Researchers Discover 'Modus Operandi' Of Heart Muscle Protein

Researchers at the University of Pennsylvania School of Medicine have discovered that a protein called leiomodin ( Lmod ) promotes the assembly of an important heart muscle protein called actin. What's more, Lmod directs assembly of actin to form the pumping unit of heart. The findings appear in this week's issue of Science .

" Very little was known about Lmod when we began this study, " says lead author Roberto Dominguez, PhD, Associate Professor of Physiology.

" It appeared that this protein was present in muscle cells but this hadn't been demonstrated directly and nobody knew what it did, " explains Dominguez. " We compared amino acid sequence of Lmod with the sequence of another protein called tropomodulin [Tmod] that was already known to bind actin filaments in muscle cells. We found that one part of Lmod was very similar to Tmod, but Lmod was a bigger protein than Tmod and contained unique features that made us suspect that it could assemble the actin filaments of heart muscle. This is exactly what we found. '

The results answer a question that scientists studying the heart have long asked: What controls assembly of the pumping unit of the heart?

Actin is most abundant protein in most animal cells and forms long polymers, or filaments, that make up the cell skeleton. In cells that make up muscles and heart, interactions of actin filaments with motor proteins produce contractions that pump blood through the body.

Actin spontaneously forms polymers in test tubes, but living cells use nucleator proteins to control time and place where actin filaments forms. " For a long time, physiologists have wondered what serves as the nucleator protein in cardiac muscle cells, " says co-author Professor Thomas Pollard, PhD, of Yale University. " It was very satisfying after all these years to discover that Lmod can serve as the nucleator protein to initiate forming of actin polymers in heart muscle cells. '

Lmod also directs actin filaments to the sarcomere, part of heart that controls contractions or pumping. When Lmod was knocked down in cardiac muscle cells by an RNA silencing technique, sarcomeres became completely disorganized and couldn't direct muscles to contract.

Proper localization of Lmod in heart cells is critical, because even moderately elevated levels promote the formation of abnormal actin bundles in nuclei of cardiac muscle cells where actin doesn't belong. A similar disorganization of actin bundles is characteristic of a disease of skeletal muscle weakness called intranuclear rod myopathy. Although this disease is caused by a mutation in a skeletal muscle-specific actin gene, similarity in appearance suggests that mutations in Lmod could cause same type of disease in cardiac muscle cells.

The Penn team is currently studying how heart regulates the level of Lmod and how Lmod might be relevant to cardiac muscle disease. In addition, team is attempting to crystallize Lmod in order to study its structure directly.

Malgorzata Boczkowska of Penn and David Chereau of Boston Biomedical Institute are co-first authors of this study. Other key contributors are Pekka Lappalainen and Aneta Skwarbek-Maruszewska of University of Helsinki; Ikuko Fujiwara of Yale; David B. Hayes of Boston Biomedical Institute; and Grzegorz Rebowski of Penn. The study was supported by grants from National Heart Lung and Blood Institute and the National Institute of General Medical Sciences.

PENN Medicine is a $3. 5 billion enterprise dedicated to the related missions of medical education, biomedical research, and excellence in patient care. PENN Medicine consists of University of Pennsylvania School of Medicine ( founded in 1765 as the nation's first medical school ) and University of Pennsylvania Health System.

Penn's School of Medicine is currently ranked #3 in nation in U. S. News & World Report's survey of top research-oriented medical schools; and, according to most recent data from National Institutes of Health, received over $379 million in NIH research funds in 2006 fiscal year. Supporting 1, 400 fulltime faculty and 700 students, School of Medicine is recognized worldwide for its superior education and training of the next generation of physician-scientists and leaders of academic medicine.

The University of Pennsylvania Health System includes three hospitals its flagship hospital, Hospital of University of Pennsylvania, rated one of the nation's " Honor Roll " hospitals by U. S. News & World Report; Pennsylvania Hospital, nation's first hospital; and Penn Presbyterian Medical Center a faculty practice plan; a primary-care provider network; two multispecialty satellite facilities; and home care and hospice.

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