Heart Failure Induced by Perinatal Ablation of Cardiac Myosin Light Chain Kinase
Yasmin F K Islam1, Ryan Joseph1, Rajib R Chowdhury1
1Department of Physiology and Functional Genomics, University of Florida Gainesville, FL, USA.
Abstract:
Background: Germline knockout mice are invaluable in understanding the function of the targeted genes. Sometimes, however, unexpected phenotypes are encountered, due in part to the activation of compensatory mechanisms. Germline ablation of cardiac myosin light chain kinase (cMLCK) causes mild cardiac dysfunction with cardiomyocyte hypertrophy, whereas ablation in adult hearts results in acute heart failure with cardiomyocyte atrophy. We hypothesized that compensation after ablation of cMLCK is dependent on developmental staging and perinatal-onset of cMLCK ablation will result in more evident heart failure than germline ablation, but less profound when compared to adult-onset ablation. Methods and Results: The floxed-Mylk3 gene was ablated at the beginning of the perinatal stage using a single intra-peritoneal tamoxifen injection of 50 mg/kg into pregnant mice on the 19th day of gestation, this being the final day of gestation. The level of cMLCK protein level could no longer be detected 3 days after the injection, with these mice hereafter denoted as the perinatal Mylk3-KO. At postnatal day 19, shortly before weaning age, these mice showed reduced cardiac contractility with a fractional shortening 22.8 ± 1.0% (n = 7) as opposed to 31.4 ± 1.0% (n = 11) in controls. The ratio of the heart weight relative to body weight was significantly increased at 6.68 ± 0.28 mg/g (n = 12) relative to the two control groups, 5.90 ± 0.16 (flox/flox, n = 11) and 5.81 ± 0.33 (wild/wild/Cre, n = 5), accompanied by reduced body weight. Furthermore, their cardiomyocytes were elongated without thickening, with a long-axis of 101.8 ± 2.4 μm (n = 320) as opposed to 87.1 ± 1.6 μm (n = 360) in the controls. Conclusion: Perinatal ablation of cMLCK produces an increase of heart weight/body weight ratio, a reduction of contractility, and an increase in the expression of fetal genes. The perinatal Mylk3-KO cardiomyocytes were elongated in the absence of thickening, differing from the compensatory hypertrophy shown in the germline knockout, and the cardomyocyte thinning shown in adult-inducible knockout.
Insights
Perinatal knockout of cardiac myosin light chain kinase (cMLCK) in mice leads to heart failure with elongated cardiomyocytes. This finding highlights developmental stage-dependent compensation in cardiac gene function.
Area of Science:
- Cardiovascular Biology
- Developmental Biology
- Genetics
Background:
- Germline gene knockouts are crucial for understanding gene function but can reveal compensatory mechanisms.
- Cardiac myosin light chain kinase (cMLCK) ablation shows varied cardiac phenotypes depending on the developmental stage of ablation.
- Hypothesis: Perinatal cMLCK ablation results in heart failure distinct from germline or adult-onset ablation.
Purpose of the Study:
- To investigate the cardiac phenotype resulting from perinatal ablation of cMLCK.
- To compare the effects of perinatal cMLCK ablation with germline and adult-onset ablation models.
Main Methods:
- Perinatal knockout of the *Mylk3* gene (encoding cMLCK) was induced in mice via tamoxifen injection on day 19 of gestation.
- Cardiac function, heart weight to body weight ratio, body weight, and cardiomyocyte morphology were assessed at postnatal day 19.
- Gene expression analysis was performed to evaluate fetal gene markers.
Main Results:
- Perinatal cMLCK knockout mice exhibited reduced cardiac contractility (fractional shortening) and increased heart weight to body weight ratio.
- These mice also showed reduced body weight and elongated cardiomyocytes without thickening.
- Increased expression of fetal genes was observed, indicating a distinct cardiac response.
Conclusions:
- Perinatal cMLCK ablation causes significant cardiac dysfunction, including reduced contractility and altered cardiomyocyte structure.
- The observed phenotype differs from both germline and adult-inducible cMLCK knockout models, emphasizing developmental timing in compensatory responses.
- This study provides insights into the role of cMLCK during perinatal cardiac development and adaptation.
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