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M Boshart

Publications and source records attributed to M Boshart.

At least 19 recordsLinked to original sources
PubMed2001

Deletion of a novel protein kinase with PX and FYVE-related domains increases the rate of differentiation of Trypanosoma brucei.

Growth control of African trypanosomes in the mammalian host is coupled to differentiation of a non-dividing life cycle stage, the stumpy bloodstream form. We show that a protein kinase with novel domain architecture is important for growth regulation. Zinc finger kinase (ZFK) has a kinase domain related to RAC and S6 kinases flanked by a FYVE-related zinc finger and a phox (PX) homology domain. To investigate the function of the kinase during cyclical development, a stable transformation procedure for bloodstream forms of differentiation-competent (pleomorphic) Trypanosoma brucei strains was established. Deletion of both allelic copies of ZFK by homologous recombination resulted in reduced growth of bloodstream-form parasites in culture, which was correlated with an increased rate of differentiation to the non-dividing stumpy form. Growth and differentiation rates were returned to wild-type level by ectopic ZFK expression. The phenotype is stage-specific, as growth of procyclic (insect form) trypanosomes was unaffected, and Deltazfk/Deltazfk clones were able to undergo full cyclical development in the tsetse fly vector. Deletion of ZFK in a differentiation-defective (monomorphic) strain of T. brucei did not change its growth rate in the bloodstream stage. This suggests a function of ZFK associated with the trypanosomes' decision between either cell cycle progression, as slender bloodstream form, or differentiation to the non-dividing stumpy form.

Amino Acid Sequence↗
PubMed2000

A developmentally regulated aconitase related to iron-regulatory protein-1 is localized in the cytoplasm and in the mitochondrion of Trypanosoma brucei.

Mitochondrial energy metabolism and Krebs cycle activities are developmentally regulated in the life cycle of the protozoan parasite Trypanosoma brucei. Here we report cloning of a T. brucei aconitase gene that is closely related to mammalian iron-regulatory protein 1 (IRP-1) and plant aconitases. Kinetic analysis of purified recombinant TbACO expressed in Escherichia coli resulted in a K(m) (isocitrate) of 3 +/- 0.4 mM, similar to aconitases of other organisms. This was unexpected since an arginine conserved in the aconitase protein family and crucial for substrate positioning in the catalytic center and for activity of pig mitochondrial aconitase (Zheng, L., Kennedy, M. C., Beinert, H., and Zalkin, H. (1992) J. Biol. Chem. 267, 7895-7903) is substituted by leucine in the TbACO sequence. Expression of the 98-kDa TbACO was shown to be lowest in the slender bloodstream stage of the parasite, 8-fold elevated in the stumpy stage, and increased a further 4-fold in the procyclic stage. The differential expression of TbACO protein contrasted with only minor changes in TbACO mRNA, indicating translational or post-translational mechanisms of regulation. Whereas animal cells express two distinct compartmentalized aconitases, mitochondrial aconitase and cytoplasmic aconitase/IRP-1, TbACO accounts for total aconitase activity in trypanosomes. By cell fractionation and immunofluorescence microscopy, we show that native as well as a transfected epitope-tagged TbACO localizes in both the mitochondrion (30%) and in the cytoplasm (70%). Together with phylogenetic reconstructions of the aconitase family, this suggests that animal IRPs have evolved from a multicompartmentalized ancestral aconitase. The possible functions of a cytoplasmic aconitase in trypanosomes are discussed.

Aconitate Hydratase↗
PubMed1999

Iron-dependent regulation of transferrin receptor expression in Trypanosoma brucei.

Transferrin is an essential growth factor for African trypanosomes. Here we show that expression of the trypanosomal transferrin receptor, which bears no structural similarity with mammalian transferrin receptors, is regulated by iron availability. Iron depletion of bloodstream forms of Trypanosoma brucei with the iron chelator deferoxamine resulted in a 3-fold up-regulation of the transferrin receptor and a 3-fold increase of the transferrin uptake rate. The abundance of expression site associated gene product 6 (ESAG6) mRNA, which encodes one of the two subunits of the trypanosome transferrin receptor, is regulated 5-fold by a post-transcriptional mechanism. In mammalian cells the stability of transferrin receptor mRNA is controlled by iron regulatory proteins (IRPs) binding to iron-responsive elements (IREs) in the 3'-untranslated region (UTR). Therefore, the role of a T. brucei cytoplasmic aconitase (TbACO) that is highly related to mammalian IRP-1 was investigated. Iron regulation of the transferrin receptor was found to be unaffected in Deltaaco::NEO/Deltaaco::HYG null mutants generated by targeted disruption of the TbACO gene. Thus, the mechanism of post-transcriptional transferrin receptor regulation in trypanosomes appears to be distinct from the IRE/IRP paradigm. The transferrin uptake rate was also increased when trypanosomes were transferred from medium supplemented with foetal bovine serum to medium supplemented with sera from other vertebrates. Due to varying binding affinities of the trypanosomal transferrin receptor for transferrins of different species, serum change can result in iron starvation. Thus, regulation of transferrin receptor expression may be a fast compensatory mechanism upon transmission of the parasite to a new host species.

Aconitate Hydratase↗
PubMed1997

Cell density triggers slender to stumpy differentiation of Trypanosoma brucei bloodstream forms in culture.

Differentiation from replicating slender forms to non-dividing stumpy bloodstream forms of T. brucei limits the parasite population size in the mammalian host in addition to and independently of the antibody response. Using a culture system for pleomorphic strains of T. brucei we show that slender forms very efficiently differentiate to stumpy forms in vitro and that the induction of differentiation is correlated to cell density. Differentiation in the host and in culture were compared using a battery of markers including cell morphology and volume, cell cycle position, the kinetics of the differentiation, expression of NADH dehydrogenase (diaphorase), expression of several differentially regulated transcripts and the kinetics of transformation to replicating procyclic forms after induction with cis-aconitate. By all available criteria, differentiation in culture reflects the natural process in the mammalian host. Time course experiments reveal a very tight temporal correlation between cell cycle arrest of bloodstream forms, appearance of a stumpy differentiation marker and the competence of a bloodstream form population to initiate transformation to procyclic forms in response to cis-aconitate. Our results show that induction of bloodstream form differentiation can occur independently of host-derived cues. We suggest a density sensing mechanism which induces differentiation to the non-dividing stumpy stage and thereby enables the parasite population to autoregulate its proliferation.

Aconitic Acid↗
PubMed1997

Trans mRNA splicing in trypanosomes: cloning and analysis of a PRP8-homologous gene from Trypanosoma brucei provides evidence for a U5-analogous RNP.

In trypanosomes all mRNAs are generated through trans mRNA splicing, requiring the functions of the small nuclear RNAs U2, U4 and U6. In the absence of conventional cis mRNA splicing, the structure and function of a U5-analogous snRNP in trypanosomes has remained an open question. In cis splicing, a U5 snRNP-specific protein component called PRP8 in yeast and p220 in man is a highly conserved, essential splicing factor involved in splice-site recognition and selection. We have cloned and sequenced a genomic region from Trypanosoma brucei, that contains a PRP8/p220-homologous gene (p277) coding for a 277 kDa protein. Using an antibody against a C-terminal region of the trypanosomal p277 protein, a small RNA of approximately 65 nucleotides could be specifically co-immunoprecipitated that appears to be identical with a U5 RNA (SLA2 RNA) recently identified by Dungan et al. (1996). Based on sedimentation, immunoprecipitation and Western blot analyses we conclude that this RNA is part of a stable ribonucleoprotein (RNP) complex and associated not only with the p277 protein, but also with the common proteins present in the other trans-spliceosomal snRNPs. Together these results demonstrate that a U5-analogous RNP exists in trypanosomes and suggest that basic functions of the U5 snRNP are conserved between cis and trans splicing.

Animals↗
PubMed1997

Differentiation of African trypanosomes is controlled by a density sensing mechanism which signals cell cycle arrest via the cAMP pathway.

Differentiation of African trypanosomes from replicating slender bloodstream forms to nondividing stumpy forms limits the parasite population size, allowing survival of the mammalian host and establishment of a stable host-parasite relationship. Using a novel in vitro culture system we have shown that slender to stumpy differentiation is induced by parasite density alone and thus is independent of host cues. Here we investigate the density sensing mechanism and show that trypanosomes release a soluble activity of low relative molecular mass, termed stumpy induction factor (SIF), which accumulates in conditioned medium. SIF activity triggers cell cycle arrest in G1/G0 phase and induces differentiation with high efficiency and rapid kinetics. Membrane-permeable derivates of cAMP or the phosphodiesterase inhibitor etazolate perfectly mimic SIF activity. Furthermore, SIF activity elicits an immediate two- to threefold elevation of intracellular cAMP content upon addition to slender forms. We conclude that SIF and hence density sensing operate through the cAMP signalling pathway. Temporal correlation of markers indicates that cell cycle arrest invariably precedes differentiation. Thus, our results indicate that the cell cycle regulation of bloodstream forms is under dominant control of cAMP signalling. Irreversible commitment to the quiescent state is elicited by a cAMP agonist within a period shorter than one complete cell cycle.

Animals↗
PubMed1996

High molecular mass agarose matrix supports growth of bloodstream forms of pleomorphic Trypanosoma brucei strains in axenic culture.

Primary axenic culture of Trypanosoma brucei bloodstream forms almost invariably requires a period of culture adaptation with cell death and clonal selection. This has been particularly difficult and in many cases unsuccessful for natural pleomorphic strains, which are characterized by their ability to differentiate from replicating long slender bloodstream forms into short stumpy forms. Here we show that a representative set of pleomorphic T. brucei strains can be cultured in vitro on agarose plates without any previous adaptation period and selection. The slender morphology was retained and the growth rate was identical to the growth rate in vivo. Long term in vitro culture for 3 months with this method did not affect the ability of the AnTat 1.1 strain to give rise to pleomorphic infections in mice. Stumpy populations emanating from these infections transformed rapidly and synchronously into dividing procyclic forms when triggered with cis-aconitate and a temperature shift. The growth supporting activity of agarose plates could be traced to high molecular mass polymeric agarose; beta-agarase digestion destroyed the activity. Membrane chamber experiments show that direct physical contact of trypanosomes with the agarose matrix is essential. In the absence of high molecular mass agarose, the cell division process is grossly impaired. We suggest that agarose mimics an interaction of trypanosomes with the host's extracellular matrix. Applications of the culture method are discussed.

Animals↗
PubMed1994

Constancy of expression of the protein kinase A regulatory subunit R1 alpha in hepatoma cell lines of different phenotypes.

In somatic hybrids between fibroblast microcells and rat hepatoma cells, tissue-specific extinguisher 1 (TSE1), localized to mouse chromosome 11, extinguishes the expression of tyrosine aminotransferase and phospho(enol)pyruvate carboxykinase. Recently, it was demonstrated that TSE1 corresponds to R1 alpha, a regulatory subunit of protein kinase A. Here, we have analyzed whether R1 alpha could play a role in differentiation of the hepatocyte. It is known that the TSE1/R1 alpha target genes belong to the group of neonatal functions that are repressed until birth. High expression of R1 alpha characterizes fetal-type BW1J hepatoma cells in which the neonatal target genes are silent. This R1 alpha is active in trans to extinguish these genes in hybrids between BW1J and Fao adult-type rat hepatoma cells. Reexpression of the target genes is correlated with loss of R1 alpha and/or overexpression of the mRNA for the hepatocyte-enriched transcription factors HNF4 and HNF3 alpha. Phenylalanine hydroxylase is shown to be another function negatively regulated by R1 alpha. In BW cells in which expression of phenylalanine hydroxylase has been activated (after either 5-aza-cytidine treatment or transfection with genomic DNA from adult-type hepatoma cells), no down-regulation of R1 alpha expression occurs: an independent mechanism overcomes R1 alpha repression. Finally, dedifferentiated derivatives of the adult-type rat hepatoma cells express neither the R1 alpha target genes nor the R1 alpha gene itself. Thus, in three different situations in which modulation of R1 alpha expression could be anticipated, it fails to occur.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Activation of the tyrosine aminotransferase gene is dependent on synergy between liver-specific and hormone-responsive elements.

Tyrosine aminotransferase (TAT; L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) gene activity is stimulated by glucocorticoids and glucagon and is repressed by insulin. Expression and responsiveness to the different signal transduction pathways are restricted to the liver, in which the gene is activated shortly after birth. Here we provide a model for the basis of this tissue specificity of the hormonal control. In the two enhancers mediating hormone induction of TAT gene activity we find the hormone response elements in combination with binding sites for constitutive liver-enriched transcription factors: proteins of the hepatocyte nuclear factor 3 family bind in the vicinity of the glucocorticoid response element located 2.5 kb upstream of the transcription start site, while hepatocyte nuclear factor 4 interacts with an essential element in the cAMP-responsive enhancer at -3.6 kb. By juxtaposing the liver-specific element and the target sequence of the signal transduction pathway the regulatory properties of either enhancer can be reconstituted. Thus, the interdependence of the respective enhancer motifs restricts the hormonal activation of the TAT gene to the liver. The coincidence of the onset of TAT gene expression around birth with the perinatal changes in the concentrations of glucocorticoids, glucagon, and insulin suggests cooperation of signal transduction pathways and cell type-specific transcription factors in the developmental activation of the TAT gene.

Animals↗
PubMed1993

Extinction of tyrosine aminotransferase gene activity in somatic cell hybrids involves modification and loss of several essential transcriptional activators.

Extinction is defined as the loss of cell type-specific gene expression that occurs in somatic cell hybrids derived by fusion of cells with dissimilar phenotypes. To explore the basis of this dominant-negative regulation, we have studied the activities of the control elements of the liver-specific gene encoding tyrosine aminotransferase (TAT) in hepatoma/fibroblast hybrid crosses. We show that extinction in complete somatic cell hybrids is accompanied by the loss of activity of all known cell type-specific control elements of the TAT gene. This inactivity is the result of first, lack of expression of genes coding for the transcriptional activators HNF4 and HNF3 beta and HNF3 gamma, which bind to essential elements of the enhancers; and second, loss of in vivo binding and activity of ubiquitous factors to these enhancers, including CREB, which is the target for repression by the tissue-specific extinguisher locus TSE1. Complete extinction of TAT gene activity is therefore a multifactorial process affecting all three enhancers controlling liver-specific and hormone-inducible expression. It results from lack of activation, rather than active repression, and involves both post-translational modification and loss of essential transcriptional activators.

Base Sequence↗
PubMed1992

The rat poly pyrimidine tract binding protein (PTB) interacts with a single-stranded DNA motif in a liver-specific enhancer.

We characterized and purified a protein from rat liver which specifically binds to a DNA motif present in a liver-specific enhancer of the rat tyrosine aminotransferase (TAT) gene, when offered as single-stranded DNA. Binding is highly sequence-specific and coincides with a region known to be essential for function of the enhancer. Microsequencing revealed that this protein is the rat homologue of the mouse and human poly Pyrimidine Tract binding protein (PTB), which has been shown to bind to premRNA and may participate in RNA splicing. This finding was corroborated by subsequent Western blot experiments using a PTB-specific antibody. These findings indicate a possible dual role for this protein in RNA processing and transcription.

Amino Acid Sequence↗
PubMed1992

Multiple mRNA isoforms of the transcription activator protein CREB: generation by alternative splicing and specific expression in primary spermatocytes.

We have characterized cDNA clones representing mouse CREB (cyclic AMP responsive element binding protein) mRNA isoforms. These include CREB delta and CREB alpha, of which the rat and human homologues have been previously identified. Both encode proteins with CRE-binding activity and identical transactivation potential. The additional CREB mRNA isoforms potentially encode CREB related proteins. From the structural organization of the mouse CREB gene we conclude that the multiple transcripts are generated by alternative splicing. Furthermore we show that specific CREB mRNA isoforms are expressed at a high level in the adult testis. Expression of these isoforms is induced after commencement of spermatogenesis. In situ hybridization suggests that this expression occurs predominantly in the primary spermatocytes. Comparison of the CREB gene with the recently isolated CREM (cAMP responsive element modulator) cDNAs illustrates that the two genes have arisen by gene duplication and have diverged to encode transcriptional activators and repressors of the cAMP signal transduction pathway.

Amino Acid Sequence↗
PubMed1992

Phosphorylation of CREB affects its binding to high and low affinity sites: implications for cAMP induced gene transcription.

Cyclic AMP treatment of hepatoma cells leads to increased protein binding at the cyclic AMP response element (CRE) of the tyrosine aminotransferase (TAT) gene in vivo, as revealed by genomic footprinting, whereas no increase is observed at the CRE of the phosphoenolpyruvate carboxykinase (PEPCK) gene. Several criteria establish that the 43 kDa CREB protein is interacting with both of these sites. Two classes of CRE with different affinity for CREB are described. One class, including the TATCRE, is characterized by asymmetric and weak binding sites (CGTCA), whereas the second class containing symmetrical TGACGTCA sites shows a much higher binding affinity for CREB. Both classes show an increase in binding after phosphorylation of CREB by protein kinase A (PKA). An in vivo phosphorylation-dependent change in binding of CREB increases the occupancy of weak binding sites used for transactivation, such as the TATCRE, while high affinity sites may have constitutive binding of transcriptionally active and inactive CREB dimers, as demonstrated by in vivo footprinting at the PEPCK CRE. Thus, lower basal level and higher relative stimulation of transcription by cyclic AMP through low affinity CREs should result, allowing finely tuned control of gene activation.

Amino Acid Sequence↗
PubMed1991

The tissue-specific extinguisher locus TSE1 encodes a regulatory subunit of cAMP-dependent protein kinase.

The tissue-specific extinguisher locus TSE1, a dominant negative regulator of transcription in somatic cell hybrids, acts via a cAMP response element (CRE) to repress activity of a hepatocyte-specific enhancer. Guided by the antagonism between TSE1 and cAMP-mediated signal transduction, we identified the regulatory subunit RI alpha of protein kinase A (PKA) as the product of the TSE1 locus. The evidence derives from concordant expression of RI alpha mRNA and TSE1 genetic activity, high resolution mapping of the RI alpha gene and TSE1 on human chromosome 17, and the ability of a transfected RI alpha cDNA to generate a phenocopy of TSE1-mediated extinction. The mechanism of TSE1/RI alpha-mediated extinction involves repression of basal PKA activity, reduced phosphorylation of CREB at Ser-133, and a corresponding reduction of in vivo protein binding at the target CRE.

Animals↗