Binding of Crumbs to the Par-6 CRIB-PDZ Module Is Regulated by Cdc42

Capture, mutual inhibition and release mechanism for aPKC-Par6 and its multisite polarity substrate Lgl

The mutually antagonistic relationship of atypical protein kinase C (aPKC) and partitioning-defective protein 6 (Par6) with the substrate lethal (2) giant larvae (Lgl) is essential for regulating polarity across many cell types. Although aPKC-Par6 phosphorylates Lgl at three serine sites to exclude it from the apical domain, aPKC-Par6 and Lgl paradoxically form a stable kinase-substrate complex, with conflicting roles proposed for Par6. We report the structure of human aPKCι-Par6α bound to full-length Llgl1, captured through an aPKCι docking site and a Par6PDZ contact. This complex traps a phospho-S663 Llgl1 intermediate bridging between aPKC and Par6, impeding phosphorylation progression. Thus, aPKCι is effectively inhibited by Llgl1pS663 while Llgl1 is captured by aPKCι-Par6. Mutational disruption of the Lgl-aPKC interaction impedes complex assembly and Lgl phosphorylation, whereas disrupting the Lgl-Par6PDZ contact promotes complex dissociation and Lgl phosphorylation. We demonstrate a Par6PDZ-regulated substrate capture-and-release model requiring binding by active Cdc42 and the apical partner Crumbs to drive complex disassembly. Our results suggest a mechanism for mutual regulation and spatial control of aPKC-Par6 and Lgl activities.

A PDZ-kinase allosteric relay mediates Par complex regulator exchange

The Par complex polarizes the plasma membrane of diverse animal cells using the catalytic activity of atypical PKC (aPKC) to pattern substrates. Two upstream regulators of the Par complex, Cdc42 and Par-3, bind separately to the complex to influence its activity in different ways. Each regulator binds a distinct member of the complex, Cdc42 to Par-6 and Par-3 to aPKC, making it unclear how they influence one another's binding. Here, we report the discovery that Par-3 binding to aPKC is regulated by aPKC autoinhibition and link this regulation to Cdc42 and Par-3 exchange. The Par-6 PDZ domain activates aPKC binding to Par-3 via a novel interaction with the aPKC kinase domain. Cdc42 and Par-3 have opposite effects on the Par-6 PDZ-aPKC kinase interaction: while the Par-6 kinase domain interaction competes with Cdc42 binding to the complex, Par-3 binding is enhanced by the interaction. The differential effect of Par-3 and Cdc42 on the Par-6 PDZ interaction with the aPKC kinase domain forms an allosteric relay that connects their binding sites and is responsible for the negative cooperativity that underlies Par complex polarization and activity.

A PDZ-kinase allosteric relay mediates Par complex regulator exchange

The Par complex polarizes the plasma membrane of diverse animal cells using the catalytic activity of atypical Protein Kinase C (aPKC) to pattern substrates. Two upstream regulators of the Par complex, Cdc42 and Par-3, bind separately to the complex to influence its activity in different ways. Each regulator binds a distinct member of the complex, Cdc42 to Par-6 and Par-3 to aPKC, making it unclear how they influence one another's binding. Here we report the discovery that Par-3 binding to aPKC is regulated by aPKC autoinhibition and link this regulation to Cdc42 and Par-3 exchange. The Par-6 PDZ domain activates aPKC binding to Par-3 via a novel interaction with the aPKC kinase domain. Cdc42 and Par-3 have opposite effects on the Par-6 PDZ-aPKC kinase interaction: while the Par-6 kinase domain interaction competes with Cdc42 binding to the complex, Par-3 binding is enhanced by the interaction. The differential effect of Par-3 and Cdc42 on the Par-6 PDZ interaction with the aPKC kinase domain forms an allosteric relay that connects their binding sites and is responsible for the negative cooperativity that underlies Par complex polarization and activity.

Human CRB1 and CRB2 form homo- and heteromeric protein complexes in the retina

Crumbs homolog 1 (CRB1) is one of the key genes linked to retinitis pigmentosa and Leber congenital amaurosis, which are characterized by a high clinical heterogeneity. The Crumbs family member CRB2 has a similar protein structure to CRB1, and in zebrafish, Crb2 has been shown to interact through the extracellular domain. Here, we show that CRB1 and CRB2 co-localize in the human retina and human iPSC-derived retinal organoids. In retina-specific pull-downs, CRB1 was enriched in CRB2 samples, supporting a CRB1-CRB2 interaction. Furthermore, novel interactors of the crumbs complex were identified, representing a retina-derived protein interaction network. Using co-immunoprecipitation, we further demonstrate that human canonical CRB1 interacts with CRB1 and CRB2, but not with CRB3, which lacks an extracellular domain. Next, we explored how missense mutations in the extracellular domain affect CRB1-CRB2 interactions. We observed no or a mild loss of CRB1-CRB2 interaction, when interrogating various CRB1 or CRB2 missense mutants in vitro. Taken together, our results show a stable interaction of human canonical CRB2 and CRB1 in the retina.

The Cyst Epithelium in Polycystic Kidney Disease Patients Displays Normal Apical-Basolateral Cell Polarity

The main characteristic of polycystic kidney disease is the development of multiple fluid-filled renal cysts. The discovery of mislocalized sodium-potassium pump (Na,K-ATPase) in the apical membrane of cyst-lining epithelia alluded to reversal of polarity as a possible explanation for the fluid secretion. The topic of apical Na,K-ATPase in cysts remains controversial. We investigated the localization of the Na,K-ATPase and assessed the apical-basolateral polarization of cyst-lining epithelia by means of immunohistochemistry in kidney tissue from six polycystic kidney disease patients undergoing nephrectomy. The Na,K-ATPase α1 subunit was conventionally situated in the basolateral membrane of all immunoreactive cysts. Proteins of the Crumbs and partitioning defective (Par) complexes were localized to the apical membrane domain in cyst epithelial cells. The apical targeting protein Syntaxin-3 also immunolocalized to the apical domain of cyst-lining epithelial cells. Proteins of the basolateral Scribble complex immunolocalized to the basolateral domain of cysts. Thus, no deviations from the typical epithelial distribution of basic cell polarity proteins were observed in the cysts from the six patients. Furthermore, we confirmed that cysts can originate from virtually any tubular segment with preserved polarity. In conclusion, we find no evidence of a reversal in apical-basolateral polarity in cyst-lining epithelia in polycystic kidney disease.

Allosteric Signaling in PDZ Energetic Networks: Embedding Error Analysis

Allosteric signaling in proteins has been known for some half a century, yet how the signal traverses the protein remains an active area of research. Recently, the importance of electrostatics to achieve long-range signaling has become increasingly appreciated. Our laboratory has been working on developing network approaches to capture such interactions. In this study, we turn our attention to the well-studied allosteric model protein, PDZ. We study the allosteric dynamics on a per-residue basis in key constructs involving the PDZ domain, its allosteric effector, and its peptide ligand. We utilize molecular dynamics trajectories to create the networks for the constructs to explore the allosteric effect by plotting the heat kernel results onto axes defined by principal components. We introduce a new metric to quantitate the volume sampled by a residue in the latent space. We relate our findings to PDZ and the greater field of allostery.

Negative cooperativity underlies dynamic assembly of the Par complex regulators Cdc42 and Par-3

The Par complex polarizes diverse animal cells through the concerted action of multiple regulators. Binding to the multi-PDZ domain containing protein Par-3 couples the complex to cortical flows that construct the Par membrane domain. Once localized properly, the complex is thought to transition from Par-3 to the Rho GTPase Cdc42 to activate the complex. While this transition is a critical step in Par-mediated polarity, little is known about how it occurs. Here, we used a biochemical reconstitution approach with purified, intact Par complex and qualitative binding assays and found that Par-3 and Cdc42 exhibit strong negative cooperativity for the Par complex. The energetic coupling arises from interactions between the second and third PDZ protein interaction domains of Par-3 and the aPKC Kinase-PBM (PDZ binding motif) that mediate the displacement of Cdc42 from the Par complex. Our results indicate that Par-3, Cdc42, Par-6, and aPKC are the minimal components that are sufficient for this transition to occur and that no external factors are required. Our findings provide the mechanistic framework for understanding a critical step in the regulation of Par complex polarization and activity.

Apical-basal polarity and the control of epithelial form and function

Epithelial cells are the most common cell type in all animals, forming the sheets and tubes that compose most organs and tissues. Apical-basal polarity is essential for epithelial cell form and function, as it determines the localization of the adhesion molecules that hold the cells together laterally and the occluding junctions that act as barriers to paracellular diffusion. Polarity must also target the secretion of specific cargoes to the apical, lateral or basal membranes and organize the cytoskeleton and internal architecture of the cell. Apical-basal polarity in many cells is established by conserved polarity factors that define the apical (Crumbs, Stardust/PALS1, aPKC, PAR-6 and CDC42), junctional (PAR-3) and lateral (Scribble, DLG, LGL, Yurt and RhoGAP19D) domains, although recent evidence indicates that not all epithelia polarize by the same mechanism. Research has begun to reveal the dynamic interactions between polarity factors and how they contribute to polarity establishment and maintenance. Elucidating these mechanisms is essential to better understand the roles of apical-basal polarity in morphogenesis and how defects in polarity contribute to diseases such as cancer.

Energetic determinants of animal cell polarity regulator Par-3 interaction with the Par complex

The animal cell polarity regulator Par-3 recruits the Par complex (consisting of Par-6 and atypical PKC, aPKC) to specific sites on the cell membrane. Although numerous physical interactions have been reported between Par-3 and the Par complex, it is unclear how each of these interactions contributes to the overall binding. Using a purified, intact Par complex and a quantitative binding assay, here, we found that the energy required for this interaction is provided by the second and third PDZ protein interaction domains of Par-3. We show that both Par-3 PDZ domains bind to the PDZ-binding motif of aPKC in the Par complex, with additional binding energy contributed from the adjacent catalytic domain of aPKC. In addition to highlighting the role of Par-3 PDZ domain interactions with the aPKC kinase domain and PDZ-binding motif in stabilizing Par-3–Par complex assembly, our results indicate that each Par-3 molecule can potentially recruit two Par complexes to the membrane during cell polarization. These results provide new insights into the energetic determinants and structural stoichiometry of the Par-3–Par complex assembly.

The discovery of 1, 3-diamino-7H-pyrrol[3, 2-f]quinazoline compounds as potent antimicrobial antifolates

The shortage of new antibiotics makes infections caused by gram-negative (G^-) bacteria a significant clinical problem. The key enzymes involved in folate biosynthesis represent important targets for drug discovery, and new antifolates with novel mechanisms are urgently needed. By targeting to dihydrofolate reductase (DHFR), a series of 1,3-diamino-7H-pyrrol[3,2-f]quinazoline (PQZ) compounds were designed, and exhibited potent antibacterial activities in vitro, especially against multi-drug resistant G^- strains. Multiple experiments indicated that PQZ compounds contain a different molecular mechanism against the typical DHFR inhibitor, trimethoprim (TMP), and the thymidylate synthase (TS) was identified as another potential but a relatively weak target. A significant synergism between the representative compound, OYYF-175, and sulfamethoxazole (SMZ) was observed with a strong cumulative and significantly bactericidal effect at extremely low concentrations (2 μg/mL for SMZ and 0.03 pg/mL for OYYF-175), which could be resulted from the simultaneous inhibition of dihydropteroate synthase (DHPS), DHFR and TS. PQZ compounds exhibited therapeutic effects in a mouse model of intraperitoneal infections caused by Escherichia coli (E. coli). The co-crystal structure of OYYF-175-DHFR was solved and the detailed interactions were provided. The inhibitors reported represent innovative chemical structures with novel molecular mechanism of action, which will benefit the generation of new, efficacious bactericidal compounds.

Pak1 and PP2A antagonize aPKC function to support cortical tension induced by the Crumbs-Yurt complex

The Drosophila polarity protein Crumbs is essential for the establishment and growth of the apical domain in epithelial cells. The protein Yurt limits the ability of Crumbs to promote apical membrane growth, thereby defining proper apical/lateral membrane ratio that is crucial for forming and maintaining complex epithelial structures such as tubes or acini. Here, we show that Yurt also increases Myosin-dependent cortical tension downstream of Crumbs. Yurt overexpression thus induces apical constriction in epithelial cells. The kinase aPKC phosphorylates Yurt, thereby dislodging the latter from the apical domain and releasing apical tension. In contrast, the kinase Pak1 promotes Yurt dephosphorylation through activation of the phosphatase PP2A. The Pak1–PP2A module thus opposes aPKC function and supports Yurt-induced apical constriction. Hence, the complex interplay between Yurt, aPKC, Pak1, and PP2A contributes to the functional plasticity of Crumbs. Overall, our data increase our understanding of how proteins sustaining epithelial cell polarization and Myosin-dependent cell contractility interact with one another to control epithelial tissue architecture.

Mechanoregulation of PDZ Proteins, An Emerging Function

Mechanical forces have emerged as essential regulators of cell organization, proliferation, migration, and polarity to regulate cellular and tissue homeostasis. Changes in forces or loss of the cellular response to them can result in abnormal embryonic development and diseases. Over the past two decades, many efforts have been put in deciphering the molecular mechanisms that convert forces into biochemical signals, allowing for the identification of many mechanotransducer proteins. Here we discuss how PDZ proteins are emerging as new mechanotransducer proteins by altering their conformations or localizations upon force loads, leading to the formation of macromolecular modules tethering the cell membrane to the actin cytoskeleton.

RhoGAP19D inhibits Cdc42 laterally to control epithelial cell shape and prevent invasion

Cdc42-GTP is required for apical domain formation in epithelial cells, where it recruits and activates the Par-6-aPKC polarity complex, but how the activity of Cdc42 itself is restricted apically is unclear. We used sequence analysis and 3D structural modeling to determine which Drosophila GTPase-activating proteins (GAPs) are likely to interact with Cdc42 and identified RhoGAP19D as the only high-probability Cdc42GAP required for polarity in the follicular epithelium. RhoGAP19D is recruited by α-catenin to lateral E-cadherin adhesion complexes, resulting in exclusion of active Cdc42 from the lateral domain. rhogap19d mutants therefore lead to lateral Cdc42 activity, which expands the apical domain through increased Par-6/aPKC activity and stimulates lateral contractility through the myosin light chain kinase, Genghis khan (MRCK). This causes buckling of the epithelium and invasion into the adjacent tissue, a phenotype resembling that of precancerous breast lesions. Thus, RhoGAP19D couples lateral cadherin adhesion to the apical localization of active Cdc42, thereby suppressing epithelial invasion.

Research Models and Gene Augmentation Therapy for CRB1 Retinal Dystrophies

Retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA) are inherited degenerative retinal dystrophies with vision loss that ultimately lead to blindness. Several genes have been shown to be involved in early onset retinal dystrophies, including CRB1 and RPE65. Gene therapy recently became available for young RP patients with variations in the RPE65 gene. Current research programs test adeno-associated viral gene augmentation or editing therapy vectors on various disease models mimicking the disease in patients. These include several animal and emerging human-derived models, such as human-induced pluripotent stem cell (hiPSC)-derived retinal organoids or hiPSC-derived retinal pigment epithelium (RPE), and human donor retinal explants. Variations in the CRB1 gene are a major cause for early onset autosomal recessive RP with patients suffering from visual impairment before their adolescence and for LCA with newborns experiencing severe visual impairment within the first months of life. These patients cannot benefit yet from an available gene therapy treatment. In this review, we will discuss the recent advances, advantages and disadvantages of different CRB1 human and animal retinal degeneration models. In addition, we will describe novel therapeutic tools that have been developed, which could potentially be used for retinal gene augmentation therapy for RP patients with variations in the CRB1 gene.

Par protein localization during the early development of Mnemiopsis leidyi suggests different modes of epithelial organization in the metazoa

In bilaterians and cnidarians, epithelial cell-polarity is regulated by the interactions between Par proteins, Wnt/PCP signaling pathway, and cell-cell adhesion. Par proteins are highly conserved across Metazoa, including ctenophores. But strikingly, ctenophore genomes lack components of the Wnt/PCP pathway and cell-cell adhesion complexes raising the question if ctenophore cells are polarized by mechanisms involving Par proteins. Here, by using immunohistochemistry and live-cell imaging of specific mRNAs, we describe for the first time the subcellular localization of selected Par proteins in blastomeres and epithelial cells during the embryogenesis of the ctenophore Mnemiopsis leidyi. We show that these proteins distribute differently compared to what has been described for other animals, even though they segregate in a host-specific fashion when expressed in cnidarian embryos. This differential localization might be related to the emergence of different junctional complexes during metazoan evolution.

Phospholipids of the Plasma Membrane - Regulators or Consequence of Cell Polarity?

Cell polarity is a key feature of many eukaryotic cells, including neurons, epithelia, endothelia and asymmetrically dividing stem cells. Apart from the specific localization of proteins to distinct domains of the plasma membrane, most of these cells exhibit an asymmetric distribution of phospholipids within the plasma membrane too. Notably, research over the last years has revealed that many known conserved regulators of apical-basal polarity in epithelial cells are capable of binding to phospholipids, which in turn regulate the localization and to some extent the function of these proteins. Conversely, phospholipid-modifying enzymes are recruited and controlled by polarity regulators, demonstrating an elaborated balance between asymmetrically localized proteins and phospholipids, which are enriched in certain (micro)domains of the plasma membrane. In this review, we will focus on our current understanding of apical-basal polarity and the implication of phospholipids within the plasma membrane during the cell polarization of epithelia and migrating cells.

Structural Features of Tight-Junction Proteins

Tight junctions are complex supramolecular entities composed of integral membrane proteins, membrane-associated and soluble cytoplasmic proteins engaging in an intricate and dynamic system of protein-protein interactions. Three-dimensional structures of several tight-junction proteins or their isolated domains have been determined by X-ray crystallography, nuclear magnetic resonance spectroscopy, and cryo-electron microscopy. These structures provide direct insight into molecular interactions that contribute to the formation, integrity, or function of tight junctions. In addition, the known experimental structures have allowed the modeling of ligand-binding events involving tight-junction proteins. Here, we review the published structures of tight-junction proteins. We show that these proteins are composed of a limited set of structural motifs and highlight common types of interactions between tight-junction proteins and their ligands involving these motifs.

New insights into apical-basal polarization in epithelia

The establishment of an apical-basal axis of polarity is essential for the organization and functioning of epithelial cells. Polarization of epithelial cells is orchestrated by a network of conserved polarity regulators that establish opposing cortical domains through mutually antagonistic interactions and positive feedback loops. While our understanding is still far from complete, the molecular details behind these interactions continue to be worked out. Here, we highlight recent findings on the mechanisms that control the activity and localization of apical-basal polarity regulators, including oligomerization and higher-order complex formation, auto-inhibitory interactions, and electrostatic interactions with the plasma membrane.

Cdc42 defines apical identity and regulates epithelial morphogenesis by promoting apical recruitment of Par6-aPKC and Crumbs

Cdc42 regulates epithelial morphogenesis together with the Par complex (Baz/Par3-Par6-aPKC), Crumbs (Crb/CRB3) and Stardust (Sdt/PALS1). However, how these proteins work together and interact during epithelial morphogenesis is not well understood. To address this issue, we used the genetically amenable Drosophila pupal photoreceptor and follicular epithelium. We show that during epithelial morphogenesis active Cdc42 accumulates at the developing apical membrane and cell-cell contacts, independently of the Par complex and Crb. However, membrane localization of Baz, Par6-aPKC and Crb all depend on Cdc42. We find that although binding of Cdc42 to Par6 is not essential for the recruitment of Par6 and aPKC to the membrane, it is required for their apical localization and accumulation, which we find also depends on Par6 retention by Crb. In the pupal photoreceptor, membrane recruitment of Par6-aPKC also depends on Baz. Our work shows that Cdc42 is required for this recruitment and suggests that this factor promotes the handover of Par6-aPKC from Baz onto Crb. Altogether, we propose that Cdc42 drives morphogenesis by conferring apical identity, Par-complex assembly and apical accumulation of Crb.

Emerging Themes in PDZ Domain Signaling: Structure, Function, and Inhibition

Post-synaptic density-95, disks-large and zonula occludens-1 (PDZ) domains are small globular protein-protein interaction domains widely conserved from yeast to humans. They are composed of ∼90 amino acids and form a classical two α-helical/six β-strand structure. The prototypical ligand is the C-terminus of partner proteins; however, they also bind internal peptide sequences. Recent findings indicate that PDZ domains also bind phosphatidylinositides and cholesterol. Through their ligand interactions, PDZ domain proteins are critical for cellular trafficking and the surface retention of various ion channels. In addition, PDZ proteins are essential for neuronal signaling, memory, and learning. PDZ proteins also contribute to cytoskeletal dynamics by mediating interactions critical for maintaining cell-cell junctions, cell polarity, and cell migration. Given their important biological roles, it is not surprising that their dysfunction can lead to multiple disease states. As such, PDZ domain-containing proteins have emerged as potential targets for the development of small molecular inhibitors as therapeutic agents. Recent data suggest that the critical binding function of PDZ domains in cell signaling is more than just glue, and their binding function can be regulated by phosphorylation or allosterically by other binding partners. These studies also provide a wealth of structural and biophysical data that are beginning to reveal the physical features that endow this small modular domain with a central role in cell signaling.

The Crumbs3 Polarity Protein

The Crumbs proteins are evolutionarily conserved apical transmembrane proteins. Drosophila Crumbs was discovered via its crucial role in epithelial polarity during fly embryogenesis. Crumbs proteins have variable extracellular domains but a highly conserved intracellular domain that can bind FERM and PDZ domain proteins. Mammals have three Crumbs genes and this review focuses on Crumbs3, the major Crumbs isoform expressed in mammalian epithelial cells. Although initial work has highlighted the role of Crumbs3 in polarity, more recent studies have found it has an important role in tissue morphogenesis functioning as a linker between the apical membrane and the actin cytoskeleton. In addition, recent publications have linked Crumbs3 to growth control via regulation of the Hippo/Yap pathway.

Regulation of Cell Polarity by Exocyst-Mediated Trafficking

One requirement for establishing polarity within a cell is the asymmetric trafficking of intracellular vesicles to the plasma membrane. This tightly regulated process creates spatial and temporal differences in both plasma membrane composition and the membrane-associated proteome. Asymmetric membrane trafficking is also a critical mechanism to regulate cell differentiation, signaling, and physiology. Many eukaryotic cell types use the eight-protein exocyst complex to orchestrate polarized vesicle trafficking to certain membrane locales. Members of the exocyst were originally discovered in yeast while screening for proteins required for the delivery of secretory vesicles to the budding daughter cell. The same eight exocyst genes are conserved in mammals, in which the specifics of exocyst-mediated trafficking are highly cell-type-dependent. Some exocyst members bind to certain Rab GTPases on intracellular vesicles, whereas others localize to the plasma membrane at the site of exocytosis. Assembly of the exocyst holocomplex is responsible for tethering these vesicles to the plasma membrane before their soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated exocytosis. In this review, we will focus on the role and regulation of the exocyst complex in targeted vesicular trafficking as related to the establishment and maintenance of cellular polarity. We will contrast exocyst function in apicobasal epithelial polarity versus front-back mesenchymal polarity, and the dynamic regulation of exocyst-mediated trafficking during cell phenotype transitions.

Structural basis for the interaction between the cell polarity proteins Par3 and Par6

Polarity is a fundamental property of most cell types. The Par protein complex is a major driving force in generating asymmetrically localized protein networks and consists of atypical protein kinase C (aPKC), Par3, and Par6. Dysfunction of this complex causes developmental abnormalities and diseases such as cancer. We identified a PDZ domain-binding motif in Par6 that was essential for its interaction with Par3 in vitro and for Par3-mediated membrane localization of Par6 in cultured cells. In fly embryos, we observed that the PDZ domain-binding motif was functionally redundant with the PDZ domain in targeting Par6 to the cortex of epithelial cells. Our structural analyses by x-ray crystallography and NMR spectroscopy showed that both the PDZ1 and PDZ3 domains but not the PDZ2 domain in Par3 engaged in a canonical interaction with the PDZ domain-binding motif in Par6. Par3 thus has the potential to recruit two Par6 proteins simultaneously, which may facilitate the assembly of polarity protein networks through multivalent PDZ domain interactions.

Distinct functions of Crumbs regulating slit diaphragms and endocytosis in Drosophila nephrocytes

Mammalian podocytes, the key determinants of the kidney's filtration barrier, differentiate from columnar epithelial cells and several key determinants of apical-basal polarity in the conventional epithelia have been shown to regulate podocyte morphogenesis and function. However, little is known about the role of Crumbs, a conserved polarity regulator in many epithelia, for slit-diaphragm formation and podocyte function. In this study, we used Drosophila nephrocytes as model system for mammalian podocytes and identified a conserved function of Crumbs proteins for cellular morphogenesis, nephrocyte diaphragm assembly/maintenance, and endocytosis. Nephrocyte-specific knock-down of Crumbs results in disturbed nephrocyte diaphragm assembly/maintenance and decreased endocytosis, which can be rescued by Drosophila Crumbs as well as human Crumbs2 and Crumbs3, which were both expressed in human podocytes. In contrast to the extracellular domain, which facilitates nephrocyte diaphragm assembly/maintenance, the intracellular FERM-interaction motif of Crumbs is essential for regulating endocytosis. Moreover, Moesin, which binds to the FERM-binding domain of Crumbs, is essential for efficient endocytosis. Thus, we describe here a new mechanism of nephrocyte development and function, which is likely to be conserved in mammalian podocytes.

Role of the Crumbs proteins in ciliogenesis, cell migration and actin organization

Epithelial cell organization relies on a set of proteins that interact in an intricate way and which are called polarity complexes. These complexes are involved in the determination of the apico-basal axis and in the positioning and stability of the cell-cell junctions called adherens junctions at the apico-lateral border in invertebrates. Among the polarity complexes, two are present at the apical side of epithelial cells. These are the Par complex including aPKC, PAR3 and PAR6 and the Crumbs complex including, CRUMBS, PALS1 and PATJ/MUPP1. These two complexes interact directly and in addition to their already well described functions, they play a role in other cellular processes such as ciliogenesis and polarized cell migration. In this review, we will focus on these aspects that involve the apical Crumbs polarity complex and its relation with the cortical actin cytoskeleton which might provide a more comprehensive hypothesis to explain the many facets of Crumbs cell and tissue properties.

The PAR proteins: from molecular circuits to dynamic self-stabilizing cell polarity

PAR proteins constitute a highly conserved network of scaffolding proteins, adaptors and enzymes that form and stabilize cortical asymmetries in response to diverse inputs. They function throughout development and across the metazoa to regulate cell polarity. In recent years, traditional approaches to identifying and characterizing molecular players and interactions in the PAR network have begun to merge with biophysical, theoretical and computational efforts to understand the network as a pattern-forming biochemical circuit. Here, we summarize recent progress in the field, focusing on recent studies that have characterized the core molecular circuitry, circuit design and spatiotemporal dynamics. We also consider some of the ways in which the PAR network has evolved to polarize cells in different contexts and in response to different cues and functional constraints.

Protein Complex Assemblies in Epithelial Cell Polarity and Asymmetric Cell Division

Asymmetric local concentration of protein complexes on distinct membrane regions is a fundamental property in numerous biological processes and is a hallmark of cell polarity. Evolutionarily conserved core polarity proteins form specific and dynamic networks to regulate the establishment and maintenance of cell polarity, as well as distinct polarity-driven cellular events. This review focuses on the molecular and structural basis governing regulated formation of several sets of core cell polarity regulatory complexes, as well as their functions in epithelial cell polarization and asymmetric cell division.