Epithelial cell polarity and cell junctions in Drosophila.

The polarized architecture of epithelial cells and tissues is a fundamental determinant of animal anatomy and physiology. Recent progress made in the genetic and molecular analysis of epithelial polarity and cellular junctions in Drosophila has led to the most detailed understanding of these processes in a whole animal model system to date. Asymmetry of the plasma membrane and the differentiation of membrane domains and cellular junctions are controlled by protein complexes that assemble around transmembrane proteins such as DE-cadherin, Crumbs, and Neurexin IV, or other cytoplasmic protein complexes that associate with the plasma membrane. Much remains to be learned of how these complexes assemble, establish their polarized distribution, and contribute to the asymmetric organization of epithelial cells.

Apical, lateral, and basal polarization cues contribute to the development of the follicular epithelium during Drosophila oogenesis.

Analysis of the mechanisms that control epithelial polarization has revealed that cues for polarization are mediated by transmembrane proteins that operate at the apical, lateral, or basal surface of epithelial cells. Whereas for any given epithelial cell type only one or two polarization systems have been identified to date, we report here that the follicular epithelium in Drosophila ovaries uses three different polarization mechanisms, each operating at one of the three main epithelial surface domains. The follicular epithelium arises through a mesenchymal-epithelial transition. Contact with the basement membrane provides an initial polarization cue that leads to the formation of a basal membrane domain. Moreover, we use mosaic analysis to show that Crumbs (Crb) is required for the formation and maintenance of the follicular epithelium. Crb localizes to the apical membrane of follicle cells that is in contact with germline cells. Contact to the germline is required for the accumulation of Crb in follicle cells. Discs Lost (Dlt), a cytoplasmic PDZ domain protein that was shown to interact with the cytoplasmic tail of Crb, overlaps precisely in its distribution with Crb, as shown by immunoelectron microscopy. Crb localization depends on Dlt, whereas Dlt uses Crb-dependent and -independent mechanisms for apical targeting. Finally, we show that the cadherin-catenin complex is not required for the formation of the follicular epithelium, but only for its maintenance. Loss of cadherin-based adherens junctions caused by armadillo (beta-catenin) mutations results in a disruption of the lateral spectrin and actin cytoskeleton. Also Crb and the apical spectrin cytoskeleton are lost from armadillo mutant follicle cells. Together with previous data showing that Crb is required for the formation of a zonula adherens, these findings indicate a mutual dependency of apical and lateral polarization mechanisms.

The hindsight gene is required for epithelial maintenance and differentiation of the tracheal system in Drosophila.

During animal development, morphogenesis of tissues and organs requires dynamic cell shape changes and movements that are accomplished without loss of epithelial integrity. Data from vertebrate and invertebrate systems have implicated several cell surface and cytoskeleton-associated molecules in the establishment and maintenance of epithelial architecture, but there has been little analysis of the genetic regulatory hierarchies that control epithelial morphogenesis in specific tissues. Here we show that the Drosophila Hindsight nuclear zinc-finger protein is required during tracheal morphogenesis for the maintenance of epithelial integrity and assembly of apical extracellular structures known as taenidia. In hindsight (hnt) mutants tracheal placodes form, invaginate, and undergo primary branching as well as early fusion events. Starting at midembryogenesis, however, the tracheal epithelium collapses or expands to give rise to sacs of tissue. While a subset of hnt mutant tracheal cells enters the apoptotic pathway, genetic suppression of apoptosis indicates that this is not the cause of the epithelial defects. Surviving hnt mutant tracheal cells retain cell-cell junctions and a normal subcellular distribution of apical markers such as Crumbs and DE-Cadherin. However, taenidia do not form on the lumenal surface of tracheal cells. While loss of epithelial integrity is a common feature of crumbs, stardust, and hnt mutants, defective assembly of taenidia is unique to hnt mutants. These data suggest that HNT is a tissue-specific factor that regulates maintenance of the tracheal epithelium as well as differentiation of taenidia.

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.

Genetic analysis of cadherin function in animal morphogenesis.

Cadherins are a superfamily of Ca(2+)-dependent adhesion molecules found in metazoans. Several classes of cadherins have been defined from which two - classic cadherins and Fat-like cadherins - have been studied by genetic approaches. Recent in vivo studies in Caenorhabditis elegans and Drosophila show that cadherins play an active role in a number of distinct morphogenetic processes. Classic cadherins function in epithelial polarization, epithelial sheet or tube fusion, cell migration, cell sorting, and axonal patterning. Fat-like cadherins are required for epithelial morphogenesis, proliferation control, and epithelial planar polarization.

DE-Cadherin is required for intercellular motility during Drosophila oogenesis.

Cadherins are involved in a variety of morphogenetic movements during animal development. However, it has been difficult to pinpoint the precise function of cadherins in morphogenetic processes due to the multifunctional nature of cadherin requirement. The data presented here indicate that homophilic adhesion promoted by Drosophila E-cadherin (DE-cadherin) mediates two cell migration events during Drosophila oogenesis. In Drosophila follicles, two groups of follicle cells, the border cells and the centripetal cells migrate on the surface of germline cells. We show that the border cells migrate as an epithelial patch in which two centrally located cells retain epithelial polarity and peripheral cells are partially depolarized. Both follicle cells and germline cells express DE-cadherin, and border cells and centripetal cells strongly upregulate the expression of DE-cadherin shortly before and during their migration. Removing DE-cadherin from either the follicle cells or the germline cells blocks migration of border cells and centripetal cells on the surface of germline cells. The function of DE-cadherin in border cells appears to be specific for migration as the formation of the border cell cluster and the adhesion between border cells are not disrupted in the absence of DE-cadherin. The speed of migration depends on the level of DE-cadherin expression, as border cells migrate more slowly when DE-cadherin activity is reduced. Finally, we show that the upregulation of DE-cadherin expression in border cells depends on the activity of the Drosophila C/EBP transcription factor that is essential for border cell migration.

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.

faint sausage encodes a novel extracellular protein of the immunoglobulin superfamily required for cell migration and the establishment of normal axonal pathways in the Drosophila nervous system.

We examined the structure of the nervous system in Drosophila embryos homozygous for a null mutation in the faint sausage (fas) gene. In the peripheral nervous system (PNS) of fas mutants, neurons fail to delaminate from the ectodermal epithelium; in the central nervous system (CNS), the positions of neuronal cell bodies and glial cells are abnormal and normal axonal pathways do not form. Sequence analysis of fas cDNAs revealed that the fas protein product has characteristics of an extracellular protein and that it is a novel member of the immunoglobulin (Ig) superfamily. In situ hybridization demonstrated that fas transcripts are expressed throughout the embryo but they are in relatively high concentrations in the lateral ectoderm, from which the peripheral nervous system delaminates and in the CNS. Antiserum directed against Fas protein was found to stain neurons but not glia in the CNS. We conclude that fas encodes a protein that, in the developing nervous system, is present on the surface of neurons and is essential for nerve cell migration and the establishment of axonal pathways.

Epithelial differentiation in Drosophila.

Our understanding of epithelial development in Drosophila has been greatly improved in recent years. Two key regulators of epithelial polarity, Crumbs and DE-cadherin, have been studied at the genetic and molecular levels and a number of additional genes are being analyzed that contribute to the differentiation of epithelial cell structure. Epithelial architecture has a profound influence on morphogenetic movements, patterning and cell-type determination. The combination of embryological and genetic/molecular tools in Drosophila will help us to elucidate the complex events that determine epithelial cell structure and how they relate to morphogenesis and other developmental processes.

Crumbs, a component of the apical membrane, is required for zonula adherens formation in primary epithelia of Drosophila.

The zonula adherens (ZA) is a cell-cell adherens junction that forms a belt in the apical most region of the lateral cell surface of many epithelia. It is composed of the cadherin-catenin complex and many associated proteins and is connected to a prominent belt of microfilaments. The ZA is believed to play an important role in the differentiation and behavior of epithelial tissues and thus contributes substantially to embryonic morphogenesis. In Drosophila embryos the ZA is formed during and shortly after gastrulation from adherens junction material that appears on the cell surface during cellularization. A ZA is present in a subset of epithelia in the Drosophila embryo called primary epithelia. A second specific marker for primary epithelia is the Crumbs protein, which in concert with the gene product of stardust is required to maintain epithelial polarity. This report shows that both genes are required for the reorganization of adherens junction material into the ZA. Using immunoelectron microscopy it is shown that Crumbs is not a component of the ZA but is distributed over the entire apical cell surface and concentrated in the immediate vicinity of the ZA. These results indicate a rather direct requirement of an apical activity for the organization of the lateral membrane domain in Drosophila primary epithelia. It is proposed that the marginal zone of the apical cell surface contains a crumbs- and stardust-dependent retention mechanism for adherens junction material that aids in the formation of the ZA.

shotgun encodes Drosophila E-cadherin and is preferentially required during cell rearrangement in the neurectoderm and other morphogenetically active epithelia.

Adhesion molecules of the cadherin superfamily have an important role during vertebrate development. The DE-cadherin homolog DE-cadherin is the first classic cadherin isolated from invertebrates. We report here that DE-cadherin is encoded by the shotgun (shg) gene. shg is expressed in most embryonic epithelia and decreases in cells that undergo epithelial-mesenchymal transitions like the mesoderm or neural precursors. Removal of both maternal and zygotic shg function leads to severe defects in all epithelia expressing shg, suggesting that DE-cadherin, similar to vertebrate classic cadherins, has a crucial role for the formation and/or maintenance of epithelial tissues. Interestingly, the analysis of different shg alleles indicates that the requirement for shg in a given epithelium depends on the degree of its morphogenetic activity. Only epithelia involved in extensive morphogenetic movements require zygotic shg function in addition to maternal expression. In support of this view we find that suppression of morphogenetic movements rescues the zygotic shg phenotype. We find that in zygotic shg nulls the level of Dalpha-catenin and Armadillo at adherens junctions is dramatically reduced, surprisingly also in epithelia that differentiate normally and possess a zonula adherens.

Proneural and neurogenic genes control specification and Morphogenesis of stomatogastric nerve cell precursors in Drosophila.

The stomatogastric nervous system (SNS) of the Drosophila embryo develops from a placode which appears in the stomodeam epithelium. Most cells of this placode invaginate as three pouches (the iSNSPs) into the interior of the embryo. After separating from the stomodeum, the SNS pouches transiently form epithelial vesicles and eventually dissociate into solid clusters of cells which migrate on the foregut epithelium and differentiate into the neurons of the SNS. Prior and during iSNSP invagination, two small subpopulations of SNSPs (dSNSPs and tSNSPs) delaminate as individual cells from the SNS placode (Hartenstein et al., 1994). The results presented in this paper show that the neurogenic and proneural genes are expressed and required during all phases of SNS development to control the number, pattern, and structural characteristics of the SNSP subpopulations. First, loss-of-function mutations of the proneural and gain-of-function mutations of the neurogenic genes result in the absence or reduction of delaminating SNSPs; loss of function of neurogenic genes leads to the overproduction of d/tSNSPs and a loss of iSNSPs. Second, both proneural and neurogenic genes are involved in the invagination and dissociation of iSNSPs. Reduction of neurogenic gene function leads to a premature dissociation of iSNSPs; gain of neurogenic gene function blocks invagination and dissociation of these cells. Since all iSNSPs form a homogenous population with regard to their differentiative fate as SNS neurons, these results indicate that lateral inhibition is not a necessary aspect of the developmental process controlled by neurogenic and proneural gene function.

Neurogenic and proneural genes control cell fate specification in the Drosophila endoderm.

The Drosophila endoderm segregates into three non-neural cell types, the principle midgut epithelial cells, the adult midgut precursors, and the interstitial cell precursors, early in development. We show that this process occurs in the absence of mesoderm and requires proneural and neurogenic genes. In neurogenic mutants the principle midgut epithelial cells are missing and the other two cell types develop in great excess. Consequently, the midgut epithelium does not form. In achaete-scute complex and daughterless mutants the interstitial cell precursors do not develop and the number of adult midgut precursors is strongly reduced. Development of the principle midgut epithelial cells and formation of the midgut epithelium is restored in neurogenic proneural double mutants. The neurogenic/proneural genes are, in contrast to the neuroectoderm, not expressed in small clusters of cells but initially homogeneously in the endoderm suggesting that no prepattern exists which determines the position of the segregating cells. Hence, the segregation pattern solely depends on neurogenic/proneural gene interaction. Proneural genes are required but not sufficient to determine specific cell fates because they are required for cell type specification in both ectoderm and endoderm. Our data also suggest that the neurogenic/proneural genes are involved in the choice between epithelial versus mesenchymal cell morphologies.

Embryonic development of the stomatogastric nervous system in Drosophila.

Using several cell-specific markers, the pattern of proliferation, morphogenesis, and neuronal differentiation of the Drosophila larval stomatogastric nervous system (SNS) was analyzed. In the late embryo, four SNS ganglia (frontal ganglion, hypocerebral ganglion, paraesophageal ganglion, ventricular ganglion) can be distinguished. In the early embryo, the precursor cells of the SNS (SNSPs), being an integral part of the anlage of the esophagus, undergo four synchronous rounds of division. Subsequently, SNSPs segregate from the esophageal epithelium in a complex and stereotyped pattern. The majority of SNSPs invaginate and transiently form three (rostral, intermediate, caudal) pouches that, after separating from the esophagus, become epithelial vesicles. At later stages, these SNSPs gradually lose their epithelial phenotype. Starting at the anterior-dorsal tip of each vesicle, SNSPs dissociate from one another and migrate to the various locations where they differentiate as neurons. Cells of the rostral and intermediate vesicle contribute to the frontal ganglion; the hypocerebral ganglion develops from the intermediate vesicle, the paraesophageal ganglion from the rostral vesicle, and the ventricular ganglion from the caudal vesicle. In addition to the invaginating SNSPs, several distinct groups of SNSPs delaminate as individual cells from the esophageal epithelium. Three clusters of SNSPs delaminate from a region anterior to the rostral pouch; a single SNSP delaminates from the tip of each pouch. All delaminating SNSPs contribute to the frontal ganglion. A significant number of SNSPs undergo cell death. In the late embryo, the stomatogastric ganglia are interconnected by the recurrent nerve and esophageal nerves. The frontal ganglion projects to the brain via the frontal connectives. Both recurrent nerve and frontal connectives are pioneered by small subpopulations of early differentiating stomatogastric neurons that most likely derive from among the dSNSPs and iSNSPs.

Embryonic origin of hemocytes and their relationship to cell death in Drosophila.

We have studied the embryonic development of Drosophila hemocytes and their conversion into macrophages. Hemocytes derive exclusively from the mesoderm of the head and disperse along several invariant migratory paths throughout the embryo. The origin of hemocytes from the head mesoderm is further supported by the finding that in Bicaudal D, a mutation that lacks all head structures, and in twist snail double mutants, where no mesoderm develops, hemocytes do not form. All embryonic hemocytes behave like a homogenous population with respect to their potential for phagocytosis. Thus, in the wild type, about 80-90% of hemocytes become macrophages during late development. In mutations with an increased amount of cell death (knirps; stardust; fork head), this figure approaches 100%. In contrast, in these mutations, the absolute number of hemocytes does not differ from that in wild type, indicating that cell death does not 'induce' the formation of hemocytes. Finally, we show that, in the Drosophila embryo, apoptosis can occur independently of macrophages, since mutations lacking macrophages (Bicaudal D; twist snail double mutants; torso4021) show abundant cell death.

Epithelium formation in the Drosophila midgut depends on the interaction of endoderm and mesoderm.

The reorganization of mesenchymal cells into an epithelial sheet is a widely used morphogenetic process in metazoans. An example of such a process is the formation of the Drosophila larval midgut epithelium that develops through a mesenchymal-epithelial transition from endodermal midgut precursors. We have studied this process in wild type and a number of mutants that show defects in midgut epithelium formation. Our results indicate that the visceral mesoderm serves as a basal substratum to which endodermal cells have to establish direct contact in order to form an epithelium. Furthermore, we have analyzed the midgut phenotype of embryos mutant for the gene shotgun, and the results suggest that shotgun directs adhesion between midgut epithelial cells, which is independent from the adhesion between endoderm and visceral mesoderm.

The development of cellular junctions in the Drosophila embryo.

The pattern and development of cellular junctions in the different tissues of the Drosophila embryo from the blastoderm stage until hatching were analyzed. The cellular junctions found include: gap junctions, two types of septate junctions, and several types of cell-cell and cell-substrate adherens junctions. During early and mid embryogenesis (stages 4 to 13) only spot adherens junctions, gap junctions, and zonulae adherentes prevail. Scattered spot adherens junctions are already formed at the blastoderm stage. During and shortly after gastrulation, spot adherens junctions become concentrated at the apical pole and fuse into continuous zonulae adherentes in the posterior endoderm and the ectoderm. In addition to the zonulae adherentes, ectodermally derived epithelia possess scattered gap junctions and form pleated septate junctions and hemiadherens junctions during late embryogenesis (stages 14 to 17). Mesenchymal tissues (i.e., all nonepithelial tissues of the embryo, including the neural primordium and, transiently, the mesoderm and endoderm) possess both spot adherens junctions and gap junctions at a low frequency. Initially, the midgut epithelium does not establish a junctional complex and possess only gap junctions and spot adherens junctions. Only late in development does a circumferential smooth septate junction develop; zonulae adherentes are missing. The various derivatives of the mesoderm express spot adherens junctions, hemiadherens junctions, and gap junctions, but never zonulae adherentes or septate junctions. After organogenesis, several different types of tissue-specific adherens junctions are formed, among them connecting hemiadherens junctions (between gut epithelium and visceral muscle and early during the formation of the muscle tendon junction), muscle tendon junctions (between somatic muscle and tendon cells), fasciae adherentes (between the cells of both the visceral muscle and the dorsal vessel), and autocellular nephrocyte junctions (in nephrocytes). Interesting exceptions to the general pattern of junctional development are provided by the outer epithelial layer of the proventriculus and the Malpighian tubules. Both tissues develop as typical ectodermal epithelia and possess zonulae adherentes. During late embryogenesis, both epithelia lose the zonulae adherentes and form smooth rather than pleated septate junctions, thereby expressing a junctional complex similar to that of the endodermally derived midgut epithelium.

Crumbs and stardust act in a genetic pathway that controls the organization of epithelia in Drosophila melanogaster.

We provide evidence that the genes crumbs (crb) and stardust (sdt) encode critical components of a pathway that acts at the apical pole of epithelial cells to control the cytoarchitecture of ectodermally derived epithelia of the Drosophila embryo. We describe the developmental defects caused by sdt mutations, which are very similar to those associated with mutations in crb. In both mutants the epithelial structure of ectodermal cells breaks down during early organogenesis, leading to the formation of irregular clusters of cells and cell death in some epithelia. Certain cells can, however, compensate for the loss of crb or sdt function in a tissue-specific manner, later reassuming an epithelial cell shape and forming small epithelial vesicles, suggesting that, besides crb and sdt, other tissue-specific components are involved in this process. The crb protein (CRB) is continuously expressed in wild-type embryos in cells of the ectoderm and ectodermally derived epithelia. In sdt mutant embryos CRB is present only during gastrulation, but becomes undetectable during germ band extension; the protein is again visible during early organogenesis, at the time when the sdt mutant phenotype becomes apparent. In sdt mutant embryos, CRB is associated with the apical membrane only in well-differentiated epithelial cells, but it is expressed diffusely in the cytoplasm of cells which have lost epithelial morphology. Our results suggest that time- and tissue-specific control mechanisms exist to establish and maintain epithelial cell structure. Mosaic experiments suggest that sdt is required cell autonomously, in contrast to crb, the requirement of which appears to be non-cell-autonomous. Double mutant combinations of crb and sdt suggest that these genes are part of a common genetic pathway (crb/sdt pathway), in which sdt acts downstream of crb and is activated by the latter.

crumbs and stardust, two genes of Drosophila required for the development of epithelial cell polarity.

Loss-of-function mutations in the Drosophila genes crumbs and stardust are embryonic lethal and cause a breakdown of ectodermally derived epithelia during organogenesis, leading to formation of irregular cell clusters and extensive cell death in some epithelia. The mutant phenotype develops gradually and affects the various epithelia to different extents. crumbs encodes a large transmembrane protein with 30 EGF-like repeats and four laminin A G-domain-like repeats in its extracellular domain, suggesting its participation in protein-protein interactions. The CRUMBS protein is exclusively expressed on the apical membrane of all ectodermally derived epithelia, the tissues affected in crumbs and stardust mutant embryos. The gene function is completely abolished by a crumbs mutation that causes production of a protein with a truncated cytoplasmic domain. Instead of being apically localized as in wild-type, the mutant CRUMBS protein is diffusely distributed in the cytoplasm; this occurs before any morphologically detectable cellular phenotype is visible, suggesting that targeting of proteins is affected in crumbs mutant embryos. Later, the protein can be detected on the apical and basolateral membranes. Mutations in stardust produce a phenotype nearly identical to that associated with crumbs mutations, suggesting that both genes are functionally related. Double mutant combinations and gene dosage studies suggest that both genes are part of a common genetic pathway, in which stardust acts downstream of crumbs.