The fragile X-related proteins FXR1P and FXR2P contain a functional nucleolar-targeting signal equivalent to the HIV-1 regulatory proteins.

Fragile X syndrome is caused by the absence of the fragile X mental-retardation protein (FMRP). FMRP and the fragile X-related proteins 1 and 2 (FXR1P and FXR2P) form a gene family with functional similarities, such as RNA binding, polyribosomal association and nucleocytoplasmic shuttling. In a previous study, we found that FMRP and FXR1P shuttle between cytoplasm and nucleoplasm, while FXR2P shuttles between cytoplasm and nucleolus. The nuclear and nucleolar-targeting properties of these proteins were investigated further. Here, we show that FXR2P contains in its C-terminal part, a stretch of basic amino acids 'RPQRRNRSRRRRFR' that resemble the nucleolar-targeting signal (NoS) of the viral protein Rev. This particular sequence is also present within exon 15 of the FXR1 gene. This exon undergoes alternative splicing and is therefore only present in some of the FXR1P isoforms. We investigated the intracellular distribution of various FXR1P isoforms with (iso-e and iso-f) and without (iso-d) the potential NoS in transfected COS cells treated with the nuclear export inhibitor leptomycin-B. Both iso-e and iso-f showed a nucleolar localization, as observed for FXR2P; iso-d was detected in the nucleo-plasm outside the nucleoli. Further, when a labelled 16-residue synthetic peptide corresponding to the NoS of FXR1P was added to human fibroblast cultures a clear nucleolar signal was observed. Based on these data we argue that the intranuclear distribution of FXR2P and FXR1P isoforms is very likely to be mediated by a similar NoS localized in their C-terminal region. This domain is absent in some FXR1P isoforms as well as in all FMRP isoforms, suggesting functional differences for this family of proteins, possibly related to RNA metabolism in different tissues.

Immunocytochemical and biochemical characterization of FMRP, FXR1P, and FXR2P in the mouse.

Fragile X syndrome is caused by the absence of expression of the FMR1 gene. Both FXR1 and FXR2 are autosomal gene homologues of FMR1. The products of the three genes are belonging to a family of RNA-binding proteins, called FMRP, FXR1P, and FXR2P, respectively, and are associated with polyribosomes as cytoplasmic mRNP particles. The aim of the present study is to obtain more knowledge about the cellular function of the three proteins (Fxr proteins) and their interrelationships in vivo. We have utilized monospecific antibodies raised against each of these proteins and performed Western blotting and immunolabeling at the light-microscopic level on tissues of wild-type and Fmr1 knockout adult mice. In addition, we have performed immunoelectron microscopy on hippocampal neurons of wild-type mice to study the subcellular distribution of the Fxr proteins. A high expression was found in brain and gonads for all three proteins. Skeletal muscle tissue showed only a high expression for Fxr1p. In the brain the three proteins were colocalized in the cytoplasm of the neurons; however, in specific neurons Fxr1p was also found in the nucleolus. Immunoelectronmicrsocopy on hippocampal neurons demonstrated the majority of the three proteins in association with ribosomes and a minority in the nucleus. The colocalization of the Fxr proteins in neurons is consistent with similar cellular functions in those specific cells. The presence of the three proteins in the nucleus of hippocampal neurons suggests a nucleocytoplasmic shuttling for the Fxr proteins. In maturing and adult testis a differential expression was observed for the three proteins in the spermatogenic cells. The similarities and differences between the distribution of the Fxr proteins have implications with respect to their normal function and the pathogenesis of the fragile X syndrome.

Immunological detection of polycystin-1 in human kidney.

Polycystin-1 is the protein product of the PKD1 gene. Mutations in this gene are responsible for most cases of polycystic kidney disease, but little is known about how these mutations lead to the development of cysts. Indeed, even less is known about the normal role of polycystin-1 in the kidney. The cellular localization of polycystin-1 has been the subject of intense investigation by many groups, including ours. In this report we describe our results and compare our data with those of others. We generated 14 different polyclonal antisera against fragments of the predicted 462-kDa polycystin-1 molecule to enable us to investigate the expression of polycystin-1 in cells and tissues by immunocytochemistry, western blotting, and immunoprecipitation. Our antibodies readily recognized a 134-kDa polycystin-1 fragment overexpressed in COS cells and stained the epithelial cells of fetal, adult, and cystic kidney sections with the same pattern as reported by others. However, further investigations revealed that this pattern was not specific for polycystin-1. We could not unequivocally detect polycystin-1 in vivo, either by immunoblotting or immunocytochemistry. Therefore our studies do not support the reported pattern of polycystin-1 expression in the kidney.

Oligomerization properties of fragile-X mental-retardation protein (FMRP) and the fragile-X-related proteins FXR1P and FXR2P.

The absence of fragile-X mental-retardation protein (FMRP) results in fragile-X syndrome. Two other fragile-X-related (FXR) proteins have been described, FXR1P and FXR2P, which are both very similar in amino acid sequence to FMRP. Interaction between the three proteins as well as with themselves has been demonstrated. The FXR proteins are believed to play a role in RNA metabolism. To characterize a possible functional role of the interacting proteins the complex formation of the FXR proteins was studied in mammalian cells. Double immunofluorescence analysis in COS cells over-expressing either FMRP ISO12/FXR1P or FMRP ISO12/FXR2P confirmed heterotypic interactions. However, Western-blotting studies on cellular homogenates containing physiological amounts of the three proteins gave different indications. Gel-filtration experiments under physiological as well as EDTA conditions showed that the FXR proteins were in complexes of >600 kDa, as parts of messenger ribonuclear protein (mRNP) particles associated with polyribosomes. Salt treatment shifted FMRP, FXR1P and FXR2P into distinct intermediate complexes, with molecular masses between 200 and 300 kDa. Immunoprecipitations of FMRP as well as FXR1P from the dissociated complexes revealed that the vast majority of the FXR proteins do not form heteromeric complexes. Further analysis by [(35)S]methionine labelling in vivo followed by immunoprecipitation indicated that no proteins other than the FXR proteins were present in these complexes. These results suggest that the FXR proteins form homo-multimers preferentially under physiological conditions in mammalian cells, and might participate in mRNP particles with separate functions.

Different targets for the fragile X-related proteins revealed by their distinct nuclear localizations.

Fragile X syndrome is caused by the absence of the fragile X mental retardation protein (FMRP). FMRP and its structural homologues FXR1P and FXR2P form a family of RNA-binding proteins (FXR proteins). The three proteins associate with polyribosomes as cytoplasmic mRNP particles. Here we show that small amounts of FMRP, FXR1P and FXR2P shuttle between cytoplasm and nucleus. Mutant FMRP of a severely affected fragile X patient (FMRPI304N) does not associate with polyribosomes and shuttles more frequently than normal FMRP, indicating that the association with polyribosomes regulates the shuttling process. Using leptomycin B we demonstrate that transport of the FXR proteins out of the nucleus is mediated by the export receptor exportin1. Finally, inactivation of the nuclear export signal in two FXR proteins shows that FMRP shuttles between cytoplasm and nucleoplasm, while FXR2P shuttles between cytoplasm and nucleolus. Therefore, molecular dissection of the shuttling routes used by the FXR proteins suggests that they transport different RNAs.

Subcellular localization of the oncoprotein MTG8 (CDR/ETO) in neural cells.

The t(8;21) translocation associated with acute myeloid leukemia (AML) disrupts two genes, the AML1 gene also known as the core binding factor A2 (CBFA2) on chromosome 21, and a gene on chromosome 8, hereafter referred to as MTG8, but also known as CDR and ETO. Extensive information is available on AML1, a member of the CBF family of transcription factors, containing a highly conserved domain, the runt box, of the Drosophila segmentation gene runt. This gene is essential for the hematopoietic development and is found disrupted in several leukemias. In contrast, the function of the MTG8 gene is poorly understood. The predicted protein sequence shows two unusual, putative zinc-fingers, three proline-rich regions, a PEST domain and several phosphorylation sites. In addition, we found a region encompassing aa 443-514 predicted to have a significant propensity to form coiled coil structures. MTG8 displays a high degree of similarity with nervy, a homeotic target gene of Drosophila, expressed in the nervous system. Human and mouse wild-type MTG8 are also highly expressed in brain relative to other tissues. For these reasons, we set out to investigate the expression and subcellular localization of the MTG8 protein in neural cells. Immunohistochemical experiments in a 12.5-day-old mouse embryo clearly showed that the protein was expressed in the neural cells of the developing brain and the spinal cord. In primary cultures of hippocampal neurons of 2-3 day-old mice, MTG8 was found in the nucleus, in the cytoplasm and as fine granules in the neurites. Cytoplasmic localization of the protein was observed in Purkinje cells of both human and mouse cerebellum. The molecular mass of MTG8 in total human and mouse brain was analysed by immunoblotting and determined to be between 70 and 90 kDa. Isoforms with the same molecular mass were demonstrated in synaptosomes isolated from mouse forebrain. The evidence of MTG8 in the nucleus and cytoplasm of neural cells suggests a specific mechanism regulating the subcellular localization of the protein.

Differential expression of FMR1, FXR1 and FXR2 proteins in human brain and testis.

Lack of expression of the fragile X mental retardation protein (FMRP) results in mental retardation and macroorchidism, seen as the major pathological symptoms in fragile X patients. FMRP is a cytoplasmic RNA-binding protein which cosediments with the 60S ribosomal subunit. Recently, two proteins homologous to FMRP were discovered: FXR1 and FXR2. These novel proteins interact with FMRP and with each other and they are also associated with the 60S ribosomal subunit. Here, we studied the expression pattern of the three proteins in brain and testis by immunohistochemistry. In adult brain, FMR1, FXR1 and FXR2 proteins are coexpressed in the cytoplasm of specific differentiated neurons only. However, we observed a different expression pattern in fetal brain as well as in adult and fetal testis, suggesting independent functions for the three proteins in those tissues during embryonic development and adult life.

The Han:SPRD rat is not a genetic model of human autosomal dominant polycystic kidney disease type 1.

Human autosomal dominant polycystic kidney disease (ADPKD) is a high incidence disorder leading to renal failure in many patients. The majority of cases results from a mutation in the PKD1 gene. The only well documented animal model of ADPKD is the Han:SPRD-Pkd strain. Its genetic basis is unknown as yet. In the current study we determined whether the disease in these rats is genetically linked to the rat homologue of the PKD1 gene. We used the protamine gene as a polymorphic marker (Prm1) of the PKD1 region. Matings of Han:SPRD-Pkd with BB rats and backcross of the offspring with BB yielded animals informative for linkage analysis. This analysis revealed random segregation of the defect and the Prm1 marker, indicating that the model is not caused by a mutation in the PKD1 gene. We conclude that the Han:SPRD-Pkd rat strain is not a genetic model of PKD1.

The fragile X syndrome.

The fragile X syndrome is caused by the amplification of a polymorphic CGG repeat in the 5' untranslated region of the FMR1 gene and is the most common form of inherited mental retardation. When the repeat is amplified beyond 200 repeat units, the repeat and the FMR1 promoter region are methylated. As a result of this methylation the gene is silenced and no FMR1 gene product (FMRP) is translated. The lack of expression of FMRP in the fragile X syndrome causes the fragile X phenotype. A mouse model for the fragile X syndrome (knockout for FMRP) has been generated to study the pathological mechanisms leading to the symptoms seen in fragile X patients. FMRP is widely expressed in different tissues and localized predominantly in the cytoplasm associated with the 60S ribosomal subunit. The protein has RNA binding properties and possibly shuttles between cytoplasm and nucleus. The target signals necessary for this intracellular transport, like a nuclear location signal and a nuclear export signal, are present in FMRP. FMRP is also able to bind to other proteins by using specific sequence domains present in the protein. The coiled-coil structures formed by these domains are known to be involved in protein-protein interaction. In this review we postulate that FMRP is involved in the transport of RNA and/or proteins from the nucleus to the cytoplasm.

Animal model for fragile X syndrome.

The fragile X syndrome, one of the most common forms of inherited mental retardation, is caused by an expansion of a polymorphic CGG repeat upstream of the coding region in the FMR1 gene. The expansion blocks expression of the FMR1 gene due to methylation of the FMR1 promoter. Functional studies on the FMR1 protein have shown that the protein can bind RNA and might be involved in transport of RNAs from the nucleus to the cytoplasm. A role of FMR1 protein on translation of certain mRNAs has been suggested. An animal model for fragile X syndrome exists and these mice show some behavioural difficulties mimicking the human fragile X syndrome phenotype. This review presents what is known about the protein and what is learned from the animal model for fragile X syndrome.

Efficacy of a peptide-based gene delivery system depends on mitotic activity.

We have developed and tested a transfection compound based on synthetic peptides. It consists of a 12 amino acid DNA binding peptide (P2) with an alkyl group added to the aminoterminus (P2lip) and a peptide derived from the hemagglutinin protein (HA). The components aggregate spontaneously to particles that proved to be an efficient, easy to use and chemically stable transfection compound. With this system we found a marked correlation between transfection efficiency and mitotic activity. Cells that are allowed to perform a mitosis after exposure to either DNA-P2lip/HA or DNA-cationic liposome complexes are transfected much more efficiently than cells arrested in the cell cycle. In search of an explanation for this phenomenon we studied transport of plasmid DNA across the nuclear membrane. Plasmid DNA injected into the cytoplasm of quiescent human fibroblasts is not expressed, in contrast to DNA injected into the nucleus. FISH analysis showed that the plasmid DNA is not transported into the nucleus efficiently. Similarly, DNA-P2lip/HA complexes are readily taken up by both proliferating and nonproliferating cells, but do not readily penetrate the nuclear membrane. We conclude that delivery of plasmid DNA to the cytoplasm is not sufficient for transfection of eukaryotic cells. The nuclear membrane is apparently an important barrier. This explains why a mitotic event is required for efficient transfection with the currently available transfection systems. The implications for the further development of transfection compounds for use in vivo, where nonproliferating cells are often the target, are discussed.

FMRP is associated to the ribosomes via RNA.

The FMR1 transcript is alternatively spliced and generates different splice variants coding for FMR1 proteins (FMRP) with a predicted molecular mass of 70-80 kDa. FMRP is widely expressed and localized in the cytoplasm. To study a possible interaction with other cellular components, FMRP was isolated and characterized under non-denaturing conditions. Under physiological salt conditions FMRP appears to have a molecular mass of > 600 kDa, indicating a binding to other cellular components. This interaction is disrupted in the presence of high salt concentrations. The dissociation conditions to free FMRP from the complex are similar to the dissociation of FMRP from RNA as shown before. The binding of FMRP from the complex is also disrupted by RNAse treatment. That the association of FMRP to a high molecular weight complex possibly occurs via RNA, is further supported by the observation that the binding of FMRP, containing an lle304Asn substitution, to the high molecular weight complex is reduced. An equal reduced binding of mutated FMRP to RNA in vitro was observed before under the same conditions. The reduced binding of FMRP with the lle304Asn substitution further indicates that the interaction to the complex indeed occurs via FMRP and not via other RNA binding proteins. In a reconstitution experiment where the low molecular mass FMRP (70-80 kDa) is mixed with a reticulocyte lysate (enriched in ribosomes) it was shown that FMRP can associate to ribosomes and that this binding most likely occurs via RNA.

Detection and subcellular localization of an AML1 chimeric protein in the t(8;21) positive acute myeloid leukemia.

AML1, a gene encoding a protein of the PEBP2/CBF family of transcription factors is disrupted by translocations associated with human leukemia. In the t(8;21) acute myelogenous leukemia (AML), AML1 was found fused to a gene on chromosome 8 that we designated CDR (also known as ETO and MTG8). Immunoprecipitation experiments followed by immunoblotting using a combination of antibodies against different epitopes of one of the predicted chimeric proteins encoded by a fully characterized fusion transcript enabled us to visualize a chimeric protein in the t(8;21) Kasumi-1 cell line. The estimated size of this protein is 64 kDa. Immunoblotting of leukemic blasts containing the t(8;21) detected a protein of the same size. Immunofluorescence experiments indicate that the chimeric protein is localized in the nucleus. A normal AML1 protein of 27 kDa was also detected in t(8;21) Kasumi-1 cells. It remains to be established by which mechanism the mutant AML1 isoform may contribute to the leukemogenesis process of t(8;21)-positive acute myeloid leukemia.

A fluorimetric enzyme assay for the diagnosis of Sanfilippo disease type A (MPS IIIA).

4-Methylumbelliferyl-alpha-D-N-sulphoglucosaminide (MU-alpha-GlcNS) was synthesized and shown to be a substrate for the lysosomal heparin sulphamidase. Sanfilippo A patients' fibroblasts (n = 42) and lymphocytes (n = 1) showed 0-3% of mean normal heparin sulphamidase activity; in total leukocytes from patients (n = 8) sulphamidase activity was clearly deficient. In fibroblasts from obligate heterozygotes for Sanfilippo A, the sulphamidase activity was reduced in 9 out of 10 cases. Heparin sulphamidase desulphates MU-alpha GlcNS to MU-alpha GlcNH2 and further hydrolysis during a second incubation is required to liberate 4-methylumbelliferone, which can be measured. Yeast alpha-glucosidase, which has low but sufficient alpha-glucosaminidase activity, was used to hydrolyse the reaction intermediate MU-alpha GlcNH2 to release 4-methylumbelliferone and free glucosamine.

Normal phenotype in two brothers with a full FMR1 mutation.

The fragile X syndrome is associated with an expanding CGG repeat in the 5' untranslated region of the first exon of the FMR1 gene. Subsequent methylation of the promoter region inhibits expression of the FMR1 gene. In two clinically normal brothers large, expanded CGG repeats and cytogenetically visible fragile sites were found. The FMR1 promoter was unmethylated and both RNA and protein could be detected. This indicates that inactivation of the FMR1 gene and not repeat expansion itself results in the fragile X phenotype. We conclude that repeat expansion does not necessarily induce methylation and that methylation is no absolute requirement for the induction of fragile sites.

Characterization of FMR1 proteins isolated from different tissues.

FMR1 protein expression was studied in different tissues. In human, monkey and murine tissues, high molecular mass FMR1 proteins (67-80 kDa) are found, as shown in lymphoblastoid cells lines. The identity of these proteins was confirmed by their absence in tissues from patients with the fragile X syndrome and a FMR1 knock-out mouse. An Ile367Asn substitution in the FMR1 protein did not alter the translation, processing and localization of FMR1 proteins in lymphoblastoid cells from a patient carrying this mutation. All the high molecular mass FMR1 proteins isolated from normal lymphoblastoid cells and cells from the patient with the Ile367Asn substitution were able to bind RNA. However, the FMR1 proteins of the patient had reduced affinity for RNA binding at high salt concentrations. In some human, monkey and murine tissues low molecular mass FMR1 proteins (39-41 kDa) were found, which had the same N terminus as the 67-90 kDa isoforms, but differ in their C terminus and are therefore most likely the result of carboxy-terminal proteolytic cleavage. These low molecular mass FMR1 proteins did not bind RNA, in contrast with the high molecular mass FMR1 proteins. The significance of these low molecular mass proteins remains to be studied.

Effects of striatal excitotoxicity on huntingtin-like immunoreactivity.

The relationship between the specific neuronal loss observed in Huntington's disease and the mutation in the IT15 gene responsible for this disease remains obscure. Using an antipeptide antibody against amino acids 3114-3141 of the human huntington protein, we demonstrate that striatal injection of quinolinic acid in mice induces increased immunoreactivity for huntington in some remaining neurons but not in glial cells. This increase is apparent in both neuronal cell bodies and in cell processes in the white matter six hours after excitotoxic challenge. This finding suggests that huntington may be involved in the response to excitotoxic stress in these neurons.

Characterization and localization of the Huntington disease gene product.

The recent identification of the Huntington's disease (HD) gene, enabled us to synthesize oligopeptides corresponding with the carboxy-terminal end of the predicted HD-gene (IT15) product. Immunobiochemcial studies with polyclonal antibodies directed against this synthetic peptide (position 3114-3141) on lymphoblastoid cells from normal individuals and patients with Huntington disease, revealed the presence of a protein (huntingtin) with a molecular mass of approximately 330 kDa. Immunocytochemical studies showed a cytoplasmic localization of huntingtin in various cell types including neurons. In most of the neuronal cells the protein was also present in the nucleus. No difference in molecular mass or intracellular localization was found between normal and mutant cells.

Characterization and localization of the FMR-1 gene product associated with fragile X syndrome.

The fragile X syndrome is the most frequent form of inherited mental retardation after Down's syndrome, having an incidence of one in 1,250 males. The fragile X syndrome results from amplification of the CGG repeat found in the FMR-1 gene. This CGG repeat shows length variation in normal individuals and is increased significantly in both carriers and patients; it is located 250 base pairs distal to a CpG island which is hypermethylated in fragile X patients. The methylation probably results in downregulation of FMR-1 gene expression. No information can be deduced about the function of the FMR-1 protein from its predicted sequence. Here we investigate the nature and function of the protein encoded by the FMR-1 gene using polyclonal antibodies raised against the predicted amino-acid sequences. Four different protein products, possibly resulting from alternative splicing, have been identified by immunoblotting in lymphoblastoid cell lines of healthy individuals. All these proteins were missing in cell lines from patients not expressing FMR-1 messenger RNA. The intracellular localization of the FMR-1 gene products was investigated by transient expression in COS-1 cells and found to be cytoplasmic. Localization was also predominantly cytoplasmic in the epithelium of the oesophagus, but in some cells was obviously nuclear.

A chicken embryo model to study the growth of human uveal melanoma.

In vitro cultured human uveal and skin melanoma cells were injected into the chicken embryonal eye at a stage when the immune system was not yet mature. The melanoma cells were accepted as part of the organism by the host. Even single melanoma cells could be traced by morphological methods as well as by immunohistochemical markers, such as S100, HMB-45, NKI/C3 and HNK-1. We found tumors in 20 and 40 percent of the embryos injected with uveal melanoma and skin melanoma, respectively. The embryos did not exhibit abnormal development of the eye as a result of the microinjection and had a high survival rate (90 and 60%, respectively) during embryogenesis. With this model for uveal melanoma the growth and possibly the metastatic behavior of human uveal melanoma cells can be studied.