The alarmin interleukin-1α triggers secondary degeneration through reactive astrocytes and endothelium after spinal cord injury.

Spinal cord injury (SCI) triggers neuroinflammation, and subsequently secondary degeneration and oligodendrocyte (OL) death. We report that the alarmin interleukin (IL)-1α is produced by damaged microglia after SCI. Intra-cisterna magna injection of IL-1α in mice rapidly induces neutrophil infiltration and OL death throughout the spinal cord, mimicking the injury cascade seen in SCI sites. These effects are abolished through co-treatment with the IL-1R1 antagonist anakinra, as well as in IL-1R1-knockout mice which demonstrate enhanced locomotor recovery after SCI. Conditional restoration of IL-1R1 expression in astrocytes or endothelial cells (ECs), but not in OLs or microglia, restores IL-1α-induced effects, while astrocyte- or EC-specific Il1r1 deletion reduces OL loss. Conditioned medium derived from IL-1α-stimulated astrocytes results in toxicity for OLs; further, IL-1α-stimulated astrocytes generate reactive oxygen species (ROS), and blocking ROS production in IL-1α-treated or SCI mice prevented OL loss. Thus, after SCI, microglia release IL-1α, inducing astrocyte- and EC-mediated OL degeneration.

Microglia are an essential component of the neuroprotective scar that forms after spinal cord injury.

The role of microglia in spinal cord injury (SCI) remains poorly understood and is often confused with the response of macrophages. Here, we use specific transgenic mouse lines and depleting agents to understand the response of microglia after SCI. We find that microglia are highly dynamic and proliferate extensively during the first two weeks, accumulating around the lesion. There, activated microglia position themselves at the interface between infiltrating leukocytes and astrocytes, which proliferate and form a scar in response to microglia-derived factors, such as IGF-1. Depletion of microglia after SCI causes disruption of glial scar formation, enhances parenchymal immune infiltrates, reduces neuronal and oligodendrocyte survival, and impairs locomotor recovery. Conversely, increased microglial proliferation, induced by local M-CSF delivery, reduces lesion size and enhances functional recovery. Altogether, our results identify microglia as a key cellular component of the scar that develops after SCI to protect neural tissue.

IL-1α Gene Deletion Protects Oligodendrocytes after Spinal Cord Injury through Upregulation of the Survival Factor Tox3.

Spinal cord injury (SCI) causes the release of danger signals by stressed and dying cells, a process that leads to neuroinflammation. Evidence suggests that inflammation plays a role in both the damage and repair of injured neural tissue. We show that microglia at sites of SCI rapidly express the alarmin interleukin (IL)-1α, and that infiltrating neutrophils and macrophages subsequently produce IL-1ß. Infiltration of these cells is dramatically reduced in both IL-1α(-/-) and IL-1ß(-/-) mice, but only IL-1α(-/-) mice showed rapid (at day 1) and persistent improvements in locomotion associated with reduced lesion volume. Similarly, intrathecal administration of the IL-1 receptor antagonist anakinra restored locomotor function post-SCI. Transcriptome analysis of SCI tissue at day 1 identified the survival factor Tox3 as being differentially regulated exclusively in IL-1α(-/-) mice compared with IL-1ß(-/-) and wild-type mice. Accordingly, IL-1α(-/-) mice have markedly increased Tox3 levels in their oligodendrocytes, beginning at postnatal day 10 (P10) and persisting through adulthood. At P10, the spinal cord of IL-1α(-/-) mice showed a transient increase in mature oligodendrocyte numbers, coinciding with increased IL-1α expression in wild-type animals. In adult mice, IL-1α deletion is accompanied by increased oligodendrocyte survival after SCI. TOX3 overexpression in human oligodendrocytes reduced cellular death under conditions mimicking SCI. These results suggest that IL-1α-mediated Tox3 suppression during the early phase of CNS insult plays a crucial role in secondary degeneration. SIGNIFICANCE STATEMENT: The mechanisms underlying bystander degeneration of neurons and oligodendrocytes after CNS injury are ill defined. We show that microglia at sites of spinal cord injury (SCI) rapidly produce the danger signal interleukin (IL)-1α, which triggers neuroinflammation and locomotor defects. We uncovered that IL-1α(-/-) mice have markedly increased levels of the survival factor Tox3 in their oligodendrocytes, which correlates with the protection of this cell population, and reduced lesion volume, resulting in unprecedented speed, level, and persistence of functional recovery after SCI. Our data suggest that central inhibition of IL-1α or Tox3 overexpression during the acute phase of a CNS insult may be an effective means for preventing the loss of neurological function in SCI, or other acute injuries such as ischemia and traumatic brain injuries.

Effects of myeloablation, peripheral chimerism, and whole-body irradiation on the entry of bone marrow-derived cells into the brain.

Understanding how bone marrow-derived cells (BMDCs) enter the central nervous system (CNS) is critical for the development of therapies for brain-related disorders using hematopoietic stem cells. We investigated the brain damages and blood-brain barrier (BBB) modification following either whole-body irradiation or a myeloablative chemotherapy regimen in mice, and the capacity for these treatments to induce the entry of BMDCs into the CNS. Neither treatment had a lasting effect on brain integrity and both were equally efficient at achieving myeloablation. Injection of bone marrow cells from green fluorescent protein (GFP) transgenic mice was able to completely repopulate the hematopoietic niche in the circulation and in hematopoietic organs (thymus and spleen). However, GFP(+) cells only entered the brain following whole-body irradiation. We conclude that myeloablation, damages to the brain integrity, or the BBB and peripheral chimerism are not responsible for the entry of BMDCs into the CNS following irradiation.

Powerful beneficial effects of macrophage colony-stimulating factor on beta-amyloid deposition and cognitive impairment in Alzheimer's disease.

Alzheimer's disease is a major cause of dementia in humans. The appearance of cognitive decline is linked to the overproduction of a short peptide called beta-amyloid (Abeta) in both soluble and aggregate forms. Here, we show that injecting macrophage colony-stimulating factor (M-CSF) to Swedish beta-amyloid precursor protein (APP(Swe))/PS1 transgenic mice, a well-documented model for Alzheimer's disease, on a weekly basis prior to the appearance of learning and memory deficits prevented cognitive loss. M-CSF also increased the number of microglia in the parenchyma and decreased the number of Abeta deposits. Senile plaques were smaller and less dense in the brain of M-CSF-treated mice compared to littermate controls treated with vehicle solution. Interestingly, a higher ratio of microglia internalized Abeta in the brain of M-CSF-treated animals and the phagocytosed peptides were located in the late endosomes and lysosomes. Less Abeta(40) and Abeta(42) monomers were also detected in the extracellular protein enriched fractions of M-CSF-treated transgenic mice when compared with vehicle controls. Finally, treating APP(Swe)/PS1 mice that were already demonstrating installed Abeta pathology stabilized the cognitive decline. Together these results provide compelling evidence that systemic M-CSF administration is a powerful treatment to stimulate bone marrow-derived microglia, degrade Abeta and prevent or improve the cognitive decline associated with Abeta burden in a mouse model of Alzheimer's disease.

Irradiation does not compromise or exacerbate the innate immune response in the brains of mice that were transplanted with bone marrow stem cells.

Microglia and invading macrophages play key roles in the brain immune response. The contributions of these two populations of cells in health and diseases have yet to be clearly established. The use of chimeric mice receiving bone marrow-derived stem cell grafts from green fluorescent protein (GFP)-expressing mice has provided an invaluable tool to distinguish between local and blood-derived monocytic populations. The validity of the method is questioned because of the possible immune alterations caused by the irradiation of the recipient mouse. In this experiment, we compared the brain expression of innate immune markers Toll-like receptor 2, interleukin-1 beta, tumor necrosis factor-alpha, and monocyte chemoattractant protein-1 in C57BL/6, GFP, and chimeric mice following an intracerebral injection of lipopolysaccharide. The endotoxin caused a marked transcriptional activation of all these innate immune genes in microglial cells across the ipsilateral side of injection. The expression patterns and signal intensity were similar in the brains of the three groups of mice. Consequently, the chimera technique is appropriate to study the role of infiltrating and resident immune cells in the brain without having immune compromised hosts. Disclosure of potential conflicts of interest is found at the end of this article.