Dataset Information

Knockdown and overexpression of Unc-45b result in defective myofibril organization in skeletal muscles of zebrafish embryos.

ABSTRACT:

Background

Unc-45 is a myosin chaperone and a Hsp90 co-chaperone that plays a key role in muscle development. Genetic and biochemical studies in C. elegans have demonstrated that Unc-45 facilitates the process of myosin folding and assembly in body wall muscles. Loss or overexpression of Unc-45 in C. elegans results in defective myofibril organization. In the zebrafish Danio rerio, unc-45b, a homolog of C. elegans unc-45, is expressed in both skeletal and cardiac muscles. Earlier studies indicate that mutation or knockdown of unc-45b expression in zebrafish results in a phenotype characterized by a loss of both thick and thin filament organization in skeletal and cardiac muscle. The effects of unc-45b knockdown on other sarcomeric structures and the phenotype of Unc-45b overexpression, however, are poorly understood in vertebrates.

Results

Both knockdown and overexpression provide useful tools to study gene function during animal development. Using such methods, we characterized the role of Unc-45b in myofibril assembly of skeletal muscle in Danio rerio. We showed that, in addition to thick and thin filament defects, knockdown of unc-45b expression disrupted sarcomere organization in M-lines and Z-lines of skeletal muscles in zebrafish embryos. Western blotting analysis showed that myosin protein levels were significantly decreased in unc-45b knockdown embryos. Similarly, embryos overexpressing Unc-45b also exhibited severely disorganized myosin thick filaments. Disruption of thick filament organization by Unc-45b overexpression depends on the C-terminal UCS domain in Unc-45b required for interaction with myosin. Deletion of the C-terminal UCS domain abolished the disruptive activity of Unc-45b in myosin thick filament organization. In contrast, deletion of the N-terminal TPR domain required for binding with Hsp90α had no effect.

Conclusion

Collectively, these studies indicate that the expression levels of Unc-45b must be precisely regulated to ensure normal myofibril organization. Loss or overexpression of Unc-45b leads to defective myofibril organization.

SUBMITTER: Bernick EP

PROVIDER: S-EPMC2954953 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:UNC-45B is a multidomain molecular chaperone that is essential for the proper folding and assembly of myosin into muscle thick filaments in vivo. It has previously been demonstrated that the UCS domain is responsible for the chaperone-like properties of the UNC-45B. To better understand the chaperoning function of the UCS domain of the UNC-45B chaperone, we engineered mutations designed to 1) disrupt chaperone-client interactions by removing and altering the structure of a putative client-interacting loop and 2) disrupt chaperone-client interactions by changing highly conserved residues in a putative client-binding groove. We tested the effect of these mutations by using a, to our knowledge, novel combination of complementary biophysical assays (circular dichroism, chaperone activity, and small-angle x-ray scattering) and in vivo tools (Caenorhabditis elegans sarcomere structure). Removing the putative client-binding loop altered the secondary structure of the UCS domain (by decreasing the α-helix content), leading to a significant change in its solution conformation and a reduced chaperoning function. Additionally, we found that mutating several conserved residues in the putative client-binding groove did not alter the UCS domain secondary structure or structural stability but reduced its chaperoning activity. In vivo, these groove mutations were found to significantly alter the structure and organization of C. elegans sarcomeres. Furthermore, we tested the effect of R805W, a mutation distant from the putative client-binding region, which in humans, has been known to cause congenital and infantile cataracts. Our in vivo data show that, to our surprise, the R805W mutation appeared to have the most drastic detrimental effect on the structure and organization of the worm sarcomeres, indicating a crucial role of R805 in UCS-client interactions. Hence, our experimental approach combining biophysical and biological tools facilitates the study of myosin-chaperone interactions in mechanistic detail.