View of the amnioserosa and lateral epidermis of a Drosophila melanogaster embryo undergoing dorsal closure, immunostained to visualize phosphotyrosine (green) and DFos (pink). DFos is cytoplasmically localized in amnioserosal cells at this stage. (Image courtesy of Bruce Reed, University of Toronto).

Jun amino-terminal kinase (JNK) is known to operate during dorsal closure, a process whereby two sheets of epidermal cells on either side of the extraembryonic amnioserosa stretch over and displace the amnioserosa, resulting in the zipper-like closure of the dorsal surface. Activation of JNK in the leading-edge cells of the epidermis induces secretion of decapentaplegic (Dpp), which causes lateral epidermal cells to elongate dorsoventrally. But Howard Lipshitz and colleagues now show that, while JNK activity remains high in the leading edge, dorsal closure also requires that JNK signalling is downregulated in the amnioserosa. The presence of this high/low JNK activity boundary probably underlies the formation of focal complexes — areas of the cell in which proteins transduce extracellular signals to influence the actin cytoskeleton — in the leading edge of the closing epidermis, thereby influencing cell shape and behaviour.

The authors showed that hindsight (Hnt), a zinc-finger transcription factor, is needed for dorsal closure using hnt mutants that expressed reduced levels of Hnt protein — most of the mutant embryos showed an 'anterior-open' or 'dorsal-hole' phenotype. Further analysis showed that JNK signalling was upregulated in the hnt mutants, suggesting that Hnt might downregulate JNK activity during dorsal closure. Hnt is expressed in the amnioserosa, but not in the epidermis, and the authors used lacZ-enhancer-trap lines that expressed Dpp and Puckered (Puc) — two downstream substrates of JNK — to confirm that JNK signalling is downregulated at, or before, the start of dorsal closure.

As a further measure of JNK activity, the authors looked at the cellular localization of the AP-1 transcription factors DJun and DFos in the amnioserosa. Before dorsal closure, both proteins localize to the nucleus but, during dorsal closure, DFos is predominantly cytoplasmic and DJun is present in both the cytoplasm and the nucleus (possibly as a consequence of its longer half-life). Predictably, DJun and DFos both persisted in the nuclei in the hnt mutants.

What effect does downregulating JNK activity have on dorsal closure? Persistent JNK signalling in the amnioserosa (as occurs in hnt mutants) prevents the accumulation of phosphotyrosine and F-actin — components of focal complexes — in the leading-edge cells.

A simple interpretation of this is that focal complexes cannot form unless there is a high/low JNK signalling boundary between the leading edge and the amnioserosa. As focal complexes comprise many proteins that can effect the dynamics of the actin cytoskeleton, it is likely that such structures mediate the ability of the epidermis to move dorsally before fusing at the dorsal midline.