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Mitochondrial DNA replication and transcription are dissociated during embryonic cardiac hypertrophy
J M Kennedy1, S R Lobacz, S W Kelley
1Department of Physiology and Biophysics, University of Illinois, Chicago 60680.
Insights
Hypothermia in embryonic chicks caused cardiac hypertrophy and reduced cytochrome-c oxidase mRNA levels. Mitochondrial DNA increased, suggesting a dissociation between gene replication and transcription in cardiac cells.
Area of Science:
- Cardiovascular Physiology
- Mitochondrial Biology
- Developmental Biology
Background:
- Cardiac hypertrophy is a significant cardiovascular adaptation.
- Mitochondrial function is crucial for cellular energy production.
- Hypothermia can impact cardiac development and cellular processes.
Purpose of the Study:
- To investigate the effects of hypothermia on cardiac hypertrophy in embryonic chicks.
- To examine the regulation of mitochondrial gene expression and replication under hypothermic conditions.
- To determine the relationship between mitochondrial DNA copy number and mRNA levels of mitochondrially encoded genes.
Main Methods:
- Induction of cardiac hypertrophy via hypothermia (32°C from day 11).
- Measurement of ventricular protein, RNA, and DNA content.
- Assay of mitochondrial enzyme activities (cytochrome-c oxidase, citrate synthase).
- Quantitative slot-blot analysis of mitochondrial DNA and CO III mRNA levels.
Main Results:
- Hypothermia induced cardiac hypertrophy, evidenced by increased ventricular mass.
- Significant reductions in cytochrome-c oxidase and citrate synthase activities were observed in hypothermic ventricles.
- A decrease in CO III mRNA concentration was found, while mitochondrial DNA concentration increased.
- No significant changes in enzyme activities were noted in pectoralis muscles.
Conclusions:
- Hypothermia-induced cardiac hypertrophy is associated with decreased mitochondrial enzyme activity.
- The reduction in CO III mRNA is not directly coupled with mitochondrial DNA copy number changes.
- This dissociation provides a model for studying mitochondrial biogenesis regulation.
Abstract:
Cardiac hypertrophy was produced in embryonic chicks by decreasing the incubation temperature from 38 degrees C to 32 degrees C on day 11. Increases in ventricular protein, RNA, and DNA support the cardiac enlargement. Cytochrome-c oxidase activity and citrate synthase activity were depressed in hypothermic ventricles by 63% and 56%, respectively. No significant differences were seen in enzyme activities in pectoralis muscles. The involvement of mitochondrial gene replication and transcription was evaluated using a cDNA clone for the mitochondrially encoded subunit III of cytochrome-c oxidase (CO III). Quantitative slot-blot analysis demonstrated that the relative CO III mRNA concentration was reduced in hypothermic ventricles. In contrast, the relative mitochondrial DNA concentration was increased in hypothermic ventricles. Taken together, these data indicate that a hypothermia-induced decrease in cytochrome-c oxidase activity is associated with a decrease in CO III mRNA, which is not coupled to a decrease in the mitochondrial DNA copy number. This dissociation of mitochondrial gene replication and transcription may provide a useful model for examining the regulation of mitochondrial biogenesis.