Phagocytic clearance of apoptotic neurons by Microglia/Brain macrophages in vitro: involvement of lectin-, integrin-, and phosphatidylserine-mediated recognition

Microglia, the tissue macrophages of the brain, play a crucial role in recognition and phagocytic removal of apoptotic neurons. The microglial receptors for recognition of apoptotic neurons are not yet characterized. Here we established a co-culture model of primary microglia and cerebellar granule neurons to examine the receptor systems involved in recognition/uptake of apoptotic neurons. Treatment with 100 microM S-nitrosocysteine induced apoptosis of cerebellar neurons as indicated by nuclear condensation and phosphatidylserine exposure to the exoplasmic leaflet of the plasma membrane. Microglial cells were added to neurons 2 h after apoptosis induction and co-cultured for 6 h in the presence of ligands that inhibit recognition by binding to respective receptors. Binding/phagocytosis was determined after combined 4', 6-diamidino-2-phenylindole/propidium iodide (for apoptotic/necrotic neurons) and lectin staining (for microglia). Uptake of apoptotic neurons was reduced by N-acetylglucosamine or galactose, suggesting that recognition involves asialoglycoprotein-like lectins. Furthermore, the inhibition of microglial binding/uptake of apoptotic neurons by RGDS peptide suggests a role of microglial vitronectin receptor. As microglia selectively bind lipid vesicles enriched in phosphatidylserine and O-phospho-L-serine interfered with the uptake of apoptotic neurons, an involvement of phosphatidylserine receptor is rather likely. Apoptotic neurons do not release soluble signals that serve to attract or activate microglia. Collectively, these results suggest that apoptotic neurons generate a complex surface signal recognized by different receptor systems on microglia.

Alpha-synuclein selectively binds to anionic phospholipids embedded in liquid-disordered domains

Previous studies indicate that binding of alpha-synuclein to membranes is critical for its physiological function and the development of Parkinson's disease (PD). Here, we have investigated the association of fluorescence-labeled alpha-synuclein variants with different types of giant unilamellar vesicles using confocal microscopy. We found that alpha-synuclein binds with high affinity to anionic phospholipids, when they are embedded in a liquid-disordered as opposed to a liquid-ordered environment. This indicates that not only electrostatic forces but also lipid packing and hydrophobic interactions are critical for the association of alpha-synuclein with membranes in vitro. When compared to wild-type alpha-synuclein, the disease-causing alpha-synuclein variant A30P bound less efficiently to anionic phospholipids, while the variant E46K showed enhanced binding. This suggests that the natural association of alpha-synuclein with membranes is altered in the inherited forms of Parkinson's disease.

Mechanochemical bond scission for the activation of drugs

Pharmaceutical drug therapy is often hindered by issues caused by poor drug selectivity, including unwanted side effects and drug resistance. Spatial and temporal control over drug activation in response to stimuli is a promising strategy to attenuate and circumvent these problems. Here we use ultrasound to activate drugs from inactive macromolecules or nano-assemblies through the controlled scission of mechanochemically labile covalent bonds and weak non-covalent bonds. We show that a polymer with a disulfide motif at the centre of the main chain releases an alkaloid-based anticancer drug from its β-carbonate linker by a force-induced intramolecular 5-exo-trig cyclization. Second, aminoglycoside antibiotics complexed by a multi-aptamer RNA structure are activated by the mechanochemical opening and scission of the nucleic acid backbone. Lastly, nanoparticle-polymer and nanoparticle-nanoparticle assemblies held together by hydrogen bonds between the peptide antibiotic vancomycin and its complementary peptide target are activated by force-induced scission of hydrogen bonds. This work demonstrates the potential of ultrasound to activate mechanoresponsive prodrug systems.

Nucleic acid amphiphiles: synthesis and self-assembled nanostructures

This review provides an overview of a relatively new class of bio-conjugates, DNA amphiphiles, which consist of oligonucleotides covalently bonded to synthetic hydrophobic units. The reader will find the basic principles for the structural design and preparation methods of the materials. Moreover, the self-assembly into superstructures of higher order will be highlighted. Finally, some potential applications will be described.

Controlled Release of Volatiles under Mild Reaction Conditions: From Nature to Everyday Products

Volatile organic compounds serve in nature as semiochemicals for communication between species, and are often used as flavors and fragrances in our everyday life. The quite limited longevity of olfactive perception has led to the development of pro-perfumes or pro-fragrances--ideally nonvolatile and odorless fragrance precursors which release the active volatiles by bond cleavage. Only a limited amount of reaction conditions, such as hydrolysis, temperature changes, as well as the action of light, oxygen, enzymes, or microorganisms, can be used to liberate the many different chemical functionalities. This Review describes the controlled chemical release of fragrances and discusses additional challenges such as precursor stability during product storage as well as some aspects concerning toxicity and biodegradability. As the same systems can be applied in different areas of research, the scope of this Review covers fragrance delivery as well as the controlled release of volatiles in general.

Receptor binding and pH stability - how influenza A virus hemagglutinin affects host-specific virus infection

Influenza A virus strains adopt different host specificities mainly depending on their hemagglutinin (HA) protein. Via HA, the virus binds sialic acid receptors of the host cell and, upon endocytic uptake, HA triggers fusion between the viral envelope bilayer and the endosomal membrane by a low pH-induced conformational change leading to the release of the viral genome into the host cell cytoplasm. Both functions are crucial for viral infection enabling the genesis of new progeny virus. Adaptation to different hosts in vitro was shown to require mutations within HA altering the receptor binding and/or fusion behavior of the respective virus strain. Human adapted influenza virus strains (H1N1, H3N2, H2N2) as well as recent avian influenza virus strains (H5, H7 and H9 subtypes) which gained the ability to infect humans mostly contained mutations in the receptor binding site (RBS) of HA enabling increased binding affinity of these viruses to human type (α-2,6 linked sialic acid) receptors. Thus, the receptor binding specificity seems to be the major requirement for successful adaptation to the human host; however, the RBS is not the only determinant of host specificity. Increased binding to a certain cell type does not always correlate with infection efficiency. Furthermore, viruses carrying mutations in the RBS often resulted in reduced viral fitness and were still unable to transmit between mammals. Recently, the pH stability of HA was reported to affect the transmissibility of influenza viruses. This review summarizes recent findings on the adaptation of influenza A viruses to the human host and related amino acid substitutions resulting in altered receptor binding specificity and/or modulated fusion pH of HA. Furthermore, the role of these properties (receptor specificity and pH stability of HA) for adaptation to and transmissibility in the human host is discussed. This article is part of a Special Issue entitled: Viral Membrane Proteins -- Channels for Cellular Networking.

Polyphenylene dendrimers with different fluorescent chromophores asymmetrically distributed at the periphery

A new synthetic approach leading to asymmetrically substituted polyphenylene dendrimers is presented. Following this method, polyphenylene dendrimers decorated with an increasing number of chromophores at the periphery have been obtained up to the second generation. Especially the synthesis of a polyphenylene dendrimer bearing three donor chromophores and one acceptor chromophore has been realized. Intramolecular energy transfer within this molecule is demonstrated by applying absorption and fluorescence measurements.

Dissipative adaptation in driven self-assembly leading to self-dividing fibrils

Out-of-equilibrium self-assembly of proteins such as actin and tubulin is a key regulatory process controlling cell shape, motion and division. The design of functional nanosystems based on dissipative self-assembly has proven to be remarkably difficult due to a complete lack of control over the spatial and temporal characteristics of the assembly process. Here, we show the dissipative self-assembly of FtsZ protein (a bacterial homologue of tubulin) within coacervate droplets. More specifically, we show how such barrier-free compartments govern the local availability of the energy-rich building block guanosine triphosphate, yielding highly dynamic fibrils. The increased flux of FtsZ monomers at the tips of the fibrils results in localized FtsZ assembly, elongation of the coacervate compartments, followed by division of the fibrils into two. We rationalize the directional growth and division of the fibrils using dissipative reaction-diffusion kinetics and capillary action of the filaments as main inputs. The principle presented here, in which open compartments are used to modulate the rates of dissipative self-assembly by restricting the absorption of energy from the environment, may provide a general route to dissipatively adapting nanosystems exhibiting life-like behaviour.

Membrane-mediated induction and sorting of K-Ras microdomain signaling platforms

The K-Ras4B GTPase is a major oncoprotein whose signaling activity depends on its correct localization to negatively charged subcellular membranes and nanoclustering in membrane microdomains. Selective localization and clustering are mediated by the polybasic farnesylated C-terminus of K-Ras4B, but the mechanisms and molecular determinants involved are largely unknown. In a combined chemical biological and biophysical approach we investigated the partitioning of semisynthetic fully functional lipidated K-Ras4B proteins into heterogeneous anionic model membranes and membranes composed of viral lipid extracts. Independent of GDP/GTP-loading, K-Ras4B is preferentially localized in liquid-disordered (l(d)) lipid domains and forms new protein-containing fluid domains that are recruiting multivalent acidic lipids by an effective, electrostatic lipid sorting mechanism. In addition, GDP-GTP exchange and, thereby, Ras activation results in a higher concentration of activated K-Ras4B in the nanoscale signaling platforms. Conversely, palmitoylated and farnesylated N-Ras proteins partition into the l(d) phase and concentrate at the l(d)/l(o) phase boundary of heterogeneous membranes. Next to the lipid anchor system, the results reveal an involvement of the G-domain in the membrane interaction process by determining minor but yet significant structural reorientations of the GDP/GTP-K-Ras4B proteins at lipid interfaces. A molecular mechanism for isoform-specific Ras signaling from separate membrane microdomains is postulated from the results of this study.

Intramolecular energy hopping and energy trapping in polyphenylene dendrimers with multiple peryleneimide donor chromophores and a terryleneimide acceptor trap chromophore

Intramolecular Förster-type excitation energy transfer (FRET) processes in a series of first-generation polyphenylene dendrimers substituted with spatially well-separated peryleneimide chromophores and a terryleneimide energy-trapping chromophore at the rim were investigated by steady-state and time-resolved fluorescence spectroscopy. Energy-hopping processes among the peryleneimide chromophores are revealed by anisotropy decay times of 50--80 ps consistent with a FRET rate constant of k(hopp) = 4.6 ns(-1). If a terryleneimide chromophore is present at the rim of the dendrimer together with three peryleneimide chromophores, more than 95% of the energy harvested by the peryleneimide chromophores is transferred and trapped in the terryleneimide. The two decay times (tau(1) = 52 ps and tau(2) = 175 ps) found for the peryleneimide emission band are recovered as rise times at the terryleneimide emission band proving that the energy trapping of peryleneimide excitation energy by the terryleneimide acceptor occurs via two different, efficient pathways. Molecular- modeling-based structures tentatively indicate that the rotation of the terryleneimide acceptor group can lead to a much smaller distance to a single donor chromophore, which could explain the occurrence of two energy-trapping rate constants. All energy-transfer processes are quantitatively describable with Förster energy transfer theory, and the influence of the dipole orientation factor in the Förster equation is discussed.

Virus-like particles templated by DNA micelles: a general method for loading virus nanocarriers

DNA amphiphile particles template formation of virus capsids and enable their loading.

Real Time Analysis of STAT3 Nucleocytoplasmic Shuttling

The transcription factor STAT3 is most important for the signal transduction of interleukin-6 and related cytokines. Upon stimulation cytoplasmic STAT3 is phosphorylated at tyrosine 705, translocates into the nucleus, and induces target genes. Notably, STAT proteins are also detectable in the nuclei of unstimulated cells. In this report we introduce a new method for the real time analysis of STAT3 nucleocytoplasmic shuttling in living cells which is based on the recently established fluorescence localization after photobleaching (FLAP) approach. STAT3 was C-terminally fused with the cyan (CFP) and yellow (YFP) variants of the green fluorescent protein. In the resulting STAT3-CFP-YFP (STAT3-CY) fusion protein the YFP can be selectively bleached using the 514-nm laser of a confocal microscope. This setting allows studies on the dynamics of STAT3 nucleocytoplasmic transport by monitoring the subcellular distribution of fluorescently labeled and selectively bleached STAT3-CY. By this means we demonstrate that STAT3-CY shuttles continuously between the cytosol and the nucleus in unstimulated cells. This constitutive shuttling does not depend on the phosphorylation of tyrosine 705 because a STAT3(Y705F)-CY mutant shuttles to the same extent as STAT3-CY. Experiments with deletion mutants reveal that the N-terminal moiety of STAT3 is essential for shuttling. Further studies suggest that a decrease in STAT3 nuclear export contributes to the nuclear accumulation of STAT3 in response to cytokine stimulation. The new approach presented in this study is generally applicable to any protein of interest for analyzing nucleocytoplasmic transport mechanisms in real time.

Structure and topology of the influenza virus fusion peptide in lipid bilayers

The secondary structure of a 20-amino acid length synthetic peptide corresponding to the N terminus of the second subunit of hemagglutinin (HA2) of influenza virus A/PR8/34 and its interaction with phospholipid bilayers are investigated using ESR, Fourier transform infrared (FTIR), and CD spectroscopy. N-terminal spin labeling of the peptide did not affect the secondary structure of the peptide either in solution or when bound to liposomes as revealed by FTIR and CD spectroscopy. ESR spectra show that the mobility of the labeled peptide is dramatically restricted in the presence of phosphatidylcholine liposomes, suggesting a strong binding to the lipid membranes. The N terminus of the peptide penetrates into the membrane and is located within the hydrophobic core. We find an oblique insertion of the peptide into the lipid bilayer with an angle of about 45 degrees between helix axis and membrane plane using FTIR spectroscopy. No gross changes of the peptide's orientation, motion, and secondary structure were observed between pH 7.4 and pH 5.0. A model of the insertion of the fusion sequence of HA2 into a lipid bilayer is presented taking into account recent investigations on the low pH conformation of HA2 (Bullough, P. A., Hughson, F. M., Skehel, J. J., and Wiley, D. C. (1994) Nature 371, 37-43).

Studies on the "insoluble" glycoprotein complex from human colon. Identification of reduction-insensitive MUC2 oligomers and C-terminal cleavage

The "insoluble" glycoprotein complex was isolated from human colonic tissue and mucin subunits were prepared following reduction. Antibodies raised against peptide sequences within MUC2 revealed that virtually all of this mucin occurs in the insoluble glycoprotein complex. In addition, reduction released a 120-kDa C-terminal MUC2 fragment, showing that proteolytic cleavage in this domain may occur and leave the fragment attached to the complex via disulfide bonds. The variable number tandem repeat region and the irregular repeat domain were isolated after trypsin digestion and shown to have molecular weights of 930,000 and 180,000, respectively, suggesting a molecular weight for the entire MUC2 monomer of approximately 1.5 million. Gel chromatography and agarose gel electrophoresis revealed several populations of MUC2 subunits, and analytical ultracentrifugation showed that these have molecular weights on the order of 2 million, 4 million, and 5 million, corresponding to monomers, dimers, and trimers, respectively. Agarose gel electrophoresis of subunits from individuals expressing both a "long" and a "short" MUC2 allele revealed a larger number of populations, consistent with the presence of short and long monomers and oligomers arising from permutations of the two types of monomers. In addition to disulfide bonds, MUC2 monomers are apparently joined by a "novel," reduction-insensitive bond.

DNA block copolymers: functional materials for nanoscience and biomedicine

We live in a world full of synthetic materials, and the development of new technologies builds on the design and synthesis of new chemical structures, such as polymers. Synthetic macromolecules have changed the world and currently play a major role in all aspects of daily life. Due to their tailorable properties, these materials have fueled the invention of new techniques and goods, from the yogurt cup to the car seat belts. To fulfill the requirements of modern life, polymers and their composites have become increasingly complex. One strategy for altering polymer properties is to combine different polymer segments within one polymer, known as block copolymers. The microphase separation of the individual polymer components and the resulting formation of well defined nanosized domains provide a broad range of new materials with various properties. Block copolymers facilitated the development of innovative concepts in the fields of drug delivery, nanomedicine, organic electronics, and nanoscience. Block copolymers consist exclusively of organic polymers, but researchers are increasingly interested in materials that combine synthetic materials and biomacromolecules. Although many researchers have explored the combination of proteins with organic polymers, far fewer investigations have explored nucleic acid/polymer hybrids, known as DNA block copolymers (DBCs). DNA as a polymer block provides several advantages over other biopolymers. The availability of automated synthesis offers DNA segments with nucleotide precision, which facilitates the fabrication of hybrid materials with monodisperse biopolymer blocks. The directed functionalization of modified single-stranded DNA by Watson-Crick base-pairing is another key feature of DNA block copolymers. Furthermore, the appropriate selection of DNA sequence and organic polymer gives control over the material properties and their self-assembly into supramolecular structures. The introduction of a hydrophobic polymer into DBCs in aqueous solution leads to amphiphilic micellar structures with a hydrophobic polymer core and a DNA corona. In this Account, we discuss selected examples of recent developments in the synthesis, structure manipulation and applications of DBCs. We present achievements in synthesis of DBCs and their amplification based on molecular biology techniques. We also focus on concepts involving supramolecular assemblies and the change of morphological properties by mild stimuli. Finally, we discuss future applications of DBCs. DBC micelles have served as drug-delivery vehicles, as scaffolds for chemical reactions, and as templates for the self-assembly of virus capsids. In nanoelectronics, DNA polymer hybrids can facilitate size selection and directed deposition of single-walled carbon nanotubes in field effect transistor (FET) devices.

Targeting Stat3 in the myeloid compartment drastically improves the in vivo antitumor functions of adoptively transferred T cells

Improving effector T-cell functions is highly desirable for preventive or therapeutic interventions of diverse diseases. Signal transducer and activator of transcription 3 (Stat3) in the myeloid compartment constrains Th1-type immunity, dampening natural and induced antitumor immune responses. We have recently developed an in vivo small interfering RNA (siRNA) delivery platform by conjugating a Toll-like receptor 9 agonist with siRNA that efficiently targets myeloid and B cells. Here, we show that either CpG triggering combined with the genetic Stat3 ablation in myeloid/B cell compartments or administration of the CpG-Stat3siRNA drastically augments effector functions of adoptively transferred CD8+ T cells. Specifically, we show that both approaches are capable of increasing dendritic cell and CD8(+) T-cell engagement in tumor-draining lymph nodes. Furthermore, both approaches can significantly activate the transferred CD8(+) T cells in vivo, upregulating effector molecules such as perforin, granzyme B, and IFN-γ. Intravital multiphoton microscopy reveals that Stat3 silencing combined with CpG triggering greatly increases killing activity and tumor infiltration of transferred T cells. These results suggest the use of CpG-Stat3siRNA, and possibly other Stat3 inhibitors, as a potent adjuvant to improve T-cell therapies.

Protein-mediated phospholipid translocation in the endoplasmic reticulum with a low lipid specificity

The outside-inside translocation rate of various amphiphilic spin-labeled phospholipids has been measured in rat liver endoplasmic reticulum vesicles. The eight spin-labels tested experienced a fast flip-flop rate with the same half-time of approximately 20 min at 37 degrees C. The stationary distribution of these phospholipid analogues was ca. 45% on the inner vesicular leaflet and 55% on the external one, showing that there is no net enrichment of some lipid in one layer under the experimental conditions used. The initial rate of translocation was reduced 4-fold if membranes were preincubated with N-ethylmaleimide (2 mM) and was about an order of magnitude lower in liposomes made from the extracted lipids. An apparent saturability of the transbilayer diffusion can be deduced from the variation of the initial velocity of the relocation kinetics vs the amount of analogue incorporated in the membrane. Moreover, translocation rates of two different spin-labeled phospholipids introduced simultaneously in the membrane were almost equally reduced by the presence of the other lipid. On the other hand, no competition between the water-soluble dibutyroylphosphatidylcholine and the amphiphilic spin-labeled phospholipids could be detected. Overall, these results suggest that phospholipid translocation in the endoplasmic reticulum is a protein-mediated process with a low specificity, which tends, in the absence of any other metabolic event, to equilibrate the phospholipid composition of the two membrane halves.