“Highlights” calls attention to exciting advances in developmental biology that have recently been reported in Developmental Dynamics. Development is a broad field encompassing many important areas. To reflect this fact, the section spotlights significant discoveries that occur across the entire spectrum of developmental events and problems: from new experimental approaches, to novel interpretations of results, to noteworthy findings utilizing different developmental organisms.
The art of negotiation (Requirements for Mediator Complex Subunits Distinguish Three Classes of Notch Target Genes at the Drosophila Wing Margin by Florence Janody, and Jessica E. Treisman, Dev Dyn240:2051–2059) The mediator complex earned its name by bringing together transcription factors and Pol II machinery, enabling transcriptional activation and elongation of target genes. Previously, the authors showed that two subunits from mediator's kinase module, Med12 and Med13, are required for cell affinity differences at the dorsal-ventral boundary of the Drosophila wing disc. Here, they test how these and the other kinase module subunits, Cdk8 and Cyclin C (CycC), affect expression of Notch targets at this position. Interestingly, Med12 and Med13 each activate two of the four wing margin target genes assayed, and repress the other two. Analysis of cdk8 or CycC mutants demonstrates that their functions are distinct from Med12 and Med13: they are only required for activation of one target. Coimmuniprecipitation and epistasis experiments suggest that kinase module subunits directly interact with the Notch transcriptional complex. The findings reveal a division of labor between the subunits, but mechanisms of target activation vs. repression remain a mystery. Perhaps in all its wisdom, mediator has avoided a labor dispute by outsourcing to differential cofactors.
Just the FAKs, ma'am (A Focal Adhesion Protein-Base Mechanochemical Checkpoint Regulates Cleft Progression During Branching Morphogenesis by William P. Daley, Joshua M. Kohn, and Melinda Larsen, Dev Dyn240:2069–2083) The story you are about to read is true. The names have been preserved to acknowledge the diligent. The scene: branching morphogenesis in the submandibular salivary gland (SMG), post cleft initiation. Previously, Larsen and colleagues found that the transition to cleft progression is gated by a mechanochemical checkpoint that is cleared when a sufficient threshold of cleft localized Rho kinase (ROCK 1)—mediated actomyosin contractility is reached. Who are the culprits behind the mysterious checkpoint, and how does it promote cell–matrix adhesions required for cleft formation? When ROCK or myosin is removed, activated integrin β1, focal adhesions (FAs), a class of cell–matrix adhesion complexes, and activated focal adhesion kinase (FAK) all come down. Inhibition of FAK itself leads to a dose-dependent decrease in branched epithelial buds and increase in initiated clefts, suggesting FAK masterminds the mechanochemical checkpoint. FAK also regulates assembly of the extracellular matrix protein fibronectin (FN) in cells adjacent to the forming cleft, a process previously shown to mediate cleft progression once the checkpoint is cleared. Here, they clarify that process by showing that inhibiting FN results in decreased FA and FAK activation. The evidence points to FA/FAK activation as a nodal point upon which inside-out (activated integrin β1 to FA/FAK) and outside-in (FN to FA/FAK) signals converge to promote and potentiate cleft progression, respectively. With the henchmen caught in a dragnet, it won't be long before they leak some secrets.
Beyond electroporation (Development of High-Concentration Lipoplexes for In Vivo Gene Function Studies in Vertebrate Embryos by Poongodi Geetha-Loganathan, Suresh Nimmagadda, Ismail Hafez, Katherine Fu, Pieter R. Cullis, and Joy M. Richman, Dev Dyn2108–2119) Chick embryologists are familiar with the frustrations of misexpressing nucleic acids. The most widely used transgenesis method, electroporation, triggers embryonic death at limb bud stages and is ineffective in certain tissues and organs. Transfection using replication competent retrovirus RCASBP is another option, but the size of vector insert is limited to a mere 2.5 kb. Here, Geetha-Loganathan et al. present their transfection reagent of choice, cationic/helper DODAC/DOPE lipids, which they find superior to other commercial and published liposomes. Their formulation can be used to express DNA, or knock down gene expression with anti-sense morpholinos, or siRNAs. It also outperforms commercial reagents in delivering plasmids to primary cultures. Induced gene expression is transient, lasting about two days, a potential short-coming possibly remedied by combining the technique with transposase systems that induce stable integration. Late stage chick embryologists take note: your realm of possibilities has just expanded.