An actomyosin-based barrier inhibits cell mixing at compartmental boundaries in Drosophila embryos.

I liked this paper because it provides a novel and unexpected explanation to a very well-known but poorly understood phenomenon of cell segregation during development of metazoan organisms. This paper supplies a very convincing demonstration that formation of actomyosin cables separating different cell compartments is principally responsible for cell segregation.

Proper morphogenesis of multicellular organisms requires physical segregation of different cell lineages into different compartments. This is essential because these compartments will eventually produce different parts of the body. It has been well known that the cells belonging to different compartments do not intermix; however, there was little information about the mechanisms responsible for this segregation. In general, it was believed that differential adhesion was responsible for this process. In this work, the authors used segmentation of the early Drosophila embryo as a model system. They found that prominent actin-myosin II cables form in the cells at the compartment boundaries and genetic, drug-mediated or even local laser-mediated loss of myosin II function results in the loss of cell compartmentalization. In the future, it will be interesting to determine the mechanisms responsible for the formation of actomyosin cables between different cell compartments. Maybe cell adhesion molecules do play an important role in this process and function in cell segregation not by providing the differential adhesion, as it was previously thought, but by establishing the spatial intracellular positioning for actomyosin cable formation.

This paper brings into focus several competing hypotheses for compartmentation in developing embryos and argues for a contractile mechanism at the parasegmental boundary as opposed to a primary role for differential adhesion.

In addition to imaging studies, chromophore assisted laser inactivation (CALI) of EFGP myosin regulatory light chain (MRLC) is employed to show that myosin based contractility at the parasegmental boundary is a key factor in maintaining cellular compartmentation in Drosophila embryos. These CALI experiments show first that cytokinesis is blocked in vivo by CALI of EGFP myosin II. Second, when myosin II at the parasegmental boundary is inactivated, cell division results in cells transiently invading the neighboring compartment. Blots from larger volumes of the embryo irradiated in bulk show that GFP and phospho MRLC are diminished after CALI, whereas control proteins are not. None of these effects were observed when CALI of a control protein, EGFP-moesin, was performed. In this case, in spite of the low efficiency of EGFP-CALI, the low time resolution (long cumulative irradiation periods) and accumulation of inactivated myosin II molecules permits a distinctive CALI phenotype which can be observed with confocal laser powers. For a News and Views article of this paper, see ref {1}.

References: {1} Martin and Wieschaus, Nat Cell Biol 2010, 12:5-7 [PMID:20027198].

The compartmentalisation of tissues is an essential feature of animal embryogenesis and organogenesis. Important mechanistic insights into appropriate compartmentalisation in this work demonstrate that parasegment (PS) boundaries in the Drosophila embryo are straightened by actomyosin cables that generate tensile forces to 'reign in' cells infringing a boundary.

Monier et al. have performed a deficiency screen aimed at identifying genes that regulate cell sorting at PS boundaries. In addition to several regulators of the Wg signalling pathway (which is known to be involved in anteroposterior compartmentalisation), the authors have recovered one deletion affecting MRLC (non-muscle myosin II regulatory light chain) and a cytoplasmic actin gene. They show that PS boundary formation is disrupted not only in actin-MRLC deletion mutants but also in embryos treated with a pharmaceutical Rho kinase inhibitor (inhibits MRLC activation) as well as in embryos expressing a dominant-negative myosin heavy chain. They further show that actomyosin cables are present along PS boundaries. These cables are transiently deformed by dividing or intercalating cells flanking the boundary, but such deformations are rapidly straightened out by contraction of the cable. Finally, local inhibition of myosin function (using chromophore-assisted laser inactivation) results in cell sorting defects in the affected area. Whereas this elegant study provides a compelling mechanistic explanation for the maintenance of boundaries, it will be interesting to see how other molecular cues involved in boundary formation (such as Eph-ephrin signalling) are integrated with cytoskeletal dynamics.

This paper identifies a novel role for a supracellular actomyosin cable in controlling the integrity of compartment boundaries in mitotically active tissues. How organisms define boundaries between populations of cells with different fates (compartments) is a fundamental problem in developmental biology, with profound implications for diseases where compartmentation is perturbed (e.g. tumor invasion).

This paper identifies a novel myosin II-based mechanism that is found at the interface between groups of cells during segmentation of the trunk in Drosophila embryos. Such compartmentalization is developmentally regulated by signals such as the Wnt pathway. Strikingly, the authors find that cells form prominent myosin II-based cables at the boundary. This is a supracellular structure, insofar as it spans the many cells at the boundary and is formed in the cells that immediately abut either side of the boundary. Using a range of approaches, notably chromophore-assisted laser inactivation (CALI) of myosin II in the cable, they show that this cable is necessary to prevent cellular intermixing and invasion across the boundary. Strikingly, dividing cells appear to be principally responsible for this intermixing and, consistent with this, the myosin cable is transiently assembled in mitotically active cells. How the cable prevents newly divided cells from intermixing is an interesting question for the future, as is the issue of how cells assemble the cable specifically at the boundary. Thus, overall, the paper identifies cell division as an important challenge for cell segregation - and this myosin II-based cable as a novel solution to that challenge.

This paper offers a rare combination -- an important technical advance and an important novel result that has eluded past efforts. The technical advance is the use of chromophore-assisted light inactivation to locally inactivate a protein and examine the consequences of such an inactivation in vivo. The important result is the finding that myosin II-based tension, rather than differential cell adhesion, is likely to play the primary role in establishing boundaries between different embryonic compartments.

Since Holtfreter proposed in the 1950s that differential adhesion is likely to mediate cell sorting in embryos most people in the field have been searching for adhesion molecules that could account for how embryos create boundaries between distinct compartments, such as Drosophila embryonic segments or wing anterior-posterior and dorsal-ventral compartments, or vertebrate hindbrain rhombomeres. Genetic analysis has failed to identify the relevant adhesion molecules, although it is thought that some adhesion molecules might help. Here, and in a parallel paper {1}, the authors show that myosin II is critical. Indeed, not only is myosin II enriched at boundaries but also its destruction or loss of activation affects boundary formation and leads to the intermixing of cells between adjacent compartments. The precise signaling cascade triggering myosin II enrichment remains to be defined. Interestingly, it has been found recently that tension positively feedbacks onto myosin II recruitment {2}. So, a small enrichment of myosin II might be further amplified by tension along the border to build a stronger boundary.

References: {1} Landsberg et al. Curr Biol 2009, 19:1950-5 [PMID:19879142]. {2} Fernandez-Gonzalez et al. Dev Cell 2009, 17:736-43 [PMID:19879198].