Morphogens and pattern formation

Tissue polarity points from cells that have higher Frizzled levels towards cells that have lower Frizzled levels

BACKGROUND

The frizzled (fz) gene of Drosophila encodes the founding member of the large family of receptors for the Wnt family of signaling molecules. It was originally studied in the adult epidermis, where it plays a key role in the generation of tissue polarity. Mutations in components of the fz signal transduction pathway disrupt tissue polarity; on the wing, hairs normally point distally but their polarity is altered by these mutations.

RESULTS

We devised a method to induce a gradient of fz expression with the highest levels near the distal wing tip. The result was a large area of proximally pointing hairs in this region. This reversal of polarity was seen when fz expression was induced just before the start of hair morphogenesis when polarity is established, suggesting that the gradient of Fz protein acted fairly directly to reverse hair polarity. A similar induction of the dishevelled (dsh) gene, which acts cell autonomously and functions downstream of fz in the generation of tissue polarity, resulted in a distinct tissue polarity phenotype, but no reversal of polarity; this argues that fz signaling was required for polarity reversal. Furthermore, the finding that functional dsh was required for the reversal of polarity argues that the reversal requires normal fz signal transduction.

CONCLUSIONS

The data suggest that cells sense the level of Fz protein on neighboring cells and use this information in order to polarize themselves. A polarizing signal is transmitted from cells with higher Fz levels to cells with lower levels. Our observations enable us to propose a general mechanism to explain how Wnts polarize target cells.