Here are the highlights from the current issue of Development:

Dronc regulates Numb and neuroblast formation

The ability of stem cells to maintain a balance between self-renewal and differentiation is crucial for development and tissue homeostasis. In Drosophila neuroblasts, the tumour suppressor Numb restricts proliferation and self-renewal in differentiating daughter cells but how its activity is regulated is unclear. Here, Bingwei Lu and colleagues reveal a novel mechanism for the control of Numb activity (see p. 2185). They show that phosphorylation of Numb at conserved sites modulates its tumour suppressor activity and that the antagonist actions of Polo kinase and protein phosphatase 2A control Numb phosphorylation. Expression of Numb-TS4D (a phosphomimetic form of Numb) abolishes Numb activity, they report, and leads to the formation of ectopic neuroblasts. They also identify the Dronc caspase as a Numb binding partner and show that Dronc overexpression suppresses the effects of Numb-TS4D in an apoptosis-independent and possibly non-catalytic manner. By contrast, reduction of Dronc activity enhances phospho-Numb-induced ectopic neuroblast formation. Together, these results provide new insights into neural stem cell homeostasis in Drosophila.

Stem cell development goes live

Stem cells are maintained by signals from their local microenvironment but it has been hard to study exactly how stem cell behaviour is controlled. Now, Lucy Morris and Allan Spradling describe a culture method for live imaging Drosophila ovarian development within the germarium and use it to test some long-held beliefs about ovarian follicle development (see p. 2207). The germarium is a structure at the anterior tip of ovarioles that produces new ovarian follicles by controlling follicle and germline stem cell (GSC) division and nurturing their developing daughters. The researchers confirm, for example, that GSC divisions are oriented with respect to the germarium’s anteroposterior axis. They also show that somatic escort cells (the glial-like partners of early germ cells) do not adhere to and migrate with GSC daughters as previously proposed, but pass the GSC daughters from one escort cell to the next using dynamic membrane activity. These and other results establish the live imaging system as a valuable tool for the study of stem cell biology.

p45NF-E2 controls intrauterine growth

In mice, the absence of the leucine zipper transcription factor p45NF-E2 results in thrombocytopenia (reduced platelet numbers), impaired placental vascularisation and intrauterine growth restriction (IUGR). It is generally assumed that the lack of embryonic platelets causes the growth problems seen in p45NF-E2-deficient embryos, but now, Berend Isermann and colleagues report that the placental defect and IUGR of p45NF-E2 null mouse embryos is unrelated to thrombocytopenia (see p. 2235). Instead, they show that p45NF-E2 is expressed in trophoblast cells where it is required for normal syncytiotrophoblast formation, placental vascularisation and embryonic growth. Expression of p45NF-E2 in labyrinthine trophoblast cells, they report, colocalises with the expression of Gcm1, a zinc-finger transcription factor crucial for syncytiotrophoblast formation. Finally, they show that p45NF-E2 cell-autonomously represses Gcm1-dependent syncytiotrophoblast formation by inhibiting acetylation in vitro and in vivo. The identification of this novel function for p45NF-E2 during placental development provides new insights into the mechanisms underlying IUGR, a poorly understood but common complication of human pregnancies.

miR165: a plant dose-dependent positional cue

Cell fate determination by positional cues occurs during both plant and animal development. Although some positional cues have dose-dependent effects in animals, this type of cue has not been identified in plants. Here, however, Keiji Nakajima and colleagues show that microRNA165 (miR165) non-cell-autonomously regulates the differentiation of multiple cell types in the Arabidopsis root in a dose-dependent manner (see p. 2303). The Arabidopsis root consists of a central stele (which contains the pericycle layer and the xylem) surrounded by layers of endodermis, cortex and epidermis. Endodermis-derived miR165/166 is known to specify xylem differentiation in the root meristem by suppressing the expression of the transcription factor PHABULOSA (PHB) in the stele. Using an inducible miR165 expression line, the researchers now reveal that endodermis-derived miR165 acts in a dose-dependent manner to establish a PHB expression gradient across the stele that controls the differentiation of two xylem cell types and the pericycle. Thus, these studies reveal that plant development requires at least one dose-dependent positional cue.

Extracellular Engrailed signals get direct

Although homeoprotein transcription factors are best known as cell-autonomous developmental regulators, several homeoproteins have direct non-cell-autonomous activities in the developing vertebrate nervous system. But do homeoproteins also act as signalling molecules during invertebrate development? On p. 2315, Alain Joliot, Florence Maschat and co-workers present the first in vivo evidence for homeoprotein signalling in Drosophila. They use detergent-free immunostaining to reveal an extracellular pool of the homeoprotein Engrailed (En) in the fly wing disc. They then use a secreted single-chain anti-En antibody to show that En is a short-range signalling molecule that participates in the development of the wing’s anterior crossvein. Finally, they report that, in contrast to the repressive effect of En on decapentaplegic (dpp) expression, where it acts intracellularly as a transcription factor, extracellular En activity helps to form the anterior crossvein by enhancing Dpp signalling. The researchers propose, therefore, that direct signalling by homeoproteins is an evolutionarily conserved mechanism that is involved in the development of multiple tissue types.

Blood vessels guide 3D lung branching

Traditionally, blood vessels are regarded as an inert network of tubes that supply tissues with nutrients and oxygen, but recent studies suggest that blood vessels play perfusion-independent roles in early development. Now, on p. 2359, Eli Keshet and co-workers report that blood vessels also determine the reproducible branch pattern of lung airways in mice. During lung development, the coordinated branching of epithelial and vascular tubes culminates in their co-alignment in the mature organ. By ablating the lung vasculature in vivo and in lung explants, the researchers show that, although the first two-dimensional round of epithelial branching proceeds at a nearly normal rate, branching events that require rotation to change the branching plane into the third dimension are selectively affected. This role of the vasculature is independent of perfusion, flow or blood-borne substances but can be partly explained by perturbation of the expression of stereospecific branching regulators such as FGF10. Together, these results reveal a novel perfusion-independent role for the vasculature in directing three-dimensional organogenes.

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Loss-of-function studies in Xenopus have been, until recently, limited to transient knockdowns by injection of morpholino antisense oligonucleotides.  In part because of X. laevis’ complex allotetraploid genome, the system lacked techniques for targeted gene disruption.  In recent years the use of the closely related Xenopus tropicalis, a true diploid with one of the smallest tetrapod genomes, has allowed the addition of genetics to the suite of molecular and embryonic techniques enjoyed by frog researchers, and in a recent PNAS paper from Richard Harland’s lab, John Young and colleagues provide the first example of targeted gene knockout in amphibians using zinc finger nucleases.

The C2H2 zinc finger, the most common DNA binding domain, was first discovered, appropriately enough in Xenopus.  Each zinc finger recognizes a specific trinucleotide sequence, and several fingers can be linked in tandem to bind a longer DNA sequence, with a high degree of specificity.  In addition it has been possible to engineer zinc finger protein domains with novel specificities and to couple them to the DNA cleavage domain of the restriction enzyme Fok1, giving the possibility of engineering zinc finger nuclease (ZFN) that targets a unique genomic locus and produces a double strand break (DSB).  In the absence of a homologous template, cells repair DSBs by non-homologous end joining which sometimes introduces small insertions or deletions. If these result in a frame-shift, and the ZFN is targeted to the coding sequence of a gene, a null or hypomorph allele may be produced.  So far ZFNs have been used to target genes in drosophila, zebrafish and rat.  The Young et. al. paper has now extended the use of the technique to Xenopus.

The authors first tested whether somatic ZFN genome editing was possible in amphibian embryos by injecting mRNA encoding a nuclease targeted against GFP into eggs heterozygous  for a single-copy ubiquitous GFP transgene.  Lower doses of mRNA resulted in a mosaic loss of fluorescence and when the mRNA concentration was increased most cells in the resulting embryos were not fluorescent.  Sequencing of the GFP locus in injected embryos showed insertions and deletions of 5-20 bp in the targeted sequence.

A panel of ZFNs targeting noggin was then designed and 6 constructs that were active in a yeast-based single stranded annealing assay were tested in Xenopus embryos.  All ZFNs were tolerated well by the embryos and produced mutant amplicons at a frequency of 10-47% with insertions/deletions ranging from 5-195 bp.  To test heritability, injected embryos carrying mutated loci were raised to sexual maturity and crossed to wild type frogs.  3 mutated noggin alleles were recovered in the next generation, including a 4bp insertion resulting in a premature stop codon, probably a null allele, and a 12 bp deletion which might be a hypomorph, showing that ZFNs can be used to produce an allelic series of any gene of interest.

An exciting future possible application of ZFNs is the ability of DSBs to increase the rates of homologous recombination, in the presence of template DNA with homologous ends, by many orders of magnitude.   Although this has not been tested in embryos yet, it appears to work well in cell-lines, raising the possibility of using ZFNs to generate knockins as well as knockouts in genes of interest.  For the time being, however, the ability to disrupt specific genes in Xenopus is an exciting development for loss-of-function studies in this model system.

Young, J., Cherone, J., Doyon, Y., Ankoudinova, I., Faraji, F., Lee, A., Ngo, C., Guschin, D., Paschon, D., Miller, J., Zhang, L., Rebar, E., Gregory, P., Urnov, F., Harland, R., & Zeitler, B. (2011). Efficient targeted gene disruption in the soma and germ line of the frog Xenopus tropicalis using engineered zinc-finger nucleases Proceedings of the National Academy of Sciences, 108 (17), 7052-7057 DOI: 10.1073/pnas.1102030108

(8 votes)

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The story of our recently released Development paper ‘FatJ acts via the Hippo mediator Yap1 to restrict the size of neural progenitor cell pools’  involves hundreds of dozens of fresh free-range eggs and not trivial amounts of time spent peering down a microscope.  I have written this with Nick van Hateren, who is the joint first author of this paper along with me.

We had recently developed a short hairpin based system for carrying out RNA interference in the chicken called the pRFPRNAi system. This was an exciting time in the lab, as there had previously been no such system to carry out functional genetics in our favourite model system, and we were looking forward to make full use of it. This was also a great chance for us to demonstrate to the community that the chicken really is an excellent model system to do RNAi screens.

During this time, RNA interference screens were all the rage, and several screens had already been carried out in Drosophila cell lines and the worm, but never in a whole vertebrate. Even though shRNA was possible in the mouse, introducing constructs into mouse embryonic tissue was not a trivial matter, and carrying out a screen even on a small scale would present significant challenges, and indeed still would. The main obstacle here was the inaccessibility of the mouse embryos as they developed within the mother. The chick embryo, on the other hand came conveniently packaged inside an egg, and transfecting tissues by electroporation is a well-established and efficient technique. The spinal cord, in particular, was ideal for our studies. It is shaped like a tube, making it easy to inject it with a DNA solution. The DNA can then be transfected to only one side of the spinal cord by electroporation, while the other side would remain as a convenient internal control.

Armed with these reassuring facts, we began to search for suitable candidates for an RNAi screen. We had previously carried out a microarray analysis of the chick spinal cord, and amongst the thousands of genes expressed there, there were 40 genes that contained cadherin domains. These appeared to be the perfect choice, since they were a reasonable number and also because the large size of the cadherins makes it difficult to carry out overexpression studies.

We decided it would be prudent to target three different regions of each gene, which meant that we would need to sub-clone 120 shRNA sequences. Even though this sounded like a daunting task, the reality was far from it. Our cloning strategy was already well optimised, and we were done sooner than we expected. It was time to get down to the interesting work…

The screen itself was carried out very systematically. We had planned out the whole week so we could get in two rounds of electroporations and end up with a batch of fixed and frozen embryos ready for sectioning and analysis. The longest part was actually sectioning the vast numbers of embryos we generated, but we were lucky that our friendly lab technician Vicky was happy to give us a hand with this.

The screen was a rollercoaster ride of emotions – ranging from euphoria to dejection when a whole batch of our antibodies went bad. By the end of it we had many more positive hits than we had expected, and the range of phenotypes was also reassuringly diverse (Examples of two are in the image below). We were very excited that this would be a wonderful showcase for the feasibility of RNAi screens in the chick embryo.

One of our most intriguing hits was FatJ, a cadherin that appeared to be important for controlling the number of a small sub-population of interneurons. Loss of FatJ caused a small but robustly reproducible increase in the number of these interneurons, and we were intrigued to understand more about this phenotype.

We found that FatJ expression is restricted to the intermediate region of the neural tube,  and we were very encouraged to find that this domain corresponded to the progenitor pools for the interneurons whose numbers were increased following FatJ knockdown. We then examined the number of cells in different progenitor pools within the FatJ expression domain. After a great many cell counts and many hours of confocal microscope time, we determined there was a corresponding increase in the number of progenitor cells within the FatJ expression domain. This gave us a valuable clue to the mechanism of FatJ action: the loss of FatJ causes an increase in the number of progenitors which then differentiate normally to produce a corresponding increase in the number of interneurons. We confirmed this by double labelling with progenitor and differentiated interneuron markers and ensuring no cells expressed both markers simultaneously.

At that time, there were relatively few studies of FatJ reported in the literature; however we noticed that FatJ was the closest vertebrate orthologue of Drosophila Fat (dFat) which was known to be involved in planar cell polarity and was upstream of the newly-discovered Hippo pathway that controls tissue size in Drosophila. Many components of this pathway are highly conserved in vertebrates so we reasoned that FatJ might act through the Hippo pathway to regulate proliferation of neural progenitors. The Hippo pathway is a MAP Kinase cascade that phosphorylates the transcriptional regulator Yorkie (Yap in vertebrates) and this prevents the expression of proliferative and anti-apoptosis genes. Our hypothesis was that, in the absence of FatJ, there was no signalling through the Hippo pathway so Yap1 was not phosphorylated and proliferative genes continued to be expressed. This would lead to an increase in the number of cells within the progenitor pool. To test this theory, we designed shRNAs to target Yap1 and Tead4 (the transcription factor partner of Yap1) and electroporated these at the same time as FatJ shRNAs. We found that loss of Yap1 or Tead4 at the same time as loss of FatJ produced a normal number of interneurons and therefore rescued the FatJ phenotype.

Around this time, a paper by Cao et al (Genes Dev. 2008 Dec 1;22(23):3320-34) was published reporting the regulation of neural progenitor pools by the Hippo pathway and that dominant repressor forms of Yap1 and Tead produce an increase in the number of Lim1/2 positive cells – the same phenotype we observed after FatJ knockdown! Crucially, the authors did not focus on the upstream signal controlling the hippo pathway, which we believed to be FatJ. To address this, we attempted to determine more directly if loss of FatJ caused a change in the phosphorylation state of Hippo pathway components. This was a time-consuming process involving many electroporations followed by sub-dissection of transfected cells and then western blot analysis with phospho-specific antibodies. Unfortunately, the anti-phosphoMst antibody (the Hippo orthologue) did not work well enough to detect a change in activity of the Hippo pathway. However, we did detect a decrease in the level of phospho-Yap1 after FatJ knockdown and this decrease was also evident by immunohistochemistry of neural tube sections. Therefore, we had confirmation that loss of FatJ causes a decrease in phosphorylation of a downstream hippo component.

This gave us a mechanism for the observed phenotype; FatJ normally acts via downstream Hippo pathway components to limit the size of specific progenitor pools in the neural tube. In the absence of FatJ, these progenitors continue to proliferate resulting in a corresponding increase in the number of the interneurons. Intriguingly, the FatJ^-/- mutant mouse phenotype displays a wider neural tube than wild-type littermates suggesting that longer-term loss of FatJ expression could lead to significant tissue overgrowth.

This brought us to the end of a long journey; starting from an RNAi screen and ending with a mechanism. Even though our screen focused on a specific group of genes, we ended with a range of phenotypes – this really highlights the usefulness of the chick as a model system and has proven that RNAi screens are indeed feasible in this system, opening up new possibilities for functional genomics in higher vertebrates.

Van Hateren, N., Das, R., Hautbergue, G., Borycki, A., Placzek, M., & Wilson, S. (2011). FatJ acts via the Hippo mediator Yap1 to restrict the size of neural progenitor cell pools Development, 138 (10), 1893-1902 DOI: 10.1242/dev.064204

(5 votes)

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Can I just bring to eveyone’s attention that the 6th International Chick Conference is now to be held at The Roslin Insititute, UK. Sept 17-20, 2011.
This forum often attracts a strong developmental biology contingent and we anticipate the 2011 conference will include many relevant themes (e.g Morphogenesis; Organogenesis; Patterning, Cell Fates and Organizers; Genetic manipulation of chickens;Imaging and Image Analysis). Not only that but this is the first conference to be held at the new and beautiful Roslin Building, in the stunning location of the Pentland hills of Edinburgh.
Please register your interest now at-
http://www.roslin.ed.ac.uk/chick6/

Thanks
Megan

(6 votes)

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Regenerative medicine and stem cell research go hand-in-hand when it comes to dreaming up future strategies for treating disease and injury in humans.  Today’s image is from a recent Development paper discussing how damaged heart tissue regenerates in zebrafish, and serves as a great model for devising strategies to help human heart attack patients.

When a person suffers a heart attack, white blood cells move into the injured area of the heart and create scar tissue.  This scar tissue is important to maintain the structural integrity of the heart, but causes long-term changes in the heart’s architecture that may lead to heart failure.   A recent paper in Development looks at this process in zebrafish, and describes how the zebrafish heart can undergo regeneration after injury to cardiac tissue.  In this paper, researchers used cryocauterization to cause localized injury to the heart that appears similar to that seen in humans after a heart attack.  Cryocauterization caused myocardial cell apoptosis within the injured area, followed by formation of scar tissue, followed by complete regeneration.  This regeneration included key cardiac tissue types, including epicardium, myocardium, endocardium and coronary vasculature.  This amazing regeneration ability of the zebrafish heart may provide a framework for how this process may be engineered for human patients after suffering heart attacks.

The images above show zebrafish heart tissue after injury (dpi = days post-injury), with bottom images showing higher magnification views of the boxed regions.  The injured area (IA) lacks tropomyosin staining (red).  Shortly after injury (A), the presence of Mlck (myosin light chain kinase, green) at the border of the injury indicates the presence of activated platelets, which promote scar formation.  After a few days, the Mlck-positive cells in the injured area (B,C) indicates the presence of smooth muscle scar tissue.  Many days after injury (D), the lack of Mlck suggests that the scar tissue has been replaced with new, healthy tissue.

For a more general description of this image, see my imaging blog within EuroStemCell, the European stem cell portal.

Gonzalez-Rosa, J., Martin, V., Peralta, M., Torres, M., & Mercader, N. (2011). Extensive scar formation and regression during heart regeneration after cryoinjury in zebrafish Development, 138 (9), 1663-1674 DOI: 10.1242/dev.060897

(5 votes)

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The Map of Life is a recently published guide to convergent evolution produced by the University of Cambridge that has been touring science festivals and events across the world. It contains hundreds of article about structures and adaptations that have evolved independently in unrelated organisms such as camera eyes in jellyfish and snails to gliding in lizards and mammals.

This project is co-ordinated by the Professor of Evolutionary Palaeobiology, Simon Conway Morris in the Department of Earth Sciences. Professor Conway Morris and his team have spent several years on the project and the depth of the Map of Life really reflects this. The articles are all interlinked to each other, making it less of a list of convergent adaptations and more of a well linked database. I found myself wandering from camera eye evolution (they evolved 7 times!) to cognition in birds!

All the information presented here comes from peer reviewed journals and other scientific literature. Although this seems primarily aimed at students and academics it is written in a way that also makes it accessible to members of the public (with some basic understanding of science).

The Map of Life does a great job in showing off the beauty (and laziness!) of evolution and how it arrives at the same or similar adaptations independently. It also tells us that evolution can be predictable when faced with similar environmental or selective pressures and interestingly, could also give us some clues about how life could evolve on other planets or moons.

The main message that the Map of Life presents is that evolution is true. Whilst there are may proponents of evolution, what really separates this message from others is that it is funded by the John Templeton Foundation, an organisation that aims to align science and religion. Whilst this organisation has been at the centre of controversy recently, it is quite refreshing to see them displaying the marvels of evolution with a resource that is so accessible and absorbing.

Links:

Map of Life – http://www.mapoflife.org
Map of Life on Facebook – http://www.facebook.com/mapoflife
Press release from Cambridge University – http://www.admin.cam.ac.uk/news/dp/2011030302

(5 votes)

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The March of Dimes Prize in Developmental Biology was jointly awarded this April to David Page, Director of the Whitehead Institute, and Patricia Ann Jacobs, professor of human genetics at Southampton University Medical School and co-director of research at the Wessex Regional Genetics Laboratory. Both Page and Jacobs specialize in research on human sex chromosomes and the development of sex disorders.

Page has been studying the human Y chromosome for nearly three decades. He and his colleagues have changed the scientific community’s perception of the Y, revealing the mechanism by which it maintains its genetic diversity through recombination at palindromic regions on the chromosome. He has also investigated the developmental effects that arise when this process doesn’t occur properly, which can lead to a loss of sperm production, sex reversal, and Turner’s syndrome.

Jacobs’ research has also played a fundamental role in establishing current understanding of human sex chromosomes. She is best known for a 1959 paper that first described Klinefelter syndrome, in which males carry an extra X chromosome.

The March of Dimes Prize has been awarded annually since 1996 to scientists whose research has advanced the understanding and treatment of birth defects. Jacobs and Page jointly received the award, worth $250,000, on May 2nd, at the annual meeting of the Pediatric Academic Societies in Denver, Colorado.

(Image from Whitehead Institute for Biomedical Research)

(3 votes)

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Congratulations to Hozana Andrade Castillo of the University of Sao Paulo in Brazil, whose image of a mouse pharyngeal arch jumped from third place to first place in the last few days of voting. Her image will appear on a cover of Development in the next few months.

Developing pharyngeal arch region of mouse embryo showing immunofluorescence detection of neurofilaments (green), DiI labeling of trigeminal ganglion (red), and nuclei detected with DAPI (blue).

Second place in this round went to the cilia-stained squid image taken by Davalyn Powell of the University of Colorado Denver, followed by the flower-like image of cell division in the slipper limpet (by Anna Franz – University of Oxford) and an ascidian muscle (by Christine Carag Krieger – University of Pennsylvania).

We had over a thousand people vote in this round as many people forwarded the link to their friends. Thanks to everyone for participating and voting!

The next round of images will be up on June 6th. Around that same time, the winning image from the first round will appear on the cover of Development, and that week also marks the start of the 2011 Woods Hole Embryology Course. We’re looking forward to the images from this year!

(1 votes)

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More and more, the central dogma is becoming well, dogged, for being a dogma at all. As humans, we have 3 billion nucleotides. Only 1% of it makes up our protein coding genes, which led to the development of the central dogma: DNA is transcribed to RNA and translated into proteins. During undergrad, we’re taught that transcription factor proteins bind to sequences in the DNA to either enhance or shut off transcription, thereby discontinuing protein production. Gene regulation seems to follow certain pathways and rules itself.

Then, 20 years ago RNAi was discovered, a whole new field where RNA can also regulate protein production. Generally, small RNAs repress protein coding genes by either causing their mRNAs to decay or inhibiting their translation into protein. They can be naturally produced for the sake of controlling the native genes, following a similar central dogmatic pathway: DNA is transcribed to a precursor RNA, which is converted into teeny RNA strands. These strands are selected by Argonaute proteins (AGOs) and used to target specific protein coding genes to halt their expression.

The progression from DNA to RNA to “something”, is like the foundation of genetics. And gene regulation is about how to control that progression. Evolution develops these elegant pathways to produce “something”, and then it evolved ways to control the pathway.

Well, not always.

Sometimes you can get “unintelligent design.”

Which is what a student joked, at a journal club meeting I’d attended today. The meeting started off as a presentation of a recently published article that assigned a role to an AGO protein in Arabidopsis. It ended up as a digression on evolution and how we perceive it, among other things.

The protein of interest was AGO10, which is somehow involved in miRNA/small RNA directed repression of target genes. But how it exactly it does this, was hazy to us, the observers. By inference, previous studies on its loss of function mutant, ago10/zwille/pinhead had deformed shoot apical meristems. After embryogenesis, the meristems at the junctions of leaves are where new ones form. In the mutants, the meristem is the size of pinhead, smaller than normal (Pictured in F, on the left, versus wild type in C, by Tucker et al., 2008). The protein expression of some transcription factors & signalling peptides changed in the ago10 mutants as well. But what does AGO10 specifically do?

(Picture: A, B = AGO10 expression in the yellow YFP areas of the embryo, at heart stage & torpedo stage. CLV, a stem-cell specific signalling peptide. its expression is weak is in the embryo ago10, as pictured in D, E. (it’s not strong in WT either though) CLV needs AGO10 to be maintained past embryogenesis, in the meristem Tucker et al. 2008).

One group discovered that AGO10 doesn’t really help in repression. It actually indirectly causes the activation of target genes. AGO10 is a decoy for the main AGO1, that performs 90% of miRNA directed repression in Arabidopsis plants. However, it’s a decoy for one miRNA alone (so it seems). It binds and sequesters miR166/165, a well studied miRNA that is deeply conserved in mosses and higher plants. By sequestering it, miR166/65 is prevented from binding to AGO1 and regulating its targets (HD ZIP proteins). Effectively it’s as if miR166/65, the silencer, has been silenced itself. By a massive protein no less.

Oddly, AGO10 is most abundant in one tissue type, the developing embryo. So it only catches up all the miR166/65 population in that one tissue. If it doesn’t, as suggested by ago10, then the embryo develops into mutant plant. 

But why would plants bother developing this? Couldn’t nature have just evolved so that miR166/65 isn’t transcribed in the embryo? So that some transcription factor or miRNA shuts it off. Like, why produce something and then evolve a whole protein to muffle its activity? It’s like..evolution forgot to turn off the tap or shut the door, and created a massive sink or club bouncer instead. Energy wise, it seems a waste though. Evidently there probably is an unknown purpose.

The authors of the paper suggest that miR166 is required in specific cells, but not all cells of the developing embryo. So miR166 is allowed to be expressed but AGO10 prevents it from being spread elsewhere..assuming that miRNAs can travel…

(A-C: WT, CLV expression is maintained throughout embryogenesis and meristem development. Without AGO10 in D-F, its expression disappears).

It’s funny. We have these commonly accepted notions about gene regulation engrained in us from years of training. With the result that when we’re faced with something altogether new, we’re not sure what to make of it. We’re not even sure if we can accept it, but there it is. It’s like realizing black swans with ghoulish red eyes exist in Australia.

Sources/References:

Lab discussion with students at the Research School of Biology at ANU, led by PIs Tony Millar and Iain Searle.

Zhu H, Hu F, Wang R, Zhou X, Sze SH, Liou LW, Barefoot A, Dickman M, & Zhang X (2011). Arabidopsis Argonaute10 Specifically Sequesters miR166/165 to Regulate Shoot Apical Meristem Development. Cell, 145 (2), 242-56 PMID: 21496644

Pictures:
Tucker MR, Hinze A, Tucker EJ, Takada S, Jürgens G, & Laux T (2008). Vascular signalling mediated by ZWILLE potentiates WUSCHEL function during shoot meristem stem cell development in the Arabidopsis embryo. Development (Cambridge, England), 135 (17), 2839-43 PMID: 18653559

Flickr CC by Kyknoord

(5 votes)

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This is a retelling of the student and post-doc workshop from the second day of the BSDB/BSCB joint spring meeting that took place in Canterbury at the University of Kent. The session emphasised the need for accurate science and scientific involvement in public communication. It ended up a bit longer than I’d intended, but this is something I’m really enthusiastic about and felt it needed to be shared in detail. I hope you find it helpful.

Panellists:

Dr Peter Wilmshurst – A consultant cardiologist, known for his refusal to falsify or withhold data in pharmaceutical studies. He was being sued for libel and slander by NMT medical until the company entered liquidation in April.

Rose Wu – A representative for the charity Sense about Science which works tirelessly despite limited funding to improve the public image of science, aids accurate reporting of scientific issues in the media and campaigns for further government support for research.

Dr Jenny Rohn – A UCL post-doc by day. Also known for her punditry, she runs the popular science communication website lablit.com has been interviewed numerous times for tv and radio. She has published numerous stories and editorials and two fictional novels Experimental Heart and The Honest Look, communicating science through the engaging and emotional personal lives of scientists. She was also central to the campaign to save UK science funding.

(more…)

(4 votes)

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