The intrinsic ability of the heart to adjust the strength of its contractions to the amount of blood filling the heart has been the subject of much interest, and even more speculation. This fundamental mechanism that regulates heart work with every single beat is vital to our existence. Breakdown of this cellular mechanism of control leads to lung congestion and heart failure. The force of contraction is essentially determined by the interactions of two large proteins, actin and myosin. Our group now demonstrates that the cellular mechanism involves sensing the spacing between the filaments of myosin and actin as the heart fills with blood. The strength of contraction is then adjusted according to the stretch of the proteins within the muscle fibres. We have been able to do this by rapidly analyzing the scatter of pulses of synchrotron x-ray radiation off the muscle fibres of the intact heart as it contracts spontaneously. Many other proteins are known to be associated with actin and myosin, and important for structural organization. Surpassing other studies of isolated heart muscle, our work shows that other proteins must be responsible for active regulation of the spacing between the muscle proteins and the strength of contractions.

BIOPHYSJ/2007/112995

Elasticity of short DNA molecules: theory and experiment for contour lengths of 0.6-7 m.

Yeonee Seol, JILA
Jinyu Li, University of Colorado at Boulder
Philip Nelson, University of Pennsylvania
Thomas T. Perkins, University of Colorado, Boulder
M. D. Betterton, University of Colorado, Boulder

Keywords: DNA elasticity; force-extension behavior; optical trapping; single-molecule; stretching DNA; worm-like chain

DNA, the biomolecule that provides the blueprint for life, has a lesser-known identity as a stretchy polymer. The authors have found a flaw in the most common model for DNA elasticity, a discovery that will improve the accuracy of single-molecule research and perhaps pave the way for DNA to become an official standard for measuring picoscale forces, a notoriously difficult challenge.

The experiments described in this paper reveal that a classic model for measuring the elasticity of double-stranded DNA leads to errors when the molecules are short. For instance, measurements are off by up to 18 percent for molecules 632 nanometers long, and by 10 percent for molecules about twice that length. (By contrast, the DNA in a single human cell, if linked together and stretched out, would be about 2 meters long.)

The old elasticity model assumes that polymers are infinitely long, whereas the most popular length for high precision single-molecule studies is 600 nm to 2 microns, coauthor Tom Perkins says. Accordingly, several university collaborators developed a new theory, the finite worm-like chain (FWLC) model, which improves accuracy by incorporating three previously neglected effects, including length.

The work described in this article is part of a NIST project studying the possible use of DNA as a picoforce standard, because enzymes build DNA with atomic precision. DNA already is used informally to calibrate atomic force microscopes. An official standard could, for the first time, enable picoscale measurements that are traceable to internationally accepted units. DNA elasticity could provide a force standard from 0.1 -10 pico-Newtons (pN), where 1 pN is the approximate weight of an E. coli cell or the force exerted by 1 milliwatt of light reflected off a mirror.

The work was supported by the Alfred P. Sloan Foundation, a Burroughs Wellcome Fund Career Award in the Biomedical Sciences, the Butcher Foundation, a W.M. Keck Grant in the RNA Sciences, NIST, and the National Science Foundation.

Non-Linear Force-Length Relationship in the ADP-Induced Contraction of Skeletal Myofibrils

Yuta Shimamoto, Waseda University
Fumiaki Kono, Waseda University
Madoka Suzuki, Consolidated Research Institute for Advanced Science and Medical Care, Waseda University
Shin'ichi Ishiwata, Waseda University

Keywords: ADP-bound cross-bridges; cooperativity; dextran; lattice spacing; length-dependent activation; sarcomere

Muscle is one of the most important organs in biological system, yet its regulatory mechanism is still not fully revealed because of its complex nature involving binding of both Ca2+ and myosin (a linear force-generating “motor”) to actin filament (the “track” protein). In this work the authors approached the mechanism of regulation by myosin molecules from a new angle. The idea was to test the effect of actin-myosin interaction on force regulation independently of Ca2+, utilizing artificial activation by ADP, a product of ATP hydrolysis, in the absence of free Ca2+. The authors carefully examined the properties of activation by simultaneously monitoring both individual sarcomeres using an inverted phase-contrast microscope, and by measuring their generated force. Contrary to the “textbook” version of the length-tension relationship, in which the generated force decreases with stretching muscle above its optimum length, a larger active force was generated at longer muscle lengths. Furthermore, the authors linked this cooperative force enhancement with the changes in three-dimensional structure of sarcomeres by monitoring the distance between the actin and myosin filaments. The authors find that the developed force is significantly modulated by tiny (less than 1 nm) changes in the interfilament distance. This property must be characteristic of striated muscles, in which the two filaments form a liquid crystal-like array. This discovery suggests a new chapter in our understanding of the regulation of muscle contraction by narrowing the focusing on the myosin molecules. This is a significant advance in the regulation of muscle contraction that was pioneered in 1972 by Professor M. Endo.

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