In the context of an international collaboration, a Parisian team* has just shown how cell nuclei are distributed in muscle fiber. This is one step further in understanding how muscles work, which will help us better address muscular disease in the future.
“We want to understand what is happening during muscle cell formation, and to understand how the muscle functions.” Nothing less. For the past ten years, the teams headed by Bruno Cadot, at the Paris Myology Institute, and Edgar Gomes, in Lisbon, have been exploring this complex mechanism. Muscle tissue is unique in two ways. On the one hand, it is made up of long fibers, rather than clearly individualized cells. Each fiber is the fusion of several primordial cells, called myoblasts. Myoblast nuclei are distributed evenly along the fiber wall. Illnesses like certain forms of muscular dystrophy are characterized in part by poor spatial distribution of the nuclei, which tend to conglomerate. On the other hand, “we have known since the 1980s that the muscle’s cytoskeleton, which is responsible for cell shape and movement, is organized on the basis of the nuclear envelope rather than being anchored to a centrosome as is the case with all other cells,” explains the researcher.
In fact, the Parisian team has just shown how microtubules, the “cables” that form the structure of the cytoskeleton, attach to the nuclear envelope when myoblasts are differentiated into fibers. More precisely, the researchers observed the phenomenon in myotubes, which exist at an intermediary stage. Indeed, they are the result of the fusion of myoblasts but have not yet acquired the capacity to contract as a muscle fiber does.
Using, among others, a new technique known as BioID, developed by a partner from Singapore, the researchers identified the partners of a specific muscle cell protein: Nesprine-1α. Nesprin-1 is built into the nuclear envelope, where it recruits different proteins from the centrosome and binds with Akap450 in particular. This protein anchors the microtubules.
The role that this phenomenon plays in nucleus distribution remained to be determined. The team called upon digital simulation, using a system developed by another partner, in Heidelberg. “We were able to simulate the movements of nuclei within the cells by tweaking all the parameters and to verify our hypothesis: microtubules were anchored to the nuclear envelope by Nesprin and Akap450, which is essential to proper nuclear distribution,” explains Cadot. The digital result was then confirmed on the workbench with cellular biology methods.
Still a Far Cry from Clinical Care
Using muscle cells from a patient suffering from muscular dystrophy caused by a mutation in the Nesprin-1a gene, the researchers observed abnormal nuclear distribution. “This suggests, once again, that the positioning of the nuclei is important for muscular function,” points out Cadot. At this stage, the laboratory is not actually searching for therapeutic targets, but is trying to understand the detailed mechanics of muscle differentiation and functioning.
A few years ago, the team showed how kinesin motor protein drives nuclear movement along the microtubules. In another article, the team has just detailed how nuclei migrate toward the muscle fiber wall. “The next step is to understand why the centrosome disappears when the muscle cell differentiates. This is important, since the proteins from the disintegrated centrosome are the ones that bond to the nuclear envelop, anchoring the microtubules,” announces the researcher. This kind of research, while it is still very basic, could still suggest pathways for understanding not only muscular diseases, but also some forms of cancer in which the cells also present a disorganized centrosome.
*Inserm unit 974/UPMC/AIM, Myology Research Center, Paris
P. Gimpel et al. Nesprin-1a-Dependent Microtubule Nucleation from the Nuclear Envelope via Akap450 is Necessary for Nuclear Positioning in Muscle Cells. Current Biology of September 27, 2017