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#apaperaday: Pharmacological inhibition of HDAC6 improves muscle phenotypes in dystrophin-deficient mice by downregulating TGF-β via Smad3 acetylation

In today’s #apaperaday, Prof. Aartsma-Rus reads and comments on the paper titled: Pharmacological inhibition of HDAC6 improves muscle phenotypes in dystrophin-deficient mice by downregulating TGF-β via Smad3 acetylation

Today’s pick is another HDAC inhibition paper, which was featured already in 2022 as well. So this will be the first repeat tweet – we’ll do the summary first and then compare notes. Paper is from @NatureComms by Osseni et al DOI: 10.1038/s41467-022-34831-3

Lack of dystrophin causes muscle fibers to be sensitive to damage, leading to chronic muscle wasting. TGFbeta signaling plays a role in the pathological production of fibrotic tissue instead of muscle regeneration. HDAC inhibitors can modify gene expression.

However, as also mentioned yesterday, HDACs come in different classes. Some operate in the nucleus by modifying the chromatin, while others operate in the cytoplasm and deacetylase proteins. HDAC6 is the latter type. Tubustatin A (TubA) is an inhibitor of only HDAC6.

HDAC6 has been shown to play a role in atrophy of muscle and HDAC6 inhibition could prevent fibrosis in an animal model for lung fibrosis. Furthermore, panHDAC inhibitor givinostat showed positive results in mdx mice and (in 2022) preliminary positive results in patients

So authors here want to test what TubA treatment can do in the mdx mouse model. Mice were treated for 30 days started at 7 week and wild type controls and vehicle treated mdx mice were taken along as controls. Authors showed increased acetylation of tubulin as expected.

Furthermore, histone acetylation did not change (although mdx mice had higher levels of acetylation than wild type – which is known and something givinostat can address). TubA treatment improved grip strength and endurance (less strength drop with repeated tests).

TubA treatment also increased expression of utrophin and reduced muscle fiber size variability and increased cross sectional areas of fast and slow muscles (so less very small fibers). Muscle atrophy markers were reduced and there were less centrally located nuclei

Reduction of central nuclei is a bit ambiguous always: was there less need or less potential for muscle regeneration? However, authors also show that the microtubules are more structured with TubA treatment, suggesting there was less need.

Neuromuscular junctions were also less fragmented (see image) which authors speculate was also due to improve tubuline function. mTOR is a key regulator of protein production and muscle growth and authors show that while mTOR levels do not change, the signaling pathway is activated.

They confirm this in C2C12 cells – which seems the wrong way around (finding an effect in disease mouse model and then validating it in control cell cultures). Finally authors show in C2C12 cells that smad2/3 phosphorylation is reduced with TubA treatment after TGFbeta exposure

They argue that smad3 was acetylated more due to the HDAC6 inhibition and that this reduced phosphorylation and thereby TGF-beta induced signaling of fibrotic pathways (only shown in cells).

Authors discuss that the TubA treatment does 3 things in mdx mice 1. stabilizing the microtubuli and the neuromuscular junction and dystrophin glycoprotein associated complex partners 2. increasing utrophin 3. inhibiting TGFbeta signally and fibrosis.

This is shown in mdx mice or only cell models for now. Furthermore, givinostat will likely do this as well but also inhibits inflammation, which will give regeneration an extra boost. Now lets see how my current assessment differed from the one in 2022: https://twitter.com/oligogirl/status/1601219452703748103