Molecular cloning, tissue distribution, and chromosomal localization of MMEL2, a gene coding for a novel human member of the neutral endopeptidase-24.11 family

Members of the neutral endopeptidase (NEP, also known as MME for membrane metallo-endopeptidase in the Human Gene Nomenclature database) family play significant roles in pain perception, arterial pressure regulation, phosphate metabolism, and homeostasis. In this paper, we report the cloning of a new human member of the NEP family that we named MMEL2 for membrane metallo-endopeptidase-like 2. The MMEL2 protein has the structural characteristics of type II transmembrane proteins, although the presence of a furin-like cleavage site in the ectodomain suggests that it may be released into the medium following proteolytic cleavage. The MMEL2 protein contains the zinc-binding consensus sequence HEXXH and all the residues known to be essential for the enzymatic activity of other members of the family. The MMEL2 mRNA was detected predominantly in testis, but weak expression also was observed in brain, kidney, and heart. The human MMEL2 gene was mapped to 1p36 by fluorescence in situ hybridization. It will be important to test whether MMEL2 defects are associated with diseases such as hereditary motor sensory neuropathy 2A, Schwartz-Jampel-Aberfeld syndrome, or neuroblastoma, which all map to this locus.

Disease-causing missense mutations in the PHEX gene interfere with membrane targeting of the recombinant protein

PHEX is homologous to the M13 zinc metallopeptidases, a class of type II membrane glycoproteins. Although more than 140 mutations in the PHEX gene have been identified in patients with X-linked hypophosphatemia (XLH), the most prevalent form of inherited rickets, the molecular consequences of disease-causing PHEX mutations have not yet been investigated. We examined the effect of PHEX missense mutations on cellular trafficking of the recombinant protein. Four mutant PHEX cDNAs were generated by PCR mutagenesis: C85R, G579R and S711R, identified in XLH patients, and E581V, previously engineered in neutral endopeptidase 24.11, where it abolished catalytic activity but not plasma membrane targeting. Wild-type and mutant PHEX cDNAs were transfected in HEK(293) cells and PHEX protein expression was characterized. In contrast to the wild-type and E581V PHEX proteins, the C85R, G579R and S711R mutants were completely sensitive to endoglycosidase H digestion, indicating that they were not fully glycosylated. Sequestration of the disease-causing mutant proteins in the endoplasmic reticulum (ER) and plasma membrane localization of wild-type and E581V PHEX proteins was demonstrated by immunofluorescence and cell surface biotinylation. Of the three mutant PHEX proteins, the S711R was the least stable and the only one that could be rescued from the ER to the plasma membrane in cells grown at 26 degrees C. The chemical chaperone glycerol failed to correct defective targeting of all three mutant proteins. Our data provide a mechanism for loss of PHEX function in XLH patients expressing the C85R, G579R and S711R mutations.

Neutral endopeptidase is expressed on the follicular granulosa cells of rabbit ovaries

Neutral endopeptidase (NEP) is a zinc metallopeptidase ubiquitously distributed in various tissues in mammals. This peptidase is involved in the post-secretory metabolism of various neuropeptides and peptide hormones in vivo, such as enkephalins, bradykinin, atrial natriuretic peptide, substance P and endothelins. In this paper we show that NEP is expressed in ovaries as a 110-kDa glycosylated integral membrane protein with enzymatic properties similar to those of the kidney protein. Using immunohistochemistry, we localize the peptidase in the granulosa cells of follicles at all stages of maturation, with the exception of atretic follicles. We also observe immunoreactive staining in the epithelia that lines the blood vessels in the medulla and the surface of the ovary. The co-localization of NEP and bioactive peptides known to be physiological substrates of NEP in other tissues suggests an important role for this protein in processes such as follicle maturation, ovulation, and/or regulation of ovarian blood flow, by modulating the physiological function of these peptides.

Two rat brain staufen isoforms differentially bind RNA

In neurones, a limited number of mRNAs is found in dendrites, including transcripts encoding the microtubule-associated protein 2 (MAP2). Recently, we identified a cis-acting dendritic targeting element (DTE) in MAP2 mRNAs. Here we used the yeast tri-hybrid system to identify potential trans-acting RNA-binding factors of the DTE. A cDNA clone was isolated that encodes a member of a mammalian protein family that is highly homologous to the Drosophila RNA-binding protein Staufen. Mammalian Staufen appears to be expressed in most tissues and brain areas. Two distinct rat brain Staufen isoforms, rStau+I6 and rStau-I6, are encoded by alternatively spliced mRNAs. Both isoforms contain four double-stranded RNA-binding domains (dsRBD). In the larger rStau+I6 isoform, six additional amino acids are inserted in the second dsRBD. Although both isoforms interacted with the MAP2-DTE and various additional RNA fragments in an in vitro north-western assay, rStau-I6 exhibited a stronger signal of bound radioactively labelled RNAs as compared with rStau+I6. Using an antibody directed against mammalian Staufen, the protein was detected in somata and dendrites of neurones of the adult rat hippocampus and cerebral cortex. Ultrastructural studies revealed that in dendrites, rat Staufen accumulates along microtubules. Thus in neurones, rat Staufen may serve to link RNAs to the dendritic microtubular cytoskeleton and may thereby regulate their subcellular localization.

A novel murine Staufen isoform modulates the RNA content of Staufen complexes

Mouse Staufen (mStau) is a double-stranded RNA-binding protein associated with polysomes and the rough endoplasmic reticulum (RER). We describe a novel endogenous isoform of mStau (termed mStau(i)) which has an insertion of six amino acids within dsRBD3, the major double-stranded RNA (dsRNA)-binding domain. With a structural change of the RNA-binding domain, this conserved and widely distributed isoform showed strongly impaired dsRNA-binding ability. In transfected cells, mStau(i) exhibited the same tubulovesicular distribution (RER) as mStau when weakly expressed; however, when overexpressed, mStau(i) was found in large cytoplasmic granules. Markers of the RER colocalized with mStau(i)-containing granules, showing that overexpressed mStau(i) could still be associated with the RER. Cotransfection of mStau(i) with mStau relocalized overexpressed mStau(i) to the reticular RER, suggesting that they can form a complex on the RER and that a balance between these isoforms is important to achieve proper localization. Coimmunoprecipitation demonstrated that the two mStau isoforms are components of the same complex in vivo. Analysis of the immunoprecipitates showed that mStau is a component of an RNA-protein complex and that the association with mStau(i) drastically reduces the RNA content of the complex. We propose that this new isoform, by forming a multiple-isoform complex, regulates the amount of RNA in mStau complexes in mammalian cells.

Developmental expression and tissue distribution of Phex protein: effect of the Hyp mutation and relationship to bone markers

Mutations in PHEX, a phosphate-regulating gene with homology to endopeptidases on the X chromosome, are responsible for X-linked hypophosphatemia (XLH). The murine Hyp homologue has the phenotypic features of XLH and harbors a large deletion in the 3' region of the Phex gene. We characterized the developmental expression and tissue distribution of Phex protein, using a monoclonal antibody against human PHEX, examined the effect of the Hyp mutation on Phex expression, and compared neprilysin (NEP), osteocalcin, and parathyroid hormone/parathyroid hormone-related protein (PTH/PTHrP) receptor gene expression in bone of normal and Hyp mice. Phex encodes a 100- to 105-kDa glycoprotein, which is present in bones and teeth of normal mice but not Hyp animals. These results were confirmed by in situ hybridization (ISH) and ribonuclease protection assay. Phex protein expression in femur and calvaria decreases with age, suggesting a correlation between Phex expression and bone formation. Immunohistochemical studies detected Phex protein in osteoblasts, osteocytes, and odontoblasts, but not in osteoblast precursors. In contrast to Phex, the abundance of NEP messenger RNA (mRNA) and protein is not significantly altered in Hyp bone. Similarly, osteocalcin and PTH/PTHrP receptor gene expression are not compromised in bone of Hyp mice. Our results are consistent with the hypothesis that loss of Phex function affects the mineralizing activity of osteoblasts rather than their differentiation.

Gene products from LUQ neurons in the abdominal ganglion are present at the renal pore of Aplysia californica

The L2-4,6 and L5 cells located in the left upper quadrant of the abdominal ganglion of Aplysia californica express the L5-67 and LUQ-1 genes, respectively, in a nonoverlapping manner. These cells send major neurites to the kidney and at least some of them were shown to innervate the renal pore closer muscle, and thereby control its function. By using in-situ hybridization and immunofluorescence, the presence of L5-67 and LUQ-1 mRNAs and peptides was studied in the kidney, with emphasis on the region of the renal pore. We detected immunoreactive materials in many small varicose nerve fibers running along the central epithelium in the inner parts of the kidney, and in neurites located within a large nerve associated with muscles inside the renal pore. Our observations represent the first direct evidence of the presence of gene products from LUQ cells at the renal pore, suggesting that they may be responsible for mediating LUQ cell signals. Furthermore, mRNAs coding for the L5-67 and LUQ-1 peptides were also found in the nerve structure inside the renal pore. Our report documents a striking example of neuropeptide mRNA targeting nerve terminals that are very distant from their cell bodies.

Expression of autocrine motility factor/phosphohexose isomerase in Cos7 cells

Autocrine motility factor (AMF) is identical to the glycolytic enzyme phosphohexose isomerase (PHI) and overexpression of AMF/PHI is associated with tumor malignancy. In order to study the overexpression of AMF/PHI, an HA-tagged AMF construct was transiently transfected into Cos7 cells. Expression of a tagged AMF-HA allowed us to determine that over a period of 16 hours only a small amount (0.1-1%) of total cellular AMF-HA was secreted into the cell medium. Cell-associated AMF-HA was exclusively cytosolic as it could be completely extracted with Triton X-100 and concentrated within actin rich pseudopodial domains. Treatment of the cells with the glycolysis inhibitor oxamate disrupted the association of AMF-HA with actin concentrations demonstrating that glycolysis regulates the formation of these AMF/PHI-associated actin-rich protrusions. AMF/PHI is a well-characterized tumor cell secreted cytokine and we identify here an alternate intracellular function for this glycolytic enzyme/cytokine in cell motility.

The double-stranded RNA-binding protein Staufen is incorporated in human immunodeficiency virus type 1: evidence for a role in genomic RNA encapsidation

Human Staufen (hStau), a double-stranded RNA (dsRNA)-binding protein that is involved in mRNA transport, is incorporated in human immunodeficiency virus type 1 (HIV-1) and in other retroviruses, including HIV-2 and Moloney murine leukemia virus. Sucrose and Optiprep gradient analyses reveal cosedimentation of hStau with purified HIV-1, while subtilisin assays demonstrate that it is internalized. hStau incorporation in HIV-1 is selective, is dependent on an intact functional dsRNA-binding domain, and quantitatively correlates with levels of encapsidated HIV-1 genomic RNA. By coimmunoprecipitation and reverse transcription-PCR analyses, we demonstrate that hStau is associated with HIV-1 genomic RNA in HIV-1-expressing cells and purified virus. Overexpression of hStau enhances virion incorporation levels, and a corresponding, threefold increase in HIV-1 genomic RNA encapsidation levels. This coordinated increase in hStau and genomic RNA packaging had a significant negative effect on viral infectivity. This study is the first to describe hStau within HIV-1 particles and provides evidence that hStau binds HIV-1 genomic RNA, indicating that it may be implicated in retroviral genome selection and packaging into assembling virions.

Molecular cloning, sequence analysis and expression distribution of an aminopeptidase in Aplysia california

We are investigating the role of membrane-bound peptidases in the inactivation of neuropeptides in Aplysia californica. Recently, we reported the biochemical characterization of a membrane-bound neuropeptide-degrading enzyme which has enzymatic characteristics similar to those of the mammalian aminopeptidase N (Bawab W, Querido E, Crine P, DesGroseillers L. Identification and characterization of aminopeptidases from Aplysia californica, Biochem J 1992;286:967-975). We now report the cloning and sequencing of a cDNA encoding an aminopeptidase enzyme (apAP) and the localization of the apAP transcript in Aplysia. The apAP cDNA encodes a putative protein of 1007 amino acids, which shows around 34% sequence identity to mammalian aminopeptidases A and N sequences. The deduced amino acid sequence suggests that apAP is a type II membrane-bound protein, with a long extracellular domain in which the consensus sequence of zinc-binding metallopeptidases (His-Glu-Xxx-Xxx-His) is found. RT-PCR and Northern blot experiments showed that the apAP gene is expressed as a single 6.8-kb transcript in the central nervous system, gill, heart, kidney and ovotestis.

Microtubule-dependent recruitment of Staufen-green fluorescent protein into large RNA-containing granules and subsequent dendritic transport in living hippocampal neurons

Dendritic mRNA transport and local translation at individual potentiated synapses may represent an elegant way to form synaptic memory. Recently, we characterized Staufen, a double-stranded RNA-binding protein, in rat hippocampal neurons and showed its presence in large RNA-containing granules, which colocalize with microtubules in dendrites. In this paper, we transiently transfect hippocampal neurons with human Staufen-green fluorescent protein (GFP) and find fluorescent granules in the somatodendritic domain of these cells. Human Stau-GFP granules show the same cellular distribution and size and also contain RNA, as already shown for the endogenous Stau particles. In time-lapse videomicroscopy, we show the bidirectional movement of these Staufen-GFP-labeled granules from the cell body into dendrites and vice versa. The average speed of these particles was 6.4 microm/min with a maximum velocity of 24. 3 microm/min. Moreover, we demonstrate that the observed assembly into granules and their subsequent dendritic movement is microtubule dependent. Taken together, we have characterized a novel, nonvesicular, microtubule-dependent transport pathway involving RNA-containing granules with Staufen as a core component. This is the first demonstration in living neurons of movement of an essential protein constituent of the mRNA transport machinery.

Cloning and characterization of Aplysia neutral endopeptidase, a metallo-endopeptidase involved in the extracellular metabolism of neuropeptides in Aplysia californica

Cell surface metallo-endopeptidases play important roles in cell communication by controlling the levels of bioactive peptides around peptide receptors. To understand the relative relevance of these enzymes in the CNS, we characterized a metallo-endopeptidase in the CNS of Aplysia californica, whose peptidergic pathways are well described at the molecular, cellular, and physiological levels. The membrane-bound activity cleaved Leu-enkephalin at the Gly3-Phe4 bond with an inhibitor profile similar to that of the mammalian neutral endopeptidase (NEP). This functional homology was supported by the molecular cloning of cDNAs from the CNS, which demonstrated that the Aplysia and mammalian NEPs share all the same amino acids that are essential for the enzymatic activity. The protein is recognized both by specific anti-Aplysia NEP (apNEP) antibodies and by the [125I]-labeled NEP-specific inhibitor RB104, demonstrating that the apNEP gene codes for the RB104-binding protein. In situ hybridization experiments on sections of the ganglia of the CNS revealed that apNEP is expressed in neurons and that the mRNA is present both in the cell bodies and in neurites that travel along the neuropil and peripheral nerves. When incubated in the presence of a specific NEP inhibitor, many neurons of the buccal ganglion showed a greatly prolonged physiological response to stimulation, suggesting that NEP-like metallo-endopeptidases may play a critical role in the regulation of the feeding behavior in Aplysia. One of the putative targets of apNEP in this behavior is the small cardioactive peptide, as suggested by RP-HPLC experiments. More generally, the presence of apNEP in the CNS and periphery may indicate that it could play a major role in the modulation of synaptic transmission in Aplysia and in the metabolism of neuropeptides close to their point of release.

Mammalian staufen is a double-stranded-RNA- and tubulin-binding protein which localizes to the rough endoplasmic reticulum

Staufen (Stau) is a double-stranded RNA (dsRNA)-binding protein involved in mRNA transport and localization in Drosophila. To understand the molecular mechanisms of mRNA transport in mammals, we cloned human (hStau) and mouse (mStau) staufen cDNAs. In humans, four transcripts arise by differential splicing of the Stau gene and code for two proteins with different N-terminal extremities. In vitro, hStau and mStau bind dsRNA via each of two full-length dsRNA-binding domains and tubulin via a region similar to the microtubule-binding domain of MAP-1B, suggesting that Stau cross-links cytoskeletal and RNA components. Immunofluorescent double labeling of transfected mammalian cells revealed that Stau is localized to the rough endoplasmic reticulum (RER), implicating this RNA-binding protein in mRNA targeting to the RER, perhaps via a multistep process involving microtubules. These results are the first demonstration of the association of an RNA-binding protein in addition to ribosomal proteins, with the RER, implicating this class of proteins in the transport of RNA to its site of translation.

Cloning and functional expression of an Aplysia 5-HT receptor negatively coupled to adenylate cyclase

Serotonin (5-HT) is involved in the control of various behaviors in Aplysia californica, including reproduction, feeding, locomotion, circadian rhythm, synaptic plasticity, and synaptic growth. The large variety of functions of 5-HT is mediated by different receptor subtypes that are coupled to different second-messenger systems. Here, we report the cloning of a cDNA coding for an Aplysia G-protein-coupled 5-HT receptor (5-HTap1). Its deduced amino acid sequence resembles those of the 5-HT1 receptor subfamily. When expressed in stable cell lines, 5-HTap1 exhibits high-affinity binding for the serotonergic radioligand [N-methyl-3H]lysergic acid diethylamide. This binding is competed by several 5-HT agonists and antagonists, and the pharmacological profile of inhibition has some similarities with those of 5-HT1 and 5-HT7 receptors. Application of 5-HT or its agonists 5-carboxamidotryptamine maleate and (+/-)-8-hydroxy-2-(di-n-propyl-amino) tetralin hydrobromide on cells transformed with 5-HTap1 produced a dose-dependent inhibition of forskolin-stimulated cAMP accumulation. 5-HTap1 is thus negatively coupled to adenylate cyclase. The production of antiserum against the 5-HTap1 receptor allowed us to examine its expression in animal tissues. The receptor protein is detected in every tissue examined, although it seems only weakly expressed in some samples. The receptor is also found in every ganglia of the nervous system, both in the sheath and in the neurons. 5-HTap1 mRNA is absent from the sheath, indicating that the protein observed there is probably located on the nerve terminals.

Pex mRNA is localized in developing mouse osteoblasts and odontoblasts

Mutations in PEX, a phosphate-regulating gene with homology to endopeptidase on the X chromosome, were recently identified in patients with X-linked hypophosphatemia (XLH), an inherited disorder of phosphate homeostasis characterized by growth retardation and rachitic and osteomalacic bone disease. To understand the mechanism by which loss of PEX function elicits the mutant phenotype, a study of its mRNA localization and ontogenesis was undertaken. Using the reverse transcriptase-nested polymerase chain reaction (RT-nested PCR) with polyA+ RNA purified from mouse testis, a 337-bp Pex cDNA fragment was generated and cloned in the pCRII plasmid. The cDNA was used to generate sense and anti-sense Pex riboprobes for in situ hybridization (ISH) and Northern analysis. To survey a large number of different tissues, sagittal sections of embryos and newborn mice were examined. ISH showed the presence of Pex mRNA in osteoblasts and odontoblasts. Pex gene expression was detectable on Day 15 of embryonic development, which coincides with the beginning of intercellular matrix deposition in bones. Finally, Northern analysis of total RNA from calvariae and teeth of 3-day-old and adult mice showed that the abundance of the 7-kb Pex transcript is decreased in adult bones and in nongrowing teeth. The present study demonstrates that Pex mRNA is expressed in bones and teeth and suggests that this putative endopeptidase plays an important role in the development of these tissues.

Alternative splicing and genomic organization of the L5-67 gene of Aplysia californica

The L5-67 gene was first identified on the basis of its high expression level in the LUQ neurons, a group of four giant cells located in the left upper quadrant of the abdominal ganglion of Aplysia californica. Its mRNA and peptides were later shown to be present in these cells, as well as in about 100 other smaller neurons in the CNS. L5-67 propeptide and/or mature peptides are also present in peripheral organs, particularly in the kidney, which is the target of most LUQ processes. Using RT-PCR, we show the presence of an alternatively spliced L5-67 transcript arising from the exclusion of the fourth exon from the mature mRNA. This alternative splicing event occurs specifically in the kidney, although we could not identify the cells in which it takes place. Translation of this transcript generates a 52 amino acid (aa) propeptide in which the first N-terminal 45 aa are identical to the original L5-67 propeptide. The last seven C-terminal aa are unrelated to the previously characterized L5-67 peptides due to a change in the open reading frame.

L5-67 and LUQ-1 peptide precursors of Aplysia californica: distribution and localization of immunoreactivity in the central nervous system and in peripheral tissues

Two genes (L5-67 and LUQ-1) that encode neuropeptide precursors have recently been shown to be expressed in a distinct and non-overlapping manner in the five left upper quadrant (LUQ) cells of the abdominal ganglion of Aplysia (Landry et al. [1992]. J. Neurobiol 23:89-101). By using wholemount immunohistochemistry and radioimmunoassay (RIA), the pattern of expression of these two genes was assessed at the protein level throughout the central nervous system (CNS) and in peripheral tissues of Aplysia californica. The distribution of LUQ-1 precursor-like immunoreactivity was fairly limited, occurring in the ventral LUQ cell (L5) and in a total of approximately 20 additional neurons in the abdominal and cerebral ganglia. L5-67 precursor-like immunoreactive material was more prevalent, appearing in a total of approximately 100 neurons distributed among each of the central ganglia. Identified L5-67-immunoreactive neurons included the four dorsal LUQ cells (L2-4 and L6) and two giant neurons (R2 and LPI1). In one group of cells, the H cluster of the cerebral ganglion, L5-67 immunofluorescence was substantially more intense in larger versus smaller animals, suggesting that this peptide precursor is subject to developmental regulation in certain neurons. Immunoelectron microscopic examination of the subcellular localization of L5-67 immunoreactivity in LUQ cell somata and axons revealed its association with dense-core vesicles (approximately 114 nm in diameter). In the periphery, L5-67-immunoreactive fibers were detected in specific regions of the circulatory system (auricle, ventricle, cristae aorta, anterior aorta) and the reproductive system (genital ganglion, large hermaphroditie duct, small hermaphroditie duct, ovotestis). The kidney and the intestine, two tissues in which considerable secretion and absorption occur, contained material immunoreactive to both L5-67 and LUQ-1 antisera. The localization of the two peptide precursors in these tissues differed substantially, with L5-67 occurring in widely ramifying varicose fibers, whereas LUQ-1 was found in restricted foci of fibers and in small spherical cells that appeared to lack processes. These results support previous findings concerning the heterogeneity of neurotransmitter phenotypes in the LUQ cells. Furthermore, they are indicative of a fairly broad role for the L5-67-derived neuropeptides, and a more limited role for the LUQ-1-derived neuropeptides, in the regulation of the visceral organ systems of Aplysia.

Localization of a human double-stranded RNA-binding protein gene (STAU) to band 20q13.1 by fluorescence in situ hybridization

Asymmetric transport of mRNA within the cells is mediated by RNA-binding proteins that form, along with the mRNAs and perhaps other small RNAs, stable ribonucleoprotein complexes. However, the nature of the protein components of these complexes in vertebrates is still unknown. In Drosophila, genetic studies have identified a number of potential genes that are necessary for localization of mRNAs in oocytes; one of the most studied is the staufen gene. The staufen protein has been shown to bind to localized mRNAs in oocytes and to be expressed in somatic cells as well. To understand the mechanism of mRNA transport in mammals and characterize its components, we recently cloned and sequenced the human staufen homolog cDNA (HGMW-approved symbol STAU). In this paper, we show that the gene is unique in the human genome and report its chromosomal localization by fluorescence in situ hybridization. The human staufen gene maps to chromosome 20q13.1, a region that is associated with certain genetic diseases.

Processing of the L5-67 precursor peptide and characterization of LUQIN in the LUQ neurons of Aplysia californica

Metabolic labeling of the dorsal Left Upper Quadrant (LUQ) cells of the abdominal ganglion of Aplysia californica and RP-HPLC separation of their peptide content allowed us to identify the L5-67 precursor and its processed peptides. Cleavage of the signal peptide occurred between amino acids 23 and 24 of the prepropeptide and generated a propeptide of 89 amino acids. Further processing by endopeptidases at the twin basic residues Lys12-Arg13 of the precursor generated a peptide of 76 amino acids, as well as an amidated decapeptide, LUQIN. The sequence of LUQIN was determined by amino acid sequencing and by its comigration with the synthetic peptide Ala-Pro-Ser-Trp-Arg-Pro-Gln-Gly-Arg-Phe-amide in three different RP-HPLC systems. The amidation of LUQIN was further demonstrated by its resistance to carboxypeptidase A digestion.

A novel actin cDNA is expressed in the neurons of Aplysia californica

A novel actin cDNA has been isolated from an abdominal ganglion cDNA library of Aplysia californica by differential screening. This cDNA of 1596 nucleotides in length encodes a putative actin protein of 41.8 kDa. This protein shows 95.2% identity with another Aplysia actin gene previously shown to be expressed in the muscular sheath of the ganglion (DesGroseillers et al. (1990) Nucleic Acids Res. 18, 3654). However, the 5' and 3' untranslated regions of these cDNAs are completely different. PCR experiments performed with mRNA isolated from dissected neurons, ovotestis or kidney reveal that the gene is expressed in the neurons of the ganglia and in other tissues as well. Southern blot analysis reveals that the neuronal actin gene is a member of a large gene family.

Analysis of genes coding for the major and minor fimbrial subunits of the Prs-like fimbriae F165(1) of porcine septicemic Escherichia coli strain 4787

Septicemic Escherichia coli 4787 of porcine origin produces a Prs-like fimbrial antigen, F165(1) which agglutinates sheep erythrocytes in a similar manner to Prs fimbriae, but unlike the latter, also agglutinates pig erythrocytes. In a previous study, we reported the cloning of the f165(1) operon and showed that it encodes a Prs-like adhesin. Here, we report the sequence of the f165(1)A, and f165(1)EFG genes. f165(1)A encodes a protein of 161 amino acids preceded by a signal peptide of 21 amino acids. A size of 19.3 kDa was calculated for the processed F165(1)A protein. The E, F, and G open reading frames potentially give rise to mature proteins of 149, 148 and 313 amino acids respectively. The F165(1)A protein showed significant similarity with the major subunit protein of P-fimbriae of F11 serotype, differing only in four positions. F165(1)E and F165(1)G were found to be closely related to the PrsE and PrsG proteins of Prs-fimbriae variants, whereas F165(1)F was found to be almost completely identical to the F protein of various P and Prs fimbriae. Our results indicated that the f165(1) is a mosaic operon consisting of sequences related to both the pap operon and the prs operon.

Identification and characterization of a neutral endopeptidase activity in Aplysia californica

Kidney plasma membranes of Aplysia californica were shown to contain an endopeptidase activity which cleaved [Leu]enkephalin (Tyr-Gly-Gly-Phe-Leu) and [Leu]enkephalinamide (Tyr-Gly-Gly-Phe-Leu-NH2) at the Gly3-Phe4 bond, as determined by reverse-phase h.p.l.c. analysis of metabolites. The optimal pH was shown to be 6.5. The bivalent cation chelating agent, 1,10-phenanthroline protected [Leu]enkephalin from degradation, suggesting that this enzyme is a metallopeptidase. The degradation of [Leu]enkephalin was also abolished by the neutral endopeptidase-24.11 inhibitors RB104 (2-[(3-iodo-4-hydroxyl)-phenylmethyl]-4-N-[3-(hydroxyamino-3-oxo-1- phenylmethyl)-propyl]amino-4-oxobutanoic acid), HABCO-Gly [(3-hydroxy-aminocarbonyl-2-benzyl-1-oxypropyl)glycine], phosphoramidon and thiorphan, with IC50 values of 1 nM, 1 microM, 20 microM and 30 microM respectively. By contrast, the angiotensin-converting enzyme inhibitor captopril and the serine proteinase inhibitor phenylmethanesulphonyl fluoride were without effect. Phase separation experiments using Triton X-114 showed that about 64% of the neutral endopeptidase activity in the Aplysia kidney membrane corresponds to an integral membrane protein. A specific radioiodinated inhibitor ([125I]RB104) was shown to bind the Aplysia endopeptidase with high affinity; the KD and Bmax. values were 21 +/- 5 pM and 20.3 +/- 5 fmol/mg of proteins respectively. This inhibitor was used to determine the molecular form of the enzyme, after separation of solubilized membrane proteins on SDS/PAGE and transfer on to nitrocellulose membranes. A single protein band with an apparent molecular mass of 140 kDa was observed. The labelling was abolished by specific neutral endopeptidase inhibitors. This study provides the first biochemical characterization of an endopeptidase with catalytic properties similar to those of neutral endopeptidase-24.11 in the mollusc Aplysia californica.

Identification and characterization of aminopeptidases from Aplysia californica

Aminopeptidase activities were identified in extracts of kidney, ovotestis, head ganglia, heart and haemolymph of Aplysia californica. These enzyme preparations hydrolysed [3H][Leu]enkephalin at the Try-1-Gly-2 bond as determined by h.p.l.c. analysis of cleavage products. In all these tissues, enkephalin-degrading aminopeptidase activities were present both in membrane-bound and cytosolic fractions. The bivalent-cation-chelating agent, 1,10-phenanthroline, inhibited kidney membrane aminopeptidase activity with an IC50 of 30 microM, suggesting that this enzyme is a metalloproteinase. The aminopeptidase inhibitor amastatin was the most potent inhibitor of [Leu]enkephalin degradation (IC50 25 nM) by membrane-bound aminopeptidase, and bacitracin, bestatin and puromycin were about 100-1000 times less potent. In contrast with membrane-bound aminopeptidase, the cytosolic form is sensitive to puromycin. Angiotensin-converting enzyme inhibitor had no effect on [Leu]enkephalin degradation by kidney membranes, while the neutral endopeptidase inhibitors were poor inhibitors of the enzymes in this preparation. The Km values of the aminopeptidase in the kidney membranes and cytosolic fractions for the [Leu]enkephalin substrate were 2.4 and 7.4 microM respectively. The aminopeptidase present in the kidney membranes also hydrolysed endogenous Phe-Met-Arg-Phe-amide peptide at the Phe-1-Met-2 bond as well as synthetic alanine p-nitroanilide and leucine p-nitroanilide. When used in a competition assay, these substrates inhibited hydrolysis of [3H][Leu]enkephalin, suggesting that the same enzyme degraded all these substrates. Taken together, these results suggest that Aplysia tissues contain both a membrane-bound aminopeptidase related to the mammalian aminopeptidase N and a cytosolic puromycin-sensitive aminopeptidase.

Differential expression of neuropeptide gene mRNA within the LUQ cells of Aplysia californica

Two neuropeptide precursor cDNAs (LUQ-1 and L5-67) have been recently isolated from the Left Upper Quadrant (LUQ) neurons of the abdominal ganglion of Aplysia californica (Shyamala, Fisher, and Scheller, 1986; Wickham and DesGroseillers, 1991). Using in situ hybridization techniques as well as dot blot and polymerase chain reaction (PCR) assays, we have studied the expression of these genes in the central nervous system (CNS) of Aplysia californica. The LUQ-1 gene was found to be expressed in neuron L5 in the abdominal ganglion, whereas the expression of the L5-67 gene was observed in the other four LUQ cells (L2-4 and L6). When in situ hybridization was performed on paraffin sections of the abdominal ganglion, clusters of smaller cells located in the left hemiganglion, were also found to express either the LUQ-1 or the L5-67 gene, never both. In many sections, the mRNAs coding for the two neuropeptides were found not only in cell bodies but also in the axon of individual LUQ neurons and even as far as the pericardial nerve. The presence of neuropeptide mRNA in axons, pericardial nerve, and kidney has been confirmed by polymerase chain reaction. A specific, although diffuse hybridization in the left upper quadrant also suggests that mRNA is present in the neuritic field. Taken together these results indicate that neuron L5 is the only giant neuron expressing the LUQ-1 gene and might therefore have a physiological function different from the other four LUQ cells. Neuropeptide mRNAs were also found in the axon and/or the neuritic field of giant neurons and could play important roles related to cell signalling in axons and nerve termini.

Nucleotide sequence of an actin cDNA gene from Aplysia californica

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Molecular aspects of egg-laying behavior in Aplysia californica

The Aplysia neuroendocrine system is a particularly advantageous model for cellular and molecular studies because of the relatively small number and large size of its component neurons. In addition, numerous anatomical and studies have resulted in the assignment of behavioral roles to individual identified neurons. Recombinant DNA techniques have been used to isolate the genes that encode the precursors of peptides involved in egg-laying behavior. The comparison of the egg-laying hormone (ELH) gene family within the genus Aplysia reveals high homologies in the overall structure of the precursors. A well-conserved tetrabasic residue has been shown to be the first endoproteolytic cleavage site of the precursor, giving rise to two intermediates, which are differentially processed and packaged. Some members of the ELH gene family are expressed specifically in the bag cell clusters or the atrial gland, respectively, providing an opportunity to study control of gene expression at the molecular level.

Important role of the long terminal repeat of the helper Moloney murine leukemia virus in Abelson virus-induced lymphoma

The helper virus has been shown to play a critical role in the development of lymphoma induced by the defective Abelson murine leukemia virus (A-MuLV). Indeed, A-MuLV pseudotyped with some viruses, such as the Moloney MuLV, has been shown to be highly lymphogenic, whereas A-MuLV pseudotyped with other viruses, such as the BALB/c endogenous N-tropic MuLV, has been shown to be devoid of lymphogenic potential (N. Rosenberg and D. Baltimore, J. Exp. Med. 147:1126-1141, 1978; C. D. Scher, J. Exp. Med. 147: 1044-1053, 1978). To map the viral DNA sequences encoding the determinant of the lymphogenic potential of Moloney MuLV when complexed with A-MuLV, we constructed chimeric helper viral DNA genomes in vitro between parental cloned infectious viral DNA genomes from Moloney MuLV and from BALB/c endogenous N-tropic MuLV. Chimeric helper MuLVs, recovered after transfection of NIH 3T3 cells were used to rescue A-MuLV, and the pseudotypes were inoculated into newborn NIH Swiss, CD-1, and SWR/J mice to test their lymphogenic potential. We found that a 0.44-kilobase-pair PstI-KpnI long terminal repeat-containing fragment from the Moloney MuLV was sufficient to confer some, but not complete, lymphogenic potential to a chimeric virus (p7M2) in NIH Swiss and SWR/J mice, but not in CD-1 mice. The addition of the 3'-end env sequences (comprising the carboxy terminus of gp70 and all p15E) to the U3 long terminal repeat sequences restored the full lymphogenic potential of the Moloney MuLV. Our data indicate that the 3'-end sequences of the helper Moloney MuLV are somehow involved in the development of lymphoma induced by A-MuLV. The same sequences have previously been found to harbor the determinant of leukemogenicity and of disease specificity of Moloney MuLV when inoculated alone.

Aplysia californica neurons express microinjected neuropeptide genes

Neuropeptide genes are expressed in specific subsets of large polyploid neurons in Aplysia californica. We have defined the transcription initiation sites of three of these neuropeptide genes (the R14, L11, and ELH genes) and determined the nucleotide sequence of the promoter regions. The genes contain the usual eucaryotic promoter signals as well as other structures of potential regulatory importance, including inverted and direct repeats. The L11 and ELH genes, which are otherwise unrelated, have homology in the promoter regions, while the R14 promoter was distinct. When cloned plasmids were microinjected into Aplysia neurons in organ culture, transitions between supercoiled, relaxed circular, and linear DNAs occurred along with ligation into high-molecular-weight species. About 20% of the microinjected neurons expressed the genes. The promoter region of the R14 gene functioned in expression of the microinjected DNA in all cells studied. When both additional 5' and 3' sequences were included, the gene was specifically expressed only in R14, suggesting that the specificity of expression is generated by a multicomponent repression system. Finally, the R14 peptide could be expressed in L11, demonstrating that it is possible to alter the transmitter phenotype of these neurons by introduction of cloned genes.

Retrovirus-induced spongiform encephalopathy: the 3'-end long terminal repeat-containing viral sequences influence the incidence of the disease and the specificity of the neurological syndrome

Using chimeric murine leukemia viruses (MuLVs) constructed in vitro with parental viral genomes from the neurotropic Cas-BR-E MuLV and the nonneurotropic amphotropic 4070-A MuLV, we previously mapped the paralysis-inducing determinant of Cas-BR-E MuLV within a pol-env region. To assess the role of the long terminal repeats (LTRs) in influencing the neurological disease, we constructed another chimeric MuLV (pNEMO-1)m harboring the gag-pol-env from Cas-BR-E MuLV and the LTR region from the strongly T-cell tropic Moloney MuLV. Although the Cas-BR-E MuLV induced mainly nonthymic leukemia, pNEMO-1 MuLV induced a thymic form of leukemia, as the parental Moloney MuLV. The pNEMO-1 MuLV induced neurological diseases less frequently than Cas-BR-E MuLV when inoculated intraperitoneally into NIH/Swiss, SIM.S, and SWR/J mice. However, it induced neurological disorders more frequently and with a shorter latency than Cas-BR-E MuLV when inoculated intrathymically. Most mice with a neurological disorder induced with pNEMO-1 MuLV showed a new clinical syndrome not usually seen with the parental Cas-BR-E MuLV: They had no lower limb paralysis but were excessively tremulous, spastic, and immobile. The topographical distribution of the spongiform degeneration in the brain of mice with this new syndrome was different from that seen in mice with lower limb paralysis induced by Cas-BR-E MuLV. These results indicate that the 1.0-kilobase-pair Cla I-Pvu I LTR-containing fragment harbors sequences influencing the incidence and the clinical manifestation of the neurological disease and suggest a specificity of LTR sequences for a new tissue (brain).

Neurotropic Cas-BR-E murine leukemia virus harbors several determinants of leukemogenicity mapping in different regions of the genome

The infectious virus derived from the molecularly cloned genome of the neurotropic ecotropic murine Cas-BR-E retrovirus was previously shown to have retained the ability to induce hind-limb paralysis and leukemia when inoculated into susceptible mice (P. Jolicoeur, N. Nicolaiew, L. DesGroseillers, and E. Rassart, J. Virol. 45:1159-1163, 1983). To map the viral sequences encoding the leukemogenic determinant(s) of this virus, we used chimeric viral genomes constructed in vitro between cloned viral DNAs from the leukemogenic Cas-BR-E murine leukemia virus (MuLV) and from the related nonleukemogenic amphotropic 4070-A MuLV. Infectious chimeric MuLVs, recovered from NIH 3T3 cells microinjected with these DNAs, were inoculated into newborn NIH Swiss, SIM.S, and SWR/J mice to test their leukemogenic potential. We found that each chimeric MuLV, harboring either the long terminal repeat, the gag-pol, or the pol-env region of the Cas-BR-E MuLV genome, was leukemogenic, indicating that this virus harbors several determinants of leukemogenicity mapping in different regions of its genome. This result suggests that the amphotropic 4070-A MuLV has multiple regions along its genome which prevent the expression of its leukemogenic phenotype, and it also shows that substitution of only one of these regions for Cas-BR-E MuLV sequences is sufficient to make it leukemogenic.

The tandem direct repeats within the long terminal repeat of murine leukemia viruses are the primary determinant of their leukemogenic potential

To map the viral sequences encoding the leukemogenic determinant(s) of nondefective murine leukemia viruses (MuLVs), we constructed chimeric viral genomes in vitro between cloned viral DNAs from the highly leukemogenic Gross passage A (Gross A) MuLV and from the related nonleukemogenic BALB/c N-tropic MuLV. Infectious chimeric MuLVs, recovered from murine cells microinjected with these DNAs, were inoculated into newborn mice to test the leukemogenic potential of these viruses. We found that the U3 long terminal repeat region from Gross A genomes was sufficient to confer an intermediate leukemogenic potential to chimeric MuLVs. Sequencing data indicated that the U3 tandem direct repeat was responsible for this effect. Adding most of the Gross A p15E-coding sequences to the Gross A U3 long terminal repeat enhanced the leukemogenic potential of chimeric viruses significantly. Adding a larger 3'-end env region (all p15E-coding sequences and 345 base pairs of the carboxy terminus of gp70) to the Gross A U3 long terminal repeat restored the full leukemogenic potential of Gross A MuLV. Chimeric viruses harboring only the Gross A 3'-end env region were, however, nonleukemogenic. Similar chimeric MuLVs, constructed with genomes from the parental weakly leukemogenic BALB/c B-tropic MuLVs and nonleukemogenic BALB/c N-tropic MuLVs, were also studied. Our data indicate that the U3 tandem direct repeat sequences appear to be necessary and sufficient to confer some leukemogenic potential to MuLV. However, env 3'-end sequences, mostly the p15E-encoding sequences, are required for the expression of fully leukemic phenotypes.

Mapping the viral sequences conferring leukemogenicity and disease specificity in Moloney and amphotropic murine leukemia viruses

The Moloney murine leukemia virus (MuLV) is a highly leukemogenic virus. To map the leukemogenic potential of Moloney MuLV, we constructed chimeric viral DNA genomes in vitro between parental cloned infectious viral DNA from Moloney and amphotropic 4070-A MuLVs. Infectious chimeric MuLVs were recovered by microinjection of recombinant DNA into NIH/3T3 cells and tested for their leukemogenic potential by inoculation into NIH/Swiss newborn mice. Parental Moloney MuLV and amphotropic 4070-A MuLV induced thymic and nonthymic leukemia, respectively, when inoculated intrathymically. With chimeric MuLVs, we found that the primary determinant of leukemogenicity of Moloney and amphotropic MuLVs lies within the 1.5-kilobase-pair ClaI-PvuI long terminal repeat (LTR)-containing fragment. The presence of additional Moloney env-pol sequences with the Moloney LTR enhanced the leukemogenic potential of a chimeric MuLV significantly, indicating that these sequences were also involved in tumor development. Since parental viruses induced different forms of leukemia, we could also map the viral sequences conferring this disease specificity. We found that the 1.5-kilobase-pair ClaI-PvuI LTR-containing fragment of Moloney MuLV was necessary and sufficient for a chimeric MuLV to induce thymic leukemia. Similarly, the same LTR-containing fragment of amphotropic MuLV was necessary and sufficient for a chimeric MuLV to induce nonthymic leukemia. Therefore, our results suggest that specific sequences within this short LTR-containing fragment determine two important viral functions: the ability to transform cells in vivo (leukemic transformation) and the selection of a specific population of cells to be transformed (disease specificity).

Physical mapping of the paralysis-inducing determinant of a wild mouse ecotropic neurotropic retrovirus

We have recently shown that a molecularly cloned ecotropic retrovirus, initially isolated from the brain of a paralyzed wild mouse, retained the ability to induce hind limb paralysis when inoculated into susceptible mice (Jolicoeur et al., J. Virol. 45:1159-1163, 1983). To map the viral DNA sequences encoding the determinant of paralysis, we constructed chimeric viral DNA genomes in vitro between parental cloned infectious viral DNA genomes from this neurotropic murine leukemia virus (MuLV) and from nonneurotropic amphotropic 4070-A MuLV. Infectious chimeric MuLVs, recovered after microinjection of NIH 3T3 cells with these recombinant DNAs, were inoculated into newborn SIM.S and SWR/J mice to test the paralysis-inducing potential. We found that the 3.9-kilobase-pair SalI-ClaI fragment of the neurotropic MuLV comprising the 3' end of pol and all env sequences was sufficient to confer the paralysis-inducing potential to chimeric viruses. Therefore, this region of the neurotropic MuLV genome most likely harbors the primary determinant of paralysis.

An X-linked gene affecting mouse cell DNA synthesis also affects production of unintegrated linear and supercoiled DNA of murine leukemia virus

To identify specific cellular factors which could be required during the synthesis of retroviral DNA, we have studied the replication of murine leukemia virus in mouse cells temperature sensitive for cell DNA synthesis (M. L. Slater and H. L. Ozer, Cell 7:289-295, 1976) and in several of their revertants. This mutation has previously been mapped on the X chromosome. We found that a short incubation of mutant cells at a nonpermissive temperature (39 degrees C) during the early part of the virus cycle (between 0- to 20-h postinfection) greatly inhibited virus production. This effect was not observed in revertant or wild-type cells. Molecular studies by the Southern transfer procedure of the unintegrated viral DNA synthesized in these cells at a permissive (33 degrees C) or nonpermissive temperature revealed that the levels of linear double-stranded viral DNA (8.8 kilobase pairs) were nearly identical in mutant or revertant cells incubated at 33 or 39 degrees C. However, the levels of two species of supercoiled viral DNA (with one or two long terminal repeats) were significantly lower in mutant cells incubated at 39 degrees C than in mutant cells incubated at 33 degrees C or in revertant cells incubated at 39 degrees C. Pulse-chase experiments showed that linear viral DNA made at 39 degrees C could not be converted into supercoiled viral DNA in mutant cells after a shift down to 33 degrees C. In contrast, such conversion was observed in revertant cells. Restriction endonuclease analysis did not detect differences in the structure of linear viral DNA made at 39 degrees C in mutant cells as compared to linear viral DNA isolated from the same cells at 33 degrees C. However, linear viral DNA made at 39 degrees C in mutant cells was poorly infectious in transfection assays. Taken together, these results strongly suggest that this X-linked gene, affecting mouse cell DNA synthesis, is operating in the early phase of murine leukemia virus replication. It seems to affect the level of production of unintegrated linear viral DNA only slightly while greatly reducing the infectivity of these molecules. In contrast, the accumulation of supercoiled viral DNA and subsequent progeny virus production are greatly reduced. Our pulse-chase experiments suggest that the apparent, but not yet identified, defect in linear viral DNA molecules might be responsible for their subsequent impaired circularization.

Physical mapping of the Fv-1 tropism host range determinant of BALB/c murine leukemia viruses

The murine leukemia viruses (MuLVs) have different host ranges and were originally designated N-tropic and B-tropic if they replicated preferentially in vitro on NIH and BALB/c fibroblasts, respectively. It was later found that N-tropic MuLVs were in fact restricted in BALB/c cells, that B-tropic MuLVs were restricted in NIH cells, and that both viruses were restricted in (BALB X NIH) F1 cells. A single gene, Fv-1, with two alleles, Fv-1b and Fv-1n, determines this dominant restriction. A virus-encoded protein seems to carry the viral host range determinant which is recognized by the Fv-1 gene product. To map the viral DNA sequences encoding this determinant, we constructed viral DNA recombinants in vitro between the cloned infectious viral DNA genomes from BALB/c N-tropic and B-tropic MuLVs. Infectious recombinant MuLVs were recovered by microinjecting these recombinant DNAs into murine Fv-1- SC-1 cells and were subsequently tested in vitro for their host ranges (N- or B-tropic). We found that a short 302-base pair 5'-end fragment was necessary and sufficient to confer a specific host range to a recombinant. Our sequencing data revealed that this fragment codes for amino acid sequences in gag p30. They also showed that only two consecutive amino acid differences, Gln-ArgN- and Thr-GluB-, in p30 are responsible for the N- and B-tropic host ranges of the BALB/c MuLVs, respectively. Therefore, it appears that the Fv-1b and Fv-1n gene products can discriminate between these two p30 amino acid sequences.

Thymotropism of murine leukemia virus is conferred by its long terminal repeat

Several murine leukemia viruses (MuLV) replicate efficiently in the thymus (T+) of the mouse, whereas others are unable to replicate (T-) in this organ. To map the region of the viral genome harboring the sequences responsible for this thymotropic phenotype, we constructed viral DNA recombinants in vitro between cloned infectious viral DNAs from T- BALB/c N-tropic MuLV and from T+ BALB/c B-tropic MuLV or AKR Gross passage A MuLV. (N- and B-tropic refer to the Fv-1 host range of MuLV.) Infectious recombinant MuLVs, recovered from murine cells microinjected with these recombinant DNAs, were injected into newborn mice to test their ability to replicate in the thymus. We found that the long terminal repeat from the T+ BALB/c B-tropic or AKR Gross passage A MuLV genome was sufficient to allow replication of recombinant MuLVs in the thymus. Our sequence data suggested that the U3 tandem direct repeat was responsible for this effect. These results suggest a new role for the U3 long terminal repeat in the replication of MuLV in specific differentiated target cells.

The high leukemogenic potential of Gross passage A murine leukemia virus maps in the region of the genome corresponding to the long terminal repeat and to the 3' end of env

The Gross passage A murine leukemia virus (MuLV) is a highly leukemogenic, ecotropic fibrotropic retrovirus. Its genome is similar to that of other nonleukemogenic ecotropic fibrotropic MuLVs but differs at the 3' end and in the long terminal repeat. To determine whether these modifications were related to its leukemogenic potential, we constructed a viral DNA recombinant in vitro with cloned infectious DNA from this highly leukemogenic Gross passage A MuLV and from a weakly leukemogenic endogenous BALB/c B-tropic MuLV. Infectious viruses, recovered after microinjection of murine cells with recombinant DNA, were injected into newborn mice. We show here that the Gross passage A 1.35-kilobase-pair KpnI fragment (harboring part of gp70, all of p15E, and the long terminal repeat) is sufficient to confer a high leukemogenic potential to this recombinant.

Molecular cloning of infectious viral DNA from ecotropic neurotropic wild mouse retrovirus

Among a mixture of amphotropic and ecotropic murine leukemia viruses (MuLVs) isolated from paralyzed wild mice, only N-tropic ecotropic MuLV, cloned by cell culture techniques, has been shown to induce paralysis after reinjection into susceptible mice (M. B. Gardner, Curr. Top. Microbiol. Immunol. 79:215-239, 1978). The viral DNA genome of one of these neurotropic MuLVs (Cas-Br-E) has been cloned in Charon 21A at the SalI site. One clone, designated NE-8, was studied in more detail. A restriction endonuclease map of this cloned DNA was derived. Cloned viral DNA microinjected into NIH 3T3 cells produced infectious MuLV which was characterized as XC+, ecotropic, and N-tropic. The virus that was recovered after the microinjection of NE-8 DNA was also injected into susceptible SIM.S and NIH Swiss mice and was found to induce lower limb paralysis in these animals. These results make it highly unlikely that other agents (which might have escaped detection and separation from ecotropic MuLV by the techniques previously used) play a role in the etiology of this disease and clearly indicate that the ecotropic MuLV genome harbors sequences responsible for this paralysis. The availability of this clone DNA would now allow us to map these sequences on the genome.

New class of leukemogenic ecotropic recombinant murine leukemia virus isolated from radiation-induced thymomas of C57BL/6 mice

We previously reported the establishment of several lymphoid cell lines from X-ray-induced thymomas of C57BL/Ka mice, and all, except one, produce retroviruses (P. Sankar-Mistry and P. Jolicoeur, J. Virol.35:270-275, 1980). Biological characterization of five of these new primary radiation leukemia viruses (RadLVs) indicated that they had a B-tropic, fibrotropic, and ecotropic host range and were leukemogenic when reinjected into C57BL/Ka newborn mice. The leukemogenic potential of one isolate (G(6)T(2)) was further assessed and shown to be retained after prolonged passaging on fibroblasts in vitro. Restriction endonuclease analysis of the DNA of four of our new RadLV isolates (G(6)T(2), Ti-7, Ti-8, and Ti-9) revealed that G(6)T(2) and Ti-7 murine leukemia virus (MuLV) genomes had identical restriction maps, whereas Ti-8 and Ti-9 genomes were different from each other and from the G(6)T(2) and Ti-7 genomes. The physical maps of these genomes were similar to that of known ecotropic MuLV genomes (including the C57BL/Ka endogenous ecotropic MuLV) within their long terminal repeats, env, the right portion of pol, and the left portion of gag. However, a region covering the end of gag and the beginning of pol was different and showed several similarities with xenotropic MuLV genomes of BALB/c, AKR, and C58 mice previously mapped. Our results suggest that these primary RadLV genomes are recombinants between the parental ecotropic MuLV genome and a nonecotropic (xenotropic) sequence. This nonecotropic gag-pol region might be important in conferring the leukemogenic potential to these isolates. Therefore, these RadLVs appear to form a new class of leukemogenic recombinant MuLVs recovered from leukemic tissues of mice. They appear to be distinct from the recombinant AKR mink cell focus-inducing MuLVs which have a dual-tropic host range and harbor xenotropic env sequences. To further study the leukemogenic potential of these RadLVs, the genome of one of them (G(6)T(2)) was cloned in Charon 21A as an infectious molecule.

Molecular cloning of viral DNA from leukemogenic Gross passage A murine leukemia virus and nucleotide sequence of its long terminal repeat

The viral DNA genome of the leukemogenic Gross passage A virus was cloned in phage Charon 21A as an infectious molecule. The virus recovered by transfection with this infectious DNA was ecotropic, N-tropic, fibrotropic, and XC+. It was leukemogenic when reinjected into newborn SIM mice, indicating that ecotropic murine leukemia virus (MuLV) from an AKR mouse thymoma can harbor leukemogenic sequences. Its restriction map was similar to that of nonleukemogenic AKR MuLV, its putative parent, but differed at the 3' end and in the long terminal repeat (LTR). The nucleotide sequence of the Gross A virus LTR was identical to the AKR MuLV LTR sequence (Van Beveren et al., J. Virol. 41:542-556, 1982) in U5, R, and part of U3. All differences between both LTRs were found in U3. Only one copy of the U3 tandem direct repeat was conserved in the Gross A virus LTR, and it was rearranged by the insertion of a 36-base-pair sequence and by five point mutations. Only one additional point mutation common to several oncogenic MuLVs was present in U3. These structural changes in the U3 LTR and at the 3' end of the genome may be related to the leukemogenicity of this virus.

Synthesis of murine leukemia viral DNA in vitro: evidence for plus-strand DNA synthesis at both ends of the genome

We studied the synthesis of B-tropic murine leukemia viral DNA in vitro by detergent-disrupted virions. The reaction products (detected by the Southern transfer technique) included full-length, infectious, double-stranded DNA and several subgenomic fragments. Restriction endonuclease analysis and hybridization and specific probes revealed two classes of subgenomic fragments: some were derived from the right end of the genome, and some were derived from the left end. Most of the fragments harbored one long terminal repeat copy at their ends, suggesting that they were initiated correctly. S1 nuclease and restriction endonuclease treatments of these fragments indicated that a single-stranded gap was present near the first initiation site of plus strong-stop DNA. The treatments also suggested the presence of a second initiation site flanked by a single-stranded gap 0.9 kilobase pairs from the right end of the genome. Our data clearly show that plus-strand DNA is synthesized at both ends of the genome, by using plus strong stop as the first initiation site and additional initiation sites.

Molecular cloning of B- and N-tropic endogenous BALB/c murine leukemia virus circular DNA intermediates: isolation and characterization of infectious recombinant clones

The unintegrated closed circular DNA intermediates from B- and N-tropic endogenous BALB/c murine leukemia virus (MuLV) were isolated from acutely infected mouse fibroblasts and cleaved at their unique HindIII sites. The linearized B- and N-tropic MuLV DNAs were then inserted in lambda vector Charon 21A and cloned. Seventeen B-tropic and ten N-tropic recombinants were analyzed. The viral insert of these 27 MuLV recombinants could be grouped into 3 classes. The first class included permuted full-length molecules of 8.8 kilobase pairs (kbp) having two long terminal repeats (LTR) (0.55 kbp) mixed with 8.25-kbp molecules. The second class included 8.25-kbp molecules which harbored only one LTR copy. The third class of inserts was made of molecules with deletions of various lengths (0.1 to 3 kbp). These deletions were localized by restriction enzyme analysis. Significantly more deletions were observed in N-tropic than in B-tropic recombinants, although these two genomes isolated from acutely infected cells seemed to differ only by the absence of a 70-base-pair (bp) sequence in the N-tropic LTR. The biological activity of each of the recombinants of the first and second classes was assayed by transfection on NIH/3T3 cells. Two recombinants, one B-tropic (lambda B-16) and one N-tropic (lambdaN-20) were found to be infectious. Both were able to give rise to replication-competent MuLV which could form large XC plaques and had kept the tropism of its progenitor. Like its progenitor in vivo DNA, lambdaN-20-cloned DNA lacked a 70-bp sequence in its unique LTR. The emerging N-tropic MuLV after transfection with lambdaN-20 was used to acutely infect NIH/3T3 cells. The viral DNA intermediates isolated were now found to have two LTR copies. They had also acquired a 70-bp sequence in each LTR. The acquisition or loss of this 70-bp sequence did not seem to affect the replication, the XC plaque formation, or the tropism of the virus.