Catching up

So I felt like I couldn’t put off the sixteen hundred articles twiddling their thumbs and tapping their feet in my RSS reader any longer. This is the first part of the crop that has accumulated since late December (yikes!). Legless axolotls, homing starfish, secretly related proteins, and more!

1. Axolotls are good at regenerating – until you make them grow up.

(Portrait of a pale lab/aquarium variety axolotl by Orizatriz, Wiki Commons.)

It’s probably not exactly obvious from my posting record, but a large part of my PhD work is about regeneration. It’s something we humans are pretty shit at, but many other vertebrates aren’t. Axolotls, these adorably dumb-faced salamanders, can easily regrow their legs. However, lab axolotls are kind of permanent babies. Although they can grow up in the sense that they are able to breed, they normally keep larval characteristics like gills throughout their lives. It’s reasonable to suspect that this influences their regenerative ability – after all, tadpoles lose their ability to regrow limbs the moment they turn into frogs.

It’s possible to make axolotls metamorphose, too, if you treat them with thyroxine (the same hormone that induces metamorphosis in “normal” amphibians). And when they turn into proper adult salamanders, they suddenly become much poorer regenerators. They can still replace a limb – kind of. But they take twice as long as non-metamorphosed axolotls of the same age and size, and they invariably wind up with small, malformed limbs, often missing bones. After amputation, new skin is slower to grow over their wounds, and the cells that gather under the new skin are sluggish to divide. Something about metamorphosis – that isn’t simply age – dramatically changes how they respond to amputation.

Reference: Monaghan JR et al. (2014) Experimentally induced metamorphosis in axolotls reduces regeneration rate and fidelity. Regeneration advance online publication, doi: 10.1002/reg2.8

*

2. Similar cells repair muscles in crustaceans and vertebrates

“Regeneration” can cover a lot of different processes. For example, depending on the creature and the organ you’ve damaged, regenerated body parts can come from totally different kinds of cells. In planarian flatworms, a single kind of stem cell can replace anything else in the body. In the eyes of newts, mature cells of the iris transform into lens cells to replace a missing lens. In our muscles, there are special cells called satellite cells that are held in reserve specifically to make new muscle cells when needed.

This recent study of a little crustacean called Parhyale hawaiensis suggests that muscle regeneration in the fingernail-sized arthropod works in much the same way. Konstantinidis and Averof shot early embryos of Parhyale with DNA encoding a fluorescent marker, which randomly integrated into the genomes of some of the cells it hit. In a few “lucky” individuals, the marker ended up labelling just one cell lineage, and the pair used these animals to figure out which cells made which tissues in a regenerated limb.

It turned out that cells in Parhyale are limited in their potential. Descendants of the ectodermal lineage could make skin and nerves but not muscle, and the mesodermal lineage built muscle but not skin or nerves. Moreover, labelled cells only contributed to regeneration near their original location – animals with their left sides labelled never regrew glowing limbs on the right side. This is starting to sound a lot like vertebrates, but it’s still a very general observation. However, the similarities don’t end there.

Like vertebrate muscles, the muscles of the little crustaceans contain satellite-like cells derived from the mesodermal lineage that sit beside mature muscle cells and express the Pax3/7 gene. When the researchers transplanted some of these cells from animals with the glowy label into leg stumps of non-glowy animals, there were glowing muscle cells in some of the regenerated limbs. So like satellite cells, these cells can turn into muscle during regeneration. There’s little question that muscle cells have a common origin in vertebrates and arthropods like Parhyale, but it’s really cool to see that their mechanisms of regeneration also might.

Reference: Konstantinidis N & Averof M (2014) A common cellular basis for muscle regeneration in arthropods and vertebrates. Science, published online 02/01/2014, doi: 10.1126/science.1243529

*

3. Convergent evolution is a poor explanation of rhodopsins

Proteins can be difficult. I mean, sometimes they do their darnedest to hide their family ties. A protein is a chain of amino acids (on average about 300 of them) often folded into a complex shape. Closely related proteins have obviously similar amino acid sequences. However, more distant relatives can be harder to identify. There are about 20 different kinds of amino acids in proteins, so the number of possible sequences is unimaginably vast. The same function can be carried out by very different sequences, and therefore enough evolution can completely erase sequence similarity.

Protein structures are generally thought to be more conserved than sequences. Like function, structure allows for a huge amount of sequence variation without significantly changing. However, theoretically, it’s possible that two unrelated proteins have similar structures because of their similar functions, not because of common ancestry. Apparently, this has been argued for the two types of rhodopsins – proteins that harvest light in systems as different as a the “solar generator” of a salt-loving microbe and the photoreceptors of our own eyes.

If Type I and Type II rhodopsins are similar despite being unrelated, one would assume that this is because they need to be that way to capture light. There are, after all, astronomical numbers of possible protein structures, and the chances of two protein families accidentally stumbling onto the same one without selection steering are slim to say the least. But, in fact, you can rearrange the structure of a rhodopsin in all kinds of cunning ways without destroying its function. This rather weakens the case for convergent evolution, and suggests that similarity of structure does indicate common ancestry here.

Reference: Mackin KA et al. (2014) An empirical test of convergent evolution in rhodopsins. Molecular Biology and Evolution 31:85-95

*

4. Starfish can see their way back home

(Blue starfish, the beast featured in the paper, in its natural habitat. Richard Ling, Wiki Commons.)

Starfish aren’t widely known as visual creatures, but they do have eyes at the tips of their arms. The eyes are a bit… basic – no lenses, just a hundred or two little units filled with photoreceptors. Garm and Nilsson set out to find out how the starfish used their eyes. They measured or calculated the eyes’ visual fields (five arm-eyes together can see pretty much everywhere around the animal), resolution (very coarse), reaction speed (slow), and their sensitivity to various wavelengths (they are colour-blind, most sensitive to ocean blue).

Then they took some poor starfish and dumped them a little way off the coral reefs they like staying on. The creatures could walk home from short distances (about 2 m or less), but if you take them too far away, they just wander around in random directions. Likewise if you take off their eyes (don’t worry, they regenerate) or do the experiment in the dark. In conclusion: starfish eyes aren’t exactly top-end cameras, but they are definitely useful to the animals. And what would a slow, brainless mopper-up of coral reef rubbish do with eagle eyes anyway?

(The paper states the walking speed of these starfish as about 4-5 cm per minute. I have a feeling this wasn’t the most exciting fieldwork these guys have done…)

Reference: Garm A & Nilsson D-E (2014) Visual navigation in starfish: first evidence for the use of vision and eyes in starfish. Proceedings of the Royal Society B 281:20133011

*

5. What makes wormies settle

OK, Shikuma et al. (2014) one isn’t so much for its own news value, but I hadn’t known that my favourite worms need bacteria to settle until I saw this paper, so I think it deserves a mention. (Besides, it has beautiful pictures of baby Hydroides in it, which I couldn’t resist posting below. They are So. Cute. Yes, I’m weird.)

Shikuma_etal2014-hydroidesBabies

Tubeworms of the serpulid family have swimming larvae which are in many ways like the acorn worm larvae mentioned in my previous post (except cuter). They are tiny, look nothing like an adult worm, have bands of cilia for swimming and feeding, and live in the plankton until they’re ready to metamorphose. When they find a place they like, they settle and turn into adult worms. And apparently, this particular species (Hydroides elegans) not only needs a specific bacterium to like a place, it needs specific proteins produced by that bacterium.

The proteins in question are the components of a nasty device bacteria probably stole from viruses and then used to poke holes in one another. But to Hydroides larvae, they appear to be necessary for metamorphosis. Put healthy bacteria together with worm babies in a dish, and you’ll get happily settled little worms. Do the same with bacteria with damage to the relevant genes, and nothing happens. Use an extract containing the proteins but not the bacteria, and you still get metamorphosing worms. Use too much, though, and they start dying. Everything in moderation…

(Maybe my dismal failure at raising happy young worms years ago could have been remedied with the right bacteria?)

Reference: Shikuma NJ et al. (2014) Marine tubeworm metamorphosis induced by arrays of bacterial phage tail-like structures. Science 343:529-533

*

6. Relative of animals does strange multicellularity with familiar genetics

Although this idea probably hasn’t reached popular perception, animals are surrounded by other multicellular lineages in the tree of life. Sure, most of them are only part-time multicellular, but that’s beside the point. What’s clear is that multicellularity, at least in its simpler forms, is rampant in our extended family. Slime moulds do it, fungi do it, our closest relatives choanoflagellates do it, and our next closest relatives, filastereans and ichthyosporeans also do it.

These latter two groups are really poorly known (the fact that only a taxonomist could like the latter’s name probably doesn’t help), but the situation is getting better with the attention they are receiving as relatives of animals. There are now genome sequences out, and some people are looking at the life cycles of the little creatures to search for clues to our own origins.

Iñaki Ruiz-Trillo recently published a paper describing an ichthyosporean that can form a weird kind of colony with many nuclei in the same membrane starting from a single cell (Suga and Ruiz-Trillo, 2013). Now his team describe a different kind of multicellularity in a filasterean, Capsaspora owczarzaki. Rather than developing from a single cell, this guy does something more akin to the slime mould way: take a load of individual cells and bring them together. (Below: a clump of Capsaspora cells from Sebé-Pedros et al. [2013]. On the right is a regular photograph of the colony. The two-coloured fluorescence on the left indicates that the colony formed by different cells coming together rather than a single cell dividing.)

Sebé-Pedros_etal2013-F4.capsasporaClump

But, interestingly, some of the genetics involved is similar to what animals use, despite the different ways in which the two groups achieve multicellularity. For example, we’ve known since all those genomes came out that the proteins animals use to glue cells together and make them talk to each other are often older than animals. Well, Ruiz-Trillo’s filasterean appears to ramp up the production of some of these when it goes multicellular. It also uses a gene regulation strategy that animals are really big on: it edits the RNA transcribed from many genes in different ways depending on cell type/life stage before it’s translated into protein.

A lot of the details are going to need further investigation, since this was a global RNA-sequencing study with a bird’s-eye view of what genes are doing. It’s still a nice reminder that, like most other innovations in evolutionary history, the multicellularity of animals didn’t spring fully formed out of nowhere.

References:

Suga H & Ruiz-Trillo I (2013) Development of ichthyosporeans sheds light on the origin of metazoan multicellularity. Development 377:284-292

Sebé-Pedros A et al. (2013) Regulated aggregative multicellularity in a close unicellular relative of metazoa. eLife 2:e01287

“Same” function, but the devil is in the details.

Aaaaaand todaaaaay, ladies and, um, other kinds of people…. Hox genes!

Considering that I did my Honours project on them and I think they are made of awesome, I’m kind of shocked by the general lack of them here*. Hmmmmmm. Well, having just found Sambrani et al. (2013), I think today is a good time to do something about that.

Hox genes in general are “what goes where” type regulators of development. In bilaterian animals, they tend to work along the head to tail axis of the embryo. (Cnidarians like sea anemones also have them, but the situation re: main body axis and Hox genes in cnidarians is a leeeetle less clear. And heaven knows what sort of weird things happened with the rest of the animals.)

Hox genes are responsible for one of the peculiarities of the insect body plan. Unlike many other arthropods, insects have leg-free abdomens. On the left below is a poor little lobster with legs or related appendages all the way down (plus a bonus clutch of eggs). (Arnstein Rønning, Wikimedia Commons). To her right is a bland, boring insect abdomen (Hans Hillewaert, Wikimedia Commons).

As I said, Hox genes are responsible for the difference. Three of them are expressed in various segments of the abdomen of a developing insect: Ultrabithorax (Ubx), Abdominal-A and Abdominal-B. I’m going to whip out that amazing fluorescent image of Hox gene expression in a fruit fly embryo from Lemons and McGinnis (2006) because aside from being cool as hell, it also happens to be a good illustration:

(The embryo is folded back on itself, so the Abd-B-expressing tail end is right next to the Hox gene-free head)

In insects, all three can turn off the expression of the leg “master” gene distal-less (dll). However, they turn out to do so through two different mechanisms. Ubx and Abd-A proteins have long been known to team up with the distantly related Extradenticle (Exd) and Homothorax (Hth). With their partners, the Hoxes can sit on a regulatory region belonging to the dll gene and prevent its activation.

Sambrani et al. were curious whether Abd-B works in the same way. Sure enough, Abd-B also represses dll wherever it shows up. However, when it comes to interacting with Exd and Hth, differences start to emerge. For starters, those two aren’t even present in the rear end of the abdomen, where Abd-B does its business. When the researchers took the regulatory region of dll and threw various combinations of proteins at it, they found that (1) Abd-B is perfectly capable of binding the DNA on its own, (2) Exd, Hth or engrailed (another Hox cofactor) didn’t improve this ability at all, (3) Hth alone or in combination with the others actually inhibited the binding of Abd-B to the dll regulatory sequence.

Interestingly, dll repression in the anterior and posterior abdominal segments requires the exact same bits of regulatory DNA even though different proteins are involved. It looks like in the posterior segments, Abd-B actually takes over an “Exd” binding site – maybe that’s how it can do the job without getting Exd itself involved.

Furthermore, while the DNA-binding ability of Abd-B is crucial to its ability to kill dll expression, the same is not the case for Ubx. The authors speculate that cooperation with Exd and Hth kind of exempts Ubx from having to bind the regulatory sequences itself, while Abd-B, being on its own, can’t afford to slack off like that. The paper illustrates the idea with such a deliciously ugly pair of drawings that I feel compelled to post it:

(I know they’re going for colour-matching with the fluorescent images, but unfortunately glowy greens and reds that look good on a black background kind of just hurt my eyes on white.)

I don’t really have a point to make here. (There doesn’t always have to be a point, right?) There’s absolutely nothing surprising about the fact that different Hox genes evolved the same overall function in different ways –  after all, they existed as separate entities long before insects lost their buttward legs. I just think Hox genes are cool, and this was an interesting look into the nuts and bolts of how they work. And that’s that.

Cheerio!

***

*Well, aside from this one I’ve written three posts about them and a couple more where they are mentioned. That’s maybe not that bad considering how many different things I’m interested in.

***

References:

Lemons D and McGinnis W (2006) Genomic evolution of Hox gene clusters. Science 313:1918-1922

Sambrani N et al. (2013) Distinct molecular strategies for Hox-mediated limb suppression in Drosophila: From cooperativity to dispensability/antagonism in TALE partnership. PLoS Genetics 9:e1003307.

The importance of minding your own business

One of the defining characteristics of life is responding to stuff that the environment throws at it. At the level of cells, such responses are often accomplished by what we call signalling pathways. These are chains of interacting proteins that detect a stimulus (chemicals, voltage differences, pressure, light, etc.) on one end, and affect gene regulation or modify the activity of cellular components on the other end. One of the most common way of passing a message from one protein to another is phosphorylation – an enzyme called a kinase attaches phosphate groups to another protein, changing its behaviour. Kinases that phosphorylate proteins are unsurprisingly called protein kinases. (Their families are named after their favourite amino acid to phosphorylate, so we have tyrosine kinases, histidine kinases, etc.)

There are shitloads of protein kinasess. Legend has it that the acronym JAK, which officially refers to the “two-faced” Janus kinases, originally stood for “Just Another Kinase”. (I guess “Just Another Damned Kinase” didn’t abbreviate so well.) Every cell encounters many different stimuli, each of which may require a different response, and a diversity of signalling pathways can provide a more sophisticated ability to handle all conceivable circumstances. And sometimes, it’s best if such pathways keep to themselves.

Capra et al. (2012) investigate a curious property of a simple signalling pathway in bacteria. This pathway reacts to a shortage of phosphate, and consists only of the histidine kinase PhoR, and the regulatory protein it phosphorylates (PhoB). (Presumably there is still enough phosphate for the enzyme to work when the reaction kicks in…) The PhoR-PhoB pathway is found in all sorts of bacteria. In each major group, the handful of amino acids that determine the specificity of the interaction are strongly conserved. However, these “specificity residues” sometimes differ markedly between groups. Their conservation within groups suggests that changing them has dire consequences. So how and, most importantly, why were they changed anyway?

The study focused on three groups of bacteria: the alpha, beta and gamma classes of proteobacteria, which include familiar bugs like E. coli. In fact, E. coli (a gamma-proteobacterium) was one of the two main experimental species, the other one being the alpha-proteobacterium Caulobacter crescentus. The alpha bugs have an odd set of PhoR specificity residues compared to other proteobacteria, and the researchers hypothesised that this isn’t accidental. Instead, they thought, it might prevent PhoR from meddling with another signalling pathway that gamma-proteobacteria like E. coli lack.

The differences certainly aren’t without consequence. PhoR from E. coli can barely phosphorylate PhoB from alpha-proteobacteria, while it works quite happily on the same protein from other gamma-proteobacteria. It also does reasonably well on PhoB from the beta class, in accordance with the greater similarity of their specificity residues. C. crescentus PhoR only really works on PhoB from its own class.

How about that hypothesised other pathway? Well, when E. coli and C. crescentus PhoR are tested on the regulatory proteins from all similar pathways in C. crescentus, one particular molecule stands out. NtrX is the member of a pathway that has been duplicated in alpha- but not gamma-proteobacteria – and E. coli PhoR phosphorylates it! Is this duplication the reason why PhoR took a strange direction in this class of bacteria?

Multiple lines of evidence indicate that the researchers’ hunch was right. Replacing just one of the three altered specificity residues in C. crescentus PhoR to match the sequence in the other classes causes it to start interacting with NtrX at the expense of its normal function. C. crescentus with such “gamma-like” PhoR grows just as lousily in a  phosphate-poor environment as C. crescentus with no PhoR at all, but only if NtrX is also present – delete the ntrx gene from the genome, and the disadvantage almost completely disappears. (NtrX isn’t disposable, though – under normal circumstances, it’s NtrX-deficient bacteria who perform badly.) A gamma-like PhoR can still interact normally with its correct target*, but it’ll simply ignore poor PhoB when NtrX is also around.

[*Which suggests to me that more than those few amino acids are involved in the PhoR-PhoB interaction, since a C. crescentus PhoR with specificity residues completely identical to those of E. coli still phosphorylates C. crescentus PhoB much better than PhoR from E. coli. However, those three do seem to be the main culprits in the NtrX mix-up.]

(In an interesting twist, it turns out that beta-proteobacteria also possess the NtrX pathway, but they tweaked NtrX instead of PhoR. The result is the same – each protein minds its own business, peace and prosperity and mad procreation ensue.)

The authors hypothesise that the Ntr pathways must have duplicated and diverged at a time when phosphate limitation didn’t come up often, given how much of a nuisance NtrX becomes to old-fashioned PhoR when phosphate is scarce. When phosphate did become a problem, the bugs were stuck with an already established NtrX pathway that they couldn’t just boot out of their genomes. Under those circumstances, any mutation getting NtrX out of PhoR’s way would have been the definition of beneficial.

Avoiding crosstalk seems to be a general feature of this kind of pathway: when you compare the specificity residues of all the signalling kinases and kinase targets from the same kind of bacterium, it’s as though they’re all doing their darnedest to be as different as possible. Capra et al. note that signalling pathways relying on a small set of amino acids to ensure specificity are very common in all life forms. They also often proliferate by gene duplication, which would make the crosstalk-avoidance issue a huge force in protein evolution. Good thing that so few mutations are needed, then – where would the complexity of the living world be if duplicated pathways all died, stuck between being redundant and screwing the organism?

***

Reference:

Capra EJ et al. (2012) Adaptive mutations that prevent crosstalk enable the expansion of paralogous signaling protein families. Cell 150:222-232