Introduction: Unconventional myosins (Myo) are intracellular mechanoenzymes located within the cytoplasm and perform directional movement along tracks of cytoplasmic actin filaments, thereby acting as cargo transporters for proteins and cell organelles. They are involved in various cellular processes including energy substrate uptake and receptor signalling. Here, we investigate the regulation of Myo5a and Myo5b in cardiomyocytes’ development and the role of Myo5b in the adult mouse heart.
Methods and Results: QRT-PCR revealed a development specific expression pattern for Myo5a and b with low Myo5b and high Myo5a mRNA in embryonic HL-1 cardiomyocytes and high Myo5b and low Myo5a expression in isolated adult mouse cardiomyocytes. To analyse the role of Myo5b in the adult heart, mice with a cardiomyocyte-specific Myo5b knockout (αMHC-Cre;Myo5bflox/flox: Myo5b-KO; Myo5bflox/flox: WT) were generated. Myo5b-KO mice were born at the expected Mendelian ratio. At the age of 12 weeks, cardiac morphology, dimension and function appeared normal in female and male Myo5b-KO mice, however, RNA-Seq of left ventricular tissue revealed 796 altered genes (p-adj. <0.05). Pathway analyses (DAVID tool) identified substantial alterations in 70 pathways including the metabolism. In addition, genes involved in glucose and fatty acid uptake (glucose transporter GLUT4 and fatty acid translocase CD36) and glycolysis and fatty acid metabolism were significantly altered in Myo5b-KO compared with WT mice. At the age of 24 weeks, all Myo5b-KO mice had developed dilated cardiomyopathy, which was more pronounced in males than females (Table). Histological analyses showed increased cardiac fibrosis and inflammation associated with enhanced long-term mortality (mean survival for Myo5-KO: 34±3 weeks vs. >12 months for corresponding WT). In female Myo5b-KO mice, heart failure was enhanced by pregnancy/postpartum stress (PP) compared with age-matched nulli pari (NP) Myo5b-KO mice (Table).
Conclusion: Our results show that Myo5a and 5b are differently regulated in cardiomyocytes during development with a switch from Myo5a to Myo5b during postnatal maturation. Cardiac Myo5b deficiency leads to impairment of the gene expression programs in young mice prior to postpartum and age-related heart failure. Thus, Myo5b plays an important role for cardiac homeostasis and physiological stress response.
Table
| |
male |
female NP |
female PP |
| |
WT
N=22 |
Myo5b-KO
N=20 |
WT
N=21 |
Myo5b-KO
N=19 |
WT
N=4 |
Myo5b-KO
N=5 |
| FAC (%) |
50±15 |
27±12*** |
52±12 |
38±13$$$# |
50±13
|
15±11&&§§ |
| LVEDA (cm2) |
24.5±3.3 |
32.1±9.4** |
19.6±3.0 |
22.6±5.4$### |
24.7±2.9 |
46±3&&&§§§ |
| LVESA (cm2) |
12.4±4.6
|
24.3±11.3***
|
9.5±3.2
|
14.6±7.1$$## |
14.3±6.1 |
40±7&&&§§§
|
*P<0.05; **P<0.01; ***P<0.005 vs. WT male; $P<0.05; $$P<0.01; $$$P<0.005 vs. WT female;
#P<0.05; ##P<0.01, ###P<0.005 vs. Myo5b-KO male; &&P<0.01, &&&P<0.001 vs. Myo5b-KO female NP; §§P<0.01, §§§P<0.001 vs. WT female PP.
Time point of analysis: 24 weeks.