May the Best Mitochondria Win

May the Best Mitochondria Win!

Written by Samuel Salamun
Illustrated by Anna Tram
“Mitochondria is the powerhouse of the cell!” This is a phrase many of us are familiar with from when we first started learning about cell biology back in high school. However, what happens when our powerhouses are defective? Do our cells have a way to prevent this from happening?

Mitochondria (the plural of mitochondrion) are very interesting organelles—miniature organ-like structures in our cells—because they carry their own DNA that is separate from ours (which is found in the nuclei of our cells). This DNA contains instructions to make the mitochondrial machinery, or proteins, needed to produce the energy that will be used by the cell in the form of adenosine triphosphate, or ATP for short. As you can imagine, any changes or mutations in the mitochondrial DNA could result in defective proteins which would lead to problems when producing energy.

Another interesting and worrying fact about mitochondria is that they don’t have robust mechanisms to prevent or repair damage to their DNA. In addition, unlike our genomic DNA that we get from both of our parents, we only inherit mitochondrial DNA from our mothers. So in this case, you did get it from your mama! Unfortunately, this could lead to the death of our species as we keep collecting mutations in our mitochondrial DNA and could eventually lose all functioning mitochondria.

All isn’t lost though! Mitochondria can fuse with each other to form long mitochondrial tubes that contain lots of mitochondrial DNA. As a result, the defective mutant mitochondria may fuse with the healthy, non-mutant wild type mitochondria and borrow their machinery to properly produce ATP. A group of researchers (including some from the University of Toronto) showed that the female germline cells—which are the cells that develop into eggs—take advantage of this to select for and only allow healthy mitochondria to be passed down to the next generation, possibly by removing the defective mitochondria.

They used a technique called fluorescence labelling to differentiate between wild type and mutant mitochondria in the germlines of female fruit flies using their DNA and observed them as the flies developed. They found that the mutant mitochondria start to get removed as the level of mitofusin, a protein that enables mitochondria to fuse, drops. By preventing the mitochondria from fusing, it forces unhealthy mitochondria to stand on their own and leaves them unable to efficiently produce ATP, causing the cell to mark them for destruction. This process, called mitochondrial DNA selection, ensures that only healthy mitochondria are present in the egg cells and passed to the next generation. In conclusion, let’s all thank our mothers (and their egg cells) for making sure that we only get the best and the healthiest mitochondria!

Sources:

  1. Toby Lieber, Swathi P. Jeedigunta, Jonathan M. Palozzi, Ruth Lehmann, Thomas R. Hurd. Mitochondrial fragmentation drives selective removal of deleterious mtDNA in the germline. Nature, 2019; DOI: 10.1038/s41586-019-1213-4
  2. NYU Langone Health / NYU School of Medicine. “How egg cells choose their best powerhouses to pass on.” ScienceDaily. ScienceDaily, 15 May 2019. <www.sciencedaily.com/releases/2019/05/190515131741.htm>.