cells, circulating monocytes, and injured muscle tissue cells to recruit MOs/MPs into injured muscles. CCL2/CCR2 signaling also up-regulates IGF-1 expression by intramuscular macrophages to promote acute skeletal muscle injury repair.

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Muscle damage and inflammation during recovery from exercise.

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References
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Journal ArticleDOI

Monocyte and macrophage heterogeneity

TL;DR: Recent studies have shown that monocyte heterogeneity is conserved in humans and mice, allowing dissection of its functional relevance: the different monocyte subsets seem to reflect developmental stages with distinct physiological roles, such as recruitment to inflammatory lesions or entry to normal tissues.
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Blood Monocytes Consist of Two Principal Subsets with Distinct Migratory Properties

TL;DR: Using a murine adoptive transfer system to probe monocyte homing and differentiation in vivo, two functional subsets among murine blood monocytes are identified: a short-lived CX(3)CR1(lo)CCR2(+)Gr1(+) subset that is actively recruited to inflamed tissues and a CX (3) CR1(hi)CCS1-dependent recruitment to noninflamed tissues.
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Mice carrying null mutations of the genes encoding insulin-like growth factor I (Igf-1) and type 1 IGF receptor (Igf1r)

TL;DR: In addition to generalized organ hypoplasia in Igf1r(-/-) embryos, including the muscles, and developmental delays in ossification, deviations from normalcy were observed in the central nervous system and epidermis.
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The Many Roles of Chemokines and Chemokine Receptors in Inflammation

TL;DR: The properties of chemokines and their receptors are discussed and the roles of these chemoattractants in selected clinical disorders are highlighted.
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Decreased lesion formation in CCR2 −/− mice reveals a role for chemokines in the initiation of atherosclerosis

TL;DR: A role for MCP-1 in the development of early atherosclerotic lesions is revealed and upregulation of this chemokine by minimally oxidized lipids is an important link between hyperlipidaemia and fatty streak formation.
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