Reflecting on the simple observation that some muscle tissues are designed for intense aerobic activities and contain a high number of mitochondria while others are specialized for rapid and powerful activities and have a low number of mitochondria, Dr Moyes' team asked a challenging biological question: what determines tissue mitochondrial number?
Fish are perfect organisms to look at the determination of muscle mitochondrial content because their muscle fiber types are separated into white fibers with low mitochondrial content and red fibers with high mitochondrial content, while mammals display mixed fiber types. Also, fish are good models to examine evolutionary variation in mitochondrial content since muscle mitochondrial capacity for energy production can vary significantly between closely related species. For example, tunas are impressive athletes that display higher mitochondrial capacity for energy production than the closely related billifishes. Anne Dalziel and collaborators examined the origin of the variation in mitochondrial content among muscle tissues and species by comparing three species of tunas and two species of billifishes. The tissues selected covered a broad range of mitochondrial content: white muscle, red muscle, cardiac muscle and the heater organ, a specialized billifish tissue that warms the eyes and brain to maintain vision in cold water.
To examine mitochondrial content variation among tissues and species, the authors measured citrate synthase (CS) activity, a marker of mitochondrial content. For all species, CS activity per gram tissue was higher in red muscle and cardiac muscle than white muscle, confirming that white muscle had the lowest numbers of mitochondria. The heater organ had the highest CS activity per gram tissue of all tissues, indicating large numbers of mitochondria. The team also confirmed that CS activity was higher in tunas than billifishes for homologous tissues, suggesting that tunas have more mitochondria than billifishes.
Once the extent of mitochondrial content variation among tissues and species was established, the group focused their efforts on finding the process underlying this variation. Their strategy was to normalize CS activity to various quantitative parameters. For example, if the difference in CS activity per gram tissue were to be abolished once the activity was normalized for DNA content, this would indicate that the difference was due to a variation in the number of nuclei per gram of tissue. Indeed, the team found that variation in DNA content was the main reason for the difference in CS activity among muscle tissues, indicating that tissues with the largest nuclei numbers per gram tissue produce the most mitochondrial enzymes.
But even after correcting for DNA content, the species difference in CS activity remained. The authors tested the idea that the species difference in CS activity stems from variation in CS transcript levels. They found that tuna CS activity remained elevated in comparison to billifishes when they normalized CS activity to CS transcript level. Therefore, tunas achieve higher levels of CS activity than billifishes for a given quantity of CS transcript,indicating that the step responsible for the species difference occurs after transcription. Indeed, the team speculated that differential regulation of translation, protein stability and mitochondrial stability might be responsible for the species difference in CS activity.
The authors now want to understand how various levels of CS activity can be achieved from a given amount of transcript since, as they conclude, this `may have broad implications for the regulation of mitochondrial content and will represent an important step in unravelling the possible molecular mechanisms by which physiologically important traits, such as mitochondrial content, may evolve.'