Microglia coordinate cellular interactions during spinal cord repair in mice

Abstract

Traumatic spinal cord injury (SCI) triggers a neuro-inflammatory response dominated by tissue-resident microglia and monocyte derived macrophages (MDMs). Since activated microglia and MDMs are morphologically identical and express similar phenotypic markers in vivo, identifying injury responses specifically coordinated by microglia has historically been challenging. Here, we pharmacologically depleted microglia and use anatomical, histopathological, tract tracing, bulk and single cell RNA sequencing to reveal the cellular and molecular responses to SCI controlled by microglia. We show that microglia are vital for SCI recovery and coordinate injury responses in CNS-resident glia and infiltrating leukocytes. Depleting microglia exacerbates tissue damage and worsens functional recovery. Conversely, restoring select microglia-dependent signaling axes, identified through sequencing data, in microglia depleted mice prevents secondary damage and promotes recovery. Additional bioinformatics analyses reveal that optimal repair after SCI might be achieved by co-opting key ligand-receptor interactions between microglia, astrocytes and MDMs.

Single cell profiling of CD45+ spinal cord cells reveals microglial and B cell heterogeneity and crosstalk following spinal cord injury

“…Notably, we uncovered several potential microglial-B cell interactions that will be worthwhile to investigate further. A recent paper by Brennan et al [7] also identified the Ccl3/Ccl4-Ccr5 pair through receptor-ligand analysis that we found to be potentially involved in microglial-B cell recruitment, although in the context of interactions between microglia (Ccl3) and macrophages (Ccr5). B cells also express Ccr5, which means they could respond to this chemokine signal to be recruited into the tissue.…”

“…In the Milich study, they specifically excluded lymphocytes from their analysis. B cells have also been found in the other SCI studies in which the cellular changes have been explored by scRNA-seq [4,7]. Our unique data set enabled us to investigate the potential interactions between these two cell types.…”

“…This ensured that we had sufficient cells to draw conclusions about the microglial responses with our chip-based approach. Several recent studies have focused on the immune cell response to SCI using scRNA-seq approaches using droplet-based approaches that allow for the analysis of a larger number of cells [3,7,42]. The study by Milich et al [3], examined almost all cell types within the spinal cord at acute phases of injury, including 1, 3, and 7 dpi.…”

“…The cellular and molecular mechanisms leading to persistent deficits after SCI are only partially understood. Recently, profiling of individual cells within the spinal cord and their temporal responses after SCI by single-cell RNA sequencing (scRNA-seq) has substantially advanced our understanding of the breadth of cellular plasticity following SCI, and revealed concurrent dynamics within specific cell types including neurons, glia, and immune cells [3][4][5][6][7]. A recent scRNA-seq study of immune cells in severe SCI showed prolonged gene expression changes in microglia (up to 3 months post-injury), which were proposed to enhance recovery [4].…”

Capacity of astrocytes to promote axon growth in the injured mammalian central nervous system

“…Their beneficial effects on axon growth are consistent with the observations of exacerbated functional deficits following disruption of astrogliosis in rodent models ( Faulkner et al, 2004 ; Okada et al, 2006 ; Herrmann et al, 2008 ). Recent gene expression data, by bulk or single-cell RNA sequencing, of reactive astrocytes collected at the lesion from acute to chronic phases of spinal cord injury will aid unbiased discovery of astrocyte-derived factors that can facilitate axon regeneration in the mature mammalian CNS ( Anderson et al, 2016 ; Milich et al, 2021 ; Wahane et al, 2021 ; Wei et al, 2021 ; Brennan et al, 2022 ; Burda et al, 2022 ; Li et al, 2022 ). Given the long-recognized diversity of astrocytes, based on origin, location, injury type, and molecular signatures ( Zhang and Barres, 2010 ; Khakh and Sofroniew, 2015 ), a future challenge will be to define context-dependent astrocyte functions, including their regulation of axon plasticity.…”