mt-Nd2 Allele of the ALR/Lt mouse confers resistance against both chemically induced and autoimmune diabetes

AIMS/HYPOTHESIS

ALR/Lt, a mouse strain with strong resistance to type 1 diabetes, is closely related to autoimmune type 1 diabetes-prone NOD/Lt mice. ALR pancreatic beta cells are resistant to the beta cell toxin alloxan, combinations of cytotoxic cytokines, and diabetogenic NOD T-cell lines. Reciprocal F1 hybrids between either ALR and NOD or ALR and NON/Lt, showed that alloxan resistance was transmitted to F1 progeny only when ALR was the maternal parent. Here we show that the mitochondrial genome (mtDNA) of ALR mice contributes resistance to diabetes.

METHODS

When F1 progeny from reciprocal outcrosses between ALR and NOD were backcrossed to NOD, a four-fold lower frequency of spontaneous type 1 diabetes development occurred when ALR contributed the mtDNA. Because of the apparent interaction between nuclear and mtDNA, the mitochondrial genomes were sequenced.

RESULTS

An ALR-specific sequence variation in the mt-Nd2 gene producing a leucine to methionine substitution at amino acid residue 276 in the NADH dehydrogenase 2 was discovered. An isoleucine to valine mutation in the mt-Co3 gene encoding COX3 distinguished ALR and NOD from NON and ALS. All four strains were distinguished by variation in a mt-encoded arginyl tRNA polyadenine tract. Shared alleles of mt-Co3 and mt-Tr comparing NOD and ALR allowed for exclusion of these two genes as candidates, implicating the mt-Nd2 variation as a potential ALR-derived type 1 diabetes protective gene.

CONCLUSIONS/INTERPRETATION

The unusual resistance of ALR mice to both ROS-mediated and autoimmune type 1 diabete stresses reflects an interaction between the nuclear and mt genomes. The latter contribution is most likely via a single nucleotide polymorphism in mt-Nd2.

Pyrosequence-based typing of alleles of the HLA-DQB1 gene

DNA typing of alleles of the highly polymorphic HLA-DQBI gene was performed by Pyrosequencing using purified DNA from the 11th International Histocompatibility Workshop human cell lines and samples from the Children's Hospital of Pittsburgh registry of diabetics and their first-degree relatives. Pyrosequencing was optimized for genotyping exon 2 of the HLA-DQB1 gene, but the procedure should be applicable to other HLA loci. The 47 HLA-DQB1 alleles were readily identifiable, as were the 1,128 potential allelic heterozygous combinations. The method required PCR conditions that specifically amplified DQB1 but not the pseudogene, DQB2. The new method of pyrosequence-based typing can be performed in 96- or 384-well format. The 61 polymorphic residues of DQB1 exon 2 were identified within four pyrosequencing reactions, obtained by a 70-nucleotide read length in each reaction, in about an hour's time. Allelic combinations of HLA-DQB1 most frequentlyfound in the population of diabetics and their immediate family members were analyzed and successfully compared to typing of the DQB1 alleles by sequence-specific oligonucleotide probe protocols. Pyrosequence-based typing is compatible with genotyping of allelic combinations expected from heterozygous individuals, resulting in nucleotide resolution of the highly polymorphic HLA system. Using pyrosequencing, more than 750 sample wells can be processed in a working day, resulting in the identification of more than 50,000 bases.

Anti-L-selectin aptamers: binding characteristics, pharmacokinetic parameters, and activity against an intravascular target in vivo

Therapeutic and diagnostic applications have been envisioned for aptamers, a class of oligonucleotide ligands that bind their target molecules with high affinity and specificity (Gold, J. Biol. Chem. 270, 13581-13584, 1995). To identify parameters that are important for the in vivo activity of aptamers acting on intravascular targets, we have studied binding characteristics in vitro, pharmacokinetic parameters in Sprague-Dawley rats, and inhibitory activity in a SCID mouse/human lymphocyte model of lymphocyte trafficking for both 2'F pyrimidine 2'OH purine RNA and ssDNA anti-human L-selectin aptamers. The data indicate that aptamers with low nanomolar affinity are suitable candidates for use as in vivo reagents and that nonspecific binding to vascular cells is not an issue for efficacy. As is often observed for other reagents, plasma clearance is biphasic. Both the distribution phase and the clearance rate strongly affect in vivo activity. Pharmacokinetic parameters and in vivo activity are significantly improved by conjugating aptamers to a carrier molecule, such as polyethylene glycol (PEG). Most active in vivo is 1d40, a 2'F pyrimidine 2'OH purine aptamer conjugated to 40 kDa PEG. At a dose of 5.4 nmol/kg body weight, its duration of effect (time to 50% inhibition) is 11.2 hours, and at 1 mg or 90 nmol/kg, its plasma clearance rate (CL) is 0.4 ml/min/kg. Its ED50 is estimated to be 80 pmol/kg in preinjection dose-response experiments, compared with 4 pmol/kg for the dimeric anti-L-selectin antibody DREG56. Further improvement of in vivo activity is expected from nucleotide modifications that increase resistance to nuclease digestion for aptamers where mass is not rate limiting for clearance. Because the relationship of clearance to conjugate molecular weight (MW) is not the same for all aptamers, it is advisable to determine the relationship at the outset of in vivo studies. In summary, the data suggest that properly formulated aptamers have the capacity to be effective therapeutic agents against intravascular targets.

Anti-L-selectin oligonucleotide ligands recognize CD62L-positive leukocytes: binding affinity and specificity of univalent and bivalent ligands

Oligonucleotide aptamers generated against purified LS-Rg, a human L-selectin/IgG fusion protein, bound human CD62L-positive leukocytes. FACS analysis of lymphocytes or neutrophils stained with fluorescently labeled aptamers indicated specificity and sensitivity for cellular L-selectin similar to that observed with anti-L-selectin antibody. Univalent aptamers were compared to bivalent aptamers as well as to the anti-adhesion, anti-L-selectin antibody DREG56. Equilibrium and kinetic binding experiments were performed to examine the affinity and kinetic binding parameters of L-selectin aptamers to evaluate their binding to CD62L-positive leukocytes and to test their potential as L-selectin antagonists. Binding experiments indicated that bivalent aptamers approached the affinity and the dissociation rate of bivalent antibody, and preferentially recognized cellular compared to soluble L-selectin, a potentially useful distinction in vivo. Anti-L-selectin aptamers also inhibited L-selectin dependent self-adhesion of neutrophils suggesting that in vitro univalent and bivalent aptamers provided anti-adhesion activity similar to that observed with blocking antibody and indicated a direct blocking mechanism of action during inhibition of L-selectin-dependent trafficking of lymphocytes observed in vivo.

Identification of an intragenic ribosome binding site that affects expression of the uncB gene of the Escherichia coli proton-translocating ATPase (unc) operon

The uncB gene codes for the a subunit of the Fo proton channel sector of the Escherichia coli F1 Fo ATPase. Control of expression of uncB appears to be exerted at some step after translational initiation. Sequence analysis by the perceptron matrices (G. D. Stormo, T. D. Schneider, L. Gold, and A. Ehrenfeucht, Nucleic Acids Res. 10:2997-3011, 1982) identified a potential ribosome binding site within the uncB reading frame preceding a five-codon reading frame which is shifted one base relative to the uncB reading frame. Elimination of this binding site by mutagenesis resulted in a four- to fivefold increase in expression of an uncB'-'lacZ fusion gene containing most of uncB. Primer extension inhibition (toeprint) analysis to measure ribosome binding demonstrated that ribosomes could form an initiation complex at this alternative start site. Two fusions of lacZ to the alternative reading frame demonstrated that this site is recognized by ribosomes in vivo. The results suggest that expression of uncB is reduced by translational frameshifting and/or a translational false start at this site within the uncB reading frame.

DNA aptamers block L-selectin function in vivo. Inhibition of human lymphocyte trafficking in SCID mice

Selectins participate in the initial events leading to leukocyte extravasation from the blood into tissues. Thus the selectins have generated much interest as targets for antiinflammatory agents. Therapeutic molecules based on the monomeric carbohydrate ligand sialyl Lewis X (SLe(X)) have low affinities and are not specific for a given selectin. Using SELEX (Systematic Evolution of Ligands by EXponential Enrichment) technology, we have generated aptamers specific for L-selectin that require divalent cations for binding and have low nanomolar affinity. In vitro, the deoxyoligonucleotides inhibit L-selectin binding to immobilized SLe(X) in static assays and inhibit L-selectin-mediated rolling of human lymphocytes and neutrophils on cytokine-activated endothelial cells in flow-based assays. These aptamers also block L-selectin-dependent lymphocyte trafficking in vivo, indicating their potential utility as therapeutics.

High-affinity RNA ligands to Escherichia coli ribosomes and ribosomal protein S1: comparison of natural and unnatural binding sites

High-affinity RNA ligands were generated against intact 30S ribosomes, S1-depleted 30S ribosomes, and purified ribosomal protein S1. Sequence analysis indicated two classes of ligand: unstructured RNAs containing a Shine-Dalgarno sequence and structured RNAs containing a pseudoknot. The Shine-Dalgarno-containing ligands were generated against S1-depleted 30S ribosomes but, surprisingly, not against intact 30S ribosomes or ribosomal protein S1. In contrast, pseudoknot-containing ligands were generated against intact ribosomes as well as purified S1 protein. The two classes of ligand exhibited specificity for their respective targets, as well as conserved sequence and secondary structure reminiscent of naturally occurring, cis-acting mRNA elements.

The bacteriophage T4 regB ribonuclease. Stimulation of the purified enzyme by ribosomal protein S1

Infection of Escherichia coli by bacteriophage T4 induces a mRNA ribonuclease activity that shows specificity for cleavage within the sequence GGAG. Substrates of the activity in vivo include a number of phage mRNAs that are cleaved at GGAGs within the Shine/Dalgarno domains of their translation initiation regions. Induction of the ribonuclease depends on the product of the T4 gene regB. We describe here the overproduction and extensive purification of the RegB protein. RegB precisely co-purifies with an activity that cleaves within the sequence GGAG in oligonucleotide and polynucleotide RNAs and is therefore likely to constitute the sequence-specific catalytic component of the observed activity. We further report that the low cleavage rate observed with our preparations of purified RegB is substantially increased (1-2 orders of magnitude) by the addition of E. coli ribosomal protein S1. We discuss the implications of this observation for the mechanism of action of the RegB ribonuclease in vitro and in vivo.

Recognition of the mRNA selenocysteine insertion sequence by the specialized translational elongation factor SELB

In Escherichia coli the unusual amino acid selenocysteine is incorporated cotranslationally at an in-frame UGA codon. Incorporation of selenocysteine relies, in part, on the interaction between a specialized elongation factor, the SELB protein, and a cis-acting element within the mRNA. Boundary and toeprint experiments illustrate that the SELB-GTP-Sec-tRNA(Sec) ternary complex binds to the selenoprotein encoding mRNAs fdhF and fdnG, serving to increase the concentration of SELB and Sec-tRNA(Sec) on these mRNAs in vivo. Moreover, toeprint experiments indicate that SELB recognizes the ribosome-bound message and that, upon binding, SELB may protrude out of the ribosomal-mRNA track so as to approach the large ribosomal subunit. The results place the mRNA-bound SELB-GTP-Sec-tRNA(Sec) ternary complex at the selenocysteine codon (as expected) and suggest a mechanism to explain the specificity of selenocysteine insertion. Cis-acting mRNA regulatory elements can tether protein factors to the translation complex during protein synthesis.

Translation initiation complex formation with 30 S ribosomal particles mutated at conserved positions in the 3'-minor domain of 16 S RNA

Escherichia coli 30 S ribosomal subunits containing in vitro (phage T7 RNA polymerase-generated) 16 S rRNA, both wild-type and mutant, were examined by toeprinting. These synthetic particles were used to compare the effects of the absence of base modification and of specific nucleotide substitutions in conserved sequence regions of the RNA on the assembly of mRNA, tRNAs and 30 S particles into a translational initiation complex. Initiation factor-3-dependent selection of tRNA(fMet) from a mixture of tRNA(fMet) and tRNA(Phe) occurred with all particles, although 20 times less initiation factor-3 was needed for the synthetic particles, including the mutants. Whereas isolated 30 S particles or those reconstituted with isolated RNA did not distinguish between tRNA(fMet) and tRNA(Phe) for ternary complex formation in the absence of initiation factor-3 (intrinsic selection ability), the synthetic particles preferred tRNA(fMet). The difference between the natural and synthetic particles appears to be due to the absence of certain base modifications, but not m2(6)A, in the synthetic RNA. Synthetic particles containing the mutation U1512C, which converts the universal U.G pair to C.G enhanced both tRNA(fMet) binding and selectivity, although other mutations at that site, namely U1512G, G1523A and U1512C/C1524U, had no such effect. Mutants U1498G and G1401C/C1501G, both located in a highly conserved single-stranded region of the 3'-minor domain, also enhanced tRNA(fMet) selectivity, in this case by reducing complex formation with elongator tRNA. Complex formation between elongator tRNA and the G1401C/C1501G mutant was reduced to almost undetectable levels. The results also indicated that the association rate for initiation complex formation for G1401C/C1501G was considerably lower than for the wild-type sequence. This result had not been detected by standard tRNA-30 S binding assays. Overall, the data suggest that (some of) the 16 S rRNA base modifications as well as the tertiary structure around the decoding site act to desensitize the intrinsic selection ability of the ribosome for tRNA(fMet).

Nature of the ribosomal mRNA track: analysis of ribosome-binding sites containing different sequences and secondary structures

The ribosomal mRNA track was investigated by toeprinting 30S ribosomes, in the presence or absence of tRNA, using a variety of different ribosome-binding sites. We found that: (1) the ribosome, by itself, recognizes the mRNA translational initiation site; (2) the ribosomal mRNA track makes extensive contact with mRNA independent of tRNA and the start codon; (3) ribosome-mRNA complexes are less stable than complexes containing tRNA; and (4) toeprinting can be used to analyze the contour of the ribosomal mRNA track, yielding information on its "height" as well as its "length" dimension. Examination of several ribosome-binding sites, including those containing very stable secondary structure, indicated that the "height" of the mRNA track is quite roomy, while the nucleotide distance between the site of Shine-Dalgarno annealing, the P site, and the 3'-edge of the mRNA track is fixed. The data suggest a mechanism for tethering regulatory elements to the ribosome during translation.

The 0.7 to 3.3 megabase chromosomes from Candida, Kluyveromyces, and Pichia provide accurate size standards for pulsed field gel electrophoresis

Pulsed field gel electrophoresis was used to size intact chromosomal DNAs from Candida albicans, Kluyveromyces lactis, Pichia scolyti, and Pichia mississippiensis by optimization methods using, as size standards, concatenated bacteriophage lambda DNA, and intact and NotI digestion products of Schizosaccharomyces pombe chromosomal DNAs. These newly sized fungal DNAs can now serve as convenient and accurate size standards for DNA molecules between 0.7 and 3.3 megabases (Mb). These size standards are valid over a wide range of different electrophoretic conditions.

Structure-function analysis of Escherichia coli translation initiation factor IF3: tyrosine 107 and lysine 110 are required for ribosome binding

Translation initiation factor IF3 is required for peptide chain initiation in Escherichia coli. IF3 binds directly to 30S ribosomal subunits ensuring a constant supply of free 30S subunits for initiation complex formation, participates in the kinetic selection of the correct initiator region of mRNA, and destabilizes initiation complexes containing noninitiator tRNAs. The roles that tyrosine 107 and lysine 110 play in IF3 function were examined by site-directed mutagenesis. Tyrosine 107 was changed to either phenylalanine (Y107F) or leucine (Y107L), and lysine 110 was converted to either arginine (K110R) or leucine (K110L). These single amino acid changes resulted in a reduced affinity of IF3 for 30S subunits. Association equilibrium constants (M-1) for 30S subunit binding were as follows: wild-type, 7.8 x 10(7); Y107F, 4.1 x 10(7); Y107L, 1 x 10(7); K110R, 5.1 x 10(6); K110L, < 1 x 10(2). The mutant IF3s were similarly impaired in their abilities to specifically select initiation complexes containing tRNA(fMet). Toeprint analysis indicated that 5-fold more Y107L or K110R protein was required for proper initiator tRNA selection. K110L protein was unable to mediate this selection even at concentrations up to 10-fold higher than wild type. The results indicate that tyrosine 107 and lysine 110 are critical components of the ribosome binding domain of IF3 and, furthermore, that dissociation of complexes containing noninitiator tRNAs requires prior binding of IF3 to the ribosomes.

The Escherichia coli chromosome contains specific, unmethylated dam and dcm sites

The Escherichia coli chromosome encodes two methylases, dam and dcm, which recognize the sequences GATC and CC(A/T)GG, respectively. Specific dam and dcm sites on the E. coli chromosome were found to be unmethylated in vivo by using pulsed-filed gel electrophoresis experiments scanning megabase regions of DNA. Some sites were totally unmethylated. The dam sites display variable methylation depending on the local sequence, and, in general, their methylation shows complex modulation by growth conditions and growth rate, suggesting multiple protection mechanisms. Sites resistant to complete dam or dcm methylation appear to be distributed throughout the chromosome. These unusual sites may identify regions of the chromosome with interesting biological functions.

Translation initiation in Escherichia coli: sequences within the ribosome-binding site

The translational roles of the Shine-Dalgarno sequence, the initiation codon, the space between them, and the second codon have been studied. The Shine-Dalgarno sequence UAAGGAGG initiated translation roughly four times more efficiently than did the shorter AAGGA sequence. Each Shine-Dalgarno sequence required a minimum distance to the initiation codon in order to drive translation; spacing, however, could be rather long. Initiation at AUG was more efficient than at GUG or UUG at each spacing examined; initiation at GUG was only slightly better than UUG. Translation was also affected by residues 3' to the initiation codon. The second codon can influence the rate of initiation, with the magnitude depending on the initiation codon. The data are consistent with a simple kinetic model in which a variety of rate constants contribute to the process of translation initiation.

Linking number anomalies in DNA under conditions close to condensation

Changes in linking number and the apparent winding angle of pBR322 DNA have been evaluated in mixed ethanol-water solvents containing either Na or Mg as the major counterion contributing to the electrostatic shielding of the duplex. The average number of superhelical turns (tau) produced in the standard electrophoresis buffer (Tris-borate-EDTA, pH 8.0) by the transfer of DNA, relaxed in 200 mM NaCl, 10 mM NaH2PO4/Na2HPO4, and 2 mM EDTA, pH 7, by calf thymus topoisomerase or ligated in 6.6 mM MgCl2, 1 mM KCl, 1 mM ATP, 1 mM dithiothreitol, and 66 mM Tris, pH 7.6, by T4 ligase, was determined as a function of the EtOH concentration. At low enzyme concentrations, the tau values became increasingly more positive in the presence of both cations as the ethanol concentration increased, indicating that the duplex structure was overwound in the ethanol solvents. Winding angle changes between 0 and 20% ethanol, calculated from these values of tau, exhibited the same correlations with CD spectral properties as had been previously observed for 100% aqueous systems containing monovalent cations [Kilkuskie, R., Wood, N., Shinn, R., Ringquist, S., & Hanlon, S. (1988) Biochemistry 27, 4377-4386]. The results at higher concentrations of ethanol (25-30%), however, were anomalous for the Mg-ligase system. The anomalies increased with higher ethanol, ligase, or Mg concentration. Gel run under these conditions showed enhanced concentrations of slow-moving components, indicative of ligation of intermolecular associated DNA species. At a 10-fold higher level of ligase, ethanol appeared to unwind the duplex, confirming the results of Lee, Mizusawa, and Kakefuda [(1981) Proc. Natl. Acad. Sci. U.S.A. 78, 2838-2842]. All of these anomalies occur under solvent conditions which are close to conditions which produce a heterogeneous dispersion of sedimenting species in ultracentrifugal experiments and compact rodlike structures, visualized by electron microscopy. The circular dichroism spectra at the onset of the formation of these structures show the characteristics of a chirally packed array of DNA duplexes. The reversal of the trend of the ethanol effect on linking number at higher enzyme and Mg(II) concentrations can be most easily explained by the promotion of the condensation phenomenon by either the ligase or a contaminating factor in the preparation. We suggest that the anomalies in the linking number and winding angle values are due to either ligation of chirally bent DNA species or a change in the helical period as the linear DNA adapts to the conformation required for collapse.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of charge modification on the helical period of duplex DNA

Supercoiled enriched PM-2 DNA has been relaxed by treating with calf thymus topoisomerase I and used in the preparation of a family of n-butylamine adducts of varying levels of substitution. The amine is cross-linked by formaldehyde to the exocyclic amino group of G when the DNA is in duplex form. These amine adducts of covalently closed relaxed (ccr) DNA, freed of the formaldehyde and n-butylamine reactants, have circular dichroism (CD) spectral properties similar to those previously reported for the adducts of calf thymus DNA [Chen, C., Kilkuskie, R., & Hanlon, S. (1981) Biochemistry 20, 4987-4995]. In both instances, the CD transformation effected by increasing levels of substituted cationic amine is similar to that induced by solvents of high electrolyte content. The adducts also exhibit greatly increased electrophoretic mobility compared to unreacted controls or a control treated only with formaldehyde. Mobility changes in the presence of variable amounts of ethidium bromide demonstrate that this phenomenon is attributable to the formation of negative supercoils and is not due to denaturation or unwinding of the duplex. Incremental increases in superhelicity due to the attachment of the amine have been measured by reference to a topoisomerase ladder of underivatized PM-2 DNA and converted to changes in winding angle. As the extent of substitution increases, the rotational strength of the positive band above 260 nm decreases, and the winding angle increases in the nonlinear manner observed previously for underivatized PM-2 DNA [Baase, W. A., & Johnson, W. C., Jr. (1979) Nucleic Acids Res. 6, 797-814]. In fact, the relationship between these two properties is the same for both the adducts and the underivatized ccr species. Thus, the attachment of the amine has the same conformational effects as the electrolyte content of the solvent. The effect can be rationalized in terms of the reduction of the electrostatic free energy of the duplex due to site-bound or localized cation binding in the minor groove.

Covalent attachment of an alkylamine prevents the B to Z transition in poly(dG-dC)

Covalent complexes of n-butylamine and double-stranded poly(dG-dC) were prepared by coupling the amine to exocyclic amino groups of guanine bases with CH2O. Neither the absorption spectrum above 230 nm nor the s020,w of the complexes in low to moderate ionic strength solvents, freed of excess unreacted reagents, differs significantly from that of unreacted poly(dG-dC) or a control which had been exposed only to CH2O. In contrast, the CD spectra are profoundly altered. The minimum at 252 nm becomes more negative, and the rotational strength of the positive band above 260 nm is reduced as a linear function of the extent of amine attachment. At 0.22 mol of amine per mole of nucleotide, the transformation is similar to that observed by others in poly(dG-dC) when complexed to core histones in reconstituted core particles or in concentrated LiCl solvents at temperatures below the B----Z transition. Sedimentation studies reveal that these changes in the circular dichroism (CD) spectra reflect secondary structural effects rather than the formation of aggregates or psi type structures. Raman spectra reveal, however, that these secondary structural changes must occur within the B family as the amine complex retains B backbone geometry. The conformation produced by the attachment of the amine is probably a higher winding angle (overwound) B variant.(ABSTRACT TRUNCATED AT 250 WORDS)