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[Identification and characterization of alternativly splicing variants for murine mater gene].

Mater encoding an oocyte-specific autoantigen,and is associated with premature autoimmune ovarian dysgenesis (AOD) in mouse. Based on RT-PCR, cDNA cloning, screening, sequencing and analysis, we have detected a total of four Mater splice variants, designated as variant B, E, F and G. All these splicing forms are in frame in terms of expected protein products. Among these, B was consistent with the previous report, whereas E, F, G belong to novel splice variants that have not been reported previously. Variant E lacks exon 6, variant F both lacks exon 10 and retains a part of intron 8, variant G lacks part of exon 14, and variant H lacks part of exon 13. The cDNA sequences at all the exon-intron boundaries confirms to the "GT-AG" splicing rule. Variant B, E, F exist in all the four strains. Variant G exists only in SWR/J. According to the cDNA sequences of these four splice variants, amimo acid sequences of the corresponding expected protein isoforms were deduced, and their potential functional effects were predicted in this thesis. Further identification and characterization of these expected protein isoforms would provide valuable information for their functional importance.

Alternative Splicing↗

Some G protein heterotrimers physically dissociate in living cells.

Heterotrimeric G proteins mediate physiological processes ranging from phototransduction to cell migration. In the accepted model of G protein signaling, Galphabetagamma heterotrimers physically dissociate after activation, liberating free Galpha subunits and Gbetagamma dimers. This model is supported by evidence obtained in vitro with purified proteins, but its relevance in vivo has been questioned. Here, we show that at least some heterotrimeric G protein isoforms physically dissociate after activation in living cells. Galpha subunits extended with a transmembrane (TM) domain and cyan fluorescent protein (CFP) were immobilized in the plasma membrane by biotinylation and cross-linking with avidin. Immobile CFP-TM-Galpha greatly decreased the lateral mobility of intracellular Gbeta1gamma2-YFP, indicating the formation of stable heterotrimers. A GTPase-deficient (constitutively active) mutant of CFP-TM-GalphaoA lost the ability to restrict Gbeta1gamma2-YFP mobility, whereas GTPase-deficient mutants of CFP-TM-Galphai3 and CFP-TM-Galphas retained this ability. Activation of cognate G protein-coupled receptors partially relieved the constraint on Gbeta1gamma2-YFP mobility induced by immobile CFP-TM-GalphaoA and CFP-TM-Galphai3 but had no effect on the constraint induced by CFP-TM-Galphas. These results demonstrate the physical dissociation of heterotrimers containing GalphaoA and Galphai3 subunits in living cells, supporting the subunit dissociation model of G protein signaling for these subunits. However, these results are also consistent with the suggestion that G protein heterotrimers (e.g., Galphas) may signal without physically dissociating.

Animals↗

Human epidermal growth factor (EGF) module-containing mucin-like hormone receptor 3 is a new member of the EGF-TM7 family that recognizes a ligand on human macrophages and activated neutrophils.

The epidermal growth factor (EGF)-TM7 subgroup of G-protein-coupled receptors is composed predominantly of leukocyte-restricted glycoproteins defined by their unique hybrid structure, in which extracellular EGF-like domains are coupled to a seven-span transmembrane moiety via a mucin-like stalk. The EGF-TM7 group comprises mouse F4/80, human EGF module-containing mucin-like hormone receptor (EMR) 1, human EMR2, and human and mouse CD97, the genes for which map to human chromosome 19p13 and the syntenic regions of the mouse genome. In this study we describe the cloning and characterization of EMR3, a novel human EGF-TM7 molecule, and show the existence of its cellular ligand. The EMR3 gene maps closely to the existing members of the EGF-TM7 family on human chromosome 19p13.1 and, in common with other EGF-TM7 genes, is capable of generating different protein isoforms through alternative splicing. Two alternative splice forms have been isolated: one encoding a 652-amino acid cell surface protein consisting of two EGF-like domains, a mucin stalk, and a putative G-protein-coupled receptor domain and the other encoding a truncated soluble form containing only two EGF-like domains. As with other members of the EGF-TM7 family, EMR3 mRNA displays a predominantly leukocyte-restricted expression pattern, with highest levels in neutrophils, monocytes, and macrophages. Through the use of soluble EMR3 multivalent probes we have shown the presence of a ligand at the surface of monocyte-derived macrophages and activated human neutrophils. These interactions suggest a potential role for EMR3 in myeloid-myeloid interactions during immune and inflammatory responses.

Alternative Splicing↗

Down-regulation of mitochondrial transcription factor A during spermatogenesis in humans.

Mitochondrial transcription factor A (mtTFA) is a key activator of mitochondrial transcription in mammals. It also has a role in mitochondrial DNA (mtDNA) replication, since transcription generates an RNA primer necessary for initiation of mtDNA replication. In the mouse, testis-specific mtTFA transcripts encode a protein isoform that is imported to the nucleus rather than into mitochondria of spermatocytes and elongating spermatids. We now report molecular characterization of human mtTFA (h-mtTFA) expression in somatic tissues and male germ cells. Similarly to the mouse, analysis of cDNAs and Northern blots identified abundant testis-specific transcript isoforms generated by use of alternate transcription initiation sites. However, unlike the mouse, none of the testis-specific transcripts predicts a nuclear protein isoform, and Western blot analysis identified only the mitochondrial form of h-mtTFA in human testis. Immunohistochemistry and in situ were used to compare the distribution of mtTFA protein, testis-specific mtTFA transcripts, mtDNA and mtRNA in sections of human testis. Our results show that the mtTFA protein and mtDNA exhibit parallel gradients with high levels in undifferentiated male germ cells and low levels or an absence in different male germ cells. Testis-specific transcripts exhibit the opposite pattern, suggesting that in both humans and mice, these testis-specific mtTFA transcripts down-regulate mtTFA protein levels in mammalian mitochondria. Our findings demonstrate that mtTFA does not have a critical role in the nucleus, suggest a mechanism for reducing mtDNA copy number during spermatogenesis and have implications for the understanding of maternal transmission of mtDNA.

Animals↗

Different functional forms of G-protein beta gamma-subunits, beta gamma-I and beta gamma-II, in bovine brain.

Heterotrimeric GTP binding regulatory proteins (G proteins) are involved in the signal transduction process in cells. We have previously demonstrated that G protein (Gi/o) in bovine brain contains two subspecies of the beta gamma-subunit, beta gamma-I and beta gamma-II, with distinct gamma subunits, i.e., gamma-I and gamma-II, but identical beta-subunit. We found that gamma-I agreed with the gamma-subunit reported elsewhere, while gamma-II was a novel gamma-subunit. In the present study we separated the fractions containing G-protein isoforms, Go*, Gi1, Go and Gi2, with a Mono Q column and they were subjected to 15% polyacrylamide gel electrophoresis. All isoforms were shown to possess both gamma subunits, gamma-I and gamma-II. The molar ratio of the two gamma-subunit isoforms was one to one, based on the relative intensity of Coomassie blue-stained bands. Differences in biological activity between G proteins composed of beta gamma-I and beta gamma-II were investigated. The amount of GTP gamma S bound to the alpha-subunit was larger in alpha beta gamma-I than in alpha beta gamma-II. The pertussis toxin-catalyzed ADP-ribosylation on alpha-subunit was enhanced by either beta gamma-subunit subspecies, but the effect was larger with beta gamma-I than with beta gamma-II. From gel filtration with a Sephacryl S 300, it appeared that all alpha- and either beta gamma-subunit, i.e., beta gamma-I or beta gamma-II, formed a trimer complex. These findings suggest the possible existence of two different functional forms in each G-protein isoform depending on the beta gamma-subunit subspecies.

Animals↗

Conditional rescue of olfactory learning and memory defects in mutants of the 14-3-3zeta gene leonardo.

Members of the ubiquitous 14-3-3 family of proteins are abundantly expressed in metazoan neurons. The Drosophila 14-3-3zeta gene leonardo is preferentially expressed in adult mushroom bodies, centers of insect learning and memory. Mutants exhibit defects in olfactory learning and memory and physiological neuroplasticity at the neuromuscular junction. Because strong mutations in this gene are lethal, we investigated the nature of the defects that precipitate the learning and memory deficit and the role of the two protein isoforms encoded by leonardo in these processes. We find that the behavioral deficit in the mutants is not caused by aberrant development, LEONARDO protein is acutely required for learning and memory, and both protein isoforms can function equivalently in embryonic development and behavioral neuroplasticity.

14-3-3 Proteins↗

Adult human subventricular, subgranular, and subpial zones contain astrocytes with a specialized intermediate filament cytoskeleton.

Human glial fibrillary acidic protein-delta (GFAP-delta) is a GFAP protein isoform that is encoded by an alternative splice variant of the GFAP-gene. As a result, GFAP-delta protein differs from the predominant splice form, GFAP-alpha, by its C-terminal protein sequence. In this study, we show that GFAP-delta protein is not expressed by all GFAP-expressing astrocytes but specifically by a subpopulation located in the subpial zone of the cerebral cortex, the subgranular zone of the hippocampus, and, most intensely, by a ribbon of astrocytes following the ependymal layer of the cerebral ventricles. Therefore, at least in the sub ventricular zone (SVZ), GFAP-delta specifically marks the population of astrocytes that contain the neural stem cells in the adult human brain. Interestingly, the SVZ astrocytes actively splice GFAP-delta transcripts, in contrast to astrocytes adjacent to this layer. Furthermore, we show that GFAP-delta protein, unlike GFAP-alpha, is not upregulated in astrogliosis. Our data therefore indicate a different functional role for GFAP-delta in astrocyte physiology. Finally, transfection studies showed that GFAP-delta protein expression has a negative effect on GFAP filament formation, and therefore could be important for modulating intermediate filament cytoskeletal properties, possibly facilitating astrocyte motility. Further studies on GFAP-delta and the cells that express it are important for gaining insights into its function during differentiation, migration and during health and disease.

Alternative Splicing↗

Blood coagulation and alternative pre-mRNA splicing: an overview.

Throughout the 20th century, great advances were made in understanding of how blood coagulation occurs, what physiological and biochemical mechanisms are responsible for its regulation, and what genes and their protein products comprise the essential components of the hemostatic network. Recently, complete sequencing of the human genome revealed that the structural diversity of higher eukaryotes cannot be solely attributed to the number of protein-encoding genes, whereas tools of molecular biology helped establish that pre-mRNAs produced by most protein-encoding genes undergo alternative splicing, a mechanism that enables production of multiple protein isoforms by a single gene. Research in the field of thrombosis and hemostasis revealed that the genes encoding several critical proteins at various junctures of the coagulation cascade produce alternatively spliced protein isoforms with distinct structural and biochemical characteristics, revealing a principally novel dimension in the regulation of blood clotting and, possibly, a few novel therapeutic approaches to treatment of abnormal hemostasis. This review summarizes recently published data pertaining to biosynthesis of the alternatively spliced isoforms of tissue factor (TF, or coagulation factor III), tissue factor pathway inhibitor (TFPI), and coagulation factor XI (FXI), and discusses future directions of this continuously evolving area of biomedical research, with an emphasis on molecular mechanics responsible for regulation of constitutive as well as alternative pre-mRNA splicing.

Alternative Splicing↗

Disruption of Netrin G1 by a balanced chromosome translocation in a girl with Rett syndrome.

We have identified a girl with characteristic features of Rett syndrome (RTT) who carries a de novo balanced translocation involving chromosomes 1 and 7. Both breakpoints were mapped by fluorescence in situ hybridization with selected genomic clones from the regions of interest. Southern blot hybridisations, utilizing probes derived from breakpoint spanning BACs, detected several aberrant fragments specific for the patient. Sequence analysis of the cloned junction fragment indicated that on chromosome 1 the predominantly brain-expressed Netrin G1 (NTNG1) gene is disrupted, whereas on chromosome 7 there was no indication for a truncated gene. The chromosome 1 breakpoint lies within the 3' part of NTNG1 and affects alternatively spliced transcripts, suggesting that the phenotype in this patient is the result of disturbed NTNG1 expression. In silico translation of the NTNG1 splice variants predicted protein isoforms with different C-termini: one membrane bound through a glycosylphosphatidylinositol anchor and the other soluble. The membrane-bound protein isoform would be affected by the breakpoint, whereas the soluble form would remain intact. Our results suggest that the central nervous system is sensitive to NTNG1 expression levels and that NTNG1 is a novel candidate disease gene for RTT.

Base Sequence↗

Evidence for a new, high-molecular weight isoform of protein kinase C in rat hippocampus.

We describe a new form of protein kinase C (PKC) with a molecular weight of 97 kDa, higher than the known forms of vertebrate PKC. This putative new high-molecular weight isoform, which we are calling PKC (HMW), is increased in the membrane fraction either upon application of phorbol esters or with afferent synaptic stimulation of Schaffer collaterals in hippocampal slices. The protein cross-reacts on immunoblot with affinity-purified polyclonal antiserum raised against a peptide derived from the carboxy-terminus of PKC eta; it does not cross-react, however, with antiserum against the amino-terminal region of PKC eta. In the tissues examined, PKC(HMW) is localized primarily in brain, in contrast to PKC eta, which is found predominantly in lung and skin.

Amino Acid Sequence↗

Cellular distribution of isoforms of protein kinase C (PKC) in pancreatic acini.

As in a previous study (Biochim, Biophys. Acta 1224 (1994) 127-138), we used quantitative immunoblot analysis and found that rat pancreatic acini possess four different isoforms of PKC-alpha, delta, epsilon and zeta. The phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) caused translocation of each isoform from the cytosol to the membrane fraction. CCK-8 increased diacylglycerol (DAG) and caused translocation of PKC-sigma and PKC-epsilon but not that of PKC-alpha or PKC-zeta. L-364,718, a CCK receptor antagonist, prevented as well as reversed the effects of CCK-8 on DAG and on translocation of PKC-sigma and PKC-epsilon. To explore the possibility that different isoforms of PKC might have different distributions in rat pancreas, we used immunocytochemistry to determine the cellular distribution of different isoforms of PKC in intact pancreas as well as pancreatic acini. In intact pancreas, PKC-alpha and PKC-sigma were detected in islet cells but not in duct or acinar cells. PKC-epsilon was detected in the apical region of acinar cells and PKC-zeta was detected over the luminal surfaces of acinar cells and the ductules that extend from the acinus. Neither PKC-epsilon nor PKC-zeta was detected in islets. In pancreatic acini PKC-alpha and PKC-sigma were detected in islets or fragments of islets that contaminated the preparation but were not detected in acinar cells. PKC-epsilon was detected in the apical region of acinar cells and adding 1 microM TPA or 1 microM CCK-8 accentuated the immunostaining but did not alter its cellular distribution. L-364,718 reversed the changes in immunostaining caused by CCK-8. PKC-zeta was detected over the luminal surface of the acinar cells. TPA, but not CCK-8 or CCK-8 followed by L-364,718, increased the number of acini that showed staining of the luminal surfaces of acinar cells. Thus, the present results demonstrate that different isoforms of PKC are distributed differently in rat pancreas and that the different patterns of distribution can explain, at least in part, the different responses to CCK-8.

Animals↗

An apolipoprotein E-based therapeutic improves outcome and reduces Alzheimer's disease pathology following closed head injury: evidence of pharmacogenomic interaction.

Apolipoprotein E (apoE) modifies glial activation and the CNS inflammatory response in an isoform-specific manner. Peptides derived from the receptor-binding region of apoE have been demonstrated to maintain the functional activity of the intact protein, and to improve histological and functional deficits after closed head injury. In the current study, APOE2, APOE3, and APOE4 targeted replacement (TR) mice expressing the human apoE protein isoforms (apoE2, apoE3 and apoE4) were used in a clinically relevant model of closed head injury to assess the interaction between the humanized apoE background and the therapeutic apoE mimetic peptide, apoE(133-149). Treatment with the apoE-mimetic peptide reduced microglial activation and early inflammatory events in all of the targeted replacement animals and was associated with histological and functional improvement in the APOE2TR and APOE3TR animals. Similarly, brain beta amyloid protein (Abeta)(1-42) levels were increased as a function of head injury in all of the targeted replacement mice, while treatment with apoE peptide suppressed Abeta(1-42) levels in the APOE2TR and APOE3TR animals. These results suggest a pharmacogenomic interaction between the therapeutic effects of the apoE mimetic peptide and the human apoE protein isoforms. Furthermore, they suggest that administration of apoE-mimetic peptides may serve as a novel therapeutic strategy for the treatment of acute and chronic neurological disease.

Alzheimer Disease↗

Identification of a program of contractile protein gene expression initiated upon skeletal muscle differentiation.

The functional diversity of skeletal muscle is largely determined by the combinations of contractile protein isoforms that are expressed in different fibers. Just how the developmental expression of this large array of genes is regulated to give functional phenotypes is thus of great interest. In the present study, we performed a comprehensive analysis of contractile protein isoform mRNA profiles in skeletal muscle systems representing each generation of fiber formed: primary, secondary, and regenerating fibers. We find that in each system examined there is a common pattern of isoform gene expression during early differentiation for 5 of the 6 gene families we have investigated: myosin light chain (MLC)1, MLC2, tropomyosin, troponin (Tn)C, and TnI. We suggest that the common isoform patterns observed together represent a genetic program of skeletal muscle differentiation that is independent of the mature fiber phenotype and is found in all newly formed myotubes. Within each of these contractile protein gene families the program is independent of the isoforms of myosin heavy chain (MHC) expressed. The maintenance of such a program may reflect a specific requirement of the initial differentiation process.

Animals↗

Abundant expression of myosin heavy-chain IIB RNA in a subset of human masseter muscle fibres.

Type IIB fast fibres are typically demonstrated in human skeletal muscle by histochemical staining for the ATPase activity of myosin heavy-chain (MyHC) isoforms. However, the monoclonal antibody specific for the mammalian IIB isoform does not detect MyHC IIB protein in man and MyHC IIX RNA is found in histochemically identified IIB fibres, suggesting that the IIB protein isoform may not be present in man; if this is not so, jaw-closing muscles, which express a diversity of isoforms, are likely candidates for their presence. ATPase histochemistry, immunohistochemistry polyacrylamide gel electrophoresis and in situ hybridization, which included a MyHC IIB-specific mRNA riboprobe, were used to compare the composition and RNA expression of MyHC isoforms in a human jaw-closing muscle, the masseter, an upper limb muscle, the triceps, an abdominal muscle, the external oblique, and a lower limb muscle, the gastrocnemius. The external oblique contained a mixture of histochemically defined type I, IIA and IIB fibres distributed in a mosaic pattern, while the triceps and gastrocnemius contained only type I and IIA fibres. Typical of limb muscle fibres, the MyHC I-specific mRNA probes hybridized with histochemically defined type I fibres, the IIA-specific probes with type IIA fibres and the IIX-specific probes with type IIB fibres. The MyHC IIB mRNA probe hybridized only with a few histochemically defined type I fibres in the sample from the external oblique; in addition to this IIB message, these fibres also expressed RNAs for MyHC I, IIA and IIX. MyHC IIB RNA was abundantly expressed in histochemical and immunohistochemical type IIA fibres of the masseter, together with transcripts for IIA and in some cases IIX. No MyHC IIB protein was detected in fibres and extracts of either the external oblique or masseter by immunohistochemistry, immunoblotting and electrophoresis. Thus, IIB RNA, but not protein, was found in the fibres of two different human skeletal muscles. It is believed this is the first report of the substantial expression of IIB mRNA in man as demonstrated in a subset of masseter fibres, but rarely in limb muscle, and in only a few fibres of the external oblique. These findings provide further evidence for the complexity of myosin gene expression, especially in jaw-closing muscles.

Abdominal Muscles↗

Glucose transporters (GLUT1, 2, & 4) in fat, muscle and liver in a rat model of endotoxic shock.

To explore the mechanisms for hypoglycemia in our rat model of septic shock, we examined whether changes occur in glucose transporter isoform protein level. Total membrane protein fractions were collected from tissues 6 to 8 hours after endotoxin injection at which time animals exhibited hypoglycemia (7.2 +/- 0.5 vs. 2.6 +/- 1.2mM) and lactacidemia (1.0 +/- 1.0 vs. 5.1 +/- 1.8mM/L) as compared to saline-treated controls. The protein level of glucose transporter isoforms GLUT1 and 4 in fat did not significantly change in septic shock when compared to control animals (126 +/- 22% and 114 +/- 79%, respectively). Likewise, no change was seen in GLUT1 or 4 in muscle (124 +/- 52% and 101 +/- 28%, respectively). The protein abundance of isoforms GLUT1 and 2 in liver were not significantly altered (123 +/- 35% and 101 +/- 23%, respectively). Septic shock induced hypoglycemia cannot be directly explained by changes in total glucose transporter protein levels.

Adipose Tissue↗

A genomewide survey of developmentally relevant genes in Ciona intestinalis. IX. Genes for muscle structural proteins.

Ascidians are simple chordates that are related to, and may resemble, vertebrate ancestors. Comparison of ascidian and vertebrate genomes is expected to provide insight into the molecular genetic basis of chordate/vertebrate evolution. We annotated muscle structural (contractile protein) genes in the completely determined genome sequence of the ascidian Ciona intestinalis, and examined gene expression patterns through extensive EST analysis. Ascidian muscle protein isoform families are generally of similar, or lesser, complexity in comparison with the corresponding vertebrate isoform families, and are based on gene duplication histories and alternative splicing mechanisms that are largely or entirely distinct from those responsible for generating the vertebrate isoforms. Although each of the three ascidian muscle types - larval tail muscle, adult body-wall muscle and heart - expresses a distinct profile of contractile protein isoforms, none of these isoforms are strictly orthologous to the smooth-muscle-specific, fast or slow skeletal muscle-specific, or heart-specific isoforms of vertebrates. Many isoform families showed larval-versus-adult differential expression and in several cases numerous very similar genes were expressed specifically in larval muscle. This may reflect different functional requirements of the locomotor larval muscle as opposed to the non-locomotor muscles of the sessile adult, and/or the biosynthetic demands of extremely rapid larval development.

Amino Acid Sequence↗

Novel meiosis-specific isoform of mammalian SMC1.

Structural maintenance of chromosomes (SMC) proteins fulfill pivotal roles in chromosome dynamics. In yeast, the SMC1-SMC3 heterodimer is required for meiotic sister chromatid cohesion and DNA recombination. Little is known, however, about mammalian SMC proteins in meiotic cells. We have identified a novel SMC protein (SMC1beta), which-except for a unique, basic, DNA binding C-terminal motif-is highly homologous to SMC1 (which may now be called SMC1alpha) and is not present in the yeast genome. SMC1beta is specifically expressed in testes and coimmunoprecipitates with SMC3 from testis nuclear extracts, but not from a variety of somatic cells. This establishes for mammalian cells the concept of cell-type- and tissue-specific SMC protein isoforms. Analysis of testis sections and chromosome spreads of various stages of meiosis revealed localization of SMC1beta along the axial elements of synaptonemal complexes in prophase I. Most SMC1beta dissociates from the chromosome arms in late-pachytene-diplotene cells. However, SMC1beta, but not SMC1alpha, remains chromatin associated at the centromeres up to metaphase II. Thus, SMC1beta and not SMC1alpha is likely involved in maintaining cohesion between sister centromeres until anaphase II.

Amino Acid Motifs↗

RNA CUG-binding protein 1 increases translation of 20-kDa isoform of CCAAT/enhancer-binding protein beta by interacting with the alpha and beta subunits of eukaryotic initiation translation factor 2.

Expression of a dominant negative 20-kDa isoform of CCAAT/enhancer-binding protein (C/EBPbeta), LIP, is increased in proliferating livers and in tumor cells. Two RNA-binding proteins, CUGBP1 and calreticulin, have been implicated in the translational regulation of C/EBPbeta. In this paper, we present evidence showing several critical steps by which liver increases translation of LIP after partial hepatectomy. At early stages after partial hepatectomy, liver activates CUGBP1 by a hyperphosphorylation. The activated CUGBP1 binds to the 5' region of C/EBPbeta mRNA and replaces calreticulin, which partially represses translation of C/EBPbeta in quiescent livers. The hyperphosphorylated CUGBP1 also interacts with the alpha and beta subunits of initiation factor eIF2. Our data demonstrate that the interaction of CUGBP1 with the eIF2alpha enhances the association of CUGBP1 with ribosomes and correlates with increased translation of LIP in the liver after partial hepatectomy. Our data support the hypothesis that CUGBP1 increases translation of LIP by the interaction with the eIF2alpha subunit. This facilitates subsequent recruitment of larger numbers of ribosomes to initiate translation of LIP.

Animals↗