Bouts of white-matter destruction in the brain and spinal cord are a hallmark of multiple sclerosis, but the inflammatory disease also ravages gray matter. Using a mouse model of cortical MS, researchers led by Doron Merkler at the University of Geneva, Thomas Misgeld of the Technical University of Munich, and Martin Kerschensteiner of Ludwig-Maximilians University, Munich, report that cortical inflammation triggers the loss of dendritic spines, suppressing neuronal activity across the cortex. The study, published January 25 in Nature Neuroscience, found that calcium overload in a small fraction of spines marked them for destruction by resident microglia and infiltrating monocytes. Crucially, these spines and synapses rebounded once inflammation subsided. A drug that dampens microglial activation kept their appetite for the spines at bay. The findings add to mounting evidence implicating neuroinflammation in the destruction of synapses, and suggest that quelling it could even reverse damage already done.
- A mouse model of cortical MS invokes inflammation throughout the brain.
- Dendritic spines shrivel; neuronal activity wanes.
- Calcium overload marks spines for removal by myeloid cells.
“These findings represent substantial promise for the clinical treatment of MS, as they suggest that early intervention with immunomodulatory therapies could work in combination with endogenous healing processes within the brain to prevent progressive MS-associated neurodegeneration,” wrote Marc Aurel Busche and Robert Ellingford of University College London. The findings may also have implications for other neurodegenerative diseases, they added.
Overzealous munching of synapses by ill-tempered microglia is a common scourge across neurodegenerative diseases, including Alzheimer’s, Huntington’s, and frontotemporal dementia (Nov 2015 conference news; Apr 2016 news; May 2016 news). In MS, an autoimmune attack on the fatty myelin sheath that surrounds axons is the predominant feature of the earlier, relapsing/remitting phase. However, as the disease progresses, it resists treatment, and gray matter in the cortex becomes increasingly damaged as well (Mahad et al., 2015; Eshaghi et al., 2018; Scalfari et al., 2018). Studies have found loss of synapses and dendritic spines in postmortem cortical samples of people who died with progressive MS, but what drives this degeneration is not understood (Jürgens et al., 2016; Albert et al., 2017).
To investigate, co-first authors Mehrnoosh Jafari, Adrian-Minh Schumacher, and Nicolas Snaidero used a mouse model of cortical MS. The researchers injected interferon-γ and tumor necrosis factor-α (TNF-α)—proinflammatory cytokines detected in the meninges of patients with MS—into the somatosensory cortices of mice that previously had been immunized with myelin–oligodendrocyte glycoprotein (MOG). These animals mobilize an autoimmune response that attacks their own myelin, much as occurs in MS. Within three days of injecting the cytokines, widespread lesions emerged across the cortex. Akin to those found in people with progressive MS, the lesions were marked by extensive myelin loss and were inundated by phagocytes and, to a lesser extent, T cells.
The researchers next zoomed in on dendritic spines—the tiny protrusions that serve as a platform for synapses. Three days after the cytokine shock, the number of spines plummeted by a third in cortical projection neurons. Oddly, surviving and dying spines often sat next to each other on the same dendrite. In addition, the researchers detected widespread loss of excitatory, but not inhibitory, synapses across the cortex. This loss of excitatory input resulted in neuronal silencing. Strikingly, within two weeks after the cytokine injection, all of these deficits were gone: spines and synapses grew back, and neuronal activity was restored.
What caused some spines to perish, while others weathered the cerebral cytokine storm? Disturbances in calcium homeostasis are known to herald spine loss, so the researchers measured calcium levels within spines using cranial windows and confocal microscopy (Jul 2008 news). Three days after tripping off the inflammatory cascade, the scientists detected a calcium overload in about 6 percent of dendritic spines. Tracking the fate of these spines over time, they found that the calcium-loaded spines were more likely to be removed. They calculated that this inflammation-induced calcium overload accounts for the loss of about 1 percent of cortical spines per hour, resulting in a 30 percent loss over the course of the acute inflammatory episode.
Going, Going, GONE. In control mice (top rows), dendritic spines were not overloaded with calcium. In cortical MS mice (bottom rows), some spines had high calcium (marked by arrows) at 0 hours (left column). Two hours later (right column), one calcium-overloaded spine (dashed arrow) was gone; the other had filled with more calcium (yellow arrow). [Courtesy of Jafari et al., Nature Neuroscience 2020.]
The work supports the decades-old hypothesis that calcium dysregulation is a driver of neurodegeneration, noted Christopher Norris of the University of Kentucky in Lexington. “The present work … is yet another example of just how vital the ‘Ca2+ hypothesis’ is after all these years, and further cements the visionary status of its early proponents.”
How were the calcium-loaded spines destroyed? Using transgenic mice in which peripheral monocytes and resident microglia can be distinguished based on different fluorescent markers, the researchers caught both types of cells red-handed, i.e., stuffed with synaptic material.
Finally, the researchers asked whether an inhibitor of colony-stimulating factor 1 receptor might stem the synaptic carnage. High doses of CSF-1R inhibitors wipe out microglia completely, but low doses stifle microglial function. Such inhibitors have been widely used in AD studies. A recent report found that blocking CSF-1R with a newly developed, highly selective, and CNS-penetrant small molecule inhibitor called sCSF1Rinh lessened axonal damage in a mouse model of progressive MS (Hagan et al., 2020).
In their model of cortical MS, Jafari and colleagues found that low doses of inhibitor halved the number of infiltrating monocytes, and reduced activation markers on brain-resident microglia. Using RNA sequencing, they found that CSF-1R inhibition kept the microglia in a homeostatic state; without it, the cells adopted a neurodegenerative signature similar to one reported in previous studies (Sep 2017 news). The treatment also dampened microglial engulfment of synapses, and prevented spine loss.
In all, the findings suggest that while inflammation goads innate immune cells to gorge on synapses, the brain has mechanisms in place to reverse the damage once inflammation subsides. This reversibility supports the idea that drugs such as CSF-1R inhibitors stand a chance at rejuvenating lost synapses, Kerschensteiner said. He plans to test the idea in a model of lower-grade, chronic cortical inflammation, which would be more akin to disease in humans, he said.
So far, the mechanisms that drive calcium overload in a subset of spines, and how that triggers their removal, remain unknown, Kerschensteiner said. He noted that the complement system could be involved in tagging the spines and marking them for removal, and/or calcium overload could lead to destabilization of the spines, beckoning phagocytes to the scene. Oleg Butovsky of Brigham and Women’s Hospital in Boston raised these possibilities as well, and added that the role of T cells in the synaptic destruction remains to be explored. He pointed out that while cytokine injection alone induced infiltration of monocytes and T cells, synaptic loss and demyelination only occurred in mice previously immunized with myelin–oligodendrocyte glycoprotein, suggesting an adaptive T cell response was somehow involved.
“This is a very exciting study, but many questions remain that require further investigation,” wrote Rebecca Gillani of Massachusetts General Hospital in Boston. Among them, Gillani wondered about the timing of synapse loss with respect to disease progression in people with MS, noting that it is possible that synapses are affected even before the transition from relapsing-remitting to progressive MS. Pinning down the beginning of this synaptic loss would help researchers know when to start treating it, she noted. She, along with other commentators, also wondered whether synaptic loss would be reversible in the context of the sustained, inflammatory assault that takes place in MS.—Jessica Shugart
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