The biomechanical basis of biased epithelial tube elongation in lung and kidney development.

During lung development, epithelial branches expand preferentially in a longitudinal direction. This bias in outgrowth has been linked to a bias in cell shape and in the cell division plane. How this bias arises is unknown. Here, we show that biased epithelial outgrowth occurs independent of the surrounding mesenchyme, of preferential turnover of the extracellular matrix at the bud tips and of FGF signalling. There is also no evidence for actin-rich filopodia at the bud tips. Rather, we find epithelial tubes to be collapsed during early lung and kidney development, and we observe fluid flow in the narrow tubes. By simulating the measured fluid flow inside segmented narrow epithelial tubes, we show that the shear stress levels on the apical surface are sufficient to explain the reported bias in cell shape and outgrowth. We use a cell-based vertex model to confirm that apical shear forces, unlike constricting forces, can give rise to both the observed bias in cell shapes and tube elongation. We conclude that shear stress may be a more general driver of biased tube elongation beyond its established role in angiogenesis. This article has an associated 'The people behind the papers' interview.

Morphological study of embryonic Chd8+/- mouse brains using light-sheet microscopy.

OBJECTIVE: Autism spectrum disorder (ASD) encompasses a group of neurodevelopmental conditions that remain poorly understood due to their genetic complexity. CHD8 is a risk allele strongly associated with ASD, and heterozygous Chd8 loss-of-function mice have been reported to exhibit macrocephaly in early postnatal stages. In this work, we sought to identify measurable brain alterations in early embryonic development. RESULTS: We performed light-sheet fluorescence microscopy imaging of N-cadherin stained and optically cleared Chd8+/- and wild-type mouse brains at embryonic day 12.5 (E12.5). We report a detailed morphometric characterization of embryonic brain shapes and cortical neuroepithelial apical architecture. While Chd8+/- characteristic expansion of the forebrain and midbrain was not observed this early in embryogenesis, a tendency for a decreased lateral ventricular sphericity and an increased intraocular distance in Chd8+/- brains was found compared to controls. This study advocates the use of high-resolution microscopy technologies and multi-scale morphometric analyses of target brain regions to explore the etiology and cellular basis of Chd8 haploinsufficiency.

Genetic manipulation of ureteric bud tip progenitors in the mammalian kidney through an Adamts18 enhancer driven tet-on inducible system.

The ureteric epithelial progenitor (UEP) population within the embryonic kidney generates the arborized epithelial network of the kidney's collecting system and plays a critical role in the expansion and induction of the surrounding nephron progenitor pool. Adamts18 shows UEP- restricted expression in the kidney and progenitor tip-restricted expression in several other organs undergoing branching epithelial growth. Adamts18 is encoded by 23 exons. Genetic removal of genomic sequence spanning exons 1 to 3 led to a specific loss of Adamts18 expression in UEPs, suggesting this region may encode a UEP-specific enhancer. Intron 2 (3 â€‹kb) was shown to have enhancer activity driving expression of the doxycycline inducible tet-on transcriptional regulator (rtTA) in an Adamts18en-rtTA transgenic mouse strain. Crossing Adamts18en-rtTA mice to a doxycycline dependent GFP reporter mouse enabled the live imaging of embryonic kidney explants. This facilitated the analysis of ureteric epithelial branching events at the cellular level. Ablation of UEPs at the initiation of ureteric bud outgrowth through the doxycycline-mediated induction of Diphtheria Toxin A (DTA) generated a range of phenotypes from complete kidneys agenesis, to duplex kidneys with double ureters. The latter outcome points to the potential of regulative processes to restore UEPs. In contrast, overexpression of YAP prior to ureteric bud outgrowth led to a complete failure of kidney development. Elevating YAP levels at later stages retarded branching growth. A similar phenotype was observed with the overexpression of MYC within the branch-tip localized UEP population. These experiments showcase the utility of the Adamts18en-rtTA transgenic model to the investigation of cellular and molecular events specific to branch tip progenitors within the mammalian kidney complementing existing CRE-dependent genetic tools. Further, the illustrative examples point to areas where new insight may be gained into the regulation of UEP programs.

Image-based modeling of kidney branching morphogenesis reveals GDNF-RET based Turing-type mechanism and pattern-modulating WNT11 feedback.

Branching patterns and regulatory networks differ between branched organs. It has remained unclear whether a common regulatory mechanism exists and how organ-specific patterns can emerge. Of all previously proposed signalling-based mechanisms, only a ligand-receptor-based Turing mechanism based on FGF10 and SHH quantitatively recapitulates the lung branching patterns. We now show that a GDNF-dependent ligand-receptor-based Turing mechanism quantitatively recapitulates branching of cultured wildtype and mutant ureteric buds, and achieves similar branching patterns when directing domain outgrowth in silico. We further predict and confirm experimentally that the kidney-specific positive feedback between WNT11 and GDNF permits the dense packing of ureteric tips. We conclude that the ligand-receptor based Turing mechanism presents a common regulatory mechanism for lungs and kidneys, despite the differences in the molecular implementation. Given its flexibility and robustness, we expect that the ligand-receptor-based Turing mechanism constitutes a likely general mechanism to guide branching morphogenesis and other symmetry breaks during organogenesis.

Wnt11 directs nephron progenitor polarity and motile behavior ultimately determining nephron endowment.

A normal endowment of nephrons in the mammalian kidney requires a balance of nephron progenitor self-renewal and differentiation throughout development. Here, we provide evidence for a novel action of ureteric branch tip-derived Wnt11 in progenitor cell organization and interactions within the nephrogenic niche, ultimately determining nephron endowment. In Wnt11 mutants, nephron progenitors dispersed from their restricted niche, intermixing with interstitial progenitors. Nephron progenitor differentiation was accelerated, kidneys were significantly smaller, and the nephron progenitor pool was prematurely exhausted, halving the final nephron count. Interestingly, RNA-seq revealed no significant differences in gene expression. Live imaging of nephron progenitors showed that in the absence of Wnt11 they lose stable attachments to the ureteric branch tips, continuously detaching and reattaching. Further, the polarized distribution of several markers within nephron progenitors is disrupted. Together these data highlight the importance of Wnt11 signaling in directing nephron progenitor behavior which determines a normal nephrogenic program.

Mathematical Approaches of Branching Morphogenesis.

Many organs require a high surface to volume ratio to properly function. Lungs and kidneys, for example, achieve this by creating highly branched tubular structures during a developmental process called branching morphogenesis. The genes that control lung and kidney branching share a similar network structure that is based on ligand-receptor reciprocal signalling interactions between the epithelium and the surrounding mesenchyme. Nevertheless, the temporal and spatial development of the branched epithelial trees differs, resulting in organs of distinct shape and size. In the embryonic lung, branching morphogenesis highly depends on FGF10 signalling, whereas GDNF is the driving morphogen in the kidney. Knockout of Fgf10 and Gdnf leads to lung and kidney agenesis, respectively. However, FGF10 plays a significant role during kidney branching and both the FGF10 and GDNF pathway converge on the transcription factors ETV4/5. Although the involved signalling proteins have been defined, the underlying mechanism that controls lung and kidney branching morphogenesis is still elusive. A wide range of modelling approaches exists that differ not only in the mathematical framework (e.g., stochastic or deterministic) but also in the spatial scale (e.g., cell or tissue level). Due to advancing imaging techniques, image-based modelling approaches have proven to be a valuable method for investigating the control of branching events with respect to organ-specific properties. Here, we review several mathematical models on lung and kidney branching morphogenesis and suggest that a ligand-receptor-based Turing model represents a potential candidate for a general but also adaptive mechanism to control branching morphogenesis during development.

Branching morphogenesis in the developing kidney is governed by rules that pattern the ureteric tree.

Metanephric kidney development is orchestrated by the iterative branching morphogenesis of the ureteric bud. We describe an underlying patterning associated with the ramification of this structure and show that this pattern is conserved between developing kidneys, in different parts of the organ and across developmental time. This regularity is associated with a highly reproducible branching asymmetry that is consistent with locally operative growth mechanisms. We then develop a class of tip state models to represent elaboration of the ureteric tree and describe rules for 'half-delay' branching morphogenesis that describe almost perfectly the patterning of this structure. Spatial analysis suggests that the observed asymmetry may arise from mutual suppression of bifurcation, but not extension, between the growing ureteric tips, and demonstrates that disruption of patterning occurs in mouse mutants in which the distribution of tips on the surface of the kidney is altered. These findings demonstrate that kidney development occurs by way of a highly conserved reiterative pattern of asymmetric bifurcation that is governed by intrinsic and locally operative mechanisms.

Luminal mitosis drives epithelial cell dispersal within the branching ureteric bud.

The ureteric bud is an epithelial tube that undergoes branching morphogenesis to form the renal collecting system. Although development of a normal kidney depends on proper ureteric bud morphogenesis, the cellular events underlying this process remain obscure. Here, we used time-lapse microscopy together with several genetic labeling methods to observe ureteric bud cell behaviors in developing mouse kidneys. We observed an unexpected cell behavior in the branching tips of the ureteric bud, which we term "mitosis-associated cell dispersal." Premitotic ureteric tip cells delaminate from the epithelium and divide within the lumen; although one daughter cell retains a basal process, allowing it to reinsert into the epithelium at the site of origin, the other daughter cell reinserts at a position one to three cell diameters away. Given the high rate of cell division in ureteric tips, this cellular behavior causes extensive epithelial cell rearrangements that may contribute to renal branching morphogenesis.

Dissecting and culturing and imaging the mouse urogenital system.

Current knowledge of the morphological and molecular events driving branching morphogenesis of the ureteric bud (UB) during development of the metanephric kidney has been greatly facilitated by the ability to explant this organ to culture. The UB can be further isolated from the mesenchyme and grown within a three-dimensional, collagen-based matrix when supplemented with the appropriate growth factors. The protocol presented here outlines the dissection and culture techniques necessary to dissect and culture the whole kidney and the isolated UB.

Isolation of high quality RNA from embryonic kidney and cells.

All the mRNAs within a cell and their relative levels are indicative of gene expression within that cell, which is essential for its structure and function in its differentiated state. Therefore, methods for the identification of the specific mRNAs and the quantitation of their levels are invaluable tools for understanding gene expression. Due to high endogenous RNase activity within virtually all living cells, the isolation of good quality RNA with minimal degradation is not a trivial task. This protocol outlines a tried and tested methodology for isolating high quality RNA from embryonic kidneys for various applications including microarray analysis and quantitative reverse transcription PCR (qRT-PCR).

Immunohistochemical staining of dpERK staining during early kidney development.

Signaling through the ERK/MAPK pathway within the Wolffian duct and ureteric bud epithelium is critical for kidney induction and branching morphogenesis. ERK signaling is activated by receptor tyrosine kinase such as RET and FGFR2. This protocol describes a method to detect the diphosphorylated form of ERK (dpERK) on paraffin embedded tissue of early mouse embryo.

The tyrosine phosphatase Shp2 acts downstream of GDNF/Ret in branching morphogenesis of the developing mouse kidney.

The tyrosine phosphatase Shp2 acts downstream of various growth factors, hormones or cytokine receptors. Mutations of the Shp2 gene are associated with several human diseases. Here we have ablated Shp2 in the developing kidneys of mice, using the ureteric bud epithelium-specific Hoxb7/Cre. Mutant mice produced a phenotype that is similar to mutations of the genes of the GDNF/Ret receptor system, that is: strongly reduced ureteric bud branching and downregulation of the Ret target genes Etv4 and Etv5. Shp2 mutant embryonic kidneys also displayed reduced cell proliferation at the branch tips and branching defects, which could not be overcome by GDNF in organ culture. We also examined compound mutants of Shp2 and Sprouty1, which is an inhibitor of receptor tyrosine kinase signaling in the kidney. Sprouty1 single mutants produce supernumerary ureteric buds, which branch excessively. Sprouty1 mutants rescued branching deficits in Ret(-/-) and GDNF(-/-) kidneys. Sprouty1; Shp2 double mutants showed no rescue of kidney branching. Our data thus indicate an intricate interplay of Shp2 and Sprouty1 in signaling downstream of receptor tyrosine kinases during kidney development. Apparently, Shp2 mediates not only GDNF/Ret but also signaling by other receptor tyrosine kinases in branching morphogenesis of the embryonic kidney.

Kidney development in the absence of Gdnf and Spry1 requires Fgf10.

GDNF signaling through the Ret receptor tyrosine kinase (RTK) is required for ureteric bud (UB) branching morphogenesis during kidney development in mice and humans. Furthermore, many other mutant genes that cause renal agenesis exert their effects via the GDNF/RET pathway. Therefore, RET signaling is believed to play a central role in renal organogenesis. Here, we re-examine the extent to which the functions of Gdnf and Ret are unique, by seeking conditions in which a kidney can develop in their absence. We find that in the absence of the negative regulator Spry1, Gdnf, and Ret are no longer required for extensive kidney development. Gdnf-/-;Spry1-/- or Ret-/-;Spry1-/- double mutants develop large kidneys with normal ureters, highly branched collecting ducts, extensive nephrogenesis, and normal histoarchitecture. However, despite extensive branching, the UB displays alterations in branch spacing, angle, and frequency. UB branching in the absence of Gdnf and Spry1 requires Fgf10 (which normally plays a minor role), as removal of even one copy of Fgf10 in Gdnf-/-;Spry1-/- mutants causes a complete failure of ureter and kidney development. In contrast to Gdnf or Ret mutations, renal agenesis caused by concomitant lack of the transcription factors ETV4 and ETV5 is not rescued by removing Spry1, consistent with their role downstream of both RET and FGFRs. This shows that, for many aspects of renal development, the balance between positive signaling by RTKs and negative regulation of this signaling by SPRY1 is more critical than the specific role of GDNF. Other signals, including FGF10, can perform many of the functions of GDNF, when SPRY1 is absent. But GDNF/RET signaling has an apparently unique function in determining normal branching pattern. In contrast to GDNF or FGF10, Etv4 and Etv5 represent a critical node in the RTK signaling network that cannot by bypassed by reducing the negative regulation of upstream signals.

Kidney development: from ureteric bud formation to branching morphogenesis.

Epithelial branching morphogenesis is critical to the formation of various organs such as the vasculature, mammary glands, lungs, and kidneys in vertebrate embryos. One fascinating aspect of branching morphogenesis is to understand how a simple epithelial tube grows by reiterative branching to form a complex epithelial tree structure. Recent studies combining mouse genetics and chimeric analysis with live imaging have uncovered the molecular networks and interactions that govern kidney branching morphogenesis. This review focuses on ureteric bud (UB) formation and epithelial branching during kidney development. The invasion of the metanephric mesenchyme by the UB is a fundamental step toward establishing the cyto-architecture of the kidney and determining the number of nephrons, which form the filtration units of the adult kidney.

Ret-dependent cell rearrangements in the Wolffian duct epithelium initiate ureteric bud morphogenesis.

While the genetic control of renal branching morphogenesis has been extensively described, the cellular basis of this process remains obscure. GDNF/RET signaling is required for ureter and kidney development, and cells lacking Ret are excluded from the tips of the branching ureteric bud in chimeric kidneys. Here, we find that this exclusion results from earlier Ret-dependent cell rearrangements in the caudal Wolffian duct, which generate a specialized epithelial domain that later emerges as the tip of the primary ureteric bud. By juxtaposing cells with elevated or reduced RET activity, we find that Wolffian duct cells compete, based on RET signaling levels, to contribute to this domain. At the same time, the caudal Wolffian duct transiently converts from a simple to a pseudostratified epithelium, a process that does not require Ret. Thus, both Ret-dependent cell movements and Ret-independent changes in the Wolffian duct epithelium contribute to ureteric bud formation.

Reduction of BMP4 activity by gremlin 1 enables ureteric bud outgrowth and GDNF/WNT11 feedback signalling during kidney branching morphogenesis.

Antagonists act to restrict and negatively modulate the activity of secreted signals during progression of embryogenesis. In mouse embryos lacking the extra-cellular BMP antagonist gremlin 1 (Grem1), metanephric development is disrupted at the stage of initiating ureteric bud outgrowth. Treatment of mutant kidney rudiments in culture with recombinant gremlin 1 protein induces additional epithelial buds and restores outgrowth and branching. All epithelial buds express Wnt11, and Gdnf is significantly upregulated in the surrounding mesenchyme, indicating that epithelial-mesenchymal (e-m) feedback signalling is restored. In the wild type, Bmp4 is expressed by the mesenchyme enveloping the Wolffian duct and ureteric bud and Grem1 is upregulated in the mesenchyme around the nascent ureteric bud prior to initiation of its outgrowth. In agreement, BMP activity is reduced locally as revealed by lower levels of nuclear pSMAD protein in the mesenchyme. By contrast, in Grem1-deficient kidney rudiments, pSMAD proteins are detected in many cell nuclei in the metanephric mesenchyme, indicative of excessive BMP signal transduction. Indeed, genetic lowering of BMP4 levels in Grem1-deficient mouse embryos completely restores ureteric bud outgrowth and branching morphogenesis. The reduction of BMP4 levels in Grem1 mutant embryos enables normal progression of renal development and restores adult kidney morphology and functions. This study establishes that initiation of metanephric kidney development requires the reduction of BMP4 activity by the antagonist gremlin 1 in the mesenchyme, which in turn enables ureteric bud outgrowth and establishment of autoregulatory GDNF/WNT11 feedback signalling.

Protease nexin 1 and its receptor LRP modulate SHH signalling during cerebellar development.

Development of the postnatal cerebellum relies on the tight regulation of cell number by morphogens that control the balance between cell proliferation, survival and differentiation. Here, we analyze the role of the serine-protease inhibitor protease nexin 1 (PN-1; SERPINE2) in the proliferation and differentiation of cerebellar granular neuron precursors (CGNPs) via the modulation of their main mitogenic factor, sonic hedgehog (SHH). Our studies show that PN-1 interacts with low-density lipoprotein receptor-related proteins (LRPs) to antagonize SHH-induced CGNP proliferation and that it inhibits the activity of the SHH transcriptional target GLI1. The binding of PN-1 to LRPs interferes with SHH-induced cyclin D1 expression. CGNPs isolated from Pn-1-deficient mice exhibit enhanced basal proliferation rates due to overactivation of the SHH pathway and show higher sensitivity to exogenous SHH. In vivo, the Pn-1 deficiency alters the expression of SHH target genes. In addition, the onset of CGNP differentiation is delayed, which results in an enlarged outer external granular layer. Furthermore, the Pn-1 deficiency leads to an overproduction of CGNPs and to enlargement of the internal granular layer in a subset of cerebellar lobes during late development and adulthood. We propose that PN-1 contributes to shaping the cerebellum by promoting cell cycle exit.

Differential regulation of gene expression in the digit forming area of the mouse limb bud by SHH and gremlin 1/FGF-mediated epithelial-mesenchymal signalling.

Spatially and temporally coordinated changes in gene expression are crucial to orderly progression of embryogenesis. We combine mouse genetics with experimental manipulation of signalling to analyze the kinetics by which the SHH morphogen and the BMP antagonist gremlin 1 (GREM1) control gene expression in the digit-forming mesenchyme of mouse limb buds. Although most mesenchymal cells respond rapidly to SHH signalling, the transcriptional upregulation of specific SHH target signals in the mesenchyme occurs with differential temporal kinetics and in a spatially restricted fashion. In particular, the expression of the BMP antagonist Grem1 is always upregulated in mesenchymal cells located distal to the SHH source and acts upstream of FGF signalling by the apical ectodermal ridge. GREM1/FGF-mediated feedback signalling is, in turn, required to propagate SHH and establish the presumptive digit expression domains of the Notch ligand jagged 1 (Jag1) and 5'Hoxd genes in the distal limb bud mesenchyme. Their establishment is significantly delayed in Grem1-deficient limb buds and cannot be rescued by specific restoration of SHH signalling in mutant limb buds. This shows that GREM1/FGF feedback signalling is required for regulation of the temporal kinetics of the mesenchymal response to SHH signalling. Finally, inhibition of SHH signal transduction at distinct time points reveals the differential temporal dependence of Grem1, Jag1 and 5'Hoxd gene expression on SHH signalling. In particular, the expression of Hoxd13 depends on SHH signal transduction significantly longer than does Hoxd11 expression, revealing that the reverse co-linear establishment, but not maintenance of their presumptive digit expression domains, depends on SHH signalling.

Gremlin-mediated BMP antagonism induces the epithelial-mesenchymal feedback signaling controlling metanephric kidney and limb organogenesis.

Epithelial-mesenchymal feedback signaling is the key to diverse organogenetic processes such as limb bud development and branching morphogenesis in kidney and lung rudiments. This study establishes that the BMP antagonist gremlin (Grem1) is essential to initiate these epithelial-mesenchymal signaling interactions during limb and metanephric kidney organogenesis. A Grem1 null mutation in the mouse generated by gene targeting causes neonatal lethality because of the lack of kidneys and lung septation defects. In early limb buds, mesenchymal Grem1 is required to establish a functional apical ectodermal ridge and the epithelial-mesenchymal feedback signaling that propagates the sonic hedgehog morphogen. Furthermore, Grem1-mediated BMP antagonism is essential to induce metanephric kidney development as initiation of ureter growth, branching and establishment of RET/GDNF feedback signaling are disrupted in Grem1-deficient embryos. As a consequence, the metanephric mesenchyme is eliminated by apoptosis, in the same way as the core mesenchymal cells of the limb bud.

Mouse limb deformity mutations disrupt a global control region within the large regulatory landscape required for Gremlin expression.

The mouse limb deformity (ld) mutations cause limb malformations by disrupting epithelial-mesenchymal signaling between the polarizing region and the apical ectodermal ridge. Formin was proposed as the relevant gene because three of the five ld alleles disrupt its C-terminal domain. In contrast, our studies establish that the two other ld alleles directly disrupt the neighboring Gremlin gene, corroborating the requirement of this BMP antagonist for limb morphogenesis. Further doubts concerning an involvement of Formin in the ld limb phenotype are cast, as a targeted mutation removing the C-terminal Formin domain by frame shift does not affect embryogenesis. In contrast, the deletion of the corresponding genomic region reproduces the ld limb phenotype and is allelic to mutations in Gremlin. We resolve these conflicting results by identifying a cis-regulatory region within the deletion that is required for Gremlin activation in the limb bud mesenchyme. This distant cis-regulatory region within Formin is also altered by three of the ld mutations. Therefore, the ld limb bud patterning defects are not caused by disruption of Formin, but by alteration of a global control region (GCR) required for Gremlin transcription. Our studies reveal the large genomic landscape harboring this GCR, which is required for tissue-specific coexpression of two structurally and functionally unrelated genes.