RNA-binding activity of hepatitis delta antigen involves two arginine-rich motifs and is required for hepatitis delta virus RNA replication

Hepatitis delta antigen (HDAg) is an RNA-binding protein with binding specificity for hepatitis delta virus (HDV) RNA (J. H. Lin, M. F. Chang, S. C. Baker, S. Govindarajan, and M. M. C. Lai, J. Virol. 64:4051-4058, 1990). By amino acid sequence homology search, we have identified within its RNA-binding domain two stretches of an arginine-rich motif (ARM), which is present in many prokaryotic and eukaryotic RNA-binding proteins. The first one is KERQDHRRRKA and the second is EDEKRERRIAG, and they are separated by 29 amino acids. Deletion of either one of these ARM sequences resulted in the total loss of the in vitro RNA-binding activity of HDAg. Thus, HDAg is different from other RNA-binding proteins in that it requires two ARM-like sequences for its RNA-binding activity. Replacement of the spacer sequence between the two ARMs with a shorter stretch of sequence also reduced RNA binding in vitro. Furthermore, site-specific mutations of the basic amino acid residues in both ARMs resulted in the total loss or reduction of RNA-binding activity. The biological significance of the RNA-binding activity was studied by examining the trans-activating activity of the RNA-binding mutants. The plasmids expressing HDAgs with various mutations in the RNA-binding motifs were cotransfected with a replication-defective HDV dimer cDNA construct into COS cells. It was found that all the HDAg mutants which had lost the in vitro RNA-binding activity also lost the ability to complement the defect of HDV RNA replication. We conclude that the trans-activating function of HDAg requires its binding to HDV RNA.

Autoimmune VH gene family: PCR-generated murine germline VH10 genes

A relatively large number of variable region genes (V) contribute, via gene rearrangements with smaller numbers of additional gene elements (D and J), to generate diversity in the immune response. While some VH gene families are thought to contain 100- 1000 members, the VH10 family has only two known functioning members with 99% sequence homology. Both members (monoclonal antibodies) are capable of binding DNA, and since they were derived from inbred mice afflicted with the lupus syndrome they are considered autoimmune antibodies. Relative uniqueness of the VH10 primary nucleotide sequence presents a model system with which to examine unrearranged VH genes and attempt to identify germline genes eventually expressed as autoantibodies. PCR amplified germline sequences of the VH10 family are highly conserved, with few base substitutions evenly distributed between both framework and CDR regions. It was determined that the PCR amplified germline sequences are highly similar to the DNA sequences of the two monoclonal VH10 antibodies, and a non-functional psuedo-germline gene was found that is identical to a non-functional cDNA derived from a hybridoma cell line. These findings indicate that the use of unique CDR DNA sequences for the identification and amplification of specific germline V genes via PCR can yield vital information that may answer fundamental questions about the origins of autoimmune anti-DNA antibodies in afflicted individuals. The nature of the germline gene populations and the possible microheterogeniety of these genes may prove to be important in understanding the role of autoimmune antibodies in normal and diseased individuals.

An internal deletion in the cytoplasmic tail reverses the apical localization of human NGF receptor in transfected MDCK cells

A cDNA encoding the full-length 75-kD human nerve growth factor receptor was transfected into MDCK cells and its product was found to be expressed predominantly (80%) on the apical membrane, as a result of vectorial targeting from an intracellular site. Apical hNGFR bound NGF with low affinity and internalized it inefficiently (6% of surface bound NGF per hour). Several mutant hNGFRs were analyzed, after transfection in MDCK cells, for polarized surface expression, ligand binding, and endocytosis. Deletionof juxta-membrane attachment sites for a cluster of O-linked sugars did not alter apical localization. A mutant receptor lacking the entire cytoplasmic tail (except for the five proximal amino acids) was also expressed on the apical membrane, suggesting that information for apical sorting was contained in the ectoplasmic or transmembrane domains. However, a 58 amino acid deletion in the hNGFR tail that moved a cytoplasmic tyrosine (Tyr 308) closer to the membrane into a more charged environment resulted in a basolateral distribution of the mutant receptor and reversed vectorial (basolateral) targeting. The basolateral mutant receptor also internalized 125I-NGF rapidly (90% of surface bound NGF per hour), exhibited a larger intracellular fraction and displayed a considerably shortened half-life (approximately 3 h). We suggest that hNGFR with the internal cytoplasmic deletion expresses a basolateral targeting signal, related to endocytic signals, that is dominant over apical targeting information in the ecto/transmembrane domains. These results apparently contradict a current model that postulates that basolateral targeting is a default mechanism.

Oculocerebrocutaneous (Delleman) syndrome: a pleiotropic disorder affecting ectodermal tissues with unilateral predominance

We present a patient with oculocerebrocutaneous syndrome. The boy shows only mild psychomotor delay in spite of rather severe appearing anomalies of the central nervous system. A primarily unilateral involvement of this syndrome is emphasized. A postzygotic/somatic mutation resulting in a mosaic state might account for the primarily ectodermal involvement, the unilateral predominance, and the sporadic nature of this syndrome. An alternative hypothesis of an environmental factor might also explain the clinical manifestations of the syndrome.

The antigen of hepatitis delta virus: examination of in vitro RNA-binding specificity

The only known protein of hepatitis delta virus (HDV), the delta antigen, is found both within virus particles and within the nucleus of the infected cell, where it has one or more roles essential for RNA genome replication. Others have demonstrated that the antigen has the ability, in vitro, to specifically bind HDV RNA species. We report a further examination of this phenomenon, using partially purified recombinant protein, expressed as a fusion with the staphylococcal protein A. From Northwestern (RNA-immunoblot) analyses with both complete and various subdomains of HDV genomic and antigenomic RNAs, we found that a necessary feature for specific binding was that the RNA be able to fold to some extent into the so-called rodlike structure; this structure is a predicted intramolecular partial base-pairing of the circular RNA, with about 70% of all bases involved, so as to produce an unbranched rodlike structure. Six different subregions of the HDV rodlike structure, three on the genomic RNA and three on its complement, the antigenomic RNA, were tested and found to be sufficient for antigen binding. However, features in addition to the rodlike structure may also be necessary for specific binding, because we found that a similar structure present in the RNA of the potato spindle tuber viroid did not allow binding.

The roles of the delta antigen in the structure and replication of hepatitis delta virus

The delta antigen is the only known protein encoded by hepatitis delta virus (HDV). The predicted protein is 195 amino acids in length, but for reasons that are not yet clear, there occurs during the replication of the HDV genome, a specific base change in the termination codon of this open reading frame. This leads to the synthesis of a form of the delta antigen that is 19 amino acids longer, with a total length of 214 amino acids. Studies are described which relate to the roles of these two forms of the delta antigen in genome replication and subsequent particle assembly.

Hepatitis delta virus genome replication: a polyadenylated mRNA for delta antigen

Hepatitis delta virus (HDV) replicates its genome in the nucleus of an infected cell. However, an unsolved problem has been the identification in the cytoplasm of a putative mRNA for the synthesis of the only virus-coded protein, the delta antigen. We now report the characterization of an 800-base RNA that is cytoplasmic, polyadenylated, and antigenomic and that should direct the translation of the delta antigen. This RNA was about 500 times less abundant than full-length genomic RNA. We mapped the predominant 5' terminus and also the 3' site at which the poly(A) is added. At a point 15 to 20 bases upstream of the poly(A) addition site is the sequence AAUAAA, which could have been used as a signal for the polyadenylation. When an infectious cDNA clone of the whole HDV genome was changed at this site to UUUAAA, the clone was no longer infectious and it was unable to direct the synthesis of the delta antigen. These findings provided additional evidence that the polyadenylated RNA was at least the predominant method for the expression of the delta antigen. Apparently the HDV RNA was processed as if it were a host mRNA polymerase II transcript, although this did not necessarily indicate that HDV RNA was transcribed with this enzyme.

A specific base transition occurs on replicating hepatitis delta virus RNA

Three independent lines of evidence showed that when an infectious clone of hepatitis delta virus of known sequence was used to initiate genome replication, up to 41% of the genomes were specifically mutated in the amber termination codon (UAG to UGG) for the open reading frame of the delta antigen, thereby increasing the length of the predicted protein from 195 to 214 amino acids. This change was detected only on molecules that participated in RNA-directed RNA synthesis.

Cloned hepatitis delta virus cDNA is infectious in the chimpanzee

A head-to-tail trimer of a full-length cDNA clone of the hepatitis delta virus (HDV) genome was examined for infectivity by direct inoculation into the liver of a chimpanzee that was already infected with hepatitis B virus. Five weeks after inoculation, a marked elevation of serum alanine aminotransferase activity was observed, followed by the appearance of high levels of HDV RNA and antigen in both liver and serum and a high level of viral particles in the serum. A transient suppression of hepatitis B virus replication was evident during the acute phase of HDV infection. Seroconversion for antibodies to delta antigen occurred 3 weeks after the onset of the disease. These results demonstrate that a typical HDV infection can be initiated by inoculation of a susceptible animal with recombinant HDV cDNA.

Gene transfer of truncated NGF receptor clones leads to cell surface expression in mouse fibroblasts

Transfection of recombinant bacteriophage clones encoding human NGF receptor sequences resulted in cell surface expression in mouse fibroblasts. Unexpectedly, receptors were expressed even after transfection with phage clones which lack 5' gene sequences. Stable transformants were purified and analyzed in detail. S1 nuclease protection and primer extension analysis revealed that an initiation site lies within an intron sequence in the middle of the receptor gene. A truncated mRNA transcript was detected that allowed for the expression of NGF receptors capable of binding to NGF. Since the original phage clones lacked the first two exons, these results suggest that the normal N-terminal sequences may not be necessary for cell surface expression and binding to NGF.

Initiation of replication of the human hepatitis delta virus genome from cloned DNA: role of delta antigen

Beginning with three partial cDNA clones of the RNA genome of human hepatitis delta virus (HDV), we assembled the complete 1,679-base sequence on a single molecule and then inserted a trimer of this into plasmid pSLV, a simian virus 40-based eucaryotic expression vector. This construct was used to transfect both monkey kidney (COS7) and human hepatocellular carcinoma (HuH7) cell lines. In this way we obtained replication of the HDV RNA genome and the appearance, in the nucleoli, of the delta antigen, the only known virus-coded protein. This proved both that the HDV genome could replicate in nonliver as well as liver cells and that there was no requirement for the presence of hepatitis B virus sequences or proteins. When the pSVL construct was made with a dimer of an HDV sequence with a 2-base-pair deletion in the open reading frame, genome replication was reduced at least 40-fold. However, when we cotransfected with a plasmid that expressed the correct delta antigen, the mutated dimer achieved a level of genome replication comparable to that of the nonmutated sequence. We thus conclude that the delta antigen can act in trans and is essential for replication of the HDV genome.

A constitutive promoter directs expression of the nerve growth factor receptor gene

Expression of nerve growth factor receptor is normally restricted to cells derived from the neural crest in a developmentally regulated manner. We analyzed promoter sequences for the human nerve growth factor receptor gene and found that the receptor promoter resembles others which are associated with constitutively expressed genes that have housekeeping and growth-related functions. Unlike these other genes, the initiation of transcription occurred at one major site rather than at multiple sites. The constitutive nature of the nerve growth factor receptor promoter may account for the ability of this gene to be transcribed in a diverse number of heterologous cells after gene transfer. The intron-exon structure of the receptor gene indicated that structural features are precisely divided into discrete domains.

Axons regulate Schwann cell expression of the major myelin and NGF receptor genes

The elaboration of myelin by Schwann cells is triggered by contact with appropriate peripheral axons. Among the most prominent features of this interaction is the activation and high-level expression of the genes encoding the major myelin proteins P0 and Myelin Basic Protein (MBP). Although the initial induction of these genes is thought to be dependent upon contact with axons, neither the inductive signal of the axon nor the receptor and associated second messenger system of the Schwann cell that transduces this signal has been identified. In this report, we demonstrate that expression of the P0 and MBP genes in rapidly myelinating Schwann cells is sharply reduced upon withdrawal of axons, but that this expression can be substantially restored by agents that raise the intracellular concentration of cyclic AMP. We further show that Schwann cell expression of a third gene, i.e. that encoding the Nerve Growth Factor receptor, is strongly activated by the withdrawal of axons, and that this activation is largely independent of cAMP.

Expression and structure of the human NGF receptor

The nucleotide sequence for the human nerve growth factor (NGF) receptor has been determined. The 3.8 kb receptor mRNA encodes a 427 amino acid protein containing a 28 amino acid signal peptide, an extracellular domain containing four 40 amino acid repeats with six cysteine residues at conserved positions followed by a serine/threonine-rich region, a single transmembrane domain, and a 155 amino acid cytoplasmic domain. The sequence of the extracellular domain of the NGF receptor predicts a highly ordered structure containing a negatively charged region that may serve as the ligand-binding site. This domain is conserved through evolution. Transfection of a full-length cDNA in mouse fibroblasts results in stable expression of NGF receptors that are recognized by monoclonal antibodies to the human NGF receptor and that bind [125I]NGF.

The nerve growth factor receptor gene is at human chromosome region 17q12-17q22, distal to the chromosome 17 breakpoint in acute leukemias

Genomic and cDNA clones for the human nerve growth factor receptor have been used in conjunction with somatic cell hybrid analysis and in situ hybridization to localize the nerve growth factor receptor locus to human chromosome region 17q12-q22. Additionally, part, if not all, of the nerve growth factor receptor locus is present on the translocated portion of 17q (17q21-qter) from a poorly differentiated acute leukemia in which the chromosome 17 breakpoint was indistinguishable cytogenetically from the 17 breakpoint observed in the t(15;17)(q22;q21) translocation associated with acute promyelocytic leukemia. Thus the nerve growth factor receptor locus may be closely distal to the acute promyelocytic leukemia-associated chromosome 17 breakpoint at 17q21.

Human alpha- and beta-globin gene transcription in mouse erythroleukaemia cells

Human beta-globin genes introduced into mouse erythroleukaemia (MEL) cells by DNA co-transformation are correctly regulated when erythroid cell differentiation is induced by dimethylsulphoxide (DMSO). In contrast, cloned human alpha-globin genes are efficiently transcribed in MEL cells before induction, and no increase in the level alpha-globin mRNA is observed when the cells differentiate. These observations suggest that the mechanisms by which alpha- and beta-globin genes are activated during erythroid cell differentiation are fundamentally different. Analysis of the transcription of hybrid human alpha-beta-globin genes in MEL cells revealed that the sequences responsible for differences in transcription of the intact alpha- and beta-globin genes are located on the 3' side of the mRNA capping site of the two genes, suggesting that cis-acting regulatory sequences are located within the structural genes.

Differences in human alpha- and beta-globin gene expression in mouse erythroleukemia cells: the role of intragenic sequences

Human beta-globin genes introduced into mouse erythroleukemia (MEL) cells by DNA cotransformation are correctly regulated when erythroid cell differentiation is induced by dimethylsulfoxide (DMSO). In contrast, cloned human alpha-globin genes are efficiently transcribed in MEL cells prior to induction, and no increase in the level of alpha-globin mRNA is observed when the cells differentiate. These observations suggest that the mechanisms by which alpha- and beta-globin genes are activated during erythroid cell differentiation are fundamentally different. Analysis of the transcription of hybrid human alpha/beta-globin genes in MEL cells revealed that the sequences responsible for differences in transcription of the intact alpha- and beta-globin genes are located on the 3' side of the mRNA capping site of the two genes, suggesting that cis-acting regulatory sequences are located within the structural genes.

The structure and function of a eukaryotic promoter

Transformation, or DNA-mediated gene transfer, permits the stable introduction of new genetic information into a cell and therefore provides an opportunity to examine the expression of exogenous DNA sequences in the transformed host. A series of deletion mutants to analyze the sequences required for accurate and quantitative in vivo transcription of the herpes simplex virus (HSV) thymidine kinase (tk) gene has been constructed. Control of the efficiency of transcription as well as rough specification of the sites of initiation reside within sequences 40-110 nucleotides from the tk structural gene. The sequences responsible for induction of tk mRNA overlap this transcriptional control region. Analysis of the conformation of the integrated tk gene in the chromosome reveals that these DNA sequences are exquisitely sensitive to nuclease attack. The major heat shock gene of Drosophila and the human beta globin gene have also been introduced into murine cells to identify sequences responsible for induction of these finely regulated genes.

Longitudinal karyotype and genetic signature analysis of cultured human colon adenocarcinoma cell lines LS180 and LS174T

Giemsa-banded chromosomes were analyzed at intervals during either 34 or 70 serial subcultivations of two cell lines, LS180 and LS174T, established from one primary human colon adenocarcinoma, and at passage 14 of autochthonous normal bowel cells, NB(LS174T). The cell lines were established and subcultured by either scraping or trypsin treatment of primary cultures; the scraped cell line was designated LS180, and the trypsin-dispersed cell line was named LS174T. Early passages of LS174T cells were composed mainly of 46,XX (38%) and 45,X (34%) karyotypes; LS180 cultures possessed cells with 46,XX (54%), 45,X (7.5%), and 47,XX+D (19.5%) chromosome modes. In both cell lines, the 45,X karyotype predominated in later subcultivations. After the fifth passage, all LS180 cells examined exhibited a translocation from the long arm of the X chromosome to the long arm of the No. 5 chromosome. Cultures from the patient's normal bowel mucosa and peripheral blood leukocytes had normal 46,XX karyotypes. Genetic signature analysis sustantiated the common genetic origin of the cell lines, and we concluded that differences observed between LS180 and LS174T were not due to contamination with other cell lines. LS180 and LS174T represent closely related cell lines differing cytogenetically in a translocation.