Editors' ChoiceDevelopmental Biology

Invasion of the T tubules

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Sci. Signal.  03 Feb 2015:
Vol. 8, Issue 362, pp. ec23
DOI: 10.1126/scisignal.aaa8147

To fly, insect flight muscles require tremendous amounts of oxygen. Peterson and Krasnow showed that as Drosophila melanogaster metamorphose from larva to adult, tracheal branches invaded the flight muscle through surface openings on the muscle that are connected to T tubules and then spread within the T-tubule network. Muscles that lacked these surface openings, such as thoracic or abdominal muscles, were not invaded by tracheal branches and instead only had tracheal branches along the muscle surface. Tracheal branching and development are controlled by Branchless [Bnl, the homolog of mammalian fibroblast growth factor (FGF)] and Breathless (Btl, the homolog of the FGF receptor). Animals heterozygous for mutations in either one or both of these genes had fewer tracheal branches within the flight muscle. Knockdown of Bnl in the flight muscle or Btl in the trachea during pupal development eliminated tracheal invasion into the flight muscle. Overexpression of Bnl in the flight muscle increased the number of tracheal branches on its surface and reduced the invasion of tracheal branches into the T tubules, suggesting that there may be a localized source that attracts the tracheal branches into the T tubules. Molecular characterization of T-tubule membranes by in situ immunofluorescence analysis indicated that these membranes were similar to the basolateral membranes and septate junctions of epithelial cells. Muscle-specific knockdown of the clathrin adaptor molecule AP-1γ, which is involved in basolateral secretion from epithelial cells, resulted in loss of invasion of tracheal branches into the T tubules and an increase in surface branches. Thus, targeted secretion of the FGF homolog Bnl appears to direct the developing muscle-associated trachea to penetrate into the T-tubule network.

S. J. Peterson, M. A. Krasnow, Subcellular trafficking of FGF controls tracheal invasion of Drosophila flight muscle. Cell 160, 313–323 (2015). [PubMed]