Staying alive: dalmation mediated blocking of apoptosis is essential for tissue maintenance

Drosophila Dalmatian combines sororin and shugoshin roles in establishment and protection of cohesion

Sister chromatid cohesion is crucial to ensure chromosome bi-orientation and equal chromosome segregation. Cohesin removal via mitotic kinases and Wapl has to be prevented in pericentromeric regions in order to protect cohesion until metaphase, but the mechanisms of mitotic cohesion protection remain elusive in Drosophila Here, we show that dalmatian (Dmt), an ortholog of the vertebrate cohesin-associated protein sororin, is required for protection of mitotic cohesion in flies. Dmt is essential for cohesion establishment during interphase and is enriched on pericentromeric heterochromatin. Dmt is recruited through direct association with heterochromatin protein-1 (HP1), and this interaction is required for cohesion. During mitosis, Dmt interdependently recruits protein phosphatase 2A (PP2A) to pericentromeric regions, and PP2A binding is required for Dmt to protect cohesion. Intriguingly, Dmt is sufficient to protect cohesion upon heterologous expression in human cells. Our findings of a hybrid system, in which Dmt exerts both sororin-like establishment functions and shugoshin-like heterochromatin-based protection roles, provide clues to the evolutionary modulation of eukaryotic cohesion regulation systems.

Naa50/San-dependent N-terminal acetylation of Scc1 is potentially important for sister chromatid cohesion

The gene separation anxiety (san) encodes Naa50/San, a N-terminal acetyltransferase required for chromosome segregation during mitosis. Although highly conserved among higher eukaryotes, the mitotic function of this enzyme is still poorly understood. Naa50/San was originally proposed to be required for centromeric sister chromatid cohesion in Drosophila and human cells, yet, more recently, it was also suggested to be a negative regulator of microtubule polymerization through internal acetylation of beta Tubulin. We used genetic and biochemical approaches to clarify the function of Naa50/San during development. Our work suggests that Naa50/San is required during tissue proliferation for the correct interaction between the cohesin subunits Scc1 and Smc3. Our results also suggest a working model where Naa50/San N-terminally acetylates the nascent Scc1 polypeptide, and that this co-translational modification is subsequently required for the establishment and/or maintenance of sister chromatid cohesion.

Mammary epithelial tubes elongate through MAPK-dependent coordination of cell migration

Mammary branching morphogenesis is regulated by receptor tyrosine kinases (RTKs). We sought to determine how these RTK signals alter proliferation and migration to accomplish tube elongation in mouse. Both behaviors occur but it has been difficult to determine their relative contribution to elongation in vivo, as mammary adipocytes scatter light and limit the depth of optical imaging. Accordingly, we utilized 3D culture to study elongation in an experimentally accessible setting. We first used antibodies to localize RTK signals and discovered that phosphorylated ERK1/2 (pERK) was spatially enriched in cells near the front of elongating ducts, whereas phosphorylated AKT was ubiquitous. We next observed a gradient of cell migration speeds from rear to front of elongating ducts, with the front characterized by both high pERK and the fastest cells. Furthermore, cells within elongating ducts oriented both their protrusions and their migration in the direction of tube elongation. By contrast, cells within the organoid body were isotropically protrusive. We next tested the requirement for proliferation and migration. Early inhibition of proliferation blocked the creation of migratory cells, whereas late inhibition of proliferation did not block continued duct elongation. By contrast, pharmacological inhibition of either MEK or Rac1 signaling acutely blocked both cell migration and duct elongation. Finally, conditional induction of MEK activity was sufficient to induce collective cell migration and ductal elongation. Our data suggest a model for ductal elongation in which RTK-dependent proliferation creates motile cells with high pERK, the collective migration of which acutely requires both MEK and Rac1 signaling.

Building and specializing epithelial tubular organs: the Drosophila salivary gland as a model system for revealing how epithelial organs are specified, form and specialize

The past two decades have witnessed incredible progress toward understanding the genetic and cellular mechanisms of organogenesis. Among the organs that have provided key insight into how patterning information is integrated to specify and build functional body parts is the Drosophila salivary gland, a relatively simple epithelial organ specialized for the synthesis and secretion of high levels of protein. Here, we discuss what the past couple of decades of research have revealed about organ specification, development, specialization, and death, and what general principles emerge from these studies.