1 2 aim

#apaperaday: Dystrophin myonuclear domain restoration governs treatment efficacy in dystrophic muscle

In today’s #apaperaday, Prof. Aartsma-Rus reads and comments on the paper titled: Dystrophin myonuclear domain restoration governs treatment efficacy in dystrophic muscle

Today’s pick is from PNAS News by Morin et al on how dystrophin distributes over muscle fibers. Very cool work if you are a dystrophin fan. DOI: 10.1073/pnas.2206324120 

Dystrophin is part of a structural complex in muscle fibers. Muscle fibers have many nuclei & each produces proteins. Dystrophin is huge & research on how it distributes & how far it travels have been done – also by my group, but never in much detail.

Here authors performed a very detailed study using a mouse model that has a green protein signal on its dystrophin protein. This means that dystrophin can be followed & studied. But if you want to study how dystrophin distributes having all nuclei express it are not informative.

So authors crossed mice with normal (colorless) dystrophin and green dystrophin. Female offspring will express green dystrophin in ~50% of the cells due to X- inactivation.

This work showed fibers had a mosaic green staining and dystrophin extended a minimum of 80 micrometer & across ~half the fiber diameter. Authors calculated that one nucleus could populate about 5000 square micrometer of muscle fiber surface with dystrophin (nuclear domain).

This was the same in an mdx background where they studied revertant fibers, and in female carrier mice. Also after genome editing the nuclear domain was the same. Authors tested exon skipping with tricyclo DNA: this resulted in 50% of dystrophin distributed all over the fibers.

Authors also showed dystrophin is enriched in myotendinous junctions and after restoration of expression dystrophin is expressed there first. Dystrophin levels can vary up to 5 fold within a fiber.

Finally they show dystrophin does not move and there is not a pool of dystrophin to replace dystrophins that are lost (bleaching experiments). Authors discuss that others have studied this in the past but then it was unclear if distribution was due to RNA or protein distribution.

Authors show the same nuclear domain in mouse, mdx mouse and cultured cells. Still to be done: 3D fiber bundles (of course there is always more to be done). Very cool work! Congratulations to the authors!

This fundamental work has implications for dystrophin restoration approaches: dystrophin is immobile once in its complex and distributes only a little from each nucleus. In case you are interested in our own work on X-inactivation and dystrophin: https://pubmed.ncbi.nlm.nih.gov/22359642/