The Cre/loxP site-specific recombination system derived from bacteriophage P1 provides a convenient tool for directed modifications of genomes in various organisms. To exploit Cre-mediated manipulation of mouse genomic sequences at the zygote stage, we have developed a transgenic mouse line carrying the CAG-cre transgene in which the cre gene is under control of the cytomegalovirus immediate early enhancer-chicken beta-actin hybrid (CAG) promoter. The activity of the Cre recombinase at early stages of development was examined by crossing the CAG-cre transgenic mice to another transgenic mouse line carrying a reporter gene construct, CAG-CAT-Z, which directs expression of the E. coli lacZ gene upon Cre-mediated excision of the loxP-flanked chloramphenicol acetyltransferase (CAT) gene located between the CAG promoter and the lacZ gene. PCR-based analysis of F1 progeny from CAG-cre males x CAG-CAT-Z females showed that transmission of the CAG-cre transgene was accompanied by the complete deletion of the CAT gene of the CAG-CAT-Z transgene in all tissues, and that this deletion was never observed in the progeny without transmission of the CAG-cre gene. On the other hand, analysis of F1 mice from CAG-CAT-Z males x CAG-cre females showed that the CAG-CAT-Z transgene had undergone complete deletion of the CAT gene in all tissues irrespective of the cotransmission of the CAG-cre gene. This Cre-mediated recombination in F1 mice occurred before the two-cell stage of embryonic development, as shown by X-gal staining. The results suggest that the CAG-cre transgene is expressed in developing oocytes of CAG-cre transgenic mice, and Cre mRNA and/or protein are retained in mature oocytes irrespective of the transmission of the CAG-cre transgene, resulting in efficient Cre-mediated recombination of paternally derived target genes upon fertilization. The CAG-cre transgenic mouse should serve as a useful tool to introduce prescribed genetic modifications into the mouse embryo at the zygote stage.

BACKGROUND

Primary and secondary prevention trials suggest that statins possess favorable effects independent of cholesterol reduction. We investigated whether statin therapy may also accelerate reendothelialization after carotid balloon injury.

RESULTS

Simvastatin treatment in 34 male Sprague-Dawley rats accelerated reendothelialization of the balloon-injured arterial segments (reendothelialized area at 2 weeks, 12.3+/-1.8 versus 5.4+/-1.1 mm2, P< 0.01) and resulted in a dose-dependent (0.2 or 1 mg/kg IP) significant reduction in neointimal thickening at 2, 3, and 4 weeks compared with saline-injected controls (n=18). To elucidate the mechanism, we investigated the contribution of bone marrow-derived endothelial progenitor cells (EPCs) by bone marrow transplantation from Tie2/lacZ mice to background mice or nude rats. X-gal staining of mouse carotid artery specimens revealed a 2.9-fold increase in the number of beta-gal-positive cells per square millimeter appearing on the carotid artery luminal surface at 2 weeks, and double-fluorescence immunohistochemistry disclosed a significant 5-fold increase in the number of double-positive cells (beta-gal, isolectin B4) on the luminal surface in carotid arteries of statin-treated nude rats (20+/-3 versus 4+/-1 cells/mm surface length, P<0.005). Statins increased circulating rat EPCs (2.4-fold at 2 weeks and 2.5-fold at 4 weeks, P<0.001) and induced adhesiveness of cultured human EPCs by upregulation of the integrin subunits alpha5, beta1, alpha(v), and beta5 of human EPCs as shown by reverse transcription-polymerase chain reaction and fluorescence-activated cell sorting.

CONCLUSIONS

These findings establish additional mechanisms by which statins may specifically preempt disordered vascular wall pathology and constitute physiological evidence that EPC mobilization represents a functionally relevant consequence of statin therapy.

A protocol based on saturation transfer difference (STD) NMR spectra was developed to characterize the binding interactions at an atom level, termed group epitope mapping (GEM). As an example we chose the well-studied system of galactose binding to the 120-kDa lectin Ricinus communis agglutinin I (RCA(120)). As ligands we used methyl beta-D-galactoside and a biantennary decasaccharide. Analysis of the saturation transfer effects of methyl beta-D-galactoside showed that the H2, H3, and H4 protons are saturated to the highest degree, giving evidence of their close proximity to protons of the RCA(120) lectin. The direct interaction of the lectin with this region of the galactose is in excellent agreement with results obtained from the analysis of the binding specificities of many chemically modified galactose derivatives (Bhattacharyya, L.; Brewer, C. F. Eur. J. Biochem. 1988, 176, 207-212). This new NMR technique can identify the binding epitope of even complex ligands very quickly, which is a great improvement over time-consuming chemical modifications. Efficient GEM benefits from a relatively high off rate of the ligand and a large excess of the ligand over the receptor. Even for a ligand like the biantennary decasaccharide with micromolar binding affinity, the binding epitopes could easily be mapped to the terminal beta-D-Gal-(1-4)-beta-D-GlcNAc (beta-D-GlcNAc = N-acetyl-D-glucosamine) residues located at the nonreducing end of the two carbohydrate chains. The binding contribution of the terminal galactose residue is stronger than those of the penultimate GlcNAc residues. We could show that the GlcNAc residues bind "edge-on" with the region from H2 to H4, making contact with the protein. Analysis of STD NMR experiments performed under competitive conditions proved that the two saccharides studied bind at the same receptor site, thereby ruling out unspecific binding.

We describe a series of plasmid vectors which contain modular features particularly useful for studying gene expression in eukaryotic systems. The vectors contain the Escherichia coli beta-galactosidase (beta Gal)-encoding region (the lacZ gene) flanked by unique polylinker segments on the 5' and 3' ends, and several combinations of a variety of modules: a selectable marker (an amber suppressor tRNA), a translational initiation region, a synthetic intron segment, the early polyadenylation signal from SV40, and 3' regions from two nematode genes. A segment encoding the nuclear localization peptide from the SV40 T antigen is incorporated into many of the constructs, leading to beta Gal accumulation in nuclei, which can facilitate identification of producing cells in complex tissues. To make functional beta Gal fusions to secreted proteins, we constructed plasmids with an alternate module encoding a synthetic transmembrane domain upstream from lacZ. This domain is designed to stop transfer of secreted proteins across the membrane during secretion, allowing the beta Gal domain of the fusion polypeptide to remain in the cytoplasm and thus function in enzymatic assays. We have used the vectors to analyze expression of several genes in the nematode Caenorhabditis elegans, and have demonstrated in these studies that lacZ can be expressed in a wide variety of different tissues and cell types. These vectors should be useful in studying gene expression both in C. elegans and in other experimental systems.

The yeast snoRNP protein, NOP1, is structurally and functionally homologous to vertebrate fibrillarin and is essential for viability. A conditionally lethal allele was constructed by placing NOP1 expression under the control of a GAL promoter. Growth on glucose medium results in the depletion of NOP1 over several generations, during which cell growth is progressively impaired. Pulse labelling of proteins shows that NOP1 depleted strains are greatly impaired in the production of cytoplasmic ribosomes, and they have a reduced level of rRNA. Northern hybridization and pulse-chase labelling of pre-rRNA show a progressive impairment of all pre-rRNA processing steps. The pathway leading to 18S rRNA is particularly affected. Methylation of pre-rRNA is concomitantly impaired and unmethylated pre-rRNA accumulates and is not processed over long periods. NOP1 depletion does not prevent the accumulation of seven snoRNAs tested including U3; the levels of two species, U14 and snR190, decline. The snoRNAs synthesized in the absence of NOP1 retain TMG cap structures. Subnuclear fractionation and immunocytochemistry indicate that they continue to be localized in the nucleolus.

Csx/Nkx2.5 is a vertebrate homeobox gene with a sequence homology to the Drosophila tinman, which is required for the dorsal mesoderm specification. Recently, heterozygous mutations of this gene were found to cause human congenital heart disease (Schott, J.-J., Benson, D. W., Basson, C. T., Pease, W., Silberbach, G. M., Moak, J. P., Maron, B. J., Seidman, C. E. and Seidman, J. G. (1998) Science 281, 108-111). To investigate the functions of Csx/Nkx2.5 in cardiac and extracardiac development in the vertebrate, we have generated and analyzed mutant mice completely null for Csx/Nkx2.5. Homozygous null embryos showed arrest of cardiac development after looping and poor development of blood vessels. Moreover, there were severe defects in vascular formation and hematopoiesis in the mutant yolk sac. Interestingly, TUNEL staining and PCNA staining showed neither enhanced apoptosis nor reduced cell proliferation in the mutant myocardium. In situ hybridization studies demonstrated that, among 20 candidate genes examined, expression of ANF, BNP, MLC2V, N-myc, MEF2C, HAND1 and Msx2 was disturbed in the mutant heart. Moreover, in the heart of adult chimeric mice generated from Csx/Nkx2.5 null ES cells, there were almost no ES cell-derived cardiac myocytes, while there were substantial contributions of Csx /Nkx2.5-deficient cells in other organs. Whole-mount &bgr;-gal staining of chimeric embryos showed that more than 20% contribution of Csx/Nkx2. 5-deficient cells in the heart arrested cardiac development. These results indicate that (1) the complete null mutation of Csx/Nkx2.5 did not abolish initial heart looping, (2) there was no enhanced apoptosis or defective cell cycle entry in Csx/Nkx2.5 null cardiac myocytes, (3) Csx/Nkx2.5 regulates expression of several essential transcription factors in the developing heart, (4) Csx/Nkx2.5 is required for later differentiation of cardiac myocytes, (5) Csx/Nkx2. 5 null cells exert dominant interfering effects on cardiac development, and (6) there were severe defects in yolk sac angiogenesis and hematopoiesis in the Csx/Nkx2.5 null embryos.

Recent evidence indicates that vascular progenitor cells may be the source of smooth muscle cells (SMCs) that accumulate in atherosclerotic lesions, but the origin of these progenitor cells is unknown. To explore the possibility of vascular progenitor cells existing in adults, a variety of tissues from ApoE-deficient mice were extensively examined. Immunohistochemical staining revealed that the adventitia in aortic roots harbored large numbers of cells having stem cell markers, e.g., Sca-1(+) (21%), c-kit(+) (9%), CD34(+) (15%), and Flk1(+) cells (4%), but not SSEA-1(+) embryonic stem cells. Explanted cultures of adventitial tissues using stem cell medium displayed a heterogeneous outgrowth, for example, islands of round-shaped cells surrounded by fibroblast-like cell monolayers. Isolated Sca-1(+) cells were able to differentiate into SMCs in response to PDGF-BB stimulation in vitro. When Sca-1(+) cells carrying the LacZ gene were transferred to the adventitial side of vein grafts in ApoE-deficient mice, beta-gal(+) cells were found in atherosclerotic lesions of the intima, and these cells enhanced the development of the lesions. Thus, a large population of vascular progenitor cells existing in the adventitia can differentiate into SMCs that contribute to atherosclerosis. Our findings indicate that ex vivo expansion of these progenitor cells may have implications for cellular, genetic, and tissue engineering approaches to vascular disease.

Glycopeptides and oligosaccharides of either the N-acetyllactosaminic or the oligomannosidic type derived from glycoproteins containing the N-glycosylamine linkage were used to define the specificity of different lectins (concanavalin A, Lens culinaris agglutinin, Vicia faba agglutinin, Pisum sativum agglutinin, Ricinus communis agglutinins, soybean agglutinin, wheat germ agglutinin, Solanum tuberosum agglutinin, Datura stramonium agglutinin, Lotus tetragonolobus agglutinin, Ulex europeus agglutinin) by studying the inhibition of human red blood cell agglutination by these structures. The results obtained show that lectins considered 'identical' in terms of monosaccharide specificity, possess the ability to recognize fine differences in more complex structures. In fact, different lectins are able to recognize different saccharidic sequences on the same glycan structure. As these sequences are likely to be common to numerous glycoproteins, including cell membrane glycoproteins, the results obtained with lectins in the study of cell surface carbohydrates have to be very carefully interpreted. Moreover, our results confirm previous data on the spatial configuration of the glycan moiety of glycoproteins deduced from the construction of molecular models: the fact that oligosaccharides bearing an alpha-NeuAc-(2 leads to 6)-Gal unit are more powerful inhibitors than oligosaccharides bearing an alpha-NeuAc-(2 leads to 3)-Gal unit could be related to the high rotational freedom of alpha-2,6 linkage; the observation that glycoasparagines, glycopeptides and glycoproteins possess a higher affinity for lectins than the related oligosaccharides could be explained by the fact that the glycan--amino acid linkage leads to structures more rigid than those of the oligosaccharides themselves.

The Mesp1 gene encodes the basic HLH protein MesP1 which is expressed in the mesodermal cell lineage during early gastrulation. Disruption of the Mesp1 gene leads to aberrant heart morphogenesis, resulting in cardia bifida. In order to study the defects in Mesp1-expressing cells during gastrulation and in the specification of mesodermal cell lineages, we introduced a (beta)-galactosidase gene (lacZ) under the control of the Mesp1 promoter by homologous recombination. The early expression pattern revealed by (beta)-gal staining in heterozygous embryos was almost identical to that observed by whole mount in situ hybridization. However, the (beta)-gal activity was retained longer than the mRNA signal, which enabled us to follow cell migration during gastrulation. In heterozygous embryos, the Mesp1-expressing cells migrated out from the primitive streak and were incorporated into the head mesenchyme and heart field. In contrast, Mesp1-expressing cells in the homozygous deficient embryos stayed in the primitive streak for a longer period of time before departure. The expression of FLK-1, an early marker of endothelial cell precursors including heart precursors, also accumulated abnormally in the posterior region in Mesp1-deficient embryos. In addition, using the Cre-loxP site-specific recombination system, we could determine the lineage of the Mesp1-expressing cells. The first mesodermal cells that ingressed through the primitive streak were incorporated as the mesodermal component of the amnion, and the next mesodermal population mainly contributed to the myocardium of the heart tube but not to the endocardium. These results strongly suggest that MesP1 is expressed in the heart tube precursor cells and is required for mesodermal cells to depart from the primitive streak and to generate a single heart tube.

BACKGROUND

Vascular endothelial growth factor (VEGF) is being investigated for therapeutic angiogenesis in ischemic myocardium. Primarily, transient delivery systems have been tested. The goal of this study was to investigate the effects of continuous expression of VEGF in myocardium by use of myoblast-mediated delivery.

RESULTS

Primary murine myoblasts (5 x 10(5) cells in 10 microL of PBS with 0.5% BSA) expressing both the murine VEGF gene and the beta-galactosidase (beta-gal) gene from a retroviral promoter were implanted in the ventricular wall of immunodeficient mice (n=11) via a subdiaphragmatic approach. Control immunodeficient mice (n=12) were injected with the same number of myoblasts expressing only the beta-gal gene. Between days 14 and 16, surviving mice were euthanized and the hearts processed for histology. In the experimental group, 11 of 11 mice demonstrated failure to thrive by day 13; 5 deaths occurred between days 8 and 15. There were no complications in the control mice. Histochemistry documented successful implantation of myoblasts (positive beta-gal reaction product) in 6 of 6 surviving experimental mice and 12 of 12 controls. Histology disclosed intramural vascular tumors resembling hemangiomas in the VEGF-myoblast-injected myocardium in 6 of 6 surviving mice. beta-Gal-expressing cells were present at the site of the vascular tumors. Immunohistochemistry localized abundant endothelial nitric oxide synthase and CD31 (platelet and endothelial cell adhesion molecule) within the lesion, consistent with the presence of endothelial cells.

CONCLUSIONS

In this model, unregulated continuous expression of VEGF is associated with (1) a high rate of failure to thrive/death and (2) formation of endothelial cell-derived intramural vascular tumors in the implantation site. These results underscore the importance of regulating VEGF expression for therapeutic angiogenesis.

The study of the expression of alpha-galactosyl epitopes on various mammalian cells is of particular interest, since as much as 1% of circulating IgG antibodies in humans interact with this carbohydrate residue. This natural antibody, designated "anti-Gal," was previously found to bind to terminal Gal alpha 1----3Gal beta 1----4GlcNAc-R on biochemically defined glycolipids (Galili, U., Macher, B. A., Buehler, J., and Shohet, S. B. (1985) J. Exp. Med. 162, 573-582; Galili, U., Buehler, J., Shohet, S. B., and Macher, B. A. (1987) J. Exp. Med. 165, 693-704). The expression of anti-Gal binding epitopes on nucleated cells from various mammalian species was studied by immunostaining with this antibody. The binding of anti-Gal to various cells was correlated with the binding of the lectin Bandeiraea (Griffonia) simplicifolia IB4 (BS lectin). The BS lectin also interacts with alpha-galactosyl residues and particularly with high affinity with Gal alpha 1----3Gal beta 1----4GlcNAc residues. We observed a striking evolutionary pattern in the expression of these epitopes on mammalian nucleated cells. Fibroblasts, epithelial cells, endothelial cells, smooth muscle cells, and lymphoid cells of nonprimate mammals, prosimians, and New World monkeys readily bound both anti-Gal and BS lectin. However, no such binding was detectable on cells of Old World monkeys, apes, and humans. Measurment of the binding of radiolabeled BS lectin to the various nucleated cells suggests that cells binding anti-Gal express 10(6) to 3.5 x 10(7) alpha-galactosyl epitopes, most of which, based on the anti-Gal specificity, seem to have the structure of Gal alpha 1----3Gal beta 1----4GlcNAc-R. The absence of these epitopes from human cells results from diminished activity of the enzyme alpha 1----3 galactosyltransferase, which catalyzes the following reaction. Gal beta 1----4GlcNAc-R + UDP-Gal(alpha 1----3-galactosyltransferase)----Gal alpha 1----3Gal beta 1----4GlcNAc-R + UDP This enzyme, which participates in the glycosylation of cell membrane glycoconjugates in nonprimate mammals, prosimians, and New World monkeys, appears to have been suppressed in Old World primates as a result of evolutionary events which occurred 20-30 million years ago. It is argued that an anomalous activity of this enzyme in man may result in initiation of autoimmune diseases because of the de novo expression of Gal alpha 1----3Gal beta 1----4GlcNAc-R epitopes recognized by anti-Gal.

Mesenchymal stem cells (MSC) have a therapeutic potential in patients with fractures to reduce the time of healing and treat nonunions. The use of MSC to treat fractures is attractive for several reasons. First, MSCs would be implementing conventional reparative process that seems to be defective or protracted. Secondly, the effects of MSCs treatment would be needed only for relatively brief duration of reparation. However, an integrated approach to define the multiple regenerative contributions of MSC to the fracture repair process is necessary before clinical trials are initiated. In this study, using a stabilized tibia fracture mouse model, we determined the dynamic migration of transplanted MSC to the fracture site, their contributions to the repair process initiation, and their role in modulating the injury-related inflammatory responses. Using MSC expressing luciferase, we determined by bioluminescence imaging that the MSC migration at the fracture site is time- and dose-dependent and, it is exclusively CXCR4-dependent. MSC improved the fracture healing affecting the callus biomechanical properties and such improvement correlated with an increase in cartilage and bone content, and changes in callus morphology as determined by micro-computed tomography and histological studies. Transplanting CMV-Cre-R26R-Lac Z-MSC, we found that MSCs engrafted within the callus endosteal niche. Using MSCs from BMP-2-Lac Z mice genetically modified using a bacterial artificial chromosome system to be beta-gal reporters for bone morphogenic protein 2 (BMP-2) expression, we found that MSCs contributed to the callus initiation by expressing BMP-2. The knowledge of the multiple MSC regenerative abilities in fracture healing will allow design of novel MSC-based therapies to treat fractures.

Human core 1 beta3-galactosyltransferase (C1beta3Gal-T) generates the core 1 O-glycan Galbeta1-3GalNAcalpha1-SerThr (T antigen), which is a precursor for many extended O-glycans in animal glycoproteins. We report here that C1beta3Gal-T activity requires expression of a molecular chaperone designated Cosmc (core 1 beta3-Gal-T-specific molecular chaperone). The human Cosmc gene is X-linked (Xq23), and its cDNA predicts a 318-aa transmembrane protein ( approximately 36.4 kDa) with type II membrane topology. The human lymphoblastoid T cell line Jurkat, which lacks C1beta3Gal-T activity and expresses the Tn antigen GalNAcalpha1-SerThr, contains a normal gene and mRNA encoding C1beta3Gal-T, but contains a mutated Cosmc with a deletion introducing a premature stop codon. Expression of Cosmc cDNA in Jurkat cells restored C1beta3Gal-T activity and T antigen expression. Without Cosmc, the C1beta3Gal-T is targeted to proteasomes. Expression of active C1beta3Gal-T in Hi-5 insect cells requires coexpression of Cosmc. Overexpression of active C1beta3Gal-T in mammalian cell lines also requires coexpression of Cosmc, indicating that endogenous Cosmc may be limiting. A small portion of C1beta3Gal-T copurifies with Cosmc from cell extracts, demonstrating physical association of the proteins. These results indicate that Cosmc acts as a specific molecular chaperone in assisting the foldingstability of C1beta3Gal-T. The identification of Cosmc, a uniquely specific molecular chaperone required for a glycosyltransferase expression in mammalian cells, may shed light on the molecular basis of acquired human diseases involving altered O-glycosylation, such as IgA nephropathy, Tn syndrome, Henoch-Schönlein purpura, and malignant transformation, all of which are associated with a deficiency of C1beta3Gal-T activity.

Oncogene-induced senescence is an important mechanism by which normal cells are restrained from malignant transformation. Here we report that the suppression of the c-Myc (MYC) oncogene induces cellular senescence in diverse tumor types including lymphoma, osteosarcoma, and hepatocellular carcinoma. MYC inactivation was associated with prototypical markers of senescence, including acidic beta-gal staining, induction of p16INK4a, and p15INK4b expression. Moreover, MYC inactivation induced global changes in chromatin structure associated with the marked reduction of histone H4 acetylation and increased histone H3 K9 methylation. Osteosarcomas engineered to be deficient in p16INK4a or Rb exhibited impaired senescence and failed to exhibit sustained tumor regression upon MYC inactivation. Similarly, only after lymphomas were repaired for p53 expression did MYC inactivation induce robust senescence and sustained tumor regression. The pharmacologic inhibition of signaling pathways implicated in oncogene-induced senescence including ATM/ATR and MAPK did not prevent senescence associated with MYC inactivation. Our results suggest that cellular senescence programs remain latently functional, even in established tumors, and can become reactivated, serving as a critical mechanism of oncogene addiction associated with MYC inactivation.

Previous studies demonstrated that the SAGA (Spt-Ada-Gcn5-Acetyltransferase) complex facilitates the binding of TATA-binding protein (TBP) during transcriptional activation of the GALGAL genes. This recruitment is dependent on both induction by galactose and the Gal4 activation domain. Furthermore, we demonstrate that another well-characterized activator, Gal4-VP16, also recruits SAGA in vivo. Finally, we provide evidence that a specific interaction between Spt3 and TBP in vivo is important for Gal4 transcriptional activation at a step after SAGA recruitment. These results, taken together with previous studies, demonstrate a dependent pathway for the recruitment of TBP to GAL gene promoters consisting of the recruitment of SAGA by Gal4 and the subsequent recruitment of TBP by SAGA.

A series of colicin-tolerant (tol) mutants of Escherichia coli K-12, which adsorbed colicins but were not killed by them, were isolated and studied genetically and physiologically. Three major classes of mutants were found: tol II, tolerant to colicins A, E1, E2, E3, and K; tol III, tolerant to A, E2, E3, and K; and tol VIII, tolerant to E1 only. The sites of tol II and tol III mutations mapped near the gal region (gene order: tol-gal-bio) and were cotransduced with gal by P1. In heterozygous diploids, tol(+) was dominant over tol; tol II and tol III gave full complementation. All the tol mutations that mapped near gal rendered the bacteria more fragile during growth and hypersensitive to deoxycholate and to ethylenediaminetetraacetic acid. The tol VIII mutation mapped between str and his. These mutants were extremely sensitive to deoxycholate and were also hypersensitive to methylene blue, acridines, and various other compounds. The sensitivity is attributed to increased uptake due to selective alteration of the permeability barrier. The colicin-tolerant mutations are interpreted as affecting some components of the cytoplasmic membrane which mediate between the adsorbed colicin molecules and the target sites of their biochemical effects in the bacterial cell.

The irreversible G1 arrest in senescent human diploid fibroblasts is probably caused by inactivation of the G1 cyclin-cyclin-dependent kinase (Cdk) complexes responsible for phosphorylation of the retinoblastoma protein (pRb). We show that the Cdk inhibitor p21(Sdi1,Cip1,Waf1), which accumulates progressively in aging cells, binds to and inactivates all cyclin E-Cdk2 complexes in senescent cells, whereas in young cells only p21-free Cdk2 complexes are active. Furthermore, the senescent-cell-cycle arrest occurs prior to the accumulation of the Cdk4-Cdk6 inhibitor p16(Ink4a), suggesting that p21 may be sufficient for this event. Accordingly, cyclin D1-associated phosphorylation of pRb at Ser-780 is lacking even in newly senescent fibroblasts that have a low amount of p16. Instead, the cyclin D1-Cdk4 and cyclin D1-Cdk6 complexes in these cells are associated with an increased amount of p21, suggesting that p21 may be responsible for inactivation of both cyclin E- and cyclin D1-associated kinase activity at the early stage of senescence. Moreover, even in the late stage of senescence when p16 is high, cyclin D1-Cdk4 complexes are persistent, albeit reduced by </=50% compared to young cells. We also provide new evidence that p21 may play a role in inactivation of the DNA replication factor proliferating cell nuclear antigen during early senescence. Finally, because p16 accumulates in parallel with the increases in senescence-associated beta-<em>Gal</em> activity and cell volume that characterize the senescent phenotype, we suggest that p16 upregulation may be part of a differentiation program that is turned on in senescent cells. Since p21 decreases after senescence is achieved, this upregulation of p16 may be essential for maintenance of the senescent-cell-cycle arrest.

Despite major advances in characterizing the eukaryotic transcriptional machinery, the function of promoter-specific transcriptional activators (activators) is still not understood. For example, in no case have the direct in vivo targets of a transcriptional activator been unambiguously identified, nor has it been resolved whether activators have a single essential target or multiple redundant targets. Here we address these issues for the prototype acidic activator yeast Gal4p. Gal4p binds to the upstream activating sequence (UAS) of <em>GAL</em>1 and several other <em>GAL</em> genes and stimulates transcription in the presence of galactose. Previous studies have shown that <em>GAL</em>1 transcription is dependent on the yeast SAGA (Spt/Ada/GCN5/acetyltransferase) complex. Using formaldehyde-based in vivo cross-linking, we show that the Gal4p activation domain recruits SAGA to the <em>GAL</em>1 UAS. If SAGA is not recruited to the UAS, the preinitiation complex (PIC) fails to assemble at the <em>GAL</em>1 core promoter, and transcription does not occur. SAGA, but not other transcription components, is also recruited by the Gal4p activation domain to a plasmid containing minimal Gal4p-binding sites. Recruitment of SAGA by Gal4p and stimulation of PIC assembly is dependent on several SAGA subunits but not the SAGA histone acetyl-transferase (HAT) GCN5. Based on these and other results, we conclude that SAGA is an essential target of Gal4p that, following recruitment to the UAS, facilitates PIC assembly and transcription.

OBJECTIVE

The G-protein-coupled receptor Gpr40 is expressed in beta-cells where it contributes to free fatty acid (FFA) enhancement of glucose-stimulated insulin secretion. However, other sites of Gpr40 expression, including the intestine, have been suggested. The transcription factor IPF1/PDX1 was recently shown to bind to an enhancer element within the 5'-flanking region of Gpr40, implying that IPF1/PDX1 might regulate Gpr40 expression. Here, we addressed whether 1) Gpr40 is expressed in the intestine and 2) Ipf1/Pdx1 function is required for Gpr40 expression.

METHODS

In the present study, Gpr40 expression was monitored by X-gal staining using Gpr40 reporter mice and by in situ hybridization. Ipf1/Pdx1-null and beta-cell specific mutants were used to investigate whether Ipf1/Pdx1 controls Gpr40 expression. Plasma insulin, glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucose levels in response to acute oral fat diet were determined in Gpr40 mutant and control mice.

RESULTS

Here, we show that Gpr40 is expressed in endocrine cells of the gastrointestinal tract, including cells expressing the incretin hormones GLP-1 and GIP, and that Gpr40 mediates FFA-stimulated incretin secretion. We also show that Ipf1/Pdx1 is required for expression of Gpr40 in beta-cells and endocrine cells of the anterior gastrointestinal tract.

CONCLUSIONS

Together, our data provide evidence that Gpr40 modulates FFA-stimulated insulin secretion from beta-cells not only directly but also indirectly via regulation of incretin secretion. Moreover, our data suggest a conserved role for Ipf1/Pdx1 and Gpr40 in FFA-mediated secretion of hormones that regulate glucose and overall energy homeostasis.

To explore the role of DNA methylation in the brain, we examined the expression pattern of de novo DNA methyltransferases Dnmt3a and Dnmt3b in the mouse central nervous system (CNS). By comparing the levels of Dnmt3a and Dnmt3b mRNAs and proteins in the CNS, we showed that Dnmt3b is detected within a narrow window during early neurogenesis, whereas Dnmt3a is present in both embryonic and postnatal CNS tissues. To determine the precise pattern of Dnmt3a and Dnmt3b gene expression, we carried out X-gal histochemistry in transgenic mice in which the lacZ marker gene is knocked into the endogenous Dnmt3a or Dnmt3b gene locus (Okano et al. [1999] Cell 99:247-257). In Dnmt3b-lacZ transgenic mice, X-gal-positive cells are dispersed across the ventricular zone of the CNS between embryonic days (E) 10.5 and 13.5 but become virtually undetectable in the CNS after E15.5. In Dnmt3a-lacZ mice, X-gal signal is initially observed primarily in neural precursor cells within the ventricular and subventricular zones between E10.5 and E17.5. However, from the newborn stage to adulthood, Dnmt3a X-gal signal was detected predominantly in postmitotic CNS neurons across all the regions examined, including olfactory bulb, cortex, hippocampus, striatum, and cerebellum. Furthermore, Dnmt3a signals in CNS neurons increase during the first 3 weeks of postnatal development and then decline to a relatively low level in adulthood, suggesting that Dnmt3a may be of critical importance for CNS maturation. Immunocytochemistry experiments confirmed that Dnmt3a protein is strongly expressed in neural precursor cells, postmitotic CNS neurons, and oligodendrocytes. In contrast, glial fibrillary acidic protein-positive astrocytes exhibit relatively weak or no Dnmt3a immunoreactivity in vitro and in vivo. Our data suggest that whereas Dnmt3b may be important for the early phase of neurogenesis, Dnmt3a likely plays a dual role in regulating neurogenesis prenatally and CNS maturation and function postnatally.

Circulating immune complexes (CICs) isolated from sera of patients with IgA nephropathy (IgAN) consist of undergalactosylated, mostly polymeric, and J chain-containing IgA1 and IgG antibodies specific for N-acetylgalactosamine (GalNAc) residues in O-linked glycans of the hinge region of IgA1 heavy chains. Antibodies with such specificity occur in sera of IgAN patients, and in smaller quantities in patients with non-IgA proliferative glomerulonephritis and in healthy controls; they are present mainly in the IgG (predominantly IgG2 subclass), and less frequently in the IgA1 isotype. Their specificity for GalNAc was determined by reactivity with IgA1 myeloma proteins with enzymatically removed N-acetylneuraminic acid (NeuNAc) and galactose (Gal); removal of the O-linked glycans of IgA1 resulted in significantly decreased reactivity. Furthermore, IgA2 proteins that lack the hinge region with O-linked glycans but are otherwise structurally similar to IgA1 did not react with IgG or IgA1 antibodies. The re-formation of isolated and acid-dissociated CICs was inhibited more effectively by IgA1 lacking NeuNAc and Gal than by intact IgA1. Immobilized GalNAc and asialo-ovine submaxillary mucin (rich in O-linked glycans) were also effective inhibitors. Our results suggest that the deficiency of Gal in the hinge region of IgA1 molecules results in the generation of antigenic determinants containing GalNAc residues that are recognized by naturally occurring IgG and IgA1 antibodies.

Major depressive disorder (MDD) is a common complex disorder with a partly genetic etiology. We conducted a genome-wide association study of the MDD2000+ sample (2431 cases, 3673 screened controls and >1 M imputed single-nucleotide polymorphisms (SNPs)). No SNPs achieved genome-wide significance either in the MDD2000+ study, or in meta-analysis with two other studies totaling 5763 cases and 6901 controls. These results imply that common variants of intermediate or large effect do not have main effects in the genetic architecture of MDD. Suggestive but notable results were (a) gene-based tests suggesting roles for adenylate cyclase 3 (ADCY3, 2p23.3) and galanin (GAL, 11q13.3); published functional evidence relates both of these to MDD and serotonergic signaling; (b) support for the bipolar disorder risk variant SNP rs1006737 in CACNA1C (P=0.020, odds ratio=1.10); and (c) lack of support for rs2251219, a SNP identified in a meta-analysis of affective disorder studies (P=0.51). We estimate that sample sizes 1.8- to 2.4-fold greater are needed for association studies of MDD compared with those for schizophrenia to detect variants that explain the same proportion of total variance in liability. Larger study cohorts characterized for genetic and environmental risk factors accumulated prospectively are likely to be needed to dissect more fully the etiology of MDD.

Analysis of several mutations in the mouse is providing useful insights into the nature of the genes required for the establishment of the left-right axis during early development. Here we describe a new targeted allele of the mouse Tg737 gene, Tg737(Delta)2-3(beta)Gal), which causes defects in left-right asymmetry and other abnormalities during embryogenesis. The Tg737 gene was originally identified based on its association with the mouse Oak Ridge Polycystic Kidney (orpk) insertional mutation, which causes polycystic kidney disease and other defects. Complementation tests between the original orpk mutation and the new targeted knock-out mutation demonstrate that Tg737(Delta)2-3(beta)Gal) behaves as an allele of Tg737. The differences in the phenotype between the two mutations suggest that the orpk mutation is a hypomorphic allele of the Tg737 gene. Unlike the orpk allele, where all homozygotes survive to birth, embryos homozygous for the Tg737(Delta)2-3(beta)Gal) mutation arrest in development at mid-gestation and exhibit neural tube defects, enlargement of the pericardial sac and, most notably, left-right asymmetry defects. At mid-gestation the direction of heart looping is randomized, and at earlier stages in development lefty-2 and nodal, which are normally expressed asymmetrically, exhibit symmetrical expression in the mutant embryos. Additionally, we determined that the ventral node cells in mutant embryos fail to express the central cilium, which is a characteristic and potentially functional feature of these cells. The expression of both Shh and Hnf3(beta) is downregulated in the midline at E8.0, indicating that there are significant alterations in midline development in the Tg737(Delta)2-3(beta)Gal) homozygous embryos. We propose that the failure of ventral node cells to fully mature alters their ability to undergo differentiation as they migrate out of the node to contribute to the developing midline structures. Analysis of this new knockout allele allows us to define a critical role for the Tg737 gene during early embryogenesis. We have named the product of the Tg737 gene Polaris, which is based on the various polarity related defects associated with the different alleles of the Tg737 gene.

Glucose repression is a global regulatory mechanism in yeast. We have investigated how glucose regulates the <em>GAL</em> genes, which are required for galactose fermentation. We found that the <em>GAL</em> genes are controlled by a transcriptional cascade. Thus, <em>GAL</em>4, which encodes an activator of the <em>GAL</em> genes, is repressed by MIG1, a zinc finger protein that binds to the <em>GAL</em>4 promoter. MIG1 has a dual role in control of the <em>GAL</em> genes, since MIG1 also binds to the promoter of <em>GAL</em>1, a gene regulated by <em>GAL</em>4. A disruption of MIG1 interacts synergistically with a disruption of <em>GAL</em>80, a gene involved in galactose induction. This suggests that the MIG1-dependent response to glucose is amplified by down-regulation of the induction pathway.