Detection in Malaysia of a Borrelia sp. From Haemaphysalis hystricis (Ixodida: Ixodidae)

Spirochetes from the Borrelia genus are known to cause diseases in humans, namely Lyme disease and relapsing fever. These organisms are commonly transmitted to humans by arthropod vectors including ticks, mite, and lice. Here, we report the molecular detection of a Borrelia sp. from a Haemaphysalis hystricis Supino tick collected from wildlife in an Orang Asli settlement in Selangor, Malaysia. Phylogenetic analyses of partial 16s rRNA and flaB gene sequences revealed that the Borrelia sp. is closely related to the relapsing fever group borreliae, Borrelia lonestari, Borrelia miyamotoi, and Borrelia theileri, as well as a number of uncharacterized Borrelia sp. from ticks in Portugal and Japan. To our knowledge, this is the first report of a Borrelia sp. detected in H. hystricis, and in Malaysia. The zoonotic potential of this Borrelia sp. merits further investigation.

An EDMD mutation in C. elegans lamin blocks muscle-specific gene relocation and compromises muscle integrity

BACKGROUND

In worms, as in other organisms, many tissue-specific promoters are sequestered at the nuclear periphery when repressed and shift inward when activated. It has remained unresolved, however, whether the association of facultative heterochromatin with the nuclear periphery, or its release, has functional relevance for cell or tissue integrity.

RESULTS

Using ablation of the unique lamin gene in C. elegans, we show that lamin is necessary for the perinuclear positioning of heterochromatin. We then express at low levels in otherwise wild-type worms a lamin carrying a point mutation, Y59C, which in humans is linked to an autosomal-dominant form of Emery-Dreifuss muscular dystrophy. Using embryos and differentiated tissues, we track the subnuclear position of integrated heterochromatic arrays and their expression. In LMN-1 Y59C-expressing worms, we see abnormal retention at the nuclear envelope of a gene array bearing a muscle-specific promoter. This correlates with impaired activation of the array-borne myo-3 promoter and altered expression of a number of muscle-specific genes. However, an equivalent array carrying the intestine-specific pha-4 promoter is expressed normally and shifts inward when activated in gut cells of LMN-1 Y59C worms. Remarkably, adult LMN-1 Y59C animals have selectively perturbed body muscle ultrastructure and reduced muscle function.

CONCLUSION

Lamin helps sequester heterochromatin at the nuclear envelope, and wild-type lamin permits promoter release following tissue-specific activation. A disease-linked point mutation in lamin impairs muscle-specific reorganization of a heterochromatic array during tissue-specific promoter activation in a dominant manner. This dominance and the correlated muscle dysfunction in LMN-1 Y59C worms phenocopies Emery-Dreifuss muscular dystrophy.

Repetitive transgenes in C. elegans accumulate heterochromatic marks and are sequestered at the nuclear envelope in a copy-number- and lamin-dependent manner

Chromatin is nonrandomly distributed in nuclear space, yet the functional significance of this remains unclear. Here, we make use of transgenes carrying developmentally regulated promoters to study subnuclear gene positioning during the development of Caenorhabditis elegans. We found that small transgenes (copy number ≤50) are randomly distributed in early embryonic nuclei, independent of promoter activity. However, in differentiated tissues, these same transgenes occupied specific subnuclear positions: When promoters are repressed, transgenes are found at the nuclear periphery, whereas active, developmentally regulated promoters are enriched in the nuclear core. The absence of specific transgene positioning in embryonic nuclei does not reflect an absence of proteins that mediate perinuclear sequestration: Embryonic nuclei are able to sequester much larger transgene arrays (copy number 300-500) at the periphery. This size-dependent peripheral positioning of gene arrays in early embryos correlates with the accumulation of heterochromatic marks (H3K9me3 and H3K27me3) on large arrays. Interestingly, depletion of nuclear lamina components caused release of arrays from the nuclear envelope and interfered with their efficient silencing. Our results suggest that developmentally silenced chromatin binds the nuclear lamina in a manner correlated with the deposition of heterochromatic marks. Peripheral sequestration of chromatin may, in turn, support the maintenance of silencing.

The spatial dynamics of tissue-specific promoters during C. elegans development

To understand whether the spatial organization of the genome reflects the cell's differentiated state, we examined whether genes assume specific subnuclear positions during Caenorhabditis elegans development. Monitoring the radial position of developmentally controlled promoters in embryos and larval tissues, we found that small integrated arrays bearing three different tissue-specific promoters have no preferential position in nuclei of undifferentiated embryos. However, in differentiated cells, they shifted stably toward the nuclear lumen when activated, or to the nuclear envelope when silent. In contrast, large integrated arrays bearing the same promoters became heterochromatic and nuclear envelope-bound in embryos. Tissue-specific activation of promoters in these large arrays in larvae overrode the perinuclear anchorage. For transgenes that carry both active and inactive promoters, the inward shift of the active promoter was dominant. Finally, induction of master regulator HLH-1 prematurely induced internalization of a muscle-specific promoter array in embryos. Fluorescence in situ hybridization confirmed analogous results for the endogenous endoderm-determining gene pha-4. We propose that, in differentiated cells, subnuclear organization arises from the selective positioning of active and inactive developmentally regulated promoters. We characterize two forces that lead to tissue-specific subnuclear organization of the worm genome: large repeat-induced heterochromatin, which associates with the nuclear envelope like repressed genes in differentiated cells, and tissue-specific promoters that shift inward in a dominant fashion over silent promoters, when they are activated.

Dual functions of Mdt1 in genome maintenance and cell integrity pathways in Saccharomyces cerevisiae

Recent evidence indicates considerable cross-talk between genome maintenance and cell integrity control pathways. The RNA recognition motif (RRM)- and SQ/TQ cluster domain (SCD)-containing protein Mdt1 is required for repair of 3'-blocked DNA double-strand breaks (DSBs) and efficient recombinational maintenance of telomeres in budding yeast. Here we show that deletion of MDT1 (PIN4/YBL051C) leads to severe synthetic sickness in the absence of the genes for the central cell integrity MAP kinases Bck1 and Slt2/Mpk1. Consistent with a cell integrity function, mdt1Delta cells are hypersensitive to the cell wall toxin calcofluor white and the Bck1-Slt2 pathway activator caffeine. An RRM-deficient mdt1-RRM0 allele shares the severe bleomycin hypersensitivity, inefficient recombinational telomere maintenance and slt2 synthetic sickness phenotypes, but not the cell wall toxin hypersensitivity with mdt1Delta. However, the mdt1-RRM(3A) allele, where only the RNA-binding site is mutated, behaves similarly to the wild-type, suggesting that the Mdt1 RRM functions as a protein-protein interaction rather than a nucleic acid-binding module. Surprisingly, in a strain background where double mutants are sick but still viable, bck1Deltamdt1Delta and slt2Deltamdt1Delta mutants differ in some of their phenotypes, consistent with the emerging concept of flexible signal entry and exit points in the Bck1-Mkk1/2-Slt2 pathway. Overall, the results indicate that Mdt1 has partially separable functions in both cell wall and genome integrity pathways.

Cell cycle-dependent phosphorylation of Rad53 kinase by Cdc5 and Cdc28 modulates checkpoint adaptation

In budding yeast the evolutionarily conserved checkpoint response varies in its sensitivity to DNA damaging agents through the cell cycle. Specifically, higher amounts of damage are needed to activate the downstream checkpoint kinase Rad53 in S-phase cells. We examined here whether phosphorylation of Rad53 itself by cell cycle-dedicated kinases regulates Rad53 activation. We found that during unperturbed growth Rad53 exhibits a small phosphorylation-dependent electrophoretic mobility shift in G(2), M and G(1) phases of the cell cycle that is lost in S phase. We show that Rad53 is phosphorylated in vitro by Cdc5, a mitotic Polo-like kinase, and by the yeast cyclin-dependent kinase, Cdc28. Consistently, the cell cycle-dependent Rad53 mobility shift requires both Cdc5 and Cdc28 activities. We mapped the in vitro targeted phosphorylation sites by mass spectrometry and confirmed with mass spectroscopy that serines 774, 789 and 791 within Rad53 are phosphorylated in vivo in M-phase arrested cells. By creating nonphosphorylatable mutations in the endogenous RAD53 gene, we confirmed that the CDK and Polo kinase target sites are responsible for the observed cell cycle-dependent shift in protein mobility. The loss of phospho-acceptor sites does not interfere with Rad53 activation but accelerates checkpoint adaptation after induction of a single irreparable double-strand break. We thus demonstrate that cell cycle-dependent phosphorylation can fine-tune the response of Rad53 to DNA damage.

ATR/Mec1: coordinating fork stability and repair

During S phase, eukaryotic cells unwind and duplicate a tremendous amount of DNA, generating structures that are very sensitive to both endogenous and exogenous insults. The collision of DNA polymerases with damaged DNA or other obstructions to fork progression generates replication stress, which can evolve into fork collapse if the replisome components are not stabilized. To ensure genome integrity, stalled replication forks are recognized by a checkpoint, whose central player is the human kinase ATR or Mec1 in S. cerevisiae. This review will discuss recent findings revealing roles of the ATR/Mec1 kinase: both in stabilizing the replisome directly and in activating the checkpoint response to regulate origin firing, DNA repair, fork restart, and cell cycle progression.

Location-specific functions of the two forkhead-associated domains in Rad53 checkpoint kinase signaling

Signaling proteins often contain multiple modular protein-protein interaction domains of the same type. The Saccharomyces cerevisiae checkpoint kinase Rad53 contains two phosphothreonine-binding forkhead-associated (FHA) domains. To investigate if the precise position of these domains relative to each other is important, we created three rad53 alleles in which FHA1 and FHA2 domains were individually or simultaneously transposed to the opposite location. All three mutants were approximately 100-fold hypersensitive to DNA lesions whose survival requires intact Rad53 FHA domain functions, but they were not hypersensitive to DNA damage that is addressed in an FHA domain-independent manner. FHA domain-transposed Rad53 could still be recruited for activation by upstream kinases but then failed to autophosphorylate and activate FHA domain-dependent downstream functions. The results indicate that precise FHA domain positions are important for their roles in Rad53, possibly via regulation of the topology of oligomeric Rad53 signaling complexes.

Mdt1 facilitates efficient repair of blocked DNA double-strand breaks and recombinational maintenance of telomeres

DNA recombination plays critical roles in DNA repair and alternative telomere maintenance. Here we show that absence of the SQ/TQ cluster domain-containing protein Mdt1 (Ybl051c) renders Saccharomyces cerevisiae particularly hypersensitive to bleomycin, a drug that causes 3'-phospho-glycolate-blocked DNA double-strand breaks (DSBs). mdt1Delta also hypersensitizes partially recombination-defective cells to camptothecin-induced 3'-phospho-tyrosyl protein-blocked DSBs. Remarkably, whereas mdt1Delta cells are unable to restore broken chromosomes after bleomycin treatment, they efficiently repair "clean" endonuclease-generated DSBs. Epistasis analyses indicate that MDT1 acts in the repair of bleomycin-induced DSBs by regulating the efficiency of the homologous recombination pathway as well as telomere-related functions of the KU complex. Moreover, mdt1Delta leads to severe synthetic growth defects with a deletion of the recombination facilitator and telomere-positioning factor gene CTF18 already in the absence of exogenous DNA damage. Importantly, mdt1Delta causes a dramatic shift from the usually prevalent type II to the less-efficient type I pathway of recombinational telomere maintenance in the absence of telomerase in liquid senescence assays. As telomeres resemble protein-blocked DSBs, the results indicate that Mdt1 acts in a novel blocked-end-specific recombination pathway that is required for the efficiency of both drug-induced DSB repair and telomerase-independent telomere maintenance.

Telomere-related functions of yeast KU in the repair of bleomycin-induced DNA damage

Bleomycins are small glycopeptide cancer chemotherapeutics that give rise to 3'-modified DNA double-strand breaks (DSBs). In Saccharomyces cerevisiae, DSBs are predominantly repaired by RAD52-dependent homologous recombination (HR) with some support by Yku70/Yku80 (KU)-dependent pathways. The main DSB repair function of KU is believed to be as part of the non-homologous end-joining (NHEJ) pathway, but KU also functions in a "chromosome healing" pathway that seals DSBs by de novo telomere addition. We report here that rad52Deltayku70Delta double mutants are considerably more bleomycin hypersensitive than rad52Deltalig4Delta cells that lack the NHEJ-specific DNA ligase 4. Moreover, the telomere-specific KU mutation yku80-135i also dramatically increases rad52Delta bleomycin hypersensitivity, almost to the level of rad52Deltayku80Delta. The results indicate that telomere-specific functions of KU play a more prominent role in the repair of bleomycin-induced damage than its NHEJ functions, which could have important clinical implications for bleomycin-based combination chemotherapies.

FHA domain-ligand interactions: importance of integrating chemical and biological approaches

Combinatorial library screens based on binding affinity may preferentially select ligands with ability for ionic interactions and miss the biologically relevant ligands that bind more weakly with predominantly hydrophobic interactions.

Rad53 Kinase Activation-independent Replication Checkpoint Function of the N-terminal Forkhead-associated (FHA1) Domain

Saccharomyces cerevisiae Rad53 has crucial functions in many aspects of the cellular response to DNA damage and replication blocks. To coordinate these diverse roles, Rad53 has two forkhead-associated (FHA) phosphothreonine-binding domains in addition to a kinase domain. Here, we show that the conserved N-terminal FHA1 domain is essential for the function of Rad53 to prevent the firing of late replication origins in response to replication blocks. However, the FHA1 domain is not required for Rad53 activation during S phase, and as a consequence of defective downstream signaling, Rad53 containing an inactive FHA1 domain is hyperphosphorylated in response to replication blocks. The FHA1 mutation dramatically hypersensitizes strains with defects in the cell cycle-wide checkpoint pathways (rad9Delta and rad17Delta) to DNA damage, but it is largely epistatic with defects in the replication checkpoint (mrc1Delta). Altogether, our data indicate that the FHA1 domain links activated Rad53 to downstream effectors in the replication checkpoint. The results reveal an important mechanistic difference to the homologous Schizosaccharomyces pombe FHA domain that is required for Mrc1-dependent activation of the corresponding Cds1 kinase. Surprisingly, despite the severely impaired replication checkpoint and also G(2)/M checkpoint functions, the FHA1 mutation by itself leads to only moderate viability defects in response to DNA damage, highlighting the importance of functionally redundant pathways.

Mdt1, a novel Rad53 FHA1 domain-interacting protein, modulates DNA damage tolerance and G(2)/M cell cycle progression in Saccharomyces cerevisiae

The Rad53 kinase plays a central role in yeast DNA damage checkpoints. Rad53 contains two FHA phosphothreonine-binding domains that are required for Rad53 activation and possibly downstream signaling. Here we show that the N-terminal Rad53 FHA1 domain interacts with the RNA recognition motif, coiled-coil, and SQ/TQ cluster domain-containing protein Mdt1 (YBl051C). The interaction of Rad53 and Mdt1 depends on the structural integrity of the FHA1 phosphothreonine-binding site as well as threonine-305 of Mdt1. Mdt1 is constitutively threonine phosphorylated and hyperphosphorylated in response to DNA damage in vivo. DNA damage-dependent Mdt1 hyperphosphorylation depends on the Mec1 and Tel1 checkpoint kinases, and Mec1 can directly phosphorylate a recombinant Mdt1 SQ/TQ domain fragment. MDT1 overexpression is synthetically lethal with a rad53 deletion, whereas mdt1 deletion partially suppresses the DNA damage hypersensitivity of checkpoint-compromised strains and generally improves DNA damage tolerance. In the absence of DNA damage, mdt1 deletion leads to delayed anaphase completion, with an elongated cell morphology reminiscent of that of G(2)/M cell cycle mutants. mdt1-dependent and DNA damage-dependent cell cycle delays are not additive, suggesting that they act in the same pathway. The data indicate that Mdt1 is involved in normal G(2)/M cell cycle progression and is a novel target of checkpoint-dependent cell cycle arrest pathways.

Diverse but overlapping functions of the two forkhead-associated (FHA) domains in Rad53 checkpoint kinase activation

Forkhead-associated (FHA) domains are phosphothreonine-binding modules prevalent in proteins with important cell cycle and DNA damage response functions. The yeast checkpoint kinase Rad53 is unique in containing two FHA domains. We have generated novel recessive rad53 alleles with abolished FHA domain functions resulting from Ala substitution of the critical phosphothreonine-binding residues Arg70 and Arg605. In asynchronous cells, inactivation of the N-terminal FHA1 domain did not impair Rad53 activation and downstream functions, whereas inactivation of the C-terminal FHA2 domain led to reduced Rad53 activation and significantly increased DNA damage sensitivity. Simultaneous inactivation of both FHA domains abolished Rad53 activation and all downstream functions and dramatically increased the sensitivity to DNA damage and replication blocks similar to kinase-defective and rad53 null alleles, but did not compromise the essential viability function of Rad53. Interestingly, in G2/M synchronized cells, mutation of either FHA domain prevented Rad53 activation and impaired the cell cycle arrest checkpoint. Our data demonstrate that both FHA domains are required for normal Rad53 functions and indicate that the two FHA domains have differential but partially overlapping roles in Rad53 activation and downstream signaling.

FHA domains as phospho-threonine binding modules in cell signaling

Forkhead-associated (FHA) domains are present in >200 diverse proteins in all phyla from bacteria to mammals and seem to be particularly prevalent in proteins with cell cycle control functions. Recent work from several laboratories has considerably improved our understanding of the structure and function of these domains that were virtually unknown a few years ago, and the first disease associations of FHA domains have now emerged. FHA domains form 11-stranded beta-sandwiches that contain some 100-180 amino acid residues with a high degree of sequence diversity. FHA domains act as phosphorylation-dependent protein-protein interaction modules that preferentially bind to phospho-threonine residues in their targets. Interestingly, point mutations in the human CHK2 gene that lead to single-residue amino acid substitutions in the FHA domain of this cell cycle checkpoint kinase have been found to cause a subset of cases of the Li-Fraumeni multi-cancer syndrome.

Posttranscriptional regulation of the RAD5 DNA repair gene by the Dun1 kinase and the Pan2-Pan3 poly(A)-nuclease complex contributes to survival of replication blocks

The yeast Dun1 kinase has complex checkpoint functions including DNA damage-dependent cell cycle arrest in G(2)/M, transcriptional induction of repair genes, and regulation of postreplicative DNA repair pathways. Here we report that the Dun1 forkhead-associated domain interacts with the Pan3 subunit of the poly(A)-nuclease complex and that dun1pan2 and dun1pan3 double mutants are dramatically hypersensitive to replicational stress. This phenotype was independent of the function of Dun1 in regulating deoxyribonucleotide levels as it was also observed in strains lacking the ribonucleotide reductase inhibitor Sml1. dun1pan2 mutants initially arrested normally in response to replication blocks but died in the presence of persistent replication blocks with considerably delayed kinetics compared with mutants lacking the Rad53 kinase, indicating that the double mutation does not compromise the intra-S phase checkpoint. Interestingly, the RAD5 gene involved in error-free postreplication repair pathways was specifically up-regulated in dun1pan2 double mutants. Moreover, inducible overexpression of RAD5 mimicked the double mutant phenotype by hypersensitizing dun1 mutants to replication blocks. The data indicate that Dun1 and Pan2-Pan3 cooperate to regulate the stoichiometry and thereby the activity of postreplication repair complexes, suggesting that posttranscriptional mechanisms complement the transcriptional response in the regulation of gene expression by checkpoint signaling pathways in Saccharomyces cerevisiae.

Role of the N-terminal forkhead-associated domain in the cell cycle checkpoint function of the Rad53 kinase

Forkhead-associated (FHA) domains are multifunctional phosphopeptide-binding modules and are the hallmark of the conserved family of Rad53-like checkpoint protein kinases. Rad53-like kinases, including the human tumor suppressor protein Chk2, play crucial roles in cell cycle arrest and activation of repair processes following DNA damage and replication blocks. Here we show that ectopic expression of the N-terminal FHA domain (FHA1) of the yeast Rad53 kinase causes a growth defect by arresting the cell cycle in G(1). This phenotype was highly specific for the Rad53-FHA1 domain and not observed with the similar Rad53-FHA2, Dun1-FHA, and Chk2-FHA domains, and it was abrogated by mutations that abolished binding to a phosphothreonine-containing peptide in vitro. Furthermore, replacement of the RAD53 gene with alleles containing amino acid substitutions in the FHA1 domain resulted in an increased DNA damage sensitivity in vivo. Taken together, these data demonstrate that the FHA1 domain contributes to the checkpoint function of Rad53, possibly by associating with a phosphorylated target protein in response to DNA damage in G(1).

FHA domain boundaries of the dun1p and rad53p cell cycle checkpoint kinases

Dun1p and Rad53p of the budding yeast Saccharomyces cerevisiae are members of a conserved family of cell cycle checkpoint protein kinases that contain forkhead-associated (FHA) domains. Here, we demonstrate that these FHA domains contain 130-140 residues, and are thus considerably larger than previously predicted by sequence comparisons (55-75 residues). In vivo, expression of the proteolytically defined Dun1p FHA domain, but not a fragment containing only the predicted domain boundaries, inhibited the transcriptional induction of repair genes following replication blocks. This indicates that the non-catalytic FHA domain plays an important role in the transcriptional function of the Dun1p protein kinase.

Genomic mapping of chromosomal region 2p15-p21 (D2S378-D2S391): integration of Genemap'98 within a framework of yeast and bacterial artificial chromosomes

The region of chromosome 2 encompassed by the polymorphic markers D2S378 (centromeric) and D2S391 (telomeric) spans an approximately 10-cM distance in cytogenetic bands 2p15-p21. This area is frequently involved in cytogenetic alterations in human cancers. It also harbors the genes for several genetic disorders, including Type I hereditary nonpolyposis colorectal cancer (HNPCC), familial male precocious puberty (FMPP), Carney complex (CNC), Doyne's honeycomb retinal dystrophy (DHRD), and one form of familial dyslexia (DYX-3). Only a handful of known genes have been mapped to 2p16. These include MSH2, which is responsible for HNPCC, FSHR, the gene responsible for FMPP, EFEMP-1, the gene mutated in DHRD, GTBP, a DNA repair gene, and SPTBN1, nonerythryocytic beta-spectrin. The genes for CNC and DYX-3 remain unknown, due to lack of a contig of this region and its underrepresentation in the existing maps. This report presents a yeast- and bacterial-artificial chromosome (YAC and BAC, respectively) resource for the construction of a sequence-ready map of 2p15-p21 between the markers D2S378 and D2S391 at the centromeric and telomeric ends, respectively. The recently published Genemap'98 lists 146 expressed sequence tags (ESTs) in this region; we have used our YAC-BAC map to place each of these ESTs within a framework of 40 known and 3 newly cloned polymorphic markers and 37 new sequence-tagged sites. This map provides an integration of genetic, radiation hybrid, and physical mapping information for the region corresponding to cytogenetic bands 2p15-p21 and is expected to facilitate the identification of disease genes from the area.

Mutations in the human Jagged1 gene are responsible for Alagille syndrome

Alagille syndrome (AGS) is an autosomal-dominant disorder characterized by intrahepatic cholestasis and abnormalities of heart, eye and vertebrae, as well as a characteristic facial appearance. Identification of rare AGS patients with cytogenetic deletions has allowed mapping of the gene of 20p12. We have generated a cloned contig of the critical region and used fluorescent in situ hybridization on cells from patients with submicroscopic deletions to narrow the candidate region to only 250 kb. Within this region we identified JAG1, the human homologue of rat Jagged1, which encodes a ligand for the Notch receptor. Cell-cell Jagged/Notch interactions are known to be critical for determination of cell fates in early development, making this an attractive candidate gene for a developmental disorder in humans. Determining the complete exon-intron structure of JAG1 allowed detailed mutational analysis of DNA samples from non-deletion AGS patients, revealing three frame-shift mutations, two splice donor mutations and one mutation abolishing RNA expression from the altered allele. We conclude that AGS is caused by haploinsufficiency of JAG1.

A 2.8-Mb clone contig of the multiple endocrine neoplasia type 1 (MEN1) region at 11q13

Multiple endocrine neoplasia type 1 (MEN1) is an autosomal dominant disorder that results in parathyroid, anterior pituitary, and pancreatic and duodenal endocrine tumors in affected individuals. The MEN1 locus is tightly linked to the marker PYGM on chromosome 11q13, and linkage analysis has placed the MEN1 gene within a 2-Mb interval flanked by D11S1883 and D11S449. As a step toward cloning the MEN1 gene, we have constructed a 2.8-Mb clone contig consisting of YAC and bacterial clones (PAC, BAC, and P1) for the D11S480 to D11S913 region. The bacterial clones alone represent nearly all of the 2.8-Mb contig. The contig was assembled based on a high-density STS-content analysis of 79 genomic clones (YAC, PAC, BAC, and P1) with 118 STSs. The STSs included 22 polymorphic markers and 20 transcripts, with the remainder primarily derived from the end sequences of the genomic clones. An independent cosmid contig for the 1-Mb PYGM-SEA region was also generated. Support for correctness of the 2.8-Mb contig map comes from an independent ordering of the clones by fiber-FISH. This sequence-ready contig will be a useful resource for positional cloning of MEN1 and other disease genes whose loci fall within this region.

Interferon-gamma downregulates the proliferative response of hapten-specific B cells stimulated by antigen and cytokines

IFN-gamma plays a role in many aspects of cellular interactions, both positive and negative. Among its functions during the immune response, the antagonistic effects of IFN-gamma and IL-4 are well documented. Observations in our laboratory suggested that IFN-gamma could also interfere with the activation of single, antigen-specific B cells by antigen and other cytokines. Closer examination revealed that IFN-gamma reduced the number of proliferating cell clones in response to antigen and a variety of cytokines, alone or in combination. Cell viability remained at the initial level and the cells were still able to produce Ig, albeit to a lesser extent than in the absence of IFN-gamma. On the other hand, the frequency of IgM secreting clones was not affected, whereas the total amount of secreted IgM was lower in the presence of IFN-gamma, probably due to the reduced cell number and a decrease in Ig production. In addition, proliferation was prevented when B cells were pre-incubated with IFN-gamma and then stimulated by other cytokines. Kinetic studies revealed that INF-gamma had to be present from the onset of culture because delayed addition did not inhibit the proliferation of the B cells. After its initial action, IFN-gamma could be removed without abolishing the negative signal for proliferation. From these results it can be concluded that IFN-gamma transmits a signal that causes B cells to stop proliferating and prevents them from forming large clones.

Effects of combinations of cytokines on murine antigen-specific B lymphocytes

Cytokines are among the most important mediators in the immune response. They act on B lymphocytes at various stages along the activation pathway. To study the effects of combinations of cytokines, we have used an antigen-specific single B cell-cloning system devised in this laboratory. We report here that IL-1 can enhance the proliferative response and IgM secretion by B cells induced by IL-6. The results of a similar study demonstrated that IL-1 can also enhance the effects of IL-5 on B cell proliferation and IgM secretion in an additive manner. Kinetic analyses showed that IL-1 had to be present from the beginning of the culture for an optimum cooperative effect, whereas the addition of IL-6 could be delayed up to 2 days without a significant reduction of the response. In contrast, IL-5 had to be added together with IL-1 at the onset of culture to promote an optimum response. The responses elicited by IL-1 plus IL-6, IL-1 plus IL-5, and IL-4 plus IL-5 were almost identical. The addition of further cytokines to these cultures gave no enhancement above the effects observed with the two-cytokine combinations.

Spontaneous and cytokine-inducible 'natural' immunoglobulin secreting cells in organized lymphoid tissues of mice

The number and frequency of spontaneous and cytokine-inducible 'natural' immunoglobulin-secreting cells (ISC) were determined in bone marrow (BM), spleen and Peyer's patch (PP), in vitro. Cells were cultured at limiting dilution in the presence or absence of exogenous recombinant cytokines and supernatants then assayed for total immunoglobulin (Ig) and Ig isotype using an ELISA. Most spontaneous ISC were found in the spleen and BM, with fewer in PP. The addition of recombinant interleukin 5 (rIL-5) promoted a marked increase in both the ISC frequency and the amount of Ig secreted/ISC whereas recombinant IL-6 (rIL-6) promoted only a marginal increase. Recombinant IL-4 (rIL-4) promoted a marginal increase in ISC frequency only. The isotype profile of ISC was in the order IgM greater than IgG2 greater than IgA greater than IgG3 greater than IgG1. The exposure of cells to 1200 rad of gamma-radiation resulted in decreased numbers of spontaneous ISC in all tissues, but the addition of rIL-5 or rIL-6 to the irradiated cells increased both the ISC frequency and Ig secreted. The Ig isotype profile was similar to that of non-irradiated ISC with a few minor exceptions. This large population of potential cytokine-inducible ISC could contribute to 'natural' Ig secretion in vivo.

The response of human B cells to interleukin 4 is determined by their stage of activation and differentiation

The effect of purified recombinant human interleukin 4 (IL-4) on proliferation and IgM secretion of normal and malignant human B cells was studied. IL-4 was found to co-stimulate the proliferation of splenic B cells in the presence of anti-Ig coupled to polyacrylamide beads (anti-Ig beads) for a period of 4 days. In contrast, IL-4 had little co-stimulatory effect on the proliferative response of splenic B cells to the more potent mitogen Staphylococcus aureus Cowan strain 1 (SAC). Moreover, IL-4 inhibited interleukin 2 (IL-2)-induced proliferation of cells co-stimulated with SAC. Mitogen-induced pre-activation of B cells in the presence of IL-4 resulted in a reduction in subsequent IL-2-induced IgM secretion without significantly affecting proliferation. Human B-cell tumours were also cultured over a 2-3 day period in the presence of anti-Ig beads plus IL-2, or IL-4 or both IL-2 and IL-4. IL-4 inhibited IL-2-induced proliferation in all cases of B-cell chronic lymphocytic leukaemia (B-CLL) and the majority of cases of low-grade lymphoma (LGL) and hairy cell leukaemia (HCL). These findings suggest that IL-4 has stimulatory actions on resting B cells, most evident in the presence of submaximal co-mitogenic signals, and inhibitory actions on activated B cells, especially antagonism of the effects of IL-2.