Reconstruction of Par-dependent polarity in apolar cells reveals a dynamic process of cortical polarization

Sensitive fluorescent biosensor reveals differential subcellular regulation of PKC

The protein kinase C (PKC) family of serine and threonine kinases, consisting of three distinctly regulated subfamilies, has been established as critical for various cellular functions. However, how PKC enzymes are regulated at different subcellular locations, particularly at emerging signaling hubs, is unclear. Here we present a sensitive excitation ratiometric C kinase activity reporter (ExRai-CKAR2) that enables the detection of minute changes (equivalent to 0.2% of maximum stimulation) in subcellular PKC activity. Using ExRai-CKAR2 with an enhanced diacylglycerol (DAG) biosensor, we uncover that G-protein-coupled receptor stimulation triggers sustained PKC activity at the endoplasmic reticulum and lysosomes, differentially mediated by Ca2+-sensitive conventional PKC and DAG-sensitive novel PKC, respectively. The high sensitivity of ExRai-CKAR2, targeted to either the cytosol or partitioning defective complexes, further enabled us to detect previously inaccessible endogenous atypical PKC activity in three-dimensional organoids. Taken together, ExRai-CKAR2 is a powerful tool for interrogating PKC regulation in response to physiological stimuli.

Artificial cell system as a tool for investigating pattern formation mechanisms of intracellular reaction-diffusion waves

Intracellular positional information is crucial for the precise control of biological phenomena, including cell division, polarity, and motility. Intracellular reaction-diffusion (iRD) waves are responsible for regulating positional information within cells as morphogens in multicellular tissues. However, iRD waves are explained by the coupling of biochemical reactions and molecular diffusion which indicates nonlinear systems under far from equilibrium conditions. Because of this complexity, experiments using defined elements rather than living cells containing endogenous factors are necessary to elucidate their pattern formation mechanisms. In this review, we summarize the effectiveness of artificial cell systems for investigating iRD waves derived from their high controllability and ability to emulate cell-size space effects. We describe how artificial cell systems reveal the characteristics of iRD waves, including the mechanisms of wave generation, mode selection, and period regulation. Furthermore, we introduce remaining open questions and discuss future challenges even in Min waves and in applying artificial cell systems to various iRD waves.

Astrocytes control quiescent NSC reactivation via GPCR signaling-mediated F-actin remodeling

The transitioning of neural stem cells (NSCs) between quiescent and proliferative states is fundamental for brain development and homeostasis. Defects in NSC reactivation are associated with neurodevelopmental disorders. Drosophila quiescent NSCs extend an actin-rich primary protrusion toward the neuropil. However, the function of the actin cytoskeleton during NSC reactivation is unknown. Here, we reveal the fine filamentous actin (F-actin) structures in the protrusions of quiescent NSCs by expansion and super-resolution microscopy. We show that F-actin polymerization promotes the nuclear translocation of myocardin-related transcription factor, a microcephaly-associated transcription factor, for NSC reactivation and brain development. F-actin polymerization is regulated by a signaling cascade composed of G protein-coupled receptor Smog, G protein αq subunit, Rho1 guanosine triphosphatase, and Diaphanous (Dia)/Formin during NSC reactivation. Further, astrocytes secrete a Smog ligand folded gastrulation to regulate Gαq-Rho1-Dia-mediated NSC reactivation. Together, we establish that the Smog-Gαq-Rho1 signaling axis derived from astrocytes, an NSC niche, regulates Dia-mediated F-actin dynamics in NSC reactivation.

Sensitive Fluorescent Biosensor Reveals Differential Subcellular Regulation of PKC

The protein kinase C (PKC) family of serine/threonine kinases, which consist of three distinctly regulated subfamilies, have long been established as critical for a variety of cellular functions. However, how PKC enzymes are regulated at different subcellular locations, particularly at emerging signaling hubs such as the ER, lysosome, and Par signaling complexes, is unclear. Here, we present a sensitive Excitation Ratiometric (ExRai) C Kinase Activity Reporter (ExRai-CKAR2) that enables the detection of minute changes in subcellular PKC activity. Using ExRai-CKAR2 in conjunction with an enhanced diacylglycerol (DAG) biosensor capable of detecting intracellular DAG dynamics, we uncover the differential regulation of PKC isoforms at distinct subcellular locations. We find that G-protein coupled receptor (GPCR) stimulation triggers sustained PKC activity at the ER and lysosomes, primarily mediated by Ca2+ sensitive conventional PKC (cPKC) and novel PKC (nPKC), respectively, with nPKC showing high basal activity due to elevated basal DAG levels on lysosome membranes. The high sensitivity of ExRai-CKAR2, targeted to either the cytosol or Par-complexes, further enabled us to detect previously inaccessible endogenous atypical PKC (aPKC) activity in 3D organoids. Taken together, ExRai-CKAR2 is a powerful tool for interrogating PKC regulation in response to physiological stimuli.

Astrocytes control quiescent NSC reactivation via GPCR signaling-mediated F-actin remodeling

The transitioning of neural stem cells (NSCs) between quiescent and proliferative states is fundamental for brain development and homeostasis. Defects in NSC reactivation are associated with neurodevelopmental disorders. Drosophila quiescent NSCs extend an actin-rich primary protrusion toward the neuropil. However, the function of the actin cytoskeleton during NSC reactivation is unknown. Here, we reveal the fine F-actin structures in the protrusions of quiescent NSCs by expansion and super-resolution microscopy. We show that F-actin polymerization promotes the nuclear translocation of Mrtf, a microcephaly-associated transcription factor, for NSC reactivation and brain development. F-actin polymerization is regulated by a signaling cascade composed of G-protein-coupled receptor (GPCR) Smog, G-protein αq subunit, Rho1 GTPase, and Diaphanous (Dia)/Formin during NSC reactivation. Further, astrocytes secrete a Smog ligand Fog to regulate Gαq-Rho1-Dia-mediated NSC reactivation. Together, we establish that the Smog-Gαq-Rho1 signaling axis derived from astrocytes, a NSC niche, regulates Dia-mediated F-actin dynamics in NSC reactivation.

Synthetic Par polarity induces cytoskeleton asymmetry in unpolarized mammalian cells

Polarized cells rely on a polarized cytoskeleton to function. Yet, how cortical polarity cues induce cytoskeleton polarization remains elusive. Here, we capitalized on recently established designed 2D protein arrays to ectopically engineer cortical polarity of virtually any protein of interest during mitosis in various cell types. This enables direct manipulation of polarity signaling and the identification of the cortical cues sufficient for cytoskeleton polarization. Using this assay, we dissected the logic of the Par complex pathway, a key regulator of cytoskeleton polarity during asymmetric cell division. We show that cortical clustering of any Par complex subunit is sufficient to trigger complex assembly and that the primary kinetic barrier to complex assembly is the relief of Par6 autoinhibition. Further, we found that inducing cortical Par complex polarity induces two hallmarks of asymmetric cell division in unpolarized mammalian cells: spindle orientation, occurring via Par3, and central spindle asymmetry, depending on aPKC activity.

Persistent cell contacts enable E-cadherin/HMR-1- and PAR-3-based symmetry breaking within a developing C. elegans epithelium

Tissue-wide patterning is essential to multicellular development, requiring cells to individually generate polarity axes and coordinate them in space and time with neighbors. Using the C. elegans intestinal epithelium, we identified a patterning mechanism that is informed by cell contact lifetime asymmetry and executed via the scaffolding protein PAR-3 and the transmembrane protein E-cadherin/HMR-1. Intestinal cells break symmetry as PAR-3 and HMR-1 recruit apical determinants into punctate "local polarity complexes" (LPCs) at homotypic contacts. LPCs undergo an HMR-1-based migration to a common midline, thereby establishing tissue-wide polarity. Thus, symmetry breaking results from PAR-3-dependent intracellular polarization coupled to HMR-1-based tissue-level communication, which occurs through a non-adhesive signaling role for HMR-1. Differential lifetimes between homotypic and heterotypic cell contacts are created by neighbor exchanges and oriented divisions, patterning where LPCs perdure and thereby breaking symmetry. These cues offer a logical and likely conserved framework for how epithelia without obvious molecular asymmetries can polarize.

Ccdc85c-Par3 condensates couple cell polarity with Notch to control neural progenitor proliferation

Polarity proteins regulate the proliferation and differentiation of neural progenitors to generate neurons during brain development through multiple signaling pathways. However, how cell polarity couples the signaling pathways remains unclear. Here, we show that coiled-coil domain-containing protein 85c (Ccdc85c) interacts with the polarity protein Par3 to regulate the proliferation of radial glial cells (RGCs) via phase separation coupled to percolation (PSCP). We find that the interaction with Ccdc85c relieves the intramolecular auto-inhibition of Par3, which leads to PSCP of Par3. Downregulation of Ccdc85c causes RGC differentiation. Importantly, the open conformation of Par3 facilitates the recruitment of the Notch regulator Numb to the Par3 condensates, which might prevent the attenuation of Notch activity to maintain RGC proliferation. Furthermore, ectopic activation of Notch signaling rescues RGC proliferation defects caused by the downregulation of Ccdc85c. These results suggest that Ccdc85c-mediated PSCP of Par3 regulates Notch signaling to control RGC proliferation during brain development.

Context matters: Lessons in epithelial polarity from the Caenorhabditis elegans intestine and other tissues

Epithelia are tissues with diverse morphologies and functions across metazoans, ranging from vast cell sheets encasing internal organs to internal tubes facilitating nutrient uptake, all of which require establishment of apical-basolateral polarity axes. While all epithelia tend to polarize the same components, how these components are deployed to drive polarization is largely context-dependent and likely shaped by tissue-specific differences in development and ultimate functions of polarizing primordia. The nematode Caenorhabditis elegans (C. elegans) offers exceptional imaging and genetic tools and possesses unique epithelia with well-described origins and roles, making it an excellent model to investigate polarity mechanisms. In this review, we highlight the interplay between epithelial polarization, development, and function by describing symmetry breaking and polarity establishment in a particularly well-characterized epithelium, the C. elegans intestine. We compare intestinal polarization to polarity programs in two other C. elegans epithelia, the pharynx and epidermis, correlating divergent mechanisms with tissue-specific differences in geometry, embryonic environment, and function. Together, we emphasize the importance of investigating polarization mechanisms against the backdrop of tissue-specific contexts, while also underscoring the benefits of cross-tissue comparisons of polarity.

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

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

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.

Re-visiting the principles of apicobasal polarity in Drosophila neural stem cells

The ability of stem cells to divide asymmetrically is crucial for cell-type diversity and tissue homeostasis. Drosophila neural stem cells, also knowns as neuroblasts, utilize asymmetric cell division to self-renew and give rise to differentiated daughter cells. Drosophila neuroblasts relies on the polarized protein complexes on the apical and basal cortex to govern cell polarity and asymmetry. Here, we review recent advances in our understanding of the neuroblast polarity focusing on how actin cytoskeleton, phosphoinositide lipids and liquid-liquid phase separation regulate the asymmetric cell division of Drosophila neuroblasts.

Phases of cortical actomyosin dynamics coupled to the neuroblast polarity cycle

The Par complex dynamically polarizes to the apical cortex of asymmetrically dividing Drosophila neuroblasts where it directs fate determinant segregation. Previously, we showed that apically directed cortical movements that polarize the Par complex require F-actin (Oon and Prehoda, 2019). Here, we report the discovery of cortical actomyosin dynamics that begin in interphase when the Par complex is cytoplasmic but ultimately become tightly coupled to cortical Par dynamics. Interphase cortical actomyosin dynamics are unoriented and pulsatile but rapidly become sustained and apically-directed in early mitosis when the Par protein aPKC accumulates on the cortex. Apical actomyosin flows drive the coalescence of aPKC into an apical cap that depolarizes in anaphase when the flow reverses direction. Together with the previously characterized role of anaphase flows in specifying daughter cell size asymmetry, our results indicate that multiple phases of cortical actomyosin dynamics regulate asymmetric cell division.

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

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

Surface wetting by kinetic control of liquid-liquid phase separation

Motivated by the observations of intracellular phase separations and the wetting of cell membranes by protein droplets, we study the nonequilibrium surface wetting by Monte Carlo simulations of a lattice gas model involving particle creation. We find that, even when complete wetting should occur in equilibrium, the fast creation of particles can hinder the surface wetting for a long time due to the bulk droplet formation. Performing molecular dynamics simulations, we show that this situation also holds in colloidal particle systems when the disorder density is sufficiently high. The results suggest an intracellular control mechanism of surface wetting by changing the speed of component synthesis.

Phase Separation and Mechanical Forces in Regulating Asymmetric Cell Division of Neural Stem Cells

Asymmetric cell division (ACD) of neural stem cells and progenitors not only renews the stem cell population but also ensures the normal development of the nervous system, producing various types of neurons with different shapes and functions in the brain. One major mechanism to achieve ACD is the asymmetric localization and uneven segregation of intracellular proteins and organelles into sibling cells. Recent studies have demonstrated that liquid-liquid phase separation (LLPS) provides a potential mechanism for the formation of membrane-less biomolecular condensates that are asymmetrically distributed on limited membrane regions. Moreover, mechanical forces have emerged as pivotal regulators of asymmetric neural stem cell division by generating sibling cell size asymmetry. In this review, we will summarize recent discoveries of ACD mechanisms driven by LLPS and mechanical forces.

Fluid flow dynamics in cellular patterning

The development of complex forms of multicellular organisms depends on the spatial arrangement of cellular architecture and functions. The interior design of the cell is patterned by spatially biased distributions of molecules and biochemical reactions in the cytoplasm and/or on the plasma membrane. In recent years, a dynamic change in the cytoplasmic fluid flow has emerged as a key physical process of driving long-range transport of molecules to particular destinations within the cell. Here, recent experimental advances in the understanding of the generation of the various types of cytoplasmic flows and contributions to intracellular patterning are reviewed with a particular focus on feedback mechanisms between the mechanical properties of fluid flow and biochemical signaling during animal cell polarization.

A balance between antagonizing PAR proteins specifies the pattern of asymmetric and symmetric divisions in C. elegans embryogenesis

Coordination between cell differentiation and proliferation during development requires the balance between asymmetric and symmetric modes of cell division. However, the cellular intrinsic cue underlying the choice between these two division modes remains elusive. Here, we show evidence in Caenorhabditis elegans that the invariable lineage of the division modes is specified by the balance between antagonizing complexes of partitioning-defective (PAR) proteins. By uncoupling unequal inheritance of PAR proteins from that of fate determinants during cell division, we demonstrate that changes in the balance between PAR-2 and PAR-6 can be sufficient to re-program the division modes from symmetric to asymmetric and vice versa in two daughter cells. The division mode adopted occurs independently of asymmetry in cytoplasmic fate determinants, cell-size asymmetry, and cell-cycle asynchrony between sister cells. We propose that the balance between PAR proteins represents an intrinsic self-organizing cue for the specification of the two division modes during development.

The polarity protein PARD3 and cancer

Tissue disorganisation is one of the main hallmarks of cancer. Polarity proteins are responsible for the arrangement of cells within epithelial tissues through the asymmetric organisation of cellular components. Partition defective 3 (PARD3) is a master regulator of the Par polarity complex primarily due to its ability to form large complexes via its self-homologous binding domain. In addition to its role in polarity, PARD3 is a scaffolding protein that binds to intracellular signalling molecules, many of which are frequently deregulated in cancer. The role of PARD3 has been implicated in multiple solid cancers as either a tumour suppressor or promoter. This dual functionality is both physiologically and cell context dependent. In this review, we will discuss PARD3's role in tumourigenesis in both laboratory and clinical settings. We will also review several of the mechanisms underpinning PARD3's function including its association with intracellular signalling pathways and its role in the regulation of asymmetric cell division.

Substrate-dependent control of the chiral orientation of mesenchymal stem cells: image-based quantitative profiling

Stem-cell (SC) chirality or left-right (LR) asymmetry is an essential attribute, observed during tissue regeneration. The ability to control the LR orientation of cells by biophysical manipulation is a promising approach for recapitulating their inherent function. Despite remarkable progress in tissue engineering, the development of LR chirality in SCs has been largely unexplored. Here, we demonstrate the role of substrate stiffness on the LR asymmetry of cultured mesenchymal stem cells (MSCs). We found that MSCs acquired higher asymmetricity when cultured on stiffer PCL/collagen matrices. To confirm cellular asymmetry, different parameters such as the aspect ratio, orientation angle and intensity of polarized proteins (Par) were investigated. The results showed a significant (p < 0.01) difference in the average orientation angle, the cellular aspect ratio, and the expression of actin and Par proteins in MSCs cultured on matrices with different stiffnesses. Furthermore, a Gaussian support-vector machine was applied to classify cells cultured on both (2% and 10% PCL/Collagen) matrices, with a resulting accuracy of 96.2%. To the best of our knowledge, this study is the first that interrelates and quantifies MSC asymmetricity with matrix properties using a simple 2D model.

Target-induced clustering activates Trim-Away of pathogens and proteins

Trim-Away is a recently developed technology that exploits off-the-shelf antibodies and the RING E3 ligase and cytosolic antibody receptor TRIM21 to carry out rapid protein depletion. How TRIM21 is catalytically activated upon target engagement, either during its normal immune function or when repurposed for targeted protein degradation, is unknown. Here we show that a mechanism of target-induced clustering triggers intermolecular dimerization of the RING domain to switch on the ubiquitination activity of TRIM21 and induce virus neutralization or drive Trim-Away. We harness this mechanism for selective degradation of disease-causing huntingtin protein containing long polyglutamine tracts and expand the Trim-Away toolbox with highly active TRIM21-nanobody chimeras that can also be controlled optogenetically. This work provides a mechanism for cellular activation of TRIM RING ligases and has implications for targeted protein degradation technologies.

FGF Pyramus Has a Transmembrane Domain and Cell-Autonomous Function in Polarity

Most fibroblast growth factors (FGFs) function as receptor ligands through their conserved FGF domain, but sequences outside this domain vary and are not well studied. This core domain of 120 amino acids (aa) is flanked in all FGFs by highly divergent amino-terminal and carboxy-terminal sequences of variable length. Drosophila has fewer FGF genes, with only three identified to date, pyramus (pyr), thisbe (ths), and branchless (bnl), and all three encoding relatively large FGF proteins (∼80 kDa). We hypothesized that the longer FGF proteins present in Drosophila and other organisms may relate to an ancestral form, in which multiple functions or regulatory properties are present within a single polypeptide. Here, we focused analysis on Pyr, finding that it harbors a transmembrane domain (TMD) and extended C-terminal intracellular domain containing a degron. The intracellular portion limits Pyr levels, whereas the TMD promotes spatial precision in the paracrine activation of Heartless FGF receptor. Additionally, degron deletion mutants that upregulate Pyr exhibit cell polarity defects that lead to invagination defects at gastrulation, demonstrating a previously uncharacterized cell-autonomous role. In summary, our data show that Pyr is the first demonstrated transmembrane FGF, that it has both extracellular and intracellular functions, and that spatial distribution and levels of this particular FGF protein are tightly regulated. Our results suggest that other FGFs may be membrane tethered or multifunctional like Pyr.

Liquid-Liquid Phase Separation in Neuronal Development and Synaptic Signaling

Formation of biomolecular condensates that are not enclosed by membranes via liquid-liquid phase separation (LLPS) is a general strategy that cells adopt to organize membraneless subcellular compartments for diverse functions. Neurons are highly polarized with elaborate branching and functional compartmentalization of their neurites, thus, raising additional demand for the proper subcellular localization of both membraneless and membrane-based organelles. Recent studies have provided evidence that several protein assemblies involved in the establishment of neuronal stem cell (NSC) polarity and in the asymmetric division of NSCs form distinct molecular condensates via LLPS. In synapses of adult neurons, molecular apparatuses controlling presynaptic neurotransmitter release and postsynaptic signaling transmission are also likely formed via LLPS. These molecular condensates, though not enclosed by lipid bilayers, directly associate with plasma membranes or membrane-based organelles, indicating that direct communication between membraneless and membrane-based organelles is a common theme in neurons and other types of cells.

Liquid-liquid phase separation in biology: mechanisms, physiological functions and human diseases

Cells are compartmentalized by numerous membrane-enclosed organelles and membraneless compartments to ensure that a wide variety of cellular activities occur in a spatially and temporally controlled manner. The molecular mechanisms underlying the dynamics of membrane-bound organelles, such as their fusion and fission, vesicle-mediated trafficking and membrane contactmediated inter-organelle interactions, have been extensively characterized. However, the molecular details of the assembly and functions of membraneless compartments remain elusive. Mounting evidence has emerged recently that a large number of membraneless compartments, collectively called biomacromolecular condensates, are assembled via liquid-liquid phase separation (LLPS). Phase-separated condensates participate in various biological activities, including higher-order chromatin organization, gene expression, triage of misfolded or unwanted proteins for autophagic degradation, assembly of signaling clusters and actin- and microtubule-based cytoskeletal networks, asymmetric segregations of cell fate determinants and formation of pre- and post-synaptic density signaling assemblies. Biomacromolecular condensates can transition into different material states such as gel-like structures and solid aggregates. The material properties of condensates are crucial for fulfilment of their distinct functions, such as biochemical reaction centers, signaling hubs and supporting architectures. Cells have evolved multiple mechanisms to ensure that biomacromolecular condensates are assembled and disassembled in a tightly controlled manner. Aberrant phase separation and transition are causatively associated with a variety of human diseases such as neurodegenerative diseases and cancers. This review summarizes recent major progress in elucidating the roles of LLPS in various biological pathways and diseases.

Par complex cluster formation mediated by phase separation

The evolutionarily conserved Par3/Par6/aPKC complex regulates the polarity establishment of diverse cell types and distinct polarity-driven functions. However, how the Par complex is concentrated beneath the membrane to initiate cell polarization remains unclear. Here we show that the Par complex exhibits cell cycle-dependent condensation in Drosophila neuroblasts, driven by liquid–liquid phase separation. The open conformation of Par3 undergoes autonomous phase separation likely due to its NTD-mediated oligomerization. Par6, via C-terminal tail binding to Par3 PDZ3, can be enriched to Par3 condensates and in return dramatically promote Par3 phase separation. aPKC can also be concentrated to the Par3N/Par6 condensates as a client. Interestingly, activated aPKC can disperse the Par3/Par6 condensates via phosphorylation of Par3. Perturbations of Par3/Par6 phase separation impair the establishment of apical–basal polarity during neuroblast asymmetric divisions and lead to defective lineage development. We propose that phase separation may be a common mechanism for localized cortical condensation of cell polarity complexes.

The evolutionarily conserved complex, the Par proteins, regulates cell polarity. Here, the authors show that in Drosophila neuroblasts, the Par complex exhibits liquid–liquid phase separation dependent on the cell cycle.

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.