Many of us remember from our early years of learning biology that mitochondria are the ‘power house of the cell’ which sounds like the kind of job that is crucial to the survival of a cell. What does being the powerhouse of the cell really mean though? In a simplistic sense, one can imagine the mitochondria as an engine for the machine that is the cell, which is working all day to generate energy in the form of adenosine triphosphate (ATP) from last night’s delicious sandwich; ATP is the cell’s main energy-carrying molecule. It is like the fuel the cell uses to get things done.
Mitochondria possess a rather intriguing evolutionary history. Since mitochondria accommodate their own, unique DNA, scientists in the mid-1900s hypothesized that several years ago mitochondria were possibly independent bacteria-like entities only to subsequently lose their autonomy upon being engulfed by an early version of the eukaryotic cell, which is basically what we are made up of. This theory was popularised by a scientist named Lynn Margulis and is referred to as the “endosymbiont theory.” The host cell somehow chose to utilize the oxygen using the capability of the ingested early mitochondria rather than destroying it. Thus began the evolutionary journey that led to the existence of today’s eukaryotic cells which make up the very essence of who we are. The endosymbiont theory continues to be researched today because some questions relating to the origin of the eukaryotic cell still remain unanswered.
The textbook representation of the mitochondrion that some of us were exposed to does not always paint a complete picture of this organelle* (* specialized part of a cell, analogous to an organ inside the body). For instance, within a cell, the mitochondria are actually present in the form of an intricate network of tubes that are constantly moving about, bumping into each other and tearing apart. This dynamic nature of the mitochondria is characteristic of any healthy, living eukaryotic cell. To enable these dynamics, separate players — molecular scissors that cut the mitochondria and molecular glues which enable the fusion of the mitochondria — are essential. In doing so, all these players come together to maintain a perfect balance of fission and fusion for the proper functioning of the cell. Or does it? Although several of the players involved in facilitating mitochondrial dynamics have been identified, the exact cellular signals that enable mitochondrial fission or fusion remain largely unknown.
Research from our lab at the Indian Institute of Science, for the first time, has shown that the cytoskeleton, which can be thought of as the skeleton inside your cells, plays an important role in controlling how much the mitochondria within a cell breaks apart. These cytoskeletal structures, also known as the “microtubules”, are a set of hollow tubes in the cell that are dynamic. They are self-organizing tubes that act as highways within cells, enable the transport of various essentials across the cells by continuously growing and shrinking. Additionally, the microtubules generate forces and thus help with cell movement during wound healing, cell division during growth, and organelle movement to maintain cell integrity, etc.
From the time of our conception to adulthood, we continuously grow physically and a lot of this growth is attributed to cell division. Cell division also comes into play during wound repair, growth, and development. When cells divide, it is crucial that the stuff inside the cells is equally distributed from the parent cell to the resulting daughter cells to ensure its survival. Cells employ various mechanisms to ensure proper distribution of your genes but it is not so clear how the cells always manage to almost equally partition mitochondria into respective daughter cells. Thereby, understanding the mechanism behind how microtubule dynamics affect mitochondrial breakage gives us insight on how this almost perfect partitioning is achieved.
In the fission yeast model organism we used in this study, mitochondria remain bound to the microtubules. We discovered that to maintain the balance between fission and fusion of mitochondria this attachment of the mitochondria to microtubules is essential. When microtubules shrink, mitochondria undergo fission, and conversely, when microtubules grow, mitochondrial fission is prevented. Why is this important? In several diseases including neurodegeneration, mitochondrial form, and therefore function, is affected. This work gives us insight into how we might be able to restore mitochondrial function in disease states by altering the microtubule dynamics.
Since the microtubules play a pivotal role during cell division and is physically attached to the mitochondria through a linker, it was the first candidate chosen to be investigated. Scientists have previously noticed that the mitochondria and microtubules usually interact with each other, however, it was not yet known if one organelle directly affected the dynamics of the other. To know if this was the case, we first had to figure out what removing the microtubules from the cell would do to the mitochondria. Using quantitative light microscopy as a tool to study this question, we exposed the cells to a drug that destroys the microtubule’s ability to form highways in the cell. Surprisingly, as soon as the microtubules shrink, the mitochondria begin to break apart as shown in the gif below.
Mitochondria are known to fragment when cells are stressed. To make sure we weren’t tearing up the mitochondria simply because of the action of the drug on the cell, we decided to check if the same is observed in cells with genetically shorter microtubules. Conversely, we checked to see how the mitochondria look in cells with longer than normal microtubules as well. It turns out that in all the different cases, the total amount of mitochondria remains similar. However, the form of the mitochondria is altered. Therefore as expected, in the cells with short microtubules, the mitochondria were shorter and larger in number while in cells with longer microtubules, the mitochondria were longer and fewer in number as shown in the gif below.
Based on the above observations it was clear that microtubules play an important role in ensuring mitochondria do not break apart too much. However, the microtubules being an integral part of the cell, play an important role in various cellular tasks. Therefore, one of the following scenarios are likely
- Microtubules play a direct role in mitochondrial fragmentation.
- Messing around with the length of the microtubules induces some form of stress to the cell which is probably why we see the mitochondria breaking apart.
To do away with the possibility of an indirect effect of manipulating the microtubules as mentioned above (b), we sought to remove the linker between the mitochondria and the microtubules. Basically, there is no longer the glue holding these organelles together which is analogous to increasing the distance between a mitochondrion from the microtubule. In these cells without the linker, the mitochondria were broken up into large numbers and were relatively much smaller in size as shown in the gif below. This proved that it was merely the lack of attachment to the microtubules that cause mitochondria to fragment. The microtubules essentially shield the mitochondria from being completely broken apart.
The question now became-why are the mitochondria breaking when they no longer have contact with the microtubules? Remember the molecular scissors from earlier? In yeast cells, these molecular scissors, (also known as “dnm-1”) pinch the mitochondria until they break apart. It seems like when the mitochondria are attached to the microtubules, it is not possible for the dnm-1 scissors to cut the mitochondria. However, when the mitochondria are free from the microtubules, like in the case of cells with short microtubules or in the absence of the microtubule linker, the mitochondria are accessible to the dnm-1 scissors to break them apart. We confirmed that this was indeed the case by looking at mitochondria in cells with long microtubules (where mitochondria showed increased attachment to microtubules) and containing excess dnm-1. We found that despite providing these cells with excess dnm-1, the mitochondria in these cells did not show increased mitochondrial breakage, indicating the protective role of the microtubules.
What is the biological significance of mitochondrial fragmentation upon loss of microtubules? When a cell is just about to divide, all the microtubules from the cell enter the nucleus (which stores DNA and is the central sphere depicted in all the gifs) to facilitate cell division. During this process, because the mitochondria are no longer attached to the microtubules, they break apart into larger numbers so as to increase their probability of ending up in equal amounts into the daughter cells. If the cell has had just one tiny mitochondria, it is more likely for it to end up in only one of the daughter cells, post division. On the other hand, if there are ten mitochondria within the parent cell, the probability of equal distribution of the mitochondria into the daughter cells increases.
It is important for cells to contain enough mitochondria for it to function properly and survive. Problematic mitochondria can result from stresses experienced by the cell as well as neurological disease states. Thus, understanding the mechanism behind how mitochondrial fragmentation is controlled could pave a way to design solutions to reverse the fragmentation phenotype and thereby possibly reducing the severity of these diseases.
In summary, an intact microtubule cytoskeletal network is required for the maintenance of normal mitochondrial dynamics in normal cells, and the mitochondrial association with microtubule prevents mitochondrial fission by possibly inhibiting the assembly of the mitochondrial scissors (dnm-1) around mitochondria. Also, upon cell division, the lack of contact between the microtubule and the mitochondria enables the mitochondria to break into enough numbers so as to distribute themselves equally into the new daughter cells.