Nutrient-dependent pathology in mitochondrial hypertrophic cardiomyopathy model

Objective

Mitochondrial translation defects are a major cause of early childhood hypertrophic cardiomyopathy (CMP). While the genetic basis of these disorders is being increasingly uncovered, the downstream molecular mechanisms driving disease pathogenesis remain poorly understood. In this study, we investigated the consequences of defects in mitochondrial ribosomal large subunit protein 44 (MRPL44), associated with infantile-onset CMP in human cardiomyocytes, in nutrient environments relevant to cardiac development.

Methods

Induced pluripotent stem cell line with MRPL44 patient mutation and controls were differentiated to cardiomyocytes and grown in glucose or lipid-enriched medium reflecting prenatal or postnatal fuel preferences, respectively. Mitochondrial, lipid metabolic and cellular characteristics were studied by immunofluorescence and cellular transcriptome by RNA-sequencing.

Results

In glucose-rich medium, patient-derived cardiomyocytes exhibit increased mitochondrial DNA (mtDNA) content and elevated mitochondrial transcripts. In contrast the lipid-enriched medium triggered both mitochondrial and endoplasmic reticulum -related stress responses, disrupted lipid and cholesterol homeostasis, accompanied by remodeling of the central biosynthetic pathway of one carbon metabolism. The cells accumulated lipids while also inducing lipid uptake and synthesis genes, suggesting maladaptive metabolic rewiring.

Conclusion

Our findings indicate that glucose and lipids, the latter being the postnatally favored cardiac fuel, exert remarkably different consequences in MRPL44 deficient cardiomyocytes. The lipid enriched medium elicited robust activation of metabolic stress responses, with chronic upregulation of anabolic biosynthesis pathways and lipid accumulation indicative of conflicting metabolic homeostasis. These observations provide a mechanistic basis for postnatal disease manifestation and highlight nutrient metabolism as a key driver in development of infantile-onset mitochondrial hypertrophic cardiomyopathy.

Highlights

• MRPL44 deficiency impairs mitochondrial translation but induces mtDNA replication and transcription in iPSC-derived cardiomyocytes.

• In glucose conditions, MRPL44 mutant cardiomyocytes upregulate mitochondrial replication and transcription program, but not translation.

• Lipid-enriched nutrient conditions exacerbate disease phenotype, inducing mitochondrial and ER stress responses in MRPL44 deficiency.

• Mitochondrial ribosome defect disrupts lipid homeostasis in cardiomyocytes causing impaired fatty acid oxidation, lipid accumulation and altered cholesterol metabolism.