Phosphorylation of cardiac myosin heavy chains (see MYH7B, 609928) and light chains (see MYL2, 160781) by a kinase, such as MYLK3, potentiates the force and rate of cross-bridge recruitment in cardiac myocytes (Chan et al., 2008).

Using microarray analysis to identify transcripts upregulated in failing human hearts, followed by 5-prime RACE, Seguchi et al. (2007) cloned MYLK3, which they called cardiac MLCK. The deduced protein contains a C-terminal serine-threonine kinase domain, including an ATP-binding site and conserved active site residues. Northern blot and database analysis indicated that MYLK3 expression is restricted to heart.

Chan et al. (2008) cloned mouse Mylk3, which encodes a deduced 795-amino acid protein containing a C-terminal calmodulin (see CALM1; 114180)-binding regulatory domain following the catalytic domain. Expression of Mylk3 increased during development from neonatal to adult stages and decreased again in aged hearts (18 to 21 months old). Immunohistochemical analysis detected diffuse Mylk3 staining in the cytoplasm of neonatal rat cardiomyocytes; however, in some areas, Mylk3 appeared to colocalize with actin (ACTC1; 102540) in a striated pattern.

Using cDNA expression analysis, Seguchi et al. (2007) found that MYLK3 was upregulated in failing human myocardia, and expression correlated with the pulmonary arterial pressure of patients with heart failure. Mylk3 expression also increased in a rat model of myocardial infarction. Recombinant murine Mylk3 showed specificity for Myl2 (Mlc2v), and phosphorylation was calcium- and calmodulin-dependent. Knockdown of Mylk3 with small interfering RNAs in cultured rat cardiomyocytes decreased Myl2 phosphorylation and impaired epinephrine-induced activation of sarcomere reassembly.

Chan et al. (2008) showed that mouse Mylk3 phosphorylated Myl2 and Myl7 (Mlc2a) in the absence of calcium or calmodulin. Mylk3 phosphorylated Myl2 in a dose-dependent manner with high affinity but relatively low catalytic efficiency. Mylk3 promoted sarcomere organization and increased contractility in neonatal rat cardiomyocytes in the absence of altered intracellular calcium. In a mouse model of myocardial infarction, the protein level of Mylk3 decreased relative to sham operated hearts, but the mRNA level was unchanged, suggesting posttranscriptional regulation of MYLK3 in aging and heart failure.

Hartz (2008) mapped the MYLK3 gene to chromosome 16q11.2 based on an alignment of the MYLK3 sequence (GenBANK AJ247087) with the genomic sequence (build 36.1). Chan et al. (2008) mapped the mouse Mylk3 gene to chromosome 8.

Seguchi et al. (2007) found that knockdown of Mylk3 in zebrafish embryos resulted in dilated cardiac ventricles and immature sarcomere structure.

Tougas et al. (2019) found that Mylk3 +/- mice had normal heart weight/body weight ratio, but they had enlarged hearts and reduced cardiac contractility compared to wildtype. Histologic analysis did not find interstitial fibrosis in either Mylk3 +/- or Mylk3 -/- mice, but isolated ventricular cardiomyocytes from Mylk3 +/- mice had larger cell surface area and displayed reductions in contractility and speed of contraction. In Mylk3 +/- mice, cardiac myosin light chain kinase (Mlck) protein, encoded by Mylk3, was reduced by approximately 75%, and cardiac Mlck mRNA was reduced by approximately half. Further analysis suggested that the ubiquitin-proteasome system (UPS) was unlikely responsible for the marked reduction of cardiac Mlck proteins in heterozygous knockout cardiomyocytes.

Williams et al. (2020) found that mouse substrain C57BL/6N (B6N) with deletion of Nnt developed dilated cardiomyopathy (CM), particularly at 12 months of age, whereas substrain C57BL/6J (B6J) with deletion of Nnt did not, suggesting that Nnt loss alone did not cause CM. Analysis of B6N hearts revealed defective cardiomyocytes and enrichment of cardiac remodeling genes. Further analysis identified a point mutation in the Mylk3 gene as the likely cause of the CM observed in the B6N substrain with a deletion of Nnt. This point mutation was present in the B6N substrain but absent in the B6J substrain. Mylk3 is expressed almost exclusively expressed in heart, and the point mutation abolished protein expression in B6N mice.