DECIPHERING THE SECRET OF SARCOMERE ASSEMBLY AND DISEASES USING THE ZEBRAFISH MODEL SYSTEM: REGULATION OF MYOFIBRILLOGENESIS BY SMYD1B AND ITS PARTNERS

Abstract

Myofibrillogenesis is a process of precise assembly of sarcomeric proteins into the highly organized sarcomeres which are essential for muscle cell differentiation and function. Myofibrillogenesis requires proper folding and assembly of newly synthesized sarcomeric proteins. Mutations of the sarcomeric proteins are known to cause skeletal and cardiac muscle diseases.

<italic>smyd1b</italic> is a skeletal and cardiac muscle-specific gene which encodes two alternatively spliced isoforms, <italic>smyd1b_tv1</italic> and <italic>smyd1b_tv2</italic>. Knockdown of <italic>smyd1b</italic> (tv1 and tv2) expression resulted in zebrafish larvae without locomotion and heart contraction. Thick filament assembly was significantly disrupted in <italic>smyd1b</italic> knockdown embryos. Yeast Two-Hybrid study showed that Smyd1 associates with another muscle-specific protein--skNAC, however, skNAC function in muscle cells is unknown.

In order to expand the understanding of <italic>smyd1b</italic> function and study the working mechanism, I further characterized the function of Smyd1b and its partners including skNAC and Hsp90&alpha1 during muscle development, and carried out mechanistic studies using zebrafish as a model system.

Our findings show that: 1) In addition to the thick filament, <italic>smyd1b</italic> plays an important role in the assembly of thin and titin filaments, as well as Z-line and M-line. 2) Knockdown of <italic>smyd1b</italic> has no effect for heart tube formation; however, it disrupts the myofibril assembly of the cardiac muscle that causes the heart defect. 3) Smyd1b_tv1, but not Smyd1b_tv2 can be localized on the M-line of sarcomeres. 4) Ser225 on Smyd1b_tv1, which is a potential phosphorylation site, is important for the M-line localization of Smyd1b_tv1. 5) Knockdown of <italic>smyd1b</italic> causes the upregulation of <italic>hsp90&alpha1</italic> and <italic>unc45b</italic> gene expression. 6) <italic>hsp90&alpha1</italic> plays an important role for myofibril assembly. 6) Knockdown of <italic>smyd1b</italic> or <italic>hsp90&alpha1</italic> causes the reduction of myosin protein accumulation. 7) Smyd1b_tv1, but not Smyd1b_tv2 associates with the myosin chaperones Hsp90&alpha1 and Unc45b. 8) <italic>sknac</italic> is required for the thick and thin filaments assembly. 9) Knockdown of <italic>sknac</italic> causes the reduction of myosin protein accumulation.

These studies provide us an in-depth characterization of <italic>smyd1b</italic> and its partners' function and expands the mechanistic understanding of how <italic>smyd1b</italic> fulfils its vital role in myofibrillogenesis. Most importantly, this study provides new insights to help us understand the complex process of myofibrillogenesis and sarcomere diseases.