Why do mitochondria fuse together?

Why do mitochondria fuse together?

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Contrary to all of the textbook images of mitochondria that I have seen over the years, I had just learned that the mitochondria within a cell form a dynamic branching network along microtubule scaffolding by fusing with each other.

Why would they need to fuse together?

When they fuse, would this involve only the outer mitochondrial membrane?

Would it not behoove a mitochondrion to remain distinct from others both physically and genetically?

Mitochondria produce a large quantity of reactive oxygen species (ROS) that damage the mitochondrial genetic material. These cell organelles do have a DNA repair mechanism similar to that of the nucleus to some extent 3 which is not entirely understood however they have multiple copies and so the fusion and fission of mitochondria work together to maintain as many viable/healthy mitochondria as possible.

When two mitochondria fuse, their genetic material can recombine and thus exchange their damaged DNA. In this way, one of them repairs its DNA at the expense of the other accumulating more damaged DNA. Mitochondria that accumulate too much damaged DNA will 'die' - get eliminated, but they can be replaced by fission of healthy ones. Take a look at this wiki page, it has useful info.

Edit: This article may also be useful. (Mitochondrial fusion and division: regulation and role in cell viability Giovanni BENARD and Mariusz KARBOWSKI, Semin Cell Dev Biol. 2009 May;20(3):365-74.)

Mitochondria are granular or filamentous organelle, which are present in the cytoplasm of a cell. They are ovoid in shape, with the presence of double membranes. Mitochondrial fusion is required to distribute mitochondrial DNA to the mitochondrial population and to maintain its respiratory competent and energized organelles. Mitochondrial division is required to efficiently distribute organelles to distal parts of cells.

  • Eukaryotic cells contain varying amounts of mitochondria, depending on the cells&rsquo energy needs.
  • Mitochondria have many features that suggest they were formerly independent organisms, including their own DNA, cell-independent division, and physical characteristics similar to alpha-proteobacteria.
  • Some mitochondrial genes transferred to the nuclear genome over time, yet mitochondria retained some genetic material for reasons not completely understood.
  • The hypothesized transfer of genes from mitochondria to the host cell&rsquos nucleus likely explains why mitochondria are not able to survive outside the host cell.
  • crista: cristae (singular crista) are the internal compartments formed by the inner membrane of a mitochondrion
  • vacuole: a large, membrane-bound, fluid-filled compartment in a cell&rsquos cytoplasm
  • endosymbiosis: when one symbiotic species is taken inside the cytoplasm of another symbiotic species and both become endosymbiotic

Mitochondrial Replication and Genetics

How do Mitochondia Replicate?

The Mitochondrial replication is diagrammed in the cartoon in the side bar and shown above in an electron micrograph. Mitochondria replicate much like bacterial cells. When they get too large, they undergo fission. This involves a furrowing of the inner and then the outer membrane as if someone was pinching the mitochondrion. Then the two daughter mitochondria split. Of course, the mitochondria must first replicate their DNA. This will be discussed in more detail in the next section. An electron micrograph depicting the furrowing process is shown in these figures. The figure above was taken from Fawcett, A Textbook of Histology, Chapman and Hall, 12th edition, 1994

Sometimes new mitochondria are synthesized de novo in centers that are rich in proteins and polyribosomes needed for their synthesis. The electron micrograph in the above figure shows such a center. It appears that the cluster of mitochondria are sitting in a matrix of proteins and other materials needed for their production. How might you prove that material in that region was making mitochondrial proteins? Return to Menu

Certain mitochondrial proteins are needed before the mitochondria can divide.

This has been shown in a study by Sorgo and Yaffe, J Cell Bio. 126: 1361-1373, 1994. They showed the result of the removal of an outer membrane protein from mitochondria called MDM10. This figure shows the results. The mitochondria are able to take in components and produce membranes and matrix enzymes. However, fission is not allowed. Thus, the result is a giant mitochondrion. This is illstrated in the micrograph below.

Mitochondrial DNA and its function.

Mitochondria have some of their own DNA, ribosomes, and can make many of their own proteins. The DNA is circular and lies in the punctate structures called "nucleoids". Each nucleoid may contain 4-5 copies of the mitochondrial DNA (mrDNA).

Human mitochondrial DNA is 16,569 bp encodes a number of mitochondrial proteins

  • Subunits 1, 2, and 3 of cytochrome oxidase
  • Subunits 6, 8,9 of the Fo ATPase
  • Apocytochrome b subunit of CoQH2-Cytochrome C reductase
  • Seven NADH-CoQ reductase subunits

The nucleus encodes the remaining proteins. Most of the lipid is imported (recall the lectures on lipid addition to membranes). This cartoon from your text shows the nuclear involvement. The highlighted labels are drugs that can be used to block the process and test the source of the mitochondrial protein.

Mitochondria also have their own ribosomes and tRNA:

(Magalhaes, PJ Andreu, AL, Schon EA, Evidence for the presence of 5 S rRNA in mammalian mitochondria Mol Biol Cell 9: 2375-2382)

The Figure to the left shows mitochondrial Ribosomes as granules in the mitochondria.

The texts still say that mitochondria have no 5S rRNA, however the recent study cited above shows evidence for 5S in carefully prepared mitochondrial fractions. These workers found 5S in highly purified mitochondria and mitoplasts (mitochondria without the outer membrane). Conclusion: 5S rRNA is imported into mitochondria, but its function is uncertain.

Visualization of mitochondrial DNA

T o visualize the structure of mitochondrial DNA, we have to extract the proteins in the matrix and reveal the DNA (arrows in the figure to the right).

One can also see ribosomes in the circles.

Alternatively, one can extract the DNA and float it on a water surface. Then, it can be picked up by a plastic coated grid, and examined in the electron microscope. Mitochondrial circular DNA is shown in the following figure. This electron micrograph is taken from Fawcett, A Textbook of Histology, Chapman and Hall, 12th edition, 1994.

Mitochondrial Inheritance

In mammals, 99.99% of mitochondrial DNA (mtDNA) is inherited from the mother. This is because the sperm carries its mitochondria around a portion of its tail and has only about 100 mitochondria compared to 100,000 in the oocyte. As the cells develop, more and more of the mtDNA from males is diluted out. Hence less than one part in 10 4 or 0.01% of the mtDNA is paternal. This means that mutations of mtDNA can be passed from mother to child. It also has implications if one does cloning of mammals with the use of somatic cells. The nuclear DNA would be from the donor cell, but the mtDNA would be from the host cell. This is how Dolly the sheep was cloned.

There is a Yeast strain, called "Petite" that have structurally abnormal mitochondria that are incapable of oxidative phosphorylation. These mitochondria have lost some or all of their DNA. Mitochondrial inheritance from yeast is biparental, and both parent cells contribute to the daughter cells when the haploid cells fuse. After meiosis and mitosis, there is random distribution of mitochondria to daughter cells. If the fusion is with yeast that are petite and yeast that are not, a certain percentage of the daughter cells will be "petite".

Mutations in mammalian mtDNA do cause diseases, because there is such a short sequence and very heavy information content in the sequence. The next lecturer on mitochondria in this series will spend a great deal of time on the mitochondrial genome. Since each cell contains hundreds of mitochondria and thousands of copies of the genome, the effects of the mutated mitochondria may be diluted out. As expected, those tissues or organs most likely to be affected would be the ones most dependent on oxidative phosphorylation (ATP production). In young persons it might not be picked up because even a person with 15% normal mitochondria might have enough to be healthy. However, aging patients may show a more severe disease phenotype.

  • Leber's hereditary optic neuropathy (degeneration of the optic nerve, accompanied by increasing blindness): caused by mutation to the gene encoding subunit 4 of the NADH-C0Q reductase.
  • "ragged muscle fibers" associated with jerky movements is caused by mutation of mitochondrial lysine tRNA.
  • Kaerns-Sayre syndrome: eye defects, abnormal heartbeat, Central nervous system degeneration. Several large deletions in mtDNA.

Can damaged mitochondrial DNA be repaired?

  • Meeusen, S, Tieu, Q, Wong E, Weiss, E, Schieltz, D, Yates, JR, and Nunnari, J. Mgm101p is a novel component of the mitochonrial nucleoid that binds DNA and is required for the repair of oxidatively damaged mitochondrial DNA. J Cell Biol 145: 291-304 (1999)
  • Mgm stands for "mitochondrial genome maintenance". It was discovered in yeast cells while searching for mutants that caused a temperature sensitive loss of mitochondrial DNA.
  • Fused Mgm101 to green fluorescent protein and found that it was localized to the punctate "nucleoid" structures. Localization overlapped with that of DNA detection systems.
  • After protein screening found the Mgm101, they studied how its loss affected respiratory competence. Clearly the protein was needed for function, but they do not know exactly what its role is at this point.
  • Looked at the COOH terminal region and saw that it was highly basic. That suggested that the Mgm101p might have the ability to bind DNA. Compared its binding to DNA cellulose columns (in high salt conditions) with another known DNA binding protein and confirmed relatively high affinity binding by both proteins.

What happens to old, worn-out mitochondria?

Mitochondrial numbers are controlled by autophagy. This is a process by which lysosomes are involved in controlling cell constituents. This Figure shows the process it is taken from Fawcett, A Textbook of Histology, Chapman and Hall, 12th edition, 1994.

The process begins by wrapping endoplasmic reticulum membranes around the mitochondrion. Then, vesicles come from the Golgi complex and join with the autophagic vacuole. These vesicles contain hydrolases attached to the mannose 6 phosphate receptors in the vesicle membranes. The lysosome web page discusses their function and fate. Recall that they fuse with the autophagic vacuole. The acid pH then allows the hydrolases to be removed from their receptors. The receptors are recycled back to the Golgi complex in other vesicles.

In the meantime, the lysosome forms as the pH drops and the cells begin to degrade the contents. Recall that lysosomes are LAMP+, but they do not carry the MPR because these have been recycled to the Golgi Complex. What coat is found around the transport vesicles going to the autophagic vacuole?

Mitochondrial ROS regulate signaling

In the mid-1990s, NADPH oxidase activity had been demonstrated to promote signaling pathways involved in cell proliferation through oxidation of particular cysteine residues in proteins, modulating their activity [14–16]. By contrast, mitochondrial ROS (mROS) were proposed to be produced only under pathological conditions to invoke damage [17]. However, in the late 1990s, mROS emerged as signaling molecules that communicate between mitochondria and the rest of the cell under physiological conditions. An early example of this retrograde signaling under physiological conditions is the observation that hypoxic conditions stimulate mitochondria to release ROS, resulting in the stabilization of hypoxia inducible factors (HIFs) and the induction of genes responsible for metabolic adaptation to low oxygen [2, 18]. Subsequently, mROS were shown to regulate cellular metabolism and tumor necrosis factor receptor signaling [19–21]. Eight sites in the mitochondrial inner membrane and matrix have been implicated in the production of ROS [22]. The factors that control mROS production include the concentration of oxygen available to mitochondria, the redox state of the different electron transport chain complexes and mitochondrial membrane potential [23]. In the past decade, mROS have been shown to regulate a wide range of biological processes, including oxygen sensing, epigenetics, autophagy, innate and adaptive immune responses, stem cell proliferation and differentiation, and hormone signaling [24–28].

A scientific colleague and friend once quipped, ‘If you don’t have a mechanism just say it is ROS’. There is some justification for this remark, since ROS have been linked to a wide variety of biological outcomes, including proliferation, differentiation, metabolic adaptation and senescence, though with no insight into the specific mROS targets required to invoke such diverse biological outcomes, or the mechanism involved. It is important to note that mROS targets that relay signaling could be localized in any or all of the mitochondrial matrix, the intermembrane space, or the cytosol. Furthermore, given the reactivity and toxicity of ROS at high levels, it seems likely that lower levels of mROS may be generated that invoke distinct biological outcomes. Control of different stem cell fates by ROS is an example of different levels of ROS invoking different biological outcomes. The two salient features of stem cells are their ability to self-renew, and their ability to differentiate into specialized tissues [29, 30]. An emerging model is that quiescent stem cells reside at low levels of ROS and slight increases in ROS are necessary signals for self-renewal and cellular differentiation [31–33]. ROS levels above those required for self-renewal or differentiation impair these critical two stem cell properties and result in stem cell hyper-proliferation, resulting in stem cell exhaustion [34]. Going forward, it will be important to systematically quantify the ROS levels generated by mitochondria and their targets that are necessary for stem cell proliferation, differentiation and exhaustion in a given stem cell model system.

One interesting development over the past two decades has been the change in perspective on the role of mROS signaling in aging. Originally the free radical theory of aging proposed that, during the aging process, damaged mitochondria produced increasing amounts of ROS leading to tissue damage [35]. However, in most studies antioxidants have not extended lifespan of model organisms and clinical trials using antioxidants in humans have not shown any beneficial effects on age-related diseases [36]. On the contrary, recent evidence in yeast, Caenorhabditis elegans, and mice suggests that increasing mitochondrial generation of ROS can activate cellular stress pathways to dampen tissue degeneration, promote healthy aging and increase lifespan [37–40]. Based on the studies from the past two decades, an emerging model of mROS and signaling suggests that low levels (picomolar to nanomolar range) of mROS are necessary to maintain homeostatic biological processes, while slightly elevated levels of mROS initiate pathways for adaptation to stress. Much higher levels of mROS trigger cell death or senescence.

Inside mitochondria and their fascinating genome

EPFL professor and biophysicist Suliana Manley with student Sofia Zaganelli and a super-resolution microscope. Credit: EPFL / Alain Herzog

Mitochondria are present in all eukaryotic cells: in our cells, in mammalian cells, in the cells of plants and even of fungi. Mitochondria produce energy for cells to function as multicellular organisms, and are known as the 'powerhouses' of the cell. Inside mitochondria lie the genetic information for making this energy.

EPFL biophysicist Suliana Manley and her team collaborated with Jean-Claude Martinou's cell biology group from the University of Geneva to look deep within living cells. Inside mitochondria there rests RNA granules that are smaller than the diffraction limit of light, i.e. smaller than one one-thousandth the width of a strand of hair. Using super-resolution microscopy, they discovered that mitochondrial RNA's are packaged into tiny liquid droplets that can fuse together and break apart. The results are published in today's issue of Nature Cell Biology.

"The organization of genetic information contained within mitochondria is highly dynamic thanks to this liquid-like aspect of its RNA granules," explains Manley. "The way they continuously exchange material gives us insight into how mitochondria are able to make sure they have the genetic information they require to produce energy within cells."

What led the scientists to inspect RNA granules is linked to the unique identity of mitochondria. In fact, the mitochondrial genome is independent of the cell's genome, so the genetic identity of the mitochondria is separate from the genetic identity of the cell and the rest of the organism. Mitochondria's genome is only around 16 thousand base pairs long whereas the DNA of the human cell, more than 100,000 times as long, consists of 3 billion base pairs. The mitochondria's genome is inherited from the maternal lineage, so the way your cells produce energy essentially comes from your mother. Mitochondria are hypothesized to have their origins in bacteria: 1.5 billion years ago during the course of evolution, bacteria may have been engulfed by another cell to start an endo-symbiotic relationship with time, the bacteria evolved to become this highly specialized organelle that produces energy for the cell.

Determining the way mitochondria work is important for understanding how the cell functions, but also for understanding how a cell malfunctions. Especially in cells that require large amounts of energy like nerve and muscle cells, dysfunctional mitochondria can have devastating consequences, resulting in severe disease.

Respiratory chain complexes and supercomplexes

The proton gradient across the cristae membrane is generated by three large membrane protein complexes of the respiratory chain in the cristae, known as complex I (NADH/ubiquinone oxidoreductase), III (cytochrome c reductase) and IV (cytochrome c oxidase) (Fig. 2). Complex I feeds electrons from the soluble carrier molecule NADH into the respiratory chain and transfers them to a quinol in the membrane. The energy released in the electron transfer reaction is utilized for pumping four protons from the matrix into the crista lumen. Complex III takes the electrons from the reduced quinol and transfers them to the small, soluble electron carrier protein cytochrome c, pumping one proton in the process. Finally, complex IV transfers the electrons from cytochrome c to molecular oxygen and contributes to the proton gradient by using up four protons per consumed oxygen molecule to make water. Complex II (succinate dehydrogenase) transfers electrons from succinate directly to quinol and does not contribute to the proton gradient.

The respiratory chain complexes have been studied in great detail for decades. High-resolution X-ray structures are available for mitochondrial complex III [44] and IV [45]. At a molecular mass of

1 megadalton (MDa), mitochondrial complex I is far larger and has more subunits than complexes III and IV put together. As yet there is no X-ray structure of the mammalian complex, but very recently a

3.6 Å X-ray structure of complex I from the obligate aerobic yeast Yarrowia lipolytica has been obtained [46]. Comparison to the high-resolution X-ray structure of the

550 kDa complex I from the thermophilic bacterium Thermus thermophilus [47] indicates that the 14 conserved core subunits have essentially the same structure in both, including three proton antiporter modules in the membrane and eight iron-sulfur clusters in the matrix arm. The mitochondrial complex has about three times as many protein subunits as its bacterial ancestor. Most functions of the extra subunits are unknown, but many of them are likely to work in assembly or the regulation of complex I function. Features that are conserved from bacteria to mitochondria include a long horizontal α-helix on the matrix side that may stabilize the membrane domain. The recent 5 Å single-particle cryo-EM structure of bovine heart complex I (Fig. 6) has resolved the proton-translocating modules, iron-sulfur clusters and long horizontal helix, and 14 of the 31 supernumerary mammalian complex I subunits have been identified [48]. However, the way in which electron transfer from NADH to ubiquinone in complex I is coupled to proton translocation is still unknown, and much else remains to be discovered.

Cryo-EM structure of bovine heart complex I. Mitochondrial complex I (

1 MDa) has a matrix arm and a membrane arm. The matrix arm contains a row of eight iron-sulfur clusters (red) that conduct electrons from NADH to ubiquinol at the junction of the matrix and membrane arms (Fig. 7). The membrane arm consists of 78 trans-membrane helices, including three proton-pumping modules. (Adapted from [51] EMDB code 2676)

Not unlike the ATP synthase, which forms dimer rows in the cristae, the proton pumps of the electron transport chain assemble into supercomplexes or ‘respirasomes’. Respiratory chain supercomplexes were first postulated on the basis of blue-native gels of yeast and bovine heart mitochondria solubilized in the mild detergent digitonin [49]. Negative-stain electron microscopy [50] and single-particle cryo-EM [51] of the 1.7 MDa bovine heart supercomplex revealed that it consists of one copy of complex I, one complex III dimer, and one complex IV monomer. X-ray structures of the component complexes were fitted to the 3D map (Fig. 7) [51], indicating the path of the electron from NADH via the iron-sulfur clusters of complex I and ubiquinol to the prosthetic groups of complex III, and finally to molecular oxygen in complex IV. Genetic evidence provides strong support for the existence of respirasomes in vivo [52], but they were long thought to be artifacts of detergent solubilization, notwithstanding their well defined structure. Recent cryo-ET work has shown that they do exist in cristae membranes of bovine heart mitochondria (Davies and Kühlbrandt, unpublished results). Saccharomyces cerevisiae, which lacks complex I, nevertheless has a respiratory chain supercomplex consisting of complex III and IV [53]. Far from being randomly distributed in the membrane, the ATP synthase and electron transport complexes of the respiratory chain thus form supramolecular assemblies in the cristae, in a way that is essentially conserved from yeast to humans (Fig. 8). A clear functional role of mitochondrial supercomplexes has not yet been established. They may make electron transfer to and from ubiquinone in complexes I and III more efficient, as the relative positions and orientations of the two complexes are precisely aligned rather than random. However, there is no direct evidence that this makes a difference. The supercomplexes may simply help to avoid random, unfavorable protein–protein interactions in the packed environment of the inner mitochondrial membrane [54]. Alternatively, they may control the ratio of respiratory chain complexes in the membrane, or aid their long-term stability.

Cryo-EM structure of the 1.7 MDa bovine heart respiratory chain supercomplex. a The supercomplex consists of one copy of NADH dehydrogenase (complex I, blue), a cytochrome b-c 1 dimer (complex III, pink), and a single copy of cytochrome c oxidase (complex IV, green). b The ubiquinol (UQ) binding sites of complexes I and III and the short distance between the cytochrome c binding sites in complexes III and IV, which would favor efficient electron transfer. Cofactors active in electron transport are marked in yellow (FMN), orange (iron–sulfur clusters), dark blue (quinols), red (hemes), and green (copper atoms). Arrows indicate the electron path through the supercomplex. (Adapted from [51])

ATP synthase dimer rows shape the mitochondrial cristae. At the cristae ridges, the ATP synthases (yellow) form a sink for protons (red), while the proton pumps of the electron transport chain (green) are located in the membrane regions on either side of the dimer rows. Guiding the protons from their source to the proton sink at the ATP synthase, the cristae may work as proton conduits that enable efficient ATP production with the shallow pH gradient between cytosol and matrix. Red arrows show the direction of the proton flow. (Adapted from [17])

A main protein component of the crista lumen is the small soluble electron carrier protein cytochrome c that shuttles electrons from complex III to complex IV. If released into the cytoplasm, cytochrome c triggers apoptosis [55]. It is imperative, therefore, that cytochrome c does not leak from the cristae and that the outer membrane remains tightly sealed during mitochondrial fission and fusion.

Materials and methods

Cell culture

Human cells derived from embryonic kidney, HEK293, hepatoma tissue culture, HTC-116, and large-cell lung cancer, NCI-H460 (American Type Culture Collection, Manassas, VA), as well as primary skin fibroblasts derived from healthy individuals and HEK293 cells expressing C. intestinalis AOX [22] or UCP1 [23,24] were cultured in DMEM medium containing 4.5 g/L glucose and 2 mM glutamine (glutamax Gibco Thermo Fisher Scientific, MA), 10% fetal calf serum, 200 μM uridine, 2 mM pyruvate, 100 U/mL each penicillin and streptomycin. The trypan blue exclusion test was used to determine the number of viable and dead HEK293 cells [25].

Immunoblot analyses and in-gel enzyme activity assays

For western blot analysis, mitochondrial proteins (50 μg) were separated by SDS–PAGE on a 12% polyacrylamide gel, transferred to a nitrocellulose membrane, and probed overnight at 4 °C with antibodies against the protein of interest, AOX 1:10,000 [26], UCP1 1:10,000 [27]. Membranes were then washed in TBST and incubated with mouse or rabbit peroxidase-conjugated secondary antibodies for 2 h at room temperature. The antibody complexes were visualized with the Western Lightning Ultra Chemiluminescent substrate kit (Perkin Elmer). For the analysis of RC complexes, mitochondrial proteins (100 μg) were extracted with 6% digitonin and separated by hrCN-PAGE on a 3.5%–12% polyacrylamide gel. Gels were stained by IGA assay detecting CI, CII, and CIV activity, as described [28].

Staining procedures and life cell imaging

Cells were seeded on glass coverslips and grown inside wells of a 12 well plate for 48 h in standard growth media at 37 °C, 5% CO2. The culture medium was replaced with prewarmed medium containing fluorescent dyes, namely 100 nM MTG (Invitrogen M7514) and 100 nM MTY [1] or 100 nM ER thermo yellow [29]. After 10 min, the staining medium was replaced with fresh prewarmed medium or PBS and cells were observed immediately by Leica TCS SP8 confocal laser microscopy.

Assay of mitochondrial RC activity

The measurement of RC activities was carried out using a Cary 50 spectrophotometer (Varian Australia, Victoria, Australia), as described in [30]. Protein was estimated using the Bradford assay.

Simultaneous spectrofluorometric, temperature, oxygen uptake assay

Detached subconfluent HEK293, NCI-H460, or HTC-116 cells (25 cm 2 flask) or trypsinized subconfluent skin fibroblasts (75 cm 2 flask) were treated for a minimum of 10 min with 100 nM MTY (or 100 nM compound A15 [1]) in 10 mL DMEM and recovered by centrifugation at 1,500 gmax for 5 min. The pellet was washed once in 1 mL PBS, then maintained as a concentrated pellet. After anaerobiosis (checked by inserting an optic fiber equipped with an oxygen-sensitive fluorescent terminal sensor [Optode device FireSting O2, Bionef, Paris, France]) was established (10 min incubation of the pellet at 38 °C), cells (1 mg prot) were added to 750 μl PBS thermostatically maintained at 38 °C. The fluorescence (excitation 542 nm, emission 562 nm for MTY excitation 559 nm, emission 581 nm for ER thermo yellow excitation 500 nm, emission 520 nm for A15), the temperature of the medium in the cuvette, and the respiration of the intact cell suspension were simultaneously measured in a magnetically stirred, 38 °C-thermostated 1-mL-quartz cell using a Xenius XC spectrofluorometer (SAFAS, Monaco). Oxygen uptake was measured with an optode device fitted to a handmade cap, ensuring either closure of the quartz cell yet allowing micro-injections (hole with 0.6-mm diameter), or leaving the quartz cell open to allow for constant oxygen replenishment. Alternatively, untreated HEK293 cells (250 μg protein) were added to 750 μL of buffer consisting of 0.25 M sucrose, 15 mM KCl, 30 mM KH2PO4, 5 mM MgCl2, 1 mM EGTA, pH 7.4, followed by the addition of rhodamine to 100 nM and digitonin to 0.01% w/v. The permeabilized cells were successively given a mitochondrial substrate (10 mM succinate) and ADP (0.1 mM) to ensure state 3 (phosphorylating) conditions, under which either 5 μM oligomycin or 0.8 mM cyanide was added.


Data are presented as mean ±SD. Statistical significance was calculated by standard unpaired one-way ANOVA with Bonferroni posttest correction a p value <0.05 was considered statistically significant (GraphPad Prism).

  • How the internal structure and organization of a cell provides an understanding of how and why a cell works
  • The role mitochondria play in the cell and why it is important for a cell to make ATP
  • How cells metabolize food to provide the molecules necessary for mitochondrial function
  • How the structure of the F1F0 ATP synthase leads to the production of ATP
  • What experimental techniques are used to investigate mitochondrial structure and function in the laboratory

The cell is a powerful case study to help us explore the functional logic of living systems. All organisms, from single-celled algae to complex multicellular organisms like us, are made up of cells. In this course, you will learn the how and why of biology by exploring the function of the molecular components of cells, and how these cellular components are organized in a complex hierarchy.

This course is designed to explore the fundamentals of cell biology. The overarching goal is for learners to understand, from a human-centered perspective, that cells are evolving ensembles of macromolecules that in turn form complex communities in tissues, organs, and multicellular organisms.

We will focus, in particular, on the mitochondrion, the organelle that powers the cell. In this context, we will look at the processes of cell metabolism. Finally, we will examine the F1F0 ATP synthase, the molecular machine that is responsible for the synthesis of most of the ATP that your cells require to do work. To underscore the importance of cell biology to our lives, we will address questions of development and disease and implications of science in society.

By the end of four weeks, we hope learners will have a deep intuition for the functional logic of a cell. Together we will ask how do things work within a cell, why do they work the way they do, and how are we impacted?

Join us as we explore the extraordinary and wonderfully dynamic world of the cell.

Scientists discover why fungi have 36,000 sexes

IF HUMANS were mushrooms, finding a date would be much easier. Whereas we muddle by with just two sexes, the fungi have 36,000, all of which can mate with each other, in a mysterious process involving underground fronds.

So why don't humans have such a varied sex life? British scientists now believe they know the answer: two sexes is exactly the right number if you reproduce by fusing cells together. Larger numbers - anywhere from a handful of sexes up to 36,000 - are only permissible if, like slime moulds or mushrooms, you keep your partners' cells at arm's - or frond's - length.

Professor Laurence Hurst of the University of Bath's biology department has now developed and tested a theoretical model that explains that the number of sexes is determined by the method of sexual reproduction. In particular, it depends on the powerhouses of the cell - the mitochondria - which sit outside the nucleus and generate energy for the rest of the cell.

"Mitochondria are necessary for the capability to convert oxygen to energy," said Professor Hurst at the British Association Festival in Sheffield yesterday. "But they have their own DNA, and they can divide all the time, unlike the cell itself. That's potentially harmful if there is a mutation in the mitochondrial DNA."

If the cells of the mating parents fuse during sex, then the mitochondrial DNA will also be brought together. If either set of DNA has harmful genes, they will be inherited by the offspring and damage its chances of survival.

The best solution for sex involving cell fusion is thus to throw away one of the sets of mitochondrial DNA - precisely what happens in humans. There, the father's mitochondria are carried on the sperm, but destroyed once inside the mother's egg.

That instantly halves the chance of inheriting bad mitochondrial genes. "The difference with mushrooms is that they never allow the two sets of mitochondria to come into the same cell space to compete," said Professor Hurst.

It means that while a mushroom can have sex with almost every companion it meets - decided by its own "sex gene" - any harmful mitochondrial mutations are restricted to the parent carrying them, and cannot easily spread through the population. This "Berlin Wall" strategy is used by many lower species that have multiple sexes, including ciliate bacteria.

However, Professor Hurst's model predicts that those without mitochondrial DNA do not need sexes - and that is what happens in the natural world.

Mitochondria are believed to be the descendants of ancient bacteria captured for their energy-producing ability by cells early in Earth's history. The process described by Professor Hurst's work suggests that cells have evolved methods to ensure the "best" mitochondria are retained.

So that explains sexes - but what about sex? What's the purpose of that? "It gets rid of bad mutations and lets good ones spread through the population," said Professor Hurst. "But basically it seems to be to avoid parasites."

Dr Martin Hall, of the Natural History Museum, seeking clues in a maggot. He says the study of maggots and corpses can tell forensic scientists the time of death and help to solve murder cases Jayne Emsley

Murderes should beware: maggots will find the remains they leave behind, says Dr Martin Hall, of the Natural History Museum.

He has been studying the life cycle of maggots to help forensic scientists determine the times of death of bodies

"For a blowfly, a human body is just as acceptable as a dead hedgehog," he said.

"We are improving our estimates of how long it takes flies to find bodies all the time. For example, it will take them longer if a body is in a third floor float than if its outside."

The study of maggots and corpses now means that scientists can predict with 95 per cent certainty how many days a body has been in the open to within 12 hours. Evidence from Maggots were first used in the UK in 1935 over the double murder by Buck Ruckston of his wife and her maid whose bodies were dumped in a ravine.

Using Oxygen to Release Energy

How does cellular respiration occur in mitochondria? The matrix is filled with water and proteins (enzymes). Those proteins take organic molecules, such as pyruvate and acetyl CoA, and chemically digest them. Proteins embedded in the inner membrane and enzymes involved in the citric acid cycle ultimately release water (H2O) and carbon dioxide (CO2) molecules from the breakdown of oxygen (O2) and glucose (C6H12O6). The mitochondria are the only places in the cell where oxygen is reduced and eventually broken down into water.

Mitochondria are also involved in controlling the concentration of calcium (Ca 2+ ) ions within the cell. They work very closely with the endoplasmic reticulum to limit the amount of calcium in the cytosol.

Watch the video: Σύνδρομο Κόπωσης Επινεφριδίων - Ινομυαλγία - Χρόνια Κόπωση (August 2022).