Journal Article

Nodal patterning without Lefty inhibitory feedback is functional but fragile

Alexander F Schier, Andrea Pauli, Deepak Reyon, J Keith Joung, James A Gagnon, Steven Zimmerman, Nathan D Lord, Katherine W Rogers, Deniz C Aksel, Shengdar Q Tsai

2017

Overview

Developmental signaling pathways often activate their own inhibitors. Such inhibitory feedback has been suggested to restrict the spatial and temporal extent of signaling or mitigate signaling fluctuations, but these models are difficult to rigorously test. Here, we determine whether the ability of the mesendoderm inducer Nodal to activate its inhibitor Lefty is required for development. We find that zebrafish lefty mutants exhibit excess Nodal signaling and increased specification of mesendoderm, resulting in embryonic lethality. Strikingly, development can be fully restored without feedback: Lethal patterning defects in lefty mutants can be rescued by ectopic expression of lefty far from its normal expression domain or by spatially and temporally uniform exposure to a Nodal inhibitor drug. While drug-treated mutants are less tolerant of mild perturbations to Nodal signaling levels than wild type embryos, they can develop into healthy adults. These results indicate that patterning without inhibitory feedback is functional but fragile. During animal development, a single fertilized cell gives rise to different tissues and organs. This ‘patterning’ process depends on signaling molecules that instruct cells in different positions in the embryo to acquire different identities. To avoid mistakes during patterning, each cell must receive the correct amount of signal at the appropriate time. In a process called ‘inhibitory feedback’, a signaling molecule instructs cells to produce molecules that block its own signaling. Although inhibitory feedback is widely used during patterning in organisms ranging from sea urchins to mammals, its exact purpose is often not clear. In part this is because feedback is challenging to experimentally manipulate. Removing the inhibitor disrupts feedback, but also increases signaling. Since the effects of broken feedback and increased signaling are intertwined, any resulting developmental defects do not provide information about what feedback specifically does. In order to examine the role of feedback, it is therefore necessary to disconnect the production of the inhibitor from the signaling process. In developing embryos, a well-known signaling molecule called Nodal instructs cells to become specific types – for example, a heart or gut cell. Nodal also promotes the production of its inhibitor, Lefty. To understand how this feedback system works, Rogers, Lord et al. first removed Lefty from zebrafish embryos. These embryos had excessive levels of Nodal signaling, did not develop correctly, and could not survive. Bathing the embryos in a drug that inhibits Nodal reduced excess signaling and allowed them to develop successfully. In these drug-treated embryos, inhibitor production is disconnected from the signaling process, allowing the role of feedback to be examined. Drug-treated embryos were less able to tolerate fluctuations in Nodal signaling than normal zebrafish embryos, which could compensate for such disturbances by adjusting Lefty levels. Overall, it appears that inhibitory feedback in this patterning system is important to compensate for alterations in Nodal signaling, but is not essential for development. Understanding the role of inhibitory feedback will be useful for efforts to grow tissues and organs in the laboratory for clinical use. The results presented by Rogers, Lord et al. also suggest the possibility that drug treatments could be developed to help correct birth defects in the womb.

• Publisher: eLife Sciences Publications, Ltd,eLife Science Publications, Ltd,eLife Sciences Publications Ltd
• Subject: Analysis / Animals / Biology / Biology (General) / Birth defects / Danio rerio / Defects / Developmental Biology and Stem Cells / developmental patterning / Ectopic expression / Embryo / Embryogenesis / Embryos / Endoderm / Endoderm - embryology / Feedback / feedback inhibition / Gene Knockout Techniques / Heart / Hybridization / Hygiene / Left-Right Determination Factors / Left-Right Determination Factors - genetics / Left-Right Determination Factors - metabolism / Lefty / Lethality / Medicine / Mesendoderm / Mesoderm / Mesoderm - embryology / Nodal / Nodal Protein / Nodal Protein - metabolism / Pattern formation / Phosphorylation / Q / QH301-705.5 / R / Science / Signal Transduction / TGFβ / Time / Zebrafish / Zebrafish - embryology / Zebrafish Proteins / Zebrafish Proteins - genetics / Zebrafish Proteins - metabolism