The effects of myelin on macrophage activation are phenotypic specific via cPLA2 in the context of spinal cord injury inflammation

Repeated trans-spinal magnetic stimulation promotes microglial phagocytosis of myelin debris after spinal cord injury through LRP-1

Spinal cord injury (SCI) is a serious trauma of the central nervous system. The clearance of myelin debris is a critical step in the functional recovery following spinal cord injury (SCI). Recent studies have begun to reveal critical roles for professional phagocytes in the central nervous system, microglia, and their receptors in the control of myelin debris in neurodegenerative disease. Repeated trans-spinal magnetic stimulation (rTSMS) has been demonstrated as a noninvasive SCI treatment that enhances tissue repair and functional recovery. In this study, we investigated the role and molecular mechanism of rTSMS on microglial phagocytosis of myelin debris in a rat SCI model. In our studies, we found that rTSMS significantly promoted the motor function recovery of SCI rats associated with the inhibition the neuroinflammation and glia scar formation. Immunofluorescence results further showed that the rTSMS promotes the clearance of myelin debris by microglia in vivo and in vitro. Additionally, receptor-associated protein (RAP), a Low-density lipoprotein receptor-related protein-1 (LRP-1) inhibitor, could cancel the accelerated microglial phagocytosis of myelin debris after rTSMS in vitro experiments. Simultaneously, Elisa's results and western blotting respectively showed that rTSMS significantly decreased the levels of soluble LRP-1(sLRP-1) and the LRP-1 splicing enzyme of ADAM17. In conclusion, rTSMS could promote the clearance of myelin debris by microglia through LRP-1 to improve the functional recovery of SCI rats.

Targeting foamy macrophages by manipulating ABCA1 expression to facilitate lesion healing in the injured spinal cord

Spinal cord injury (SCI) triggers a complex cascade of events, including myelin loss, neuronal damage, neuroinflammation, and the accumulation of damaged cells and debris at the injury site. Infiltrating bone marrow derived macrophages (BMDMϕ) migrate to the epicenter of the SCI lesion, where they engulf cell debris including abundant myelin debris to become pro-inflammatory foamy macrophages (foamy Mϕ), participate neuroinflammation, and facilitate the progression of SCI. This study aimed to elucidate the cellular and molecular mechanisms underlying the functional changes in foamy Mϕ and their potential implications for SCI. Contusion at T10 level of the spinal cord was induced using a New York University (NYU) impactor (5 g rod from a height of 6.25 mm) in male mice. ABCA1, an ATP-binding cassette transporter expressed by Mϕ, plays a crucial role in lipid efflux from foamy cells. We observed that foamy Mϕ lacking ABCA1 exhibited increased lipid accumulation and a higher presence of lipid-accumulated foamy Mϕ as well as elevated pro-inflammatory response in vitro and in injured spinal cord. We also found that both genetic and pharmacological enhancement of ABCA1 expression accelerated lipid efflux from foamy Mϕ, reduced lipid accumulation and inhibited the pro-inflammatory response of foamy Mϕ, and accelerated clearance of cell debris and necrotic cells, which resulted in functional recovery. Our study highlights the importance of understanding the pathologic role of foamy Mϕ in SCI progression and the potential of ABCA1 as a therapeutic target for modulating the inflammatory response, promoting lipid metabolism, and facilitating functional recovery in SCI.

Bromocriptine protects perilesional spinal cord neurons from lipotoxicity after spinal cord injury

Recent studies have revealed that lipid droplets accumulate in neurons after brain injury and evoke lipotoxicity, damaging the neurons. However, how lipids are metabolized by spinal cord neurons after spinal cord injury remains unclear. Herein, we investigated lipid metabolism by spinal cord neurons after spinal cord injury and identified lipid-lowering compounds to treat spinal cord injury. We found that lipid droplets accumulated in perilesional spinal cord neurons after spinal cord injury in mice. Lipid droplet accumulation could be induced by myelin debris in HT22 cells. Myelin debris degradation by phospholipase led to massive free fatty acid production, which increased lipid droplet synthesis, β-oxidation, and oxidative phosphorylation. Excessive oxidative phosphorylation increased reactive oxygen species generation, which led to increased lipid peroxidation and HT22 cell apoptosis. Bromocriptine was identified as a lipid-lowering compound that inhibited phosphorylation of cytosolic phospholipase A2 by reducing the phosphorylation of extracellular signal-regulated kinases 1/2 in the mitogen-activated protein kinase pathway, thereby inhibiting myelin debris degradation by cytosolic phospholipase A2 and alleviating lipid droplet accumulation in myelin debris-treated HT22 cells. Motor function, lipid droplet accumulation in spinal cord neurons and neuronal survival were all improved in bromocriptine-treated mice after spinal cord injury. The results suggest that bromocriptine can protect neurons from lipotoxic damage after spinal cord injury via the extracellular signal-regulated kinases 1/2-cytosolic phospholipase A2 pathway.

The role of foam cells in spinal cord injury: challenges and opportunities for intervention

Spinal cord injury (SCI) results in a large amount of tissue cell debris in the lesion site, which interacts with various cytokines, including inflammatory factors, and the intrinsic glial environment of the central nervous system (CNS) to form an inhibitory microenvironment that impedes nerve regeneration. The efficient clearance of tissue debris is crucial for the resolution of the inhibitory microenvironment after SCI. Macrophages are the main cells responsible for tissue debris removal after SCI. However, the high lipid content in tissue debris and the dysregulation of lipid metabolism within macrophages lead to their transformation into foamy macrophages during the phagocytic process. This phenotypic shift is associated with a further pro-inflammatory polarization that may aggravate neurological deterioration and hamper nerve repair. In this review, we summarize the phenotype and metabolism of macrophages under inflammatory conditions, as well as the mechanisms and consequences of foam cell formation after SCI. Moreover, we discuss two strategies for foam cell modulation and several potential therapeutic targets that may enhance the treatment of SCI.

Immunopathology of Extracellular Vesicles in Macrophage and Glioma Cross-Talk

Glioblastomas (GBM) are a devastating disease with extremely poor clinical outcomes. Resident (microglia) and infiltrating macrophages are a substantial component of the tumor environment. In GBM and other cancers, tumor-derived extracellular vesicles (EVs) suppress macrophage inflammatory responses, impairing their ability to identify and phagocytose cancerous tissues. Furthermore, these macrophages then begin to produce EVs that support tumor growth and migration. This cross-talk between macrophages/microglia and gliomas is a significant contributor to GBM pathophysiology. Here, we review the mechanisms through which GBM-derived EVs impair macrophage function, how subsequent macrophage-derived EVs support tumor growth, and the current therapeutic approaches to target GBM/macrophage EV crosstalk.

Immune Regulatory Functions of Macrophages and Microglia in Central Nervous System Diseases

Macrophages can be characterized as a very multifunctional cell type with a spectrum of phenotypes and functions being observed spatially and temporally in various disease states. Ample studies have now demonstrated a possible causal link between macrophage activation and the development of autoimmune disorders. How these cells may be contributing to the adaptive immune response and potentially perpetuating the progression of neurodegenerative diseases and neural injuries is not fully understood. Within this review, we hope to illustrate the role that macrophages and microglia play as initiators of adaptive immune response in various CNS diseases by offering evidence of: (1) the types of immune responses and the processes of antigen presentation in each disease, (2) receptors involved in macrophage/microglial phagocytosis of disease-related cell debris or molecules, and, finally, (3) the implications of macrophages/microglia on the pathogenesis of the diseases.

Regulation of microglia function by neural stem cells

Neural stem and precursor cells (NPCs) build and regenerate the central nervous system (CNS) by maintaining their pool (self-renewal) and differentiating into neurons, astrocytes, and oligodendrocytes (multipotency) throughout life. This has inspired research into pro-regenerative therapies that utilize transplantation of exogenous NPCs or recruitment of endogenous adult NPCs for CNS regeneration and repair. Recent advances in single-cell RNA sequencing and other "omics" have revealed that NPCs express not just traditional progenitor-related genes, but also genes involved in immune function. Here, we review how NPCs exert immunomodulatory function by regulating the biology of microglia, immune cells that are present in NPC niches and throughout the CNS. We discuss the role of transplanted and endogenous NPCs in regulating microglia fates, such as survival, proliferation, migration, phagocytosis and activation, in the developing, injured and degenerating CNS. We also provide a literature review on NPC-specific mediators that are responsible for modulating microglia biology. Our review highlights the immunomodulatory properties of NPCs and the significance of these findings in the context of designing pro-regenerative therapies for degenerating and diseased CNS.

Autologous Platelet-Rich Growth Factor Reduces M1 Macrophages and Modulates Inflammatory Microenvironments to Promote Sciatic Nerve Regeneration

The failure of peripheral nerve regeneration is often associated with the inability to generate a permissive molecular and cellular microenvironment for nerve repair. Autologous therapies, such as platelet-rich plasma (PRP) or its derivative platelet-rich growth factors (PRGF), may improve peripheral nerve regeneration via unknown mechanistic roles and actions in macrophage polarization. In the current study, we hypothesize that excessive and prolonged inflammation might result in the failure of pro-inflammatory M1 macrophage transit to anti-inflammatory M2 macrophages in large nerve defects. PRGF was used in vitro at the time the unpolarized macrophages (M0) macrophages were induced to M1 macrophages to observe if PRGF altered the secretion of cytokines and resulted in a phenotypic change. PRGF was also employed in the nerve conduit of a rat sciatic nerve transection model to identify alterations in macrophages that might influence excessive inflammation and nerve regeneration. PRGF administration reduced the mRNA expression of tumor necrosis factor-α (TNFα), interleukin-1β (IL-1β), and IL-6 in M0 macrophages. Increased CD206 substantiated the shift of pro-inflammatory cytokines to the M2 regenerative macrophage. Administration of PRGF in the nerve conduit after rat sciatic nerve transection promoted nerve regeneration by improving nerve gross morphology and its targeted gastrocnemius muscle mass. The regenerative markers were increased for regrown axons (protein gene product, PGP9.5), Schwann cells (S100β), and myelin basic protein (MBP) after 6 weeks of injury. The decreased expression of TNFα, IL-1β, IL-6, and CD68⁺ M1 macrophages indicated that the inflammatory microenvironments were reduced in the PRGF-treated nerve tissue. The increase in RECA-positive cells suggested the PRGF also promoted angiogenesis during nerve regeneration. Taken together, these results indicate the potential role and clinical implication of autologous PRGF in regulating inflammatory microenvironments via macrophage polarization after nerve transection.

Clickable Biomaterials for Modulating Neuroinflammation

Crosstalk between the nervous and immune systems in the context of trauma or disease can lead to a state of neuroinflammation or excessive recruitment and activation of peripheral and central immune cells. Neuroinflammation is an underlying and contributing factor to myriad neuropathologies including neurodegenerative diseases like Alzheimer’s disease and Parkinson’s disease; autoimmune diseases like multiple sclerosis; peripheral and central nervous system infections; and ischemic and traumatic neural injuries. Therapeutic modulation of immune cell function is an emerging strategy to quell neuroinflammation and promote tissue homeostasis and/or repair. One such branch of ‘immunomodulation’ leverages the versatility of biomaterials to regulate immune cell phenotypes through direct cell-material interactions or targeted release of therapeutic payloads. In this regard, a growing trend in biomaterial science is the functionalization of materials using chemistries that do not interfere with biological processes, so-called ‘click’ or bioorthogonal reactions. Bioorthogonal chemistries such as Michael-type additions, thiol-ene reactions, and Diels-Alder reactions are highly specific and can be used in the presence of live cells for material crosslinking, decoration, protein or cell targeting, and spatiotemporal modification. Hence, click-based biomaterials can be highly bioactive and instruct a variety of cellular functions, even within the context of neuroinflammation. This manuscript will review recent advances in the application of click-based biomaterials for treating neuroinflammation and promoting neural tissue repair.

Bone Marrow Mesenchymal Stem Cells (BMSCs) Expressing Netrin-1 Alleviates Spinal Cord Injury (SCI)-Induced Inflammation

Spinal cord injury (SCI) is a common central nervous system (CNS) injury. Bone marrow mesenchymal stem cells (BMSCs) transplantation is a potential treatment for traumatic SCI. However, the role and mechanism of BMSCs with high expression of Netrin-1 on the repair and inflammation of spinal cord injury cells remains unclear. Our study intends to assess the effect of BMSCs with high Netrin-1 level on the repair of SCI cells. BMSCs or Netrin-1 transfected BMSCs were co-cultured with mechanically injured nerve cells followed by analysis of the differentiation of BMSCs by light microscope, apoptosis activity, expression of TLR-4 and NF-κB, and the TNF-α and IL-1β content in cell supernatant by ELISA. BMSCs with high Netrin-1 expression promoted the proliferation of BMSCs, inhibited apoptosis, and promoted the differentiation of nerve cells along with increased ALK activity, and the expression of GFAP and BDNF. Co-culture with BMSCs or BMSCs with high Netrin-1 expression increased mechanically damaged nerve cell proliferation, decreased apoptosis, downregulated TLR-4 and NF-κB (P < 0.05) with more significant changes after co-culture with BMSCs with high Netrin-1 expression. In conclusion, Netrin-1 can promote BMSCs proliferation and differentiation, and inhibit apoptosis. By inhibiting inflammation, it can promote damaged nerve cell proliferation and repair.

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

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.

Macrophage phagocytosis after spinal cord injury: when friends become foes

After spinal cord injury (SCI), macrophages can exert either beneficial or detrimental effects depending on their phenotype. Aside from their critical role in inflammatory responses, macrophages are also specialized in the recognition, engulfment, and degradation of pathogens, apoptotic cells, and tissue debris. They promote remyelination and axonal regeneration by removing inhibitory myelin components and cellular debris. However, excessive intracellular presence of lipids and dysregulated intracellular lipid homeostasis result in the formation of foamy macrophages. These develop a pro-inflammatory phenotype that may contribute to further neurological decline. Additionally, myelin-activated macrophages play a crucial role in axonal dieback and retraction. Here, we review the opposing functional consequences of phagocytosis by macrophages in SCI, including remyelination and regeneration versus demyelination, degeneration, and axonal dieback. Furthermore, we discuss how targeting the phagocytic ability of macrophages may have therapeutic potential for the treatment of SCI.