Still Heart Encodes a Structural HMT, SMYD1b, with Chaperone-Like Function during Fast Muscle Sarcomere Assembly

Abstract

The vertebrate sarcomere is a complex and highly organized contractile structure whose assembly and function requires the coordination of hundreds of proteins. Proteins require proper folding and incorporation into the sarcomere by assembly factors, and they must also be maintained and replaced due to the constant physical stress of muscle contraction. Zebrafish mutants affecting muscle assembly and maintenance have proven to be an ideal tool for identification and analysis of factors necessary for these processes. The still heart mutant was identified due to motility defects and a nonfunctional heart. The cognate gene for the mutant was shown to be smyd1b and the still heart mutation results in an early nonsense codon. SMYD1 mutants show a lack of heart looping and chamber definition due to a lack of expression of heart morphogenesis factors gata4, gata5 and hand2. On a cellular level, fast muscle fibers in homozygous mutants do not form mature sarcomeres due to the lack of fast muscle myosin incorporation by SMYD1b when sarcomeres are first being assembled (19hpf), supporting SMYD1b as an assembly protein during sarcomere formation.

A compact unc45b‐promoter drives muscle‐specific expression in zebrafish and mouse

“…SMYD1 is exclusively expressed in cardiac and skeletal muscle cells from zebrafish and mice (Just et al ., 2011; Nagandla et al ., 2016) where it locates to the cell nucleus, as expected for a histone methyltransferase (HMT), and to the sarcomeric M‐band (Just et al ., 2011). In zebrafish, Smyd1b was shown to be crucial to orchestrate thick filament assembly in cardiomyocytes and fast‐twitch skeletal muscle cells and when defective leads to severe cardiac and skeletal muscle dysfunction due to impaired myofibrillogenesis (Just et al ., 2011; Li et al ., 2013; Prill et al ., 2015; Tan et al ., 2006). Consistently, nullizygous SMYD1 mice die at embryonic day E10.5 due to severe heart malformations and cardiac dysfunction (Gottlieb et al ., 2002).…”

Defective sarcomere assembly in smyd1a and smyd1b zebrafish mutants

“…Interestingly, Prill and colleagues noted that slow myofibrils become disrupted in still heart tm123a ( sth ), another smyd1b mutant allele at 48 hpf (13). The still heart tm123a ( sth ) zebrafish mutant carries an insertion of 9 nt that contain an in‐frame stop codon at position 45 (13). All these mutants contained premature stop codons in the smyd1b gene, thus unlikely producing a functional protein.…”

“…Previous studies using MO knockdown and zebrafish mutant reported contradictory findings regarding Smyd1b function in the slow muscle of zebrafish embryos (7)(8)(9)13). To decipher these contradictory findings, we decided to knock out smyd1b using CRISPR.…”

“…Recent studies have demonstrated that zebrafish Smyd1b and mouse Smyd1 play a vital role in muscle cell differentiation and sarcomere assembly in skeletal and cardiac muscles (6)(7)(8)(9)(10)(11)(12)(13)(14)(15). Smyd1 is a SET domain containing lysine methyltransferase specifically expressed in skeletal and cardiac muscles (6,7).…”

“…Genetic studies demonstrated that Smyd1 is required for both early myogenesis and later myofiber maturation during muscle development in mice and zebrafish (7,8,(12)(13)(14). Muscle-specific deletion of Smyd1 in mouse embryos using Myf5 cre impaired myoblast differentiation and resulted in fewer myofibers and perinatal death (12).…”