Myeloid Cell Responses to Contraction-induced Injury Differ in Muscles of Young and Old Mice

Area of Science:

• Muscle regeneration and aging research
• Immunology and cell biology
• Skeletal muscle physiology

Background:

• Myeloid cells are crucial for muscle repair after injury.
• Aging may impair the myeloid cell response, affecting muscle healing.
• Understanding age-related changes in myeloid cells is vital for muscle health.

Purpose of the Study:

• To investigate age-related differences in myeloid cell accumulation and function following muscle injury.
• To test the hypothesis that old mice exhibit elevated neutrophils and reduced/delayed macrophages in injured muscle.

Main Methods:

• Inducing muscle injury via lengthening contractions in young and old mice.
• Analyzing neutrophil (Gr-1+) and macrophage (CD68+, CD163+) content at 2 and 5 days post-injury.
• Measuring messenger RNA (mRNA) levels of macrophage-associated genes.

Main Results:

• Both young and old mice showed increased neutrophils and macrophages by Day 2.
• Neutrophils decreased by Day 5, while total macrophages increased, but were lower in old mice.
• M2 macrophages (CD163+) increased in both groups, with higher levels in old mice by Day 5.
• Age-related changes in macrophage gene expression suggest altered function in older mice.

Conclusions:

• Contrary to hypothesis, neutrophils did not persist, and M2 macrophages accumulated in old muscle.
• Total macrophage accumulation was lower in old mice after injury.
• Altered myeloid cell responses, particularly macrophage dynamics and gene expression, may contribute to impaired muscle regeneration in aging.