Microglia contribute to the postnatal development of cortical somatostatin-positive inhibitory cells and to whisker-evoked cortical activity

Lactate reprograms glioblastoma immunity through CBX3-regulated histone lactylation

Glioblastoma (GBM), an aggressive brain malignancy with a cellular hierarchy dominated by GBM stem cells (GSCs), evades antitumor immunity through mechanisms that remain incompletely understood. Like most cancers, GBMs undergo metabolic reprogramming toward glycolysis to generate lactate. Here, we show that lactate production by patient-derived GSCs and microglia/macrophages induces tumor cell epigenetic reprogramming through histone lactylation, an activating modification that leads to immunosuppressive transcriptional programs and suppression of phagocytosis via transcriptional upregulation of CD47, a "don't eat me" signal, in GBM cells. Leveraging these findings, pharmacologic targeting of lactate production augments efficacy of anti-CD47 therapy. Mechanistically, lactylated histone interacts with the heterochromatin component chromobox protein homolog 3 (CBX3). Although CBX3 does not possess direct lactyltransferase activity, CBX3 binds histone acetyltransferase (HAT) EP300 to induce increased EP300 substrate specificity toward lactyl-CoA and a transcriptional shift toward an immunosuppressive cytokine profile. Targeting CBX3 inhibits tumor growth by both tumor cell-intrinsic mechanisms and increased tumor cell phagocytosis. Collectively, these results suggest that lactate mediates metabolism-induced epigenetic reprogramming in GBM that contributes to CD47-dependent immune evasion, which can be leveraged to augment efficacy of immuno-oncology therapies.

The chemokine Cxcl14 regulates interneuron differentiation in layer I of the somatosensory cortex

Spontaneous and sensory-evoked activity sculpts developing circuits. Yet, how these activity patterns intersect with cellular programs regulating the differentiation of neuronal subtypes is not well understood. Through electrophysiological and in vivo longitudinal analyses, we show that C-X-C motif chemokine ligand 14 (Cxcl14), a gene previously characterized for its association with tumor invasion, is expressed by single-bouquet cells (SBCs) in layer I (LI) of the somatosensory cortex during development. Sensory deprivation at neonatal stages markedly decreases Cxcl14 expression. Additionally, we report that loss of function of this gene leads to increased intrinsic excitability of SBCs-but not LI neurogliaform cells-and augments neuronal complexity. Furthermore, Cxcl14 loss impairs sensory map formation and compromises the in vivo recruitment of superficial interneurons by sensory inputs. These results indicate that Cxcl14 is required for LI differentiation and demonstrate the emergent role of chemokines as key players in cortical network development.

Interneuron Diversity: How Form Becomes Function

A persistent question in neuroscience is how early neuronal subtype identity is established during the development of neuronal circuits. Despite significant progress in the transcriptomic characterization of cortical interneurons, the mechanisms that control the acquisition of such identities as well as how they relate to function are not clearly understood. Accumulating evidence indicates that interneuron identity is achieved through the interplay of intrinsic genetic and activity-dependent programs. In this work, we focus on how progressive interactions between interneurons and pyramidal cells endow maturing interneurons with transient identities fundamental for their function during circuit assembly and how the elimination of transient connectivity triggers the consolidation of adult subtypes.

Early-life maturation of the somatosensory cortex: sensory experience and beyond

Early life experiences shape physical and behavioral outcomes throughout lifetime. Sensory circuits are especially susceptible to environmental and physiological changes during development. However, the impact of different types of early life experience are often evaluated in isolation. In this mini review, we discuss the specific effects of postnatal sensory experience, sleep, social isolation, and substance exposure on barrel cortex development. Considering these concurrent factors will improve understanding of the etiology of atypical sensory perception in many neuropsychiatric and neurodevelopmental disorders.

Maternal immunoglobulins are distributed in the offspring's brain to support the maintenance of cortical interneurons in the postnatal period

It is known that maternal immunoglobulins (Igs) are transferred to the offspring across the placenta. However, receiving maternal Igs, especially before the blood-brain barrier (BBB) is formed in the offspring's brain, carries the risk of transferring some brain-reactive Igs. It is thus hypothesized that there may be some unknown benefit to the offspring's brain that overweighs this risk. In this study, we show that the Ig detected in the embryonic/perinatal mouse brain is IgG not produced by the pups themselves, but is basically transferred from the mother across the placenta using the neonatal Fc receptor (FcRn) during embryonic stages. The amount of IgG in the brain gradually decreases after birth, and almost disappears within 3 weeks postnatally. IgG is detected on axon bundles, microglia, and some meningeal cells, including border-associated macrophages (BAMs), endothelial cells, and fibroblasts. Using Fcer1g knock-out (KO) mice, we show that BAMs and microglia receive maternal IgG in an Fc receptor γ chain (FcRγ)-dependent manner, but IgG on other meningeal cells and axon bundles is received independently of the FcRγ. These results suggest that maternal IgG may be used in multiple ways by different mechanisms. In maternal IgG-deficient mice, the number of interneurons in the cerebral cortex is not altered around birth but is reduced postnatally, suggesting that receipt of maternal IgG is necessary for the maintenance of cortical interneurons in the postnatal period. These data suggest that maternal IgG has an important function in the developing brain, where neither obvious inflammation nor infection is observed.

Microglia as integrators of brain-associated molecular patterns

Microglia are brain-resident macrophages that play key roles in brain development and experience dependent plasticity. In this review we discuss recent findings regarding the molecular mechanisms through which mammalian microglia sense the unique molecular patterns of the homeostatic brain. We propose that microglial function is acutely controlled in response to 'brain-associated molecular patterns' (BAMPs) that function as indicators of neuronal activity and neural circuit remodeling. A further layer of regulation comes from instructive cytokine cues that define unique microglial functional states. A systematic investigation of the receptors and signaling pathways that mediate these two regulatory axes may begin to define a functional code for microglia-neuron interactions.

Reorganization of adolescent prefrontal cortex circuitry is required for mouse cognitive maturation

Most cognitive functions involving the prefrontal cortex emerge during late development. Increasing evidence links this delayed maturation to the protracted timeline of prefrontal development, which likely does not reach full maturity before the end of adolescence. However, the underlying mechanisms that drive the emergence and fine-tuning of cognitive abilities during adolescence, caused by circuit wiring, are still unknown. Here, we continuously monitored prefrontal activity throughout the postnatal development of mice and showed that an initial activity increase was interrupted by an extensive microglia-mediated breakdown of activity, followed by the rewiring of circuit elements to achieve adult-like patterns and synchrony. Interfering with these processes during adolescence, but not adulthood, led to a long-lasting microglia-induced disruption of prefrontal activity and neuronal morphology and decreased cognitive abilities. These results identified a nonlinear reorganization of prefrontal circuits during adolescence and revealed its importance for adult network function and cognitive processing.

Contactomics of Microglia and Intercellular Communication

Microglia represent the main immunocompetent cell type in the parenchyma of the brain and the spinal cord, with roles extending way beyond their immune functions. While emerging data show the pivotal role of microglia in brain development, brain health and brain diseases, the exact mechanisms through which microglia contribute to complex neuroimmune interactions are still largely unclear. Understanding the communication between microglia and other cells represents an important cornerstone of these interactions, which may provide novel opportunities for therapeutic interventions in neurological or psychiatric disorders. As such, in line with studying the effects of the numerous soluble mediators that influence neuroimmune processes, attention on physical interactions between microglia and other cells in the CNS has increased substantially in recent years. In this chapter, we briefly summarize the latest literature on "microglial contactomics" and its functional implications in health and disease.

Microglia and Microbiome-Gut-Brain Axis

The mammalian gut contains a community of microorganisms called gut microbiome. The gut microbiome is integrated into mammalian physiology, contributing to metabolism, production of metabolites, and promoting immunomodulatory actions. Microglia, the brain's resident innate immune cells, play an essential role in homeostatic neurogenesis, synaptic remodeling, and glial maturation. Microglial dysfunction has been implicated in the pathogenesis of several neuropsychiatric disorders. Recent findings indicate that microglia are influenced by the gut microbiome and their derived metabolites throughout life. The pathways by which microbiota regulate microglia have only started to be understood, but this discovery has the potential to provide valuable insights into the pathogenesis of brain disorders associated with an altered microbiome. Here, we discuss the recent literature on the role of the gut microbiome in modulating microglia during development and adulthood and summarize the key findings on this bidirectional crosstalk in selected examples of neuropsychiatric and neurodegenerative disorders. We also highlight some current caveats and perspectives for the field.

epDevAtlas: Mapping GABAergic cells and microglia in postnatal mouse brains

During development, brain regions follow encoded growth trajectories. Compared to classical brain growth charts, high-definition growth charts could quantify regional volumetric growth and constituent cell types, improving our understanding of typical and pathological brain development. Here, we create high-resolution 3D atlases of the early postnatal mouse brain, using Allen CCFv3 anatomical labels, at postnatal days (P) 4, 6, 8, 10, 12, and 14, and determine the volumetric growth of different brain regions. We utilize 11 different cell type-specific transgenic animals to validate and refine anatomical labels. Moreover, we reveal region-specific density changes in γ-aminobutyric acid-producing (GABAergic), cortical layer-specific cell types, and microglia as key players in shaping early postnatal brain development. We find contrasting changes in GABAergic neuronal densities between cortical and striatal areas, stabilizing at P12. Moreover, somatostatin-expressing cortical interneurons undergo regionally distinct density reductions, while vasoactive intestinal peptide-expressing interneurons show no significant changes. Remarkably, microglia transition from high density in white matter tracks to gray matter at P10, and show selective density increases in sensory processing areas that correlate with the emergence of individual sensory modalities. Lastly, we create an open-access web-visualization (https://kimlab.io/brain-map/epDevAtlas) for cell-type growth charts and developmental atlases for all postnatal time points.

Chandelier cells shine a light on the formation of GABAergic synapses

Uncovering the wiring rules employed by neurons during development represents a formidable challenge with important repercussions for neurodevelopmental disorders. Chandelier cells (ChCs) are a singular GABAergic interneuron type, with a unique morphology, that have recently begun to shed light on the rules that drive the formation and plasticity of inhibitory synapses. This review will focus on the wealth of recent data charting the emergence of synapses formed by ChCs onto pyramidal cells, from the molecules involved to the plasticity of these connections during development.

Quantification of the density and morphology of dendritic spines and synaptic protein distribution using Thy1-YFP transgenic mice

Synaptopathy, which encompasses morphological deficits and any abnormal protein distribution of synapses, is a critical feature of many neurological diseases. We here provide a protocol using mice stably expressing a Thy1-YFP transgene to assess synaptic features in vivo. We describe steps for recording the entire morphology of projection neurons using confocal microscopy based on YFP signals. We detail assessment of the density and size of dendritic spines and the distributions of synaptic proteins using ImageJ and statistical analysis using Prism. For complete details on the use and execution of this protocol, please refer to Shih et al. (2020).¹.

Multifaceted microglia during brain development: Models and tools

Microglia, the brain resident macrophages, are multifaceted glial cells that belong to the central nervous and immune systems. As part of the immune system, they mediate innate immune responses, regulate brain homeostasis and protect the brain in response to inflammation or injury. At the same time, they can perform a wide array of cellular functions that relate to the normal functioning of the brain. Importantly, microglia are key actors of brain development. Indeed, these early brain invaders originate outside of the central nervous system from yolk sac myeloid progenitors, and migrate into the neural folds during early embryogenesis. Before the generation of oligodendrocytes and astrocytes, microglia thus occupy a unique position, constituting the main glial population during early development and participating in a wide array of embryonic and postnatal processes. During this developmental time window, microglia display remarkable features, being highly heterogeneous in time, space, morphology and transcriptional states. Although tremendous progress has been made in our understanding of their ontogeny and roles, there are several limitations for the investigation of specific microglial functions as well as their heterogeneity during development. This review summarizes the current murine tools and models used in the field to study the development of these peculiar cells. In particular, we focus on the methodologies used to label and deplete microglia, monitor their behavior through live-imaging and also discuss the progress currently being made by the community to unravel microglial functions in brain development and disorders.

Fractalkine/CX3CR1-Dependent Modulation of Synaptic and Network Plasticity in Health and Disease

CX3CR1 is a G protein-coupled receptor that is expressed exclusively by microglia within the brain parenchyma. The only known physiological CX3CR1 ligand is the chemokine fractalkine (FKN), which is constitutively expressed in neuronal cell membranes and tonically released by them. Through its key role in microglia-neuron communication, the FKN/CX3CR1 axis regulates microglial state, neuronal survival, synaptic plasticity, and a variety of synaptic functions, as well as neuronal excitability via cytokine release modulation, chemotaxis, and phagocytosis. Thus, the absence of CX3CR1 or any failure in the FKN/CX3CR1 axis has been linked to alterations in different brain functions, including changes in synaptic and network plasticity in structures such as the hippocampus, cortex, brainstem, and spinal cord. Since synaptic plasticity is a basic phenomenon in neural circuit integration and adjustment, here, we will review its modulation by the FKN/CX3CR1 axis in diverse brain circuits and its impact on brain function and adaptation in health and disease.