Curved EFC/F-BAR-domain dimers are joined end to end into a filament for membrane invagination in endocytosis

Characterization of atypical BAR domain-containing proteins coded by Toxoplasma gondii

Toxoplasma gondii, the causative agent of toxoplasmosis, infects cells and replicates inside via the secretion of factors stored in specialized organelles (rhoptries, micronemes, dense granules) and the capture of host materials. The genesis of the secretory organelles and the processes of secretion and endocytosis depend on vesicular trafficking events whose molecular bases remain poorly known. Notably, there is no characterization of the BAR (Bin/Amphiphysin/Rvs) domain-containing proteins expressed by T. gondii and other apicomplexans, although such proteins are known to play critical roles in vesicular trafficking in other eukaryotes. Here, by combining structural analyses with in vitro assays and cellular observations, we have characterized TgREMIND (REgulators of Membrane Interacting Domains), involved in the genesis of rhoptries and dense granules, and TgBAR2 found at the parasite cortex. We establish that TgREMIND comprises an F-BAR domain that can bind curved neutral membranes with no strict phosphoinositide requirement and exert a membrane remodeling activity. Next, we establish that TgREMIND contains a new structural domain called REMIND, which negatively regulates the membrane-binding capacities of the F-BAR domain. In parallel, we report that TgBAR2 contains a BAR domain with an extremely basic membrane-binding interface able to deform anionic membranes into very narrow tubules. Our data show that T. gondii codes for two atypical BAR domain-containing proteins with very contrasting membrane-binding properties, allowing them to function in two distinct regions of the parasite trafficking system.

Molecular Dynamics Reveal Key Steps in BAR-Related Membrane Remodeling

Endocytosis plays a complex role in pathogen-host interactions. It serves as a pathway for pathogens to enter the host cell and acts as a part of the immune defense mechanism. Endocytosis involves the formation of lipid membrane vesicles and the reshaping of the cell membrane, a task predominantly managed by proteins containing BAR (Bin1/Amphiphysin/yeast RVS167) domains. Insights into how BAR domains can remodel and reshape cell membranes provide crucial information on infections and can aid the development of treatment. Aiming at deciphering the roles of the BAR dimers in lipid membrane bending and remodeling, we conducted extensive all-atom molecular dynamics simulations and discovered that the presence of helix kinks divides the BAR monomer into two segments-the "arm segment" and the "core segment"-which exhibit distinct movement patterns. Contrary to the prior hypothesis of BAR domains working as a rigid scaffold, we found that it functions in an "Arms-Hands" mode. These findings enhance the understanding of endocytosis, potentially advancing research on pathogen-host interactions and aiding in the identification of new treatment strategies targeting BAR domains.

Mechanistic divergences of endocytic clathrin-coated vesicle formation in mammals, yeasts and plants

Clathrin-coated vesicles (CCVs), generated by clathrin-mediated endocytosis (CME), are essential eukaryotic trafficking organelles that transport extracellular and plasma membrane-bound materials into the cell. In this Review, we explore mechanisms of CME in mammals, yeasts and plants, and highlight recent advances in the characterization of endocytosis in plants. Plants separated from mammals and yeast over 1.5 billion years ago, and plant cells have distinct biophysical parameters that can influence CME, such as extreme turgor pressure. Plants can therefore provide a wider perspective on fundamental processes in eukaryotic cells. We compare key mechanisms that drive CCV formation and explore what these mechanisms might reveal about the core principles of endocytosis across the tree of life. Fascinatingly, CME in plants appears to more closely resemble that in mammalian cells than that in yeasts, despite plants being evolutionarily further from mammals than yeast. Endocytic initiation appears to be highly conserved across these three systems, requiring similar protein domains and regulatory processes. Clathrin coat proteins and their honeycomb lattice structures are also highly conserved. However, major differences are found in membrane-bending mechanisms. Unlike in mammals or yeast, plant endocytosis occurs independently of actin, highlighting that mechanistic assumptions about CME across different systems should be made with caution.

Distinct Bin/Amphiphysin/Rvs (BAR) family proteins may assemble on the same tubule to regulate membrane organization in vivo

Intracellular membrane tubules play a crucial role in diverse cellular processes, and their regulation is facilitated by Bin-Amphiphysin-Rvs (BAR) domain-containing proteins. This study investigates the roles of Drosophila ICA69 (dICA69) (an N-BAR protein) and Drosophila CIP4 (dCIP4) (an F-BAR protein), focusing on their impact on in vivo membrane tubule organization. In contrast to the prevailing models of BAR-domain protein function, we observed colocalization of endogenous dICA69 with dCIP4-induced tubules, indicating their potential recruitment for tubule formation and maintenance. Moreover, actin-regulatory proteins such as Wasp, SCAR, and Arp2/3 were recruited at the site of CIP4-induced tubule formation. An earlier study indicated that F-BAR proteins spontaneously segregate from the N-BAR domain proteins during membrane tubule formation. In contrast, our observation supports a model in which different BAR-domain family members can associate with the same tubule and cooperate to fine-tune the tubule width, possibly by recruiting actin modulators during the generation of tubules. Our data suggests that cooperative activities of distinct BAR-domain family proteins may determine the length and width of the membrane tubule in vivo.

Phosphorylation of the F-BAR protein Hof1 drives septin ring splitting in budding yeast

A double septin ring accompanies cytokinesis in yeasts and mammalian cells. In budding yeast, reorganisation of the septin collar at the bud neck into a dynamic double ring is essential for actomyosin ring constriction and cytokinesis. Septin reorganisation requires the Mitotic Exit Network (MEN), a kinase cascade essential for cytokinesis. However, the effectors of MEN in this process are unknown. Here we identify the F-BAR protein Hof1 as a critical target of MEN in septin remodelling. Phospho-mimicking HOF1 mutant alleles overcome the inability of MEN mutants to undergo septin reorganisation by decreasing Hof1 binding to septins and facilitating its translocation to the actomyosin ring. Hof1-mediated septin rearrangement requires its F-BAR domain, suggesting that it may involve a local membrane remodelling that leads to septin reorganisation. In vitro Hof1 can induce the formation of intertwined septin bundles, while a phosphomimetic Hof1 protein has impaired septin-bundling activity. Altogether, our data indicate that Hof1 modulates septin architecture in distinct ways depending on its phosphorylation status.

GRAF1 Acts as a Downstream Mediator of Parkin to Regulate Mitophagy in Cardiomyocytes

Cardiomyocytes rely on proper mitochondrial homeostasis to maintain contractility and achieve optimal cardiac performance. Mitochondrial homeostasis is controlled by mitochondrial fission, fusion, and mitochondrial autophagy (mitophagy). Mitophagy plays a particularly important role in promoting the degradation of dysfunctional mitochondria in terminally differentiated cells. However, the precise mechanisms by which this is achieved in cardiomyocytes remain opaque. Our study identifies GRAF1 as an important mediator in PINK1-Parkin pathway-dependent mitophagy. Depletion of GRAF1 (Arhgap26) in cardiomyocytes results in actin remodeling defects, suboptimal mitochondria clustering, and clearance. Mechanistically, GRAF1 promotes Parkin-LC3 complex formation and directs autophagosomes to damaged mitochondria. Herein, we found that these functions are regulated, at least in part, by the direct binding of GRAF1 to phosphoinositides (PI(3)P, PI(4)P, and PI(5)P) on autophagosomes. In addition, PINK1-dependent phosphorylation of Parkin promotes Parkin-GRAF1-LC3 complex formation, and PINK1-dependent phosphorylation of GRAF1 (on S668 and S671) facilitates the clustering and clearance of mitochondria. Herein, we developed new phosphor-specific antibodies to these sites and showed that these post-translational modifications are differentially modified in human hypertrophic cardiomyopathy and dilated cardiomyopathy. Furthermore, our metabolic studies using serum collected from isoproterenol-treated WT and GRAF1CKO mice revealed defects in mitophagy-dependent cardiomyocyte fuel flexibility that have widespread impacts on systemic metabolism. In summary, our study reveals that GRAF1 co-regulates actin and membrane dynamics to promote cardiomyocyte mitophagy and that dysregulation of GRAF1 post-translational modifications may underlie cardiac disease pathogenesis.

EHBP1 Is Critically Involved in the Dendritic Arbor Formation and Is Coupled to Factors Promoting Actin Filament Formation

The coordinated action of a plethora of factors is required for the organization and dynamics of membranous structures critically underlying the development and function of cells, organs, and organisms. The evolutionary acquisition of additional amino acid motifs allows for expansion and/or specification of protein functions. We identify a thus far unrecognized motif specific for chordata EHBP1 proteins and demonstrate that this motif is critically required for interaction with syndapin I, an F-BAR domain-containing, membrane-shaping protein predominantly expressed in neurons. Gain-of-function and loss-of-function studies in rat primary hippocampal neurons (of mixed sexes) unraveled that EHBP1 has an important role in neuromorphogenesis. Surprisingly, our analyses uncovered that this newly identified function of EHBP1 did not require the domain responsible for Rab GTPase binding but was strictly dependent on EHBP1's syndapin I binding interface and on the presence of syndapin I in the developing neurons. These findings were underscored by temporally and spatially remarkable overlapping dynamics of EHBP1 and syndapin I at nascent dendritic branch sites. In addition, rescue experiments demonstrated the necessity of two additional EHBP1 domains for dendritic arborization, the C2 and CH domains. Importantly, the additionally uncovered critical involvement of the actin nucleator Cobl in EHBP1 functions suggested that not only static association with F-actin via EHBP1's CH domain is important for dendritic arbor formation but also actin nucleation. Syndapin interactions organize ternary protein complexes composed of EHBP1, syndapin I, and Cobl, and our functional data show that only together these factors give rise to proper cell shape during neuronal development.

Toxoplasma membrane inositol phospholipid binding protein TgREMIND is essential for secretory organelle function and host infection

Apicomplexan parasites possess specialized secretory organelles called rhoptries, micronemes, and dense granules that play a vital role in host infection. In this study, we demonstrate that TgREMIND, a protein found in Toxoplasma gondii, is necessary for the biogenesis of rhoptries and dense granules. TgREMIND contains a Fes-CIP4 homology-Bin/Amphiphysin/Rvs (F-BAR) domain, which binds to membrane phospholipids, as well as a novel uncharacterized domain that we have named REMIND (regulator of membrane-interacting domain). Both the F-BAR domain and the REMIND are crucial for TgREMIND functions. When TgREMIND is depleted, there is a significant decrease in the abundance of dense granules and abnormal transparency of rhoptries, leading to a reduction in protein secretion from these organelles. The absence of TgREMIND inhibits host invasion and parasite dissemination, demonstrating that TgREMIND is essential for the proper function of critical secretory organelles required for successful infection by Toxoplasma.

The SH3 binding site in front of the WH1 domain contributes to the membrane binding of the BAR domain protein endophilin A2

The Bin-Amphiphysin-Rvs (BAR) domain of endophilin binds to the cell membrane and shapes it into a tubular shape for endocytosis. Endophilin has a Src-homology 3 (SH3) domain at their C-terminal. The SH3 domain interacts with the proline-rich motif (PRM) that is found in proteins such as neural Wiskott-Aldrich syndrome protein (N-WASP). Here, we re-examined the binding sites of the SH3 domain of endophilin in N-WASP by machine learning-based prediction and identified the previously unrecognized binding site. In addition to the well-recognized PRM at the central proline-rich region, we found a PRM in front of the N-terminal WASP homology 1 (WH1) domain of N-WASP (NtPRM) as a binding site of the endophilin SH3 domain. Furthermore, the diameter of the membrane tubules in the presence of NtPRM mutant was narrower and wider than that in the presence of N-WASP and in its absence, respectively. Importantly, the NtPRM of N-WASP was involved in the membrane localization of endophilin A2 in cells. Therefore, the NtPRM contributes to the binding of endophilin to N-WASP in membrane remodeling.

A CIE change in our understanding of endocytic mechanisms

The past six decades have seen major advances in our understanding of endocytosis, ranging from descriptive studies based on electron microscopy to biochemical and genetic characterization of factors required for vesicle formation. Most studies focus on clathrin as the major coat protein; indeed, clathrin-mediated endocytosis (CME) is the primary pathway for internalization. Clathrin-independent (CIE) pathways also exist, although mechanistic understanding of these pathways remains comparatively elusive. Here, we discuss how early studies of CME shaped our understanding of endocytosis and describe recent advances in CIE, including pathways in model organisms that are poised to provide key insights into endocytic regulation.

Membrane Proteins and Membrane Curvature: Mutual Interactions and a Perspective on Disease Treatments

The pathogenesis of various diseases often involves an intricate interplay between membrane proteins and membrane curvature. Understanding the underlying mechanisms of this interaction could offer novel perspectives on disease treatment. In this review, we provide an introduction to membrane curvature and its association with membrane proteins. Furthermore, we delve into the impact and potential implications of this interaction in the context of disease treatment. Lastly, we discuss the prospects and challenges associated with harnessing these interactions for effective disease management, aiming to provide fresh insights into therapeutic strategies.

Fusion pore dynamics of large secretory vesicles define a distinct mechanism of exocytosis

Exocrine cells utilize large secretory vesicles (LSVs) up to 10 μm in diameter. LSVs fuse with the apical surface, often recruiting actomyosin to extrude their content through dynamic fusion pores. The molecular mechanism regulating pore dynamics remains largely uncharacterized. We observe that the fusion pores of LSVs in the Drosophila larval salivary glands expand, stabilize, and constrict. Arp2/3 is essential for pore expansion and stabilization, while myosin II is essential for pore constriction. We identify several Bin-Amphiphysin-Rvs (BAR) homology domain proteins that regulate fusion pore expansion and stabilization. We show that the I-BAR protein Missing-in-Metastasis (MIM) localizes to the fusion site and is essential for pore expansion and stabilization. The MIM I-BAR domain is essential but not sufficient for localization and function. We conclude that MIM acts in concert with actin, myosin II, and additional BAR-domain proteins to control fusion pore dynamics, mediating a distinct mode of exocytosis, which facilitates actomyosin-dependent content release that maintains apical membrane homeostasis during secretion.

Membrane shapers from two distinct superfamilies cooperate in the development of neuronal morphology

Membrane-shaping proteins are driving forces behind establishment of proper cell morphology and function. Yet, their reported structural and in vitro properties are noticeably inconsistent with many physiological membrane topology requirements. We demonstrate that dendritic arborization of neurons is powered by physically coordinated shaping mechanisms elicited by members of two distinct classes of membrane shapers: the F-BAR protein syndapin I and the N-Ank superfamily protein ankycorbin. Strikingly, membrane-tubulating activities by syndapin I, which would be detrimental during dendritic branching, were suppressed by ankycorbin. Ankycorbin's integration into syndapin I-decorated membrane surfaces instead promoted curvatures and topologies reflecting those observed physiologically. In line with the functional importance of this mechanism, ankycorbin- and syndapin I-mediated functions in dendritic arborization mutually depend on each other and on a surprisingly specific interface mediating complex formation of the two membrane shapers. These striking results uncovered cooperative and interdependent functions of members of two fundamentally different membrane shaper superfamilies as a previously unknown, pivotal principle in neuronal shape development.

Self-assembly of CIP4 drives actin-mediated asymmetric pit-closing in clathrin-mediated endocytosis

Clathrin-mediated endocytosis is pivotal to signal transduction pathways between the extracellular environment and the intracellular space. Evidence from live-cell imaging and super-resolution microscopy of mammalian cells suggests an asymmetric distribution of actin fibres near the clathrin-coated pit, which induces asymmetric pit-closing rather than radial constriction. However, detailed molecular mechanisms of this 'asymmetricity' remain elusive. Herein, we used high-speed atomic force microscopy to demonstrate that CIP4, a multi-domain protein with a classic F-BAR domain and intrinsically disordered regions, is necessary for asymmetric pit-closing. Strong self-assembly of CIP4 via intrinsically disordered regions, together with stereospecific interactions with the curved membrane and actin-regulating proteins, generates a small actin-rich environment near the pit, which deforms the membrane and closes the pit. Our results provide mechanistic insights into how disordered and structured domain collaboration promotes spatio-temporal actin polymerisation near the plasma membrane.

FNBP1 Facilitates Cervical Cancer Cell Survival by the Constitutive Activation of FAK/PI3K/AKT/mTOR Signaling

Cervical cancer is the most prevalent gynecological tumor among women worldwide. Although the incidence and mortality of cervical cancer have been declining thanks to the wide-scale implementation of cytological screening, it remains a major challenge in clinical treatment. High viability is one of the leading causes of the chemotherapeutic resistance in cervical cancers. Formin-binding protein 1 (FNBP1) could stimulate F-actin polymerization beneath the curved plasma membrane in the cell migration and endocytosis, which had previously been well defined. Here, FNBP1 was also demonstrated to play a crucial role in cervical cancer cell survival, and the knockdown of which could result in the attenuation of FAK/PI3K/AKT signaling followed by significant apoptotic accumulation and proliferative inhibition. In addition, the epidermal growth factor (hrEGF) abrogated all the biological effects mediated by the silencing of FNBP1 except for the cell adhesion decrease. These findings indicated that FNBP1 plays a key role in maintaining the activity of focal adhesion kinase (FAK) by promoting cell adhesion. The activated FAK positively regulated downstream PI3K/AKT/mTOR signaling, which is responsible for cell survival. Promisingly, FNBP1 might be a potential target against cervical cancer in combination therapy.

Complex regulation of alarmins S100A8/A9 and secretion via gasdermin D pores exacerbates autoinflammation in familial Mediterranean fever

BACKGROUND

Familial Mediterranean fever (FMF), caused by mutations in the pyrin-encoding MEFV gene, is characterized by uncontrolled caspase-1 activation and IL-1β secretion. A similar mechanism drives inflammation in cryopyrin-associated periodic fever syndrome (CAPS) caused by mutations in NLRP3. CAPS and FMF, however, result in largely different clinical manifestations, pointing to additional, autoinflammatory pathways involved in FMF. Another hallmark of FMF is extraordinarily high expression of S100A8 and S100A9. These alarmins are ligands of Toll-like receptor 4 and amplifiers of inflammation. However, the relevance of this inflammatory pathway for the pathogenesis of FMF is unknown.

OBJECTIVE

This study investigated whether mutations in pyrin result in specific secretion of S100A8/A9 alarmins through gasdermin D pores' amplifying FMF pathology.

METHODS

S100A8/A9 levels in FMF patients were quantified by enzyme-linked immunosorbent assay. In vitro models with knockout cell lines and specific protein inhibitors were used to unravel the S100A8/A9 secretion mechanism. The impact of S100A8/A9 to the pathophysiology of FMF was analyzed with FMF (MEFVV726A/V726A) and S100A9-/- mouse models. Pyrin-S100A8/A9 interaction was investigated by coimmunoprecipitation, immunofluorescence, and enzyme-linked immunosorbent assay studies.

RESULTS

The S100A8/A9 complexes directly interacted with pyrin. Knocking out pyrin, caspase-1, or gasdermin D inhibited the secretion of these S100 alarmins. Inflammatory S100A8/A9 dimers were inactivated by tetramer formation. Blocking this inactivation by targeted S100A9 deletion in a murine FMF model demonstrated the relevance of this novel autoinflammatory pathway in FMF.

CONCLUSION

This is the first proof that members of the S100 alarmin family are released in a pyrin/caspase-1/gasdermin D-dependent pathway and directly drive autoinflammation in vivo.

GxcM-Fbp17/RacC-WASP signaling regulates polarized cortex assembly in migrating cells via Arp2/3

The actin-rich cortex plays a fundamental role in many cellular processes. Its architecture and molecular composition vary across cell types and physiological states. The full complement of actin assembly factors driving cortex formation and how their activities are spatiotemporally regulated remain to be fully elucidated. Using Dictyostelium as a model for polarized and rapidly migrating cells, we show that GxcM, a RhoGEF localized specifically in the rear of migrating cells, functions together with F-BAR protein Fbp17, a small GTPase RacC, and the actin nucleation-promoting factor WASP to coordinately promote Arp2/3 complex-mediated cortical actin assembly. Overactivation of this signaling cascade leads to excessive actin polymerization in the rear cortex, whereas its disruption causes defects in cortical integrity and function. Therefore, apart from its well-defined role in the formation of the protrusions at the cell front, the Arp2/3 complex-based actin carries out a previously unappreciated function in building the rear cortical subcompartment in rapidly migrating cells.

Small GTPase Cdc42, WASP, and scaffold proteins for higher-order assembly of the F-BAR domain protein

The higher-order assembly of Bin-amphiphysin-Rvs (BAR) domain proteins, including the FCH-BAR (F-BAR) domain proteins, into lattice on the membrane is essential for the formation of subcellular structures. However, the regulation of their ordered assembly has not been elucidated. Here, we show that the higher ordered assembly of growth-arrested specific 7 (GAS7), an F-BAR domain protein, is regulated by the multivalent scaffold proteins of Wiskott-Aldrich syndrome protein (WASP)/neural WASP, that commonly binds to the BAR domain superfamily proteins, together with WISH, Nck, the activated small guanosine triphosphatase Cdc42, and a membrane-anchored phagocytic receptor. The assembly kinetics by fluorescence resonance energy transfer monitoring indicated that the GAS7 assembly on liposomes started within seconds and was further increased by the presence of these proteins. The regulated GAS7 assembly was abolished by Wiskott-Aldrich syndrome mutations both in vitro and in cellular phagocytosis. Therefore, Cdc42 and the scaffold proteins that commonly bind to the BAR domain superfamily proteins promoted GAS7 assembly.

Amphipathic peptide-phospholipid nanofibers: Kinetics of fiber formation and molecular transfer between assemblies

Understanding the kinetics of nano-assembly formation is important to elucidate the biological processes involved and develop novel nanomaterials with biological functions. In the present study, we report the kinetic mechanisms of nanofiber formation from a mixture of phospholipids and the amphipathic peptide 18A[A11C], carrying cysteine substitution of the apolipoprotein A-I-derived peptide 18A at residue 11. 18A[A11C] with acetylated N-terminus and amidated C-terminus can associate with phosphatidylcholine to form fibrous aggregates at neutral pH and lipid-to-peptide molar ratio of ∼1, although the reaction pathways of self-assembly remain unclear. Here, the peptide was added to giant 1-palmitoyl-2-oleoyl phosphatidylcholine vesicles to monitor nanofiber formation under fluorescence microscopy. The peptide initially solubilized the lipid vesicles into particles smaller than the resolution of optical microscope, and fibrous aggregates appeared subsequently. Transmission electron microscopy and dynamic light scattering analyses revealed that the vesicle-solubilized particles were spherical or circular, measuring ∼10-20 nm in diameter. The rate of nanofiber formation of 18A with 1,2-dipalmitoyl phosphatidylcholine from the particles was proportional to the square of lipid-peptide concentration in the system, suggesting that the association of particles, accompanied by conformational changes, was the rate-limiting step. Moreover, molecules in the nanofibers could be transferred between aggregates faster than those in the lipid vesicles. These findings provide useful information for the development and control of nano-assembling structures using peptides and phospholipids.

A Comparative Analysis of the Membrane Binding and Remodeling Properties of Two Related Sorting Nexin Complexes Involved in Autophagy

The sorting nexin (SNX) proteins, Atg20 and Atg24, are involved in nonselective autophagy, are necessary for efficient selective autophagy, and are required for the cytoplasm-to-vacuole transport pathway. However, the specific roles of these proteins in autophagy are not well understood. Atg20 and Atg24 each contain a Phox homology domain that facilitates phosphoinositide binding. They also each contain an SNX-Bin/Amphiphysin/Rvs domain that forms a cup-shaped dimer, capable of binding to curved membranes and remodeling those membranes in some cases. Atg20 and Atg24 form two distinct complexes, an Atg24/Atg24 homodimer and an Atg20/Atg24 heterodimer. Despite the presence of Atg24 in both complexes, it is currently unclear if these complexes have different membrane binding and remodeling properties. Therefore, in this study, we explored the membrane binding and shaping properties of these two dimeric complexes. We found that Atg24/Atg24 and Atg20/Atg24 have distinct membrane binding preferences. Both dimers recognized membranes containing phosphatidylinositol 3-phosphate [PI(3)P] and phosphatidylinositol 3,5-bisphosphate, but Atg20/Atg24 bound to a broader array of liposomes, including those lacking phosphorylated phosphatidylinositol. In addition, we discovered that while both complexes bound to autophagosomal-like liposomes containing at least 5% PI(3)P, Atg20/Atg24 was capable of binding to autophagosomal-like liposomes lacking PI(3)P. Lastly, we observed that the Atg20/Atg24 heterodimer tubulates PI(3)P-containing and autophagosomal-like liposomes, but the Atg24/Atg24 homodimer could not tubulate these liposomes. Our findings suggest that these two dimers contain distinct membrane binding and shaping properties.

Curvature dependence of BAR protein membrane association and dissociation kinetics

BAR (Bin/Amphiphysin/Rvs) domain containing proteins function as lipid bilayer benders and curvature sensors, and they contribute to membrane shaping involved in cell signaling and metabolism. The mechanism for their membrane shape sensing has been investigated by both equilibrium binding and kinetic studies. In prior research, stopped-flow spectroscopy has been used to deduce a positive dependence on membrane curvature for the binding rate constant, k_on, of a BAR protein called endophilin. However, the impact of bulk diffusion of endophilin, on the kinetic binding parameters has not been thoroughly considered. Employing similar methods, and using lipid vesicles of multiple sizes, we obtained a linear dependence of k_on on vesicle curvature. However, we found that the observed relation can be explained without considering the local curvature sensing ability of endophilin in the membrane association process. In contrast, the diffusion-independent unbinding rate constant (k_off) obtained from stopped-flow measurements shows a negative dependence on membrane curvature, which is controlled/mediated by endophilin-membrane interactions. This latter dependency, in addition to protein-protein interactions on the membrane, explains the selective binding of BAR proteins to highly curved membranes in equilibrium binding experiments.

Spontaneous local membrane curvature induced by transmembrane proteins

The (local) curvature of cellular membranes acts as a driving force for the targeting of membrane-associated proteins to specific membrane domains, as well as a sorting mechanism for transmembrane proteins, e.g., by accumulation in regions of matching spontaneous curvature. The latter measure was previously experimentally employed to study the curvature induced by the potassium channel KvAP and by aquaporin AQP0. However, the direction of the reported spontaneous curvature levels as well as the molecular driving forces governing the membrane curvature induced by these integral transmembrane proteins could not be addressed experimentally. Here, using both coarse-grained and atomistic molecular dynamics (MD) simulations, we report induced spontaneous curvature values for the homologous potassium channel Kv 1.2/2.1 Chimera (KvChim) and AQP0 embedded in unrestrained lipid bicelles that are in very good agreement with experiment. Importantly, the direction of curvature could be directly assessed from our simulations: KvChim induces a strong positive membrane curvature (≈0.036 nm^-1) whereas AQP0 causes a comparably small negative curvature (≈-0.019 nm^-1). Analyses of protein-lipid interactions within the bicelle revealed that the potassium channel shapes the surrounding membrane via structural determinants. Differences in shape of the protein-lipid interface of the voltage-gating domains between the extracellular and cytosolic membrane leaflets induce membrane stress and thereby promote a protein-proximal membrane curvature. In contrast, the water pore AQP0 displayed a high structural stability and an only faint effect on the surrounding membrane environment that is connected to its wedge-like shape.

PSTPIP1-LYP phosphatase interaction: structural basis and implications for autoinflammatory disorders

Mutations in the adaptor protein PSTPIP1 cause a spectrum of autoinflammatory diseases, including PAPA and PAMI; however, the mechanism underlying these diseases remains unknown. Most of these mutations lie in PSTPIP1 F-BAR domain, which binds to LYP, a protein tyrosine phosphatase associated with arthritis and lupus. To shed light on the mechanism by which these mutations generate autoinflammatory disorders, we solved the structure of the F-BAR domain of PSTPIP1 alone and bound to the C-terminal homology segment of LYP, revealing a novel mechanism of recognition of Pro-rich motifs by proteins in which a single LYP molecule binds to the PSTPIP1 F-BAR dimer. The residues R228, D246, E250, and E257 of PSTPIP1 that are mutated in immunological diseases directly interact with LYP. These findings link the disruption of the PSTPIP1/LYP interaction to these diseases, and support a critical role for LYP phosphatase in their pathogenesis.

Exploring Cell Surface-Nanopillar Interactions with 3D Super-Resolution Microscopy

Plasma membrane topography has been shown to strongly influence the behavior of many cellular processes such as clathrin-mediated endocytosis, actin rearrangements, and others. Recent studies have used three-dimensional (3D) nanostructures such as nanopillars to imprint well-defined membrane curvatures (the "nano-bio interface"). In these studies, proteins and their interactions were probed by two-dimensional fluorescence microscopy. However, the low resolution and limited axial detail of such methods are not optimal to determine the relative spatial position and distribution of proteins along a 100 nm-diameter object, which is below the optical diffraction limit. Here, we introduce a general method to explore the nanoscale distribution of proteins at the nano-bio interface with 10-20 nm precision using 3D single-molecule super-resolution (SR) localization microscopy. This is achieved by combining a silicone-oil immersion objective and 3D double-helix point spread function microscopy. We carefully adjust the objective to minimize spherical aberrations between quartz nanopillars and the cell. To validate the 3D SR method, we imaged the 3D shape of surface-labeled nanopillars and compared the results with electron microscopy measurements. Turning to transmembrane-anchored labels in cells, the high quality 3D SR reconstructions reveal the membrane tightly wrapping around the nanopillars. Interestingly, the cytoplasmic protein AP-2 involved in clathrin-mediated endocytosis accumulates along the nanopillar above a specific threshold of 1/R (the reciprocal of the radius) membrane curvature. Finally, we observe that AP-2 and actin preferentially accumulate at positive Gaussian curvature near the pillar caps. Our results establish a general method to investigate the nanoscale distribution of proteins at the nano-bio interface using 3D SR microscopy.

Sphingomyelin-Sequestered Cholesterol Domain Recruits Formin-Binding Protein 17 for Constricting Clathrin-Coated Pits in Influenza Virus Entry

Influenza A virus (IAV) is a global health threat. The cellular endocytic machineries harnessed by IAV remain elusive. Here, by tracking single IAV particles and quantifying the internalized IAV, we found that the sphingomyelin (SM)-sequestered cholesterol, but not the accessible cholesterol, is essential for the clathrin-mediated endocytosis (CME) of IAV. The clathrin-independent endocytosis of IAV is cholesterol-independent. Whereas, the CME of transferrin depends on SM-sequestered cholesterol and accessible cholesterol. Furthermore, three-color single-virus tracking and electron microscopy showed that the SM-cholesterol complex nanodomain is recruited to the IAV-containing clathrin-coated structure (CCS) and facilitates neck constriction of the IAV-containing CCS. Meanwhile, formin-binding protein 17 (FBP17), a membrane-bending protein which activates actin nucleation, is recruited to IAV-CCS complex in a manner dependent on the SM-cholesterol complex. We propose that the SM-cholesterol nanodomain at the neck of CCS recruits FBP17 to induce neck constriction by activating actin assembly. These results unequivocally show the physiological importance of the SM-cholesterol complex in IAV entry. Importance: IAV infects the cells by harnessing cellular endocytic machineries. Better understanding of the cellular machineries used for its entry might lead to the development of antiviral strategies, and would also provide important insights into physiological endocytic processes. This work demonstrated that a special pool of cholesterol in plasma membrane, SM-sequestered cholesterol, recruits FBP17 for the constriction of clathrin-coated pits in IAV entry. Meanwhile, the clathrin-independent cell entry of IAV is cholesterol-independent. The internalization of transferrin, the gold-standard cargo endocytosed solely via CME, is much less dependent on the SM-cholesterol complex. These results would provide new insights into IAV infection and pathway/cargo-specific involvement of cholesterol pool(s).

Revisiting I-BAR Proteins at Central Synapses

Dendritic spines, the distinctive postsynaptic feature of central nervous system (CNS) excitatory synapses, have been studied extensively as electrical and chemical compartments, as well as scaffolds for receptor cycling and positioning of signaling molecules. The dynamics of the shape, number, and molecular composition of spines, and how they are regulated by neural activity, are critically important in synaptic efficacy, synaptic plasticity, and ultimately learning and memory. Dendritic spines originate as outward protrusions of the cell membrane, but this aspect of spine formation and stabilization has not been a major focus of investigation compared to studies of membrane protrusions in non-neuronal cells. We review here one family of proteins involved in membrane curvature at synapses, the BAR (Bin-Amphiphysin-Rvs) domain proteins. The subfamily of inverse BAR (I-BAR) proteins sense and introduce outward membrane curvature, and serve as bridges between the cell membrane and the cytoskeleton. We focus on three I-BAR domain proteins that are expressed in the central nervous system: Mtss2, MIM, and IRSp53 that promote negative, concave curvature based on their ability to self-associate. Recent studies suggest that each has distinct functions in synapse formation and synaptic plasticity. The action of I-BARs is also shaped by crosstalk with other signaling components, forming signaling platforms that can function in a circuit-dependent manner. We discuss another potentially important feature-the ability of some BAR domain proteins to impact the function of other family members by heterooligomerization. Understanding the spatiotemporal resolution of synaptic I-BAR protein expression and their interactions should provide insights into the interplay between activity-dependent neural plasticity and network rewiring in the CNS.

Mechanism of negative membrane curvature generation by I-BAR domains

The membrane sculpting ability of BAR domains has been attributed to the intrinsic curvature of their banana-shaped dimeric structure. However, there is often a mismatch between this intrinsic curvature and the diameter of the membrane tubules generated. I-BAR domains are especially mysterious since they are almost flat but generate high negative membrane curvature. Here, we use atomistic implicit-solvent computer modeling to show that the membrane bending of the IRSp53 I-BAR domain is dictated by its higher oligomeric structure, whose curvature is completely unrelated to the intrinsic curvature of the dimer. Two other I-BARs give similar results, whereas a flat F-BAR sheet develops a concave membrane-binding interface, consistent with its observed positive membrane curvature generation. Laterally interacting helical spirals of I-BAR dimers on tube interiors are stable and have an enhanced binding energy that is sufficient for membrane bending to experimentally observed tubule diameters at a reasonable surface density.

Enrichment of membrane curvature-sensing proteins from Escherichia coli using spherical supported lipid bilayers

Bacteria display dynamically organized curved membrane structures, especially during cell division. The importance of membrane curvature-sensing (MCS) proteins for the recognition and regulation of biological membrane morphologies has predominately been investigated in eukaryotic cells. Recently, a technique for screening MCS proteins from solutions that contain peripheral membrane proteins was developed, and MCS protein candidates were identified from mammalian cells. The technique uses differently sized spherical supported lipid bilayers (SSLBs), which consist of spherical SiO₂ particles covered with a lipid bilayer. To discriminate between proteins possessing the MCS property, SSLBs with the same surface area were used in a comparative sedimentation assay with shotgun proteome analysis. In this study, to prove that the technique could be applied to other samples, MCS proteins in Escherichia coli were investigated. Through a comparative proteomic study, 35 and 47 proteins were enriched as candidate MCS proteins preferentially bound to SSLBs of 100 nm and 1000 nm, respectively. Among the identified MCS candidate proteins, FtsZ and SecA were further examined for their MCS properties using the two SSLB sizes, which revealed a high binding affinity for the low membrane curvature (large SSLB). This is the first study to explore MCS proteins in prokaryotic cells and the MCS property of the SecA protein. The results demonstrate a method to enrich MCS proteins that could be utilized to better elucidate membrane dynamics and protein function expression on curved membrane structures in prokaryotic cells.

Lysine acetylation regulates the interaction between proteins and membranes

Lysine acetylation regulates the function of soluble proteins in vivo, yet it remains largely unexplored whether lysine acetylation regulates membrane protein function. Here, we use bioinformatics, biophysical analysis of recombinant proteins, live-cell fluorescent imaging and genetic manipulation of Drosophila to explore lysine acetylation in peripheral membrane proteins. Analysis of 50 peripheral membrane proteins harboring BAR, PX, C2, or EHD membrane-binding domains reveals that lysine acetylation predominates in membrane-interaction regions. Acetylation and acetylation-mimicking mutations in three test proteins, amphiphysin, EHD2, and synaptotagmin1, strongly reduce membrane binding affinity, attenuate membrane remodeling in vitro and alter subcellular localization. This effect is likely due to the loss of positive charge, which weakens interactions with negatively charged membranes. In Drosophila, acetylation-mimicking mutations of amphiphysin cause severe disruption of T-tubule organization and yield a flightless phenotype. Our data provide mechanistic insights into how lysine acetylation regulates membrane protein function, potentially impacting a plethora of membrane-related processes.

Lysine acetylation regulates the function of soluble proteins in vivo, yet it remains largely unexplored whether lysine acetylation regulates the function of membrane proteins. Here, the authors map lysine acetylation predominantly in membrane-interaction regions in peripheral membrane proteins and show with three candidate proteins how lysine acetylation is a regulator of membrane protein function.

A bacterial membrane sculpting protein with BAR domain-like activity

Bin/Amphiphysin/RVS (BAR) domain proteins belong to a superfamily of coiled-coil proteins influencing membrane curvature in eukaryotes and are associated with vesicle biogenesis, vesicle-mediated protein trafficking, and intracellular signaling. Here, we report a bacterial protein with BAR domain-like activity, BdpA, from Shewanella oneidensis MR-1, known to produce redox-active membrane vesicles and micrometer-scale outer membrane extensions (OMEs). BdpA is required for uniform size distribution of membrane vesicles and influences scaffolding of OMEs into a consistent diameter and curvature. Cryo-TEM reveals that a strain lacking BdpA produces lobed, disordered OMEs rather than membrane tubules or narrow chains produced by the wild-type strain. Overexpression of BdpA promotes OME formation during planktonic growth of S. oneidensis where they are not typically observed. Heterologous expression results in OME production in Marinobacter atlanticus and Escherichia coli. Based on the ability of BdpA to alter membrane architecture in vivo, we propose that BdpA and its homologs comprise a newly identified class of bacterial BAR domain-like proteins.

FBP17-mediated finger-like membrane protrusions in cell competition between normal and RasV12-transformed cells

At the initial stage of carcinogenesis, cell competition often occurs between newly emerging transformed cells and the neighboring normal cells, leading to the elimination of transformed cells from the epithelial layer. For instance, when RasV12-transformed cells are surrounded by normal cells, RasV12 cells are apically extruded from the epithelium. However, the underlying mechanisms of this tumor-suppressive process still remain enigmatic. We first show by electron microscopic analysis that characteristic finger-like membrane protrusions are projected from both normal and RasV12 cells at their interface. In addition, FBP17, a member of the F-BAR proteins, accumulates in RasV12 cells, as well as surrounding normal cells, which plays a positive role in the formation of finger-like protrusions and apical elimination of RasV12 cells. Furthermore, cdc42 acts upstream of these processes. These results suggest that the cdc42/FBP17 pathway is a crucial trigger of cell competition, inducing “protrusion to protrusion response” between normal and RasV12-transformed cells.

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EM analysis shows finger-like membrane protrusions between normal and RasV12 cells

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Cdc42/FBP17 regulate the formation of the finger-like membrane protrusions

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Cdc42/FBP17-mediated finger-like protrusions promote elimination of RasV12 cells

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‘Protrusion to protrusion response’ triggers cell competition

Investigating the morphological dynamics of the plasma membrane by high-speed atomic force microscopy

Despite numerous recent developments in bioimaging techniques, nanoscale and live-cell imaging of the plasma membrane has been challenging because of the insufficient z-resolution of optical microscopes, as well as the lack of fluorescent probes to specifically label small membrane structures. High-speed atomic force microscopy (HS-AFM) is a powerful tool for visualising the dynamics of a specimen surface and is therefore suitable for observing plasma membrane dynamics. Recent developments in HS-AFM for live-cell imaging have enabled the visualisation of the plasma membrane and the network of cortical actin underneath the membrane in a living cell. Furthermore, correlative imaging with fluorescence microscopy allows for the direct visualisation of morphological changes of the plasma membrane together with the dynamic assembly or disassembly of proteins during the entire course of endocytosis in a living cell. Here, we review these recent advances in HS-AFM in order to analyse various cellular events occurring at the cell surface.

The phosphoinositide code is read by a plethora of protein domains

INTRODUCTION

The proteins that decipher nucleic acid- and protein-based information are well known, however, those that read membrane-encoded information remain understudied. Here we report 70 different human, microbial and viral protein folds that recognize phosphoinositides (PIs), comprising the readers of a vast membrane code.

AREAS COVERED

Membrane recognition is best understood for FYVE, PH and PX domains, which exemplify hundreds of PI code readers. Comparable lipid interaction mechanisms may be mediated by kinases, adjacent C1 and C2 domains, trafficking arrestin, GAT and VHS modules, membrane-perturbing annexin, BAR, CHMP, ENTH, HEAT, syntaxin and Tubby helical bundles, multipurpose FERM, EH, MATH, PHD, PDZ, PROPPIN, PTB and SH2 domains, as well as systems that regulate receptors, GTPases and actin filaments, transfer lipids and assembled bacterial and viral particles.

EXPERT OPINION

The elucidation of how membranes are recognized has extended the genetic code to the PI code. Novel discoveries include PIP-stop and MET-stop residues to which phosphates and metabolites are attached to block phosphatidylinositol phosphate (PIP) recognition, memteins as functional membrane protein apparatuses, and lipidons as lipid "codons" recognized by membrane readers. At least 5% of the human proteome senses such membrane signals and allows eukaryotic organelles and pathogens to operate and replicate.

Recent developments in membrane curvature sensing and induction by proteins

BACKGROUND

Membrane-bound intracellular organelles have characteristic shapes attributed to different local membrane curvatures, and these attributes are conserved across species. Over the past decade, it has been confirmed that specific proteins control the large curvatures of the membrane, whereas many others due to their specific structural features can sense the curvatures and bind to the specific geometrical cues. Elucidating the interplay between sensing and induction is indispensable to understand the mechanisms behind various biological processes such as vesicular trafficking and budding.

SCOPE OF REVIEW

We provide an overview of major classes of membrane proteins and the mechanisms of curvature sensing and induction. We then discuss the importance of membrane elastic characteristics to induce the membrane shapes similar to intracellular organelles. Finally, we survey recently available assays developed for studying the curvature sensing and induction by many proteins.

MAJOR CONCLUSIONS

Recent theoretical/computational modeling along with experimental studies have uncovered fascinating connections between lipid membrane and protein interactions. However, the phenomena of protein localization and synchronization to generate spatiotemporal dynamics in membrane morphology are yet to be fully understood.

GENERAL SIGNIFICANCE

The understanding of protein-membrane interactions is essential to shed light on various biological processes. This further enables the technological applications of many natural proteins/peptides in therapeutic treatments. The studies of membrane dynamic shapes help to understand the fundamental functions of membranes, while the medicinal roles of various macromolecules (such as proteins, peptides, etc.) are being increasingly investigated.

Functional interdependence of the actin nucleator Cobl and Cobl-like in dendritic arbor development

Local actin filament formation is indispensable for development of the dendritic arbor of neurons. We show that, surprisingly, the action of single actin filament-promoting factors was insufficient for powering dendritogenesis. Instead, this required the actin nucleator Cobl and its only evolutionary distant ancestor Cobl-like acting interdependently. This coordination between Cobl-like and Cobl was achieved by physical linkage by syndapins. Syndapin I formed nanodomains at convex plasma membrane areas at the base of protrusive structures and interacted with three motifs in Cobl-like, one of which was Ca²⁺/calmodulin-regulated. Consistently, syndapin I, Cobl-like’s newly identified N terminal calmodulin-binding site and the single Ca²⁺/calmodulin-responsive syndapin-binding motif all were critical for Cobl-like’s functions. In dendritic arbor development, local Ca²⁺/CaM-controlled actin dynamics thus relies on regulated and physically coordinated interactions of different F-actin formation-promoting factors and only together they have the power to bring about the sophisticated neuronal morphologies required for neuronal network formation in mammals.

Role of the F-BAR Family Member PSTPIP2 in Autoinflammatory Diseases

Proline-serine-threonine-phosphatase-interacting protein 2 (PSTPIP2) belongs to the Fes/CIP4 homology-Bin/Amphiphysin/Rvs (F-BAR) domain family. It exhibits lipid-binding, membrane deformation, and F-actin binding activity, suggesting broader roles at the membrane–cytoskeleton interface. PSTPIP2 is known to participate in macrophage activation, neutrophil migration, cytokine production, and osteoclast differentiation. In recent years, it has been observed to play important roles in innate immune diseases and autoinflammatory diseases (AIDs). Current research indicates that the protein tyrosine phosphatase PTP-PEST, Src homology domain-containing inositol 5’-phosphatase 1 (SHIP1), and C‐terminal Src kinase (CSK) can bind to PSTPIP2 and inhibit the development of AIDs. However, the mechanisms underlying the function of PSTPIP2 have not been fully elucidated. This article reviews the research progress and mechanisms of PSTPIP2 in AIDs. PSTPIP2 also provides a new therapeutic target for the treatment of AIDs.

Mechanical Principles Governing the Shapes of Dendritic Spines

Dendritic spines are small, bulbous protrusions along the dendrites of neurons and are sites of excitatory postsynaptic activity. The morphology of spines has been implicated in their function in synaptic plasticity and their shapes have been well-characterized, but the potential mechanics underlying their shape development and maintenance have not yet been fully understood. In this work, we explore the mechanical principles that could underlie specific shapes using a minimal biophysical model of membrane-actin interactions. Using this model, we first identify the possible force regimes that give rise to the classic spine shapes-stubby, filopodia, thin, and mushroom-shaped spines. We also use this model to investigate how the spine neck might be stabilized using periodic rings of actin or associated proteins. Finally, we use this model to predict that the cooperation between force generation and ring structures can regulate the energy landscape of spine shapes across a wide range of tensions. Thus, our study provides insights into how mechanical aspects of actin-mediated force generation and tension can play critical roles in spine shape maintenance.

Physical Properties and Reactivity of Microdomains in Phosphatidylinositol-Containing Supported Lipid Bilayer

We characterized the size, distribution, and fluidity of microdomains in a lipid bilayer containing phosphatidylinositol (PI) and revealed their roles during the two-dimensional assembly of a membrane deformation protein (FBP17). The morphology of the supported lipid bilayer (SLB) consisting of PI and phosphatidylcholine (PC) on a mica substrate was observed with atomic force microscope (AFM). Single particle tracking (SPT) was performed for the PI+PC-SLB on the mica substrate by using the diagonal illumination setup. The AFM topography showed that PI-derived submicron domains existed in the PI+PC-SLB. The spatiotemporal dependence of the lateral lipid diffusion obtained by SPT showed that the microdomain had lower fluidity than the surrounding region and worked as the obstacles for the lipid diffusion. We observed the two-dimensional assembly of FBP17, which is one of F-BAR family proteins included in endocytosis processes and has the function generating lipid bilayer tubules in vitro. At the initial stage of the FBP17 assembly, the PI-derived microdomain worked as a scaffold for the FBP17 adsorption, and the fluid surrounding region supplied FBP17 to grow the FBP17 domain via the lateral molecular diffusion. This study demonstrated an example clearly revealing the roles of two lipid microregions during the protein reaction on a lipid bilayer.

The state of F-BAR domains as membrane-bound oligomeric platforms

Fes/Cip4 homology Bin/amphiphysin/Rvs (F-BAR) domains, like all BAR domains, are dimeric units that oligomerize and bind membranes. F-BAR domains are generally coupled to additional domains that function in protein binding or have enzymatic activity. Because of their crescent shape and ability to oligomerize, F-BAR domains have been traditionally viewed as membrane-deformation modules. However, multiple independent studies have provided no evidence that certain F-BAR domains are able to tubulate membrane. Instead, a growing body of literature featuring structural, biochemical, biophysical, and microscopy-based studies supports the idea that the F-BAR domain family can be unified only by their ability to form oligomeric assemblies on membranes to provide platforms for molecular assembly.

Structural insights into membrane remodeling by SNX1

The sorting nexin (SNX) family of proteins deform the membrane to generate transport carriers in endosomal pathways. Here, we elucidate how a prototypic member, SNX1, acts in this process. Performing cryoelectron microscopy, we find that SNX1 assembles into a protein lattice that consists of helical rows of SNX1 dimers wrapped around tubular membranes in a crosslinked fashion. We also visualize the details of this structure, which provides a molecular understanding of how various parts of SNX1 contribute to its ability to deform the membrane. Moreover, we have compared the SNX1 structure with a previously elucidated structure of an endosomal coat complex formed by retromer coupled to a SNX, which reveals how the molecular organization of the SNX in this coat complex is affected by retromer. The comparison also suggests insight into intermediary stages of assembly that results in the formation of the retromer-SNX coat complex on the membrane.

Protein-dependent membrane remodeling in mitochondrial morphology and clathrin-mediated endocytosis

Cellular membranes can adopt a plethora of complex and beautiful shapes, most of which are believed to have evolved for a particular physiological reason. The closely entangled relationship between membrane morphology and cellular physiology is strikingly seen in membrane trafficking pathways. During clathrin-mediated endocytosis, for example, over the course of a minute, a patch of the more or less flat plasma membrane is remodeled into a highly curved clathrin-coated vesicle. Such vesicles are internalized by the cell to degrade or recycle plasma membrane receptors or to take up extracellular ligands. Other, steadier, membrane morphologies can be observed in organellar membranes like the endoplasmic reticulum or mitochondria. In the case of mitochondria, which are double membrane-bound, ubiquitous organelles of eukaryotic cells, especially the mitochondrial inner membrane displays an intricated ultrastructure. It is highly folded and consequently has a much larger surface than the mitochondrial outer membrane. It can adopt different shapes in response to cellular demands and changes of the inner membrane morphology often accompany severe diseases, including neurodegenerative- and metabolic diseases and cancer. In recent years, progress was made in the identification of molecules that are important for the aforementioned membrane remodeling events. In this review, we will sum up recent results and discuss the main players of membrane remodeling processes that lead to the mitochondrial inner membrane ultrastructure and in clathrin-mediated endocytosis. We will compare differences and similarities between the molecular mechanisms that peripheral and integral membrane proteins use to deform membranes.

The Role of BAR Proteins and the Glycocalyx in Brain Endothelium Transcytosis

Within the brain, endothelial cells lining the blood vessels meticulously coordinate the transport of nutrients, energy metabolites and other macromolecules essential in maintaining an appropriate activity of the brain. While small molecules are pumped across specialised molecular transporters, large macromolecular cargos are shuttled from one side to the other through membrane-bound carriers formed by endocytosis on one side, trafficked to the other side and released by exocytosis. Such a process is collectively known as transcytosis. The brain endothelium is recognised to possess an intricate vesicular endosomal network that mediates the transcellular transport of cargos from blood-to-brain and brain-to-blood. However, mounting evidence suggests that brain endothelial cells (BECs) employ a more direct route via tubular carriers for a fast and efficient transport from the blood to the brain. Here, we compile the mechanism of transcytosis in BECs, in which we highlight intracellular trafficking mediated by tubulation, and emphasise the possible role in transcytosis of the Bin/Amphiphysin/Rvs (BAR) proteins and glycocalyx (GC)-a layer of sugars covering BECs, in transcytosis. Both BAR proteins and the GC are intrinsically associated with cell membranes and involved in the modulation and shaping of these membranes. Hence, we aim to summarise the machinery involved in transcytosis in BECs and highlight an uncovered role of BAR proteins and the GC at the brain endothelium.

Phase plates in the transmission electron microscope: operating principles and applications

In this paper, we review the current state of phase plate imaging in a transmission electron microscope. We focus especially on the hole-free phase plate design, also referred to as the Volta phase plate. We discuss the implementation, operating principles and applications of phase plate imaging. We provide an imaging theory that accounts for inelastic scattering in both the sample and in the hole-free phase plate.

Comparative Study of Curvature Sensing Mediated by F-BAR and an Intrinsically Disordered Region of FBP17

Membrane curvature has emerged as an intriguing physical principle underlying biological signaling and membrane trafficking. The CIP4/FBP17/Toca-1 F-BAR subfamily is unique in the BAR family because its structurally folded F-BAR domain does not contain any hydrophobic motifs that insert into membrane. Although widely assumed so, whether the banana-shaped F-BAR domain alone can sense curvature has never been experimentally demonstrated. Using a nanobar-supported lipid bilayer system, we found that the F-BAR domain of FBP17 displayed minimal curvature sensing in vitro. In comparison, an alternatively spliced intrinsically disordered region (IDR) adjacent to the F-BAR domain has the membrane curvature-sensing ability greatly exceeding that of F-BAR domain alone. In living cells, the presence of the IDR delayed the recruitment of FBP17 in curvature-coupled cortical waves. Collectively, we propose that contrary to the common belief, FBP17's curvature-sensing capability largely originates from IDR, and not the F-BAR domain alone.

Sorting nexins: A novel promising therapy target for cancerous/neoplastic diseases

Sorting nexins (SNXs) are a diverse group of cytoplasmic- and membrane-associated phosphoinositide-binding proteins containing the PX domain proteins. The function of SNX proteins in regulating intracellular protein trafficking consists of endocytosis, endosomal sorting, and endosomal signaling. Dysfunctions of SNX proteins are demonstrated to be involved in several cancerous/neoplastic diseases. Here, we review the accumulated evidence of the molecular structure and biological function of SNX proteins and discuss the regulatory role of SNX proteins in distinct cancerous/neoplastic diseases. SNX family proteins may be a valuable potential biomarker and therapeutic strategy for diagnostics and treatment of cancerous/neoplastic diseases.

A multi-ethnic meta-analysis identifies novel genes, including ACSL5, associated with amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating progressive motor neuron disease that affects people of all ethnicities. Approximately 90% of ALS cases are sporadic and thought to have multifactorial pathogenesis. To understand the genetics of sporadic ALS, we conducted a genome-wide association study using 1,173 sporadic ALS cases and 8,925 controls in a Japanese population. A combined meta-analysis of our Japanese cohort with individuals of European ancestry revealed a significant association at the ACSL5 locus (top SNP p = 2.97 × 10⁻⁸). We validated the association with ACSL5 in a replication study with a Chinese population and an independent Japanese population (1941 ALS cases, 3821 controls; top SNP p = 1.82 × 10⁻⁴). In the combined meta-analysis, the intronic ACSL5 SNP rs3736947 showed the strongest association (p = 7.81 × 10⁻¹¹). Using a gene-based analysis of the full multi-ethnic dataset, we uncovered additional genes significantly associated with ALS: ERGIC1, RAPGEF5, FNBP1, and ATXN3. These results advance our understanding of the genetic basis of sporadic ALS.

Gen Sobue, Masao Nagasaki and colleagues report a genome-wide association study for amyotrophic lateral sclerosis (ALS) in a large, multi-ethnic cohort comprising Japanese, Chinese, and European ancestry populations. They find a significant association to variants within the ACSL5 gene and identify novel associations with 4 additional genes using a gene-based approach.

PAPA spectrum disorders

Pyogenic arthritis, pyoderma gangrenosum (PG) and acne (PAPA) syndrome is an autosomal dominant autoinflammatory syndrome due to mutations in proline-serine-threonine phosphatase interacting protein 1 (PSTPIP1) gene and presenting with cutaneous and articular manifestations. Other autoinflammatory syndromes caused by mutations in PSTPIP1 gene or characterized by clinical findings overlapping with those found in PAPA syndrome have been recently included in the group of PAPA spectrum disorders. These disorders are PASH (PG, acne and hidradenitis suppurativa [HS]), PAPASH (PASH associated with pyogenic sterile arthritis), PsAPASH (PASH combined with psoriatic arthritis [PsA], PASS (PG, acne, ankylosing spondylitis, with or without HS), PAC (PG, acne and ulcerative colitis [UC]) and PAMI syndrome (PSTPIP1-associated myeloid-related-proteinemia inflammatory syndrome). Except for PAPA and PAMI, no specific pathogenetic mutations have been identified in these syndromes. Dermatologists should be aware that PG, acne and HS may represent cutaneous signs hiding the presence of these rare entities. Systemic corticosteroids, a number of immunosuppressants and biologics, such as interleukin (IL)-1 antagonists and tumour necrosis factor (TNF) α inhibitors, are nowadays therapy for these diseases. A pathogenesis-driven treatment is the near future in the management of these conditions.

Calcineurin Aβ-Specific Anchoring Confers Isoform-Specific Compartmentation and Function in Pathological Cardiac Myocyte Hypertrophy

BACKGROUND

The Ca²⁺/calmodulin-dependent phosphatase calcineurin is a key regulator of cardiac myocyte hypertrophy in disease. An unexplained paradox is how the β isoform of the calcineurin catalytic A-subunit (CaNAβ) is required for induction of pathological myocyte hypertrophy, despite calcineurin Aα expression in the same cells. It is unclear how the pleiotropic second messenger Ca²⁺ drives excitation-contraction coupling while not stimulating hypertrophy by calcineurin in the normal heart. Elucidation of the mechanisms conferring this selectivity in calcineurin signaling should reveal new strategies for targeting the phosphatase in disease.

METHODS

Primary adult rat ventricular myocytes were studied for morphology and intracellular signaling. New Förster resonance energy transfer reporters were used to assay Ca²⁺ and calcineurin activity in living cells. Conditional gene deletion and adeno-associated virus-mediated gene delivery in the mouse were used to study calcineurin signaling after transverse aortic constriction in vivo.

RESULTS

CIP4 (Cdc42-interacting protein 4)/TRIP10 (thyroid hormone receptor interactor 10) was identified as a new polyproline domain-dependent scaffold for CaNAβ2 by yeast 2-hybrid screen. Cardiac myocyte-specific CIP4 gene deletion in mice attenuated pressure overload-induced pathological cardiac remodeling and heart failure. Blockade of CaNAβ polyproline-dependent anchoring using a competing peptide inhibited concentric hypertrophy in cultured myocytes; disruption of anchoring in vivo using an adeno-associated virus gene therapy vector inhibited cardiac hypertrophy and improved systolic function after pressure overload. Live cell Förster resonance energy transfer biosensor imaging of cultured myocytes revealed that Ca²⁺ levels and calcineurin activity associated with the CIP4 compartment were increased by neurohormonal stimulation, but minimally by pacing. Conversely, Ca²⁺ levels and calcineurin activity detected by nonlocalized Förster resonance energy transfer sensors were induced by pacing and minimally by neurohormonal stimulation, providing functional evidence for differential intracellular compartmentation of Ca²⁺ and calcineurin signal transduction.

CONCLUSIONS

These results support a structural model for Ca²⁺ and CaNAβ compartmentation in cells based on an isoform-specific mechanism for calcineurin protein-protein interaction and localization. This mechanism provides an explanation for the specific role of CaNAβ in hypertrophy and its selective activation under conditions of pathologic stress. Disruption of CaNAβ polyproline-dependent anchoring constitutes a rational strategy for therapeutic targeting of CaNAβ-specific signaling responsible for pathological cardiac remodeling in cardiovascular disease deserving of further preclinical investigation.