August 24, 2005
Spiders Help Scientists Discover How Muscles Relax
Using muscle tissue from tarantulas, an HHMI international research
scholar and his colleagues have figured out the detailed structure and
arrangement of the miniature molecular motors that control movement.
Their work, which takes advantage of a new technique for visualizing
tissues in their natural state, provides new insights into the
molecular basis of muscle relaxation, and perhaps its activation
too.
“We have solved the structure of the array of miniature motors
that form our muscles and found out how they are switched off,”
said Raúl Padrón, a HHMI international research scholar
in the Department of Structural Biology at the Venezuelan Institute for
Scientific Research (Instituto Venezolano de Investigaciones
Científicas or IVIC) in Caracas, Venezuela.
“Solving the structure of the relaxed state will allow us to investigate how these filaments are activated when they are switched on.”
Raúl A. Padrón
The findings are reported in the August 25, 2005, issue of the
journal Nature.
 | | | | |  |  | | | | |  |  | Atomic model of the thick filament from tarantula striated muscle
The surface of the three-dimensional reconstruction of the thick filament is shown in gray, together with the atomic models of two myosin molecules, indicating the intramolecular and intermolecular interactions.
Image and Video: John Woodhead
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Padrón and his colleagues focused their studies on striated
muscle—the type of muscle that controls skeletal movement and
contractions of the heart. Striated muscles are made of long
cylindrical cells called muscle fibers. Within the fibers, millions of
units known as sarcomeres give rise to movement of skeletal muscles.
Sarcomeres are composed mainly of thick filaments of myosin, the most
common protein in muscle cells, responsible for their elastic and
contractile properties. The thick filaments are arranged in parallel
with thin filaments of another muscle protein, actin. When the actin
and myosin filaments slide along one another, the muscle contracts or
relaxes.
Padrón's study focused on the long, rod-shaped myosin of the
thick filaments. The heads of these myosin rods project outward from
the thick filament to connect with and move actin filaments during
contraction of a muscle.
The structural studies were done using tarantula striated muscle,
which the team has been studying since the 1980s. Striated muscles from
the large, hairy spiders contain filaments that are particularly well
ordered, making them easier to study structurally than the more
disorganized filaments found in vertebrate striated muscle,
Padrón explained.
Padrón and Lorenzo Alamo at IVIC partnered with Roger Craig,
John Woodhead and Fa-Qing Zhao at the University of Massachusetts
Medical School to use cryo-electron microscopy to answer questions
about the thick filament's structure, questions that could not be
answered with existing electron microscopy techniques.
Standard electron microscopy requires dehydration and staining of a
tissue sample, which modifies the structure of the specimen and
distorts its shape. Cryo-electron microscopy avoids these problems by
rapidly freezing the sample. Using the new technique, the researchers
were able to visualize the muscle tissue in a form closer to its
structure in the body than had previously been possible.
It took several years to refine the techniques required to preserve
the thick muscle filaments in their relaxed state. Even then, the
researchers faced mathematical difficulties in calculating a
three-dimensional map of the filaments. In 2004, using a new approach
that Edward Egelman at the University of Virginia Health Sciences
Center had developed to create the map, they soon had their structure.
“The new reconstruction was very detailed; we were all amazed
with the level of detail that it showed,” Padrón said.
The structure provides crucial new details. A twisting, symmetrical
arrangement of myosin heads spaced around the filament's circumference
surrounds a backbone made up of 12 parallel strands or sub-filaments.
“This is the first time that the structure of the backbone has
been clearly seen in any thick filament reconstruction,” the
researchers wrote.
“The structure reveals how the helices of the myosin heads are
formed and maintained and how the filaments are switched off due to
interactions between the myosin heads Padrón explained.
“It also opens the way to understanding how the thick filaments
are activated.”
The details of their model permitted the researchers to explain how,
in relaxed muscle, the heads of each myosin molecule are inhibited from
interacting with actin by interacting with each other instead. When
muscle is activated, they suggest, the bonds between the myosin heads
are broken. This frees each head to interact with actin and cause
muscle contraction.
“We have focused on the relaxed muscle to understand the
structure of the thick filaments when they are not involved in
contraction, but rather fully ordered—a state more amenable to
understanding their structure,” Padrón said.
“Solving the structure of the relaxed state will allow us to
investigate how these filaments are activated when they are switched
on.”
The scientists were surprised to find that the atomic structure of
isolated myosin molecules from vertebrate smooth muscle—the type
of muscle found in the digestive tract, bladder, arteries, and
veins—closely matched their invertebrate striated muscle myosin
filament. Kenneth Taylor's research group at Florida State University
reported the atomic structure of the smooth muscle myosin
molecules.
The similarity suggests, Padrón said, that the interacting
head structure may be common to relaxed-state myosin for smooth and
striated muscle and among varied species. “This model is
applicable across the whole animal kingdom and all muscle types, and
that's exciting,” he remarked.
Padrón said he hopes to apply the research to muscle diseases
that arise from malfunctioning of the muscles' on/off switches. One
such disease is hypertrophic cardiomyopathy, in which the wall of the
left ventricle of the heart becomes enlarged, causing sudden death. It
is caused by mutations in certain genes that encode several muscle
proteins—some of which are related specifically to the myosin
that Padrón studies.
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Cindy Fox Aisen
