MAR 03, 2015 10:49 AM PST

Filaments in Heart Muscle Cells Don't Automatically Keep the Beat

WRITTEN BY: Judy O'Rourke
Two hearts, said Keats, can beat as one; but a study led by Weizmann Institute scientists in collaboration with researchers from the University of Pennsylvania shows that sometimes a single heart muscle cell can beat as more than two dozen. The findings, reported recently in Nature Communications, provide an extremely detailed glimpse into the mechanisms behind normal and irregular heart muscle cell contractions. The study may help define the limitations of existing therapies for abnormal heartbeat and, in the future, suggest ways of designing new ones.

Each heart muscle cell consists of numerous parallel filaments comprising repeated subunits. When the heart beats, each individual filament contracts to produce muscle cell contractions.
A chicken heart muscle cell under a fluorescent microscope; the filaments consist of repeated subunits (bright dotted lines). The schematic representation shows three neighboring filaments; the black lines are the boundaries of their subunits, such that the lower filament is aligned with the middle one, while the upper one is not.
Optimally, all the filaments should contract in a synchronized manner, thus ensuring the greatest amplitude of contraction for each muscle cell, and ultimately, the strongest and most effective beating of the entire heart.

However, a new theoretical model proposed and analyzed by professor Samuel Safran, PhD, and postdoctoral fellow Kinjal Dasbiswas, PhD, suggests that the filaments contract together only when their subunits, and subunit boundaries, are aligned with one another.

Since such alignment usually only happens among a limited number of neighboring filaments, aligned filaments contract together as a bundle, but each such bundle contracts out of phase with others. Therefore, a heart cell does not necessarily beat as a single uniform entity; rather, the number of different beating entities in the cell depends on the bundle number, which may reach more than two dozen.

The theory further predicted that the alignment of the filaments in the heart muscle cell depends on the cell's physical environment; more specifically on the elasticity of the supporting structure called the extracellular matrix.

The new understanding may one day help design improved treatments for heart disease. For example, in the future, if new heart cells are grown to replace diseased ones, their growth environment may be manipulated so that their structure is well-ordered and, to paraphrase Keats, all their filaments beat as one.

[Source: Weizmann Institute of Science]
About the Author
Bachelor's
Judy O'Rourke worked as a newspaper reporter before becoming chief editor of Clinical Lab Products magazine. As a freelance writer today, she is interested in finding the story behind the latest developments in medicine and science, and in learning what lies ahead.
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