Size-Dependent Segregation Controls Macrophage Phagocytosis of Antibody-Opsonized Targets.

Macrophages protect the body from damage and disease by targeting antibody-opsonized cells for phagocytosis. Though antibodies can be raised against antigens with diverse structures, shapes, and sizes, it is unclear why some are more effective at triggering immune responses than others. Here, we define an antigen height threshold that regulates phagocytosis of both engineered and cancer-specific antigens by macrophages. Using a reconstituted model of antibody-opsonized target cells, we find that phagocytosis is dramatically impaired for antigens that position antibodies >10 nm from the target surface. Decreasing antigen height drives segregation of antibody-bound Fc receptors from the inhibitory phosphatase CD45 in an integrin-independent manner, triggering Fc receptor phosphorylation and promoting phagocytosis. Our work shows that close contact between macrophage and target is a requirement for efficient phagocytosis, suggesting that therapeutic antibodies should target short antigens in order to trigger Fc receptor activation through size-dependent physical segregation.

Chloroplast Sec14-like 1 (CPSFL1) is essential for normal chloroplast development and affects carotenoid accumulation in Chlamydomonas.

Plastid isoprenoid-derived carotenoids serve essential roles in chloroplast development and photosynthesis. Although nearly all enzymes that participate in the biosynthesis of carotenoids in plants have been identified, the complement of auxiliary proteins that regulate synthesis, transport, sequestration, and degradation of these molecules and their isoprenoid precursors have not been fully described. To identify such proteins that are necessary for the optimal functioning of oxygenic photosynthesis, we screened a large collection of nonphotosynthetic (acetate-requiring) DNA insertional mutants of Chlamydomonas reinhardtii and isolated cpsfl1 The cpsfl1 mutant is extremely light-sensitive and susceptible to photoinhibition and photobleaching. The CPSFL1 gene encodes a CRAL-TRIO hydrophobic ligand-binding (Sec14) domain protein. Proteins containing this domain are limited to eukaryotes, but some may have been retargeted to function in organelles of endosymbiotic origin. The cpsfl1 mutant showed decreased accumulation of plastidial isoprenoid-derived pigments, especially carotenoids, and whole-cell focused ion-beam scanning-electron microscopy revealed a deficiency of carotenoid-rich chloroplast structures (e.g., eyespot and plastoglobules). The low carotenoid content resulted from impaired biosynthesis at a step prior to phytoene, the committed precursor to carotenoids. The CPSFL1 protein bound phytoene and ß-carotene when expressed in Escherichia coli and phosphatidic acid in vitro. We suggest that CPSFL1 is involved in the regulation of phytoene synthesis and carotenoid transport and thereby modulates carotenoid accumulation in the chloroplast.

Quantitative biophysical analysis defines key components modulating recruitment of the GTPase KRAS to the plasma membrane.

The gene encoding the GTPase KRAS is frequently mutated in pancreatic, lung, and colorectal cancers. The KRAS fraction in the plasma membrane (PM) correlates with activation of the mitogen-activated protein kinase (MAPK) pathway and subsequent cellular proliferation. Understanding KRAS's interaction with the PM is challenging given the complexity of the cellular environment. To gain insight into key components necessary for KRAS signal transduction at the PM, we used synthetic membranes such as liposomes and giant unilamellar vesicles. Using surface plasmon resonance (SPR) spectroscopy, we demonstrated that KRAS and Raf-1 proto-oncogene Ser/Thr kinase (RAF1) domains interact with these membranes primarily through electrostatic interactions with negatively charged lipids reinforced by additional interactions involving phosphatidyl ethanolamine and cholesterol. We found that the RAF1 region spanning RBD through CRD (RBDCRD) interacts with the membrane significantly more strongly than the isolated RBD or CRD domains and synergizes KRAS partitioning to the membrane. We also found that calmodulin and phosphodiesterase 6 delta (PDE6δ), but not galectin3 previously proposed to directly interact with KRAS, passively sequester KRAS and prevent it from partitioning into the PM. RAF1 RBDCRD interacted with membranes preferentially at nonraft lipid domains. Moreover, a C-terminal O-methylation was crucial for KRAS membrane localization. These results contribute to a better understanding of how the KRAS-membrane interaction is tuned by multiple factors whose identification could inform drug discovery efforts to disrupt this critical interaction in diseases such as cancer.

Forming giant vesicles with controlled membrane composition, asymmetry, and contents.

Growing knowledge of the key molecular components involved in biological processes such as endocytosis, exocytosis, and motility has enabled direct testing of proposed mechanistic models by reconstitution. However, current techniques for building increasingly complex cellular structures and functions from purified components are limited in their ability to create conditions that emulate the physical and biochemical constraints of real cells. Here we present an integrated method for forming giant unilamellar vesicles with simultaneous control over (i) lipid composition and asymmetry, (ii) oriented membrane protein incorporation, and (iii) internal contents. As an application of this method, we constructed a synthetic system in which membrane proteins were delivered to the outside of giant vesicles, mimicking aspects of exocytosis. Using confocal fluorescence microscopy, we visualized small encapsulated vesicles docking and mixing membrane components with the giant vesicle membrane, resulting in exposure of previously encapsulated membrane proteins to the external environment. This method for creating giant vesicles can be used to test models of biological processes that depend on confined volume and complex membrane composition, and it may be useful in constructing functional systems for therapeutic and biomaterials applications.

Integrating molecular and network biology to decode endocytosis.

The strength of network biology lies in its ability to derive cell biological information without a priori mechanistic or molecular knowledge. It is shown here how a careful understanding of a given biological pathway can refine an interactome approach. This permits the elucidation of additional design principles and of spatio-temporal dynamics behind pathways, and aids in experimental design and interpretation.

Antibody:CD47 ratio regulates macrophage phagocytosis through competitive receptor phosphorylation.

Cancer immunotherapies often modulate macrophage effector function by introducing either targeting antibodies that activate Fcγ receptors (FcγRs) or blocking antibodies that disrupt inhibitory SIRPα-CD47 engagement. However, how these competing signals are integrated is poorly understood, raising questions about how to effectively titrate immune responses. Here, we find that macrophage phagocytic decisions are regulated by the ratio of activating ligand to inhibitory ligand over a broad range of absolute molecular densities. Using both endogenous and chimeric receptors, we show that activating:inhibitory ligand ratios of at least 10:1 are required to promote phagocytosis of model antibody-opsonized CD47-inhibited targets and that lowering that ratio reduces FcγR phosphorylation because of inhibitory phosphatases recruited to CD47-bound SIRPα. We demonstrate that ratiometric signaling is critical for phagocytosis of tumor cells and can be modified by blocking SIRPα, indicating that balancing targeting and blocking antibodies may be important for controlling macrophage phagocytosis in cancer immunotherapy.

A viral fusogen hijacks the actin cytoskeleton to drive cell-cell fusion.

Cell-cell fusion, which is essential for tissue development and used by some viruses to form pathological syncytia, is typically driven by fusogenic membrane proteins with tall (>10 nm) ectodomains that undergo conformational changes to bring apposing membranes in close contact prior to fusion. Here we report that a viral fusogen with a short (<2 nm) ectodomain, the reptilian orthoreovirus p14, accomplishes the same task by hijacking the actin cytoskeleton. We show that phosphorylation of the cytoplasmic domain of p14 triggers N-WASP-mediated assembly of a branched actin network. Using p14 mutants, we demonstrate that fusion is abrogated when binding of an adaptor protein is prevented and that direct coupling of the fusogenic ectodomain to branched actin assembly is sufficient to drive cell-cell fusion. This work reveals how the actin cytoskeleton can be harnessed to overcome energetic barriers to cell-cell fusion.

Role of the AP2 beta-appendage hub in recruiting partners for clathrin-coated vesicle assembly.

Adaptor protein complex 2 alpha and beta-appendage domains act as hubs for the assembly of accessory protein networks involved in clathrin-coated vesicle formation. We identify a large repertoire of beta-appendage interactors by mass spectrometry. These interact with two distinct ligand interaction sites on the beta-appendage (the "top" and "side" sites) that bind motifs distinct from those previously identified on the alpha-appendage. We solved the structure of the beta-appendage with a peptide from the accessory protein Eps15 bound to the side site and with a peptide from the accessory cargo adaptor beta-arrestin bound to the top site. We show that accessory proteins can bind simultaneously to multiple appendages, allowing these to cooperate in enhancing ligand avidities that appear to be irreversible in vitro. We now propose that clathrin, which interacts with the beta-appendage, achieves ligand displacement in vivo by self-polymerisation as the coated pit matures. This changes the interaction environment from liquid-phase, affinity-driven interactions, to interactions driven by solid-phase stability ("matricity"). Accessory proteins that interact solely with the appendages are thereby displaced to areas of the coated pit where clathrin has not yet polymerised. However, proteins such as beta-arrestin (non-visual arrestin) and autosomal recessive hypercholesterolemia protein, which have direct clathrin interactions, will remain in the coated pits with their interacting receptors.

Force-induced transcellular tunnel formation in endothelial cells.

The endothelium serves as a protective semipermeable barrier in blood vessels and lymphatic vessels. Leukocytes and pathogens can pass directly through the endothelium by opening holes in endothelial cells, known as transcellular tunnels, which are formed by contact and self-fusion of the apical and basal plasma membranes. Here we test the hypothesis that the actin cytoskeleton is the primary barrier to transcellular tunnel formation using a combination of atomic force microscopy and fluorescence microscopy of live cells. We find that localized mechanical forces are sufficient to induce the formation of transcellular tunnels in HUVECs. When HUVECs are exposed to the bacterial toxin EDIN, which can induce spontaneous transcellular tunnels, less mechanical work is required to form tunnels due to the reduced cytoskeletal stiffness and thickness of these cells, similar to the effects of a ROCK inhibitor. We also observe actin enrichment in response to mechanical indentation that is reduced in cells exposed to the bacterial toxin. Our study shows that the actin cytoskeleton of endothelial cells provides both passive and active resistance against transcellular tunnel formation, serving as a mechanical barrier that can be overcome by mechanical force as well as disruption of the cytoskeleton.

Defining residues involved in human rhinovirus 2A proteinase substrate recognition.

The 2A proteinase (2A(pro)) of human rhinoviruses (HRVs) initiates proteolytic processing by cleaving between the C-terminus of VP1 and its own N-terminus. It subsequently cleaves the host protein eIF4GI. HRV2 and HRV14 2A(pro) cleave at IITTA *GPSD and DIKSY *GLGP on their respective polyproteins. The HRV2 2A(pro) cleavage site on eIF4GI is TLSTR *GPPR. We show that HRV2 2A(pro) can self-process at the eIF4GI cleavage sequence whereas HRV14 2A(pro) cannot, due to the presence of the arginine residue at P1. The mutations A104C or A104S in HRV14 2A(pro) restored cleavage when arginine was present at P1, although not to wild-type levels. These experiments define residues which determine substrate recognition in rhinoviral 2A(pro).

Size-dependent protein segregation at membrane interfaces.

Membrane interfaces formed at cell-cell junctions are associated with characteristic patterns of membrane protein organization, such as E-cadherin enrichment in epithelial junctional complexes and CD45 exclusion from the signaling foci of immunological synapses. To isolate the role of protein size in these processes, we reconstituted membrane interfaces in vitro using giant unilamellar vesicles decorated with synthetic binding and non-binding proteins. We show that size differences between binding and non-binding proteins can dramatically alter their organization at membrane interfaces in the absence of active contributions from the cytoskeleton, with as little as a ~5 nm increase in non-binding protein size driving its exclusion from the interface. Combining in vitro measurements with Monte Carlo simulations, we find that non-binding protein exclusion is also influenced by lateral crowding, binding protein affinity, and thermally-driven membrane height fluctuations that transiently limit access to the interface. This simple, sensitive, and highly effective means of passively segregating proteins has implications for signaling at cell-cell junctions and protein sorting at intracellular contact points between membrane-bound organelles.

Reconstitution of proteins on electroformed giant unilamellar vesicles.

In vitro reconstitution of simplified biological systems from molecular parts has proven to be a powerful method for investigating the biochemical and biophysical principles underlying cellular processes. In recent years, there has been a growing interest in reconstitution of protein-membrane interactions to understand the critical role played by membranes in organizing molecular-scale events into micron-scale patterns and protrusions. However, while all reconstitution experiments depend on identifying and isolating an essential set of soluble biomolecules, such as proteins, DNA, and RNA, reconstitution of membrane-based processes involves the additional challenge of forming and working with lipid bilayer membranes with composition, fluidity, and mechanical properties appropriate for the process at hand. Here we discuss a selection of methods for forming synthetic lipid bilayer membranes and present a versatile electroformation protocol that our lab uses for reconstituting proteins on giant unilamellar vesicles. This synthetic membrane-based approach to reconstitution offers the ability to study protein organization and activity at membranes under more cell-like conditions, addressing a central challenge to accomplishing the grand goal of "building the cell."

Membrane bending by protein-protein crowding.

Curved membranes are an essential feature of dynamic cellular structures, including endocytic pits, filopodia protrusions and most organelles. It has been proposed that specialized proteins induce curvature by binding to membranes through two primary mechanisms: membrane scaffolding by curved proteins or complexes; and insertion of wedge-like amphipathic helices into the membrane. Recent computational studies have raised questions about the efficiency of the helix-insertion mechanism, predicting that proteins must cover nearly 100% of the membrane surface to generate high curvature, an improbable physiological situation. Thus, at present, we lack a sufficient physical explanation of how protein attachment bends membranes efficiently. On the basis of studies of epsin1 and AP180, proteins involved in clathrin-mediated endocytosis, we propose a third general mechanism for bending fluid cellular membranes: protein-protein crowding. By correlating membrane tubulation with measurements of protein densities on membrane surfaces, we demonstrate that lateral pressure generated by collisions between bound proteins drives bending. Whether proteins attach by inserting a helix or by binding lipid heads with an engineered tag, protein coverage above ~20% is sufficient to bend membranes. Consistent with this crowding mechanism, we find that even proteins unrelated to membrane curvature, such as green fluorescent protein (GFP), can bend membranes when sufficiently concentrated. These findings demonstrate a highly efficient mechanism by which the crowded protein environment on the surface of cellular membranes can contribute to membrane shape change.

Solitary and repetitive binding motifs for the AP2 complex alpha-appendage in amphiphysin and other accessory proteins.

Adaptor protein (AP) complexes bind to transmembrane proteins destined for internalization and to membrane lipids, so linking cargo to the accessory internalization machinery. This machinery interacts with the appendage domains of APs, which have platform and beta-sandwich subdomains, forming the binding surfaces for interacting proteins. Proteins that interact with the subdomains do so via short motifs, usually found in regions of low structural complexity of the interacting proteins. So far, up to four motifs have been identified that bind to and partially compete for at least two sites on each of the appendage domains of the AP2 complex. Motifs in individual accessory proteins, their sequential arrangement into motif domains, and partial competition for binding sites on the appendage domains coordinate the formation of endocytic complexes in a temporal and spatial manner. In this work, we examine the dominant interaction sequence in amphiphysin, a synapse-enriched accessory protein, which generates membrane curvature and recruits the scission protein dynamin to the necks of coated pits, for the platform subdomain of the alpha-appendage. The motif domain of amphiphysin1 contains one copy of each of a DX(F/W) and FXDXF motif. We find that the FXDXF motif is the main determinant for the high affinity interaction with the alpha-adaptin appendage. We describe the optimal sequence of the FXDXF motif using thermodynamic and structural data and show how sequence variation controls the affinities of these motifs for the alpha-appendage.

The conserved isoleucine-valine-phenylalanine motif couples activation state and endocytic functions of beta-arrestins.

Beta-arrestins (betaarrs) play a central role in the regulation of G-protein-coupled receptors (GPCRs). Their binding to phosphorylated activated GPCRs induces a conformational transition to an active state resulting in the release of their flexible C-terminal tail. Binding sites for clathrin and the adaptor protein (AP)-2 clathrin adaptor complex are then unmasked, which drive the recruitment of betaarrs-GPCR complexes into clathrin-coated pits (CCPs). A conserved isoleucine-valine-phenylalanine (IVF) motif of the C-terminal tail controls betaarr activation through intramolecular interactions. Here, we provide structural, biochemical and functional evidence in living cells that the IVF motif also controls binding to AP-2. While the F residue is directly involved in AP-2 binding, substitutions of I and V residues, markedly enhanced affinity for AP-2 resulting in active betaarr mutants, which are constitutively targeted to CCPs in the absence of any GPCR activation. Conformational change and endocytic functions of betaarrs thus appear to be coordinated via the complex molecular interactions established by the IVF motif.

High prevalence of long-term cardiovascular, neurological and psychosocial morbidity after treatment for craniopharyngioma.

INTRODUCTION: The treatment of craniopharyngiomas is associated with long-term morbidity. AIM OF THE STUDY: To assess the long-term functional outcome and mortality rates after treatment for craniopharyngiomas, we audited our data with special focus on cardiovascular, neurological and psychosocial morbidity. PATIENTS AND METHODS: Between 1965 and 2002, 54 consecutive patients underwent surgery for craniopharyngiomas at the Leiden University Medical Centre (LUMC). Fifteen patients (25%) received additional postoperative radiation therapy. The median follow-up period was 10 years (range 1-37 years). RESULTS: Long-term cure rate was 82% and long-term recurrence rate 18%. Visual fields/visual acuity stabilized or improved in 74% of cases. The long-term prevalence rate of hypopituitarism was 89%. In addition, long-term cardiovascular, neurological and psychosocial morbidity rates were high: 22% (risk factors 57%), 49% and 47%, respectively. Female sex was an independent predictor of increased cardiovascular, neurological and psychosocial morbidity (odds ratio 3.78, P = 0.031). Ten patients (18%) died during an 828 person-year follow-up. The actuarial patient survival rates 5, 10 and 20 years after the initial operation were 95, 85 and 85%, respectively. The standardized mortality ratio (SMR) was 2.88 [95% confidence interval (CI) 1.35-4.99]. CONCLUSION: Craniopharyngioma is associated with excessive long-term multisystem morbidity and mortality, especially in female patients, despite a high cure rate. These observations indicate that dedicated long-term follow-up of these patients is required. The purpose of the follow-up should be: first, to look for recurrences and to ensure appropriate endocrine replacement, especially oestrogen replacement in premenopausal females; and second, to achieve intensive control of glucose, lipids, blood pressure and weight, as in any other patient with increased risk for cardiovascular disease.

Human rhinovirus 2 2Apro recognition of eukaryotic initiation factor 4GI. Involvement of an exosite.

The 2A proteinase (2Apro) of human rhinovirus 2 is a cysteine proteinase with a unique chymotrypsin-like fold. During viral replication, 2Apro performs self-processing by cleaving between its own N terminus and the C terminus of the preceding protein, VP1. Subsequently, 2Apro cleaves the two isoforms of the cellular protein, eukaryotic initiation factor (eIF) 4G. We have previously shown that HRV2 2Apro can directly bind to eIF4G isoforms. Here we demonstrate using deletion mutants of eIF4GI that HRV2 2Apro requires eIF4GI amino acids 600-674 for binding; however, the amino acids at the cleavage site, Arg681 downward arrow Gly, are not required. The HRV2 2Apro binding domain for eIF4GI was identified by site-directed mutagenesis. Specifically, mutations Leu17 --> Arg and Asp35 --> Glu severely impaired HRV2 2Apro binding and thus processing of eIF4GI in rabbit reticulocyte lysates; self-processing, however, was not affected. Alanine scanning analysis further identified the loop containing residues Tyr32, Ser33, and Ser34 as important for eIF4GI binding. Although Asp35 is part of the catalytic triad, most of the eIF4GI binding domain lies in a unique exosite structure absent from other chymotrypsin-like enzymes and is distinct from the substrate binding cleft. The exosite represents a novel virulence determinant that may allow the development of specific inhibitors for HRV2 2Apro.

Evolving nature of the AP2 alpha-appendage hub during clathrin-coated vesicle endocytosis.

Clathrin-mediated endocytosis involves the assembly of a network of proteins that select cargo, modify membrane shape and drive invagination, vesicle scission and uncoating. This network is initially assembled around adaptor protein (AP) appendage domains, which are protein interaction hubs. Using crystallography, we show that FxDxF and WVxF peptide motifs from synaptojanin bind to distinct subdomains on alpha-appendages, called 'top' and 'side' sites. Appendages use both these sites to interact with their binding partners in vitro and in vivo. Occupation of both sites simultaneously results in high-affinity reversible interactions with lone appendages (e.g. eps15 and epsin1). Proteins with multiple copies of only one type of motif bind multiple appendages and so will aid adaptor clustering. These clustered alpha(appendage)-hubs have altered properties where they can sample many different binding partners, which in turn can interact with each other and indirectly with clathrin. In the final coated vesicle, most appendage binding partners are absent and thus the functional status of the appendage domain as an interaction hub is temporal and transitory giving directionality to vesicle assembly.