Methods & results: Myosin-5a and -5b heavy chains, encoded by MYO5a and MYO5b, were contrariwise regulated during cardiomyocyte development with MYO5a being predominantly expressed during foetal and MYO5b during adult stages. This reciprocal expression pattern was also present in the pathophysiological stress situation induced by myocardial infarction (MI). WT hearts 3/7 days post-MI revealed an upregulation of MYO5a (3d: +122%, p<0.05; 7d: +76%, p<0.01) and a down-regulation of MYO5b (3d: -58%; 7d: -39%, p<0.01). Moreover, human end-stage failing hearts revealed a reduced myosin-5b protein expression as well (-28%, p<0.05). Mice with a cardiomyocyte-specific knockout (KO) of MYO5b (KO: αMHC-Cre;Myo5b^flox/flox; WT: Myo5b^flox/flox) developed normally up to the age of 3 months (M) with their LV function comparable with WT mice, even though impairments in electrical conduction such as intermittent bundle branch blocks or supraventricular extra-systoles were already present at this age. The subsequent development of age-related heart failure (6M male: WT 50±15, KO 27±12 %FAC, p<0.001) was concomitant with the development of fibrosis and inflammation, cardiomyocyte hypertrophy, sarcomeric and metabolic impairments and premature mortality (all KO mice died before 37 weeks of age). RNA-Seq analyses revealed deregulated gene expression in KO mice prior to the development of heart failure, including genes involved in metabolism, sarcomere composition and function, Ca²⁺ homeostasis and electrical conduction. A number of these mRNAs was merely reduced in the cytoplasm and not in the nucleus of isolated KO cardiomyocytes. Immunohistochemistry and co-immunoprecipitation experiments revealed binding of myosin-5b to ribosomes. Subsequent analysis of myosin-5b-associated ribonucleoprotein particles revealed mRNAs coding for genes involved in fatty acid and glucose metabolism, sarcomere structure and function, ion channel subunits and Ca²⁺ homeostasis.

Conclusion: The functionally and structurally related motor proteins myosin-5a and -5b display an expression pattern similar to MYH6 and MYH7 as they are reciprocally regulated during development with a switch in expression from MYO5a to MYO5b during the maturation from foetal to adult cardiomyocytes and in response to pathophysiological stress (MI). The KO of MYO5b in cardiomyocytes triggers abnormalities in cardiac electrical conduction, cardiac dysfunction and premature death, possibly due to dysfunctional transport of mRNA loaded ribosomes containing mRNAs important for sarcomere function, Ca²⁺ homeostasis, cardiac conductance and energy uptake.