Scientists Scope Out Huntington Disease

Abstract

AIDED BY AN INNOVATIVE MICROscope that makes it possible to track the fate of individual neurons, researchers have uncovered evidence that could put an end to the longstanding controversy over the role of abnormal protein deposits in the brains of patients with Huntington disease. For years, researchers have debated whether inclusion bodies—aggregates of abnormal deposits of a protein called huntingtin—cause nerve cells to die or are protective or incidental to neuronal death. Now, Steven Finkbeiner, MD, PhD, and colleagues at the Gladstone Institute of Neurological Disease at the University of California, San Francisco, have discovered that inclusion body formation is a beneficial coping response of diseased neurons (Nature. 2004;431:805-810). Mutant huntingtin protein contains characteristically long stretches of the amino acid glutamine. In Finkbeiner’s study with rat neurons, a fluorescent protein was fused to mutant huntingtin protein, allowing the researchers to peer into living neurons to see whether they contained inclusion bodies (aggregated huntingtin) or diffuse huntingtin. Levels of diffuse mutant huntingtin were “very predictive” of how rapidly a neuron died, he noted. In addition, cells died more rapidly if they did not develop inclusion bodies, demonstrating that inclusion body formation was not required for cell death, Finkbeiner explained. In fact, inclusion bodies appear to protect the neurons: almost immediately after they formed, the rate of nerve cell death decreased nearly to the level observed in control cells. Moreover, within 24 to 48 hours, diffuse huntingtin decreased sharply to nearly undetectable levels. “We suspect that neurons somehow sequester diffuse forms of huntingtin within inclusion bodies, kind of like mothballing them,” he explained. Finkbeiner noted that the only way to resolve the controversy about the role of inclusion bodies was to develop a microscope that would allow him to “follow individual neurons over time and directly observe intracellular events as they unfolded.” Conventional microscopes were unsuitable for this purpose because with no way to repeatedly position a plate in exactly the same place, even minute shifts or tilts make it virtually impossible to return to the same neuron to monitor it over time. To solve this problem, Finkbeiner wrote a computer program that allows his microscope to capture a series of images and compare each to an image collected on the first day of imaging. The program makes it possible to determine when the plate is in exactly the same position as it was on that first day, enabling researchers to repeatedly identify and monitor the same neurons. Finkbeiner and colleagues are continuing to use his microscope to study pathological events in Huntington disease, such as whether the proteasome, an organelle that functions as the cell’s trash can, is dysfunctional in neurons with mutant huntingtin. The cellstalking approach could also be applied to the study of many diseases, he noted, particularly those in which researchers are attempting to sort out cell death and other pathological changes that occur within cells. As a physician-researcher, Finkbeiner sees parallels in his work to the clinic—but insteadofbringingbenchdiscoveries to the bedside, he sees his approach as applying bedside medicine to the laboratory bench. “Essentially, what we did was treat every cell as a patient,” he explained. “You have individual people, you follow what happens to them, you record their stats and things like that. And that’s all we’re doing to identify risk factors for neuronal death.”