To Branch, Or Not To Branch – Plant Hormones Help Turn A Stem Into A Bush

When we hear the expression “stem cells”, we tend to think of cells from animals or patients that are used to treat diseases or promote regeneration. However, stem cells are also present in plants. The growing tips of plants are called meristems and they are reservoirs of plant stem cells. A meristem is formed at the base of each leaf and can remain dormant as a small bud or be activated and give rise to a whole new branch. Gardeners know that pruning leaves can activate the buds and help transform a single stem plant into a multi-branched bush, but the exact mechanisms that govern branch formation are not fully understood.


The recent paper “Strigolactone Can Promote or Inhibit Shoot Branching by Triggering Rapid Depletion of the Auxin Efflux Protein PIN1 from the Plasma Membrane” published in PLOS Biology by Naoki Shinohara and colleagues has uncovered an important novel pathway that regulates the formation of branches in plants. The researchers based their work on an existing model which states that the plant growth hormone auxin is a central regulator of branch formation. Auxin levels are highest in activated buds because this is where auxin is produced. Auxin then flows to the roots, where auxin levels are low (“auxin sinks“). The removal of auxin from the activated bud allows for further auxin production and thus creates a continuous auxin flow pattern. This is thought to establish a positive feedback loop for the activated bud, which then ultimately results in the formation of branch emanating from the activated bud. This model is called the “auxin transport canalization” and is explained in an excellent accompanying article “Transforming a Stem into a Bush” by Amy Coombs, also published in PLOS Biology.

Once an activated bud initiates the positive auxin feedback loop, it also becomes necessary to inhibit the branch formation from other buds. If all the buds in a plant started making branches at once, the plant’s resources would probably become depleted very quickly, possibly resulting in the chaotic formation of too many suboptimal branches. There is a clearly a need for a system that allows some activated buds to go on to make branches, while putting the brake on other buds so that they bide their time. The details of such a fine-tuned balance of selected activation and inhibition have been a bit of a mystery, but the work by Shinohara and colleagues is a major step forward in unraveling this puzzle.

The researchers show that the plant hormone strigolactone removes the auxin export protein PIN1 from the cell surface and increases its degradation. Therefore, a plant without strigolactone would have more PIN1, sustain greater auxin flux and thus increase branching. Genetically engineered plants that do not produce strigolactone did indeed show more branch formation. When the researchers added back synthetic strigolactone (called GR24), they were able to suppress the excessive branch formation. However, the researchers also obtained a somewhat counter-intuitive result: When they gave GR24 to plants with defective auxin transport, low doses of GR24 actually helped branch formation and only higher doses suppressed branch formation. The problem with these results is that the synthetic strigolactone also severely impacted the general growth of the plants (not just branch formation) in the auxin transport mutants, and it is difficult to interpret whether the subtle differences between low and high doses were just generalized effects due to reduced overall plant health or whether they were truly related to aberrant branching.

The oddly opposite results obtained with low dose and high dose GR24 treatment are probably going to raise some controversy, and as Amy Coombs pointed out, not all scientists agree with the auxin transport canalization theory of branch formation in plants. This is not the first study to propose an interaction between strigolactone and auxin as regulators of plant branch formation, but it is one of the most comprehensive papers in this area. It includes a mathematical model of the interaction between these two regulators, tests the model with experiments and identifies a novel cellular mechanism for how strigolactone reduces PIN1. These results do suggest that plants have a very finely-tuned system involving at least two hormones, auxin and strigolactone, that act together to promote branch formation in some buds, while suppressing bud formation in others. As a stem cell biologist who works with mammalian stem cells, I am quite intrigued by this fascinating interplay between activating and suppressing hormones in plants that permit a self-organized branch formation. In mammals, we still do not fully understand how during development, some embryonic stem cells commit to one lineage and form organs such as a heart, while also preventing other stem cells in the developing embryo to form a second or third heart in other areas. It is quite likely that developing mammalian embryonic stem cells also depend on positive feedback loops and inhibitory systems, similar to what the researchers found in the plants. Many major discoveries in cell biology and molecular biology are first made in plants and we then discover similar principles of regulation in animals and humans.


Image credit: Panel from Figure 5 of the PLOS Biology (2013) paper by Shinohara N., et al, Green indicates the PIN protein and magenta shows the autofluorescence of chloroplasts

Shinohara, N., Taylor, C., & Leyser, O. (2013). Strigolactone Can Promote or Inhibit Shoot Branching by Triggering Rapid Depletion of the Auxin Efflux Protein PIN1 from the Plasma Membrane PLoS Biology, 11 (1) DOI: 10.1371/journal.pbio.1001474

Flipping the Switch: Using Optogenetics to Treat Seizures

Optogenetics is emerging as one of the most exciting new tools in biomedical research. This method is based on introducing genes that encode for light-sensitive proteins into cells. A laser beam can then be used to activate the light-sensitive proteins. Many of the currently used optogenetic proteins respond to the laser activation by changing the membrane voltage potential inside the cells. This is the reason why neurons and other cells that can be excited by electrical impulses, are ideally suited for studying optogenetic responses.

The recent paper “On-demand optogenetic control of spontaneous seizures in temporal lobe epilepsy” by Esther Krook-Magnuson and colleagues in Nature Communications (published online on January 22, 2013) applies the optogenetic approach to treat seizures in mice. The researchers used mice that had been genetically modified to express the inhibitory light sensitive protein halorhodopsin (normally only found in single cell organisms but not in mammals) in neurons. They placed an optical fiber to deliver the laser light to an area of the brain where they chemically induced a specific type of seizures (temporal lobe epilepsy or TLE) in the mice.

The results were quite remarkable. Activation of the laser light reduced the seizure duration by half within just five seconds. Krook-Magnuson and colleagues then also chose a second optogenetic approach to treat the seizures. Instead of using mice that contained the inhibitory light-sensitive protein halorhodopsin, they opted for mice with the excitatory (activating) light-sensitive protein channelrhodopsin (Chr2). This may seem a bit counter-intuitive, since the problem in epilepsy is that there is too much activation of neurons. One would not necessarily want to introduce activating light-sensitive proteins into neurons that are already too active. The key to understanding their strategy is the choice of the target: a subset of GABAergic cells, which can inhibit the seizure activity in neighboring neurons. This second approach was just as effective as the first approach, which used the halorhodopsin protein.

This means that one can substantially cut down seizure duration by more than half, either by directly inhibiting seizing neurons, or by activating inhibitory neurons. This research shows that there is tremendous potential for developing novel optogenetic treatments for epilepsy. Specifically targeting selected neurons that are involved in seizure activity would be preferable to generalized treatment with medications that affect global neuronal activity and could cause side effects (as is often the case with current epilepsy medications).

It is not yet clear whether this treatment can be easily applied to humans. The linchpin of the experiment was genetically introducing the light-sensitive proteins into selected neurons of the mice. This type of targeted neuronal gene therapy would be far more difficult in humans. The other obstacle is that the light activation in the mice required implantation of an optical fiber which directed the light into a specific area of the brain. Performing such an invasive procedure in patients could carry potential risks that would need to be carefully balanced with the risks and benefits of just continuing to use anti-seizure medications. Hopefully, future improvements in gene therapy methods and light stimulation will be able to help overcome these obstacles and pave the way for a whole new class of optogenetics-based therapies in patients with epilepsy and other neurological disorders.


Image credit: Confocal image of an eGFP filled striatal medium spiny neuron, National Institutes of Health (NIH), Margaret I. Davis
Krook-Magnuson, E., Armstrong, C., Oijala, M., & Soltesz, I. (2013). On-demand optogenetic control of spontaneous seizures in temporal lobe epilepsy Nature Communications, 4 DOI: 10.1038/ncomms2376

Is Cannabis Usage “Related” to Strokes?

Any research related to cannabis is bound to be sensationalized or politicized because people have strong emotional and political views about its usage. A few months ago, my fellow Scilogs blogger Suzi Gage wrote an excellent blog post about a study that investigated the link between cannabis usage and intelligence. That study had many critical flaws which were often ignored when the research was reported and discussed in the media. All research should be conducted and reported cautiously. However, when research touches on highly controversial topics, it is even more important that researchers clarify whether their research establishes statistically rigorous associations and true cause-effect relationships or whether they are merely pointing out important observations that require further research to derive definitive conclusions.

The recent paper entitled “Cannabis-related Stroke: Myth or Reality?” published by Wolff and colleagues in the journal Stroke investigates whether cannabis usage is related to stroke, concludes:


“In regard to the literature, cannabis-related stroke is not a myth, and a likely mechanism of stroke in most cannabis users is the presence of reversible MIS induced by this drug. The reality of the relationship between cannabis and stroke is, however, complex because other confounding factors have to be considered (ie, lifestyle and genetic factors). To confirm that cannabis may be a precipitating factor of RCVS with severe complications, an epidemiological study to determine the incidence of MIS, complicated or not by stroke, in the general population and in the cannabis users is necessary.”


The abbreviation MIS stands for multifocal intracranial stenosis, and refers to the presence of multiple blockages in the blood vessels of the brain that are impeding the blood flow and causing the stroke. RCVS stands for reversible cerebral vasoconstriction syndrome and describes a transient spasm of the blood vessels that briefly interrupts the blood flow to the brain, thus causing the stroke. The novelty of the paper by Wolff and colleagues is their idea that cannabis usage causes reversible MIS, i.e. that cannabis transiently causes multiple blockages that are reversed when cannabis is removed.

This is indeed an interesting idea, but unfortunately, they do not present any convincing data to back up this intriguing hypothesis. The data in their paper consists of a table listing 59 cases of stroke in patients who used cannabis, combining their own data with data that has been published by others. The authors admit that many of the patients who used cannabis were also active users of tobacco and alcohol, which makes it difficult to attribute the stroke to cannabis as opposed to these other confounding factors.

Most of the patients had some sort of brain imaging performed to diagnose the stroke, but less than half of them had a follow up scan later on to see if the blockage was still present. In those few cases where follow up imaging was performed, most of them did show some degree of reversibility of the blockages in the brain blood vessels. However, they do not present data on whether strokes in non-cannabis users also show similar reversibility patterns.

Wolff and colleagues also reference an older 2001 paper “Triggering Myocardial Infarction by Marijuana” by Mittleman et al and state that their current findings are in accordance with the conclusions of the 2001 paper. The paper by Mittleman and colleagues studied heart attacks and not strokes, but both diseases are caused by reduced blood flow, so it is not unreasonable to compare the data. The 2001 paper stated that the risk of having a heart attack is increased 4.8 fold within an hour of using cannabis. However, one has to bear in mind that the 2001 paper studied heart attacks in 3882 patients, of whom only 3.2% used cannabis and only 9 patients had used cannabis within an hour of the heart attack. The 4.8 fold risk determination was therefore based on this tiny sample of 9 patients!

In summary, the paper by Wolff and colleagues does not really answer the question of whether cannabis-related stroke is myth or reality. The small sample size, the observational nature of the data, the lack of follow up imaging on all the patients and the lack of controlling for confounding risk factors such as tobacco (which has a very strong association with stroke) make it difficult to draw definitive conclusions. All we can say is that Wolff and colleagues have presented an intriguing hypothesis that cannabis might cause strokes by inducing transient blockages or spasms of blood vessels in the brain. We need more definitive data to determine how cannabis usage is “related” to strokes: Is it a true cause of stroke or is it just an indicator of other more established risk factors, such as tobacco usage.

Wolff, V., Armspach, J., Lauer, V., Rouyer, O., Bataillard, M., Marescaux, C., & Geny, B. (2012). Cannabis-related Stroke: Myth or Reality? Stroke, 44 (2), 558-563 DOI: 10.1161/STROKEAHA.112.671347

Using Viagra To Burn Fat

Mammals have two types of fat tissue: Brown Adipose Tissue (BAT or “brown fat”) and White Adipose Tissue (WAT or “white fat”). Brown fat cells are packed with many small fat droplets and mitochondria, which is why they appear “brown” under the microscope. Their mitochondria contain high levels of the protein UCP-1 (uncoupling protein 1), which “uncouples” fat metabolism from the generation of chemical energy molecules (ATP) for the cell. Instead, brown fat cells release the energy contained in the fat in the form of heat. This explains why brown fat is primarily found in hibernating animals or in newborns that need to generate heat. They burn their fat to maintain their body temperature. White fat, on the other hand, cannot be burned off so easily and also seems to responsible for many of the deleterious effects associated with obesity, such as diabetes and inflammation.

Recent studies in humans have shown that even adult humans have small stores of brown fat, primarily located in the neck or near big blood vessels. Occasional small islands of brown fat cells can be found amid the large white fat tissue in the adult. Since brown fat is generally considered to be much healthier than white fat, scientists have tried to develop methods to convert white fat into brown fat. Some have used genetic approaches in mouse models of obesity, others have exposed human subjects to a few hours of cold temperatures. The paper “Increased cGMP promotes healthy expansion and browning of white adipose tissue” published in the FASEB Journal by Michaela Mitschke and colleagues (Online publication January 9, 2013) uses a rather unusual approach to induce the “browning” of white fat.

The researchers treated mice with Viagra (sildenafil), a drug that is normally used for erectile dysfunction. They found that only seven days of Viagra treatment increased the levels of the brown fat protein UCP-1 and that the white fat began showing the presence of “beige” (not quite white and not fully brown) fat. The choice of Viagra was not quite arbitrary, because they also showed that cultured fat cells contain cGMP-dependent protein kinase I (PKGI), which is part of the signaling pathway targeted by Viagra, and that increasing the levels of PKGI converted these cells into thermogenic brown fat cells. The researchers did not observe any weight loss in the mice, but they attributed this to the fact that they purposefully chose a very short treatment time in order to investigate fat conversion in the absence of fat loss. Their data suggests that longer treatment would lead to even more white-to-brown conversion of fat and to an actual weight loss, because the generated “beige” or brown-like fat cells could be easily burned off. A prior study that treated mice for 12 weeks with Viagra did indeed show some evidence of weight loss with Viagra treatment.

This new study is quite interesting and may have important practical implications because it uses an approved drug that is commonly available for human studies. Treating obese patients with Viagra would be much easier than trying to genetically convert their white fat to brown fat or to expose them to long periods of cold. However, overweight people should not expect that “Super-Size” orders at their favorite fast food joint will come with a Viagra pill. They also should not run to their physicians to ask for Viagra prescriptions at this point. One has to bear in mind that there are a number of caveats when trying to apply these findings in mice to humans. Chronic Viagra treatment in humans may be associated with some significant side effects and there is no consensus that “browning” of fat in adult humans will necessarily improve their health. Most of the data on the benefits of creating brown fat are based on animal studies. We therefore still need to await future studies, both in animals and in humans, that study the impact of long-term Viagra treatment on weight loss and associated health benefits as well as potential side effects, before definitive conclusions can be drawn. In the mean time, there will be plenty of opportunities to milk this research finding for humorous quips at late night talk shows.


Image credit: Co-culture of pre-adipocytes with mouse endothelial cells, via Wikimedia, Authors: Alexes Daquinag, Glauco Souza

Mitschke, M., Hoffmann, L., Gnad, T., Scholz, D., Kruithoff, K., Mayer, P., Haas, B., Sassmann, A., Pfeifer, A., & Kilic, A. (2013). Increased cGMP promotes healthy expansion and browning of white adipose tissue The FASEB Journal DOI: 10.1096/fj.12-221580

Beautiful Animations of Cellular Processes

The professional animator and molecular biologist Janet Iwasa at Harvard Medical School is generating beautiful animations of cellular processes such as proteasome structure and function or endocytosis. Importantly, she has published these on her website with a Creative Commons license so that everyone has access to them. She has been interviewed by EarthSky, where she explains why she became a molecular animator.

Movie about the proteasome structure:

Movie about chromosome segregation:

Movie about protein translocation (movement of proteins across membranes):

There are plenty of other beautiful animations and illustrations on her website and I highly recommend that anyone with an interest in cell biology should explore those.

Dual Identity and Radicalism Among Immigrants

The majority of my scientific colleagues with whom I work in the United States are either immigrants or children of immigrants. Most of them are American citizens, but they also retain strong cultural bonds with their ancestral homelands. This does not seem to constitute much of a problem for them. America is the land of immigrants where one is surrounded by people who are quite comfortable with their hyphenated dual identities. Irish-Americans or Chinese-Americans can be proud of their respective Irish and Chinese heritages without feeling that this makes them “less” American.

The situation was rather different when I lived in Germany. I was the only German with Pakistani roots among all the medical students at my university in Munich, and I did not know of any other hyphenated Germans at the research institute where I worked in the early 1990s. We had visiting scientists or PhD students, such as a grad student from Taiwan in our laboratory, but it was generally understood that they would go back to whatever country they came from. Their colleagues did not expect them to ever “feel German” and the visiting scientists or students also did not intend to “become German”. My Taiwanese lab-mate learned enough German to complete his PhD dissertation, but had no long-term plans of living in Germany. He clearly identified with being Taiwanese and his goal was to return to Taiwan.

Twenty years later, the situation in Germany has changed quite a bit. Hyphenated Germans are becoming far more common at universities. This is in part due to the fact that it has become much easier to become a permanent resident or acquire German citizenship, both for adult immigrants as well as their children. Children from immigrant families have grown up in Germany and they now have to grapple with their dual identities. Unlike the United States, where dual identities have been around for centuries and there is a broad acceptance of having two or even more identities, German society and the emerging hyphenated Germans are still trying to figure out what it means to be a Turkish-German, Nigerian-German or Russian-German.

In my own experience, having a dual identity has been an asset, allowing me to interact with and learn from a broad spectrum of fellow humans. However, I also realize that having a dual identity can also be somewhat problematic. The recent study “When Dual Identity Becomes a Liability : Identity and Political Radicalism Among Migrants” by Bernd Simon and colleagues in the journal Psychological Science (advanced online publication on January 14, 2013) attempted to study the impact of having a dual identity on political radicalism. The researchers recruited university students in Germany with either Turkish or Russian immigrant backgrounds and asked them to answer a set of questions about their personal experiences with having a dual identity using a Web-based questionnaire. Roughly half of the students were German citizens and the average percentage of lifetime spent in Germany was 66%.
Four questions assessed the extent of having a dual identity:


1) “I feel I belong to both the Turks/Russians and the Germans”

2) “Sometimes I feel more like a German and sometimes more like a Turk/Russian—it depends on the situation”

3) “I have many similarities with Germans as well as Turks/Russians”

4) “I feel good in the Turkish/Russian as well as the German culture.”
The students were also asked to address whether they felt there was an incompatibility of their ethnic and their German identity:

“I have the feeling that I would have to give up my Turkish/ Russian identity if I wanted to become German”
The participants also answered a number of additional questions about their cultural, national and religious identity.
In addition to these questions, the researchers assessed whether the students expressed support or understanding for “radical” (illegal, violent) actions, such as participating in an illegal demonstration, participating in a violent demonstration, blocking a road, occupying houses or offices, writing a political slogan on a public wall, and damaging other people’s property. The students also indicated whether they agreed with the following statements:

“I would participate even in a protest action that may involve a confrontation with the police”

“If the police and the courts can’t provide justice, you sometimes have to bring about justice yourself.”

Rating scales for these eight items (the six “radical” actions and the two “radical” statements) ranged from 0 (no understanding/not true at all ) to 4 (total understanding/absolutely true) and these eight items were used to determine their sympathy for radical action.

The results of the study are quite interesting. Having a dual identity by itself was only correlated with sympathy for radical action, if the respondents felt that the two identities were incompatible, i.e. becoming German would mean having to give up their Turkish or Russian identity. Students who felt that the German identity was quite compatible with their ethnic identity did not express any significant sympathy for radical action. Furthermore, among students with a Turkish immigrant background, sympathy for radical action decreased when religious identification became stronger and among Russian immigrant background students, it decreased when ethnocultural identification became stronger. This may come as a surprise to people who assume that stronger religious identification encourages sympathy for radical actions.

There are some limitations of this study. It only assessed sympathy for radical action by self-report without necessarily measuring actual participation in radical action, and one can debate whether showing support for writing political graffiti or participating in an illegal demonstration is as “radical” as endorsing actual violence. The study was purely correlational, without being able to determine cause-effect relationships and it also did not assess why some participants felt that their dual identities were incompatible. Nevertheless, this research suggests that possible sympathy for radical actions is not associated with having a dual identity, but with the perception of incompatibility between the two identities. One can surmise that reducing the perception of incompatibility could potentially reduce the sympathy for radical (illegal and violent) actions, but this would have to be addressed in future studies that investigate the effect of such an intervention.

How does one overcome the perception of incompatibility? My own experience is that multiple identities are not only compatible, but they are also complementary. I do not think that I have to sacrifice one cultural identity for another one. I notice that there are numerous parallels and similarities between German, Pakistani and American cultures in terms of the core values and goals of people. However, people often tend to focus on the differences and rare incompatibilities, because they make for sensationalist stories or political rallying cries that are far more marketable than the “boring” realization of how similar we all are. Societies in other countries such as the United States have also had periods in time during which dual identities were eyed with suspicion and their citizens felt that their identities were incompatible, but they were able to overcome these conflicts. This makes me optimistic that in Germany we will also reach a point when dual identities will be widely accepted and perhaps even seen as an important part of the fabric of the future German society.

Image credit: Germany flag map via Wikimedia Commons / Public Domain – shows all the states of Germany with their individual flags

Radical Tails: Antioxidants Can Prevent Regeneration

Amphibians such as frogs or salamanders have a remarkable ability to regenerate amputated limbs and tails. The regenerative process involves the formation of endogenous pluripotent stem cells, which then expand and differentiate into the tissue types that give rise to the regenerated body part. The complex interplay of the cell types and signals involved in this regenerative response to the injury are not fully known and there is considerable interest in identifying all the necessary steps. The ultimate hope is that by identifying the specific mechanisms of injury response and regeneration, one might be able to activate similar repair processes in humans, who lack the extraordinary regenerative capacity of amphibians.

The recent paper “Amputation-induced reactive oxygen species are required for successful Xenopus tadpole tail regeneration” by Nick Love and colleagues published online in the journal Nature Cell Biology on January 13, 2013 elegantly demonstrates that reactive oxygen species (ROS), also known as oxygen radicals or oxidants, play a critical role in the regeneration of amphibian tails. Using a rather elegant approach, the researchers generated Xenopus tadpoles with a genetically integrated sensor of the oxidant-sensitive protein HyPerYFP that emits fluorescence upon contact with ROS, and is thought to be rather specific for the oxidant H2O2, more commonly known as hydrogen peroxide. This allowed them to study the hydrogen peroxide levels in all cells of the live tadpole, while it was responding to an injury. They found that within 6 hours after the tail amputation, the residual tail tissue was flooded with high levels of the hydrogen peroxide and that as the tail started growing back, the regenerative edge of the growing tail continued to show high levels of this oxidant.

After excluding the possible confounding phenomenon that the increase in ROS was merely a bystander effect of increases in inflammatory cells, the researchers then performed a pivotal set of experiments in which they used anti-oxidants to see if these would impact the tail regeneration. The researchers first utilized pharmacological inhibitors that reduce the production of oxidants as well as the therapeutic antioxidant MCI-186 (its trade-name is Edaravone and is marketed for use in patients in Japan). These pharmacological agents were all very effective in terms of lowering the hydrogen peroxide levels in the regenerating tail, but they also significantly impaired the regeneration itself. In another intriguing set of experiments, the researchers treated the tadpoles with these agents immediately after the injury and then withdrew them after three days, to see if the regeneration would set in after their removal. Interestingly, when the tails were exposed to agents that prevented the generation of the oxidants, the regenerative program remained blocked even when they were removed. On the other hand, the antioxidant scavenger that soaks up oxidants being produced did not permit regeneration while it was present, but regeneration resumed after the antioxidant was removed.

The researchers also performed complementary genetic experiments in which they reduced oxidant revels by suppressing the enzymes that produce oxidants. The results all point to an important conclusion: There is a burst of oxidants that are released after injury and that are necessary to initiate the regenerative program. The exact molecular targets of the oxidant hydrogen peroxide that enable regeneration remain unknown, but some of the data in the paper points to the Wnt protein pathway as a potential oxidant-sensitive regenerative signal in the tadpole tail.

One has to bear in mind that this work was performed in tadpoles and may not be necessarily fully applicable to the human setting, but Wnt is a key regulator of stem cell renewal, differentiation and regeneration in human tissues. This does suggest that there may be some key similarities between the tadpole regeneration pathways and those found in humans. Despite the shared Wnt signals in tadpoles and humans, building a bridge from this work in Xenopus tadpoles to research and therapeutic applications in humans will be quite challenging. After all, the elegance of this study lies in the genetically integrated oxidant sensor that allows live tracking of oxidants as well as the fact that tadpoles can regenerate whole limbs and tails. Current tools do not permit real-time tracking of human oxidant levels in tissues and humans can usually only regenerate very small amounts of tissue, such as superficial skin injury.

Nevertheless, this work is an important milestone in understanding the role of oxidants as promoters of regeneration and it is very likely that at least some similar pro-regenerative role of oxidants may also be present in human tissues. One of the most important take home messages of this work is that we need get rid of the common “oxidants are bad guys and antioxidants are good guys” myth. Oxidants can be harmful in some context, but they can also serve as important regenerative signals. Indiscriminate use of antioxidants can actually impair these important endogenous signals. Instead of consuming large quantities of non-specific antioxidants, we need to use antioxidants in a very targeted, context-specific and perhaps time-limited manner so that they only prevent oxidative damage without affecting beneficial oxidant signaling.


Image credit: Image of a Xenopus hybrid from Figure S1 in Narbonne P, Simpson D, Gurdon J (2011). “Deficient Induction Response in a Xenopus Nucleocytoplasmic Hybrid“. PLOS Biology.

Love, N., Chen, Y., Ishibashi, S., Kritsiligkou, P., Lea, R., Koh, Y., Gallop, J., Dorey, K., & Amaya, E. (2013). Amputation-induced reactive oxygen species are required for successful Xenopus tadpole tail regeneration Nature Cell Biology DOI: 10.1038/ncb2659

Immune Cells Can Remember Past Lives

The generation of induced pluripotent stem cells (iPSCs) is one of the most fascinating discoveries in the history of stem cell biology. John Gurdon and Shinya Yamanaka received the 2012 Nobel Prize for showing that adult cells could be induced to become embryonic-like stem cells (iPSCs). Many stem cell laboratories now routinely convert skin cells or blood cells from an adult patient into iPSCs. The stem cell properties of the generated iPSCs then allow researchers to convert them into a desired cell type, such as heart cells (cardiomyocytes) or brain cells (neurons), which can then be used for cell-based therapies or for the screening of novel drugs. The initial conversion of adult cells to iPSCs is referred to as “reprogramming” and is thought to represent a form of rejuvenation, because the adult cell appears to lose its adult cell identity and reverts to an immature embryonic-like state. However, we know surprisingly little about the specific mechanisms that allow adult cells to become embryonic-like. For example, how does a blood immune cell such as a lymphocyte lose its lymphocyte characteristics during the reprogramming process? Does the lymphocyte that is converted into an immature iPSC state “remember” that it used to be a lymphocyte? If yes, does this memory affect what types of cells the newly generated iPSCs can be converted into, i.e. are iPSCs derived from lymphocytes very different from iPSCs that are derived from skin cells?

There have been a number of recent studies that have tried to address the question of the “memory” in iPSCs, but two recent papers published in the January 3, 2013 issue of the journal Cell Stem Cell provide some of the most compelling proofs of an iPSC “memory” and also show that this “memory” could be used for therapeutic purposes. In the paper “Regeneration of Human Tumor Antigen-Specific T Cells from iPSCs Derived from Mature CD8+ T Cells“, Vizcardo and colleagues studied the reprogramming of T-lymphocytes derived from the tumor of a melanoma patient. Mature T-lymphocytes are immune cells that can recognize specific targets, depending on what antigen they have been exposed to. The tumor infiltrating cells used by Vizcardo and colleagues have been previously shown to recognize the melanoma tumor antigen MART-1. The researchers were able to successfully generate iPSCs from the T-lymphocytes, and they then converted the iPSCs back to T-lymphocytes. What they found was that the newly generated T-lymphocytes expressed a receptor that was specific for the MART tumor antigen. Even though the newly generated T-lymphocytes had not been exposed to the tumor, they had retained their capacity to respond to the melanoma antigen. The most likely explanation for this is that the generated iPSCs “remembered” their previous exposure to the tumor in their past lives as T-lymphocytes before they had been converted to embryonic-like iPSCs and then “reborn” as new T-lymphocytes. The iPSC reprogramming apparently did not wipe out their “memory”.

This finding has important therapeutic implications. One key problem that the immune system faces when fighting a malignant tumor is that the demand for immune cells outpaces their availability. The new study suggests that one can take activated immune cells from a cancer patient, convert them to the iPSC state, differentiate them back into rejuvenated immune cells, expand them and inject them back into the patient. The expanded and rejuvenated immune cells would retain their prior anti-tumor memory, be primed to fight the tumor and thus significantly augment the ability of the immune system to slow down the tumor growth.

The paper by Vizcardo and colleagues did not actually show the rejuvenation and anti-tumor efficacy of the iPSC-derived T-lymphocytes and this needs to be addressed in future studies. However, the paper “Generation of Rejuvenated Antigen-Specific T Cells by Reprogramming to Pluripotency and Redifferentiation” by Nishimura and colleagues in the same issue of Cell Stem Cell, did address the rejuvenation question, albeit in a slightly different context. This group of researchers obtained T-lymphocytes from a patient with HIV, then generated iPSC and re-differentiated the iPSCs back into T-lymphocytes. Similar to what Vizcardo and colleagues had observed, Nishimura and colleagues found that their iPSC derived T-lymphocytes retained an immunological memory against HIV antigens. Importantly, the newly derived T-lymphocytes were highly proliferative and had longer telomeres. The telomeres are chunks of DNA that become shorter as cells age, so the lengthening of telomeres and the high growth rate of the iPSC derived T-lymphocytes were both indicators that the iPSC reprogramming process had made the cells younger while also retaining their “memory” or ability to respond to HIV.

Further studies are now needed to test whether adding the rejuvenated cells back into the body does actually help prevent tumor growth and can treat HIV infections. There is also a need to ensure that the cells are safe and the rejuvenation process itself did not cause any harmful genetic changes. Long telomeres have been associated with the formation of tumors and one has to make sure that the iPSC-derived lymphocytes do not become malignant. These two studies represent an exciting new development in iPSC research. They not only clearly document that iPSCs retain a memory of the original adult cell type they are derived from but they also show that this memory can be put to good use. This is especially true for immune cells, because retaining an immunological memory allows rejuvenated iPSC-derived immune cells to resume the fight against a tumor or a virus.


Image credit: “Surface of HIV infected macrophage” by Sriram Subramaniam at the National Cancer Institute (NCI) via National Institutes of Health Image Bank

Inspired By Snake Venom

When I remember the 80s, I think of Nena’s 99 Luftballons, Duran Duran’s Wild Boys and ….snake venom. Back in those days, I used to be a typical high school science nerd. My science nerdiness interfered with my ability to socialize with non-nerds and it was characterized by an unnecessary desire to read science books and articles that I did not really understand, just so that I could show off with some fancy science terminology. I did not have much of an audience to impress, because my class-mates usually ignored me. My high school biology teacher, Herr Sperr, was the only one who had the patience to listen to me. One of the science books that I purchased was called “Gehirn und Nervensystem” (i.e. “Brain and Nervous System”), published by Spektrum der Wissenschaft, the German publisher of Scientific American. It was a collection of Scientific American articles in the field of neuroscience that had been translated into German. I was thumbing through it, looking for some new neurobiology idea or expression that I could use to impress Herr Sperr. While browsing the book, I came across the article “Der Nervenwachstumsfaktor” (originally published in Scientific American as “The Nerve-Growth Factor” in 1979) by Rita Levi-Montalcini and Pietro Calissano.

My curiosity was piqued by this article, because I did not realize that nerves had “growth factors” and because one of the authors, Rita Levi-Montalcini, had just won the Nobel Prize in the preceding year. I started reading the article and loved it, reading it over and over again. I liked the article so much, that I did not even try to show off about it and kept the newly discovered inspiration to myself. There are many reasons why I loved the article and I will just mention two of them:

1. Scientific discovery is an exciting journey, starting and ending with unanswered questions

Levi-Montalcini and Calissano started off by describing the state of knowledge and the unanswered questions in the field of developmental neurobiology and neuronal differentiation in the 1940s, when Levi-Montalcini was about to launch her career as a scientist. They commented on how the simple yet brilliant idea to test whether tumors could influence the growth of nerves sparked a whole new field of investigation. They narrated a beautiful story of scientific discovery, from postulating a “nerve growth factor” to actually isolating and sequencing it. Despite all the advances that Levi-Montalcini and her colleagues had made, the article ended with a new mystery, that the role of the nerve growth factor may be much bigger than all the researchers suspected. The nerve growth factor was able to act on cells that were not neurons and it was unclear why this was the case. By hinting at these yet to be defined roles, the article made it clear that so much more work was necessary and I felt that an invitation was being extended to the readers to participate in the future discovery.

2. Scientific tools can harbor surprises and important clues

The article mentioned one important coincidence that helped shape the progress of discovering the sequence of the nerve growth factor. To assess whether the putative nerve growth factor contained nucleic acids, Levi-Montalcini and her colleagues exposed the “soup” that was inducing the growth of nerves to snake venom. The rationale was that snake venom (by the way, the German expression “Schlangengift” sounds even more impressive than the English “snake venom”) would degrade nucleic acids and if the growth enhancing properties disappeared, it would mean that the nerve growth inducing factor contained nucleic acids. It turned out that the snake venom unexpectedly magnified the nerve growth enhancing effects, because the venom contained large quantities of the nerve growth factor itself. This unexpected finding made it much easier for the researchers to sequence the nerve growth factor, because the snake venom now provided access to a large source of the nerve growth factor and it resulted in a new mystery: Why would snake venom contain a nerve growth factor?

In the subsequent decades, as I embarked on my own career as a scientist, I often thought about this article that I read back in high school. It inspired me to become a cell biologist and many of the projects in my laboratory today focus on the effects of growth factors on blood vessels and stem cells. The article also made me think about the importance of continuously re-evaluating the tools that we use. Sometimes our tools are not as neutral or straight-forward as we think, and this lesson is just as valid today as it was half a century ago. For example, a recent paper in Cell found that the virus used for reprogramming adult cells into stem cells is not merely a tool that allows entry of the reprogramming factors, as was previously thought. The virus tool can actually activate the stem cell reprogramming itself, reminiscent of how the “snake venom” tool was able to induce nerve growth.

Rita Levi-Montalcini was one of the world’s greatest biologists and passed away on December 30, 2012. In addition to her outstanding scientific work, she was also a shining example of an activist scientist with a conscience, who fought for education and research. I never had the opportunity to meet her in person, but I was inspired by her work and I will always see her as a role model.

Image credit: Cover of the book “Gehirn und Nervensystem” by Spektrum der Wissenschaft