MYLK3

MYLK3 (cMLCK) is a cardiac serine/threonine kinase that maintains sarcomere integrity and contractility by phosphorylating ventricular myosin regulatory light chain 2 (MLC2v/MYL2) in cardiomyocytes (PMID:23095280). This phosphorylation keeps myosin biased away from the energy-sparing superrelaxed (SRX) state; loss of cMLCK shifts myosin toward SRX, disorganizes the sarcomere, and impairs contraction, while restoring cMLCK by AAV9_MYLK3 delivery normalizes the SRX/DRX ratio and rescues contractile function in mouse and human iPSC-derived cardiomyocyte models (PMID:37128901). cMLCK is acutely and continuously required: inducible ablation in adult hearts produces rapid heart failure with sarcomeric disorganization and cardiomyocyte atrophy within days (PMID:27025239), and its phenotypic role is developmentally regulated across germline, perinatal, and adult onset (PMID:27833563). Loss-of-function and truncating MYLK3 mutations that abolish kinase activity and reduce cMLCK protein cause familial dilated cardiomyopathy in humans (PMID:29235529, PMID:30690923), a link reinforced by a null Mylk3 allele driving the C57BL/6N substrain cardiomyopathy with BAC transgenic rescue (PMID:32213617). cMLCK abundance is tightly controlled post-transcriptionally and is destabilized by the ubiquitin-proteasome system under pressure overload (PMID:23095280), repressed by miR-1 targeting the MYLK3 3'UTR (PMID:22719074), and degraded via SQSTM1/p62-driven autophagy in drug-toxicity contexts, where reduced MYLK3 suppresses CAMK2-PLN-SERCA2a Ca2+ handling and provokes arrhythmia (PMID:40568844). Transcriptionally, GATA4 dephosphorylation represses MYLK3 in osimertinib cardiotoxicity, defining a GATA4-MYLK3-MYL2 axis (PMID:41330421).

1. 2012 High

   Established that cMLCK is the principal kinase phosphorylating MLC2v in vivo and that its proteasomal loss precipitates decompensated heart failure, defining the core kinase-substrate axis and a disease-relevant degradation mechanism.

   Evidence Reciprocal Mylk3 knockout and cardiomyocyte-specific overexpression in mice with pressure overload and proteasome inhibition

   PMID:23095280

   Open questions at the time

   • Did not resolve which E3 ligase targets cMLCK
   • Mechanism coupling MLC2v phosphorylation to contractile output not yet defined at the myosin level
2. 2012 Medium

   Identified miR-1 as a negative post-transcriptional regulator of cMLCK, explaining how MYLK3 abundance and downstream MLC2v phosphorylation can be tuned independent of gene dosage.

   Evidence Cardiac-specific miR-1 transgenic mice, MYLK3 3'UTR reporter assays, and anti-miR rescue

   PMID:22719074

   Open questions at the time

   • Did not establish physiological contexts where endogenous miR-1 controls cMLCK
   • Other regulatory miRNAs not surveyed
3. 2016 High

   Demonstrated that cMLCK is acutely required in mature cardiomyocytes for sarcomere integrity, distinguishing maintenance roles from developmental ones.

   Evidence Tamoxifen-inducible cardiomyocyte-specific Mylk3 knockout with echocardiography, histology, and electron microscopy

   PMID:27025239

   Open questions at the time

   • Did not define the molecular basis of cardiomyocyte atrophy versus hypertrophy
   • Reversibility of acute ablation not tested
4. 2016 Medium

   Showed that cMLCK's effect on cardiomyocyte morphology is developmentally staged, with perinatal loss yielding an intermediate phenotype between germline and adult ablation.

   Evidence Perinatal inducible Mylk3 knockout with morphometry and fetal gene expression analysis

   PMID:27833563

   Open questions at the time

   • Mechanism underlying stage-dependent morphological responses unresolved
   • Single-lab observation
5. 2017 High

   Connected human MYLK3 loss-of-function mutations to familial dilated cardiomyopathy via reduced protein and reduced MLC2v phosphorylation.

   Evidence Whole exome sequencing, segregation analysis, and in vitro phosphorylation assays in DCM families

   PMID:29235529

   Open questions at the time

   • Did not establish per-mutation effects on enzyme kinetics
   • Genotype-phenotype variability across carriers not detailed
6. 2019 High

   Quantified that a truncating cMLCK mutation produces complete loss of kinase activity, providing direct enzymatic confirmation of pathogenicity.

   Evidence Phos-tag SDS-PAGE and ADP-Glo kinase assays with kinetic parameters for wild-type versus mutant

   PMID:30690923

   Open questions at the time

   • Did not assess in vivo consequences of this specific allele
   • Effect on protein stability versus catalysis not dissected
7. 2019 Medium

   Revealed post-transcriptional control of cMLCK protein stability, since heterozygous loss reduced protein ~75% despite only ~50% mRNA reduction, linking dosage to mild contractile deficit.

   Evidence Heterozygous Mylk3 knockout mice with echocardiography, qPCR, and Western blot

   PMID:31244672

   Open questions at the time

   • Did not identify the stability-controlling factors
   • Single-lab model
8. 2020 High

   Confirmed Mylk3 loss as causal for cardiomyopathy by attributing the C57BL/6N substrain phenotype to a null allele rescued by BAC transgenesis.

   Evidence Substrain comparison, RNAseq, variant calling, and BAC transgenic rescue with echocardiography

   PMID:32213617

   Open questions at the time

   • Did not map downstream remodeling effectors
   • Strain-specific modifiers not excluded
9. 2023 High

   Mechanistically tied cMLCK to the myosin superrelaxed state and demonstrated therapeutic rescue, establishing both the biophysical readout and translational strategies.

   Evidence Knock-in mice, human iPSC-CMs, AAV9-MYLK3 delivery, CRISPR correction, SRX/DRX measurement, and a small-molecule cMLCK activator

   PMID:37128901

   Open questions at the time

   • Durability and safety of AAV9 and activator approaches not established
   • Structural basis of activator action on Vmax not resolved
10. 2025 Medium

    Linked SQSTM1/p62-mediated autophagic degradation of MYLK3 to CAMK2-PLN-SERCA2a Ca2+ dyshomeostasis, extending cMLCK biology to drug-induced arrhythmia.

    Evidence iPSC-CMs with SQSTM1-MYLK3 co-immunoprecipitation, autophagy flux and phosphorylation assays, and rescue by MYLK3 overexpression or omecamtiv mecarbil

    PMID:40568844

    Open questions at the time

    • Single Co-IP without reciprocal validation of the SQSTM1-MYLK3 interaction
    • Direct mechanism coupling cMLCK to CAMK2 phosphorylation not defined
11. 2026 Medium

    Defined a GATA4-MYLK3-MYL2 transcriptional axis underlying osimertinib cardiotoxicity, broadening regulation of cMLCK to transcriptional repression.

    Evidence iPSC-CMs, TAC mouse model, single-nucleus RNA sequencing, and omecamtiv rescue

    PMID:41330421

    Open questions at the time

    • GATA4-MYLK3 link not confirmed by direct promoter binding assay
    • Generalizability beyond osimertinib not established

• How cMLCK abundance is integrated across competing proteasomal, autophagic, microRNA, and transcriptional inputs to set MLC2v phosphorylation in different physiological and stress contexts remains unresolved.
• No unified model of cMLCK turnover control
• E3 ligase mediating proteasomal degradation unidentified
• Direct kinase-level link between cMLCK and CAMK2 signaling unestablished