Microglia and Astrocytes in Disease: Dynamic Duo or Partners in Crime?

Microglia are resident immune cells of the brain, which derive from a different cell lineage to all other cells in the brain. They are highly motile cells, constantly patrolling the brain parenchyma. Astrocytes are the largest cell component of the brain and develop from a common progenitor along with neurons and oligodendrocytes. They tile the entire brain and do not migrate during normal physiology. These two cell types are important for normal mammalian brain development and respond rapidly to disease, infection, and trauma. Microglia and astrocytes interact via contact-dependent and secreted factors to modulate their function during normal health and in disease. Microglia can drive reactivity in astrocytes via the release of specific cytokines, while astrocytes can drive dysfunction in microglia by withholding cholesterol. Many tools exist to manipulate both microglia and astrocytes, however, complete removal of astrocytes is currently impossible as this results in death. scRNASeq experiments must be both adequately powered and take into account possible artifacts as a result of subsampling when disseminating results. Ideally, cluster-specific differentially expressed genes should be validated using visualization methods (in situ hybridization or spatial transcriptomic approaches) and functional assays. Caution should be taken in the nomenclature of different ‘activation’ states of both microglia and astrocytes. While no method is perfect, the field needs to clearly state what constitutes a subset of cells: biologically relevant and functionally characterized descriptions will be the most beneficial. Microglia–astrocyte interactions represent a delicate balance affecting neural cell functions in health and disease. Tightly controlled to maintain homeostasis during physiological conditions, rapid and prolonged departures during disease, infection, and following trauma drive multiple outcomes: both beneficial and detrimental. Recent sequencing studies at the bulk and single-cell level in humans and rodents provide new insight into microglia–astrocyte communication in homeostasis and disease. However, the complex changing ways these two cell types functionally interact has been a barrier to understanding disease initiation, progression, and disease mechanisms. Single cell sequencing is providing new insights; however, many questions remain. Here, we discuss how to bridge transcriptional states to specific functions so we can develop therapies to mediate negative effects of altered microglia–astrocyte interactions. Microglia–astrocyte interactions represent a delicate balance affecting neural cell functions in health and disease. Tightly controlled to maintain homeostasis during physiological conditions, rapid and prolonged departures during disease, infection, and following trauma drive multiple outcomes: both beneficial and detrimental. Recent sequencing studies at the bulk and single-cell level in humans and rodents provide new insight into microglia–astrocyte communication in homeostasis and disease. However, the complex changing ways these two cell types functionally interact has been a barrier to understanding disease initiation, progression, and disease mechanisms. Single cell sequencing is providing new insights; however, many questions remain. Here, we discuss how to bridge transcriptional states to specific functions so we can develop therapies to mediate negative effects of altered microglia–astrocyte interactions. mouse model of Alzheimer’s disease based on amyloid deposition. The mouse contains five familial Alzheimer’s disease mutations [three in amyloid precursor protein (APP) and two in presenilin-1 (PSEN1) under the control of neuronal-specific Thy1 promoter] driving amyloid production and deposition. mouse model of AD based on amyloid deposition, similar to 5xFAD. APP/PS1 (Thy1) mice express single familial APP and single PSEN1 mutations under the control of neuronal-specific Thy1 promoter. large, postmitotic macroglia comprising ~50% of cells in the CNS in mammals. terminal processes of astrocytes that interact with blood vessels. They have a high concentration of water channels. (complement component 1q) complex glycoprotein complement component protein that associates with C1r and C1s to form the C1 complex. The C1 complex triggers the classical complement pathway. In the CNS, C1q also labels synapses for removal by microglia. selection of proteins (the complement system) in the innate immune system; incorporates antibodies and phagocytic cells to clear cell debris, pathogens, and promotes inflammation. In the CNS, complement is also used for promoting synapse pruning by microglia during development and in neurodegenerative disease. protein encoded by Csf1r: type III receptor tyrosine kinase binding CSF1 and IL-34. Essential for the survival and proliferation of many myeloid cells, including microglia. single cell transcriptomic methods using microfluidics to partition single cells or nuclei into nanoliter droplets (e.g., Drop-seq; 10X Genomics). umbrella term referring to a number of neurodegenerative diseases characterized by degeneration of the frontal and temporal lobes of the brain. protein encoded by Gfap; intermediate filament protein present in astrocytes in the CNS, but also in ependymal and radial glial cells during development. pathological process in which excess glutamate is not cleared from the synaptic cleft, causing damage and death of neurons. protein encoded by Aif1; calcium binding protein present in microglia and macrophages, commonly used for visualization of microglia. small, motile, resident innate immune cell comprising around 5–10% of the CNS in humans and mice. (single molecule fluorescence in situ hybridization) method enabling visualization of single RNA molecules using targeted probes. Used to provide important validation and spatial context for scRNASeq data. tumor of the brain or spinal cord; formed by oligodendrocytes. miniaturized version of an organ, made from differentiated stem cells and maintained in a culture dish. Organoids of nervous tissue can include combinations of neurons and glia. prions are small misfolded proteins that can propagate their misfolded shape. Prion infection refers to infection of an organ or tissue with a misfolded prion protein. response of microglia or astrocytes to an external stimulus, normally noxious in nature (e.g., bacterial or viral infection, acute trauma, or pathology associated with disease). method that allows measurement of expression of genes in individual cells or nuclei (snRNASeq). removal of excess synapses by microglia and astrocytes. Most evident during development or disease.