Development and Function of the Drosophila Tracheal System

Matrix metalloproteinase 1 modulates invasive behavior of tracheal branches during entry into Drosophila flight muscles

Tubular networks like the vasculature extend branches throughout animal bodies, but how developing vessels interact with and invade tissues is not well understood. We investigated the underlying mechanisms using the developing tracheal tube network of Drosophila indirect flight muscles (IFMs) as a model. Live imaging revealed that tracheal sprouts invade IFMs directionally with growth-cone-like structures at branch tips. Ramification inside IFMs proceeds until tracheal branches fill the myotube. However, individual tracheal cells occupy largely separate territories, possibly mediated by cell-cell repulsion. Matrix metalloproteinase 1 (MMP1) is required in tracheal cells for normal invasion speed and for the dynamic organization of growth-cone-like branch tips. MMP1 remodels the CollagenIV-containing matrix around branch tips, which show differential matrix composition with low CollagenIV levels, while Laminin is present along tracheal branches. Thus, tracheal-derived MMP1 sustains branch invasion by modulating the dynamic behavior of sprouting branches as well as properties of the surrounding matrix.

Centrosomes in Branching Morphogenesis

The centrosome, a major microtubule organizer, has important functions in regulating the cytoskeleton as well as the position of cellular structures and orientation of cells within tissues. The centrosome serves as the main cytoskeleton-organizing centre in the cell and is the classical site of microtubule nucleation and anchoring. For these reasons, centrosomes play a very important role in morphogenesis, not just in the early stages of cell divisions but also in the later stages of organogenesis. Many organs such as lung, kidney and blood vessels develop from epithelial tubes that branch into complex networks. Cells in the nervous system also form highly branched structures in order to build complex neuronal networks. During branching morphogenesis, cells have to rearrange within tissues though multicellular branching or through subcellular branching, also known as single-cell branching. For highly branched structures to be formed during embryonic development, the cytoskeleton needs to be extensively remodelled. The centrosome has been shown to play an important role during these events.

Two-step regulation of trachealess ensures tight coupling of cell fate with morphogenesis in the Drosophila trachea

During organogenesis, inductive signals cause cell differentiation and morphogenesis. However, how these phenomena are coordinated to form functional organs is poorly understood. Here, we show that cell differentiation of the Drosophila trachea is sequentially determined in two steps and that the second step is synchronous with the invagination of the epithelial sheet. The master gene trachealess is dispensable for the initiation of invagination, while it is essential for maintaining the invaginated structure, suggesting that tracheal morphogenesis and differentiation are separately induced. trachealess expression starts in bipotential tracheal/epidermal placode cells. After invagination, its expression is maintained in the invaginated cells but is extinguished in the remaining sheet cells. A trachealess cis-regulatory module that shows both tracheal enhancer activity and silencer activity in the surface epidermal sheet was identified. We propose that the coupling of trachealess expression with the invaginated structure ensures that only invaginated cells canalize robustly into the tracheal fate.

Cells in developing organs have two important decisions to make: where to be and what cell type to become. If cells end up in the wrong places, they can stop an organ from working, so it is vital that one decision depends upon the other. The so-called progenitor cells responsible for forming the trachea, for example, can either become part of a flat sheet or part of a tube. The cells on the sheet need to become epidermal cells, while the cells in the tube need to become tracheal cells. Work on fruit flies found that a gene called 'trachealess' plays an important role in this process. Without it, developing flies cannot make a trachea at all.

At the start of trachea development, some of the cells form thickened structures called placodes. The progenitor cells in the placodes start to divide, and the structures buckle inwards to form pockets. These pockets then lengthen into tubes. The trachealess gene codes for a protein that works as a genetic switch. It turns other genes on or off, helping the progenitor cells inside the pockets to become tracheal cells. But, it is not clear whether trachealess drives the formation of the pockets: the progenitor cells first decide what to be; or whether pocket formation tells the cells to use trachealess: the progenitor cells first decide where to be.

To find out, Kondo and Hayashi imaged developing fly embryos and saw that the trachealess gene does not start pocket formation, but that it is essential to maintain the pockets. Flies without the gene managed to form pockets, but they did not last long. Looking at embryos with defects in other genes involved in pocket formation revealed why. In these flies, some of the progenitor cells using trachealess got left behind when the pockets started to form. But rather than forming pockets of their own (as they might if trachealess were driving pocket formation), they turned their trachealess gene off. Progenitor cells in the fly trachea seem to decide where to be before they decide what cell type to become. This helps to make sure that trachea cells do not form in the wrong places.

A question that still remains is how do the cells know when they are inside a pocket? It is possible that the cells are sensing different mechanical forces or different chemical signals. Further research could help scientists to understand how organs form in living animals, and how they might better recreate that process in the laboratory.

Yorkie controls tube length and apical barrier integrity during airway development

Skouloudaki et al. identify an alternative role of the transcriptional coactivator Yorkie (Yki) in controlling water impermeability and tube size of developing Drosophila airways. Tracheal impermeability is triggered by Yki-mediated transcriptional regulation of δ-aminolevulinate synthase (Alas), whereas tube elongation is controlled by binding of Yki to the actin-severing factor Twinstar.

Epithelial organ size and shape depend on cell shape changes, cell–matrix communication, and apical membrane growth. The Drosophila melanogaster embryonic tracheal network is an excellent model to study these processes. Here, we show that the transcriptional coactivator of the Hippo pathway, Yorkie (YAP/TAZ in vertebrates), plays distinct roles in the developing Drosophila airways. Yorkie exerts a cytoplasmic function by binding Drosophila Twinstar, the orthologue of the vertebrate actin-severing protein Cofilin, to regulate F-actin levels and apical cell membrane size, which are required for proper tracheal tube elongation. Second, Yorkie controls water tightness of tracheal tubes by transcriptional regulation of the δ-aminolevulinate synthase gene (Alas). We conclude that Yorkie has a dual role in tracheal development to ensure proper tracheal growth and functionality.

Veins and Arteries Build Hierarchical Branching Patterns Differently: Bottom-Up versus Top-Down

A tree-like hierarchical branching structure is present in many biological systems, such as the kidney, lung, mammary gland, and blood vessels. Most of these organs form through branching morphogenesis, where outward growth results in smaller and smaller branches. However, the blood vasculature is unique in that it exists as two trees (arterial and venous) connected at their tips. Obtaining this organization might therefore require unique developmental mechanisms. As reviewed here, recent data indicate that arterial trees often form in reverse order. Accordingly, initial arterial endothelial cell differentiation occurs outside of arterial vessels. These pre-artery cells then build trees by following a migratory path from smaller into larger arteries, a process guided by the forces imparted by blood flow. Thus, in comparison to other branched organs, arteries can obtain their structure through inward growth and coalescence. Here, new information on the underlying mechanisms is discussed, and how defects can lead to pathologies, such as hypoplastic arteries and arteriovenous malformations.

Single-cell branching morphogenesis in the Drosophila trachea

The terminal cells of the tracheal epithelium in Drosophila melanogaster are one of the few known cell types that undergo subcellular morphogenesis to achieve a stable, branched shape. During the animal's larval stages, the cells repeatedly sprout new cytoplasmic processes. These grow very long, wrapping around target tissues to which the terminal cells adhere, and are hollowed by a gas-filled subcellular tube for oxygen delivery. Our understanding of this ramification process remains rudimentary. This review aims to provide a comprehensive summary of studies on terminal cells to date, and attempts to extrapolate how terminal branches might be formed based on the known genetic and molecular components. Next to this cell-intrinsic branching mechanism, we examine the extrinsic regulation of terminal branching by the target tissue and the animal's environment. Finally, we assess the degree of similarity between the patterns established by the branching programs of terminal cells and other branched cells and tissues from a mathematical and conceptual point of view.

How high resolution 3-dimensional imaging changes our understanding of postnatal lung development

During the last 10 + years biologically and clinically significant questions about postnatal lung development could be answered due to the application of modern cutting-edge microscopic and quantitative histological techniques. These are in particular synchrotron radiation based X-ray tomographic microscopy (SRXTM), but also ³Helium Magnetic Resonance Imaging, as well as the stereological estimation of the number of alveoli and the length of the free septal edge. First, the most important new finding may be the following: alveolarization of the lung does not cease after the maturation of the alveolar microvasculature but continues until young adulthood and, even more important, maybe reactivated lifelong if needed to rescue structural damages of the lungs. Second, the pulmonary acinus represents the functional unit of the lung. Because the borders of the acini could not be detected in classical histological sections, any investigation of the acini requires 3-dimensional (imaging) methods. Based on SRXTM it was shown that in rat lungs the number of acini stays constant, meaning that their volume increases by a factor of ~ 11 after birth. The latter is very important for acinar ventilation and particle deposition.