Are cells guaranteed to get at least one mitochondrion when they divide?

Are cells guaranteed to get at least one mitochondrion when they divide?

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If mitochondria exist at random within a cell, isn't there a possibility that cell division will result in a daughter cell with no mitochondria? If not, what is the process for guaranteeing at least one is present in each daughter cell? If so, what happens to that cell?

Isn't there a possibility that cell division will result in a daughter cell with no mitochondria?

Yes, there is always the possibility. However, there must be a strong negative selection pressure against eukaryotic life that cannot achieve the proper partitioning of mitochondria, so you can imagine that there are mechanisms in place to prevent this case.

Mitochondria are both passively and actively partitioned to daughter cells. This is understood to occur through the cytoskeleton and with the control of mitochondrial fusion and fission at key stages of the cell cycle, prior to mitosis and cytokinesis!

Here is a great review from several years ago that addresses your question well.

In addition to S Pr's excellent example, I wanted to point out that some very recent research describes some special behavior in oocyte development specifically related to mitochondria selection.

Here's a easy-to-read version:

Here's the original version in Nature:

Specifically, during meiosis, the oocyte specifically "puts the mitochondria to the test" by separating all of them (fragmentation) and having each of them operate independently. (Typically mitochondria act in concert, each one potentially making up for deficiencies in their peers). Any that do not "make the cut" are eliminated, and the result is an egg cell that has the best mitochondria to pass along to the next generation.

A typical animal cell has 1000-2000 mitochondria. From a statistical point of view, assuming a random distribution of the mitochondria and that the cell splits in half, the probability of having 0 mitochondria is (1/2)^1000 or 9e-302. This makes it an impossibility for all practical purposes.

With enough mitochondria, a process to ensure the cell splits roughly in half and a somewhat random distribution of mitochondria would be sufficient to get at least one mitochondria in each daughter cell.

To address the assumptions:

  1. Random distribution of mitochondria - assumed in the question
  2. Cells split roughly in half - source on dividing assymetries "In somatic divisions, however, cell size asymmetry is mild and, only rarely, one daughter cell is more than double the size of the other."

Lab study solves textbook problem: How cells know their size

New research describes how cells judge their size to know when to stop dividing. In this digital microscopy image, waves of cell division sweep through a fruit fly embryo to reduce cell size. Credit: Amodeo Lab/Dartmouth College

Scientists have searched for years to understand how cells measure their size. Cell size is critical. It's what regulates cell division in a growing organism. When the microscopic structures double in size, they divide. One cell turns into two. Two cells turn into four. The process repeats until an organism has enough cells. And then it stops. Or at least it is supposed to.

The complete chain of events that causes cell division to stop at the right time is what has confounded scientists. Beyond being a textbook problem, the question relates to serious medical challenges: Cells that stop dividing too soon can cause defects in growing organisms. Uncontrolled cell growth can lead to cancers or other disorders.

A study from Dartmouth, published in Current Biology, provides a new answer to the question by tackling the problem in reverse: The research focused on large cells that reduce their size through division until enough cells are formed to move to other stages of development.

"The early embryo is an ideal place to study cell size control," said Amanda Amodeo, an assistant professor of biology at Dartmouth and the lead researcher. "The cells we work with are eggs that are visible to the eye. They don't need to grow before dividing, so it allows us to look at connections that are obscured in adult cells."

According to the study, a set amount of the protein histone H3 is loaded into an embryo before fertilization and is used up as the embryo divides into more cells. As histones are consumed to accommodate the growing number of nuclei, they release the enzyme Chk1 to bind with another protein, CDC25, to stop the multiplication of cells.

The research is technical, but the mechanism is relatively straightforward: With histone H3 out of the way in a growing cell, the stop enzyme Chk1 finds and disables the protein that triggers cell cycle progression, CDC25.

"The key to our research result was coming up with the possibility that unusually large amounts of histone H3 may feed into the stop enzyme," said Yuki Shindo a postdoctoral research fellow at Dartmouth and first author of the paper. "Once we noticed that, we were able to test this idea in our living test tube, fruit fly eggs."

The new research builds on earlier studies which found that a biological constant exists between the size of a genome and the size of a cell. Researchers knew that once a balance point was achieved, cells would stop duplicating, but didn't understand how cells could determine the ratio.

To find the answer to the long-running question, the research team studied fruit fly eggs. Because of their large size compared to other cells, the team was able to get a different perspective on the cell cycle.

"We've had all of the pieces for years but couldn't quite get them to fit together," said Amodeo. "Once we recognized that H3 interacts directly with both DNA and Chk1, the work went very fast. Everything worked the first time, which is a good sign that the hypothesis is right."

Since the same molecules that control cell division—histone H3, CDC25 and Chk1—are all identified in cancer and other ailments, the finding can help researchers that are seeking answers to questions related to development and disease.

"We were originally curious about a basic biological question on how cells in a growing egg make a decision to stop at the correct timing," said Shindo. "We are now excited that our findings may also have an important implication for a broader context such as disease."

Imaging method highlights new role for cellular 'skeleton' protein

A cancer cell labeled for actin (red) and mitochondria (cyan). The scientists designed novel probes that specifically monitor interactions between actin and mitochondria. Credit: Salk Institute/Waitt Advanced Biophotonics Center

While your skeleton helps your body to move, fine skeleton-like filaments within your cells likewise help cellular structures to move. Now, Salk researchers have developed a new imaging method that lets them monitor a small subset of these filaments, called actin.

"Actin is the most abundant protein in the cell, so when you image it, it's all over the cell," says Uri Manor, director of Salk's Biophotonics Core facility and corresponding author of the paper. "Until now, it's been really hard to tell where individual actin molecules of interest are, because it's difficult to separate the relevant signal from all the background."

With the new imaging technique, the Salk team has been able to home in on how actin mediates an important function: helping the cellular "power stations" known as mitochondria divide in two. The work, which appeared in the journal Nature Methods on August 10, 2020, could provide a better understanding of mitochondrial dysfunction, which has been linked to cancer, aging, and neurodegenerative diseases.

Mitochondrial fission is the process by which these energy-generating structures, or organelles, divide and multiply as part of normal cellular maintenance the organelles divide not only when a cell itself is dividing, but also when cells are under high amounts of stress or mitochondria are damaged. However, the exact way in which one mitochondrion pinches off into two mitochondria has been poorly understood, particularly how the initial constriction happens. Studies have found that removing actin from a cell entirely, among many other effects, leads to less mitochondrial fission, suggesting a role for actin in the process. But destroying all the actin causes so many cellular defects that it's hard to study the protein's exact role in any one process, the researchers say.

So, Manor and his colleagues developed a new way to image actin. Rather than tag all the actin in the cell with fluorescence, they created an actin probe targeted to the outer membrane of mitochondria. Only when actin is within 10 nanometers of the mitochondria does it attach to the sensor, causing the fluorescence signal to increase.

Rather than see actin scattered haphazardly around all mitochondrial membranes, as they might if there were no discrete interactions between actin and the organelles, Manor's team saw bright hotspots of actin. And when they looked closely, the hotspots were located at the same locations where another organelle called the endoplasmic reticulum crosses the mitochondria, previously found to be fission sites. Indeed, as the team watched actin hotspots light up and disappear over time, they discovered that 97 percent of mitochondrial fission sites had actin fluorescing around them. (They speculate that there was also actin at the other 3 percent of fission sites, but that it wasn't visible).

"This is the clearest evidence I've ever seen that actin is accumulating at fission sites," says Cara Schiavon, co-first author of the paper and a joint postdoctoral fellow in the labs of Uri Manor and Salk Professor Gerald Shadel. "It's much easier to see than when you use any other actin marker."

By altering the actin probe so that it attached to the endoplasmic reticulum membrane rather than the mitochondria, the researchers were able to piece together the order in which different components join the mitochondrial fission process. The team's results suggest that the actin attaches to the mitochondria before it reaches the endoplasmic reticulum. This lends important insight towards how the endoplasmic reticulum and mitochondria work together to coordinate mitochondrial fission.

In additional experiments described in a pre-print manuscript available on bioRxiv, Manor's team also reports that the same accumulation of endoplasmic reticulum-associated actin is seen at the sites where other cellular organelles—including endosomes, lysosomes and peroxisomes—divide. This suggests a broad new role for a subset of actin in organelle dynamics and homeostasis (physiological equilibrium).

In the future, the team hopes to look at how genetic mutations known to alter mitochondrial dynamics might also affect actin's interactions with the mitochondria. They also plan to adapt the actin probes to visualize actin that's close to other cellular membranes.

"This is a universal tool that can now be used for many different applications," says Tong Zhang, a light microscopy specialist at Salk and co-first author of the paper. "By switching out the targeting sequence or the nanobody, you can address other fundamental questions in cell biology."

"We're in a golden age of microscopy, where new instruments with ever higher resolution are always being invented but in spite of that there are still major limitations to what you can see," says Manor. "I think combining these powerful microscopes with new methods that select for exactly what you want to see is the next generation of imaging."

These worms' stem cells are developmental shapeshifters

A microscopy image showing planarian neoblasts (blue) and differentiating cells (yellow). Some of the neoblasts and differentiating cells are expressing a gene needed to create intestinal cells, represented by pink dots. Credit: Amelie Raz/Whitehead Institute

Planarians are small water-dwelling worms known for their regenerative capacity. If you chop one into ten pieces, you'll end up with ten fully-formed worms.

While humans have pools of specialized stem cells that can create our regenerative body parts like hair and skin, these worms owe their regenerative superpowers to a special kind of stem cell called a neoblast. At least some of these cells are "pluripotent," meaning that they can divide to create almost any cell type in a worm's body at any time. Neoblasts are actually the only dividing cells in planarians—fully committed cells like those in the eyes or intestines cannot divide again.

"The big question for us is, how does a neoblast go from being able to make anything, to making one particular thing?" says Amelie Raz, a postdoctoral researcher at Whitehead Institute who conducted her graduate research in the lab of Whitehead Institute Member Peter Reddien. "How do they go from being able to make anything in the body to being, say, an intestine cell that's going to stay an intestine cell until it dies?"

Now, in a paper published online April 20 in the journal Cell Stem Cell, researchers at Whitehead Institute lay out a new model for how these stem cells commit to their fates and go on to create fully differentiated cells. The process of cellular differentiation is often viewed as a hierarchy, with one special stem cell at the top which can take a number of potential paths to arrive at a specialized state. This is generally thought to take place over a series of cell divisions in which each generation's fate is gradually restricted.

"We're proposing something happens that is very different from the conventional view," says senior author Reddien, who is also a professor of biology at Massachusetts Institute of Technology and an investigator with the Howard Hughes Medical Institute. "We think that stem cells can make broad jumps in state without going through a series of fate-restricting divisions. We call it the single-step fate model."

In the new model, neoblasts that are on a path toward creating skin cells or intestine cells can produce progeny cells that can switch fates to create cells of other types. The work is a step in the long road to understanding these worms' regenerative capacities, and could possibly inform regenerative medicine approaches far in the future.

"The ability of planarian stem cells to essentially switch their fate is really, really powerful," says Raz, the first author of the paper. "Obviously this is a long way off, but theoretically the concept of stem cell fate switching could be applied to regenerative medicine, with human stem cell programming."

Upturning the hierarchy

Neoblasts can be sorted into many "classes." For example, one class of neoblasts contains all the materials to make skin cells, and others have the necessary toolkit to form the worms' primitive kidneys or their intestines. According to the hierarchical model, these specialized neoblasts are intermediaries between a pluripotent cell at the top of the hierarchy, and the non-dividing body cells.

"You can imagine that the special cell at the top is a blank slate with no predisposition towards any cell type—it can make anything," says Raz. "This is how we've often imagined development works."

But Raz, Reddien and Omri Wurtzel, a former postdoc in the Reddien lab now at Tel Aviv University, started to question this assumption after noticing a few mysterious properties of planarian cells.

First of all, researchers have observed in the past that when a planarian is treated with radiation to kill all existing stem cells, a single neoblast can rescue the animal by forming a colony containing many different classes of neoblasts. If, as previous theories suggested, there was a single class of neoblast that gave rise to all these types, Raz and Reddien reasoned that that class should be a common resident in every colony that formed. After creating many of these colonies and analyzing their composition, however, the researchers saw that this was not the case. "For every class we looked at, there were plenty of colonies that lacked that class altogether," says Reddien. "There was no unique class present in all colonies."

Another sticking point: the researchers began to realize that, when applying the hierarchy model, the math of planarian cell divisions and potency just didn't add up. In a prior cell transplantation study, the Reddien lab found that many of the neoblasts they tested were pluripotent —in this study they found that proportion to be larger than what they would expect if only non-specialized neoblasts were pluripotent. "When you add up all the different kinds of specialized neoblasts, it's at least three quarters of the neoblast population, and almost certainly higher than that." says Raz. Therefore, the researchers wondered if some specialized neoblasts could be pluripotent as well.

Another study from the Reddien lab showed that skin-specialized neoblasts did not retain skin fate through more than one cell division. Also, in about half of all cell divisions in planarians, the two daughter cells will be different from one another. This raised the possibility that specialized neoblasts can divide asymmetrically as a possible route to stem cells changing fate.

Furthermore, the timeline for regeneration was off—the rate at which planarians were able to regrow body parts didn't allow for several rounds of fate-restricting divisions.

After conducting experiments to study these different situations, Raz, Wurtzel, and Reddien were able to create a case for their new model of cell differentiation. "What we think is happening is that planarians have a ton of plasticity in their general stem cell population, where individual cells can move in and out of different specialized stages through the process of cell division in order to give rise to what is required," Raz says.

"This is just the beginning of exploring this process, even though we've been studying it for many years," Reddien says. "Focusing on the model, we're suggesting that the cells can choose one fate, and then through the process of a division with an asymmetric outcome, one of the daughter cells can now divide again and choose a different fate. That fate switching process might be fundamental to explaining pluripotency."

Reddien's lab will continue investigating the mechanisms of neoblast fate specification, including how specialization lines up with the timing of the cell cycle.

"Understanding the structure of cell lineage and how fate choices are made is fundamental to understanding adult stem cell biology, and how in the context of injury and repair, new cells can be brought about," says Reddien. "Do they have to go through long, complex lineage trajectories? Or can they make big jumps in state from stem cells to the final state? How flexible is that? All of these things have potential implications for understanding stem cell biology broadly, and we hope that the work will highlight some of these mechanisms and provide opportunities to explore general principles in the future."

What remission means

Remission is a word doctors often use when talking about cancer. It means that after treatment there is no sign of the cancer.

You might hear your doctor talk about complete remission and partial remission.

Complete remission

This means that the cancer can't be detected on scans, x-rays, blood tests or other tests. Doctors sometimes call this a complete response or they might say there is no evidence of disease.

Partial remission

This means the treatment has killed some of the cells, but not all. The cancer has shrunk, but can still be seen on scans and doesn't appear to be growing.

The treatment might have stopped the cancer from growing. Or the treatment could have made the cancer smaller so that other treatments are more likely to help, such as surgery or radiotherapy. This is sometimes called a partial response.

Another term doctors use is stable disease. This can mean that the cancer has stayed the same size or it might even have grown by a small amount.

Fast spiking axons take mitochondria for a ride

Mitochondrial inside axons. Credit:

(Medical Xpress)—One of the most incredible instruments you might ever get to play with is a fiberoptic imaging wand that you hold against the underside of your tongue. Through a semi-mysterious optical arrangement, the device lets you see your own red blood cells squeezing through tiny capillaries on a screen in front of you—live and in real time. It is now fairly well established that to understand the function of neurons, we need to understand their mitochondria. To be able to watch them inside living creatures in the same way we can now watch our blood cells would be a remarkable advance. Marija Sajic, and her collegues at University College London have now done just that. In a recent paper in Plos Biology, they use their technique to dispel some of the confusion surrounding one of the most pressing issues in neurobiology: what is the effect of spiking activity on mitochondria?

On friday, we looked into some of the mechanisms by which spiking activity controls myelination, at least in cultured dorsal root ganglion cells (DRGs). It is difficult to study the full effects of spikes on mitochondrial dynamics in whole cultured nerves because the behavior of interest plays out on a large scale. In humans for example, we could be talking axons of up to meter, in blue whales, several meters. Marija and her group were able to isolate DRG axons in the saphenous nerve of intact mice. The saphenous nerve ennervates sensory cells, including capscacian-sensitive Merkle Disks in parts of the legs. With this preparation, they could activate the specialized dendrite and track mitochondria over long distances. In DRG cells, this dendrite is really an axon, only the spikes that it conducts proceed "antidromically" (toward the cell body) in the tortured nomenclature we still seemingly use even for these uniquely structured, invertebrate-like neural relics.

But what is so important about their mitochondrial dynamics?

The brain has always been described in terms of the prevailing technology of the day. Descartes concepts of hydraulic nerves survived up through the era of steam, gears, and clocks, but ultimately gave way to more modern analogy with telephone lines, and then later, the computer. Today we know that neurons are actually just fancy Keurig coffee making machines that mostly produce ATP instead of caffeine. The "K-cups" themselves, the mitochondria, sprinkle this tender like benevolent mob bosses as they cruise the neural byways. Like the cups, they come in variety packs with similar functionality, that expire quickly with use and are ultimately destined to be recycled.

The metaphor stalls with real mitochondria in that their actual path to demise is not such predictable, inevitable, timed destruction. In the massively-parallelized Keurig-like neuron, the thousands of mitochondria continually recreate their own unique molecular blends through fusions and fissions (perhaps more aptly called mass equilibrations) which mix and replenish their contents. But there is a little bit more to this picture. As mitochondria move about the axon like huge container ships bidirectionaly distributing goods, these constant mass equilibrations have a net effect not just of transporting, but also sorting and filtering labelled packages from the nucleus. These products include perhaps 1500 nuclear-encoded proteins, and an untold number of nuclear mRNAs that hitch a ride along with the larger membrane-associated mitochondrial entourage.

Marija's group found that as they stimulated DRG neurons at up to 50hz, both the number and velocity of mitochondria moving in the anterograde (toward the Merkel disk synapses) was increased. Lower frequency stimulation increased mitochondrial motility as well, but did it equally so in both directions. It was noted that the stationary pool of mitochondria tended to decrease in length, presumably as a result of the fissions needed to fortify the mobile pool. It may be worthwhile to note here that the extent to which mitochondria might actually grow, in the absence of fusion, is difficult to gauge. When regular cells divide, they generally undergo a what might be called a system reboot, and quickly begin to put on mass. In mitochondria, however, fission is not intimately linked with replication of their multiple nucleoid mtDNA strands.

The idea that a "stem" population, or a master mitochondrial reservoir, might exist somewhere in a metabolically quiescent (and therefore intact, undamaged state) is appealing, but difficult to prove. A stemlike daughter mitochondrion resulting from some kind of asymmetric division, could in theory remain in the nuclear vicinity where it could readily harvest nuclear instructions and pump out reinforcements. The total mass of mitochondria present in these axons, and the relative change in total mass are important to know because they would give some indication of synaptic demand. Previous observations suggested that worn out mitochondria with collapsed membrane potentials tended to be transported back to the cell for degradation, although new research has now indicated that is not the whole story. Recently, we mentioned the work of Mark Ellisman, which showed mitochondria being transported down axons, extravasated at synapses, and then taken up by other cells. Clearly a theory, perhaps an energy equation, that can take into account all these kinds of behaviors is sorely needed.

As Marija observes, it is difficult to imagine that increased activity can rapidly target mitochondria to all of the axon's synapses. Recalling here that for DRG cells, these are actually postsynaptic signal generators of the 200 or more follicles, or Merkel disks in its domain. With redundant ennervation, this might amount to over 1000 branches comprising a meter or so of cutaneous neurite material. One possibility here might be that if spike activity can selectively solvate the cytoplasm of the particular branches that were stimulated, a path of lesser mechanical resistance could, in theory, be transiently maintained. Actual measurements of the heat and motions evolved during spikes suggest that not just axons, but maybe even myelin, can carry some of the energy of the action potential through phase changes. As far as the synapses, it may seem obvious that the presynaptic compliment needs a fair share of ATP to fuel vessicle dynamics, but less obvious is the nature of the demand at the postsynapse. However, as we indicated above, mitochondria have a lot more to offer than just ATP, and this may in part explain their abundance here.

Marija notes that many papers on neural energetics take ample account of the energy for restoring membrane potential after spikes, but make only light mention of the ATP demand of axonal transport. ATP generators based on the enzyme GADPH have been shown to take care of motorized vessicle transport in axons, but mitochondria (not surprisingly perhaps) seem to require mitochondrial ATP for transport. We have previously suggested that one organizational principle of nervous systems might be that neurons tend to match the energy sunk into axons, ie. for spikes and transport, with the return on investment that they get at all their synapses. That idea might go a long way towards explaining the observed low release probabilities for many synapses.

The close association of spiking activity with synapse-directed mitochondrial translocation is a vindication of many common sense intuitions about the operations of neurons. Phase-changes and pressure waves associated with spikes may provide additional mechanisms whereby untethered mitochondria can progress quickly down an axon. Cooperative, but congested motor protein transporters, plying along crowded microtubule and actin tracks, are the dogmatic means of transport, but proving that they are they only game in town may await better methods. Imaging of the exchange of myosin, kinesin and dynamin motors in mitochondria with more accurate labels may permit better correlation of direction changes to these controllers.

Back when neurons were still just simple computers that electrically beeped untold bits to each other over cold axon wires, spikes were not seen as the hierarchical synthesis of every activity in the cell down to the molecular scale that we might say they are today. In other words, spikes were just a summary report of inputs to be integrated with the current state, and passed on. In comprehending the intimate relationships of mitochondria to spikes (and other molecular dignitaries like calcium) we might now more broadly interpret them as synced messages that a neuron sends to itself, and by implication its spatially extended inhabitants. Synapses weigh this information heavily but ultimately, but like the electoral college, fold in a heavy dose of local administration to their output. The sizes and positions within the cell to which mitochondria are deployed can not be idealized or anthropomorphized to be those metrics that the neuron decides are best for itself, but rather what is thermodynamically demanded.

Many important experiments might now be cued up to further investigate live mitochondrial dynamics in intact systems. It would be useful to look directly at more traditional axons, like those of motor neurons, and also purely dendritic systems, like those of Purkinje cells. The simplistic "electrotonic" conceptualization of the massive Purkinje tree, for example, seems to make little sense if the informational output of tens of thousands of dendritic synapses is lost by bottlenecking through the severely limited bandwidth of the single primary shaft. A similar bottleneck might be said to exist at the axon, where a single spike train must coordinate thousands of end effectors, many of which make redundant connections on the same target. If instead if we view these neuritic mazes as the proving grounds for the tiny geometrical bundles of membrane-pipelined metabolism we call mitochondria, we may hint at their larger functions.

Matching energy supply and demand is critical in the bioenergetic homeostasis of all cells. This is a special problem in neurons where high levels of energy expenditure may occur at sites remote from the cell body, given the remarkable length of axons and enormous variability of impulse activity over time. Positioning mitochondria at areas with high energy requirements is an essential solution to this problem, but it is not known how this is related to impulse conduction in vivo. Therefore, to study mitochondrial trafficking along resting and electrically active adult axons in vivo, confocal imaging of saphenous nerves in anaesthetised mice was combined with electrical and pharmacological stimulation of myelinated and unmyelinated axons, respectively. We show that low frequency activity induced by electrical stimulation significantly increases anterograde and retrograde mitochondrial traffic in comparison with silent axons. Higher frequency conduction within a physiological range (50 Hz) dramatically further increased anterograde, but not retrograde, mitochondrial traffic, by rapidly increasing the number of mobile mitochondria and gradually increasing their velocity. Similarly, topical application of capsaicin to skin innervated by the saphenous nerve increased mitochondrial traffic in both myelinated and unmyelinated axons. In addition, stationary mitochondria in axons conducting at higher frequency become shorter, thus supplying additional mitochondria to the trafficking population, presumably through enhanced fission. Mitochondria recruited to the mobile population do not accumulate near Nodes of Ranvier, but continue to travel anterogradely. This pattern of mitochondrial redistribution suggests that the peripheral terminals of sensory axons represent sites of particularly high metabolic demand during physiological high frequency conduction. As the majority of mitochondrial biogenesis occurs at the cell body, increased anterograde mitochondrial traffic may represent a mechanism that ensures a uniform increase in mitochondrial density along the length of axons during high impulse load, supporting the increased metabolic demand imposed by sustained conduction.

Are mitochondria the main contributor of reactive oxygen species in cells?

Physiologists often assume that mitochondria are the main producers of reactive oxygen species (ROS) in cells. Consequently, in biomedicine, mitochondria are considered as important targets for therapeutic treatments, and in evolutionary biology, they are considered as mediators of life-history tradeoffs. Surprisingly, data supporting such an assumption are lacking, at least partially due to the technical difficulties in accurately measuring the level of ROS produced by different subcellular compartments in intact cells. In this Commentary, we first review three potential reasons underlying the misassumption of mitochondrial dominance in the production of cellular ROS. We then introduce some other major sites/enzymes responsible for cellular ROS production. With the use of a recently developed cell-based assay, we further discuss the contribution of mitochondria to the total rate of ROS release in cell lines and primary cells of different species. In these cells, the contribution of mitochondria varies between cell types but mitochondria are never the main source of cellular ROS. This indicates that although mitochondria are one of the significant sources of cellular ROS, they are not necessarily the main contributor under normal conditions. Intriguingly, similar findings were also observed in cells under a variety of stressors, life-history strategies and pathological stages, in which the rates of cellular ROS production were significantly enhanced. Finally, we make recommendations for designing future studies. We hope this paper will encourage investigators to carefully consider non-mitochondrial sources of cellular ROS in their study systems or models.

What Are Mitochondria?

If you remember your days in freshman biology, you might recall that mitochondria are the "powerhouses" of your cells. The analogy is apt the role of mitochondria is to transform the food we eat into cellular energy. In fact, mitochondria produce about 90 percent of the energy that our cells need to survive.

But mitochondria don&apost stop at making energy they&aposre also essential in triggering cell death, an essential function that, when hindered, can lead to tumor growth and cancer.

And these are not the only health problems brought about by dysfunctional mitochondria. As David Asprey, founder of਋ulletproof, explains, mitochondrial dysfunction "appears to be at the heart of most illness and chronic disease."

“You can get mitochondrial dysfunction if you don’t have enough mitochondria, if the ones you have aren’t working well or if you don’t produce them consistently,” he explains. 𠇍isturbingly, research suggests half of people under the age of 40 have early onset mitochondrial dysfunction.”

Circular Mitochondrial Genome

As noted, one of the core arguments for endosymbiosis points to its circular genome. What is often not noted, however, are the cases where eukaryotic mitochondria have linear genomes with eukaryotic telomeres (Rycovska et al., 2004 Nosek et al., 1998 Fukuhara et al., 1993). Indeed, two strains of the same species of yeast differ with respect to the linearity or circularity of their mitochondrial genome (Fukuhara et al., 1993 Drissi et al., 1994).

In the case of linear chromosomes, the DNA polymerase enzymes are unable to replicate right to the end of the chromosome. This is because the enzymes are unable to replace the lagging strand’s terminal RNA primer. Unless there is a mechanism for circumventing this, it will result in the chromosomes shortening after each round of replication (in eukaryotes, the enzyme telomerase attaches extra DNA to the chromosomal ends). This means that the transition from genome circularity to linearity — a fete in itself given the changes that have to be made to the mode of replication — must happen in concert with the evolution of a mechanism to prevent progressive chromosomal shortening. Such an evolutionary transition is far from trivial. Biologist Albert de Roos writes,

[I]n linear mitochondrial chromosomes various different mechanisms to “prevent” shortening exist, ranging from hairpin loops and self-priming to protein-assisted primer synthesis (see here). The telomeric regions of mitochondrial chromosomes do not seem to have a direct phylogenetic relation since they use other proteins and mechanisms than nuclear telomeres. Thus, it is difficult to deduce evolutionary pathways purely based on phylogenetic data on telomeres and mechanisms for end replication.

Furthermore, mitochondrial genes often do possess introns (Lang et al., 2007). These are particularly prevalent in the mtDNA of fungi and plants. The mitochondrial genetic code may also be slightly different from that of bacteria (Jukes and Osawa, 1990).

Better together: Mitochondrial fusion supports cell division

IMAGE: The dark patches in these images from the Patti laboratory are mitochondria. From left, the mitochondria in a non-dividing (quiescent) cell, fused mitochondria in a dividing cell, and mitochondria prevented. view more

Credit: Patti laboratory (Washington University in St. Louis) and eLife

Mitochondria are the powerhouses of the cell. And for mitochondria, much like for double-header engines stacked together in a steam train, working in multiples has its benefits.

New research from Washington University in St. Louis shows that when cells divide rapidly, their mitochondria are fused together. In this configuration, the cell is able to more efficiently use oxygen for energy. Fused mitochondria also churn out a biochemical byproduct, aspartate, that is key to cell division.

This work by researchers in the lab of Gary Patti, the Michael and Tana Powell Associate Professor of Chemistry in Arts & Sciences, was reported in a recent publication in the journal eLife. It illuminates the inner workings of dividing cells and shows how mitochondria combine to help cells to multiply in unexpected ways.

Given that cancer cells are known for dividing at a runaway pace, the new findings may have implications for cancer diagnosis and treatment.

"Most studies of proliferating cells are conducted in the context of cancer, where scientists are comparing a cancer tissue that's rapidly growing with normal tissue that surrounds the tumor or a normal tissue from a different patient," said Conghui Yao, a PhD candidate in Patti's lab at Washington University and first author of the new study. "These kinds of comparisons are physiologically relevant but have some disadvantages.

"A tumor is a very complicated thing, not only because it is made up of different kinds of cells, but also because the environment of a tumor is different from that of healthy tissue," she added.

For example, a tumor needs nutrients to grow, but it doesn't have the blood vessel infrastructure that typically supply other healthy tissues in the body. As a result, tumors are often starved for oxygen.

But even in the presence of abundant oxygen, cancer cells get energy through a relatively inefficient fermentation process. Instead of using oxygen to burn glucose in their mitochondria to get their juice, cancer cells use an "aerobic glycolysis" process that turns their glucose into lactate. This process is called the Warburg effect.

Although the phenomenon has been observed in rapidly dividing cells for more than 90 years, scientists still don't fully understand it. The earliest of explanations suggested that mitochondria in cancer cells are damaged in a way that prevents them from producing energy normally.

Yao was familiar with the Warburg effect and its implications. So when she set up an experimental system that allowed her to turn cell division on and off, she was surprised to see that her dividing cells were consuming a lot of oxygen.

"Much of the literature had suggested that dividing cells would do the opposite," Yao said. "So we looked into not only why our dividing cells were consuming more oxygen, but also how they were able to consume more oxygen."

Part of the beauty of Yao's initial experiment was its simplicity: She was able to measure metabolism in one specific type of cell under two distinct conditions -- when the cell was dividing and when it was not dividing. That's also how she was able to hone in on the particular structural change to mitochondria which was driving the efficiencies she observed.

"The dividing cells had the same amount of mitochondria per protein or per mass, compared to non-dividing cells," said Patti, whose research is focused on the biochemical reactions that underlie metabolism. "But we did notice when we imaged mitochondria in these dividing cells that they are significantly longer."

Longer because some adjacent mitochondria had fused into one -- making multiple, adjoined mitochondria into bigger, more efficient, energy-generating machines.

The other notable thing that "mega-mitochondria" are particularly good at creating, Yao discovered, is a molecule called aspartate that is essential for cells to replicate.

"Recent work from other labs has taught us that one of the most important reasons that dividing cells need to consume oxygen is to make aspartate. So it made sense to us that mitochondrial fusion in dividing cells would increase aspartate production," Yao said.

Yao and Patti are not the first to observe mitochondrial fusion. But they are among the first to interrogate mitochondrial fusion with sophisticated metabolomic technologies, enabling a molecular-level understanding of the process as it relates to cell division. The biochemical alterations they observed may represent processes that can be targeted in malignant cancer cells.

"It is often stated that rapidly dividing cancer cells increase fermentation at the expense of decreasing oxygen consumption for mitochondrial activity," Patti said. "Our results suggest that at least some rapidly dividing cells increase both processes under normal oxygenated conditions.

"Since utilization of nutrients by rapidly dividing cancer cells is the basis for various drugs and diagnostic tests, these findings may have important clinical significance and may represent a metabolic vulnerability in cancer," Patti added.

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