Increased Autolysis of µ-Calpain in Skeletal Muscles of Chronic Alcohol-Fed Rats.

BACKGROUND: Proteolysis can proceed via several distinct pathways such as the lysosomal, calcium-dependent, and ubiquitin-proteasome-dependent pathways. Calpains are the main proteases that cleave a large variety of proteins, including the giant sarcomeric proteins, titin and nebulin. Chronic ethanol feeding for 6 weeks did not affect the activities of µ-calpain and m-calpain in the m. gastrocnemius. In our research, changes in µ-calpain activity were studied in the m. gastrocnemius and m. soleus of chronically alcohol-fed rats after 6 months of alcohol intake. METHODS: SDS-PAGE analysis was applied to detect changes in titin and nebulin contents. Titin phosphorylation analysis was performed using the fluorescent dye Pro-Q Diamond. Western blotting was used to determine µ-calpain autolysis as well as µ-calpain and calpastatin contents. The titin and nebulin mRNA levels were assessed by real-time PCR. RESULTS: The amounts of the autolysed isoform (78 kDa) of full-length µ-calpain (80 kDa) increased in the m. gastrocnemius and m. soleus of alcohol-fed rats. The calpastatin content increased in m. gastrocnemius. Decreased intact titin-1 (T1) and increased T2-proteolytic fragment contents were found in the m. gastrocnemius and m. soleus of the alcohol-fed rats. The nebulin content decreased in the rat gastrocnemius muscle of the alcohol-fed group. The phosphorylation levels of T1 and T2 were increased in the m. gastrocnemius and m. soleus, and decreased titin and nebulin mRNA levels were observed in the m. gastrocnemius. The nebulin mRNA level was increased in the soleus muscle of the alcohol-fed rats. CONCLUSIONS: In summary, our data suggest that prolonged chronic alcohol consumption for 6 months resulted in increased autolysis of µ-calpain in rat skeletal muscles. These changes were accompanied by reduced titin and nebulin contents, titin hyperphosphorylation, and development of hindlimb muscle atrophy in the alcohol-fed rats.

Nuances of electrophoresis study of titin/connectin.

Almost 40 years has passed since the discovery of giant elastic protein titin (also known as connectin) of striated and smooth muscles using gel electrophoresis. Sodium dodecyl sulfate polyacrylamide gel electrophoresis is a major technique for studying the isoform composition and content of titin. This review provides historical insights into the technical aspects of the electrophoresis methods used to identify titin and its isoforms. We particularly focus on the nuances of the technique that improve the preservation of its primary structure so that its high molecular weight isoforms can be visualized.

Smooth muscle titin forms in vitro amyloid aggregates.

Amyloids are insoluble fibrous protein aggregates, and their accumulation is associated with amyloidosis and many neurodegenerative diseases, including Alzheimer's disease. In the present study, we report that smooth muscle titin (SMT; 500 kDa) from chicken gizzard forms amyloid aggregates in vitro This conclusion is supported by EM data, fluorescence analysis using thioflavin T (ThT), Congo red (CR) spectroscopy and X-ray diffraction. Our dynamic light scattering (DLS) data show that titin forms in vitro amyloid aggregates with a hydrodynamic radius (Rh) of approximately 700-4500 nm. The initial titin aggregates with Rh approximately 700 nm were observed beyond first 20 min its aggregation that shows a high rate of amyloid formation by this protein. We also showed using confocal microscopy the cytotoxic effect of SMT amyloid aggregates on smooth muscle cells from bovine aorta. This effect involves the disorganization of the actin cytoskeleton and result is cell damage. Cumulatively, our results indicate that titin may be involved in generation of amyloidosis in smooth muscles.

Isoform composition and gene expression of thick and thin filament proteins in striated muscles of mice after 30-day space flight.

Changes in isoform composition, gene expression of titin and nebulin, and isoform composition of myosin heavy chains as well as changes in titin phosphorylation level in skeletal (m. gastrocnemius, m. tibialis anterior, and m. psoas) and cardiac muscles of mice were studied after a 30-day-long space flight onboard the Russian spacecraft "BION-M" number 1. A muscle fibre-type shift from slow-to-fast and a decrease in the content of titin and nebulin in the skeletal muscles of animals from "Flight" group was found. Using Pro-Q Diamond staining, an ~3-fold increase in the phosphorylation level of titin in m. gastrocnemius of mice from the "Flight" group was detected. The content of titin and its phosphorylation level in the cardiac muscle of mice from "Flight" and "Control" groups did not differ; nevertheless an increase (2.2 times) in titin gene expression in the myocardium of flight animals was found. The observed changes are discussed in the context of their role in the contractile activity of striated muscles of mice under conditions of weightlessness.

Prion protein inhibits microtubule assembly by inducing tubulin oligomerization.

A growing body of evidence points to an association of prion protein (PrP) with microtubular cytoskeleton. Recently, direct binding of PrP to tubulin has also been found. In this work, using standard light scattering measurements, sedimentation experiments, and electron microscopy, we show for the first time the effect of a direct interaction between these proteins on tubulin polymerization. We demonstrate that full-length recombinant PrP induces a rapid increase in the turbidity of tubulin diluted below the critical concentration for microtubule assembly. This effect requires magnesium ions and is weakened by NaCl. Moreover, the PrP-induced light scattering structures of tubulin are cold-stable. In preparations of diluted tubulin incubated with PrP, electron microscopy revealed the presence of approximately 50 nm disc-shaped structures not reported so far. These unique tubulin oligomers may form large aggregates. The effect of PrP is more pronounced under the conditions promoting microtubule formation. In these tubulin samples, PrP induces formation of the above oligomers associated with short protofilaments and sheets of protofilaments into aggregates. Noticeably, this is accompanied by a significant reduction of the number and length of microtubules. Hence, we postulate that prion protein may act as an inhibitor of microtubule assembly by inducing formation of stable tubulin oligomers.

Reconstitution of ventricular myosin with atrial light chains 1 improves its functional properties.

Atrial light chain 1 (ALC-1) is expressed in embryonic and hypertrophied human ventricles but not in normal adult human ventricles. We investigated the effects of recombinant human atrial light chains (hALC-1) on the structure and enzymatic activity of synthetic filaments of ventricular myosin. The endogenous ventricular myosin light chain 1 (VLC-1) was partially replaced by recombinant hALC-1 yielding hALC-1 levels of 12%, 24% and 42%. This reconstitution of ventricular myosin with hALC-1 did not change the length of synthetic myosin filaments but led to more rounded myosin heads in comparison with those of control filaments. Actin-activated ATPase activity of myosin, a parameter of functional activity of molecular motor, amounted to 79.5 nmol P(i)/mg per min in control myosin filaments. Reconstitution with hALC-1 caused a profound increase of the actin-activated myosin ATPase activity in a dose dependent manner, for example, synthetic myosin filaments formed with 12%, 24% and 42% hALC-1 reconstituted myosin revealed the actin-activated ATPase activity increased by 18%, 26% and 36%, respectively, as compared to control. These results strongly suggest that in vivo expression of ALC-1 enhances ventricular myosin function, thereby contributing to cardiac compensation.

Visualization of caldesmon binding to synthetic filaments of smooth muscle myosin.

We have used synthetic filaments of unphosphorylated chicken gizzard myosin with a compact, highly ordered structure under relaxing conditions (in the absence of Ca2+ and in the presence of ATP) to visualize the mode of caldesmon binding to myosin filaments by negative staining and immunogold electron microscopy. We demonstrate that the addition of caldesmon to preformed myosin filaments leads to the appearance of numerous smooth projections curving out from the filament surface. The addition of caldesmon or its N-terminal fragment resulted in the partial masking of myosin filament periodicity. However, it did not change the inner structure of the filaments. It is demonstrated that most caldesmon molecules bind to myosin filaments through the N-terminal part, while the C-terminal parts protrude from the filament surface, as confirmed by immunoelectron microscopy visualization. Together with the available biochemical data on caldesmon binding to both actin and myosin and electron microscopic observations on the mode of caldesmon attachment to actin filaments with the C-termini of the molecules curving out from the filaments, the visualization of caldesmon attachment to myosin filaments completes the scenario of actin to myosin tethering by caldesmon.