Scientists Discover an “Impossible” Way Muscle Filaments Can Grow

Researchers have found an unexpected way muscle cells might rebuild actin filaments. These filaments need to stay a precise length for muscles to work, but they also constantly age and need replacement. For decades, scientists didn't know how cells managed both.

Biophysicists at Emory University have now identified a key mechanism. Their study, published in Nature Communications, shows that actin can grow from an end previously thought unable to support growth. This discovery could help us better understand muscle disorders like dilated cardiomyopathy, a major cause of heart failure.

Shashank Shekhar, a physics professor at Emory and the study's senior author, said this is a major step in understanding how the cell's internal skeleton, especially in muscle cells, is put together. The findings challenge a model that has been used for over 40 years to describe how actin filaments form and keep their length.

Shekhar noted that their work overturns a long-held belief by showing a new way the protein leiomodin builds actin filaments in muscle. Sudipta Biswas, an Emory PhD candidate and the study's first author, added that they also explained how problems with leiomodin can disrupt how the heart's contracting parts are assembled.

For example, a genetic change affecting leiomodin has been linked to dilated cardiomyopathy. This disease gradually weakens heart muscle and makes it harder for the heart to pump blood effectively.

Muscle Actin Defies the Usual Growth Rule

Shekhar's lab studies how actin, a very common and versatile protein in the body, is regulated mechanically and chemically. Actin forms filaments that make up the internal skeleton of cells, helping them keep their shape and move. As these filaments grow, they can push against a cell membrane, moving the cell forward. This process also helps cells change shape, divide, and allows immune cells to attack bacteria.

In many cells, actin filaments form loose, mesh-like networks. But muscle cells need a much more organized setup. In muscles, actin and myosin are arranged into straight, stable structures called sarcomeres. These are the smallest units responsible for muscle contraction.

Inside a sarcomere, actin filaments slide past myosin towards the center. This shortens the sarcomere, like pulling a drawstring, while the individual filaments themselves stay the same length. Shekhar explained that muscle contraction is controlled by the length of these actin filaments, which remains constant from birth to death.

However, even though their overall length stays the same, the filaments themselves are not static. They constantly need to replace old material as proteins break down. This raised a puzzle: how do filaments maintain their length while their proteins are continuously replaced?

Each actin filament has two distinct ends: a pointed (minus) end and a barbed (plus) end. For decades, the standard idea was that actin units are lost from the minus end, while new units are added at the plus end. This process, called "treadmilling," makes the filament seem to move forward as new parts are added.

Muscle cells complicate this model. In sarcomeres, actin filament length is tightly controlled, and the barbed (plus) end is capped by a protein that stops new molecules from being added there. Shekhar said that the actin filaments in your muscles today are not the same ones you were born with, yet their length remains constant. No one knew how they kept assembling and remodeling themselves.

A New Route for Growth

A hint came from earlier work by Shekhar and his colleagues at Ohio State University. They studied a toxin from Vibrio cholerae, the bacterium that causes cholera. In non-muscle cells, this toxin can take over actin machinery and reverse normal treadmilling, causing filaments to grow from the minus end instead.

This unusual behavior led Shekhar and Biswas to wonder if healthy muscle cells might use a similar mechanism. If so, something would need to help actin grow from the minus end. Biswas focused on leiomodin 2, a type of leiomodin found near the pointed ends of actin filaments in heart muscle cells. She noted that it has parts similar to the Vibrio toxin.

Previous research had already suggested that leiomodin 2 affects filament length. Biswas added that earlier lab experiments showed that if leiomodin 2 is removed from heart muscle cells, their actin filaments become shorter. If there's too much leiomodin 2, the filaments grow longer than normal.

To understand what the protein was doing at a molecular level, Biswas used microfluidic-assisted total internal reflection fluorescence microscopy (mf-TIRF). This specialized method allows researchers to watch individual protein molecules as actin filaments assemble and disassemble. Shekhar's lab is one of only a few worldwide that use mf-TIRF for this purpose. Researchers attach different fluorescent colors to individual proteins and track their movements.

Single Molecules Revealed Pointed End Growth

Biswas first attached leiomodin 2 molecules to the bottom of a microfluidic chamber. She then labeled individual actin molecules with red fluorescence and added them. The actin began forming filaments that were fixed at their pointed ends by the anchored leiomodin 2, appearing like tiny, growing red fluorescent worms.

The key question was whether new actin was actually being added at these pointed (minus) ends. Biswas tested this by introducing another supply of actin molecules, this time labeled green. The green molecules gathered at the leiomodin base. With each addition of green actin, the existing red sections of the filaments moved farther from the base in the direction of fluid flow.

This color shift directly showed that new building blocks were being added at the pointed end, not just at the opposite end. Biswas stated that they provided the first direct molecular evidence that actin filaments grow from their pointed ends, proving wrong those who thought this was impossible.

These findings also explain why changes in Leiomodin2 can lead to either shorter, thin actin filaments or abnormally long, thin filaments in heart muscle cells. Biswas noted that understanding how a genetic change causes a disease is often the first step toward finding ways to treat or prevent it.

Deep Dive & References

Leiomodin 2 is a processive pointed-end elongator of actin filaments - Nature Communications, 2026