Neural Stem Cell Protein CCN1 Guides Microglial Distribution During Development

The development of the central nervous system (CNS) depends on the coordinated interactions between neural stem cells (NSCs) and resident immune cells to regulate neurogenesis, tissue organization, and brain maturation. Among these immune cells, microglia play essential roles in maintaining developmental homeostasis, yet the molecular mechanisms governing their spatial distribution remain poorly understood.

In a recent study published in Genes & Diseases, researchers from Tongji University, Shanghai Institute of Stem Cell Research and Clinical Translation, Tsinghua University, and Sycamore Research Institute of Life Sciences investigated the role of cellular communication network factor 1 (CCN1), a matricellular protein secreted by neural stem cells, in regulating microglial localization through region-specific cellular interactions during brain development.

Using CNS-specific Ccn1 knockout (cKO) mice generated via the Nestin-Cre system, the researchers integrated single-cell RNA sequencing (scRNA-seq) and high-resolution spatial transcriptomics to elucidate the molecular mechanisms governing these multicellular dynamics.

Single-cell analysis at embryonic stage E17.5 and postnatal day P2 revealed that Ccn1 is predominantly expressed in NSPCs, including NSCs and astrocyte precursor cells, making it a critical factor in the germinal regions of the telencephalon. Ccn1 deletion led to a significant reduction in the overall proportion of microglia in the brain. Immunostaining and FACS analysis confirmed that this decrease was spatially restricted, primarily occurring within the ventricular zone (VZ) at both late embryonic and early postnatal stages, while microglia accumulated in the rostral lateral septal region (LSR). Furthermore, Ccn1 deficiency induced substantial transcriptional alterations in microglia, particularly the upregulation of genes associated with autophagy, apoptosis, and immune activation, such as Atg7, Ctsl, Ms4a6d, and Cd86.

This region-specific redistribution was accompanied by an increased expression of activation-associated gene modules and markers such as Spp1 and Lpl in LSR microglia. The authors identified that Ccn1 deletion in NSCs-rather than astrocytes-was the primary driver of this phenomenon, as NSCs exhibit region-specific dysregulation of key signaling ligands, namely Csf1 and Il2.

Ccn1 deficiency in NSCs increases Il2 expression and alters regional Csf1 signaling, with activation in the LSR and suppression in the VZ, accompanied by an elevated expression of their corresponding receptors and downstream target genes in microglia. These changes affect NSC-microglia communication through the CSF1-CSF1R and IL2-CD53 pathways, leading to transcriptional remodeling and abnormal microglial distribution. Reduced CSF1 secretion in the VZ and its accumulation in the LSR promote microglial migration toward the LSR, causing microglial depletion in the VZ. Concurrently, elevated IL2 expression in the LSR drives microglial overactivation and enhanced autophagy, contributing to region-specific alterations in microglial function and localization. These findings highlight the importance of neural stem cell-derived molecular cues in establishing region-specific developmental microenvironments and demonstrate that neural stem cells actively regulate immune cell behavior through spatially restricted signaling networks during brain development.

In conclusion, this study identifies CCN1 as a critical regulator of neural cell-microglia communication, demonstrating that its loss disrupts region-specific microglial distribution and activation through altered Csf1 and Il2 signaling. These findings advance the understanding of neuroimmune regulation during brain development and provide a molecular framework for investigating how disrupted spatial cellular interactions may contribute to neurodevelopmental and neurological disorders.