Gatekeeper of brain steroid signals boosts emotional resilience to stress

PHILADELPHIA - A cellular protein called HDAC6, newly characterized as a gatekeeper of steroid biology in the brain, may provide a novel target for treating and preventing stress-linked disorders, such as depression and post-traumatic stress disorder (PTSD), according to research from the Perelman School of Medicine at the University of Pennsylvania. Glucocorticoids are natural steroids secreted by the body during stress. A small amount of these hormones helps with normal brain function, but their excess is a precipitating factor for stress-related disorders. Glucocorticoids exert their effects on mood by acting on receptors in the nucleus of emotion–regulating neurons, such as those producing the neurotransmitter serotonin. For years, researchers have searched for ways to prevent deleterious effects of stress by blocking glucocorticoids in neurons. However, this has proved difficult to do without simultaneously interfering with other functions of these hormones, such as the regulation of immune function and energy metabolism. In a recent Journal of Neuroscience paper, the lab of Olivier Berton, PhD, assistant professor of Psychiatry, shows how a regulator of glucocorticoid receptors may provide a path towards resilience to stress by modulating glucocorticoid signaling in the brain. The protein HDAC6, which is particularly enriched in serotonin pathways, as well as in other mood-regulatory regions in both mice and humans, is ideally distributed in the brain to mediate the effect of glucocorticoids on mood and emotions. HDAC6 likely does this by controlling the interactions between glucocorticoid receptors and hormones in these serotonin circuits ceus for social workers Experiments that first alerted Berton and colleagues to a peculiar role of HDAC6 in stress adaptation came from an approach that reproduces certain clinical features of traumatic stress and depression in mice. The animals are exposed to brief bouts of aggression from trained "bully" mice. In most aggression-exposed mice this experience leads to the development of a lasting form of social aversion that can be treated by chronic administration of antidepressants. In contrast, a portion of mice exposed to chronic aggression consistently express spontaneous resilience to the stress and do not develop any symptoms. By comparing gene expression in the brains of spontaneously resilient and vulnerable mice, Berton and colleagues discovered that reducing HDAC6 expression is a hallmark of naturally resilient animals. While aggression also caused severe changes in the shape of serotonin neurons and their capacity to transmit electrical signals in vulnerable mice, stress-resilient mice, in contrast, escaped most of these neurobiological changes. To better understand the link between HDAC6 and the development of stress resilience, Berton and colleagues devised a genetic approach to directly manipulate HDAC6 levels in neurons: Deletion of HDAC6 in serotonin neurons -- the densest HDAC6-expressing cell group in the mouse brain -- dramatically reduced social and anxiety symptoms in mice exposed to bullies and also fully prevented neurobiological changes due to stress, fully mimicking a resilient phenotype. Using biochemical assays, Berton's team showed it is by promoting reversible chemical changes onto a heat shock chaperone protein, Hsp90, that HDAC6 deletion is able to literally switch off the effects of glucocorticoid hormones on social and anxiety behaviors. Chaperones are proteins that help with the folding or unfolding and the assembly or disassembly of protein complexes. The way in which glucocorticoid receptor chaperoning and stress are linked is not well understood. Yet, genetic variations in certain components of the glucocorticoid receptor chaperone complex have been associated with the development of stress-related disorders and individual variability in therapeutic responses to antidepressants. "We provide pharmacological and genetic evidence indicating that HDAC6 controls certain aspects of Hsp90 structure and function in the brain, and thereby modulates protein interactions, as well as hormone- and stress-induced glucocorticoid receptor signaling and behavior," explains Berton. Together, these results identify HDAC6 as a possible stress vulnerability biomarker and point to pharmacological inhibition of HDAC6 as a potential new strategy for antidepressant interventions through regulation of Hsp90 in glucocorticoid signaling in serotonin neurons. Co-first-authors are Julie Espallergues and Sarah L. Teegarden, along with Avin Veerakumar, Janette Boulden, Collin Challis, Jeanine Jochems, Michael Chan, Tess Petersen, Chang-Gyu Hahn, Irwin Lucki, and Sheryl G. Beck, all from Penn. Other authors are Evan Deneris, from Case Western Reserve University, Cleveland, Ohio, and Patrick Matthias, Miescher Institute for Biomedical Research, Basel, Switzerland. This work was funded by the National Institute of Mental Health grants MH087581 and MH0754047 and grants from the International Mental Health Research Organization and the National Alliance for Research on Schizophrenia and Depression. Penn Medicine is one of the world's leading academic medical centers, dedicated to the related missions of medical education, biomedical research, and excellence in patient care. Penn Medicine consists of the Raymond and Ruth Perelman School of Medicine at the University of Pennsylvania (founded in 1765 as the nation's first medical school) and the University of Pennsylvania Health System, which together form a $4.3 billion enterprise. The Perelman School of Medicine is currently ranked #2 in U.S. News & World Report's survey of research-oriented medical schools. The School is consistently among the nation's top recipients of funding from the National Institutes of Health, with $479.3 million awarded in the 2011 fiscal year. The University of Pennsylvania Health System's patient care facilities include: The Hospital of the University of Pennsylvania -- recognized as one of the nation's top 10 hospitals by U.S. News & World Report; Penn Presbyterian Medical Center; and Pennsylvania Hospital - the nation's first hospital, founded in 1751. Penn Medicine also includes additional patient care facilities and services throughout the Philadelphia region. Penn Medicine is committed to improving lives and health through a variety of community-based programs and activities. In fiscal year 2011, Penn Medicine provided $854 million to benefit our community.

Brain Wiring a No-Brainer?

The brain appears to be wired more like the checkerboard streets of New York City than the curvy lanes of Columbia, Md., suggests a new brain imaging study. The most detailed images, to date, reveal a pervasive 3D grid structure with no diagonals, say scientists funded by the National Institutes of Health.

“Far from being just a tangle of wires, the brain’s connections turn out to be more like ribbon cables -- folding 2D sheets of parallel neuronal fibers that cross paths at right angles, like the warp and weft of a fabric,” explained Van Wedeen, M.D., of Massachusetts General Hospital (MGH), A.A. Martinos Center for Biomedical Imaging and the Harvard Medical School. “This grid structure is continuous and consistent at all scales and across humans and other primate species.”

Wedeen and colleagues report new evidence of the brain’s elegant simplicity March 30, 2012 in the journal Science. The study was funded, in part, by the NIH’s National Institute of Mental Health (NIMH), the Human Connectome Project of the NIH Blueprint for Neuroscience Research, and other NIH components.

“Getting a high resolution wiring diagram of our brains is a landmark in human neuroanatomy,” said NIMH Director Thomas R. Insel, M.D. “This new technology may reveal individual differences in brain connections that could aid diagnosis and treatment of brain disorders.”

Knowledge gained from the study helped shape design specifications for the most powerful brain scanner of its kind, which was installed at MGH’s Martinos Center last fall. The new Connectom diffusion magnetic resonance imaging (MRI) scanner can visualize the networks of crisscrossing fibers – by which different parts of the brain communicate with each other – in 10-fold higher detail than conventional scanners, said Wedeen.

“This one-of-a-kind instrument is bringing into sharper focus an astonishingly simple architecture that makes sense in light of how the brain grows,” he explained. “The wiring of the mature brain appears to mirror three primal pathways established in embryonic development.”

As the brain gets wired up in early development, its connections form along perpendicular pathways, running horizontally, vertically and transversely. This grid structure appears to guide connectivity like lane markers on a highway, which would limit options for growing nerve fibers to change direction during development. If they can turn in just four directions: left, right, up or down, this may enforce a more efficient, orderly way for the fibers to find their proper connections – and for the structure to adapt through evolution, suggest the researchers.

Obtaining detailed images of these pathways in human brain has long eluded researchers, in part, because the human cortex, or outer mantle, develops many folds, nooks and crannies that obscure the structure of its connections. Although studies using chemical tracers in neural tracts of animal brains yielded hints of a grid structure, such invasive techniques could not be used in humans.

Wedeen’s team is part of a Human Connectome Project Harvard/MGH-UCLA consortium that is optimizing MRI technology to more accurately to image the pathways. In diffusion imaging, the scanner detects movement of water inside the fibers to reveal their locations. A high resolution technique called diffusion spectrum imaging (DSI) makes it possible to see the different orientations of multiple fibers that cross at a single location – the key to seeing the grid structure ceus for social workers

In the current study, researchers performed DSI scans on postmortem brains of four types of monkeys – rhesus, owl, marmoset and galago – and in living humans. They saw the same 2D sheet structure containing parallel fibers crossing paths everywhere in all of the brains – even in local path neighborhoods. The grid structure of cortex pathways was continuous with those of lower brain structures, including memory and emotion centers. The more complex human and rhesus brains showed more differentiation between pathways than simpler species.

Among immediate implications, the findings suggest a simplifying framework for understanding the brain’s structure, pathways and connectivity.

The technology used in the current study was able to see only about 25 percent of the grid structure in human brain. It was only apparent in large central circuitry, not in outlying areas where the folding obscures it. But lessons learned were incorporated into the design of the newly installed Connectom scanner, which can see 75 percent of it, according to Wedeen.

Much as a telescope with a larger mirror or lens provides a clearer image, the new scanner markedly boosts resolving power by magnifying magnetic fields with magnetically stronger copper coils, called gradients. Gradients make it possible to vary the magnetic field and get a precise fix on locations in the brain. The Connectom scanner’s gradients are seven times stronger than those of conventional scanners. Scans that would have previously taken hours – and, thus would have been impractical with living human subjects – can now be performed in minutes.

“Before, we had just driving directions. Now, we have a map showing how all the highways and byways are interconnected,” said Wedeen. “Brain wiring is not like the wiring in your basement, where it just needs to connect the right endpoints. Rather, the grid is the language of the brain and wiring and re-wiring work by modifying it.”


Detail from DSI scan shows fabric-like 3D grid structure of connections in monkey brain.

Source: Van Wedeen, M.D., Martinos Center and Dept. of Radiology, Massachusetts General Hospital and Harvard University Medical School


Curvature in this DSI image of a whole human brain turns out to be folding of 2D sheets of parallel neuronal fibers that cross paths at right angles. This picture came from the new Connectom scanner.
Source: Van Wedeen, M.D., Martinos Center and Dept. of Radiology, Massachusetts General Hospital and Harvard University Medical School

Reference

Wedeen VJ, Rosene DL, Ruopeng W, Guangping D, Mortazavi F, Hagmann P, Kass JH, Tseng W-YI. The Geometric Structure of the Brain Fiber Pathways: A Continuous Orthogonal Grid. March 30, 2012 Science.

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The mission of the NIMH is to transform the understanding and treatment of mental illnesses through basic and clinical research, paving the way for prevention, recovery and cure. For more information, visit the NIMH website.

The NIH Blueprint for Neuroscience Research is a cooperative effort among the NIH Office of the Director and the 15 NIH Institutes and Centers that support research on the nervous system. By pooling resources and expertise, the Blueprint supports transformative neuroscience research, and the development of new tools, training opportunities, and other resources to assist neuroscientists.

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit the NIH website.