Single-molecule detection technologies in miniaturized high-throughput screening: fluorescence intensity distribution analysis

Single-molecule detection technologies are becoming a powerful readout format to support ultra-high-throughput screening. These methods are based on the analysis of fluorescence intensity fluctuations detected from a small confocal volume element. The fluctuating signal contains information about the mass and brightness of the different species in a mixture. The authors demonstrate a number of applications of fluorescence intensity distribution analysis (FIDA), which discriminates molecules by their specific brightness. Examples for assays based on brightness changes induced by quenching/dequenching of fluorescence, fluorescence energy transfer, and multiple-binding stoichiometry are given for important drug targets such as kinases and proteases. FIDA also provides a powerful method to extract correct biological data in the presence of compound fluorescence.     

Nicotinic autoreceptor function in rat brain during maturation and aging: possible differential sensitivity to organophosphorus anticholinesterases

Acetylcholine (ACh) release is modulated pre-synaptically by both muscarinic and nicotinic receptor-mediated processes. While muscarinic autoreceptors inhibit ACh release, nicotinic autoreceptors enhance ACh release and thus disruption of these processes could potentially affect cholinergic toxicity following exposure to anticholinesterases. Marked age-related differences in sensitivity to some organophosphorus (OP) anticholinesterases have been reported. We compared nicotinic autoreceptor function (NAF) during maturation and aging and evaluated its potential modulation by the common OP insecticide, chlorpyrifos (CPF). Cortical synaptosomes were pre-loaded with [3H]choline, superfused (0.6 ml/min) with physiological buffer and [3H]ACh release was evoked with potassium (KCl, 9 mM), with or without co-addition of exogenous ACh to stimulate nicotinic autoreceptors. Fractions of perfusate were subsequently collected and area under the curve (AUC) for [3H] was analyzed by scintillation counting. The difference in evoked release due to co-addition of exogenous ACh was defined as NAF. Under these conditions, atropine (ATR, 0.1 microM) appeared requisite for NAF; thus this muscarinic antagonist was subsequently added to all perfusion buffers. In synaptosomes from adult tissues, exogenous ACh (3-100 microM) significantly increased release in a concentration-dependent manner. The nicotinic antagonist mecamylamine (MEC, 100 microM) substantially reduced the potassium-evoked release elicited by co-addition of ACh (10 microM). Interestingly, the nicotinic agonists nicotine (NIC) and dimethylphenylpiperazinium (DMPP; 0.1-10 microM) had no effect on release. The active metabolite of CPF (i.e. chlorpyrifos oxon (CPO), 1-10 microM) inhibited NAF in vitro. Maturation-related expression of NAF was noted (AUC with co-addition of 10 microM ACh: 7-day rats, 7+/-6; 21-day rats, 44+/-6; 90-day rats, 196+/-37; 24-month rats, 173+/-52). NAF was substantially reduced (67-91%) 96 h after maximum tolerated dosages of CPF in adult and aged rats (279 mg/kg, sc) but not in juveniles (127 mg/kg, sc), even though AChE inhibition was similar among the age groups (>80%). Together these data suggest that NAF is differentially expressed during maturation and that this neuromodulatory process may be selectively altered by some OP insecticides, potentially contributing to age-related differences in response to AChE inhibitors. As NAF has been postulated to be activated under conditions of 'impaired' cholinergic function, selective alteration of this pre-synaptic process by OP anticholinesterases may be also important in age-related conditions associated with cholinergic hypofunction.     

DNA sequence variation and molecular genotyping of natural killer leukocyte immunoglobulin-like receptor,LILRA3

Leukocyte immunoglobulin-like receptors (LILRs) resemble killer cell immunoglobulin-like receptors (KIR) in structure and function and the KIR and LILR gene families form the major part of the leukocyte receptor cluster (LRC) of human chromosome 19q13.4. Unlike KIR, the LILR gene clusters do not vary in gene number. However, some individuals lack expression of LILRA3. This null allele has a 6.7-kb deletion, which encompasses the first six translated exons. This haplotype enabled unambiguous direct sequencing of LILRA3 alleles using genomic DNA from individuals heterozygous for the deletion. We have performed nucleotide sequencing of a 2.5-kb region within LILRA3 and identified eight bi-allelic substitutions, four of which were non-synonymous. Two from four previously identified LILRA3 cDNA sequences were confirmed and a further six alleles characterised, of which four will encode unique peptides. At least one of the polymorphic positions identified (encoding residue 84 of the first Ig domain) is likely to directly influence ligand binding. A PCR-SSP molecular genotyping system was developed and used to describe a panel of 172 Caucasoid individuals from South-East England. Six alleles were present in this group but they were unevenly distributed, as three alleles accounted for 88% of the studied chromosomes.     

p21+/+ (CDKN1A+/+) and p21-/- (CDKN1A-/-) human colorectal carcinoma cells display equivalent amounts of thermal radiosensitization

The mechanism of thermal radiosensitization is related to the inhibition of repair of radiation-induced DNA damage by heat. Due to the interaction of the gene p21/WAF1/CIP1 (now known as CDKN1A) with a variety of DNA repair proteins, its involvement in thermal radiosensitization was investigated. Two isogenetic human colorectal cancer cell lines with wild-type TP53 status were used. The 80S4 cell line was deficient in CDKN1A and the HCT116 cells were CDKN1A proficient. Both cell lines were significantly more sensitive to 44 degrees C than 42 degrees C heating (P < 0.01), and both cell lines expressed thermotolerance for heating times longer than about 2 h at the lower temperature. There were no significant differences in the X-radiation response of the two cell lines. Further, the two cell lines displayed similar cell survival levels after hyperthermia given before or after X radiation for both hyperthermia temperatures. Comparison of thermal enhancement ratios confirmed that there was no difference in the amount of thermal radiosensitization induced in the two cell lines. The induction and subsequent repair of DNA double-strand breaks, as measured by clamped homogeneous gel electrophoresis, was also the same in both cell lines. These findings strongly suggest that the gene CDKN1A does not play an important role in the expression of thermal radiosensitization.     

Identification and characterization of novel surface proteins in Lactobacillus johnsonii and Lactobacillus gasseri

We have identified and sequenced the genes encoding the aggregation-promoting factor (APF) protein from six different strains of Lactobacillus johnsonii and Lactobacillus gasseri. Both species harbor two apf genes, apf1 and apf2, which are in the same orientation and encode proteins of 257 to 326 amino acids. Multiple alignments of the deduced amino acid sequences of these apf genes demonstrate a very strong sequence conservation of all of the genes with the exception of their central regions. Northern blot analysis showed that both genes are transcribed, reaching their maximum expression during the exponential phase. Primer extension analysis revealed that apf1 and apf2 harbor a putative promoter sequence that is conserved in all of the genes. Western blot analysis of the LiCl cell extracts showed that APF proteins are located on the cell surface. Intact cells of L. johnsonii revealed the typical cell wall architecture of S-layer-carrying gram-positive eubacteria, which could be selectively removed with LiCl treatment. In addition, the amino acid composition, physical properties, and genetic organization were found to be quite similar to those of S-layer proteins. These results suggest that APF is a novel surface protein of the Lactobacillus acidophilus B-homology group which might belong to an S-layer-like family.     

Ubiquitin conjugate dynamics in the gut and liver of hibernating ground squirrels

Protein synthesis is severely depressed in hibernating mammals. In the absence of significant protein synthesis, the continued turnover of proteins as a function of normal cellular activity would result in the net depletion of protein pools. We measured levels of ubiquitylated proteins in the gut of thirteen-lined ground squirrels ( Spermophilus tridecemlineatus) and liver of golden-mantled ground squirrels ( Spermophilus lateralis). In both tissues, ubiquitin conjugate concentrations increased during entrance into torpor and were elevated 2-3 fold by late torpor compared with levels in active animals. The data are consistent with a depression of proteolysis with a resultant high level of ubiquitylated proteins during the natural hypothermia of torpor. The periodic returns to euthermy during the hibernation season allow for degradation of these conjugated proteins and may serve to restore protein pools.     

Targeting protein phosphatase 1 (PP1) to the actin cytoskeleton: the neurabin I/PP1 complex regulates cell morphology

Neurabin I, a neuronal actin-binding protein, binds protein phosphatase 1 (PP1) and p70 ribosomal S6 protein kinase (p70S6K), both proteins implicated in cytoskeletal dynamics. We expressed wild-type and mutant neurabins fused to green fluorescent protein in Cos7, HEK293, and hippocampal neurons. Biochemical and cellular studies showed that an N-terminal F-actin-binding domain dictated neurabin I localization at actin cytoskeleton and promoted disassembly of stress fibers. Deletion of the C-terminal coiled-coil and sterile alpha motif domains abolished neurabin I dimerization and induced filopodium extension. Immune complex assays showed that neurabin I recruited an active PP1 via a PP1-docking sequence,(457)KIKF(460). Mutation of the PP1-binding motif or PP1 inhibition by okadaic acid and calyculin A abolished filopodia and restored stress fibers in cells expressing neurabin I. In vitro and in vivo studies suggested that the actin-binding domain attenuated protein kinase A (PKA) phosphorylation of neurabin I. Modification of a major PKA site, serine-461, impaired PP1 binding. Finally, p70S6K was excluded from neurabin I/PP1 complexes and required the displacement of PP1 for recruitment to neurabin I. These studies provided new insights into the assembly and regulation of a neurabin I/PP1 complex that controls actin rearrangement to promote spine development in mammalian neurons.     

Centrosome reorientation in wound-edge cells is cell type specific

The reorientation of the microtubule organizing center during cell migration into a wound in the monolayer was directly observed in living wound-edge cells expressing gamma-tubulin tagged with green fluorescent protein. Our results demonstrate that in CHO cells, the centrosome reorients to a position in front of the nucleus, toward the wound edge, whereas in PtK cells, the centrosome lags behind the nucleus during migration into the wound. In CHO cells, the average rate of centrosome motion was faster than that of the nucleus; the converse was true in PtK cells. In both cell lines, centrosome motion was stochastic, with periods of rapid motion interspersed with periods of slower motion. Centrosome reorientation in CHO cells required dynamic microtubules and cytoplasmic dynein/dynactin activity and could be prevented by altering cell-to-cell or cell-to-substrate adhesion. Microtubule marking experiments using photoactivation of caged tubulin demonstrate that microtubules are transported in the direction of cell motility in both cell lines but that in PtK cells, microtubules move individually, whereas their movement is more coherent in CHO cells. Our data demonstrate that centrosome reorientation is not required for directed migration and that diverse cells use distinct mechanisms for remodeling the microtubule array during directed migration.     

Assessment of the accuracy of the Conconi test in determining gas analysis anaerobic threshold

The primary objective of this study was to assess the validity of the Conconi method of assessing anaerobic threshold (AT). Twenty-two competitive cyclists (11 men, 11 women) performed an incremental, maximal oxygen uptake (Vdot;o(2)max) test on an electrically braked cycle ergometer. AT was assessed by a nonlinear increase in the carbon dioxide to oxygen consumption ratio (V-slope method). AT was also measured by a computer-assessed break point in heart rate from linearity (Conconi method). Mean values for heart rate at AT were 157.3 and 163.3 b x min(-1) for the Conconi and V-slope methods, respectively. Although there was no significant difference (t = 1.07, p = 0.30) in the 2 methods, a rather low correlation coefficient (r = 0.458), high standard error of estimate (SEE = 10.7 b x min(-1)), and high total error (TE = 16.7 b x min(-1)) would preclude the use of the Conconi method as a practical tool for assessing AT.