Establishment of the PAR-1 cortical gradient by the aPKC-PRBH circuit

Oligomerization and positive feedback on membrane recruitment encode dynamically stable PAR-3 asymmetries in the C. elegans zygote

Studies of PAR polarity have emphasized a paradigm in which mutually antagonistic PAR proteins form complementary polar domains in response to transient cues. A growing body of work suggests that the oligomeric scaffold PAR-3 can form unipolar asymmetries without mutual antagonism, but how it does so is largely unknown. Here we combine single molecule analysis and modeling to show how the interplay of two positive feedback loops promote dynamically stable unipolar PAR-3 asymmetries in early C. elegans embryos. First, the intrinsic dynamics of PAR-3 membrane binding and oligomerization encode negative feedback on PAR-3 dissociation. Second, membrane-bound PAR-3 promotes its own recruitment through a mechanism that requires the anterior polarity proteins CDC-42, PAR-6 and PKC-3. Using a kinetic model tightly constrained by our experimental measurements, we show that these two feedback loops are individually required and jointly sufficient to encode dynamically stable and locally inducible unipolar PAR-3 asymmetries in the absence of posterior inhibition. Given the central role of PAR-3, and the conservation of PAR-3 membrane-binding, oligomerization, and core interactions with PAR-6/aPKC, these results have widespread implications for PAR-mediated polarity in metazoa.

Optimized PAR-2 RING dimerization mediates cooperative and selective membrane binding for robust cell polarity

Cell polarity networks are defined by quantitative features of their constituent feedback circuits, which must be tuned to enable robust and stable polarization, while also ensuring that networks remain responsive to dynamically changing cellular states and/or spatial cues during development. Using the PAR polarity network as a model, we demonstrate that these features are enabled by the dimerization of the polarity protein PAR-2 via its N-terminal RING domain. Combining theory and experiment, we show that dimer affinity is optimized to achieve dynamic, selective, and cooperative binding of PAR-2 to the plasma membrane during polarization. Reducing dimerization compromises positive feedback and robustness of polarization. Conversely, enhanced dimerization renders the network less responsive due to kinetic trapping of PAR-2 on internal membranes and reduced sensitivity of PAR-2 to the anterior polarity kinase, aPKC/PKC-3. Thus, our data reveal a key role for a dynamically oligomeric RING domain in optimizing interaction affinities to support a robust and responsive cell polarity network, and highlight how optimization of oligomerization kinetics can serve as a strategy for dynamic and cooperative intracellular targeting.

Temporally distinct roles of Aurora A in polarization of the C. elegans zygote

During asymmetric cell division, cell polarity is coordinated with the cell cycle to allow proper inheritance of cell fate determinants and the generation of cellular diversity. In the Caenorhabditis elegans zygote, polarity is governed by evolutionarily conserved Partitioning-defective (PAR) proteins that segregate to opposing cortical domains to specify asymmetric cell fates. Timely establishment of PAR domains requires a cell cycle kinase, Aurora A (AIR-1 in C. elegans). Aurora A depletion by RNAi causes a spectrum of phenotypes including reversed polarity, excess posterior domains and no posterior domain. How depletion of a single kinase can cause seemingly opposite phenotypes remains obscure. Using an auxin-inducible degradation system and drug treatments, we found that AIR-1 regulates polarity differently at different times of the cell cycle. During meiosis I, AIR-1 acts to prevent later formation of bipolar domains, whereas in meiosis II, AIR-1 is necessary to recruit PAR-2 onto the membrane. Together, these data clarify the origin of multiple polarization phenotypes in RNAi experiments and reveal multiple roles of AIR-1 in coordinating PAR protein localization with cell cycle progression.

Temporally distinct roles of Aurora A in polarization of the C. elegans zygote

During asymmetric cell division, coordination of cell polarity and the cell cycle is critical for proper inheritance of cell fate determinants and generation of cellular diversity. In Caenorhabditis elegans (C. elegans), polarity is established in the zygote and is governed by evolutionarily conserved Partitioning defective (PAR) proteins that localize to distinct cortical domains. At the time of polarity establishment, anterior and posterior PARs segregate to opposing cortical domains that specify asymmetric cell fates. Timely establishment of these PAR domains requires a cell cycle kinase, Aurora A (AIR-1 in C.elegans). Aurora A depletion by RNAi causes a spectrum of phenotypes including no posterior domain, reversed polarity, and excess posterior domains. How depletion of a single kinase can cause seemingly opposite phenotypes remains obscure. Using an auxin-inducible degradation system, drug treatments, and high-resolution microscopy, we found that AIR-1 regulates polarity via distinct mechanisms at different times of the cell cycle. During meiosis I, AIR-1 acts to prevent the formation of bipolar domains, while in meiosis II, AIR-1 is necessary to recruit PAR-2 onto the membrane. Together these data clarify the origin of the multiple polarization phenotypes observed in RNAi experiments and reveal multiple roles of AIR-1 in coordinating PAR protein localization with the progression of the cell cycle.

Single-molecule FISH in C. elegans embryos reveals early embryonic expression dynamics of par-2 , lgl-1 and chin-1 and possible differences between hyper-diverged strains

Wild C. elegans strains harbor natural variation in developmental pathways, but investigating these differences requires precise and well-powered phenotyping methods. Here we employ a microfluidics platform for single-molecule FISH to simultaneously visualize the transcripts of three genes in embryos of two distinct strains. We capture transcripts at high resolution by developmental stage in over one hundred embryos of each strain and observe wide-scale conservation of expression, but subtle differences in par-2 and chin-1 abundance and rate of change. As both genes reside in a genomic interval of hyper-divergence, these results may reflect consequences of pathway evolution over long timescales.

Minimal functional domains of the core polarity regulator Dlg

The compartmentalized domains of polarized epithelial cells arise from mutually antagonistic actions between the apical Par complex and the basolateral Scrib module. In Drosophila, the Scrib module proteins Scribble (Scrib) and Discs-large (Dlg) are required to limit Lgl phosphorylation at the basolateral cortex, but how Scrib and Dlg could carry out such a ‘protection’ activity is not clear. We tested Protein Phosphatase 1α (PP1) as a potential mediator of this activity, but demonstrate that a significant component of Scrib and Dlg regulation of Lgl is PP1 independent, and found no evidence for a Scrib-Dlg-PP1 protein complex. However, the Dlg SH3 domain plays a role in Lgl protection and, in combination with the N-terminal region of the Dlg HOOK domain, in recruitment of Scrib to the membrane. We identify a ‘minimal Dlg’ comprised of the SH3 and HOOK domains that is both necessary and sufficient for Scrib localization and epithelial polarity function in vivo.

This article has an associated First Person interview with the first author of the paper.

Summary: A minimal SH3-HOOK fragment of Dlg is sufficient to support epithelial polarity through mechanisms independent of the PP1 phosphatase.

The Drosophila anterior-posterior axis is polarized by asymmetric myosin activation

The Drosophila anterior-posterior axis is specified at mid-oogenesis when the Par-1 kinase is recruited to the posterior cortex of the oocyte, where it polarizes the microtubule cytoskeleton to define where the axis determinants, bicoid and oskar mRNAs, localize. This polarity is established in response to an unknown signal from the follicle cells, but how this occurs is unclear. Here we show that the myosin chaperone Unc-45 and non-muscle myosin II (MyoII) are required upstream of Par-1 in polarity establishment. Furthermore, the myosin regulatory light chain (MRLC) is di-phosphorylated at the oocyte posterior in response to the follicle cell signal, inducing longer pulses of myosin contractility at the posterior that may increase cortical tension. Overexpression of MRLC-T21A that cannot be di-phosphorylated or treatment with the myosin light-chain kinase inhibitor ML-7 abolishes Par-1 localization, indicating that the posterior of MRLC di-phosphorylation is essential for both polarity establishment and maintenance. Thus, asymmetric myosin activation polarizes the anterior-posterior axis by recruiting and maintaining Par-1 at the posterior cortex. This raises an intriguing parallel with anterior-posterior axis formation in C. elegans, where MyoII also acts upstream of the PAR proteins to establish polarity, but to localize the anterior PAR proteins rather than Par-1.

Constitutive activation of β-catenin in odontoblasts induces aberrant pulp calcification in mouse incisors

During dentin formation, odontoblast polarization ensures that odontoblasts directionally secrete dentin matrix protein, leading to tubular dentin formation; however, little is known about the major features and regulatory mechanisms of odontoblast polarization. In a study of epithelial cell polarization, β-catenin was shown to serve as a structural component of cadherin-based adherens junctions to initiate cell polarity. However, the role of β-catenin in odontoblast polarization has not been well investigated. In this study, we explored whether β-catenin participated in odontoblast polarization to regulate the secretion of mineralization proteins. We established Col1-CreErt2; β-catenin exon3fl/fl (CA-β-catenin) mice, which constitutively activate β-catenin in odontoblasts. CA-β-catenin mice exhibited disorganization and depolarization of incisor odontoblasts. Moreover, the incisor dentin was hypomineralized, and ectopic calcification was found in mouse incisor pulp. In addition, by constitutive activation of β-catenin, the expression levels of the core polarity molecule Cdc42 and its downstream polarity protein complex Par3-Par6-aPKC were decreased in the incisors of CA-β-catenin mice. These findings suggest that β-catenin plays an essential role in dentin formation by regulating odontoblast polarization.

Construction of intracellular asymmetry and asymmetric division in Escherichia coli

The design principle of establishing an intracellular protein gradient for asymmetric cell division is a long-standing fundamental question. While the major molecular players and their interactions have been elucidated via genetic approaches, the diversity and redundancy of natural systems complicate the extraction of critical underlying features. Here, we take a synthetic cell biology approach to construct intracellular asymmetry and asymmetric division in Escherichia coli, in which division is normally symmetric. We demonstrate that the oligomeric PopZ from Caulobacter crescentus can serve as a robust polarized scaffold to functionalize RNA polymerase. Furthermore, by using another oligomeric pole-targeting DivIVA from Bacillus subtilis, the newly synthesized protein can be constrained to further establish intracellular asymmetry, leading to asymmetric division and differentiation. Our findings suggest that the coupled oligomerization and restriction in diffusion may be a strategy for generating a spatial gradient for asymmetric cell division.

Mechanochemical Control of Symmetry Breaking in the Caenorhabditis elegans Zygote

Cell polarity is the asymmetric organization of cellular components along defined axes. A key requirement for polarization is the ability of the cell to break symmetry and achieve a spatially biased organization. Despite different triggering cues in various systems, symmetry breaking (SB) usually relies on mechanochemical modulation of the actin cytoskeleton, which allows for advected movement and reorganization of cellular components. Here, the mechanisms underlying SB in Caenorhabditis elegans zygote, one of the most popular models to study cell polarity, are reviewed. A zygote initiates SB through the centrosome, which modulates mechanics of the cell cortex to establish advective flow of cortical proteins including the actin cytoskeleton and partitioning defective (PAR) proteins. The chemical signaling underlying centrosomal control of the Aurora A kinase–mediated cascade to convert the organization of the contractile actomyosin network from an apolar to polar state is also discussed.

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.

Cellular self-organization: generating order from the abyss

While the organization of inanimate systems such as gases or liquids is predominantly thermodynamically driven—a mixture of two gases will tend to mix until they reach equilibrium—biological systems frequently exhibit organization that is far from a well-mixed equilibrium. The anisotropies displayed by cells are evident in some of the dynamic processes that constitute life including cell development, movement, and division. These anisotropies operate at different length-scales, from the meso- to the nanoscale, and are proposed to reflect self-organization, a characteristic of living systems that is becoming accessible to reconstitution from purified components, and thus a more thorough understanding. Here, some examples of self-organization underlying cellular anisotropies at the cellular level are reviewed, with an emphasis on Rho-family GTPases operating at the plasma membrane. Given the technical challenges of studying these dynamic proteins, some of the successful approaches that are being employed to study their self-organization will also be considered.

Biomimetic niches reveal the minimal cues to trigger apical lumen formation in single hepatocytes

The symmetry breaking of protein distribution and cytoskeleton organization is an essential aspect for the development of apicobasal polarity. In embryonic cells this process is largely cell autonomous, while differentiated epithelial cells collectively polarize during epithelium formation. Here, we demonstrate that the de novo polarization of mature hepatocytes does not require the synchronized development of apical poles on neighbouring cells. De novo polarization at the single-cell level by mere contact with the extracellular matrix and immobilized cadherin defining a polarizing axis. The creation of these single-cell liver hemi-canaliculi allows unprecedented imaging resolution and control and over the lumenogenesis process. We show that the density and localization of cadherins along the initial cell-cell contact act as key triggers of the reorganization from lateral to apical actin cortex. The minimal cues necessary to trigger the polarization of hepatocytes enable them to develop asymmetric lumens with ectopic epithelial cells originating from the kidney, breast or colon.

Distinct activities of Scrib module proteins organize epithelial polarity

A polarized architecture is central to both epithelial structure and function. In many cells, polarity involves mutual antagonism between the Par complex and the Scribble (Scrib) module. While molecular mechanisms underlying Par-mediated apical determination are well-understood, how Scrib module proteins specify the basolateral domain remains unknown. Here, we demonstrate dependent and independent activities of Scrib, Discs-large (Dlg), and Lethal giant larvae (Lgl) using the Drosophila follicle epithelium. Our data support a linear hierarchy for localization, but rule out previously proposed protein-protein interactions as essential for polarization. Cortical recruitment of Scrib does not require palmitoylation or polar phospholipid binding but instead an independent cortically stabilizing activity of Dlg. Scrib and Dlg do not directly antagonize atypical protein kinase C (aPKC), but may instead restrict aPKC localization by enabling the aPKC-inhibiting activity of Lgl. Importantly, while Scrib, Dlg, and Lgl are each required, all three together are not sufficient to antagonize the Par complex. Our data demonstrate previously unappreciated diversity of function within the Scrib module and begin to define the elusive molecular functions of Scrib and Dlg.

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

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

Basic-hydrophobic sites are localized in conserved positions inside and outside of PH domains and affect localization of Dictyostelium myosin 1s

Myosin 1s have critical roles in linking membranes to the actin cytoskeleton via direct binding to acidic lipids. Lipid binding may occur through PIP3/PIP2-specific PH domains or nonspecific ionic interactions involving basic-hydrophobic (BH) sites but the mechanism of myosin 1s distinctive lipid targeting is poorly understood.  Now we show that PH domains occur in all Dictyostelium myosin 1s and that the BH sites of Myo1A, B, C, D, and F are in conserved positions near the β3/β4 loops of their PH domains. In spite of these shared lipid-binding sites, we observe significant differences in myosin 1s highly dynamic localizations. All myosin 1s except Myo1A are present in macropinocytic structures but only Myo1B and Myo1C are enriched at the edges of macropinocytic cups and associate with the actin in actin waves.  In contrast, Myo1D, E, and F are enclosed by the actin wave.  Mutations of BH sites affect localization of all Dictyostelium myosin 1s. Notably, mutation of the BH site located within the PH domains of PIP3-specific Myo1D and Myo1F completely eradicates membrane binding. Thus, BH sites are important determinants of motor targeting and may have a similar role in the localization of other myosin 1s.

Switching states: dynamic remodelling of polarity complexes as a toolkit for cell polarization

Polarity is defined by the segregation of cellular components along a defined axis. To polarize robustly, cells must be able to break symmetry and subsequently amplify these nascent asymmetries. Finally, asymmetric localization of signaling molecules must be translated into functional regulation of downstream effector pathways. Central to these behaviors are a diverse set of cell polarity networks. Within these networks, molecules exhibit varied behaviors, dynamically switching among different complexes and states, active versus inactive, bound versus unbound, immobile versus diffusive. This ability to switch dynamically between states is intimately connected to the ability of molecules to generate asymmetric patterns within cells. Focusing primarily on polarity pathways governed by the conserved PAR proteins, we discuss strategies enabled by these dynamic behaviors that are used by cells to polarize. We highlight not only how switching between states is linked to the ability of polarity proteins to localize asymmetrically, but also how cells take advantage of 'state switching' to regulate polarity in time and space.

Spatial regulation of the polarity kinase PAR-1 by parallel inhibitory mechanisms

The MARK/PAR-1 family of kinases are conserved regulators of cell polarity that share a conserved C-terminal kinase-associated domain (KA1). Localization of MARK/PAR-1 kinases to specific regions of the cell cortex is a hallmark of polarized cells. In Caenorhabditiselegans zygotes, PAR-1 localizes to the posterior cortex under the influence of another polarity kinase, aPKC/PKC-3. Here, we report that asymmetric localization of PAR-1 protein is not essential, and that PAR-1 kinase activity is regulated spatially. We find that, as in human MARK1, the PAR-1 KA1 domain is an auto-inhibitory domain that suppresses kinase activity. Auto-inhibition by the KA1 domain functions in parallel with phosphorylation by PKC-3 to suppress PAR-1 activity in the anterior cytoplasm. The KA1 domain also plays an additional role that is essential for germ plasm maintenance and fertility. Our findings suggest that modular regulation of kinase activity by redundant inhibitory inputs contributes to robust symmetry breaking by MARK/PAR-1 kinases in diverse cell types.