Take the nasty bacterium that causes cholera, delete the gene for its wellstudied toxin, and you should end up with a harmless microbe that can immunize people against the real thing. It sounded like a good plan, recalls Alessio Fasano of the University of Maryland School of Medicine in Baltimore.
The scheme failed, however. A decade ago, when Fasano and his colleagues used a genetically stripped Vibrio cholerae to vaccinate 10 people, half the volunteers developed mild diarrhea. The altered bacterium obviously had some bite left in it.
The effort to understand what went wrong in that vaccine experiment has shifted the interests of Fasano and his colleagues to topics far from cholera. In 1991, they identified a second V. cholerae toxin that contributes to the diarrhea of the disease. Since then, they've learned that this toxin mimics a protein that the body uses to regulate the permeability of the seams between cells within the intestine, brain, heart, and other tissues.
Fasano sees many uses for this human protein. By helping drugs slip out of the intestine and into the bloodstream, the protein may become a component of an insulin pill that replaces a diabetic person's periodic shots. A similar approach could enable scientists to sneak medications past the formidable blood-brain barrier.
Moreover, the discovery of the way the body normally controls tight junctions, the specialized seams between cells, may help investigators explain why the junctions sometimes aren't tight enough. Leaky junctions occur in disorders including diabetes, celiac disease, and even some brain illnesses.
"It's the typical example of scientific serendipity. We were looking for one thing and came up with something totally different," chuckles Fasano.
If the cells that line the intestine represent bricks in a wall, tight junctions are the mortar that binds them together, or so scientists long thought. More recently, as they've identified some of the proteins that make up a tight junction, investigators have begun to realize that the bricks-and-mortar analogy is misleading. "Our molecular understanding of the junction complex is just exploding now," notes Mark Donowitz of the Johns Hopkins Medical Institutions in Baltimore.
Tight junctions have proven to be far more flexible and dynamic than mortar. Although the junctions in the intestine usually prevent proteins, water, and other molecules from passing between adjoining cells, they occasionally permit some molecules to slip through. In fact, cells can signal the tight junctions to alter their permeability, says Fasano.
The cholera toxin that his group discovered has now led the researchers to a regulator of the junction. The cholera toxin identified earlier directly increases secretions from the cells lining the intestine, resulting in diarrhea. Fasano's toxin instead opens tight junctions, enabling water and other molecules from blood to seep between the cells and into the intestine.
Drawing on the scientific name for tight junctions, the investigators dubbed the bacterial protein zonula occludens toxin, or Zot. They also identified the protein on intestinal cells to which Zot binds, triggering the loosening of the tight junctions.
Fasano and his colleagues immediately wondered if there was a human protein resembling the bacterial toxin. After all, because of their intimate and long-term association with the organisms they infect, bacteria are among the best cell biologists around. The microbes frequently make proteins that mimic some of the host's molecules.
Sure enough, the investigators found a Zotlike human protein, which they named zonulin. Zonulin, too, makes the intestine "leakier," says Fasano. To confirm that the human intestine produces this protein, the scientists obtained various tissues from a cadaver and screened them for zonulin by using antibodies that latch onto Zot. As expected, intestinal tissue contained the protein, but so did heart, brain, and a few other tissues.
This briefly perplexed the researchers, until they remembered that tight junctions join cells in many different regions of the body. Indeed, next to the intestinal lining, the best-known locale for tight junctions is the blood-brain barrier. This sheet of cells lines the blood vessels that snake through the brain, creating a barricade that protects the delicate nerve tissue from potentially damaging molecules within the bloodstream.
In the January JOURNAL OF NEUROCHEMISTRY, Fasano's group confirms that brain tissue contains the cell-surface protein that zonulin binds to induce a cell to open its tight junctions. Furthermore, the scientists have confirmed that in pieces of the blood-brain barrier grown in dishes, both zonulin and Zot open the tight junctions.
"We have precious little understanding of the cell biology of blood-brain-barrier tight junctions. This adds to that. It's a nice piece of work," says William M. Pardridge of the University of California, Los Angeles.
Fasano calls zonulin a key that opens a gate between cells. He notes that the human body actually has several similar keys. Zonulin made in the intestine differs by a few amino acids from the ones produced in the brain or in other tissues. The subtle alterations seem to prevent zonulin meant for one tissue from altering the tight junctions of another.
"It makes sense," says Fasano. "[The body] may have a need to make the intestine leakier but leave the blood-brain barrier unaffected, or vice-versa."
While intrigued by zonulin's potential to help elucidate the workings of tight junctions, Fasano is far more excited by the prospect of using the protein to improve human health. He envisions physicians administering zonulin or Zot to deliver drugs through the intestine into the bloodstream. This technique might eliminate the need for injecting many drugs, he says.
"We can make the intestine leakier to the point that we can allow passage of molecules not normally absorbable by the intestine. The implications of this are tremendous," says Fasano.
An obvious candidate for such a strategy is insulin, the hormone that people with diabetes take to control their blood sugar. Like many other proteins with medical uses, insulin molecules are too large to be absorbed by intestinal cells or to slip past the tight junctions. So, oral insulin stays in the digestive tract, where it gets broken apart. That's why people must inject themselves with the drug, an onerous task, especially for kids.
In a 1997 mouse study, Fasano's team found that Zot dramatically increases the intestine's ability to absorb insulin. Moreover, mice given oral-insulin doses with Zot controlled their glucose levels just as well as mice receiving insulin injections did. Recently, the same approach has controlled the blood-glucose concentrations of diabetic monkeys, adds Fasano.
Wouldn't the toxin trigger diarrhea? None of the animals experienced that side effect or suffered any other problems, says Fasano.
Water from the bloodstream does seep into the intestine, but the colon, where Zot has no effect, apparently reabsorbs it. This suggests that Zot alone can't trigger diarrhea and that other molecules made by V. cholerae must contribute to that symptom. Moreover, the Zot-induced change in intestinal tight-junction permeability reverses itself within a half hour, Fasano adds.
The University of Maryland, which owns several patents stemming from the zonulin research, has assigned most of them to Zone Therapeutics, a firm in Rockville, Md. With the success of the animal testing, the company now plans to ask the Food and Drug Administration for permission to start testing the insulin-Zot or insulin-zonulin strategy on people.
There's certainly a strong demand for an insulin pill. When Fasano and his colleagues published their study of mice, people with diabetes besieged his office with requests for oral insulin.
The investigators believe that Zotlike compounds will also let them breach the blood-brain barrier. This barricade has frustrated drug developers for many years. "More than 98 percent of all drugs do not cross the blood-brain barrier," notes Pardridge.
Because of this obstacle, physicians targeting the brain must directly inject most drugs through the skull, a risky and infection-prone procedure. Working with mice and a tracer that mimics drugs, Fasano's group has tried something far simpler--with success.
"When we administer both Zot and a tracer, we see the tracer reaching the brain," says the researcher. Buoyed by these still unpublished results, he suggests this strategy could help physicians get chemotherapy drugs to brain tumors.
Pardridge expresses skepticism about this approach, however. "If you're going to deliver drugs by disrupting the bloodbrain barrier, you're going to let in everything else in the bloodstream," he warns, pointing out that other compounds that disrupt the barrier have induced seizures in animals.
Fasano counters that those compounds irreversibly destroy the barrier, whereas Zot or zonulin would open tight junctions of the blood-brain barrier just long enough to allow transport of a drug across it. Preliminary tests with rodents support this belief, he says.
"If well used, zonulin should not cause any side effects, since the process is quick and reversible within a few minutes," asserts Fasano.
Researchers other than Pardridge also worry that Fasano is overly optimistic at this early stage in his research. They note that he hasn't yet announced the finding of the gene for zonulin or provided a detailed description of the protein in a scientific journal.
"The story sounds wonderful, [but] there's information that needs to be filled in: the precise identity of the protein, where it's made [in the body], and how it works," cautions Jeffrey B. Matthews of Beth Israel Deaconess Medical Center in Boston, who studies intestinal permeability.
Fasano says that his group has such data but waited for the University of Maryland to patent zonulin and its uses. A number of publications should appear soon, he says.
Fasano predicts that besides delivering drugs, zonulin will help explain facets of several diseases. Take celiac disease, an enigmatic intestinal disorder seemingly triggered by abnormal sensitivity to gluten, a component of wheat, barley, and other grains. People with celiac disease suffer a broad array of symptoms, including diarrhea, weight loss, and malnutrition. Curiously, people with celiac disease also seem to face an unusually high risk of developing autoimmune disorders, such as type I, or juvenile-onset, diabetes.
Scientists have yet to develop a good explanation for what goes wrong in celiac disease, but Fasano's team may have found an important clue. It's been known for some time that intestinal tight junctions become more permeable in celiac disease, especially when the disorder flares up. "These gates are stuck open, and we don't know why," says Fasano.
He and his colleagues recently reported at a meeting of pediatricians in Agricento, Italy, that intestinal tissue from people with acute celiac disease contains much more zonulin than does similar tissue from people free of the disease. The investigators hypothesize that gluten, for a still unknown reason, stimulates inappropriate zonulin production, leading to the condition's varied symptoms.
They also speculate that the leaky tight junctions in a person with celiac disease may allow molecules to leak from the intestine into the blood and trigger autoimmune reactions. To support this idea, Fasano has examined laboratory rats that spontaneously develop diabetes because their immune systems begin attacking insulin-producing cells in the pancreas. These rodents, he finds, always have an increase in intestinal permeability several weeks before the diabetes symptoms appear.
As scientists pay more attention to zonulin and its ability to regulate tight junctions throughout the body, they may link the protein to additional illnesses, Fasano predicts. Although no one has yet implicated zonulin in Alzheimer's disease, one of the symptoms of the disorder is a breakdown of the blood-brain barrier.
"We have also identified an important heart disease in which zonulin seems to be involved," says Fasano, hinting at a new line of investigation.
Not bad for a failed vaccine experiment.
Ah, summer nights! The heat and humidity mingle with the sweet scent of citronella candles, the blue glow of a bug zapper, and the sticky feel of mosquito repellent. Some unfortunate people need this entire antimosquito arsenal to avoid getting eaten alive, while others hardly attract the pesky creatures at all.
Scientists have known for decades that mosquitoes are drawn to carbon dioxide, exhaled in abundance by the animals that hungry mosquitoes favor. Carbon dioxide doesn't tell the whole story, though.
Mosquitoes, after all, tend to bite people on the arms and legs. "I can't imagine that much carbon dioxide coming off of someone's hand," says Ulrich R. Bernier, a chemist at the U.S. Department of Agriculture's Agricultural Research Service (ARS) in Gainesville, Fla. "And how often do mosquitoes fly into your mouth or nose?"
Carbon dioxide is clearly important, but the skin must give off other attractants, too, he reasons.
In 1968, ARS scientists discovered that lactic acid, which emanates from human skin, acts as an attractant. Now, Bernier and his colleagues are trying to find other compounds emitted by skin that lure mosquitoes to their meals of blood. Using sensitive analytical techniques, they have catalogued more than 340 candidates. In the lab, the scientists are now offering some of these substances to mosquitoes to gauge their attractiveness.
The researchers' ultimate goal is to develop better bug repellents and traps, a goal made more urgent by the threat of mosquito-borne diseases, says Bernier. Other scientists might also find biochemical, medical, or forensic applications for the results, he says.
In the past, testing mosquito attractants required a brave volunteer to place an arm into a cage full of the insects. Luckily, the ARS group learned about 20 years ago that when a person handles glass, a residue that includes mosquito attractants is transferred from the skin. So instead of a live arm, researchers can often substitute a glass petri dish, which will attract mosquitoes for up to 6 hours.
Bernier wanted to find out what's in that mosquito-tempting residue. To collect samples, he and his colleagues asked volunteers to rub a handful of small glass beads between their palms for several minutes. The researchers then loaded the beads into a gas chromatograph-mass spectrometer, a device that separates mixtures of chemicals and identifies each component.
The glass beads enable researchers to trap oily skin emanations without capturing a lot of perspiration that would overwhelm the gas chromatograph with water. The technique also minimizes the collection of squalene, a precursor to cholesterol that's abundant on skin. In this way, the method unmasks the scarcer chemicals, Bernier says. He and his colleagues Matthew M. Booth and Richard A. Yost of the University of Florida in Gainesville described the technique in the Jan. 1, 1999 ANALYTICAL CHEMISTRY.
The glass-bead sampling, however, doesn't capture the skin's most volatile components, which simply evaporate. Moreover, many water-soluble molecules don't show up in the samples because little water is collected. To get around these problems, the researchers would also wrap each volunteer's arm in a plastic bag and then drew out the humid air for a complementary analysis.
With these methods, Bernier and his colleagues have doubled the number of chemicals known to emanate from human skin. One day, more sensitive detection methods could further lengthen the list, he says.
"There could be well over 1,000 trace chemicals that I can't detect," he speculates. He and his colleagues present their latest findings in the Feb. 15 ANALYTICAL CHEMISTRY.
To gauge the mosquitoes' reaction to chemicals, the researchers use a simple setup that they call an olfactometer. It consists of three clear plastic cages stacked on top of one another, each connected to a pair of cylindrical ports. Screens divide the attractants from the insects. Inside each cage, 75 female mosquitoes wait to be enticed. Only females bite; they need blood to nourish their developing eggs.
During a test, the researchers place potential attractants in the ports and blow gentle streams of air into the cage. If the mosquitoes like what they
smell, they fly upwind toward the attractant. The researchers then tally the number of insects in the port.
To account for differences in mosquito mood, the Gainesville group tests the three cages of mosquitoes in a random order at three different times of the day. "On some days, the mosquitoes don't respond as well. Other days, they're very active," says Bernier. The researchers run many trials and analyze their results statistically.
Mosquitoes show the best response to combinations of compounds, and it's time-consuming to find the right mix of attractants. Bernier winnowed attractants from the list of 346 chemicals by methodically mixing the skin's most abundant compounds with lactic acid. He has found that a blend of three particular chemicals attracts at least 90 percent of the mosquitoes in the olfactometer every time. Bernier's own arm and hand usually draw about 70 percent.
"It's a delicate situation," Bernier says. If the proportions aren't right, the mix of compounds leaves the mosquitoes unimpressed. Also, some compounds mask the real attractants, rendering them useless. "If you mix 30 compounds together, the mosquitoes don't even know [the attractant] is there," Bernier says.
The group has filed a patent on the blend of three compounds, which Bernier declines to identify.
In addition to pure chemicals, Bernier and his colleagues ran tests with skin residue from four people, all USDA employees, who varied in their mosquito appeal. The scent on glass beads handled by the tastiest individual lured 77 percent, while the most repellent person drew only 23 percent of the mosquitoes.
Similarly, chemical analysis shows differences between the people.
Some smells that repel people lure mosquitoes, it turns out. Several years ago, Willem Takken and his colleagues at Wageningen Agricultural University in the Netherlands found that a species called Anopheles gambiae loves both stinky feet and Limburger cheese. A bacterium used to make Limburger is also found on the human foot, accounting for the similarity in odor, Takken says. The microorganism probably produces acids that draw the mosquitoes near.
ARS entomologist Daniel L. Kline tested the attractiveness of his own feet to mosquitoes in the lab and outdoors. He wore the same pair of socks 12 hours each day for 3 days. Alone, the socks did not draw many mosquitoes. When combined with carbon dioxide, however, the socks lured many mosquitoes, including disease-carrying species.
The researchers have never observed an attractant to draw 100 percent of the mosquitoes in the olfactometer, Bernier says, although recently one blend came close. The last mosquito began flying toward the attractant but made a U-turn at the last second and flew back into the cage.
A combination of compounds that can beat out the scent of a person could lead to better mosquito traps. In many parts of the world, mosquitoes transmit diseases such as malaria, West Nile fever (SN: 12/11/99, p. 378), and encephalitis.
The Gainesville group always works with Aedes aegypti, also known as the yellow fever mosquito, because it's an easy species to study. Bernier says, "They'll bite anytime."
Other species, such as the malaria-carrying Anopheles quadrimaculatus, turn up their noses--or proboscises, rather--at human scents in the lab.
It's still too early to know whether the findings will yield useful products. To lure mosquitoes to a trap, an attractant must offer more appeal than the people nearby, which is a tall order to fill. "It's very hard to approximate the human body. We don't know enough yet," says Bernier.
Although by themselves some of the blends attract more mosquitoes than Bernier's arm, if both an arm and a blend are presented at the same time, the arm wins hands down.
However, if the researchers add a masking compound to the port with the human hand, the mosquitoes fly over to the port with the chemical blend. Perhaps a masking chemical could make a person less appealing than a nearby attractant baited trap.
It will be difficult to find something to compete with the commercial repellent DEET, or N,N-diethyl-3-methyl-benzamide (SN: 4/27/96, p. 270). "There's a push for natural repellents, but nothing compares to DEET in duration and effectiveness right now," Bernier says. "We're trying."
DEET is a contact repellent, so the mosquito has to land on the chemical for it to work. A repellent that acts while in the air would be desirable but is "a lot harder to come up with," says Bernier.
Another problem with DEET is that it doesn't work well against all mosquito species, Bernier notes. And although DEET is considered safe, some concerns linger about its effects on people (SN: 11/30/96, p. 347). Repellents based on naturally occurring compounds could minimize those fears, Bernier says.
The researchers' results have suggested some futuristic applications. The catalog of skin emanations could provide new ways to analyze criminal evidence, for instance. Chemicals in people's fingerprints seem to vary. Identifying prints' components could contribute information about the people involved in a crime (see sidebar).
In a medical setting, the skin chemicals might give useful clues about health problems and drug use.
Bernier, however, focuses on insect control. One day, he says, people may be able to take a pill that acts as a "systemic repellent" by altering the compounds given off by skin. The pill could cut back the production of attractive compounds or step up the production of ones that mask an enticing scent.
People then could stroll around with their own antimosquito force fields. They could turn off the bug zapper, snuff out the citronella candles, throw out the sprays, and just enjoy the summer nights.
RELATED ARTICLE: Chemicals in fingerprints could help solve crimes
The chemicals released by skin not only lead mosquitoes to their prey, but they might also lead police officers to criminals.
Chemist Michelle V. Buchanan and her colleagues at Oak Ridge (Tenn.) National Laboratory (ORNL) are identifying the chemical compounds in fingerprints. The researchers are looking for patterns in the chemical composition of sample Prints from 300 volunteers. This type of information could help law enforcement officers assemble profiles of criminal suspects.
Buchanan's foray into fingerprints began with an observation made in 1993 by Art Bohanan, a detective with the Knoxville Police Department. While gathering evidence related to the abduction and murder of a 3-year-old girl, police could find none of the girl's fingerprints in the primary suspect's car. The suspect's prints, however, were all over the vehicle. A similar situation had occurred during an earlier child-abduction case.
Bohanan suspected that the kids' fingerprints had somehow disappeared, so he decided to conduct an experiment. He asked several children and adults to handle clean glass and plastic soda bottles. He kept half of the bottles in his cool basement and the other half in the back seat of his car. Each day for a month, he removed bottles from both places and dusted for fingerprints. In the warm car, the children's prints began fading right away and often disappeared within 24 hours. In the basement, all the prints remained longer. Under both sets of conditions, the adult fingerprints lasted, on average, four times as long as the children's prints.
Bohanan asked ORNL scientists for help in solving this mystery. Buchanan and her colleagues had 50 volunteers shake open-ended vials of rubbing alcohol between thumb and forefinger, thus collecting samples of compounds from their fingertips. Half of the volunteers were between 4 and 12 years old, and the other half, between 17 and 46 years old. The scientists then used gas chromatography and mass spectrometry to separate and identify the chemicals dissolved in the alcohol.
They found dramatic differences. The adult fingers gave off more oily, long-lasting compounds such as fatty acid esters than the children's did. The kids' samples contained more cholesterol and volatile chemicals, such as free fatty acids, than the adults' did. Most of the components released by kid's fingers simply evaporate, Buchanan realized.
Subsequent tests also turned up estrogen and testosterone. One of the adult samples showed nicotine.
An upcoming ORNL study will collect skin compounds with a technique similar to the one used by the Agricultural Research Service in Gainesville, Fla. Volunteers will rub between their hands some gglass beads, the kind sold in hobby shops for flower vases. Buchanan and her group will then analyze the composition of the residue left on the beads.
If patterns arise, police and forensic scientists might be able to assemble rough profiles of suspects based on the chemical composition of fingerprints. Analyzing skin chemicals might also be a noninvasive way to test for medical disorders or drug use, says Buchanan (SN: 2/7/98, p. 88).
"Your skin is such a huge organ. With all the pores that you have, it's a major way for your body to get rid of things," she notes.
The work could also lead to more effective dyes for fingerprint dusting. At the very least, the findings have taught Bohanan and his fellow officers to look for kids' prints as soon as possible. That way, missing fingerprints won't worsen the pain of parents who are missing a child.
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|Title Annotation:||a method of drug delivery through the intestines|
|Date:||Apr 22, 2000|
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