Tracheal occlusion increases the rate of epithelial branching of embryonic mouse lung via the FGF10-FGFR2b-Sprouty2 pathway

Vangl-dependent mesenchymal thinning shapes the distal lung during murine sacculation

The planar cell polarity (PCP) complex is speculated to function in murine lung development, where branching morphogenesis generates an epithelial tree whose distal tips expand dramatically during sacculation. Here, we show that PCP is dispensable in the airway epithelium for sacculation. Rather, we find a Celsr1-independent role for the PCP component Vangl in the pulmonary mesenchyme: loss of Vangl1/2 inhibits mesenchymal thinning and expansion of the saccular epithelium. Further, loss of mesenchymal Wnt5a mimics sacculation defects observed in Vangl2-mutant lungs, implicating mesenchymal Wnt5a/Vangl signaling as a key regulator of late lung morphogenesis. A computational model predicts that sacculation requires a fluid mesenchymal compartment. Lineage-tracing and cell-shape analyses are consistent with the mesenchyme acting as a fluid tissue, suggesting that loss of Vangl1/2 impacts the ability of mesenchymal cells to exchange neighbors. Our data thus identify an explicit function for Vangl and the pulmonary mesenchyme in actively shaping the saccular epithelium.

ERK-mediated curvature feedback regulates branching morphogenesis in lung epithelial tissue

Intricate branching patterns emerge in internal organs due to the recurrent occurrence of simple deformations in epithelial tissues. During murine lung development, epithelial cells in distal tips of the single tube require fibroblast growth factor (FGF) signals emanating from their surrounding mesenchyme to form repetitive tip bifurcations. However, it remains unknown how the cells employ FGF signaling to convert their behaviors to achieve the recursive branching processes. Here, we show a mechano-chemical regulatory system underlying lung branching morphogenesis, orchestrated by extracellular signal-regulated kinase (ERK) as a downstream driver of FGF signaling. We found that tissue-scale curvature regulated ERK activity in the lung epithelium using two-photon live cell imaging and mechanical perturbations. ERK activation occurs specifically in epithelial tissues exhibiting positive curvature, regardless of whether the change in curvature was attributable to morphogenesis or perturbations. Moreover, ERK activation accelerates actin polymerization preferentially at the apical side of cells, mechanically contributing to the extension of the apical membrane, culminating in a reduction of epithelial tissue curvature. These results indicate the existence of a negative feedback loop between tissue curvature and ERK activity that transcends spatial scales. Our mathematical model confirms that this regulatory mechanism is sufficient to generate the recursive branching processes. Taken together, we propose that ERK orchestrates a curvature feedback loop pivotal to the self-organized patterning of tissues.

Mechanobiological approaches to synthetic morphogenesis: learning by building

Tissue morphogenesis occurs in a complex physicochemical microenvironment with limited experimental accessibility. This often prevents a clear identification of the processes that govern the formation of a given functional shape. By applying state-of-the-art methods to minimal tissue systems, synthetic morphogenesis aims to engineer the discrete events that are necessary and sufficient to build specific tissue shapes. Here, we review recent advances in synthetic morphogenesis, highlighting how a combination of microfabrication and mechanobiology is fostering our understanding of how tissues are built.

Lung Development in a Dish: Models to Interrogate the Cellular Niche and the Role of Mechanical Forces in Development

Over the past decade, emphasis has been placed on recapitulating in vitro the architecture and multicellular interactions found in organs in vivo [1, 2]. Whereas traditional reductionist approaches to in vitro models enable teasing apart the precise signaling pathways, cellular interactions, and response to biochemical and biophysical cues, model systems that incorporate higher complexity are needed to ask questions about physiology and morphogenesis at the tissue scale. Significant advancements have been made in establishing in vitro models of lung development to understand cell-fate specification, gene regulatory networks, sexual dimorphism, three-dimensional organization, and how mechanical forces interact to drive lung organogenesis [3-5]. In this chapter, we highlight recent advances in the rapid development of various lung organoids, organ-on-a-chip models, and whole lung ex vivo explant models currently used to dissect the roles of these cellular signals and mechanical cues in lung development and potential avenues for future investigation (Fig. 3.1).

Intraluminal chloride regulates lung branching morphogenesis: involvement of PIEZO1/PIEZO2

BACKGROUND

Clinical and experimental evidence shows lung fluid volume as a modulator of fetal lung growth with important value in treating fetal lung hypoplasia. Thus, understanding the mechanisms underlying these morphological dynamics has been the topic of multiple investigations with, however, limited results, partially due to the difficulty of capturing or recapitulating these movements in the lab. In this sense, this study aims to establish an ex vivo model allowing the study of lung fluid function in branching morphogenesis and identify the subsequent molecular/ cellular mechanisms.

METHODS

Ex vivo lung explant culture was selected as a model to study branching morphogenesis, and intraluminal injections were performed to change the composition of lung fluid. Distinct chloride (Cl^-) concentrations (5.8, 29, 143, and 715 mM) or Cl^- channels inhibitors [antracene-9-carboxylic acid (A9C), cystic fibrosis transmembrane conductance regulator inhibitor172 (CFTRinh), and calcium-dependent Cl^- channel inhibitorA01 (CaCCinh)] were injected into lung lumen at two timepoints, day0 (D0) and D2. At D4, morphological and molecular analyses were performed in terms of branching morphogenesis, spatial distribution (immunofluorescence), and protein quantification (western blot) of mechanoreceptors (PIEZO1 and PIEZO2), neuroendocrine (bombesin, ghrelin, and PGP9.5) and smooth muscle [alpha-smooth muscle actin (α-SMA) and myosin light chain 2 (MLC2)] markers.

RESULTS

For the first time, we described effective intraluminal injections at D0 and D2 and demonstrated intraluminal movements at D4 in ex vivo lung explant cultures. Through immunofluorescence assay in in vivo and ex vivo branching morphogenesis, we show that PGP9.5 colocalizes with PIEZO1 and PIEZO2 receptors. Fetal lung growth is increased at higher [Cl^-], 715 mM Cl^-, through the overexpression of PIEZO1, PIEZO2, ghrelin, bombesin, MLC2, and α-SMA. In contrast, intraluminal injection of CFTRinh or CaCCinh decreases fetal lung growth and the expression of PIEZO1, PIEZO2, ghrelin, bombesin, MLC2, and α-SMA. Finally, the inhibition of PIEZO1/PIEZO2 by GsMTx4 decreases branching morphogenesis and ghrelin, bombesin, MLC2, and α-SMA expression in an intraluminal injection-independent manner.

CONCLUSIONS

Our results identify PIEZO1/PIEZO2 expressed in neuroendocrine cells as a regulator of fetal lung growth induced by lung fluid.

FGF10 ameliorates lipopolysaccharide-induced acute lung injury in mice via the BMP4-autophagy pathway

Introduction: Damage to alveolar epithelial cells caused by uncontrolled inflammation is considered to be the main pathophysiological change in acute lung injury. FGF10 plays an important role as a fibroblast growth factor in lung development and lung diseases, but its protective effect against acute lung injury is unclear. Therefore, this study aimed to investigate protective effect and mechanism of FGF10 on acute lung injury in mice. Methods: ALI was induced by intratracheal injection of LPS into 57BL/6J mice. Six hours later, lung bronchoalveolar lavage fluid (BALF) was acquired to analyse cells, protein and the determination of pro-inflammatory factor levels, and lung issues were collected for histologic examination and wet/dry (W/D) weight ratio analysis and blot analysis of protein expression. Results: We found that FGF10 can prevent the release of IL-6, TNF-α, and IL-1β, increase the expression of BMP4 and autophagy pathway, promote the regeneration of alveolar epithelial type Ⅱ cells, and improve acute lung injury. BMP4 gene knockdown decreased the protective effect of FGF10 on the lung tissue of mice. However, the activation of autophagy was reduced after BMP4 inhibition by Noggin. Additionally, the inhibition of autophagy by 3-MA also lowered the protective effect of FGF10 on alveolar epithelial cells induced by LPS. Conclusions: These data suggest that the protective effect of FGF10 is related to the activation of autophagy and regeneration of alveolar epithelial cells in an LPS-induced ALI model, and that the activation of autophagy may depend on the increase in BMP4 expression.

Trans-epithelial fluid flow and mechanics of epithelial morphogenesis

Active fluid transport across epithelial monolayers is emerging as a major driving force of tissue morphogenesis in a variety of healthy and diseased systems, as well as during embryonic development. Cells use directional transport of ions and osmotic gradients to drive fluid flow across the cell surface, in the process also building up fluid pressure. The basic physics of this process is described by the osmotic engine model, which also underlies actin-independent cell migration. Recently, the trans-epithelial fluid flux and the hydraulic pressure gradient have been explicitly measured for a variety of cellular and tissue model systems across various species. For the kidney, it was shown that tubular epithelial cells behave as active mechanical fluid pumps: the trans-epithelial fluid flux depends on the hydraulic pressure difference across the epithelial layer. When a stall pressure is reached, the fluid flux vanishes. Hydraulic forces generated from active fluid pumping are important in tissue morphogenesis and homeostasis, and could also underlie multiple morphogenic events seen in other developmental contexts. In this review, we highlight findings that examined the role of trans-epithelial fluid flux and hydraulic pressure gradient in driving tissue-scale morphogenesis. We also review organ pathophysiology due to impaired fluid pumping and the loss of hydraulic pressure sensing at the cellular scale. Finally, we draw an analogy between cellular fluidic pumps and a connected network of water pumps in a city. The dynamics of fluid transport in an active and adaptive network is determined globally at the systemic level, and transport in such a network is best when each pump is operating at its optimal efficiency.

Focal sources of FGF-10 promote the buckling morphogenesis of the embryonic airway epithelium

During airway branching morphogenesis, focal regions of FGF-10 expression in the pulmonary mesenchyme are thought to provide a local guidance cue, which promotes chemotactically the directional outgrowth of the airway epithelium. Here, however, we show that an ectopic source of FGF-10 induces epithelial buckling morphogenesis and the formation of multiple new supernumerary buds. FGF-10-induced budding can be modulated by altered epithelial tension and luminal fluid pressure. Increased tension suppresses the formation of ectopic branches, while a collapse of the embryonic airway promotes more expansive buckling and additional FGF-10-induced supernumerary buds. Our results indicate that a focal source of FGF-10 can promote epithelial buckling and suggest that the overall branching pattern cannot be explained entirely by the templated expression of FGF-10. Both FGF-10-mediated cell behaviors and exogenous mechanical forces must be integrated to properly shape the bronchial tree.

Stress ball morphogenesis: How the lizard builds its lung

The function of the lung is closely coupled to its structural anatomy, which varies greatly across vertebrates. Although architecturally simple, a complex pattern of airflow is thought to be achieved in the lizard lung due to its cavernous central lumen and honeycomb-shaped wall. We find that the wall of the lizard lung is generated from an initially smooth epithelial sheet, which is pushed through holes in a hexagonal smooth muscle meshwork by forces from fluid pressure, similar to a stress ball. Combining transcriptomics with time-lapse imaging reveals that the hexagonal meshwork self-assembles in response to circumferential and axial stresses downstream of pressure. A computational model predicts the pressure-driven changes in epithelial topology, which we probe using optogenetically driven contraction of 3D-printed engineered muscle. These results reveal the physical principles used to sculpt the unusual architecture of the lizard lung, which could be exploited as a novel strategy to engineer tissues.

Negative Transpulmonary Pressure Disrupts Airway Morphogenesis by Suppressing Fgf10

Mechanical forces are increasingly recognized as important determinants of cell and tissue phenotype and also appear to play a critical role in organ development. During the fetal stages of lung morphogenesis, the pressure of the fluid within the lumen of the airways is higher than that within the chest cavity, resulting in a positive transpulmonary pressure. Several congenital defects decrease or reverse transpulmonary pressure across the developing airways and are associated with a reduced number of branches and a correspondingly underdeveloped lung that is insufficient for gas exchange after birth. The small size of the early pseudoglandular stage lung and its relative inaccessibility in utero have precluded experimental investigation of the effects of transpulmonary pressure on early branching morphogenesis. Here, we present a simple culture model to explore the effects of negative transpulmonary pressure on development of the embryonic airways. We found that negative transpulmonary pressure decreases branching, and that it does so in part by altering the expression of fibroblast growth factor 10 (Fgf10). The morphogenesis of lungs maintained under negative transpulmonary pressure can be rescued by supplementing the culture medium with exogenous FGF10. These data suggest that Fgf10 expression is regulated by mechanical stress in the developing airways. Understanding the mechanical signaling pathways that connect transpulmonary pressure to FGF10 can lead to the establishment of novel non-surgical approaches for ameliorating congenital lung defects.

Conserved Mechanisms in the Formation of the Airways and Alveoli of the Lung

Branching is an intrinsic property of respiratory epithelium that can be induced and modified by signals emerging from the mesenchyme. However, during stereotypic branching morphogenesis of the airway, the relatively thick upper respiratory epithelium extrudes through a mesenchymal orifice to form a new branch, whereas during alveologenesis the relatively thin lower respiratory epithelium extrudes to form sacs or bubbles. Thus, both branching morphogenesis of the upper airway and alveolarization in the lower airway seem to rely on the same fundamental physical process: epithelial extrusion through an orifice. Here I propose that it is the orientation and relative stiffness of the orifice boundary that determines the stereotypy of upper airway branching as well as the orientation of individual alveolar components of the gas exchange surface. The previously accepted dogma of the process of alveologenesis, largely based on 2D microscopy, is that alveoli arise by erection of finger-like interalveolar septae to form septal clefts that subdivide pre-existing saccules, a process for which the contractile properties of specialized alveolar myofibroblasts are necessary. Here I suggest that airway tip splitting and stereotypical side domain branching are actually conserved processes, but modified somewhat by evolution to achieve both airway tip splitting and side branching of the upper airway epithelium, as well as alveologenesis. Viewed in 3D it is clear that alveolar “septal tips” are in fact ring or purse string structures containing elastin and collagen that only appear as finger like projections in cross section. Therefore, I propose that airway branch orifices as well as alveolar mouth rings serve to delineate and stabilize the budding of both airway and alveolar epithelium, from the tips and sides of upper airways as well as from the sides and tips of alveolar ducts. Certainly, in the case of alveoli arising laterally and with radial symmetry from the sides of alveolar ducts, the mouth of each alveolus remains within the plane of the side of the ductal lumen. This suggests that the thin epithelium lining these lateral alveolar duct buds may extrude or “pop out” from the duct lumen through rings rather like soap or gum bubbles, whereas the thicker upper airway epithelium extrudes through a ring like toothpaste from a tube to form a new branch.

Organ-Specific Branching Morphogenesis

A common developmental process, called branching morphogenesis, generates the epithelial trees in a variety of organs, including the lungs, kidneys, and glands. How branching morphogenesis can create epithelial architectures of very different shapes and functions remains elusive. In this review, we compare branching morphogenesis and its regulation in lungs and kidneys and discuss the role of signaling pathways, the mesenchyme, the extracellular matrix, and the cytoskeleton as potential organ-specific determinants of branch position, orientation, and shape. Identifying the determinants of branch and organ shape and their adaptation in different organs may reveal how a highly conserved developmental process can be adapted to different structural and functional frameworks and should provide important insights into epithelial morphogenesis and developmental disorders.

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.

A multiscale computational model of angiogenesis after traumatic brain injury, investigating the role location plays in volumetric recovery

Vascular endothelial growth factor (VEGF) is a key protein involved in the process of angiogenesis. VEGF is of particular interest after a traumatic brain injury (TBI), as it re-establishes the cerebral vascular network in effort to allow for proper cerebral blood flow and thereby oxygenation of damaged brain tissue. For this reason, angiogenesis is critical in the progression and recovery of TBI patients in the days and weeks post injury. Although well established experimental work has led to advances in our understanding of TBI and the progression of angiogenisis, many constraints still exist with existing methods, especially when considering patient progression in the days following injury. To better understand the healing process on the proposed time scales, we develop a computational model that quickly simulates vessel growth and recovery by coupling VEGF and its interactions with its associated receptors to a physiologically inspired fractal model of the microvascular re-growth. We use this model to clarify the role that diffusivity, receptor kinetics and location of the TBI play in overall blood volume restoration in the weeks post injury and show that proper therapeutic angiogenesis, or vasculogenic therapies, could speed recovery of the patient as a function of the location of injury.

The control of lung branching morphogenesis

Branching morphogenesis generates epithelial trees which facilitate gas exchange, filtering, as well as secretion processes with their large surface to volume ratio. In this review, we focus on the developmental mechanisms that control the early stages of lung branching morphogenesis. Lung branching morphogenesis involves the stereotypic, recurrent definition of new branch points, subsequent epithelial budding, and lung tube elongation. We discuss current models and experimental evidence for each of these steps. Finally, we discuss the role of the mesenchyme in determining the organ-specific shape.

The Lung Vasculature: A Driver or Passenger in Lung Branching Morphogenesis?

Multiple cellular, biochemical, and physical factors converge to coordinate organogenesis. During embryonic development, several organs such as the lung, salivary glands, mammary glands, and kidneys undergo rapid, but intricate, iterative branching. This biological process not only determines the overall architecture, size and shape of such organs but is also a pre-requisite for optimal organ function. The lung, in particular, relies on a vast surface area to carry out efficient gas exchange, and it is logical to suggest that airway branching during lung development represents a rate-limiting step in this context. Against this background, the vascular network develops in parallel to the airway tree and reciprocal interaction between these two compartments is critical for their patterning, branching, and co-alignment. In this mini review, we present an overview of the branching process in the developing mouse lung and discuss whether the vasculature plays a leading role in the process of airway epithelial branching.

Fgf10/Fgfr2b Signaling Orchestrates the Symphony of Molecular, Cellular, and Physical Processes Required for Harmonious Airway Branching Morphogenesis

Airway branching morphogenesis depends on the intricate orchestration of numerous biological and physical factors connected across different spatial scales. One of the key regulatory pathways controlling airway branching is fibroblast growth factor 10 (Fgf10) signaling via its epithelial fibroblast growth factor receptor 2b (Fgfr2b). Fine reviews have been published on the molecular mechanisms, in general, involved in branching morphogenesis, including those mechanisms, in particular, connected to Fgf10/Fgfr2b signaling. However, a comprehensive review looking at all the major biological and physical factors involved in branching, at the different scales at which branching operates, and the known role of Fgf10/Fgfr2b therein, is missing. In the current review, we attempt to summarize the existing literature on airway branching morphogenesis by taking a broad approach. We focus on the biophysical and mechanical forces directly shaping epithelial bud initiation, branch elongation, and branch tip bifurcation. We then shift focus to more passive means by which branching proceeds, via extracellular matrix remodeling and the influence of the other pulmonary arborized networks: the vasculature and nerves. We end the review by briefly discussing work in computational modeling of airway branching. Throughout, we emphasize the known or speculative effects of Fgfr2b signaling at each point of discussion. It is our aim to promote an understanding of branching morphogenesis that captures the multi-scalar biological and physical nature of the phenomenon, and the interdisciplinary approach to its study.

Human induced pluripotent stem cell-derived lung organoids in an ex vivo model of the congenital diaphragmatic hernia fetal lung

Three‐dimensional lung organoids (LOs) derived from pluripotent stem cells have the potential to enhance our understanding of disease mechanisms and to enable novel therapeutic approaches in neonates with pulmonary disorders. We established a reproducible ex vivo model of lung development using transgene‐free human induced pluripotent stem cells generated from fetuses and infants with Bochdalek congenital diaphragmatic hernia (CDH), a polygenic disorder associated with fetal lung compression and pulmonary hypoplasia at birth. Molecular and cellular comparisons of CDH LOs revealed impaired generation of NKX2.1⁺ progenitors, type II alveolar epithelial cells, and PDGFRα⁺ myofibroblasts. We then subjected these LOs to disease relevant mechanical cues through ex vivo compression and observed significant changes in genes associated with pulmonary progenitors, alveolar epithelial cells, and mesenchymal fibroblasts. Collectively, these data suggest both primary cell‐intrinsic and secondary mechanical causes of CDH lung hypoplasia and support the use of this stem cell‐based approach for disease modeling in CDH.

Complementary Effect of Maternal Sildenafil and Fetal Tracheal Occlusion Improves Lung Development in the Rabbit Model of Congenital Diaphragmatic Hernia

OBJECTIVE

To evaluate the effect of combining antenatal sildenafil with fetal tracheal occlusion (TO) in fetal rabbits with surgically induced congenital diaphragmatic hernia (CDH).

BACKGROUND

Although antenatal sildenafil administration rescues vascular abnormalities in lungs of fetal rabbits with CDH, it only partially improves airway morphometry. We hypothesized that we could additionally stimulate lung growth by combining this medical treatment with fetal TO.

METHODS

CDH was created on gestational day (GD)23 (n=54). Does were randomized to receive either sildenafil 10 mg/kg/d or placebo by subcutaneous injection from GD24 to GD30. On GD28, fetuses were randomly assigned to TO or sham neck dissection. At term (GD30) fetuses were delivered, ventilated, and finally harvested for histological and molecular analyses. Unoperated littermates served as controls.

RESULTS

The lung-to-body-weight ratio was significantly reduced in sham-CDH fetuses either (1.2 ± 0.3% vs 2.3 ± 0.3% in controls, P=0.0003). Sildenafil had no effect on this parameter, while CDH fetuses undergoing TO had a lung-to-body-weight ratio comparable to that of controls (2.5 ± 0.8%, P<0.0001). Sildenafil alone induced an improvement in the mean terminal bronchiolar density (2.5 ± 0.8 br/mm vs 3.5 ± 0.9 br/mm, P=0.043) and lung mechanics (static elastance 61 ± 36 cmH2O /mL vs 113 ± 40 cmH2O/mL, P=0.008), but both effects were more pronounced in fetuses undergoing additional TO (2.1 ± 0.8 br/mm, P=0.001 and 31 ± 9 cmH2O/mL, P<0.0001 respectively). Both CDH-sham and CDH-TO fetuses treated with placebo had an increased medial wall thickness of peripheral pulmonary vessels (41.9 ± 2.9% and 41.8 ± 3.2%, vs 24.0 ± 2.9% in controls, P<0.0001). CDH fetuses treated with sildenafil, either with or without TO, had a medial thickness in the normal range (29.4% ± 2.6%). Finally, TO reduced gene expression of vascular endothelial growth factor and surfactant protein A and B, but this effect was counteracted by sildenafil.

CONCLUSION

In the rabbit model for CDH, the combination of maternal sildenafil and TO has a complementary effect on vascular and parenchymal lung development.

The Effect of Extended Continuous Positive Airway Pressure on Changes in Lung Volumes in Stable Premature Infants: A Randomized Controlled Trial

OBJECTIVE

To compare changes in lung volumes, as measured by functional residual capacity (FRC), through to discharge in stable infants randomized to 2 weeks of extended continuous positive airway pressure CPAP (eCPAP) vs CPAP discontinuation (dCPAP).

STUDY DESIGN

Infants born at ≤32 weeks of gestation requiring ≥24 hours of CPAP were randomized to 2 weeks of eCPAP vs dCPAP when meeting CPAP stability criteria. FRC was measured with the nitrogen washout technique. Infants were stratified by gestational age (<28 and ≥ 28 weeks) and twin gestation. A linear mixed-effects model was used to evaluate the change in FRC between the 2 groups. Data were analyzed blinded to treatment group allocation.

RESULTS

Fifty infants were randomized with 6 excluded, for a total of 44 infants. Baseline characteristics were similar in the 2 groups. The infants randomized to eCPAP vs dCPAP had a greater increase in FRC from randomization through 2 weeks (12.6 mL vs 6.4 mL; adjusted 95% CI, 0.78-13.47; P = .03) and from randomization through discharge (27.2 mL vs 17.1 mL; adjusted 95% CI, 2.61-17.59; P = .01).

CONCLUSIONS

Premature infants randomized to eCPAP had a significantly greater increase in FRC through discharge compared with those randomized to dCPAP. An increased change in FRC may lead to improved respiratory health.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT02249143.

Electrical stimulation of developmental forces reveals the mechanism of limb formation in vertebrate embryos

Current knowledge on limbs development lacks a physical description of the forces leading to formation of the limbs precursors or "buds". Earlier stages of development are driven by large scale morphogenetic movements, such as dipolar vortical flows and mechanical buckling, pulled by rings of cells. It is a natural hypothesis that similar phenomena occur during limb formation. However it is difficult to experiment on the developmental forces, in such a complex dynamic system. Here, we report a physical study of hindlimb bud formation in the chicken embryo. We use electrical stimulation to enhance the physical forces present in the tissue, prior to limb bud formation. By triggering the physical forces in a rapid and amplified pattern, we reveal the mechanism of formation of the hindlimbs: the early presumptive embryonic territory is composed of a set of rings encased like Russian dolls. Each ring constricts in an excitable pattern of force, and the limb buds are generated by folding at a pre-existing boundary between two rings, forming the dorsal and ventral ectoderms. The amniotic sac buckles at another boundary. Physiologically, the actuator of the excitable force is the tail bud pushing posteriorly along the median axis. The developmental dynamics suggests how animals may evolve by modification of the magnitude of these forces, within a common broken symmetry. On a practical level, localized electrical stimulation of morphogenetic forces opens the way to in vivo electrical engineering of tissues.

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.

Many organs develop through branching morphogenesis, but whether the underlying mechanisms are shared is unknown. Here, the authors show that a ligand-receptor based Turing mechanisms, similar to that observed in lung development, likely underlies branching morphogenesis of the kidney.

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.

The mechanosensitive ion channel TRPV4 is a regulator of lung development and pulmonary vasculature stabilization

Introduction –

Clinical observations and animal models suggest a critical role for the dynamic regulation of transmural pressure and peristaltic airway smooth muscle contractions for proper lung development. However, it is currently unclear how such mechanical signals are transduced into molecular and transcriptional changes at the cell level. To connect these physical findings to a mechanotransduction mechanism, we identified a known mechanosensor, TRPV4, as a component of this pathway.

Methods –

Embryonic mouse lung explants were cultured on membranes and in submersion culture to modulate explant transmural pressure. Time-lapse imaging was used to capture active changes in lung biology, and whole-mount images were used to visualize the organization of the epithelial, smooth muscle, and vascular compartments. TRPV4 activity was modulated by pharmacological agonism and inhibition.

Results –

TRPV4 expression is present in the murine lung with strong localization to the epithelium and major pulmonary blood vessels. TRPV4 agonism and inhibition resulted in hyper- and hypoplastic airway branching, smooth muscle differentiation, and lung growth, respectively. Smooth muscle contractions also doubled in frequency with agonism and were reduced by 60% with inhibition demonstrating a functional role consistent with levels of smooth muscle differentiation. Activation of TRPV4 increased the vascular capillary density around the distal airways, and inhibition resulted in a near complete loss of the vasculature.

Conclusions –

These studies have identified TRPV4 as a potential mechanosensor involved in transducing mechanical forces on the airways to molecular and transcriptional events that regulate the morphogenesis of the three essential tissue compartments in the lung.

Mechanical Tension Drives Elongational Growth of the Embryonic Gut

During embryonic development, most organs are in a state of mechanical compression because they grow in a confined and limited amount of space within the embryo’s body; the early gut is an exception because it physiologically herniates out of the coelom. We demonstrate here that physiological hernia is caused by a tensile force transmitted by the vitelline duct on the early gut loop at its attachment point at the umbilicus. We quantify this tensile force and show that applying tension for 48 h induces stress-dependent elongational growth of the embryonic gut in culture, with an average 90% length increase (max: 200%), 65% volume increase (max: 160%), 50% dry mass increase (max: 100%), and 165% cell number increase (max: 300%); this mechanical cue is required for organ growth as guts not subject to tension do not grow. We demonstrate that growth results from increased cell proliferation when tension is applied. These results outline the essential role played by mechanical forces in shaping and driving the proliferation of embryonic organs.

Integrating Mechanical Force into Lung Development

In this issue of Developmental Cell, Tang et al. (2018) and Li et al. (2018) combine genetic manipulation, mechanical perturbation, and live imaging to show how mechanical forces and local growth factors intersect to influence epithelial behavior and cell fate specification within the developing lung.

Tissue geometry may govern lung branching mode selection

Lung branching morphogenesis proceeds in three stereotyped modes (domain, planar, and orthogonal branching). Much is known about the molecular players, including growth factors such as fibroblast growth factor 10 but it is unknown how these signals could actuate the different branching patterns. With the aim of identifying mechanisms that may determine the different branching modes, we developed a computational model of the epithelial lung bud and its surrounding mesenchyme. We studied transport of morphogens and localization of morphogen flux at lobe surfaces and lobe edges. We find that a single simple mechanism is theoretically capable of directing an epithelial tubule to elongate, bend, flatten, or bifurcate, depending solely on geometric ratios of the tissues in the vicinity of a growing tubule tip. Furthermore, the same simple mechanism is capable of generating orthogonal or planar branching, depending only on the same geometric ratios.

Microfluidic chest cavities reveal that transmural pressure controls the rate of lung development

Mechanical forces are increasingly recognized to regulate morphogenesis, but how this is accomplished in the context of the multiple tissue types present within a developing organ remains unclear. Here, we use bioengineered 'microfluidic chest cavities' to precisely control the mechanical environment of the fetal lung. We show that transmural pressure controls airway branching morphogenesis, the frequency of airway smooth muscle contraction, and the rate of developmental maturation of the lungs, as assessed by transcriptional analyses. Time-lapse imaging reveals that branching events are synchronized across distant locations within the lung, and are preceded by long-duration waves of airway smooth muscle contraction. Higher transmural pressure decreases the interval between systemic smooth muscle contractions and increases the rate of morphogenesis of the airway epithelium. These data reveal that the mechanical properties of the microenvironment instruct crosstalk between different tissues to control the development of the embryonic lung.

Comparative analysis of the mechanical signals in lung development and compensatory growth

This review compares the manner in which physical stress imposed on the parenchyma, vasculature and thorax and the thoraco-pulmonary interactions, drive both developmental and compensatory lung growth. Re-initiation of anatomical lung growth in the mature lung is possible when the loss of functioning lung units renders the existing physiologic-structural reserves insufficient for maintaining adequate function and physical stress on the remaining units exceeds a critical threshold. The appropriate spatial and temporal mechanical interrelationships and the availability of intra-thoracic space, are crucial to growth initiation, follow-on remodeling and physiological outcome. While the endogenous potential for compensatory lung growth is retained and may be pharmacologically augmented, supra-optimal mechanical stimulation, unbalanced structural growth, or inadequate remodeling may limit functional gain. Finding ways to optimize the signal-response relationships and resolve structure-function discrepancies are major challenges that must be overcome before the innate compensatory ability could be fully realized. Partial pneumonectomy reproducibly removes a known fraction of functioning lung units and remains the most robust model for examining the adaptive mechanisms, structure-function consequences and plasticity of the remaining functioning lung units capable of regeneration. Fundamental mechanical stimulus-response relationships established in the pneumonectomy model directly inform the exploration of effective approaches to maximize compensatory growth and function in chronic destructive lung diseases, transplantation and bioengineered lungs.

Quantifying cellular and subcellular stretches in embryonic lung epithelia under peristalsis: where to look for mechanosensing

Peristalsis begins in the lung as soon as the smooth muscle (SM) forms, and persists until birth. As the prenatal lung is filled with liquid, SM action can, through lumen pressure, deform tissues far from the immediately adjacent tissues. Stretching of embryonic tissues has been shown to have potent morphogenetic effects. We hypothesize that these effects are at work in lung morphogenesis. In order to refine that broad hypothesis in a quantitative framework, we geometrically analyse cell shapes in an epithelial tissue, and individual cell deformations resulting from peristaltic waves that completely occlude the airway. Typical distortions can be very large, with opposite orientations in the stalk and tip regions. Apical distortions are always greater than basal distortions. We give a quantitative estimate of the relationship between length of occluded airway and the resulting tissue stretch in the distal tip. We refine our analysis of cell stresses and strains from peristalsis with a simple mechanical model of deformation of cells within an epithelium, which accounts for basic subcellular geometry and material properties. The model identifies likely stress concentrations near the nucleus and at the apical cell-cell junction. The surprisingly large strains of airway peristalsis may serve to rearrange cells and stimulate other mechanosensitive processes by repeatedly aligning cytoskeletal components and/or breaking and reforming lateral cell-cell adhesions. Stress concentrations between nuclei of adjacent cells may serve as a mechanical control mechanism guiding the alignment of nuclei as an epithelium matures.

Pulling together: Tissue-generated forces that drive lumen morphogenesis

Mechanical interactions are essential for bending and shaping tissues during morphogenesis. A common feature of nearly all internal organs is the formation of a tubular network consisting of an epithelium that surrounds a central lumen. Lumen formation during organogenesis requires precisely coordinated mechanical and biochemical interactions. Whereas many genetic regulators of lumen formation have been identified, relatively little is known about the mechanical cues that drive lumen morphogenesis. Lumens can be shaped by a variety of physical behaviors including wrapping a sheet of cells around a hollow core, rearranging cells to expose a lumenal cavity, or elongating a tube via cell migration, though many of the details underlying these movements remain poorly understood. It is essential to define how forces generated by individual cells cooperate to produce the tissue-level forces that drive organogenesis. Transduction of mechanical forces relies on several conserved processes including the contraction of cytoskeletal networks or expansion of lumens through increased fluid pressure. The morphogenetic events that drive lumen formation serve as a model for similar mechanical processes occurring throughout development. To understand how lumenal networks arise, it will be essential to investigate how biochemical and mechanical processes integrate to generate complex structures from comparatively simple interactions.

Tissue crosstalk in lung development

Lung development follows a stereotypic program orchestrated by key interactions among epithelial and mesenchymal tissues. Deviations from this developmental program can lead to pulmonary diseases including bronchopulmonary dysplasia and pulmonary hypertension. Significant efforts have been made to examine the cellular and molecular basis of the tissue interactions underlying these stereotypic developmental processes. Genetically engineered mouse models, lung organ culture, and advanced imaging techniques are a few of the tools that have expanded our understanding of the tissue interactions that drive lung development. Intimate crosstalk has been identified between the epithelium and mesenchyme, distinct mesenchymal tissues, and individual epithelial cells types. For interactions such as the epithelial-mesenchymal crosstalk regulating lung specification and branching morphogenesis, the key molecular players, FGF, BMP, WNT, and SHH, are well established. Additionally, VEGF regulation underlies the epithelial-endothelial crosstalk that coordinates airway branching with angiogenesis. Recent work also discovered a novel role for SHH in the epithelial-to-mesenchymal (EMT) transition of the mesothelium. In contrast, the molecular basis for the crosstalk between upper airway cartilage and smooth muscle is not yet known. In this review we examine current evidence of the tissue interactions and molecular crosstalk that underlie the stereotypic patterning of the developing lung and mediate injury repair.

Localized Smooth Muscle Differentiation Is Essential for Epithelial Bifurcation during Branching Morphogenesis of the Mammalian Lung

The airway epithelium develops into a tree-like structure via branching morphogenesis. Here, we show a critical role for localized differentiation of airway smooth muscle during epithelial bifurcation in the embryonic mouse lung. We found that during terminal bifurcation, changes in the geometry of nascent buds coincided with patterned smooth muscle differentiation. Evaluating spatiotemporal dynamics of α-smooth muscle actin (αSMA) in reporter mice revealed that αSMA-expressing cells appear at the basal surface of the future epithelial cleft prior to bifurcation and then increase in density as they wrap around the bifurcating bud. Disrupting this stereotyped pattern of smooth muscle differentiation prevents terminal bifurcation. Our results reveal stereotyped differentiation of airway smooth muscle adjacent to nascent epithelial buds and suggest that localized smooth muscle wrapping at the cleft site is required for terminal bifurcation during airway branching morphogenesis.

Measuring the micromechanical properties of embryonic tissues

Local mechanical properties play an important role in directing embryogenesis, both at the cell (differentiation, migration) and tissue level (force transmission, organ formation, morphogenesis). Measuring them is a challenge as embryonic tissues are small (μm to mm) and soft (0.1-10 kPa). We describe here how glass fiber cantilevers can be fabricated, calibrated and used to apply small forces (0.1-10 μN), measure contractile activity and assess the bulk tensile elasticity of embryonic tissue. We outline how pressure (hydrostatic or osmotic) can be applied to embryonic tissue to quantify stiffness anisotropy. These techniques can be assembled at low cost and with a minimal amount of equipment. We then present a protocol to prepare tissue sections for local elasticity and adhesion measurements using the atomic force microscope (AFM). We compare AFM nanoindentation maps of native and formaldehyde fixed embryonic tissue sections and discuss how the local elastic modulus obtained by AFM compares to that obtained with other bulk measurement methods. We illustrate all of the techniques presented on the specific example of the chick embryonic digestive tract, emphasizing technical issues and common pitfalls. The main purpose of this report is to make these micromechanical measurement techniques accessible to a wide community of biologists and biophysicists.

Quantifying stretch and secretion in the embryonic lung: Implications for morphogenesis

Branching in the embryonic lung is controlled by a variety of morphogens. Mechanics is also believed to play a significant role in lung branching. The relative roles and interactions of these two broad factors are challenging to determine. We considered three hypotheses for explaining why tracheal occlusion triples branching with no overall increase in size. Both hypotheses are based on tracheal occlusion blocking the exit of secretions. (H1) Increased lumen pressure stretches tissues; stretch receptors at shoulders of growing tips increase local rate of branching. (H2) Blocking exit of secretions blocks advective transport of morphogens, leading to (H2a) increased overall concentration of morphogens or (H2b) increased flux of morphogens at specific locations. We constructed and analyzed computational models of tissue stretch and solute transport in a 3D lung geometry. Observed tissue stresses and stretches were predominantly in locations unrelated to subsequent branch locations, suggesting that tissue stretch (H1) is not the mechanism of enhancement of branching. Morphogen concentration in the mesenchyme (H2a) increased with tracheal occlusion, consistent with previously reported results. Morphogen flux at the epithelial surface (H2b) completely changed its distribution pattern when the trachea was occluded, tripling the number of locations at which it was elevated. Our results are consistent with the hypothesis that tracheal occlusion blocks outflow of secretions, leading to a higher number of high-flux locations at branching tips, in turn leading to a large increase in number of branching locations.

Morphogenetic implications of peristaltic fluid-tissue dynamics in the embryonic lung

Peristalsis begins in the lung as soon as the smooth muscle forms, and persists until birth. Since the prenatal lung is liquid-filled, smooth muscle action can deform tissues and transport fluid far from the immediately adjacent tissues. Stretching of embryonic tissues and sensation of internal fluid flows have been shown to have potent morphogenetic effects. We hypothesize that these effects are at work in lung morphogenesis. To place that hypothesis in a quantitative framework, we analyze a model of the fluid-structure interactions between embryonic tissues and lumen fluid resulting from peristaltic waves that partially occlude the airway. We find that if the airway is closed, deformations are synchronized; by contrast, if the trachea is open, maximal occlusion precedes maximal pressure. We perform a parametric analysis of how occlusion, stretch, and flow depend on tissue stiffnesses, smooth muscle force, tissue shape and size, and fluid viscosity. We find that most of these relationships are governed by simple ratios.

An interplay of geometry and signaling enables robust lung branching morphogenesis

Early branching events during lung development are stereotyped. Although key regulatory components have been defined, the branching mechanism remains elusive. We have now used a developmental series of 3D geometric datasets of mouse embryonic lungs as well as time-lapse movies of cultured lungs to obtain physiological geometries and displacement fields. We find that only a ligand-receptor-based Turing model in combination with a particular geometry effect that arises from the distinct expression domains of ligands and receptors successfully predicts the embryonic areas of outgrowth and supports robust branch outgrowth. The geometry effect alone does not support bifurcating outgrowth, while the Turing mechanism alone is not robust to noisy initial conditions. The negative feedback between the individual Turing modules formed by fibroblast growth factor 10 (FGF10) and sonic hedgehog (SHH) enlarges the parameter space for which the embryonic growth field is reproduced. We therefore propose that a signaling mechanism based on FGF10 and SHH directs outgrowth of the lung bud via a ligand-receptor-based Turing mechanism and a geometry effect.

Smad7 modulates TGFβ signaling during cranial suture development to maintain suture patency

Craniosynostosis, the premature fusion of one or more sutures between the calvarial bones, is a common birth defect. Mutations in genes encoding receptors for the transforming growth factor-beta (TGFβ) family of signaling molecules have been associated with craniosynostosis, but how TGFβ signaling is regulated during suture development is not known. In the present study, we found that expression of Smad2 and Smad3, intracellular mediators of canonical TGFβ signaling, gradually increases during early postnatal suture development in rat in both the coronal suture (CS), which remains patent throughout life, and the posterior frontal suture (PFS), which undergoes programmed closure by postnatal day 22. The amounts of phosphorylated Smad2 and Smad3 proteins showed a similar gradual increase in the PFS and CS, but in the CS, Smad2/3 activation was suppressed after neonatal day 10. The suppression of Smad2/3 activation in the CS correlated with upregulation of Smad7 expression. We demonstrate that siRNA-mediated knockdown of Smad7 caused increased phosphorylation of Smad2 and Smad3 and induced osseous obliteration of the CS from postnatal days 10 to 22. The Smad7 siRNA-induced CS closure was associated with significantly increased levels of Fgf10 and phosphorylated ERK1/2 in the suture mesenchyme. Moreover, addition of the Erk1/2 inhibitor U0126 partially blocked Smad7-siRNA-induced CS closure. These findings suggest that canonical TGFβ signaling induces suture closure at least in part through activation of FGF and ERK signaling and that Smad7 plays an important role in maintaining suture patency by suppressing canonical TGFβ signaling during suture development.

The control of branching morphogenesis

Many organs of higher organisms are heavily branched structures and arise by an apparently similar process of branching morphogenesis. Yet the regulatory components and local interactions that have been identified differ greatly in these organs. It is an open question whether the regulatory processes work according to a common principle and how far physical and geometrical constraints determine the branching process. Here, we review the known regulatory factors and physical constraints in lung, kidney, pancreas, prostate, mammary gland and salivary gland branching morphogenesis, and describe the models that have been formulated to analyse their impacts.

[Congenital cystic adenomatoid malformations of the lung: diagnosis, treatment, pathophysiological hypothesis]

Congenital cystic adenomatoid malformations (CCAM) of the lung are the most frequent congenital lung malformations. Their diagnosis is based on histological features. CCAM consist of bronchopulmonary cystic lesions which are classified according to the presence and cysts size. Type I CCAM are composed of large cysts (>2 cm) lined by a columnar pseudostratified epithelium. Type II CCAM contain multiple small cystic lesions (<1 cm) lined by a flattened cuboidal epithelium. Type III CCAM are more solid and contain immature structures resembling the pseudoglandular stage of lung development. Ultrasonography (US) allows early detection during the second trimester of pregnancy as cystic, and/or hyperechoic fetal lung lesions. Although most CCAM remain asymptomatic, CCAM can cause polyhydramnios or fetal hydrops, respiratory distress at birth, infections and pneumothoraces during infancy, and may give rise to malignancies. Serial US allow detection of complications, and planification of delivery. Complicated forms require an urgent treatment. In fetuses with a macrocystic life-threatening lesion, a thoraco-amniotic shunt can be placed. Microcystic compressive forms may respond to prenatal steroids. Post-natal symptomatic lesions require early surgery. The treatment of asymptomatic forms remains controversial. Some recommend a non-operative approach with a long-term clinical and radiological following, whereas other favour a preventive surgical excision. The origin of CCAM remains unknown. Recent advances suggest a transient and focal abnormality in lung development which may result from an airway obstruction. This article reviews the diagnosis, treatment, and pathophysiology of CCAM.

Apical constriction initiates new bud formation during monopodial branching of the embryonic chicken lung

Branching morphogenesis sculpts the airway epithelium of the lung into a tree-like structure to conduct air and promote gas exchange after birth. In the avian lung, a series of buds emerges from the dorsal surface of the primary bronchus via monopodial branching to form the conducting airways; anatomically, these buds are similar to those formed by domain branching in the mammalian lung. Here, we show that monopodial branching is initiated by apical constriction of the airway epithelium, and not by differential cell proliferation, using computational modeling and quantitative imaging of embryonic chicken lung explants. Both filamentous actin and phosphorylated myosin light chain were enriched at the apical surface of the airway epithelium during monopodial branching. Consistently, inhibiting actomyosin contractility prevented apical constriction and blocked branch initiation. Although cell proliferation was enhanced along the dorsal and ventral aspects of the primary bronchus, especially before branch formation, inhibiting proliferation had no effect on the initiation of branches. To test whether the physical forces from apical constriction alone are sufficient to drive the formation of new buds, we constructed a nonlinear, three-dimensional finite element model of the airway epithelium and used it to simulate apical constriction and proliferation in the primary bronchus. Our results suggest that, consistent with the experimental results, apical constriction is sufficient to drive the early stages of monopodial branching whereas cell proliferation is dispensable. We propose that initial folding of the airway epithelium is driven primarily by apical constriction during monopodial branching of the avian lung.

Alveolarization genes modulated by fetal tracheal occlusion in the rabbit model for congenital diaphragmatic hernia: a randomized study

Background

The mechanisms by which tracheal occlusion (TO) improves alveolarization in congenital diaphragmatic hernia (CDH) are incompletely understood. Therefore transcriptional and histological effects of TO on alveolarization were studied in the rabbit model for CDH. The question of the best normalization strategy for gene expression analysis was also addressed.

Methods

Fetal rabbits were randomized for CDH or sham operation on gestational day 23/31 and for TO or sham operation on day 28/31 resulting in four study groups. Untouched littermates were added. At term and before lung harvest, fetuses were subjected to mechanical ventilation or not. Quantitative real-time PCR was performed on lungs from 4–5 fetuses of each group with and without previous ventilation. Stability of ten housekeeping genes (HKGs) and optimal number of HKGs for normalization were determined, followed by assessment of HKG expression levels. Expression levels of eleven target genes were studied in ventilated lungs, including genes regulating elastogenesis, cell-environment interactions, and thinning of alveolar walls. Elastic staining, immunohistochemistry and Western blotting completed gene analysis.

Results

Regarding HKG expression, TO increased β-actin and β-subunit of ATP synthase. Mechanical ventilation increased β-actin and β2-microglobulin. Flavoprotein subunit of succinate dehydrogenase and DNA topoisomerase were the most stable HKGs. CDH lungs showed disorganized elastin deposition with lower levels for tropoelastin, fibulin-5, tenascin-C, and α6-integrin. After TO, CDH lungs displayed a normal pattern of elastin distribution with increased levels for tropoelastin, fibulin-5, tenascin-C, α6-integrin, ß1-integrin, lysyl oxidase, and drebrin. TO increased transcription and immunoreactivity of tissue inhibitor of metalloproteinase-1.

Conclusions

Experimental TO might improve alveolarization through the mechanoregulation of crucial genes for late lung development. However part of the transcriptional changes involved genes that were not affected in CDH, raising the question of TO-induced disturbances of alveolar remodeling. Attention should also be paid to selection of HKGs for studies on mechanotransduction-mediated gene expressions.

Evo-devo of the mammary gland

We propose a new scenario for mammary evolution based on comparative review of early mammary development among mammals. Mammary development proceeds through homologous phases across taxa, but evolutionary modifications in early development produce different final morphologies. In monotremes, the mammary placode spreads out to form a plate-like mammary bulb from which more than 100 primary sprouts descend into mesenchyme. At their distal ends, secondary sprouts develop, including pilosebaceous anlagen, resulting in a mature structure in which mammary lobules and sebaceous glands empty into the infundibula of hair follicles; these structural triads (mammolobular-pilo-sebaceous units or MPSUs) represent an ancestral condition. In marsupials a flask-like mammary bulb elongates as a sprout, but then hollows out; its secondary sprouts include hair and sebaceous anlagen (MPSUs), but the hairs are shed during nipple formation. In some eutherians (cat, horse, human) MPSUs form at the distal ends of primary sprouts; pilosebaceous components either regress or develop into mature structures. We propose that a preexisting structural triad (the apocrine-pilo-sebaceous unit) was incorporated into the evolving mammary structure, and coupled to additional developmental processes that form the mammary line, placode, bulb and primary sprout. In this scenario only mammary ductal trees and secretory tissue derive from ancestral apocrine-like glands. The mammary gland appears to have coopted signaling pathways and genes for secretory products from even earlier integumentary structures, such as odontode (tooth-like) or odontode-derived structures. We speculate that modifications in signal use (such as PTHrP and BMP4) may contribute to taxonomic differences in MPSU development.

Signalling through mechanical inputs: a coordinated process

There is growing awareness that mechanical forces - in parallel to electrical or chemical inputs - have a central role in driving development and influencing the outcome of many diseases. However, we still have an incomplete understanding of how such forces function in coordination with each other and with other signalling inputs in vivo. Mechanical forces, which are generated throughout the organism, can produce signals through force-sensitive processes. Here, we first explore the mechanisms through which forces can be generated and the cellular responses to forces by discussing several examples from animal development. We then go on to examine the mechanotransduction-induced signalling processes that have been identified in vivo. Finally, we discuss what is known about the specificity of the responses to different forces, the mechanisms that might stabilize cells in response to such forces, and the crosstalk between mechanical forces and chemical signalling. Where known, we mention kinetic parameters that characterize forces and their responses. The multi-layered regulatory control of force generation, force response and force adaptation should be viewed as a well-integrated aspect in the greater biological signalling systems.

Systems biology of lung development and regeneration: current knowledge and recommendations for future research

The lung begins as a simple outpouching of the foregut and develops by stages into a highly complex organ, the proper function of which is essential to life for terrestrial mammals. Interruption of normal lung development can result in death or chronic disease. Conversely, repair after lung injury, as well as many acquired diseases, involves recapitulation, often aberrant, of developmental pathways. The principal paradigms in lung development are branching morphogenesis and alveolar septation, but others, such as vasculogenesis, are critical. These are partially understood at the level of cellular differentiation and molecular signaling, but a true systems biology analysis of lung development and lung repair/regeneration, including bioinformatics analysis and integration of data from unbiased and complementary '-omics' level studies, is still lacking. The past decade has seen increasing numbers of genomic, proteomic, metabolomics, and epigenomic studies of lung development and lung remodeling. In many cases, these studies have confirmed the importance of pathways uncovered painstakingly through single-molecule approaches, but they have also uncovered novel and unexpected pathways and new paradigms such as noncoding RNA. Future studies will need to combine data from multiple repositories and apply novel mathematical and computational models in order to establish a systems-level understanding of this remarkable organ.

Branching geometry induced by lung self-regulated growth

Branching morphogenesis is a widely spread phenomenon in nature. In organogenesis, it results from the inhomogeneous growth of the epithelial sheet, leading to its repeated branching into surrounding mesoderm. Lung morphogenesis is an emblematic example of tree-like organogenesis common to most mammals. The core signalling network is well identified, notably the Fgf10/Shh couple, required to initiate and maintain branching. In a previous study, we showed that the restriction by SHH of Fgf10 expression domain to distal mesenchyme spontaneously induces differential epithelial proliferation leading to branching. A simple Laplacian model qualitatively reproduced FGF10 dynamics in the mesenchyme and the spontaneous self-avoiding branching morphogenesis. However, early lung geometry has several striking features that remain to be addressed. In this paper, we investigate, through simulations and data analysis, if the FGF10-diffusion scenario accounts for the following aspects of lung morphology: size dispersion, asymmetry of branching events, and distal epithelium-mesothelium equilibrium. We report that they emerge spontaneously in the model, and that most of the underlying mechanisms can be understood as dynamical interactions between gradients and shape. This suggests that specific regulation may not be required for the emergence of these striking geometrical features.

A role for mesenchyme dynamics in mouse lung branching morphogenesis

Mammalian airways are highly ramified tree-like structures that develop by the repetitive branching of the lung epithelium into the surrounding mesenchyme through reciprocal interactions. Based on a morphometric analysis of the epithelial tree, it has been recently proposed that the complete branching scheme is specified early in each lineage by a programme using elementary patterning routines at specific sites and times in the developing lung. However, the coupled dynamics of both the epithelium and mesenchyme have been overlooked in this process. Using a qualitative and quantitative in vivo morphometric analysis of the E11.25 to E13.5 mouse whole right cranial lobe structure, we show that beyond the first generations, the branching stereotypy relaxes and both spatial and temporal variations are common. The branching pattern and branching rate are sensitive to the dynamic changes of the mesoderm shape that is in turn mainly dependent upon the volume and shape of the surrounding intrathoracic organs. Spatial and temporal variations of the tree architecture are related to local and subtle modifications of the mesoderm growth. Remarkably, buds never meet after suffering branching variations and continue to homogenously fill the opening spaces in the mesenchyme. Moreover despite inter-specimen variations, the growth of the epithelial tree and the mesenchyme remains highly correlated over time at the whole lobe level, implying a long-range regulation of the lung lobe morphogenesis. Together, these findings indicate that the lung epithelial tree is likely to adapt in real time to fill the available space in the mesenchyme, rather than being rigidly specified and predefined by a global programme. Our results strongly support the idea that a comprehensive understanding of lung branching mechanisms cannot be inferred from the branching pattern or behavior alone. Rather it needs to be elaborated upon with the reconsideration of mesenchyme-epithelium coupled growth and lung tissues mechanics.

Simulations demonstrate a simple network to be sufficient to control branch point selection, smooth muscle and vasculature formation during lung branching morphogenesis

Proper lung functioning requires not only a correct structure of the conducting airway tree, but also the simultaneous development of smooth muscles and vasculature. Lung branching morphogenesis is strongly stereotyped and involves the recursive use of only three modes of branching. We have previously shown that the experimentally described interactions between Fibroblast growth factor (FGF)10, Sonic hedgehog (SHH) and Patched (Ptc) can give rise to a Turing mechanism that not only reproduces the experimentally observed wildtype branching pattern but also, in part counterintuitive, patterns in mutant mice. Here we show that, even though many proteins affect smooth muscle formation and the expression of Vegfa, an inducer of blood vessel formation, it is sufficient to add FGF9 to the FGF10/SHH/Ptc module to successfully predict simultaneously the emergence of smooth muscles in the clefts between growing lung buds, and Vegfa expression in the distal sub-epithelial mesenchyme. Our model reproduces the phenotype of both wildtype and relevant mutant mice, as well as the results of most culture conditions described in the literature.

Shape self-regulation in early lung morphogenesis

The arborescent architecture of mammalian conductive airways results from the repeated branching of lung endoderm into surrounding mesoderm. Subsequent lung’s striking geometrical features have long raised the question of developmental mechanisms involved in morphogenesis. Many molecular actors have been identified, and several studies demonstrated the central role of Fgf10 and Shh in growth and branching. However, the actual branching mechanism and the way branching events are organized at the organ scale to achieve a self-avoiding tree remain to be understood through a model compatible with evidenced signaling. In this paper we show that the mere diffusion of FGF10 from distal mesenchyme involves differential epithelial proliferation that spontaneously leads to branching. Modeling FGF10 diffusion from sub-mesothelial mesenchyme where Fgf10 is known to be expressed and computing epithelial and mesenchymal growth in a coupled manner, we found that the resulting laplacian dynamics precisely accounts for the patterning of FGF10-induced genes, and that it spontaneously involves differential proliferation leading to a self-avoiding and space-filling tree, through mechanisms that we detail. The tree’s fine morphological features depend on the epithelial growth response to FGF10, underlain by the lung’s complex regulatory network. Notably, our results suggest that no branching information has to be encoded and that no master routine is required to organize branching events at the organ scale. Despite its simplicity, this model identifies key mechanisms of lung development, from branching to organ-scale organization, and could prove relevant to the development of other branched organs relying on similar pathways.