Interaction between the Msh2 and Msh6 Nucleotide-binding Sites in the Saccharomyces cerevisiae Msh2-Msh6 Complex

Indirect evidence has suggested that the Msh2-Msh6 mispair-binding complex undergoes conformational changes upon binding of ATP and mispairs, resulting in the formation of Msh2-Msh6 sliding clamps and licensing the formation of Msh2-Msh6-Mlh1-Pms1 ternary complexes. Here, we have studied eight mutant Msh2-Msh6 complexes with defective responses to nucleotide binding and/or mispair binding and used them to study the conformational changes required for sliding clamp formation and ternary complex assembly. ATP binding to the Msh6 nucleotide-binding site results in a conformational change that allows binding of ATP to the Msh2 nucleotide-binding site, although ATP binding to the two nucleotide-binding sites appears to be uncoupled in some mutant complexes. The formation of Msh2-Msh6-Mlh1-Pms1 ternary complexes requires ATP binding to only the Msh6 nucleotide-binding site, whereas the formation of Msh2-Msh6 sliding clamps requires ATP binding to both the Msh2 and Msh6 nucleotide-binding sites. In addition, the properties of the different mutant complexes suggest that distinct conformational states mediated by communication between the Msh2 and Msh6 nucleotide-binding sites are required for the formation of ternary complexes and sliding clamps.     

The RAINFOR database: monitoring forest biomass and dynamics

Problem: Data from over 100 permanent sample plots which have been studied for 10–20 years need a suitable system for storage which allows simple data manipulation and retrieval for analysis. Methods: A relational database linking tree records, taxonomic nomenclature and corresponding environmental data has been built in MS Access as part of the RAINFOR project. Conclusion: The database allows flexible and long‐term use of a large amount of data: more than 100 tree plots across Amazonia, incorporating over 80 000 records of individual trees and over 300 000 total records of tree diameter measurements from successive censuses. The database is designed to enable linkages to existing soil, floristic or plant‐trait databases. This database will be a useful tool for exploring the impact of environmental factors on forest structure and dynamics at local to continental scales, and long term changes in forest ecology. As an early example of its potential, we explore the impact of different methodological assumptions on estimates of tropical forest biomass and carbon storage.     

Nitroimidazole conjugates of bis(thiosemicarbazonato)Cu(II) - Potential combination agents for the PET imaging of hypoxia

Combination agents comprising two different pharmacophores with the same biological target have the potential to show additive or synergistic activity. Bis(thiosemicarbazonato)copper(II) complexes (e.g. ⁶⁴Cu-ATSM) and nitroimidazoles (e.g. ¹⁸F-MISO) are classes of tracer used for the delineation of tumor hypoxia by positron emission tomography (PET). Three nitroimidazole-bis(thiosemicarbazonato)copper(II) conjugates were produced in order to investigate their potential as combination hypoxia imaging agents. Two were derived from the known bifunctional bis(thiosemicarbazone) H₂ATSM/A and the third from the new precursor diacetyl-2-(4-N-methyl-3-thiosemicarbazone)-3-(4-N-ethylamino-3-thiosemicarbazone) - H₂ATSM/en. Oxygen-dependent uptake studies were performed using the ⁶⁴Cu radiolabelled complexes in EMT6 carcinoma cells. All the complexes displayed appreciable hypoxia selectivity, with the nitroimidazole conjugates displaying greater selectivity than a simple propyl derivative used as a control. Participation of the nitroimidazole group in the trapping mechanism is indicated by the increased hypoxic uptake of the 2- vs. the 4-substituted ⁶⁴Cu-ATSM/A derivatives. The 2-nitroimidazole derivative of ⁶⁴Cu-ATSM/en demonstrated superior hypoxia selectivity to ⁶⁴Cu-ATSM over the range of oxygen concentrations tested. Biodistribution of the radiolabelled 2-nitroimidazole conjugates was carried out in EMT6 tumor-bearing mice. The complexes showed significantly different uptake trends in comparison to each other and previously studied Cu-ATSM derivatives. Uptake of the Cu-ATSM/en conjugate in non-target organs was considerably lower than for derivatives based on Cu-ATSM/A.     

The existence of an optimal range of cytosolic free calcium for insulin-stimulated glucose transport in rat adipocytes

We have examined the effects of extracellular and intracellular Ca2+ concentrations upon basal and insulin-stimulated 2-deoxyglucose uptake in isolated rat adipocytes. In the absence of extracellular Ca2+, both basal and insulin-stimulated glucose uptake were significantly reduced. Insulin-stimulated glucose transport was optimal at 1 and 2 mM Ca2+. Further increases in extracellular Ca2+ concentration (3 mM) significantly diminished insulin-stimulated glucose uptake. When intracellular Ca2+ concentrations were augmented by ionomycin (1 microM), insulin-stimulated glucose uptake was significantly reduced at extracellular Ca2+ concentrations of 2 and 3 mM. The levels of intracellular free Ca2+ concentrations were then measured with Ca2+ indicator fura-2. The correlation between the levels of intracellular free Ca2+ and the magnitude of insulin-stimulated glucose uptake revealed that the optimal effect of insulin is observed at Ca2+ levels between 140 and 370 nM. At both extremes outside of this window, both low and high levels of intracellular Ca2+ result in diminished cellular responsiveness to insulin. These data suggest that intracellular calcium concentrations may exert a dual role in the regulation of cellular sensitivity to insulin. First, there must exist a minimal concentration of intracellular calcium to promote insulin action. Second, increased levels of intracellular calcium may provide a critical signal for diminution of insulin action.     

Inhibition of GSK-3β Rescues the Impairments in Bone Formation and Mechanical Properties Associated with Fracture Healing in Osteoblast Selective Connexin 43 Deficient Mice

Connexin 43 (Cx43) is the most abundant gap junction protein in bone and is required for osteoblastic differentiation and bone homeostasis. During fracture healing, Cx43 is abundantly expressed in osteoblasts and osteocytes, while Cx43 deficiency impairs bone formation and healing. In the present study we selectively deleted Cx43 in the osteoblastic lineage from immature osteoblasts through osteocytes and tested the hypothesis that Cx43 deficiency results in delayed osteoblastic differentiation and impaired restoration of biomechanical properties due to attenuated β-catenin expression relative to wild type littermates. Here we show that Cx43 deficiency results in alterations in the mineralization and remodeling phases of healing. In Cx43 deficient fractures the mineralization phase is marked by delayed expression of osteogenic genes. Additionally, the decrease in the RankL/ Opg ratio, osteoclast number and osteoclast size suggest decreased osteoclast bone resorption and remodeling. These changes in healing result in functional deficits as shown by a decrease in ultimate torque at failure. Consistent with these impairments in healing, β-catenin expression is attenuated in Cx43 deficient fractures at 14 and 21 days, while Sclerostin (Sost) expression, a negative regulator of bone formation is increased in Cx43cKO fractures at 21 days, as is GSK-3β, a key component of the β-catenin proteasomal degradation complex. Furthermore, we show that alterations in healing in Cx43 deficient fractures can be rescued by inhibiting GSK-3β activity using Lithium Chloride (LiCl). Treatment of Cx43 deficient mice with LiCl restores both normal bone formation and mechanical properties relative to LiCl treated WT fractures. This study suggests that Cx43 is a potential therapeutic target to enhance fracture healing and identifies a previously unknown role for Cx43 in regulating β-catenin expression and thus bone formation during fracture repair.     

Chromosomal assignment of amplified genes in hydroxyurea-resistant hamster cells

We have shown previously that cDNAs for the M1 and M2 subunits of ribonucleotide reductase, ornithine decarboxylase (ODC), and p5-8, a 55,000-Dalton protein, hybridize to amplified genomic sequences in a highly hydroxyurea-resistant hamster cell line. We have extended these observations to include two additional, independently isolated, hydroxyurea-resistant cell lines: SC8, a single-step hamster ovary cell line, and KH450, a multistep human myeloid leukemic cell line, have also undergone genomic amplification for sequences homologous to ODC and p5-8 cDNAs. However, neither SC8 nor KH450 contains amplified genomic sequences homologous to an M1 cDNA probe. A panel of mouse-hamster somatic cell hybrids was used to map sequences homologous to M1, M2, ODC, and 5-8 cDNAs in the hamster genome. The M2, ODC, and p5-8 cDNAs hybridized to DNA fragments that segregated with hamster chromosome 7. In contrast, M1 cDNA hybridized to DNA fragments that segregated with hamster chromosome 3. These data suggest that the genes RRM2, (M2), ODC, and p5-8, but not RRMI (M1), are linked and may have been co-amplified in the selection of the hydroxyurea-resistant hamster and human cell lines.