The developmental origins of adult diseases

I weighed 7 pounds, 7 ounces when I was born on Saturday, 9 March 1957, at 7:07 in the morning. I know this because all the 7s were memorable, but mainly because this is what doctors and nurses do: they document everything.

You know this. Everytime we visit a doctor, they write down our weight, our height, our blood pressure, every parameter they can squeeze out of us. I can go online right now and read the doctor’s notes on every medical visit I’ve made in the last 20-some years — every prescription, every measurement, all of my complaints, every recommendation, every vaccination…it’s all there. Doctors are obsessive record keepers. There is so much medical data stored away that I sometimes wonder how anyone can extract useful information from it.

But they have! One attempt that has had significant influence was to correlate birth weight data in infants with their adult history of cardiovascular disease. Surprise, your weight on the day you were born is associated with your blood pressure, 60 years later (in a broad statistical sense, of course — this is a population-level correlation.) This led David Barker to make the specific hypothesis that “poor nutrition, health and development among girls and young women is the origin of high death rates from cardiovascular disease in the next generation.” This idea has since been broadened to form the developmental origin of adult disease hypothesis, that all kinds of medical phenomena have their origns in fetal development, and in the environmental effects that have influenced that development.

Credit where credit is due, the original exploration of the hypothesis was thanks to careful records kept by one midwife, Margaret Burnside, who assisted in the birth of over 15,000 babies in Hertfordshire between 1911 and 1930, and also the records of over 2000 births at Jessop Hospital in Sheffield between 1907 and 1924. They then compared birth records with death certificates in the 1950s-1990s to extract the first hints of associations.*

There’s a huge industry of papers being turned out now that look at correlations between birth weight and adult medical conditions. We’re also seeing more complex connections between disease and growth rate in the first year.

Some of them are very well established associations with low birth weight, like hypertension, coronary artery disease, non-insulin dependent diabetes, stroke, dislipidaemia, elevated clotting factors, and impaired neurodevelopment. Other ‘problems’ have been associated with low birth weight in a small number of studies — there really are amazing numbers of papers where researchers mine the medical data for connections, some of them possibly spurious. So small babies may be more likely to develop issues with chronic lung disease, depression, schizophrenia, and general behavioral problems. They may have reduced uterine and ovarian size and precocious pubarche. They might be more prone to breast and testicular cancer.

Surprisingly, they may also marry later, if at all, be left-handed, and have denser fingerprint whorls. You can find it all in the scientific literature.

If you are thinking that you were a plump, fat baby, so you have nothing to worry about, think again. There are correlations between large birth weight and breast cancer (everything seems to cause breast cancer,) prostate cancer, childhood leukemia, and polycystic ovary disease.

This week in my eco devo course, we talked about this hypothesis, and I also handed out a bunch of papers, a different one for each student (there are so many papers in this field!), and today we’re going to have the students assess the literature. It should be fun! The goal is to get a feel for how strong or how valid the various correlations actually are. We’ve also discussed the Dutch famine data. The Nazis starved much of Holland, including the major cities of Rotterdam, Amsterdam, and Leiden, for 7 months in 1945, until the country was liberated by the Allies.

Wasn’t that nice of Nazis to do a massive experiment on a whole nation of 9 million people for us? They let women in each trimester of their pregnancy subsist on 580 calories/day, and then went away and let us analyze the effects. Maternal malnutrition in the third trimester turns out to be bad for babies, who knew? Anyway, the subtext for this week, as it should be for every week, is that Nazis are bad***.

The bigger message is, of course, that development matters and has lifelong consequences, and good, responsible governments provide adequate nutrition to pregnant women and children.

*All these records were handwritten on pieces of paper! The effort to transcribe everything and extract the information in a computational form must have been daunting.**

**My daughter is currently involved in a research project to use natural language processing to synthesize information stored in modern medical records at UW Madison Department of Medicine. That’s useful for a lot of reasons, including drilling down through years of impenetrable treatment notes.

***I hope that overtly political message doesn’t get me in trouble with the university administration.

Betelgeuse, Betelgeuse, Betelgeuse

Something funny is going on 650 light years away…or should I use the past tense? Something funny was going on 650 years ago. The star Betelgeuse is/was acting up, dimming and then brightening (well, it’s always been flickering a bit, but this was a greater reduction in brightness than usual.) And now some people are saying it’s about to go supernova! There is a real-time deathwatch on YouTube. “LIVE Betelgeuse Supernova Explosion Is Finally HAPPENING NOW!” it says.

That’s a bit much, and I hope no one is staring at a YouTube page hoping to catch the instant when a rare cosmic event happens. You might be waiting a lifetime. Or maybe seeing it in the next few minutes, but not likely.

Here’s a less sensationalistic perspective.

“Our best models indicate that Betelgeuse is in the stage when it’s burning helium to carbon and oxygen in its core,” Morgan MacLeod, a postdoctoral fellow in theoretical astrophysics at Harvard University and lead author of a recent study about Betelgeuse’s Great Dimming, told Space.com. “That means it’s still tens of thousands or maybe a hundred thousand years from exploding, if those models are correct.”

Awww, but it sounds like it will be spectacular when we do get the Giant Space Kablooiee, and not spectacularly dangerous, the best kind of spectacular there is.

“When it happens, the star will become as bright as the full moon, except that it will be concentrated in a single point,” Montargès said. “For maybe two months, it will be so bright that if you shut down all the lights in a city and have no clouds, you would be able to read a book in the light of the supernova. It will be so bright that it will be visible in the daylight, too. There will be another star shining in the sky during the day.”

Fortunately, although close enough to provide such a spectacle, Betelgeuse is too far away from Earth for its explosion to be dangerous to us. Astronomers think that a giant star would have to blow up within 160 light-years from our planet for us to feel the explosion’s effect, according to EarthSky.

Don’t get your hopes up, though. I do wonder if that guy running the live video feed is prepared to keep it going for 10,000 years. How can you be interested in astronomy and not be aware of the scale of the events you’re interested in?

What are all these plastics doing to us?

In my eco devo course, we’ve been looking at increasingly subtle effects. We started out the semester examining obviously devastating agents in the environment — think thalidomide, stuff that outright kills embryos or causes gross distortions of developmental processes. Then we spent a few weeks looking at endocrine disruptors, agents that perturb developmental signaling and produce embryos with, for example, fertility problems or changes in sexual differentiation. There are a lot of ways chemistry can screw you up short of wrecking external morphology!

This past week we also looked at micro- and nanoplastics, which I personally find have the potential to be a colossal nightmare. The US is producing about 75 million tons of plastic waste each year, and that crap doesn’t go away. You can throw it in a landfill or dump it in the ocean, but it is just physically eroded down into smaller and smaller fragments, allowing it to infiltrate ever deeper into us and our world. Did you know that archaeologists are finding microplastics drifting down into soils 7 meters down, and that they’re finding them in thousand year old sites? We are filling the world with these novel stable polymers, and we have a poor idea of what they’re doing to us.

So we read a paper by Pederson et al. (2020) about the effect of nanoplastics on zebrafish embryos. Like every paper on this kind of topic, it has to tell us about the magnitude of the problem.

Plastic pollution is ubiquitous and an emerging concern in both freshwater and marine environments. Since mass production began in the 1940s, plastic manufacturing has increased rapidly, with 348 million tons produced globally in 2018. Large amounts end up in the oceans, which are now predicted to contain more than five trillion individual pieces of plastic materials (equaling 250,000 tons) in the first 20 m of the water column. Plastics have been identified virtually everywhere: from arctic sea ice to ocean sediments. In freshwater systems, plastics have been identified in large quantities in lakes, rivers, and basins, especially in areas near dense human populations. Their ubiquity has allowed for potential human exposure to plastics through the consumption of aquatic organisms and via drinking water, especially due to the inability of drinking water facilities to entirely remove anthropogenic particles sourced through freshwater environments. In 2019, the World Health Organization (WHO) called for a greater assessment of plastics in the environment after 90% of bottled water was found to contain small plastic particles (World Health Organization). In addition, anthropogenic particles, many of which are likely plastic fragments and fibers, have been detected in over 81% of tap water sources, allowing for an average of 5800 particles to be ingested annually per person.

This paper isn’t even talking about familiar microplastics — it’s all about nanoplastics, particles less than 1µm in diameter. Eventually, all plastics will be broken down to that degree, but we give these an additional boost by intentionally synthesizing these for use in toothpastes and cosmetics and cleansers, and we’ve added <1 parts per billion (ppb) to tens of thousands of ppb to freshwater and marine ecosystems. We don’t have a practical way to remove this stuff. Go ahead, take a swig of that water bottled in plastic, you’ll just absorb those exotic polymers, they’ll be circulating in your bloodstream and getting incorporated into your tissues. You’ll hardly notice.

Zebrafish embryos and larvae swimming in a solution of up to 10000 ppb nanoplastics didn’t seem to mind. There was no effect on mortality, no change in growth rate, no apparent deformities at all. Maybe we’ll all be OK after all.

Except…they do visibly accumulate the plastics in their tissues (they used plastics that fluoresce in the UV).

And then they looked at gene expression in various known pathways — metabolic genes, genes involved in nervous system function, the cardiovascular system — and whoa, they’re just shifted all over the place. It’s a sign of how robust development is that the organism was looking so normal to human eyes. We are all loaded with compensatory developmental mechanisms to make our construction more reliable, and it always impresses me how much damage and insult an embryo can take and still emerge fairly recognizable.

Heatmap indicating predicted upregulation or downregulation in subpathways based on z-scores. (Red is upregulated, blue is downregulated)

One disappointment in the paper is that the behavioral assays were fairly crude, but that’s not the investigators’ fault. They’re working with 5-day old larvae, which, while zebrafish are remarkable little sensory processing machines at that age, they’re still kind of stupid, with a limited behavioral repertoire. The authors looked at spontaneous motor activity, and the fish exhibited a dose-dependent increase in burst swimming. They’re twitchier. More hyperactive. Their brains are being randomly modified chemically, and we’re seeing changes that I’d expect to be more apparent with more sensitive assays.

The message I’m trying to get across to the students is that there is a wide range of phenomena that environmental factors are causing, and we don’t know most of them. It took us decades to get corporations to remove lead from our gasoline, despite the obvious ways it was perturbing our growth and behavior. Are plastics going to be the leaded gasoline of the 21st century?

There is a solution: make biodegradable plastics, ones that don’t reduce to dead stable particles, but instead are digestible by organisms and can be metabolized. Progress is being made in that direction!

An attractive solution to mitigate the environmental impact of microplastics is to develop plastics that do not generate persistent microplastics as part of their normal life cycle. Even plastics that are properly collected and recycled generate microplastics as part of the normal wear from everyday use or as a consequence of recycling or washing processes. Thus, to prevent the accumulation of microplastics, new plastic materials must be developed that are completely biodegradable so that any particles generated from these products will quickly degrade in the environment. Biodegradation is the process by which microbes break down polymers into simpler molecules that can be used as a source of carbon to produce biomass. This requires that the polymer contains chemical bonds, most notably in the polymer’s primary backbone structure, that are physically accessible to enzymes that naturally recognize these bonds as substrates, and that the underlying monomer molecules that are released through this enzymatic cleavage can be consumed by microorganisms. In natural environments, this process is typically performed by consortia of microbes, including bacteria and fungi, secreting hydrolytic enzymes, which sever the polymer to release a variety of monomers and oligomers that can then be utilized as a carbon nutrient source by the microbes. Catabolism of these polymer-derived oligomers and monomers leads to the generation of organismal biomass and CO2 via respiration.

Why would we want structural materials that inevitably break down? Well, maybe you don’t, but I think if we whisper “planned obsolescence” into the ears of corporate executives, maybe they’ll force us to accept them.

Pedersen AF, Meyer DN, Petriv A-MV, Soto AL, Shields JN, Akemann C, Baker BB, Tsou W-L,
Zhang Y, Baker TR (2020) Nanoplastics impact the zebrafish (Danio rerio) transcriptome: Associated developmental and neurobehavioral consequences. Environmental Pollution https://doi.org/10.1016/j.envpol.2020.115090.

How’s your sperm count doin’, guys?

Humans are going to go extinct, says the BBC. “Spermageddon!”, says the Daily Mail. Mankind, specifically, faces doom. If it’s not for disappearing Y chromosomes, it’s our plummeting sperm counts. I don’t know, can women do it all alone?

I had to dig through all this garbage last weekend, as I was preparing to spend another week plowing through the endocrine disruption literature, and considering the effects of things like BPA and pesticides on male developmental biology. In particular, the average sperm count has been declining for the past 50 years. The newspapers were all revved up a while ago over this one paper by Levine et al. (2017), “Temporal trends in sperm count: a systematic review and meta-regression analysis”. It really was one paper that triggered it all, despite all the other papers on the subject, because the author was fond of saying things like, “Eventually we may have a problem, and with reproduction in general, and it may be the extinction of the human species.” It is a serious concern, but hyperbole doesn’t help.

Part of the problem was that all of the secondary sources were using the same or similar over-simplified graph from the paper. This one:

(a) Meta-regression model for mean sperm concentration by fertility and geographic groups, adjusted for potential confounders. (b) Meta-regression model for mean total sperm count by fertility and geographic groups, adjusted for potential confounders. Meta-regression model weighted by sperm concentration (SC) SE, adjusted for fertility group, time × fertility group interaction, geographic group, time × geographic group interaction, age, abstinence time, semen collection method reported, counting method reported, having more than one sample per men, indicators for study selection of population and exclusion criteria (some vasectomy candidates, some semen donor candidates, exclusion of men with chronic diseases, exclusion by other reasons not related to fertility, selection by occupation not related to fertility), whether year of collection was estimated, whether arithmetic mean of SC was estimated, whether SE of SC was estimated and indicator variable to denote studies with more than one estimate. Total sperm count (TSC) meta-regression models weighted by TSC SE, adjusted for similar covariates and method used to assess semen volume.

It’s terrible. Here we have a single parameter, sperm count, that can be easily modified by a host of variables: subject age, whether they smoke, time of last ejaculate, disease state, etc., etc., all in many different observations with different protocols, and they’ve crunched it all down to a straight line. I did not believe it. Where are the error bars, for Onan’s sake?

It’s particularly annoying, because when I worked my way back to the original paper, it included this better figure:

(a) Mean sperm concentration by year of sample collection in 244 estimates collected in 1973–2011 and simple linear regression. (b) Mean total sperm count by year of sample collection in 244 estimates collected in 1973–2011 and simple linear regression.

You could make the valid point that this version is more complicated and doesn’t include all the corrections and adjustments made in the first one, but I’d argue that this one is stripped of the biases of the authors’ interpretations. It still makes the point that sperm counts are going down, but now I can see how noisy the data are.

I also went looking for other articles that assessed the phenomenon, too. For instance, here’s a different 2017 meta-analysis by Sengupta et al. that does a better job of visualizing the data. This, for example, is a bubble plot that illustrates the sample size of each of the constituent data sets.

Temporal decline in sperm concentration (×106/ml) from 1965 to 2015, bubble size corresponds to the number of men in the study.

Look at all that variation! You can see that the earlier studies had much smaller sample sizes, and that the studies ballooned in recent years. If you like more conventional statistical analyses, here’s a box & whisker plot.

Box and whisker plot of sperm concentration data of European men of the past 50 years.

That doesn’t lend itself as well to hysterical over-interpretation, of course. That says instead that we should be carefully studying this real problem, rather than freaking out over the imminent extinction of the human species, which isn’t really happening. There’s so much variation in these numbers that we can console ourselves with the fact that even if we personally are functionally sterilized by the chemical bath we’re living in, there are plenty of men still pumping out lots of sperm to step in and fertilize womankind (which is really a terrible perspective on it all.) One paper I read found that rural men were more strongly affected, but that the men of New York Citaay maintained a robust sperm count. That urban men will do the job of maintaining humanity’s numbers is probably not reassuring to readers of the Daily Mail, though.

The point here is not to diminish the reality of the problem — BPA, atrazine, various pesticides, and fracking chemicals are all doing unpleasant things to our masculine (and feminine!) bits, and we should do something about it. We are being poisoned, it’s just not going to drive us to extinction in the near future. Eventually, yes.

Hagai Levine, Niels Jørgensen, Anderson Martino-Andrade, Jaime Mendiola, Dan Weksler-Derri, Irina Mindlis, Rachel Pinotti, Shanna H. Swan (2017) Temporal trends in sperm count: a systematic review and meta-regression analysis. Human Reproduction Update, pp. 1–14, 2017.

P Sengupta, E Borges, Jr, S Dutta, E Krajewska-Kulak (2017) Decline in sperm count in European men during the past 50 years. Human & Experimental Toxicology, https://doi.org/10.1177/09603271177036.

Wilson’s Principles of Teratology

It’s another busy week of EcoDevo, and even though the campus was closed I still had to give a lecture on endocrine disruptors. I started by laying out Wilson’s Principles of Teratology…wait, what? You don’t know them? I guess I’d better explain them to the internet at large.

These principles are a bit like Koch’s Principles, only for teratology — you better know them if you want to figure out the causes of various problems at birth, and you do: about 3% of all human births express a defect serious enough for concern. Here’s the list:

  1. Susceptibility to teratogenesis depends on the genotype of the conceptus and the manner in which this interacts with adverse environmental factors.
  2. Susceptibility to teratogenesis varies with the developmental stage at the time of exposure to an adverse influence. There are critical periods of susceptibility to agents and organ systems affected by these agents.
  3. Teratogenic agents act in specific ways on developing cells and tissues to initiate sequences of abnormal developmental events.
  4. The access of adverse influences to developing tissues depends on the nature of the influence. Several factors affect the ability of a teratogen to contact a developing conceptus, such as the nature of the agent itself, route and degree of maternal exposure, rate of placental transfer and systemic absorption, and composition of the maternal and embryonic/fetal genotypes.
  5. There are four manifestations of deviant development (death, malformation, growth retardation and functional defect).
  6. Manifestations of deviant development increase in frequency and degree as dosage increases from the No Observable Adverse Effect Level (NOAEL) to a dose producing 100% lethality (LD100).

The first two tell you what is tricky about teratology. There are multiple variables that affect the response: genetic variability in the conceptus (and, I would suggest, maternal variations), and also timing is critical. A drug might do terrible things to an embryo at 4 weeks, but at 3 months the fetus shrugs it off.

Ultimately, though, the teratogen is having some specific effect (3) on a developing tissue. We just have to figure out what it is, while keeping in mind that that effect might be hiding in a maze of genetics (1) and time (2).

Another complication is that in us mammals the embryo is sheltered deep inside the mother, who has defense mechanisms. The agent has to somehow get in (4). A complication within a complication: sometimes the teratogenic agent is harmless until Mom chemically modifies it as part of her defense, and instead creates a more potent poison.

#5 is just listing the terrible outcomes of screwing with development.

#6 I do not trust. It’s saying the effect is going to follow a common sense increase with increasing dosage, but even that isn’t always true. There is a phenomenon called the inverted-U response where the effect increases with dosage, then plateaus, and then drops off at high concentrations. We’re dealing with complex regulatory phenomena with multiple molecular actors that may have unpredictable interactions. There are teratogens that do terrible things to embryos at low concentrations, but do nothing at ridiculously high concentrations — as if the high dose triggers effective defense mechanisms that the low dose sidesteps.

I had to review these principles in class yesterday, because although I’d also discussed them earlier in the semester, we are currently dealing with teratogens of monstrous subtlety, these compounds that mimic our own normal developmental signals, the same signals our bodies use to assemble critical organ systems. It’s as if some joker were placing inappropriate traffic signals along a busy highway — most would do no harm, but some may totally confuse travelers who then end up detouring up into the kidneys rather than down the genitals, as they preferred, or they end up crashing into the thyroid.

Unfortunately, in this case the responsible jokers are mainly gigantic megacorporations who are spewing these dangerous signals all over the countryside…and then we get to wait until the people swimming in them try to have children, and then the teratologists get to say “death, malformation, growth retardation and functional defect”.

In case you were wondering, Wilson didn’t come up with his list first — a 19th century scientist named Gabriel Madeleine Camille Dareste did it first. No, not first. Lots of people have been documenting these developmental problems as long as there’s been writing, like on this Chaldean tablet:

When a woman gives birth to an infant:
With the ears of a lion There will be a powerful king
That wants the right ear The days of the king will be prolonged
That wants both ears There will be mourning in the country
Whose ears are both deformed The country will perish and the enemy rejoice
That has no mouth The mistress of the house will die
Whose nostrils are absent The country will be in affliction and the house of the man will be ruined
That has no tongue The house of the man will be ruined
That has no right hand The country will be convulsed by an earthquake
That has no fingers The town will have no births
That has the heart open with no skin The country will suffer from calamities
That has no penis The master of the house shall be enriched by the harvest of his field
Whose anus is closed The country shall suffer from want of nourishment
Whose right foot is absent His house will be ruined and there will be abundance in that of the neighbor
That has no feet The canals of the country will be cut and the house ruined
If a queen gives birth to:
An infant with teeth already cut The days of the king will be prolonged
A son and a daughter at the same time The land will be enlarged
An infant with the face of a lion The king will not have a rival
An infant with 6 toes on both feet The king shall rule the enemies’ country

Nowadays we’re more interested in causes than imagined consequences, I hope.

Endocrine disruptors — you’re soaking in them

A human embryo at the 4th week of development is just a tiny bean with a length measured in millimeters, but at this time all kinds of remarkable features are starting to develop. This week in class I talked about urogenital development, which involves forming an array of incredibly delicate, thin tubes from a structure called the urogenital ridge, a thickening of an embryonic membrane which will eventually form a succession of kidneys, the pronephros, mesonephros, and metanephros, only the last persisting into adulthood. The key feature for the story I was telling, though, is that they formed something called the mesonephric duct, and then the paramesonephric duct which parallels it. Another name for the mesonephric duct is the Wolffian duct, and the paramesonephric duct is called the Müllerian duct (personally, I don’t care for the self-serving names given to critical bits of the developing embryo by 19th century men, but it’s what still persists in the embryo. So it goes.)

Both of these ducts are associated with the bipotential or indifferent gonad. There are no sexual differences in embryos this young.

The sex differences emerge later, in response to differential signals. The Müllerian ducts degenerate in males, while the Wolffian ducts persist. In females, the Müllerian ducts persist, while the Wolffian ducts fade away. The bipotential gonad associates with the remaining duct and differentiates into testes or ovaries.

I’ll refrain from delving deeper into the details. My point is that these minuscule ducts and tissues form very early, and are going to expand to form critical, elaborate structures necessary for human fertility. They’re fragile. You really don’t want to perturb the signals and processes going on in a one month old embryo, especially since you may not see the consequences for 15 or 20 years.

In 1941, pharmaceutical companies started to market a synthetic drug with properties similar to estrogen, called diethylstilbesterol, or DES. It wasn’t patented, so anyone could make and sell it, and they pushed it hard to pregnant women. There was weak evidence that it could help sustain pregnancies in women with low progesterone levels, so sure, let’s market it as “routine prophylaxis in ALL pregnancies.” About 4 million pregnant women took this stuff at the suggestion of their doctors, between 1941 and 1971, when it was finally banned.

Think about that. This was an endocrine disruptor, term that wasn’t invented until the 1990s, but everyone knew then that it would have some kind of effect, since it was a functional analog of estrogen. So they gave it to pregnant women, and by that means delivered a potent hormonal signal to their embryos at a time when they were carefully assembling those delicate little tubes. Worse, they knew that high doses given to mice and hamsters caused mammary, cervical, vaginal, and uterine cancers in adult females, and that adult males developed lung cancers, which ought to have set off all kinds of alarm bells. Any tissue that was sensitive to estrogen could be provoked to turn cancerous with DES.

Just for dessert, it was determined in 1953 that DES did nothing to maintain at risk pregnancies. They continued to prescribe the stuff. Just in case, you know.

For additional profit, they also marketed it as a growth hormone for livestock. That continued until it was eventually banned for that purpose in 1979.

Here’s the structure of this potent little molecule.

A is DES; B is estrogen; C is BPA, the common, heavily used plasticizer that we now know is an endocrine disruptor.

You might be wondering what happened to those 4 million women who took the drug. They were fine! Humans and other primates seem to be more resistant to the carcinogenic effects of DES, and they were taking much lower doses than those poor rodents in testing labs who were given massive doses of the drug.

And what about the millions of boomer babies who were doped with it in utero? Again, mostly fine — this is the thing about endocrine disruptors, they tend not to have the gross teratogenic effects we associate with chemicals that cause significant birth defects, like thalidomide. They’re more subtle. They perturb the balance of internal organ systems, and in this case, cause problems in the physiology of reproductive organs, which may lead to fertility issues or some kinds of cancers. I emphasize may because I know DES-exposed people who have had children and are cancer-free; it’s more a matter of letting their gynecologists know to keep an eye on potential warning signs.

But it can go very wrong.

DES is still used in experimental studies because it’s such an interesting molecule. Regular readers probably know about the importance of Hox genes; these are genes expressed along the body axis in pretty much all animals that defined anterior-posterior structures. The same genes also get re-expressed to define the proximal-distal axis of the tetrapod limb. They seem to be a handy-dandy molecular tool for establishing tissue identities along a line.

Here’s another instance of Hox genes defining position on an organ: they’re re-expressed in the Müllerian ducts, which become the fallopian tubes of adult women.

Hoxa9 is expressed throughout the oviduct, Hoxa13 in only the cervix, and Hoxa11 and Hoxa10 in between, forming a kind of positional coding system. This is really neat! I like finding examples of molecular recycling in the evolution of developing systems.

What isn’t so neat is that DES downregulates Hoxa10 by inhibiting an important signaling molecule, Wnt7a, creating coding ambiguities in the structure of that delicate little tube. That leads to poor cell specification and disorganized tissue, erasing what should be clear, sharp boundaries in the organ, which may then be expressed in dysplasias, increasing the odds of cancer.

As if that weren’t enough, we don’t really know what perturbing these signaling pathways does to other developing organs, like the brain. Also, DES affects methylation/demethylation of the genome, so it may have transgenerational effects — pregnant women who took DES may have messed up their children, but there is some evidence (weak, I think) that it also affects their grandchildren.

But wait! It’s banned, so we shouldn’t have to worry about it anymore! That’s partly true, but look at the diagram of the molecules above. Estrogen and DES share similarities to another molecule, bisphenol A, a ubiquitous plasticizer used to make plastic materials less brittle. BPA is found in your food packaging. It lines the interior of aluminum cans. Any plastic you have that is at all flexible has been treated with plasticizers, like BPA or phthalates. It’s leaching into your food and your general environment, and it does not go away. The US has banned its use in baby bottles and baby formula packaging, but not from all your snack food packages and your phones.

If you’re of a certain age, you might recall those commercials for a dishwashing detergent that announced, “you’re soaking in it!“, as if that meant the stuff must be safe. We ought to be aware that capitalist industries have us all soaking in a gentle bath of toxic chemicals right now, and it’s not safe.

You know, alcohol is not good for children and other growing things

A few weeks ago, I had an absolutely delicious stout at a brew pub in Alexandria. I’m going to have to remember it, because it may have been the last time I let alcohol pass these lips. Why? Because I’m slowly turning into one of those snooty teetotalers who tut-tut over every tiny sin. It started with vegetarianism, now it’s giving up alcohol, where will it end? Refusing caffeine, turning down the enticements of naked women, refusing to dance? The bluenose in me is emerging as I get older. I shall become a withered, juiceless old Puritan with no joy left in me.

It didn’t help that last week I was lecturing on alcohol teratogenesis in my eco devo course, and it was reminding me of what a pernicious, sneaky molecule it is. I’ve known a lot of this stuff for years, but there’s a kind of blindness brought on by familiarity that led me to dismiss many of the problems. You know the phenomenon: “it won’t affect me, I only drink in moderation” and other excuses. Yeah, no. There are known mechanisms for how alcohol affects you, besides the obvious ones of inebriation.

  1. It induces cell death.
  2. It affects neural crest cell migration.
  3. It downregulates sonic hedgehog, essential for midline differentiation.
  4. It downregulates Sox5 and Ngn1, genes responsible for neuron growth and maturation.
  5. It weakens L1-modulated cell adhesion.

I already knew all about those first four — I’ve done experiments in zebrafish like these done in mice.

Take a normal, healthy embryo like the one in A, expose it to alcohol, and stain the brain for cell death with any of a number of indicator dyes, like Nile Blue sulfate in this example B (I’ve used acridine orange, it works the same way). That brain is speckled with dead cells, killed by alcohol. If you do it just right, you can also see selective cell death in neural crest cell populations, so you’re specifically killing cells involved in the formation of the face and the neurons that innervate it. In C, you can see the rescuing effects of superoxide dismutase, a free radical scavenger, and that tells you that one of the mechanisms behind the cell death is the cell-killing consequences of free radicals. I could get a similar reduction in the effects with megadoses of vitamin C, but that doesn’t mean a big glass of orange juice will save you from your whisky bender.

I was routinely generating one-eyed jawless fish, a consequence of the double-whammy of knocking out sonic hedgehog and cell death in the cells that make branchial arches.

You can wave away these results by pointing out those huge concentrations of alcohol we use to get those observable effects, but we only do that because we don’t have the proper sensitivity to detect subtle variations in the faces of mice or fish. So we crank up the dosage to get a big, undeniable effect.

I only just learned about the L1 effects, and that’s a case where we have a sensitive assay for alcohol’s effects. L1 is a cell surface adhesion molecule — it helps appropriate populations of cells stick together in the nervous system. It also facilitates neurite growth. It’s good for happy growing brains.

It also makes for a relatively easy and quantitative assay. Put neuronal progenitors that express L1 in a dish, and they clump together, as they should in normal development. Add a little alcohol to the medium, and they become less sticky, and the clumps disperse.

What’s troubling about this is the dosage. Adhesion is significantly reduced at concentration of 7mM, which is what the human blood alcohol level reaches after a single drink. The fetal brain may not be forming as robustly when Mom does a little social drinking that doesn’t leave her impaired at all, not even a slight buzz.

Maybe you console yourself by telling yourself a little bit does no harm, your liver soaks up most of the damage (and livers are self-repairing!), that it’s only binge drinkers who have to worry about fetal alcohol syndrome, etc., etc., etc. We have lots of excuses handy. Humans are actually surprisingly sensitive to environmental insults, we have mechanisms to compensate, but there’s no denying that we’re modifying our biochemistry and physiology in subtle ways by exposure to simple molecules.

Now maybe you also tell yourself that you’re a grown-up, I’m talking about fetal tissues, and you also don’t intend to get pregnant in the near future or ever. I’m also a great big fully adult person who is definitely not ever going to get pregnant, but development is a life-long process, and we’re all fragile creatures who nonetheless soak up all kinds of interesting and dangerous chemicals during our existence. We know alcohol will kill adult brain cells, but what else does it do? Do you want to be a guinea pig? I think that, as I age, I am becoming increasingly aware of all the bad stuff I did to myself in my heedless youth, and am starting to think that maybe I need to be a little more careful, belatedly.

Oh, you want some reassuring information? Next week we’re discussing endocrine disruptors in my class — DDT, DES, BPA, PCB, etc. — all these wonderful products of plastics and petrochemical technology. You’re soaking in them right now. They never go away. How’s your sperm count looking? Any weird glandular dysplasias? Ethanol looks pretty good compared to chlorinated and brominated biphenyls.

Am I creepy? Kooky? Altogether ooky?

There may be something wrong with me. I just spent a happy hour and twenty minutes watching a video about brown recluse spiders, and my only regret was that we don’t have any Loxosceles living anywhere near me. We don’t have any medically significant venomous spiders in this region — it’s one of my only regrets about living in west central Minnesota.

See? Fascinating. Good bit on horizontal gene transfer of the sphingomyelin toxin, lots of practical advice on brown recluse bites, and the spiders are all gentle and generally kind. It tickles my brain in all the right spots. Is that weird?

And then, the best essay I’ve read this week is all about bats and white-nose syndrome. You too can grieve for all the beautiful animals, and you should find them beautiful, that are succumbing to this terrible epidemic.

If you know where and when to look, you can find bats all over the midwest. We’ve got a bunch nesting over our garage, and we put up a bat house near our deck — we’d be thrilled to have even more.

Bats and spiders, and more generally any invertebrate that has a freaky number of legs or eyes — I’m beginning to wonder if maybe I’ve got some kind of exotic disease…a Halloween infection, or Addams syndrome, or something similarly diagnosable.

Of course, one of they symptoms of this syndrome is that I don’t want to be cured. Give me more.

(By the way, I’m teaching a course in science essay writing in the Fall, and am collecting samples. That bat article is going right into the folder. I might be planning to infect impressionable young students with my disease.)

Today is climate change day in the classroom

As I’ve mentioned before, one of the things I’m doing in my Eco Devo class is to throw more of the burden of learning on the students. It would be too easy for me to just get up and lecture, telling them what they should know, and it is often hard for me to just shut up and let the students talk. I’ve split up the course so that Monday is when I start talking and dominate the classroom, Wednesday I ask the students to answer questions about Monday’s lecture and the book chapter, and on Fridays they’re given a paper to analyze.

This week’s paper is Morphological plasticity of the coral skeleton under CO2-driven seawater acidification by Tambutté and others. The context is that we’ve been talking about cellular physiology and development, and responses to environmental stresses, so I figured a primary research article about the effect of rising CO2 levels would be appropriate.

(Answer: more CO2 is not good for corals. Decreasing pH leads to a cnidarian version of osteoporosis.)

(a) Representative longitudinal sections; (b) transverse sections. pH treatment is indicated in the top left corner of each image. Scale bar, 1 mm.