Brains & Bourbon Ep.2: neuronal plasticity, religion, and green Chartreuse

A new week, means a new episode of Brains and Bourbon! Our guest this week is George Vidal, a 4th year graduate student in Carla Shatz's lab here at Stanford, who talks to us about neuronal plasticity, and the intersection between science and religion, and shares with us his favorite cocktail -- green Chartreuse.

This week, we have two versions: one shorter, one longer (including additional and expanded sections).

The Shorter Version

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The Longer Version

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You can also stream or download the shorter version of the episode here: Brains & Bourbon Ep2 Plasticity/Chartreuse/George Vidal SHORT or the longer version of the show here: Brains & Bourbon Ep2 Plasticity/Chartreuse/George Vidal FULL You can subscribe to "Brains and Bourbon," and all of the Neuwrite West podcasts, by searching for "Neuwritewest" at the iTunes store and subscribing to our channel.
Thanks for listening! Erica Seigneur Forrest Collman Nick Weiler

Vampire Worms

What comes to mind when I say the word "worm"? If you're not acquainted with invertebrate neurobiology, it's probably that squiggly, segmented creature with five hearts that you accidentally cut in half with a spade once while digging around in your garden. If you're familiar with the nematode Caenorhabditis elegans, you may instead think of a tiny roundworm with exactly 959 somatic cells that develop in the same way every time in every worm.

Nope. What you should really be thinking is 'fangs.' Or, more precisely, 'tooth-like denticles,' though unfortunately, 'worms with tooth-like denticles' doesn't quite conjure up the same imagery as 'worms with fangs.'

To be fair, not every worm has a fang. But we can take advantage of the similarity between nematodes with and without fangs to learn how variations in their neural circuitry may contribute to differences in their feeding behavior.

C. elegans is a bacterivore. It crawls around in soil, foraging for bacteria and gulping them down into its bicameral pharynx. When the bacteria get to the posterior chamber, they run into a grinder, a hard structure that breaks them down mechanically, like the stones in the gizzards of chickens and herbivorous saurians.

Pristionchus pacificus, a predatory cousin of C. elegans, instead develops one or two teeth akin to the fangs of a snake. It can then use its dorsal tooth to puncture another worm and suck out its viscera. To help digest its prey in the absence of a grinder, P. pacificus plays host to a set of gut bacteria that do the work for it.

Now here’s the fun stuff: in the first 20-30 seconds of the video below, you can see the back-and-forth pumping motion as the pharynx moves food down the gut. In the last 30 seconds, when the C. elegans stops moving, the tooth and movement of the mouth muscles are more clear. Warning: graphic nematode-on-nematode violence.

If I were a nematode, a cannibalistic vampire worm would certainly be the stuff of nightmares. Luckily, I am 1600 times taller than P. pacificus is long.

Since these two types of feeding behaviors require different sets of muscle movements, it stands to reason that the neurons of the pharynx might be wired somewhat differently. Daniel Bumbarger, a postdoctoral fellow in the Sommer lab at the Max Planck Institute in Tübingen, borrowed from Google and graph theory to compare the anatomical circuitry of these two species. He took advantage of the fact that the pharyngeal nervous system in both of these species is almost a closed system, connected to the rest of the worm’s sensory and motor functions by only a single neuron. In a marvelous stroke of luck that probably made up for the tedious work of creating a 3000-slice EM reconstruction of the P. pacificus pharynx, it turned out that the identities and locations of the 20 C. elegans pharyngeal neurons are preserved almost perfectly in P. pacificus – it’s only their connectivity that differs (although we can argue about how connectivity may define identity, neuron identity in C. elegans is defined by lineage as well as function). The muscle cells pm1 and pm3, which contract rhythmically in predatory feeding as P. pacificus punctures its prey, are more highly innervated in P. pacificus. Conversely, pm7, the cell that normally drives the grinder in C. elegans, exists but receives no neural input at all in P. pacificus.

It's a classical comparative neuroanatomy study, but on a much smaller scale. The qualitative information is interesting, but what else can we do with a map of anatomical connections between neurons? The authors decided that they might be able to tease something more out of this connectivity map by integrating mathematical algorithms from other disciplines. In this case, the field they drew from was that of the search engine – specifically, Google. The PageRank system, named not for the fact that it ranks webpages, but for Google co-founder Larry Page, is meant to rank the importance of any given node in the network, based recursively on the number and weight of other nodes that link to it. To put it simply, if important people think you’re important, then you’re considered more important, which in turn affects the rank of people you think are important.

In comparing the two worm neural networks, the authors found that neurons controlling the behavior of the anterior pharynx (where the tooth is) were more important in P. pacificus, while neurons controlling the posterior pharynx (where the grinder is) were more important in C. elegans. What does this actually mean? Well, it suggests that more information is flowing through that particular part of the circuit, and that the physical behavior of the worm is most dominated by what those particular neurons say.

To look at where these specific interneurons were getting information from, the researchers then used a measure of focused centrality from graph theory (a branch of mathematics that focuses on the links between pairs of objects in a network). This revealed that neurons I1 and I2 receive a lot of indirect input from the motor neuron M4, and send out a whole lot of indirect input to the muscle cell pm4. The authors suggest that this higher proportion of indirect information flow in P. pacificus as opposed to C. elegans may correlate with more complex functions and the ability to switch between different behaviors.

It is a little difficult to understand what some of these methods could actually teach us about the system – do we really need to do a closeness centrality analysis to find that muscle cells that move the tooth receive more inputs in P. pacificus? We could just compare the number of synapses onto each of those cells between the two species. There is still a lot of work to be done here in finding what kinds of analyses might actually yield biologically relevant insights, but once we've identified the best methods in a small, isolated system such as this, we could expand their use to understanding indirect information flow through larger networks of neurons. We might also find something interesting if we take a look at the characteristics of the information being passed to these important-looking neurons. We can integrate this kind of information flow analysis with knowledge of whether the synapses are excitatory or inhibitory, the strength of each synapse, the changes that might arise with learning, and the effect of neuromodulators like dopamine and serotonin (which are especially important in regulating worm feeding behavior and which play large but poorly understood roles in the human alimentary system as well).

I'm hopeful that we will soon see functional data to correlate with the behavior, from ablation studies, optogenetic inhibition, in vivo electrophysiology, or even imaging with voltage indicators. In addition to testing the predictions made about the importance of certain cells, such data could shed light on various unanswered observations, such as why pm4, a muscle in the middle of the pharynx, and the gland cell, whose function is unknown, seem to be so central and important in the network analyses. And maybe we can even solve the mystery of why worms with fangs are so cool.

References

Bumbarger, D. J., Riebesell, M., Rödelsperger, C. & Sommer, R. J. System-wide Rewiring Underlies Behavioral Differences in Predatory and Bacterial-Feeding Nematodes. Cell 152, 109–119 (2013).

Bento, G., Ogawa, A. & Sommer, R. J. Co-option of the hormone-signalling module dafachronic acid-DAF-12 in nematode evolution. Nature 466, 494–497 (2010).

Can neurofeedback go deeper?

Feedback, a word with more than 100 years of history, is one of the most crucial concepts to understanding how the brain works. Feedback occurs both at the cellular level – neurons are wired to feed information forward and back – as well as at a more behavioral level. We understand feedback colloquially as the reactions we get from the people around us that help to shape our behavior, but the brain uses feedback at multiple levels, down to individual cells and circuits. For example, you use visual feedback to correct the position of your hand if you can not find the light switch at first try, or mostly tactile feedback if the room is dark. At a more cognitive level, negative feedback from people around us is discouraging, while positive feedback encourages us to repeat the same behavior. Inside the brain, one can find signals that reflect something like this higher-level behavioral feedback information from the outside world. One famous example is the dopaminergic neurons in midbrain encoding reward and the lack thereof. The brain, a highly dynamic structure fed by these feedback information, changes in order to optimize actions that maximize reward. This continuous learning process provides adaptation to our changing environment.

To get a sense of what might be happening at the neural level, let's go back to the first example of the motor system's ability to control your arm and hand as you flip a light switch. The motor cortex controls the basics of intentionally moving your hand to the correct location, but if you miss the switch for some reason, the cerebellum takes charge of solving the mysterious discrepancy by comparing outgoing motor commands against sensory feedback, which acts as an error signal. This feedback controller continuously compares the desired output ("hit switch") with the actual result ("there's no switch here!"), and makes adjustments until the desired movement is achieved.

Scientists feed back to the disordered nervous system

Increasingly, clinical applications take advantage of the feedback mechanisms that are extensively used by the brain. The most prominent clinical applications so far are brain-computer interfaces (BCI) using implanted electrodes, such as those used to control neuroprosthetics, and EEG neurofeedback (NFB). BCI is a kind of brain computer interaction where a device provides direct communication between brain and computer. This approach is being used for neuroprosthetics that might substitute sensory, motor or cognitive modalities. There are devices, for example, that assist individuals with quadriplegia. These invasive devices decode the neural activity in motor cortex that are received from implanted electrodes, while the patients think about moving their limbs in a certain way and command a robotic limb that executes the movement. While they get trained for the device, individuals learn to use their neural activity to manipulate computer cursors on screen in order to command the device perform simple motor tasks just by thinking about the task and seeing the visual feedback. Thus, visual feedback provides the cues for the accuracy of the aimed movement, just like the real thing.

EEG NFB uses brainwaves recorded from electrodes placed on scalp and conveys them to a computer, which decodes these signals. The signals are then visualized on the screen so that the individual learns to self-regulate the underlying neural signals picked up by the electrodes. Thus, NFB is distinguished from the BCI approach in several aspects. Unlike the implanted BCI electrodes used for neuroprosthetics, NFB is not invasive, relying instead on EEG electrodes placed on the skull. While implanted electrodes can record from multiple neurons in a very defined region, EEG recordings reflect an aggregate activity, recorded from a rather large, indistinct region, resulting in low spatial resolution. Similarly, temporal resolution of implanted electrodes are better than the EEG electrodes. However, the invasiveness makes neuroprosthetics available only to patients whose conditions are not treatable with any other method, while NFB is basically available to everyone.

BCI and NFB are used for different clinical purposes. While BCI has mainly been developed to assist individuals with severe neurological deficits, ongoing NFB research often aims to ameliorate various psychiatric conditions such as addiction, ADHD, depression, autism and anxiety, besides initial successes with Parkinson’s disease, tremor and dystonia. Neuroprosthetics with implanted electrodes can be considered as a last resort for the patients whose lives are severely affected by the disease, and who cannot benefit from any other therapy. In the case of psychiatric disorders, medication and psychotherapy can generally keep the disease under control, or the patients can live a somewhat normal life without intervention. As a result, individuals with these disorders cannot yet benefit from the more advanced, but also more invasive methods used with BCIs.

In recent years, researchers have begun experimenting on invasive methods for the treatment of pychiatric conditions. One example where implanted electrodes have been used to treat psychiatric disorders is deep brain stimulation (DBS) which has been successfully used to treat major depressive disorder in recent years and found to be effective in many conditions that are otherwise incurable . In addition, there are clinical trials ongoing for DBS treatment of obsessive compulsive disorder. DBS requires implantation of deep brain electrodes as in the BCI approach, but does not require any feedback training as the treatment is comprised of continuous stimulation of an affected brain area.

Neurofeedback for DBS patients

Stimulation of specific brain regions seems to be effective against some psychiatric disorders, but these approaches have not yet harnessed the power of feedback that has been so effective in EEG NFB treatments. Here, I would like to argue that we can, and maybe should, take advantage of these invasive electrodes that are implanted for DBS, and perform neurofeedback. This would be an invaluable opportunity for those individuals to focus on how to feel better (less anxious, less sad, more attentive etc). Currently, DBS is thought to work through altering the abnormal activity in affected regions in a way that decreases symptoms. The patient has no control over the stimulation or its influence on the symptoms. I propose that it might be better to have patients learn to self-regulate this activity in real time rather than just applying standard stimulation. In the development of neuroprosthetics, we have become aware of the importance of feedback to the patient to allow self-regulation. Currently, for instance, a paralyzed patient can move a robotic arm by just thinking about it, but because there is no sensory feedback, the simplest movements are at best slow and clumsy, leaving patients with a lot of frustration. Researchers now are working on providing sensory feedback to make the robotic arm closer to a real one. Similarly, neural feedback can be applied to DBS patients to improve the quality of the improvement.

An important note here is that psychiatric problems are inherently more complex than the neurologic ones such as paralysis in terms of the subjective awareness of the existing problem and capacity to imagine what it would be like to be “normal”. During BCI training, a paralyzed patient needs to imagine herself moving a limb, which is a very easy task. Additionally, research shows that the brain activity is similar when a particular movement is performed vs. imagined. However, let's say for a patient experiencing depression, imagining to be happy or motivated might not be as intuitive. Similarly for an individual with ADHD, being more focused and attentive is a difficult state to attain. Revealing neural correlates of these deficits have thus been crucial in providing the connection between the brain activity and what the patient experiences. NFB shows that patients can self-regulate the brain activity if it is presented to them in the form of a sensory stimulus.

There are two steps that need to be taken to make invasive NFB possible: First is to explore how the neural activity recorded by DBS electrodes correlates with the symptoms of the condition of interest. There are several human imaging and animal electrophysiology studies that begin to address this, but we need to be able to precisely interpret the correlation of symptoms to neural activity in real time. The success of EEG NFB in various psychiatric conditions suggests that this is an achievable goal. Secondly, the electrode placement for invasive NFB might be slightly different than DBS, which would require more research. Additionally, there are technical challenges, such as the need to both stimulate and record from the same electrodes, that will eventually need to be addressed.

Incorporating neurofeedback to DBS not only has the potential to improve the quality of the treatment, but it might give rise to longer term, persistent effects. If the patients learn to control the abnormal activity, or even just get a feedback on how it is changing with stimulation, they might eventually learn to suppress or change the symptoms by themselves. This obviously opens a whole new avenue of research about how plasticity and learning may come about with neurofeedback.

To conclude, it is crucial to benefit from the existing technologies as much as possible. neuroprosthetics, DBS and NFB are rapidly developing, promising techniques and I believe that invasive NFB is worth exploring with the hope of open a new avenue for long incurable psychiatric conditions.

Inflammatory comments on ageing

None of us is getting any younger. In fact, most of us can expect to live longer than ever. The world’s oldest man died just last week aged 116 and, as global life expectancy continues to increase, we’re going to have to tackle age-related frailty and functional fragility to avoid unbearable healthcare costs and provide quality of life for a growing population of centenarians. With rates of dementia as high as 60% in the over 85s¹, neurodegeneration is a big issue; but treating age-related diseases in isolation may not be the most effective solution. Scientists now think of ageing as an intricate process during which tissues and cells change the way they communicate and interact, resulting in a gradual loss of function. Not exactly a simple cause-effect pairing that can be hit with a youth pill.  

So how can we even start to pick apart the elaborate tapestry of ageing? What exactly happens when a cell changes the way it communicates? To perform effectively, neurons need to pick up signals from the outside world. Indeed all cells, tissues and organs function by responding to external signals, thus allowing an organism to respond and adapt to changes in environment. It is difficult to show in a static diagram the dynamic nature of these interactions. But I tried anyway. ↓

 

The majority of signals are chemical compounds, floating around outside a cell, waiting to contact their receptor protein. Once in range, the signal and the receptor are drawn closer and dock together. In response to the signal, the receptor twists and changes; not only on the surface, but also through the cell membrane and into the cytoplasm – the inner world of the cell. The shifting and gyrating receptor can hit other proteins, slapping on new chemical groups, removing others and sending these messengers on through the internal cellular space on a collision course with target proteins of their own. This molecular line of dominoes almost always ends at the cell’s control centre – the nucleus. It is here that messages are converted into changes in gene expression, protein production and, in turn, cellular behaviour. The proteins that directly interact with DNA to bring about such changes are called transcription factors.

Different signal molecules and receptors trigger distinct domino effects in the cell, which scientists refer to as signalling pathways. Though different cells are tuned to pick up different signals, many of the intracellular proteins used to transmit the message have been the same for millions of years. Throughout evolution, cells have taken the Heath Robinson* approach to signalling. Rather than inventing new proteins every time a new function had to be performed, cells used the tools already at their disposal. As a result, the same proteins can be found in different types of cells, across species and even across kingdoms. You share molecular signalling pathways with almost everything – from bears, to bananas to bacteria.

*Americans see Rube Goldberg

One versatile and highly abundant transcription factor, and the focus of a new study in Nature², is NFkB (nuclear factor kappa B). This molecule is a hallmark of inflammation and once activated, perpetuates inflammatory responses. However, many different cells use this molecule to transmit all sorts of messages and, as a consequence, its role in a particular signalling pathway is often difficult to pin down. In their new study, Zhang and colleagues used several tricks to see just what NFkB is up to in the brain and how its activity relates to ageing. By linking it to a fluorescent molecule, the authors were able to visualise NFkB activity in the brain. The more active the transcription factor, the brighter the signal. In this case, brains got brighter with age supporting the idea that inflammation in the brain increases as we get older. While not always bad, inflammation in the brain is a sign of altered equilibrium and contributes to several neurodegenerative diseases^3-6. By injecting a non-functional version of the protein, the authors were able to interrupt NFkB activation in the brains of old mice. Interestingly, reducing NFkB activity not only reduced inflammation, but also improved cognitive function (measured by maze navigation), decreased muscle degeneration (measured by grip tests) and even extended life span.

So why is inflammation increased in older brains? It turns out that in mouse middle age, the majority of NFkB activity comes from the microglia –the brain’s resident immune cells. A change in the calibration of these cells over time increases their sensitivity in old age. Instead of waiting for an infection, the microglia are active all the time and mount an inappropriate inflammatory response. This is when NFkB first gets activated and it makes the microglia produce other inflammatory molecules, which perpetuate the rumour that there is some kind of infection and that inflammation is necessary. Neurons take up these signals from the microglia and NFkB inside these cells also becomes active. This form of molecular baton passing between the immune microglia and the neurons of the hypothalamus is possible because they share the same signalling molecules, and is likely to bounce back and forth in an escalating feedback loop. In this way, an inappropriate response by one cell type (the microglia) can result in another cell type getting the wrong idea. As the signals feedback, the neurons change their behaviour. One consequence is that they gradually reduce production of an important molecule called gonadotropin-releasing hormone (GnRH). The decline of GnRH is responsible (along with other things) for the loss of reproductive function with age; but this new study showed that GnRH injection can also improve neural regeneration in old mice and strengthen cognitive and muscle function. In addition, by specifically removing a gene controlling NFkB in microglia, the authors were able to restrain inflammatory microglia in middle-aged mice. This nipped inflammation in the bud, prevented the involvement of neurons in the inflammatory process, reduced GnRH decline and extended lifespan. Hurray! A new miracle drug, right? Well, maybe not quite yet.

Unfortunately, we’re still a long way from using NFkB inhibition or GnRH replacement to extend human lifespan. The fact that many cells are using the same molecules to transmit messages makes targeting specific molecules very difficult. During evolution, different cell types adopted different roles as they became more specialised. However, having come from the same ancestral cell, they use the same old machinery to perform new and different tasks. Now, when a signal from one type of cell gets picked up by another, there is a level of cross-communication that can have unexpected and sometimes undesirable outcomes. If we try and remove a protein that is misbehaving in one cell type, we risk interrupting its routine function in another. The complex crosstalk between biological molecules is a caveat worth remembering whenever a new protein or gene hits the headlines as the root of all ageing or instigator of disease. These genes and proteins did not evolve to make us old and sick. They evolved to perform a task that is almost certainly essential for survival. It is the misuse of a protein or its appearance in the wrong cell at the wrong time that causes problems. A “disease-causing” protein is, in all probability, performing an essential role elsewhere in the body so cannot just be eliminated.

It will take incredibly sophisticated techniques to iron out exactly how best to recalibrate cellular function following an undesirable change like the increase in NFkB activity in older neurons. Unravelling the mysteries of ageing is not an easy task and is likely to involve the integration of many different scientific disciplines. By identifying how different cells and organs communicate, we hope to get a better idea of when and how signalling wires get crossed and so identify the right target to prevent disease and maybe even slow the ageing process.

1)         Alzheimer’s Association (2013). 2012 Alzheimer’s Disease Facts and Figures. http://www.alz.org/downloads/facts_figures_2012.pdf OPEN ACCESS!

2)         Zhang et al. (2013). Hypothalamic programming of systemic ageing involving IKK-β, NF-κB and GnRH. Nature 497: 211-216.

3)         Wright et al. (2013) Neuroinflammation and Neuronal Loss Precede Aβ Plaque Deposition in the hAPP-J20 Mouse Model of Alzheimer’s Disease. PLoS ONE 8(4): e59586. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0059586 OPEN ACCESS!

4)             Ugur Tufekci et al. (2012). Chapter Four – Inflammation in Parkinson's Disease. Advances in Protein Chemistry and Structural Biology Vol. 88 pp. 69–132. http://dx.doi.org/10.1016/B978-0-12-398314-5.00004-0

5)         Holmes (2013). Systemic inflammation and Alzheimer's disease. Neuropathology and Applied Neurobiology 39 (1): 1365-2990 http://dx.doi.org/10.1111/j.1365-2990.2012.01307.x

6)             Hauser and Oksenberg (2006). The Neurobiology of Multiple Sclerosis: Genes, Inflammation, and Neurodegeneration. Neuron 52 (1): 61–76 http://dx.doi.org/10.1016/j.neuron.2006.09.011

Linky and the Brain: Science and Art edition

Hi folks. Please excuse the brevity of this post - the adrenaline/endorphine one-two punch of tonight's softball game victory has worn off, and I'm ready to catch up on my sleep. But, before I pop off, here's some links I've been collecting over the past couple of weeks to share with you all.

Science? Art? Yes.

Via Laughing Squid, visualizations of animal sounds. The work of software designer and artist Mark Fischer, these eerily evocative illustrations are generated by passing animal calls through a wavelet analysis. Part work of art, part complex data visualization, I thoroughly enjoyed viewing these images. My particular favorite is the Blue-Crowned Manakin. Visit Aguasonic Acoustics for additional interpretations.

Again, courtesy of Laughing Squid, entries in Princeton University's 2013 Art of Science competition. 44 images selected from 170 submissions from 24 Princeton University departments, images were produced as part of scientific research. I vaguely remember Stanford University hosting such a competition; can someone more knowledgeable comment in some information? Shout out to my c.elegans homies (Hi, Sammy): I loved this picture of a swarm of worms on an agar plate. Also, here's a picture entitled "Worm Water Slide". Best title, or best title ever?

Over at the Smithsonian, a brief news report on research into how the natural frequency at which an individuals skull vibrates (this varies from person to person, from 35 to 65 times per second), can (moderately) predict what type of music the person does not enjoy listening to. The Unique Vibrations of Your Skull Affect How Your Hear Music.

Science, in the trenches.

I enjoyed reading this blog post, on the site Small Pond Science, that talks about the familiar situation of having a set of data that perfectly answers a hypothesis... that you didn't initially set out to test. Pretending you planned to test that hypothesis the whole time.

Slight/moderate facepalm moment: Cornell researcher and blogger Zhana Vrangalova writes up a post-game analysis of the media coverage of her recent paper, "Birds of a feather? Not when it comes to sexual permissiveness".

For pure delight, nothing can beat this, via The Atlantic: the second author of the recent Nature paper describing the collection of fluid isolated in the Earth's crust in the Precambrian era, "took one for the team" and drank some of their ~1 billion year old water. It doesn't taste very good.

Public and Private Sectors

A write up of a recent NSF-funded workshop on priorities for the Brain Initiative. (via nsf.gov)

Mozilla launches a new online resource for scientists, Science Lab, with the initial vision of encouraging researchers and members of the open web community to "share ideas, tools and best practices for using next-generation web solutions to solve real problems in science, and explore ways to make research faster, more agile and collaborative". (Press release, via The Next Web)

And that's all I've got. Until next time!

Ask Stanford Medicine: Dr. Josef Parvizi discusses drug-resistant epilepsy

As part of the Stanford School of Medicine's Ask Stanford Med series, Stanford neurologist Dr. Joseph Parvizi will be responding to questions on the subject of drug resistant epilepsy, and a surgical procedure to help treat the devastating condition. Dr. Parvizi, MD, PhD, is the director of the Stanford Program for Drug-Resistant Epilepsies. Dr. Parvizi will be taking questions both via twitter, or via the School of Medicine's Scope blog. Questions will be collected until Wednesday (June 19) at 5 PM Pacific Time. Both neuroscientists and members of the general public are encouraged to ask a question, and to join the conversation at the Scope blog.

To find out more about the event, or to ask a question or to join the conversation, visit the Scope blog.

To ask a question via Twitter, be sure to include #AskSUMed in your tweet.

The Cruel Irony of Saxitoxin

It was summer of 1997 at Cape Blanc on the western coast of Africa and some two hundred monk seals, nearly two-thirds of the population in the region, lay dead (1, 2). The cause of death was saxitoxin, a highly potent neurotoxin, whose existence is one of Nature’s cruelest ironies. Saxitoxin is a chemical compound, an alkaloid to be precise, that binds to sodium and potassium channels in the brain and muscle (3). This binding obstructs the flow of sodium and potassium ions across the cell membrane, impedes the ability of brain cells and muscle cells to send electrical signals, and can cause death by respiratory failure (3). Certain species of marine invertebrates and fish can accumulate saxitoxin to high levels and then pass it on to their predators. A possible mechanism for this is a difference in the sodium and potassium channels between predators and prey. Interestingly, some species of clam have a mutation in a single amino acid of their sodium channels that makes them resistant to saxitoxin (4), to the detriment of many animals up the food chain, including monk seals and humans. Ecologists estimate that during the fateful summer of 1997, the Cape Blanc monk seals could have been ingesting nearly lethal doses of saxitoxin every day from the fish they consumed (2). And every year, seafood contaminated with saxitoxin kills nearly 300 people and sickens another 1700 (3).

Where does saxitoxin come from? The villains in this story are marine microorganims called dinoflagellates. These microorganisms belong to the kingdom Protista, the dusty attic of taxonomy where biologists place eukaryotic life forms that they really don’t understand. Dinoflagellates are about as closely related to humans as to plants and fungi and incredibly diverse within themselves. About 4000 species have been described, 90% of which live in the world’s oceans and survive either by preying on other single-celled organisms or by doing photosynthesis (5). Most of these species do not produce saxitoxin but do have other curious characteristics, like genomes up to 100 times larger than the human genome, permanently condensed chromosomes in all stages of the life cycle (instead of just during cell division, like in other eukaryotes), and chloroplasts acquired independently of land plants (6). Three genera of dinoflagellates that do produce saxitoxin, Gymnodinium, Pyrodinium, and Alexandrium, get consumed by filter-feeding fish and marine invertebrates that effectively concentrate the poison in their viscera (2, 3). The monk seals at Cape Blanc had the misfortune to be near a bloom of Gymnodinium catenatum and possibly Pyrodinium bahamense (2). Such blooms occur seasonally and may account for other unexplained mass mortalities of marine wildlife (2), though, luckily for humans, many countries have monitoring programs for edible mussels, clams, gastropods, crustaceans, and fish that minimize public health risk (3).

Why do dinoflagellates produce saxitoxin? It is unlikely that they intend to kill marine mammals because the saxitoxin-producing species are photosynthetic (7). Arguably, they would derive no benefit from killing high-level predators in their ecosystems and may even be harmed by an increase in lower-level predators that would result. The large size of dinoflagellate genomes, the difficulty of culturing dinoflagellates in the lab, and their extreme divergence from all organisms with sequenced genomes makes genomic and evolutionary studies of saxitoxin production in dinoflagellates challenging, and information is incomplete (8). The most popular and widely studied hypothesis is that saxitoxin deters the immediate predators of photosynthetic dinoflagellates, such as cocepods and mollusks (3). However, evidence is inconclusive because though saxitoxin production correlates with decreased consumption of dinoflagellates by cocepods, many studies have found no effect of saxitoxin on the survival of mollusks, and other studies found that even dinoflagellate species that do not produce saxitoxin could kill their predators by an as-yet-unidentified mechanism (3). Another hypothesis is that saxitoxin acts as a pheromone and regulates mating and other social behavior in dinoflagellate colonies, though any hypothesis that proposes a non-toxic role for saxitoxin must account for how dinoflagellates that do not produce it perform the same tasks (3). Nonetheless, at present, it is a good bet that dinoflagellates use saxitoxin for their own purposes and that its effect on the brain is a cruel irony.

 Sources

1. The IUCN Red List of Threatened Species. http://www.iucnredlist.org/details/13653/0
2. Reyero M et al (1999). Evidence of Saxitoxin Derivatives as Causative Agents in the 1997 Mass Mortality of Monk Seals in the Cape Blanc Peninsula. Natural Toxins. 7: 311-315. Open access.
3. Cusick KD and GS Sayler (2013). An Overview on the Marine Neurotoxin, Saxitoxin: Genetics, Molecular Targets, Methods of Detection and Ecological Functions. Marine Drugs. 11: 991-1018. Open access.
4. Bricelj VM et al (2005). Sodium channel mutation leading to saxitoxin resistance in clams increases risk of PSP. Nature 434:763-767. Paywall.
5. Dinoflagellates. Smithsonian Museum of Natural History. http://www.mnh.si.edu/highlight/sem/dinoflagellates.html
6. Wisecaver JH and JD Hackett (2013). Dinoflagellate Genome Evolution. Annual review of microbiology. 65: 369-387. Paywall.
7. Algae and Human Affairs edited by CA Lembi and JR Waaland and published in 1989.
8. Hackett JD et al (2013). Evolution of saxitoxin synthesis in cyanobacteria and dinoflagellates. Molecular biology and evolution. 30: 70-78. Open access.