CSF1R ligands promote microglial proliferation but are not the sole regulators of developmental microglial proliferation

Abstract

Microglia - the predominant immune cells of the brain and spinal cord - perform essential functions for the development and maintenance of the central nervous system, contingent upon the regulated developmental proliferation of microglia. However, the factor(s) that regulate microglial proliferation remain unclear. Here, we confirmed the timeline of developmental proliferation and used bioinformatics to identify potential signalling onto microglia in mouse from datasets collected at an age of high developmental microglial proliferation. Of the predicted factors, we found that colony stimulating factor 1 receptor (CSF1R) ligands boosted proliferation in vitro and were increasingly expressed in the brain across development with each displaying a distinct regional and temporal expression pattern. However, we did not observe a coincident alteration to CSF1R ligand levels in a model of abnormal developmental proliferation. Together, although CSF1R ligands can promote microglial proliferation in culture, their developmental expression patterns suggest that they function alongside other unknown factors to regulate developmental microglial proliferation.

Keywords: Brain; CSF1; CSF1R; Development; IL34; Microglia; Mouse; Proliferation.

Fig. 1.

Developing microglia proliferate within the first two postnatal weeks. (A) Representative images of IBA1⁺ microglia (red) and KI67⁺ proliferative cells (white) within P0, P7 and P30 somatosensory cortex. Scale bars: 50 µm. (B) Plot of the mean percentage of the cortical microglia that are proliferating at each developmental time point. (C) Plot of cortical microglial densities across development. (D) Representative images of IBA1⁺ microglia (red) and KI67⁺ proliferative cells (white) in P0, P7 and P30 CA1 region of the hippocampus. Scale bars: 50 µm. (E) Plot of the mean percentage of CA1 microglia that are proliferating at each developmental time point. (F) Plot of CA1 microglial densities across development. In B, C, E, F, bars represent mean±s.e.m.; n=8 mice per time point. Counts of microglia and proliferating microglia were averaged from three or four images in either the somatosensory cortex or CA1. *P<0.05, **P<0.01, ***P<0.001, ****P<0.0001 (one-way ANOVA, Tukey post-hoc).

Fig. 2.

CellChat predicts potential ligand–receptor signalling onto microglia in the P7 mouse cortex and hippocampus. (A) A UMAP plot depicting the cellular populations of the P7 prefrontal cortex using data from Joglekar et al. (2021). Dots represent individual cells clustered based on transcriptional similarity, while colours indicate distinct cell lineages or cellular states based on gene expression patterns. (B) A CellChat plot depicting the ligands (blue) secreted from CNS cell populations predicted to signal onto receptors on microglia (orange) in the P7 mouse prefrontal cortex. (C) A UMAP plot depicting the cellular populations of the P7 hippocampus using data from Joglekar et al. (2021). Dots represent individual cells clustered based on transcriptional similarity, while colours indicate distinct cell lineages or cellular states based on gene expression patterns. (D) A CellChat plot depicting the ligands (blue) secreted from CNS cell populations predicted to signal onto receptors on microglia (orange) in the P7 mouse hippocampus.

Fig. 3.

CSF1R ligands boost microglial proliferation in serum-free primary microglial culture. (A) Schematic of immunopanning for the isolation and culture of murine microglia. I. V., in vitro. Schematic created in BioRender by Ho, M. 2025. https://BioRender.com/zvx72w4. This figure was sublicensed under CC-BY 4.0 terms. (B) A plot of the percentage of total Cd11b⁺ microglia that were KI67⁺ after a 72-h treatment of 10 ng/ml of each predicted signalling factor in microglial base media. (C) Representative images of proliferative microglia (CD11b⁺KI67⁺) at 1000 ng/ml. Scale bars: 100 µm. (D) A plot of the mean percentage of microglia that are proliferative at tenfold increasing concentrations of CSF1 or IL34. (E) A plot of the mean percentage of viable microglial sustained by tenfold increasing concentrations of CSF1 or IL34. BM here indicates a microglial base media control supplemented with 2 ng/ml TGFβ2; GM indicates microglial growth media control containing both 2 ng/ml TGFβ2 and 10 ng/ml CSF1. (F) A plot of the percentage of live microglia sustained by BM containing either 1 ng/ml or 10 ng/ml CSF1. Bars represent mean±s.e.m.; B: n=2, D-F: n=3, independent microglial cultures with treatments in triplicate or quadruplicate. ****P<0.0001 (one-way ANOVA, Tukey post-hoc). ns, not significant.

Fig. 4.

CSF1 and IL34 protein increase throughout development and display distinct spatiotemporal transcriptional expression patterns. (A,B) IL34 (A) and CSF1 (B) protein levels in whole-brain homogenates as measured by ELISA. (C) Representative brain microscopy image with regions of interest outlined in red. Scale bar: 100 µm. (D,F) Representative images of Il34 transcript visualized with RNAscope™ in CA1 (D) and somatosensory cortex (F). Scale bars: 40 µm. (E,G) Plots of the mean percentage of total cells of CA1 (E) and somatosensory cortex (G) that are Il34⁺ across development. (H,J) Representative images of Csf1 transcript visualized with RNAscope™ in CA1 (H) and somatosensory cortex (J). Scale bars: 40 µm. (I,K) Plots of the mean percentage of total cells of CA1 (I) and somatosensory cortex (K) that are Csf1⁺ across development. (L) Density maps depicting the spatiotemporal expression patterns of Il34 and Csf1 transcripts in RNAscope™ across development. Warmer colours indicate greater, while cooler colours indicate lower, transcript density. Bars represent mean±s.e.m.; A,B: n=5 mice per time point; E: n=3-5 mice per time point, G: n=4-5 mice per time point, I,K: n=2-3 mice per time point. *P<0.05, ****P<0.0001 (one-way ANOVA, Tukey post-hoc).

Fig. 5.

CSF1R signalling is largely restricted to microglia and border-associated macrophages in the P7 hippocampus. (A) UMAP plot depicting the cellular populations of the P7 hippocampus. Dots represent individual cells clustered based on transcriptional similarity, while colours indicate distinct cell lineages or cellular states based on gene expression patterns. (B) UMAP plot depicting expression of Csf1r in the P7 hippocampus. (C) UMAP plot depicting expression of Csf1 in the P7 hippocampus. (D) UMAP plot depicting expression of Il34 in the P7 hippocampus. (E) Dot plot depicting the cell population expression levels of Csf1r, Csf1 and Il34 in the P7 hippocampus. (F) CellChat plot predicting the dominant sender populations that secrete CSF1 and IL34 and dominant receiver populations that receive signalling via CSF1R in the P7 hippocampus. (G) Violin plot depicting the expression of microglial Csf1r across the mouse lifespan. (H) UMAP plot depicting inhibitory neurons, excitatory neurons and non-neuronal cells of the adult hippocampus. Dots represent individual cells clustered based on transcriptional similarity, while colours indicate distinct cell lineages or cellular states based on gene expression patterns. (I) UMAP plot depicting expression of Il34 in the adult hippocampus. (J) Violin plot comparing the expression level of Il34 between inhibitory neurons, excitatory neurons and non-neuronal cells of the adult hippocampus. Datasets for A-F: Joglekar et al., 2021; G: Hammond et al., 2019; H-J: Yao et al., 2021. G,J: ****P<0.0001 (one-way ANOVA, Tukey post-hoc).

Fig. 6.

Unaltered Il34 and Csf1 levels of IL1α and IL1β knockout mice are insufficient to explain aberrant developmental microglial proliferation. (A) Representative whole brain microscopy image with analysed regions of interest outlined in red. (B) Representative images of IBA1⁺ microglia (red) and KI67⁺ proliferative cells (white) in the P10 hippocampus and cortex of IL1α and IL1β knockout mice. Scale bars: 40 µm. (C,D) Plots of the mean percentage of hippocampal microglia (C) and somatosensory cortex microglia (D) that are proliferating in wild-type (WT), IL1α and IL1β knockout mice across development. (E,F) Plots of mean hippocampal microglial densities (E) and mean somatosensory cortex microglial densities (F) in WT, IL1α and IL1β knockout mice. (G,H) Plots of the mean percentage of total cells of CA1 that are Il34⁺ (G) or Csf1⁺ (H) at P10 in WT, IL1α and IL1β knockout mice. (I,J) Plots of the mean percentage of total cells of somatosensory cortex that are Il34⁺ (I) or Csf1⁺ (J) at P10 in WT, IL1α and IL1β knockout mice. Bars represent mean±s.e.m.; C-F: n=6-8 mice per time point; G,I: n=5 mice per genotype; H,J: n=3-5 mice per genotype. *P<0.05, **P<0.01, ***P<0.001, ****P<0.0001 (C-F: two-way ANOVA, Tukey post-hoc; G-J: one-way ANOVA, Tukey post-hoc).