Diversity in the signals required for nuclear accumulation of U snRNPs and variety in the pathways of nuclear transport.

The requirements for nuclear targeting of a number of U snRNAs have been studied by analyzing the behavior of in vitro-generated transcripts after microinjection into the cytoplasm of Xenopus oocytes. Like the previously studied U1 snRNA, U2 snRNA is excluded from the nucleus when it does not have the 2,2,7mGpppN cap structure typical of the RNA polymerase II (pol II)-transcribed U snRNAs. Surprisingly, two other pol II-transcribed U snRNAs, U4 and U5, have a much less stringent requirement for the trimethyl cap structure. The gamma-monomethyl triphosphate cap structure of the RNA polymerase III-transcribed U6 snRNA, on the other hand, is shown not to play a role in nuclear targeting. Wheat germ agglutinin, which is known to prevent the import of many proteins into the nucleus, inhibits nuclear uptake of U6, but not of U1 or U5 snRNAs. Conversely, a 2,2,7mGpppG dinucleotide analogue of the trimethyl cap structure inhibits transport of the pol II U snRNAs, but does not detectably affect the transport of either U6 snRNA or a karyophilic protein. From these results it can be deduced that U6 enters the nucleus by a pathway similar or identical to that used by karyophilic proteins. The composite nuclear localization signals of the trimethyl cap-containing U snRNPs, however, do not function in the same way as previously defined nuclear targeting signals.

Activation of RET as a dominant transforming gene by germline mutations of MEN2A and MEN2B.

Multiple endocrine neoplasia types 2A and 2B (MEN2A and MEN2B) and familial medullary thyroid carcinoma are dominantly inherited cancer syndromes. All three syndromes are associated with mutations in RET, which encodes a receptor-like tyrosine kinase. The altered RET alleles were shown to be transforming genes in NIH 3T3 cells as a consequence of constitutive activation of the RET kinase. The MEN2A mutation resulted in RET dimerization at steady state, whereas the MEN2B mutation altered RET catalytic properties both quantitatively and qualitatively. Oncogenic conversion of RET in these neoplastic syndromes establishes germline transmission of dominant transforming genes in human cancer.

Activated RET oncogene products induce maturation of xenopus oocytes.

The RET proto-oncogene encodes a transmembrane receptor of the tyrosine kinase family, recently found to be the gene responsible for the multiple endocrine neoplasia type 2A and 2B syndromes. RET was found specifically activated, by gene rearrangement, in human thyroid carcinomas of the papillary subtype. In most cases the activation consisted of an in frame fusion of the RET tyrosine-kinase domain, at the carboxy-terminus, with heterologous genes at the amino-terminus. These chimeric oncogenes are collectively named RET/PTC. Two forms of these gene products, RET/PTC1 and RET/PTC3, have been tested for their ability to induce meiotic maturation in Xenopus oocytes. Injection of RET/PTC mRNAs into immature oocytes induced maturation-promoting-factor (MPF) activation and germinal vesicle breakdown (GVBD). The injected oocytes expressed polypeptides recognized by an anti-RET gene product antibody as well as by an antiphosphotyrosine antibody, indicating activation of the tyrosine-kinase domain. The RET/PTC induced maturation was dependent on endogenous ras; in fact, the coinjection of RET/PTC mRNA with a neutralizing anti-ras antibody blocked oocytes maturation without interfering with the accumulation and tyrosine-phosphorylation of the RET/PTC protein.

Molecular characterization of RET/PTC3; a novel rearranged version of the RETproto-oncogene in a human thyroid papillary carcinoma.

The RET proto-oncogene encodes a transmembrane receptor of the tyrosine kinase family and has frequently been found activated in human thyroid carcinomas of the papillary subtype. In most cases the activation consisted of the fusion of its tyrosine-kinase domain with the 5'-terminal region of a gene designated H4 or D10S170. We have named the resulting H4/RET chimeric oncogene RET/PTC. Another activated form of the RET oncogene has subsequently been found in a thyroid carcinoma and is now referred to as RET/PTC2. Here we report the identification and cloning of a novel rearranged version of the RET oncogene in a human thyroid papillary carcinoma. In this case the tyrosine-kinase domain of RET was fused to a sequence 790 bp long belonging to a new gene that we have named RFG (RET Fused Gene). This novel chimeric oncogene has been designated RET/PTC3. In order to have more insights into the function of RFG we have completely cloned and sequenced its cDNA. RFG predicted amino-acid sequence does not have any significant homology to any already known genes and is ubiquitously expressed in human and mouse tissues. Finally we provide evidence indicating that the rearrangement leading to the generation of RET/PTC3 occurred in vivo in the original tumor DNA.

Activated RET/PTC oncogene elicits immediate early and delayed response genes in PC12 cells.

The expression of the receptor-like tyrosine kinase RET is associated with tumors, tissues or cell lines of neural crest origin. In addition RET products (Ret) are involved in determining cell fate during the differentiation of the enteric nervous system and during renal organogenesis. However, as yet, no direct evidence exists to indicate that the Ret kinase activity might interfere in a specific way with cellular differentiation, or proliferation, of a neural crest derived cell line. By using two constitutively activated forms of RET (RET/PTC1 and RET/PTC3) in transient transfection experiments, we have obtained evidence that active RET could reprogramme the gene expression pattern in the rat pheochromocytoma PC12 cell line. Transcription driven by gene promoters, such as NGFI-A and vgf, which belong, respectively, to primary and delayed response genes to nerve growth factor (NGF), and by the neuron-specific enolase (NSE) promoter, is rapidly induced by the expression of activated RET oncogenes. This induction is not elicited in other non neural derived cell types tested. We also demonstrate that endogenous ras activity is required for RET induction of these neural markers. Finally, in the RET/PTC transfected PC12 cells, NGF is unable to induce further their transcription. This suggests that RET/PTC could share an intracellular signalling pathway with the NGF-receptor.

Only the substitution of methionine 918 with a threonine and not with other residues activates RET transforming potential.

Specific point-mutations of the RET receptor tyrosine kinase protooncogene are responsible for the inheritance of multiple endocrine neoplasia type 2A (MEN2A) and 2B (MEN2B), and familial medullary thyroid carcinoma (FMTC). MEN2B is caused by the substitution of methionine 918 by a threonine in the tyrosine kinase (TK) domain of RET. This mutation converts RET into a dominant transforming oncogene. We have substituted Met918 with four different residues and found that RET acquired transforming activity only when Met918 was substituted with a threonine. However, also when serine and valine, but not leucine or phenylalanine, were inserted in position 918, the RET TK function was activated and induced, especially in the case of the RET(918Ser), immmediate-early response genes. We conclude that the preservation of Met918 is critical for the control of RET kinase. However, only when a threonine residue is present in position 918, does RET efficiently couple with a transforming pathway.

Functional analysis of mutant Xenopus U2 snRNAs.

Methods for studying pre-mRNA splicing in Xenopus oocytes have been improved to allow simultaneous analysis of the splicing reaction and the formation of splicing complexes in vivo. The number, order of appearance, and dependence on intact U1 and U2 snRNPs of complexes formed in vivo on a pre-mRNA substrate are similar but not identical to those observed in vitro. The migration on native gels of the complexes formed in vivo and in vitro is, however, dissimilar. RNAase H-mediated inhibition of splicing caused by oligonucleotide microinjection can be overcome by coinjection of a gene encoding the U snRNA that is targeted for cleavage. Transcripts from the injected gene complement the defect in splicing by assembling into functionally active U snRNPs. Using this assay, mutant U2 snRNAs have been tested for their ability to function in splicing and in splicing complex formation. The results indicate that much of the U2 snRNA, including regions essential for detectable binding of the U2-specific proteins A' and B", is dispensable for splicing.

Changing the RNA polymerase specificity of U snRNA gene promoters.

The promoter of a Xenopus tropicalis U6 gene can be transcribed by both RNA polymerases II and III. Two distinct elements, a TATA-like sequence and the region of transcription initiation, are only required for transcription by RNA polymerase III, while further common elements are required for transcription by both polymerases. Based on the unusually stringent requirement for a purine at the normal position of polymerase III transcription initiation and on the properties of mutants in this region, we suggest that RNA polymerase III itself may recognize the site of transcription initiation and thus be directly involved in efficient promoter selection. We have used the information obtained on U6 promoter structure to manufacture a U6 promoter that is RNA polymerase II-specific and to change the Xenopus U2 gene promoter specificity from RNA polymerase II to RNA polymerase III.

Anti-OTF-1 antibodies inhibit NFIII stimulation of in vitro adenovirus DNA replication.

HeLa cell OTF-1 has been purified on the basis of its DNA binding activity and used to raise a polyclonal rabbit antiserum. This antiserum is shown to recognize both native and denatured OTF-1 from both human and a similar protein from Xenopus culture cells, but to react either more weakly or not at all with the lymphoid cell-specific OTF-2. Separately, NFIII has been purified on the basis of its ability to stimulate Adenovirus DNA replication in vitro. On denaturing polyacrylamide gels OTF-1 and NFIII exhibit identical mobility. Anti-OTF-1 antiserum recognizes NFIII and neutralizes its stimulatory effect on DNA replication. Moreover, OTF-1 can functionally replace NFIII. Taken together with previously published DNA binding data, this indicates that OTF-1 and NFIII are either very closely related or identical.

Major determinants of the specificity of interaction between small nuclear ribonucleoproteins U1A and U2B'' and their cognate RNAs.

The basis of the specificity of interaction of U1 and U2 small nuclear (sn)RNAs and their cognate binding proteins, U1A and U2B'', has been examined. The U1A protein recognizes U1 snRNA on its own, whereas U2B'' binds specifically to U2 snRNA only in the presence of a second protein, U2A'. Exchange of two nucleotides between the two RNAs or of eight amino acids between the two proteins reverses binding specificity.

The U2B'' RNP motif as a site of protein-protein interaction.

The U2 snRNP contains two specific proteins, U2B'' and U2A'. Neither of these proteins, on its own, is capable of specific interactions with U2 RNA. Here, a complex between U2B'' and U2A' that forms in the absence of RNA is identified. Analysis of mutant forms of U2B'' shows that the smallest fragment able to bind specifically U2 RNA (amino acids 1-88) is also the minimal region required for complex formation with U2A', and implies that this region must be largely structurally intact for U2A' interaction. Although this truncated U2B'' fragment is capable of making specific protein--RNA and protein-protein interactions its structure, as measured by the ability to bind to U2A'', appears to depend on the rest of the protein. Hybrids between U2B'' and the closely related U1A protein are used to localize U2B'' specific amino acids involved in protein-protein interaction. These can be divided into two functional groups. U2A' interaction with U2B'' amino acids 37-46 permits binding to U2 RNA whereas interaction with U2B'' specific amino acids between positions 14 and 25 reduces non-specific binding to U1 RNA. These two proteins may serve as a general example of how RNA binding may be modulated by protein-protein interaction in the assembly of RNPs, particularly since the region of U2'' involved in interaction with U2A' consists mainly of a conserved RNP motif.

Multiple domains of U1 snRNA, including U1 specific protein binding sites, are required for splicing.

Domains of U1 snRNA which are functionally important have been identified using a splicing complementation assay in Xenopus oocytes. Mutations in, and deletions of, all three of the hairpin loop structures near the 5' end of the RNA are strongly deleterious. Similarly, mutation of the Sm binding site abolishes complementation activity. Analysis of the protein binding properties of the mutant U1 snRNAs reveals that three of the functionally important domains, the first two hairpin loops and the Sm binding site, are required for interaction with U1 snRNP proteins. The fourth functionally important domain does not detectably affect snRNP protein binding and is not evolutionarily conserved. All of the deleterious mutations are shown to have similar effects on in vivo splicing complex formation.

A mutation in the RET proto-oncogene in Hirschsprung's disease affects the tyrosine kinase activity associated with multiple endocrine neoplasia type 2A and 2B.

We demonstrate that a Hirschsprung (HSCR) mutation in the tyrosine kinase domain of the RET proto-oncogene abolishes in cis the tyrosine-phosphorylation associated with the activating mutation in multiple endocrine neoplasia type 2A (MEN2A) in transiently transfected Cos cells. Yet the double mutant RET2AHS retains the ability to form stable dimers, thus dissociating the dimerization from the phosphorylation potential. Co-transfection experiments with single and double mutants carrying plasmids RET2A and RET2AHS in different ratios drastically reduced the phosphorylation levels of the RET2A protein, suggesting a dominant-negative effect of the HSCR mutation. Also, the phosphorylation associated with the multiple endocrine neoplasia type 2B (MEN2B) allele was affected in experiments with single and double mutants carrying plasmids co-transfected under the same conditions. Finally, analysis of the enzymic activity of MEN2A and MEN2B tumours confirmed the relative levels of tyrosine phosphorylation observed in Cos cells, indicating that this condition, in vivo, may account for the RET transforming potential.

Identification of the RNA binding segment of human U1 A protein and definition of its binding site on U1 snRNA.

The interaction between the U1 snRNP-specific U1 A protein and U1 snRNA has been analysed. The binding site for the protein on the RNA is shown to be in hairpin II, which extends from positions 48 to 91 in the RNA. Within this hairpin the evolutionarily conserved loop sequence is crucial for interaction with U1 A protein. U1 A protein can also bind the loop sequence when it is part of an artificial RNA which cannot form a stable hairpin structure. The region of the protein required to bind to U1 snRNA consists of a conserved 80 amino acid motif, previously identified in many ribonucleoprotein (RNP) proteins, together with (maximally) 11 N-terminal and 10 C-terminal flanking amino acids. Point mutations introduced into two of the most highly conserved regions of this motif abolish RNA binding. U1 snRNA mutants from which the U1 A binding site has been deleted are shown to be capable of assembly into RNP particles which are immunoprecipitable by patient antisera which recognize U1 A protein. The role of RNA-protein and protein-protein interactions in U snRNP assembly are discussed.

Developmental expression of the RET protooncogene.

The RET protooncogene encodes a transmembrane protein of the receptor-type tyrosine kinase family whose ligand has not yet been identified. Its activation in vivo is restricted to human carcinomas of the thyroid. In order to learn more about the possible role played by RET during normal development, we have examined its expression by performing in situ hybridization experiments on mouse embryos. Here, we show that the RET protooncogene is expressed during mouse embryogenesis in an unusual temporal and spatial manner. In fact, its expression was first detected around day 10 of gestation in the basal plate of the neural tube and in the developing encephalic ganglia, and later its pattern of expression was definitely established in neural structures, mostly in neural crest derivatives (spinal and encephalic ganglia). As far as the central nervous system is concerned, RET expression was confined to the ventral part of the midbrain from 12.5 days postcoitum (dpc) until birth. RET was also found to be expressed within structures of sensory organs such as the ganglial layer of the retina and the olfactory epithelium. A peculiar pattern of RET expression was clearly observed in the wall of the gut and in the nephrogenic zone of the developing kidney cortex, specifically in the metanephrogenic vesicles. Finally, RET was found to be expressed in the liver mostly between 12.5 dpc and 14.5 dpc. In conclusion, its expression in the early stages of embryogenesis suggests that RET may play a role in the differentiation of specific neural structures and the excretory system.

A weak interaction between the U2A' protein and U2 snRNA helps to stabilize their complex with the U2B" protein.

The U2 snRNP complex contains two specific proteins, U2B" and U2A'. We have analysed the interaction of U2A' with U2B" and with U2 RNA. U2A' can form an weak but detectable RNA-protein complex with U2 RNA and a stable protein complex with U2B". This protein-protein complex binds efficiently and specifically to U2 RNA. Binding experiments with mutant forms of U2A' shows that the region of U2A' essential for binding to U2B" is extensive, being located between amino acid position 1-164. The behaviour of the wild type U2A' protein, and in particular of a mutant version of the protein in which amino acids 3, 4 and 5 are mutated, suggests that U2A' forms a weak interaction with U2 RNA which helps to stabilize the U2A'-U2B"-U2 RNA complex. Mutants of U2 RNA were used to localize the region of U2 RNA important for interaction with U2A'. The results show that U2A' interacts with the stem of hairpin IV.

Ligand stimulation of a Ret chimeric receptor carrying the activating mutation responsible for the multiple endocrine neoplasia type 2B.

Inherited activating mutations of Ret, a receptor tyrosine kinase, predispose to multiple endocrine neoplasia (MEN) types 2A and 2B and familial medullary thyroid carcinoma. To investigate the effects induced by acute stimulation of Ret, we transfected both PC12 and NIH 3T3 cells with a molecular construct in which the ligand-binding domain of the epidermal growth factor receptor was fused to the catalytic domain of Ret. Acute stimulation of the chimeric receptor induced PC12 cells to express a neuronal-like phenotype. Moreover, we introduced the dominant mutation, responsible for the multiple endocrine neoplasia type 2B, in the catalytic domain of the Ret chimera. Expression of the mutant chimera, in the absence of ligand stimulation, induces the PC12 cells to acquire a flat morphology with short neuritic processes and transforms the NIH 3T3 cells. Stimulation of the mutant chimera with epidermal growth factor causes a drastic overgrowth of long neuritic processes, with the induction of the suc1-associated protein tyrosine phosphorylation in PC12 cells and higher transforming efficiency in NIH 3T3 cells. These data indicate that the gain-of-function MEN2B mutation does not abrogate ligand responsiveness of Ret and suggest that the presence of Ret ligand could play a role in the pathogenesis of the MEN2B syndrome.