Myelin and non-myelin debris contribute to foamy macrophage formation after spinal cord injury

• Published in: Neurobiology of disease Vol. 163; p. 105608
• Main Authors: Ryan, Christine B., Choi, James S., Al-Ali, Hassan, Lee, Jae K.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01-02-2022; Elsevier
• Subjects: Animals; Cytokines - metabolism; Disease Models, Animal; Female; Inflammation; Inflammation - metabolism; Inflammation - pathology; Lipids; Macrophage Activation - physiology; Macrophages; Macrophages - metabolism; Macrophages - pathology; Male; Mice; Myelin Sheath - metabolism; Myelin Sheath - pathology; Myeloid; Spinal Cord - metabolism; Spinal Cord - pathology; Spinal Cord Injuries - metabolism; Spinal Cord Injuries - pathology; spinal cord injury; knockout; intraperitoneal; quantitative polymerase chain reaction; Lipids; Inflammation; Myeloid; myelin basic protein; Macrophages; heterozygote; days post injury; central nervous system; matrix metalloprotease; subcutaneous; Hanks Balanced Salt Solution; High Content Screening; phosphate buffered saline; Oil Red O; Tumor Necrosis Factor; bone-marrow-derived macrophages; Damage Associated Molecular Pattern
• ISSN: 0969-9961; 1095-953X
• DOI: 10.1016/j.nbd.2021.105608

Tissue damage after spinal cord injury (SCI) elicits a robust inflammatory cascade that fails to resolve in a timely manner, resulting in impaired wound healing and cellular regeneration. This inflammatory response is partly mediated by infiltrating immune cells, including macrophages. As professional phagocytes, macrophages initially play an important role in debris clearance at the injury site, which would be necessary for proper tissue regeneration. After SCI, most macrophages become filled with lipid droplets due to excessive uptake of lipid debris, assuming a “foamy” phenotype that is associated with a proinflammatory state. Myelin has been assumed to be the main source of lipid that induces foamy macrophage formation after injury given its abundance in the spinal cord. This assumption has led to the widespread use of purified myelin treatment to model foamy macrophage formation in vitro. However, the assumption that myelin is necessary for foamy macrophage formation remains untested. To this end, we developed a novel foamy macrophage assay utilizing total spinal cord homogenate to include all sources of lipid present at the injury site. Using the myelin basic protein knockout (MBP KO, i.e., Shiverer) mice that lack myelin, we investigated lipid accumulation in foamy macrophages. Primary macrophages treated with myelin-deficient spinal cord homogenate still formed large lipid droplets typically observed in foamy macrophages, although to a lesser degree than cells treated with normal homogenate. Similarly, MBP KO mice subjected to contusive spinal cord injury also formed foamy macrophages that exhibited reduced lipid content and associated with improved histological outcomes and reduced immune cell infiltration. Therefore, the absence of myelin does not preclude foamy macrophage formation, indicating that myelin is not the only major source of lipid that contributes this pathology, even though myelin may alter certain aspects of its inflammatory profile.
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•Development of high content assay for primary foamy macrophages treated with spinal cord homogenate.•Myelin and non-myelin sources of exogenous lipids drive foamy macrophage formation after SCI.•Myelin-deficient mice exhibit decreased lipid content and immune cell infiltration after SCI.