Gradients of Rac1 Nanoclusters Support Spatial Patterns of Rac1 Signaling

Differential Lipid Binding Specificities of RAP1A and RAP1B are Encoded by the Amino Acid Sequence of the Membrane Anchors

RAP1 proteins belong to the RAS family of small GTPases that operate as molecular switches by cycling between GDP-bound inactive and GTP-bound active states. The C-terminal anchors of RAP1 proteins are known to direct membrane localization, but how these anchors organize RAP1 on the plasma membrane (PM) has not been investigated. Using high-resolution imaging, we show that RAP1A and RAP1B form spatially segregated nanoclusters on the inner leaflet of the PM, with further lateral segregation between GDP-bound and GTP-bound proteins. The C-terminal polybasic anchors of RAP1A and RAP1B differ in their amino acid sequences and exhibit different lipid binding specificities, which can be modified by single-point mutations in the respective polybasic domains (PBD). Molecular dynamics simulations reveal that single PBD mutations substantially reduce the interactions of the membrane anchors with the PM lipid phosphatidylserine. In summary, we show that aggregate lipid binding specificity encoded within the C-terminal anchor determines PM association and nanoclustering of RAP1A and RAP1B. Taken together with previous observations on RAC1 and KRAS, the study reveals that the PBD sequences of small GTPase membrane anchors can encode distinct lipid binding specificities that govern PM interactions.

Intrinsically Disordered Membrane Anchors of Rheb, RhoA, and DiRas3 Small GTPases: Molecular Dynamics, Membrane Organization, and Interactions

Protein structure has been well established to play a key role in determining function; however, intrinsically disordered proteins and regions (IDPs and IDRs) defy this paradigm. IDPs and IDRs exist as an ensemble of structures rather than a stable 3D structure yet play essential roles in many cell-signaling processes. Nearly all Ras superfamily GTPases are tethered to membranes by a lipid tail at the end of a flexible IDR. The sequence of the IDR is a key determinant of membrane localization, and interaction between the IDR and the membrane has been shown to affect signaling in RAS proteins through the modulation of dynamic membrane organization. Here, we utilized atomistic molecular dynamics simulations to study the membrane interaction, conformational dynamics, and lipid sorting of three IDRs from small GTPases Rheb, RhoA, and DiRas3 in model membranes representing their physiological target membranes. We found that complementarity between the lipidated IDR sequence and target membrane lipid composition is a determinant of conformational plasticity. We also show that electrostatic interactions between anionic lipids and basic residues on IDRs are correlated with sampling of semistable conformational substates, and lack of these interactions is associated with greater conformational diversity. Finally, we show that small GTPase IDRs with a polybasic domain alter local lipid composition by segregating anionic lipids and, in some cases, excluding other lipids from their immediate vicinity in favor of anionic lipids.

Decreased Tiam1-mediated Rac1 activation is responsible for impaired directional persistence of chondrocyte migration in microtia

The human auricle has a complex structure, and microtia is a congenital malformation characterized by decreased size and loss of elaborate structure in the affected ear with a high incidence. Our previous studies suggest that inadequate cell migration is the primary cytological basis for the pathogenesis of microtia, however, the underlying mechanism is unclear. Here, we further demonstrate that microtia chondrocytes show a decreased directional persistence during cell migration. Directional persistence can define a leading edge associated with oriented movement, and any mistakes would affect cell function and tissue morphology. By the screening of motility-related genes and subsequent confirmations, active Rac1 (Rac1-GTP) is identified to be critical for the impaired directional persistence of microtia chondrocytes migration. Moreover, Rho guanine nucleotide exchange factors (GEFs) and Rho GTPase-activating proteins (GAPs) are detected, and overexpression of Tiam1 significantly upregulates the level of Rac1-GTP and improves directional migration in microtia chondrocytes. Consistently, decreased expression patterns of Tiam1 and active Rac1 are found in microtia mouse models, Bmp5se/J and Prkralear-3J/GrsrJ. Collectively, our results provide new insights into microtia development and therapeutic strategies of tissue engineering for microtia patients.

Implication of Rac1 GTPase in molecular and cellular mitochondrial functions

Rac1 is a member of the Rho GTPase family which plays major roles in cell mobility, polarity and migration, as a fundamental regulator of actin cytoskeleton. Signal transduction by Rac1 occurs through interaction with multiple effector proteins, and its activity is regulated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). The small protein is mainly anchored to the inner side of the plasma membrane but it can be found in endocellular compartments, notably endosomes and cell nuclei. The protein localizes also into mitochondria where it contributes to the regulation of mitochondrial dynamics, including both mitobiogenesis and mitophagy, in addition to signaling processes via different protein partners, such as the proapoptotic protein Bcl-2 and chaperone sigma-1 receptor (σ-1R). The mitochondrial form of Rac1 (mtRac1) has been understudied thus far, but it is as essential as the nuclear or plasma membrane forms, via its implication in regulation of oxidative stress and DNA damages. Rac1 is subject to diverse post-translational modifications, notably to a geranylgeranylation which contributes importantly to its mitochondrial import and its anchorage to mitochondrial membranes. In addition, Rac1 contributes to the mitochondrial translocation of other proteins, such as p53. The mitochondrial localization and functions of Rac1 are discussed here, notably in the context of human diseases such as cancers. Inhibitors of Rac1 have been identified (NSC-23766, EHT-1864) and some are being developed for the treatment of cancer (MBQ-167) or central nervous system diseases (JK-50561). Their effects on mtRac1 warrant further investigations. An overview of mtRac1 is provided here.

SP-R210 isoforms of Myosin18A modulate endosomal sorting and recognition of influenza A virus infection in macrophages

Influenza A virus (IAV) infection causes acute and often lethal inflammation in the lung. The role of macrophages in this adverse inflammation is partially understood. The surfactant protein A receptor 210 (SP-R210) consists of two isoforms, a long (L) SP-R210L and a short (S) SP-R210S isoform encoded by alternative splicing of the myosin 18A gene. We reported that disruption of SP-R210L enhances cytosolic and endosomal antiviral response pathways. Here, we report that SP-R210L antagonizes type I interferon β (IFNβ), as depletion of SP-R210L potentiates IFNβ secretion. SP-R210 antibodies enhance and attenuate IFNβ secretion in SP-R210L replete and deficient macrophages, respectively, indicating that SP-R210 isoform stoichiometry alters macrophage function intrinsically. This reciprocal response is coupled to unopposed and restricted expression of viral genes in control and SP-R210L-deficient macrophages, respectively. Human monocytic cells with sub-stoichiometric expression of SP-R210L resist IAV infection, whereas alveolar macrophages with increased abundance of SP-R210L permit viral gene expression similar to murine macrophages. Uptake and membrane binding studies show that lack of SP-R210 isoforms does not impair IAV binding and internalization. Lack of SP-R210L, however, results in macropinocytic retention of the virus that depends on both SP-R210S and interferon-inducible transmembrane protein-3 (IFITM3). Mass spectrometry and Western blot analyses indicate that SP-R210 isoforms modulate differential recruitment of the Rho-family GTPase RAC1 and guanine nucleotide exchange factors. Our study suggests that SP-R210 isoforms modulate RAC-dependent macropinosomal sorting of IAV to discrete endosomal and lysosomal compartments that either permit or prevent endolysosomal escape and inflammatory sensing of viral genomes in macrophages.

The ubiquitous nanocluster: A molecular scale organizing principle that governs cellular information flow

The language of biology at the scale of the cell is constituted of alphabets represented by biomolecules. These are stitched together in a variety of ways to create meaning. We argue that the phrases of this language are nanoscale molecular assemblies or nano-hubs for the purpose of information flow. At the cell surface information is sensed and processed via membrane receptors, often configured as multimers. These nano-assemblies serve as receiver nano-hubs, which are flexibly configured with additional nano-hubs that we term modifiers and transducers. This framework serves to process information that is transmitted for execution inside the cell. Here, we explore some examples about how nano-hubs are built and how they may contribute to cellular information flow.

Piezo1 Is Required for Myoblast Migration and Involves Polarized Clustering in Association with Cholesterol and GM1 Ganglioside

A specific plasma membrane distribution of the mechanosensitive ion channel Piezo1 is required for cell migration, but the mechanism remains elusive. Here, we addressed this question using WT and Piezo1-silenced C2C12 mouse myoblasts and WT and Piezo1-KO human kidney HEK293T cells. We showed that cell migration in a cell-free area and through a porous membrane decreased upon Piezo1 silencing or deletion, but increased upon Piezo1 activation by Yoda1, whereas migration towards a chemoattractant gradient was reduced by Yoda1. Piezo1 organized into clusters, which were preferentially enriched at the front. This polarization was stimulated by Yoda1, accompanied by Ca2+ polarization, and abrogated by partial cholesterol depletion. Piezo1 clusters partially colocalized with cholesterol- and GM1 ganglioside-enriched domains, the proportion of which was increased by Yoda1. Mechanistically, Piezo1 activation induced a differential mobile fraction of GM1 associated with domains and the bulk membrane. Conversely, cholesterol depletion abrogated the differential mobile fraction of Piezo1 associated with clusters and the bulk membrane. In conclusion, we revealed, for the first time, the differential implication of Piezo1 depending on the migration mode and the interplay between GM1/cholesterol-enriched domains at the front during migration in a cell-free area. These domains could provide the optimal biophysical properties for Piezo1 activity and/or spatial dissociation from the PMCA calcium efflux pump.

Membrane nanodomains: Dynamic nanobuilding blocks of polarized cell growth

Cell polarity is intimately linked to numerous biological processes, such as oriented plant cell division, particular asymmetric division, cell differentiation, cell and tissue morphogenesis, and transport of hormones and nutrients. Cell polarity is typically initiated by a polarizing cue that regulates the spatiotemporal dynamic of polarity molecules, leading to the establishment and maintenance of polar domains at the plasma membrane. Despite considerable progress in identifying key polarity regulators in plants, the molecular and cellular mechanisms underlying cell polarity formation have yet to be fully elucidated. Recent work suggests a critical role for membrane protein/lipid nanodomains in polarized morphogenesis in plants. One outstanding question is how the spatiotemporal dynamics of signaling nanodomains are controlled to achieve robust cell polarization. In this review, we first summarize the current state of knowledge on potential regulatory mechanisms of nanodomain dynamics, with a special focus on Rho-like GTPases from plants. We then discuss the pavement cell system as an example of how cells may integrate multiple signals and nanodomain-involved feedback mechanisms to achieve robust polarity. A mechanistic understanding of nanodomains' roles in plant cell polarity is still in the early stages and will remain an exciting area for future investigations.

A planar polarized MYO6-DOCK7-RAC1 axis promotes tissue fluidification in mammary epithelia

Tissue fluidification and collective motility are pivotal in regulating embryonic morphogenesis, wound healing, and tumor metastasis. These processes frequently require that each cell constituent of a tissue coordinates its migration activity and directed motion through the oriented extension of lamellipodium cell protrusions, promoted by RAC1 activity. While the upstream RAC1 regulators in individual migratory cells or leader cells during invasion or wound healing are well characterized, how RAC1 is controlled in follower cells remains unknown. Here, we identify a MYO6-DOCK7 axis essential for spatially restricting RAC1 activity in a planar polarized fashion in model tissue monolayers. The MYO6-DOCK7 axis specifically controls the extension of cryptic lamellipodia required to drive tissue fluidification and cooperative-mode motion in otherwise solid and static carcinoma cell collectives.

Voltage-gated sodium channels: from roles and mechanisms in the metastatic cell behavior to clinical potential as therapeutic targets

During the second half of the last century, the prevalent knowledge recognized the voltage-gated sodium channels (VGSCs) as the proteins responsible for the generation and propagation of action potentials in excitable cells. However, over the last 25 years, new non-canonical roles of VGSCs in cancer hallmarks have been uncovered. Their dysregulated expression and activity have been associated with aggressive features and cancer progression towards metastatic stages, suggesting the potential use of VGSCs as cancer markers and prognostic factors. Recent work has elicited essential information about the signalling pathways modulated by these channels: coupling membrane activity to transcriptional regulation pathways, intracellular and extracellular pH regulation, invadopodia maturation, and proteolytic activity. In a promising scenario, the inhibition of VGSCs with FDA-approved drugs as well as with new synthetic compounds, reduces cancer cell invasion in vitro and cancer progression in vivo. The purpose of this review is to present an update regarding recent advances and ongoing efforts to have a better understanding of molecular and cellular mechanisms on the involvement of both pore-forming α and auxiliary β subunits of VGSCs in the metastatic processes, with the aim at proposing VGSCs as new oncological markers and targets for anticancer treatments.

RAS nanoclusters are cell surface transducers that convert extracellular stimuli to intracellular signalling

Mutations of rat sarcoma virus (RAS) oncogenes (HRAS, KRAS and NRAS) can contribute to the development of cancers and genetic disorders (RASopathies). The spatiotemporal organization of RAS is an important property that warrants further investigation. In order to function, wild-type or oncogenic mutants of RAS must be localized to the inner leaflet of the plasma membrane (PM), which is driven by interactions between their C-terminal membrane-anchoring domains and PM lipids. The isoform-specific RAS-lipid interactions promote the formation of nanoclusters on the PM. As main sites for effector recruitment, these nanoclusters are biologically important. Since the spatial distribution of lipids is sensitive to changing environments, such as mechanical and electrical perturbations, RAS nanoclusters act as transducers to convert external stimuli to intracellular mitogenic signalling. As such, effective inhibition of RAS oncogenesis requires consideration of the complex interplay between RAS nanoclusters and various cell surface and extracellular stimuli. In this review, we discuss in detail how, by sorting specific lipids in the PM, RAS nanoclusters act as transducers to convert external stimuli into intracellular signalling.

Cholesterol and Sphingomyelin Polarize at the Leading Edge of Migrating Myoblasts and Involve Their Clustering in Submicrometric Domains

Myoblast migration is crucial for myogenesis and muscular tissue homeostasis. However, its spatiotemporal control remains elusive. Here, we explored the involvement of plasma membrane cholesterol and sphingolipids in this process. In resting C2C12 mouse myoblasts, those lipids clustered in sphingomyelin/cholesterol/GM1 ganglioside (SM/chol/GM1)- and cholesterol (chol)-enriched domains, which presented a lower stiffness than the bulk membrane. Upon migration, cholesterol and sphingomyelin polarized at the front, forming cholesterol (chol)- and sphingomyelin/cholesterol (SM/chol)-enriched domains, while GM1-enriched domains polarized at the rear. A comparison of domain proportion suggested that SM/chol- and GM1-enriched domains originated from the SM/chol/GM1-coenriched domains found at resting state. Modulation of domain proportion (through cholesterol depletion, combined or not with actin polymerization inhibition, or sphingolipid synthesis inhibition) revealed that the higher the chol- and SM/chol-enriched domains, the higher the myoblast migration. At the front, chol- and SM/chol-enriched domains were found in proximity with F-actin fibers and the lateral mobility of sphingomyelin in domains was specifically restricted in a cholesterol- and cytoskeleton-dependent manner while domain abrogation impaired F-actin and focal adhesion polarization. Altogether, we showed the polarization of cholesterol and sphingomyelin and their clustering in chol- and SM/chol-enriched domains with differential properties and roles, providing a mechanism for the spatial and functional control of myoblast migration.

Molecular principles of bidirectional signalling between membranes and small GTPases

Most small GTPases actuate their functions on subcellular membranes, which are increasingly seen as integral components of small GTPase signalling. In this review, we used the highly studied regulation of Arf GTPases by their GEFs to categorize the molecular principles of membrane contributions to small GTPase signalling, which have been highlighted by integrated structural biology combining in vitro reconstitutions in artificial membranes and high-resolution structures. As an illustration of how this framework can be harnessed to better understand the cooperation between small GTPases, their regulators and membranes, we applied it to the activation of the small GTPase Rac1 by DOCK-ELMO, identifying novel contributions of membranes to Rac1 activation. We propose that these structure-based principles should be considered when interrogating the mechanisms whereby small GTPase systems ensure spatial and temporal control of cellular signalling on membranes.

RhoA regulation in space and time

RhoGTPases are well known for being controllers of cell cytoskeleton and share common features in the way they act and are controlled. These include their switch from GDP to GTP states, their regulations by different guanine exchange factors (GEFs), GTPase-activating proteins and guanosine dissociation inhibitors (GDIs), and their similar structure of active sites/membrane anchors. These very similar features often lead to the common consideration that the differences in their biological effects mainly arise from the different types of regulators and specific effectors associated with each GTPase. Focusing on data obtained through biosensors, live cell microscopy and recent optogenetic approaches, we highlight in this review that the regulation of RhoA appears to depart from Cdc42 and Rac1 modes of regulation through its enhanced lability at the plasma membrane. RhoA presents a high dynamic turnover at the membrane that is regulated not only by GDIs but also by GEFs, effectors and a possible soluble conformational state. This peculiarity of RhoA regulation may be important for the specificities of its functions, such as the existence of activity waves or its putative dual role in the initiation of protrusions and contractions.

The E3 ubiquitin ligase MG53 inhibits hepatocellular carcinoma by targeting RAC1 signaling

Ras-related C3 botulinum toxin substrate 1 (RAC1) overexpressiosn and hyperactivation are correlated with aggressive growth and other malignant characteristics in a wide variety of cancers including hepatocellular carcinoma (HCC). However, the regulatory mechanism of RAC1 expression and activation in HCC is not fully understood. Here, we demonstrated that E3 ubiquitin ligase MG53 (also known as tripartite motif 72, TRIM72) acted as a direct inhibitor of RAC1, and it catalyzed the ubiquitination of RAC1 and further inhibited RAC1 activity in HCC cells. Mechanistically, MG53 directly bound with RAC1 through its coiled-coil domain and suppressed RAC1 activity by catalyzing the Lys48 (K48)-linked polyubiquitination of RAC1 at Lys5 residue in HCC cells. We further demonstrated that MG53 significantly suppressed the malignant behaviors of HCC cells and enhanced the chemosensitivity of HCC cells to sorafenib treatment by inhibiting RAC1-MAPK signaling axis. In summary, we identified MG53 as a novel RAC1 inhibitor and tumor suppressor in HCC, and it suppressed HCC progression by inducing K48-linked polyubiquitination of RAC1 and further inhibiting the RAC1-MAPK signaling. Altogether, our investigation provided a new therapeutic strategy for RAC1 overactivated tumors by modulating MG53.

The Intracellular Distribution of the Small GTPase Rho5 and Its Dimeric Guanidine Nucleotide Exchange Factor Dck1/Lmo1 Determine Their Function in Oxidative Stress Response

Rho5, the yeast homolog of human Rac1, is a small GTPase which regulates the cell response to nutrient and oxidative stress by inducing mitophagy and apoptosis. It is activated by a dimeric GEF composed of the subunits Dck1 and Lmo1. Upon stress, all three proteins rapidly translocate from the cell surface (Rho5) and a diffuse cytosolic distribution (Dck1 and Lmo1) to mitochondria, with translocation of the GTPase depending on both GEF subunits. We here show that the latter associate with mitochondria independent from each other and from Rho5. The trapping of Dck1-GFP or GFP-Lmo1 to the mitochondrial surface by a specific nanobody fused to the transmembrane domain (TMD) of Fis1 results in a loss of function, mimicking the phenotypes of the respective gene deletions, dck1 or lmo1. Direct fusion of Rho5 to Fis1TMD, i.e., permanent attachment to the mitochondria, also mimics the phenotypes of an rho5 deletion. Together, these data suggest that the GTPase needs to be activated at the plasma membrane prior to its translocation in order to fulfill its function in the oxidative stress response. This notion is substantiated by the observation that strains carrying fusions of Rho5 to the cell wall integrity sensor Mid2, confining the GTPase to the plasma membrane, retained their function. We propose a model in which Rho5 activated at the plasma membrane represses the oxidative stress response under standard growth conditions. This repression is relieved upon its GEF-mediated translocation to mitochondria, thus triggering mitophagy and apoptosis.

Formation of self-organizing functionally distinct Rho of plants domains involves a reduced mobile population

Rho family proteins are central to the regulation of cell polarity in eukaryotes. Rho of Plants-Guanyl nucleotide Exchange Factor (ROPGEF) can form self-organizing polar domains following co-expression with an Rho of Plants (ROP) and an ROP GTPase-Activating Protein (ROPGAP). Localization of ROPs in these domains has not been demonstrated, and the mechanisms underlying domain formation and function are not well understood. Here we show that six different ROPs form self-organizing domains when co-expressed with ROPGEF3 and GAP1 in Nicotiana benthamiana or Arabidopsis (Arabidopsis thaliana). Domain formation was associated with ROP-ROPGEF3 association, reduced ROP mobility, as revealed by time-lapse imaging and Fluorescence Recovery After Photobleaching beam size analysis, and was independent of Rho GTP Dissociation Inhibitor mediated recycling. The domain formation depended on the ROPs' activation/inactivation cycles and interaction with anionic lipids via a C-terminal polybasic domain. Coexpression with the microtubule-associated protein ROP effector INTERACTOR OF CONSTITUTIVELY ACTIVE ROP 1 (ICR1) revealed differential function of the ROP domains in the ability to recruit ICR1. Taken together, the results reveal mechanisms underlying self-organizing ROP domain formation and function.

Signaling in the crowded cell

High-resolution technologies have clarified some of the principles underlying cellular actions. However, understanding how cells receive, communicate, and respond to signals is still challenging. Questions include how efficient regulation of assemblies, which execute cell actions at the nanoscales, transmits productively at micrometer scales, especially considering the crowded environment, and how the cell organization makes it happen. Here, we describe how cells can navigate long-range diffusion-controlled signaling via association/dissociation of spatially proximal entities. Dynamic clusters can span the cell, engaging in most signaling steps. Effective local concentration, allostery, scaffolding, affinities, and the chemical and mechanical properties of the macromolecules and the environment play key roles. Signaling strength and duration matter, for example, deciding if a mutation promotes cancer or developmental syndromes.

Targeting small GTPases and their downstream pathways with intracellular macromolecule binders to define alternative therapeutic strategies in cancer

The RAS superfamily of small GTPases regulates major physiological cellular processes. Mutation or deregulation of these small GTPases, their regulators and/or their effectors are associated with many diseases including cancer. Hence, targeting these classes of proteins is an important therapeutic strategy in cancer. This has been recently achieved with the approval of the first KRASG12C covalent inhibitors for the clinic. However, many other mutants and small GTPases are still considered as 'undruggable' with small molecule inhibitors because of a lack of well-defined pocket(s) at their surface. Therefore, alternative therapeutic strategies have been developed to target these proteins. In this review, we discuss the use of intracellular antibodies and derivatives - reagents that bind their antigen inside the cells - for the discovery of novel inhibitory mechanisms, targetable features and therapeutic strategies to inhibit small GTPases and their downstream pathways. These reagents are also versatile tools used to better understand the biological mechanisms regulated by small GTPases and to accelerate the drug discovery process.

Small GTPases all over invadosomes

Cell invasion is associated with numerous patho-physiologic states including cell development and metastatic dissemination. This process couples the activation of cell motility with the capacity to degrade the extracellular matrix, thereby permitting cells to pass through basal membranes. Invasion is sustained by the actions of invadosomes, an ensemble of subcellular structures with high functional homology. Invadosomes are 3D acto-adhesive structures that can also mediate local extracellular matrix degradation through the controlled delivery of proteases. Intracellular RHO GTPases play a central role in the regulation of invadosomes where their complex interplay regulates multiple invadosome functions. This review aims to provide an overview of the synergistic activities of the small GTPases in invadosome biology. This broad-based review also reinforces the importance of the spatiotemporal regulation of small GTPases and the impact of this process on invadosome dynamics.

Intrinsically disordered proteins and membranes: a marriage of convenience for cell signalling?

The structure-function paradigm has guided investigations into the molecules involved in cellular signalling for decades. The peripheries of this paradigm, however, start to unravel when considering the co-operation between proteins and the membrane in signalling processes. Intrinsically disordered regions hold distinct advantages over folded domains in terms of their binding promiscuity, sensitivity to their particular environment and their ease of modulation through post-translational modifications. Low sequence complexity and bias towards charged residues are also favourable for the multivalent electrostatic interactions that occur at the surfaces of lipid bilayers. This review looks at the principles behind the successful marriage between protein disorder and membranes in addition to the role of this partnership in modifying and regulating signalling in cellular processes. The HVR (hypervariable region) of small GTPases is highlighted as a well-studied example of the nuanced role a short intrinsically disordered region can play in the fine-tuning of signalling pathways.

Post-Translational Modification and Subcellular Compartmentalization: Emerging Concepts on the Regulation and Physiopathological Relevance of RhoGTPases

Cells and tissues are continuously exposed to both chemical and physical stimuli and dynamically adapt and respond to this variety of external cues to ensure cellular homeostasis, regulated development and tissue-specific differentiation. Alterations of these pathways promote disease progression-a prominent example being cancer. Rho GTPases are key regulators of the remodeling of cytoskeleton and cell membranes and their coordination and integration with different biological processes, including cell polarization and motility, as well as other signaling networks such as growth signaling and proliferation. Apart from the control of GTP-GDP cycling, Rho GTPase activity is spatially and temporally regulated by post-translation modifications (PTMs) and their assembly onto specific protein complexes, which determine their controlled activity at distinct cellular compartments. Although Rho GTPases were traditionally conceived as targeted from the cytosol to the plasma membrane to exert their activity, recent research demonstrates that active pools of different Rho GTPases also localize to endomembranes and the nucleus. In this review, we discuss how PTM-driven modulation of Rho GTPases provides a versatile mechanism for their compartmentalization and functional regulation. Understanding how the subcellular sorting of active small GTPase pools occurs and what its functional significance is could reveal novel therapeutic opportunities.

Super resolution microscopy reveals DHA-dependent alterations in glioblastoma membrane remodelling and cell migration

Brain fatty acid binding protein (FABP7; B-FABP) promotes glioblastoma (GBM) cell migration and is associated with tumor infiltration, properties associated with a poor prognosis in GBM patients. FABP7-expressing neural stem-like cells are known to drive tumor migration/infiltration and resistance to treatment. We have previously shown that FABP7's effects on cell migration can be reversed when GBM cells are cultured in medium supplemented with the omega-3 fatty acid, docosahexaenoic acid (DHA). Here, we use super-resolution imaging on patient-derived GBM stem-like cells to examine the importance of FABP7 and its fatty acid ligands in mitigating GBM cell migration. As FABPs are involved in fatty acid transport from membrane to cytosol, we focus on the effect of FABP7 and its ligand DHA on GBM membrane remodeling, as well as FABP7 nanoscale domain formation on GBM membrane. Using quantitative plasma membrane lipid order imaging, we show that FABP7 expression in GBM cells correlates with increased membrane lipid order, with DHA dramatically decreasing lipid order. Using super-resolution stimulated emission depletion (STED) microscopy, we observe non-uniform distribution of FABP7 on the surface of GBM cells, with FABP7 forming punctate nanoscale domains of ∼100 nm in diameter. These nanodomains are particularly enriched at the migrating front of GBM cells. Interestingly, FABP7 nanodomains are disrupted when GBM cells are cultured in DHA-supplemented medium. We demonstrate a tight link between cell migration, a higher membrane lipid order and increased FABP7 nanoscale domains. We propose that DHA-mediated disruption of membrane lipid order and FABP7 nanodomains forms the basis of FABP7/DHA-mediated inhibition of cell migration in GBM.

Molecular motion and tridimensional nanoscale localization of kindlin control integrin activation in focal adhesions

Focal adhesions (FAs) initiate chemical and mechanical signals involved in cell polarity, migration, proliferation and differentiation. Super-resolution microscopy revealed that FAs are organized at the nanoscale into functional layers from the lower plasma membrane to the upper actin cytoskeleton. Yet, how FAs proteins are guided into specific nano-layers to promote interaction with given targets is unknown. Using single protein tracking, super-resolution microscopy and functional assays, we link the molecular behavior and 3D nanoscale localization of kindlin with its function in integrin activation inside FAs. We show that immobilization of integrins in FAs depends on interaction with kindlin. Unlike talin, kindlin displays free diffusion along the plasma membrane outside and inside FAs. We demonstrate that the kindlin Pleckstrin Homology domain promotes membrane diffusion and localization to the membrane-proximal integrin nano-layer, necessary for kindlin enrichment and function in FAs. Using kindlin-deficient cells, we show that kindlin membrane localization and diffusion are crucial for integrin activation, cell spreading and FAs formation. Thus, kindlin uses a different route than talin to reach and activate integrins, providing a possible molecular basis for their complementarity during integrin activation.

Function of membrane domains in rho-of-plant signaling

In a crowded environment, establishing interactions between different molecular partners can take a long time. Biological membranes have solved this issue, as they simultaneously are fluid and possess compartmentalized domains. This nanoscale organization of the membrane is often based on weak, local, and multivalent interactions between lipids and proteins. However, from local interactions at the nanoscale, different functional properties emerge at the higher scale, and these are critical to regulate and integrate cellular signaling. Rho of Plant (ROP) proteins are small guanosine triphosphate hydrolase enzymes (GTPases) involved in hormonal, biotic, and abiotic signaling, as well as fundamental cell biological properties such as polarity, vesicular trafficking, and cytoskeleton dynamics. Association with the membrane is essential for ROP function, as well as their precise targeting within micrometer-sized polar domains (i.e. microdomains) and nanometer-sized clusters (i.e. nanodomains). Here, we review our current knowledge about the formation and the maintenance of the ROP domains in membranes. Furthermore, we propose a model for ROP membrane targeting and discuss how the nanoscale organization of ROPs in membranes could determine signaling parameters like signal specificity, amplification, and integration.

FilGAP, a GAP protein for Rac, regulates front-rear polarity and tumor cell migration through the ECM

Migrating tumor cells are characterized by a sustained front-rear asymmetry, with a front enriched in filamentous actin, which is induced by Rho small GTPase Rac. Regulation of Rac activity by its regulators should be required for effective motility. Here, we show that FilGAP, a GTPase-activating protein (GAP) for Rac, controls front-rear polarity and contributes to maintain effective tumor cell migration through the extracellular matrix (ECM). Overexpression of FilGAP in breast cancer cells induced polarized morphology and led to increased migration speed in collagen matrices, while depletion of FilGAP impaired the cell polarity and migration. FilGAP localizes to the cell front through its pleckstrin-homology (PH) domain in a phosphatidylinositol 3,4,5-trisphosphate (PIP3)-dependent manner and appears to inactivate Rac at its site. We found that the affinity of PH domain to PIP3 is critically involved in the maintenance of cell polarity. Moreover, small GTPase ADP-ribosylation factor 6 (Arf6), which binds to the FilGAP PH domain, also regulates FilGAP-mediated cell polarity and migration of breast cancer cells. We propose that FilGAP regulates front-rear polarity through its PIP3 and Arf6 binding in tumor cell migration through the ECM.

Single-Protein Tracking to Study Protein Interactions During Integrin-Based Migration

Cell migration is a complex biophysical process which involves the coordination of molecular assemblies including integrin-dependent adhesions, signaling networks and force-generating cytoskeletal structures incorporating both actin polymerization and myosin activity. During the last decades, proteomic studies have generated impressive protein-protein interaction maps, although the subcellular location, duration, strength, sequence, and nature of these interactions are still concealed. In this chapter we describe how recent developments in superresolution microscopy (SRM) and single-protein tracking (SPT) start to unravel protein interactions and actions in subcellular molecular assemblies driving cell migration.

A Plasma Membrane Nanodomain Ensures Signal Specificity during Osmotic Signaling in Plants

In the course of their growth and development, plants have to constantly perceive and react to their environment. This is achieved in cells by the coordination of complex combinatorial signaling networks. However, how signal integration and specificity are achieved in this context is unknown. With a focus on the hyperosmotic stimulus, we use live super-resolution light imaging methods to demonstrate that a Rho GTPase, Rho-of-Plant 6 (ROP6), forms stimuli-dependent nanodomains within the plasma membrane (PM). These nanodomains are necessary and sufficient to transduce production of reactive oxygen species (ROS) that act as secondary messengers and trigger several plant adaptive responses to osmotic constraints. Furthermore, osmotic signal triggers interaction between ROP6 and two NADPH oxidases that subsequently generate ROS. ROP6 nanoclustering is also needed for cell surface auxin signaling, but short-time auxin treatment does not induce ROS accumulation. We show that auxin-induced ROP6 nanodomains, unlike osmotically driven ROP6 clusters, do not recruit the NADPH oxidase, RBOHD. Together, our results suggest that Rho GTPase nano-partitioning at the PM ensures signal specificity downstream of independent stimuli.

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.

Activation of Cdc42 GTPase upon CRY2-Induced Cortical Recruitment Is Antagonized by GAPs in Fission Yeast

The small GTPase Cdc42 is critical for cell polarization in eukaryotic cells. In rod-shaped fission yeast Schizosaccharomyces pombe cells, active GTP-bound Cdc42 promotes polarized growth at cell poles, while inactive Cdc42-GDP localizes ubiquitously also along cell sides. Zones of Cdc42 activity are maintained by positive feedback amplification involving the formation of a complex between Cdc42-GTP, the scaffold Scd2, and the guanine nucleotide exchange factor (GEF) Scd1, which promotes the activation of more Cdc42. Here, we use the CRY2-CIB1 optogenetic system to recruit and cluster a cytosolic Cdc42 variant at the plasma membrane and show that this leads to its moderate activation also on cell sides. Surprisingly, Scd2, which binds Cdc42-GTP, is still recruited to CRY2-Cdc42 clusters at cell sides in individual deletion of the GEFs Scd1 or Gef1. We show that activated Cdc42 clusters at cell sides are able to recruit Scd1, dependent on the scaffold Scd2. However, Cdc42 activity is not amplified by positive feedback and does not lead to morphogenetic changes, due to antagonistic activity of the GTPase activating protein Rga4. Thus, the cell architecture is robust to moderate activation of Cdc42 at cell sides.

Rac1 conditional deletion attenuates retinal ganglion cell apoptosis by accelerating autophagic flux in a mouse model of chronic ocular hypertension

Autophagy has a fundamental role in maintaining cell homeostasis. Although autophagy has been implicated in glaucomatous pathology, how it regulates retinal ganglion cell (RGC) injury is largely unknown. In the present work, we found that biphasic autophagy in RGCs occurred in a mouse model of chronic ocular hypertension (COH), accompanied by activation of Rac1, a member of the Rho family. Rac1 conditional knockout (Rac1 cKO) in RGCs attenuated RGC apoptosis, in addition to blocking the increase in the number of autophagosomes and the expression of autophagy-related proteins (Beclin1, LC3-II/I, and p62) in COH retinas. Electron micrograph and double immunostaining of LAMP1 and LC3B showed that Rac1 cKO accelerated autolysosome fusion in RGC axons of COH mice. Inhibiting the first autophagic peak with 3-methyladenine or Atg13 siRNA reduced RGC apoptosis, whereas inhibiting the second autophagic peak with 3-MA or blocking autophagic flux by chloroquine increased RGC apoptosis. Furthermore, Rac1 cKO reduced the number of autophagosomes and apoptotic RGCs induced by rapamycin injected intravitreally, which suggests that Rac1 negatively regulates mTOR activity. Moreover, Rac1 deletion decreased Bak expression and did not interfere with the interaction of Beclin1 and Bcl-2 or Bak in COH retinas. In conclusion, autophagy promotes RGC apoptosis in the early stages of glaucoma and results in autophagic cell death in later stages. Rac1 deletion alleviates RGC damage by regulating the cross talk between autophagy and apoptosis through mTOR/Beclin1-Bak. Interfering with the Rac1/mTOR signaling pathway may provide a new strategy for treating glaucoma.

Auxin-induced signaling protein nanoclustering contributes to cell polarity formation

Cell polarity is fundamental to the development of both eukaryotes and prokaryotes, yet the mechanisms behind its formation are not well understood. Here we found that, phytohormone auxin-induced, sterol-dependent nanoclustering of cell surface transmembrane receptor kinase 1 (TMK1) is critical for the formation of polarized domains at the plasma membrane (PM) during the morphogenesis of cotyledon pavement cells (PC) in Arabidopsis. Auxin-induced TMK1 nanoclustering stabilizes flotillin1-associated ordered nanodomains, which in turn promote the nanoclustering of ROP6 GTPase that acts downstream of TMK1 to regulate cortical microtubule organization. In turn, cortical microtubules further stabilize TMK1- and flotillin1-containing nanoclusters at the PM. Hence, we propose a new paradigm for polarity formation: A diffusive signal triggers cell polarization by promoting cell surface receptor-mediated nanoclustering of signaling components and cytoskeleton-mediated positive feedback that reinforces these nanodomains into polarized domains.

The Functional Role of Voltage-Gated Sodium Channel Nav1.5 in Metastatic Breast Cancer

Voltage-gated sodium channels (VGSCs), which are abnormally expressed in various types of cancers such as breast cancer, prostate cancer, lung cancer, and cervical cancer, are involved in the metastatic process of invasion and migration. Nav1.5 is a pore-forming α subunit of VGSC encoded by SCN5A. Various studies have demonstrated that Nav1.5, often as its neonatal splice form, is highly expressed in metastatic breast cancer cells. Abnormal activation and expression of Nav1.5 trigger a variety of cellular mechanisms, including changing H+ efflux, promoting epithelial-to-mesenchymal transition (EMT) and the expression of cysteine cathepsin, to potentiate the metastasis and invasiveness of breast cancer cells in vitro and in vivo. Here, we systematically review the latest available data on the pro-metastatic effect of Nav1.5 and its underlying mechanisms in breast cancer. We summarize the factors affecting Nav1.5 expression in breast cancer cells, and discuss the potential of Nav1.5 blockers serving as candidates for breast cancer treatment.

Palmitoylated Proteins in Dendritic Spine Remodeling

Activity-responsive changes in the actin cytoskeleton are required for the biogenesis, motility, and remodeling of dendritic spines. These changes are governed by proteins that regulate the polymerization, depolymerization, bundling, and branching of actin filaments. Thus, processes that have been extensively characterized in the context of non-neuronal cell shape change and migration are also critical for learning and memory. In this review article, we highlight actin regulatory proteins that associate, at least transiently, with the dendritic plasma membrane. All of these proteins have been shown, either in directed studies or in high-throughput screens, to undergo palmitoylation, a potentially reversible, and stimulus-dependent cysteine modification. Palmitoylation increases the affinity of peripheral proteins for the membrane bilayer and contributes to their subcellular localization and recruitment to cholesterol-rich membrane microdomains.

Actin dynamics in cell migration

Cell migration is an essential process, both in unicellular organisms such as amoeba and as individual or collective motility in highly developed multicellular organisms like mammals. It is controlled by a variety of activities combining protrusive and contractile forces, normally generated by actin filaments. Here, we summarize actin filament assembly and turnover processes, and how respective biochemical activities translate into different protrusion types engaged in migration. These actin-based plasma membrane protrusions include actin-related protein 2/3 complex-dependent structures such as lamellipodia and membrane ruffles, filopodia as well as plasma membrane blebs. We also address observed antagonisms between these protrusion types, and propose a model - also inspired by previous literature - in which a complex balance between specific Rho GTPase signaling pathways dictates the protrusion mechanism employed by cells. Furthermore, we revisit published work regarding the fascinating antagonism between Rac and Rho GTPases, and how this intricate signaling network can define cell behavior and modes of migration. Finally, we discuss how the assembly of actin filament networks can feed back onto their regulators, as exemplified for the lamellipodial factor WAVE regulatory complex, tightly controlling accumulation of this complex at specific subcellular locations as well as its turnover.

How Diffusion Impacts Cortical Protein Distribution in Yeasts

Proteins associated with the yeast plasma membrane often accumulate asymmetrically within the plane of the membrane. Asymmetric accumulation is thought to underlie diverse processes, including polarized growth, stress sensing, and aging. Here, we review our evolving understanding of how cells achieve asymmetric distributions of membrane proteins despite the anticipated dissipative effects of diffusion, and highlight recent findings suggesting that differential diffusion is exploited to create, rather than dissipate, asymmetry. We also highlight open questions about diffusion in yeast plasma membranes that remain unsolved.

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.

Voltage-dependent activation of Rac1 by Nav 1.5 channels promotes cell migration

Ion channels can regulate the plasma membrane potential (Vm ) and cell migration as a result of altered ion flux. However, the mechanism by which Vm regulates motility remains unclear. Here, we show that the Nav 1.5 sodium channel carries persistent inward Na+ current which depolarizes the resting Vm at the timescale of minutes. This Nav 1.5-dependent Vm depolarization increases Rac1 colocalization with phosphatidylserine, to which it is anchored at the leading edge of migrating cells, promoting Rac1 activation. A genetically encoded FRET biosensor of Rac1 activation shows that depolarization-induced Rac1 activation results in acquisition of a motile phenotype. By identifying Nav 1.5-mediated Vm depolarization as a regulator of Rac1 activation, we link ionic and electrical signaling at the plasma membrane to small GTPase-dependent cytoskeletal reorganization and cellular migration. We uncover a novel and unexpected mechanism for Rac1 activation, which fine tunes cell migration in response to ionic and/or electric field changes in the local microenvironment.

Statistical Tests for Force Inference in Heterogeneous Environments

We devise a method to detect and estimate forces in a heterogeneous environment based on experimentally recorded stochastic trajectories. In particular, we focus on systems modeled by the heterogeneous overdamped Langevin equation. Here, the observed drift includes a "spurious” force term when the diffusivity varies in space. We show how Bayesian inference can be leveraged to reliably infer forces by taking into account such spurious forces of unknown amplitude as well as experimental sources of error. The method is based on marginalizing the force posterior over all possible spurious force contributions. The approach is combined with a Bayes factor statistical test for the presence of forces. The performance of our method is investigated analytically, numerically and tested on experimental data sets. The main results are obtained in a closed form allowing for direct exploration of their properties and fast computation. The method is incorporated into TRamWAy, an open-source software platform for automated analysis of biomolecule trajectories.

Optogenetic Rac1 engineered from membrane lipid-binding RGS-LOV for inducible lamellipodia formation

We report the construction of a single-component optogenetic Rac1 (opto-Rac1) to control actin polymerization by dynamic membrane recruitment. Opto-Rac1 is a fusion of wildtype human Rac1 small GTPase to the C-terminal region of BcLOV4, a LOV (light-oxygen-voltage) photoreceptor that rapidly binds the plasma membrane upon blue-light activation via a direct electrostatic interaction with anionic membrane phospholipids. Translocation of the fused wildtype Rac1 effector permits its activation by GEFs (guanine nucleotide exchange factors) and consequent actin polymerization and lamellipodia formation, unlike in existing single-chain systems that operate by allosteric photo-switching of constitutively active Rac1 or the heterodimerization-based (i.e. two-component) membrane recruitment of a Rac1-activating GEF. Opto-Rac1 induction of lamellipodia formation was spatially restricted to the patterned illumination field and was efficient, requiring sparse stimulation duty ratios of ∼1-2% (at the sensitivity threshold for flavin photocycling) to cause significant changes in cell morphology. This work exemplifies how the discovery of LOV proteins of distinct signal transmission modes can beget new classes of optogenetic tools for controlling cellular function.

The Vibrio cholerae MARTX toxin silences the inflammatory response to cytoskeletal damage before inducing actin cytoskeleton collapse

Multifunctional autoprocessing repeats-in-toxin (MARTX) toxins are pore-forming bacterial toxins that translocate multiple functionally independent effector domains into a target eukaryotic cell. Vibrio cholerae colonizes intestinal epithelial cells (IECs) and uses a MARTX toxin with three effector domains-an actin cross-linking domain (ACD), a Rho inactivation domain (RID), and an α/β hydrolase domain (ABH)-to suppress innate immunity and enhance colonization. We investigated whether these multiple catalytic enzymes delivered from a single toxin functioned in a coordinated manner to suppress intestinal innate immunity. Using cultured human IECs, we demonstrated that ACD-induced cytoskeletal collapse activated extracellular signal-regulated kinase, p38, and c-Jun amino-terminal kinase mitogen-activated protein kinase (MAPK) signaling to elicit a robust proinflammatory response characterized by the secretion of interleukin-8 (IL-8; also called CXCL8) and the expression of CXCL8, tumor necrosis factor (TNF), and other proinflammatory genes. However, RID and ABH, which are naturally delivered together with ACD, blocked MAPK activation through Rac1 and thus prevented ACD-induced inflammation. RID also abolished IL-8 secretion induced by heat-killed bacteria, TNF, or latrunculin A. Thus, MARTX toxins use enzymatic multifunctionality to silence the host response to bacterial factors and to the damage caused by the toxins. Furthermore, these data show how V. cholerae MARTX toxin suppresses intestinal inflammation and contributes to cholera being classically defined as a noninflammatory diarrheal disease.

Intracellular organization in cell polarity - placing organelles into the polarity loop

Many studies have investigated the processes that support polarity establishment and maintenance in cells. On the one hand, polarity complexes at the cell cortex and their downstream signaling pathways have been assigned as major regulators of polarity. On the other hand, intracellular organelles and their polarized trafficking routes have emerged as important components of polarity. In this Review, we argue that rather than trying to identify the prime 'culprit', now it is time to consider all these players as a collective. We highlight that understanding the intimate coordination between the polarized cell cortex and the intracellular compass that is defined by organelle positioning is essential to capture the concept of polarity. After briefly reviewing how polarity emerges from a dynamic maintenance of cellular asymmetries, we highlight how intracellular organelles and their associated trafficking routes provide diverse feedback for dynamic cell polarity maintenance. We argue that the asymmetric organelle compass is an indispensable element of the polarity network.

Mapping spatio-temporal dynamics of single biomolecules in living cells

We present a Bayesian framework for inferring spatio-temporal maps of diffusivity and potential fields from recorded trajectories of single molecules inside living cells. The framework naturally lets us regularise the high-dimensional inference problem using prior distributions in order to obtain robust results. To overcome the computational complexity of inferring thousands of map parameters from large single particle tracking datasets, we developed a stochastic optimisation method based on local mini-batches and parsimonious gradient calculation. We quantified the gain in convergence speed on numerical simulations, and we demonstrated for the first time temporal regularisation and aligned values of the inferred potential fields across multiple time segments. As a proof-of-concept, we mapped the dynamics of HIV-1 Gag proteins involved in the formation of virus-like particles (VLPs) on the plasma membrane of live T cells at high spatial and temporal resolutions. We focused on transient aggregation events lasting only on tenth of the time required for full VLP formation. The framework and optimisation methods are implemented in the TRamWAy open-source software platform for analysing single biomolecule dynamics.

Spatiotemporal Control of Intracellular Membrane Trafficking by Rho GTPases

As membrane-associated master regulators of cytoskeletal remodeling, Rho GTPases coordinate a wide range of biological processes such as cell adhesion, motility, and polarity. In the last years, Rho GTPases have also been recognized to control intracellular membrane sorting and trafficking steps directly; however, how Rho GTPase signaling is regulated at endomembranes is still poorly understood. In this review, we will specifically address the local Rho GTPase pools coordinating intracellular membrane trafficking with a focus on the endo- and exocytic pathways. We will further highlight the spatiotemporal molecular regulation of Rho signaling at endomembrane sites through Rho regulatory proteins, the GEFs and GAPs. Finally, we will discuss the contribution of dysregulated Rho signaling emanating from endomembranes to the development and progression of cancer.

Structural and signaling role of lipids in plasma membrane repair

The plasma membrane forms the physical barrier between the cytoplasm and extracellular space, allowing for biochemical reactions necessary for life to occur. Plasma membrane damage needs to be rapidly repaired to avoid cell death. This relies upon the coordinated action of the machinery that polarizes the repair response to the site of injury, resulting in resealing of the damaged membrane and subsequent remodeling to return the injured plasma membrane to its pre-injury state. As lipids comprise the bulk of the plasma membrane, the acts of injury, resealing, and remodeling all directly impinge upon the plasma membrane lipids. In addition to their structural role in shaping the physical properties of the plasma membrane, lipids also play an important signaling role in maintaining plasma membrane integrity. While much attention has been paid to the involvement of proteins in the membrane repair pathway, the role of lipids in facilitating plasma membrane repair remains poorly studied. Here we will discuss the current knowledge of how lipids facilitate plasma membrane repair by regulating membrane structure and signaling to coordinate the repair response, and will briefly note how lipid involvement extends beyond plasma membrane repair to the tissue repair response.

In Situ Subcellular Detachment of Cells Using a Cell-Friendly Photoresist and Spatially Modulated Light

Dynamic adhesion and detachment of subcellular regions occur during cell migration, thus a technique allowing precise control of subcellular detachment of cells will be useful for cell migration study. Previous methods for cell detachment were developed either for harvesting cells or cell sheets attached on surfaces with low resolution patterning capability, or for detaching subcellular regions located on predefined electrodes. In this paper, a method that allows in situ subcellular detachment of cells with ≈1.5 µm critical feature size while observing cells under a fluorescence microscope is introduced using a cell-friendly photoresist and spatially modulated light. Using this method, a single cell, regions in cell sheets, and a single focal adhesion complex within a cell are successfully detached. Furthermore, different subcellular regions of migrating cells are detached and changes in cell polarity and migration direction are quantitatively analyzed. This method will be useful for many applications in cell detachment, in particular when subcellular resolution is required.

The Regulatory Role of Rac1, a Small Molecular Weight GTPase, in the Development of Diabetic Retinopathy

Diabetic retinopathy, a microvascular complication of diabetes, remains the leading cause of vision loss in working age adults. Hyperglycemia is considered as the main instigator for its development, around which other molecular pathways orchestrate. Of these multiple pathways, oxidative stress induces many metabolic, functional and structural changes in the retinal cells, leading to the development of pathological features characteristic of this blinding disease. An increase in cytosolic reactive oxygen species (ROS), produced by cytosolic NADPH oxidase 2 (Nox2), is an early event in the pathogenesis of diabetic retinopathy, which leads to mitochondrial damage and retinal capillary cell apoptosis. Activation of Nox2 is mediated through an obligatory small molecular weight GTPase, Ras-related C3 botulinum toxin substrate 1 (Rac1), and subcellular localization of Rac1 and its activation are regulated by several regulators, rendering it a complex biological process. In diabetes, Rac1 is functionally activated in the retina and its vasculature, and, via Nox2-ROS, contributes to mitochondrial damage and the development of retinopathy. In addition, Rac1 is also transcriptionally activated, and epigenetic modifications play a major role in this transcriptional activation. This review focusses on the role of Rac1 and its regulation in the development and progression of diabetic retinopathy, and discusses some possible avenues for therapeutic interventions.

Developmental control of plant Rho GTPase nano-organization by the lipid phosphatidylserine

Rho guanosine triphosphatases (GTPases) are master regulators of cell signaling, but how they are regulated depending on the cellular context is unclear. We found that the phospholipid phosphatidylserine acts as a developmentally controlled lipid rheostat that tunes Rho GTPase signaling in Arabidopsis Live superresolution single-molecule imaging revealed that the protein Rho of Plants 6 (ROP6) is stabilized by phosphatidylserine into plasma membrane nanodomains, which are required for auxin signaling. Our experiments also revealed that the plasma membrane phosphatidylserine content varies during plant root development and that the level of phosphatidylserine modulates the quantity of ROP6 nanoclusters induced by auxin and hence downstream signaling, including regulation of endocytosis and gravitropism. Our work shows that variations in phosphatidylserine levels are a physiological process that may be leveraged to regulate small GTPase signaling during development.