Cholesterol modulates cell signaling and protein networking by specifically interacting with PDZ domain-containing scaffold proteins

MINDIN Exerts Protumorigenic Actions on Primary Prostate Tumors via Downregulation of the Scaffold Protein NHERF-1

Advanced prostate cancer preferential metastasis to bone is associated with osteomimicry. MINDIN is a secreted matrix protein upregulated in prostate tumors that overexpresses bone-related genes during prostate cancer progression. Na+/H+ exchanger regulatory factor (NHERF-1) is a scaffold protein that has been involved both in tumor regulation and osteogenesis. We hypothesize that NHERF-1 modulation is a mechanism used by MINDIN to promote prostate cancer progression. We analyzed the expression of NHERF-1 and MINDIN in human prostate samples and in a premetastatic prostate cancer mouse model, based on the implantation of prostate adenocarcinoma TRAMP-C1 (transgenic adenocarcinoma of the mouse prostate) cells in immunocompetent C57BL/6 mice. The relationship between NHERF-1 and MINDIN and their effects on cell proliferation, migration, survival and osteomimicry were evaluated. Upregulation of MINDIN and downregulation of NHERF-1 expression were observed both in human prostate cancer samples and in the TRAMP-C1 model. MINDIN silencing restored NHERF-1 expression to control levels in the mouse model. Stimulation with MINDIN reduced NHERF-1 expression and triggered its mobilization from the plasma membrane to the cytoplasm in TRAMP-C1 cells. MINDIN-dependent downregulation of NHERF-1 promoted tumor cell migration and proliferation without affecting osteomimicry and adhesion. We propose that MINDIN downregulates NHERF-1 expression leading to promotion of processes involved in prostate cancer progression.

Study of PDZ-Peptide and PDZ-Lipid Interactions by Surface Plasmon Resonance/BIAcore

Surface plasmon resonance (SPR)/BIAcore technology enables the characterization of molecular interactions, including determination of affinities and kinetics. In BIAcore, one of the interaction partners (the ligand) is immobilized on a chip and the other (the analyte) is provided in solution. BIAcore allows to study association and dissociation rates in real time without the use of labeling. BIAcore can be applied to molecular interactions involving small compounds and biological macromolecules such as proteins, lipids, nucleic acids, or carbohydrates. Here we describe protocols for the measurements of PDZ domain-peptide (oriented biotinylated peptides), PDZ domain-liposomes (lipid membranes), and PDZ-lipid-peptide tripartite interactions.

Prostate cancer extracellular vesicles mediate intercellular communication with bone marrow cells and promote metastasis in a cholesterol-dependent manner

Primary tumours can establish long-range communication with distant organs to transform them into fertile soil for circulating tumour cells to implant and proliferate, a process called pre-metastatic niche (PMN) formation. Tumour-derived extracellular vesicles (EV) are potent mediators of PMN formation due to their diverse complement of pro-malignant molecular cargo and their propensity to target specific cell types (Costa-Silva et al., 2015; Hoshino et al., 2015; Peinado et al., 2012; Peinado et al., 2017). While significant progress has been made to understand the mechanisms by which pro-metastatic EVs create tumour-favouring microenvironments at pre-metastatic organ sites, comparatively little attention has been paid to the factors intrinsic to recipient cells that may modify the extent to which pro-metastatic EV signalling is received and transduced. Here, we investigated the role of recipient cell cholesterol homeostasis in prostate cancer (PCa) EV-mediated signalling and metastasis. Using a bone metastatic model of enzalutamide-resistant PCa, we first characterized an axis of EV-mediated communication between PCa cells and bone marrow that is marked by in vitro and in vivo PCa EV uptake by bone marrow myeloid cells, activation of NF-κB signalling, enhanced osteoclast differentiation, and reduced myeloid thrombospondin-1 expression. We then employed a targeted, biomimetic approach to reduce myeloid cell cholesterol in vitro and in vivo prior to conditioning with PCa EVs. Reducing myeloid cell cholesterol prevented the uptake of PCa EVs by recipient myeloid cells, abolished NF-κB activity and osteoclast differentiation, stabilized thrombospondin-1 expression, and reduced metastatic burden by 77%. These results demonstrate that cholesterol homeostasis in bone marrow myeloid cells regulates pro-metastatic EV signalling and metastasis by acting as a gatekeeper for EV signal transduction.

Activation of WNT signaling restores the facial deficits in a zebrafish with defects in cholesterol metabolism

Inborn errors of cholesterol metabolism occur as a result of mutations in the cholesterol synthesis pathway (CSP). Although mutations in the CSP cause a multiple congenital anomaly syndrome, craniofacial abnormalities are a hallmark phenotype associated with these disorders. Previous studies have established that mutation of the zebrafish hmgcs1 gene (Vu57 allele), which encodes the first enzyme in the CSP, causes defects in craniofacial development and abnormal neural crest cell (NCC) differentiation. However, the molecular mechanisms by which the products of the CSP disrupt NCC differentiation are not completely known. Cholesterol is known to regulate the activity of WNT signaling, an established regulator of NCC differentiation. We hypothesized that defects in cholesterol synthesis are associated with reduced WNT signaling, consequently resulting in abnormal craniofacial development. To test our hypothesis we performed a combination of pharmaceutical inhibition, gene expression assays, and targeted rescue experiments to understand the function of the CSP and WNT signaling during craniofacial development. We demonstrate reduced expression of four canonical WNT downstream target genes in homozygous carriers of the Vu57 allele and reduced axin2 expression, a known WNT target gene, in larvae treated with Ro-48-8071, an inhibitor of cholesterol synthesis. Moreover, activation of WNT signaling via treatment with WNT agonist I completely restored the craniofacial defects present in a subset of animals carrying the Vu57 allele. Collectively, these data suggest interplay between the CSP and WNT signaling during craniofacial development.

Inclusion of endophenotypes in a standard GWAS facilitate a detailed mechanistic understanding of genetic elements that control blood lipid levels

Dyslipidemia is the primary cause of cardiovascular disease, which is a serious human health problem in large parts of the world. Therefore, it is important to understand the genetic and molecular mechanisms that regulate blood levels of cholesterol and other lipids. Discovery of genetic elements in the regulatory machinery is often based on genome wide associations studies (GWAS) focused on end-point phenotypes such as total cholesterol level or a disease diagnosis. In the present study, we add endophenotypes, such as serum levels of intermediate metabolites in the cholesterol synthesis pathways, to a GWAS analysis and use the pig as an animal model. We do this to increase statistical power and to facilitate biological interpretation of results. Although the study population was limited to ~ 300 individuals, we identify two genome-wide significant associations and ten suggestive associations. Furthermore, we identify 28 tentative associations to loci previously associated with blood lipids or dyslipidemia associated diseases. The associations with endophenotypes may inspire future studies that can dissect the biological mechanisms underlying these previously identified associations and add a new level of understanding to previously identified associations.

Roles of Farnesyl-Diphosphate Farnesyltransferase 1 in Tumour and Tumour Microenvironments

Farnesyl-diphosphate farnesyltransferase 1 (FDFT1, squalene synthase), a membrane-associated enzyme, synthesizes squalene via condensation of two molecules of farnesyl pyrophosphate. Accumulating evidence has noted that FDFT1 plays a critical role in cancer, particularly in metabolic reprogramming, cell proliferation, and invasion. Based on these advances in our knowledge, FDFT1 could be a potential target for cancer treatment. This review focuses on the contribution of FDFT1 to the hallmarks of cancer, and further, we discuss the applicability of FDFT1 as a cancer prognostic marker and target for anticancer therapy.

Cholesterol Interaction with the Trimeric HIV Fusion Protein gp41 in Lipid Bilayers Investigated by Solid-State NMR Spectroscopy and Molecular Dynamics Simulations

HIV-1 entry into cells is mediated by the fusion protein gp41. Cholesterol plays an important role in this virus-cell fusion, but molecular structural information about cholesterol-gp41 interaction is so far absent. Here, we present experimental and computational data about cholesterol complexation with gp41 in lipid bilayers. We focus on the C-terminal region of the protein, which comprises a membrane-proximal external region (MPER) and the transmembrane domain (TMD). We measured peptide-cholesterol contacts in virus-mimetic lipid bilayers using solid-state NMR spectroscopy, and augmented these experimental data with all-atom molecular dynamics simulations. 2D 19F NMR spectra show correlation peaks between MPER residues and the cholesterol isooctyl tail, indicating that cholesterol is in molecular contact with the MPER-TMD trimer. 19F-13C distance measurements between the peptide and 13C-labeled cholesterol show that C17 on the D ring and C9 at the intersection of B and C rings are ~7.0 Å from the F673 side-chain 4-19F. At high peptide concentrations in the membrane, the 19F-13C distance data indicate three cholesterol molecules bound near F673 in each trimer. Mutation of a cholesterol recognition amino acid consensus motif did not change these distances, indicating that cholesterol binding does not require this sequence motif. Molecular dynamics simulations further identify two hotspots for cholesterol interactions. Taken together, these experimental data and simulations indicate that the helix-turn-helix conformation of the MPER-TMD is responsible for sequestering cholesterol. We propose that this gp41-cholesterol interaction mediates virus-cell fusion by recruiting gp41 to the boundary of the liquid-disordered and liquid-ordered phases to incur membrane curvature.

GM1 as Adjuvant of Innovative Therapies for Cystic Fibrosis Disease

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) protein is expressed at the apical plasma membrane (PM) of different epithelial cells. The most common mutation responsible for the onset of cystic fibrosis (CF), F508del, inhibits the biosynthesis and transport of the protein at PM, and also presents gating and stability defects of the membrane anion channel upon its rescue by the use of correctors and potentiators. This prompted a multiple drug strategy for F508delCFTR aimed simultaneously at its rescue, functional potentiation and PM stabilization. Since ganglioside GM1 is involved in the functional stabilization of transmembrane proteins, we investigated its role as an adjuvant to increase the effectiveness of CFTR modulators. According to our results, we found that GM1 resides in the same PM microenvironment as CFTR. In CF cells, the expression of the mutated channel is accompanied by a decrease in the PM GM1 content. Interestingly, by the exogenous administration of GM1, it becomes a component of the PM, reducing the destabilizing effect of the potentiator VX-770 on rescued CFTR protein expression/function and improving its stabilization. This evidence could represent a starting point for developing innovative therapeutic strategies based on the co-administration of GM1, correctors and potentiators, with the aim of improving F508del CFTR function.

The bidirectional relationship between CFTR and lipids

Cystic Fibrosis (CF) is the most common life-shortening genetic disease among Caucasians, resulting from mutations in the gene encoding the Cystic Fibrosis Transmembrane conductance Regulator (CFTR). While work to understand this protein has resulted in new treatment strategies, it is important to emphasize that CFTR exists within a complex lipid bilayer — a concept largely overlooked when performing structural and functional studies. In this review we discuss cellular lipid imbalances in CF, mechanisms by which lipids affect membrane protein activity, and the specific impact of detergents and lipids on CFTR function.

In this review, Cottrill et al. discuss how the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) interacts with the membrane into which it is inserted. They summarize recent insight into the ways lipids are imbalanced in CF epithelia and how the lipid environment affects CFTR.

Single-Virus Tracking: From Imaging Methodologies to Virological Applications

Uncovering the mechanisms of virus infection and assembly is crucial for preventing the spread of viruses and treating viral disease. The technique of single-virus tracking (SVT), also known as single-virus tracing, allows one to follow individual viruses at different parts of their life cycle and thereby provides dynamic insights into fundamental processes of viruses occurring in live cells. SVT is typically based on fluorescence imaging and reveals insights into previously unreported infection mechanisms. In this review article, we provide the readers a broad overview of the SVT technique. We first summarize recent advances in SVT, from the choice of fluorescent labels and labeling strategies to imaging implementation and analytical methodologies. We then describe representative applications in detail to elucidate how SVT serves as a valuable tool in virological research. Finally, we present our perspectives regarding the future possibilities and challenges of SVT.

Effects of immunisation against PCSK9 in mice bearing melanoma

INTRODUCTION

Inhibition of proprotein convertase subtilisin/kexin 9 (PCSK9) is an established modality for the treatment of hypercholesterolaemia. However, the impact of PCSK9 inhibition in other situations such as cancer remains largely unknown. The current study was conducted to study the effects of PCSK9 inhibition on cancer endpoints in mice bearing melanoma.

MATERIAL AND METHODS

To generate antiPCSK9 antibody in vivo, a nanoliposomal antiPCSK9 vaccine adsorbed to 0.4% Alum adjuvant was subcutaneously injected in C57BL/6 mice four times with bi-weekly intervals. Two weeks after the last immunisation, mice were subcutaneously inoculated with B16F0 melanoma cells. After a tumour mass was palpable (approximately 10 mm³), the mice were randomly divided into four groups and subjected to different treatment protocols: (1) PBS (untreated control), (2) vaccine group, (3) the combination of vaccine and a single dose of liposomal doxorubicin (Doxil^®), and (4) liposomal doxorubicin (positive control) group. To determine therapeutic efficacy, mouse body weight, tumour size, and survival were monitored every three days for 36 days.

RESULTS

The nanoliposomal antiPCSK9 vaccine was found to efficiently induce specific antibodies against PCSK9 in C57BL/6 mice, thereby reducing plasma levels and function of PCSK9. Tumour volumes in the vaccinated group were not significantly different from those in the liposomal doxorubicin, combination, and control groups. The time to reach endpoint (TTE) values of the vaccine (28 ±5 days), combination (30 ±6 days), liposomal doxorubicin (34 ±2 days), and control (31 ±2 days) groups were not significantly different, either. Furthermore, the tumour growth delay (TGD) values of the vaccine (-11.5 ±15.4%), liposomal doxorubicin (7.75 ±6.5%), combination (-6 ±20.77%), and control (0 ±7.5) groups were not significantly different. Finally, there was no significant difference between the median survival time and lifespan of the vaccinated versus other tested groups.

CONCLUSIONS

The nanoliposomal PCSK9 vaccine did not adversely affect the growth of melanoma tumour nor the survival of tumour-bearing mice.

Interaction of Cholesterol With the Human SLC1A5 (ASCT2): Insights Into Structure/Function Relationships

The human SLC1A5 commonly known as ASCT2 is a sodium-dependent neutral amino acid antiporter involved in transmembrane traffic of glutamine that is exchanged through the cell membrane with smaller amino acids such as serine or threonine. Due to the strong overexpression in human cancers, ASCT2 is widely studied for its relevance to human health. Of special interest are the aspects related to the regulation of its function. The role of cholesterol as a modulator of the transport activity has been studied using a combined strategy of computational and experimental approaches. The effect of cholesterol on theNa ex + -[3H]glutamineex/glutaminein antiport in proteoliposomes has been evaluated by adding cholesteryl hemisuccinate. A strong stimulation of transport activity was observed in the presence of 75 μg cholesteryl hemisuccinate per mg total lipids. The presence of cholesterol did not influence the proteoliposome volume, in a wide range of tested concentration, excluding that the stimulation could be due to effects on the vesicles. cholesteryl hemisuccinate, indeed, improved the incorporation of the protein into the phospholipid bilayer to some extent and increased about three times the Vmax of transport without affecting the Km for glutamine. Docking of cholesterol into the hASCT2 trimer was performed. Six poses were obtained some of which overlapped the hypothetical cholesterol molecules observed in the available 3D structures. Additional poses were docked close to CARC/CRAC motifs (Cholesterol Recognition/interaction Amino acid Consensus sequence). To test the direct binding of cholesterol to the protein, a strategy based on the specific targeting of tryptophan and cysteine residues located in the neighborhood of cholesterol poses was employed. On the one hand, cholesterol binding was impaired by modification of tryptophan residues by the Koshland's reagent. On the other hand, the presence of cholesterol impaired the interaction of thiol reagents with the protein. Altogether, these results confirmed that cholesterol molecules interacted with the protein in correspondence of the poses predicted by the docking analysis.

Sterol Carrier Protein Inhibition-Based Control of Mosquito Vectors: Current Knowledge and Future Perspectives

Cholesterol is one of the most vital compounds for animals as it is involved in various biological processes and acts as the structural material in the body. However, insects do not have some of the essential enzymes in the cholesterol biosynthesis pathway and this makes them dependent on dietary cholesterol. Thus, the blocking of cholesterol uptake may have detrimental effects on the survival of the insect. Utilizing this character, certain phytochemicals can be used to inhibit mosquito sterol carrier protein-2 (AeSCP-2) activity via competitive binding and proven to have effective insecticidal activities against disease-transmitting mosquitoes and other insect vectors. A range of synthetic compounds, phytochemicals, and synthetic analogs of phytochemicals are found to have AeSCP-2 inhibitory activity. Phytochemicals such as alpha-mangostin can be considered as the most promising group of compounds when considering the minimum environmental impact and availability at a low cost. Once the few limitations such as very low persistence in the environment are addressed successfully, these chemicals may be used as an effective tool for controlling mosquitoes and other disease-transmitting vector populations.

Structural and signaling role of lipids in plasma membrane repair

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

Nanoscale dynamics of cholesterol in the cell membrane

Cholesterol constitutes ∼30-40% of the mammalian plasma membrane, a larger fraction than of any other single component. It is a major player in numerous signaling processes as well as in shaping molecular membrane architecture. However, our knowledge of the dynamics of cholesterol in the plasma membrane is limited, restricting our understanding of the mechanisms regulating its involvement in cell signaling. Here, we applied advanced fluorescence imaging and spectroscopy approaches on in vitro (model membranes) and in vivo (live cells and embryos) membranes as well as in silico analysis to systematically study the nanoscale dynamics of cholesterol in biological membranes. Our results indicate that cholesterol diffuses faster than phospholipids in live membranes, but not in model membranes. Interestingly, a detailed statistical diffusion analysis suggested two-component diffusion for cholesterol in the plasma membrane of live cells. One of these components was similar to a freely diffusing phospholipid analogue, whereas the other one was significantly faster. When a cholesterol analogue was localized to the outer leaflet only, the fast diffusion of cholesterol disappeared, and it diffused similarly to phospholipids. Overall, our results suggest that cholesterol diffusion in the cell membrane is heterogeneous and that this diffusional heterogeneity is due to cholesterol's nanoscale interactions and localization in the membrane.

Agonists that stimulate secretion promote the recruitment of CFTR into membrane lipid microdomains

The cystic fibrosis transmembrane conductance regulator (CFTR) is a tightly regulated anion channel that mediates secretion by epithelia and is mutated in the disease cystic fibrosis. CFTR forms macromolecular complexes with many proteins; however, little is known regarding its associations with membrane lipids or the regulation of its distribution and mobility at the cell surface. We report here that secretagogues (agonists that stimulate secretion) such as the peptide hormone vasoactive intestinal peptide (VIP) and muscarinic agonist carbachol increase CFTR aggregation into cholesterol-dependent clusters, reduce CFTR lateral mobility within and between membrane microdomains, and trigger the fusion of clusters into large (3.0 µm2) ceramide-rich platforms. CFTR clusters are closely associated with motile cilia and with the enzyme acid sphingomyelinase (ASMase) that is constitutively bound on the cell surface. Platform induction is prevented by pretreating cells with cholesterol oxidase to disrupt lipid rafts or by exposure to the ASMase functional inhibitor amitriptyline or the membrane-impermeant reducing agent 2-mercaptoethanesulfonate. Platforms are reversible, and their induction does not lead to an increase in apoptosis; however, blocking platform formation does prevent the increase in CFTR surface expression that normally occurs during VIP stimulation. These results demonstrate that CFTR is colocalized with motile cilia and reveal surprisingly robust regulation of CFTR distribution and lateral mobility, most likely through autocrine redox activation of extracellular ASMase. Formation of ceramide-rich platforms containing CFTR enhances transepithelial secretion and likely has other functions related to inflammation and mucosal immunity.

Novel screening system revealed that intracellular cholesterol trafficking can be a good target for colon cancer prevention

In conventional research methods for cancer prevention, cell proliferation and apoptosis have been intensively targeted rather than the protection of normal or benign tumor cells from malignant transformation. In this study, we aimed to identify candidate colon cancer chemopreventive drugs based on the transcriptional activities of TCF/LEF, NF-κB and NRF2, that play important roles in the process of malignant transformation. We screened a “validated library” consisting of 1280 approved drugs to identify hit compounds that decreased TCF/LEF and NF-κB transcriptional activity and increased NRF2 transcriptional activity. Based on the evaluation of these 3 transcriptional activities, 8 compounds were identified as candidate chemopreventive drugs for colorectal cancer. One of those, itraconazole, is a clinically used anti-fungal drug and was examined in the Min mouse model of familial adenomatous polyposis. Treatment with itraconazole significantly suppressed intestinal polyp formation and the effects of itraconazole on transcriptional activities may be exerted partly through inhibition of intracellular cholesterol trafficking. This screen represents one of the first attempts to identify chemopreventive agents using integrated criteria consisting of the inhibition of TCF/LEF, NF-κB and induction of NRF2 transcriptional activity.

Microbial Reduction of Cholesterol to Coprostanol: An Old Concept and New Insights

The gut microbiota plays a key role in cholesterol metabolism, mainly through the reduction of cholesterol to coprostanol. The latter sterol exhibits distinct physicochemical properties linked to its limited absorption in the gut. Few bacteria were reported to reduce cholesterol into coprostanol. Three microbial pathways of coprostanol production were described based on the analysis of reaction intermediates. However, these metabolic pathways and their associated genes remain poorly studied. In this review, we shed light on the microbial metabolic pathways related to coprostanol synthesis. Moreover, we highlight current strategies and future directions to better characterize these microbial enzymes and pathways.

The role of cholesterol metabolism in cancer

Cholesterol plays an important role in cancer development. Both clinical and experimental studies have found that hypercholesterolemia and a high-fat high-cholesterol diet can affect cancer development. External cholesterol can directly activate the oncogenic Hedgehog pathway, and internal cholesterol can induce mTORC1 signaling. Cholesterol is a key component of lipid rafts, which are the major platforms for signaling regulation in cancer, and chelating membrane cholesterol is an effective anti-cancer strategy that disrupts the functions of lipid rafts. Cholesterol metabolism is often reprogrammed in cancer cells. Targeting cholesterol metabolism as a new therapeutic approach has received increasing attention. Here, we summarize some key molecular mechanisms supporting the use of anti-cholesterol therapy for cancer treatment.

Emerging Themes in PDZ Domain Signaling: Structure, Function, and Inhibition

Post-synaptic density-95, disks-large and zonula occludens-1 (PDZ) domains are small globular protein-protein interaction domains widely conserved from yeast to humans. They are composed of ∼90 amino acids and form a classical two α-helical/six β-strand structure. The prototypical ligand is the C-terminus of partner proteins; however, they also bind internal peptide sequences. Recent findings indicate that PDZ domains also bind phosphatidylinositides and cholesterol. Through their ligand interactions, PDZ domain proteins are critical for cellular trafficking and the surface retention of various ion channels. In addition, PDZ proteins are essential for neuronal signaling, memory, and learning. PDZ proteins also contribute to cytoskeletal dynamics by mediating interactions critical for maintaining cell-cell junctions, cell polarity, and cell migration. Given their important biological roles, it is not surprising that their dysfunction can lead to multiple disease states. As such, PDZ domain-containing proteins have emerged as potential targets for the development of small molecular inhibitors as therapeutic agents. Recent data suggest that the critical binding function of PDZ domains in cell signaling is more than just glue, and their binding function can be regulated by phosphorylation or allosterically by other binding partners. These studies also provide a wealth of structural and biophysical data that are beginning to reveal the physical features that endow this small modular domain with a central role in cell signaling.

Modes of Cholesterol Binding in Membrane Proteins: A Joint Analysis of 73 Crystal Structures

Cholesterol is a highly asymmetric lipid molecule. As an essential constituent of the cell membrane, cholesterol plays important structural and signaling roles in various biological processes. The first high-resolution crystal structure of a transmembrane protein in complex with cholesterol was a human β₂-adrenergic receptor structure deposited to the Protein Data Bank in 2007. Since then, the number of the cholesterol-bound crystal structures has grown considerably providing an invaluable resource for obtaining insights into the structural characteristics of cholesterol binding. In this work, we examine the spatial and orientation distributions of cholesterol relative to the protein framework in a collection of 73 crystal structures of membrane proteins. To characterize the cholesterol-protein interactions, we apply singular value decomposition to an array of interatomic distances, which allows us to systematically assess the flexibility and variability of cholesterols in transmembrane proteins. Together, this joint analysis reveals the common characteristics among the observed cholesterol structures, thereby offering important guidelines for prediction and modification of potential cholesterol binding sites in transmembrane proteins.

Cholesterol Effects on the Physical Properties of Lipid Membranes Viewed by Solid-state NMR Spectroscopy

In this chapter, we review the physical properties of lipid/cholesterol mixtures involving studies of model membranes using solid-state NMR spectroscopy. The approach allows one to quantify the average membrane structure, fluctuations, and elastic deformation upon cholesterol interaction. Emphasis is placed on understanding the membrane structural deformation and emergent fluctuations at an atomistic level. Lineshape measurements using solid-state NMR spectroscopy give equilibrium structural properties, while relaxation time measurements study the molecular dynamics over a wide timescale range. The equilibrium properties of glycerophospholipids, sphingolipids, and their binary and tertiary mixtures with cholesterol are accessible. Nonideal mixing of cholesterol with other lipids explains the occurrence of liquid-ordered domains. The entropic loss upon addition of cholesterol to sphingolipids is less than for glycerophospholipids, and may drive formation of lipid rafts. The functional dependence of ²H NMR spin-lattice relaxation (R _1Z) rates on segmental order parameters (S _CD) for lipid membranes is indicative of emergent viscoelastic properties. Addition of cholesterol shows stiffening of the bilayer relative to the pure lipids and this effect is diminished for lanosterol. Opposite influences of cholesterol and detergents on collective dynamics and elasticity at an atomistic scale can potentially affect lipid raft formation in cellular membranes.

Microbial impact on cholesterol and bile acid metabolism: current status and future prospects

Recently, the gut microbiota has emerged as a crucial factor that influences cholesterol metabolism. Ever since, significant interest has been shown in investigating these host-microbiome interactions to uncover microbiome-mediated functions on cholesterol and bile acid (BA) metabolism. Indeed, changes in gut microbiota composition and, hence, its derived metabolites have been previously reported to subsequently impact the metabolic processes and have been linked to several diseases. In this context, associations between a disrupted gut microbiome, impaired BA metabolism, and cholesterol dysregulation have been highlighted. Extensive advances in metagenomic and metabolomic studies in this field have allowed us to further our understanding of the role of intestinal bacteria in metabolic health and disease. However, only a few have provided mechanistic insights into their impact on cholesterol metabolism. Identifying the myriad functions and interactions of these bacteria to maintain cholesterol homeostasis remain an important challenge in such a field of research. In this review, we discuss the impact of gut microbiota on cholesterol metabolism, its association with disease settings, and the potential of modulating gut microbiota as a promising therapeutic target to lower hypercholesterolemia.

Transcriptome analysis of hopanoid deficient mutant of Rhodopseuodomonas palustris TIE-1

All three domains of life have an ordered plasma membrane which is pivotal in the selective fitness of primitive life. Like cholesterol in eukaryotes, hopanoids are important in bacteria to modulate membrane order. Hopanoids are pentacyclic triterpenoid lipids biosynthesised in many eubacteria, few ferns and lichens. Hopanoid modulates outer membrane order and hopanoid deficiency results in the weakened structural integrity of the membrane which may in turn affect the other structures within or spanning the cell envelope and contributing to various membrane functions. Hence, to decipher the role of hopanoid, genome-wide transcriptome of wild-type and Δshc mutant of Rhodopseudomonas palustris TIE-1 was studied which indicated 299 genes were upregulated and 306 genes were downregulated in hopanoid deficient mutant, representing ∼11.5% of the genome. Thirty-eight genes involved in chemotaxis, response to stimuli and signal transduction were differentially regulated and impaired motility in hopanoid deficient mutant showed that hopanoid plays a crucial role in chemotaxis. The docking study demonstrated that diguanylate cyclase which catalyses the synthesis of secondary messenger exhibited the capability to interact with hopanoids and might be confederating in chemotaxis and signal transduction. Seventy-four genes involved in membrane transport were differentially expressed and cell assays also explicit that the multidrug transport is compromised in Δshc mutant. Membrane transport is reliant on hopanoids which may explain the basis for previous observations linking hopanoids to antibiotic resistance. Disturbing the membrane order by targeting lipid synthesis can be a possible novel approach in developing new antimicrobials and hopanoid biosynthesis could be a potential target.

Cyclodextrins reduce the ability of Pseudomonas aeruginosa outer-membrane vesicles to reduce CFTR Cl- secretion

Pseudomonas aeruginosa secretes outer-membrane vesicles (OMVs) that fuse with cholesterol-rich lipid rafts in the apical membrane of airway epithelial cells and decrease wt-CFTR Cl^- secretion. Herein, we tested the hypothesis that a reduction of the cholesterol content of CF human airway epithelial cells by cyclodextrins reduces the inhibitory effect of OMVs on VX-809 (lumacaftor)-stimulated Phe508del CFTR Cl^- secretion. Primary CF bronchial epithelial cells and CFBE cells were treated with vehicle, hydroxypropyl-β-cyclodextrin (HPβCD), or methyl-β-cyclodextrin (MβCD), and the effects of OMVs secreted by P. aeruginosa on VX-809 stimulated Phe508del CFTR Cl^- secretion were measured in Ussing chambers. Neither HPβCD nor MβCD were cytotoxic, and neither altered Phe508del CFTR Cl^- secretion. Both cyclodextrins reduced OMV inhibition of VX-809-stimulated Phe508del-CFTR Cl^- secretion when added to the apical side of CF monolayers. Both cyclodextrins also reduced the ability of P. aeruginosa to form biofilms and suppressed planktonic growth of P. aeruginosa. Our data suggest that HPβCD, which is in clinical trials for Niemann-Pick Type C disease, and MβCD, which has been approved by the U.S. Food and Drug Administration for use in solubilizing lipophilic drugs, may enhance the clinical efficacy of VX-809 in CF patients when added to the apical side of airway epithelial cells, and reduce planktonic growth and biofilm formation by P. aeruginosa. Both effects would be beneficial to CF patients.

Cholesterol Regulation of Pulmonary Endothelial Calcium Homeostasis

Cholesterol is a key structural component and regulator of lipid raft signaling platforms critical for cell function. Such regulation may involve changes in the biophysical properties of lipid microdomains or direct protein-sterol interactions that alter the function of ion channels, receptors, enzymes, and membrane structural proteins. Recent studies have implicated abnormal membrane cholesterol levels in mediating endothelial dysfunction that is characteristic of pulmonary hypertensive disorders, including that resulting from long-term exposure to hypoxia. Endothelial dysfunction in this setting is characterized by impaired pulmonary endothelial calcium entry and an associated imbalance that favors production vasoconstrictor and mitogenic factors that contribute to pulmonary hypertension. Here we review current knowledge of cholesterol regulation of pulmonary endothelial Ca²⁺ homeostasis, focusing on the role of membrane cholesterol in mediating agonist-induced Ca²⁺ entry and its components in the normal and hypertensive pulmonary circulation.

Quantitative Lipid Imaging Reveals a New Signaling Function of Phosphatidylinositol-3,4-Bisphophate: Isoform- and Site-Specific Activation of Akt

Activation of class I phosphatidylinositol 3-kinase (PI3K) leads to formation of phosphatidylinositol-3,4,5-trisphophate (PIP₃) and phosphatidylinositol-3,4-bisphophate (PI34P₂), which spatiotemporally coordinate and regulate a myriad of cellular processes. By simultaneous quantitative imaging of PIP₃ and PI34P₂ in live cells, we here show that they have a distinctively different spatiotemporal distribution and history in response to growth factor stimulation, which allows them to selectively induce the membrane recruitment and activation of Akt isoforms. PI34P₂ selectively activates Akt2 at both the plasma membrane and early endosomes, whereas PIP₃ selectively stimulates Akt1 and Akt3 exclusively at the plasma membrane. These spatiotemporally distinct activation patterns of Akt isoforms provide a mechanism for their differential regulation of downstream signaling molecules. Collectively, our studies show that different spatiotemporal dynamics of PIP₃ and PI34P₂ and their ability to selectively activate key signaling proteins allow them to mediate class I PI3K signaling pathways in a spatiotemporally specific manner.

Cellular phosphatase activity of C1-Ten/Tensin2 is controlled by Phosphatidylinositol-3,4,5-triphosphate binding through the C1-Ten/Tensin2 SH2 domain

Regulation of tyrosine phosphorylation on insulin receptor substrate-1 (IRS-1) is essential for insulin signaling. The protein tyrosine phosphatase (PTP) C1-Ten/Tensin2 has been implicated in the regulation of IRS-1, but the molecular basis of this dephosphorylation is not fully understood. Here, we demonstrate that the cellular phosphatase activity of C1-Ten/Tensin2 on IRS-1 is mediated by the binding of the C1-Ten/Tensin2 Src-homology 2 (SH2) domain to phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P₃). We show that the role of C1-Ten/Tensin2 is dependent on insulin-induced phosphoinositide 3-kinase activity. The C1-Ten/Tensin2 SH2 domain showed strong preference and high affinity for PtdIns(3,4,5)P₃. Using site-directed mutagenesis, we identified three basic residues in the C1-Ten/Tensin2 SH2 domain that were critical for PtdIns(3,4,5)P₃ binding but were not involved in phosphotyrosine binding and PTP activity. Using a PtdIns(3,4,5)P₃ binding-deficient mutant, we showed that the specific binding of the C1-Ten/Tensin2 SH2 domain to PtdIns(3,4,5)P₃ allowed C1-Ten/Tensin2 to function as a PTP in cells. Collectively, our findings suggest that the interaction between the C1-Ten/Tensin2 SH2 domain and PtdIns(3,4,5)P₃ produces a negative feedback loop of insulin signaling through IRS-1.

The utrophin-beta 2 syntrophin complex regulates adipocyte lipid droplet size independent of adipogenesis

Utrophin is a widely expressed cytoskeleton protein and is associated with lipid droplets (LDs) in adipocytes. The scaffold protein beta 2 syntrophin (SNTB2) controls signaling events by recruiting distinct membrane and cytoskeletal proteins, and binds to utrophin. Here we show that SNTB2 forms a complex with utrophin in adipocytes. SNTB2 protein is strongly diminished when utrophin is low. Of note, knock-down of utrophin or SNTB2 enhances LD growth during adipogenesis. SNTB2 reduction has no effect on basal and induced lipolysis, and insulin-stimulated phosphorylation of Akt is normal. The antilipolytic activity of insulin is enhanced in adipocytes with low SNTB2, while knock-down of utrophin has no effect. Uptake of exogenously supplied oleate and linoleate is comparable in scrambled and SNTB2 siRNA-treated cells. In the fibroblasts, diminished SNTB2 is associated with lower proliferation. CCAAT/enhancer-binding protein alpha and sterol regulatory element-binding proteins which are critical transcription factors for adipogenesis are normally expressed. Consequently, maturation of cells with SNTB2 knock-down is not grossly impaired. In fibroblasts, SNTB2 is localized to filamentous and vesicular structures which are distinct from beta actin, alpha tubulin, endoplasmic reticulum, early endosomes, lysosomes and mitochondria. Collectively, our data provide evidence that the utrophin-SNTB2 complex regulates LD size without affecting adipogenesis.

Cholesterol accumulation by suppression of SMT1 leads to dwarfism and improved drought tolerance in herbaceous plants

Dwarfism and drought tolerance are 2 valuable traits in breeding of many crops. In this study, we report the novel physiological roles of cholesterol in regulation of plant growth and drought tolerance. Compared with the wild type, sterol-C24-methyltransferase 1 (SMT1) gene transcript was greatly reduced in a bermudagrass mutant with dwarfism and enhanced drought tolerance, accompanied with cholesterol accumulation, elevated transcript levels of a small group of genes including SAMDC, and increased concentrations of putrescine (Put), spermidine (Spd), and spermine (Spm). Knock-down of OsSMT1 expression by RNA interference resulted in similar phenotypic changes in transgenic rice. Moreover, exogenously applied cholesterol also led to elevated transcripts of a similar set of genes, higher levels of Put, Spd, and Spm, improved drought tolerance, and reduced plant height in both bermudagrass and rice. We revealed that it is Spm, but not Spd, that is responsible for the height reduction in bermudagrass and rice. In conclusion, we suggest that cholesterol induces expression of SAMDC and leads to dwarfism and elevated drought tolerance in plants as a result of the promoted Spd and Spm synthesis.

Dynamic pattern generation in cell membranes: Current insights into membrane organization

It has been two decades since the lipid raft hypothesis was first presented. Even today, whether these nanoscale cholesterol-rich domains are present in cell membranes is not completely resolved. However, especially in the last few years, a rich body of literature has demonstrated both the presence and the importance of non-random distribution of biomolecules on the membrane, which is the focus of this review. These new developments have pushed the experimental limits of detection and have brought us closer to observing lipid domains in the plasma membrane of live cells. Characterization of biomolecules associated with lipid rafts has revealed a deep connection between biological regulation and function and membrane compositional heterogeneities. Finally, tantalizing new developments in the field have demonstrated that lipid domains might not just be associated with the plasma membrane of eukaryotes but could potentially be a ubiquitous membrane-organizing principle in several other biological systems. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.

NHERF1 Between Promises and Hopes: Overview on Cancer and Prospective Openings

Na+/H+ exchanger regulatory factor 1 (NHERF1) is a scaffold protein, with two tandem PDZ domains and a carboxyl-terminal ezrin-binding (EB) region. This particular sticky structure is responsible for its interaction with different molecules to form multi-complexes that have a pivotal role in a lot of diseases. In particular, its involvement during carcinogenesis and cancer progression has been deeply analyzed in different tumors. The role of NHERF1 is not unique in cancer; its activity is connected to its subcellular localization. The literature data suggest that NHERF1 could be a new prognostic/predictive biomarker from breast cancer to hematological cancers. Furthermore, the high potential of this molecule as therapeutical target in different carcinomas is a new challenge for precision medicine. These evidences are part of a future view to improving patient clinical management, which should allow different tumor phenotypes to be treated with tailored therapies. This article reviews the biology of NHERF1, its engagement in different signal pathways and its involvement in different cancers, with a specific focus on breast cancer. It also considers NHERF1 potential role during inflammation related to most human cancers, designating new perspectives in the study of this "Janus-like" protein.

Structure function relations in PDZ-domain-containing proteins: Implications for protein networks in cellular signalling

Protein scaffolds as essential backbones for organization of supramolecular signalling complexes are a recurrent theme in several model systems. Scaffold proteins preferentially employ linear peptide binding motifs for recruiting their interaction partners. PDZ domains are one of the more commonly encountered peptide binding domains in several proteins including those involved in scaffolding functions. This domain is known for its promiscuity both in terms of ligand selection, mode of interaction with its ligands as well as its association with other protein interaction domains. PDZ domains are subject to several means of regulations by virtue of their functional diversity. Additionally, the PDZ domains are refractive to the effect of mutations and maintain their three-dimensional architecture under extreme mutational load. The biochemical and biophysical basis for this selectivity as well as promiscuity has been investigated and reviewed extensively. The present review focuses on the plasticity inherent in PDZ domains and its implications for modular organization as well as evolution of cellular signalling pathways in higher eukaryotes.

Membrane cholesterol depletion as a trigger of Nav1.9 channel-mediated inflammatory pain

Cholesterol is a major lipid component of the mammalian plasma membrane. While much is known about its metabolism, its transport, and its role in atherosclerotic vascular disease, less is known about its role in neuronal pathophysiology. This study reveals an unexpected function of cholesterol in controlling pain transmission. We show that inflammation lowers cholesterol content in skin tissue and sensory DRG culture. Pharmacological depletion of cellular cholesterol entails sensitization of nociceptive neurons and promotes mechanical and thermal hyperalgesia through the activation of voltage-gated Nav1.9 channels. Inflammatory mediators enhance the production of reactive oxygen species and induce partitioning of Nav1.9 channels from cholesterol-rich lipid rafts to cholesterol-poor non-raft regions of the membrane. Low-cholesterol environment enhances voltage-dependent activation of Nav1.9 channels leading to enhanced neuronal excitability, whereas cholesterol replenishment reversed these effects. Consistently, we show that transcutaneous delivery of cholesterol alleviates hypersensitivity in animal models of acute and chronic inflammatory pain. In conclusion, our data establish that membrane cholesterol is a modulator of pain transmission and shed a new light on the relationship between cholesterol homeostasis, inflammation, and pain.

Mesoscale organization of domains in the plasma membrane - beyond the lipid raft

The plasma membrane is compartmentalized into several distinct regions or domains, which show a broad diversity in both size and lifetime. The segregation of lipids and membrane proteins is thought to be driven by the lipid composition itself, lipid-protein interactions and diffusional barriers. With regards to the lipid composition, the immiscibility of certain classes of lipids underlies the "lipid raft" concept of plasmalemmal compartmentalization. Historically, lipid rafts have been described as cholesterol and (glyco)sphingolipid-rich regions of the plasma membrane that exist as a liquid-ordered phase that are resistant to extraction with non-ionic detergents. Over the years the interest in lipid rafts grew as did the challenges with studying these nanodomains. The term lipid raft has fallen out of favor with many scientists and instead the terms "membrane raft" or "membrane nanodomain" are preferred as they connote the heterogeneity and dynamic nature of the lipid-protein assemblies. In this article, we will discuss the classical lipid raft hypothesis and its limitations. This review will also discuss alternative models of lipid-protein interactions, annular lipid shells, and larger membrane clusters. We will also discuss the mesoscale organization of plasmalemmal domains including visible structures such as clathrin-coated pits and caveolae.

A High-Throughput Fluorometric Assay for Lipid-Protein Binding

An increasing number of intracellular and extracellular proteins are shown to interact with membrane lipids under physiological conditions. For rapid and robust quantitative measurement of lipid-protein interaction, we developed a sensitive fluorescence quenching-based assay that is universally applicable to all proteins and lipids. The assay employs fluorescence protein (FP)-tagged proteins whose fluorescence emission intensity is decreased when they bind vesicles containing quenching lipids. This simple assay can be performed with a fluorescence plate reader or a spectrofluorometer and optimized for different proteins with various combinations of FPs and quenching lipids. The assay allows a rapid, sensitive, and accurate determination of lipid specificity and affinity for various lipid-binding proteins, and high-throughput screening of molecules that modulate their membrane binding.

Role of the PDZ-scaffold protein NHERF1/EBP50 in cancer biology: from signaling regulation to clinical relevance

The transmission of cellular information requires fine and subtle regulation of proteins that need to interact in a coordinated and specific way to form efficient signaling networks. The spatial and temporal coordination relies on scaffold proteins. Thanks to protein interaction domains such as PDZ domains, scaffold proteins organize multiprotein complexes enabling the proper transmission of cellular information through intracellular networks. NHERF1/EBP50 is a PDZ-scaffold protein that was initially identified as an organizer and regulator of transporters and channels at the apical side of epithelia through actin-binding ezrin-moesin-radixin proteins. Since, NHERF1/EBP50 has emerged as a major regulator of cancer signaling network by assembling cancer-related proteins. The PDZ-scaffold EBP50 carries either anti-tumor or pro-tumor functions, two antinomic functions dictated by EBP50 expression or subcellular localization. The dual function of NHERF1/EBP50 encompasses the regulation of several major signaling pathways engaged in cancer, including the receptor tyrosine kinases PDGFR and EGFR, PI3K/PTEN/AKT and Wnt-β-catenin pathways.

A Ligand-Binding Assay to Measure the Affinity and Specificity of Sterol-Binding Proteins In Vitro

Sterols are major constituents of the plasma membrane of eukaryotic cells and serve as a precursor for several classes of signaling molecules, including steroids and hydroxy sterols. They maintain the functionality and permeability barrier of the plasma membrane through lipid-lipid and lipid-protein interactions. The S. cerevisiae pathogen-related yeast proteins 1, 2, and 3 (Pry) belong to a large protein superfamily known as CAP/SCP/TAPS. Members of this superfamily have been implicated in a wide variety of processes, including immune defense in mammals and plants, pathogen virulence, sperm maturation and fertilization, venom toxicity, and prostate and brain cancer. Pry proteins bind and export sterols in vivo and the purified Pry1 protein binds sterols and related small hydrophobic compounds in vitro. Here we describe a method to determine lipid binding of a purified protein in vitro.

Orthogonal lipid sensors identify transbilayer asymmetry of plasma membrane cholesterol

Controlled distribution of lipids across various cell membranes is crucial for cell homeostasis and regulation. We developed an imaging method that allows simultaneous in situ quantification of cholesterol in two leaflets of the plasma membrane (PM) using tunable orthogonal cholesterol sensors. Our imaging revealed marked transbilayer asymmetry of PM cholesterol (TAPMC) in various mammalian cells, with the concentration in the inner leaflet (IPM) being ∼12-fold lower than that in the outer leaflet (OPM). The asymmetry was maintained by active transport of cholesterol from IPM to OPM and its chemical retention at OPM. Furthermore, the increase in the IPM cholesterol level was triggered in a stimulus-specific manner, allowing cholesterol to serve as a signaling lipid. We found excellent correlation between the IPM cholesterol level and cellular Wnt signaling activity, suggesting that TAPMC and stimulus-induced PM cholesterol redistribution are crucial for tight regulation of cellular processes under physiological conditions.

Fluorescence-Based In Situ Quantitative Imaging for Cellular Lipids

Membrane lipids are dynamic molecules and their local concentrations serve as regulatory signals for diverse biological processes. To achieve quantitative in situ imaging of various lipids, we developed a ratiometric analysis using fluorescence biosensors, each of which is composed of an engineered lipid-binding protein and a covalently attached solvatochromic fluorophore. To cover a wide range of lipid concentration, lipid-binding proteins are engineered to have variable dynamic ranges. These tunable sensors allow robust and sensitive in situ quantitative lipid imaging in mammalian cells, providing new insight into the spatiotemporal dynamics and fluctuation of key signaling lipids. The sensor strategy is also applicable to in situ quantification of multiple cellular lipids or a single lipid in the opposing leaflets of cell membranes.

Sarcolemmal dependence of cardiac protection and stress-resistance: roles in aged or diseased hearts

Disruption of the sarcolemmal membrane is a defining feature of oncotic death in cardiac ischaemia-reperfusion (I-R), and its molecular makeup not only fundamentally governs this process but also affects multiple determinants of both myocardial I-R injury and responsiveness to cardioprotective stimuli. Beyond the influences of membrane lipids on the cytoprotective (and death) receptors intimately embedded within this bilayer, myocardial ionic homeostasis, substrate metabolism, intercellular communication and electrical conduction are all sensitive to sarcolemmal makeup, and critical to outcomes from I-R. As will be outlined in this review, these crucial sarcolemmal dependencies may underlie not only the negative effects of age and common co-morbidities on myocardial ischaemic tolerance but also the on-going challenge of implementing efficacious cardioprotection in patients suffering accidental or surgically induced I-R. We review evidence for the involvement of sarcolemmal makeup changes in the impairment of stress-resistance and cardioprotection observed with ageing and highly prevalent co-morbid conditions including diabetes and hypercholesterolaemia. A greater understanding of membrane changes with age/disease, and the inter-dependences of ischaemic tolerance and cardioprotection on sarcolemmal makeup, can facilitate the development of strategies to preserve membrane integrity and cell viability, and advance the challenging goal of implementing efficacious 'cardioprotection' in clinically relevant patient cohorts. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

Frizzled 7 and PIP2 binding by syntenin PDZ2 domain supports Frizzled 7 trafficking and signalling

PDZ domain-containing proteins work as intracellular scaffolds to control spatio-temporal aspects of cell signalling. This function is supported by the ability of their PDZ domains to bind other proteins such as receptors, but also phosphoinositide lipids important for membrane trafficking. Here we report a crystal structure of the syntenin PDZ tandem in complex with the carboxy-terminal fragment of Frizzled 7 and phosphatidylinositol 4,5-bisphosphate (PIP2). The crystal structure reveals a tripartite interaction formed via the second PDZ domain of syntenin. Biophysical and biochemical experiments establish co-operative binding of the tripartite complex and identify residues crucial for membrane PIP2-specific recognition. Experiments with cells support the importance of the syntenin-PIP2 interaction for plasma membrane targeting of Frizzled 7 and c-jun phosphorylation. This study contributes to our understanding of the biology of PDZ proteins as key players in membrane compartmentalization and dynamics.

Lipids Regulate Lck Protein Activity through Their Interactions with the Lck Src Homology 2 Domain

Lymphocyte-specific protein-tyrosine kinase (Lck) plays an essential role in T cell receptor (TCR) signaling and T cell development, but its activation mechanism is not fully understood. To explore the possibility that plasma membrane (PM) lipids control TCR signaling activities of Lck, we measured the membrane binding properties of its regulatory Src homology 2 (SH2) and Src homology 3 domains. The Lck SH2 domain binds anionic PM lipids with high affinity but with low specificity. Electrostatic potential calculation, NMR analysis, and mutational studies identified the lipid-binding site of the Lck SH2 domain that includes surface-exposed basic, aromatic, and hydrophobic residues but not the phospho-Tyr binding pocket. Mutation of lipid binding residues greatly reduced the interaction of Lck with the ζ chain in the activated TCR signaling complex and its overall TCR signaling activities. These results suggest that PM lipids, including phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate, modulate interaction of Lck with its binding partners in the TCR signaling complex and its TCR signaling activities in a spatiotemporally specific manner via its SH2 domain.