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In time of revision: of Wingless and morphogens

Posted by , on 27 December 2013

In time of revision: of Wingless and morphogens

Alfonso Martinez Arias

The recent publication of the important work of C. Alexandre, LA. Baena and JP. Vincent on the molecular requirements for Wingless signalling in Drosophila (http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12879.html) offers an opportunity to consider the relationship between ideas and facts in modern developmental biology.

The work reports the surprising finding that fruit flies whose only source of Wingless is a membrane tethered form of the protein are viable and, except for a few minor defects in growth, morphologically normal. This is a very surprising result and, although earlier work had shown some hints in this direction during wing development, there is much that invites thought and reflection in this incisive and beautiful piece of work. However, to understand this, we need a small amount of background.

Wingless is the Drosophila homologue of Wnt-1, a member of a major family of signalling molecules that play significant roles in development and disease. Wnt signalling became to prominence because of work in Drosophila which, in the 90s, was deep in the hunt for Morphogens, molecules postulated to be able to instruct the patterning of fields of cells. There are two definitions of morphogens –and as critical results begin to emerge, the differences between the two becomes more and more important. The original one by Alan Turing refers to molecules that can trigger the generation form. A second definition, more popular and the one most widely known, emerged in the 70s and is associated with Lewis Wolpert notion of Positional Information (PI). According to this a Morphogen is seen as a diffusible substance that specifies pattern in a field of cells in a concentration dependent manner i.e. cells can ‘interpret’ different concentrations in different manners and activate different genes. It is this second definition that has served as a guide to interpret much of the molecular genetics of developmental processes in Drosophila. Wingless signals through an elaborate molecular device of which only one element needs to be considered in this discussion: like all Wnt proteins, Wingless effects its signalling activity through the nuclear activity of ß-catenin (Armadillo in Drosophila) i.e. Wingless can signal to other cells, but in the cells that see Wingless, what matters is the state of Armadillo/ß-catenin.

In the 90s, the finding that many of the molecules uncovered by the genetic analysis of pattern formation were diffusible led to identify many of them as “morphogens”, in the wolpertian sense. Naturally one of these was Wingless and much of the effort to make it a Morphogen focused on its function during the development of the adult, in particular within a structure called the wing imaginal disc, that will give rise to the wing and thorax of the adult Drosophila. For most of the development of this structure (over 40 of the 96 hours that lasts its development and patterning), Wingless is expressed in a thin stripe bisecting the growing disc. As ill defined (timewise) removal of Wingless generated defects in the growth and patterning of the wing, the notion emerged that it was the diffusion of Wingless from the stripe that was responsible for the pattern. A number of experiments were designed to test this hypothesis, including testing the effects of diffusible Wingless in contrast to membrane tethered Wingless and to those of its effector Armadillo/ß-catenin. Key in these experiments, as it is to any test of a wolpertian Morphogen, was the identification of direct response targets to the signalling activity of Wingless. Following a tradition, three were identified that during the patterning of the wing disc could be interpreted in this light; from high to low response thresholds: senseless, Distalless and vestigial. The experiments were interpreted to suggest (I am being careful in how I phrase this: ‘interpreted to suggest’ i.e. there was a fair amount of wishful thinking here) that there was a functional gradient of Wingless in the wing disc and that indeed high levels of Wingless triggered senseless expression, intermediate did Distalless and low elicited vestigial. Furthermore, membrane tethered Wingless could only signal to adjacent cells and Armadillo/ß-catenin could only elicit a response in the cells where it was expressed. And everybody, or almost everybody bought into it. In characteristic style Nature, Cell and Science broadcasted the news: Wingless was –and by the way still is- a Morphogen. However, looking at the data some of us had problems with these readings, the design of the experiments and the interpretation of the results. NCS was not interested and some of this questioning can be found in other journals where results have, on average, a longer shelf life and more information (see Development or Developmental Biology) as opposed to ‘cool experiments’ for morphogene enthusiasts, perpetuated in reviews. Part of the reason for these doubts was the existence of another view of the function of Wingless in the wing that, even though ignored, had a firmer base on the experimental results.

In the alternative view, Wingless did not (and DOES NOT) act as a Morphogen –in the wolpertian sense. Furthermore, a large number of experiments suggested that there was not much of a relationship between the long range diffusion of Wingless and the patterning of the wing. This work has been summarized elsewhere and in more general terms (for some details see http://amapress.gen.cam.ac.uk/?p=1191 and references therein). The gist of it is that

  1. There are different phases of Wingless expression in the wing and that there is a need to correlate specific functions with this different phases. Particularly during a crucial phase in the growth of the wing disc, all cells appears to express low levels of wingless.
  2. That removal of the stripe of Wingless expression had little or no effect on the growth of the wing.
  3. That Wingless acted at different times by creating some sort of a memory (implemented by Vestigial and Distalless) that would be pass on to the next stage.

The new work of Alexandre et al is a very rigorous confirmation of these observations in the wing but goes beyond them, showing in a most convincing manner, that there is no requirement for Wingless long range diffusion at all during the development of Drosophila and providing details and important hints of how Wingless works during wing development. It also shows that this might be a theme for the way Wingless works in Drosophila. Using elegant novel engineering technology they substitute the wildtype copy of Wingless for a membrane tethered form, and show that a fly with this genotype is viable. From the point of view of morphology it is a very good looking fly, though it has some small defects on growth, physiology and reproduction; but, for all practical purposes -certainly those that concern Drosophila pattern formation buffs- it is good. The defects should make people think and opened many interesting questions about Wnt signalling. Significantly, the authors go on to show that most of the growth of the wing disc is associated with ubiquitous low levels of Wingless expression in all the cells of the disc during the early/mid part of imaginal development (second and early third larval instars to the experts) and that therefore all cells have access to the levels of Wingless that they need, when they need it. This is consistent with previous suggestions that to understand Wingless in the wing disc one has to take into consideration its different patterns of expression and focus on early events. Its extension to the rest of development and other tissues of the fly is extremely important and invites much thought.

These observations are, indeed, surprising. However, rather than rejecting them and looking for small holes in the experiment (there is no perfect experiment in Biology) maybe we should simply use them to.reconsider our views of Wnt signalling. After all, none of the people who are raising caveats about this work –and there are a few- fluttered an eye brow when thinking about Wingless as a Morphogen, disregarding experiments to the contrary and boosting weak results in journals of wide readership, to favour an idea which was only an idea (see http://amapress.gen.cam.ac.uk/?p=1191) . The new work is not without issues but they are minor and do not have to do with the techniques, the membrane tetheres Wingless or the experimental design (all of them fine), but rather with what the results tell us about Wingless and Wnt signalling. Importantly: the work does not imply that Wingless does not diffuse in Drosophila; it does!. The work does not say that diffusible Wingless might not play a role, in Drosophila or other organisms; it might!. All that the work says is that in Drosophila, during normal development, there is not a major need for Wingless to diffuse a long range to perform its function. One of the reasons why it is important to take stock of this, is because there are few experiments (any?) in vertebrates that show a requirement for ling range diffusion of Wnt proteins in pattern formation. The accolade of Wingless as a Morphogen in vertebrates is a shallow extrapolation of the statements about Wingless in Drosophila, mostly in NCS but also, by mimicry, in other journals. Let us hope that this work leads to a more careful and rigorous analysis of the function of these molecules (NB this does not mean that reviewers should ask for endless lists of experiments, it simply says that we should be a bit more critical of what we have and a bit more thoughtful in the interpretation of the results).

One of the reasons for the use of Wingless in Drosophila might have to do with the rapid and robust development of the organism. Computation of signals take time and the wing disc (as all development of the fruit fly) is under extreme temporal constrains that have led it to evolve rapid mechanisms built around deeply interlocked gene regulatory networks (behold the early segmentation cascade). Thus, in evolutionary time it might be easier to reengineer the system, on the basis of the Gene Regulatory Networks, than to change the properties of the molecules (in the case in hand here, the diffusibility of Wingless). Thus the situation in the wing.

 

The twilight of the Morphogen?

The notion of a Morphogen is very tightly linked to pattern formation and signalling and in the light of the XXI century, it might be good to look at its roots and maybe return to the Turing version instead of the wolpertian PI version. Many people would be happier. While it is becoming increasingly clear that signalling molecules can elicit concentration dependent responses, it is not at all clear what the role of these responses are in vivo. We find correlations between concentrations and differential patterning but as one often gets in reviewers comments: correlation is not causation. In these considerations we should not forget the issue of time integrals and their relationship to spatial concentration. It seems that we need to divorce ourselves from simple naïve notions based on qualitative models. The Wnt version might just be one.

In the end rather than hanging on to notions for which, let us remember, there was precious little evidence for, we should look at the new results and think about what they tell us about the system rather than about a molecule. There is much in the work of Alexandre et al about this and we should take it beyond the simple ‘Morphogen or no Morphogen’. A twist to some famous words of Jean Rostand come to mind: theories pass, the wing remains.

 

 

 




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11 thoughts on “In time of revision: of Wingless and morphogens”

  1. Whoa. What an emotional diatribe.

    I liked both the article by JP Vincent and the News&Views by Morata & Struhl, which puts things in perspective.

    The published evidence that Wg can and does diffuse over a long range to activate signalling is actually pretty solid. Seems odd that this author doesn’t find that evidence convincing.

    The fact that tethered Wg rescues much, but not all, of development simply shows that short-range signalling is more important than previously appreciated. The early wing pouch specification and late wing margin specification are both short-range signalling events. This leaves only the growth function of Wg (which is mild) to depend on long-range signalling.

    I’m keen to know what happens to the belts of naked cuticle in the embryo when Wg is tethered. Same goes for other roles of Wg.

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    1. Thank you for the comment.

      The ‘reader’ appears to have a very different view of Wingless function during the development of the wing from that commonly held, which is good. However, if the only issue here was an underappreciated role of short range signalling and a solution to the ‘mild’ role of Wingless in the growth of the wing, there would not be a manuscript as the one just published and certainly not two news and views articles, raising some issues about it. The reason for the’noise’ and the ‘emotion’ in my post and in some of the Views in Nature, is that the published results clearly affect much in the field, in the notion of Morphogen and in the views of some people. There is little question that the work of Alexandre et al. addresses (and challenges) a widely held view that the function of Wingless in wing development is associated with its role as a Morphogen controlling, through long range signalling, growth and patterning. The manuscript contains more but this is one important issue that it addresses.

      I am pleased to hear that the ‘reader’, together with some of us, believes that the function of Wingless on growth is mild. These are the facts and one hopes that they will spread and that we start talking about Wingless and Wnt signalling in a different light. As for the short range function of Wingless, it was clear to some of us in 1994 (Couso, J.P., Bishop, S. Martinez Arias, A. (1994). The wingless signalling pathway and the patterning of the wing margin. Development. 120, 621-636) so I can only agree with the ‘reader’ that this function has been ‘underappreciated’. Perhaps the ‘reader’ can understand and forgive my emotion that something like that is noticed after almost 20 years of being overlooked in favour of a Morphogen view.

      One final thing, the work of Alexandre et al is about much more than this and one hopes that having established how Wingless works in the wing, people can move to the other issues that emerge from this work.

      As for the denticles in the tethered Wingless larvae, I agree that it is an interesting question and that the flies will soon be available to look into this and more.

      Once again, thank you for the comment

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    2. I have been looking for references to the notion that Wingless controls the growth of the wing through a long range mechanism, just in case I had missed something. Have found many but there is a good and succinct review that might be of interest to the ‘reader’. It is from Barry Thompson, one of the authorities on Wingless signaling in Drosophila, who has written about the role of Wingless as a morphogen. In a review on “Developmental control of cell growth and division in Drosophila’ (Current Opinion in Cell Biology 2010, 22:788–794) he is very explicit when summarizing the state of the field:

      “By comparing the development of the fly wing, leg and eye, it is clear that the pattern of expression of key ‘organising’ signals (such as BMP Dpp, the cytokine Unpaired, the Wnt Wingless and ligands for Notch) and the range over which they spread prefigures the size and shape of the tissue (Figure 2 ). The range of these signals increases during tissue growth and altering the range can cause a corresponding change in the size of the adult tissue it gives rise to”

      There are some surprising statements here but the reference to Wingless cannot be missed and summarizes the believe in the field. Incidentally and in case you cannot get the ms, Figure 2 highlights the DV stripe of Wingless that, according to the author (and most of the field) acts as the source of the morphogen.

      I am glad to hear that the ‘reader’ holds a different view and believes that the role of Wingless in growth is ‘mild’ -though this is debatable as the issue is not the degree of input but, as Alexandre et al make clear, the mechanism.

      I hope this is helpful.

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    3. In my view, saying that this new work “simply shows that short-range signalling is more important than previously appreciated” is a very elegant understatement.

      If the Wg phenotype found in the 70s was the one described by Alexandre, Baena-Lopez & Vincent instead of the Wg[1] wing-to-notum ‘homeotic’ transformation, would it have been pursued like it was? Would Wg signalling have been considered so central? I honestly do not think so.

      Nobody is denying that Wg forms a gradient, nor that it has graded signalling activity. But this work shows clearly that, for the case of Wg, diffusion is not required for the bulk of its role in morphogenesis. Now, can we still call Wg a Morphogen without muddling the concept? That is the question. [Note I write ‘Wg’, not ‘Wnt’].

      This sentence from the News and Views by Ginés Morata says it all: “Considering the many functions of Wg during embryogenesis and during larval and adult life, and the essential role assigned to the protein’s spread, any expert would have confidently predicted that a fly with only tethered Wg would not develop.”

      Happy New Year!

      Conflict of interest disclosure: as an ex-postdoc of Alfonso Martinez Arias, I had the pleasure of publicly, and sometimes heatedly, disagreeing with him roughly half of the time for over 5 years.

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  2. Joaquin de Navascues (with whom indeed I had some useful disagreements over the years; the ‘ex’meaning only that he has deservedly moved on) emphasizes the important points.

    First. that the visibility and significance of the work is because by and large Wingless is still considered a morphogen and, in the knowledge of being repetitive, the notion of Wnt proteins acting as morphogens in vertebrates is an extrapolation from the experiments in Drosophila. Thus, if the notion in Drosophila is as dramatically challenged -as it is in the work of Alexandre, Baena and Vincent- it is important that the conclusion is stated loud and clear.

    The second point is summarized in the quotation from the News and Views of G. Morata, which brings me back to the original point of the “reader’, as people should not be misled by anonymous comments (well not totally anonymous since there is a picture of this person in the post and some of you might know the identity; I don’t). To my knowledge, in addition to the work I have referred to many times, the only other earlier challenges to the notion of Wingless as a morphogen in the wing come from the work of A. Garcia Bellido, some of it with LA Baena, one of the authors of the recent manuscript. The ‘reader’ is definitely not Antonio but reads like someone who has gone quickly through the three stages of new and surprising findings: 1. It is impossible if not wrong; 2. It is possible but not important and, finally 3. It is right but not new.
    I also take this opportunity to emphasize that the work under discussion has a lot more, and probably more important, than the issue of the morphogen.

  3. What ho Alfonso!

    in addition to the hispanic papers you have referred to, there is at least one other one. JP Vincent and I found that, in mosaic embryos made by nuclear transplanation, Wg only seemed to be able to drive expression of engrailed in the next door cell, we were rather surprised at that at the time. Cell 1994 vol 77, p 909
    Here is the abstract:

    The wingless protein is secreted, but it is not known whether it acts only on cells near to its site of synthesis or whether it has a longer range. Here, we use mosaic Drosophila embryos to estimate the range of wingless as it acts to maintain expression of the engralled gene. We find that expression of engrailed is often sustained in those wingless− cells that are located near a wild-type patch of tissue, but this is not invariably so. Also, the numbers of cells maintaining engrailed expression are small. From these findings, we argue that wingless-expressing cells sustain engrailed expression only in adjoining cells. The wingless gene is also needed for maintenance of its own expression; using mosaics, we find that the range of this action is short as well.

    By the way i can see the identity of the”reader” from his picture. Doesnt that picture ring any bells with you?

    Peter

    1. Thank you, Peter, for your contribution. This is an important issue.

      I agree that there are lots of questions about the embryo and I had overlooked your work with Jean Paul (who I should emphasize, has been pursuing these issues for a long time). As for ‘a reader’, some private ‘correspondents’ have made suggestions to me, and all coincide, but I am not good at these things so will remain agnostic. If it is the person that many of you think, his statement is some U-turn. But this person might be the kind that just likes to go with the flow of the time; it is understandable.

      Nobody can accuse you of that!

      Happy New Year to you

  4. Thanks Alfonso for starting this and all for the exchanges. In Fig 3 ( Activity of the wingless promoter in the prospective wing) the authors also show the developmental expression pattern of Wg in the presumptive notum. Here, unlike in the wing pouch, the domain of Wg transcription at 84h is narrow and is not preceded by a broader earlier domain. The presumptive-notum expression (in addition to patterning the bristles, which appear normal) are needed ( from earlier work in our lab) for the expression of Sr in the muscles attachment sites and for expression of Vg in myoblasts. I would love to get the Nrt-Wg flies and look at both these. But, if the flies can’t fly, that’s a start.
    Cheers
    Vijay

  5. Thanks for the perpective.

    I am not an expert in drosophila genetics and wing development, but I remembered some work looked at the role of HSPG and Wingless diffusion.

    Notably, interferring with enzymes of the HSPG (dally, dally-like, notum) synthesis pathway led to modified distribution of Wingless. This was presented as an evidence in favor of the morphogen/diffusing Wingless model (even if HSPG might also influence Wingless activity).

    How do you explain these results in the light of Alexandre et al. paper ?

  6. Thanks for prompting a discussion on these very interesting new data.

    I would like to add 2 notes:

    1. the N&V that accompanies the article by Alexandre et al fails to confront the apparent discrepancy between the earlier paper from Zecca et al 95 on membrane tethered Wg, and the new results from Alexandre et al. The 2 papers used exactly the same membrane tethered Wg construct (called NrtWg). Zecca et al reported that NrtWg only acts in the expressing cells in the immediately adjacent cells. Alexandre et al show that NrtWg acts at a longer range (see their Figure 2). Why is this? Close examination of the data from Alexandre et al provides a simple explanation and a new interpretation of the Zecca et al data. In Figure 2c of Alexandre et al, we see mutant clones generated in a NrtWg fly: the NrtWg expressing cells show autonomous high levels of the Dll target, and there is non autonomous expression of Dll at a lower level at a distance (the surprising result). In Zecca et al, NrtWg expressing clones of cells similarly show autonomous high expression of Dll (and in the immediate neighbouring cell). If non autonomous low expression of Dll also occurs, it would not be possible to see this because the flies still contain endogenous Wg and consequently a low of Dll in the tissue. In other words, endogenous Wg activity and the low expression of Dll in the tissue masks the non-autonomous effect of NrtWg in Zecca et al. The elegant knock in strategy of Alexandre et al unmasks this non autonomous effect. I would conclude that Zecca et al could not conclude correctly because of endogenous Wg activity. Doing the same experiment in a wg mutant background would test this directly.

    2. What could explain then the non-autonomous effect of NrtWg clones? Alexandre et al discuss the possible contribution of low Wg expression at earlier stages, and memory. Memory refers to the possibility that signaling and/or transcriptional activation of the targets is maintained through cell division to the progeny, thereby allowing clonal propagation of the response in the tissue, at a distance, an idea first proposed in 96 by Lecuit et al for Dpp). The N&V asserts that this cannot be due to memory on the basis that NrtWg does not have long range effect as reported in Zecca et al. The point made above in 1) regarding the interpretation of the data in this paper leads one to be more cautious. There is most certainly a non-autonomous effect in Zecca et al but the design of the experiment cannot reveal it, contrary to Alexandre et al. So memory, whatever its molecular incarnation might be, is an interesting hypothesis to consider. Given how little is quantitatively known about the temporal dynamics of Wg signaling, we have to be agnostic and do further experiments to conclude.
    Another plausible scenario is cytonemes which lead us to reconsider the ‘geometry’ of a cell and the notion of autonomy/non-autonomy in a quite dramatic way. In the case of Notch signaling, cellular protrusions (not quite like cytonemes) do seem to account for signaling ‘at a distance’.

    Being more quantitative, using reversible on/off strategies to induce/shut off signaling (i.e.. through optogenetics), and developing further the cell biology of signaling will undoubtedly pave the way for a test of these various models.

    It is exciting to see that current genetic manipulations allow profound reconsideration/deepening of developmental concepts.

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    1. Thomas, thank you for your thoughts. I can only suggest that you read Alexandre et al again, carefully rather than under the influence of the News and Views. You will then see that there is no non-autonomous effect of Nrt-Wg beyond the neighbouring cells in the experiments of Alexandre, Baena and Vincent. I am not going to unwrap the issues that you raise because I trust that when you read the paper afresh you will see that you have missed the experiments.

      Please do not be misled: there is no long range nonautonomous effect of Wg in the Nrt-Wg flies. The results are very clear. The memory is well proven, as is the ubiquitous expression when the wing is growing. We agree that the new technology is central to this. As for Zecca et al. 1996, I have pointed out a few things in http://amapress.gen.cam.ac.uk/?p=1336

      It is disaapointing that Alexadre, Baena and Vincent do this definitive set of experiments, publish them in a high visibility place and the News and Views and some readers miss the point. If some people had looked at the original experiments with half of the rigour than they are looking at this work, Wingless would have never got the accolade of Morphogen.

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