SR-B1 drives endothelial cell LDL transcytosis via DOCK4 to promote atherosclerosis.

Atherosclerosis, which underlies life-threatening cardiovascular disorders such as myocardial infarction and stroke1, is initiated by passage of low-density lipoprotein (LDL) cholesterol into the artery wall and its engulfment by macrophages, which leads to foam cell formation and lesion development2,3. It is unclear how circulating LDL enters the artery wall to instigate atherosclerosis. Here we show in mice that scavenger receptor class B type 1 (SR-B1) in endothelial cells mediates the delivery of LDL into arteries and its accumulation by artery wall macrophages, thereby promoting atherosclerosis. LDL particles are colocalized with SR-B1 in endothelial cell intracellular vesicles in vivo, and transcytosis of LDL across endothelial monolayers requires its direct binding to SR-B1 and an eight-amino-acid cytoplasmic domain of the receptor that recruits the guanine nucleotide exchange factor dedicator of cytokinesis 4 (DOCK4)4. DOCK4 promotes internalization of SR-B1 and transport of LDL by coupling the binding of LDL to SR-B1 with activation of RAC1. The expression of SR-B1 and DOCK4 is increased in atherosclerosis-prone regions of the mouse aorta before lesion formation, and in human atherosclerotic arteries when compared with normal arteries. These findings challenge the long-held concept that atherogenesis involves passive movement of LDL across a compromised endothelial barrier. Interventions that inhibit the endothelial delivery of LDL into artery walls may represent a new therapeutic category in the battle against cardiovascular disease.

Loss of the transforming growth factor-ß effector ß2-Spectrin promotes genomic instability.

Exposure to genotoxins such as ethanol-derived acetaldehyde leads to DNA damage and liver injury and promotes the development of cancer. We report here a major role for the transforming growth factor ß/mothers against decapentaplegic homolog 3 adaptor ß2-Spectrin (ß2SP, gene Sptbn1) in maintaining genomic stability following alcohol-induced DNA damage. ß2SP supports DNA repair through ß2SP-dependent activation of Fanconi anemia complementation group D2 (Fancd2), a core component of the Fanconi anemia complex. Loss of ß2SP leads to decreased Fancd2 levels and sensitizes ß2SP mutants to DNA damage by ethanol treatment, leading to phenotypes that closely resemble those observed in animals lacking both aldehyde dehydrogenase 2 and Fancd2 and resemble human fetal alcohol syndrome. Sptbn1-deficient cells are hypersensitive to DNA crosslinking agents and have defective DNA double-strand break repair that is rescued by ectopic Fancd2 expression. Moreover, Fancd2 transcription in response to DNA damage/transforming growth factor ß stimulation is regulated by the ß2SP/mothers against decapentaplegic homolog 3 complex. CONCLUSION: Dysfunctional transforming growth factor ß/ß2SP signaling impacts the processing of genotoxic metabolites by altering the Fanconi anemia DNA repair pathway. (Hepatology 2017;65:678-693).

Identification of roles for H264, H306, H439, and H635 in acid-dependent lipoprotein release by the LDL receptor.

Lipoproteins internalized by the LDL receptor (LDLR) are released from this receptor in endosomes through a process that involves acid-dependent conformational changes in the receptor ectodomain. How acidic pH promotes this release process is not well understood. Here, we assessed roles for six histidine residues for which either genetic or structural data suggested a possible role in the acid-responsiveness of the LDLR. Using assays that measured conformational change, acid-dependent lipoprotein release, LDLR recycling, and net lipoprotein uptake, we show that H635 plays important roles in acid-dependent conformational change and lipoprotein release, while H264, H306, and H439 play ancillary roles in the response of the LDLR to acidic pH.

Quantitative fluorescence imaging reveals point of release for lipoproteins during LDLR-dependent uptake.

The LDL receptor (LDLR) supports efficient uptake of both LDL and VLDL remnants by binding lipoprotein at the cell surface, internalizing lipoprotein through coated pits, and releasing lipoprotein in endocytic compartments before returning to the surface for further rounds of uptake. While many aspects of lipoprotein binding and receptor entry are well understood, it is less clear where, when, and how the LDLR releases lipoprotein. To address these questions, the current study employed quantitative fluorescence imaging to visualize the uptake and endosomal processing of LDL and the VLDL remnant ß-VLDL. We find that lipoprotein release is rapid, with most release occurring prior to entry of lipoprotein into early endosomes. Published biochemical studies have identified two mechanisms of lipoprotein release: one that involves the ß-propeller module of the LDLR and a second that is independent of this module. Quantitative imaging comparing uptake supported by the normal LDLR or by an LDLR variant incapable of ß-propeller-dependent release shows that the ß-propeller-independent process is sufficient for release for both lipoproteins but that the ß-propeller process accelerates both LDL and ß-VLDL release. Together these findings define where, when, and how lipoprotein release occurs and provide a generalizable methodology for visualizing endocytic handling in situ.

S-nitrosylation of ARH is required for LDL uptake by the LDL receptor.

The LDL receptor (LDLR) relies upon endocytic adaptor proteins for internalization of lipoproteins. The results of this study show that the LDLR adaptor autosomal recessive hypercholesterolemia protein (ARH) requires nitric oxide to support LDL uptake. Nitric oxide nitrosylates ARH at C199 and C286, and these posttranslational modifications are necessary for association of ARH with the adaptor protein 2 (AP-2) component of clathrin-coated pits. In the absence of nitrosylation, ARH is unable to target LDL-LDLR complexes to coated pits, resulting in poor LDL uptake. The role of nitric oxide on LDLR function is specific for ARH because inhibition of nitric oxide synthase activity impairs ARH-supported LDL uptake but has no effect on other LDLR-dependent lipoprotein uptake processes, including VLDL remnant uptake and dab2-supported LDL uptake. These findings suggest that cells that depend upon ARH for LDL uptake can control which lipoproteins are internalized by their LDLRs through changes in nitric oxide.

Sterol-dependent nuclear import of ORP1S promotes LXR regulated trans-activation of apoE.

Oxysterol binding protein related protein 1S (ORP1S) is a member of a family of sterol transport proteins. Here we present evidence that ORP1S translocates from the cytoplasm to the nucleus in response to sterol binding. The sterols that best promote nuclear import of ORP1S also activate the liver X receptor (LXR) transcription factors and we show that ORP1S binds to LXRs, promotes binding of LXRs to LXR response elements (LXREs) and specifically enhances LXR-dependent transcription via the ME.1 and ME.2 enhancer elements of the apoE gene. We propose that ORP1S is a cytoplasmic sterol sensor, which transports sterols to the nucleus and promotes LXR-dependent gene transcription through select enhancer elements.

Guide to assembling a successful K99/R00 application.

The National Institutes of Health's (NIH) K99/R00 Pathway to Independence Award offers promising postdoctoral researchers and clinician-scientists an opportunity to receive research support at both the mentored and the independent levels with the goal of facilitating a timely transition to a tenure-track faculty position. This transitional program has been generally successful, with most K99/R00 awardees successfully securing R01-equivalent funding by the end of the R00 period. However, often highly promising proposals fail because of poor grantsmanship. This overview provides guidance from the perspective of long-standing members of the National Heart, Lung, and Blood Institute's Mentored Transition to Independence study section for the purpose of helping mentors and trainees regarding how best to assemble competitive K99/R00 applications.

Role of an intramolecular contact on lipoprotein uptake by the LDL receptor.

The LDL receptor (LDLR) is an endocytic receptor that plays a major role in the clearance of atherogenic lipoproteins from the circulation. During the endocytic process, the LDLR first binds lipoprotein at the cell surface and then traffics to endosomes, where the receptor releases bound lipoprotein. Release is acid-dependent and correlates with the formation of an intramolecular contact within the receptor. Human mutations at residues that form the contact are associated with familial hypercholesterolemia (FH) and the goal of the present study was to determine the role of contact residues on LDLR function. We show that mutations at nine contact residues reduce the ability of the LDLR to support lipoprotein uptake. Unexpectedly, only four of the mutations (W515A, W541A, H562Y and H586Y) impaired acid-dependent lipoprotein release. The remaining mutations decreased the lipoprotein-binding capacity of the LDLR through either reduction in the number of surface receptors (H190Y, K560W, H562Y and K582W) or reduction in the fraction of surface receptors that were competent to bind lipoprotein (W144A and W193A). We also examined three residues, distal to the contact, which were predicted to be necessary for the LDLR to adopt the acidic conformation. Of the three mutations we tested (G293S, F362A and G375S), one mutation (F362A) reduced lipoprotein uptake. Together, these data suggest that the intramolecular interface plays multiple roles in LDLR function.

Nanospring behaviour of ankyrin repeats.

Ankyrin repeats are an amino-acid motif believed to function in protein recognition; they are present in tandem copies in diverse proteins in nearly all phyla. Ankyrin repeats contain antiparallel alpha-helices that can stack to form a superhelical spiral. Visual inspection of the extrapolated structure of 24 ankyrin-R repeats indicates the possibility of spring-like behaviour of the putative superhelix. Moreover, stacks of 17-29 ankyrin repeats in the cytoplasmic domains of transient receptor potential (TRP) channels have been identified as candidates for a spring that gates mechanoreceptors in hair cells as well as in Drosophila bristles. Here we report that tandem ankyrin repeats exhibit tertiary-structure-based elasticity and behave as a linear and fully reversible spring in single-molecule measurements by atomic force microscopy. We also observe an unexpected ability of unfolded repeats to generate force during refolding, and report the first direct measurement of the refolding force of a protein domain. Thus, we show that one of the most common amino-acid motifs has spring properties that could be important in mechanotransduction and in the design of nanodevices.

Disruption of LDL but not VLDL clearance in autosomal recessive hypercholesterolemia.

Genetic defects in LDL clearance result in severe hypercholesterolemia and premature atherosclerosis. Mutations in the LDL receptor (LDLR) cause familial hypercholesterolemia (FH), the most severe form of genetic hypercholesterolemia. A phenocopy of FH, autosomal recessive hypercholesterolemia (ARH), is due to mutations in an adaptor protein involved in LDLR internalization. Despite comparable reductions in LDL clearance rates, plasma LDL levels are substantially lower in ARH than in FH. To determine the metabolic basis for this difference, we examined the synthesis and catabolism of VLDL in murine models of FH (Ldlr(-/-)) and ARH (Arh(-/-)). The hyperlipidemic response to a high-sucrose diet was greatly attenuated in Arh(-/-) mice compared with Ldlr(-/-) mice despite similar rates of VLDL secretion. The rate of VLDL clearance was significantly higher in Arh(-/-) mice than in Ldlr(-/-) mice, suggesting that LDLR-dependent uptake of VLDL is maintained in the absence of ARH. Consistent with these findings, hepatocytes from Arh(-/-) mice (but not Ldlr(-/-) mice) internalized beta-migrating VLDL (beta-VLDL). These results demonstrate that ARH is not required for LDLR-dependent uptake of VLDL by the liver. The preservation of VLDL remnant clearance attenuates the phenotype of ARH and likely contributes to greater responsiveness to statins in ARH compared with FH.

Wbox2: A clathrin terminal domain-derived peptide inhibitor of clathrin-mediated endocytosis.

Clathrin-mediated endocytosis (CME) occurs via the formation of clathrin-coated vesicles from clathrin-coated pits (CCPs). Clathrin is recruited to CCPs through interactions between the AP2 complex and its N-terminal domain, which in turn recruits endocytic accessory proteins. Inhibitors of CME that interfere with clathrin function have been described, but their specificity and mechanisms of action are unclear. Here we show that overexpression of the N-terminal domain with (TDD) or without (TD) the distal leg inhibits CME and CCP dynamics by perturbing clathrin interactions with AP2 and SNX9. TDD overexpression does not affect clathrin-independent endocytosis or, surprisingly, AP1-dependent lysosomal trafficking from the Golgi. We designed small membrane-permeant peptides that encode key functional residues within the four known binding sites on the TD. One peptide, Wbox2, encoding residues along the W-box motif binding surface, binds to SNX9 and AP2 and potently and acutely inhibits CME.

The role of calcium in lipoprotein release by the low-density lipoprotein receptor.

The LDL receptor (LDLR) mediates efficient endocytosis of VLDL, VLDL remnants, and LDL. As part of the uptake process, the LDLR releases lipoproteins in endosomes. Released lipoproteins are subsequently trafficked to lysosomes for degradation, while the LDLR recycles back to the cell surface for further rounds of uptake. Endosomes have at least two features that can promote lipoprotein release: an acidic pH and low concentrations of free calcium. The relative contributions of acidic pH and low free calcium to lipoprotein release are not known. Here, we generated fibroblasts that express either normal LDLR or an LDLR variant that is unable to employ the acid-dependent release mechanism to determine the relative contributions of acidic pH and low free calcium on lipoprotein release. We show that endosomal concentrations of free calcium can drive lipoprotein release at rates that are similar to those of acid-dependent release and that the calcium-dependent and acid-dependent mechanisms can cooperate during lipoprotein release. Assessment of lipoprotein uptake by these two cell lines showed that LDL uptake requires the acid-dependent mechanism, while uptake of the VLDL remnant, beta-VLDL, does not. We propose that endosomes use both the acid-dependent and calcium-dependent release mechanisms to drive lipoprotein release and that the acid-dependent process is only required for LDL release.

TGF-ß/ß2-spectrin/CTCF-regulated tumor suppression in human stem cell disorder Beckwith-Wiedemann syndrome.

Beckwith-Wiedemann syndrome (BWS) is a human stem cell disorder, and individuals with this disease have a substantially increased risk (~800-fold) of developing tumors. Epigenetic silencing of ß2-spectrin (ß2SP, encoded by SPTBN1), a SMAD adaptor for TGF-ß signaling, is causally associated with BWS; however, a role of TGF-ß deficiency in BWS-associated neoplastic transformation is unexplored. Here, we have reported that double-heterozygous Sptbn1+/- Smad3+/- mice, which have defective TGF-ß signaling, develop multiple tumors that are phenotypically similar to those of BWS patients. Moreover, tumorigenesis-associated genes IGF2 and telomerase reverse transcriptase (TERT) were overexpressed in fibroblasts from BWS patients and TGF-ß-defective mice. We further determined that chromatin insulator CCCTC-binding factor (CTCF) is TGF-ß inducible and facilitates TGF-ß-mediated repression of TERT transcription via interactions with ß2SP and SMAD3. This regulation was abrogated in TGF-ß-defective mice and BWS, resulting in TERT overexpression. Imprinting of the IGF2/H19 locus and the CDKN1C/KCNQ1 locus on chromosome 11p15.5 is mediated by CTCF, and this regulation is lost in BWS, leading to aberrant overexpression of growth-promoting genes. Therefore, we propose that loss of CTCF-dependent imprinting of tumor-promoting genes, such as IGF2 and TERT, results from a defective TGF-ß pathway and is responsible at least in part for BWS-associated tumorigenesis as well as sporadic human cancers that are frequently associated with SPTBN1 and SMAD3 mutations.

Quantitative fluorescence co-localization to study protein-receptor complexes.

Fluorescence microscopy can be used to assess quantitatively the interaction between a ligand and its receptor, between two macromolecules, or between a macromolecule and a particular intracellular compartment by co-localization analysis. In general, this analysis involves tagging potential interacting partners with distinct fluorophores-by direct labeling of a small ligand, by expression of fluorescent cDNA constructs, by immunofluorescence labeling, or by some combination of these methods. Pairwise comparison of the fluorescence intensity of the two fluorophores at each pixel in a two channel digital image of the sample reveals regions where both are present. With appropriate protocols, the image data can be interpreted to indicate where the potential interacting partners are co-localized. Keeping in mind the limited resolution of the light microscope, co-localization is often used to support the claim that two molecules are interacting.All quantitative methods for evaluating co-localization begin with identifying the pixels where the intensities of both color channels are above background. Typically this involves two sequential image segmentation steps: the first to exclude pixels where neither channel is above background, and the second to set overlap thresholds that exclude pixels where only one color channel is present. Following segmentation, various quantitative measures can be computed to describe the remaining subset of pixels where the two color channels overlap. These metrics range from simple calculation of the fraction of pixels where overlap occurs to more sophisticated image correlation metrics. Additional constraints may be employed to distinguish true co-localization from random overlap. Finally, an image map showing only the co-localized pixels may be displayed as an additional image channel in order to visualize the spatial distribution of co-localized pixels. Several commercial and open source software solutions provide this type of co-localization analysis, making image segmentation and calculation of metrics relatively straightforward. As an example, we provide a protocol for the time-dependent co-localization of fluorescently tagged lipoproteins with LDL receptor (LDLR) and with the early endosome marker EEA1.

Cavin-3 dictates the balance between ERK and Akt signaling.

Cavin-3 is a tumor suppressor protein of unknown function. Using both in vivo and in vitro approaches, we show that cavin-3 dictates the balance between ERK and Akt signaling. Loss of cavin-3 increases Akt signaling at the expense of ERK, while gain of cavin-3 increases ERK signaling at the expense Akt. Cavin-3 facilitates signal transduction to ERK by anchoring caveolae to the membrane skeleton of the plasma membrane via myosin-1c. Caveolae are lipid raft specializations that contain an ERK activation module and loss of the cavin-3 linkage reduces the abundance of caveolae, thereby separating this ERK activation module from signaling receptors. Loss of cavin-3 promotes Akt signaling through suppression of EGR1 and PTEN. The in vitro consequences of the loss of cavin-3 include induction of Warburg metabolism (aerobic glycolysis), accelerated cell proliferation, and resistance to apoptosis. The in vivo consequences of cavin-3 knockout are increased lactate production and cachexia. DOI:http://dx.doi.org/10.7554/eLife.00905.001.

Localization and structure of the ankyrin-binding site on beta2-spectrin.

Spectrins are tetrameric actin-cross-linking proteins that form an elastic network, termed the membrane skeleton, on the cytoplasmic surface of cellular membranes. At the plasma membrane, the membrane skeleton provides essential support, preventing loss of membrane material to environmental shear stresses. The skeleton also controls the location, abundance, and activity of membrane proteins that are critical to cell and tissue function. The ability of the skeleton to modulate membrane stability and function requires adaptor proteins that bind the skeleton to membranes. The principal adaptors are the ankyrin proteins, which bind to the beta-subunit of spectrin and to the cytoplasmic domains of numerous integral membrane proteins. Here, we present the crystal structure of the ankyrin-binding domain of human beta2-spectrin at 1.95 A resolution together with mutagenesis data identifying the binding surface for ankyrins on beta2-spectrin.

The epidermal growth factor homology domain of the LDL receptor drives lipoprotein release through an allosteric mechanism involving H190, H562, and H586.

The low density lipoprotein (LDL) receptor (LDLR) mediates efficient endocytosis of VLDL, VLDL remnants, and LDL. As part of the endocytic process, the LDLR releases lipoproteins in endosomes. The release process correlates with an acid-dependent conformational change in the receptor from an extended, "open" state to a compact, "closed" state. The closed state has an intramolecular contact involving H190, H562, and H586. The current model for lipoprotein release holds that protonation of these histidines drives the conformational change that is associated with release. We tested the roles of H190, H562, and H586 on LDLR conformation and on lipoprotein binding, uptake, and release using variants in which the three histidines were replaced with alanine (AAA variant) or in which the histidines were replaced with charged residues that can form ionic contacts at neutral pH (DRK variant). Contrary to expectation, both the AAA and the DRK variants exhibited normal acid-dependent transitions from open to closed conformations. Despite this similarity, both the AAA and DRK mutations modulated lipoprotein release, indicating that H190, H562, and H586 act subsequent to the conformational transition. These observations also suggest that the intramolecular contact does not drive release through a competitive mechanism. In support of this possibility, mutagenesis experiments showed that beta-VLDL binding was inhibited by mutations at D203 and E208, which are exposed in the closed conformation of the LDLR. We propose that H190, H562, and H586 are part of an allosteric mechanism that drives lipoprotein release.

Identification of a VLDL-induced, FDNPVY-independent internalization mechanism for the LDLR.

The low-density lipoprotein (LDL) receptor (LDLR) binds to and internalizes lipoproteins that contain apolipoproteinB100 (apoB100) or apolipoproteinE (apoE). Internalization of the apoB100 lipoprotein ligand, LDL, requires the FDNPVY(807) sequence on the LDLR cytoplasmic domain, which binds to the endocytic machinery of coated pits. We show here that inactivation of the FDNPVY(807) sequence by mutation of Y807 to cysteine prevented the uptake of LDL; however, this mutation did not prevent LDLR-dependent uptake of the apoE lipoprotein ligand, beta-VLDL. Comparison of the surface localization of the LDLR-Y807C using LDLR-immunogold, LDL-gold and beta-VLDL-gold probes revealed enrichment of LDLR-Y807C-bound beta-VLDL in coated pits, suggesting that beta-VLDL binding promoted the internalization of the LDLR-Y807C. Consistent with this possibility, treatment with monensin, which traps internalized LDLR in endosomes, resulted in the loss of surface LDLR-Y807C only when beta-VLDL was present. Reconstitution experiments in which LDLR variants were introduced into LDLR-deficient cells showed that the HIC(818) sequence is involved in beta-VLDL uptake by the LDLR-Y807C. Together, these experiments demonstrate that the LDLR has a very low-density lipoprotein (VLDL)-induced, FDNPVY-independent internalization mechanism.

Cholesterol-regulated translocation of NPC1L1 to the cell surface facilitates free cholesterol uptake.

Although NPC1L1 is required for intestinal cholesterol absorption, data demonstrating mechanisms by which this protein facilitates the process are few. In this study, a hepatoma cell line stably expressing human NPC1L1 was established, and cholesterol uptake was studied. A relationship between NPC1L1 intracellular trafficking and cholesterol uptake was apparent. At steady state, NPC1L1 proteins localized predominantly to the transferrin-positive endocytic recycling compartment, where free cholesterol also accumulated as revealed by filipin staining. Interestingly, acute cholesterol depletion induced with methyl-beta-cyclodextrin stimulated relocation of NPC1L1 to the plasma membrane, preferentially to a newly formed "apical-like" subdomain. This translocation was associated with a remarkable increase in cellular cholesterol uptake, which in turn was dose-dependently inhibited by ezetimibe, a novel cholesterol absorption inhibitor that specifically binds to NPC1L1. These findings define a cholesterol-regulated endocytic recycling of NPC1L1 as a novel mechanism regulating cellular cholesterol uptake.

The modular adaptor protein autosomal recessive hypercholesterolemia (ARH) promotes low density lipoprotein receptor clustering into clathrin-coated pits.

Autosomal recessive hypercholesterolemia is characterized by a cell type-specific defect in low density lipoprotein receptor (LDLR) endocytosis. LDLR-mediated uptake of LDL is impaired in the liver, but not in fibroblasts of subjects with this disorder. The disease is caused by mutations in ARH, which encodes a putative adaptor protein that interacts with the cytoplasmic tail of the LDLR, phospholipids, and two components of the clathrin endocytic machinery, clathrin and adaptor protein-2 (AP-2) in vitro. To determine the physiological relevance of these interactions, we examined the effect of mutations in the ARH on LDLR location and function in polarized hepatocytes (WIF-B). The integrity of the FDNPVY sequence in the LDLR cytoplasmic tail was required for ARH-associated LDLR clustering into clathrin-coated pits. The phosphotyrosine binding domain of ARH plus either the clathrin box or the AP-2 binding region were required for both clustering and internalization of the LDLR. Parallel studies performed in vivo with the same recombinant forms of ARH in livers of Arh(-/-) mice confirmed the relevance of the cell culture findings. These results demonstrate that ARH must bind the LDLR tail and either clathrin or AP-2 to promote receptor clustering and internalization of LDL.