p53 dependence of early apoptotic and proliferative responses within the mouse intestinal epithelium following gamma-irradiation

p53 is now well characterized as a tumour suppressor gene, with loss of normal p53 function being recorded as the commonest genetic event associated with human malignancy. In particular, its involvement with tumorigenesis within the intestine is well established. Normal p53 function has been shown to be crucial for the induction of apoptosis in tumour cell lines, murine thymocytes and murine haematopoietic cells following DNA damage. To elucidate further the role of p53 in the cellular response to DNA damage we have investigated the response to gamma-irradiation of crypt cells in vivo from the small and large intestine of mice bearing a constitutive p53 deletion. Four hours after gamma-irradiation, a time point at which wild type crypt cells show abundant apoptosis, crypt cells from p53-deficient mice differed in that they were completely resistant to the induction of apoptosis. The p53 dose dependence of this phenomenon was clearly shown by the intermediate level of apoptosis observed in p53 heterozygotes. Analysis of the mitotic index and the bromodeoxyuridine labelling index showed that two other responses of wild type crypts to gamma-irradiation, namely the G2 block and the reduction in bromodeoxyuridine incorporation, were both largely intact in p53 deficient animals. These observations demonstrate that p53 function is essential for a major component of the normal response to gamma-irradiation induced DNA damage in intestinal mucosal cells, and suggest that p53 deficiency permits a population of cells bearing DNA damage to escape the normal process of deletion.

Murine genetic models of human disease

Until recently, good animal models of human disease have been available only in limited numbers, largely because of technical difficulties associated with transgenesis. As a consequence of recent rapid advances principally, but not exclusively, focused around the use of embryonic stem cells, it is now theoretically possible to model the genetic lesion underlying any human disease in the mouse. This has led not only to a better understanding of complex disease processes, such as those associated with malignancy, but, as in the cases of cystic fibrosis and duchenne muscular dystrophy, is now allowing the development of novel therapy regimes.

The stability and hydrophobicity of cytosolic and mitochondrial malate dehydrogenases and their relation to chaperonin-assisted folding

mMDH and cMDH are structurally homologous enzymes which show very different responses to chaperonins during folding. The hydrophilic and stable cMDH is bound by cpn60 but released by Mg-ATP alone, while the hydrophobic and unstable mMDH requires both Mg-ATP and cpn10. Citrate equalises the stability of the native state of the two proteins but has no effect on the co-chaperonin requirement, implying that hydrophobicity, and not stability, is the determining factor. The yield and rate of folding of cMDH is unaffected while that of mMDH is markedly increased by the presence of cpn60, cpn10 and Mg-ATP. In 200 mM orthophosphate, chaperonins do not enhance the rate of folding of mMDH, but in low phosphate concentrations chaperonin-assisted folding is 3-4-times faster.

Engineering surface loops of proteins--a preferred strategy for obtaining new enzyme function

A prerequisite for the rational redesign of enzymes is that altering amino acids in an attempt to obtain new biological function does not unexpectedly alter the globular, natural framework of the native protein on which the design is being executed. The results of combinatorial-mutagenesis strategies suggest that random variation of amino acid sequence is most easily tolerated at the solvent-exposed surfaces of a protein. This review analyses effective redesigns of Bacillus stearothermophilus lactate dehydrogenase (bsLDH), in which all residue variations are at solvent-exposed surfaces. The majority of these variations were located within surface loops, which interconnect stable secondary structures traversing the globular core of the protein.

129/Ola mice carrying a null mutation in PrP that abolishes mRNA production are developmentally normal

The neural membrane glycoprotein PrP is implicated in the pathogenesis of the transmissible spongiform encephalopathies; however, the normal function of PrP and its precise role in disease are not understood. Recently, gene targeting has been used to produce mice with neo/PrP fusion transcripts, but no detectable PrP protein in the brain (1). Here we report the use of a different targeting strategy, to produce inbred mice with a complete absence of both PrP protein and mRNA sequences. At 7 mo of age, these mice show no overt phenotypic abnormalities despite the normal high levels of expression of PrP during mouse development. The mice are being used in experiments designed to address the role of PrP in the pathogenesis of scrapie and the replication of infectivity.

Substitution of the amino acid at position 102 with polar and aromatic residues influences substrate specificity of lactate dehydrogenase

The Gln residue at amino acid position 102 of Bacillus stearothermophilus lactate dehydrogenase was replaced with Ser, Thr, Tyr, or Phe to investigate the effect on substrate recognition. The Q102S and Q102T mutant enzymes were found to have a broader range of substrate specificity (measured by kcat/Km) than the wild-type enzyme. However, it is evident that either Ser or Thr at position 102 are of a size able to accommodate a wide variety of substrates in the active site and substrate specificity appears to rely largely on size discrimination in these mutants. The Q102F and Q102Y mutant enzymes have low catalytic efficiency and do not show this relaxed substrate specificity. However, their activities are restored by the presence of an aromatic substrate. All of the enzymes have a very low catalytic efficiency with branched chain aliphatic substrates.

Transgenic approaches to cancer biology

Transgenic animal models have played a major role in advancing our understanding of tumorigenesis. The most important recent advance has been the production of animals bearing targeted mutations generated by homologous recombination. For the first time, we can ask questions about loss of gene function and the consequences of gene alterations in situ. Perhaps most significantly, this approach has been applied to two of the tumour suppressor genes, Rb and p53. Homologous recombination has helped to clarify not only the normal roles of these genes, but also the mechanisms by which their dysfunction may lead to tumorigenesis.

Impaired interleukin-3 response in Pim-1-deficient bone marrow-derived mast cells

The mouse Pim-1 gene encodes two cytoplasmic serine-threonine-specific protein kinases. The gene has been found to be activated (overexpressed) by retroviral insertion in hematopoietic tumors in mice. Transgenic mice that overexpress Pim-1 (E mu-Pim-1) have a low incidence of spontaneous T-cell lymphomas and an increased susceptibility to Moloney murine leukemia virus and N-ethyl-N-nitrosourea-induced lymphomas. Apart from a slight enlargement of the spleen, no abnormalities were found in prelymphomatous transgenic mice. Inactivation of the Pim-1 gene in the germline of mice resulted in mice with a surprisingly subtle phenotype. Therefore, we investigated whether subtle effects of the absence of Pim-1 could be made visible during in vitro culturing of hematopoietic cells. We found that bone marrow-derived mast cells (BMMC) lacking Pim-1 had a distinct growth disadvantage when grown on interleukin (IL)-3, but not when stimulated by the factors IL-4, IL-9, or Steel factor (SF). This indicates a role for Pim-1 as a modulator of the IL-3 signal transduction pathway.

Age-dependent selection against hypoxanthine phosphoribosyl transferase-deficient cells in mouse haematopoiesis

The basis of a previously observed difference in the level of contribution of hypoxanthine phosphoribosyltransferase-deficient cells between the haematopoietic and non-haematopoietic tissues of chimaeric and heterozygous mice has been clarified by studying two populations of female mice that differ only in that one is heterozygous for a null allele at the hprt locus and the other is wild type at this locus. Both populations are heterozygous for an electrophoretic variant allele at the X-linked Pgk-1 locus, so that X-chromosome inactivation generates cells expressing different isozymes of phosphoglycerate kinase which can be assayed to monitor cell selection. The results show that hypoxanthine phosphoribosyltransferase deficiency itself, rather than an effect of another X-linked gene, causes a reduced level of contribution to haematopoietic tissues. Further, the extent of the depletion increases significantly with age, and this effect is due to a progressive reduction in the level of contribution to haematopoietic tissues rather than to an increase in the level of contribution to non-haematopoietic tissues.

Thymocyte apoptosis induced by p53-dependent and independent pathways

Death by apoptosis is characteristic of cells undergoing deletion during embryonic development, T- and B-cell maturation and endocrine-induced atrophy. Apoptosis can be initiated by various agents and may be a result of expression of the oncosuppressor gene p53 (refs 6-8). Here we study the dependence of apoptosis on p53 expression in cells from the thymus cortex. Short-term thymocyte cultures were prepared from mice constitutively heterozygous or homozygous for a deletion in the p53 gene introduced into the germ line after gene targeting. Wild-type thymocytes readily undergo apoptosis after treatment with ionizing radiation, the glucocorticoid methylprednisolone, or etoposide (an inhibitor of topoisomerase II), or after Ca(2+)-dependent activation by phorbol ester and a calcium ionophore. In contrast, homozygous null p53 thymocytes are resistant to induction of apoptosis by radiation or etoposide, but retain normal sensitivity to glucocorticoid and calcium. The time-dependent apoptosis that occurs in untreated cultures is unaffected by p53 status. Cells heterozygous for p53 deletion are partially resistant to radiation and etoposide. Our results show that p53 exerts a significant and dose-dependent effect in the initiation of apoptosis, but only when it is induced by agents that cause DNA-strand breakage.

The energetics and cooperativity of protein folding: a simple experimental analysis based upon the solvation of internal residues

The reversible unfolding of two dissimilar proteins, phosphoglycerate kinase from Bacillus stearothermophilus (PGK) and Staphylococcus aureus nuclease (SAN), was induced with two denaturants, urea and guanidinium chloride (GuHCl). For each protein, structural transitions were monitored by intrinsic fluorescence intensity changes arising from a unique tryptophan residue. In the case of SAN the single, native tryptophan residue was used, whereas for PGK two versions, one with a tryptophan at position 315 and one at 379, were constructed genetically. The resultant folding curves were analyzed by considering the change in the solvation free energy of internal amino acid residues as the denaturant concentration was varied. We derive the following simple relationship: -RT ln K = delta Gw + n delta Gs,m[D]/Kden. + [D]) where K is the equilibrium constant describing the distribution of folded and unfolded forms at a given denaturant concentration [D], delta Gw is the free energy change for the transition in the absence of denaturant, and n is the number of internal side chains becoming exposed. delta Gs,m and Kden. are constants derived empirically from the solvation energies of model compounds and represent the behavior of an average internal side chain between 0 and 6 M GuHCl and 0 and 8 M urea. For proteins of known structure these values can easily be derived, and for others, average values in guanidinium chloride (delta Gs,m = 0.775 kcal/mol and Kden. = 5.4 M) or urea (delta Gs,m = 1.198 kcal/mol and Kden. = 25.25 M) can be used in the analysis. Results show that the parameters n and delta Gw are independent of the denaturant used for all 12 transitions studied. This supports the hypothesis that the unfolding activity of urea and GuHCl can be accounted for by their effect on the solvation energy of amino acid side chains which are buried in the folded but exposed in the unfolded protein. This simple analytical treatment allows the "cooperativity" of protein folding to be interpreted in terms of the number of side chains becoming exposed to the solvent in a given step and allows accurate estimation of the free energy irrespective of the denaturant concentration needed to induce the transition.

Binding and hydrolysis of nucleotides in the chaperonin catalytic cycle: implications for the mechanism of assisted protein folding

Cpn60 was labeled with pyrene maleimide in order to follow structural rearrangements in the protein triggered by the binding of nucleotides and cpn10. The conjugate binds ATP, AMP-PNP, and ADP(P(i)) with pyrene fluorescence enhancements of 60%, 60%, and 15%, respectively. In each case, binding is cooperative with half-saturation (K1/2) occurring at 10 microM, 290 microM, and 2500 microM and Hill constants (nH) of 4, 3, and 3, respectively. Inclusion of the co-protein, cpn10, tightens the binding of ATP, AMP-PNP, and ADP(P(i)) to give K1/2 values of 6 microM, 100 microM, and < 0.07 microM, respectively, and cooperativity is increased. Titration of the cpn60/ADP (14-mer) complex with cpn10 (7-mer) gives a stoichiometry of 14:7 with respect to subunits, confirming the molecular asymmetry shown by electron microscopy. Transient kinetics demonstrate that ATP initially forms a weak collision complex with cpn60 (Kd = 4 mM) which isomerizes to the strongly binding state at a rate of 180 s-1. We suggest that the slow structural rearrangement driven by ATP binding is the same event which lowers the affinity of the chaperonin for protein substrates; a suggestion reinforced by the loss of AMP-PNP binding affinity in the presence of an unstructured polypeptide. As such, this rearrangement of cpn60 is analogous to a force-generating step in energy transduction. Measurements of ATP hydrolysis (pH 7.5, 25 degrees C) show that it is slow (0.04 s-1) compared both with the structural rearrangement and with the dissociation of products. This defines the steady-state complex as cpn60/ATP, a form of the chaperonin which binds substrate proteins weakly. The rate of hydrolysis of ATP is stimulated 20-fold upon binding unfolded lactate dehydrogenase, and the yield of folded enzyme is increased even in the absence of cpn10. Addition of this co-protein inhibits hydrolysis on only half of the sites in cpn60 and leads to a faster release of folded LDH. A mechanism for the action of chaperonins is proposed which depends upon cpn60 being cycled between states which have, alternately, low and high affinity for unfolded proteins. This cycle is driven by the binding and hydrolysis of ATP.

Dissecting the contributions of a specific side-chain interaction to folding and catalysis of Bacillus stearothermophilus lactate dehydrogenase

X-ray crystallography predicts hydrogen-bonding interactions between the side chains of Thr198 and two other amino acid residues, Glu194 (adjacent to the catalytic His195) and Ser318 (on the alpha-H helix which rearranges on substrate binding). In order to investigate the contribution of this conserved amino acid residue, Thr198, two mutants of Bacillus stearothermophilus lactate dehydrogenase were created (Val198 and Ile198). The steady-state kinetic parameters for both mutant enzymes were very similar with increased substrate Km and reduced kcat when compared with the wild-type enzyme. The mutation Val198 allowed non-productive binding of pyruvate to the unprotonated form of His195. Steady-state kinetic parameters determined for the Val198 mutant enzyme in high solvent viscosity suggested both an altered rate-limiting step in catalysis and implicated Thr198 in allosteric activation by the effector fructose 1,6-bisphosphate (Fru1,6P2). A shift in the Fru1,6P2 activation constant for the Val198 mutant enzyme suggested that Thr198 stabilises the catalytically competent (Fru1,6P2-activated) form of the enzyme by 6.6 kJ/mol. However, Thr198 was not important for maintaining the thermal stability of the Fru1,6P2-activated form. Equilibrium unfolding in guanidinium chloride indicated that Thr198 contributes 17.2 kJ/mol subunits towards the tertiary structural stability. The results emphasise the importance of the side chain-hydroxyl group of Thr198 which is required for (a) productive substrate binding, (b) allosteric activation and (c) protein conformational stability. The characteristics of the B. stearothermophilus lactate dehydrogenase mutations reported here were significantly different from those of the same mutations made in the corresponding position of the analogous enzyme Thermus flavus malate dehydrogenase [Nishiyama, M., Shimada, K., Horinouchi, S., & Beppu, T. (1991) J. Biol. Chem. 266, 14294-14299].

T cell receptor delta gene mutant mice: independent generation of alpha beta T cells and programmed rearrangements of gamma delta TCR genes

T cells bearing T cell receptor (TCR) gamma and delta chain heterodimers are first generated early in ontogeny. They form distinct subsets that differ in their TCR repertoires and tissue distribution. Disruption of the mouse TCR C delta gene segment by a gene targeting method caused the complete loss of T cells bearing TCR gamma delta chains, but had little or no effect on the development of T cells bearing TCR alpha beta chains. The analyses of TCR gamma and delta genes in the mutant mice suggest that intracellular mechanisms acting at the level of DNA rearrangement play key roles in the differential gamma and delta gene rearrangements and in the generation of the highly restricted junctional sequences during fetal thymic development.

Mutations in T-cell antigen receptor genes alpha and beta block thymocyte development at different stages

Analysis of mice carrying mutant T-cell antigen receptor (TCR) genes indicates that TCR-beta gene rearrangement or expression is critical for the differentiation of CD4-CD8- thymocytes to CD4+CD8+ thymocytes, as well as for the expansion of the pool of CD4+CD8+ cells. TCR-alpha is irrelevant in these developmental processes. The development of gamma delta T cells does not depend on either TCR-alpha or TCR-beta.

The importance of arginine 102 for the substrate specificity of Escherichia coli malate dehydrogenase

The malate dehydrogenase from Escherichia coli has been specifically altered at a single amino acid residue by using site-directed mutagenesis. The conserved Arg residue at amino acid position 102 in the putative substrate binding site was replaced with a Gln residue. The result was the loss of the high degree of specificity for oxaloacetate. The difference in relative binding energy for oxaloacetate amounted to about 7 kcal/mol and a difference in specificity between oxaloacetate and pyruvate of 8 orders of magnitude between the wild-type and mutant enzymes. These differences may be explained by the large hydration potential of Arg and the formation of a salt bridge with a carboxylate group of oxaloacetate.

Requirement for a functional Rb-1 gene in murine development

Human retinoblastomas can occur both as hereditary and as sporadic cases. Knudson's proposal that they result from two mutational events, of which one is present in the germ line in hereditary cases, has been confirmed by more recent molecular analysis, which has shown both events to involve loss or mutational inactivation of the same gene, RB-1 (ref. 2). RB-1 heterozygosity also predisposes to osteosarcoma, and RB-1 allele losses are seen in sporadic lung, breast, prostate and bladder carcinomas. RB-1 is expressed in most, if not all, tissues and codes for a nuclear phosphoprotein which becomes hypophosphorylated in the G0 growth arrest state and in the G1 phase of the cell cycle. To gain a further insight into the role of RB-1 we and other groups have generated mice carrying an inactivated allele of the homologous gene, Rb-1 (ref. 10), by gene targeting. We report here that young heterozygous mice do not appear abnormal and do not develop retinoblastoma at a detectable frequency. However, homozygous mutant embryos fail to reach term and show a number of abnormalities in neural and haematopoietic development. Broadly similar results are reported by the other groups.

Cystic fibrosis in the mouse by targeted insertional mutagenesis

Cystic fibrosis is a fatal genetic disorder which afflicts 50,000 people worldwide. A viable animal model would be invaluable for investigating and combating this disease. The mouse cystic fibrosis transmembrane conductance regulator gene was disrupted in embryonal stem cells using an insertional gene targeting vector. Germ-line chimaeras were derived and the offspring of heterozygous crosses studied. These homozygous mutant mice survive beyond weaning. In vivo electrophysiology demonstrates the predicted defect in chloride ion transport in these mice and can distinguish between each genotype. Histological analysis detects important hallmarks of human disease pathology, including abnormalities of the colon, lung and vas deferens. This insertional mouse mutation provides a valid model system for the development and testing of therapies for cystic fibrosis patients.

Design of a specific phenyllactate dehydrogenase by peptide loop exchange on the Bacillus stearothermophilus lactate dehydrogenase framework

Restriction sites were introduced into the gene for Bacillus stearothermophilus lactate dehydrogenase which enabled a region of the gene to be excised which coded for a mobile surface loop of polypeptide (residues 98-110) which normally seals the active site vacuole from bulk solvent and is a major determinant of substrate specificity. Oligonucleotide-overlap extension (using the polymerase chain reaction) was used to obtain double-stranded DNA regions which coded for different length and sequence loops and which also contained the same restriction sites. The variable length and sequence loops were inserted into the cut gene and used to synthesize hydroxyacid dehydrogenases with altered substrate specificities. Loops which were longer and shorter than the original were made. The substrate specificities of enzymes with these new loops were considerably altered. For many poor enzyme-substrate pairs, the effect of fructose 1,6-bisphosphate on the steady-state kinetic parameters suggested that the substrate was mainly bound in a nonproductive mode. With one longer loop construction (BL1), activity with pyruvate was reduced one-million-fold but activity with phenylpyruvate was largely unaltered. A switch in specificity (kcat/KM) of 390,000-fold was achieved. The 1700:1 selectivity of enzyme BL1 for phenylpyruvate over pyruvate is that required in a phenyllactate dehydrogenase to be used in monitoring phenylpyruvate in the urine of patients with phenylketonuria consuming an apparently phenylalanine-free diet.