Functional nuclear compartmentalization in skeletal muscle cells

Functional nuclear compartmentalization in skeletal muscle cells

A muscle fiber consists of just one cell, but many nuclei. A research team has now shown just how varied these nuclei are. The study, which has been published in Nature Communications, can help us better understand muscle diseases such as Duchenne muscular dystrophy.

Usually, each cell has exactly one nucleus. But the cells of our skeletal muscles are different: These long, fibrous cells have a comparatively large cytoplasm that contains hundreds of nuclei. But up to now, we have known very little about the extent to which the nuclei of a single muscle fiber differ from each other in terms of their gene activity, and what effect this has on the function of the muscle.

A research team has now unlocked some of the secrets contained in these muscle cell nuclei. As the researchers report in the journal Nature Communications, the team investigated the gene expression of cell nuclei using a still quite novel technique called single-nucleus RNA sequencing - and in the process, they came across an unexpectedly high variety of genetic activity.

"Due to the heterogeneity of its nuclei, a single muscle cell can act almost like a tissue, which consists of a variety of very different cell types," explains one of the two lead authors of the study. "This enables the cell to fulfill its numerous tasks, like communicating with neurons or producing certain muscle proteins."

The researchers began by studying the gene expression of several thousand nuclei from ordinary muscle fibers of mice, as well as nuclei from muscle fibers that were regenerating after an injury. The team genetically labeled the nuclei and isolated them from the cells. "We wanted to find out whether a difference in gene activity could be observed between the resting and the growing muscle," says the author.

And they did indeed find such differences. For example, the researchers observed that the regenerating muscle contained more active genes responsible for triggering muscle growth. "What really astonished us, however, was the fact that, in both muscle fiber types, we found a huge variety of different types of nuclei, each with different patterns of gene activity," explains the author.

Before the study, it was already known that different genes are active in nuclei located in the vicinity of a site of neuronal innervation than in the other nuclei. "However, we have now discovered many new types of specialized nuclei, all of which have very specific gene expression patterns," says another author. Some of these nuclei are located in clusters close to other cells adjacent to the muscle fiber: for example, cells of the tendon or perimysium - a connective tissue sheath that surrounds a bundle of muscle fibers.

"Other specialized nuclei seem to control local metabolism or protein synthesis and are distributed throughout the muscle fiber," the author explains. However, it is not yet clear what exactly the active genes in the nuclei do: "We have come across hundreds of genes in previously unknown small groups of nuclei in the muscle fiber that appear to be activated," reports the author.

In a next step, the team studied the muscle fiber nuclei of mice with Duchenne muscular dystrophy. This disease is the most common form of hereditary muscular dystrophy (muscle wasting) in humans. It is caused by a mutation on the X chromosome, which is why it mainly affects boys. Patients with this disease lack the protein dystrophin, which stabilizes the muscle fibers. This results in the cells gradually dying off.

"In this mouse model, we observed the loss of many types of cell nuclei in the muscle fibers," reports the author. Other types were no longer organized into clusters, as the team had previously observed, but scattered throughout the cell. "I couldn't believe this when I first saw it," the author recounts. "I asked my team to repeat the single-nucleus sequencing immediately before we investigated the finding any further." But the results remained the same.

"We also found some disease-specific nuclear subtypes," reports the author. Some of these are nuclei that only transcribe genes to a small extent and are in the process of dying off. Others are nuclei that contain genes that actively repair damaged myofibers. "Interestingly, we also observed this increase in gene activity in muscle biopsies of patients with muscle diseases provided by Professor Simone Spuler's Myology Lab at the MDC," says the author. "It seems this is how the muscle tries to counteract the disease-related damage."

"With our study, we are presenting a powerful method for investigating pathological mechanisms in the muscle and for testing the success of new therapeutic approaches," concludes the author. As muscular malfunction is also observed in a variety of other diseases, such as diabetes and age- or cancer-related muscle atrophy, the approach can be used to better research these changes too.