Development and dynamics of cell polarity at a glance

Atypical Protein Kinase C Promotes its own Asymmetric Localisation by Phosphorylating Cdc42 in the C. elegans zygote

Atypical protein kinase C (aPKC) is a major regulator of cell polarity. Acting in conjunction with Par6, Par3 and the small GTPase Cdc42, aPKC becomes asymmetrically localised and drives the polarisation of cells. aPKC activity is crucial for its own asymmetric localisation, suggesting a hitherto unknown feedback mechanism contributing to polarisation. Here we show in the C. elegans zygote that the feedback relies on aPKC phosphorylation of Cdc42 at serine 71. The turnover of CDC-42 phosphorylation ensures optimal aPKC asymmetry and activity throughout polarisation by tuning Par6/aPKC association with Par3 and Cdc42. Moreover, turnover of Cdc42 phosphorylation regulates actomyosin cortex dynamics that are known to drive aPKC asymmetry. Given the widespread role of aPKC and Cdc42 in cell polarity, this form of self-regulation of aPKC may be vital for the robust control of polarisation in many cell types.

A mathematical framework for understanding the spontaneous emergence of complexity applicable to growing multicellular systems

In embryonic development and organogenesis, cells sharing identical genetic codes acquire diverse gene expression states in a highly reproducible spatial distribution, crucial for multicellular formation and quantifiable through positional information. To understand the spontaneous growth of complexity, we constructed a one-dimensional division-decision model, simulating the growth of cells with identical genetic networks from a single cell. Our findings highlight the pivotal role of cell division in providing positional cues, escorting the system toward states rich in information. Moreover, we pinpointed lateral inhibition as a critical mechanism translating spatial contacts into gene expression. Our model demonstrates that the spatial arrangement resulting from cell division, combined with cell lineages, imparts positional information, specifying multiple cell states with increased complexity-illustrated through examples in C.elegans. This study constitutes a foundational step in comprehending developmental intricacies, paving the way for future quantitative formulations to construct synthetic multicellular patterns.

Diverse functions and pathogenetic role of Crumbs in retinopathy

The Crumbs protein (CRB) family plays a crucial role in maintaining the apical-basal polarity and integrity of embryonic epithelia. The family comprises different isoforms in different animals and possesses diverse structural, localization, and functional characteristics. Mutations in the human CRB1 or CRB2 gene may lead to a broad spectrum of retinal dystrophies. Various CRB-associated experimental models have recently provided mechanistic insights into human CRB-associated retinopathies. The knowledge obtained from these models corroborates the importance of CRB in retinal development and maintenance. Therefore, complete elucidation of these models can provide excellent therapeutic prospects for human CRB-associated retinopathies. In this review, we summarize the current animal models and human-derived models of different CRB family members and describe the main characteristics of their retinal phenotypes.

Glucose stockpile in the intestinal apical brush border in C. elegans

Revealing the mechanisms of glucose transport is crucial for studying pathological diseases caused by glucose toxicities. Numerous studies have revealed molecular functions involved in glucose transport in the nematode Caenorhabditis elegans, a commonly used model organism. However, the behavior of glucose in the intestinal lumen-to-cell remains elusive. To address that, we evaluated the diffusion coefficient of glucose in the intestinal apical brush border of C. elegans by using fluorescent glucose and fluorescence recovery after photobleaching. Fluorescent glucose taken in the intestine of worms accumulates in the apical brush border, and its diffusion coefficient of ∼10-8 cm2/s is two orders of magnitude slower than that in bulk. This result indicates that the intestinal brush border is a viscous layer. ERM-1 point mutations at the phosphorylation site, which shorten the microvilli length, did not significantly affect the diffusion coefficient of fluorescent glucose in the brush border. Our findings imply that glucose enrichment is dominantly maintained by the viscous layer composed of the glycocalyx and molecular complexes on the apical surface.

The alveolus: Our current knowledge of how the gas exchange unit of the lung is constructed and repaired

The mammalian lung completes its last step of development, alveologenesis, to generate sufficient surface area for gas exchange. In this process, multiple cell types that include alveolar epithelial cells, endothelial cells, and fibroblasts undergo coordinated cell proliferation, cell migration and/or contraction, cell shape changes, and cell-cell and cell-matrix interactions to produce the gas exchange unit: the alveolus. Full functioning of alveoli also involves immune cells and the lymphatic and autonomic nervous system. With the advent of lineage tracing, conditional gene inactivation, transcriptome analysis, live imaging, and lung organoids, our molecular understanding of alveologenesis has advanced significantly. In this review, we summarize the current knowledge of the constituents of the alveolus and the molecular pathways that control alveolar formation. We also discuss how insight into alveolar formation may inform us of alveolar repair/regeneration mechanisms following lung injury and the pathogenic processes that lead to loss of alveoli or tissue fibrosis.

Shaping epithelial lumina under pressure

The formation of fluid- or gas-filled lumina surrounded by epithelial cells pervades development and disease. We review the balance between lumen pressure and mechanical forces from the surrounding cells that governs lumen formation. We illustrate the mechanical side of this balance in several examples of increasing complexity, and discuss how recent work is beginning to elucidate how nonlinear and active mechanics and anisotropic biomechanical structures must conspire to overcome the isotropy of pressure to form complex, non-spherical lumina.

Assembly, dynamics and remodeling of epithelial cell junctions throughout development

Cell junctions play key roles in epithelial integrity. During development, when epithelia undergo extensive morphogenesis, these junctions must be remodeled in order to maintain mechanochemical barriers and ensure the cohesion of the tissue. In this Review, we present a comprehensive and integrated description of junctional remodeling mechanisms in epithelial cells during development, from embryonic to adult epithelia. We largely focus on Drosophila, as quantitative analyses in this organism have provided a detailed characterization of the molecular mechanisms governing cell topologies, and discuss the conservation of these mechanisms across metazoans. We consider how changes at the molecular level translate to tissue-scale irreversible deformations, exploring the composition and assembly of cellular interfaces to unveil how junctions are remodeled to preserve tissue homeostasis during cell division, intercalation, invagination, ingression and extrusion.

Comprehensive and quantitative analysis of intracellular structure polarization at the apical-basal axis in elongating Arabidopsis zygotes

A comprehensive and quantitative evaluation of multiple intracellular structures or proteins is a promising approach to provide a deeper understanding of and new insights into cellular polarity. In this study, we developed an image analysis pipeline to obtain intensity profiles of fluorescent probes along the apical-basal axis in elongating Arabidopsis thaliana zygotes based on two-photon live-cell imaging data. This technique showed the intracellular distribution of actin filaments, mitochondria, microtubules, and vacuolar membranes along the apical-basal axis in elongating zygotes from the onset of cell elongation to just before asymmetric cell division. Hierarchical cluster analysis of the quantitative data on intracellular distribution revealed that the zygote may be compartmentalized into two parts, with a boundary located 43.6% from the cell tip, immediately after fertilization. To explore the biological significance of this compartmentalization, we examined the positions of the asymmetric cell divisions from the dataset used in this distribution analysis. We found that the cell division plane was reproducibly inserted 20.5% from the cell tip. This position corresponded well with the midpoint of the compartmentalized apical region, suggesting a potential relationship between the zygote compartmentalization, which begins with cell elongation, and the position of the asymmetric cell division.

Membrane microdomains: Structural and signaling platforms for establishing membrane polarity

Cell polarity results from the asymmetric distribution of cellular structures, molecules, and functions. Polarity is a fundamental cellular trait that can determine the orientation of cell division, the formation of particular cell shapes, and ultimately the development of a multicellular body. To maintain the distinct asymmetric distribution of proteins and lipids in cellular membranes, plant cells have developed complex trafficking and regulatory mechanisms. Major advances have been made in our understanding of how membrane microdomains influence the asymmetric distribution of proteins and lipids. In this review, we first give an overview of cell polarity. Next, we discuss current knowledge concerning membrane microdomains and their roles as structural and signaling platforms to establish and maintain membrane polarity, with a special focus on the asymmetric distribution of proteins and lipids, and advanced microscopy techniques to observe and characterize membrane microdomains. Finally, we review recent advances regarding membrane trafficking in cell polarity establishment and how the balance between exocytosis and endocytosis affects membrane polarity.

Viral manipulation of cell polarity signalling

Cell polarity refers to the asymmetric distribution of biomacromolecules that enable the correct orientation of a cell in a particular direction. It is thus an essential component for appropriate tissue development and function. Viral infections can lead to dysregulation of polarity. This is associated with a poor prognosis due to viral interference with core cell polarity regulatory scaffolding proteins that often feature PDZ (PSD-95, DLG, and ZO-1) domains including Scrib, Dlg, Pals1, PatJ, Par3 and Par6. PDZ domains are also promiscuous, binding to several different partners through their C-terminal region which contain PDZ-binding motifs (PBM). Numerous viruses encode viral effector proteins that target cell polarity regulators for their benefit and include papillomaviruses, flaviviruses and coronaviruses. A better understanding of the mechanisms of action utilised by viral effector proteins to subvert host cell polarity sigalling will provide avenues for future therapeutic intervention, while at the same time enhance our understanding of cell polarity regulation and its role tissue homeostasis.

A common variant SNP rs1937810 in the MPP7 gene contributes to the susceptibility of breast cancer in the Chinese Han population

BACKGROUND

Breast cancer (BC) is common cancer caused by environmental factors and genetic ones. Previous evidence has linked gene MAGUK P55 Scaffold Protein 7 (MPP7) to BC, despite that there has been no research evaluating the relationship between MPP7 genetic polymorphisms and BC susceptibility. We aimed to investigate the potential association of the MPP7 gene with the susceptibility to BC in Han Chinese individuals.

METHODS

In total, 1390 patients with BC and 2480 controls were enrolled. For genotyping, 20 tag SNPs were chosen. The serum levels of protein MPP7 were measured in all subjects using an enzyme-linked immunosorbent assay. Genetic association analysis was performed in both genotypic and allelic modes, and the relationship between BC patients' clinical features and genotypes of relevant SNPs was examined. The functional implications of significant markers were also evaluated.

RESULTS

After adjusting for Bonferroni correction, SNP rs1937810 was found to be significantly associated with the risk of BC (p = 1.19 × 10-4 ). The odds ratio of CC genotypes in BC patients was 49% higher than in controls (1.49 [1.23-1.81]). Serum MPP7 protein levels were significantly higher in BC patients than in controls (p < 0.001). The protein level of the CC genotype was the highest, and that of the CT and TT genotypes decreased in turn (both p < 0.001).

CONCLUSIONS

Our results linked SNP rs1937810 to the susceptibility of BC and the clinical features of BC patients. This SNP is also proved to be significantly related to the serum level of protein MPP7 in both BC patients and controls.

Coordination of tissue homeostasis and growth by the Scribble-α-Catenin-Septate junction complex

Maintaining apicobasal polarity (ABP) is crucial for epithelial integrity and homeostasis during tissue development. Although intracellular mechanisms underlying ABP establishment have been well studied, it remains to be addressed how the ABP coordinates tissue growth and homeostasis. By studying Scribble, a key ABP determinant, we address molecular mechanisms underlying ABP-mediated growth control in the Drosophila wing imaginal disc. Our data reveal that genetic and physical interactions between Scribble, Septate junction complex and α-Catenin appear to be key for sustaining ABP-mediated growth control. Cells with conditional scribble knockdown instigate the loss of α-Catenin, ultimately leading to the formation of neoplasia accompanying with activation of Yorkie. In contrast, cells expressing wild type scribble progressively restore ABP in scribble hypomorphic mutant cells in a non-autonomous manner. Our findings provide unique insights into cellular communication among optimal and sub-optimal cells to regulate epithelial homeostasis and growth.

The emerging roles of the cytoskeleton in intestinal epithelium homeostasis

The intestinal epithelium must absorb many nutrients and water while forming a barrier that is impermeable to pathogens present in the external environment. Concurrently to fulfill this dual role, the intestinal epithelium is challenged by a rapid renewal of cells and forces resulting from digestion. Hence, intestinal homeostasis requires precise control of tissue integrity, tissue renewal, cell polarity, and force generation/transmission. In this review, we highlight the contribution of the cell cytoskeleton- actin, microtubules, and intermediate filaments- to intestinal epithelium homeostasis. With a focus on enterocytes, we first discuss the role of these networks in the formation and maintenance of cell-cell and cell-matrix junctions. Then, we cover their role in intracellular trafficking related to the apicobasal polarity of enterocytes. Finally, we report on the cytoskeletal changes that occur during tissue renewal. In conclusion, the importance of the cytoskeleton in maintaining intestinal homeostasis is emerging, and we think this field will keep evolving.

Prominin-1 promotes restitution of the murine extrahepatic biliary luminal epithelium following cholestatic liver injury

BACKGROUND AND AIMS

Restitution of the extrahepatic biliary luminal epithelium in cholangiopathies is poorly understood. Prominin-1 (Prom1) is a key component of epithelial ciliary body of stem/progenitor cells. Given that intrahepatic Prom1-expressing progenitor cells undergo cholangiocyte differentiation, we hypothesized that Prom1 may promote restitution of the extrahepatic bile duct (EHBD) epithelium following injury.

APPROACH AND RESULTS

Utilizing various murine biliary injury models, we identified Prom1-expressing cells in the peribiliary glands of the EHBD. These Prom1-expressing cells are progenitor cells which give rise to cholangiocytes as part of the normal maintenance of the EHBD epithelium. Following injury, these cells proliferate significantly more rapidly to re-populate the biliary luminal epithelium. Null mutation of Prom1 leads to significantly >10-fold dilated peribiliary glands following rhesus rotavirus-mediated biliary injury. Cultured organoids derived from Prom1 knockout mice are comprised of biliary progenitor cells with altered apical-basal cellular polarity, significantly fewer and shorter cilia, and decreased organoid proliferation dynamics consistent with impaired cell motility.

CONCLUSIONS

We, therefore, conclude that Prom1 is involved in biliary epithelial restitution following biliary injury in part through its role in supporting cell polarity.

Pals1a and aPKCλ are not essential for Schwann cell migration, division or myelination in zebrafish

BACKGROUND

Schwann cells (SCs) are specialized glial cells of the peripheral nervous system that produce myelin and promote fast action potential propagation. In order to myelinate, SCs engage in a series of events that include migration and division along axons, followed by extensive cytoskeletal rearrangements that ensure axonal ensheathment and myelination. SCs are polarized and extend their processes along an abaxonal-adaxonal axis. Here, we investigate the role of the apical polarity proteins, Pals1a, and aPKCλ, in SC behavior during zebrafish development.

RESULTS

We analyzed zebrafish nok and has mutants deficient for pals1a and aPKCλ function respectively. Using live imaging, transmission electron microscopy and whole mount immunostaining, we show that SCs can migrate and divide appropriately, exhibit normal radial sorting, express myelin markers and ensheath axons on time in has and nok mutants.

CONCLUSIONS

Pals1a and aPKCλ are not essential for SC migration, division or myelination in zebrafish.

Cell polarity in the protist-to-animal transition

A signature feature of the animal kingdom is the presence of epithelia: sheets of polarized cells that both insulate the organism from its environment and mediate interactions with it. Epithelial cells display a marked apico-basal polarity, which is highly conserved across the animal kingdom, both in terms of morphology and of molecular regulators. How did this architecture first evolve? Although the last eukaryotic common ancestor almost certainly possessed a simple form of apico-basal polarity (marked by the presence of one or several flagella at a single cellular pole), comparative genomics and evolutionary cell biology reveal that the polarity regulators of animal epithelial cells have a surprisingly complex and stepwise evolutionary history. Here, we retrace their evolutionary assembly. We suggest that the "polarity network" that polarized animal epithelial cells evolved by integration of initially independent cellular modules that evolved at distinct steps of our evolutionary ancestry. The first module dates back to the last common ancestor of animals and amoebozoans and involved Par1, extracellular matrix proteins, and the integrin-mediated adhesion complex. Other regulators, such as Cdc42, Dlg, Par6 and cadherins evolved in ancient unicellular opisthokonts, and might have first been involved in F-actin remodeling and filopodial dynamics. Finally, the bulk of "polarity proteins" as well as specialized adhesion complexes evolved in the metazoan stem-line, in concert with the newly evolved intercellular junctional belts. Thus, the polarized architecture of epithelia can be understood as a palimpsest of components of distinct histories and ancestral functions, which have become tightly integrated in animal tissues.

Overview of junctional complexes during mammalian early embryonic development

Cell-cell junctions form strong intercellular connections and mediate communication between blastomeres during preimplantation embryonic development and thus are crucial for cell integrity, polarity, cell fate specification and morphogenesis. Together with cell adhesion molecules and cytoskeletal elements, intercellular junctions orchestrate mechanotransduction, morphokinetics and signaling networks during the development of early embryos. This review focuses on the structure, organization, function and expressional pattern of the cell-cell junction complexes during early embryonic development. Understanding the importance of dynamic junction formation and maturation processes will shed light on the molecular mechanism behind developmental abnormalities of early embryos during the preimplantation period.

Evaluation of establishment and maintenance of primary cell cultures from several strains of common carp (Cyprinus carpio L.)

As a surrogate for the whole organism, primary cultures and cell lines serve as valuable tools for investigating exogenous and endogenous cytopathy. Studying cell responsiveness to diseases and contaminants is considered a less demanding and more readily accessible research approach that minimizes animal distress and provides more specific data. In the current work, the authors established primary cultures from several different organs and tissues of common carp (Cyprinus carpio L.) for subsequent use in other applications. They investigated the technical challenges in obtaining successful and durable carp-derived tissue cultures. The trials indicate that the type of tissue grown, carp strain and fish age impact equally upon culturing success, as do the cultivating conditions. Cells from gill epithelia, head and trunk kidneys, spleen, skin, gonads and ocular tissue were successfully established and maintained for further use in in-vitro testing. The primary cultures were, therefore, used to investigate and assess pathogens and pollutants emerging in carp's environment.

In silico testing of the universality of epithelial tissue growth

The universality of interfacial roughness in growing epithelial tissue has remained a controversial issue. Kardar-Parisi-Zhang (KPZ) and molecular beam epitaxy (MBE) universality classes have been reported among other behaviors including a total lack of universality. Here, we simulate tissues using the cellsim3d kinetic division model for deformable cells to investigate cell-colony scaling. With seemingly minor model changes, it can reproduce both KPZ- and MBE-like scaling in configurations that mimic the respective experiments. Tissue growth with strong cell-cell adhesion in a linear geometry is KPZ like, while weakly adhesive tissues in a radial geometry are MBE like. This result neutralizes the apparent scaling controversy.

pTINCR microprotein promotes epithelial differentiation and suppresses tumor growth through CDC42 SUMOylation and activation

The human transcriptome contains thousands of small open reading frames (sORFs) that encode microproteins whose functions remain largely unexplored. Here, we show that TINCR lncRNA encodes pTINCR, an evolutionary conserved ubiquitin-like protein (UBL) expressed in many epithelia and upregulated upon differentiation and under cellular stress. By gain- and loss-of-function studies, we demonstrate that pTINCR is a key inducer of epithelial differentiation in vitro and in vivo. Interestingly, low expression of TINCR associates with worse prognosis in several epithelial cancers, and pTINCR overexpression reduces malignancy in patient-derived xenografts. At the molecular level, pTINCR binds to SUMO through its SUMO interacting motif (SIM) and to CDC42, a Rho-GTPase critical for actin cytoskeleton remodeling and epithelial differentiation. Moreover, pTINCR increases CDC42 SUMOylation and promotes its activation, triggering a pro-differentiation cascade. Our findings suggest that the microproteome is a source of new regulators of cell identity relevant for cancer.

Use of a 3D inkjet-printed model to access dust particle toxicology in the human alveolar barrier

Fine dust particles in the air travel into our body via the airway tract and cause severe respiratory diseases. Thus, the analysis of the effects of dust particles on the respiratory system has been receiving significant research interest. However, most studies on the toxicity of dust particles involve two-dimensional (2D) cell cultures, animal models, and epidemiology. Here, we inkjet-printed an three-dimensional (3D) alveolar barrier model to study how dust particles cause respiratory diseases. The three-layered in vitro model was exposed to A2 fine test dust with varying concentrations and exposure durations. The results highlighted the destruction of the tissue architecture along with apoptosis in the bioprinted alveolar barrier. The damage at the cellular level induced an increase in the amount of pro-inflammatory cytokines secreted, followed by triggering of the signal transduction pathway and activation of transcription factors. As a consequence of the release of cytokines, the extracellular matrix was degraded, which led to the collapse of the cell structure, loss of cell polarity, and a decrease in the barrier tightness. Further, the pulmonary surfactant protein-related genes in the dust-treated alveolar tissue were investigated to evaluate the possible role of dust particles in pulmonary surfactant dysfunction. This study demonstrated the use of 3D-printed tissue model to evaluate the physiological impact of fine dust particles on cytotoxicity, alveolar barrier rigidity, and surfactant secretion of an alveolar barrier. This article is protected by copyright. All rights reserved.

Protein polarization: Spatiotemporal precisions in cell division and differentiation

Specification of cell polarity is vital to normal cell growth, morphogenesis, and function. As other eukaryotes, plants generate cellular polarity that is coordinated with tissue polarity and organ axes. In development, new cell types are generated by stem-cell division and differentiation, a process often involving proteins that are polarized to cortical domains at the plasma membrane. In the past decade, pioneering work using the model plant Arabidopsis identified multiple proteins that are polarized in dividing cells to instruct divisional behaviors and/or specify cell fates. In this review, we use these polarized cell-division regulators as example to summarize key mechanisms underlying protein polarization in plant cells. Recent progress underscores that self-organizing amplification processes are commonly involved in establishing cell polarity, and cellular polarity is influenced by both tissue-level and local mechanochemical cues. In addition, protein polarization during asymmetric cell division shows a distinct feature of temporal control in the stomatal lineage. We further discuss possible coordination between protein polarization and the progression of cell cycle in this developmental context.

A functional circuit formed by the autonomic nerves and myofibroblasts controls mammalian alveolar formation for gas exchange

Alveolar formation increases the surface area for gas exchange. A molecular understanding of alveologenesis remains incomplete. Here, we show that the autonomic nerve and alveolar myofibroblast form a functional unit in mice. Myofibroblasts secrete neurotrophins to promote neurite extension/survival, whereas neurotransmitters released from autonomic terminals are necessary for myofibroblast proliferation and migration, a key step in alveologenesis. This establishes a functional link between autonomic innervation and alveolar formation. We also discover that planar cell polarity (PCP) signaling employs a Wnt-Fz/Ror-Vangl cascade to regulate the cytoskeleton and neurotransmitter trafficking/release from the terminals of autonomic nerves. This represents a new aspect of PCP signaling in conferring cellular properties. Together, these studies offer molecular insight into how autonomic activity controls alveolar formation. Our work also illustrates the fundamental principle of how two tissues (e.g., nerves and lungs) interact to build alveoli at the organismal level.

Identification of Arhgef12 and Prkci as genetic modifiers of retinal dysplasia in the Crb1rd8 mouse model

Mutations in the apicobasal polarity gene CRB1 lead to diverse retinal diseases, such as Leber congenital amaurosis, cone-rod dystrophy, retinitis pigmentosa (with and without Coats-like vasculopathy), foveal retinoschisis, macular dystrophy, and pigmented paravenous chorioretinal atrophy. Limited correlation between disease phenotypes and CRB1 alleles, and evidence that patients sharing the same alleles often present with different disease features, suggest that genetic modifiers contribute to clinical variation. Similarly, the retinal phenotype of mice bearing the Crb1 retinal degeneration 8 (rd8) allele varies with genetic background. Here, we initiated a sensitized chemical mutagenesis screen in B6.Cg-Crb1^rd8/Pjn, a strain with a mild clinical presentation, to identify genetic modifiers that cause a more severe disease phenotype. Two models from this screen, Tvrm266 and Tvrm323, exhibited increased retinal dysplasia. Genetic mapping with high-throughput exome and candidate-gene sequencing identified causative mutations in Arhgef12 and Prkci, respectively. Epistasis analysis of both strains indicated that the increased dysplastic phenotype required homozygosity of the Crb1^rd8 allele. Retinal dysplastic lesions in Tvrm266 mice were smaller and caused less photoreceptor degeneration than those in Tvrm323 mice, which developed an early, large diffuse lesion phenotype. At one month of age, Müller glia and microglia mislocalization at dysplastic lesions in both modifier strains was similar to that in B6.Cg-Crb1^rd8/Pjn mice but photoreceptor cell mislocalization was more extensive. External limiting membrane disruption was comparable in Tvrm266 and B6.Cg-Crb1^rd8/Pjn mice but milder in Tvrm323 mice. Immunohistological analysis of mice at postnatal day 0 indicated a normal distribution of mitotic cells in Tvrm266 and Tvrm323 mice, suggesting normal early development. Aberrant electroretinography responses were observed in both models but functional decline was significant only in Tvrm323 mice. These results identify Arhgef12 and Prkci as modifier genes that differentially shape Crb1-associated retinal disease, which may be relevant to understanding clinical variability and underlying disease mechanisms in humans.

Inherited eye diseases affect roughly 1:1,000 individuals worldwide. Although these diseases are often linked to variants of a single gene, it is increasingly recognized that a second variant in other genes may modify disease characteristics, including age of onset, severity, and lesion appearance. Identifying such modifier genes in humans is difficult. In this study, two modifiers of a gene associated with retinal damage leading to childhood blindness in humans (CRB1) were identified in mice. Retinal damage caused by Crb1 mutation alone was less severe than in the presence of Arhgef12 or Prkci mutations. Furthermore, the modifier gene mutations caused retinal damage only in the presence of the Crb1 mutation. Our results point to a role of mouse Crb1 and the modifying effects of Arhgef12 and Prkci in a biological network that controls adhesive interactions between cells. The variation in disease severity, lesion appearance, and visual responses in these mice provide a dramatic example of modifier gene influence. This work may lead to an improved understanding of the molecular basis of CRB1-associated retinal disease, with possible relevance to diagnostic and therapeutic intervention in humans.

Energetic constraints on filament-mediated cell polarization

Cell polarization underlies many cellular processes, such as differentiation, migration, and budding. Many living cells, such as budding yeast and fission yeast, use cytoskeletal structures to actively transport proteins to one location on the membrane and create a high-density spot of membrane-bound proteins. Yet, the thermodynamic constraints on filament-based cell polarization remain unknown. We show by mathematical modeling that cell polarization requires detailed balance to be broken, and we quantify the free-energy cost of maintaining a polarized state of the cell. Our study reveals that detailed balance cannot only be broken via the active transport of proteins along filaments but also via a chemical modification cycle, allowing detailed balance to be broken by the shuttling of proteins between the filament, membrane, and cytosol. Our model thus shows that cell polarization can be established via two distinct driving mechanisms, one based on active transport and one based on nonequilibrium binding. Furthermore, the model predicts that the driven binding process dissipates orders of magnitude less free energy than the transport-based process to create the same membrane spot. Active transport along filaments may be sufficient to create a polarized distribution of membrane-bound proteins, but an additional chemical modification cycle of the proteins themselves is more efficient and less sensitive to the physical exclusion of proteins on the transporting filaments, providing insight in the design principles of the Pom1/Tea1/Tea4 system in fission yeast and the Cdc42 system in budding yeast.

Stimulatory role of nectin-4 and p95-ErbB2 in multilayered T47D cell proliferation

Multilayered proliferation in an adherent culture as well as proliferation in a suspension culture is a characteristic feature of cancer cells. We previously showed using T47D human mammary cancer cells that nectin-4, upregulated in many cancer cells, cis-interacts with ErbB2 and its trastuzumab-resistant splice variants, p95-ErbB2 and ErbB2ΔEx16, and enhances DNA synthesis mainly through the PI3K-AKT pathway in an adherent culture. We showed here that only the combination of nectin-4 and p95-ErbB2, but not that of nectin-4 and ErbB2 or that of nectin-4 and ErbB2ΔEx16, cooperatively enhanced multilayered T47D cell proliferation through the Hippo pathway-mediated SOX2 gene expression in an adherent culture. T47D cells expressed the components of the apical junctional complex (AJC) consisting of adherens junctions (AJs) and tight junctions and cell polarity molecules, but not the AJ component afadin. The AJC and apicobasal polarity were disorganized in T47D cells in a monolayer and T47D cells stably expressing both nectin-4 and p95-ErbB2 in multilayers. These results indicate that nectin-4 and p95-ErbB2 play a stimulatory role in multilayered proliferation in an adherent culture.

Establishing neuronal polarity: microtubule regulation during neurite initiation

The initiation of nascent projections, or neurites, from the neuronal cell body is the first stage in the formation of axons and dendrites, and thus a critical step in the establishment of neuronal architecture and nervous system development. Neurite formation relies on the polarized remodelling of microtubules, which dynamically direct and reinforce cell shape, and provide tracks for cargo transport and force generation. Within neurons, microtubule behaviour and structure are tightly controlled by an array of regulatory factors. Although microtubule regulation in the later stages of axon development is relatively well understood, how microtubules are regulated during neurite initiation is rarely examined. Here, we discuss how factors that direct microtubule growth, remodelling, stability and positioning influence neurite formation. In addition, we consider microtubule organization by the centrosome and modulation by the actin and intermediate filament networks to provide an up-to-date picture of this vital stage in neuronal development.

Neuronal Polarity Pathways as Central Integrators of Cell-Extrinsic Information During Interactions of Neural Progenitors With Germinal Niches

Germinal niche interactions and their effect on developing neurons have become the subject of intense investigation. Dissecting the complex interplay of cell-extrinsic and cell-intrinsic factors at the heart of these interactions reveals the critical basic mechanisms of neural development and how it goes awry in pediatric neurologic disorders. A full accounting of how developing neurons navigate their niches to mature and integrate into a developing neural circuit requires a combination of genetic characterization of and physical access to neurons and their supporting cell types plus transformative imaging to determine the cell biological and gene-regulatory responses to niche cues. The mouse cerebellar cortex is a prototypical experimental system meeting all of these criteria. The lessons learned therein have been scaled to other model systems and brain regions to stimulate discoveries of how developing neurons make many developmental decisions. This review focuses on how mouse cerebellar granule neuron progenitors interact with signals in their germinal niche and how that affects the neuronal differentiation and cell polarization programs that underpin lamination of the developing cerebellum. We show how modeling of these mechanisms in other systems has added to the growing evidence of how defective neuronal polarity contributes to developmental disease.

Simultaneous stabilization of actin cytoskeleton in multiple nephron-specific cells protects the kidney from diverse injury

Chronic kidney diseases and acute kidney injury are mechanistically distinct kidney diseases. While chronic kidney diseases are associated with podocyte injury, acute kidney injury affects renal tubular epithelial cells. Despite these differences, a cardinal feature of both acute and chronic kidney diseases is dysregulated actin cytoskeleton. We have shown that pharmacological activation of GTPase dynamin ameliorates podocyte injury in murine models of chronic kidney diseases by promoting actin polymerization. Here we establish dynamin's role in modulating stiffness and polarity of renal tubular epithelial cells by crosslinking actin filaments into branched networks. Activation of dynamin's crosslinking capability by a small molecule agonist stabilizes the actomyosin cortex of the apical membrane against injury, which in turn preserves renal function in various murine models of acute kidney injury. Notably, a dynamin agonist simultaneously attenuates podocyte and tubular injury in the genetic murine model of Alport syndrome. Our study provides evidence for the feasibility and highlights the benefits of novel holistic nephron-protective therapies.

Gradients of PI(4,5)P2 and PI(3,5)P2 Jointly Participate in Shaping the Back State of Dictyostelium Cells

Polarity, which refers to the molecular or structural asymmetry in cells, is essential for diverse cellular functions. Dictyostelium has proven to be a valuable system for dissecting the molecular mechanisms of cell polarity. Previous studies in Dictyostelium have revealed a range of signaling and cytoskeletal proteins that function at the leading edge to promote pseudopod extension and migration. In contrast, how proteins are localized to the trailing edge is not well understood. By screening for asymmetrically localized proteins, we identified a novel trailing-edge protein we named Teep1. We show that a charged surface formed by two pleckstrin homology (PH) domains in Teep1 is necessary and sufficient for targeting it to the rear of cells. Combining biochemical and imaging analyses, we demonstrate that Teep1 interacts preferentially with PI(4,5)P2 and PI(3,5)P2in vitro and simultaneous elimination of these lipid species in cells blocks the membrane association of Teep1. Furthermore, a leading-edge localized myotubularin phosphatase likely mediates the removal of PI(3,5)P2 from the front, as well as the formation of a back-to-front gradient of PI(3,5)P2. Together our data indicate that PI(4,5)P2 and PI(3,5)P2 on the plasma membrane jointly participate in shaping the back state of Dictyostelium cells.

Regulation of Cell Polarity by Posttranslational Protein Palmitoylation

Cell polarity is a common feature of many living cells, especially epithelial cells, and plays important roles in development, tissue homeostasis, and diseases. Therefore, the signaling pathways involved in establishing and maintaining cell polarity are tightly controlled. Protein S-palmitoylation has been recently recognized as an important posttranslational modification involved in cell polarity, via dynamic covalent attachment of fatty acyl groups to the cysteine residues of cell polarity proteins. Here, we describe the methods to study the function and regulation of S-palmitoylation of cell polarity proteins.

Cellular Polarity Transmission to the Nucleus

Polarity is an intrinsic and fundamental property of unicellular organisms and, as well, of single cells in multicellular ones. It can be defined as asymmetric cell organization that is self-reinforced and maintained by appropriate signaling. While cellular polarity is widely studied at the membrane and cytoplasmic level, if and how it is transmitted to the nucleus is still a matter of research and discussion. However, there is growing evidence of polarity transmission from the cell to the nucleus. In this chapter, we discuss recent reports on nuclear polarity and involvement of potential molecular players including emerin, nesprins, and nuclear F-actin which may play a significant role in establishment of this phenomenon.

Autonomous epithelial folding induced by an intracellular mechano-polarity feedback loop

Epithelial tissues form folded structures during embryonic development and organogenesis. Whereas substantial efforts have been devoted to identifying mechanical and biochemical mechanisms that induce folding, whether and how their interplay synergistically shapes epithelial folds remains poorly understood. Here we propose a mechano-biochemical model for dorsal fold formation in the early Drosophila embryo, an epithelial folding event induced by shifts of cell polarity. Based on experimentally observed apical domain homeostasis, we couple cell mechanics to polarity and find that mechanical changes following the initial polarity shifts alter cell geometry, which in turn influences the reaction-diffusion of polarity proteins, thus forming a feedback loop between cell mechanics and polarity. This model can induce spontaneous fold formation in silico, recapitulate polarity and shape changes observed in vivo, and confer robustness to tissue shape change against small fluctuations in mechanics and polarity. These findings reveal emergent properties of a developing epithelium under control of intracellular mechano-polarity coupling.

Transcriptional Control of Apical-Basal Polarity Regulators

Cell polarity is essential for many functions of cells and tissues including the initial establishment and subsequent maintenance of epithelial tissues, asymmetric cell division, and morphogenetic movements. Cell polarity along the apical-basal axis is controlled by three protein complexes that interact with and co-regulate each other: The Par-, Crumbs-, and Scrib-complexes. The localization and activity of the components of these complexes is predominantly controlled by protein-protein interactions and protein phosphorylation status. Increasing evidence accumulates that, besides the regulation at the protein level, the precise expression control of polarity determinants contributes substantially to cell polarity regulation. Here we review how gene expression regulation influences processes that depend on the induction, maintenance, or abolishment of cell polarity with a special focus on epithelial to mesenchymal transition and asymmetric stem cell division. We conclude that gene expression control is an important and often neglected mechanism in the control of cell polarity.

How cells tell up from down and stick together to construct multicellular tissues - interplay between apicobasal polarity and cell-cell adhesion

Polarized epithelia define a topological inside and outside, and hence constitute a key evolutionary innovation that enabled the construction of complex multicellular animal life. Over time, this basic function has been elaborated upon to yield the complex architectures of many of the organs that make up the human body. The two processes necessary to yield a polarized epithelium, namely regulated adhesion between cells and the definition of the apicobasal (top-bottom) axis, have likewise undergone extensive evolutionary elaboration, resulting in multiple sophisticated protein complexes that contribute to both functions. Understanding how these components function in combination to yield the basic architecture of a polarized cell-cell junction remains a major challenge. In this Review, we introduce the main components of apicobasal polarity and cell-cell adhesion complexes, and outline what is known about their regulation and assembly in epithelia. In addition, we highlight studies that investigate the interdependence between these two networks. We conclude with an overview of strategies to address the largest and arguably most fundamental unresolved question in the field, namely how a polarized junction arises as the sum of its molecular parts.

Cdc42 activity in Sertoli cells is essential for maintenance of spermatogenesis

Sertoli cells are highly polarized testicular supporting cells that simultaneously nurture multiple stages of germ cells during spermatogenesis. Proper localization of polarity protein complexes within Sertoli cells, including those responsible for blood-testis barrier formation, is vital for spermatogenesis. However, the mechanisms and developmental timing that underlie Sertoli cell polarity are poorly understood. We investigate this aspect of testicular function by conditionally deleting Cdc42, encoding a Rho GTPase involved in regulating cell polarity, specifically in Sertoli cells. Sertoli Cdc42 deletion leads to increased apoptosis and disrupted polarity of juvenile and adult testes but does not affect fetal and postnatal testicular development. The onset of the first wave of spermatogenesis occurs normally, but it fails to progress past round spermatid stages, and by young adulthood, conditional knockout males exhibit a complete loss of spermatogenic cells. These findings demonstrate that Cdc42 is essential for Sertoli cell polarity and for maintaining steady-state sperm production.

Sertoli cells of the testicular seminiferous tubule must be highly polarized to simultaneously sustain multiple stages of germ cells during spermatogenesis. Heinrich et al. use a Sertoli-specific conditional deletion mouse model to address the roles of CDC42-mediated apicobasal cell polarity in promoting testis development and spermatogenesis.

Role of RhoC in cancer cell migration

Migration is one of the five major behaviors of cells. Although RhoC—a classic member of the Rho gene family—was first identified in 1985, functional RhoC data have only been widely reported in recent years. Cell migration involves highly complex signaling mechanisms, in which RhoC plays an essential role. Cell migration regulated by RhoC—of which the most well-known function is its role in cancer metastasis—has been widely reported in breast, gastric, colon, bladder, prostate, lung, pancreatic, liver, and other cancers. Our review describes the role of RhoC in various types of cell migration. The classic two-dimensional cell migration cycle constitutes cell polarization, adhesion regulation, cell contraction and tail retraction, most of which are modulated by RhoC. In the three-dimensional cell migration model, amoeboid migration is the most classic and well-studied model. Here, RhoC modulates the formation of membrane vesicles by regulating myosin II, thereby affecting the rate and persistence of amoeba-like migration. To the best of our knowledge, this review is the first to describe the role of RhoC in all cell migration processes. We believe that understanding the detail of RhoC-regulated migration processes will help us better comprehend the mechanism of cancer metastasis. This will contribute to the study of anti-metastatic treatment approaches, aiding in the identification of new intervention targets for therapeutic or genetic transformational purposes.

Eng2, a new player involved in feedback loop regulation of Cdc42 activity in fission yeast

Cell polarity and morphogenesis are regulated by the small GTPase Cdc42. Even though major advances have been done in the field during the last years, the molecular details leading to its activation in particular cellular contexts are not completely understood. In fission yeast, the β(1,3)-glucanase Eng2 is a "moonlighting protein" with a dual function, acting as a hydrolase during spore dehiscence, and as component of the endocytic machinery in vegetative cells. Here, we report that Eng2 plays a role in Cdc42 activation during polarized growth through its interaction with the scaffold protein Scd2, which brings Cdc42 together with its guanine nucleotide exchange factor (GEF) Scd1. eng2Δ mutant cells have defects in activation of the bipolar growth (NETO), remaining monopolar during all the cell cycle. In the absence of Eng2 the accumulation of Scd1 and Scd2 at the poles is reduced, the levels of Cdc42 activation decrease, and the Cdc42 oscillatory behavior, associated with bipolar growth in wild type cells, is altered. Furthermore, overexpression of Eng2 partially rescues the growth and polarity defects of a cdc42-L160S mutant. Altogether, our work unveils a new factor regulating the activity of Cdc42, which could potentially link the polarity and endocytic machineries.

Ion Channels in Epithelial Dynamics and Morphogenesis

Mechanosensitive ion channels mediate the neuronal sensation of mechanical signals such as sound, touch, and pain. Recent studies point to a function of these channel proteins in cell types and tissues in addition to the nervous system, such as epithelia, where they have been little studied, and their role has remained elusive. Dynamic epithelia are intrinsically exposed to mechanical forces. A response to pull and push is assumed to constitute an essential part of morphogenetic movements of epithelial tissues, for example. Mechano-gated channels may participate in sensing and responding to such forces. In this review, focusing on Drosophila, we highlight recent results that will guide further investigations concerned with the mechanistic role of these ion channels in epithelial cells.

Inheritance and flexibility of cell polarity: a clue for understanding human brain development and evolution

Cell polarity is fundamentally important for understanding brain development. Here, we hypothesize that the inheritance and flexibility of cell polarity during neocortex development could be implicated in neocortical evolutionary expansion. Molecular and morphological features of cell polarity may be inherited from one type of progenitor cell to the other and finally transmitted to neurons. Furthermore, key cell types, such as basal progenitors and neurons, exhibit a highly flexible polarity. We suggest that both inheritance and flexibility of cell polarity are implicated in the amplification of basal progenitors and tangential dispersion of neurons, which are key features of the evolutionary expansion of the neocortex.

Summary: We suggest that the inheritance and flexibility of cell polarity are implicated in the evolutionary expansion of the developing neocortex by promoting the amplification of neural progenitors and tangential migration of neurons.

Chemokine-biased robust self-organizing polarization of migrating cells in vivo

Bleb-driven cell migration plays important roles in diverse biological processes. Here, we present the mechanism for polarity establishment and maintenance in blebbing cells in vivo. We show that actin polymerization defines the leading edge, the position where blebs form. We show that the cell front can direct the formation of the rear by facilitating retrograde flow of proteins that limit the generation of blebs at the opposite aspect of the cell. Conversely, localization of bleb-inhibiting proteins at one aspect of the cell results in the establishment of the cell front at the opposite side. These antagonistic interactions result in robust polarity that can be initiated in a random direction, or oriented by a chemokine gradient.

To study the mechanisms controlling front-rear polarity in migrating cells, we used zebrafish primordial germ cells (PGCs) as an in vivo model. We find that polarity of bleb-driven migrating cells can be initiated at the cell front, as manifested by actin accumulation at the future leading edge and myosin-dependent retrograde actin flow toward the other side of the cell. In such cases, the definition of the cell front, from which bleb-inhibiting proteins such as Ezrin are depleted, precedes the establishment of the cell rear, where those proteins accumulate. Conversely, following cell division, the accumulation of Ezrin at the cleavage plane is the first sign for cell polarity and this aspect of the cell becomes the cell back. Together, the antagonistic interactions between the cell front and back lead to a robust polarization of the cell. Furthermore, we show that chemokine signaling can bias the establishment of the front-rear axis of the cell, thereby guiding the migrating cells toward sites of higher levels of the attractant. We compare these results to a theoretical model according to which a critical value of actin treadmilling flow can initiate a positive feedback loop that leads to the generation of the front-rear axis and to stable cell polarization. Together, our in vivo findings and the mathematical model, provide an explanation for the observed nonoriented migration of primordial germ cells in the absence of the guidance cue, as well as for the directed migration toward the region where the gonad develops.

The ependymal cell cytoskeleton in the normal and injured spinal cord of mice

The cytoskeleton of ependymal cells is fundamental to organize and maintain the normal architecture of the central canal (CC). However, little is known about the plasticity of cytoskeletal components after spinal cord injury. Here, we focus on the structural organization of the cytoskeleton of ependymal cells in the normal and injured spinal cord of mice (both females and males) using immunohistochemical and electron microscopy techniques. We found that in uninjured animals, the actin cytoskeleton (as revealed by phalloidin staining) was arranged following the typical pattern of polarized epithelial cells with conspicuous actin pools located in the apical domain of ependymal cells. Transmission electron microscopy images showed microvilli tufts, long cilia, and characteristic intercellular membrane specializations. After spinal cord injury, F-actin rearrangements paralleled by fine structural modifications of the apical domain of ependymal cells were observed. These changes involved disruptions of the apical actin pools as well as fine structural modifications of the microvilli tufts. When comparing the control and injured spinal cords, we also found modifications in the expression of vimentin and glial fibrillary acidic protein (GFAP). After injury, vimentin expression disappeared from the most apical domains of ependymal cells but the number of GFAP-expressing cells within the CC increased. As in other polarized epithelia, the plastic changes in the cytoskeleton may be critically involved in the reaction of ependymal cells following a traumatic injury of the spinal cord.

Measurement of the Adipose Stem Cells Cell Sheets Transmittance

In the field of cell therapy, the interest in cell sheet technology is increasing. To determine the cell sheet harvesting time requires experience and practice, and different factors could change the harvesting time (variability among donors and culture media, between cell culture dishes, initial cell seeding density). We have developed a device that can measure the transmittance of the multilayer cell sheets, using a light emitting diode and a light detector, to estimate the harvesting time. The transmittance of the adipose stromal cells cell sheets (ASCCS) was measured every other day as soon as the cells were confluent, up to 12 days. The ASCCS, from three different initial seeding densities, were harvested at 8, 10, and 12 days after seeding. Real-time PCR and immunostaining confirmed the expression of specific cell markers (CD29, CD73, CD90, CD105, HLA-A, HLA-DR), but less than the isolated adipose stromal cells. The number of cells per cell sheets, the average thickness per cell sheet, and the corresponding transmittance showed no correlation. Decrease of the transmittance seems to be correlated with the cell sheet maturation. For the first time, we are reporting the success development of a device to estimate ASCCS harvesting time based on their transmittance.

Apical PAR complex proteins protect against programmed epithelial assaults to create a continuous and functional intestinal lumen

Sustained polarity and adhesion of epithelial cells is essential for the protection of our organs and bodies, and this epithelial integrity emerges during organ development amidst numerous programmed morphogenetic assaults. Using the developing Caenorhabditis elegans intestine as an in vivo model, we investigated how epithelia maintain their integrity through cell division and elongation to build a functional tube. Live imaging revealed that apical PAR complex proteins PAR-6/Par6 and PKC-3/aPkc remained apical during mitosis while apical microtubules and microtubule-organizing center (MTOC) proteins were transiently removed. Intestine-specific depletion of PAR-6, PKC-3, and the aPkc regulator CDC-42/Cdc42 caused persistent gaps in the apical MTOC as well as in other apical and junctional proteins after cell division and in non-dividing cells that elongated. Upon hatching, gaps coincided with luminal constrictions that blocked food, and larvae arrested and died. Thus, the apical PAR complex maintains apical and junctional continuity to construct a functional intestinal tube.

Orchestration of tissue-scale mechanics and fate decisions by polarity signalling

Eukaryotic development relies on dynamic cell shape changes and segregation of fate determinants to achieve coordinated compartmentalization at larger scale. Studies in invertebrates have identified polarity programmes essential for morphogenesis; however, less is known about their contribution to adult tissue maintenance. While polarity-dependent fate decisions in mammals utilize molecular machineries similar to invertebrates, the hierarchies and effectors can differ widely. Recent studies in epithelial systems disclosed an intriguing interplay of polarity proteins, adhesion molecules and mechanochemical pathways in tissue organization. Based on major advances in biophysics, genome editing, high-resolution imaging and mathematical modelling, the cell polarity field has evolved to a remarkably multidisciplinary ground. Here, we review emerging concepts how polarity and cell fate are coupled, with emphasis on tissue-scale mechanisms, mechanobiology and mammalian models. Recent findings on the role of polarity signalling for tissue mechanics, micro-environmental functions and fate choices in health and disease will be summarized.

Long time behavior and stable patterns in high-dimensional polarity models of asymmetric cell division

Asymmetric cell division is one of the fundamental processes to create cell diversity in the early stage of embryonic development. During this process, the polarity formation in the cell membrane has been considered as a key process by which the entire polarity formation in the cytosol is controlled, and it has been extensively studied in both experiments and mathematical models. Nonetheless, a mathematically rigorous analysis of the polarity formation in the asymmetric cell division has been little explored, particularly for bulk-surface models. In this article, we deal with polarity models proposed for describing the PAR polarity formation in the asymmetric cell division of a C. elegans embryo. Using a simpler but mathematically consistent model, we exhibit the long time behavior of the polarity formation of a bulk-surface cell. Moreover, we mathematically prove the existence of stable polarity solutions of the model equation in an arbitrary high-dimensional domain and analyse how the boundary position of polarity domain is determined. Our results propose that the existence and dynamics of the polarity in the asymmetric cell division can be understood universally in terms of basic mathematical structures.

Membrane contact site-dependent cholesterol transport regulates Na+/K+-ATPase polarization and spermiogenesis in Caenorhabditis elegans

Spermiogenesis in nematodes is a process whereby round and quiescent spermatids differentiate into asymmetric and crawling spermatozoa. The molecular mechanism underlying this symmetry breaking remains uncharacterized. In this study, we revealed that sperm-specific Na⁺/K⁺-ATPase (NKA) is evenly distributed on the plasma membrane (PM) of Caenorhabditis elegans spermatids but is translocated to and subsequently enters the invaginated membrane of the spermatozoa cell body during sperm activation. The polarization of NKA depends on the transport of cholesterol from the PM to membranous organelles (MOs) via membrane contact sites (MCSs). The inositol 5-phosphatase CIL-1 and the MO-localized PI4P phosphatase SAC-1 may mediate PI4P metabolism to drive cholesterol countertransport via sterol/lipid transport proteins through MCSs. Furthermore, the NKA function is required for C. elegans sperm motility and reproductive success. Our data imply that the lipid dynamics mediated by MCSs might play crucial roles in the establishment of cell polarity. eGraphical abstract.

Phosphorylation of Pal2 by the protein kinases Kin1 and Kin2 modulates HAC1 mRNA splicing in the unfolded protein response in yeast

During cellular stress in the budding yeast Saccharomyces cerevisiae, an endoplasmic reticulum (ER)-resident dual kinase and RNase Ire1 splices an intron from HAC1 mRNA in the cytosol, thereby releasing its translational block. Hac1 protein then activates an adaptive cellular stress response called the unfolded protein response (UPR) that maintains ER homeostasis. The polarity-inducing protein kinases Kin1 and Kin2 contribute to HAC1 mRNA processing. Here, we showed that an RNA-protein complex that included the endocytic proteins Pal1 and Pal2 mediated HAC1 mRNA splicing downstream of Kin1 and Kin2. We found that Pal1 and Pal2 bound to the 3' untranslated region (3'UTR) of HAC1 mRNA, and a yeast strain lacking both Pal1 and Pal2 was deficient in HAC1 mRNA processing. We also showed that Kin1 and Kin2 directly phosphorylated Pal2, and that a nonphosphorylatable Pal2 mutant could not rescue the UPR defect in a pal1Δ pal2Δ strain. Thus, our work uncovers a Kin1/2-Pal2 signaling pathway that coordinates HAC1 mRNA processing and ER homeostasis.

Apical Restriction of the Planar Cell Polarity Component VANGL in Pancreatic Ducts Is Required to Maintain Epithelial Integrity

Cell polarity is essential for the architecture and function of numerous epithelial tissues. Here, we show that apical restriction of planar cell polarity (PCP) components is necessary for the maintenance of epithelial integrity. Using the mammalian pancreas as a model, we find that components of the core PCP pathway, such as the transmembrane protein Van Gogh-like (VANGL), become apically restricted over a period of several days. Expansion of VANGL localization to the basolateral membranes of progenitors leads to their death and disruption of the epithelial integrity. VANGL basolateral expansion does not affect apico-basal polarity but acts in the cells where Vangl is mislocalized by reducing Dishevelled and its downstream target ROCK. This reduction in ROCK activity culminates in progenitor cell egression, death, and eventually pancreatic hypoplasia. Thus, precise spatiotemporal modulation of VANGL-dependent PCP signaling is crucial for proper pancreatic morphogenesis.

Leep1 interacts with PIP3 and the Scar/WAVE complex to regulate cell migration and macropinocytosis

Polarity is essential for diverse functions in many cell types. Establishing polarity requires targeting a network of specific signaling and cytoskeleton molecules to different subregions of the cell, yet the full complement of polarity regulators and how their activities are integrated over space and time to form morphologically and functionally distinct domains remain to be uncovered. Here, by using the model system Dictyostelium and exploiting the characteristic chemoattractant-stimulated translocation of polarly distributed molecules, we developed a proteomic screening approach, through which we identified a leucine-rich repeat domain–containing protein we named Leep1 as a novel polarity regulator. We combined imaging, biochemical, and phenotypic analyses to demonstrate that Leep1 localizes selectively at the leading edge of cells by binding to PIP₃, where it modulates pseudopod and macropinocytic cup dynamics by negatively regulating the Scar/WAVE complex. The spatiotemporal coordination of PIP₃ signaling, Leep1, and the Scar/WAVE complex provides a cellular mechanism for organizing protrusive structures at the leading edge.