Tag: Academic research

A study in mice headed by NYU Langone Health researchers has found that cells long thought to play a secondary role in brain function build their own far-reaching connections. These pathways appear to connect distant regions in ways that had not been mapped before.

Experts usually describe the brain as a network of nerve cells (neurons) that send each other signals to pass along information. These neurons are maintained by another kind of brain cell, the star-shaped astrocyte, which ferries in nutrients and carries away waste.

The newly reported study, headed by Melissa Cooper, PhD, a postdoctoral fellow in the department of neuroscience at NYU Grossman School of Medicine, revealed that, like neurons, astrocytes form organized webs, which enable them to communicate with other specific astrocytes across the brain rather than only sending local, generalized signals. In some cases, the pathways were found to link areas that were not already joined together by neurons.

“For more than a century, neuroscientists have thought of neurons as the main actors in the brain,” said Cooper. “Yet our findings suggest that astrocytes, which are usually viewed as merely support cells, are also running their own widespread signaling pathway, adding another layer to how brain regions stay connected.” The team suggests that while their study was carried out in mice, not humans, the findings form the basis for future studies investigating how astrocyte networks might link with injury, disease, or aging and to learning and memory.”

Cooper is first and co-corresponding author of the team’s published work in Nature, titled “Astrocytes connect specific brain regions through plastic networks,” in which the researchers stated, “Astrocyte networks can directly link brain regions that are not connected by neurons, suggesting that previously unassociated brain regions communicate with one another through gap junction-coupled astrocytes.”

“Neuronal axons have traditionally been considered to be the primary mediators of functional connectivity among brain regions,” the authors wrote, and the role of communication mediated by astrocytes has been largely underappreciated. “This communication occurs through gap junctions—membrane channels that connect the cytoplasm of neighboring cells, enabling them to redistribute resources and share biochemical signals,” the team continued. “Studies using mice lacking astrocyte gap junctions have shown that these gap junctions are necessary for memory formation, synaptic plasticity, coordination of neuronal signaling, and closing the visual and motor critical periods.”

In earlier work, Cooper reported that in a mouse model of the visual neurodegenerative disease glaucoma, astrocytes can redistribute resources from astrocytes around healthy neurons to damaged neurons. Yet the team had no way to see whether this kind of support-cell network extended across the entire brain.

Cooper said the newly reported study is the first to map active, brain-wide communication networks built by astrocytes and to show that these pathways are highly specific. The research relied on a custom-built tracing tool that let the team follow the cells’ connections in far greater detail than had been possible using past methods. “Despite the importance of astrocyte gap junctional networks, studying them has been challenging,” the investigators noted. “Current methods such as slice electrophysiology disrupt network connectivity and introduce artefacts due to tissue damage.”

For their study, the researchers used a harmless virus to deliver “network tracers” into astrocytes in selected brain regions of lab mice. These tracers tagged small molecules as the molecules passed through the gap junctions linking one astrocyte to another, allowing the team to see which cells were part of the same signaling pathway.

The scientists then made the mice’s brains transparent and used a specialized microscope to capture three-dimensional images of every tagged astrocyte. By doing this across hundreds of mice, they could map astrocyte webs across brain areas. “These networks selectively connect specific regions, rather than diffusing indiscriminately, and vary in size and organization,” they reported. “We observe local networks that are confined to single brain regions and long-range networks that robustly interconnect multiple regions across hemispheres, often exhibiting patterns distinct from known neuronal networks.”

The tracing tool and brain-clearing method were designed to be relatively low-cost and easy to reproduce so that other labs could use them to study the networks in many brain diseases.

In another part of the study, the team assessed mice that were genetically engineered with astrocytes that lacked gap junctions. The communication networks largely disappeared, suggesting that the pathways are active and depend on these physical bridges.

“By challenging our understanding of how the brain communicates over long distances, our results may offer fresh insight into how it develops, ages, and behaves in conditions such as Alzheimer’s and Parkinson’s diseases,” said study co-senior author Shane A. Liddelow, PhD, an associate professor in the neuroscience and ophthalmology departments at NYU Grossman School of Medicine.

Another key finding was that astrocyte networks are dynamic. When the team trimmed whiskers on one side of the mice’s faces—“this manipulation is known to induce robust structural remodeling in neurons,” the team noted—a pathway from the region that processes whisker touch got smaller and reconnected to different astrocyte partners.

“The fact that astrocyte networks shrink and reroute after a loss of sensory signals suggests they may be shaped by experience,” said study co-senior author Moses V. Chao, PhD, a professor in the cell biology, neuroscience, and psychiatry departments at NYU Grossman School of Medicine. “It also raises the possibility that each of us has a somewhat unique pattern of connections molded by what our brains have learned and lived through.”

The authors plan to investigate which molecules move through the networks and to apply their tracing tool to models of brain disorders. They also hope to examine how these webs change during development and aging, said Chao.

Liddelow emphasized that while gap junctions and astrocytes exist in humans, it remains unknown whether the networks link the same regions in the same way as in mice. Nevertheless, in their paper, the team concluded that their findings “… establish foundation for future exploration of how astrocyte network structure and function are shaped by injury, disease, development, aging and experience-dependent processes such as learning and memory.”

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