Researchers at the Sanford Burnham Prebys Medical Discovery Institute (SBP) say they have identified an astrocyte subpopulation as the dominant active cell type in vivo in a neuroinflammatory disease setting. Their early activation inspired a new name: ieAstrocytes (immediate early astrocytes).
The team’s study (“A functionally defined in vivo astrocyte population identified by c-Fos activation in a mouse model of multiple sclerosis modulated by S1P signaling: immediate-early astrocytes [ieAstrocytes]”) appears in eNeuro.
“Astrocytes have prominent roles in central nervous system (CNS) function and disease, with subpopulations defined primarily by morphologies and molecular markers often determined in cell culture. Here, we identify an in vivo astrocyte subpopulation termed “ieAstrocytes (immediate-early astrocytes)” that is defined by functional c-Fos activation during CNS disease development. An unbiased screen for CNS cells showing c-Fos activation during experimental autoimmune encephalomyelitis (EAE), a mouse model for multiple sclerosis (MS), was developed by using inducible, TetTag c-Fos reporter mice that label activated cells with a temporally stable, nuclear green fluorescent protein (GFP). Four-dimensional (3D over time) c-Fos activation maps in the spinal cord were produced by combining tissue clearing (iDISCO) and confocal microscopy that identified onset and expansion of GFP+ cell populations during EAE,” write the investigators.
“Over 95% of the GFP+ cells showed glial fibrillary acidic protein (GFAP) immunoreactivity—in contrast to absent or rare labeling of neurons, microglia, and infiltrating immune cells—which constituted ieAstrocytes that linearly increased in number with progression of EAE. ieAstrocyte formation was reduced by either astrocyte-specific genetic removal of sphingosine 1-phosphate receptor 1 (S1P1) or pharmacological inhibition by fingolimod (FTY720), an FDA-approved MS medicine that can functionally antagonize S1P1. ieAstrocytes thus represent a functionally defined subset of disease-linked astrocytes that are the first and predominant CNS cell population activated during EAE, and that track with disease severity in vivo. Their reduction by a disease-modifying agent supports their therapeutic relevance to MS and potentially other neuroinflammatory and neurodegenerative diseases.”
“There is an urgent need for treatments of brain inflammation disorders that are involved in many diseases, including MS and Alzheimer's disease,” says Jerold Chun, M.D., Ph.D., senior author of the paper and professor and senior vice president of neuroscience drug discovery at SBP. “Developing therapies that prevent the formation of ieAstrocytes or reduce their activation levels in the brain could offer new approaches for treating neuroinflammatory and neurodegenerative diseases.”
The scientists identified the new type of astrocyte using an unbiased, fluorescent labeling technique to visualize the most active brain cells that express an activity-dependent transcription factor, cFos. Cells that were “turned on” glowed green, allowing the researchers to track activated cells over time and space. Applying this method to a mouse model of brain inflammation allowed visualization of which cells were activated as the disease progressed.
“We expected to see immune cells light up—but surprisingly, they weren't activated. Neither were neurons or microglia,” says Dr. Chun. “ieAstrocytes were the first and predominant cells activated during disease initiation and progression, suggesting that they are a key gatekeeper and mediator of disease. This is a departure from our previous understanding that astrocytes are spectator cells, only 'moving to the dark side' once initial damage has occurred.”
ieAstrocytes increased in number as brain inflammation progressed, indicating they play a key role in disease. FDA-approved drug for MS, GILENYA® (fingolimod), reduced ieAstrocyte formation, further implicating their role in disease and identifying direct brain effects of the drug.
“Greater understanding of ieAstroycytes could unlock more of the brain's mysteries,” says Dr. Chun. “Defining these cells through their in vivo activity is an important first step, as it can help to guide therapeutic development using a readout that tracks with a brain disease.”
Dr. Chun's team is already working on their next step: characterizing these astrocytes at the molecular level, particularly the specific genes that are activated.
