Transfer/watercolor by Karen Korell, http://www.kkorellworks.com
The brain, despite its meager weigh-in at two percent of the body's mass, is our most voracious organ. Our brains consume 60 percent of the sugar coursing through our bloodstreams, a total of about 450 calories each day, a couple candy bars worth of energy. And because the brain can't store energy as fat or glycogen—a storage molecule made of glucose—like other parts of the body can, it needs a continuous supply of fuel. Demand surges when we are faced with complicated mental tasks, like puzzling over a tax form or grasping for the perfect word to complete the perfect sentence.
Neuroscientist Ian Simpson has spent the past 10 years understanding what and how our brains eat. The "what" is easy. Basically the brain is picky, like a kid who eats only green peas, and only if they're arranged in neat rows on the tines of a fork. Simpson puts it a different way: "Like every cell in the body, brain cells use glucose for energy, but that's all they'll eat," he says. It's one of the few molecules allowed to pass through the protective barrier that separates the brain from the bloodstream. (The brain does have an "alternative mechanism" for getting energy, Simpson concedes. In the absence of glucose—during a period of starvation or when someone is on the Atkins diet or in the case of a breastfed infant—the body will convert fatty acids to acids known as ketone bodies that can pass into the brain and serve as food.)
Much less is known about how the brain eats, how it regulates supply and demand of energy. "The brain is this little engine that burns through an enormous amount of calories," says Simpson, "but we're still not sure what it does with all that energy." And when something goes wrong—in the case of diabetes, for example—that balance of supply and demand gets particularly complicated.
The hallmark of diabetes is this: The blood is too rich in glucose and yet muscle cells are stressed and starving. The muscle cells can't get enough energy because the diabetic body can't properly provide or use insulin—the hormone that stimulates the uptake of glucose into muscle cells. And when diabetics take insulin for treatment, they often get "too much stimulation and too much uptake," says Simpson, which leaves them with too little glucose in their bloodstreams.
While brain cells don't rely on insulin to get energy, the brain still has to deal with high and low levels of glucose in the blood-stream. Very low levels can put diabetics on the edge of a brain shut-down. This can cause what Simpson describes as "diabetic accidents": passing out behind the wheel of a car, for example. And high levels of glucose in the blood make diabetics susceptible to all sorts of physical and neurological problems, like memory loss and stroke.
Simpson, a researcher at Penn State's College of Medicine, thinks regulating glucose in the diabetic brain all comes down to better understanding how the brain gets and uses its energy—during periods of feast and famine, good health and poor health. He is focusing on the rather complex mechanism by which glucose is transported across that border-control system known as the blood-brain barrier.
Simpson himself is diabetic—late-onset, Type I—but he insists that's not exactly why he has spent his entire career studying diabetes. "The story is a little more interesting than that," he says, chuckling.
He was in his late twenties—about to get married, finishing his doctorate in bio-chemistry back in his native England, and seeking new scientific challenges. Then came the diagnosis of diabetes. "The physician who diagnosed me, a very well known researcher in Britain, said to me, 'Why don't you contribute something useful and study your disease?'"
Instead, Simpson moved to Germany to study adrenaline receptors in red blood cells at the University of Würzburg. "It was paradise for a diabetic," Simpson says. "It's the one place in Germany where they don't make sweet wine."
Still Simpson's physician, Harry Keen, persisted. "He told me there were many more opportunities and better funding for diabetes researchers." Keen put Simpson in contact with a colleague in the United States at the National Institutes of Health. Two years after his diagnosis, Simpson held a post-doc position at the NIH's National Institute of Diabetes, Digestive, and Kidney Diseases in Bethesda, Maryland. He stayed there for 20 years, eventually becoming associate chief of the Experimental Diabetes, Metabolism and Nutrition Section.
He spent his first decade at NIH focusing on the role of the insulin in the transport of glucose in muscle and fat cells. His transition into neuroscience began one day in the early 1990s over a beer with colleague Dick Hennerberry. "We'd go to the pub after work. He'd been complaining about the neurons he was growing in the lab. They would grow beautifully for nine days, and on the tenth day they would die," Simpson remembers.
Simpson asked Hennerberry if the neurons were getting enough glucose. Simpson speculated that the cells were using up all the glucose in the growth medium in the first nine days. On day ten, with no sugar to sustain them, they died. "At the time, very little was known about how the brain obtained its glucose," he explains.
This much was known: All cells—brain, muscle, liver, or otherwise—have to work for their food. Glucose cannot simply diffuse from the bloodstream into cells; specialized proteins are needed to transport it across cellular membranes. In muscle, heart and fat cells, insulin recruits these transporters to the cell membrane. What those cells can't use is stored as fat or glycogen.
The brain works differently. No insulin. No fat storage. And a tighter border to cross. The blood brain barrier is just that—a physical barrier meant to protect the brain from potential toxins in the blood. The tiny blood vessels that snake through the brain are made up of tightly linked endothelial cells that form a two-ply membrane. Somehow, glucose molecules have to navigate both of these layers to get into the brain,where neurons—the 10 percent of nerve cells that actually transmit signals to and from the brain—and glial cells—often called the "glue" that makes up the remaining 90 percent—are waiting to be fed.
Soon after their conversation, Simpson joined Hennerberry and began growing neurons and studying their eating habits. "We published a paper together showing that these neurons had two different types of glucose transporters," he says. Hennerberry had known about one type, called GLUT1, but not the other—"the one that was really responsible for transporting all the glucose," Simpson says. That particular transporter, called GLUT3, is unique to cells that need a tremendous amount of energy very quickly, like platelets, sperm cells, and, of course, neurons.
"I thought, 'geez these transporters are different,' and I got interested," says Simpson. "That was truly my first venture into the brain."
Six months later, a post-doctoral researcher named Susan Vannucci joined Simpson's group at NIH. Vannucci had studied diabetic fat cells for her Ph.D. at Penn State, and was prepared to continue these studies at NIH. Before she earned her Ph.D., she had spent many years doing brain research with her husband, Robert Vannucci, a pediatric neurologist at Penn State College of Medicine.
"Susan and I made the switch from the study of fat cells to the study of glucose transport into brain cells together," says Simpson. Thus began a ten-year research partnership that continues today. "Our families have become good friends, and we always spend Christmas together," he says. "And if you look at my publication list you'll see lots of Vannucci and Simpson papers."
One of their first joint ventures was to characterize all the different kinds of glucose transporters that exist in the brain. "We wanted to know what the brain had to work with," Simpson says. In addition to the transporters that Simpson had previously identified in the blood-brain barrier, they found others in the neurons and glial cells.
As Simpson explains it, the two membranes that make up the blood-brain barrier are very different. The inside surface, against which blood flows, is called the luminal side. The outside surface, which faces the brain, is called the abluminal side. Specialized transporters are embedded in both membranes. To enter the brain, sugar molecules bind to transporters on the luminal side and are pumped across the luminal membrane and into the endothelial cell. There, another transporter, positioned in the abluminal membrane, grabs the sugar molecule and ferries it into the brain. To deal with contingencies, Simpson continues, there is a sort of auxiliary crew of transporters floating around between the membranes—inside the endothelial cells—that can be recruited to either side of the membrane in response to a sugar imbalance.
At NIH, Simpson and Vannucci did experiments to see how abnormal levels of glucose in the blood affect the number of transporter cells the brain makes. First, they looked at what happens in the case of hypo-glycemia, or low blood sugar. They gave healthy rats a constant high dose of insulin, which caused the muscle cells to take up all of the glucose, leaving very little for the brain. Soon, Simpson reports, the rats showed the classic signs of low blood sugar: They were woozy and couldn't function very well. They "bonked"—like people sometimes do during a long run or an intense workout at the gym. After about two or three days, however, something interesting happened: The rats recovered. They started running around like nothing was wrong. Their blood glucose levels were still very low—at the same level that was originally making them sick—but, "somehow they were getting enough glucose into their brains that they could function normally,"says Simpson. "There was compensation."
Turns out the blood-brain barrier, over time, was able to sense the deficiency in blood glucose in these otherwise healthy rats. In response, the endothelial cells began to create more glucose transporters: 25 percent more. Half of those new transporters were assigned to the luminal side of the membrane to help transport as much glucose as possible from the blood.
This change in the number of transporters, Simpson stresses, happened only in the endothelial cells. He and Vannucci looked at other cells inside the brain—at neurons and astrocytes, a type of glial cell—and saw no increase. "That was a little bit of a surprise to us," says Simpson. It meant the blood-brain barrier alone was responsible for regulating the amount of sugar going to cells inside the brain.
Next they looked at the case of diabetes, where the level of sugar in the blood is abnormally high. After inducing diabetes in rats with a drug called Streptozoticin, they expected to see a decrease in the number of transporters produced in the rats' brain cells.
"We expected the cells to say, 'We don't need to transport as much because we sense we already have too much glucose,'" Simpson remembers. Instead, there was no change in the number of glucose transporters. "We still don't understand this," Simpson says. "We were just trying to confirm the work of other groups—who suggested that too much glucose caused a reduction in the number of transporters—but we couldn't confirm it.
In late 1999, Simpson left NIH to take an academic position in the neuroscience and anatomy department at Penn State College of Medicine. "I'm still getting used to how much time students take up," he smiles. Three or four gargoyles perch on shelves around his office—"to ward away evil spirits," he says, still smiling—and next to his computer leans a colorful cross-stitch of an old woman in a bathrobe on a scale, made by his secretary at NIH. It reads, "Brain cells may come and go, but fat cells live forever."
At Penn State, Simpson has continued his fundamental studies of the bloodbrainbarrier. His work was helped considerably when one of his collaborators, Richard A. Hawkins at the University of Chicago Medical School, developed a method for separating the two membranes of the endothelial cells and counting the number of transporters in both. "There is a specific radioactive tag that will bind exclusively to transporters," Simpson explains. The tags allow researchers to count how many transporters are in each membrane.
Using this technique, Simpson and his colleagues are taking apart brain cells and looking at how transporters migrate between the two membranes of the blood-brain barrier. "We want to try to understand the mechanism by which transporters are recruited to one side of the membrane or the other," he explains. "If we can understand the mechanism responsible, we might be able to come up with a way of triggering that movement," an intervention which could help diabetics better regulate the balance of glucose in both the blood and the brain.
Simpson has also become interested in the relationship between diabetes and stroke. According to the National Institutes of Health, diabetics are two to four times more likely than non-diabetics to suffer a stroke. And, diabetic brains have a much harder time repairing the damage a stroke can cause.
Stroke is the third leading cause of death and the leading cause of disability in the U.S. Sixteen million Americans have diabetes, Simpson notes. The stakes are high. Still, the details of how diabetes exacerbates stroke have yet to be sorted out.
Recently, he and Vannucci (now at Columbia University) received support from the American Diabetes Association to study how diabetic brains deal with this catastrophic event. Part of the reason diabetics are more susceptible to stroke, Simpson acknowledges, is that they tend to be overweight and have high blood pressure and heart problems. Beyond these well-known risk factors, however, "there's something else," he argues. "Something that puts diabetics at higher risk. But we haven't figured it out yet."
Right now, Simpson is most interested in what happens after a stroke. "The combined effects of both diabetes and stroke on the brain haven't been looked at as closely as the effects of stroke on the brain," he says. "But we know that recovery from stroke in diabetics is far worse than it is in non-diabetics. This is becoming more of an issue, as there are more and more diabetics."
During recovery from a stroke the tissue surrounding the area where the blockage occurred needs to obtain a supply of glucose and oxygen in order to repair the damage. Knowing this, "We have started focusing on the process of wound healing,"Simpson says. "Maybe it's the same process that happens in other parts of the body when there's a wound." In diabetics, this process is seriously impaired, which is one reason why diabetics who incur minor foot wounds, for example are at higher risk for gangrene and amputation.
"If the same proces of impaired wound-healing applies to the brain, then we need to know which cells are involved. It's probably a complex problem of which the blood brain barrier is just one part," he speculates.
During recovery from stroke in the "normal" brain, animal models show, there's an increase in the number of transporters both at the blood-brain barrier and in the astrocytes around the damaged area. "The astrocytes and microglia, which are trying to clean up all of the mess, require more energy, too," Simpson explains.
If the same thing happens in a diabetic brain, Simpson suggests, then the same sorts of therapies that are used to promote wound healing in other parts of the body might work in the brain. Drugs containing growth factors might help the cells surrounding the wound get the extra nutrients they need.
"But first," Simpson says, "we have to show that impaired wound-healing is indeed what is happening."
Ian Simpson, Ph.D., is professor of neural and behavioral sciences in the College of Medicine, H109 Neural and Behavioral Sciences, Hershey Medical Center, Hershey, PA 17033; (717) 531-4156; ixs10@psu.edu. His research is funded by the National Institutes of Health and the American Diabetes Association. He is also a member of the Penn State University Juvenile Diabetes Research Foundation Diabetic Retinopathy Center.