Eukaryotic cells have many specialized complexes and organelles that perform crucial functions. Mitochondria are well known organelles that are critical energy generators, often called the powerhouses of the cell. Mitochondria are also unique; they carry their own little genomes and produce their own proteins. In animals, and almost all eukaryotes, mitochondrial DNA is only passed down to offspring from the mother, except in very rare cases. Scientists have now learned more about this process.
It's been thought that soon after a sperm fertilized an oocyte, the mitochondrial DNA (mtDNA) within that sperm cell was eliminated. However, new work has shown that although mature sperm do contain a handful of mitochondria, those mitochondria lack intact mtDNA. The findings have been reported in Nature Genetics.
"We found that each sperm cell does bring 100 or so mitochondria as organelles when it fertilizes an egg, but there is no mtDNA in them," said study co-author Shoukhrat Mitalipov, PhD, director of the Center for Embryonic Cell and Gene Therapy at Ohio Health & Science University.
This study showed that the mitochondria in mature sperm do not carry mtDNA, and they also lack a protein that is required for the maintenance of mtDNA, called mitochondrial transcription factor A (TFAM).
One unsolved mystery is why sperm do not pass down mtDNA. The researchers suggested that it may relate to the heavy use of mitochondrial energy by sperm as it pursues its primary objective of fertilizing an egg. Oocytes, however, are able to use energy from the cells around them and don't have to use the energy generated by their own mitochondria. The mtDNA in oocytes is often in a very healthy state.
"Eggs pass on really good mtDNA at least partly because they don't use mitochondria as a source of energy," explained Mitalipov.
Egg cells are quite large compared to sperm, and the sperm's roughly 100 organelles are overwhelmed by hundreds of thousands of mitochondria found in every oocyte, each of which carries intact mtDNA. This maternal mtDNA may provide an evolutionary advantage since the risk of mtDNA mutations is reduced. When those mutations arise, they can lead to very serious diseases that impact organs that use a lot of energy, such as the brain and heart. Right now, there are studies in Europe investigating the potential of mitochondrial replacement therapy to prevent diseases caused by the inheritance of mutated mtDNA.
This work could also be relevant for the treatment of infertility, noted corresponding author Dmitry Temiakov, PhD, a molecular biologist with Thomas Jefferson University. "Understanding the role of TFAM during sperm maturation and its function during fertilization may hold keys to our ability to treat certain infertility disorders, and increase the efficiency of assisted reproductive technologies."
Sources: Oregon Health & Science University, Nature Genetics