Thread by @EricTWang

We're excited to describe our research on how gene products are distributed in skeletal muscle! We began these studies with the goal of identifying distinct mRNA localization patterns in muscle, anticipating that mRNAs encoding sarcomeric genes might be locally translated

https://twitter.com/biorxivpreprint/status/1365489383978205185

at the Z-disk. To our surprise, other mRNAs like Gapdh and Histone 1h1 were also there, and a great deal of nascent translation occurred along the sarcomere.

The biggest surprise, however, was that mRNAs in mature skeletal myofibers are COMPLETELY dependent on microtubules to disperse throughout the cytoplasm. In cultured cells, a significant proportion of mRNA molecules undergo diffusive movement, but this type of motion essentially

disappears during sarcomere maturation. As observed by many others, the microtubule network in myofibers is impressive and reminiscent of roads in Manhattan - you can imagine mRNAs traveling along microtubule avenues before finding Z-disk streets at which to dock.

When you remove the roads, (e.g. depolymerize microtubules), RNAs and nascent proteins accumulate in the perinuclear space, often forming giant blobs. After allowing the roads to reform, these blobs dissipate, and RNAs can once again disperse throughout the cytoplasm.

Live cell tracking in mature C2C12 myotubes showed that most RNAs are essentially immobile, but that some undergo directed runs, consistent with our observations in myofibers. We classified these as "low mobility" and "directed" states, respectively.

We incorporated these states into a discrete time Markov chain simulation, and saw that if you allow RNAs to only enter the "low mobility" state, they cannot distribute efficiently through the cytoplasm -

but that addition of the "directed" state to the simulation recapitulates the uniform distribution of RNAs that is observed throughout the cytoplasm of real myofibers.

Although studies of RNA localization in skeletal muscle have been relatively rare as compared to those in neurons, we were struck by shared principles and the possibility that some molecules may play similar roles in both cell types. An important question is just how many RNAs

might be "localized"; in neurons, estimates continue to rise. At least in muscle, hitchhiking on a vehicle may be the norm rather than the exception. Perhaps there are preferences for modes of transport, e.g. subway vs. car, and these remain important topics of future study.

This work was a pleasure to conduct with students Lance Denes and Chase Kelley, both of whom combined a pure joy of scientific exploration with countless hours of careful, meticulous execution. Thx to @cziscience & @NINDSfunding for funding! #myotwitter @UF @UFHealth @RNASociety