Defective development of the embryonic liver in N-myc-deficient mice

The mechanisms involved in the formation and the differentiation of the liver remain unclear despite extensive studies. To investigate these events in mouse hepatic development, we have taken advantage of the N-myc mutant mouse line which exhibits abnormal liver development. N-myc mutant embryos die between 11.5 and 12.5 days postcoitum most probably from heart failure. In the present study, we report that at 11.5 days of gestation, extensive apoptosis restricted to the hepatocytes occurred in N-myc mutant liver when compared to wild-type samples. Moreover, the number of hematopoietic cells is reduced in the mutant liver. During early liver organogenesis, the N-myc gene is expressed in tissues involved in the induction and the differentiation of the hepatocytes. At 11.5 days postcoitum, both c-myc and N-myc genes are expressed in the liver. While c-myc is expressed at a high level in the organ per se, N-myc expression is mostly confined to the peripheral layer of the liver which will generate the Glisson's capsule. Taken together, the expression pattern of N-myc in the liver and the specific apoptosis of hepatocytes observed in N-myc mutants indicate that N-myc is required for hepatocyte survival and suggest that it is involved in the genetic cascade leading to normal liver development.

Induction of thymocyte deletion by purified single peptide/major histocompatibility complex ligands

We report a novel in vitro approach that allows study of the consequences of TCR ligation on thymocytes in the absence of thymic stromal cells. Hence, thymocytes were incubated either in the presence of recombinant antigenic peptide/MHC complexes, which represent ligands of physiologic affinities, or with anti-TCR mAb, a ligand of supraphysiologic affinity. Whereas TCR cross-linking with mAb led to thymocyte deletion, incubation with peptide/MHC ligands did not trigger cellular apoptosis. However, the addition of a costimulatory signal (provided by anti-CD28 mAb) allowed the induction of apoptosis following TCR binding to peptide/MHC ligands, and it increased the levels of cell death obtained through mAb-mediated TCR cross-linking. Requirement for accessory signals seen with TCR stimulation by peptide/MHC complexes argues in favor of qualitative differences between TCR engagement by ligands of either physiologic or supraphysiologic affinity.

Induction of thymocyte deletion by purified single peptide/major histocompatibility complex ligands

We report a novel in vitro approach that allows study of the consequences of TCR ligation on thymocytes in the absence of thymic stromal cells. Hence, thymocytes were incubated either in the presence of recombinant antigenic peptide/MHC complexes, which represent ligands of physiologic affinities, or with anti-TCR mAb, a ligand of supraphysiologic affinity. Whereas TCR cross-linking with mAb led to thymocyte deletion, incubation with peptide/MHC ligands did not trigger cellular apoptosis. However, the addition of a costimulatory signal (provided by anti-CD28 mAb) allowed the induction of apoptosis following TCR binding to peptide/MHC ligands, and it increased the levels of cell death obtained through mAb-mediated TCR cross-linking. Requirement for accessory signals seen with TCR stimulation by peptide/MHC complexes argues in favor of qualitative differences between TCR engagement by ligands of either physiologic or supraphysiologic affinity.

Generation of normal lymphocytes derived from N-myc-deficient embryonic stem cells

Myc family proteins are thought to be transcription factors involved in regulation of cell growth and differentiation. N-myc is expressed at the pre-B cell stage of B cell differentiation and is dramatically induced by the pre-B cell growth factor, IL-7. To test the idea that N-myc plays an important role in lymphocyte development, we assayed the effect of a null N-myc mutation on the differentiation of B and T lineage cells. Homozygous, mutant embryonic stem (ES) cells were injected into blastocysts derived from recombination activating gene (RAG-2)-deficient mice. Since RAG-2 mutant mice fail to develop mature lymphocytes, later-stage lymphocytes that are present in chimeric mice are ES cell derived. Surprisingly, nearly normal numbers of mature T and B cells derived from N-myc-deficient ES cells were found in peripheral lymphoid organs of chimeric mice. Lymphocytes were judged to be functional based on responses to mitogens and production of serum IgM and multiple IgG isotypes in chimeric animals. We discuss these findings in relation to N-myc function in lymphocyte development and possible redundancy with other myc genes.

Specification of axial identity in the mouse: role of the Hoxa-5 (Hox1.3) gene

Numerous lines of study have suggested that the Hox genes, encoding putative transcription factors, are key genes in the establishment of the body plan of the mammalian embryo. To examine the role of Hoxa-5 (Hox1.3) gene during development, we have used targeted mutagenesis in embryonic stem cells to produce a strain of mice carrying a disrupted Hoxa-5 allele. The viability of homozygous mutant mice is markedly reduced, with 50% of the mutant animals dying at birth or shortly thereafter. Analysis of the skeleton of Hoxa-5 mutants reveals a number of homeotic transformations restricted to the cervical and thoracic regions. Of these, one of the most frequent morphological abnormalities is the posterior transformation of the seventh cervical vertebra into the likeness of a thoracic vertebra complete with a pair of ribs. These results demonstrate that the Hoxa-5 gene has an important role in the establishment of the skeleton during development and contributes to the process whereby the axial structures are determined.

Embryonic lethality in mice homozygous for a targeted disruption of the N-myc gene

The N-myc gene encodes a putative transcription factor that is thought to function in the regulation of gene expression during cell differentiation and/or growth. To examine the role of N-myc during development, we have used targeted mutagenesis in embryonic stem cells to produce a mouse line that carries an N-myc null allele. Mice homozygous for the mutation died between 10.5 and 12.5 days of gestation. Histological analysis of mutant embryos revealed that organs and tissues expected at these stages of development were present. However, multiple defects were observed, primarily in tissues and organs that normally express N-myc. In particular, mutant hearts were underdeveloped, often retaining the S-shape more typical of 9-day-old embryos. In addition, cranial and spinal ganglia were reduced in size and/or cellularity. Most of the noted defects were more consistent with a role of N-myc in proliferation of precursor populations than with a block in differentiation per se, at least at these early stages. These results demonstrate that N-myc plays an essential role during development and clearly confirm that N-myc has a physiological function that is distinct from that of the other myc-family genes.

RAG-2-deficient mice lack mature lymphocytes owing to inability to initiate V(D)J rearrangement

We have generated mice that carry a germline mutation in which a large portion of the RAG-2 coding region is deleted. Homozygous mutants are viable but fail to produce mature B or T lymphocytes. Very immature lymphoid cells were present in primary lymphoid organs of mutant animals as defined by surface marker analyses and Abelson murine leukemia virus (A-MuLV) transformation assays. However, these cells did not rearrange their immunoglobulin or T cell receptor loci. Lack of V(D)J recombination activity in mutant pre-B cell lines could be restored by introduction of a functional RAG-2 expression vector. Therefore, loss of RAG-2 function in vivo results in total inability to initiate V(D)J rearrangement, leading to a novel severe combined immune deficient (SCID) phenotype. Because the SCID phenotype was the only obvious abnormality detected in RAG-2 mutant mice, RAG-2 function and V(D)J recombinase activity, per se, are not required for development of cells other than lymphocytes.

Isokinetic rehabilitation after arthroscopic meniscectomy

The aim of this study was to assess the effects in humans of early (2 weeks) and delayed (6 weeks) isokinetic strength training in the recovery of muscle strength following an arthroscopic partial meniscectomy. The peak torque developed in the quadriceps and hamstrings and the torque developed at a knee angle of 1.05 rad were evaluated in 16 subjects, pre-operatively (pre-op), and 2, 6, and 10 weeks post-operatively (post-op), on an isokinetic device at four different velocities (1.05, 2.09, 3.14, and 4.19 rad.s-1). The fatigue characteristics of the muscles were evaluated by having the subject perform 15 maximal contractions at 3.14 rad.s-1. Training was done on the same device (three times a week for 1-2 months), beginning either 2 or 6 weeks post-op. A repeated measures analysis of variance demonstrated a time effect but no differences between groups and no interactions. Torques developed by the knee flexors and extensors were significantly smaller 2 weeks post-op than pre-op, at all velocities tested. Torques developed in the quadriceps recovered to their pre-op values by 6 weeks, and further gained significantly in strength from 6 to 10 weeks. Quadriceps torques remained weaker than the contralateral side at 10 weeks. Hamstrings torques were either higher or similar to pre-op values by 6 weeks, and demonstrated increases from 6 to 10 weeks post-op at 1.05 and 4.19 rad.s-1 only. Total work and average power developed by the quadriceps and hamstrings during the fatigue protocol changed with time in a similar manner to torque.(ABSTRACT TRUNCATED AT 250 WORDS)

High-frequency disruption of the N-myc gene in embryonic stem and pre-B cell lines by homologous recombination

Identification of gene function has often relied on isolation of mutant cells in which expression of the gene was inactivated. Gene targeting by homologous recombination in tissue culture now may provide a technology to rapidly and directly produce such mutant mammalian cells. We demonstrate that selection of embryonic stem and pre-B cell lines for expression of a promoterless construct containing murine N-myc genomic sequences fused to a gene encoding neomycin resistance allows highly efficient recovery of variants in which the endogenous N-myc gene is disrupted. The high frequency of N-myc gene disruption by this method should permit targeted disruption of both allelic N-myc copies in various cell lines to study N-myc function.

Independent glucocorticoid induction and repression of two contiguous responsive genes

Specific DNA sequence elements which contain binding sites for the glucocorticoid receptor mediate the action of glucocorticoid hormones on gene transcription. In glucocorticoid-inducible genes, these glucocorticoid-responsive elements behave as hormone-inducible enhancers of transcription. We have taken advantage of the bovine papillomavirus (BPV) system to test the stringency of glucocorticoid regulation of transcription. BPV episomes were constructed to contain two hormone-regulated transcription units in close proximity; one transcription unit is under control of a glucocorticoid-inducible promoter (mouse mammary tumor virus) while the other is under control of a glucocorticoid-inhibited promoter (pro-opiomelanocortin). Glucocorticoids independently regulated transcription of the two physically linked transcription units, irrespective of their relative orientation and of their proximity on the BPV episomes. This result contrasts with the so-called position-independent activity of enhancers and suggests that the multicomponent organization of eucaryotic promoters restricts the action of hormone-responsive regulatory elements to a specific transcription unit, thus accounting for the stringency of hormonal regulation observed in vivo.

Tissue-specific activity of the pro-opiomelanocortin (POMC) gene and repression by glucocorticoids

The gene encoding pro-opiomelanocortin (POMC) is specifically expressed in two different cell types of the pituitary gland. We have defined the regulatory DNA sequences of the POMC gene that are responsible for this cell-specific expression. In addition, we have defined a regulatory element, located in the proximal region of the POMC promoter, that confers glucocorticoid repression in the anterior pituitary. Using DNA-mediated gene transfer into transgenic mice and tissue culture cells, the POMC regulatory sequences required for cell-specific expression and glucocorticoid repression were localized within a 543-bp fragment in the 5'-flanking region of the gene. Multiple regulatory elements that bind nuclear proteins are present within this region. In particular, a sequence that binds the glucocorticoid receptor and behaves as a "negative glucocorticoid response element" (nGRE) also binds nuclear proteins of the COUP (chicken ovalbumin upstream promoter) family of transcription factors. Thus, glucocorticoid repression of POMC transcription may result from the mutually exclusive binding of the glucocorticoid receptor and the COUP transcription factor to the POMC nGRE.

Pro-opiomelanocortin gene: a model for negative regulation of transcription by glucocorticoids

The gene encoding pro-opiomelanocortin (POMC) offers an interesting model system to study negative control of transcription in eucaryotes. Indeed, glucocorticoid hormones specifically inhibit transcription of the POMC gene in the anterior pituitary. The POMC gene is predominantly expressed in the anterior and intermediate lobes of the pituitary. However, only anterior pituitary POMC transcription is inhibited by glucocorticoids and stimulated by corticotropin-releasing hormone (CRH). Rat POMC promoter sequences required for anterior pituitary-specific expression were localized between positions -480 and -34 base pairs (bp) by DNA-mediated gene transfer into the POMC-expressing tumor cells. AtT-20. These POMC promoter sequences also confer glucocorticoid inhibition of transcription. While two of the six in vitro binding sites for purified glucocorticoid receptor identified in the rat POMC gene are within these sequences, only one is required for glucocorticoid inhibition; this binding site is located at position -63 bp in the promoter and overlaps a putative CCAAT box sequence. The DNA sequence of the POMC -63 bp receptor binding site is homologous to receptor binding sites identified in the glucocorticoid responsive element (GRE) of glucocorticoid-inducible genes. However, DNA sequence divergencies between these sites, in particular within the conserved hexanucleotide sequence 5'-TGTYCT-3', may be involved in their opposite transcriptional activity. Alternatively, binding of the receptor in the promoter proximal region of the POMC gene may inhibit transcription by a hormone-dependent repressor mechanism.

Glucocorticoid inhibition of transcription from episomal proopiomelanocortin gene promoter

Glucocorticoid hormones alter transcription of specific genes. Glucocorticoid-stimulated genes have been especially useful in unraveling molecular events responsible for positive gene regulation in mammals. The gene encoding proopiomelanocortin (POMC), which is under feedback inhibition by glucocorticoids, provides a model system to study negative gene regulation. Using an episomal bovine papilloma virus vector, we now demonstrate that a 769-base-pair fragment containing the rat POMC promoter is sufficient to confer glucocorticoid inhibition. Transcription from the episomal POMC promoter starts at the same site and is inhibited by glucocorticoids to the same extent as POMC transcription in the anterior pituitary. Glucocorticoid inhibition is specific for POMC transcripts; neither bovine papilloma virus nor cellular actin mRNAs are affected by glucocorticoids. Thus, the episomal bovine papilloma virus/POMC system can be used to study the relationship between negative regulation of POMC transcription and chromatin structure.

Structure of the rat pro-opiomelanocortin (POMC) gene

The gene encoding pro-opiomelanocortin (POMC) presents unique regulatory features. In particular, glucocorticoids inhibit transcription of the POMC gene in the anterior pituitary, but not in the intermediate pituitary. In order to study the mechanism leading to transcriptional inhibition of POMC by glucocorticoid and the interaction of the glucocorticoid receptor complex with specific DNA sequences along the POMC gene, we have cloned the rat POMC gene and determined its structure. The gene is composed of three exons and appears to be present at a single copy per haploid genome. Besides the usual regulatory signals like 'TATA' and 'CCAAT' boxes, the upstream region contains sequences homologous to known enhancer sequences and to the glucocorticoid receptor binding site observed in glucocorticoid-responsive genes.

Herpes simplex virus ribonucleotide reductase induced in infected BHK-21/C13 cells: biochemical evidence for the existence of two non-identical subunits, H1 and H2

In nearly all systems studied, ribonucleotide reductase consists of two non-identical subunits. We present here the results of our study on herpes simplex virus (HSV) ribonucleotide reductase in favour of the existence of two subunits, H1 and H2, different from the mammalian subunits, M1 and M2. First, although the viral subunits could not be separated by Blue Sepharose chromatography (unlike mammalian subunits), they seemed to dissociate at very low protein concentration as suggested by the non-linear relationship between activity and low protein concentration. Second, pyridoxal phosphate (Pyr.P)-NaBH4 treatment and 4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone (MAIQ) treatment of partially purified extract of mammalian ribonucleotide reductase which inactivated M1 and M2 respectively also inhibited the HSV ribonucleotide reductase. This activity could be restored by mixing Pyr.P-NaBH4-treated extracts with MAIQ-treated extracts of viral ribonucleotide reductase, suggesting that each treated extract contains one active subunit. Moreover, the addition of exogenous M1 or M2 subunits to one or the other of these two treated extracts did not produce any detectable reductase activity. Our interpretation of these results is that the two subunits H1 and H2 which could dissociate upon treatment did not form enzymically active hybrids with the mammalian subunits. Also, the higher degree of resistance to heat inactivation and to hydroxyurea of the viral reductase as compared to the mammalian enzyme suggests that H1 differs from M1 and H2 from M2.

Fertilizing capacity and sperm antibodies in vasovasostomized men

In order to explain the discrepancy between the patency rate (80%) and the pregnancy rate (46%) in a series of vasovasostomies, attention was focused on a group of patients who became normospermic. The mean age at vasectomy, the duration of vasobstruction, and the parameters of semen analysis were not different for those couples who achieved a pregnancy (n = 8), compared with those couples without pregnancy (n = 7). In the group with pregnancy, six of the eight patients had low titers of serum agglutinins (absent to 1:32), and the fertilizing capacity of their spermatozoa was normal. None had immobilizing antibodies. In the group without pregnancy, six of the seven patients had elevated serum agglutinins (greater than 1:256), and four had agglutinating antibodies in their seminal plasma as well as serum immobilizing antibodies. The spermatozoa of seven patients failed to fertilize zona-free hamster ova. It is concluded that a loss of fertilizing ability of the spermatozoa due to sperm antibodies is an important cause of infertility in vasovasostomized men.

Carcinoma of the sigmoid: a complication of ureterosigmoidostomy

The case of a 58-year-old man who had carcinomatous change in the sigmoid colon many years after ureterosigmoidostomy prompted the authors to review the French and English literature on the subject. They found 47 cases in addition to their own, and noted a dramatic increase in the last 10 years, even though ureterosigmoidostomy is now seldom used. The latency of this complication makes it pertinent today. In the hope of determining the etiology of the complication, the authors studied the relation between the initial lesion for which the diversion was carried out and the nature of the colonic tumour that ensued, and have attempted to draw some conclusions. None of the current theories of pathogenesis seem satisfactory. By underlying the early clinical manifestations, the authors attempt to promote earlier diagnosis and better chances of survival for patients with carcinoma of the colon after ureterosigmoidostomy.

Bromodeoxyuridine resistance in CHO cells occurs in three discrete steps

Four independent mutants were isolated from mutagenized cultures of CHO cells by sib selection on the basis of resistance to a low concentration (2.6 x 10(-5) M) of BrdU. All four lines were stable, but all had about 100% of the wild-type (WT) specific activity of thymidine kinase (TK). None of the four yielded derivatives resistant to a high level of BrdU (2 x 10(-4) M) in one step even after mutagenesis, but variants resistant to 4-6 x 10(-5) M BrdU could be isolated at frequencies of about 2 x 10(-5)/cell. At frequencies of 10(-4)-10(-5), the second-step mutants gave colonies resistant to 2 x 10(-4) M BrdU. The second and third steps of resistance were correlated with partial and complete reduction, respectively, in the specific activity of TK, suggesting that the variants may be genotypically heterozygous and homozygous-negative at the tk locus. The first step of BrdU resistance was dominant and appeared to result from a mutation in the gene from ribonucleotide reductase, since in vitro assays on partially purified preparations showed that the reductase activity in mutant cells was less sensitive to BrdUTP than in WT cells.

Analysis of deoxycytidine (dC) deaminase activity in herpes simplex virus-infected or HSV TK-transformed cells: association with mycoplasma contamination but not with virus infection

Deoxycytidine (dC) deaminase activity has been previously reported to be induced in herpes simplex virus (HSV)-infected cells (Chan, 1977). In contrast, we report here that HSV infection of either hamster cells naturally deficient in this enzyme activity or mouse cells containing a low level of activity never resulted in appearance of stimulation of dC deaminase, whereas thymidine kinase (TK) was always induced. Surprisingly, dC deaminase activity, which differed by electrophoretic mobility from the mouse or human cell enzyme, was discovered in some cells selected for the presence of HSV TK after infection with u.v.-irradiated HSV. Evidence is presented which suggests that the appearance of this new enzyme was not due to the presence of virus genes but rather to mycoplasma contamination.