Dynamic actin-mediated nano-scale clustering of CD44 regulates its meso-scale organization at the plasma membrane

Semisynthesis of segmentally isotope-labeled and site-specifically palmitoylated CD44 cytoplasmic tail

CD44, a ubiquitously expressed transmembrane receptor, plays a crucial role in cell growth, migration, and tumor progression. Dimerization of CD44 is a key event in signal transduction and has emerged as a potential target for anti-tumor therapies. Palmitoylation, a posttranslational modification, disrupts CD44 dimerization and promotes CD44 accumulation in ordered membrane domains. However, the effects of palmitoylation on the structure and dynamics of CD44 at atomic resolution remain poorly understood. Here, we present a semisynthetic approach combining solid-phase peptide synthesis, recombinant expression, and native chemical ligation to investigate the impact of palmitoylation on the cytoplasmic domain (residues 669-742) of CD44 (CD44ct) by NMR spectroscopy. A segmentally isotope-labeled and site-specifically palmitoylated CD44 variant enabled NMR studies, which revealed chemical shift perturbations and indicated local and long-range conformational changes induced by palmitoylation. The long-range effects suggest altered intramolecular interactions and potential modulation of membrane association patterns. Semisynthetic, palmitoylated CD44ct serves as the basis for studying CD44 clustering, conformational changes, and localization within lipid rafts, and could be used to investigate its role as a tumor suppressor and to explore its therapeutic potential.

Lipid nanodomains and receptor signaling: From actin-based organization to membrane mechanics

The plasma membrane serves as the primary barrier between the cell's interior and its external surroundings, which places it at the forefront of intercellular communication, receptor signal transduction and the integration of mechanical forces from outside. Most of these signals are largely dependent on the plasma membrane heterogeneity which relies on lipid-lipid and lipid-protein interactions and the lateral nano-distribution of lipids organized by the dynamic network of cortical actin. In this review, we undertake an in-depth exploration of recent discoveries, which contribute significantly to the evolution from raft model to lipid nanodomains. Specifically, we will focus on their role in membrane receptor-mediated signaling in the context of cell membrane mechanics.

GLT-1a glutamate transporter nanocluster localization is associated with astrocytic actin and neuronal Kv2 clusters at sites of neuron-astrocyte contact

Introduction: Astrocytic GLT-1 glutamate transporters ensure the fidelity of glutamic neurotransmission by spatially and temporally limiting glutamate signals. The ability to limit neuronal hyperactivity relies on the localization and diffusion of GLT-1 on the astrocytic surface, however, little is known about the underlying mechanisms. We show that two isoforms of GLT-1, GLT-1a and GLT-1b, form nanoclusters on the surface of transfected astrocytes and HEK-293 cells. Methods: We used both fixed and live cell super-resolution imaging of fluorescent protein and epitope tagged proteins in co-cultures of rat astrocytes and neurons. Immunofluorescence techniques were also used. GLT1 diffusion was assessed via single particle tracking and fluorescence recovery after photobleach (FRAP). Results: We found GLT-1a, but not GLT-1b, nanoclusters concentrated adjacent to actin filaments which was maintained after addition of glutamate. GLT-1a nanocluster concentration near actin filaments was prevented by expression of a cytosolic GLT-1a C-terminus, suggesting the C-terminus is involved in the localization adjacent to cortical actin. Using super-resolution imaging, we show that astrocytic GLT-1a and actin co-localize in net-like structures around neuronal Kv2.1 clusters at points of neuron/astrocyte contact. Conclusion: Overall, these data describe a novel relationship between GLT-1a and cortical actin filaments, which localizes GLT-1a near neuronal structures responsive to ischemic insult.

Quantitative live imaging reveals a direct interaction between CD44v6 and MET in membrane domains upon activation with both MET ligands, HGF and internalin B

Deregulation of the receptor tyrosine kinase MET/hepatocyte growth factor (HGF) pathway results in several pathological processes involved in tumor progression and metastasis. In a different context, MET can serve as an entry point for the bacterium Listeria monocytogenes, when activated by the internalin B (InlB) protein during infection of non-phagocytic cells. We have previously demonstrated that MET requires CD44v6 for its ligand-induced activation. However, the stoichiometry and the steps required for the formation of this complex, are still unknown. In this work, we studied the dynamics of the ligand-induced interaction of CD44v6 with MET at the plasma membrane. Using Förster resonance energy transfer-based fluorescence lifetime imaging microscopy in T-47D cells, we evidenced a direct interaction between MET and CD44v6 promoted by HGF and InlB in live cells. In the absence of MET, fluorescence correlation spectroscopy experiments further showed the dimerization of CD44v6 and the increase of its diffusion induced by HGF and InlB. In the presence of MET, stimulation of the cells by HGF or InlB significantly decreased the diffusion of CD44v6, in line with the formation of a ternary complex of MET with CD44v6 and HGF/InlB. Finally, similarly to HGF/InlB, disruption of liquid-ordered domains (Lo) by methyl-β-cyclodextrin increased CD44v6 mobility suggesting that these factors induce the exit of CD44v6 from the Lo domains. Our data led us to propose a model for MET activation, where CD44v6 dimerizes and diffuses rapidly out of Lo domains to form an oligomeric MET/ligand/CD44v6 complex that is instrumental for MET activation.

A Survey of Models of Cell Membranes: Toward a New Understanding of Membrane Organization

The cell membrane, the boundary that separates living cells from their environment, has been the subject of study for over a century. The fluid-mosaic model of Singer and Nicolson in 1972 proposed the plasma membrane as a two-dimensional fluid composed of lipids and proteins. Fifty years hence, advances in biophysical and biochemical tools, particularly optical imaging techniques, have allowed for a better understanding of the physical nature, organization, and composition of cell membranes. This has been made possible by visualizing membrane heterogeneities and their dynamics and appreciating the asymmetrical arrangement of lipids in living cell membranes. Despite these advances, mechanisms underlying the local spatiotemporal organization of membrane components remain unclear. This review surveys various models of membrane organization, culminating in a new model that incorporates nonequilibrium processes and forces exerted by interactions with extramembrane elements such as the actin cytoskeleton. The proposed model provides a comprehensive understanding of membrane organization, taking into account the dynamic nature of the cell membrane and its interactions with its immediate environment.

Molecular weight of hyaluronic acid crosslinked into biomaterial scaffolds affects angiogenic potential

While hyaluronic acid (HA)-based hydrogels have been used clinically for decades, the mechanisms by which HA exerts molecular weight-dependent bioactivity and how chemical modification and crosslinking may affect molecular weight-dependent bioactivity remain poorly understood. This knowledge gap presents a significant barrier to designing HA hydrogels with predictable bioactivities. As HA has been widely reported to have molecular weight-dependent effects on endothelial cells (ECs), we investigated how the molecular weight of HA in either soluble or crosslinked forms affects angiogenesis and interrogated CD44 clustering on the surface of endothelial cells as a candidate mechanism for these affects. Using soluble HA, our results show high molecular weight (HMW) HA, but not low molecular weight (LMW) HA, increased viability and tube formation in cultured human cerebral microvascular ECs (HCMVECs). No size of HA affected proliferation. When HCMVECs were cultured with crosslinked HA of varying molecular weights in the form of HA-based microporous annealed particle scaffold (HMAPS), the cell response was comparable to when cultured with soluble HA. Similarly, when implanted subcutaneously, HMAPS with HMW HA were more vascularized than those with LMW HA. We also show that antibody-mediated CD44 clustering resulted in HCMVECs with increased viability and tube-like structure formation in a manner comparable to exposure to HMW HA, suggesting that HMW acts through CD44 clustering. STATEMENT OF SIGNIFICANCE: Biomaterials based on hyaluronic acid (HA), a bioactive extracellular matrix polysaccharide, have been used in clinical products for several years. Despite the knowledge that HA molecular weight heavily influences its bioactivity, molecular weight has been largely ignored in the development of HA-based biomaterials. Given the high viscosity of high molecular weight HA typically found in native tissues, lower molecular weight polysaccharides have been used most commonly for biomaterial fabrication. By comparing the ability of injectable, microporous annealed particle scaffolds (MAPS) fabricated from variably sized HA to promote angiogenesis, this study demonstrates that MAPS with high molecular weight HA better support vascularization, likely through an unique ability to induce clustering of CD44 receptors on endothelial cells.

Heterogeneous biological membranes regulate protein partitioning via fluctuating diffusivity

Cell membranes phase separate into ordered L o and disordered L d domains depending on their compositions. This membrane compartmentalization is heterogeneous and regulates the localization of specific proteins related to cell signaling and trafficking. However, it is unclear how the heterogeneity of the membranes affects the diffusion and localization of proteins in L o and L d domains. Here, using Langevin dynamics simulations coupled with the phase-field (LDPF) method, we investigate several tens of milliseconds-scale diffusion and localization of proteins in heterogeneous biological membrane models showing phase separation into L o and L d domains. The diffusivity of proteins exhibits temporal fluctuations depending on the field composition. Increases in molecular concentrations and domain preference of the molecule induce subdiffusive behavior due to molecular collisions by crowding and confinement effects, respectively. Moreover, we quantitatively demonstrate that the protein partitioning into the L o domain is determined by the difference in molecular diffusivity between domains, molecular preference of domain, and molecular concentration. These results pave the way for understanding how biological reactions caused by molecular partitioning may be controlled in heterogeneous media. Moreover, the methodology proposed here is applicable not only to biological membrane systems but also to the study of diffusion and localization phenomena of molecules in various heterogeneous systems.

Broadband Plasmonic Nanoantennas for Multi-Color Nanoscale Dynamics in Living Cells

Recently, the implementation of plasmonic nanoantennas has opened new possibilities to investigate the nanoscale dynamics of individual biomolecules in living cells. However, studies so far have been restricted to single molecular species as the narrow wavelength resonance of gold-based nanostructures precludes the simultaneous interrogation of different fluorescently labeled molecules. Here, broadband aluminum-based nanoantennas carved at the apex of near-field probes are exploited to resolve nanoscale-dynamic molecular interactions on living cell membranes. Through multicolor excitation, the authors simultaneously recorded fluorescence fluctuations of dual-color labeled transmembrane receptors known to form nanoclusters. Fluorescence cross-correlation studies revealed transient interactions between individual receptors in regions of ≈60 nm. Moreover, the high signal-to-background ratio provided by the antenna illumination allowed the authors to directly detect fluorescent bursts arising from the passage of individual receptors underneath the antenna. Remarkably, by reducing the illumination volume below the characteristic receptor nanocluster sizes, the molecular diffusion within nanoclusters is resolved and distinguished from nanocluster diffusion. Spatiotemporal characterization of transient interactions between molecules is crucial to understand how they communicate with each other to regulate cell function. This work demonstrates the potential of broadband photonic antennas to study multi-molecular events and interactions in living cell membranes with unprecedented spatiotemporal resolution.

Glioblastoma Spheroid Invasion through Soft, Brain-Like Matrices Depends on Hyaluronic Acid-CD44 Interactions

Increased secretion of hyaluronic acid (HA), a glycosaminoglycan abundant in the brain extracellular matrix (ECM), correlates with worse clinical outcomes for glioblastoma (GBM) patients. GBM cells aggressively invade the brain parenchyma while encountering spatiotemporal changes in their local ECM, including HA concentration. To investigate how varying HA concentrations affect GBM invasion, patient-derived GBM cells are cultured within a soft, 3D matrix in which HA concentration is precisely varied and cell migration observed. Data demonstrate that HA concentration can determine the invasive activity of patient-derived GBM cells in a biphasic and highly sensitive manner, where the absolute concentration of HA at which cell migration peaked is specific to each patient-derived line. Furthermore, evidence that this response relies on phosphorylated ezrin, which interacts with the intracellular domain of HA-engaged CD44 to effectively link the actin cytoskeleton to the local ECM is provided. Overall, this study highlights CD44-HA binding as a major mediator of GBM cell migration that acts independently of integrins and focal adhesion complexes and suggests that targeting HA-CD44-ezrin interactions represents a promising therapeutic strategy to prevent tumor cell invasion in the brain.

Caveolin-1 dolines form a distinct and rapid caveolae-independent mechanoadaptation system

In response to different types and intensities of mechanical force, cells modulate their physical properties and adapt their plasma membrane (PM). Caveolae are PM nano-invaginations that contribute to mechanoadaptation, buffering tension changes. However, whether core caveolar proteins contribute to PM tension accommodation independently from the caveolar assembly is unknown. Here we provide experimental and computational evidence supporting that caveolin-1 confers deformability and mechanoprotection independently from caveolae, through modulation of PM curvature. Freeze-fracture electron microscopy reveals that caveolin-1 stabilizes non-caveolar invaginations-dolines-capable of responding to low-medium mechanical forces, impacting downstream mechanotransduction and conferring mechanoprotection to cells devoid of caveolae. Upon cavin-1/PTRF binding, doline size is restricted and membrane buffering is limited to relatively high forces, capable of flattening caveolae. Thus, caveolae and dolines constitute two distinct albeit complementary components of a buffering system that allows cells to adapt efficiently to a broad range of mechanical stimuli.

Fluorescence microscopy imaging of a neurotransmitter receptor and its cell membrane lipid milieu

Hampered by the diffraction phenomenon, as expressed in 1873 by Abbe, applications of optical microscopy to image biological structures were for a long time limited to resolutions above the ∼200 nm barrier and restricted to the observation of stained specimens. The introduction of fluorescence was a game changer, and since its inception it became the gold standard technique in biological microscopy. The plasma membrane is a tenuous envelope of 4 nm-10 nm in thickness surrounding the cell. Because of its highly versatile spectroscopic properties and availability of suitable instrumentation, fluorescence techniques epitomize the current approach to study this delicate structure and its molecular constituents. The wide spectral range covered by fluorescence, intimately linked to the availability of appropriate intrinsic and extrinsic probes, provides the ability to dissect membrane constituents at the molecular scale in the spatial domain. In addition, the time resolution capabilities of fluorescence methods provide complementary high precision for studying the behavior of membrane molecules in the time domain. This review illustrates the value of various fluorescence techniques to extract information on the topography and motion of plasma membrane receptors. To this end I resort to a paradigmatic membrane-bound neurotransmitter receptor, the nicotinic acetylcholine receptor (nAChR). The structural and dynamic picture emerging from studies of this prototypic pentameric ligand-gated ion channel can be extrapolated not only to other members of this superfamily of ion channels but to other membrane-bound proteins. I also briefly discuss the various emerging techniques in the field of biomembrane labeling with new organic chemistry strategies oriented to applications in fluorescence nanoscopy, the form of fluorescence microscopy that is expanding the depth and scope of interrogation of membrane-associated phenomena.

Novel Antibody Exerts Antitumor Effect through Downregulation of CD147 and Activation of Multiple Stress Signals

CD147 is an immunoglobulin-like receptor that is highly expressed in various cancers and involved in the growth, metastasis, and activation of inflammatory pathways via interactions with various functional molecules, such as integrins, CD44, and monocarboxylate transporters. Through screening of CD147-targeting antibodies with antitumor efficacy, we discovered a novel rat monoclonal antibody ^#147D. This humanized IgG4-formatted antibody, h4^#147D, showed potent antitumor efficacy in xenograft mouse models harboring the human PDAC cell line MIA PaCa-2, HCC cell line Hep G2, and CML cell line KU812, which featured low sensitivity to the corresponding standard-of-care drugs (gemcitabine, sorafenib, and imatinib, respectively). An analysis of tumor cells derived from MIA PaCa-2 xenograft mice treated with h4^#147D revealed that cell surface expression of CD147 and its binding partners, including CD44 and integrin α3β1/α6β1, was significantly reduced by h4^#147D. Inhibition of focal adhesion kinase (FAK), activation of multiple stress responsible signal proteins such as c-JunN-terminal kinase (JNK) and mitogen-activated protein kinase p38 (p38MAPK), and expression of SMAD4, as well as activation of caspase-3 were obviously observed in the tumor cells, suggesting that h4^#147D induced tumor shrinkage by inducing multiple stress responsible signals. These results suggest that the anti-CD147 antibody h4^#147D offers promise as a new antibody drug candidate.

A Uniform and Isotropic Cytoskeletal Tiling Fills Dendritic Spines

Dendritic spines are submicron, subcellular compartments whose shape is defined by actin filaments and associated proteins. Accurately mapping the cytoskeleton is a challenge, given the small size of its components. It remains unclear whether the actin-associated structures analyzed in dendritic spines of neurons in vitro apply to dendritic spines of intact, mature neurons in situ. Here, we combined advanced preparative methods with multitilt serial section electron microscopy (EM) tomography and computational analysis to reveal the full three-dimensional (3D) internal architecture of spines in the intact brains of male mice at nanometer resolution. We compared hippocampal (CA1) pyramidal cells and cerebellar Purkinje cells in terms of the length distribution and connectivity of filaments, their branching-angles and absolute orientations, and the elementary loops formed by the network. Despite differences in shape and size across spines and between spine heads and necks, the internal organization was remarkably similar in both neuron types and largely homogeneous throughout the spine volume. In the tortuous mesh of highly branched and interconnected filaments, branches exhibited no preferred orientation except in the immediate vicinity of the cell membrane. We found that new filaments preferentially split off from the convex side of a bending filament, consistent with the behavior of Arp2/3-mediated branching of actin under mechanical deformation. Based on the quantitative analysis, the spine cytoskeleton is likely subject to considerable mechanical force in situ.

Mapping the Nicotinic Acetylcholine Receptor Nanocluster Topography at the Cell Membrane with STED and STORM Nanoscopies

The cell-surface topography and density of nicotinic acetylcholine receptors (nAChRs) play a key functional role in the synapse. Here we employ in parallel two labeling and two super-resolution microscopy strategies to characterize the distribution of this receptor at the plasma membrane of the mammalian clonal cell line CHO-K1/A5. Cells were interrogated with two targeted techniques (confocal microscopy and stimulated emission depletion (STED) nanoscopy) and single-molecule nanoscopy (stochastic optical reconstruction microscopy, STORM) using the same fluorophore, Alexa Fluor 647, tagged onto either α-bungarotoxin (BTX) or the monoclonal antibody mAb35. Analysis of the topography of nanometer-sized aggregates (“nanoclusters”) was carried out using STORMGraph, a quantitative clustering analysis for single-molecule localization microscopy based on graph theory and community detection, and ASTRICS, an inter-cluster similarity algorithm based on computational geometry. Antibody-induced crosslinking of receptors resulted in nanoclusters with a larger number of receptor molecules and higher densities than those observed in BTX-labeled samples. STORM and STED provided complementary information, STED rendering a direct map of the mesoscale nAChR distribution at distances ~10-times larger than the nanocluster centroid distances measured in STORM samples. By applying photon threshold filtering analysis, we show that it is also possible to detect the mesoscale organization in STORM images.

A condensate dynamic instability orchestrates actomyosin cortex activation

A key event at the onset of development is the activation of a contractile actomyosin cortex during the oocyte-to-embryo transition^1-3. Here we report on the discovery that, in Caenorhabditis elegans oocytes, actomyosin cortex activation is supported by the emergence of thousands of short-lived protein condensates rich in F-actin, N-WASP and the ARP2/3 complex^4-8 that form an active micro-emulsion. A phase portrait analysis of the dynamics of individual cortical condensates reveals that condensates initially grow and then transition to disassembly before dissolving completely. We find that, in contrast to condensate growth through diffusion⁹, the growth dynamics of cortical condensates are chemically driven. Notably, the associated chemical reactions obey mass action kinetics that govern both composition and size. We suggest that the resultant condensate dynamic instability¹⁰ suppresses coarsening of the active micro-emulsion¹¹, ensures reaction kinetics that are independent of condensate size and prevents runaway F-actin nucleation during the formation of the first cortical actin meshwork.

Enhancing search efficiency through diffusive echo

Despite having been studied for decades, first passage processes remain an active area of research. In this contribution we examine a particle diffusing in an annulus with an inner absorbing boundary and an outer reflective boundary. We obtain analytic expressions for the joint distribution of the hitting time and the hitting angle in two and three dimensions. For certain configurations we observe a ``diffusive echo", i.e. two well-defined maxima in the first passage time distribution to a targeted position on the absorbing boundary. This effect, which results from the interplay between the starting location and the environmental constraints, may help to significantly increase the efficiency of the random search by generating a high, sustained flux to the targeted position over a short period. Finally, we examine the corresponding one-dimensional system for which there is no well-defined echo. In a confined system, the flux integrated over all target positions always displays a shoulder. This does not, however, guarantee the presence of an echo in the joint distribution.

Active emulsions in living cell membranes driven by contractile stresses and transbilayer coupling

The spatiotemporal organization of proteins and lipids on the cell surface has direct functional consequences for signaling, sorting, and endocytosis. Earlier studies have shown that multiple types of membrane proteins, including transmembrane proteins that have cytoplasmic actin binding capacity and lipid-tethered glycosylphosphatidylinositol-anchored proteins (GPI-APs), form nanoscale clusters driven by active contractile flows generated by the actin cortex. To gain insight into the role of lipids in organizing membrane domains in living cells, we study the molecular interactions that promote the actively generated nanoclusters of GPI-APs and transmembrane proteins. This motivates a theoretical description, wherein a combination of active contractile stresses and transbilayer coupling drives the creation of active emulsions, mesoscale liquid order (lo) domains of the GPI-APs and lipids, at temperatures greater than equilibrium lipid phase segregation. To test these ideas, we use spatial imaging of molecular clustering combined with local membrane order, and we demonstrate that mesoscopic domains enriched in nanoclusters of GPI-APs are maintained by cortical actin activity and transbilayer interactions and exhibit significant lipid order, consistent with predictions of the active composite model.

Membrane nanodomains modulate formin condensation for actin remodeling in Arabidopsis innate immune responses

The assembly of macromolecules on the plasma membrane concentrates cell surface biomolecules into nanometer- to micrometer-scale clusters (nano- or microdomains) that help the cell initiate or respond to signals. In plant-microbe interactions, the actin cytoskeleton undergoes rapid remodeling during pathogen-associated molecular pattern-triggered immunity (PTI). The nanoclustering of formin-actin nucleator proteins at the cell surface has been identified as underlying actin nucleation during plant innate immune responses. Here, we show that the condensation of nanodomain constituents and the self-assembly of remorin proteins enables this mechanism of controlling formin condensation and activity during innate immunity in Arabidopsis thaliana. Through intrinsically disordered region-mediated remorin oligomerization and formin interaction, remorin gradually recruits and condenses formins upon PTI activation in lipid bilayers, consequently increasing actin nucleation in a time-dependent manner postinfection. Such nanodomain- and remorin-mediated regulation of plant surface biomolecules is expected to be a general feature of plant innate immune responses that creates spatially separated biochemical compartments and fine tunes membrane physicochemical properties for transduction of immune signals in the host.

Triple-color single-molecule imaging for analysis of the role of receptor oligomers in signal transduction

Membrane receptors provide interfaces of various extracellular stimuli to transduce the signal into the cell. Receptors are required to possess such conflicting properties as high sensitivity and noise reduction for the cell to keep its homeostasis and appropriate responses. To understand the mechanisms by which these functions are achieved, single-molecule monitoring of the motilities of receptors and signaling molecules on the plasma membrane is one of the most direct approaches. This review article introduces several recent single-molecule imaging studies of receptors, including the author’s recent work on triple-color single-molecule imaging of G protein-coupled receptors. Based on these researches, advantages and perspectives of the single-molecule imaging approach to solving the mechanisms of receptor functions are illustrated.

Excitation energy migration to study protein oligomerization and amyloid formation

Excitation energy migration via homo-FRET (Förster resonance energy transfer) is a unique variant of traditional FRET that involves a non-radiative energy transfer between the dipoles of two or more chemical identical fluorophores in close proximity and with an overlap between its excitation and emission spectra. Such energy migrations between chemically identical fluorophores within the Förster distance having their dipoles oriented over a wide angular spread results in the depolarization of fluorescence anisotropy depending on the local density of the fluorophores. Therefore, this methodology can be employed to study protein oligomerization and amyloid fibril formation. The conceptual framework involves extracting structural information by identifying proximal and distal locations in supramolecular assemblies by monitoring the efficiency of homo-FRET between fluorophore-conjugated protein molecules within these supramolecular assemblies. This review highlights two such cases in which excitation energy migration via homo-FRET was used to characterize the formation of membrane-mediated β-sheet rich oligomers of the prion protein as well as to construct a site-specific 2D-proximity correlation map to probe inter-residue proximities within the highly organized amyloid fibrils of α-synuclein. Energy migration studies will find applications in studying a wide range of biomolecular assemblies such as lipid-protein complexes, oligomers, amyloids, and phase-separated condensates.

Formin nanoclustering-mediated actin assembly during plant flagellin and DSF signaling

Plants respond to bacterial infection acutely with actin remodeling during plant immune responses. The mechanisms by which bacterial virulence factors (VFs) modulate plant actin polymerization remain enigmatic. Here, we show that plant-type-I formin serves as the molecular sensor for actin remodeling in response to two bacterial VFs: Xanthomonas campestris pv. campestris (Xcc) diffusible signal factor (DSF), and pathogen-associated molecular pattern (PAMP) flagellin in pattern-triggered immunity (PTI). Both VFs regulate actin assembly by tuning the clustering and nucleation activity of formin on the plasma membrane (PM) at the nano-sized scale. By being integrated within the cell-wall-PM-actin cytoskeleton (CW-PM-AC) continuum, the dynamic behavior and function of formins are highly dependent on each scaffold layer's composition within the CW-PM-AC continuum during both DSF and PTI signaling. Our results reveal a central mechanism for rapid actin remodeling during plant-bacteria interactions, in which bacterial signaling molecules fine tune plant formin nanoclustering in a host mechanical-structure-dependent manner.

Impact of Glycans on Lipid Membrane Dynamics at the Nanoscale Unveiled by Planar Plasmonic Nanogap Antennas and Atomic Force Spectroscopy

Lateral compartmentalization of the plasma membrane is a prominent feature present at multiple spatiotemporal scales that regulates key cellular functions. The extracellular glycocalyx matrix has recently emerged as an important player that modulates the organization of specific receptors and patterns the lipid bilayer itself. However, experimental limitations in investigating its impact on the membrane nanoscale dynamics have hampered detailed studies. Here, we used photonic nanoantenna arrays combined with fluorescence correlation spectroscopy to investigate the influence of hyaluronic acid (HA), a prominent glycosaminoglycan, on the nanoscale organization of mimetic lipid bilayers. Using atomic force microscopy and force spectroscopy, we further correlated our dynamic measurements with the morphology and mechanical properties of bilayers at the nanoscale. Overall, we find that HA has a profound effect on the dynamics, nanoscale organization, and mechanical properties of lipid bilayers that are enriched in sphingolipids and/or cholesterol, such as those present in living cells.

Proteolytic Processing of CD44 and Its Implications in Cancer

CD44 is a transmembrane glycoprotein expressed in several healthy and tumor tissues. Modifications in its structure contribute differently to the activity of this molecule. One modification that has provoked interest is the consecutive cleavage of the CD44 extracellular ectodomain by enzymes that belong mainly to the family of metalloproteases. This process releases biologically active substrates, via alternative splice forms of CD44, that generate CD44v3 or v6 isoforms which participate in the transcriptional regulation of genes and proteins associated to signaling pathways involved in the development of cancer. These include the protooncogene tyrosine-protein kinase Src (c-Src)/signal transducer and activator of transcription 3 (STAT3), the epithelial growth factor receptor, the estrogen receptor, Wnt/βcatenin, or Hippo signaling pathways all of which are associated to cell proliferation, differentiation, or cancer progression. Whereas CD44 still remains as a very useful prognostic cell marker in different pathologies, the main topic is that the generation of CD44 intracellular fragments assists the regulation of transcriptional proteins involved in the cell cycle, cell metabolism, and most importantly, the regulation of some stem cell-associated markers.

Dynamic Plasma Membrane Organization: A Complex Symphony

Membrane protein organization is essential for proper cellular functioning and the result of a dynamic exchange between protein monomers, nanoscale protein clusters, and microscale higher-order structures. This exchange is affected by both lipid bilayer intrinsic factors, such as lipid rafts and tetraspanins, and extrinsic factors, such as cortical actin and galectins. Because membrane organizers act jointly like instruments in a symphony, it is challenging to define the 'key' organizers. Here, we posit, for the first time, definitions of key intrinsic and extrinsic membrane organizers. Tetraspanin nanodomains are key organizers that are often overlooked. We discuss how different key organizers can collaborate, which is important to get a full grasp of plasma membrane biology.

Toward a new picture of the living plasma membrane

Our understanding of the plasma membrane structure has undergone a major change since the proposal of the fluid mosaic model of Singer and Nicholson in the 1970s. In this model, the membrane, composed of over thousand lipid and protein species, is organized as a well-equilibrated two-dimensional fluid. Here, the distribution of lipids is largely expected to reflect a multicomponent system, and proteins are expected to be surrounded by an annulus of specialized lipid species. With the recognition that a multicomponent lipid membrane is capable of phase segregation, the membrane is expected to appear as patchwork quilt pattern of membrane domains. However, the constituents of a living membrane are far from being well equilibrated. The living cell membrane actively maintains a trans-bilayer asymmetry of composition, and its constituents are subject to a number of dynamic processes due to synthesis, lipid transfer as well as membrane traffic and turnover. Moreover, membrane constituents engage with the dynamic cytoskeleton of a living cell, and are both passively as well as actively manipulated by this engagement. The extracellular matrix and associated elements also interact with membrane proteins contributing to another layer of interaction. At the nano- and mesoscale, the organization of lipids and proteins emerge from these encounters, as well as from protein-protein, protein-lipid, and lipid-lipid interactions in the membrane. New methods to study the organization of membrane components at these scales have also been developed, and provide an opportunity to synthesize a new picture of the living cell surface as an active membrane composite.

Analysis of sparse molecular distributions in fibrous arrangements based on the distance to the first neighbor in single molecule localization microscopy

Single Molecule Localization Microscopy (SMLM) currently attains a lateral resolution of around 10 nm approaching molecular size. Together with increasingly specific fluorescent labeling, it opens the possibility to quantitatively analyze molecular organization. When the labeling density is high enough, SMLM provides clear images of the molecular organization. However, either due to limited labeling efficiency or due to intrinsically low molecular abundance, SMLM delivers a small set of sparse and highly precise localizations. In this work, we introduce a correlation analysis of molecular locations based on the functional dependence of the complementary cumulative distribution function (CCDF) of the distance to the first neighbor (r₁). We demonstrate that the log(-log(CCDF(r₁))) vs. log(r₁) is characterized by a scaling exponent n that takes extreme values of 2 for a random 2D distribution and 1 for a strictly linear arrangement, and find that n is a robust and sensitive metric to distinguish characteristics of the underlying structure responsible for the molecular distribution, even at a very low labeling density. The method enables the detection of fibrillary organization and the estimation of the diameter of host fibers under conditions where a visual inspection provides no clue.