Mitochondrial morphology dynamics and ROS regulate apical polarity and differentiation in Drosophila follicle cells

Mitochondrial morphology dynamics regulate signaling pathways during epithelial cell formation and differentiation. The mitochondrial fission protein Drp1 affects the appropriate activation of EGFR and Notch signaling-driven differentiation of posterior follicle cells in Drosophila oogenesis. The mechanisms by which Drp1 regulates epithelial polarity during differentiation are not known. In this study, we show that Drp1-depleted follicle cells are constricted in early stages and present in multiple layers at later stages with decreased levels of apical polarity protein aPKC. These defects are suppressed by additional depletion of mitochondrial fusion protein Opa1. Opa1 depletion leads to mitochondrial fragmentation and increased reactive oxygen species (ROS) in follicle cells. We find that increasing ROS by depleting the ROS scavengers, mitochondrial SOD2 and catalase also leads to mitochondrial fragmentation. Further, the loss of Opa1, SOD2 and catalase partially restores the defects in epithelial polarity and aPKC, along with EGFR and Notch signaling in Drp1-depleted follicle cells. Our results show a crucial interaction between mitochondrial morphology, ROS generation and epithelial cell polarity formation during the differentiation of follicle epithelial cells in Drosophila oogenesis.

Atypical Protein Kinase C Promotes its own Asymmetric Localisation by Phosphorylating Cdc42 in Polarising Cells

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 C. elegans zygotes that the feedback relies on CDC-42 phosphorylation at serine 71 by aPKC, which in turn results in aPKC dissociation from CDC-42. The dissociated aPKC then associates with PAR-3 clusters, which are transported anteriorly by actomyosin-based cortical flow. Moreover, the turnover of aPKC-mediated CDC-42 phosphorylation regulates the organisation of the actomyosin cortex that drives 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 polarisation of many cell types.

Apical-basal polarity precisely determines intestinal stem cell number by regulating Prospero threshold

Apical-basal polarity and cell-fate determinants are crucial for the cell fate and control of stem cell numbers. However, their interplay leading to a precise stem cell number remains unclear. Drosophila pupal intestinal stem cells (pISCs) asymmetrically divide, generating one apical ISC progenitor and one basal Prospero (Pros)+ enteroendocrine mother cell (EMC), followed by symmetric divisions of each daughter before adulthood, providing an ideal system to investigate the outcomes of polarity loss. Using lineage tracing and ex vivo live imaging, we identify an interlocked polarity regulation network precisely determining ISC number: Bazooka inhibits Pros accumulation by activating Notch signaling to maintain stem cell fate in pISC apical daughters. A threshold of Pros promotes differentiation to EMCs and avoids ISC-like cell fate, and over-threshold of Pros inhibits miranda expression to ensure symmetric divisions in pISC basal daughters. Our work suggests that a polarity-dependent threshold of a differentiation factor precisely controls stem cell number.

Hyaluronan driven by epithelial aPKC deficiency remodels the microenvironment and creates a vulnerability in mesenchymal colorectal cancer

Mesenchymal colorectal cancer (mCRC) is microsatellite stable (MSS), highly desmoplastic, with CD8+ T cells excluded to the stromal periphery, resistant to immunotherapy, and driven by low levels of the atypical protein kinase Cs (aPKCs) in the intestinal epithelium. We show here that a salient feature of these tumors is the accumulation of hyaluronan (HA) which, along with reduced aPKC levels, predicts poor survival. HA promotes epithelial heterogeneity and the emergence of a tumor fetal metaplastic cell (TFMC) population endowed with invasive cancer features through a network of interactions with activated fibroblasts. TFMCs are sensitive to HA deposition, and their metaplastic markers have prognostic value. We demonstrate that in vivo HA degradation with a clinical dose of hyaluronidase impairs mCRC tumorigenesis and liver metastasis and enables immune checkpoint blockade therapy by promoting the recruitment of B and CD8+ T cells, including a proportion with resident memory features, and by blocking immunosuppression.

Untangling interactions in the PAR cell polarity system

Animal cells establish polarity via the partitioning-defective protein system. Although the core of this system comprises only four proteins, a huge number of reported interactions between these members has made it difficult to understand how the system is organized and functions at the molecular level. In a recent JBC article, the Prehoda group has succeeded in reconstituting some of these interactions in vitro, resulting in a much clearer and simpler picture of partitioning-defective complex assembly.

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.

aPKC regulates apical constriction to prevent tissue rupture in the Drosophila follicular epithelium

Apical-basal polarity is an essential epithelial trait controlled by the evolutionarily conserved PAR-aPKC polarity network. Dysregulation of polarity proteins disrupts tissue organization during development and in disease, but the underlying mechanisms are unclear due to the broad implications of polarity loss. Here, we uncover how Drosophila aPKC maintains epithelial architecture by directly observing tissue disorganization after fast optogenetic inactivation in living adult flies and ovaries cultured ex vivo. We show that fast aPKC perturbation in the proliferative follicular epithelium produces large epithelial gaps that result from increased apical constriction, rather than loss of apical-basal polarity. Accordingly, we can modulate the incidence of epithelial gaps by increasing and decreasing actomyosin-driven contractility. We traced the origin of these large epithelial gaps to tissue rupture next to dividing cells. Live imaging shows that aPKC perturbation induces apical constriction in non-mitotic cells within minutes, producing pulling forces that ultimately detach dividing and neighboring cells. We further demonstrate that epithelial rupture requires a global increase of apical constriction, as it is prevented by the presence of non-constricting cells. Conversely, a global induction of apical tension through light-induced recruitment of RhoGEF2 to the apical side is sufficient to produce tissue rupture. Hence, our work reveals that the roles of aPKC in polarity and actomyosin regulation are separable and provides the first in vivo evidence that excessive tissue stress can break the epithelial barrier during proliferation.

The Roles of Par3, Par6, and aPKC Polarity Proteins in Normal Neurodevelopment and in Neurodegenerative and Neuropsychiatric Disorders

Normal neural circuits and functions depend on proper neuronal differentiation, migration, synaptic plasticity, and maintenance. Abnormalities in these processes underlie various neurodevelopmental, neuropsychiatric, and neurodegenerative disorders. Neural development and maintenance are regulated by many proteins. Among them are Par3, Par6 (partitioning defective 3 and 6), and aPKC (atypical protein kinase C) families of evolutionarily conserved polarity proteins. These proteins perform versatile functions by forming tripartite or other combinations of protein complexes, which hereafter are collectively referred to as "Par complexes." In this review, we summarize the major findings on their biophysical and biochemical properties in cell polarization and signaling pathways. We next summarize their expression and localization in the nervous system as well as their versatile functions in various aspects of neurodevelopment, including neuroepithelial polarity, neurogenesis, neuronal migration, neurite differentiation, synaptic plasticity, and memory. These versatile functions rely on the fundamental roles of Par complexes in cell polarity in distinct cellular contexts. We also discuss how cell polarization may correlate with subcellular polarization in neurons. Finally, we review the involvement of Par complexes in neuropsychiatric and neurodegenerative disorders, such as schizophrenia and Alzheimer's disease. While emerging evidence indicates that Par complexes are essential for proper neural development and maintenance, many questions on their in vivo functions have yet to be answered. Thus, Par3, Par6, and aPKC continue to be important research topics to advance neuroscience.

Structural Organization of Human Full-Length PAR3 and the aPKC-PAR6 Complex

The tripartite partition defect (PAR) polarity complex, which includes the proteins PAR3, atypical protein kinase C (aPKC), and PAR6, is a major regulator of cellular polarity. It is highly conserved and expressed in various tissues. Its largest component, PAR3, controls protein–protein interactions of the PAR complex with a variety of interaction partners, and PAR3 self-association is critical for the formation of filament-like structures. However, little is known about the structure of the PAR complex. Here, we purified non-filamentous PAR3 and the aPKC–PAR6 complex and characterized them by single-particle electron microscopy (EM). We expressed and purified an oligomerization-deficient form of PAR3, PAR3_V13D,D70K, and the active aPKC–PAR6 dimer. For PAR3, engineering at two positions is sufficient to form stable single particles with a maximum dimension of 20 nm. aPKC–PAR6 forms a complex with a maximum dimension of 13.5 nm that contains single copies of aPKC. Thus, the data present a basis for further high-resolution studies of PAR proteins and PAR complex formation.

Depletion of the apical endosome in response to viruses and bacterial toxins provides cell-autonomous host defense at mucosal surfaces

Polarized epithelial cells form an essential barrier against infection at mucosal surfaces. Many pathogens breach this barrier to cause disease, often by co-opting cellular endocytosis mechanisms to enter the cell through the lumenal (apical) cell surface. We recently discovered that the loss of the cell polarity gene PARD6B selectively diminishes apical endosome function. Here, we find that in response to the entry of certain viruses and bacterial toxins into the epithelial cells via the apical membrane, PARD6B and aPKC, two components of the PARD6B-aPKC-Cdc42 apical polarity complex, undergo rapid proteasome-dependent degradation. The perturbation of apical membrane glycosphingolipids by toxin- or virus-binding initiates degradation of PARD6B. The loss of PARD6B causes the depletion of apical endosome function and renders the cell resistant to further infection from the lumenal cell surface, thus enabling a form of cell-autonomous host defense.

Characterization of Smoothened Phosphorylation and Activation

The GPCR-family protein Smoothened (Smo) is an obligatory signal transducer of the Hedgehog (Hh) signaling pathway. Binding of Hh to its receptor Patched (Ptc) alleviates Ptc-mediated inhibition of Smo, allowing Smo to activate the Cubitus interruptus (Ci)/Gli family of zinc finger transcription factors. The activation of Smo is an early and crucial event in Hh signal transduction. Studies have shown that Hh induces cell surface/ciliary accumulation and phosphorylation of Smo by multiple kinases, including protein kinase A (PKA), casein kinase 1 (CK1), casein kinase 2 (CK2), G protein-coupled receptor kinase 2 (Gprk2/GRK2), and atypical PKC (aPKC). Here, we describe the assays used to examine the phosphorylation and activity of Smo, including in vitro kinase assay, phospho-specific antibodies, luciferase reporter assay, cell surface accumulation, and ciliary localization assays. These assays provide powerful tools to study Smo phosphorylation and activation, leading to mechanistic insight into Smo regulation.

Scribble downregulation in adenomyosis compromises endometrial stromal decidualization by decreasing FOXO1 expression

STUDY QUESTION

Does Scribble (SCRIB) contribute to aberrant decidualization of endometrial stromal cells (ESC) in adenomyosis?

SUMMARY ANSWER

SCRIB knockdown impairs decidualization of ESC by decreasing Fork-head box O1A (FOXO1) expression through the protein kinase B (AKT) and atypical protein kinase C (aPKC) activated pathways.

WHAT IS KNOWN ALREADY

Stromal SCRIB is required for primary decidual zone formation and pregnancy success in mice. In our previous studies, decidualization was dampened in ESC isolated from adenomyosis patients, yet the underlying molecular mechanisms remain elusive.

STUDY DESIGN, SIZE, DURATION

Eutopic endometrium tissue samples from diffuse adenomyosis and non-adenomyosis patients in proliferative, early-secretory and mid-secretory phase (n = 10 per phase for each group) were explored. In parallel, in vitro decidualization studies were carried out in ESC isolated from non-adenomyosis women (n = 8).

PARTICIPANTS/MATERIALS, SETTING, METHODS

The endometrial SCRIB expression was analyzed using immunohistochemistry staining and western blot. Quantitative RT-PCR (qRT-PCR), western blot and immunofluorescence staining were used to explore the expression of SCRIB in ESC during in vitro decidualization. siRNA-mediated SCRIB knockdown followed by decidual markers expression analysis, flow cytometry for cell cycle analysis and phalloidin staining for morphological analysis were performed to examine the function of SCRIB in ESC decidualization. RNA-sequencing was performed to examine the SCRIB-mediated transcriptional changes in decidualized ESC (DSC). Rescue experiments using an AKT inhibitor MK2206 and aPKC inhibitor NSC37044 were used to investigate the signaling pathways through which could mediate SCRIB-regulated FOXO1 protein expression and ESC decidualization.

MAIN RESULTS AND THE ROLE OF CHANCE

We found that the expression of SCRIB in the mid-secretory phase eutopic endometrial stroma of adenomyosis patients was significantly lower than that of non-adenomyosis. SCRIB knockdown reduced the expression of decidual markers, abrogated the epithelioid-like morphological changes, inhibited the mesenchymal-to-epithelial transitions process and promoted the cell cycle progression of ESC during in vitro decidualization. SCRIB knockdown-induced decidualization defects were attributed to a decrease in expression of transcription factor FOXO1, known to regulate decidualization. Furthermore, we found that SCRIB knockdown induced the aberrant activation of AKT and aPKC, which led to FOXO1 phosphorylation and degradation. Rescue assay confirmed that restoring the expression of FOXO1 effectively reversed the decidualization defects and cell cycle progression caused by SCRIB knockdown.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

In this study, it was demonstrated that SCRIB knockdown mediated the activation of AKT and aPKC, contributing to FOXO1 degradation and aberrant decidualization, however, the molecular link between AKT and aPKC signaling was not determined, and still requires further exploration.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings support the hypothesis that adenomyosis interferes with embryo implantation due to insufficient endometrial receptivity. Abnormal decidualization of the endometrial stroma may clarify the possible association between adenomyosis and infertility. Our findings may be clinically useful for counseling and treatment of infertile adenomyosis patients.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by the National Natural Science Foundation of China (82001523 and 82171639). The authors have no conflicts of interest to disclose.

TRIAL REGISTRATION NUMBER

N/A.

A picket fence function for adherens junctions in epithelial cell polarity

Adherens junctions are a defining feature of all epithelial cells, providing cell-cell adhesion and contractile ring formation that is essential for cell and tissue morphology. In Drosophila, adherens junctions are concentrated between the apical and basolateral plasma membrane domains, defined by aPKC-Par6-Baz and Lgl/Dlg/Scrib, respectively. Whether adherens junctions contribute to apical-basal polarization itself has been unclear because neuroblasts exhibit apical-basal polarization of aPKC-Par6-Baz and Lgl in the absence of adherens junctions. Here we show that, upon disruption of adherens junctions in epithelial cells, apical polarity determinants such as aPKC can still segregate from basolateral Lgl, but lose their sharp boundaries and also overlap with Dlg and Scrib - similar to neuroblasts. In addition, control of apical versus basolateral domain size is lost, along with control of cell shape, in the absence of adherens junctions. Manipulating the levels of apical Par3/Baz or basolateral Lgl polarity determinants in experiments and in computer simulations confirms that adherens junctions provide a 'picket fence' diffusion barrier that restricts the spread of polarity determinants along the membrane to enable precise domain size control. Movement of adherens junctions in response to mechanical forces during morphogenetic change thus enables spontaneous adjustment of apical versus basolateral domain size as an emergent property of the polarising system.

Electrostatic plasma membrane targeting contributes to Dlg function in cell polarity and tumorigenesis

Discs large (Dlg) is an essential polarity protein and a tumor suppressor originally characterized in Drosophila but also well conserved in vertebrates. Like the majority of polarity proteins, plasma membrane (PM)/cortical localization of Dlg is required for its function in polarity and tumorigenesis, but the exact mechanisms targeting Dlg to the PM remain to be fully elucidated. Here, we show that, similar to recently discovered polybasic polarity proteins such as Lgl and aPKC, Dlg also contains a positively charged polybasic domain that electrostatically binds the PM phosphoinositides PI4P and PI(4,5)P2 Electrostatic targeting by the polybasic domain contributes significantly to the PM localization of Dlg in follicular and early embryonic epithelial cells, and is crucial for Dlg to regulate both polarity and tumorigenesis. The electrostatic PM targeting of Dlg is controlled by a potential phosphorylation-dependent allosteric regulation of its polybasic domain, and is specifically enhanced by the interactions between Dlg and another basolateral polarity protein and tumor suppressor, Scrib. Our studies highlight an increasingly significant role of electrostatic PM targeting of polarity proteins in regulating cell polarity.

Apical-basal polarity regulators are essential for slit diaphragm assembly and endocytosis in Drosophila nephrocytes

Apical-basal polarity is a key feature of most epithelial cells and it is regulated by highly conserved protein complexes. In mammalian podocytes, which emerge from columnar epithelial cells, this polarity is preserved and the tight junctions are converted to the slit diaphragms, establishing the filtration barrier. In Drosophila, nephrocytes show several structural and functional similarities with mammalian podocytes and proximal tubular cells. However, in contrast to podocytes, little is known about the role of apical-basal polarity regulators in these cells. In this study, we used expansion microscopy and found the apical polarity determinants of the PAR/aPKC and Crb-complexes to be predominantly targeted to the cell cortex in proximity to the nephrocyte diaphragm, whereas basolateral regulators also accumulate intracellularly. Knockdown of PAR-complex proteins results in severe endocytosis and nephrocyte diaphragm defects, which is due to impaired aPKC recruitment to the plasma membrane. Similar, downregulation of most basolateral polarity regulators disrupts Nephrin localization but had surprisingly divergent effects on endocytosis. Our findings suggest that morphology and slit diaphragm assembly/maintenance of nephrocytes is regulated by classical apical-basal polarity regulators, which have distinct functions in endocytosis.

WWC Proteins: Important Regulators of Hippo Signaling in Cancer

Simple Summary

The conserved Hippo pathway regulates cell proliferation and apoptosis via a complex interplay of transcriptional activities, post-translational protein modifications, specific protein–protein interactions and cellular transport processes. Deregulating this highly balanced system can lead to hyperproliferation, organ overgrowth and cancer. Although WWC proteins are known as components of the Hippo signaling pathway, their association with tumorigenesis is often neglected. This review aims to summarize the current knowledge on WWC proteins and their contribution to Hippo signaling in the context of cancer.

Abstract

The Hippo signaling pathway is known to regulate cell differentiation, proliferation and apoptosis. Whereas activation of the Hippo signaling pathway leads to phosphorylation and cytoplasmic retention of the transcriptional coactivator YAP, decreased Hippo signaling results in nuclear import of YAP and subsequent transcription of pro-proliferative genes. Hence, a dynamic and precise regulation of the Hippo signaling pathway is crucial for organ size control and the prevention of tumor formation. The transcriptional activity of YAP is controlled by a growing number of upstream regulators including the family of WWC proteins. WWC1, WWC2 and WWC3 represent cytosolic scaffolding proteins involved in intracellular transport processes and different signal transduction pathways. Earlier in vitro experiments demonstrated that WWC proteins positively regulate the Hippo pathway via the activation of large tumor suppressor kinases 1/2 (LATS1/2) kinases and the subsequent cytoplasmic accumulation of phosphorylated YAP. Later, reduced WWC expression and subsequent high YAP activity were shown to correlate with the progression of human cancer in different organs. Although the function of WWC proteins as upstream regulators of Hippo signaling was confirmed in various studies, their important role as tumor modulators is often overlooked. This review has been designed to provide an update on the published data linking WWC1, WWC2 and WWC3 to cancer, with a focus on Hippo pathway-dependent mechanisms.

Epidermal PAR-6 and PKC-3 are essential for larval development of C. elegans and organize non-centrosomal microtubules

The cortical polarity regulators PAR-6, PKC-3, and PAR-3 are essential for the polarization of a broad variety of cell types in multicellular animals. In C. elegans, the roles of the PAR proteins in embryonic development have been extensively studied, yet little is known about their functions during larval development. Using inducible protein degradation, we show that PAR-6 and PKC-3, but not PAR-3, are essential for postembryonic development. PAR-6 and PKC-3 are required in the epidermal epithelium for animal growth, molting, and the proper pattern of seam-cell divisions. Finally, we uncovered a novel role for PAR-6 in organizing non-centrosomal microtubule arrays in the epidermis. PAR-6 was required for the localization of the microtubule organizer NOCA-1/Ninein, and defects in a noca-1 mutant are highly similar to those caused by epidermal PAR-6 depletion. As NOCA-1 physically interacts with PAR-6, we propose that PAR-6 promotes non-centrosomal microtubule organization through localization of NOCA-1/Ninein.

Phosphorylation and dephosphorylation of Ser852 and Ser889 control the clustering, localization and function of PAR3

Cell polarity is essential for various asymmetric cellular events, and the partitioning defective (PAR) protein PAR3 (encoded by PARD3 in mammals) plays a unique role as a cellular landmark to establish polarity. In epithelial cells, PAR3 localizes at the subapical border, such as the tight junction in vertebrates, and functions as an apical determinant. Although we know a great deal about the regulators of PAR3 localization, how PAR3 is concentrated and localized to a specific membrane domain remains an important question to be clarified. In this study, we demonstrate that ASPP2 (also known as TP53BP2), which controls PAR3 localization, links PAR3 and protein phosphatase 1 (PP1). The ASPP2-PP1 complex dephosphorylates a novel phosphorylation site, Ser852, of PAR3. Furthermore, Ser852- or Ser889-unphosphorylatable PAR3 mutants form protein clusters, and ectopically localize to the lateral membrane. Concomitance of clustering and ectopic localization suggests that PAR3 localization is a consequence of local clustering. We also demonstrate that unphosphorylatable forms of PAR3 exhibited a low molecular turnover and failed to coordinate rapid reconstruction of the tight junction, supporting that both the phosphorylated and dephosphorylated states are essential for the functional integrity of PAR3.

Sex differences in the role of atypical PKC within the basolateral nucleus of the amygdala in a mouse hyperalgesic priming model

Though sex differences in chronic pain have been consistently described in the literature, their underlying neural mechanisms are poorly understood. Previous work in humans has demonstrated that men and women differentially invoke distinct brain regions and circuits in coping with subjective pain unpleasantness. The goal of the present work was to elucidate the molecular mechanisms in the basolateral nucleus of the amygdala (BLA) that modulate hyperalgesic priming, a pain plasticity model, in males and females. We used plantar incision as the first, priming stimulus and prostaglandin E₂ (PGE₂) as the second stimulus. We sought to assess whether hyperalgesic priming can be prevented or reversed by pharmacologically manipulating molecular targets in the BLA of male or female mice. We found that administering ZIP, a cell-permeable inhibitor of aPKC, into the BLA attenuated aspects of hyperalgesic priming induced by plantar incision in males and females. However, incision only upregulated PKCζ/PKMζ immunoreactivity in the BLA of male mice, and deficits in hyperalgesic priming were seen only when we restricted our analysis to male Prkcz^-/- mice. On the other hand, intra-BLA microinjections of pep2m, a peptide that interferes with the trafficking and function of GluA2-containing AMPA receptors, a downstream target of aPKC, reduced mechanical hypersensitivity after plantar incision and disrupted the development of hyperalgesic priming in both male and female mice. In addition, pep2m treatment reduced facial grimacing and restored aberrant behavioral responses in the sucrose splash test in male and female primed mice. Immunofluorescence results demonstrated upregulation of GluA2 expression in the BLA of male and female primed mice, consistent with pep2m findings. We conclude that, in a model of incision-induced hyperalgesic priming, PKCζ/PKMζ in the BLA is critical for the development of hyperalgesic priming in males, while GluA2 in the BLA is crucial for the expression of both reflexive and affective pain-related behaviors in both male and female mice in this model. Our findings add to a growing body of evidence of sex differences in molecular pain mechanisms in the brain.

PP1-Mediated Dephosphorylation of Lgl Controls Apical-basal Polarity

Apical-basal polarity is a common trait that underlies epithelial function. Although the asymmetric distribution of cortical polarity proteins works in a functioning equilibrium, it also retains plasticity to accommodate cell division, during which the basolateral determinant Lgl is released from the cortex. Here, we investigated how Lgl restores its cortical localization to maintain the integrity of dividing epithelia. We show that cytoplasmic Lgl is reloaded to the cortex at mitotic exit in Drosophila epithelia. Lgl cortical localization depends on protein phosphatase 1, which dephosphorylates Lgl on the serines phosphorylated by aPKC and Aurora A kinases through a mechanism that relies on the regulatory subunit Sds22 and a PP1-interacting RVxF motif of Lgl. This mechanism maintains epithelial polarity and is of particular importance at mitotic exit to couple Lgl cortical reloading with the polarization of the apical domain. Hence, PP1-mediated dephosphorylation of Lgl preserves the apical-basal organization of proliferative epithelia.

The Role of pkc-3 and Genetic Suppressors in Caenorhabditis elegans Epithelial Cell Junction Formation

Epithelial cells form intercellular junctions to strengthen cell-cell adhesion and limit diffusion, allowing epithelia to function as dynamic tissues and barriers separating internal and external environments. Junctions form as epithelial cells differentiate; clusters of junction proteins first concentrate apically, then mature into continuous junctional belts that encircle and connect each cell. In mammals and Drosophila, atypical protein kinase C (aPKC) is required for junction maturation, although how it contributes to this process is poorly understood. A role for the Caenorhabditis elegans aPKC homolog PKC-3 in junction formation has not been described previously. Here, we show that PKC-3 is essential for junction maturation as epithelia first differentiate. Using a temperature-sensitive allele of pkc-3 that causes junction breaks in the spermatheca and leads to sterility, we identify intragenic and extragenic suppressors that render pkc-3 mutants fertile. Intragenic suppressors include an unanticipated stop-to-stop mutation in the pkc-3 gene, providing evidence for the importance of stop codon identity in gene activity. One extragenic pkc-3 suppressor is a loss-of-function allele of the lethal(2) giant larvae homolog lgl-1, which antagonizes aPKC within epithelia of Drosophila and mammals, but was not known previously to function in C. elegans epithelia. Finally, two extragenic suppressors are loss-of-function alleles of sups-1-a previously uncharacterized gene. We show that SUPS-1 is an apical extracellular matrix protein expressed in epidermal cells, suggesting that it nonautonomously regulates junction formation in the spermatheca. These findings establish a foundation for dissecting the role of PKC-3 and interacting genes in epithelial junction maturation.

Stepwise polarisation of developing bilayered epidermis is mediated by aPKC and E-cadherin in zebrafish

The epidermis, a multilayered epithelium, surrounds and protects the vertebrate body. It develops from a bilayered epithelium formed of the outer periderm and underlying basal epidermis. How apicobasal polarity is established in the developing epidermis has remained poorly understood. We show that both the periderm and the basal epidermis exhibit polarised distribution of adherens junctions in zebrafish. aPKC, an apical polarity regulator, maintains the robustness of polarisation of E-cadherin- an adherens junction component- in the periderm. E-cadherin in one layer controls the localisation of E-cadherin in the second layer in a layer non-autonomous manner. Importantly, E-cadherin controls the localisation and levels of Lgl, a basolateral polarity regulator, in a layer autonomous as well non-autonomous manner. Since periderm formation from the enveloping layer precedes the formation of the basal epidermis, our analyses suggest that peridermal polarity, initiated by aPKC, is transduced in a stepwise manner by E-cadherin to the basal layer.

Dysfunctional Mechanotransduction through the YAP/TAZ/Hippo Pathway as a Feature of Chronic Disease

In order to ascertain their external environment, cells and tissues have the capability to sense and process a variety of stresses, including stretching and compression forces. These mechanical forces, as experienced by cells and tissues, are then converted into biochemical signals within the cell, leading to a number of cellular mechanisms being activated, including proliferation, differentiation and migration. If the conversion of mechanical cues into biochemical signals is perturbed in any way, then this can be potentially implicated in chronic disease development and processes such as neurological disorders, cancer and obesity. This review will focus on how the interplay between mechanotransduction, cellular structure, metabolism and signalling cascades led by the Hippo-YAP/TAZ axis can lead to a number of chronic diseases and suggest how we can target various pathways in order to design therapeutic targets for these debilitating diseases and conditions.

Mutations in ANKLE2, a ZIKA Virus Target, Disrupt an Asymmetric Cell Division Pathway in Drosophila Neuroblasts to Cause Microcephaly

The apical Par complex, which contains atypical protein kinase C (aPKC), Bazooka (Par-3), and Par-6, is required for establishing polarity during asymmetric division of neuroblasts in Drosophila, and its activity depends on L(2)gl. We show that loss of Ankle2, a protein associated with microcephaly in humans and known to interact with Zika protein NS4A, reduces brain volume in flies and impacts the function of the Par complex. Reducing Ankle2 levels disrupts endoplasmic reticulum (ER) and nuclear envelope morphology, releasing the kinase Ballchen-VRK1 into the cytosol. These defects are associated with reduced phosphorylation of aPKC, disruption of Par-complex localization, and spindle alignment defects. Importantly, removal of one copy of ballchen or l(2)gl suppresses Ankle2 mutant phenotypes and restores viability and brain size. Human mutational studies implicate the above-mentioned genes in microcephaly and motor neuron disease. We suggest that NS4A, ANKLE2, VRK1, and LLGL1 define a pathway impinging on asymmetric determinants of neural stem cell division.

Non-muscle myosin II activation: adding a classical touch to ROCK

Non-muscle myosin II molecules are actin-binding proteins with ATPase activity, this latter capacity providing the energy required for actin filament cross-linking and contraction. The activation of these molecular motors relies on direct phosphorylation at conserved sites through different protein kinases, including the Rho-associated coiled coil-containing kinase (ROCK). In the light of some recent results found in our lab, we comment on the necessity of additional regulatory mechanisms to control the subcellular distribution of non-muscle myosin II proteins to ensure their full activation.

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.

Nup358 regulates microridge length by controlling SUMOylation-dependent activity of aPKC in zebrafish epidermis

The Par polarity complex, consisting of Par3, Par6 and atypical protein kinase C (aPKC), plays a crucial role in the establishment and maintenance of cell polarity. Although activation of aPKC is critical for polarity, how this is achieved is unclear. The developing zebrafish epidermis, along with its apical actin-based projections, called microridges, offers a genetically tractable system for unraveling the mechanisms of the cell polarity control. The zebrafish aPKC regulates elongation of microridges by controlling levels of apical Lgl, which acts as a pro-elongation factor. Here, we show that the nucleoporin Nup358 (also known as RanBP2) - a component of the nuclear pore complex and a part of cytoplasmic annulate lamellae (AL) - SUMOylates zebrafish aPKC. Nup358-mediated SUMOylation controls aPKC activity to regulate Lgl-dependent microridge elongation. Our data further suggest that cytoplasmic AL structures are the possible site for Nup358-mediated aPKC SUMOylation. We have unraveled a hitherto unappreciated contribution of Nup358-mediated aPKC SUMOylation in cell polarity regulation.This article has an associated First Person interview with the first author of the paper.

Structural insights into the aPKC regulatory switch mechanism of the human cell polarity protein lethal giant larvae 2

Metazoan cell polarity is controlled by a set of highly conserved proteins. Lethal giant larvae (Lgl) functions in apical-basal polarity through phosphorylation-dependent interactions with several other proteins as well as the plasma membrane. Phosphorylation of Lgl by atypical protein kinase C (aPKC), a component of the partitioning-defective (Par) complex in epithelial cells, excludes Lgl from the apical membrane, a crucial step in the establishment of epithelial cell polarity. We present the crystal structures of human Lgl2 in both its unphosphorylated and aPKC-phosphorylated states. Lgl2 adopts a double β-propeller structure that is unchanged by aPKC phosphorylation of an unstructured loop in its second β-propeller, ruling out models of phosphorylation-dependent conformational change. We demonstrate that phosphorylation controls the direct binding of purified Lgl2 to negative phospholipids in vitro. We also show that a coil-helix transition of this region that is promoted by phosphatidylinositol 4,5-bisphosphate (PIP₂) is also phosphorylation-dependent, implying a highly effective phosphorylative switch for membrane association.

Regulation of programmed cell death ligand 1 expression by atypical protein kinase C lambda/iota in cutaneous angiosarcoma

The expression of immune checkpoint proteins such as programmed cell death protein 1 (PD‐1) and its ligand (PD‐L1) has been shown to correlate with patient prognosis in many malignant cancers. The expression of PD‐L1 is controlled by c‐Myc; however, further upstream regulation of PD‐L1 expression is largely unknown. We have previously shown that atypical protein kinase C lambda/iota (aPKCλ) phosphorylates the Forkhead box protein O1 (FoxO1) transcription factor at Ser218 to suppress its DNA‐binding ability, thereby regulating c‐Myc expression and controlling physiologic and pathologic endothelial proliferation. The presence of phosphorylation of FoxO1 at Ser218 (pSer218 FoxO1) in cutaneous angiosarcoma (CAS) strongly correlates with poor patient prognosis. Here, we reported that patients with PD‐L1⁺ cells in CAS lesions showed significantly worse prognosis compared to those that were PD‐L1⁻. Expression of PD‐L1 correlated with that of aPKCλ or the presence of pSer218FoxO1. Moreover, suppression of aPKCλ expression or inhibition of its activity in HUVECs or AS‐M, an established human angiosarcoma cell line, resulted in decreased PD‐L1 expression. Our results suggest that combined treatment with immune checkpoint inhibitors and aPKCλ inhibitors could be a novel treatment strategy for CAS patients.

Increased Serine and One-Carbon Pathway Metabolism by PKCλ/ι Deficiency Promotes Neuroendocrine Prostate Cancer

Increasingly effective therapies targeting the androgen receptor have paradoxically promoted the incidence of neuroendocrine prostate cancer (NEPC), the most lethal subtype of castration-resistant prostate cancer (PCa), for which there is no effective therapy. Here we report that protein kinase C (PKC)λ/ι is downregulated in de novo and during therapy-induced NEPC, which results in the upregulation of serine biosynthesis through an mTORC1/ATF4-driven pathway. This metabolic reprogramming supports cell proliferation and increases intracellular S-adenosyl methionine (SAM) levels to feed epigenetic changes that favor the development of NEPC characteristics. Altogether, we have uncovered a metabolic vulnerability triggered by PKCλ/ι deficiency in NEPC, which offers potentially actionable targets to prevent therapy resistance in PCa.

Light-Induced Protein Clustering for Optogenetic Interference and Protein Interaction Analysis in Drosophila S2 Cells

Drosophila Schneider 2 (S2) cells are a simple and powerful system commonly used in cell biology because they are well suited for high resolution microscopy and RNAi-mediated depletion. However, understanding dynamic processes, such as cell division, also requires methodology to interfere with protein function with high spatiotemporal control. In this research study, we report the adaptation of an optogenetic tool to Drosophila S2 cells. Light-activated reversible inhibition by assembled trap (LARIAT) relies on the rapid light-dependent heterodimerization between cryptochrome 2 (CRY2) and cryptochrome-interacting bHLH 1 (CIB1) to form large protein clusters. An anti-green fluorescent protein (GFP) nanobody fused with CRY2 allows this method to quickly trap any GFP-tagged protein in these light-induced protein clusters. We evaluated clustering kinetics in response to light for different LARIAT modules, and showed the ability of GFP-LARIAT to inactivate the mitotic protein Mps1 and to disrupt the membrane localization of the polarity regulator Lethal Giant Larvae (Lgl). Moreover, we validated light-induced co-clustering assays to assess protein-protein interactions in S2 cells. In conclusion, GFP-based LARIAT is a versatile tool to answer different biological questions, since it enables probing of dynamic processes and protein-protein interactions with high spatiotemporal resolution in Drosophila S2 cells.

The Septate Junction Protein Tsp2A Restricts Intestinal Stem Cell Activity via Endocytic Regulation of aPKC and Hippo Signaling

Hippo signaling and the activity of its transcriptional coactivator, Yorkie (Yki), are conserved and crucial regulators of tissue homeostasis. In the Drosophila midgut, after tissue damage, Yki activity increases to stimulate stem cell proliferation, but how Yki activity is turned off once the tissue is repaired is unknown. From an RNAi screen, we identified the septate junction (SJ) protein tetraspanin 2A (Tsp2A) as a tumor suppressor. Tsp2A undergoes internalization to facilitate the endocytic degradation of atypical protein kinase C (aPKC), a negative regulator of Hippo signaling. In the Drosophila midgut epithelium, adherens junctions (AJs) and SJs are prominent in intestinal stem cells or enteroblasts (ISCs or EBs) and enterocytes (ECs), respectively. We show that when ISCs differentiate toward ECs, Tsp2A is produced, participates in SJ assembly, and turns off aPKC and Yki-JAK-Stat activity. Altogether, our study uncovers a mechanism allowing the midgut to restore Hippo signaling and restrict proliferation once tissue repair is accomplished.

LAP2 Proteins Chaperone GLI1 Movement between the Lamina and Chromatin to Regulate Transcription

Understanding transcription factor navigation through the nucleus remains critical for developing targeted therapeutics. The GLI1 transcription factor must maintain maximal Hedgehog pathway output in basal cell carcinomas (BCCs), and we have previously shown that resistant BCCs increase GLI1 deacetylation through atypical protein kinase Cι/λ (aPKC) and HDAC1. Here we identify a lamina-associated polypeptide 2 (LAP2) isoform-dependent nuclear chaperoning system that regulates GLI1 movement between the nuclear lamina and nucleoplasm to achieve maximal activation. LAP2β forms a two-site interaction with the GLI1 zinc-finger domain and acetylation site, stabilizing an acetylation-dependent reserve on the inner nuclear membrane (INM). By contrast, the nucleoplasmic LAP2α competes with LAP2β for GLI1 while scaffolding HDAC1 to deacetylate the secondary binding site. aPKC functions to promote GLI1 association with LAP2α, promoting egress off the INM. GLI1 intranuclear trafficking by LAP2 isoforms represents a powerful signal amplifier in BCCs with implications for zinc finger-based signal transduction and therapeutics.

Atypical protein kinase C isoforms differentially regulate directional keratinocyte migration during wound healing

BACKGROUND

The epidermis possesses regenerative properties that become apparent only after wounding. Atypical protein kinase C (aPKC) isoforms aPKCζ and aPKCλ form a ternary complex with Par3 and Par6, and play crucial roles in establishing and maintaining epithelial cell polarity. The epidermal loss of aPKCλ results in progressive depletion of hair follicle stem cells. However, it is unclear whether aPKCs have equivalent activities in epidermal regeneration.

OBJECTIVES

To clarify functional differences between aPKCζ and aPKCλ in cutaneous wound healing.

METHODS

We compared cutaneous wound healing processes in vivo using mutant mice with genetic deletion of each aPKC isoform. We also analyzed functional differences between aPKCζ and aPKCλ in cell proliferation, directional cell migration, and formation of microtubules in vitro using primary keratinocytes established from each mutant mouse.

RESULTS

Wound healing was significantly retarded in epidermis-specific aPKCλ knockout mice. In aPKCλ-deleted keratinocytes, the correct orientation of cell protrusions toward the wound was disrupted through the destabilization of Par6β. The elongation of stabilized β-tubulin was also deteriorated in aPKCλ-deleted keratinocytes, leading to defects in cell spreading. Conversely, wound healing and directional cell migration in aPKCζ-deleted mice were comparable to those in their control littermates.

CONCLUSIONS

aPKCs are not functionally equivalent; aPKCλ, but not aPKCζ, plays a primary role in cutaneous wound healing.

Modulation of the Hippo pathway and organ growth by RNA processing proteins

The Hippo tumor-suppressor pathway regulates organ growth, cell proliferation, and stem cell biology. Defects in Hippo signaling and hyperactivation of its downstream effectors-Yorkie (Yki) in Drosophila and YAP/TAZ in mammals-result in progenitor cell expansion and overgrowth of multiple organs and contribute to cancer development. Deciphering the mechanisms that regulate the activity of the Hippo pathway is key to understanding its function and for therapeutic targeting. However, although the Hippo kinase cascade and several other upstream inputs have been identified, the mechanisms that regulate Yki/YAP/TAZ activity are still incompletely understood. To identify new regulators of Yki activity, we screened in Drosophila for suppressors of tissue overgrowth and Yki activation caused by overexpression of atypical protein kinase C (aPKC), a member of the apical cell polarity complex. In this screen, we identified mutations in the heterogeneous nuclear ribonucleoprotein Hrb27C that strongly suppressed the tissue defects induced by ectopic expression of aPKC. Hrb27C was required for aPKC-induced tissue growth and Yki target gene expression but did not affect general gene expression. Genetic and biochemical experiments showed that Hrb27C affects Yki phosphorylation. Other RNA-binding proteins known to interact with Hrb27C for mRNA transport in oocytes were also required for normal Yki activity, although they suppressed Yki output. Based on the known functions of Hrb27C, we conclude that Hrb27C-mediated control of mRNA splicing, localization, or translation is essential for coordinated activity of the Hippo pathway.

The Rb tumor suppressor regulates epithelial cell migration and polarity

Altered cell polarity and migration are hallmarks of cancer and metastases. Here we show that inactivation of the retinoblastoma gene (Rb) tumor suppressor causes defects in tissue closure that reflect the inability of Rb null epithelial cells to efficiently migrate and polarize. These defects occur independently of pRB's anti-proliferative role and instead correlate with upregulation of RhoA signaling and mislocalization of apical-basal polarity proteins. Notably, concomitant inactivation of tp53 specifically overrides the motility defect, and not the aberrant polarity, thereby uncovering previously unappreciated mechanisms by which Rb and tp53 mutations cooperate to promote cancer development and metastases.

aPKC: the Kinase that Phosphorylates Cell Polarity

Establishing and maintaining cell polarity are dynamic processes that necessitate complicated but highly regulated protein interactions. Phosphorylation is a powerful mechanism for cells to control the function and subcellular localization of a target protein, and multiple kinases have played critical roles in cell polarity. Among them, atypical protein kinase C (aPKC) is likely the most studied kinase in cell polarity and has the largest number of downstream substrates characterized so far. More than half of the polarity proteins that are essential for regulating cell polarity have been identified as aPKC substrates. This review covers mainly studies of aPKC in regulating anterior-posterior polarity in the worm one-cell embryo and apical-basal polarity in epithelial cells and asymmetrically dividing cells (for example, Drosophila neuroblasts). We will go through aPKC target proteins in cell polarity and discuss various mechanisms by which aPKC phosphorylation controls their subcellular localizations and biological functions. We will also review the recent progress in determining the detailed molecular mechanisms in spatial and temporal control of aPKC subcellular localization and kinase activity during cell polarization.

aPKC is a key polarity determinant in coordinating the function of three distinct cell polarities during collective migration

Apical-basal polarity is a hallmark of epithelia and needs to be remodeled when epithelial cells undergo morphogenetic cell movements. Here, we analyze border cells in the Drosophila ovary to address how apical-basal polarity is remodeled and turned into front-back and inside-outside as well as apical-basal polarities, during collective migration. We find that the Crumbs (Crb) complex is required for the generation of the three distinct but interconnected cell polarities of border cells. Specifically, the Crb complex, together with the Par complex and the endocytic recycling machinery, ensures the strict distribution of two distinct populations of aPKC at the inside apical junction and near the outside lateral membrane. Interestingly, aPKC distributed near the outside lateral membrane interacts with Sif and promotes Rac-induced protrusions, whereas alteration of the aPKC distribution pattern changes the pattern of protrusion formation, leading to disruption of all three polarities. Therefore, we demonstrate that aPKC, spatially controlled by the Crb complex, is a key polarity molecule coordinating the generation of three distinct but interconnected cell polarities during collective migration.

α-Tocopherol promotes HaCaT keratinocyte wound repair through the regulation of polarity proteins leading to the polarized cell migration

In many developed countries including Japan, how to care the bedridden elderly people with chronic wounds such as decubitus becomes one of the most concerned issues. Although antioxidant micronutrients including vitamin E, especially α-tocopherol (α-Toc), are reported to shorten a period of wound closure, the promoting effect of α-Toc on wound healing independent of its antioxidant activity remains to be fully elucidated. The aim of this study was to examine whether α-Toc affects wound-mediated HaCaT keratinocyte polarization process including the recruitment of polarity regulating proteins, leading to wound repair independently of its antioxidant activity. We investigated the effects of α-Toc and other antioxidants such as Trolox, a cell-permeable α-Toc analog on the migration, proliferation, and cell polarization of HaCaT keratinocytes after wounding. We analyzed the localization and complex formation of polarity proteins, partitioning defective 3 (Par3), and atypical protein kinase C (aPKC), and aPKC activity by immunohistochemistry, immunoprecipitation analyses, and in vitro kinase assays, respectively. α-Toc but not other antioxidants enhanced the wound closure and cell polarization in HaCaT keratinocytes after wounding. α-Toc regulated the localization and complex formation of Par3 and aPKC during wound healing. Knockdown of aPKC or Par3 abrogated α-Toc-mediated promotion of the wound closure and cell polarization in HaCaT keratinocytes. Furthermore, aPKC kinase activity was significantly increased in α-Toc-treated cells through activation of phosphatidylinositol 3-kinase/Akt signaling pathway. These results suggest that α-Toc promotes HaCaT keratinocyte wound repair by regulating the aPKC kinase activity and the formation of aPKC-Par3 complex. © 2017 BioFactors, 44(2):180-191, 2018.

Pak1 Kinase Maintains Apical Membrane Identity in Epithelia

Epithelial cells are polarized along their apical-basal axis by the action of the small GTPase Cdc42, which is known to activate the aPKC kinase at the apical domain. However, loss of aPKC kinase activity was reported to have only mild effects on epithelial cell polarity. Here, we show that Cdc42 also activates a second kinase, Pak1, to specify apical domain identity in Drosophila and mammalian epithelia. aPKC and Pak1 phosphorylate an overlapping set of polarity substrates in kinase assays. Inactivating both aPKC kinase activity and the Pak1 kinase leads to a complete loss of epithelial polarity and morphology, with cells losing markers of apical polarization such as Crumbs, Par3/Bazooka, or ZO-1. This function of Pak1 downstream of Cdc42 is distinct from its role in regulating integrins or E-cadherin. Our results define a conserved dual-kinase mechanism for the control of apical membrane identity in epithelia.

Par3 and aPKC regulate BACE1 endosome-to-TGN trafficking through PACS1

The cleavage of amyloid precursor protein (APP) by β-site APP cleaving enzyme 1 (BACE1) is the rate-limiting step in beta amyloid generation during Alzheimer's disease (AD) pathogenesis. In AD brains, BACE1 is abnormally accumulated in endocytic compartments, where the acidic pH is optimal for its activity. However, mechanisms regulating the endosome-to-trans-Golgi network (TGN) retrieval of BACE1 remain unclear. Here, we show that partitioning defective 3 (Par3) facilitates BACE1 retrograde trafficking from endosomes to the TGN. Par3 functions through aPKC-mediated phosphorylation of BACE1 on Ser498, which in turn promotes the interaction between BACE1 and phosphofurin acidic cluster sorting protein 1 and facilitates the retrograde trafficking of BACE1 to the TGN. In human AD brains, there is a significant decrease in Ser498 phosphorylation of BACE1 suggesting that defective phosphorylation-dependent retrograde transport of BACE1 is important in AD pathogenesis. Together, our studies provide mechanistic insight into a novel role for Par3 and aPKC in regulating the retrograde endosome-to-TGN trafficking of BACE1 and shed light on the mechanisms of AD pathogenesis.

Lack of an association between the aPKCλ/ι expression in prostate cancer and the patient outcomes

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This is the first study to assess aPKCλ/ι expression in primary prostate cancer with metastatic disease.

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A total of 43 patients with prostate cancer and its metastasis to the lymph node and/or bone were analyzed in this study.

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We found no strong association between aPKCλ/ι expression and the prognosis of the patients.

Complement C5aR1 Signaling Promotes Polarization and Proliferation of Embryonic Neural Progenitor Cells through PKCζ

The complement system, typically associated with innate immunity, is emerging as a key controller of nonimmune systems including in development, with recent studies linking complement mutations with neurodevelopmental disease. A key effector of the complement response is the activation fragment C5a, which, through its receptor C5aR1, is a potent driver of inflammation. Surprisingly, C5aR1 is also expressed during early mammalian embryogenesis; however, no clearly defined function is ascribed to C5aR1 in development. Here we demonstrate polarized expression of C5aR1 on the apical surface of mouse embryonic neural progenitor cells in vivo and on human embryonic stem cell-derived neural progenitors. We also show that signaling of endogenous C5a during mouse embryogenesis drives proliferation of neural progenitor cells within the ventricular zone and is required for normal brain histogenesis. C5aR1 signaling in neural progenitors was dependent on atypical protein kinase C ζ, a mediator of stem cell polarity, with C5aR1 inhibition reducing proliferation and symmetric division of apical neural progenitors in human and mouse models. C5aR1 signaling was shown to promote the maintenance of cell polarity, with exogenous C5a increasing the retention of polarized rosette architecture in human neural progenitors after physical or chemical disruption. Transient inhibition of C5aR1 during neurogenesis in developing mice led to behavioral abnormalities in both sexes and MRI-detected brain microstructural alterations, in studied males, demonstrating a requirement of C5aR1 signaling for appropriate brain development. This study thus identifies a functional role for C5a-C5aR1 signaling in mammalian neurogenesis and provides mechanistic insight into recently identified complement gene mutations and brain disorders.SIGNIFICANCE STATEMENT The complement system, traditionally known as a controller of innate immunity, now stands as a multifaceted signaling family with a broad range of physiological actions. These include roles in the brain, where complement activation is associated with diseases, including epilepsy and schizophrenia. This study has explored complement regulation of neurogenesis, identifying a novel relationship between the complement activation peptide C5a and the neural progenitor proliferation underpinning formation of the mammalian brain. C5a was identified as a regulator of cell polarity, with inhibition of C5a receptors during embryogenesis leading to abnormal brain development and behavioral deficits. This work demonstrates mechanisms through which dysregulation of complement causes developmental disease and highlights the potential risk of complement inhibition for therapeutic purposes in pregnancy.

Nuf and Rip11 requirement for polarity determinant recycling during Drosophila development

A tight relationship between apico-basal polarity and trafficking is essential for epithelial physiology and tissue homeostasis. Recent studies have described how some Rab GTPases, key components of the intracellular traffic machinery, contribute to the establishment of cell polarity in vertebrates. We have demonstrated a novel connection between cell polarity and trafficking: in Drosophila epithelia, the apical determinant aPKC is recycled via Rab11-Nuf-recycling endosomes to maintain cell polarity. Furthermore, the phosphorylation of Nuf by aPKC allows aPKC to control the sub-cellular localization of Nuf and its own membrane accumulation. Here we review these data and show the different contribution of the 2 Drosophila Rab11 adaptor proteins, Nuf and Rip11, to the maintenance of Drosophila embryonic ectoderm polarity.

14-3-3ζ promotes lung cancer cell invasion by increasing the Snail protein expression through atypical protein kinase C (aPKC)/NF-κB signaling

14-3-3ζ has been identified as a putative oncogene in several cancers, including non-small cell lung cancer (NSCLC). However, the mechanisms underlying its functions remain undefined. In this study, we show that overexpression of 14-3-3ζ was frequently detected in lung adenocarcinoma (LuAC) tissues and was significantly associated with lymph node metastasis and poor outcome. Functional studies demonstrated that 14-3-3ζ promoted migration and invasion in A549 cells, both of which were effectively inhibited when 14-3-3ζ was silenced with short hairpin RNA (shRNA). Furthermore, 14-3-3ζ-mediated invasion of cancer cells was found to upregulate Snail through the activation of atypical protein kinase C (aPKC). Activation of aPKCζ mediates this effect by stimulating NF-κB signaling. Our results identify a specific pathway by which 14-3-3ζ induces tumor invasion and provide insight into potential therapeutic approaches to target 14-3-3ζ-associated lung adenocarcinoma.

Spatiotemporal phosphoregulation of Lgl: Finding meaning in multiple on/off buttons

Intracellular asymmetries, often termed cell polarity, determine how cells organize and divide to ultimately control cell fate and shape animal tissues. The tumor suppressor Lethal giant larvae (Lgl) functions at the core of the evolutionarily conserved cell polarity machinery that controls apico-basal polarization. This function relies on its restricted basolateral localization via phosphorylation by aPKC. Here, we summarize the spatial and temporal control of Lgl during the cell cycle, highlighting two ideas that emerged from our recent findings: 1) Aurora A directly phosphorylates Lgl during symmetric division to couple reorganization of epithelial polarity with the cell cycle; 2) Phosphorylation of Lgl within three conserved serines controls its localization and function in a site-specific manner. Considering the importance of phosphorylation to regulate the concentration of Lgl at the plasma membrane, we will further discuss how it may work as an on-off switch for the interaction with cortical binding partners, with implications on epithelial polarization and spindle orientation.

Nuclear fallout provides a new link between aPKC and polarized cell trafficking

Background

Cell polarity, essential for cell physiology and tissue coherence, emerges as a consequence of asymmetric localization of protein complexes and directional trafficking of cellular components. Although molecules required in both processes are well known their relationship is still poorly understood.

Results

Here we show a molecular link between Nuclear Fallout (Nuf), an adaptor of Rab11-GTPase to the microtubule motor proteins during Recycling Endosome (RE) trafficking, and aPKC, a pivotal kinase in the regulation of cell polarity. We demonstrate that aPKC phosphorylates Nuf modifying its subcellular distribution. Accordingly, in aPKC mutants Nuf and Rab11 accumulate apically indicating altered RE delivery. We show that aPKC localization in the apico-lateral cortex is dynamic. When we block exocytosis, by means of exocyst-sec mutants, aPKC accumulates inside the cells. Moreover, apical aPKC concentration is reduced in nuf mutants, suggesting aPKC levels are maintained by recycling.

Conclusions

We demonstrate that active aPKC interacts with Nuf, phosphorylating it and, as a result, modifying its subcellular distribution. We propose a regulatory loop by which Nuf promotes aPKC apical recycling until sufficient levels of active aPKC are reached. Thus, we provide a novel link between cell polarity regulation and traffic control in epithelia.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0253-6) contains supplementary material, which is available to authorized users.

Establishing and maintaining cell polarity with mRNA localization in Drosophila

How cell polarity is established and maintained is an important question in diverse biological contexts. Molecular mechanisms used to localize polarity proteins to distinct domains are likely context-dependent and provide a feedback loop in order to maintain polarity. One such mechanism is the localized translation of mRNAs encoding polarity proteins, which will be the focus of this review and may play a more important role in the establishment and maintenance of polarity than is currently known. Localized translation of mRNAs encoding polarity proteins can be used to establish polarity in response to an external signal, and to maintain polarity by local production of polarity determinants. The importance of this mechanism is illustrated by recent findings, including orb2-dependent localized translation of aPKC mRNA at the apical end of elongating spermatid tails in the Drosophila testis, and the apical localization of stardust A mRNA in Drosophila follicle and embryonic epithelia.

Kibra and aPKC regulate starvation-induced autophagy in Drosophila

Autophagy is a bulk degradation system that functions in response to cellular stresses such as metabolic stress, endoplasmic reticulum stress, oxidative stress, and developmental processes. During autophagy, cytoplasmic components are captured in double-membrane vesicles called autophagosomes. The autophagosome fuses with the lysosome, producing a vacuole known as an autolysosome. The cellular components are degraded by lysosomal proteases and recycled. Autophagy is important for maintaining cellular homeostasis, and the process is evolutionarily conserved. Kibra is an upstream regulator of the hippo signaling pathway, which controls organ size by affecting cell growth, proliferation, and apoptosis. Kibra is mainly localized in the apical membrane domain of epithelial cells and acts as a scaffold protein. We found that Kibra is required for autophagy to function properly. The absence of Kibra caused defects in the formation of autophagic vesicles and autophagic degradation. We also found that the well-known cell polarity protein aPKC interacts with Kibra, and its activity affects autophagy upstream of Kibra. Constitutively active aPKC decreased autophagic vesicle formation and autophagic degradation. We confirmed the interaction between aPKC and Kibra in S2 cells and Drosophila larva. Taken together, our data suggest that Kibra and aPKC are essential for regulating starvation-induced autophagy.

Functional Analysis of Keratin-Associated Proteins in Intestinal Epithelia: Heat-Shock Protein Chaperoning and Kinase Rescue

A growing body of evidence from several laboratories points at nonmechanical functions of keratin intermediate filaments (IF), such as control of apoptosis, modulation of signaling, or regulation of innate immunity, among others. While these functions are generally assigned to the ability of IF to scaffold other proteins, direct mechanistic causal relationships between filamentous keratins and the observed effects of keratin knockout or mutations are still missing. We have proposed that the scaffolding of chaperones such as Hsp70/40 may be key to understand some IF nonmechanical functions if unique features or specificity of the chaperoning activity in the IF scaffold can be demonstrated. The same criteria of uniqueness could be applied to other biochemical functions of the IF scaffold. Here, we describe a subcellular fractionation technique based on established methods of keratin purification. The resulting keratin-enriched fraction contains several proteins tightly associated with the IF scaffold, including Hsp70/40 chaperones. Being nondenaturing, this fractionation method enables direct testing of chaperoning and other enzymatic activities associated with IF, as well as supplementation experiments to determine the need for soluble (cytosolic) proteins. This method also permits to analyze inhibitory activity of cytosolic proteins at independently characterized physiological concentrations. When used as complementary approaches to knockout, knockdown, or site-directed mutagenesis, these techniques are expected to shed light on molecular mechanisms involved in the effects of IF loss of function.