Blowing up a balloon seems straightforward: pump in gas and let the changing air pressure do the rest. But when it comes to blowing up nature's own balloons — lung airways — things are a little more complicated.
The lung consists of an elaborate network of branching tubes; the tubes themselves are made of a layer of epithelial cells wrapped around a hollow, air-filled core, much like the thin rubber of a balloon surrounds the air inside. For the tubes to fit together, and for the right amount of gas to pass through them, they must have precisely the correct dimensions. Studies of developing embryos have revealed several molecular players that contribute to the construction of the lung-tube network1. Now, in a paper published in Developmental Cell, Tsarouhas et al.2 tackle the long-standing question of how the tubes expand, and provide breathtaking movies of this dynamic process in vivo.
In biological tubes, epithelial cells sit in a uniform pattern. The top of the cells (the apical surface) is in contact with the tube core (the lumen) — and, in the case of lung tubes, with the air taken in — whereas the base of the cells abuts underlying blood vessels3. While the tubes are developing, their lumen is very narrow, with the apical surfaces of opposing cells close together. As the tubes mature, the lumen diameter increases to the required size. Until now, it has been largely unclear how this expansion occurs and which molecules are involved in its regulation.
Filming lung-tube expansion in developing fruitfly embryos, Tsarouhas et al.2 provide a striking visual demonstration that this process involves three main, rapid events in a precise timeframe (Fig. 1). First, around 10.5 hours after eggs are laid, epithelial cells lining the tube spit out large amounts of protein into the tube lumen. Next, over the following 30 minutes, the tubes' diameter expands 2.5–3-fold. Finally, some 7.5 hours later, much of the material in the tube lumen is cleared out by the same epithelial cells. Shortly before the embryo is ready to hatch, the liquid remaining in the lung is exchanged for gas.
To identify the molecular regulators of lung-tube expansion, the authors studied mutant flies in which the expansion of the tubes was compromised. Their search converged on several molecules involved in an evolutionarily conserved biological process, biosynthetic vesicle formation, which mediates intracellular transport. Proteins are synthesized inside cells, and before they can be transported to their final destination (such as the lung-tube lumen) they must be loaded in membrane-enclosed vesicles. Such vesicles form by budding off from the membrane of the Golgi complex — an intracellular packaging station analogous to a central post office.
The authors find that when the synthesis of proteins that regulate the budding of a particular class of vesicle is perturbed, or mutant proteins are synthesized, the transport of proteins destined for the lumen is reduced, and tube expansion is disrupted. These findings highlight a previously unknown function of the vesicular transport machinery in tube luminal expansion.
Protein-filled tubes, however, are of little use as airways, and so the authors studied the regulators of the subsequent protein clearance. Endocytosis, a process in which a small portion of the cell membrane is sucked in to form a pouch and then pinched off to create an internal vesicle, is a central mechanism by which cells take up liquid and protein from their environment4. Endocytosis of proteins is also akin to organizing trash: what can be recycled is recycled, and what can't is sent to disposal compartments in the cell for degradation.
Tsarouhas et al. show that when the synthesis of many proteins involved in endocytosis is disrupted, or mutant proteins are synthesized, the clearance of material from tube lumina is largely abolished, and the tubes fail to mature into airways. Thus, the epithelial cells lining the tubes seem to have a more active role in tube expansion than was previously appreciated, as they coordinate both mass release of proteins to expand the developing tube lumen and subsequent 'swallowing' of this material to clear the mature lumen.
Static images of tracheal tube development have previously allowed many insights into lung development1. But the power of these latest findings2 lies in the fact that they combined live imaging with the genetic malleability and transparent development of fruitfly embryos. The films they have recorded are a testament to the idea that 'seeing is believing' — and, indeed, understanding — as they reveal many aspects of the airway-maturation process in vivo, an achievement that remains technically challenging in mammals.
Lumen expansion, especially in vertebrates, is probably controlled by various mechanisms. For example, chloride-ion secretion is involved in determining the diameter of the lumen in the thyroid gland5, but so far there is little evidence for mass release of proteins into the lumen of most types of vertebrate tube. Expansion of the mammalian lung and the intestinal lumen in zebrafish6,7 is apparently controlled by the movement of fluid into the lumen. So it is possible that the role of protein secretion, as seen by Tsarouhas et al., is to deliver protein regulators of osmotic pressure to the apical surface of epithelial cells, which in turn allow the tubes to expand by filling with liquid.
In terrestrial animals, from flies to humans, air-carrying tubes must be cleared of liquid as the embryo transfers from the liquid environment of the egg, uterus or similar developmental domain, to breathing air. Mammalian lungs accomplish this by very rapid fluid absorption at birth6. Little protein needs to be removed from normal lungs, but after acute lung injury large amounts of protein accumulate and must be cleared from the lumen of the lung airways to restore their function. This may involve vigorous endocytosis by epithelial cells that is similar to the process described by Tsarouhas and colleagues.
Their work, along with previous studies, indicates that nature has given living organisms several ways to blow up their 'balloons'. But despite this, both across species and in different organs of the same organism, at least one principle remains the same — the need to reshape and remodel tubes to suit changing organ physiology.
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Molecular Biology of the Cell (2011)