Group of heart cells that send out signals that make the heart muscle contract

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May 3, 2016

At a Glance

  • Researchers determined that microtubules, a type of molecular “strut,” play a key role in regulating contractile function in mouse and rat heart cells. 
  • Microtubule alterations that impair contraction were found in diseased human hearts, suggesting that modifying microtubules may help restore heart function.

The heart is an organ made of muscle cells. The normal adult human heart beats from 60 to 100 times per minute at rest. Each individual heart cell, known as a cardiomyocyte, contains contractile units called sarcomeres that power its beating. Cells contract, or shorten, in unison during each beat to pump blood through the heart and out through the body. In between the beats, the cells stretch back out as the heart fills with blood.

An organized network called the cytoskeleton helps cells maintain their shape and organization. Microtubules (MTs) are a major component of this cellular support structure. MTs can transmit mechanical signals and, like rods or struts, resist compression in contracting heart cells. How they perform these roles has been unclear.

A team led by Dr. Benjamin Prosser at the University of Pennsylvania set out to determine the role that MTs play in heart function. Their work was supported in part by NIH’s National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), National Heart, Lung, and Blood Institute (NHLBI), and National Institute of Biomedical Imaging and Bioengineering (NIBIB). Results were published online on April 22, 2016, in Science.

The group used an advanced imaging approach to observe fluorescently tagged MTs inside beating mouse and rat heart cells in real time. They noted that as the cells contracted, the MTs “buckled” under the force in a pattern that coincided with contraction of the sarcomeres. Like a spring, the MTs returned to their normal shape between beats.

The scientists found that MTs are linked to sarcomeres through a lattice-like structure. This connection could be altered by removing the amino acid tyrosine from MTs. With more tyrosine attached, the MTs appeared to slide rather than buckle. This allowed the force-generating machinery inside the cells to shorten further and faster. When tyrosine was removed from MTs by a natural process called detyrosination, the MTs formed tighter, stiffer connections with the sarcomeres and restricted contraction.

To determine if this change might be associated with impaired heart function in humans, the team analyzed tissue from healthy donors and patients with heart disease. They found a correlation between lower MT tyrosine levels and decreased heart function.

“Essentially we found that excess detyrosination promotes the interaction between microtubules and the sarcomere, increasing resistance to contraction, and so may contribute to reductions in cardiac function in certain disease states,” Prosser explains.

The team plans to study whether genetic alterations or drugs that decrease MT detyrosination can improve the function of diseased heart muscles.

—by Carol Torgan, Ph.D.

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  • Stem cell therapy rebuilds heart muscle in primates
  • How the heart works

References: Detyrosinated microtubules buckle and bear load in contracting cardiomyocytes. Robison P, Caporizzo MA, Ahmadzadeh H, Bogush AI, Chen CY, Margulies KB, Shenoy VB, Prosser BL. Science. 2016 Apr 22;352(6284):aaf0659. doi: 10.1126/science.aaf0659. PMID: 27102488.

Funding: NIH’s National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), National Heart, Lung, and Blood Institute (NHLBI), National Institute of Biomedical Imaging and Bioengineering (NIBIB); and the National Science Foundation.

AB
cardiovascular system The body system that consists of the heart, blood vessels, and blood, and that carries needed substances to cells and carries waste products away from cells
heart A hollow, muscular organ that pumps blood throughout the body
atrium Each of the two upper chambers of the heart that receives blood that comes into the heart.
ventricle A lower chamber of the heart that pumps blood out to the lungs and body
valve A flap of tissue in the heart or a vein that prevents blood from flowing backward.
pacemaker A group of cells located in the right atrium that sends out signals that make the heart muscle contract and that regulates heartbeat rate
artery A blood vessel that carries blood away from the heart
capillary A tiny blood vessel where substances are exchanged between the blood and the body cells
vein A blood vessel that carries blood back to the heart
aorta A blood vessel that carries blood back to the heart
force A push or a pull exerted on an object
coronary artery An artery that supplies blood to the heart itself.
diffusion The process by which molecules move from an area in which they are highly concentrated to an area in which they are less concentrated.
pressure The force that something exerts over a given area
blood pressure The pressure that is exerted by the blood against the walls of blood vessels
sphygmomanometer An instrument that measures blood pressure
plasma The liquid part of blood
red blood cell A cell in the blood that takes up oxygen in the lungs and delivers it to cells elsewhere in the body
hemoglobin An iron-containing protein that binds chemically to oxygen molecules and makes up most of a red blood cell
white blood cell A blood cell that fights disease
platlet A cell fragment that plays an important part in forming blood clots
fibrin A chemical that is important in blood clotting because it forms a net of tiny fibers that traps red blood cells
blood transfusion The transference of blood from one person to another
lymphatic system A network of veinlike vessels that returns the fluid that leaks out of blood vessels to the bloodstream
lymph The fluid that the lymphatic system collects and returns to the bloodstream
lymph node A small knob of tissue in the lymphatic system that filters lymph.
atheroscerosis A condition in which an artery wall thickens as a result of the buildup of fatty materials
heart attack A condition in which blood flow to a part of the heart muscle is blocked, which causes heart cells to die
hypertension A disorder in which a person’s blood pressure is consistently higher than normal

What sends out signals that make the heart muscle contract?

The heart's pacemaker sends out an electrical signal (impulse) that spreads throughout the heart along electrical pathways. These pathways transmit the signal from the upper to the lower chambers of the heart, which causes the heart muscle to contract.

What is a group of cells in the right atrium that signals the heart muscle to contract?

SA node (sinoatrial node) – known as the heart's natural pacemaker. The impulse starts in a small bundle of specialized cells located in the right atrium, called the SA node. The electrical activity spreads through the walls of the atria and causes them to contract. This forces blood into the ventricles.