The Excess of Pro-Inflammatory Macrophages in Aged Sarcopenic Muscle
In innate immune cells known as macrophages are resident in tissues throughout the body in significant numbers. In the brain very similar cells called microglia are found. Macrophages (and microglia) are capable of undertaking a range of very different roles. On the one hand they can participate in the immune response, drive inflammatory signaling, and attack and destroy pathogens and cancerous, senescent, or otherwise errant cells. On the other hand they can be involved in dampening inflammation, clearing molecular waste and the debris of dead cells, coordinating regeneration from injury, and assisting other cell types in the growth and replacement needed to maintain functional tissues.
Which tasks a macrophage undertakes is reflected by its polarization and surface markers. M1 macrophages are pro-inflammatory and destructive, while M2 macrophages are anti-inflammatory helpers that assist in regeneration and tissue maintenance. While the reality of macrophage state is considerably less binary than this model, it is a helpful framework for the ways in which macrophages can be both help and hindrance.
Aging is characterized by a growing degree of chronic, unresolved inflammatory signaling. Culprits include the secreted cytokines of lingering senescent cells, but also a maladaptive reaction on the part of innate immune cells such as macrophages to the many forms of molecular damage that occur in cells and tissues with advancing age. Whether misplaced mitochondrial DNA fragments, misfolded protein aggregates, or the debris of dead cells, this all makes a contribution. Greater inflammation means more macrophages shifted into the M1 state, and that in turn has consequences when it comes to the maintenance of tissues.
Age-related decline in skeletal muscle mass and function is a multifactorial phenomenon, characterized by the loss of muscle fibers and the atrophy of remaining fibers. Previous studies have highlighted the role of immune function changes in aging muscle, but the specific alterations in the immune microenvironment of skeletal muscle with age have yet to be fully understood. In this study, we utilized single nucleus RNA-seq analysis in combination with bulk RNA-seq to investigate the differences in cell composition and immune microenvironment between young and aged skeletal muscle tissue. Since macrophages were found to be the predominant immune cell population, we further identified a specific marker gene LYVE1 for macrophages and clustered them into subgroups. Additionally, we explored changes in cell-cell interactions between aged and young muscle. Furthermore, by analyzing bulk RNA-seq data, we examined the gene expression signature, functional differences, and infiltration characteristics of the young, aged, and sarcopenia groups, as well as the LYVE1-high and LYVE1-low groups.
Our analysis revealed that the sarcopenia group exhibited upregulation of several immune-related pathways, such as JAK-STAT, ERBB, and IL-2/IL-4 signaling pathways, in comparison to young controls. Previous studies have also linked these pro-inflammatory pathways to age-related muscle atrophy. We also discovered a crosstalk between immune cells and various non-immune cells within the muscle microenvironment. The results indicated that immune cells received signals from fibroblasts, endothelial cells, and other cell types, underscoring the significance of immune cells in the skeletal muscle microenvironment. Macrophages were found to be the predominant immune cell type in damaged muscles and played a crucial role in both the inflammation process and its resolution, contributing significantly to muscle repair. In response to anti-inflammatory cytokines, M1 macrophages transform into M2 macrophages in the later stages of skeletal muscle injury repair. Subsequently, these M2 macrophages along with resident M2 macrophages, facilitate the repair and regeneration processes by enhancing myoblast differentiation and vascularization, and stimulating fibro-adipogenic progenitor (FAP) cells to generate extracellular matrix. But macrophages in aged muscle are inclined to be polarized to a proinflammatory-phenotype, thereby negatively impacting the repair and regeneration capabilities of damaged muscle.