Phagocytosis mediated by scavenger receptor class BI promotes macrophage transition during skeletal muscle regeneration [Immunology]

August 28th, 2019 by Jing Zhang, Chao Qu, Taotoa Li, Wei Cui, Xiaonan Wang, Jie Du

Macrophages play an essential role in skeletal muscle regeneration. The phagocytosis of muscle cell debris induces a switch of pro-inflammatory macrophages into an anti-inflammatory phenotype, but the cellular receptors mediating this phagocytosis are still unclear. In this paper, we reported novel roles for SRB1 (scavenger receptor class BI) in regulating macrophage phagocytosis and macrophage phenotypic transitions for skeletal muscle regeneration. In a mouse model of cardiotoxin-induced muscle injury/regeneration, infiltrated macrophages expressed high level of SRB1. Using SRB1 knockout mice, we observed muscle regeneration was impaired along with decreased myogenin expression and increased matrix deposit. Bone marrow transplantation experiments indicated that SRB1 deficiency in bone marrow cells was responsible for impaired muscle regeneration. Compared with WT mice, SRB1 deficiency increased pro-inflammatory macrophage number and pro-inflammatory gene expression, and decreased anti-inflammatory macrophage number and anti-inflammatory gene expression in injured muscle. In vitro, SRB1 deficiency led to the strong decrease in macrophage phagocytic activity on myoblast debris. SRB1 deficiency macrophages easily acquired an M1 phenotype and failed to acquire an M2 phenotype in LPS/myoblast debris activation. Furthermore, SRB1 deficiency promoted activation of ERK1/2 MAPK signaling in macrophages stimulated with LPS/myoblast debris. Taken together, SRB1 in macrophages regulates phagocytosis and promotes M1 switch into M2 macrophages, contributing to muscle regeneration.