InteractiveFly: GeneBrief

kugelei: Biological Overview | References


Gene name - kugelei

Synonyms - fat-like, fat2

Cytological map position - 76E1-76E1

Function - receptor

Keywords - oocyte, follicle cells, planar cell polarity, tracheal epithelia

Symbol - kug

FlyBase ID: FBgn0261574

Genetic map position - 3L:20,001,935..20,017,707 [+]

Classification - EGF-like domain, Laminin G domain, Cadherin repeat domain

Cellular location - transmembrane



NCBI links: | EntrezGene

Recent literature
Aurich, F. and Dahmann, C. (2016). A mutation in fat2 uncouples tissue elongation from global tissue rotation. Cell Rep [Epub ahead of print]. PubMed ID: 26972006
Summary:
Global tissue rotation was proposed as a morphogenetic mechanism controlling tissue elongation. In Drosophila ovaries, global tissue rotation of egg chambers coincides with egg chamber elongation. Egg chamber rotation has been put forward to result in circumferential alignment of extracellular fibers. These fibers serve as molecular corsets to restrain growth of egg chambers perpendicular to the anteroposterior axis, thereby leading to the preferential egg chamber elongation along this axis. The atypical cadherin Fat2 is required for egg chamber elongation, rotation, and the circumferential alignment of extracellular fibers. This study generated a truncated form of Fat2 that lacks the entire intracellular region. fat2 mutant egg chambers expressing this truncated protein fail to rotate yet display normal extracellular fiber alignment and properly elongate. These data suggest that global tissue rotation, even though coinciding with tissue elongation, is not a necessary prerequisite for elongation.

Squarr, A. J., Brinkmann, K., Chen, B., Steinbacher, T., Ebnet, K., Rosen, M. K. and Bogdan, S. (2016). Fat2 acts through the WAVE regulatory complex to drive collective cell migration during tissue rotation. J Cell Biol 212: 591-603. PubMed ID: 26903538
Summary:
Directional cell movements during morphogenesis require the coordinated interplay between membrane receptors and the actin cytoskeleton. The WAVE regulatory complex (WRC; see Drosophila Arp2/3 component Actin-related protein 2/3 complex) is a conserved actin regulator. This study found that the atypical cadherin Fat2 recruits the WRC to basal membranes of tricellular contacts where a new type of planar-polarized whip-like actin protrusion is formed. Loss of either Fat2 function or its interaction with the WRC disrupts tricellular protrusions and results in the formation of nonpolarized filopodia. Evidence is provided for a molecular network in which the receptor tyrosine phosphatase Dlar interacts with the WRC to couple the extracellular matrix, the membrane, and the actin cytoskeleton during egg elongation. The data uncover a mechanism by which polarity information can be transduced from a membrane receptor to a key actin regulator to control collective follicle cell migration during egg elongation. 4D-live imaging of rotating MCF10A mammary acini further suggests an evolutionary conserved mechanism driving rotational motions in epithelial morphogenesis.
Barlan, K., Cetera, M. and Horne-Badovinac, S. (2017). Fat2 and Lar define a basally localized planar signaling system controlling collective cell migration. Dev Cell 40(5): 467-477.e465. PubMed ID: 28292425
Summary:
Collective migration of epithelial cells underlies diverse tissue-remodeling events, but the mechanisms that coordinate individual cell migratory behaviors for collective movement are largely unknown. Studying the Drosophila follicular epithelium, this study shows that the cadherin Fat2 and the receptor tyrosine phosphatase Lar function in a planar signaling system that coordinates leading and trailing edge dynamics between neighboring cells. Fat2 signals from each cell's trailing edge to induce leading edge protrusions in the cell behind, in part by stabilizing Lar's localization in these cells. Conversely, Lar signals from each cell's leading edge to stimulate trailing edge retraction in the cell ahead. Fat2/Lar signaling is similar to planar cell polarity signaling in terms of sub-cellular protein localization; however, Fat2/Lar signaling mediates short-range communication between neighboring cells instead of transmitting long-range information across a tissue. This work defines a key mechanism promoting epithelial migration and establishes a different paradigm for planar cell-cell signaling.
Chen, D. Y., Crest, J. and Bilder, D. (2017). A cell migration tracking tool supports coupling of tissue rotation to elongation. Cell Rep 21(3): 559-569. PubMed ID: 29045826
Summary:
Cell migration is indispensable to morphogenesis and homeostasis. Live imaging allows mechanistic insights, but long-term observation can alter normal biology, and tools to track movements in vivo without perturbation are lacking. This study developed a tool called M-TRAIL (matrix-labeling technique for real-time and inferred location), which reveals migration histories in fixed tissues. Using clones that overexpress GFP-tagged extracellular matrix (ECM) components, motility trajectories are mapped based on durable traces deposited onto basement membrane. M-TRAIL was applied to Drosophila follicle rotation, comparing in vivo and ex vivo migratory dynamics. The rate, trajectory, and cessation of rotation in wild-type (WT) follicles measured in vivo and ex vivo were identical, as was rotation failure in fat2 mutants. However, follicles carrying intracellularly truncated Fat2, previously reported to lack rotation ex vivo, in fact rotate in vivo at a reduced speed, thus revalidating the hypothesis that rotation is required for tissue elongation. The M-TRAIL approach could be applied to track and quantitate in vivo cell motility in other tissues and organisms.
Viktorinova, I., Henry, I. and Tomancak, P. (2017). Epithelial rotation is preceded by planar symmetry breaking of actomyosin and protects epithelial tissue from cell deformations. PLoS Genet 13(11): e1007107. PubMed ID: 29176774
Summary:
Symmetry breaking is involved in many developmental processes that form bodies and organs. One of them is the epithelial rotation of developing tubular and acinar organs. However, how epithelial cells move, how they break symmetry to define their common direction, and what function rotational epithelial motions have remains elusive. This study identified a dynamic actomyosin network that breaks symmetry at the basal surface of the Drosophila follicle epithelium of acinar-like primitive organs, called egg chambers, and may represent a candidate force-generation mechanism that underlies the unidirectional motion of this epithelial tissue. Evidence is provided that the atypical cadherin Fat2, a key planar cell polarity regulator in Drosophila oogenesis, directs and orchestrates transmission of the intracellular actomyosin asymmetry cue onto a tissue plane in order to break planar actomyosin symmetry, facilitate epithelial rotation in the opposite direction, and direct the elongation of follicle cells. In contrast, loss of this rotational motion results in anisotropic non-muscle Myosin II pulses that are disorganized in plane and causes cell deformations in the epithelial tissue of Drosophila eggs. Our work demonstrates that atypical cadherins play an important role in the control of symmetry breaking of cellular mechanics in order to facilitate tissue motion and model epithelial tissue. It is proposed that their functions may be evolutionarily conserved in tubular/acinar vertebrate organs.

BIOLOGICAL OVERVIEW

Planar cell polarity is an important characteristic of many epithelia. In the Drosophila wing, eye and abdomen, establishment of planar cell polarity requires the core planar cell polarity genes and two cadherins, Fat and Dachsous. Drosophila Fat2 is a cadherin related to Fat; however, its role during planar cell polarity has not been studied. In this study mutations were generated in fat2, and it was shown that Fat2 is required for the planar polarity of actin filament orientation at the basal side of ovarian follicle cells. Defects in actin filament orientation correlate with a failure of egg chambers to elongate during oogenesis. Using a functional fosmid-based fat2-GFP transgene, it was show that the distribution of Fat2 protein in follicle cells is planar polarized and that Fat2 localizes where basal actin filaments terminate. Mosaic analysis demonstrates that Fat2 acts non-autonomously in follicle cells, indicating that Fat2 is required for the transmission of polarity information. These results suggest a principal role for Fat-like cadherins during the establishment of planar cell polarity (Viktorinová, 2009).

The polarization of cells within the plane of the tissue is an important characteristic of many epithelia. Examples include the orientation of stereocilia in the inner ear, oriented outgrowth such as hair, and oriented cell divisions and tissue movements. A molecular pathway controlling planar cell polarity was first delineated in Drosophila melanogaster. Establishment of planar cell polarity in the wing, eye and abdomen of the fly requires an evolutionarily conserved set of 'core' planar-cell-polarity genes and their effectors. Fat and Dachsous, two members of the cadherin superfamily of Ca2+-dependent cell-adhesion molecules that provide molecular links between neighboring cells, were shown to be important for establishing planar cell polarity in these epithelia. Four Fat homologs (Fat1-4) have been identified in vertebrates (Tanoue, 2005), and a requirement has been shown for Fat4 during the establishment of planar cell polarity has recently been shown (Saburi, 2008; Viktorinová, 2009 and references therein).

A second excellent system in which to study planar cell polarity is the Drosophila ovarian follicle epithelium. Follicle cells display actin filaments at their basal side that are oriented perpendicular to the anteroposterior (long) axis of the developing egg chamber. These actin filaments resemble stress fibers, which are bundles of actin filaments observed at the basal side of some cultured epithelial and fibroblast cells. The formation of stress fibers is influenced by integrins, transmembrane proteins composed of heterodimers of α and β subunits, that connect the actin cytoskeleton to the extracellular matrix at focal adhesions. Like stress fibers, the ends of actin filaments within follicle cells are associated with integrins (PSβ-integrin), and integrins are required for the proper polarized orientation of these actin filaments. In addition, proper actin filament orientation requires the receptor tyrosine phosphatase Lar, which is involved in signaling between the extracellular matrix and the actin cytoskeleton, a receptor for extracellular matrix proteins called Dystroglycan, Dystrophin, a cytoplasmic protein binding to Dystroglycan, and the Pak family serine/threonine kinase. The functions of these proteins in signaling between the extracelluar matrix and the actin cytoskeleton suggest an important role for cell-to-matrix interactions in the establishment of planar cell polarity in the follicle epithelium (Viktorinová, 2009).

The first mutations shown to disrupt the polarized actin filament orientation in follicle cells were in the gene kugelei (also known as kugel) (Gutzeit, 1991). The analysis of kugelei mutants also first showed a link between the planar polarity of actin filaments in follicle cells and overall egg shape (Gutzeit, 1991). Whereas normal eggs are elongated along their anteroposterior axis, kugelei mutants produce eggs that are spherical in shape. Based on these observations, it was proposed that the planar-polarized actin filaments provide a 'molecular corset' that restrains the increase in size of the growing egg chamber perpendicular to the anteroposterior axis and, thereby, contributes to the elongation of the egg chamber (Gutzeit, 1991). Even though kugelei mutants were isolated several decades ago, the product of the kugelei gene has not been identified (Viktorinová, 2009). In Drosophila, Fat2 (also known as Fat-like) is highly related to Drosophila Fat as well as to the vertebrate cadherins Fat1, Fat2 and Fat3 (Castillejo-Lopez, 2004). A recent study, which used RNA interference to knock down fat2 function, has revealed a role for Drosophila Fat2 during tubulogenesis in the embryo (Castillejo-Lopez, 2004). However, whether Fat2 has a role during planar cell polarity has not been reported (Viktorinová, 2009).

This study shows that Drosophila fat2 is essential for the planar polarity of basal actin filaments in follicle cells and the elongation of egg chambers. fat2 is allelic to kugelei. Moreover, the distribution of a Fat2-GFP fusion protein is polarized within the plane of the follicle epithelium and it accumulates on cell membranes where the planar oriented actin filaments terminate. Finally, Fat2 is shown to act non-cell-autonomously to establish planar cell polarity and proper egg chamber shape. These results suggest that cell-to-cell interactions, mediated by Fat2, play an important role in the establishment of planar cell polarity in the follicle epithelium (Viktorinová, 2009).

fat2 mutants share defects with Lar and integrin mutants (mys, mew, if) in establishing planar cell polarity in the follicle epithelium. However, Lar and integrin mutants display additional phenotypes in follicle cells not observed in fat2 mutants, indicating that Fat2 acts independently of Lar and integrins in various processes that might not be related to planar cell polarity. Lar mutants, for example, are associated with oocyte polarity defects and defects in the number and localization of polar cells. Although Fat2-GFP is detected in the oocyte and polar cells, both oocyte polarity, polar cell number and localization appear normal in fat2 mutants. Integrin mutants, in addition to failing to properly organize basal actin filaments, display apical-basal defects and multi-layering of the follicle epithelium, defects not observed in fat2 mutants. fat2, Dystroglycan and Dystrophin mutants, however, all display defects in the formation of the posterior cross vein in wings (Christoforou, 2008), indicating that Fat2, Dystroglycan and Dystrophin also might play a common role during wing development (Viktorinová, 2009).

Planar-polarized orientation of basal actin filaments arises gradually during stages 5-6 of egg chamber development and is fully established by stage 7. Establishment of planar-polarized actin filament orientation parallels a redistribution of Fat2, as visualized by Fat2-GFP. Fat2-GFP, at stage 5, is initially enriched at the tricellular junctions between follicle cells. By stages 6 and 7, however, Fat2-GFP is preferentially distributed along the cellular junctions at which the oriented actin filaments terminate. This result, taken together with the observation that actin filament orientation fails to be established in fat2 mutant egg chambers, indicates that Fat2 plays a role in the initial establishment of planar-polarized actin filament orientation. The localization of Fat2 to sites where actin filaments terminate is consistent with a mechanism whereby Fat2 directs actin filament orientation by interacting, directly or indirectly, with actin filaments. Of note, mammalian Fat1, which is required for renal slit junction formation and normal development of the eye and forebrain (Ciani, 2003), has previously been shown to control actin polymerization by binding to Ena/vasodilator-stimulated phosphoprotein (VASP) (Moeller, 2004; Tanoue, 2004). The binding sites for the Ena/VASP homology 1 (EVH1) domain of Ena/VASP proteins, present in mammalian Fat1, are, however, not conserved in Drosophila Fat2 (Viktorinová, 2009).

Members of the cadherin superfamily can form homophilic or heterophilic interactions through their extracellular cadherin repeats with cadherin molecules on neighboring cell membranes at cellular junctions (Pokutta, 2007). By using mosaic expression of Fat2-GFP in follicle cells, this study found that Fat2-GFP was detectable at the lateral plasma membrane only on one of the two sides of follicle cells where the basal actin filaments terminate. This result, therefore, is consistent with the view that Fat2 does not form homophilic interactions between neighboring follicle cells at the basal side of the lateral membrane. As Fat2-GFP appears to localize to the same side of each cell throughout the tissue, this data furthermore suggest that a unique direction perpendicular to the anteroposterior axis is specified in the follicle epithelium early during oogenesis (Viktorinová, 2009).

In Lar and myospheroid (mys, encoding PSβ-integrin) mosaic mutants, both mutant and neighboring wild-type cells display abnormal actin filament orientation. This non-cell-autonomous behaviour is also observed in mosaic fat2 mutants, indicating that Fat2, like Lar and PSβ-integrin, is required for the transmission of polarity information (Viktorinová, 2009).

Two observations indicate that Fat2 is not required for the local transmission of polarity information. First, the polarized orientation of actin filaments remains normal within small fat2 mutant follicle cell clones. Secondly, in fat2 mutants, orientation of actin filaments is not randomized, but neighboring cells frequently display a parallel organization of actin filaments. The observation that the fraction of wild-type follicle cells to fat2 mutant follicle cells is important for actin filament orientation, indicates that the planar-polarized orientation of basal actin filaments involves the orchestrated action of a large number of follicle cells, and that Fat2 is required in this process (Viktorinová, 2009).

Non-autonomy and local coordination of planar cell polarity are also two features of mutant clones of planar cell polarity genes such as fat or frizzled in the Drosophila wing. These observations indicate that wing cells and follicle cells might use a conserved molecular logic to communicate planar polarity information (Viktorinová, 2009).

In wings, pathways including Fat and Dachsous, and the core planar cell polarity proteins Frizzled, Dishevelled, Diego, Prickle and Strabismus/Van Gogh are required for the planar-polarized orientation of hairs. Fat2 and Lar are dispensable for this process. By contrast, Fat2 and Lar are required for the planar-polarized orientation of actin filaments in follicle cells. By using mutant analysis, no evidence was found for a role of Fat, Dachsous, and the core planar cell polarity proteins Dishevelled, Diego, Prickle and Strabismus/Van Gogh, in establishing the planar-polarized orientation of actin filaments at the basal side of follicle cells or in the elongation of the egg chamber. Thus, it appears that there are at least two largely distinct pathways required for the establishment of planar cell polarity in wings and follicle cells. One pathway, dependent on Frizzled, Dishevelled and other core planar cell polarity proteins, is required to establish planar cell polarity in the wing. A second pathway, involving Lar, integrins and Dystroglycan, establishes planar cell polarity in the follicle cells. The only proteins known to act in the establishment of planar cell polarity in both wings and follicle cells are Fat-like cadherins. These findings suggest that Fat-like cadherins in general play an important role in the establishment of planar cell polarity (Viktorinová, 2009).

Integrins, Lar, and the Dystroglycan complexes are known to interact both with extracellular matrix proteins and the actin cytoskeleton, suggesting an important role for interactions between the extracellular matrix and the actin cytoskeleton for the planar polarization of follicle cells. Furthermore, similar to basal actin filaments, Laminin A, a component of the extracellular matrix, is polarized perpendicular to the long axis of the egg chamber, and mutations in LanA, the gene encoding Laminin A, result in spherical eggs. The finding that Fat2, a member of the cadherin superfamily of proteins, is required for planar-polarized orientation of actin filaments in follicle cells suggests that also cell-to-cell interactions are important for establishing planar cell polarity in the follicle epithelium (Viktorinová, 2009).

In addition to its role in planar-polarizing wing hairs, Fat is also required for the proper shape of the wing. Likewise, this study reports that fat2 is also required for normal planar cell polarity and tissue shape in the ovary. It is intriguing to speculate that Fat-like cadherins provide a common mechanistic link between tissue shape and planar cell polarity in both tissues (Viktorinová, 2009).

In summary, Fat2, like Fat, is required to establish a planar polarity of cells, indicating that Fat-like cadherins may play a principle role in this process. It will, therefore, be interesting to test whether vertebrate Fat1, Fat2 and Fat3, which are related to Drosophila Fat2, are also involved in establishing planar cell polarity (Viktorinová, 2009).

Symmetry breaking in an edgeless epithelium by Fat2-regulated microtubule polarity

Planar cell polarity (PCP) information is a critical determinant of organ morphogenesis. While PCP in bounded epithelial sheets is increasingly well understood, how PCP is organized in tubular and acinar tissues is not known. Drosophila egg chambers (follicles) are an acinus-like "edgeless epithelium" and exhibit a continuous, circumferential PCP that does not depend on pathways active in bounded epithelia; this follicle PCP directs formation of an ellipsoid rather than a spherical egg. This study uses an imaging algorithm to 'unroll' the entire 3D tissue surface and comprehensively analyze PCP onset. This approach traces chiral symmetry breaking to plus-end polarity of microtubules in the germarium, well before follicles form and rotate. PCP germarial microtubules provide chiral information that predicts the direction of whole-tissue rotation as soon as independent follicles form. Concordant microtubule polarity, but not microtubule alignment, requires the atypical cadherin Fat2, which acts at an early stage to translate plus-end bias into coordinated actin-mediated collective cell migration. Because microtubules are not required for PCP or migration after follicle rotation initiates, while dynamic actin and extracellular matrix are, polarized microtubules lie at the beginning of a handoff mechanism that passes early chiral PCP of the cytoskeleton to a supracellular planar polarized extracellular matrix and elongates the organ (Chen, 2016).

This work identify a central symmetry-breaking role for microtubule polarity in PCP of an 'edgeless' epithelial organ. Microtubules are the earliest PCP molecule during follicle development, germarial microtubule polarity predicts the chirality of subsequent follicle PCP events, and disruption of either microtubule alignment or polarity in the germarium prevents all subsequent aspects of follicle PCP, including the coordinated cell motility that initiates follicle rotation. These requirements for microtubules are not due to secondary effects on actin, which retains its organization in germaria with disrupted microtubules. Importantly, unlike actin, which is required acutely and constantly for collective cell migration, microtubules are not strictly required for follicle cell motility once rotation has initiated. It is therefore proposed that microtubules provide the initial source of PCP information in the early follicle and that actin, through its role in promoting tissue rotation, serves to amplify and propagate PCP (Chen, 2016).

The current data demonstrate that the microtubule polarity bias in the forming follicle predicts the chirality of PCP tissue rotation that initiates 10 hr later. It was recently reported that, in stage 4 follicles, microtubule plus-end orientation anticorrelates with rotation direction at stage 7. However, in that work, stage 4 and earlier follicles were thought to represent pre-rotation stages, contrary to the identification in this study of rotation initiation at stage 2 and that of another study that placed it at stage 1. Since both microtubule alignment and polarity are present in the germarium, the stage 5 correlation is not predictive and reflects pre-existing PCP information rather than revealing its source. In the absence of an independent and direct manipulation of microtubule plus-end orientation, it cannot be conclusively stated that the microtubule polarity that was document in the germarium is instructive for rotational direction. Nevertheless, the strong correlation between this chirality and the subsequent direction of follicle rotation, along with disruption of both in the absence of fat2, point to a model in which coordination of microtubule polarity in cells across the follicle is required for a unidirectional consensus among individual motile cells to initiate productive rotation (Chen, 2016).

The data thus suggest that the atypical cadherin Fat2, a key regulator of follicle PCP, rotation, and elongation, acts through effects on early microtubule polarity. Through analyses of middle stages of follicle development, it has been argued that Fat2 regulates global PCP alignment of the cytoskeleton, as does the canonical PCP regulator Fat in the developing Drosophila wing. However, in fat2 germaria, actin and microtubule alignment are maintained; it is coordinated microtubule polarity that is lost. These phenotypes, along with its strong genetic interaction with CLASP, argue that Fat2 promotes rotation initiation and follicle PCP via its effects on microtubule polarity. An additional role in actin regulation other than polarized alignment has not been excluded; the requirement for Fat2 and actin, but not microtubules, to maintain rotation, as well as the direct binding of actin regulators by vertebrate Fat1, a possible ortholog of Drosophila Fat2, suggests such a role. Moreover, while this work was in revision, Squarr (2016) showed that Fat2 can directly influence the actin cytoskeleton via binding to the WAVE complex (Chen, 2016).

As with PCP in the developing Drosophila wing disc, the initial PCP bias provided by microtubule polarity within the early follicle precursors is mild but becomes more robust as organogenesis progresses. A mechanism for amplification in the follicle involves whole-tissue rotation. Preventing rotation by disrupting actin or integrins causes a rapid loss of all PCP organization primed in the germarium. Interestingly, just as microtubules are largely dispensable for PCP after actin PCP becomes established, actin PCP is dispensable after circumferentially aligned ECM becomes established. Hence, PCP transitions from highly dynamic and intracellular microtubules, to longer-lasting and sometimes juxtacellular actin filaments, and then finally to the durable ECM fibrils that span multiple cells. PCP information in the follicle is therefore passed along by a 'handoff' mechanism to increasingly stable as well as larger-scale components that can ultimately biophysically influence the shape of the organ (Chen, 2016).

Non-centrosomal microtubule arrays, and in particular their regulated polarized organization, have previously been implicated as central governors of global PCP in tissues such as Drosophila wings, zebrafish gastrulae, and mammalian airway epithelia. In 'edged' PCP tissues in Drosophila, a 'global PCP' module molecularly controlled by Fat is thought to use gradients of positional information from specific sources to bring individual cell PCP in alignment with overall body axes. In the circumferential 'edgeless' PCP axis of the follicle epithelium, where no such graded information is known, Fat2 seems to similarly coordinate the PCP of individual cells. That both contexts involve important roles for polarized microtubules and are controlled by related atypical cadherins raises the possibility of ancient links between the modes of epithelial PCP organization (Chen, 2016).

Oriented basement membrane fibrils provide a memory for F-actin planar polarization via the Dystrophin-Dystroglycan complex during tissue elongation

How extracellular matrix participates to tissue morphogenesis is still an open question. In the Drosophila ovarian follicle, it has been proposed that after Fat2-dependent planar polarization of the follicle cell basal domain, oriented basement membrane (BM) fibrils and F-actin stress fibers constrain follicle growth, promoting its axial elongation. However, the relationship between BM fibrils and stress fibers and their respective impact on elongation are unclear. This study found that Dystroglycan (Dg) and Dystrophin (Dys) are involved in BM fibril deposition. Moreover, they also orient stress fibers, by acting locally and in parallel to Fat2. Importantly, Dg-Dys complex-mediated cell autonomous control of F-actin fibers orientation relies on the previous BM fibril deposition, indicating two distinct but interdependent functions. Thus, the Dg-Dys complex works as a critical organizer of the epithelial basal domain, regulating both F-actin and BM. Furthermore, BM fibrils act as a persistent cue for the orientation of stress fibers that are the main effector of elongation (Cerqueira Campos, 2020).

Deciphering the mechanisms underlying tissue morphogenesis is crucial for fundamental understanding of development and also for regenerative medicine. Building organs generally requires the precise modeling of a basement membrane extracellular matrix (ECM), which in turn can influence tissue shape. However, the mechanisms driving the assembly of a specific basement membrane (BM) and how this BM then feeds forward on morphogenesis are still poorly understood. Drosophila oogenesis offers one of the best tractable examples in which such a morphogenetic process can be studied. Each ovarian follicle, which is composed of a germline cyst surrounded by the somatic follicular epithelium, undergoes a dramatic growth, associated with tissue elongation, starting from a little sphere and ending with an egg in which the anteroposterior (AP) axis is 3-fold longer than the mediolateral (ML) axis. This elongation is roughly linear from the early to the late stages, but can be separated in at least two mechanistically distinct phases. The first phase (from stage 3 to stage 8; hereby 'early stages') requires a double gradient of JAK-STAT pathway activity that emanates from each pole and that controls myosin II-dependent apical pulsations. In the second phase, from stage 7-8, elongation depends on the atypical cadherin Fat2 that is part of a planar cell polarity (PCP) pathway orienting the basal domain of epithelial follicle cells. Earlier during oogenesis, Fat2 gives a chirality to the basal domain cytoskeleton in the germarium, the structure from which new follicles bud. This chirality is required to set up a process of oriented collective cell migration perpendicularly to the elongation axis that induces follicle revolutions from stage 1 to stage 8. From each migrating cell, Fat2 also induces, in the rear adjacent cell, the formation of planar-polarized protrusions that are required for rotation. These rotations allow the polarized deposition of BM fibrils, which involves a Rab10-dependent secretion route targeted to the lateral domain of the cells. These BM fibrils are detectable from stage 4 onwards and persist until late developmental stages. Follicle rotation also participates in the planar cell polarization of integrin-dependent basal stress fibers that are oriented perpendicularly to the AP axis. Moreover, at stage 7-8, a gradient of matrix stiffness controlled by the JAK-STAT pathway and Fat2 contributes to elongation. Then, from stage 9, the epithelial cell basal domain undergoes anisotropic oscillations, as a result of periodic contraction of the oriented stress fibers, which also promotes follicle elongation . To explain the impact of fat2 mutations on tissue elongation, it is generally accepted that oriented stress fibers and BM fibrils act as a molecular corset that constrains follicle growth in the ML axis and promotes its elongation along the AP axis. However, the exact contribution of F-actin versus BM to this corset is still unclear, as is whether the orientations of stress fibers and of BM fibrils are causally linked (Cerqueira Campos, 2020).

This analyzed the function of Dystrophin (Dys) and Dystroglycan (Dg) during follicle elongation. Dys and Dg are the two main components of the Dystrophin-associated protein complex (DAPC), an evolutionarily conserved transmembrane complex that links the ECM (via Dg) to the F-actin cytoskeleton (via Dys). This complex is expressed in a large variety of tissues and is implicated in many congenital degenerative disorders. Loss-of-function studies in model organisms have revealed an important morphogenetic role for Dg during development, usually linked to defects in ECM secretion, assembly or remodeling . A developmental role for Dys is less clear, possibly because of the existence of several paralogs in vertebrates. As Drosophila has only one Dg and one Dys gene, it is a promising model for their functional study during development and morphogenesis (Cerqueira Campos, 2020).

Dg and Dys were found to be required for follicle elongation and proper BM fibril formation early in fly oogenesis. During these early stages, DAPC loss and hypomorphic fat2 conditions similarly delay stress fiber orientation. However, DAPC promotes this alignment more locally than Fat2. Moreover, DAPC genetically interacts with fat2 in different tissues, suggesting that they belong to a common morphogenetic network. Later in oogenesis, Dg and Dys are required for stress fiber orientation in a cell-autonomous manner. This is the period when the main elongation defect is seen in these mutants, arguing for a more determinant role for stress fibers compared with BM fibrils in the elongation process. Nonetheless, this latter function depends on the earlier DAPC function in BM fibril deposition. It is proposed that BM fibrils serve as a PCP memory for the late stages that are used as a template by the DAPC for F-actin stress fiber alignment (Cerqueira Campos, 2020).

Genetic data has already demonstrated that follicle elongation relies on at least two different and successive mechanisms. The first is controlled by JAK-STAT and involves the follicle cell apical domain, whereas the second is controlled by Fat2 and involves the basal domain and the BM. Between these phases, around stage 7-8, JAK-STAT and Fat2 seem to be integrated in a third mechanism based on a BM stiffness gradient. Interestingly, a very recent report suggests that this gradient may not directly influence tissue shape but rather do so by modifying the properties of the follicle cells underneath. This study shows that the DAPC influences elongation mainly at very late stages, suggesting the existence of a fourth mechanistic elongation phase. Of note, elongation at these late stages is also defective in fat2 mutants. This is consistent with the fact that rotation is required for polarized BM fibril deposition, and that this deposition depends on and is required for DAPC function. The existence of multiple and interconnected mechanisms to induce elongation, a process that initially appeared to be very simple, highlights the true complexity of morphogenesis, and the necessity to explore it in simple models (Cerqueira Campos, 2020).

Fat2 is clearly part of the upstream signal governing the basal planar polarization. However, how this polarization leads to tissue elongation is still debated. It has been proposed that elongation relies on a molecular corset that could be formed, non-exclusively, by BM fibrils or F-actin stress fibers. The initial observation that rotation is required for both elongation and BM fibrils favored a direct mechanical role for these structures. Recent data showing that BM fibrils are stiffer than the surrounding ECM supports this view. Moreover, increasing the BM fibril number and size can lead to hyper-elongation. Finally, addition of collagenase induces follicle rounding, at least at some stages, and genetic manipulation of the ECM protein levels also influences elongation. However, these experiments did not discriminate between the function of the fibril fraction and a general BM effect. Moreover, they do not demonstrate whether their impact on elongation is direct and mechanic or, indirect by a specific response of the epithelial cells. Fat2 and rotation are also required for the proper orientation of the stress fibers. The F-actin molecular corset is dynamic with follicle cells undergoing basal pulsations, and perturbation of both these oscillations and of the stress fiber structure affect elongation. In the DAPC mutants, a faint but significant elongation defect was observed during mid-oogenesis and a stronger one after stage 12. These defects are clearly correlated with the stress fiber orientation defects observed in the same mutants, both temporally and in terms of intensity. Moreover, although overexpression of Rab10 in a Dys loss-of-function mutant restores BM fibrils, it does not rescue elongation, indicating that stress fiber orientation is instrumental (Cerqueira Campos, 2020).

Thus, if the role of the BM fibrils as a direct mechanical corset appears limited, what is their function? One possibility could have been that they promote rotation, acting by positive feedback and explaining the speed increase over time. However, the rotation reaches the same speed in WT and DAPC mutants, excluding this possibility. Similarly, increasing the fibril fraction also has no effect on rotation speed (Cerqueira Campos, 2020).

The results strongly argue that BM fibrils act as a cue for the orientation of stress fibers, which then generate the mechanical strain for elongation. This appears clear in late stages when the function of the DAPC for stress fiber orientation is dependent on the previous BM fibril deposition. Although it is unknown why the cells lose their orientation from stages 10 to 12, the BM fibrils provide the long-term memory of the initial PCP of the tissue, allowing stress fiber reorientation. Such a mechanism appears to be a very efficient way to memorize positional cues, and could represent a general BM function in many developmental processes (Cerqueira Campos, 2020).

DAPC was found to impact the two key actors at the basal domain of the follicle cells: the BM and the stress fibers linked to the BM. All the defects of Dg null mutants were also observed in Dys mutants, demonstrating a developmental and morphogenetic role for this gene. In vertebrates, at least in some tissues, Dg presence appears to be essential for BM assembly. However, BM formation on the follicular epithelium does not require Dg, suggesting the existence of alternative platforms for its general assembly. The genetic data suggest that Rab10 is epistatic to Dg for BM fibril deposition. The usual interpretation of such a result would be that Dg is involved in the targeting of ECM secretion upstream of Rab10 rather than in ECM assembly in the extracellular space. In Caenorhabditis elegans, Dg acts as a diffusion barrier to define a precise subcellular domain for ECM remodeling. One could imagine that the DAPC has a similar function in follicle cells, by defining the position where the Rab10 secretory route is targeted. However, in DAPC loss of function, some ECM is still secreted between cells, suggesting that the lateral Rab10 route is not affected. Moreover, ECM proteins do not abnormally accumulate between cells in such mutants, suggesting that they are able to leave this localization but without forming BM fibrils. Therefore, the functional interplay between Rab10 and the DAPC is still unclear (Cerqueira Campos, 2020).

As mentioned before, Dg has often been proposed to act as a scaffold to promote BM assembly in mice. Deletion of the Dg intracellular domain is only sub-lethal in mice, whereas complete loss of this protein is lethal very early during development, indicating that the abolishment of Dg's interaction with Dys affects its function only partially. In these mice, laminin assembly can still be observed, for instance in the brain and retina. Similar results were also obtained in cultured mammary epithelial cells. Thus, despite the existence of Dys paralogs that could mask some effects on ECM and the fact that the same ECM alteration was observed in Dg or Dys mutant fly follicles, not all the Dg functions related to ECM assembly or secretion involve Dys. It is possible that Dys is required when Dg needs a very specific subcellular targeting for its function, whereas a more general role in ECM assembly would be independent of Dys. The results suggest that some specific effects of Dys on ECM could have been underestimated and this could help to explain the impact of its loss of function on tissue integrity maintenance. For instance, as it has been reported that Dg influences ECM organization in fly embryonic muscles, it would be interesting to determine whether this also involves Dys (Cerqueira Campos, 2020).

The DAPC is involved in planar polarization of the basal stress fibers and its ability to read ECM structure to orchestrate integrin-dependent adhesion could play a role in many developmental and physiological contexts. The link between the ECM and F-actin provided by this complex is likely required for this function, although this remains to be formally demonstrated. BM fibrils could provide local and oriented higher density of binding sites for Dg, and the alignment could then be transmitted to the actin cytoskeleton. Alternatively, DAPC function could rely on sensing the mechanical ECM properties. The hypothesis that the DAPC could act as a mechanosensor is a long-standing proposal, partly due to the presence of spectrin repeats in Dys. The basal domain of the follicle cells may offer an amenable model to combine genetics and cell biology approaches to decipher such function (Cerqueira Campos, 2020).

Altogether, this work provides important insights on the role of the BM during morphogenesis, by acting as a static PCP cue retaining spatial information while cells are highly dynamic. It also reveals important functions of the DAPC, including Dys, that may be broadly involved during animal development and physiology (Cerqueira Campos, 2020).

The fat-like gene of Drosophila is the true orthologue of vertebrate fat cadherins and is involved in the formation of tubular organs

Fat cadherins constitute a subclass of the large cadherin family characterized by the presence of 34 cadherin motifs. To date, three mammalian Fat cadherins have been described; however, only limited information is known about the function of these molecules. This paper describes the second fat cadherin in Drosophila, fat-like (ftl). ftl is the true orthologue of vertebrate fat-like genes, whereas the previously characterized tumor suppressor cadherin, fat, is more distantly related as compared with ftl. Ftl is a large molecule of 4705 amino acids. It is expressed apically in luminal tissues such as trachea, salivary glands, proventriculus, and hindgut. Silencing of ftl results in the collapse of tracheal epithelia giving rise to breaks, deletions, and sac-like structures. Other tubular organs such as proventriculus, salivary glands, and hindgut are also malformed or missing. These data suggest that Ftl is required for morphogenesis and maintenance of tubular structures of ectodermal origin and underline its similarity in function to a reported lethal mouse knock-out of fat1 where glomerular epithelial processes collapse. Based on these results, a model is proposed where Ftl acts as a spacer to keep tubular epithelia apart rather than the previously described adhesive properties of the cadherin superfamily (Castillejo-Lopez, 2004).

Analysis of the structure and amino acid composition of ftl outlines a subfamily of 34-cadherin repeat genes characterized by the presence of two conserved cysteines at the beginning of repeat 6 and at the end of repeat 28. During embryogenesis, the transcript is expressed in salivary glands and tracheal cells, and the protein is localized on the apical side of the epithelia. Silencing by RNAi reveals abnormalities in the development of tubular structures. Knocked down embryos, using ubiquitous driver lines, show a wide range of morphological defects ranging from small deletions of the trachea to complete lack of tubular structures. In principle, this variation could be explained by incomplete penetrance of the silencing effect and/or variation in the maternal contribution. An alternative explanation might reside in the function of the gene that could be compensated by other molecules. The latter alternative if favored, first because classical mutants affecting tube formation also show variable phenotypes, thereby reflecting the view that different processes and various environments interplay with each other in tube development. Second, in the mfat1 knock-out mouse, only some mutants showed distinctive morphological defects as holo-prosencephaly. In particular, the variation in the eye phenotype reported in the same study was remarkable, often showing severe eye phenotypes on one side and normal eye development on the other side (Castillejo-Lopez, 2004).

Comparative analysis clearly shows that Ftl is more similar to vertebrate fat cadherins than Fat itself. This is best documented in the dendrogram where Drosophila Fat-like clusters, together with Anopheles Fat-like, are in a group closer to vertebrates Fat cadherins and distinct from Fat. The C. elegans fat-like-related molecules, Cdh-3 and Cdh-4, are poorly clustered suggesting that the onset of development of modern Fat cadherins may have occurred about 430 million years ago when insects emerged (Castillejo-Lopez, 2004).

The conserved feature of a 34-cadherin repeat array associated with EGF and laminin G domains in the membrane-proximal region make fat and fat-like cadherins unique. Cadherins of similar size exist in Drosophila, but they lack EGF and laminin G domains such as the Dachsous protein, or the number of cadherin domains is lower as was shown for DN-cadherin which harbors only 18 cadherin repeats. The expression of ftl in tubular tissues resembles the expression of vertebrate Fat cadherins in cells adjacent to ventricular zones. Mouse fat1 is expressed, among other sites, in the respiratory epithelium, kidney glomeruli, and in cells next to the ventricles of the central nervous system. Rat fat2 shows a more restricted expression in the postnatal cerebellum, and rat fat3 also shows expression in fetal central nervous system, in particular to the spinal cord, suggesting that this cadherin modulates the extracellular space of the axons. These neural expression patterns are clearly not observed in Drosophila ftl or fat (Castillejo-Lopez, 2004).

In light of the tumor suppressor role for Fat, it was surprising to note that the structurally similar Ftl was found in tissues that give rise to tubular structures where morphogenesis is mainly due to changes in cellular shape rather than to cellular proliferation. In the functional analyses with ftl, no tumor formation was observed. The previously proposed tumor suppressor role of vertebrate Fat cadherins was only based on sequence similarity with Fat. Just recently, this issue has been addressed by functional studies to show that mice lacking mFAT1 did not reveal any changes in cellular overgrowth (Castillejo-Lopez, 2004).

In order to investigate whether fat-like has a function in imaginal disc patterning or cell polarity similar to fat, several larval GAL4 driver lines were tested. By crossing these lines to the UAS-ds-ftl silencing lines, so far no phenotype was detected in any larval or adult tissue. Therefore, it seems unlikely that ftl plays a function during growth of imaginal disks or during the establishment of planar polarity. It should also be noted that although no ftl mRNA was detected in disks, ftl expression may have been low and may have escaped detection (Castillejo-Lopez, 2004).

The adhesion role of classical cadherins is well established. Overexpression in cell cultures of the full-length rat MEGF1/fat2 indicated a homophilic binding activity. However, overproduction of protein made it difficult to estimate the relevance of the binding strength. In homophilic interactions, the presence of a cleavable pro-sequence might be crucial for transport of the protein to the surface. It has been suggested that the absence of this pro-sequence in Fat cadherins makes strong homophilic adhesions unlikely. In any case, they would extend a very large distance from the plasma membrane and presumably be incompatible with strong intercellular adhesion. An estimation of the length of the Ftl ectodomain, based on crystallization data from five consecutive cadherin repeats, shows that the tip of Flt could well extend 160 nm away from the cell membrane. The same study also showed that the array of cadherin repeats is quite linear and rigid which would reduce the calculated length of the Ftl ectodomain only very little. Although Ftl is in fact a huge molecule, the tips would hardly reach the other side of the trachea lumen taking into account that an average lumen measures about 2000 nm in diameter. These topological restrictions make homophilic interactions through the lumen unlikely, at least when the trachea is at full size. Other weaker adhesive roles cannot be excluded and perhaps are important for cell migration or for recognizing cues in the surrounding environment especially during early tissue formation (Castillejo-Lopez, 2004).

The adhesion properties of Ftl was tested in cultured Drosophila Schneider S2 cells by transfection experiments using dsRNA against ftl. Despite the fact that the mRNA level was substantially reduced, no changes were observedin the cell shape or aggregation behavior compared with cells transfected with control dsRNA. Although these results are not conclusive, they may indicate that Ftl does not play a major role in intercellular adhesion or adhesion to the ECM in S2 cells. Similarly, the adhesion complexes in the skin of mFAT1 knock-out mice were not altered, and the epidermal cells did not show any morphological changes (Castillejo-Lopez, 2004).

The basic organization of trachea consists of a simple monolayer epithelium with an apical cuticle and a lamina on the basal side. The cuticle contains regular folds known as taenidia. These are arranged in a helical pattern and are thought to maintain a certain flexibility within tracheal epithelia and at the same time keep the lumen open. A number of genes are responsible for integrity and maintenance of this epithelium such as crumbs, a large EGF protein, a DE-cadherin, and the transcription factor hindsight. hindsight has been shown to be involved mostly in cell death of the amnioserosa and tracheal epithelia formation. The hindsight mutants show irregular taenidia folds leading to collapse of the tracheae. The trachea displays sac-like structures, a feature also observed in ftl silencing. This suggests that hindsight might be involved in the same signaling pathway as ftl (Castillejo-Lopez, 2004).

According to expression studies it is obvious that Ftl is not a molecule involved in the primary establishment of tracheal structures, rather it appears active at a time point when the tracheal anlagen is already established. The RNA appears specifically polarized within cells at the side where new branches are to be established. It is tempting to speculate that the luminal content of the trunk is somehow projected toward the secondary branches, and this process might be mediated by ftl (Castillejo-Lopez, 2004).

Recently, two interesting molecules residing in the luminal ECM of tracheae have been characterized, dumpy. They are involved in maintenance and formation of the tracheal tube. In addition, they also showed adhesive properties in other tissues such as the wing. Both proteins are secreted apically, and it has been proposed that Dumpy, through its enormous size of 800 nm, acts as a ruler setting the diameter of secondary branches. It is possible that these molecules interact with Ftl directly or through the ECM. Silencing analysis suggests, however, that Ftl is necessary for keeping epithelia apart as a spacer, rather than determining the tube diameter. Consistent with this proposal is the finding that the Ftl-ED antiserum stains the lumen (Castillejo-Lopez, 2004).

Expression studies and functional analysis of mFat1 in mice reveal valuable insights as to the function of its true orthologue in Drosophila. In the mammalian kidney, slit junctions provide the gaps between the product processes in the glomeruli. The expression of mouse fat1 at the slit junction suggests a role as both an adhesion molecule and as a spacer of the junction. In the mouse knock-out the slit junctions disappear, generating a flattened sheet overlying the basement membrane. A similar behavior is inferred in Drosophila ftl mutants whereby tracheal epithelia fuse because of degeneration of the luminal space. Further subcellular analysis using transmission electron microscopy and fourth-dimensional confocal microscopy might help to shed some light on this issue (Castillejo-Lopez, 2004).


REFERENCES

Search PubMed for articles about Drosophila Kugelei/fat-like

Castillejo-Lopez, C., Arias, W. M. and Baumgartner, S. (2004). The fat-like gene of Drosophila is the true orthologue of vertebrate fat cadherins and is involved in the formation of tubular organs. J. Biol. Chem. 279(23): 24034-43. PubMed ID: 15047711

Cerqueira Campos, F., Dennis, C., Alegot, H., Fritsch, C., Isabella, A., Pouchin, P., Bardot, O., Horne-Badovinac, S. and Mirouse, V. (2020). Oriented basement membrane fibrils provide a memory for F-actin planar polarization via the Dystrophin-Dystroglycan complex during tissue elongation. Development. PubMed ID: 32156755

Chen, D.Y., Lipari, K.R., Dehghan, Y., Streichan, S.J. and Bilder, D. (2016). Symmetry breaking in an edgeless epithelium by Fat2-regulated microtubule polarity. Cell Rep 15(6):1125-33. PubMed ID: 27134170

Christoforou C. P., et al. (2008). The detached locus encodes Drosophila Dystrophin, which acts with other components of the Dystrophin Associated Protein Complex to influence intercellular signalling in developing wing veins. Dev. Biol. 313: 519-532. PubMed ID: 18093579

Ciani L., Patel A., Allen N. D. and ffrench-Constant C. (2003). Mice lacking the giant protocadherin mFAT1 exhibit renal slit junction abnormalities and a partially penetrant cyclopia and anophthalmia phenotype. Mol. Cell. Biol. 23: 3575-3582. PubMed ID: 12724416

Gutzeit H. O., Eberhardt W. and Gratwohl E. (1991). Laminin and basement membrane-associated microfilaments in wild-type and mutant Drosophila ovarian follicles. J. Cell Sci. 100: 781-788. PubMed ID: 1814932

Moeller M. J., et al. (2004). Protocadherin FAT1 binds Ena/VASP proteins and is necessary for actin dynamics and cell polarization. EMBO J. 23: 3769-3779. PubMed ID: 15343270

Pokutta S. and Weis W. I. (2007). Structure and mechanism of cadherins and catenins in cell-cell contacts. Annu. Rev. Cell Dev. Biol. 23: 237-261. PubMed ID: 17539752

Saburi S., et al. (2008). Loss of Fat4 disrupts PCP signaling and oriented cell division and leads to cystic kidney disease. Nat. Genet. 40: 1010-1015. PubMed ID: 18604206

Squarr, A. J., Brinkmann, K., Chen, B., Steinbacher, T., Ebnet, K., Rosen, M. K. and Bogdan, S. (2016). Fat2 acts through the WAVE regulatory complex to drive collective cell migration during tissue rotation. J Cell Biol 212: 591-603. PubMed ID: 26903538

Tanoue T. and Takeichi M. (2004). Mammalian Fat1 cadherin regulates actin dynamics and cell-cell contact. J. Cell Biol. 165: 517-528. PubMed ID: 15148305

Tanoue T. and Takeichi, M. (2005). New insights into Fat cadherins. J. Cell Sci. 118: 2347-2353. PubMed ID: 15923647

Viktorinová, I., König, T., Schlichting, K. and Dahmann, C. (2009). The cadherin Fat2 is required for planar cell polarity in the Drosophila ovary. Development 136(24): 4123-32. PubMed ID: 19906848


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