Altered Macrophage Phenotype Transition Impairs Skeletal Muscle Regeneration

Abstract

Monocyte/macrophage polarization in skeletal muscle regeneration is ill defined. We used CD11b-diphtheria toxin receptor transgenic mice to transiently deplete monocytes/macrophages at multiple stages before and after muscle injury induced by cardiotoxin. Fat accumulation within regenerated muscle was maximal when ablation occurred at the same time as cardiotoxin-induced injury. Early ablation (day 1 after cardiotoxin) resulted in the smallest regenerated myofiber size together with increased residual necrotic myofibers and fat accumulation. However, muscle regeneration after late (day 4) ablation was similar to controls. Levels of inflammatory cells in injured muscle following early ablation and associated with impaired muscle regeneration were determined by flow cytometry. Delayed, but exaggerated, monocyte [CD11b(+)(CD90/B220/CD49b/NK1.1/Ly6G)(-)(F4/80/I-Ab/CD11c)(-)Ly6C(+/-)] accumulation occurred; interestingly, Ly6C(+) and Ly6C(-) monocytes were present concurrently in ablated animals and control mice. In addition to monocytes, proinflammatory, Ly6C(+) macrophage accumulation following early ablation was delayed compared to controls. In both groups, CD11b(+)F4/80(+) cells exhibited minimal expression of the M2 markers CD206 and CD301. Nevertheless, early ablation delayed and decreased the transient accumulation of CD11b(+)F4/80(+)Ly6C(-)CD301(-) macrophages; in control animals, the later tissue accumulation of these cells appeared to correspond to that of anti-inflammatory macrophages, determined by cytokine production and arginase activity. In summary, impairments in muscle regeneration were associated with exaggerated monocyte recruitment and reduced Ly6C(-) macrophages; the switch of macrophage/monocyte subsets is critical to muscle regeneration.

Regulation of inflammation and fibrosis by macrophages in lymphedema

“…We used the CD11b-DTR mice as our model system to enable systemic, chronic macrophage depletion. These animals have been previously characterized and are transgenic animals that express the simian DTR gene under the regulation of the CD11b promoter, a gene that is highly expressed on macrophages (6,31). Because the primate DTR is 100,000 times more avid at binding DT than the murine equivalent, administration of minute doses of the toxin results in targeted cell death and systemic depletion of CD11b-expressing cells.…”

Caveolin-1 deletion exacerbates cardiac interstitial fibrosis by promoting M2 macrophage activation in mice after myocardial infarction

“…Arginase activity was quantified by measuring the conversion of l -arginine into urea as previously reported [19]. In brief, 10 mM MnCl 2 in 50 mM Tris–HCl was added to the lysed sample and the mixture was then incubated at 56 °C to activate the arginase enzyme.…”

Highly Dynamic Transcriptional Signature of Distinct Macrophage Subsets during Sterile Inflammation, Resolution, and Tissue Repair

“…This result is a clear departure from the widely held assumption that the Ly6C pos/neg status is the predominant defining feature of these macrophages with strong predictive power for their molecular signature/functions. A highly related issue is the polarization status of the Ly6C pos/neg subsets, which have been assumed to correspond, as in other tissues, to M1 and M2 macrophages, respectively (13,16,23). We show that the damage-associated Ly6C pos macrophages exhibited a specific inflammatory profile, independent of STAT1, which only partially overlapped with the broad M1 gene expression pattern and displayed a unique inflammatory signature during the early response to muscle injury (days 1-2), likely reporting sterile inflammation (see later).…”

“…Previous studies showed that Ly6C pos macrophages express higher levels of TNF-a and IL-1b and lower levels of TGF-b and IL-10 than Ly6C neg macrophages (12,16,23). Therefore, they were considered as proinflammatory and anti-inflammatory macrophages, respectively.…”