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RESEARCH BRIEFS
Disease-related Protein Domain Steers Cell FateSeveral cancers, Alzheimer's, and other diseases are characterized by abnormal intracellular levels of beta-catenin, a multifunctional molecule regulated by proteins of the Wnt signaling pathway, such as disheveled (Dvl). In the Oct. 17 Nature, researchers report on some unusual properties of a conserved segment of Dvl--the DIX domain--which influences beta-catenin stability, ultimately determining whether a cell develops normally or becomes cancerous.
In Xenopus frog embryos, overexpression of Dvl with a normal DIX domain induces axis duplication (left) while overexpression of Dvl with a mutated DIX domain fails to do so (right). The finding suggests that the domain is necessary for embryogenesis and may determine cell fate. (Images courtesy of Xi He) The researchers introduced mutant versions of Dvl into mammalian cells and discovered that protein constructs with a DIX domain localized to actin fibers and cytoplasmic vesicles. Constructs without the domain failed to associate at all and, in fact, did not regulate beta-catenin levels, which is necessary for normal embryonic development. "The cell spinning out of control is inevitable when communication goes awry in the Wnt system, a major signaling pathway seen in mammalian, frog, and fruit fly cells. Our findings allow us to look into the cellular biological aspects of Wnt signaling that control embryogenesis and human cancers," explained Xi He, a senior author and HMS assistant professor of neurology at Children's Hospital. Genetic mutations associated with colorectal, prostate, and brain tumors have been mapped to the DIX domain of another Wnt pathway protein, dubbed axin tumor suppressor, suggesting that DIX domains may hold promise for future anticancer therapy. When the Dvl proteins were introduced into Xenopus frog embryos, the team, including Raymond Habas, co-author and a postdoctoral fellow in the He lab, and colleagues from the University of Colorado Health Sciences Center observed a similar finding. In cells that had constructs with a mutated DIX domain, the Dvl protein failed to bind to intracellular vesicles, induce axis formation, and regulate embryogenesis. The researchers also investigated the NMR structure of the helical DIX domain, identifying the motifs responsible for binding to actin fibers and cytoplasmic vesicles. The disheveled gene was first cloned in Drosophila melanogaster in the laboratory of Norbert Perrimon, HMS professor of genetics. Its name refers to the messy appearance of embryos with a mutation in the gene.
High-intensity Exercise Reduces Heart RiskThough increased physical activity has been correlated with a reduced risk for coronary heart disease, the leading cause of death in the U.S., the effective parameters of exercise--type, intensity, and duration--have remained unclear. Recently, a team of HSPH and HMS researchers has discovered that weight training is a significant protective factor. They also found that during the same length of time, exercising with a higher level of energy is considerably more effective at reducing risk. The results are based on answers from surveys conducted through the Health Professionals' Follow-up Study. At two-year intervals between 1986 and 1998, 44,452 men, aged 40 through 75 years, completed detailed questionnaires eliciting their diet, lifestyle, and medical history. Lead author Mihaela Tanasescu, a former HSPH student, and colleagues studied each participant using a metabolic equivalent tasks score, known as MET. One MET equals the energy spent sitting quietly. Total MET-hours reflected the cumulative energy expended for all activities. "Not surprisingly, we found a strong inverse association between physical activity and coronary heart disease in this large cohort of men. However, in addition, we found that this benefit extended to brisk walking and weight training activities, which previously have not been consistently linked to risk reduction," said study co-author Eric Rimm, HSPH associate professor of nutrition and epidemiology. Given daily half-hour walks, a more robust pace produced greater benefits, specifically, an 18 percent diminished risk. Running for an hour per week resulted in a 42 percent decrease in risk. Men who trained with weights or strength-building machines for 30 minutes a week had a 23 percent lower risk compared with men who did not practice resistance athletics. Senior author and HSPH associate professor of nutrition and epidemiology Frank Hu explained, "Weight training had been thought to be beneficial, but we now can see for certain that it is. The best way for men to reduce the risk for coronary heart disease is to increase the amount and intensity level of their physical activity as well as incorporate weight training into their exercise program." The paper appears in the Oct. 23 Journal of the American Medical Association. --Trang Au
Gene Identified for Severe Nervous System DisorderAn international team including HMS researchers has identified the gene causing peripheral neuropathy associated with agenesis of the corpus callosum (ACCPN), a devastating neurologic syndrome characterized by mental retardation, anatomic malformations, and death at an average age of 33. Although rare in most of the world, ACCPN afflicts about one in 2,100 children born in two regions of Quebec. The disease displays autosomal recessive inheritance, and in 1996, linkage analysis mapped its locus to chromosome 15q. The new study, published in the November Nature Genetics, was led by researchers at McGill University in Montreal. David Mount, HMS instructor in medicine, and Adriana Mercado, research fellow in medicine, both at Brigham and Women's Hospital, are co-authors, along with collaborators at Vanderbilt University; the Universities of Münster, Germany and Verona, Italy; and two Quebec hospitals. In 1999 Mount, then at Vanderbilt, cloned SLC12A6, a candidate gene for ACCPN. It was found to encode KCC3, a novel potassium- chloride cotransporter. Guided by Mount's genomic structure of SLC12A6, the McGill investigators identified two loss-of-function mutations in ACCPN families from the two Quebec regions. All but one of 81 French-Canadian patients had a guanine deletion in exon 18 of the SLC12A6 gene, resulting in a KCC3 protein missing the last 336 amino acids. Their parents, "obligate carriers," were found to be heterozygous for the mutation, as were four of 110 control subjects. (The latter finding is not unexpected, since one in 23 people in the regions are estimated to carry the deleterious allele.) One patient had the exon 18 mutation from one parent and a different SLC12A6 mutation from the other parent. Two non-French Canadian patients had different SLC12A6 mutations. While this genetic investigation evolved, the researchers localized the expression of the KCC3 protein to white matter tracts and large neurons in the central nervous system--an early indication that SLC12A6 might indeed be the ACCPN gene. They then made Slc12a6 knockout mice, which showed locomotor abnormalities similar to those found in ACCPN patients. They dragged their hind legs and performed poorly on tests demanding balance and limb strength. Anatomical studies of these mice showed axonal swelling and defects in myelination in the peripheral nerves--also similar to affected humans. Functional studies showed that the major Quebec mutation abolishes transport function in KCC3. But it remains unclear just how defects in the protein cause disease. "A major paradox is that KCC3 expression is minimal in the peripheral nervous system, yet both the mice and patients have a profound peripheral neuropathy," said Mount. "Sensorineural and behavioral defects in the mice are reminiscent of psychotic features in ACCPN and are consistent with genetic linkage of SLC12A6 to both schizophrenia and bipolar disorder. It will be some time before we unravel the pathophysiology of KCC3 in the peripheral and central nervous system; however, I suspect that the insights will be relevant to other disorders besides ACCPN." --Tom Reynolds |
