Unsung brain cells play key role in neurons' development
Ben Barres' research has led to a greater appreciation of glial cells, which comprise 90 percent of the brain.
Ben Barres has, on occasion, kept a brain in his office to share with visitors. That’s not so strange—he’s a brain scientist, after all. What sets him apart from most other neurobiologists is the part of the brain that he’s most interested in. While many of his colleagues have devoted their careers to studying neurons, Barres has become a world authority on a set of cells called “glia” and is at the forefront of research efforts showing how these cells play a pivotal role in brain function.
The two terms “glia” and “glial cells” are used interchangeably to denote the matter that accounts for 90 percent of the brain’s cells and more than half its volume. Only in recent years, however, have scientists realized that they are worthy of serious study. Even now, most continue to concentrate on neurons— the “talented tenth” of the human brain whose aptitude for high-speed, long-distance communication makes them the nervous system’s premier information-processors. In fact, neurons send electrical impulses whizzing down their long, tubular projections, called axons, which hook up to other neurons via junctions called synapses. A single neuron may interconnect with as many as tens of thousands of other neurons, forging circuits of dazzling complexity. A huge part of neurobiology is focused on trying to figure out the significance of all those circuits.
Astrocytes in green) can envelop the body of a neuron (in red).
“When you think about cells in the brain, pretty much the first thing you think about is neurons,” said Barres, MD, PhD, professor and chair of neurobiology, in a recent interview. But it was glial cells that riveted his attention several decades ago. “During my training as a neurologist, I looked at a lot of disease-affected brains under the microscope,” he recalled. “Every time I looked, I could see that the glia were altered—that got me curious.” He has been working on them ever since. And lately he and other neuroscientists have begun to discern how these cells play pivotal roles in sickness and health, in neuronal development and degeneration.
“There’s a loud sucking sound in this field as it draws in neuroscientists who, like neurons themselves, find themselves increasingly enmeshed with glial cells,” said Bruce Ransom, MD, PhD, who heads the University of Washington’s neurobiology department.
In 1986, Ransom, then an assistant professor of neurobiology at Stanford, got the idea of starting a journal called Glia. At the time, the glial-cell field was still considered almost disreputable — “like parapsychology,” he said. The first issue of Glia came out in 1988. “That year, we barely managed to publish 300 pages. Now we could publish 4,000 pages a year if we wanted to, but we’re holding the line at about 2,000.”
Guardian angels of the synapses
Glial cells’ activities are subtler than those of neurons—unlike their flashier electronic cousins, they converse in biochemical whispers—so learning their language has been tough. But a slew of research in just the past few years offers compelling evidence that astrocytes, one of the three major glial-cell types, play a significant role in determining where, precisely, synapses snap into place, which of them flourish and which die. The placement and relative strengths of the 100 trillion-plus synapses in our brain are intimately associated with what defines us—learning, thinking, feeling, remembering and forgetting. And, it turns out, our very synaptic architecture is profoundly influenced by the glial cells that envelop, ensheathe and communicate with neurons everywhere one looks in the brain.
Besides astrocytes, two other glial-cell types have important, if not exotic, functions in the brain. Oligodendrocytes, which account for no more than 40 percent of the cells in the human brain, extrude a flagship fatty product, myelin, that insulates neuron’s surfaces and speeds signals along their wirelike axons. Microglial cells (or “microglia”), comprising about 5 percent of the brain’s cells, serve an immune function in our brains.
But recent findings about the astrocyte—probably the most common glial cell type, constituting about half of all human brain cells—probably have done the most to change scientists’ attitudes about glia. Astrocytes are so named because their many long, thin projections cause them to appear star-shaped under a microscope. It’s been known for some time that astrocytes take care of rather passive housekeeping functions such as feeding neurons (supplying nutrients, precursors for specialized chemicals, and energy-rich molecules) or soaking up spent substances secreted into the synapses so that the next signal will be a clean one.
“We wondered, is that all?” said Barres. He had a hunch that they did much more.
Divide and conquer
To answer that question, researchers needed to study how all the brain’s different cell types work together as a system. And to do that, they first had to be able to tease them apart. Accomplishing that task was no mean feat. But Barres was persistent. Since his graduate school days in neurobiology, Barres (now aided by numerous postdocs and grad students) has figured out how to separate the brain into its cellular building blocks.
That purification advance allows for gene profiling. By extracting genetic material from purified cultures of single cell types and pouring the material over a “gene chip”—a microarray device that can quickly quantify the extent to which genes are turned on (or “expressed”) within cells—Barres and his colleagues have succeeded in demonstrating that the genes expressed by the different brain-cell types vary drastically. This has allowed the Barres group to harvest very specific markers for all those cell types.
“We can label a brain section and see where all of the astrocytes are, not just some of them. And they’re all over,” said Barres. Visualizations permitted by these markers show that astrocytes envelop neurons’ synapses. One astrocyte can envelop thousands of them, even tens of thousands, at a time.
In a study published in 2005, Barres and his co-investigators showed astrocytes are necessary for synapse generation. To show this, they cultured highly purified mouse neurons in the total absence of glial cells. “We added all the proper growth factors and so forth to prevent cultured neurons from dying,” Barres explained. “They made axons, they even fired electrical signals. Everything looked good. Except for one thing—they were hardly making any synapses. They produced what at least looked like synapses, but only at one-tenth the number normally seen. And those that were produced didn’t work very well.”
Adding astrocytes, or solutions in which astrocytes had been bathing (which presumably carried soluble factors these cells secrete), reversed the mouse neurons’ overall synaptic deficiency. Barres and his team have since demonstrated that adding a single protein secreted by astrocytes—it’s called thrombospondin—to the medium in which these purified neurons were growing boosted their synapse construction immensely.
“Thrombospondin’s role in the brain had never been studied before,” Barres said. He found that astrocytes produce it only during brain development, just at a time and a place synapses are sprouting up all over. Then, when brain maturation is completed, thrombospondin expression shuts down everywhere in the brain—except in the hippocampus, the part of the brain where new memories are formed, and one of the few places in the adult brain where constant large-scale synapse formation still happens.
What’s more, the researchers discovered another exception to the cessation of thrombospondin expression. “When the brain is injured, the neighborhood astrocytes go into a completely altered state,” Barres said. “They take on totally new properties. One of those is that they turn thrombospondin expression back on.” Barres asked: Could those astrocytes be playing a part in inducing and repairing synapses in the injured brain?
Interestingly, thrombospondin is one of only two genes are that far more highly expressed in human brains than in those of other primates. Barres and colleagues have been continuing to examine the role of astrocytes’ expression of thrombospondin and will soon publish findings on how it triggers neuronal receptors to initiate synapse formation.
A role in brain disease?
A key feature in healthy young developing brains—the very ones in which it’s so critical for synapses to form—is, ironically, synapse death. That’s because the developing brain generates far more synapses than it needs.
“The ‘good’ synapses stay, and some ‘bad’ extra synapses are pruned away,” said Barres, who has found evidence that astrocytes are playing a very active role here, too. In a study published in 2007, he and his colleagues showed that astrocytes cause some synapses to be covered with a protein called C1q not previously thought to be expressed in brain tissue. On the far side of the blood-brain barrier, whenever C1q coats the surface of bacterial or human cells it marks them for destruction by the body’s immune system. It seems to be marking synapses in the brain for destruction, too.
C1q-mediated synapse loss may turn out to be an important feature of such neurodegenerative disorders as Alzheimer’s disease, Lou Gehrig’s disease and glaucoma. The cardinal feature of all these diseases is synapse loss. Barres thinks the C1q pathway may be involved.
“Expression of this protein shuts off in adult brain,” he said. “But in neurodegenerative disease, it turns back on. Alzheimer’s disease is characterized by massive synapse loss. By the time of even the earliest detection of cognitive loss, as many as 80 percent of synapses may have already disappeared from some brain regions.”
Barres believes opportunities for putting this insight to practical use abound. “There’ve been a thousand failed clinical trials for stroke, all of them focused on keeping the neurons alive. But we know that the astrocytes make the chemical signals that keep the neurons alive. I keep hammering on this. If you’re going to keep one cell type alive in the stroke treatment, focus on the astrocytes!”
Every day, it’s a safer bet that glial cells are more than packing peanuts to keep our neurons from jiggling as we jog. As one ascends the scale of evolutionary complexity, an increasing proportion of the brain’s cells are glial. In the simple nematode worm, they’re sparse; in a fruit fly, they’re up to 25 percent; in a mouse, about 65 percent. In a human brain, behind every great neuron stand nine great glial cells.
There are three main types of glial cells. Oligodendrocytes (1) send projections that wrap axons (2) – long, signal-carrying portions of neurons (3) – in sheathes of a fatty substance called myelin (4), speeding signal conduction. Microglia (5) are, essentially, the brain’s immune cells, but they also monitor neighboring brain cells for damage and gobble up debris, and they probably have other functions, too. Astrocytes (6) carry on a host of activities. Their long extensions can monitor levels of neuronal activity either along axons at synapses (7) – junctions that relay signals from one neuron to the next – and, when those activity levels are high, signal to local blood vessels (8) to dilate, increasing blood supply to hard-working neurons. Astrocytes also produce and secrete substances that have a major influence on the formation and elimination of synapses.
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