Structure of staphylococcal alpha-hemolysin, a heptameric transmembrane pore

The structure of the Staphylococcus aureus alpha-hemolysin pore has been determined to 1.9 A resolution. Contained within the mushroom-shaped homo-oligomeric heptamer is a solvent-filled channel, 100 A in length, that runs along the sevenfold axis and ranges from 14 A to 46 A in diameter. The lytic, transmembrane domain comprises the lower half of a 14-strand antiparallel beta barrel, to which each protomer contributes two beta strands, each 65 A long. The interior of the beta barrel is primarily hydrophilic, and the exterior has a hydrophobic belt 28 A wide. The structure proves the heptameric subunit stoichiometry of the alpha-hemolysin oligomer, shows that a glycine-rich and solvent-exposed region of a water-soluble protein can self-assemble to form a transmembrane pore of defined structure, and provides insight into the principles of membrane interaction and transport activity of beta barrel pore-forming toxins.

Adult mesenchymal stem cells: characterization, differentiation, and application in cell and gene therapy

A considerable amount of retrospective data is available that describes putative mesenchymal stem cells (MSCs). However, there is still very little knowledge available that documents the properties of a MSC in its native environment. Although the precise identity of MSCs remains a challenge, further understanding of their biological properties will be greatly advanced by analyzing the mechanisms that govern their self-renewal and differentiation potential. This review begins with the current state of knowledge on the biology of MSCs, specifically with respect to their existence in the adult organism and postulation of their biological niche. While MSCs are considered suitable candidates for cell-based strategies owing to their intrinsic capacity to self-renew and differentiate, there is currently little information available regarding the molecular mechanisms that govern their stem cell potential. We propose here a model for the regulation of MSC differentiation, and recent findings regarding the regulation of MSC differentiation are discussed. Current research efforts focused on elucidating the mechanisms regulating MSC differentiation should facilitate the design of optimal in vitro culture conditions to enhance their clinical utility cell and gene therapy.

Complete genome sequence of an M1 strain of Streptococcus pyogenes

The 1,852,442-bp sequence of an M1 strain of Streptococcus pyogenes, a Gram-positive pathogen, has been determined and contains 1,752 predicted protein-encoding genes. Approximately one-third of these genes have no identifiable function, with the remainder falling into previously characterized categories of known microbial function. Consistent with the observation that S. pyogenes is responsible for a wider variety of human disease than any other bacterial species, more than 40 putative virulence-associated genes have been identified. Additional genes have been identified that encode proteins likely associated with microbial "molecular mimicry" of host characteristics and involved in rheumatic fever or acute glomerulonephritis. The complete or partial sequence of four different bacteriophage genomes is also present, with each containing genes for one or more previously undiscovered superantigen-like proteins. These prophage-associated genes encode at least six potential virulence factors, emphasizing the importance of bacteriophages in horizontal gene transfer and a possible mechanism for generating new strains with increased pathogenic potential.

Photobleaching kinetics of fluorescein in quantitative fluorescence microscopy

An investigation on the photobleaching behavior of fluorescein in microscopy was carried out through a systematic analysis of photobleaching mechanisms. The individual photochemical reactions of fluorescein were incorporated into a theoretical analysis and mathematical simulation to study the photochemical processes leading to photobleaching of fluorescein in microscopy. The photobleaching behavior of free and bound fluorescein has also been investigated by experimental means. Both the theoretical simulation and experimental data show that photobleaching of fluorescein in microscopy is, in general, not a single-exponential process. The simulation suggests that the non-single-exponential behavior is caused by the oxygen-independent, proximity-induced triplet-triplet or triplet-ground state dye reactions of bound fluorescein in microscopy. The single-exponential process is a special case of photobleaching behavior when the reactions between the triplet dye and molecular oxygen are dominant.

Designed protein pores as components for biosensors

BACKGROUND

There is a pressing need for new sensors that can detect a variety of analytes, ranging from simple ions to complex compounds and even microorganisms. The devices should offer sensitivity, speed, reversibility and selectivity. Given these criteria, protein pores, remodeled so that their transmembrane conductances are modulated by the association of specific analytes, are excellent prospects as components of biosensors.

RESULTS

Structure-based design and a separation method that employs targeted chemical modification have been used to obtain a heteromeric form of the bacterial pore-forming protein staphylococcal alpha-hemolysin, in which one of the seven subunits contains a binding site for a divalent metal ion, M(II), which serves as a prototypic analyte. The single-channel current of the heteromer in planar bilayers is modulated by nanomolar Zn(II). Other M(II)s modulate the current and produce characteristic signatures. In addition, heteromers containing more than one mutant subunit exhibit distinct responses to M(II)s Hence, a large collection of responsive pores can be generated through subunit diversity and combinatorial assembly.

CONCLUSIONS

Engineered pores have several advantages as potential sensor elements: sensitivity is in the nanomolar range; analyte binding is rapid (diffusion limited in some cases) and reversible; strictly selective binding is not required because single-channel recordings are rich in information; and for a particular analyte, the dissociation rate constant, the extent of channel block and the voltage-dependence of these parameters are distinguishing, while the frequency of partial channel block reflects the analyte concentration. A single sensor element might, therefore, be used to quantitate more than one analyte at once. The approach described here can be generalized for additional analytes.

Ceramic thin-film formation on functionalized interfaces through biomimetic processing

Processing routes have been developed for the production of thin ceramic films through precipitation from aqueous solutions. The techniques are based on crystal nucleation and growth onto functionalized interfaces. Surface functionalization routes have been developed by the mimicking of schemes used by organisms to produce complex ceramic composites such as teeth, bones, and shells. High-quality, dense polycrystalline films of oxides, hydroxides, and sulfides have now been prepared from "biomimetic" synthesis techniques. Ceramic films can be synthesized on plastics and other materials at temperatures below 100 degrees C. As a low-temperature process in which water rather than organic solvents is used, this synthesis is environmentally benign. Nanocrystalline ceramics can be produced, sometimes with preferred crystallite orientation. The direct deposition of high-resolution patterned films has also been demonstrated. The process is well suited to the production of organic-inorganic composites.

Cloning and characterization of KCC3 and KCC4, new members of the cation-chloride cotransporter gene family

The K+-Cl- cotransporters (KCCs) belong to the gene family of electroneutral cation-chloride cotransporters, which also includes two bumetanide-sensitive Na+-K+-2Cl- cotransporters and a thiazide-sensitive Na+-Cl- cotransporter. We have cloned cDNAs encoding mouse KCC3, human KCC3, and human KCC4, three new members of this gene family. The KCC3 and KCC4 cDNAs predict proteins of 1083 and 1150 amino acids, respectively. The KCC3 and KCC4 proteins are 65-71% identical to the previously characterized transporters KCC1 and KCC2, with which they share a predicted membrane topology. The four KCC proteins differ at amino acid residues within key transmembrane domains and in the distribution of putative phosphorylation sites within the amino- and carboxyl-terminal cytoplasmic domains. The expression of mouse KCC3 in Xenopus laevis oocytes reveals the expected functional characteristics of a K+Cl- cotransporter: Cl--dependent uptake of 86Rb+ which is strongly activated by cell swelling and weakly sensitive to furosemide. A direct functional comparison of mouse KCC3 to rabbit KCC1 indicates that KCC3 has a much greater volume sensitivity. The human KCC3 and KCC4 genes are located on chromosomes 5p15 and 15q14, respectively. Although widely expressed, KCC3 transcripts are the most abundant in heart and kidney, and KCC4 is expressed in muscle, brain, lung, heart, and kidney. The unexpected molecular heterogeneity of K+-Cl- cotransport has implications for the physiology and pathophysiology of a number of tissues.

Subunit stoichiometry of staphylococcal alpha-hemolysin in crystals and on membranes: a heptameric transmembrane pore

Elucidation of the accurate subunit stoichiometry of oligomeric membrane proteins is fraught with complexities. The interpretations of chemical cross-linking, analytical ultracentrifugation, gel filtration, and low-resolution electron microscopy studies are often ambiguous. Staphylococcal alpha-hemolysin (alpha HL), a homooligomeric toxin that forms channels in cell membranes, was believed to possess six subunits arranged around a sixfold axis of symmetry. Here, we report that analysis of x-ray diffraction data and chemical modification experiments indicate that the alpha HL oligomer is a heptamer. Self-rotation functions calculated using x-ray diffraction data from single crystals of alpha HL oligomers show a sevenfold axis of rotational symmetry. The alpha HL pore formed on rabbit erythrocyte membranes was determined to be a heptamer by electrophoretic separation of alpha HL heteromers formed from subunits with the charge of wild-type alpha HL and subunits with additional negative charge generated by targeted chemical modification of a single-cysteine mutant. These data establish the heptameric oligomerization state of the alpha HL transmembrane pore both in three-dimensional crystals and on a biological membrane.

Influence of the triplet excited state on the photobleaching kinetics of fluorescein in microscopy

The investigation in this report aimed at providing photophysical evidence that the long-lived triplet excited state plays an important role in the non-single-exponential photobleaching kinetics of fluorescein in microscopy. Experiments demonstrated that a thiol-containing reducing agent, mercaptoethylamine (MEA or cysteamine), was the most effective, among other commonly known radical quenchers or singlet oxygen scavengers, in suppressing photobleaching of fluorescein while not reducing the fluorescence quantum yield. The protective effect against photobleaching of fluorescein in the bound state was also found in microscopy. The antibleaching effect of MEA let to a series of experiments using time-delayed fluorescence spectroscopy and nanosecond laser flash photolysis. The combined results showed that MEA directly quenched the triplet excited state and the semioxidized radical form of fluorescein without affecting the singlet excited state. The triplet lifetime of fluorescein was reduced upon adding MEA. It demonstrated that photobleaching of fluorescein in microscopy is related to the accumulation of the long-lived triplet excited state of fluorescein and that by quenching the triplet excited state and the semioxidized form of fluorescein to restore the dye molecules to the singlet ground state, photobleaching can be reduced.

Functional comparison of the K+-Cl- cotransporters KCC1 and KCC4

The K(+)-Cl(-) cotransporters (KCCs) are members of the cation-chloride cotransporter gene family and fall into two phylogenetic subgroups: KCC2 paired with KCC4 and KCC1 paired with KCC3. We report a functional comparison in Xenopus oocytes of KCC1 and KCC4, widely expressed representatives of these two subgroups. KCC1 and KCC4 exhibit differential sensitivity to transport inhibitors, such that KCC4 is much less sensitive to bumetanide and furosemide. The efficacy of these anion inhibitors is critically dependent on the concentration of extracellular K(+), with much higher inhibition in 50 mm K(+) versus 2 mm K(+). KCC4 is also uniquely sensitive to 10 mm barium and to 2 mm trichlormethiazide. Kinetic characterization reveals divergent affinities for K(+) (K(m) values of approximately 25.5 and 17.5 mm for KCC1 and KCC4, respectively), probably due to variation within the second transmembrane segment. Although the two isoforms have equivalent affinities for Cl(-), they differ in the anion selectivity of K(+) transport (Cl(-) > SCN(-) = Br(-) > PO(4)(-3) > I(-) for KCC1 and Cl(-) > Br(-) > PO(4)(-3) = I(-) > SCN(-) for KCC4). Both KCCs express minimal K(+)-Cl(-) cotransport under isotonic conditions, with significant activation by cell swelling under hypotonic conditions. The cysteine-alkylating agent N-ethylmaleimide activates K(+)-Cl(-) cotransport in isotonic conditions but abrogates hypotonic activation, an unexpected dissociation of N-ethylmaleimide sensitivity and volume sensitivity. Although KCC4 is consistently more volume-sensitive, the hypotonic activation of both isoforms is critically dependent on protein phosphatase 1. Overall, the functional comparison of these cloned K(+)-Cl(-) cotransporters reveals important functional, pharmacological, and kinetic differences with both physiological and mechanistic implications.

Purification and characterization of carboxypeptidase D, a novel carboxypeptidase E-like enzyme, from bovine pituitary

Carboxypeptidase E (CPE) is involved in the biosynthesis of most neuropeptides and peptide hormones. Until recently, CPE was the only intracellular carboxypeptidase thought to be involved in neuroendocrine peptide processing. However, the finding that fat/fat mice, which have a mutation within the CPE gene that inactivates the enzyme, are capable of a reduced amount of insulin processing suggests that another carboxypeptidase is present within the secretory pathway. We have detected a CPE-like enzyme, designated CPD, which has many properties in common with those of CPE. Like CPE, CPD is a metallocarboxypeptidase that has a pH optimum of 5.5-6. The Km and Kcat values for a series of short peptide substrates show only minor differences between CPD and CPE. Several active site-directed inhibitors also show generally similar potency toward the two enzymes, although guanidinoethylmercaptosuccinic acid is approximately 10-fold more potent, and hippuryl-Arg is approximately 100-fold more potent as an inhibitor of CPD than of CPE. A major difference between the two enzymes is the molecular masses; CPE is 50,000-56,000, whereas CPD is approximately 180,000. Also, CPD does not elute from a substrate affinity column when the pH is raised to 8, which elutes CPE, although CPD can subsequently be eluted by arginine. Both CPE and CPD are present in purified bovine anterior pituitary secretory vesicles, but the tissue distribution of CPD is more uniform than that of CPE. Antisera to the N- and C-terminal regions of CPE do not recognize CPD. The partial N-terminal amino acid sequence of bovine CPD shows 30-40% homology with an N-terminal region of bovine and rat CPE and 70% homology with a duck protein known as gp180, a hepatitis B virus particle binding protein that shows 47% homology to CPE. Taken together, these results suggest that CPD is a novel secretory pathway enzyme that may be the bovine homologue of gp180.

Protein catalysis of the retinal subpicosecond photoisomerization in the primary process of bacteriorhodopsin photosynthesis

The rate of retinal photoisomerization in wild-type bacteriorhodopsin (wt bR) is compared with that in a number of mutants in which a positively charged (Arg(82)), a negatively charged (Asp(85) or Asp(212)), or neutral hydrogen bonding (Asp(115) or Tyr(185)) amino acid residue known to be functionally important within the retinal cavity is replaced by a neutral, non-hydrogen bonding one. Only the replacements of the charged residues reduced the photoisomerization rate of the 13-cis and all-trans isomers present in these mutants by factors of approximately 1/4 and approximately 1/20, respectively. Retinal photo- and thermal isomerization catalysis and selectivity in wt bR by charged residues is discussed in terms of the known protein structure, the valence-bond wave functions of the ground and excited state of the retinal, and the electrostatic stabilization interactions within the retinal cavity.

Yeast farnesyl-diphosphate synthase: site-directed mutagenesis of residues in highly conserved prenyltransferase domains I and II

Prenyltransferases that catalyze the fundamental chain elongation reaction in the isoprenoid biosynthetic pathway contain several highly conserved amino acids, including two aspartate-rich regions thought to be involved in substrate binding and catalysis. We report a study of site-directed mutants for yeast farnesyl-diphosphate synthase (FPPSase; geranyl-diphosphate:isopentenyl-diphosphate, EC 2.5.1.10), a prenyltransferase that catalyzes the sequential 1'-4 coupling of isopentenyl diphosphate (IPP) with dimethylallyl diphosphate and geranyl diphosphate. A recombinant form of FPPSase extended by a C-terminal -Glu-Glu-Phe alpha-tubulin epitope (EEF in single-letter amino acid code) was engineered to facilitate rapid purification of the enzyme by immunoaffinity chromatography and to remove traces of contaminating activity from wild-type FPPSase in the Escherichia coli host. Ten site-directed mutants were constructed in FPPSase::EEF. The six aspartates in domain I (at positions 100, 101, and 104) and domain II (at positions 240, 241, and 244) were changed to alanine (mutants designated D100A, D101A, D104A, D240A, D241A, and D244A); three arginine residues were changed, Arg-109 and Arg-110 to glutamine and Arg-350 to alanine (mutants designated R109Q, R110Q, and R350A); and Lys-254 was converted to alanine (mutant designated K254A). Mutations of the aspartatic residues and nearby arginine residues in domain I and Asp-240 and Asp-241 in domain II drastically lowered the catalytic activity of FPPSase::EEF. The D244A and K254A mutants were substantially less active, while kcat and the Michaelis constants for the R350A mutant were similar to those of FPPSase::EEF. Addition of an -EEF epitope to the C terminus of wild-type FPPSase resulted in a 14-fold increase of KmIPP and a 12-fold decrease of kcat, suggesting that the conserved hydrophilic C terminus of the enzyme may have a role in substrate binding and catalysis.

Development, characterization, and anti-microbial efficacy of hydroxyapatite-chlorhexidine coatings produced by surface-induced mineralization

The surface-induced mineralization (SIM) technique was used to produce hydroxyapatite (HAP) coatings on external fixation pins with the antimicrobial agent, chlorhexidine, incorporated within the coating. The SIM process involved surface modification of the substrate with organic functional groups followed by immersion in aqueous supersaturated calcium phosphate solutions. X-ray diffraction spectra confirmed that hydroxyapatite coatings were formed. Chlorhexidine was incorporated into the coating by placing the substrate into various chlorhexidine solutions in between mineralization cycles. Total uptake was measured by dissolution of the coating into a 0.1 M nitric acid solution and measuring the chlorhexidine concentration using UV spectroscopy at 251 nm. Release rates were measured by submersion of coated substrates into saline solutions and measuring chlorhexidine UV absorbency at 231 nm as a function of time. Results show an initial rapid release followed by a period of slower sustained release. The anti-microbial efficacy of the HAP-chlorhexidine coatings was evaluated in vitro using a Staphylococcus aureus cell culture. Initial results show a large "inhibition zone" formed around the chlorhexidine/HAP coating vs. coatings with HAP only. This preliminary work clearly demonstrates that SIM HAP coatings have great potential to locally deliver antimicrobial agents such as chlorhexidine at implantation sites, which may greatly reduce the incidence of pin tract infection that occurs in external fixation.

Nicastrin binds to membrane-tethered Notch

The presenilins and nicastrin, a type 1 transmembrane glycoprotein, form high molecular weight complexes that are involved in cleaving the beta-amyloid precursor protein (betaAPP) and Notch in their transmembrane domains. The former process (termed gamma-secretase cleavage) generates amyloid beta-peptide (Abeta), which is involved in the pathogenesis of Alzheimer's disease. The latter process (termed S3-site cleavage) generates Notch intracellular domain (NICD), which is involved in intercellular signalling. Nicastrin binds both full-length betaAPP and the substrates of gamma-secretase (C99- and C83-betaAPP fragments), and modulates the activity of gamma-secretase. Although absence of the Caenorhabditis elegans nicastrin homologue (aph-2) is known to cause an embryonic-lethal glp-1 phenotype, the role of nicastrin in this process has not been explored. Here we report that nicastrin binds to membrane-tethered forms of Notch (substrates for S3-site cleavage of Notch), and that, although mutations in the conserved 312-369 domain of nicastrin strongly modulate gamma-secretase, they only weakly modulate the S3-site cleavage of Notch. Thus, nicastrin has a similar role in processing Notch and betaAPP, but the 312-369 domain may have differential effects on these activities. In addition, we report that the Notch and betaAPP pathways do not significantly compete with each other.

Targeting SPARC expression decreases glioma cellular survival and invasion associated with reduced activities of FAK and ILK kinases

Secreted protein acidic and rich in cysteine (SPARC) is an extracellular glycoprotein expressed in several solid cancers, including malignant gliomas, upon adoption of metastatic or invasive behaviors. SPARC expression in glioma cells promotes invasion and survival under stress, the latter process dependent on SPARC activation of AKT. Here we demonstrate that downregulation of SPARC expression with short interfering RNA (siRNA) in glioma cells decreased tumor cell survival and invasion. SPARC siRNA reduced the activating phosphorylation of AKT and two cytoplasmic kinases, focal adhesion kinase (FAK) and integrin-linked kinase (ILK). We determined the contributions of FAK and ILK to SPARC effects using SPARC protein and cell lines engineered to overexpress SPARC. SPARC activated FAK and ILK in glioma cells previously characterized as responsive to SPARC. Downregulation of either FAK or ILK expression inhibited SPARC-mediated AKT phosphorylation, and targeting both FAK and ILK attenuated AKT activation more potently than targeting either FAK or ILK alone. Decreased SPARC-mediated AKT activation correlated with a reduction in SPARC-dependent invasion and survival upon the downregulation of FAK and/or ILK expression. These data further demonstrate the role of SPARC in glioma tumor progression through the activation of intracellular kinases that may provide novel therapeutic targets for advanced cancers.

Astrocyte elevated gene-1 is a proliferation promoter in breast cancer via suppressing transcriptional factor FOXO1

We have previously reported that astrocyte elevated gene-1 (AEG-1) was upregulated in human breast cancer. However, the biological function of AEG-1 in the development and progression of breast cancer remains to be clarified. In this study, we examined the effect of AEG-1 on cell proliferation and found that AEG-1 upregulation was significantly linked to increased Ki67 (P<0.001). Ectopic expression of AEG-1 in MCF-7 and MDA-MB-435 breast cancer cells dramatically enhanced cell proliferation and their ability of anchorage-independent growth, whereas silencing endogenous AEG-1 with shRNAs inhibited cell proliferation and colony-forming ability of the cells on soft agar. Furthermore, these proliferative effects were significantly associated with decreases of p27Kip1 and p21Cip1 two key cell-cycle inhibitors. Moreover, we further demonstrated that AEG-1 could downregulate the transcriptional activity of FOXO1 by inducing its phosphorylation through the PI3K/Akt signaling pathway. These observations were further confirmed in clinical human primary breast cancer specimens, in which high-level expression of AEG-1 was inversely correlated with the expression of FOXO1. Taken together, our results provide the first demonstration of a novel mechanism by which AEG-1 induces proliferation of breast cancer cell, and our findings suggest that AEG-1 might play an important role in tumorigenesis of breast cancer.

Age-related effects in T cell activation and proliferation

Age-associated thymic involution manifests its effects in a variety of ways that are related to a loss of T cell function. These include the appearance of a non-functional subset of T cells that increase in representation with age. Moreover there is a loss of T cell proliferative ability, a decline in the synthesis and release of interleukin-2 (IL-2), a decline in the ability of the T cell to express the IL-2 receptor, and a loss of control activity. This loss of control is demonstrated by the age-related appearance of autoantibodies and an increase in the elaboration of inflammatory cytokines such as TNF, IFN, IL-6, and TGF. A major part of the basis for the loss of T cell function is an inability of the T cell to respond to activation signals that are transmitted through the membrane binding of specific stimulatory signals. Transduction events, differentiation signals, and a loss of control mechanisms are all parts of a complicated picture of age-related immune deficiencies.

Bisindolylmaleimide VIII facilitates Fas-mediated apoptosis and inhibits T cell-mediated autoimmune diseases

Fas-mediated apoptosis is essential for the elimination of cells, and impaired apoptosis can have severe detrimental consequences. Bisindolylmaleimide VIII potentiated Fas-mediated apoptosis in human astrocytoma 1321N1 cells and in Molt-4T cells, both of which were devoid of apoptosis induced by anti-Fas antibody in the absence of bisindolylmaleimide VIII, and in Jurkat and CEM-6 T cells, which showed slight and moderate apoptotic responses, respectively, to low levels of Fas stimulation. Potentiation of Fas-mediated apoptosis by bisindolylmaleimide VIII was selective for activated, rather than non-activated, T cells, and was Fas-dependent, as it was not observed in T cells from Fas-deficient lpr/lpr mice. Administration of bisindolylmaleimide VIII to rats during autoantigen stimulation prevented the development of symptoms of T cell-mediated autoimmune diseases in two models, the Lewis rat model of experimental allergic encephalitis and the Lewis adjuvant arthritis model. Thus, the use of agents such as bisindolylmaleimide VIII may be therapeutically useful for supporting more effective elimination of detrimental cells through enhancement of Fas-dependent apoptosis signaling.

alpha-Hemolysin, gamma-hemolysin, and leukocidin from Staphylococcus aureus: distant in sequence but similar in structure

alpha-Hemolysin from Staphylococcus aureus assembles from a water-soluble, monomeric species to a membrane-bound heptamer on the surface of target cells, creating water-filled channels that lead to cell death and lysis. Staphylococcus aureus also produces the gamma-hemolysin and leukocidin toxins, which function as two component toxins in the disruption and lysis of erythrocytes and leukocytes. Analysis of the aligned sequences of alpha-hemolysin, gamma-hemolysin, and leukocidin in the context of the alpha-hemolysin heptamer structure supports the conclusion that even though the level of sequence identity between alpha-hemolysin and the gamma-hemolysin and leukocidin toxins is in the so-called twilight zone, the three-dimensional structures of the protomers are probably conserved. By analogy with alpha-hemolysin, gamma-hemolysin and leukocidin may also form oligomeric, transmembrane channels in which an antiparallel beta-barrel constitutes the primary membrane-embedded domain.