Cell type-specific recruitment of Drosophila Lin-7 to distinct MAGUK-based protein complexes defines novel roles for Sdt and Dlg-S97

Stardust (Sdt) and Discs-Large (Dlg) are membrane-associated guanylate kinases (MAGUKs) involved in the organization of supramolecular protein complexes at distinct epithelial membrane compartments in Drosophila. Loss of either Sdt or Dlg affects epithelial development with severe effects on apico-basal polarity. Moreover, Dlg is required for the structural and functional integrity of synaptic junctions. Recent biochemical and cell culture studies have revealed that various mammalian MAGUKs can interact with mLin-7/Veli/MALS, a small PDZ-domain protein. To substantiate these findings for their in vivo significance with regard to Sdt- and Dlg-based protein complexes, the subcellular distribution of Drosophila Lin-7 (DLin-7) was analyzed, and genetic and biochemical assays were performed to characterize its interaction with either of the two MAGUKs. In epithelia, Sdt mediates the recruitment of DLin-7 to the subapical region, while at larval neuromuscular junctions, a particular isoform of Dlg, Dlg-S97, is required for postsynaptic localization of DLin-7. Ectopic expression of Dlg-S97 in epithelia, however, was not sufficient to induce a redistribution of DLin-7. These results imply that the recruitment of DLin-7 to MAGUK-based protein complexes is defined by cell-type specific mechanisms and that DLin-7 acts downstream of Sdt in epithelia and downstream of Dlg at synapses (Bachmann, 2004).

This study has shown that the single fly homologue of Lin-7 is a component of different MAGUK-based protein complexes in epithelia and at synaptic junctions. This finding is in line with the previously reported association of LIN-7 and mLin-7 with various membrane specializations in worms or mammals, respectively. Nonetheless, these results deviate from these earlier reports in several regards and thereby imply novel roles for Sdt and Dlg-S97. Most notably, the requirement for either MAGUK to recruit DLin-7 to distinct membrane domains was not simply predictable from studies on their homologues in other species (Bachmann, 2004).

Pals1, a putative mammalian homologue of Sdt, binds mLin-7 in vitro (Kamberov, 2000). The physiological significance of this finding remains unclear since Pals1 localizes to tight junctions of epithelial cells, whereas a basolateral localization for mLin-7 was emphasized in several other reports. In Madin-Darby canine kidney cells, however, mLin-7 has also been detected at tight junctions. The current results now indicate that the interaction between the fly orthologues of Pals1 and mLin-7 is employed in epithelia for the recruitment of DLin-7 to the Crb-Sdt complex within the SAR (Bachmann, 2004).

The virtual absence of DLin-7 from basolateral plasma membrane compartments in Drosophila imaginal disc epithelia is in striking contrast to the situation in both mammals and nematodes. Differences in the expression, subcellular localization and binding capacity of potential interaction partners may account for this discrepancy. Two types of evolutionary conserved proteins have been implicated in the basolateral membrane recruitment of LIN-7 and mLin-7: the MAGUKs LIN-2/mLin-2 (CASK) and β-catenin. The latter was found to recruit mLin-7 to cadherin-based epithelial junctions via its C-terminal PDZ-binding motif (Perego, 2000). Although this motif (tDTDL) is conserved in C. elegans β-catenin, it is aberrant in the fly orthologue, Armadillo (tDTDC). In fact, a direct interaction between DLin-7 and Armadillo was not detectable in a yeast two-hybrid assay. Hence it appears unlikely that DLin-7 and Armadillo exhibit a mode of interaction similar to that of their counterparts in mammals. In contrast, DLin-2 (Caki/CamGUK) and DLin-7 displayed strong interaction in the yeast two-hybrid assay. Therefore DLin-2 would be expected to compete with Sdt for binding to the L27 domain of DLin-7 when expressed in epithelia. An epithelial expression of DLin-2, however, has not yet been documented and, instead, both immunostainings and mRNA analyses revealed that DLin-2 is predominantly expressed in the CNS (Bachmann, 2004).

Sdt is not expressed at detectable levels at larval NMJs and thus cannot contribute to the postsynaptic enrichment of DLin-7 at these junctions. Instead it was demonstrated that Dlg-S97 is required for the recruitment of DLin-7 to scaffolding complexes within the subsynaptic reticulum (SSR) around type I boutons. Severe mutations in dlg cause a decrease in the length of the SSR to about 40%. In immunofluorescence analyses, however, the reduction of both endogeneous or Flag-tagged DLin-7 at dlgXI-2 mutant NMJs appeared clearly more dramatic, suggesting that impaired recruitment of DLin-7 is not simply due to reduced SSR complexity. This reasoning is supported by co-immunoprecipitation experiments that revealed a physical linkage between DLin-7 and Dlg-S97. This linkage is most likely indirect, as implied by the failure of EGFP-Dlg-S97 to recruit cytosolic DLin-7 in epithelia. Although an interaction between DLin-7 and the N-terminal domain of Dlg-S97 was monitored in yeast, it was also noted that this interaction is much weaker compared with those displayed by DLin-7 in combination with DLin-2 or Sdt. In accordance with recent biochemical studies and cell culture assays, which imply the coupling of SAP97 and mLin-7 via MAGUKs such as mLin-2 or MPP3, it is therefore proposed that Dlg-S97 and DLin-7 are linked via an intermediate protein factor. In fact, both the N-terminal domain of Dlg-S97 and DLin-7 can bind to L27 domains of DLin-2 in vitro . The presence of DLin-2 at larval NMJs, however, remains questionable. Unfortunately it was not possible to employ an antibody against DLin-2 to address this issue in further detail. Third instar larvae that are homo- or hemizygous for the DLin-2 mutant allele cakix-307 exhibit normal levels of both DLin7- and Dlg-S97-specific immunofluorescence. This allele has been characterized as a deletion that removes large portions of the gene including the region encoding the PDZ-, SH3- and GUK domains of DLin-2. Nonetheless some residual function might be displayed by a truncated DLin-2x-307 mutant isoform. Thus, the observations strongly argue against, but do not completely rule out, an involvement of DLin-2 in the recruitment of DLin-7 to NMJs. It should be noted, however, that Dlg-S97, DLin-2 and DLin-7 could co-assemble into synaptic protein complexes in the CNS where they are found equally enriched within the neuropil regions (Bachmann, 2004).

In light of recent work, which has revealed complex intramolecular interactions displayed by SAP97, one should also consider the possibility that the SAP97-type N-terminus is only accessible upon binding of tissue- or compartment-specific factors to other domains within Dlg-S97. This mode of regulation, however, would not apply to Dlg-S97N-EGFP and thus cannot explain its inability to induce nuclear targeting of DLin-7 in epithelia (Bachmann, 2004).

It has been proposed that the targeting of SAP97 to epithelial membranes depends on mLin-2. This hypothesis was based on the finding that the expression of truncated mLin-2 exerts a dominant-negative effect on the localization of SAP97 in cultured epithelial cells. It is stressed that this hierarchical mode does not apply to the respective fly homologues, since Dlg-A, which lacks the SAP97-type N-terminus, is efficiently targeted to epithelial septate junctions and to NMJs. Moreover, the recruitment of Dlg-S97 by a DLin-7 binding MAGUK could hardly explain the Dlg-S97-dependent recruitment of DLin-7 (Bachmann, 2004).

These analyses strongly suggest that the interactions between DLin-7 and Sdt or Dlg-S97 take place within the respective submembraneous target regions. In addition, these interactions could play a role during the trafficking of DLin-7. In mammalian neurons mLin-7 was found in a complex with mLin-2, mLin-10 and the NMDA-type glutamate receptor subunit NR2B on dendritic vesicles, which are transported along microtubules. Likewise, the subcellular targeting of Dlg-like MAGUKs involves the association with vesicles and/or intracellular membrane compartments and depends on microtubular transport (Bachmann, 2004).

In vertebrates, mLin-7 isoforms have been detected in axonal and dendritic compartments. The postsynaptic colocalization of DLin-7 and Dlg-S97 is reminiscent of the association of mLin-7 with PSD-95/SAP90, a prominent Dlg-like MAGUK present in postsynaptic densities of vertebrate neurons. Interestingly, a recently discovered isoform of PSD-95 (PSD-95β) exhibits a SAP97-type N-terminus with conserved binding properties. In light of the current findings it is speculated that PSD-95β, as opposed to conventional PSD-95, is involved in the postsynaptic recruitment of mLin-7. SAP97 could also serve this role, although a physical association of SAP97 and mLin-7 at synaptic junctions has not yet been reported. A possible association of DLin-7 with the presynaptic membrane of synaptic boutons can hardly be resolved by confocal microscopy in the presence of strong postsynaptic immunoreactivity. Targeted expression of Flag-DLin-7 in motorneurons did not yield considerable immunofluorescence signals at NMJs, suggesting that DLin-7 is barely targeted to presynaptic nerve terminals. It should be noted, however, that the relative pre-versus postsynaptic abundance of a protein does not necessarily reflect its functional impact on either side of the synaptic cleft. For instance, while Dlg, Scrib and D-VAP-33A are clearly enriched postsynaptically at larval NMJs, genetic rescue and gain-of-function experiments have highlighted the importance of the minor presynaptic component in all three cases (Bachmann, 2004).

The roles of DLin-7 within the SAR and at synapses remain elusive. Overexpression of Flag-DLin-7 did not result in easily detectable phenotypes within epithelia or at NMJs. This analyses led to the prediction that DLin-7 acts downstream of Sdt or Dlg-S97. Accordingly, loss-of-function alleles of DLin-7 are expected to mimic previously described or yet concealed phenotypical aspects of sdt and dlg mutants. The partial reduction of DLin-7 as achieved by transgenic expression of dsRNA had no obvious effect on the shape of boutons or on epithelial polarity. Current studies are therefore aimed at both the generation of complete loss-of-function alleles and monitoring more subtle phenotypes. In accordance with previous studies in other species, DLin-7 can be expected to bind at least one ligand via its single PDZ domain. Thereby it may help to retain this ligand within the respective compartment and/or to regulate its endosomal sorting (Bachmann, 2004).

Dynein regulates epithelial polarity and the apical localization of stardust A mRNA

Intense investigation has identified an elaborate protein network controlling epithelial polarity. Although precise subcellular targeting of apical and basolateral determinants is required for epithelial architecture, little is known about how the individual determinant proteins become localized within the cell. Through a genetic screen for epithelial defects in the Drosophila follicle cells, it was found that the cytoplasmic Dynein motor is an essential regulator of apico-basal polarity. The data suggest that Dynein acts through the cytoplasmic scaffolding protein Stardust (Sdt) to localize the transmembrane protein Crumbs, in part through the apical targeting of specific sdt mRNA isoforms. The sdt mRNA localization signal maps to an alternatively spliced coding exon. Intriguingly, the presence or absence of this exon corresponds to a developmental switch in sdt mRNA localization in which apical transcripts are found only during early stages of epithelial development, while unlocalized transcripts predominate in mature epithelia. This work represents the first demonstration that Dynein is required for epithelial polarity and suggests that mRNA localization may have a functional role in the regulation of apico-basal organization. A unique mechanism is introduced in this study in which alternative splicing of a coding exon is used to control mRNA localization during development (Horne-Badovinac, 2008).

Dynein's role in MT-based apical transport has been studied in the epithelia of both mammals and flies, but an explicit link between Dynein and apico-basal polarity has not been found. One cause for this deficiency may be that Dynein is required for a number of essential cellular processes, making it difficult to study this motor under strong loss-of-function conditions. For instance, previous studies of Dynein function in Drosophila embryos have relied on combinations of hypomorphic Dhc alleles or injection of anti-Dhc antibodies which only partially block Dynein function. Notably, these manipulations have failed to produce epithelial polarity phenotypes. This study shows that strong loss of Dynein function in the FCs disrupts both molecular and morphological aspects of apico-basal polarity, providing the first direct evidence for the role of this motor in this key cell biological process. It is currently unclear why the FCs tolerate strong loss of Dynein function better than other tissues, but this property provides a unique opportunity to begin dissecting the many roles that Dynein is likely to play in epithelial organization (Horne-Badovinac, 2008).

How does Dynein regulate apico-basal polarity? Since Dynein is a minus-end directed MT motor and minus ends are apically oriented in epithelia, whether Dynein might ferry components of the Baz and/or the Crb complexes to their sites of action at the apical surface was investigated. Two pieces of data indicate that the Baz complex is not the primary Dynein cargo contributing to FC polarity. Although Baz is occasionally relocalized to the lateral FC surface in Dhc clones, most mutant cells display significant amounts of apical Baz. Furthermore, ECs in which the entire epithelium is mutant for baz or aPKC display multilayering at both EC poles, a phenotype that is distinct from the posterior multilayering in Dhc mutant epithelia. These observations are consistent with recent studies in the embryonic blastoderm, where it was shown that Dynein does play a role in Baz localization, but that it is not the only means by which this protein is targeted to the apical domain. Together, these data indicate that, while Dynein likely does play a role in the apical targeting of Baz in both the embryo and the FCs, Dynein's major contribution to FC apico-basal polarity corresponds to a different cargo (Horne-Badovinac, 2008).

The results favor a model in which Dynein works primarily through the Crb complex to influence epithelial polarity. When Dynein function is reduced in the FCs, Crb and Sdt disappear from the apical surface, even in Dhc cells that remain cuboidal. Moreover, the morphology of an egg chamber in which the entire epithelium is mutant for Dhc is quite similar to that seen for crb and sdt mutant egg chambers, as all three display multilayering predominantly in the posterior. A major challenge comes in deciphering which Crb complex components are specifically transported by Dynein. This study focused on the relationship between Dynein and Sdt because genetic interaction and molecular epistasis experiments indicate that loss of apical Sdt can account for many aspects of the Dhc polarity phenotype. The finding that the apical localization of sdt transcripts is Dynein-dependent suggests a mechanism by which Dynein could localize Sdt, in part, through the apical targeting and localized translation of its mRNA. sdt transcripts cannot be the only Crb complex component transported by Dynein, however, as the phenotype of sdtEH681 mutant FCs does not recapitulate all aspects of the polarity phenotype of Dhc mutant clones. Interestingly, crb transcripts are also targeted to the apical FC cytoplasm in a Dynein-dependent manner, although at a later stage than sdt. This finding raises the possibility that, in addition to Baz, crb mRNA and/or Crb complex proteins represent other Dynein cargoes required for full epithelial polarization. Future work will be required to more finely dissect Dynein's complex contributions to the apical targeting of the Crb complex (Horne-Badovinac, 2008).

While investigating whether sdt mRNA was likely to be a primary Dynein cargo contributing to apico-basal polarity, the surprising discovery was made that the apical targeting of sdt transcripts is regulated through the alternative splicing of a coding exon, exon 3. Only two other genes are known in which transcript localization is regulated in this way. In both instances, however, the signal lies within the 3'UTR, so the splicing event does not affect protein structure. It is curious that alternative splicing of the sdt mRNA localization signal also deletes 433 amino acids from the protein. The role of the amino acids encoded by exon 3 in Sdt function is not yet known. These amino acids are not conserved in vertebrate Sdt homologs. Moreover, Sdt A and Sdt B bind the intracellular domain of Crb with equal efficiency in vitro and this work with the sdtEH681 allele, as well as over-expression studies with the sdt A and sdt B transgenes indicate that both protein isoforms stabilize Crb in vivo. Although the possibility that exon 3 regulates both mRNA localization and protein function cannot be ruled out, together these observations suggest that the splicing of exon 3 may primarily regulate mRNA localization (Horne-Badovinac, 2008).

A potential role for Dynein-dependent apical targeting of sdt mRNA in epithelial polarity is supported by analysis of the relative contributions of the sdt A and sdt B isoforms. Although the lack of Sdt A in sdtEH681 FC clones reduces Crb at the apical membrane, this deficit leads to relatively mild effects on other aspects of polarity. This apparent discrepancy can be explained by the extrinsic cue for apical identity that is provided to the FCs through their direct contact with the germline, which may compensate for reduced Crb complex function in this tissue. However, in the embryo, where no such cue is available, sdtEH681 has a nearly null phenotype, which is rescued much more efficiently by sdt A than sdt B. Overall, these data suggest that the apical targeting of sdt transcripts may represent an important mechanism contributing to apico-basal polarity (Horne-Badovinac, 2008).

Interestingly, the apical targeting of sdt mRNA is developmentally regulated in both embryonic and adult epithelia. Specifically, it was shown that apically localized sdt transcripts are found only during early stages, while unlocalized transcripts predominate at later stages. Why is sdt transcript localization regulated in this way? It is tempting to speculate that apical transcripts are required primarily for the establishment of apico-basal polarity but not its maintenance. In reality, however, the functional distinction between the two phases of sdt mRNA localization is almost certainly more subtle. When apical sdt A transcripts are present, the epithelia are relatively immature; they tend to be proliferative, display incomplete junctional structures and have yet to adopt their final cell morphology. By contrast, when unlocalized sdt B transcripts predominate, the epithelia are more likely to be post-mitotic and highly differentiated. These observations raise the possibility that a concentrated pool of Sdt protein, generated by localized translation of apical transcripts, could function to stabilize Crb primarily during periods when cell polarity is labile, but that this dynamic regulatory mechanism would be dispensable in fully differentiated cells (Horne-Badovinac, 2008).

Protein Interactions

A possible direct interaction between Sdt and Crumbs was tested using the yeast two-hybrid system. A fusion protein consisting of the MAGUK domain of Sdt and the transactivation domain of GAL4 was co-expressed in yeast cells with various forms of the Crb cytoplasmic domain, fused to the DNA-binding domain of GAL4. The full-length intracellular domain of Crb and those with mutations in a region close to the membrane (Crb-intraWT, Crb-intraY10A and Crb-intra E16A) showed strong interaction with the MAGUK domain of Sdt. In contrast, lack of the four C-terminal amino acids ERLI (Crb-intraDERLI) completely abolishes the interaction. This suggests that binding of the MAGUK domain of Sdt depends on the four C-terminal amino acids of Crb (Bachmann, 2001).

Polarized cells contain numerous membrane domains, but it is unclear how the formation of these domains is coordinated to create a single integrated cell architecture. Genetic screens of Drosophila embryos have identified three complexes, each containing one of the PDZ domain proteins -- Stardust (Sdt), Bazooka (Baz) and Scribble (Scrib) -- that control epithelial polarity and formation of zonula adherens. These complexes can be ordered into a single regulatory hierarchy that is initiated by cell adhesion-dependent recruitment of the Baz complex to the zonula adherens. The Scrib complex represses apical identity along basolateral surfaces by antagonizing Baz-initiated apical polarity. The Sdt-containing Crb complex is recruited apically by the Baz complex to counter antagonistic Scrib activity. Thus, a finely tuned balance between Scrib and Crb complex activity sets the limits of the apical and basolateral membrane domains and positions cell junctions. These data suggest a model in which the maturation of epithelial cell polarity is driven by integration of the sequential activities of PDZ-based protein complexes (Bilder, 2003).

Computer modelling in combination with in vitro studies reveals similar binding affinities of Drosophila Crumbs for the PDZ domains of Stardust and DmPar-6

Formation of multiprotein complexes is a common theme to pattern a cell, thereby generating spatially and functionally distinct entities at specialised regions. Central components of these complexes are scaffold proteins, which contain several protein-protein interaction domains and provide a platform to recruit a variety of additional components. There is increasing evidence that protein complexes are dynamic structures and that their components can undergo various interactions depending on the cellular context. However, little is known so far about the factors regulating this behaviour. One evolutionarily conserved protein complex, which can be found both in Drosophila and mammalian epithelial cells, is composed of the transmembrane protein Crumbs/Crb3 and the scaffolding proteins Stardust/Pals1 and DPATJ/PATJ, respectively, and localises apically to the zonula adherens. In vitro analysis shows that, similar as in vertebrates, the single PDZ domain of Drosophila Par-6 can bind to the four C-terminal amino acids (ERLI) of the transmembrane protein Crumbs. To further evaluate the binding capability of Crumbs to Par-6 and the MAGUK protein Stardust, analysis of the PDZ structural database and modelling of the interactions between the C-terminus of Crumbs and the PDZ domains of these two proteins were performed. The results suggest that both PDZ domains bind Crumbs with similar affinities. These data are supported by quantitative yeast two-hybrid interactions. In vivo analysis performed in cell cultures and in the Drosophila embryo show that the cytoplasmic domain of Crumbs can recruit Par-6 and DaPKC to the plasma membrane. The data presented here are discussed with respect to possible dynamic interactions between these proteins (Kempkens, 2006).

Homology modelling and energy calculations indicate that the PDZ domains of Sdt and Par-6 both show a high affinity for Crb. According to a classification of PDZ domains, the PDZ domains of Par-6 and Sdt fall into different categories: n (neutral) – h (hydrophic) for Par-6 and Sp (small and polar) – h (hydrophobic) for Sdt. The distinction is based on the nature of the amino acids in two critical positions, those immediately following the second β-strand and at the beginning of the second α-helix. However, this classification, based exclusively on the amino acids at two positions, does not adequately describe the complexity of the PDZ family and is not sufficient to predict the specificity of binding. This is borne out by the results presented in this study (Kempkens, 2006).

The crystal structure used as a template in this study (D. melanogaster Par-6, 1RZX.PDB) includes a canonical type I ligand (ESLV), and this can easily be replaced in the structure by the C-terminal tetrapeptide of Crb (ERLI). Although better binding of the Crb C-terminus to Par-6 than to Sdt is predicted, this conclusion should be treated with caution, since the predicted structure of the Crb-Sdt complex is based solely on homology modelling. The analysis of interactions between Crb and PDZ domains showed minor van der Waals clashes (<0.3 kcal/mol), as well as strong polar and hydrophobic ligand/PDZ-domain interactions. An important component of the interaction is electrostatic. This could favour fast association (Kon) and dissociation rates (Koff), which would facilitate flexible regulation of complex formation. These values, which are slightly better for Par-6, could account for the affinity and specificity observed in the experimental interaction studies. The predicted Kd for interaction between the domains considered in this study fall in the usual range for experimental determinations of PDZ-ligand binding affinities reported by many authors. The theoretical determination of binding interactions by FoldX has been tested for many domain-peptide interactions (i.e. SH3, SH2, PDZ, Ras-Rab, etc.), is validated by available experimental data, and shows a high degree of accuracy. In this sense, the theoretical cut-off for non-binders can be reasonably approximated to that used for experimental procedures, normally 1E−4M. Thus, all putative ligands with interaction energies below 1E−4M are considered to be non-binders (Kempkens, 2006).

It has been shown that the PDZ domain of vertebrate Par-6 can bind not only to the C-terminus of CRB3, but also to the C-terminus of the N-type Ca2+ channel and the C-terminus of neurexin, and that the PDZ domains of vertebrate and Drosophila Par-6 proteins can interact with an internal region in the N-terminal portion of Pals1/Sdt. These data support the view that PDZ domains are promiscuous and can bind different ligands, thanks to the plasticity of the carboxylate-binding loop. This conformational flexibility can be modulated by interaction with other proteins. For example, binding of Cdc42 to Par-6 significantly enhances its affinity for the C-terminal ligand, while it has no effect on the interaction with the internal binding sequence of Pals1. To what extent other components facilitate loop rearrangement to accommodate internal ligands to the Par-6 PDZ binding pocket in vivo remains to be determined. Interestingly, although the PDZ domains of both Par-6 and Sdt bind to the C-terminus of Crb, only the PDZ domain of Par-6 interacts with the internal sequence in Sdt (Kempkens, 2006).

Since the binding affinities of the PDZ domains of Sdt and Par-6 for the C-terminus of Crb are in the same range, and all three proteins are expressed in the same cells, other factors must be considered that might influence their interactions. These could include, for example, (1) temporal differences in expression, (2) protein modifications, and (3) interactions with other factors. Par-6 is present in the embryo before Sdt and Crb appear, since it is expressed maternally. In the genetic hierarchy controlling cell polarity and ZA assembly, components of the Baz/Par-6/DaPKC complex appear to form the top tier, with Baz acting very early during cellularisation to establish the apical domain in an adherens junction-independent manner. At this stage, Par-6 is diffusely distributed in the cytoplasm. It becomes localised apically only after cellularisation is complete. Localisation of Par-6 requires an interaction with the monomeric GTPase Cdc42, which, like its vertebrate homologue, binds Par-6 via a conserved CRIB (Cdc42/Rac interactive binding) domain. Thus, overexpression of a dominant-negative form of Cdc42 prevents apical localisation of Par-6, and a mutant version of Par-6 that cannot interact with Cdc42 (Par-6-ΔP) remains diffusely distributed in the cytoplasm (Kempkens, 2006).

Binding of the PDZ domain of Sdt to the C-terminus of Crb may also depend on additional contacts mediated by the other interaction domains of Sdt. Sdt is a member of the MAGUK protein family, with two L27 domains, one SH3, one PDZ and one guanylate kinase (GUK) domain. The L27 domains of Pals1 and Sdt bind to PATJ/DPATJ and mLin-7/DLin-7. No partner(s) for the SH3 and GUK domains of Sdt have been described so far. Recently, a direct interaction was shown to occur between the SH3/Hook domain of MPP5/Pals1 and the GUK domain of another MAGUK family member, MPP4, in mouse photoreceptor cells. Furthermore, yeast two-hybrid experiments have uncovered an interaction between the SH3 and the Hook/GUK domains of Sdt. In other members of the MAGUK family, such as human Discs large (hDlg), CASK and p55, these two domains participate in intra- as well as intermolecular interactions, with the former being preferred, at least in vitro. Currently, the data do not allow a decision as to whether the observed SH3-Hook/GUK interaction is intra- or intermolecular, nor to what extent this might influence the capacity of the PDZ domain to bind to Crb in vivo. Similarly, at present it is only possible to speculate on the possible influence on the binding specificity/activity of the phosphorylation of the intracellular domain of Crb, which has been demonstrated by in vitro assays, but also occurs under certain in vivo conditions. Future experiments will contribute to the elucidation of the dynamic interactions among these proteins within the cell and help in the understanding of how they establish and maintain the polarised phenotype (Kempkens, 2006).

Domain-specific early and late function of Patj in Drosophila photoreceptor cells

The formation and maintenance of cell polarity is essential for epithelial morphogenesis. Patj (the Drosophila homolog of mammalian Patj) is multi-PDZ domain protein that localizes to the apical cell membrane and form a protein complex with cell polarity proteins, Crumbs (Crb) and Stardust (St). Whereas Crb and Sdt are known to be required for the organization of adherens junctions (AJs) and rhabdomeres in differentiating photoreceptors, the in vivo function of Dpatj as a member of the Crb complex in developing eye has been unclear due to the lack of loss-of-function mutations specifically affecting the patj gene. Genetic analysis of hypomorph, null, and RNA inerference reveals distinct dual functions of Patj in developing and mature photoreceptors. The C-terminal region (PDZ domains 2-4) of Patj is not essential for development of the animal but is required to prevent late-onset photoreceptor degeneration. In contrast, the N-terminal region of Patj is essential for animal viability and photoreceptor morphgenesis during development. The localization and maintenance of Crb and Sdt in the apical photoreceptor membrane are strongly affected by reduced level of Patj. Patj is necessary for proper positioning of AJs and the integrity of photoreceptors in the developing retina as well as for the maintenance of adult photoreceptors. This study provides evidence that Drosophila Patj has domain-specific early and late functions in regulating the localization and stability of the Crb-Sdt complex in photoreceptor cells (Nam, 2006).

Dynamic changes take place in developing photoreceptors to reorganize the apical cell membrane during pupal stage. Crb and Sdt are required for growth and maintenance of rhabdomeres and AJs during this time of photoreceptor morphogenesis. Patj binds Sdt to form a conserved heterotrimeric Crb complex (Roh, 2002), but its function in vivo has been unclear. In this study, a key question whether Patj is an essential member of the Crb complex was addressed in vivo (Nam, 2006).

This study made three new observations that demonstrate important functions of Patj: (1) hypomorphic patj shows severe late-onset degeneration of photoreceptor cells in adult eye although the mutant eyes develop relatively normally; (2) analysis of patj null mutant patj RNAi demonstrate that Patj is essential for early development of the animal and for morphogenesis of AJ and apical membrane domains of photoreceptor cells during pupal development (Nam, 2006).

Consistent with these results on the late-onset degeneration of photoreceptor cells in hypomorphic adult eyes, (3) degeneration of adult photoreceptor cells was found. However, despite the findings on the phenotypes of the hypomorphic mutant, it is worth noting that there are important differences between these two studies. First, a second study on patj function (Richard, 2005) is limited to the analysis of a viable hypomorphic condition patj that shows no obvious defects in the pattern of AJs and rhabdomeres in the eyes until approximately 70% late pupal development. In contrast, the current study with the newly generated null mutant and RNAi reveals important developmental functions of Patj in the eye as well as animal viability. Second, the hypomorpic mutant is not only deleted in the PDZ2-4 domain portion of the dpatj gene but also deficient for JTBR and partially CG32327 that are adjacent to the 3' end of patj. In this study, it was shown that patj RNAi causes similar phenotypes of patj null mutant in the eye, suggesting that JTBR and CG32327 do not affect eye development and, thus, have no detectable influence on analysis of hypomorphic and null mutants (Nam, 2006).

The complete loss of patj causes several significant defects in early pupal eyes at approximately 45% pupal development. (1) Both Crb and Sdt are strongly reduced or mislocalized in the absence of Patj, although the loss of patj shows slightly weaker phenotypes than those of crb and sdt mutants. Furthermore, and in contrast to the hypomorphic mutant, (2) null clones or the eyes expressing patj RNAi reveal striking basolateral displacement or expansion of AJ markers such as Arm and Baz. These results suggest that the N- and C-terminal region of Patj protein may have distinct functions in the photoreceptor cells. Because the hypomorphic mutant shows relatively normal development of photoreceptors until later pupal stage or eclosion, the PDZ2-4 domains are not required during early photoreceptor development but are necessary for the maintenance of fully differentiated photoreceptors during very late pupal and adult stage. Of interest, a significant level of Crb and Sdt are apically localized in 45% pupal photoreceptors in hypomorphic mutant eyes (Richard, 2005) but are lost later in adult eyes (Richard, 2005), suggesting that PDZ2-4 domains are required to maintain the Crb complex in late pupal and adult stages. In contrast, the N-terminal region of Patj containing MRE and PDZ1 is crucial for morphogenesis of AJ and rhabdomere during the first half of pupal stage. The MRE motif of Patj interacts with the L27 domain of Sdt. Thus, it is likely that the N-terminal region expressed in hypomorphic mutants might be sufficient for interaction with Sdt, allowing normal development. Conversely, Sdt cannot bind Patj in the null mutant, resulting in the failure of normal development as in crb or sdt mutants (Nam, 2006).

Recently, it has been shown that reduction of Patj by RNAi in MDCK epithelial cell culture leads to delayed tight junction formation and defects in cell polarization. Similarly, mammalian Patj is required for localizing tight junction proteins and stabilizing the Crb3 complex in human intestinal cells. Thus, the findings on the important function of Drosophila Patj in developing photoreceptors are consistent with the results from these mammalian studies, and further provide evidence for the developmental function of Patj in the organization of apical cell membrane in the in vivo animal model. Mouse Patj is closely related to Drosophila Patj, but it has 10 PDZ domains in contrast to the presence of 4 PDZ domains in Drosophila Patj. It will be interesting to see whether the mammalian Patj also show distinct functions of the N-terminal MRE and the multiple PDZ domains in organizing cell polarity and maintaining the stability of the Crb complex, respectively (Nam, 2006).

Mutations in the human CRB1 gene cause retinal diseases such as retinitis pigmentosa 12 (RP12) and Leber congenital amaurosis. The current results that Patj has dual functions in photoreceptor morphogenesis and maintenance in developing and adult animals, respectively, suggest that like CRB1, human Patj may be a target for early- and late-onset retinal diseases. It has been shown that the extracellular domain of Crb is required for preventing photoreceptor degeneration in ageing adult fly eyes. Results from this study and Richard (2005) indicate that PDZ2-4 domains of Drosophila Patj are required for blocking late-onset photoreceptor degeneration. It is currently unknown whether the requirement of Crb extracellular domain and the intracellular function of Drosophila Patj PDZ2-4 domains are related events in the maintenance of adult photoreceptors (Nam, 2006).

Patj plays a role in retinal morphogenesis and protects against light-dependent degeneration of photoreceptor cells in the Drosophila eye

The establishment of apicobasal polarity in epithelial cells is a prerequisite for their function. Drosophila photoreceptor cells derive from epithelial cells, and their apical membranes undergo elaborate differentiation during pupal development, forming photosensitive rhabdomeres and associated stalk membranes. Crumbs (Crb), a transmembrane protein involved in the maintenance of epithelial polarity in the embryo, defines the stalk as a subdomain of the apical membrane. Crb organizes a complex composed of several PDZ domain-containing proteins, including Patj (formerly known as Discs lost). Taking advantage of a Patj mutant line in which only a truncated form of the protein is synthesized, Patj was demonstrated to be necessary for the stability of the Crb complex at the stalk membrane and is crucial for stalk membrane development and rhabdomere maintenance during late pupal stages. Moreover, Patj protects against light-induced photoreceptor degeneration (Richard, 2006).

This study presents evidence to show that Patj plays an important role in stabilizing the Crb complex in photoreceptor cells (PRCs). A truncated form of Patj, consisting of the N-terminal L27 and the first PDZ domain, is produced in Patj mutant eyes. It is further shown that the truncated Patj protein fails to stabilize Crb and Sdt at the stalk membrane during late pupal development and in adult eyes. It has been demonstrated that Crb is required for proper localization of Sdt and Patj at the stalk membrane of PRCs and that, in sdtXP96, the maintenance of Crb and Patj is compromised. These data together with the results presented in this study support the view that lack of any component of the Crb complex leads to mislocalization and/or dysfunction of the whole complex in the Drosophila eye (Richard, 2006).

To understand how the Crb complex may ensure a proper morphogenesis of photoreceptor cells, two major events during photoreceptor development have to be considered. In the first half of pupal development, stabilization of the ZAs is essential to maintain adhesion between PRCs during the tremendous cell shape changes that take place later when the cells undergo elongation. Furthermore, from 37% pupal development onward, the apical membrane differentiates into the rhabdomere and the stalk (Richard, 2006).

In crb and sdt mutants, the rhabdomeres are shorter and thicker, suggesting a failure to stabilize adhesion in early stages of pupal development, which in turn prevents proper elongation. This interpretation is consistent with the observation that, in eyes lacking crb function, the continuity of the ZA is interrupted at early stages of pupal development. Patj mutants do not exhibit any obvious defects in ZA development or PRC elongation, which can be explained by the fact that components of the Crb complex are still correctly localized until 70% pupal development. This timing contrasts with crb or sdt mutant PRCs, in which the integrity of the Crb complex is lost at an early stage of pupal development. Several explanations may account for this different behavior of Patj mutant PRCs. (1) The N-terminal portion of Patj may still retain some function during early pupal development, which stabilizes the Crb complex and, hence, the ZA. (2) Alternatively, Patj does not play a major function for the stability of the complex at early stages. (3) Finally, additional factors may interact with Patj and stabilize the Crb complex at early stages of development, and these interactions still occur with a truncated Patj. In fact, recent in vitro studies have suggested direct interactions between Patj and either Drosophila Par-6 or Drosophila PKC, two members of the other apically localized protein complex, which is essential for epithelial cell polarity in the embryo. However, it can be excluded that the suggested interaction between the third PDZ domain of Patj and the N-terminal domain of DmPar-6 plays any role in the stabilization of the complex during the first half of pupal development. The truncated Patj protein studied here lacks the third PDZ domain, yet it remains localized at the apical membrane at this stage (Richard, 2006).

The other major aspect of PRC maturation -- the differentiation of the apical membrane into rhabdomere and stalk -- is affected in crb, sdtXP96, and Patj mutant eyes, suggesting that all three components are necessary for this process. These mutations result in a shortening of the stalk membrane. The weaker phenotype of the Patj mutant relative to that of crb mutants is probably due to the hypomorphic nature of the former. Separation of the apical membrane of PRCs into two distinct domains, the rhabdomere and the stalk, becomes manifest at approximately 55% pupal development and coincides with the restriction of Crb and its associated proteins to this region. No other mutant affecting the length of stalk membrane has been described to date, although some mutants affect individual aspects of the crb or Patj morphogenetic phenotype, displaying thicker (bifocal; DSec61), malformed (Glued; WASp) or missing (overexpression of amphiphysin) rhabdomeres. Thus, the regulation of stalk membrane development seems to be a unique function for members of the Crb complex (Richard, 2006).

One phenotype of Patj mutants observed in this study, the progressive resorption of rhabdomeric microvilli, has not been described to date for any other mutant of the Crb complex. This raises the question whether Patj is involved in other processes in addition to those that are controlled by crb and sdt. The rhabdomere is composed of microvilli, each of which is supported by actin filaments. Rhabdomere morphogenesis and integrity depend on constant renewal of the membrane and on a highly organized actin cytoskeleton. Thus, it is not surprising that mutations in proteins involved in endo- or exocytosis, such as dynamin, Rab1, Rab6, Rab11, Sec6, Sec61, or Sunglasses, affect the integrity of the rhabdomere. It has been suggested that the addition of new membrane occurs at the base of the rhabdomere in Drosophila, while shedding occurs at the distal tip in tipulids. The further analysis of the function of these genes, the subcellular distribution of the respective proteins and their possible interactions with members of the Crb complex will be required to determine any involvement of the Crb complex in these processes. Rhabdomere integrity is also affected in eyes lacking proteins involved in actin structure and remodeling, such as NinaC, Chaoptin, Glued, Moesin or Rac1 but also in mutants for rhodopsin itself, which plays a structural role in addition to its function in signal transduction. In this scenario, Patj could help to stabilize the cytoskeleton and thereby maintain the integrity of the rhabdomere. Alternatively, the four PDZ domains in Patj may mediate the assembly of additional proteins. The identification and functional characterization of these proteins will shed light on the process by which Patj controls the stability of PRCs (Richard, 2006).

At present, the possibility that the defects observed in pigment cells in Patj mutant eyes contribute to the mutant phenotype observed in PRCs cannot be excluded. In vertebrate eyes, the pigment epithelium plays an active role in the renewal of rhodopsin, and defects in the pigment epithelium can lead to degeneration of PRCs. It is not yet known whether pigment cells in the Drosophila eye have a comparable function, although they certainly serve to insulate the PRCs of individual ommatidia from the light impinging on their neighbors. The accumulation and fusion of pigment granules in Patj mutant eyes may point to a defect in vesicular biogenesis and/or secretion. Whether this defect also affects interactions with the PRCs, in other words, whether pigment cells play an active role in the maintenance of the rhabdomeres or PRC function, and if so, whether Patj is involved in this process, is not known (Richard, 2006).

Finally, the results of this study demonstrate that Patj, like Crb, protects PRCs from the deleterious effects of excess light. The degeneration of PRCs observed in Patj mutant eyes may be a direct consequence of the failure to stabilize the components of the Crb complex at the stalk membrane. Previously published data have shown that the absence of crb in Drosophila eyes leads to retinal degeneration under similar lighting conditions. Similarly, mice deficient for CRB1 display signs of retinal degeneration upon exposure to light, which are reminiscent of defects seen in patients bearing mutations in the CRB1 gene. However, the penetrance of degeneration observed in crb eyes is much higher than that observed in Patj eyes, although the cellular features of degeneration observed in both mutants are similar. Taking into account that the crb clones were produced in a white background, whereas Patj eyes are red (due the transgenes that were introduced), it is not unlikely that the pigments could play a protective role in the latter case, as also shown previously for white mutants. Preliminary experiments suggest that the presence of pigments in crb mutant ommatidia indeed slows down the light-dependent degeneration (Richard, 2006).

Taken together, these results extend the knowledge of the genes involved in controlling retinal morphogenesis and preventing light-dependent PRC degeneration in the fly. Mutations in human CRB1 lead to RP12 and LCA, two severe forms of retinal dystrophy, raising the question whether mutations in the homologues of the other members of the complex might result in similar phenotypes. Understanding the molecular mechanisms leading to the mutant phenotype in the fly will certainly contribute to unraveling the pathogenesis of these retinal dystrophies in humans (Richard, 2006).

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

Drosophila Stardust (Sdt), a member of the MAGUK family of scaffolding proteins, is a constituent of the evolutionarily conserved Crumbs-Stardust (Crb-Sdt) complex that controls epithelial cell polarity in the embryo and morphogenesis of photoreceptor cells (PRCs). Although apical localisation is a hallmark of the complex in all cell types and in all organisms analysed, only little is known about how individual components are targeted to the apical membrane. A structure-function analysis of Sdt was performed by constructing transgenic flies that express altered forms of Sdt to determine the roles of individual domains for localisation and function in photoreceptor cells. The results corroborate the observation that the organisation of the Crb-Sdt complex is differentially regulated in pupal and adult photoreceptors. In pupal photoreceptors, only the PDZ domain of Sdt - the binding site of Crb - is required for apical targeting. In adult photoreceptors, by contrast, targeting of Sdt to the stalk membrane, a distinct compartment of the apical membrane between the rhabdomere and the zonula adherens, depends on several domains, and seems to be a two-step process. The N-terminus, including the two ECR domains and a divergent N-terminal L27 domain that binds the multi-PDZ domain protein PATJ in vitro, is necessary for targeting the protein to the apical pole of the cell. The PDZ-, the SH3- and the GUK-domains are required to restrict the protein to the stalk membrane. Drosophila PATJ or Drosophila Lin-7 are stabilised whenever a Sdt variant that contains the respective binding site is present, independently of where the variant is localised. By contrast, only full-length Sdt, confined to the stalk membrane, stabilises and localises Crb, although only in reduced amounts. The amount of Crumbs recruited to the stalk membrane correlates with its length. These results highlight the importance of the different Sdt domains and point to a more intricate regulation of the Crb-Sdt complex in adult photoreceptor cells (Bulgakova, 2008).

Data presented in this study corroborate the view that distinct mechanisms control localisation of the Crb-Sdt complex in PRCs at different developmental stages. This conclusion is further supported by the observation that a truncated PATJ protein, consisting of only L27 and the first PDZ domain, is localised correctly during the first half of pupal development, but is delocalised in adult PRCs. The stability of the complex at pupal stages seems to depend only on Crb. In pupae, all core components of the complex are mislocalised in crb-mutant PRCs, whereas the absence of sdt, PATJ or Lin-7 does not affect apical localisation of the others. Accordingly, Sdt localisation at this stage only depends on its PDZ domain that binds the cytoplasmic tail of Crb. Neither the non-canonical L27N domain of Sdt, which is responsible for binding PATJ, nor the other protein-protein interaction domains are required for Sdt localisation in pupal PRCs (Bulgakova, 2008).

In the adult Drosophila eye, localisation of Crb-Sdt-complex core proteins to the stalk membrane is mutually dependent, with the exception of Lin-7, which is not required to localise other components. Similarly, in zebrafish the levels of the Crb orthologous proteins require the function of the Sdt orthologue Nagie oko. In the fly eye, changes observed at different developmental stages point to a transition in the mechanisms regulating the building and stability of the complex. This transition occurs gradually in the second half of pupal development. At the same time, Bazooka, which is associated with the adherens junctions in the first half of pupal development, accumulates in the cytoplasm. The transition also correlates with the formation of stalk membrane, which initiates around 55% pupal development and ultimately separates the apical plasma domain into two distinct compartments. This process seems to require additional, more complex control mechanisms, as reflected by the fact that several Sdt domains are required for its proper localisation at later stages. It is very possible that other, yet unknown components contribute to the stability and/or restriction of Sdt at the stalk membrane (Bulgakova, 2008).

Results presented in this study also suggest that in the adult Drosophila eye, localisation of Sdt occurs in several steps that rely on different domains. In the first step, Sdt is brought close to the apical membrane. This function is mediated by the N-terminus, including the two ECR domains and the N-terminal L27 domain. Since Par-6, a known binding partner of the ECR motifs, is localised basolaterally in adult PRCs, PATJ binding is more likely to be crucial for apical recruitment of Sdt. In fact, no localised Sdt is detected in PATJ-mutant adult PRCs. In the absence of all other domains besides the N-terminus (with the exception of L27C), Sdt proteins accumulate at the rhabdomere base, a specialised region that seems to have an important role in PRCs. Many proteins involved in morphogenesis, phototransduction or endocytosis, such as Drosophila moesin, TRPL (transient receptor potential-like) and Rab11, to mention just a few, are enriched there. The final step, recruitment of Sdt to the stalk membrane, requires the PDZ-, the SH3- and the GUK-domain. Whereas the PDZ-domain binds Crb, no binding partners for the SH3- and the GUK-domain are known. It was shown that these two domains can bind each other in vitro. Similar interactions between corresponding domains of the human MAGUK CASK were reported to occur either intramolecularly or intermolecularly between the GUK domain of human CASK and the SH3 domain of hDLG. In the MAGUK PSD-93, binding of a ligand to the PDZ domain releases intramolecular inhibition of the GUK domain by the SH3 domain. This possible complexity currently does not distinguish whether the failure to recruit Sdt to the stalk membrane upon removal of one of these domains is due to either the lack of binding additional partner(s) or the lack of intramolecular interactions, or both (Bulgakova, 2008).

Whereas Sdt is not required to restrict components of the Crb-Sdt complex to the apical membrane in pupal PRCs, the apical localisation of Par-6, a member of the Par-protein network, depends on Sdt at this developmental stage. Recently, several studies suggested a direct interaction between the Crb-Sdt and the PAR complex, but the proposed interactions differ with respect to the partners mediating the link. Results obtained from in vitro analysis have suggested a number of interactions: aPKC with both PATJ and the intracellular domain of Crb; the PDZ domain of Par-6 with either the N-terminus of Sdt and/or Pals1 or the C-terminus of CRB1 or CRB3; and the N-terminus of Par-6 with the third PDZ domain of PATJ. The observations that neither Crb nor PATJ localisation is affected in sdt-mutant pupal PRCs and that expression of Sdt-B2 in sdt-mutant PRCs completely restores Par-6 apical localisation, strongly suggests that in pupal PRCs the interaction between the Crb complex and Par-6 is mediated by the ECR motifs of Sdt. Sdt-A, which carries an additional 433 amino-acid-long stretch between ECR1 and ECR2, only partially restored apical recruitment of Par-6, suggesting that separation of ECR1 from ECR2 interferes with efficient interactions between the two proteins (Bulgakova, 2008).

The results show that in adult PRCs, sdt controls localisation and stability of Crb, PATJ and Lin-7 but the mechanisms differ. Whenever a Sdt protein is expressed that contains binding domains for PATJ or Lin-7, the amount of the latter is, independently of localisation, restored to wild-type levels. By contrast, Crb protein is stabilised only when Sdt is associated with the stalk membrane (expression of Sdt-A, Sdt-B2, Sdt-βL27C and Sdt-βN). Interestingly, none of the constructs used, including the two full-length variants, rescued Crb protein to wild-type levels. One possible explanation is that other, yet uncharacterised Sdt isoforms are expressed in the eye, which, together with Sdt-B2 and/or unknown interaction partners of the Crb-Sdt complex, regulate the amount of Crb at the stalk membrane. Additional Sdt isoforms are predicted by Flybase to exist. They mainly differ from the known forms in their N-termini, which suggests alternative interaction partners (Bulgakova, 2008).

One striking phenotype observed in PRCs mutant for crb, sdt or PATJ is the reduction of stalk-membrane length. This raises questions about how the Crb-Sdt complex regulates the size of this distinct apical membrane compartment. The results provide evidence that the amount of Crb protein is a crucial determinant of stalk-membrane length. This agrees with the observation that Crb overexpression increases stalk-membrane length. Interestingly, overexpression of a Crb protein that lacks the cytoplasmic domain and, hence, the binding site for Sdt, is sufficient to cause this increase. This suggests that either the transmembrane and/or extracellular domain of Crb regulates stalk-membrane growth. Sdt contributes to the stabilisation of Crb at the stalk and, hence, is indirectly involved in the control of stalk-membrane length. It will be interesting to explore the mechanism by which Crb regulates stalk-membrane length (Bulgakova, 2008).



Affinity-purified rabbit antibodies against a glutathione S-transferase (GST) fusion of the C-terminal 200 amino acids common to SdtA and SdtB clearly outlines epithelial cells in wild-type embryos, but not in zygotic XN05 or XP96 mutant embryos. The speckles seen in mutant as well as wild-type embryos are background nonspecific staining. From the onset of gastrulation (stage 5 or 6), Sdt is expressed around the apical cell boundaries and the apical region of the lateral cortex. In late gastrulation, coincident with formation of the zonula adherens (ZA), Sdt starts to form a solid continuous belt around the apical boundary of epithelial cells, and is also enriched at the apical surface. Sdt distribution overlaps with adherens junction markers Armadillo (Arm) and DE-cadherin (DE-cad), although Sdt seems to localize slightly apical to adherens junctions as well. When septate junctions form at the basolateral side of epithelial cells (embryonic stage 13 or 14), Sdt retains its apical localization and association with the ZA, and does not overlap with the septate junction marker Coracle (Cor). The tracheal system, foregut and hindgut, which are part of the primary epithelia, also express Sdt at their apical surface. Apical lateral subcellular distribution of Sdt persists throughout embryonic development in epithelia, although the expression gradually decrease in stages 16 and 17. Sdt is also expressed in sensory organs. For example, in antennomaxillary complexes and chordotonal organs, strong Sdt staining is seen on the dendritic tips of sensory neurons and scolopale lumen formed by support scolopale cells (Hong, 2001).

In the fly embryo, sdt transcripts can be detected from the cellular blastoderm onwards, where they are restricted to the anlage of the ectoderm (prospective ectoderm) and excluded from that of the mesoderm. Later on, sdt transcripts are present in epithelial derivatives of the ectoderm, that is, the epidermis, the foregut and hindgut, the tracheae, the salivary glands and the Malpighian tubules, where they are localized apically. At no time is sdt RNA detectable in neuroblasts or other cells of the central nervous system. Both the tissue distribution and the subcellular localization of sdt transcripts are similar to those of crb transcripts (Bachmann, 2001).

An antiserum was raised against two synthetic peptides of the Sdt GUK domain, which therefore should recognize both Sdt-MAGUK1 and Sdt-GUK1/2. It is specific for Sdt, because no staining was observed in several sdt alleles. Apical staining is detected from gastrulation (stage 7) onwards in all epithelia derived from the ectoderm. Hence, Sdt shows the same temporal and spatial expression pattern as Crb. Sdt is colocalized with Crb in the SAC, apical to the ZA. In crb mutants, Sdt protein is lost from the plasma membrane, whereas some residual ZA components show a scattered distribution on all membranes. Sdt in crb mutants thus behaves similarly to Crb protein in sdt mutants. Overexpression of Crb results in the depletion of Sdt from the apical surface (Bachmann, 2001).

Effects of Mutation or Deletion

The mutations bazooka (baz) and sdt belong to a group in which mutant embryos show severe abnormalities in the differentiation of the larval cuticles, including the genes crumbs (crb) and shotgun (shg). During cellularization, the formation of cell membranes begins with the generation of cleavage furrows extending from the periphery of the embryo in a radial direction, thus establishing the normal boundaries between cells in the ectoderm. New plasma membranes are assembled until a monolayer cell sheet has formed that shares many features with epithelial cell monolayers, including epithelial cell functions and polarized membrane transport. A major redistribution of Arm and PY occurs during cellularization and early gastrulation. At mid-cellularization, staining becomes stronger on the apical aspect than in the more basal part of the lateral domain. By the end of cellularization, Arm and PY staining are strongly reduced at the basal part of the cleavage furrow that separates nuclei of cells in the blastoderm. bazooka and stardust have been identified as X-linked genes required for the formation of the ZA in the embryo. Mutations in either of these genes leads to a zygotic embryonic lethal phenotype, which is characterized by either severe malformation or an absence of the embryonic epidermis (Wieschaus, 1984 and Tepass, 1993).

Morphogenetic movements of epithelia during development underlie the normal elaboration of the final body plan. The tissue integrity critical for these movements is conferred by anchorage of the cytoskeleton by adherens junctions, initially spot and later belt-like, zonular structures, which encircle the apical side of the cell. Loss-of-function mutations in the Drosophila genes crumbs and stardust lead to the loss of cell polarity in most ectodermally derived epithelia, followed in some, such as the epidermis, by extensive apoptosis. Both mutants fail to establish proper zonulae adherentes in the epidermis. These results suggest that the two genes are involved in different aspects of this process. Further, they are compatible with the hypothesis that crumbs delimits the apical border, where the zonula adherens usually forms and where Crumbs protein is normally most abundant. In contrast, stardust seems to be required at an earlier stage for the assembly of the spot adherence junctions. In both mutants, the defects observed at the ultrastructural level are preceded by a misdistribution of Armadillo and DE-cadherin, the homologs of beta-catenin and E-cadherin, respectively, which are two constituents of the vertebrate adherens junctions. Strikingly, expansion of the apical membrane domain in epidermal cells by overexpression of crumbs also abolishes the formation of adherens junctions and results in the disruption of tissue integrity, but without loss of membrane polarity. This result supports the view that membrane polarity is independent of the formation of adherens junctions in epidermal cells (Grawe, 1996).

The zonula adherens (ZA) belongs to a family of actin-associated cell junctions called adherens junctions. Antibodies specific to cellular junctions and nascent plasma membranes have been used to study the formation of the zonula adherens in relation to the establishment of basolateral membrane polarity. The same approach was then used as a test system to identify X-linked zygotically active genes required for ZA formation. ZA formation begins during cellularization; the basolateral membrane domain is established at mid-gastrulation. By creating deficiencies for defined regions of the X chromosome, genes have been identified that are required for the formation of the ZA and the generation of basolateral membrane polarity. Embryos mutant for both stardust (sdt) and bazooka (baz) fail to form a ZA. In addition to the failure to establish the ZA, the formation of the monolayered epithelium is disrupted after cellularization, resulting in the formation of a multilayered sheet of cells by mid-gastrulation. Electron microscope analysis of mutant embryos reveals a conversion of cells exhibiting epithelial characteristics into cells exhibiting mesenchymal characteristics. To investigate how mutations that affect an integral component of the ZA itself influence ZA formation, embryos with reduced maternal and zygotic supply of wild-type Armadillo protein were studied. These embryos, like embryos mutant for both sdt and baz, exhibit an early disruption of ZA formation. These results suggest that early stages in the assembly of the ZA are critical for the stability of the polarized blastoderm epithelium (Müller, 1996).

The mutations baz and sdt belong to a group in which mutant embryos show severe abnormalities in the differentiation of the larval cuticles, including the genes crumbs (crb) and shotgun (shg). Although the similarity in the late phenotypes of these mutants shows that the respective genes are all required for the same process, i.e., epithelial differentiation, it is difficult to determine whether all these genes act in a common pathway. Nevertheless, the genes crb and sdt show an interesting genetic interaction. Using chromosomal duplications, it has been shown that the phenotype of crb (null) embryos can be rescued by an additional copy of sdt but not vice versa (Tepass, 1993). Based on these findings, a model has been proposed that positions sdt downstream of crb in a regulatory hierarchy (Tepass, 1993). This model is complicated by the fact that sdt regulates Crb protein distribution (Tepass, 1993). A more attractive model might be that sdt functions in a parallel pathway, and, in sufficient dosage, bypasses the requirement for crb (Muller, 1996).

It is equally complicated to arrange sdt and baz in a linear pathway, given that the double mutant of zygotic null alleles shows a stronger phenotype than the single mutants. The product of the baz gene is provided maternally and zygotically. Although maternal baz may rescue the hemizygous baz phenotype to a certain extent, it is difficult to explain the enhancement of the presumed null phenotype of sdt that occurs when baz is removed, assuming the two genes function in a strict pathway. In summary, it is suggested that although baz, crb and sdt are important for the same process, it is most likely that they act in different, but related pathways (Muller, 1996)

Hemizygous baz single mutants show normal Arm and NT staining up to stage 10 of embryogenesis. Even at this rather late stage, the ZA is disordered only locally while large regions of the epidermis are still normal. Similarly, sdt single mutant embryos exhibit abnormal ZA morphology only late in development. The effects of the baz;sdt double mutant are much more severe. In postgastrula embryos, adherens junction plaques are strongly reduced and often absent; these plaques are normally present at the apical-lateral junction, indicating the presence of the ZA. The relatively weak zygotic phenotype of baz can be explained by a strong maternal component for the expression of baz. baz null embryos were produced to analyze the ZA phenotype of embryos that lack both maternal and zygotic baz activity. baz null embryos show early disruption of ZA formation like that seen in baz;sdt zygotic double mutants. In particular, the concentration of Arm in the apical region of cell contact at the beginning of gastrulation is absent in both baz null mutants and the double mutants. In addition to these early alterations in ZA formation, two morphological features of both mutant phenotypes are very similar: (1) when cellularization is complete, cell shapes in the mutant embryos are aberrant; instead of the highly columnar cell shape in the epithelium of wild-type embryos, the cells have irregular outlines and some appear bottle-shaped; (2) the regular hexagonal pattern of Arm staining seen on the surface of wild-type embryos is distorted in the double mutants; the apical cell surfaces appear to vary in size, indicating that the apical domains are expanded in some cells and constricted in others. No ZAs are found in baz null mutant embryos. Germ-line null armadillo mutants exhibit a phenotype similar to baz;sdb mutants, showing that Armadillo is also required for ZA formation. It is suggested that early stages in the assembly of the ZA are critical for the assembly and/or stability of the polarized blastoderm epithelium (Muller, 1996).

Mutations in 17 genes (26 deficiencies) were characterized that interact with Armover and/or Armunder. Interaction strength varies from deficiency to point mutation, suggesting that several genes in the original deficiencies could have contributed to, or modified, the interaction. Only for 7 of the 17 genes have interactions been identical between the point mutation and the corresponding starting deficiency. The 17 genes were sorted into four groups. Group 2 consists of genes required for cell adhesion: This group includes shotgun (which encodes DE-cadherin), as expected. Also uncovered were fat (ft) and dachsous (ds). These two genes encode nonclassical cadherin characterized by a huge extracellular domain containing up to 35 cadherin repeats and a bipartite Arm binding site. Interactions with these two mutants are similar to those observed with shotgun (DE-cadherin), the only difference being that ft interacts more weakly than shg with Armover. In addition to genes encoding cadherins (classical and nonclassical), interactions have been observed with some of the genes known to be essential (directly or indirectly) for the assembly or maintenance of adherens junctions -- stardust (sdt), discs-large (dlg), and crumbs (crb). These interact in the same direction as shg; however, the suppression of Armunder is always weaker and only dlgM52 enhances Armover to the same extent as zw3M11 (Greaves, 1999).

Crb is the earliest zygotically expressed apical transmembrane protein, but nothing is known about the cis-regulatory sequences that target it to the apical face of the cell nor the mechanisms and proteins required for this process. Nothing is known about the function of the large extracellular domain; its overexpression in a secreted or membrane-anchored form (lacking the cytoplasmic domain) does not induce any mutant phenotype. Embryos devoid of maternal Dlt fail to localize Crb. Since the blastoderm epithelium of these embryos itself lacks cell polarity, however, all other defects, including improper Crb localization, could be regarded as secondary effects. In embryos mutant for stardust, Crb is first expressed apically, but during germ band extension it is no longer detectable, making stardust a likely regulator for the maintenance of apical localization of Crb. In agreement with this, stardust mutant embryos develop a phenotype nearly identical to that of crb mutant embryos (Klebes, 2000).

To identify X chromosomal genes required for salivary gland development in the Drosophila embryo, embryos hemizygous for EMS-induced lethal mutations were screened to find mutations causing gross morphological defects in salivary gland development. The parental strain carried a lac Z transgene on the second chromosome, which was specifically expressed in the salivary glands so the mutations could be unambiguously identified. Embryos from 3,383 lines were tested for salivary gland abnormalities following lacZ staining. From 63 lines exhibiting aberrant salivary gland phenotypes, 52 stable lines were established containing mutations affecting salivary gland development. From these, 39 lines could be assigned to nine complementation groups: armadillo, brinker, folded gastrulation, giant, hindsight, Notch, runt, stardust and twisted gastrulation (Lammel, 2000).

The identified X chromosomal genes with respect to their possible contributions to salivary gland development is discussed here. For mutations in giant and Notch, severe defects or the absence of the salivary glands have been described previously. In giant mutant larvae, the gnathocephalic structures are affected, which correspond to the labial segment. This is the region where most of the salivary gland anlage originates. giant is required for the expression of Sex combs reduced (Scr), the master regulator for salivary gland development. Scr expression is absent in the labial lobe in giant mutations. The small glands seen in the absence of giant may form from remaining cells of PS 2 still expressing Scr protein. Notch is involved in the formation of epidermal cells, and in its absence neural precursor cells are formed instead. As a result, in Notch mutants a coherent sheet of epidermal cells is found only in the most dorsal position, outside the neurogenic ectoderm. Only a few ventrolateral epidermal cells are left, which may fail to form a salivary gland anlage. Similar explanations may hold for mutations such as stardust, which affect other general aspects of epithelial development, since in the presence of these mutations the maintenance of coherent epidermal sheets is disrupted (Lammel, 2000 and references therein).

Apicobasal cell polarity is crucial for morphogenesis of photoreceptor rhabdomeres and adherens junctions (AJs) in the Drosophila eye. Crumbs (Crb) is specifically localized to the apical membrane of photoreceptors, providing a positional cue for the organization of rhabdomeres and AJs. The Crb complex consisting of Crb, Stardust (Sdt) and Discs-lost (Dlt) colocalizes with another protein complex containing Par-6 and atypical protein kinase C (aPKC) in the rhabdomere stalk of photoreceptors. Loss of each component of the Crb complex causes age-dependent mislocalization of Par-6 complex proteins, and ectopic expression of Crb intracellular domain is sufficient to recruit the Par-6 complex. The absence of Par-6 complex proteins results in severe mislocalization and loss of Crb complex. Dlt directly binds to Par-6, providing a molecular basis for the mutual dependence of the two complexes. These results suggest that the interaction of Crb and Par-6 complexes is required for the organization and maintenance of apical membranes and AJs of photoreceptors (Nam, 2003).

The strong dependence of Crb localization on Sdt and Dlt suggests that Crb may be destabilized or may not be targeted to the membrane in the absence of Sdt or Dlt. It is intriguing that Sdt and Dlt are lost only partially in the absence of Crb. The findings of a direct interaction between Dlt and Par-6 suggest that Sdt-Dlt can still be targeted to the membrane in the absence of Crb through the binding of Dlt to the Par-6 complex. However, it is important to note that Dlt is essentially lost in sdt mutant clones and vice versa. This raises an intriguing possibility that Dlt or Sdt are dependent on each other in vivo to be targeted to the apical membrane via binding to either Crb or Par-6. This mutual dependency between Dlt and Sdt may explain why Dlt and Sdt are lost in the absence of the other, rather than being associated with the Par-6 complex (Nam, 2003).

The interaction between the Crb and Par-6 complexes is mediated by the PDZ3 region of Dlt and the N-terminal domain of Par-6. The N-terminal domain of Par-6 is also used for binding aPKC. Therefore, a potential function of Dlt is to bind Par-6 in competition with aPKC or to facilitate the interaction of Par-6 with aPKC or other Par-6 binding proteins. Mutant analysis indicates that loss of Dlt and Sdt in sdt- clones causes mislocalization of both Crb and Par-6 complex proteins. This suggests that Sdt-Dlt interaction provides a scaffold to recruit Crb complex to the Par-6 complex and enhance the stability of these two complexes rather than functioning as a competitor for aPKC (Nam, 2003).

Proteins in Crb and Par-6 complexes consist of multiple functional domains which may be involved in diverse protein-protein interactions. A recent study has shown that in mammalian cell culture systems the PDZ domain of Par-6 binds not only Par-3 but also the N terminus of Pals1. These results suggest that the crosstalk between the Crb and Par-6 complexes is mediated by multiple domain-specific interactions. Evidence from genetic analysis using mutants suggests that the crosstalk between the two complexes is mutually required for normal organization of apical membranes and AJs in vivo, and also provides a basis for partial redundancy of these complexes in the organization of photoreceptor cell polarity. Interestingly, when either Crb or Sdt is lost, mislocalization or elimination of other associated components including Par-6 complex proteins becomes more severe in the age-dependent manner. This suggests that the Crb complex may be required for the maintenance rather than the formation of the Par-6 complex. The age-dependent degenerative phenotype may be related to the requirement of extensive apical membrane growth to make rhabdomeres and AJs along the growing axis of photoreceptors during pupal stage. Loss of any one component of the Crb complex is likely to be increasingly more detrimental as the process of membrane reorganization proceeds. In crb- or sdt- mutants, significant fractions of Par-6 complex proteins remain in the membrane despite the age-dependent and progressive mislocalization of apical markers. By contrast, loss of Par-6 or aPKC results in mislocalization of Dlt from the apical membrane. This suggests that the Par-6 complex plays essential functions for membrane localization of Crb complex proteins. Furthermore, both Par-6 and aPKC seem to be important for survival and/or proliferation of retinal cells because mutant clones were very small compared with adjacent twin spots and often completely disrupted, probably due to cell death. This is consistent with the findings of frequent apoptosis in aPKC- or par-6- embryos (Nam, 2003).

An important distinction of Par-6 complex in the photoreceptors from other epithelia is the localization of Baz. Baz localizes with Crb complex in the subapical membrane or both the subapical region and AJ in the Drosophila embryonic epithelia. Vertebrate Par-3 also localizes to the apical tight junction in vertebrate epithelial cells. By contrast, Baz in the photoreceptors is specifically positioned in the AJs basal to the all other proteins in the Crb/Par-6 complexes. Baz and Arm are recruited together to ectopic membrane sites by misexpression of CrbJM, suggesting that Baz is an integral component of AJ. However, Baz is not recruited by CrbPBM, whereas Par-6 and aPKC can be ectopically recruited by CrbPBM rather than CrbJM. Therefore, Baz appears to be recruited to AJ independently of Par-6/aPKC (Nam, 2003).

Intriguingly, despite its specific localization to AJs, loss of Baz results in most severe disruption of AJ as well as the more apical Dlt domain. It has been proposed that the Par-6/aPKC cassette is recruited to the site of cell-cell contact and then moves along the most apical zone of the developing cell-cell contact. In this process, an important step for cell polarity formation is to tether the cytoplasmic Par-6/aPKC complex to the site of cell-cell contact at the membrane, which is mediated by the interaction of Par-3 and a membrane protein JAM. Therefore, the results that baz mutation causes loss of Dlt and AJs support the crucial role of Baz in the initial step of cell polarization. However, the distinct localization of Baz from Par-6 and aPKC in the photoreceptors suggests that the mode of Baz localization varies in different systems. In photoreceptors, Baz may be targeted to the membrane with Par-6 but be sorted out from Par-6 in subsequent steps of polarization to remain in the AJs, whereas Par-6-aPKC-Baz cassette remains together in the complex in other epithelia. In contrast to Baz, aPKC localizes to both rhabdomere stalk and AJ, suggesting that Baz and Par-6 are completely separated during polarization while aPKC is not sorted from both Par-6 and Baz. The critical function of Baz in the localization of Crb complex in the rhabdomere stalk is consistent with the requirement of Baz for Crb localization in embryonic epithelia. However, the requirement of Baz in the embryo appears to be dependent on the stage of development since Crb distribution in the absence of Baz becomes normal in late embryos. On the contrary, such stage-dependent recovery of Crb complex localization has not been observed in baz- photoreceptor cells (Nam, 2003).

Recent studies have shown that mutations in human CRB1 cause RP12 and LCA, severe recessive retinal diseases, emphasizing the importance of Crb family proteins in the eyes of mammals including humans. The Drosophila Crb and human CRB1 are localized in analogous subcellular membrane domains of photoreceptors, the rhabdomere stalk and the inner segment in Drosophila and human photoreceptors, respectively. Besides similar subcellular localization, Crb and human CRB1 are functionally conserved. Age-dependent photoreceptor defects in the crb mutant also provide analogy to age-dependent retinal degeneration in RP12/LCA patients. These studies here imply that hCRB1 may function as a protein complex with homologs of Sdt and Dlt and such a complex may interact with a homologous Par-6 complex. Whether such homologous human genes are the targets of inherited retinal diseases such as RP remains to be studied (Nam, 2003).


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stardust: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation

date revised: 2 January 2016

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