Chevron formation of the zebrafish muscle segments.

The muscle segments of fish have a folded shape, termed a chevron, which is thought to be optimal for the undulating body movements of swimming. However, the mechanism shaping the chevron during embryogenesis is not understood. Here, we used time-lapse microscopy of developing zebrafish embryos spanning the entire somitogenesis period to quantify the dynamics of chevron shape development. By comparing such time courses with the start of movements in wildtype zebrafish and analysing immobile mutants, we show that the previously implicated body movements do not play a role in chevron formation. Further, the monotonic increase of chevron angle along the anteroposterior axis revealed by our data constrains or rules out possible contributions by previously proposed mechanisms. In particular, we found that muscle pioneers are not required for chevron formation. We put forward a tension-and-resistance mechanism involving interactions between intra-segmental tension and segment boundaries. To evaluate this mechanism, we derived and analysed a mechanical model of a chain of contractile and resisting elements. The predictions of this model were verified by comparison with experimental data. Altogether, our results support the notion that a simple physical mechanism suffices to self-organize the observed spatiotemporal pattern in chevron formation.

Patched regulates Smoothened trafficking using lipoprotein-derived lipids.

Hedgehog (Hh) is a lipoprotein-borne ligand that regulates both patterning and proliferation in a wide variety of vertebrate and invertebrate tissues. When Hh is absent, its receptor Patched (Ptc) represses Smoothened (Smo) signaling by an unknown catalytic mechanism that correlates with reduced Smo levels on the basolateral membrane. Ptc contains a sterol-sensing domain and is similar to the Niemann-Pick type C-1 protein, suggesting that Ptc might regulate lipid trafficking to repress Smo. However, no endogenous lipid regulators of Smo have yet been identified, nor has it ever been shown that Ptc actually controls lipid trafficking. Here, we show that Drosophila Ptc recruits internalized lipoproteins to Ptc-positive endosomes and that its sterol-sensing domain regulates trafficking of both lipids and Smo from this compartment. Ptc utilizes lipids derived from lipoproteins to destabilize Smo on the basolateral membrane. We propose that Ptc normally regulates Smo degradation by changing the lipid composition of endosomes through which Smo passes, and that the presence of Hh on lipoproteins inhibits utilization of their lipids by Ptc.

Lipoprotein-heparan sulfate interactions in the Hh pathway.

The Drosophila lipoprotein particle, Lipophorin, bears lipid-linked morphogens on its surface and is required for long-range signaling activity of Wingless and Hedgehog. Heparan sulfate proteoglycans are also critical for trafficking and signaling of these morphogens. Here we show that Lipophorin interacts with the heparan sulfate moieties of the glypicans Dally and Dally-like. Membrane-associated glypicans can recruit Lipophorin to disc tissue, and remain associated with these particles after they are released from the membrane by cleavage of their gpi anchors. The released form of Dally colocalizes with Patched, Hedgehog, and Lipophorin in endosomes and increases Hedgehog signaling efficiency without affecting its distribution. These data suggest that heparan sulfate proteoglycans may influence lipid-linked morphogen signaling, at least in part, by binding to Lipophorin. They further suggest that the complement of proteins present on lipoprotein particles can regulate the activity of morphogens.