Gene That Changes the Brain’s Response to Stress Identified

Gene That Changes the Brain’s Response to Stress Identified

  9:41:00 AM    Science Lover

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Stress can literally warp your brain, reshaping some brain structures that help cope with life's pressures. In the short term, the stress response can be helpful -- i.e., fight or flight -- but over time it leads to a wear and tear that can cause disease in both the brain and other parts of the body. Digging deeper into what underlies these potentially harmful changes, new research has identified a key protein involved in remodeling the brain under stress. Experiments have found that the brains of mice with an inadequate amount of this protein, called brain-derived neurotrophic factor (BDNF), look similar to those of normal mice that have been under stress for long periods.

Reduced reach. Tracings of neurons from mice 
with different levels of BDNF show that a short 
supply of the protein causes relatively shrunken 
neurons (right) in some parts of the hippocampus. 
(Credit: Image courtesy of Rockefeller University)

The experiments homed in on the gene for a protein that, among other things, enhances the adaptability of neurons in the hippocampus, a brain region that plays a key role in mood, cognition and memory. When normal mice are exposed to chronic stress (simulated by confinement in a wire mesh restraint), there is a significant retraction in the projections, or dendrites, of some of the neurons in the hippocampus, which shrinks in overall volume as well. The new experiments, reported recently in Hippocampus, looked at mice that had only one instead of the usual two copies of the gene that produces BDNF. The researchers, from Rockefeller University and Weill Cornell Medical College, found that these mice had brains resembling those of normal mice after extended stress. In other words, stress did not have any effect on the experimental mice.

"The findings suggest that BDNF is one of the proteins that play a role in mediating the brain's plasticity," says Bruce S. McEwen, head of Rockefeller's Harold and Margaret Milliken Hatch Laboratory of Neuroendocrinology.

If researchers can find a way to deplete or supplement BDNF in adult mice, they may be able to answer the question of when in development, or even in adult life, it has the greatest impact. "What we're seeing is that there may be a developmental window for BDNF's role, or also that there may be a floor and a ceiling for the right amount of the protein that helps enable adaptive plasticity," McEwen says.

The new work adds to the understanding of BDNF's interaction with hormones in the brain. McEwen's lab recently discovered that a variant of the BDNF gene is a likely contender for a role in premenstrual disorders, changing mice's performance on certain memory tasks according to their stage of the estrous cycle.

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Elusive Protein Points to Mechanism Behind Hearing Loss

Elusive Protein Points to Mechanism Behind Hearing Loss

  12:37:00 AM    Science Lover

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A serendipitous discovery of deaf zebra fish larvae has helped narrow down the function of an elusive protein necessary for hearing and balance. The work, led by Rockefeller University's A. James Hudspeth, suggests that hearing loss may arise from a faulty pathway that translates sound waves into electrical impulses the brain can understand.

All ears. Electron micrographs of two hair cell bundles in the zebra fish ear show the difference between those born with (left) and without (right) the protein Tmie. (Credit: Image courtesy of Rockefeller University)

"These zebrafish larvae were originally pegged for another study, but then we saw that one-fourth of them failed to respond to acoustic stimuli and made erratic spiraling movements,

and that suggested that they were born deaf," says first author Michelle R. Gleason, who spearheaded the project. "So we took this opportunity to examine what could be responsible for this extreme hearing loss."

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New Nucleotide In DNA Could Revolutionize Epigenetics

New Nucleotide In DNA Could Revolutionize Epigenetics

  8:13:00 PM    Science Lover

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Chemical structure of cytosine, one of the four nucleotide bases that make up DNA.
New research shows that two additional nucleotides -- 5-methylcytosine and 5-hydroxymethylcytosine -- can sometimes replace cytosine in the DNA
double helix to regulate which genes are expressed. (Credit: Wikimedia Commons)

Anyone who studied a little genetics in high school has heard of adenine, thymine, guanine and cytosine – the A, T, G and C that make up the DNA code. But those are not the whole story. The rise of epigenetics in the past decade has drawn attention to a fifth nucleotide, 5-methylcytosine (5-mC), that sometimes replaces cytosine in the famous DNA double helix to regulate which genes are expressed. And now there's a sixth: 5-hydroxymethylcytosine.

In experiments to be published online April 16 by Science, researchers reveal an additional character in the mammalian DNA code, opening an entirely new front in epigenetic research.

The work, conducted in Nathaniel Heintz's Laboratory of Molecular Biology at The Rockefeller University, suggests that a new layer of complexity exists between our basic genetic blueprints and the creatures that grow out of them. "This is another mechanism for regulation of gene expression and nuclear structure that no one has had any insight into," says Heintz, who is also a Howard Hughes Medical Institute investigator. "The results are discrete and crystalline and clear; there is no uncertainty. I think this finding will electrify the field of epigenetics."

Genes alone cannot explain the vast differences in complexity among worms, mice, monkeys and humans, all of which have roughly the same amount of genetic material. Scientists have found that these differences arise in part from the dynamic regulation of gene expression rather than the genes themselves. Epigenetics, a relatively young and very hot field in biology, is the study of nongenetic factors that manage this regulation.

One key epigenetic player is DNA methylation, which targets sites where cytosine precedes guanine in the DNA code. An enzyme called DNA methyltransferase affixes a methyl group to cytosine, creating a different but stable nucleotide called 5-methylcytosine. This modification in the promoter region of a gene results in gene silencing.

Some regional DNA methylation occurs in the earliest stages of life, influencing differentiation of embryonic stem cells into the different cell types that constitute the diverse organs, tissues and systems of the body. Recent research has shown, however, that environmental factors and experiences, such as the type of care a rat pup receives from its mother, can also result in methylation patterns and corresponding behaviors that are heritable for several generations. Thousands of scientific papers have focused on the role of 5-methylcytosine in development.

The discovery of a new nucleotide may make biologists rethink their approaches to investigating DNA methylation. Ironically, the latest addition to the DNA vocabulary was found by chance during investigations of the level of 5-methylcytosine in the very large nuclei of Purkinje cells, says Skirmantas Kriaucionis, a postdoctoral associate in the Heintz lab, who did the research. "We didn't go looking for this modification," he says. "We just found it."

Kriaucionis was working to compare the levels of 5-methylcytosine in two very different but connected neurons in the mouse brain — Purkinje cells, the largest brain cells, and granule cells, the most numerous and among the smallest. Together, these two types of cells coordinate motor function in the cerebellum. After developing a new method to separate the nuclei of individual cell types from one another, Kriaucionis was analyzing the epigenetic makeup of the cells when he came across substantial amounts of an unexpected and anomalous nucleotide, which he labeled 'x.'

It accounted for roughly 40 percent of the methylated cytosine in Purkinje cells and 10 percent in granule neurons. He then performed a series of tests on 'x,' including mass spectrometry, which determines the elemental components of molecules by breaking them down into their constituent parts, charging the particles and measuring their mass-to-charge ratio. He repeated the experiments more than 10 times and came up with the same result: x was 5-hydroxymethylcytosine, a stable nucleotide previously observed only in the simplest of life forms, bacterial viruses. A number of other tests showed that 'x' could not be a byproduct of age, DNA damage during the cell-type isolation procedure or RNA contamination. "It's stable and it's abundant in the mouse and human brain," Kriaucionis says. "It's really exciting."

What this nucleotide does is not yet clear. Initial tests suggested that it may play a role in demethylating DNA, but Kriaucionis and Heintz believe it may have a positive role in regulating gene expression as well. The reason that this nucleotide had not been seen before, the researchers say, is because of the methodologies used in most epigenetic experiments. Typically, scientists use a procedure called bisulfite sequencing to identify the sites of DNA methylation. But this test cannot distinguish between 5-hydroxymethylcytosine and 5-methylcytosine, a shortcoming that has kept the newly discovered nucleotide hidden for years, the researchers say. Its discovery may force investigators to revisit earlier work. The Human Epigenome Project, for example, is in the process of mapping all of the sites of methylation using bisulfite sequencing. "If it turns out in the future that (5-hydroxymethylcytosine and 5-methylcytosine) have different stable biological meanings, which we believe very likely, then epigenome mapping experiments will have to be repeated with the help of new tools that would distinguish the two," says Kriaucionis.

Providing further evidence for their case that 5-hydroxymethylcytosine is a serious epigenetic player, a second paper to be published in Science by an independent group at Harvard reveals the discovery of genes that produce enzymes that specifically convert 5-methylcytosine into 5-hydroxymethylcytosine. These enzymes may work in a way analogous to DNA methyltransferase, suggesting a dynamic system for regulating gene expression through 5-hydroxymethylcytosine. Kriaucionis and Heintz did not know of the other group's work, led by Anjana Rao, until earlier this month. "You look at our result, and the beautiful studies of the enzymology by Dr. Rao's group, and realize that you are at the tip of an iceberg of interesting biology and experimentation," says Heintz, a neuroscientist whose research has not focused on epigenetics in the past. "This finding of an enzyme that can convert 5-methylcytosine to 5-hydroxymethylcytosine establishes this new epigenetic mark as a central player in the field."

Kriaucionis is now mapping the sites where 5-hydroxymethylcytosine is present in the genome, and the researchers plan to genetically modify mice to under- or overexpress the newfound nucleotide in specific cell types in order to study its effects. "This is a major discovery in the field, and it is certain to be tied to neural function in a way that we can decipher," Heintz says.

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Journal reference:

1. Skirmantas Kriaucionis and Nathaniel Heintz. The Nuclear DNA Base 5-Hydroxymethylcytosine Is Present in Purkinje Neurons and the Brain. Science, 2009; DOI: 10.1126/science.1169786

Adapted from materials provided by Rockefeller University, via EurekAlert!, a service of AAAS.

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