How muscle cells grow and stay healthy

Biophysicists unraveled a mystery of how muscles form at the molecular level and how they maintain their function. Nature Communications published the discovery, which may help in the design of treatments for muscle diseases such as dilated cardiomyopathy, one of the leading causes of heart failure.

“We’ve made a fundamental advance in understanding how the cellular cytoskeleton is assembled, especially in muscle cells,” says the senior author of the study.

The researchers upended a model relied on for more than four decades to explain how filaments of actin, a protein vital to cellular movement and other functions, form and maintain their length.

“Our work challenges a long-standing paradigm by uncovering a new mechanism, previously thought impossible, by which the protein leimodin builds actin filaments in muscle,” the author says.

“We also provide a molecular explanation for how defects in leimodin can disrupt the assembly of the contractile machinery of the heart,” says the first author of the paper.

A genetic mutation in leimodin, for example, is linked to dilated cardiomyopathy, a condition which progressively weakens the heart muscles and impairs its ability to pump blood effectively.

The Lab studies the mechanical and chemical regulation of actin, one of the most abundant and versatile proteins in the body.

Actin assembles into filaments that are part of the skeleton of living cells, giving them shape. It is also essential to the mobility of cells. As actin filaments elongate inside a cell they push against its membrane, causing the cell to roll forward. These same forces generated by the dynamics of actin assemblies allow a cell to shape-shift and divide into two daughter cells. Or an immune cell to engulf and kill an invading bacterium.

In many of the cells in the body, actin filaments form into loose, mesh-like bundles. In muscle cells, however, actin and the protein myosin form straight, highly stable arrays known as sarcomeres — the smallest, organized units of muscle cells.

Actin in muscle cells is highly specialized to facilitate contraction. The actin filaments slide past the myosin filaments towards the middle of the sarcomere, gathering up the length of the sarcomere like a drawstring without any change in the length of the filaments. 

“How well your muscles contract is controlled by the length of these actin filaments,” the senior author explains. “From birth, to death, the length of the actin filaments in muscle cells stays the same. It’s very tightly controlled.”

While the length of the actin filaments remains the same, however, actin filaments still need to grow and replenish themselves, so they can keep forming anew as they age, turn over and degrade.

How the filaments did this in muscles has been a mystery.

All actin filaments have two ends: one is called the pointed, or minus, end; the other is called the barbed, or plus, end. For decades, scientists have long known that the minus end of an actin filament is where the filaments depolymerize, or degrade and break off. And the plus end is where freshly energized actin subunits are added to the filament, like forming a new step in a staircase.

This process — losing an actin subunit from the minus end while gaining one at the plus end — is called “treadmilling,” since the filament appears to move forward as new building blocks are added.

This fundamental process for actin filaments, however, does not work the same way in muscle cells. Actin filaments in sarcomeres strictly control their length. Their barbed, plus end is capped with a protein that prevents the addition of new molecules.

“And yet, the actin filaments in the sarcomeres you’re born with are not the same ones in your sarcomeres today,” the senior author says. “No one knew how they keep assembling and remodeling themselves.”

In previous research, the authors found an interesting effect of a toxin from Vibrio cholerae bacteria on actin filaments in non-muscle cells. Vibrio, the pathogen that causes cholera, can hijack the machinery of actin filaments, reversing the treadmilling process so that growth occurs on the minus end.

The authors wondered if something similar could be occurring in healthy actin filaments of muscle cells. Could some catalyst spark growth on the minus end?

The first author centered the investigation on leiomodin 2 — a particular type of leiomodin found near the pointed ends of actin filaments in cardiac muscle cells. “It has domains similar to the Vibrio toxin,” she says.

“Previous laboratory dish experiments,” the author adds, “showed that if you delete leiomodin 2 from cardiac muscle cells, their actin filaments get shorter. And if you have too much leiomodin 2, the filaments grow longer than normal.”

The author designed experiments using the highly specialized technique of microfluidic-assisted total internal reflection fluorescence microscopy (mf-TIRF) for studying the dynamics of how actin filaments remodel themselves.

The Lab is one of a handful in the world using mf-TIRF to study how actin filaments grow and disassemble. The technique allows researchers to attach different colors of fluorescent dyes to single protein molecules then introduce them to a microfluidic system. The light from these dyes reveals movements of single molecules.

The author anchored molecules of leimodin 2 to the base of a microfluidic chamber and then tagged single actin molecules with red fluorescence and introduced them to the chamber.  Like tiny, red fluorescent worms growing, actin filaments began to assemble, remaining anchored to the base by the leimodin, which attaches to the pointed end of the filaments.

But were the filaments growing from their pointed, minus ends?

To find out, the author introduced more actin molecules into the system, this time tagged green and watched to see what would happen. The green molecules became anchored at the leimodin base. As each new green molecule got added to the base, the red molecules in the filaments gradually became displaced, moving away from the base in the direction of the flow.

“We provided the first direct molecular evidence that actin filaments grow from their pointed ends,” the author says. “We proved those who thought this wasn’t possible wrong.” 

The work provides a mechanistic explanation for why a mutation in Leiomodin2 leads to shorter, thin filaments or abnormally long, thin filaments in heart muscle cells.

“Understanding how a mutation causes a disease is often the first step towards finding ways to treat or prevent a disease,” the author says.

https://www.nature.com/articles/s41467-026-74809-z

https://www.nature.com/articles/s41467-026-74810-6

https://sciencemission.com/Leiomodin-2-is-a-processive-pointed-end-elongator

https://sciencemission.com/Mechanism-of-actin-thin-filament