Aproteoglycan helps zebrafish cells orient, elongate, and squeeze between each other to lengthen the early embryo, according to work by Lilianna Solnica-Krezel (Vanderbilt University, Nashville, TN) and colleagues.
The cells are participating in a process called convergent extension as part of gastrulation. They first orient and elongate along the ventral to dorsal axis; these processes create enough space for the cells to squeeze between each other, thus narrowing the dorso–ventral dimension and lengthening the embryo.
Solnica-Krezel and colleagues find that this process is defective in zebrafish that are mutant for the knypek gene. Cells lacking Knypek can perform other embryonic movements at normal speeds, but cells that would normally undergo convergent extension are less elongated than normal and poorly polarized.
The knypek locus encodes a heparan sulfate proteoglycan, glypican, that is probably anchored to the cell membrane. Complex interactions with a wnt11 mutation suggest that glypican may help in transmission of the Wnt11 signal, perhaps by stabilizing the Wnt11 ligand on the cell surface.
Mutant phenotypes suggest that glypican may also be involved in a Wnt5 pathway of head cartilage formation that, like convergent extension, involves cell elongation and packing. Glypicans “might be very specific to certain pathways,” says Solnica-Krezel. “It is very intriguing that they may be factors contributing to specificity of signaling.” ▪