In zebrafish, primordial germ cells (PGCs) are guided towards the developing gonad by the chemokine SDF-1. The G-protein-coupled receptor CXCR4b is known to transduce the SDF-1 signal, but exactly how directional migration is achieved is poorly understood. A new study in Developmental Cell shows that, unlike several other types of migrating cells in which actin polymerization is enriched underneath the expanding membrane, PGC advance involves the generation of bleb-like protrusions.

Membrane blebbing occurs when a patch of membrane detaches from the submembranous cytoskeleton or cortex and is often observed during apoptosis and cytokinesis. Erez Raz's group now report that actin-free protrusions are also observed at the leading edge of migrating PGCs.

It has previously been suggested that local myosin contraction generates internal hydrostatic pressure, which could push the cytosol into cell blebs or lead to breaks in the cell cortex, leading to a decrease in adhesion with the membrane thereby initiating bleb formation. Indeed, fluorescence resonance energy transfer (FRET) experiments indicate that myosin activity is highest at the leading edge of migrating PGCs, and a cytoplasmic flow was observed to enter the cortex-free PGC protrusions. When myosin activity was increased by expressing constitutively active Rho kinase 2 (ROK2), the plasma membrane separated further from the cortex and gave rise to broad protrusions around the entire cell perimeter. Conversely, expression of dominant-negative ROK2 or treatment with the myosin inhibitor blebbistatin caused PGCs to become rounded and inhibit their migration towards their target in vivo. These results suggest that PGC protrusion is largely dependent on myosin activation.

SDF-1 signalling through CXCR4b causes an increase in intracellular calcium at the cell front, and the authors show that this localized calcium elevation is required for PGC migration. Calcium buffering inhibits PGC polarization, whereas local elevations of calcium, induced by expressing a mutant form of the calcium sensor STIM1, led to the formation of blebs at these sites. Furthermore, it appears that calcium regulates myosin activity as inhibition of myosin suppressed the STIM1-induced protrusive activity.

The authors propose that CXCR4 activation by SDF-1 stimulates calcium-dependent myosin activity locally and loosens membrane-cortex attachments at the leading edge, thereby promoting bleb formation. A flow of cytoplasm, rather than actin polymerization, then drives the protrusion forward. Zebrafish are an ideal model to examine cell migration in vivo at high resolution, and future studies may find other instances of this alternative mode of generating protrusions.

Author: Monica Hoyos-Flight