Decreased cytochrome c oxidase subunit VIIa in aged rat heart mitochondria: immunocytochemistry
Hisashi Fujioka1, Shadi Moghaddas, Deborah G Murdock
1Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106, USA. hisashi.fujioka@case.edu
Abstract:
Aging decreases oxidative phosphorylation through cytochrome oxidase (COX) in cardiac interfibrillar mitochondria (IFM) in 24-month old (aged) rats compared to 6-month old adult Fischer 344 rats, whereas subsarcolemmal mitochondria (SSM) located beneath the plasma membrane remain unaffected. Immunoelectron microscopy (IEM) reveals in aged rats a 25% reduction in cardiac COX subunit VIIa in cardiac IFM, but not in SSM. In contrast, the content of subunit IV remains unchanged in both SSM and IFM, irrespective of age. These subunits are localized mainly on cristae membranes. In contrast, semi-quantitative immunoblotting, which detects denatured protein, indicates that the content of COX VIIa is similar in IFM and SSM from both aged and adult hearts. IEM provides a sensitive method for precise localizing and quantifying specific mitochondrial proteins. The lack of immunoreaction of COX VIIa subunit by IEM in aged IFM is not explained by a reduction in protein, but rather by a masking phenomenon or by an in situ change in protein structure affecting COX activity.
Insights
Aging impairs cardiac energy production in interfibrillar mitochondria (IFM) but not subsarcolemmal mitochondria (SSM). This involves changes in cytochrome oxidase (COX) subunit VIIa, affecting mitochondrial function in older rats.
Area of Science:
- Mitochondrial biology
- Aging research
- Cardiovascular physiology
Background:
- Aging is associated with decreased cellular energy production.
- Cardiac mitochondria are crucial for heart function.
- Mitochondrial subpopulations (IFM and SSM) may exhibit differential aging responses.
Purpose of the Study:
- To investigate age-related changes in cardiac mitochondria, specifically focusing on cytochrome oxidase (COX).
- To compare the aging effects on interfibrillar mitochondria (IFM) versus subsarcolemmal mitochondria (SSM).
- To elucidate the mechanisms behind altered COX activity in aged cardiac mitochondria.
Main Methods:
- Comparative analysis of cardiac mitochondria from aged (24-month) and adult (6-month) Fischer 344 rats.
- Immunoelectron microscopy (IEM) for precise localization and quantification of COX subunits.
- Semi-quantitative immunoblotting to assess total protein content.
Main Results:
- Aging reduced oxidative phosphorylation in cardiac IFM but not SSM.
- IEM showed a 25% decrease in COX subunit VIIa in aged IFM, but not SSM.
- Total COX VIIa protein levels were unchanged in aged hearts, suggesting a masking or structural change rather than reduced expression.
Conclusions:
- Cardiac IFM are more susceptible to age-related functional decline than SSM.
- The reduced COX activity in aged IFM is likely due to altered protein structure or masking of COX VIIa, not decreased protein content.
- IEM is a valuable tool for studying in situ mitochondrial protein changes during aging.
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