Myosin post-translational modifications and function in the presence of myopathy-linked truncating MYH2 mutations

Increased cardiac myosin super-relaxation as an energy saving mechanism in hibernating grizzly bears

AIM

The aim of the present study was to define whether cardiac myosin contributes to energy conservation in the heart of hibernating mammals.

METHODS

Thin cardiac strips were isolated from the left ventricles of active and hibernating grizzly bears; and subjected to loaded Mant-ATP chase assays, X-ray diffraction and proteomics.

MAIN FINDINGS

Hibernating grizzly bears displayed an unusually high proportion of ATP-conserving super-relaxed cardiac myosin molecules that are likely due to altered levels of phosphorylation and rod region stability.

CONCLUSIONS

Cardiac myosin depresses the heart's energetic demand during hibernation by modulating its function.

Post-translational modifications of vertebrate striated muscle myosin heavy chains

Post-translational modifications (PTMs) play a crucial role in regulating the function of many sarcomeric proteins, including myosin. Myosins comprise a family of motor proteins that play fundamental roles in cell motility in general and muscle contraction in particular. A myosin molecule consists of two myosin heavy chains (MyHCs) and two pairs of myosin light chains (MLCs); two MLCs are associated with the neck region of each MyHC's N-terminal head domain, while the two MyHC C-terminal tails form a coiled-coil that polymerizes with other MyHCs to form the thick filament backbone. Myosin undergoes extensive PTMs, and dysregulation of these PTMs may lead to abnormal muscle function and contribute to the development of myopathies and cardiovascular disorders. Recent studies have uncovered the significance of PTMs in regulating MyHC function and showed how these PTMs may provide additional modulation of contractile processes. Here, we discuss MyHC PTMs that have been biochemically and/or functionally studied in mammals' and rodents' striated muscle. We have identified hotspots or specific regions in three isoforms of myosin (MYH2, MYH6, and MYH7) where the prevalence of PTMs is more frequent and could potentially play a significant role in fine-tuning the activity of these proteins.

Myosin heavy chain 2 (MYH2) expression in hypertrophic chondrocytes of soft callus provokes endochondral bone formation in fracture

AIMS

Muscle-bone interactions during fracture healing are rarely known. Here we investigated the presence and significance of myosin heavy chain 2 (MYH2), a component of myosin derived from muscles, in fracture healing.

MAIN METHODS

We collected five hematoma and seven soft callus tissues from patients with distal radius fractures patients, randomly selected three of them, and performed a liquid chromatography-mass spectrometry (LC-MS) proteomics analysis. Proteomic results were validated by histological observation, immunohistochemistry, and immunofluorescence for MYH2 expression. These findings were further confirmed in a murine femoral fracture model in vivo and investigated using various methods in vitro.

KEY FINDINGS

The LC-MS proteomics analysis showed that MYH proteins were enriched in human soft calluses compared to hematoma. Notably, MYH2 protein is upregulated as high rank in each soft callus. The histological examination showed that MYH2 expression was elevated in hypertrophic chondrocytes within the human soft callus. Consistent with human data, Myh2 were significantly co-localized with Sox9 in hypertrophic chondrocytes of murine femoral fracture, in comparison to pre-hypertrophic and proliferating chondrocytes. Soluble MYH2 protein treatment increased MMP13 and RUNX2 expression in chondrocytes. In soluble MYH2 treatment, proliferation of chondrocytes was not altered, but the osteogenic and chondrogenic features of chondrocytes increased and decreased during differentiation, respectively.

SIGNIFICANCE

These findings indicate the potential of soluble MYH2 protein as a promising therapeutic strategy for promoting endochondral bone formation in chondrocytes following fracture.

Abnormal myosin post-translational modifications and ATP turnover time associated with human congenital myopathy-related RYR1 mutations

AIM

Conditions related to mutations in the gene encoding the skeletal muscle ryanodine receptor 1 (RYR1) are genetic muscle disorders and include congenital myopathies with permanent weakness, as well as episodic phenotypes such as rhabdomyolysis/myalgia. Although RYR1 dysfunction is the primary mechanism in RYR1-related disorders, other downstream pathogenic events are less well understood and may include a secondary remodeling of major contractile proteins. Hence, in the present study, we aimed to investigate whether congenital myopathy-related RYR1 mutations alter the regulation of the most abundant contractile protein, myosin.

METHODS

We used skeletal muscle tissues from five patients with RYR1-related congenital myopathy and compared those with five controls and five patients with RYR1-related rhabdomyolysis/myalgia. We then defined post-translational modifications on myosin heavy chains (MyHCs) using LC/MS. In parallel, we determined myosin relaxed states using Mant-ATP chase experiments and performed molecular dynamics (MD) simulations.

RESULTS

LC/MS revealed two additional phosphorylations (Thr1309-P and Ser1362-P) and one acetylation (Lys1410-Ac) on the β/slow MyHC of patients with congenital myopathy. This method also identified six acetylations that were lacking on MyHC type IIa of these patients (Lys35-Ac, Lys663-Ac, Lys763-Ac, Lys1171-Ac, Lys1360-Ac, and Lys1733-Ac). MD simulations suggest that modifying myosin Ser1362 impacts the protein structure and dynamics. Finally, Mant-ATP chase experiments showed a faster ATP turnover time of myosin heads in the disordered-relaxed conformation.

CONCLUSIONS

Altogether, our results suggest that RYR1 mutations have secondary negative consequences on myosin structure and function, likely contributing to the congenital myopathic phenotype.