Wednesday, February 9, 2011

Turning Bacteria Against Themselves

Bacteria often attack with toxins designed to hijack or even kill host cells. To avoid self-destruction, bacteria have ways of protecting themselves from their own toxins.
Now, researchers at Washington University School of Medicine in St. Louis have described one of these protective mechanisms, potentially paving the way for new classes of antibiotics that cause the bacteria's toxins to turn on themselves.
Scientists determined the structures of a toxin and its antitoxin in Streptococcus pyogenes, common bacteria that cause infections ranging from strep throat to life-threatening conditions like rheumatic fever. In Strep, the antitoxin is bound to the toxin in a way that keeps the toxin inactive.
"Strep has to express this antidote, so to speak," says Craig L. Smith, PhD, a postdoctoral researcher and first author on the paper that appears Feb. 9 in the journal Structure. "If there were no antitoxin, the bacteria would kill itself."
With that in mind, Smith and colleagues may have found a way to make the antitoxin inactive. They discovered that when the antitoxin is not bound, it changes shape.
"That's the Achilles' heel that we would like to exploit," says Thomas E. Ellenberger, DVM, PhD, the Raymond H. Wittcoff Professor and head of the Department of Biochemistry and Molecular Biophysics at the School of Medicine. "A drug that would stabilize the inactive form of the immunity factor would liberate the toxin in the bacteria."
In this case, the toxin is known as Streptococcus pyogenes beta-NAD+ glycohydrolase, or SPN. Last year, coauthor Michael G. Caparon, PhD, professor of molecular microbiology, and his colleagues in the Center for Women's Infectious Disease Research showed that SPN's toxicity stems from its ability to use up all of a cell's stores of NAD+, an essential component in powering cell metabolism. The antitoxin, known as the immunity factor for SPN, or IFS, works by blocking SPN's access to NAD+, protecting the bacteria's energy supply system.
With the structures determined, researchers can now test possible drugs that might force the antitoxin to remain unbound to the toxin, thereby leaving the toxin free to attack its own bacteria.
"The most important aspect of the structure is that it tells us a lot about how the antitoxin blocks the toxin activity and spares the bacterium," says Ellenberger.
Understanding how these bacteria cause disease in humans is important in drug design.
"There is a war going on between bacteria and their hosts," Smith says. "Bacteria secrete toxins and we have ways to counterattack through our immune systems and with the help of antibiotics. But, as bacteria develop antibiotic resistance, we need to develop new generations of antibiotics."
Many types of bacteria have evolved this toxin-antitoxin method of attacking host cells while protecting themselves. But today, there are no classes of drugs that take aim at the protective action of the bacteria's antitoxin molecules.
"Obviously they could evolve resistance once you target the antitoxin," Ellenberger says. "But this would be a new target. Understanding structures is a keystone of drug design."

Brain's 'Radio Stations' Have Much to Tell Scientists

Like listeners adjusting a high-tech radio, scientists at Washington University School of Medicine in St. Louis have tuned in to precise frequencies of brain activity to unleash new insights into how the brain works.
"Analysis of brain function normally focuses on where brain activity happens and when," says Eric C. Leuthardt, MD. "What we've found is that the wavelength of the activity provides a third major branch of understanding brain physiology."
Researchers used electrocorticography, a technique for monitoring the brain with a grid of electrodes temporarily implanted directly on the brain's surface. Clinically, Leuthardt and other neurosurgeons use this approach to identify the source of persistent, medication-resistant seizures in patients and to map those regions for surgical removal. With the patient's permission, scientists can also use the electrode grid to experimentally monitor a much larger spectrum of brain activity than they can via conventional brainwave monitoring.
Scientists normally measure brainwaves with a process called electroencephalography (EEG), which places electrodes on the scalp. Brainwaves are produced by many neurons firing at the same time; how often that firing occurs determines the activity's frequency or wavelength, which is measured in hertz, or cycles per second. Neurologists have used EEG to monitor consciousness in patients with traumatic injuries, and in studies of epilepsy and sleep.
In contrast to EEG, electrocorticography records brainwave data directly from the brain's surface.
"We get better signals and can much more precisely determine where those signals come from, down to about one centimeter," Leuthardt, assistant professor of neurosurgery, of neurobiology and of biomedical engineering, says. "Also, EEG can only monitor frequencies up to 40 hertz, but with electrocorticography we can monitor activity up to 500 hertz. That really gives us a unique opportunity to study the complete physiology of brain activity."
Leuthardt and his colleagues have used the grids to watch consciousness fade under surgical anesthesia and return when the anesthesia wears off. They found each frequency gave different information on how different circuits changed with the loss of consciousness, according to Leuthardt.
"Certain networks of brain activity at very slow frequencies did not change at all regardless of how deep under anesthesia the patient was," Leuthardt says. "Certain relationships between high and low frequencies of brain activity also did not change, and we speculate that may be related to some of the memory circuits."
Their results also showed a series of changes that occurred in a specific order during loss of consciousness and then repeated in reverse order as consciousness returned. Activity in a frequency region known as the gamma band, which is thought to be a manifestation of neurons sending messages to other nearby neurons, dropped and returned as patients lost and regained consciousness.
The results appeared in December in the Proceedings of the National Academy of Sciences.
In another paper that will publish Feb. 9 in The Journal of Neuroscience, Leuthardt and his colleagues have shown that the wavelength of brain signals in a particular region can be used to determine what function that region is performing at that time. They analyzed brain activity by focusing on data from a single electrode positioned over a number of different regions involved in speech. Researchers could use higher-frequency bands of activity in this brain area to tell whether patients:
  • had heard a word or seen a word
  • were preparing to say a word they had heard or a word they had seen
  • were saying a word they had heard or a word they had seen.
"We've historically lumped the frequencies of brain activity that we used in this study into one phenomenon, but our findings show that there is true diversity and non-uniformity to these frequencies," he says. "We can obtain a much more powerful ability to decode brain activity and cognitive intention by using electrocorticography to analyze these frequencies."
Source: Daily Science