Macrophage PPARγ, a Lipid Activated Transcription Factor Controls the Growth Factor GDF3 and Skeletal Muscle Regeneration

Abstract

SUMMARY Tissue regeneration requires inflammatory and reparatory activity of macrophages. Macrophages detect and eliminate the damaged tissue and subsequently promote regeneration. This dichotomy requires the switch of effector functions of macrophages coordinated with other cell types inside the injured tissue. The gene regulatory events supporting the sensory and effector functions of macrophages involved in tissue repair are not well understood. Here we show that the lipid activated transcription factor, PPARγ, is required for proper skeletal muscle regeneration, acting in repair macrophages. PPARγ controls the expression of the transforming growth factor-β (TGF-β) family member, GDF3, which in turn regulates the restoration of skeletal muscle integrity by promoting muscle progenitor cell fusion. This work establishes PPARγ as a required metabolic sensor and transcriptional regulator of repair macrophages. Moreover, this work also establishes GDF3 as a secreted extrinsic effector protein acting on myoblasts and serving as an exclusively macrophage-derived regeneration factor in tissue repair.

In vivo GDF3 administration abrogates aging related muscle regeneration delay following acute sterile injury

“…We selected GDF3 for a proof‐of‐concept experiment to evaluate whether the observed impaired phenotypic transition in macrophage phenotype from inflammatory to repair type (Patsalos et al, 2017) can contribute to age‐related delay in muscle regeneration. In line with previous findings (Varga et al, 2016), GDF3 protein expression in whole‐muscle lysates of CTX‐injured young mice showed a pronounced induction, which peaked at Day 4 (Figure 2a), at the time when inflammation subsides, and regenerative processes start to dominate within the injured muscle. In controlled in vitro conditions, addition of recombinant GDF3 (using either an in‐house recombinant protein or a commercially available one) in primary myoblasts induced a robust effect in myotube formation (Figure 2b upper panel) and a pronounced increase in their fusion index (Figure 2b lower panel).…”

“…Several members of the TGFβ family (Egerman et al, 2015) are known regulators of muscle regeneration, whose members are secreted by repair macrophages acting in a paracrine manner (Massague, Cheifetz, Endo, & Nadal‐Ginard, 1986; McPherron, Lawler, & Lee, 1997), including GDF3 (Varga et al, 2016). We selected GDF3 for a proof‐of‐concept experiment to evaluate whether the observed impaired phenotypic transition in macrophage phenotype from inflammatory to repair type (Patsalos et al, 2017) can contribute to age‐related delay in muscle regeneration.…”

Metabolic regulation of macrophages during tissue repair: insights from skeletal muscle regeneration

“…As for AMPK, we used LysM‐Cre;PPARγ fl/fl mice to address the role of macrophagic PPARγ during muscle regeneration in collaboration with L. Nagy's laboratory (Debrecen University, Hungary and Sanford Burnham Prebys Discovery, Orlando, FL, USA). Mice deficient for PPARγ in macrophages, as well as wild‐type mice transplanted with PPARγ‐KO bone marrow, show impaired muscle regeneration with lasting inflammation and strongly reduced size of the new myofibers, even several weeks after injury . However, on the contrary to what is observed for macrophages deficient for AMPK, there is no alteration of the kinetics of macrophage phenotype during muscle regeneration in vivo in LysM‐Cre;PPARγ fl/fl mice.…”

“…However, Ly6C neg macrophages do not express canonical M2 (IL-4) markers and their expression profile does not match with any of the in vitro 28 activation status tested so far [51]. Moreover, muscle regeneration is not altered in animals deficient for STAT6, the master regulator of IL-4 signaling [62]. In conclusion, kinetic analysis reveals a highly dynamic nature of gene expression of macrophages, identifying important metabolism changes associated with the key steps during muscle regeneration (Fig.…”

PPAR-γ in innate and adaptive lung immunity

“…Indeed, LysM‐cre/Pparg lox/lox mice lacking PPAR‐γ specifically in myeloid cells develop insulin resistance . M2 macrophages are also involved in tissue repair and PPAR‐γ in skin macrophages was found to regulate wound healing by mediating phagocytosis of apoptotic cells and repair of skeletal muscle following injury . Data suggesting an important role of M2 macrophages in browning of white adipose tissue and thermogenesis are controversial .…”