Mammalian PAR-1 determines epithelial lumen polarity by organizing the microtubule cytoskeleton

Polarity-regulating kinase partitioning-defective 1/microtubule affinity-regulating kinase 2 negatively regulates development of dendrites on hippocampal neurons

Neurons are highly polarized cells that possess two morphologically and functionally different types of protrusions, axons and dendrites, that function in the transmission and reception of neural signals, respectively. A great deal of attention has been paid to the specification and guidance of axons, but the mechanism of dendrite development remains mostly unknown. We report here that a polarity-regulating kinase, partitioning-defective 1 (Par1b)/microtubule affinity-regulating kinase 2 (MARK2), specifically regulates development of dendrites in hippocampal neurons. Ectopic expression of Par1b/MARK2 shortens the length and decreases branching of dendrites without significant effects on axons. Knockdown of endogenous Par1b/MARK2 by RNA interference stimulates dendrite development. Wnt stimulation and Dishevelled expression, both of which are known to induce dendrite development, induced recruitment of Par1b/MARK2 to the membrane fraction. Expression of a Par1b/MARK2 mutant, that contains a myristoylation signal and accumulates exclusively in membranes, does not affect dendrite development. In addition, Par1b/MARK2 efficiently phosphorylated MAP2, which is localized mainly in dendrites. These results indicate that Par1b/MARK2 negatively regulates dendrite development through phosphorylation of MAP2.

Roles of P21-activated kinases and associated proteins in epithelial wound healing

The primary function of epithelia is to provide a barrier between the extracellular environment and the interior of the body. Efficient epithelial repair mechanisms are therefore crucial for homeostasis. The epithelial wound-healing process involves highly regulated morphogenetic changes of epithelial cells that are driven by dynamic changes of the cytoskeleton. P21-activated kinases are serine/threonine kinases that have emerged as important regulators of the cytoskeleton. These kinases, which are activated downsteam of the Rho GTPases Rac and cd42, were initially mostly implicated in the regulation of cell migration. More recently, however, these kinases were shown to have many additional functions that are relevant to the regulation of epithelial wound healing. Here, we provide an overview of the morphogenetic changes of epithelial cells during wound healing and the many functions of p21-activated kinases in these processes.

PAR1 specifies ciliated cells in vertebrate ectoderm downstream of aPKC

Partitioning-defective 1 (PAR1) and atypical protein kinase C (aPKC) are conserved serine/threonine protein kinases implicated in the establishment of cell polarity in many species from yeast to humans. Here we investigate the roles of these protein kinases in cell fate determination in Xenopus epidermis. Early asymmetric cell divisions at blastula and gastrula stages give rise to the superficial (apical) and the deep (basal) cell layers of epidermal ectoderm. These two layers consist of cells with different intrinsic developmental potential, including superficial epidermal cells and deep ciliated cells. Our gain- and loss-of-function studies demonstrate that aPKC inhibits ciliated cell differentiation in Xenopus ectoderm and promotes superficial cell fates. We find that the crucial molecular substrate for aPKC is PAR1, which is localized in a complementary domain in superficial ectoderm cells. We show that PAR1 acts downstream of aPKC and is sufficient to stimulate ciliated cell differentiation and inhibit superficial epidermal cell fates. Our results suggest that aPKC and PAR1 function sequentially in a conserved molecular pathway that links apical-basal cell polarity to Notch signaling and cell fate determination. The observed patterning mechanism may operate in a wide range of epithelial tissues in many species.

Analysis of detergent-resistant membranes of Helicobacter pylori infected gastric adenocarcinoma cells reveals a role for MARK2/Par1b in CagA-mediated disruption of cellular polarity

Detergent-resistant membranes of eukaryotic cells are enriched in many important cellular signalling molecules and frequently targeted by bacterial pathogens. To learn more about pathogenic mechanisms of Helicobacter pylori and to elucidate novel effects on host epithelial cells, we investigated how bacterial co-cultivation changes the protein composition of detergent-resistant membranes of gastric adenocarcinoma (AGS) tissue culture cells. Using iTRAQ (isobaric tags for relative and absolute quantification) analysis we identified several cellular proteins, which are potentially related to H. pylori virulence. One of the proteins, which showed a significant infection-dependent increase in detergent resistance, was the polarity-associated serine/threonine kinase MARK2 (EMK1/Par-1b). We demonstrate that H. pylori causes the recruitment of MARK2 from the cytosol to the plasma membrane, where it colocalizes with the bacteria and interacts with CagA. Using Mardin Darby Canine Kidney (MDCK) monolayers and a three-dimensional MDCK tissue culture model we showed that association of CagA with MARK2 not only causes disruption of apical junctions, but also inhibition of tubulogenesis and cell differentiation.

Tight junction and polarity interaction in the transporting epithelial phenotype

Development of tight junctions and cell polarity in epithelial cells requires a complex cellular machinery to execute an internal program in response to ambient cues. Tight junctions, a product of this machinery, can act as gates of the paracellular pathway, fences that keep the identity of plasma membrane domains, bridges that communicate neighboring cells. The polarization internal program and machinery are conserved in yeast, worms, flies and mammals, and in cell types as different as epithelia, neurons and lymphocytes. Polarization and tight junctions are dynamic features that change during development, in response to physiological and pharmacological challenges and in pathological situations like infection.

Dynein modifiers in C. elegans: light chains suppress conditional heavy chain mutants

Cytoplasmic dynein is a microtubule-dependent motor protein that functions in mitotic cells during centrosome separation, metaphase chromosome congression, anaphase spindle elongation, and chromosome segregation. Dynein is also utilized during interphase for vesicle transport and organelle positioning. While numerous cellular processes require cytoplasmic dynein, the mechanisms that target and regulate this microtubule motor remain largely unknown. By screening a conditional Caenorhabditis elegans cytoplasmic dynein heavy chain mutant at a semipermissive temperature with a genome-wide RNA interference library to reduce gene functions, we have isolated and characterized twenty dynein-specific suppressor genes. When reduced in function, these genes suppress dynein mutants but not other conditionally mutant loci, and twelve of the 20 specific suppressors do not exhibit sterile or lethal phenotypes when their function is reduced in wild-type worms. Many of the suppressor proteins, including two dynein light chains, localize to subcellular sites that overlap with those reported by others for the dynein heavy chain. Furthermore, knocking down any one of four putative dynein accessory chains suppresses the conditional heavy chain mutants, suggesting that some accessory chains negatively regulate heavy chain function. We also identified 29 additional genes that, when reduced in function, suppress conditional mutations not only in dynein but also in loci required for unrelated essential processes. In conclusion, we have identified twenty genes that in many cases are not essential themselves but are conserved and when reduced in function can suppress conditionally lethal C. elegans cytoplasmic dynein heavy chain mutants. We conclude that conserved but nonessential genes contribute to dynein function during the essential process of mitosis.

Junctional adhesion molecule-A is critical for the formation of pseudocanaliculi and modulates E-cadherin expression in hepatic cells

Hepatocytes are polarized epithelial cells whose function depends upon their ability to distinguish between the apical and basolateral surfaces that are located at intercellular tight junctions. It has been proposed that the signaling cascades originating at these junctions influence cellular activity by controlling gene expression in the cell nucleus. To assess the validity of this proposal with regard to hepatocytes, we depleted expression of the tight junction protein junctional adhesion molecule-A (JAM-A) in the HepG2 human hepatocellular carcinoma cell line. Reduction of JAM-A resulted in a striking change in cell morphology, with cells forming sheets 1-2 cells thick instead of the normal multilayered clusters. In the absence of JAM-A, other tight junction proteins were mislocalized, and pseudocanaliculi, which form the apical face of the hepatocyte, were consequently absent. There was a strong transcriptional induction of the adherens junction protein E-cadherin in cells with reduced levels of JAM-A. This increase in E-cadherin was partially responsible for the observed alterations in cell morphology and mislocalization of tight junction proteins. We therefore propose the existence of a novel mechanism of cross-talk between specific components of tight and adherens junctions that can be utilized to regulate adhesion between hepatic cells.

Suppression of tubulin polymerization by the LKB1-microtubule-associated protein/microtubule affinity-regulating kinase signaling

LKB1, a tumor suppressor gene mutated in the Peutz-Jeghers syndrome, encodes a serine/threonine protein kinase. Recent biochemical studies have shown that LKB1 activates 14 AMP-activated protein kinase-related kinases including MARKs (microtubule-associated protein/microtubule affinity-regulating kinases) that regulate microtubule dynamics. Here we show in vitro that LKB1 phosphorylates and activates MARK2, which in turn phosphorylates microtubule-associated protein Tau at the KXGS motif and suppresses tubulin polymerization. In cells, forced expression of LKB1 suppresses microtubule regrowth, whereas LKB1 knockdown accelerates it. We further show that the phosphorylation of Tau by the LKB1-MARK signaling triggers proteasome-mediated degradation of Tau. These results indicate that LKB1 is involved in the regulation of microtubule dynamics through the activation of MARKs.

Helicobacter pylori CagA targets PAR1/MARK kinase to disrupt epithelial cell polarity

Helicobacter pylori cagA-positive strains are associated with gastritis, ulcerations and gastric adenocarcinoma. CagA is delivered into gastric epithelial cells and, on tyrosine phosphorylation, specifically binds and activates the SHP2 oncoprotein, thereby inducing the formation of an elongated cell shape known as the 'hummingbird' phenotype. In polarized epithelial cells, CagA also disrupts the tight junction and causes loss of apical-basolateral polarity. We show here that H. pylori CagA specifically interacts with PAR1/MARK kinase, which has an essential role in epithelial cell polarity. Association of CagA inhibits PAR1 kinase activity and prevents atypical protein kinase C (aPKC)-mediated PAR1 phosphorylation, which dissociates PAR1 from the membrane, collectively causing junctional and polarity defects. Because of the multimeric nature of PAR1 (ref. 14), PAR1 also promotes CagA multimerization, which stabilizes the CagA-SHP2 interaction. Furthermore, induction of the hummingbird phenotype by CagA-activated SHP2 requires simultaneous inhibition of PAR1 kinase activity by CagA. Thus, the CagA-PAR1 interaction not only elicits the junctional and polarity defects but also promotes the morphogenetic activity of CagA. Our findings revealed that PAR1 is a key target of H. pylori CagA in the disorganization of gastric epithelial architecture underlying mucosal damage, inflammation and carcinogenesis.

Anchoring of protein kinase A-regulatory subunit IIalpha to subapically positioned centrosomes mediates apical bile canalicular lumen development in response to oncostatin M but not cAMP

Oncostatin M and cAMP signaling stimulate apical surface-directed membrane trafficking and apical lumen development in hepatocytes, both in a protein kinase A (PKA)-dependent manner. Here, we show that oncostatin M, but not cAMP, promotes the A-kinase anchoring protein (AKAP)-dependent anchoring of the PKA regulatory subunit (R)IIalpha to subapical centrosomes and that this requires extracellular signal-regulated kinase 2 activation. Stable expression of the RII-displacing peptide AKAP-IS, but not a scrambled peptide, inhibits the association of RIIalpha with centrosomal AKAPs and results in the repositioning of the centrosome from a subapical to a perinuclear location. Concomitantly, common endosomes, but not apical recycling endosomes, are repositioned from a subapical to a perinuclear location, without significant effects on constitutive or oncostatin M-stimulated basolateral-to-apical transcytosis. Importantly, however, the expression of the AKAP-IS peptide completely blocks oncostatin M-, but not cAMP-stimulated apical lumen development. Together, the data suggest that centrosomal anchoring of RIIalpha and the interrelated subapical positioning of these centrosomes is required for oncostatin M-, but not cAMP-mediated, bile canalicular lumen development in a manner that is uncoupled from oncostatin M-stimulated apical lumen-directed membrane trafficking. The results also imply that multiple PKA-mediated signaling pathways control apical lumen development and that subapical centrosome positioning is important in some of these pathways.

A calcium- and calmodulin-dependent kinase Ialpha/microtubule affinity regulating kinase 2 signaling cascade mediates calcium-dependent neurite outgrowth

Calcium is a critical regulator of neuronal differentiation and neurite outgrowth during development, as well as synaptic plasticity in adulthood. Calcium- and calmodulin-dependent kinase I (CaMKI) can regulate neurite outgrowth; however, the signal transduction cascades that lead to its physiological effects have not yet been elucidated. CaMKIalpha was therefore used as bait in a yeast two-hybrid assay and microtubule affinity regulating kinase 2 (MARK2)/Par-1b was identified as an interacting partner of CaMKI in three independent screens. The interaction between CaMKI and MARK2 was confirmed in vitro and in vivo by coimmunoprecipitation. CaMKI binds MARK2 within its kinase domain, but only if it is activated by calcium and calmodulin. Expression of CaMKI and MARK2 in Neuro-2A (N2a) cells and in primary hippocampal neurons promotes neurite outgrowth, an effect dependent on the catalytic activities of these enzymes. In addition, decreasing MARK2 activity blocks the ability of the calcium ionophore ionomycin to promote neurite outgrowth. Finally, CaMKI phosphorylates MARK2 on novel sites within its kinase domain. Mutation of these phosphorylation sites decreases both MARK2 kinase activity and its ability to promote neurite outgrowth. Interaction of MARK2 with CaMKI results in a novel, calcium-dependent pathway that plays an important role in neuronal differentiation.

LKB1 and AMPK maintain epithelial cell polarity under energetic stress

LKB1 is mutated in both familial and spontaneous tumors, and acts as a master kinase that activates the PAR-1 polarity kinase and the adenosine 5′monophosphate–activated kinase (AMPK). This has led to the hypothesis that LKB1 acts as a tumor suppressor because it is required to maintain cell polarity and growth control through PAR-1 and AMPK, respectively. However, the genetic analysis of LKB1–AMPK signaling in vertebrates has been complicated by the existence of multiple redundant AMPK subunits. We describe the identification of mutations in the single Drosophila melanogaster AMPK catalytic subunit AMPKα. Surprisingly, ampkα mutant epithelial cells lose their polarity and overproliferate under energetic stress. LKB1 is required in vivo for AMPK activation, and lkb1 mutations cause similar energetic stress–dependent phenotypes to ampkα mutations. Furthermore, lkb1 phenotypes are rescued by a phosphomimetic version of AMPKα. Thus, LKB1 signals through AMPK to coordinate epithelial polarity and proliferation with cellular energy status, and this might underlie the tumor suppressor function of LKB1.

Par1b promotes hepatic-type lumen polarity in Madin Darby canine kidney cells via myosin II- and E-cadherin-dependent signaling

Kidney-derived Madin Darby canine kidney (MDCK) cells form lumina at their apices, and target luminal proteins to an intracellular vacuolar apical compartment (VAC) when prevented from polarizing. Hepatocytes, by contrast, organize their luminal surfaces (the bile canaliculi; BC) between their lateral membranes, and, when nonpolarized, they display an intracellular luminal compartment that is distinct from the VACs of MDCK cells. Overexpression of the serine/threonine kinase Par1b/EMK1/MARK2 induces BC-like lateral lumina and a hepatic-type intracellular luminal compartment in MDCK cells, suggesting a role for Par1b in the branching decision between kidney- and hepatic-type epithelial phenotypes. Here, we report that Par1b promotes lateral lumen polarity in MDCK cells independently of Ca(2+)-mediated cell-cell adhesion by inhibiting myosin II in a rho kinase-dependent manner. Polarization was inhibited by E-cadherin depletion but promoted by an adhesion-defective E-cadherin mutant. By contrast, apical surface formation in control MDCK cells required Ca(2+)-dependent cell-cell adhesion, but it occurred in the absence of E-cadherin. We propose that E-cadherin, when in an adhesion-incompetent state at the lateral domain, serves as targeting patch for the establishment of lateral luminal surfaces. E-cadherin depletion also reverted the hepatic-type intracellular luminal compartment in Par1b-MDCK cells to VACs characteristic of control MDCK cells, indicating a novel link between E-cadherin and luminal protein targeting.

Loss of the Par-1b/MARK2 polarity kinase leads to increased metabolic rate, decreased adiposity, and insulin hypersensitivity in vivo

Obesity is a major factor central to the development of insulin resistance and type 2 diabetes. The identification and characterization of genes involved in regulation of adiposity, insulin sensitivity, and glucose uptake are key to the design and development of new drug therapies for this disease. In this study, we show that the polarity kinase Par-1b/MARK2 is required for regulating glucose metabolism in vivo. Mice null for Par-1b were lean, insulin hypersensitive, resistant to high-fat diet-induced weight gain, and hypermetabolic. (18)F-FDG microPET and hyperinsulinemic-euglycemic clamp analyses demonstrated increased glucose uptake into white and brown adipose tissue, but not into skeletal muscle of Par-1b null mice relative to wild-type controls. Taken together, these data indicate that Par-1b is a regulator of glucose metabolism and adiposity in the whole animal and may be a valuable drug target for the treatment of both type 2 diabetes and obesity.

An oskar-dependent positive feedback loop maintains the polarity of the Drosophila oocyte

The localization of oskar mRNA to the posterior of the Drosophila oocyte defines the site of assembly of the pole plasm, which contains the abdominal and germline determinants 1, 2, 3. oskar mRNA localization requires the polarization of the microtubule cytoskeleton, which depends on the recruitment of PAR-1 to the posterior cortex in response to a signal from the follicle cells, where it induces an enrichment of microtubule plus ends 4, 5, 6, 7. Here, we show that overexpressed oskar mRNA localizes to the middle of the oocyte, as well as the posterior. This ectopic localization depends on the premature translation of Oskar protein, which recruits PAR-1 and microtubule-plus-end markers to the oocyte center instead of the posterior pole, indicating that Oskar regulates the polarity of the cytoskeleton. Oskar also plays a role in the normal polarization of the oocyte; mutants that disrupt oskar mRNA localization or translation strongly reduce the posterior recruitment of microtubule plus ends. Thus, oskar mRNA localization is required to stabilize and amplify microtubule polarity, generating a positive feedback loop in which Oskar recruits PAR-1 to the posterior to increase the microtubule cytoskeleton's polarization, which in turn directs the localization of more oskar mRNA.

Analysis of aPKClambda and aPKCzeta reveals multiple and redundant functions during vertebrate retinogenesis

Retinal lamination is known to depend on cell polarity and localized signaling. In vertebrates, atypical protein kinase C proteins, aPKClambda/iota and aPKCzeta, are essential for apical-basal cell polarity. However, it is not known to what extent functional redundancy has precluded a comprehensive functional characterization of aPKC signaling during vertebrate retinogenesis. Here, we show that aPKCs lambda and zeta are functionally redundant for multiple aspects of retinogenesis including mitotic division location and orientation, cell-type positioning, and retinal pigment epithelial (RPE) and photoreceptor cell morphogenesis. Genetic mosaic analyses demonstrate a cell-autonomous requirement of aPKCs for RPE and photoreceptor development, and a cell-non-cell-autonomous function that is intrinsic to the neural retina for cell-type positioning. Our observations uncover a previously unappreciated involvement of aPKCzeta during zebrafish retinogenesis and suggest that aPKC signaling primes the retinal environment for appropriate cell migration of post-mitotic progenitor cells but is not essential for correct cell-type specification.

Regulation of epithelial tight junction assembly and disassembly by AMP-activated protein kinase

AMP-activated protein kinase (AMPK) is a serine/threonine protein kinase that plays an important role in maintaining cellular energy balance. The activity of AMPK is modulated both by the cellular AMP-to-ATP ratio and by upstream kinases. Recently, AMPK was shown to be phosphorylated and activated by LKB1, a protein kinase that plays a conserved role in epithelial polarity regulation in mammals and Drosophila. Here, we investigate the involvement of AMPK in the regulation of epithelial tight junction assembly and cell polarization in MDCK cells. We show that the level of AMPK phosphorylation increases during calcium-induced tight junction assembly and cell polarization and that this increase depends on the kinase activity of LKB1. Expression of a kinase-dead mutant of AMPK inhibits tight junction assembly as indicated by measurement of transepithelial resistance and analysis of ZO-1 localization to the tight junction after calcium switch. Conversely, 5-aminoimidizole-4-carboxamide riboside, an activator of AMPK, promotes transepithelial resistance development and tight junction assembly upon calcium switch. Furthermore, 5-aminoimidizole-4-carboxamide riboside partially protects the tight junctions from disassembly induced by calcium depletion. These results support an important role of AMPK in the regulation of epithelial tight junction assembly and disassembly and suggest an intriguing link between cellular energy status and tight junction function.

Molecular Mechanism of Liver Development and Regeneration

The liver is the central organ for metabolism and has strong regenerative capability. Although the liver has been studied mostly biochemically and histopathologically, genetic studies using gene-targeting technology have identified a number of cytokines, intracellular signaling molecules, and transcription factors involved in liver development and regeneration. In addition, various in vitro systems such as fetal liver explant culture and primary culture of fetal liver cells have been established, and the combination of genetic and in vitro studies has accelerated investigation of liver development. Identification of the cell-surface molecules of liver progenitors has made it possible to identify and isolate liver progenitors, making the liver a unique model for stem cell biology. In this review, we summarize progresses in understanding liver development and regeneration.

Derepression of pathological cardiac genes by members of the CaM kinase superfamily

In response to pathologic stresses such as hypertension or myocardial infarction, the heart undergoes a remodeling process that is characterized by myocyte hypertrophy, myocyte death and fibrosis, resulting in impaired cardiac function and heart failure. Cardiac remodeling is associated with derepression of genes that contribute to disease progression. This review focuses on evidence linking members of the Ca(2+)/calmodulin-dependent protein kinase (CaMK) superfamily, specifically CaMKII, protein kinase D (PKD) and microtubule associated kinase (MARK), to stress-induced derepression of pathological cardiac gene expression through their effects on class IIa histone deacetylases (HDACs).

Extending from PARs in Caenorhabditis elegans to homologues in Haemonchus contortus and other parasitic nematodes

Signal transduction molecules play key roles in the regulation of developmental processes, such as morphogenesis, organogenesis and cell differentiation in all organisms. They are organized into 'pathways' that represent a coordinated network of cell-surface receptors and intracellular molecules, being involved in sensing environmental stimuli and transducing signals to regulate or modulate cellular processes, such as gene expression and cytoskeletal dynamics. A particularly important group of molecules implicated in the regulation of the cytoskeleton for the establishment and maintenance of cell polarity is the PAR proteins (derived from partition defective in asymmetric cell division). The present article reviews salient aspects of PAR proteins involved in the early embryonic development and morphogenesis of the free-living nematode Caenorhabditis elegans and some other organisms, with an emphasis on the molecule PAR-1. Recent advances in the knowledge and understanding of PAR-1 homologues from the economically important parasitic nematode, Haemonchus contortus, of small ruminants is summarized and discussed in the context of exploring avenues for future research in this area for parasitic nematodes.

AMP-activated protein kinase regulates the assembly of epithelial tight junctions

AMP activated protein kinase (AMPK), a sensor of cellular energy status in all eukaryotic cells, is activated by LKB1-dependent phosphorylation. Recent studies indicate that activated LKB1 induces polarity in epithelial cells and that this polarization is accompanied by the formation of tight junction structures. We wished to determine whether AMPK also contributes to the assembly of tight junctions in the epithelial cell polarization process. We found that AMPK is activated during calcium-induced tight junction assembly. Activation of AMPK by 5-aminoimidazole-4-carboxamide ribonucleoside facilitates tight junction assembly under conditions of normal extracellular Ca2+ concentrations and initiates tight junction assembly in the absence of Ca2+ as revealed by the relocation of zonula occludens 1, the establishment of transepithelial electrical resistance, and the paracellular flux assay. Expression of a dominant negative AMPK construct inhibits tight junction assembly in MDCK cells, and this defect in tight junction assembly can be partially ameliorated by rapamycin. These results suggest that AMPK plays a role in the regulation of tight junction assembly.

Distinct expression pattern of microtubule-associated protein/microtubule affinity-regulating kinase 4 in differentiated neurons

Protein kinases of the microtubule affinity-regulating kinase (MARK) family were originally discovered because of their ability to phosphorylate tau protein and related microtubule-associated proteins (MAPs), and their role in the establishment of cell polarity in different contexts. Recent papers have indicated that microtubule affinity-regulating kinase 4 (MARK4) is a gene that is finely regulated at transcriptional level and expressed in two spliced isoforms called MARK4L and MARK4S. We here describe the characterization of the mouse orthologue of the human MARK4 gene. Interestingly, MARK4S is predominantly expressed in the brain, whereas MARK4L shows lower transcript levels in this organ. Using MARK4 antibodies specific for each isoform, we found that both isoforms have an identical expression pattern in the mouse CNS, and are present in a number of neuronal populations. We also found that human microtubule affinity-regulating kinase 4S (hMARK4S), whose expression is not detectable in human neural progenitor cells (HNPCs) and NTera2 (NT2) cells, is up-regulated in both cell systems from the very early stages of neuronal differentiation. This indicates that neuronal commitment is marked by MARK4S up-regulation. In conclusion, this study provides the first direct evidence suggesting that MARK4 is a neuron-specific marker in the CNS, and the up-regulation of MARK4S during neuronal differentiation suggests that it plays a specialized role in neurons.

Signaling from MARK to Tau: Regulation, Cytoskeletal Crosstalk, and Pathological Phosphorylation

The hyperphosphorylation of tau is an early step in the degeneration of neurons in Alzheimer's disease and other tauopathies. Of particular importance is the phosphorylation of tau in the repeat domain which detaches tau from microtubules. This makes microtubules dynamic for their role in differentiation and neurite outgrowth, and it controls the level of tau on the microtubule surface which keeps the tracks clear for axonal transport. However, the detachment of tau from microtubules can also initiate the reactions that lead to pathological aggregation into neurofibrillary tangles. Phosphorylation of tau in the repeat domain is achieved by the kinase MARK/Par-1, a member of the calcium/calmodulin-dependent protein kinase group of kinases. In this report, we focus on the modes of MARK regulation. MARK contains several domains which offer multiple ways of regulation by posttranslational modification (e.g. phosphorylation), interactions with scaffolding proteins and subcellular targeting (e.g. 14-3-3), and interactions with other proteins. We consider in particular the interactions between MARK and other kinases, notably MARKK/TAO-1 and PAK5. MARKK (a member of the Ste20 family of kinases) activates MARK by phosphorylating it at a critical threonine residue within the activation loop. Activated MARK in turn phosphorylates tau, causes its detachment from microtubules and renders them labile. PAK5 inactivates MARK, not by phosphorylation, but by binding to the catalytic domain. PAK5 contributes to microtubule stability by preventing the MARK-induced phosphorylation of tau; conversely, PAK5 contributes to actin dynamics, presumably through the activation of cofilin, an F-actin severing protein. Thus, MARK and its regulators MARKK and PAK5 appear to mediate the crosstalk between the actin and microtubule cytoskeleton in an antagonistic fashion.

Spatial regulation of cytokinesis by the Kin1 and Pom1 kinases in fission yeast

Cytokinesis requires a tight spatio-temporal coordination with mitosis to ensure proper segregation of the genetic information during cell division. In fission yeast, an actomyosin contractile ring is assembled in mitosis and dictates the site of cytokinesis. Here we investigated the functions of Kin1 and Pom1, two conserved fission yeast kinases, in cell division. We found that kin1Delta is synthetically lethal with pom1Delta because double mutant cells fail to spatially organize the actomyosin ring during mitosis, leading to aberrant septum synthesis and accumulation of post-mitotic nuclei in the same cell compartment. Assembly of an Rlc1-GFP ring in the cell center at mitosis is also compromised. Similar cytokinetic defects are observed in a tea1Delta kin1Delta mutant. Furthermore, aberrant septation and nuclear accumulation are observed in a pom1Delta strain in which the Kin1 level is either down or up-regulated. Thus, a tight control of Kin1 level is critical for ensuring accurate cell division in a pom1Delta background. Since none of the kinases can substitute for each other, Kin1 and Pom1 have distinct complementary functions. We show that Kin1 is required for F-actin polarization in interphase and after completion of mitosis and this function may be essential for cytokinesis in a pom1Delta background.

Regulation of the polarity kinases PAR-1/MARK by 14-3-3 interaction and phosphorylation

Members of the PAR-1/MARK kinase family play critical roles in polarity and cell cycle control and are regulated by 14-3-3 scaffolding proteins, as well as the LKB1 tumour suppressor kinase and atypical protein kinase C (PKC). In this study, we initially investigated the mechanism underlying the interaction of mammalian MARK3 with 14-3-3. We demonstrate that 14-3-3 binding to MARK3 is dependent on phosphorylation, and necessitates the phosphate-binding pocket of 14-3-3. We found that interaction with 14-3-3 was not mediated by the previously characterised MARK3 phosphorylation sites, which led us to identify 15 novel sites of phosphorylation. Single point mutation of these sites, as well as the previously identified LKB1-(T211) and the atypical PKC sites (T564/S619), did not disrupt 14-3-3 binding. However, a mutant in which all 17 phosphorylation sites had been converted to alanine residues (termed 17A-MARK3), was no longer able to bind 14-3-3. Wild-type MARK3 was present in both the cytoplasm and plasma membrane, whereas the 17A-MARK3 mutant was strikingly localised at the plasma membrane. We provide data indicating that the membrane localisation of MARK3 required a highly conserved C-terminal domain, which has been termed kinase-associated domain-1 (KA-1). We also show that dissociation of 14-3-3 from MARK3 did not affect catalytic activity, and that a MARK3 mutant, which could not interact with 14-3-3, was normally active. Finally, we establish that there are significant differences in the subcellular localisation of MARK isoforms, as well as in the impact that atypical PKC overexpression has on 14-3-3 binding and localisation. Collectively, these results indicate that 14-3-3 binding to MARK isoforms is mediated by multiple phosphorylation sites, and serves to anchor MARK isoforms in the cytoplasm.

Regulatory dissociation of Tctex-1 light chain from dynein complex is essential for the apical delivery of rhodopsin

Post-Golgi to apical surface delivery in polarized epithelial cells requires the cytoplasmic dynein motor complex. However, the nature of dynein-cargo interactions and their underlying regulation are largely unknown. Previous studies have shown that the apical surface targeting of rhodopsin requires the dynein light chain, Tctex-1, which binds directly to both dynein intermediate chain (IC) and rhodopsin. In this report, we show that the S82E mutant of Tctex-1, which mimics Tctex-1 phosphorylated at serine 82, has a reduced affinity for dynein IC but not for rhodopsin. Velocity sedimentation experiments further suggest that S82E is not incorporated into the dynein complex. The dominant-negative effect of S82E causes rhodopsin mislocalization in polarized Madin-Darby canine kidney (MDCK) cells. The S82A mutant, which mimics dephosphorylated Tctex-1, can be incorporated into dynein complex but is impaired in its release. Expression of S82A also causes disruption of the apical localization of rhodopsin in MDCK cells. Taken together, these results suggest that the dynein complex disassembles to release cargo due to the specific phosphorylation of Tctex-1 at the S82 residue and that this process is critical for the apical delivery of membrane cargoes.

Apical targeting of syntaxin 3 is essential for epithelial cell polarity

In polarized epithelial cells, syntaxin 3 localizes to the apical plasma membrane and is involved in membrane fusion of apical trafficking pathways. We show that syntaxin 3 contains a necessary and sufficient apical targeting signal centered around a conserved FMDE motif. Mutation of any of three critical residues within this motif leads to loss of specific apical targeting. Modeling based on the known structure of syntaxin 1 revealed that these residues are exposed on the surface of a three-helix bundle. Syntaxin 3 targeting does not require binding to Munc18b. Instead, syntaxin 3 recruits Munc18b to the plasma membrane. Expression of mislocalized mutant syntaxin 3 in Madin-Darby canine kidney cells leads to basolateral mistargeting of apical membrane proteins, disturbance of tight junction formation, and loss of ability to form an organized polarized epithelium. These results indicate that SNARE proteins contribute to the overall specificity of membrane trafficking in vivo, and that the polarity of syntaxin 3 is essential for epithelial cell polarization.

The PAR-aPKC system: lessons in polarity

Ten years ago, par-1 and par-3 were cloned as two of the six par genes essential for the asymmetric division of the Caenorhabditis elegans zygote. PAR-1 is a protein kinase, whereas PAR-3 is a PDZ-domain-containing scaffold protein. Work over the past decade has shown that they are part of an evolutionarily conserved PAR-aPKC system involved in cell polarity in various biological contexts. Recent progress has illustrated the common principle that the PAR-aPKC system is the molecular machinery that converts initial polarity cues in the establishment of complementary membrane domains along the polarity axis. In most cases, this is achieved by mutually antagonistic interactions between the aPKC-PAR-3-PAR-6 complex and PAR-1 or PAR2 located opposite. However, accumulating evidence has also revealed that mechanisms by which the asymmetrically localized components of the PAR-aPKC system are linked with other cellular machinery for developing polarity are divergent depending on the cell type.

MARK2/EMK1/Par-1Balpha phosphorylation of Rab11-family interacting protein 2 is necessary for the timely establishment of polarity in Madin-Darby canine kidney cells

Rab11a, myosin Vb, and the Rab11-family interacting protein 2 (FIP2) regulate plasma membrane recycling in epithelial cells. This study sought to characterize more fully Rab11-FIP2 function by identifying kinase activities modifying Rab11-FIP2. We have found that gastric microsomal membrane extracts phosphorylate Rab11-FIP2 on serine 227. We identified the kinase that phosphorylated Rab11-FIP2 as MARK2/EMK1/Par-1Balpha (MARK2), and recombinant MARK2 phosphorylated Rab11-FIP2 only on serine 227. We created stable Madin-Darby canine kidney (MDCK) cell lines expressing enhanced green fluorescent protein-Rab11-FIP2 wild type or a nonphosphorylatable mutant [Rab11-FIP2(S227A)]. Analysis of these cell lines demonstrates a new role for Rab11-FIP2 in addition to that in the plasma membrane recycling system. In calcium switch assays, cells expressing Rab11-FIP2(S227A) showed a defect in the timely reestablishment of p120-containing junctional complexes. However, Rab11-FIP2(S227A) did not affect localization with recycling system components or the normal function of apical recycling and transcytosis pathways. These results indicate that phosphorylation of Rab11-FIP2 on serine 227 by MARK2 regulates an alternative pathway modulating the establishment of epithelial polarity.

The C. elegans MELK ortholog PIG-1 regulates cell size asymmetry and daughter cell fate in asymmetric neuroblast divisions

In the nematode Caenorhabditis elegans, neurons are generated from asymmetric divisions in which a mother cell divides to produce daughters that differ in fate. Here, we demonstrate that the gene pig-1 regulates the asymmetric divisions of neuroblasts that divide to produce an apoptotic cell and either a neural precursor or a neuron. In pig-1 mutants, these neuroblasts divide to produce daughters that are more equal in size, and their apoptotic daughters are transformed into their sisters, leading to the production of extra neurons. PIG-1 is orthologous to MELK, a conserved member of the polarity-regulating PAR-1/Kin1/SAD-1 family of serine/threonine kinases. Although MELK has been implicated in regulating the cell cycle, our data suggest that PIG-1, like other PAR-1 family members, regulates cell polarity.

PAR1b promotes cell-cell adhesion and inhibits dishevelled-mediated transformation of Madin-Darby canine kidney cells

Mammalian Par1 is a family of serine/threonine kinases comprised of four homologous isoforms that have been associated with tumor suppression and differentiation of epithelial and neuronal cells, yet little is known about their cellular functions. In polarizing kidney epithelial (Madin-Darby canine kidney [MDCK]) cells, the Par1 isoform Par1b/MARK2/EMK1 promotes the E-cadherin-dependent compaction, columnarization, and cytoskeletal organization characteristic of differentiated columnar epithelia. Here, we identify two functions of Par1b that likely contribute to its role as a tumor suppressor in epithelial cells. 1) The kinase promotes cell-cell adhesion and resistance of E-cadherin to extraction by nonionic detergents, a measure for the association of the E-cadherin cytoplasmic domain with the actin cytoskeleton, which is critical for E-cadherin function. 2) Par1b attenuates the effect of Dishevelled (Dvl) expression, an inducer of wnt signaling that causes transformation of epithelial cells. Although Dvl is a known Par1 substrate in vitro, we determined, after mapping the PAR1b-phosphorylation sites in Dvl, that PAR1b did not antagonize Dvl signaling by phosphorylating the wnt-signaling molecule. Instead, our data suggest that both proteins function antagonistically to regulate the assembly of functional E-cadherin-dependent adhesion complexes.

PAR proteins and the cytoskeleton: a marriage of equals

The PAR proteins are a group of widely conserved regulators of polarity, many of which are asymmetrically localized in polarized cells. Recent work shows that distinct modes of actomyosin- and microtubule-based transport contribute to the establishment of PAR asymmetries in different cell types. Cross-regulatory interactions among PAR proteins and with other conserved polarity complexes stabilize asymmetries once they form, and shape the evolution of PAR protein distributions in response to cytoskeletal transport or other polarizing inputs. The PAR proteins in turn modulate the actomyosin and microtubule cytoskeletons. In some cases, this is a form of feedback control, central to the establishment and maintenance of PAR asymmetries. In others, it underlies the elaboration of functional cell polarity.

Similarities and differences between the Wnt and reelin pathways in the forming brain

One of the key features in development is the reutilization of successful signaling pathways. Here, we emphasize the involvement of the Wnt pathway, one of the five kinds of signal transduction pathway predominating early embryonic development of all animals, in regulating the formation of brain structure. We discuss the interrelationships between the Wnt and reelin pathways in the regulation of cortical layering. We summarize data emphasizing key molecules, which, when mutated, result in abnormal brain development. This integrated view, which is based on conservation of pathways, reveals the relative position of participants in the pathway, points to control mechanisms, and allows raising testable working hypotheses. Nevertheless, although signaling pathways are highly conserved from flies to humans, the overall morphology is not. We propose that future studies directed at understanding of diversification will provide fruitful insights on mammalian brain formation.

Tight junctions: molecular architecture and function

Tight junctions are the most apical component of the epithelial junctional complex and are crucial for the formation and functioning of epithelial and endothelial barriers. They regulate selective diffusion of ions and solutes along the paracellular pathway and restrict apical/basolateral intramembrane diffusion of lipids. Research over the past years provided much insight into the molecular composition of tight junctions, and we are starting to understand the mechanisms that permit selective paracellular diffusion. Moreover, a complex network of proteins has been identified at tight junctions that is based on cytoskeleton-linked adaptors that recruit and thereby often regulate different types of signaling components that regulate epithelial proliferation, differentiation, and polarization.

Mammalian tight junctions in the regulation of epithelial differentiation and proliferation

Tight junctions are important for the permeability properties of epithelial and endothelial barriers as they restrict diffusion along the paracellular space. Recent observations have revealed that tight junctions also function in the regulation of epithelial proliferation and differentiation. They harbour evolutionarily conserved protein complexes that regulate polarisation and junction assembly. Tight junctions also recruit signalling proteins that participate in the regulation of cell proliferation and differentiation. These signalling proteins include components that affect established signalling cascades and dual localisation proteins that can associate with junctions as well as travel to the nucleus where they regulate gene expression.

How to induce non-polarized cells of hepatic origin to express typical hepatocyte polarity: generation of new highly polarized cell models with developed and functional bile canaliculi

Few in vitro models expressing complex hepatocyte polarity are available. We used the unpolarized rat Fao cell line to isolate the polarized WIF-B line. These complex rat-human hybrid cells form functional simple bile canaliculi. To obtain Fao-derived polarized models with a simpler chromosome content and developed bile canaliculi, we employed two approaches. Partial success was achieved with monochromosomal hybrids. As shown by the immunolocalization of apical, basolateral, and tight-junctional proteins, monochromosomal hybrid 11-3 cells were polarized. They formed simple functional bile canaliculi and transiently expressed the typical polarity of simple epithelial cells. One subclone blocked in this polarity state was isolated. A more robust approach was provided by spheroid culture, a three-dimensional system that strengthens cell-cell contacts. Transient spheroid culture induced irreversible polarization of Fao cells. This induction occurred in most spheroids (approximately 1% of the cells). From populations enriched in stably polarized cells, we generated new polarized cell models, designated Can. Can 3-1 cells formed simple functional bile canaliculi when plated at high density. Regardless of plating density, Can 9 and Can 10 cells formed long tubular branched canaliculi competent for vectorial transport of organic anions and bile acids, and involving several dozen adjacent cells. Thus, we have generated new cell models stably expressing typical hepatocyte polarity. Among these models, Can 9 and Can 10 are the first capable of forming functional, highly developed bile canaliculi similar to those formed in vivo.

Rab11a and myosin Vb are required for bile canalicular formation in WIF-B9 cells

Hepatocytes polarize by forming functionally distinct sinusoidal (basolateral) and canalicular (apical) plasma membrane domains. Two distinct routes are used for delivery of membrane proteins to the canaliculus. Proteins having glycosylphosphatidylinositol anchors or single transmembrane domains are targeted to the sinusoidal plasma membrane from where they transcytose to the canalicular domain. In contrast, apical ATP-binding-cassette (ABC) transporters, which are required for energy-dependent biliary secretion of bile acids (ABCB11), phospholipids (ABCB4), and nonbile acid organic anions (ABCC2), lack initial residence in the basolateral plasma membrane and traffic directly from Golgi membranes to the canalicular membrane. While investigating mechanisms of apical targeting in WIF-B9 cells, a polarized hepatic epithelial cell line, we observed that rab11a is required for canalicular formation. Knockdown of rab11a or overexpression of the rab11a-GDP locked form prevented canalicular formation as did overexpression of the myosin Vb motorless tail domain. In WIF-B9 cells, which lack bile canaliculi, apical ABC transporters colocalized with transcytotic membrane proteins in rab11a-containing endosomes and, unlike the transcytotic markers, did not distribute to the plasma membrane. We propose that polarization of hepatocytes (i.e., canalicular biogenesis) requires recruitment of rab11a and myosin Vb to intracellular membranes that contain apical ABC transporters and transcytotic markers, permitting their targeting to the plasma membrane. In this model, polarization is initiated upon delivery of rab11a-myosin Vb-containing membranes to the surface, which causes plasma membrane at the site of delivery to differentiate into apical domain (bile canaliculus).

Differential roles for actin polymerization and a myosin II motor in assembly of the epithelial apical junctional complex

Differentiation and polarization of epithelial cells depends on the formation of the apical junctional complex (AJC), which is composed of the tight junction (TJ) and the adherens junction (AJ). In this study, we investigated mechanisms of actin reorganization that drive the establishment of AJC. Using a calcium switch model, we observed that formation of the AJC in T84 intestinal epithelial cells began with the assembly of adherens-like junctions followed by the formation of TJs. Early adherens-like junctions and TJs readily incorporated exogenous G-actin and were disassembled by latrunculin B, thus indicating dependence on continuous actin polymerization. Both adherens-like junctions and TJs were enriched in actin-related protein 3 and neuronal Wiskott-Aldrich syndrome protein (N-WASP), and their assembly was prevented by the N-WASP inhibitor wiskostatin. In contrast, the formation of TJs, but not adherens-like junctions, was accompanied by recruitment of myosin II and was blocked by inhibition of myosin II with blebbistatin. In addition, blebbistatin inhibited the ability of epithelial cells to establish a columnar phenotype with proper apico-basal polarity. These findings suggest that actin polymerization directly mediates recruitment and maintenance of AJ/TJ proteins at intercellular contacts, whereas myosin II regulates cell polarization and correct positioning of the AJC within the plasma membrane.

Organization of vesicular trafficking in epithelia

Experiments using mammalian epithelial cell lines have elucidated biosynthetic and recycling pathways for apical and basolateral plasma-membrane proteins, and have identified components that guide apical and basolateral proteins along these pathways. These components include apical and basolateral sorting signals, adaptors for basolateral signals, and docking and fusion proteins for vesicular trafficking. Recent live-cell-imaging studies provide a real-time view of sorting processes in epithelial cells, including key roles for actin, microtubules and motors in the organization of post-Golgi trafficking.

Golgi positioning: are we looking at the right MAP?

One of the characteristics of the mammalian Golgi is its position adjacent to the nucleus. This characteristic is maintained through the action of the microtubule (MT) minus end–directed motor dynein and MT-associated proteins (MAPs). Recent findings suggest that GMAP-210, a member of the golgin family of proteins, may help to link Golgi membranes and vesicles with the MT cytoskeleton. However, there are good grounds to doubt that either GMAP-210 or its yeast homologue Rud3p is a MAP. Instead, they appear to function in vesicle trafficking events at the Golgi together with the GTPase ARF1 and a small membrane protein, Erv14. As such, the interesting question of how the Golgi interacts with MTs may well remain open to further investigation.

The yeast par-1 homologs kin1 and kin2 show genetic and physical interactions with components of the exocytic machinery

Kin1 and Kin2 are Saccharomyces cerevisiae counterparts of Par-1, the Caenorhabditis elegans kinase essential for the establishment of polarity in the one cell embryo. Here, we present evidence for a novel link between Kin1, Kin2, and the secretory machinery of the budding yeast. We isolated KIN1 and KIN2 as suppressors of a mutant form of Rho3, a Rho-GTPase acting in polarized trafficking. Genetic analysis suggests that KIN1 and KIN2 act downstream of the Rab-GTPase Sec4, its exchange factor Sec2, and several components of the vesicle tethering complex, the Exocyst. We show that Kin1 and Kin2 physically interact with the t-SNARE Sec9 and the Lgl homologue Sro7, proteins acting at the final stage of exocytosis. Structural analysis of Kin2 reveals that its catalytic activity is essential for its function in the secretory pathway and implicates the conserved 42-amino acid tail at the carboxy terminal of the kinase in autoinhibition. Finally, we find that Kin1 and Kin2 induce phosphorylation of t-SNARE Sec9 in vivo and stimulate its release from the plasma membrane. In summary, we report the finding that yeast Par-1 counterparts are associated with and regulate the function of the exocytic apparatus via phosphorylation of Sec9.

Epithelial trafficking: new routes to familiar places

Research carried out in mammalian epithelial cell systems over the past 25 years has delineated pathways and sorting signals involved in polarized delivery of plasma membrane proteins. Recently some progress has been made in the identification of mechanisms underlying this polarized trafficking and in the visualization of trafficking routes in live cells. A promising area of research is the study of trafficking functions of novel polarity genes identified in Drosophila and Caenorhabditis elegans.

Polarized sorting in epithelial cells: raft clustering and the biogenesis of the apical membrane

Polarized cells establish and maintain functionally distinct surface domains by an elaborate sorting process, which ensures accurate delivery of biosynthetic cargo to different parts of the plasma membrane. This is particularly evident in polarized epithelial cells, which have been used as a model system for studies of sorting mechanisms. The clustering of lipid rafts through the oligomerization of raft components could be utilized for segregating apical from basolateral cargo and for the generation of intracellular transport carriers. Besides functioning in polarized sorting in differentiated cells, raft clustering might also play an important role in the biogenesis of apical membrane domains during development.

MARK/PAR1 kinase is a regulator of microtubule-dependent transport in axons

Microtubule-dependent transport of vesicles and organelles appears saltatory because particles switch between periods of rest, random Brownian motion, and active transport. The transport can be regulated through motor proteins, cargo adaptors, or microtubule tracks. We report here a mechanism whereby microtubule associated proteins (MAPs) represent obstacles to motors which can be regulated by microtubule affinity regulating kinase (MARK)/Par-1, a family of kinases that is known for its involvement in establishing cell polarity and in phosphorylating tau protein during Alzheimer neurodegeneration. Expression of MARK causes the phosphorylation of MAPs at their KXGS motifs, thereby detaching MAPs from the microtubules and thus facilitating the transport of particles. This occurs without impairing the intrinsic activity of motors because the velocity during active movement remains unchanged. In primary retinal ganglion cells, transfection with tau leads to the inhibition of axonal transport of mitochondria, APP vesicles, and other cell components which leads to starvation of axons and vulnerability against stress. This transport inhibition can be rescued by phosphorylating tau with MARK.

Par-1 promotes a hepatic mode of apical protein trafficking in MDCK cells

Simple (i.e., nonstratified) epithelial cells use two different routes to target their newly synthesized luminal plasma membrane proteins to the cell surface: a direct route from the Golgi complex, as in the kidney-derived MDCK cell line, or an indirect route that involves a intermediate stop at the ab-luminal (basolateral) membrane, as in hepatocytes. The mechanisms or proteins responsible for these different protein targeting strategies are not known. Here, we show that increased expression of EMK1, a mammalian ortholog of Caenorhabditis elegans Par-1, in MDCK cells promotes a switch from a direct to a transcytotic mode of apical protein delivery and other trafficking changes typical of hepatocytes. These results, together with our recent demonstration that PAR-1 promotes morphological features of hepatocytes in MDCK cells, indicate that Par-1 modulates the developmental decision to build a columnar versus a hepatic epithelial cell. To our knowledge, Par-1 is the first gene assigned to this task in epithelial morphogenesis.

aPKC Acts Upstream of PAR-1b in Both the Establishment and Maintenance of Mammalian Epithelial Polarity

BACKGROUND

aPKC and PAR-1 are required for cell polarity in various contexts. In mammalian epithelial cells, aPKC localizes at tight junctions (TJs) and plays an indispensable role in the development of asymmetric intercellular junctions essential for the establishment and maintenance of apicobasal polarity. On the other hand, one of the mammalian PAR-1 kinases, PAR-1b/EMK1/MARK2, localizes to the lateral membrane in a complimentary manner with aPKC, but little is known about its role in apicobasal polarity of epithelial cells as well as its functional relationship with aPKC.

RESULTS

We demonstrate that PAR-1b is essential for the asymmetric development of membrane domains of polarized MDCK cells. Nonetheless, it is not required for the junctional localization of aPKC nor the formation of TJs, suggesting that PAR-1b works downstream of aPKC during epithelial cell polarization. On the other hand, aPKC phosphorylates threonine 595 of PAR-1b and enhances its binding with 14-3-3/PAR-5. In polarized MDCK cells, T595 phosphorylation and 14-3-3 binding are observed only in the soluble form of PAR-1b, and okadaic acid treatment induces T595-dependent dissociation of PAR-1b from the lateral membrane. Furthermore, T595A mutation induces not only PAR-1b leakage into the apical membrane, but also abnormal development of membrane domains. These results suggest that in polarized epithelial cells, aPKC phosphorylates PAR-1b at TJs, and in cooperation with 14-3-3, promotes the dissociation of PAR-1b from the lateral membrane to regulate PAR-1b activity for the membrane domain development.

CONCLUSIONS

These results suggest that mammalian aPKC functions upstream of PAR-1b in both the establishment and maintenance of epithelial cell polarity.