Natalia A. Bulgakova

The Crumbs complex: from epithelial-cell polarity to retinal degeneration.

The evolutionarily conserved Crumbs protein complex is a key regulator of cell polarity and cell shape in both invertebrates and vertebrates. The important role of this complex in normal cell function is illustrated by the finding that mutations in one of its components, Crumbs, are associated with retinal degeneration in humans, mice and flies. Recent results suggest that the Crumbs complex plays a role in the development of other disease processes that are based on epithelial dysfunction, such as tumorigenesis or the formation of cystic kidneys. Localisation of the complex is restricted to a distinct region of the apical plasma membrane that abuts the zonula adherens in epithelia and photoreceptor cells of invertebrates and vertebrates, including humans. In addition to the core components, a variety of other proteins can be recruited to the complex, depending on the cell type and/or developmental stage. Together with diverse post-transcriptional and post-translational mechanisms that regulate the individual components, this provides an enormous functional diversity and flexibility of the complex. In this Commentary, we summarise findings concerning the organisation and modification of the Crumbs complex, and the conservation of its constituents from flies to mammals. In addition, we discuss recent results that suggest its participation in various human diseases, including blindness and tumour formation.

Unraveling the Genetic Complexity of Drosophila stardust During Photoreceptor Morphogenesis and Prevention of Light-Induced Degeneration

Drosophila Stardust, a membrane-associated guanylate kinase (MAGUK), recruits the transmembrane protein Crumbs and the cytoplasmic proteins DPATJ and DLin-7 into an apically localized protein scaffold. This evolutionarily conserved complex is required for epithelial cell polarity in Drosophila embryos and mammalian cells in culture. In addition, mutations in Drosophila crumbs and DPATJ impair morphogenesis of photoreceptor cells (PRCs) and result in light-dependent retinal degeneration. Here we show that stardust is a genetically complex locus. While all alleles tested perturb epithelial cell polarity in the embryo, only a subset of them affects morphogenesis of PRCs or induces light-dependent retinal degeneration. Alleles retaining particular postembryonic functions still express some Stardust protein in pupal and/or adult eyes. The phenotypic complexity is reflected by the expression of distinct splice variants at different developmental stages. All proteins expressed in the retina contain the PSD95, Discs Large, ZO-1 (PDZ), Src homology 3 (SH3), and guanylate kinase (GUK) domain, but lack a large region in the N terminus encoded by one exon. These results suggest that Stardust-based protein scaffolds are dynamic, which is not only mediated by multiple interaction partners, but in addition by various forms of the Stardust protein itself.

Cell adhesion in Drosophila: versatility of cadherin and integrin complexes during development.

We highlight recent progress in understanding cadherin and integrin function in the model organism Drosophila. New functions for these adhesion receptors continue to be discovered in this system, emphasising the importance of cell adhesion within the developing organism and showing that the requirement for cell adhesion changes between cell types. New ways to control adhesion have been discovered, including controlling the expression and recruitment of adhesion components, their posttranslational modification, recycling and turnover. Importantly, even ubiquitous adhesion components can function differently in distinct cellular contexts.

Multiple domains of Stardust differentially mediate localisation of the Crumbs-Stardust complex during photoreceptor development in Drosophila

Drosophila Stardust (Sdt), a member of the MAGUK family of scaffolding proteins, is a constituent of the evolutionarily conserved Crumbs-Stardust (Crb-Sdt) complex that controls epithelial cell polarity in the embryo and morphogenesis of photoreceptor cells. Although apical localisation is a hallmark of the complex in all cell types and in all organisms analysed, only little is known about how individual components are targeted to the apical membrane. We have performed a structure-function analysis of Sdt by constructing transgenic flies that express altered forms of Sdt to determine the roles of individual domains for localisation and function in photoreceptor cells. The results corroborate the observation that the organisation of the Crb-Sdt complex is differentially regulated in pupal and adult photoreceptors. In pupal photoreceptors, only the PDZ domain of Sdt – the binding site of Crb – is required for apical targeting. In adult photoreceptors, by contrast, targeting of Sdt to the stalk membrane, a distinct compartment of the apical membrane between the rhabdomere and the zonula adherens, depends on several domains, and seems to be a two-step process. The N-terminus, including the two ECR domains and a divergent N-terminal L27 domain that binds the multi-PDZ domain protein PATJ in vitro, is necessary for targeting the protein to the apical pole of the cell. The PDZ-, the SH3- and the GUK-domains are required to restrict the protein to the stalk membrane. Drosophila PATJ or Drosophila Lin-7 are stabilised whenever a Sdt variant that contains the respective binding site is present, independently of where the variant is localised. By contrast, only full-length Sdt, confined to the stalk membrane, stabilises and localises Crb, although only in reduced amounts. The amount of Crumbs recruited to the stalk membrane correlates with its length. Our results highlight the importance of the different Sdt domains and point to a more intricate regulation of the Crb-Sdt complex in adult photoreceptor cells.

Dynamic microtubules produce an asymmetric E-cadherin–Bazooka complex to maintain segment boundaries

The combined action of dynamic microtubules and Rho signaling determines the level and asymmetric distribution of a mobile E-cadherin–Bazooka complex during the generation of a patterned epithelium.

Microtubule organization is determined by the shape of epithelial cells

Interphase microtubule organization is critical for cell function and tissue architecture. In general, physical mechanisms are sufficient to drive microtubule organization in single cells, whereas cells within tissues are thought to utilize signalling mechanisms. By improving the imaging and quantitation of microtubule alignment within developing Drosophila embryos, here we demonstrate that microtubule alignment underneath the apical surface of epithelial cells follows cell shape. During development, epidermal cell elongation and microtubule alignment occur simultaneously, but by perturbing cell shape, we discover that microtubule organization responds to cell shape, rather than the converse. A simple set of microtubule behaviour rules is sufficient for a computer model to mimic the observed responses to changes in cell surface geometry. Moreover, we show that microtubules colliding with cell boundaries zip-up or depolymerize in an angle-dependent manner, as predicted by the model. Finally, we show microtubule alignment responds to cell shape in diverse epithelia.

Antagonistic Functions of Two Stardust Isoforms in Drosophila Photoreceptor Cells

Two Stardust isoforms are expressed in adult Drosophila photoreceptors, which associate with Crumbs and PATJ, but form distinct complexes. Sdt-H and Sdt-D have antagonistic functions on stalk membrane length and light-dependent retinal degeneration, suggesting a fine-tuned balance of different Crumbs complexes regulating photoreceptor homeostasis.

The BAF chromatin remodelling complex is an epigenetic regulator of lineage specification in the early mouse embryo

Dynamic control of gene expression is essential for the development of a totipotent zygote into an embryo with defined cell lineages. The accessibility of genes responsible for cell specification to transcriptional machinery is dependent on chromatin remodelling complexes such as the SWI\SNF (BAF) complex. However, the role of the BAF complex in early mouse development has remained unclear. Here, we demonstrate that BAF155, a major BAF complex subunit, regulates the assembly of the BAF complex in vivo and regulates lineage specification of the mouse blastocyst. We find that associations of BAF155 with other BAF complex subunits become enriched in extra-embryonic lineages just prior to implantation. This enrichment is attributed to decreased mobility of BAF155 in extra-embryonic compared with embryonic lineages. Downregulation of BAF155 leads to increased expression of the pluripotency marker Nanog and its ectopic expression in extra-embryonic lineages, whereas upregulation of BAF155 leads to the upregulation of differentiation markers. Finally, we show that the arginine methyltransferase CARM1 methylates BAF155, which differentially influences assembly of the BAF complex between the lineages and the expression of pluripotency markers. Together, our results indicate a novel role of BAF-dependent chromatin remodelling in mouse development via regulation of lineage specification. Summary: Associations of BAF155 with other BAF complex subunits are enriched in extra-embryonic lineages prior to implantation, while changes in BAF155 levels modulate the expression of early developmental markers.

Pulsed laser ablation of solids and critical phenomena

We consider the possible manifestations of critical phenomena under pulsed laser ablation (PLA). The mechanism of phase explosion under nanosecond laser ablation is considered and the possibility of estimating the critical temperature from PLA experiments is discussed. A model based on the Euler equations and generalized van der Waals equation is developed to describe rarefaction shock waves (RSW) in near-critical matter. For a near-surface slab of a gold target heated above the critical point and expanding freely in vacuum, the evolution of the RSW has been studied. The possibility of RSW formation in stellar matter is discussed.

Drosophila p120-catenin is crucial for endocytosis of the dynamic E-cadherin-Bazooka complex

ABSTRACT The intracellular functions of classical cadherins are mediated through the direct binding of two catenins: β-catenin and p120-catenin (also known as CTNND1 in vertebrates, and p120ctn in Drosophila). Whereas β-catenin is crucial for cadherin function, the role of p120-catenin is less clear and appears to vary between organisms. We show here that p120-catenin has a conserved role in regulating the endocytosis of cadherins, but that its ancestral role might have been to promote endocytosis, followed by the acquisition of a new inhibitory role in vertebrates. In Drosophila, p120-catenin facilitates endocytosis of the dynamic E-cadherin–Bazooka subcomplex, which is followed by its recycling. The absence of p120-catenin stabilises this subcomplex at the membrane, reducing the ability of cells to exchange neighbours in embryos and expanding cell–cell contacts in imaginal discs. Summary: In Drosophila, p120-catenin facilitates endocytosis of the dynamic E-cadherin, which is followed by its recycling. Consequently, p120-catenin regulates cell shape and promotes the ability of cells to exchange neighbours.