Ulrich Tepass

Adherens junctions: from molecules to morphogenesis

How adhesive interactions between cells generate and maintain animal tissue structure remains one of the most challenging and long-standing questions in cell and developmental biology. Adherens junctions (AJs) and the cadherin–catenin complexes at their core are therefore the subjects of intense research. Recent work has greatly advanced our understanding of the molecular organization of AJs and how cadherin–catenin complexes engage actin, microtubules and the endocytic machinery. As a result, we have gained important insights into the molecular mechanisms of tissue morphogenesis.

Cadherins in embryonic and neural morphogenesis

Cadherins not only maintain the structural integrity of cells and tissues but also control a wide array of cellular behaviours. They are instrumental for cell and tissue polarization, and they regulate cell movements such as cell sorting, cell migration and cell rearrangements. Cadherins may also contribute to neurite outgrowth and pathfinding, and to synaptic specificity and modulation in the central nervous system.

Crumbs, the Drosophila homologue of human CRB1/RP12, is essential for photoreceptor morphogenesis

The apical transmembrane protein Crumbs is a central regulator of epithelial apical–basal polarity in Drosophila. Loss-of-function mutations in the human homologue of Crumbs, CRB1 (RP12), cause recessive retinal dystrophies, including retinitis pigmentosa. Here we show that Crumbs and CRB1 localize to corresponding subdomains of the photoreceptor apical plasma membrane: the stalk of the Drosophila photoreceptor and the inner segment of mammalian photoreceptors. These subdomains support the morphogenesis and orientation of the photosensitive membrane organelles: rhabdomeres and outer segments, respectively. Drosophila Crumbs is required to maintain zonula adherens integrity during the rapid apical membrane expansion that builds the rhabdomere. Crumbs also regulates stalk development by stabilizing the membrane-associated spectrin cytoskeleton, a function mechanistically distinct from its role in epithelial apical–basal polarity. We propose that Crumbs is a central component of a molecular scaffold that controls zonula adherens assembly and defines the stalk as an apical membrane subdomain. Defects in such scaffolds may contribute to human CRB1-related retinal dystrophies.

Interactions between the crumbs, lethal giant larvae and bazooka pathways in epithelial polarization

Several protein complexes that are involved in epithelial apicobasal polarity have been identified. However, the mechanism by which these complexes interact to form an integrated polarized cell morphology remains unclear. Crumbs (Crb) and Lethal giant larvae (Lgl) are components of distinct complexes that regulate epithelial polarization in Drosophila melanogaster, but may not interact directly as they localize to the apical and basolateral membrane, respectively. Nevertheless, a genetic screen identifies marked functional interactions between crb and lgl. These interactions extend to other genes within the crb (stardust, sdt) and lgl (discs large, dlg; scribble, scrib) pathways. Our findings suggest that the crb and lgl pathways function competitively to define apical and basolateral surfaces. They also suggest that in the absence of lgl pathway activity, the crb pathway is not required to maintain epithelial polarity. Moreover, we show that crb and lgl cooperate in zonula adherens formation early in development. At later stages, epithelial cells in these mutants acquire normal polarity, indicating the presence of compensatory mechanisms. We find that bazooka (baz) functions redundantly with crb/sdt to support apical polarity at mid- to late-embryogenesis. Despite regaining cell polarity, however, epithelial cells in crb and lgl pathway mutants fail to re-establish normal overall tissue architecture, indicating that the timely acquisition of polarized cell structure is essential for normal tissue organization.

Drosophila oocyte localization is mediated by differential cadherin-based adhesion

In a Drosophila follicle the oocyte always occupies a posterior position among a group of sixteen germline cells. Although the importance of this cell arrangement for the subsequent formation of the anterior–posterior axis of the embryo is well documented, the molecular mechanism responsible for the posterior localization of the oocyte was unknown. Here we show that the homophilic adhesion molecule DE-cadherin mediates oocyte positioning. During follicle biogenesis, DE-cadherin is expressed in germline (including oocyte) and surrounding follicle cells, with the highest concentration of DE-cadherin being found at the interface between oocyte and posterior follicle cells. Mosaic analysis shows that DE-cadherin is required in both germline and follicle cells for correct oocyte localization, indicating that germline–soma interactions may be involved in this process. By analysing the behaviour of the oocyte in follicles with a chimaeric follicular epithelium, we find that the position of the oocyte is determined by the position of DE-cadherin-expressing follicle cells, to which the oocyte attaches itself selectively. Among the DE-cadherin positive follicle cells, the oocyte preferentially contacts those cells that express higher levels of DE-cadherin. On the basis of these data, we propose that in wild-type follicles the oocyte competes successfully with its sister germline cells for contact to the posterior follicle cells, a sorting process driven by different concentrations of DE-cadherin. This is, to our knowledge, the first in vivo example of a cell-sorting process that depends on differential adhesion mediated by a cadherin.

The Apical Polarity Protein Network in Drosophila Epithelial Cells: Regulation of Polarity, Junctions, Morphogenesis, Cell Growth, and Survival

Epithelial tissue formation and function requires the apical-basal polarization of individual epithelial cells. Apical polarity regulators (APRs) are an evolutionarily conserved group of key factors that govern polarity and several other aspects of epithelial differentiation. APRs compose a diverse set of molecules including a transmembrane protein (Crumbs), a serine/threonine kinase (aPKC), a lipid phosphatase (PTEN), a small GTPase (Cdc42), FERM domain proteins (Moesin, Yurt), and several adaptor or scaffolding proteins (Bazooka/Par3, Par6, Stardust, Patj). These proteins form a dynamic cooperative network that is engaged in negative-feedback regulation with basolateral polarity factors to set up the epithelial apical-basal axis. APRs support the formation of the apical junctional complex and the segregation of the junctional domain from the apical membrane. It is becoming increasingly clear that APRs interact with the cytoskeleton and vesicle trafficking machinery, regulate morphogenesis, and modulate epithelial cell growth and survival. Not surprisingly, APRs have multiple fundamental links to human diseases such as cancer and blindness.

Cdc42 and Par proteins stabilize dynamic adherens junctions in the Drosophila neuroectoderm through regulation of apical endocytosis

Cell rearrangements require dynamic changes in cell–cell contacts to maintain tissue integrity. We investigated the function of Cdc42 in maintaining adherens junctions (AJs) and apical polarity in the Drosophila melanogaster neuroectodermal epithelium. About one third of cells exit the epithelium through ingression and become neuroblasts. Cdc42-compromised embryos lost AJs in the neuroectoderm during neuroblast ingression. In contrast, when neuroblast formation was suppressed, AJs were maintained despite the loss of Cdc42 function. Loss of Cdc42 function caused an increase in the endocytotic uptake of apical proteins, including apical polarity factors such as Crumbs, which are required for AJ stability. In addition, Cdc42 has a second function in regulating endocytotic trafficking, as it is required for the progression of apical cargo from the early to the late endosome. The Par complex acts as an effector for Cdc42 in controlling the endocytosis of apical proteins. This study reveals functional interactions between apical polarity proteins and endocytosis that are critical for stabilizing dynamic basolateral AJs.

Sinuous is a Drosophila claudin required for septate junction organization and epithelial tube size control.

Epithelial tubes of the correct size and shape are vital for the function of the lungs, kidneys, and vascular system, yet little is known about epithelial tube size regulation. Mutations in the Drosophila gene sinuous have previously been shown to cause tracheal tubes to be elongated and have diameter increases. Our genetic analysis using a sinuous null mutation suggests that sinuous functions in the same pathway as the septate junction genes neurexin and scribble, but that nervana 2, convoluted, varicose, and cystic have functions not shared by sinuous. Our molecular analyses reveal that sinuous encodes a claudin that localizes to septate junctions and is required for septate junction organization and paracellular barrier function. These results provide important evidence that the paracellular barriers formed by arthropod septate junctions and vertebrate tight junctions have a common molecular basis despite their otherwise different molecular compositions, morphologies, and subcellular localizations.

Cdc42 and Vesicle Trafficking in Polarized Cells

Cdc42, a highly conserved small GTPase of the Rho family, acts as a molecular switch to modulate a wide range of signaling pathways. Vesicle trafficking and cell polarity are two processes Cdc42 is known to regulate. Although the trafficking and polarity machineries are each well understood, how they interact to cross‐regulate each other in cell polarization is still a mystery. Cdc42 is an interesting candidate that may integrate these two networks within the cell. Here we review findings on the interplay between Cdc42 and trafficking in yeast, Caenorhabditis elegans, Drosophila and mammalian cell culture systems, and discuss recent advances in our understanding of the function of Cdc42 and two of its effectors, the WASp–Arp2/3 and Par complexes, in regulating polarized traffic. Work in yeast suggests that the polarized distribution of Cdc42, which acts here as a key polarity determinant, requires input from multiple processes including endocytosis and recycling. In metazoan cells, Cdc42 can regulate several steps in the biosynthetic as well as endocytotic and recycling pathways. The recent discovery that the Par polarity complex co‐operates with Cdc42 in the regulation of endocytosis and recycling opens exciting possibilities for the integration of polarity protein function and endocytotic machinery.

Cell sorting in animal development: signalling and adhesive mechanisms in the formation of tissue boundaries

The organisation of the animal body into distinct tissues requires adhesive mechanisms that promote and maintain the physical segregation, the sorting, of different cell populations. Signals that control differential cell affinities across tissue boundaries have been identified, including Hedgehog, Notch, and EGF receptor signalling. Further, several examples demonstrate that cell sorting in vivo can be driven by Eph/ephrin signalling and by the differential expression of cadherins that modulate cell adhesion and motility.