Here are the highlights of the current issue of Development:

Auxin biosynthesis: the root of xylem patterning

In the Arabidopsis root, xylem is organised into a central file of metaxylem that is flanked by protoxylem. Xylem fate is determined by HD-ZIP III transcription factors; high levels promote metaxylem formation whereas low levels specify protoxylem. The factors that downregulate HD-ZIP III levels are known but those that promote HD-ZIP III expression have remained elusive. Yunde Zhao, Ykä Helariutta, Jan Dettmer and colleagues now show that auxin biosynthesis promotes HD-ZIP III expression and metaxylem specification in Arabidopsis (p. 1250). The authors first isolate mutants that display defective xylem patterning and HD-ZIP III gene downregulation. These mutants harbour mutations in the gene encoding TRYPTOPHAN SYNTHASE BETA-SUBUNIT 1. Tryptophan is a precursor in the auxin biosynthesis pathway and, accordingly, the mutants exhibit aberrant auxin levels. They also show that metaxylem development and HD-ZIP III levels are defective in other auxin biosynthesis mutants. Based on their findings, the authors propose that tryptophan-dependent auxin biosynthesis is required for metaxylem formation.

 

From Hippo to planarians

The transcriptional co-factor Yorkie (Yki; YAP in vertebrates) is a key effector of the Hippo signalling pathway and has been implicated in growth control and patterning, as well as stem cell regulation and regeneration, in flies and vertebrates. Now, on p. 1197, Alexander Lin and Bret Pearson investigate the role of Yki in planarian flatworms. The researchers report that planarians have a single orthologue of yki and, using RNAi, they show that yki carries out pleiotropic functions. For example, they report that ykiis required for homeostasis of the planarian excretory system, which is analogous to the vertebrate kidney. The researchers also show that, in contrast to its role in flies and vertebrates, yki functions to limit stem cell proliferation and hence the size of the stem cell population. In addition, yki plays a role in axial patterning, where it acts synergistically with Wnt signalling to supress head formation. These, together with other findings, demonstrate that yki plays diverse yet non-overlapping roles in planarian biology.

 

Following the leader

Collective cell migration occurs in many developmental contexts but a full understanding of this process has been hampered by a lack of quantitative analyses in 3D in vivo contexts. Using the zebrafish lateral line primordium as a model, Darren Gilmour and co-workers set out to tackle this problem (p. 1282). The researchers develop a method to simultaneously live-label microtubules, centrosomes and nuclei, allowing them to map cell polarity and orientation across the migrating population. Using this method, they identify the transition between leader cells and followers within the collective and show that this transition is marked by changes in cell-cell adhesion; cadherin 2 is expressed across the tissue but is only assembled into adherens junctions (AJs) in the transition zone and further down the leader-follower axis. A tandem fluorescent protein timer-based approach reveals that AJs become progressively more stable along the leader-follower axis. Finally, they show that AJ assembly, but not maintenance, requires dynamic microtubules, revealing a key role for microtubules during leader-to-follower transitions within migrating collectives.

 

Dbx1 crosses the (mid)line

During nervous system development, navigating axons ‘decide’ whether or not to cross the midline. Various factors that influence axon guidance and midline crossing have been identified but it remains unclear if any one transcription factor can drive the complete midline crossing transcriptional programme. Here (p. 1260), Yasuyuki Inamata and Ryuichi Shirasaki report that a single homeodomain transcription factor, Dbx1, assigns midline-crossing identity at the progenitor stage in mice. The researchers show that Dbx1 is expressed in a subset of neural progenitors in the dorsal midbrain. Lineage tracing demonstrates that midbrain commissural neurons, which cross the midline, are generated selectively from Dbx1-positive progenitors. Furthermore, gain- and loss-of-function experiments show that Dbx1 is necessary and sufficient for midline crossing. Finally, the authors show that Dbx1 controls a molecular programme that controls the expression of Robo3, an essential regulator of midline crossing, on commissural neurons while repressing the ipsilateral neuron genetic programme. These findings reveal an unanticipated regulatory layer within the transcriptional cascade that controls nervous system wiring.

 

Lymphangiogenesis: of mice and fish

In mice, the formation of lymphatic vessels (lymphangiogenesis) requires the homeodomain transcription factor Prox1. Here, Stefan Schulte-Merker and colleagues examine whether the role of Prox1 is conserved in zebrafish (p. 1228). Using a novel transgenic reporter line, the researchers show that, in contrast to the situation seen in mice, zebrafish Prox1 is initially not expressed in all lymphatic precursor cells and reliably marks this population only during later stages of lymphangiogenesis, arguing against a role for Prox1 in lymphatic specification. In addition, targeted mutagenesis demonstrates that lymphangiogenesis can proceed in the complete absence of Prox1. Finally, they show that the functionally related transcription factors Coup-TFII and Sox18, which are implicated in lymphatic specification in mice, are also dispensable for zebrafish lymphangiogenesis. The authors conclude that an alternative lymphatic specification mechanism is present in zebrafish and propose that differences in the timing of lymphangiogenesis between mice and fish can explain this divergence.

 

Fresh air in the mesenchyme

The mesenchymal compartment of the lung plays a crucial role during lung development but, unlike its epithelial counterpart, its regulation is largely uncharacterised. Now, Gianni Carraro, Saverio Bellusci and colleagues report that miR-142-3p modulates WNT signalling to balance mesenchymal cell proliferation and differentiation during mouse lung development (p. 1272). Using microarray analyses, the researchers identify miR-142-3p as highly expressed in the embryonic lung mesenchyme. Importantly, loss-of-function assays demonstrate that miR-142-3pregulates cell proliferation specifically in the mesenchyme; in the absence of miR-142-3p, progenitor cells prematurely differentiate. They also show that miR-142-3p binds to and regulates the expression of mRNA encoding APC, a negative regulator of WNT signalling. Accordingly, miR-142-3p knockdown can be rescued by activating WNT or reducing APC expression in the mesenchyme. Based on their findings, the authors propose that miR-142-3p adds an extra layer of control to the WNT-FGF feedback loop that operates in the lung mesenchyme to correctly balance cell proliferation and differentiation.

 

PLUS…

 

Growing older gracefully - a review of the 10th edition of Developmental Biology

First published in 1985, Developmental Biology by Scott F. Gilbert has been an influential textbook for a generation of scientists. Here, Timothy Weil provides a brief review of the latest (10th) edition of the series. He comments on updates to the text, highlights details of particular interest to lecturers, and compares this book with other resources available in the internet era. See the Spotlight article on p. 1177

 

RNA polymerase II pausing during development

The transcription of developmental control genes by RNA polymerase II (Pol II) is commonly regulated at the transition to productive elongation, resulting in the promoter-proximal accumulation of transcriptionally engaged but paused Pol II. In their poster article, Bjoern Gaertner and Julia Zeitlinger review the mechanisms and possible functions of Pol II pausing during development. See the Development at a Glance article on p. 1179

 

Shared signaling systems in myeloid cell-mediated muscle regeneration

Much of the focus in muscle regeneration has been placed on identifying and delivering stem cells to promote regenerative capacity. As these efforts have advanced, we have learned that complex features of the microenvironment in which regeneration occurs can determine the success of these approaches . Here, James Tidball and colleagues discuss how myeloid cells can influence muscle regeneration, focussing on how processes in muscle and myeloid cells are co-regulated. See the Review on p. 1184

 

 

 

Here are the highlights from the current issue of Development:

 

Time to update the mammalian CV

The dorsal aorta (DA) and the cardinal vein (CV) are the first vascular pair to form during development. Although it is generally accepted that the DA derives from angioblasts, the cellular origin of the mammalian CV is currently unknown. Now, on p. 1120, Rong Wang and colleagues reveal that the mammalian CV is formed, at least in part, from endothelial cells that originate from within the DA. Using markers specific to either aortic or venous-fated endothelial cells, the authors show that the DA contains a mixed population of endothelial cells with either venous or arterial identity, but that the number of venous-fated endothelial cells decreases over time. The authors use time-lapse microscopy to visualise the migration of endothelial cells away from the DA and to the CV, and further show that this process requires ephrin B2/EphB4 signalling. The authors propose a mechanism whereby ephrin B2/EphB4-mediated cell repulsion drives the segregation of venous-fated endothelial cells away from the DA, facilitating their movement to the CV.

 

Mammary stratification caught on camera

During development, select epithelial cells must undergo asymmetric division in order to generate a stratified epithelium. Previous studies have shown that basally positioned epithelial stem cells orchestrate this process, but the mammary epithelium has two distinct cell layers inside the basement membrane, so it remains unclear which cell type is responsible for stratification. Using elegant time-lapse imaging of three-dimensional mouse mammary epithelial cultures, Andrew Ewald and colleagues now reveal (p. 1085) that it is the apical luminal epithelial cells that divide vertically to generate the stratified mammary epithelium. The authors show that this is accompanied by the loss of tight junctions, as marked by ZO-1, as well as loss of apicobasal polarity in the new daughter cells. Importantly, the authors also demonstrate that this mechanism of stratification and loss of polarity operates during early oncogenesis in the mouse mammary epithelium. These data uncover a common cellular mechanism that underpins the developmental and oncogenic stratification of mammary tissue.

 

An epigenetic bag of TRX

The Drosophila Trithorax (TRX) protein plays a key role in maintaining active transcription of many master cell fate regulatory genes. Previously, TRX was thought to function mainly at the promoter by depositing the histone H3K4 trimethylation mark (H3K4me3) via its catalytic SET domain. However, recent findings have challenged this view, prompting new investigations into the function of TRX. Now, on p. 1129, Peter Harte, Feng Tie and colleagues reveal that TRX, along with TRX-related (TRR), is responsible for histone H3K4 monomethylation (H3K4me1), and not trimethylation, suggesting a role for TRX in stimulating enhancer-dependent transcription. In vivo studies support these findings, as a catalytically inactive form of TRX results in reduced H3K4me1, but no change in H3K4me3, in Drosophila embryos. The authors also show that TRX collaborates directly with CREB-binding protein (CBP) to promote robust H3K27 acetylation, which antagonises Polycomb silencing and may also stimulate enhancer-dependent transcription. These data provide exciting new insights into the mechanisms of epigenetic regulation in developing organisms.

 

Oct4: the plot thickens

The transcription factor Oct4 is well known for its role in maintaining pluripotency in vitro and for preventing ectopic differentiation of early embryos in vivo. Recent evidence also suggests a role for Oct4 in lineage specification; however, little is known about how this occurs and the manner in which Oct4 is required. Now, on p. 1001, Jennifer Nichols and colleagues report a non-cell-autonomous requirement for sustained Oct4 expression during primitive endoderm (PrE) specification. The authors confirm that maternal and zygotic Oct4 are not required for development to the blastocyst stage, but that conditional inactivation of Oct4 in the early blastocyst results in reduced expression of PrE markers Sox17 and Gata4, and a failure to generate PrE-derived tissue in chimeric assays. Surprisingly, the formation of PrE can be rescued if the conditionally inactivated Oct4 mutants are injected at the pre-blastocyst stage with wild-type embryonic stem cells, suggesting that Oct4 is not required to operate cell-autonomously in order to specify the PrE.

 

RAd migration in the cortex

The generation of layer-specific cortical neurons is fundamental to the circuitry of the developing and adult brain. Although the intrinsic drivers of neuronal specification are becoming increasingly understood, the extrinsic signals that guide migration and consolidate post-mitotic neuronal identity are less clear. In this issue (p. 1151), Shanthini Sockanathan and colleagues investigate the role of endogenous retinoic acid (RA) signalling in regulating the radial migration and laminar fate of post-mitotic cortical neurons. Using a dominant-negative RA receptor construct, the authors show that ablation of RA signalling in mouse embryos in utero not only delays migration of subsets of cortical neurons, but also results in a failure to maintain their correct regional identity following migration. This phenotype can be partially rescued by stabilised β-catenin, which the authors show is normally maintained by RA signalling. This work sheds light on the extrinsic mechanisms that control cortical neuronal development and has important implications for disorders in which cortical neuronal circuitry is de-regulated.

 

Maternal undernutrition stimulates embryo endocytosis

The production of healthy offspring depends on many factors spanning from intrinsic genetic elements to variations in the in uteroenvironment. Poor maternal diet is associated with increased risk of cardiovascular, metabolic and behavioural disorders during later life of the offspring, but how the developing embryo copes with maternal dietary stress has not been well characterised. Now, on p. 1140, Tom Fleming and colleagues investigate the compensatory mechanisms that are activated when the early embryo is challenged by poor maternal nutrition. Using quantitative imaging techniques and extensive marker analyses, the authors show that a restricted-protein maternal diet results in stimulated endocytosis within both the trophectoderm and the primitive endoderm of the early mouse embryo to overcome the shortfall in nutrient supply. The authors show that enhanced trophectoderm endocytosis occurs in response to reduced branched-chain amino acids and is mediated via RhoA GTPase signalling. This exciting finding is an important step in uncovering the cellular mechanisms that underpin disorders caused by poor maternal nutrition.

 

PLUS…

 

Cell competition: how to eliminate your neighbours

It has been known for years, based on studies of Drosophila,  that viable cells can be eliminated by their neighbours through a process termed cell competition. New studies in mammals have revealed that this process is universal and that many factors and mechanisms are conserved.  Here, Amoyel and Bach provide an overview of the mechanistic steps involved in cell competition and discuss recent advances in the field, which have shed light on how and why cell competition exists in developing and adult organisms. See the Review on p. 988

 

The Mediator complex: a master coordinator of transcription and cell lineage development

Mediator is a multiprotein complex that is required for gene transcription by RNA polymerase II. Here, Yin and Wang describe the most recent advances in understanding the mechanisms of Mediator function, with an emphasis on its role during development and disease. See the Primer on p. 977

 

 

Here are the highlights from the current issue of Development:

 

Integrating integrin signals for neocortical growth

Within the developing neocortex, multiple progenitor cell types contribute to neuronal production, and the properties and relative abundance of these populations help to define the extent of neocortical growth. As well as apically localised radial glial cells (RGCs) that comprise the stem cell compartment, various populations of basal progenitors (BPs) exist - including basal RGCs, with both self-renewal and proliferative capacity, and more restricted progenitors. The stem cell-like properties of RGCs are linked to the fact that these cells are connected to the basal lamina, from which they receive proliferative signals via integrins. Now, Denise Stenzel et al. investigate in rodents the role of integrin α_vβ₃ in regulating the proliferative capacity of BPs (p. 795). They find that activation of this integrin induces BP proliferation both in hemisphere culture and in vivo, promoting proliferative division at the expense of neurogenic division. Moreover, they show that the α_vβ₃ ligand thyroid hormone also regulates BP proliferation via this integrin - providing pro-proliferative signals to progenitor cells that are not connected to the basal lamina.

 

Tracing the origin of epidermal immune cells

Langerhans cells (LCs) are antigen-presenting cells of the epidermis, and play a key role in detecting pathogens in the skin and coordinating the immune response. However, the precursors of LCs during embryonic development are poorly characterised, particularly in humans. Here (p. 807), Adelheid Elbe-Bürger and colleagues analyse the characteristics of early LC precursors in human first-trimester foetal skin, and explore the potential of different populations of haematopoietic precursors to colonise the developing epidermis. They find an unexpected diversity in LC precursors in terms of cell-surface markers, contrasting with the homogeneous adult LC population. The authors then used in vitro skin equivalent cultures to test the ability of different cord blood-derived precursor populations to colonise the epidermis and differentiate as LCs. Various precursor subtypes show LC potential, and the data suggest that the skin equivalents produce those signals necessary to direct LC differentiation. Together, these data suggest a heterogeneous origin for human embryonic LC precursors during early stages of development.

 

How to make a heart: insights from machine learning

The Drosophila heart provides a relatively simple system for the analysis of gene regulatory networks (GRNs), as it comprises just two cell types - contractile cardial cells (CCs) and non-muscle pericardial cells (PCs). Moreover, many transcription factors (TFs) that regulate Drosophila heart development have been identified and shown to play conserved roles in mammals. On p. 878, Alan Michelson and co-workers take a machine learning approach to identify new cardiac enhancers. The methodology, which integrates chromatin immunoprecipitation data with TF motif mapping, is highly successful at predicting enhancers that drive expression in cardiac cells, including PC- or CC-specific enhancers. Among the highly over-represented TF motifs in putative cardiac enhancers are motifs for the Myb and Su(H) TFs; the authors show that Myb regulates progenitor cell division, while Su(H) controls the PC/CC lineage decision. This work demonstrates the utility of machine learning to identify novel players regulating organ formation, and to piece together the GRNs underlying cell fate specification.

 

Controlling meristem activity

The shoot apical meristem (SAM) of higher plants contains the stem cell population that contributes to all above-ground organs. During vegetative growth, leaf primordia emerge from the periphery of the SAM, and the SAM transitions to an inflorescence meristem to initiate the reproductive phase. The WUSCHEL transcription factor specifies stem cell fate, and hence controls the size and activity of the SAM. A gene network involving the CLAVATA signalling pathway, the microRNA miR166g and HD-ZIPIII transcription factors is responsible for regulating WUSCHEL levels. On p. 830, Leor Eshed Williams and colleagues add another layer to this regulatory system, defining a role for the ERECTA receptor kinase in restricting WUSCHEL expression. Plants overexpressing miR166g and carrying a mutation in ERECTA display a massively enlarged SAM, disrupted phyllotaxis patterns and defects in floral meristem formation. ERECTA acts independently of the CLAVATA system, and although the mechanisms by which it functions are not fully clear, this work adds another important player to the complex regulatory network underlying meristem activity.

 

Mcr connects cell connections with innate immunity

The Drosophila Mcr protein is a member of the thioester protein (TEP) family of proteins, which includes key regulators of innate immunity. Mcr is an unusual member of this family, possessing a putative transmembrane domain and lacking a key residue in the thioester motif. It has, nevertheless, been shown to be involved in phagocytic uptake in cultured Drosophila cells, although its putative immune functions have not been assessed in vivo. Two papers, from Stefan Luschnig and colleagues (p. 899) and Robert Ward and colleagues (p. 889), now identify Mcr as a new component of septate junctions (SJs), the invertebrate equivalent of tight junctions that form the paracellular barrier in epithelia.

Both studies identify lethal EMS mutations inMcr in independent genetic screens, finding mutant phenotypes typical of SJ components. Consistent with a putative role in SJ formation, the protein colocalises with other SJ proteins at the lateral membrane. They further find that Mcr localisation is dependent on core SJ components and that, conversely, SJ proteins are mislocalised in Mcr mutants. At the morphological and ultrastructural level, SJs are disrupted in the absence of Mcr, and functional assays demonstrate that Mcr is required to form an effective paracellular barrier. As well as identifying a new SJ protein essential for barrier integrity, these two studies suggest an intriguing link between SJs and innate immunity. The epithelial barrier represents the first line of defence against pathogen invasion, andDrosophila haemocytes are known to undergo an epithelialisation-like process when encapsulating pathogens in the haemolymph. The identification of Mcr as a protein involved in both SJ formation and innate immunity now provides a molecular connection, and opens up new avenues for investigating potential functional links, between these two seemingly disparate processes.

 

PLUS…

Cytonemes as specialized signaling filopodia

 

Thomas Kornberg and Sougate Roy review the evidence that establishes a fundamental and essential role for cytonemes as specialized filopodia that transport signaling proteins between signaling cells. See the Primer article on p. 729

 

Pax genes: regulators of lineage specification and progenitor cell maintenance

Pax genes encode a family of transcription factors that orchestrate complex processes of lineage determination in the developing embryo. Here, Judith Blake and Melanie Ziman review the molecular functions of Pax genes during development and detail the regulatory mechanisms by which they specify and maintain progenitor cells across various tissue lineages. See the Primer article on p. 737

 

How to make an intestine

James Wells and Jason Spence review how recent advances in developmental and stem cell biology have made it possible to generate complex, three-dimensional, human intestinal tissues in vitro through directed differentiation of human pluripotent stem cells. See the Primer on p. 752