Role of CELF2 in ferroptosis: Potential targets for cancer therapy (Review)

Ferroptosis is a novel form of regulated cellular necrosis that plays a critical role in promoting cancer progression and developing drug resistance. The main characteristic of ferroptosis is iron‑dependent lipid peroxidation caused by excess intracellular levels of reactive oxygen species. CUGBP ELAV‑like family number 2 (CELF2) is an RNA‑binding protein that is downregulated in various types of cancer and is associated with poor patient prognoses. CELF2 can directly bind mRNA to a variety of ferroptosis control factors; however, direct evidence of the regulatory role of CELF2 in ferroptosis is currently limited. The aim of the present review was to summarise the findings of previous studies on CELF2 and its role in regulating cellular redox homeostasis. The present review may provide insight into the possible mechanisms through which CELF2 affects ferroptosis and to provide recommendations for future studies.

CircLIFR Inhibits Non-small Cell Lung Cancer Progression by Acting as a miR-429 Sponge to Enhance CELF2 Expression

Lung cancer is the most commonly diagnosed cancer and the leading reason for tumor-related mortality, while non-small cell lung cancer (NSCLC) is the most usual type of lung cancer. Circular RNAs (circRNAs) have emerged as vital regulators in the development of human cancers, including NSCLC. We aimed to explore the functions of circRNA leukemia inhibitory factor receptor (circLIFR) in NSCLC progression. Real-time quantitative polymerase chain reaction (RT-qPCR) was used to quantify the expression of circLIFR, microRNA-429 (miR-429), and Elav-like family member 2 (CELF2) in NSCLC tissues and cells. Cell proliferation capability of NSCLC cells was determined by Cell Counting Kit-8 (CCK-8) and colony formation assays. The flow cytometry assay was performed to evaluate cell-cycle distribution and apoptosis of NSCLC cells. The abilities of migration and invasion were measured by transwell assay. In addition, the activities of caspase 3 and caspase 9 were measured by the assay kits. The interaction relationship between miR-429 and circLIFR or CELF2 was analyzed by dual-luciferase reporter, RNA immunoprecipitation (RIP), and RNA pull-down assays. The expression levels of related proteins were examined by Western Blot assay. The xenograft experiment was established to explore the role of circLIFR in vivo. CircLIFR, circular, and stable transcript in NSCLC cells, was decreased more than 2 folds in NSCLC tissues and cells than controls (P < 0.0001). Importantly, overexpression of circLIFR impeded cell proliferation, migration, invasion, and inactivated protein kinase B (AKT)/phosphatase and tensin homolog (PTEN)-signaling pathways while enhanced apoptosis and cell-cycle arrest in NSCLC cells, which was overturned by upregulation of miR-429 or silencing of CELF2. Furthermore, the upregulation of circLIFR inhibited NSCLC tumor growth in vivo. Overexpression of circLIFR could suppress NSCLC progress by acting as a sponge of miR-429 to regulate the expression of CELF2 and PTEN/AKT-signaling pathways in NSCLC.

CELF4 regulates translation and local abundance of a vast set of mRNAs, including genes associated with regulation of synaptic function

RNA–binding proteins have emerged as causal agents of complex neurological diseases. Mice deficient for neuronal RNA–binding protein CELF4 have a complex neurological disorder with epilepsy as a prominent feature. Human CELF4 has recently been associated with clinical features similar to those seen in mutant mice. CELF4 is expressed primarily in excitatory neurons, including large pyramidal cells of the cerebral cortex and hippocampus, and it regulates excitatory but not inhibitory neurotransmission. We examined mechanisms underlying neuronal hyperexcitability in Celf4 mutants by identifying CELF4 target mRNAs and assessing their fate in the absence of CELF4 in view of their known functions. CELF4 binds to at least 15%–20% of the transcriptome, with striking specificity for the mRNA 3′ untranslated region. CELF4 mRNA targets encode a variety of proteins, many of which are well established in neuron development and function. While the overall abundance of these mRNA targets is often dysregulated in Celf4 deficient mice, the actual expression changes are modest at the steady-state level. In contrast, by examining the transcriptome of polysome fractions and the mRNA distribution along the neuronal cell body-neuropil axis, we found that CELF4 is critical for maintaining mRNA stability and availability for translation. Among biological processes associated with CELF4 targets that accumulate in neuropil of mutants, regulation of synaptic plasticity and transmission are the most prominent. Together with a related study of the impact of CELF4 loss on sodium channel Na_v1.6 function, we suggest that CELF4 deficiency leads to abnormal neuronal function by combining a specific effect on neuronal excitation with a general impairment of synaptic transmission. These results also expand our understanding of the vital roles RNA–binding proteins play in regulating and shaping the activity of neural circuits.

Epilepsy is a devastating brain disorder whereby a loss of regulation of electrochemical signals between neurons causes too much excitation and ultimately results in an “electrical storm” known as a seizure. Epilepsy can be heritable, but it is usually genetically complex, resulting from a collaboration of many genes. It is also a frequent feature of other common brain diseases, such as autism spectrum disorder and intellectual disability, likely because these diseases have a similar dysregulation of neuronal communication. To understand more about how the brain regulates electrical activity, we focused on an RNA–binding protein called CELF4, because a) mice that lack CELF4 have a complex form of epilepsy that includes features of other neurological diseases and b) this kind of protein has the potential to be a master regulator. We show that CELF4 binds to a vast array of mRNAs, and without CELF4 these mRNAs accumulate in the wrong places and can produce the wrong amount of protein. Moreover, many of these mRNAs encode key players in electrochemical signaling between neurons. Although the defects in individual mRNAs are modest, like a genetically complex disease, together these alterations collude to cause neurological symptoms including recurrent seizures.

Genome-wide association of familial late-onset Alzheimer's disease replicates BIN1 and CLU and nominates CUGBP2 in interaction with APOE

Late-onset Alzheimer's disease (LOAD) is the most common form of dementia in the elderly. The National Institute of Aging-Late Onset Alzheimer's Disease Family Study and the National Cell Repository for Alzheimer's Disease conducted a joint genome-wide association study (GWAS) of multiplex LOAD families (3,839 affected and unaffected individuals from 992 families plus additional unrelated neurologically evaluated normal subjects) using the 610 IlluminaQuad panel. This cohort represents the largest family-based GWAS of LOAD to date, with analyses limited here to the European-American subjects. SNPs near APOE gave highly significant results (e.g., rs2075650, p = 3.2×10⁻⁸¹), but no other genome-wide significant evidence for association was obtained in the full sample. Analyses that stratified on APOE genotypes identified SNPs on chromosome 10p14 in CUGBP2 with genome-wide significant evidence for association within APOE ε4 homozygotes (e.g., rs201119, p = 1.5×10⁻⁸). Association in this gene was replicated in an independent sample consisting of three cohorts. There was evidence of association for recently-reported LOAD risk loci, including BIN1 (rs7561528, p = 0.009 with, and p = 0.03 without, APOE adjustment) and CLU (rs11136000, p = 0.023 with, and p = 0.008 without, APOE adjustment), with weaker support for CR1. However, our results provide strong evidence that association with PICALM (rs3851179, p = 0.69 with, and p = 0.039 without, APOE adjustment) and EXOC3L2 is affected by correlation with APOE, and thus may represent spurious association. Our results indicate that genetic structure coupled with ascertainment bias resulting from the strong APOE association affect genome-wide results and interpretation of some recently reported associations. We show that a locus such as APOE, with large effects and strong association with disease, can lead to samples that require appropriate adjustment for this locus to avoid both false positive and false negative evidence of association. We suggest that similar adjustments may also be needed for many other large multi-site studies.

Genetic factors are well-established to play a role in risk of Alzheimer's disease (AD). However, it has been difficult to find genes that are involved in AD susceptibility, other than a small number of genes that play a role in early-onset, high-penetrant disease risk, and the APOE ε4 allele, which increases risk of late-onset disease. Here we use a European-American family-based sample to examine the role of common genetic variants on late-onset AD. We show that variants in CUGBP2 on chromosome 10p, along with nearby variants, are associated with AD in those highest-risk APOE ε4 homozygotes. We have replicated this interaction in an independent sample. CUGBP2 has one isoform that is expressed predominantly in neurons, and identification of such a new risk locus is important because of the severity of AD. We also provide support for recently proposed associated variants (BIN1, CLU, and partly CR1) and show that there are markers throughout the genome that are correlated with APOE. This emphasizes the need to adjust for APOE for such markers to avoid false associations and suggests that there may be confounding for other diseases with similar strong risk loci.

RNA binding protein CUGBP2/CELF2 mediates curcumin-induced mitotic catastrophe of pancreatic cancer cells

BACKGROUND

Curcumin inhibits the growth of pancreatic cancer tumor xenografts in nude mice; however, the mechanism of action is not well understood. It is becoming increasingly clear that RNA binding proteins regulate posttranscriptional gene expression and play a critical role in RNA stability and translation. Here, we have determined that curcumin modulates the expression of RNA binding protein CUGBP2 to inhibit pancreatic cancer growth.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we show that curcumin treated tumor xenografts have a significant reduction in tumor volume and angiogenesis. Curcumin inhibited the proliferation, while inducing G2-M arrest and apoptosis resulting in mitotic catastrophe of various pancreatic cancer cells. This was further confirmed by increased phosphorylation of checkpoint kinase 2 (Chk2) protein coupled with higher levels of nuclear cyclin B1 and Cdc-2. Curcumin increased the expression of cyclooxygenase-2 (COX-2) and vascular endothelial growth factor (VEGF) mRNA, but protein levels were lower. Furthermore, curcumin increased the expression of RNA binding proteins CUGBP2/CELF2 and TIA-1. CUGBP2 binding to COX-2 and VEGF mRNA was also enhanced, thereby increasing mRNA stability, the half-life changing from 30 min to 8 h. On the other hand, silencer-mediated knockdown of CUGBP2 partially restored the expression of COX-2 and VEGF even with curcumin treatment. COX-2 and VEGF mRNA levels were reduced to control levels, while proteins levels were higher.

CONCLUSION/SIGNIFICANCE

Curcumin inhibits pancreatic tumor growth through mitotic catastrophe by increasing the expression of RNA binding protein CUGBP2, thereby inhibiting the translation of COX-2 and VEGF mRNA. These data suggest that translation inhibition is a novel mechanism of action for curcumin during the therapeutic intervention of pancreatic cancers.

1H, 13C and 15N NMR assignments of RNA recognizing motifs 1 and 2 of BRUNOL-3 protein from human involved in myotonic dystrophy

BRUNOL-3 protein, an alternate splicing factor, has been known for playing a major role in myotonic dystrophy. It binds to the cTNT m-RNA and prevents splicing of exon-5 region, leading to translation of troponin protein having differential affinity for Ca(2+). Here, we report sequence-specific (1)H, (13)C, and (15)N resonance assignments for RNA recognition motifs 1 and 2 of BRUNOL-3 protein.

Ethanol inhibited apoptosis-related RNA binding protein, Napor-3 gene expression in the prenatal rat brain

BACKGROUND

Cell death and differentiations are the critical processes in developing fetal brain, where ethanol induces lots of changes in gene expression patterns of fetal nervous system leading to fetal alcohol syndrome (FAS). The objective of the present study was to observe whether maternal ethanol exposure can alter gene expression pattern in mother and in fetus during mid and late prenatal stage.

MATERIAL/METHODS

10% ethanol was orally applied to female Spraque-Dawley rats and fetuses were sacrificed on gestational day (GD) 19.5 and 21.5. Total mRNA was isolated for differential-display PCR (DD-PCR) and sequence was analyzed to find out the homologous gene(s) using GenBank database of the BLAST program. Finally, the gene expression pattern in different maternal and fetal brain areas of the control and the ethanol treated groups were studied by RNase protection assay (RPA) and in situ hybridization.

RESULTS

Out of several differentially expressed genes, apoptosis-related RNA binding protein (RBP), 'Napor-3' mRNA expression was significantly inhibited by ethanol in fetal rat fore-, mid- and hind- brain, and adult rat cortex and hippocampus when compared with the untreated control. The cDNA analysis was further supported our result (accession: AF090697, 95% sequence homology).

CONCLUSIONS

The age and area dependent suppression of apoptosis-related RBP, Napor-3 gene expression in proliferating fetal brain by maternal ethanol suggesting high susceptibility towards ethanol intake at the time of neuronal cell development and proliferation. Further, ethanol also affects maternal brain tissues that may be one of the reasons for ethanol-induce irreversible damage of the developing brain. The present study for the first time provides the evidence that Napor-3 suppression by ethanol during mid and late stage fetal brain converts natural physiological event apoptosis into pathological process, which may be useful as a novel therapeutic approach towards FAS-associated developmental brain damage as a consequence of maternal drinking behavior.

Mice deficient for RNA-binding protein brunol1 show reduction of spermatogenesis but are fertile

RNA-binding proteins are involved in post-transcriptional processes like mRNA stabilization, alternative splicing, and transport. Brunol1 is a novel mouse gene related to elav/Bruno family of genes encoding for RNA-binding proteins. We report here the expression and functional analysis of murine Brunol1. Expression analysis of Brunol1 during embryogenesis by RT-PCR showed that Brunol1 expression starts at 9.5 dpc and continues to the later stages of embryonic development. In adult mice, the Brunol1 expression is restricted to brain and testis. We also analyzed the Brunol1 expression in testes of different mutants with spermatogenesis defects: W/W(V), Tfm/y, Leyl(-/-), olt/olt, and qk/qk. Brunol1 transcript was detectable in Leyl(-/-), olt/olt, and qk/qk mutant but not in W/W(V) and Tfm/y mutants. We also showed by transfection of a fusion protein of green fluorescent protein and Brunol1 protein into NIH3T3 cells, that Brunol1 is localized in cytoplasm and nucleus. In order to elucidate the function of the Brunol1 protein in spermatogenesis, we disrupted the Brunol1 locus in mouse by homologous recombination, which resulted in a complete loss of the Brunol1 transcript. Male and female Brunol1(+/-) and Brunol1(-/-) mice from genetic backgrounds C57BL/6J x 129/Sv hybrid and 129X1/SvJ when inbred exhibited normal phenotype and are fertile, although the number and motility of sperms are significantly reduced. An intensive phenotypic analysis showed no gross abnormalities in testis morphology. Collectively our results demonstrate that Brunol1 might be nonessential protein for mouse embryonic development and spermatogenesis.

Complex seizure disorder caused by Brunol4 deficiency in mice

Idiopathic epilepsy is a common human disorder with a strong genetic component, usually exhibiting complex inheritance. We describe a new mouse mutation in C57BL/6J mice, called frequent-flyer (Ff), in which disruption of the gene encoding RNA-binding protein Bruno-like 4 (Brunol4) leads to limbic and severe tonic–clonic seizures in heterozygous mutants beginning in their third month. Younger heterozygous adults have a reduced seizure threshold. Although homozygotes do not survive well on the C57BL/6J background, on mixed backgrounds homozygotes and some heterozygotes also display spike-wave discharges, the electroencephalographic manifestation of absence epilepsy. Brunol4 is widely expressed in the brain with enrichment in the hippocampus. Gene expression profiling and subsequent analysis revealed the down-regulation of at least four RNA molecules encoding proteins known to be involved in neuroexcitability, particularly in mutant hippocampus. Genetic and phenotypic assessment suggests that Brunol4 deficiency in mice results in a complex seizure phenotype, likely due to the coordinate dysregulation of several molecules, providing a unique new animal model of epilepsy that mimics the complex genetic architecture of common disease.

Epilepsy is a very common brain disorder characterized by recurrent seizures, resulting from abnormal nerve cell activity in the brain. Some cases of epilepsy are caused by brain trauma, such as stroke, infection, tumor, or head injury. Others—so called “idiopathic”—do not have a clear cause. Many idiopathic epilepsies run in families, but the inheritance patterns and complex seizure types suggest that they are not due to a single defective gene but instead are caused by multiple gene defects that are inherited simultaneously in a patient. This complex inheritance makes it difficult to pinpoint the underlying defects. Here, we describe a new mutant mouse, called “frequent-flyer,” which has several different types of seizures. Although these seizures are caused by a mutation in a single gene, because this gene regulates the expression of many other genes, which, in turn, cause abnormal nerve cell activity, frequent-flyer mice provide a unique animal model of epilepsy—mimicking the complex genetic architecture of common disease.

Expression, localization and tau exon 10 splicing activity of the brain RNA-binding protein TNRC4

Elucidating the mechanisms of alternative splicing in the brain is a prerequisite to the understanding of the pathogenesis of major neurological diseases linked to impairment of pre-mRNA alternative splicing. The gene trinucleotide repeat-containing 4 (TNRC4) is predicted to encode a member of the CELF (CUG-BP- and ETR-3-like factors) family of RNA-binding proteins containing a 15-18-residue polyglutamine sequence. The TNRC4 transcript is selectively expressed in the brain. Using an anti-peptide antibody against the predicted sequence, we establish the presence of TNRC4 as a approximately 50 kDa protein in the brain. Full-length TNRC4 displays nuclear and cytoplasmic localizations in transfected cells, whereas a C-terminally truncated mutant is essentially confined to the cytoplasm. TNRC4 is not recruited into inclusions formed by polyglutamine-expanded ataxin-1 or huntingtin. TNRC4 activates tau exon 10 (E10) inclusion at high efficiency in transfected cells. TNRC4 contains two consecutive N-terminal RNA recognition motifs (RRMs) separated from the C-terminal RRM. Deletion and point mutant analysis show that the activity of TNRC4 on tau E10 splicing is mainly mediated by the RNA-binding activity of the second RRM and involves an intronic element of the tau pre-mRNA. The polyglutamine sequence has no effect on the activity of TNRC4 on tau E10 splicing. This study represents the first characterization of TNRC4 and provides further insight into the mechanisms of brain-specific alternative splicing and their possible pathological implications.

Functional antagonism between RNA binding proteins HuR and CUGBP2 determines the fate of COX-2 mRNA translation

BACKGROUND AND AIMS

Cyclooxygenase-2 (COX-2) expression is regulated at the levels of messenger RNA (mRNA) stability and translation by AU-rich elements (ARE) located in its 3' untranslated region (3'UTR). Although structurally homologous RNA binding proteins HuR and CUGBP2 stabilize COX-2 mRNA, HuR induces whereas CUGBP2 inhibits COX-2 mRNA translation. This study aimed to determine the antagonism between these proteins on COX-2 expression.

METHODS

COX-2 ARE binding activity was determined by nitrocellulose filter binding and UV cross-linking assays using recombinant glutathione S-transferase (GST)/HuR and GST/CUGBP2. Protein:protein interactions were determined by GST pull-down, yeast 2-hybrid, and immunocytochemistry assays. Nucleocytoplasmic shutting was determined by heterokaryon analyses. The effect of CUGBP2 and HuR on COX-2 ARE-dependent translation was shown by a chimeric luciferase mRNA containing COX-2 3'UTR. HT-29 cells were subjected to 12 Gy gamma-irradiation in a cesium irradiator.

RESULTS

CUGBP2 and HuR bind with similar affinities to COX-2 ARE, but CUGBP2 competes with HuR for binding. In vitro, HuR and CUGBP2 heterodimerize. Furthermore, FLAG-tagged HuR and myc-tagged CUGBP2 colocalize in the nucleus of HCT-116 cells. Moreover, both proteins shuttled between the nucleus and cytoplasm. In vitro, HuR enhanced whereas CUGBP2 inhibited translation of the chimeric luciferase COX-2 3'UTR mRNA. Furthermore, CUGBP2 competitively inhibited HuR-mediated translation of the transcript. In HT-29 cells transfected with HuR and CUGBP2, a switch in COX-2 mRNA binding from predominantly HuR to CUGBP2 occurred after radiation treatment, which was coupled with increased silencing of the COX-2 mRNA.

CONCLUSIONS

CUGBP2 overrides HuR and suppresses COX-2 mRNA translation.

ApoB mRNA editing is mediated by a coordinated modulation of multiple apoB mRNA editing enzyme components

Apolipoprotein (apo)B mRNA editing is accomplished by a large multiprotein complex. How these proteins interact to achieve the precise single-nucleotide change induced by this complex remains unclear. We investigated the relationship between altered apoB mRNA editing and changes in editing enzyme components to evaluate their roles in editing regulation. In the mouse fetal small intestine, we found that the dramatic developmental upregulation of apoB mRNA editing from approximately 3% to 88% begins with decreased levels of inhibitory CUG binding protein 2 (CUGBP2) expression followed by increased levels of apoB mRNA editing enzyme (apobec)-1 and apobec-1 complementation factor (ACF) (4- and 8-fold) and then by decreased levels of the inhibitory components glycine-arginine-tyrosine-rich RNA binding protein (GRY-RBP) and heterogeneous nuclear ribonucleoprotein (hnRNP)-C1 (75% and 56%). In contrast, the expression of KH-type splicing regulatory protein (KSRP), apobec-1 binding protein (ABBP)1, ABBP2, and Bcl-2-associated athanogene 4 (BAG4) were unaltered. In the human intestinal cell line Caco-2, the increase of apoB mRNA editing from approximately 1.7% to approximately 23% was associated with 6- and 3.2-fold increases of apobec-1 and CUGBP2, respectively. In the mouse large intestine, the editing was 48% and had a 2.7-fold relatively greater CUGBP2 level. Caco-2 and the large intestine thus have increased instead of decreased CUGBP2 and a lower level of editing, suggesting that inhibitory CUGBP2 may play a critical role in the magnitude of editing regulation. Short interfering RNA-mediated gene-specific knockdown of CUGBP2, GRY-RBP, and hnRNP-C1 resulted in increased editing in Caco-2 cells, consistent with their known inhibitory function. These data suggest that a coordinated expression of editing components determines the magnitude and specificity of apoB mRNA editing.

ETR-3 represses Tau exons 2/3 inclusion, a splicing event abnormally enhanced in myotonic dystrophy type I

Altered splicing of transcripts, including the insulin receptor (IR) and the cardiac troponin (cTNT), is a key feature of myotonic dystrophy type I (DM1). CELF and MBNL splicing factor members regulate the splicing of those transcripts. We have previously described an alteration of Tau exon 2 splicing in DM1 brain, resulting in the favored exclusion of exon 2. However, the factors required for alternative splicing of Tau exon 2 remain undetermined. Here we report a decreased expression of CELF family member and MBNL transcripts in DM1 brains as assessed by RT-PCR. By using cellular models with a control- or DM1-like splicing pattern of Tau transcripts, we demonstrate that ETR-3 promotes selectively the exclusion of Tau exon 2. These results together with the analysis of Tau exon 6 and IR exon 11 splicing in brain, muscle, and cell models suggest that DM1 splicing alteration of several transcripts involves various factors.

Cardiac tissue-specific repression of CELF activity disrupts alternative splicing and causes cardiomyopathy

Members of the CELF family of RNA binding proteins have been implicated in alternative splicing regulation in developing heart. Transgenic mice that express a nuclear dominant-negative CELF protein specifically in the heart (MHC-CELFDelta) develop cardiac hypertrophy and dilated cardiomyopathy with defects in alternative splicing beginning as early as 3 weeks after birth. MHC-CELFDelta mice exhibit extensive cardiac fibrosis, severe cardiac dysfunction, and premature death. Interestingly, the penetrance of the phenotype is greater in females than in males despite similar levels of dominant-negative expression, suggesting that there is sex-specific modulation of splicing activity. The cardiac defects in MHC-CELFdelta mice are directly attributable to reduced levels of CELF activity, as crossing these mice with mice overexpressing CUG-BP1, a wild-type CELF protein, rescues defects in alternative splicing, the severity and incidence of cardiac hypertrophy, and survival. We conclude that CELF protein activity is required for normal alternative splicing in the heart in vivo and that normal CELF-mediated alternative splicing regulation is in turn required for normal cardiac function.

Identification of CELF splicing activation and repression domains in vivo

CUG-BP and ETR-3 like factor (CELF) proteins are regulators of pre-mRNA alternative splicing. We created a series of truncation mutants to identify the regions of CELF proteins that are required to activate and to repress alternative splicing of different exons. This analysis was performed in parallel on two CELF proteins, ETR-3 (CUG-BP2, NAPOR, BRUNOL3) and CELF4 (BRUNOL4). We identified a 20-residue region of CELF4 required for repression or activation, in contrast to ETR-3, for which the required residues are more disperse. For both ETR-3 and CELF4, distinct regions were required to activate splicing of two different alternative exons, while regions required for repression of an additional third exon overlapped with regions required for activation. Our results suggest that activation of different splicing events by individual CELF proteins requires separable regions, implying the nature of the protein–protein interactions required for activation are target-dependent. The finding that residues required for activation and repression overlap suggests either that the same region interacts with different proteins to mediate different effects or that interactions with the same proteins can have different effects on splicing due to yet-to-be defined downstream events. These results provide a foundation for identifying CELF-interacting proteins involved in activated and/or repressed splicing.

Identification of putative new splicing targets for ETR-3 using sequences identified by systematic evolution of ligands by exponential enrichment

ETR-3 (also know as BRUNOL3, NAPOR, and CUGBP2) is one of six members of the CELF (CUG-BP1- and ETR-3-like factor) family of splicing regulators. ETR-3 regulates splicing by direct binding to the pre-mRNA. We performed systematic evolution of ligands by exponential enrichment (SELEX) to identify the preferred binding sequence of ETR-3. After five rounds of SELEX, ETR-3 selected UG-rich sequences, in particular UG repeats and UGUU motifs. Either of these selected motifs was able to restore ETR-3 binding and responsiveness to a nonresponsive splicing reporter in vivo. Moreover, this effect was not specific to ETR-3 since minigenes containing either of the two motifs were responsive to two other CELF proteins (CUG-BP1 and CELF4), indicating that different members of the CELF family can mediate their effects via a common binding site. Using the SELEX-identified motifs to search the human genome, we identified several possible new ETR-3 targets. We created minigenes for two of these genes, the CFTR and MTMR1 genes, and confirmed that ETR-3 regulates their splicing patterns. For the CFTR minigene this regulation was demonstrated to be dependent on the presence of the putative binding site identified in our screen. These results validate this approach to search for new targets for RNA processing proteins.

Expression profiling in spinal muscular atrophy reveals an RNA binding protein deficit

Spinal muscular atrophy is a common neuromuscular disorder caused by deletions or mutations within the survival motor neuron gene. The reason for specific motor neuron loss within the disease is still unclear. Expression profiling has been carried out in two models of spinal muscular atrophy; the heterozygote mouse model and human primary muscle cultures from a spinal muscular atrophy patient. A group of RNA binding proteins are up-regulated in spinal muscular atrophy motor neurons. One such protein, BRUNOL3, is highly expressed within spinal cord and muscle and also at the same developmental stage as survival motor neuron. The differential expression of Brunol3 has been confirmed with real-time RT-PCR in spinal cord and muscle of three different models of spinal muscular atrophy. BRUNOL3 has been shown to co-localise with survival motor neuron in the nuclei of neuronal cells and to co-immunoprecipitate with Smn in mouse brain. This is the first time that a link has been established between RNA binding proteins and survival motor neuron within motor neurons.

Dynamic antagonism between RNA-binding protein CUGBP2 and cyclooxygenase-2-mediated prostaglandin E2 in radiation damage

Damage to intestinal epithelium limits the use of ionizing radiation (IR) in cancer therapy. Prostaglandins (PGs), generated through the action of cyclooxygenase-1 (COX-1) and COX-2 protect the intestinal stem cells from IR. In previous studies, we demonstrated that the RNA-binding protein CUGBP2 regulates the stability and translation of COX-2 mRNA by interacting with AU-rich sequences in 3' UTR. Here, we demonstrate a dynamic antagonistic relationship between CUGBP2 and COX-2. Both CUGBP2 and COX-2 are rapidly induced after IR in intestinal crypt epithelial cells in mice, but CUGBP2 protein expression is observed immediately and COX-2 protein expression is delayed. In contrast, administration of bacterial lipopolysaccharide induced COX-2 expression and PGE(2), resulting in the inhibition of CUGBP2 expression and radioprotection of the intestine. These effects were reversed by NS398, a COX-2-specific inhibitor, suggesting that lipopolysaccharide-mediated inhibition of CUGBP2 is a PG-dependent mechanism. Furthermore, CUGBP2 expression is higher in COX-1(-/-) and COX-2(-/-) mice than wild-type controls at basal conditions, which is further increased after IR.

CUGBP2 plays a critical role in apoptosis of breast cancer cells in response to genotoxic injury

Posttranscriptional control of gene expression plays a key role in regulating gene expression in cells undergoing apoptosis. Cyclooxygenase-2 (COX-2) is a crucial enzyme in the conversion of arachidonic acid to prostaglandin E2 (PGE(2)) and is significantly upregulated in many types of adenocarcinomas. COX-2 overexpression leads to increased PGE(2) production, resulting in increased cellular proliferation. PGE(2) enhances the resistance of cells to ionizing radiation. Accordingly, understanding mechanisms regulating COX-2 expression may lead to important therapeutic advances. Besides transcriptional control, COX-2 expression is significantly regulated by mRNA stability and translation. We have previously demonstrated that RNA binding protein CUGBP2 binds AU-rich sequences to regulate COX-2 mRNA translation. In the current study, we have determined that expression of both COX-2 mRNA and CUGBP2 mRNA are induced in MCF-7 cells, a breast cancer cell line, following exposure to 12 Gy gamma-irradiation. However, only CUGBP2 protein is induced, but COX-2 protein levels were not altered. Silencer RNA (siRNA)-mediated inhibition of CUGBP2 reversed the block in COX-2 protein expression. Furthermore, MCF-7 cells underwent apoptosis in response to radiation injury, which was also reversed by CUGBP2 siRNAs. These data suggest that CUGBP2 is a critical regulator of the apoptotic response to genotoxic injury in breast cancer cells.

CELF6, a Member of the CELF Family of RNA-binding Proteins, Regulates Muscle-specific Splicing Enhancer-dependent Alternative Splicing

We previously described a family of five RNA-binding proteins: CUG-binding protein, embryonic lethal abnormal vision-type RNA-binding protein 3, and the CUG-binding protein and embryonic lethal abnormal vision-type RNA-binding protein 3-like factors (CELFs) 3, 4, and 5. We demonstrated that all five of these proteins specifically activate exon inclusion of cardiac troponin T minigenes in vivo via muscle-specific splicing enhancer (MSE) sequences. We also predicted that a sixth family member, CELF6, was located on chromosome 15. Here, we describe the isolation and characterization of CELF6. Like the previously described CELF proteins, CELF6 shares a domain structure containing three RNA-binding domains and a divergent domain of unknown function. CELF6 is strongly expressed in kidney, brain, and testis and is expressed at very low levels in most other tissues. In the brain, expression is widespread and maintained from the fetus to the adult. CELF6 activates exon inclusion of a cardiac troponin T minigene in transient transfection assays in an MSE-dependent manner and can activate inclusion via multiple copies of a single element, MSE2. These results place CELF6 in a functional subfamily of CELF proteins that includes CELFs 3, 4, and 5. CELF6 also promotes skipping of exon 11 of insulin receptor, a known target of CELF activity that is expressed in kidney.

ETR-3 and CELF4 protein domains required for RNA binding and splicing activity in vivo

Members of the CUG-BP and ETR-3 like factor (CELF) protein family bind within conserved intronic elements (called MSEs) flanking the cardiac troponin T (cTNT) alternative exon 5 and promote exon inclusion in vivo and in vitro. Here we use a comparative deletion analysis of two family members (ETR-3 and CELF4) to identify separate domains required for RNA binding and splicing activity in vivo. CELF proteins contain two adjacent RNA binding domains (RRM1 and RRM2) near the N-terminus and one RRM (RRM3) near the C-terminus, which are separated by a 160-230 residue divergent domain of unknown function. Either RRM1 or RRM2 of CELF4 are necessary and sufficient for binding MSE RNA and RRM2 plus an additional 66 amino acids of the divergent domain are as effective as full-length protein in activating MSE-dependent splicing in vivo. Non-overlapping N- and C-terminal regions of ETR-3 containing either RRM1 and RRM2 or RRM3 plus segments of the adjacent divergent domain activate MSE-dependent exon inclusion demonstrating an unusual functional redundancy of the N- and C-termini of the protein. These results identify specific regions of ETR-3 and CELF4 that are likely targets of protein-protein interactions required for splicing activation.

Muscleblind protein, MBNL1/EXP, binds specifically to CHHG repeats

Myotonic dystrophy (DM) type 1 is caused by an expansion of a CTG repeat in the DMPK gene and type 2 by a CCTG repeat in the ZNF9 gene. Previous reports have suggested that transcripts containing expanded CUG/CCUG repeats might have toxic gain-of-function effects, probably affecting the function of RNA-binding proteins in the pathogenesis of DM. Here, it was attempted to compare the RNA-binding properties of three proteins, CUG-BP, MBNL1/EXP and PKR, which have previously been suggested to interact with CUG repeats. MBNL1, but not CUG-BP or PKR, interacted with both CUG and CCUG repeats in a yeast three-hybrid system. By using various synthetic RNAs, it was found that MBNL1 specifically interacts with repetitive sequences summarized as CHHG and CHG repeats, where H is A, U or C. Interestingly, MBNL1 did not interact with a genuine double-stranded RNA comprising CAG/CUG repeats, suggesting that MBNL1 prefers bulge-containing double-stranded RNAs. Deletion analysis indicates a difference in RNA-binding abilities among splice variants of MBNL1. It was also found that MBNL1 can bind to repetitive motifs in ZNF9, which contain a minimal length of CCUG repeats with non-CCUG insertions.

Cell-specific RNA-binding proteins in human disease

Cell-specific RNA-binding proteins (RBPs) are involved in a variety of processes that are critical for appropriate protein expression (e.g., alternative splicing of messenger RNAs and translational control). Perturbation of the normal functions of RBPs has been implicated in a number of clinical disorders. Disease-related RBPs include the CELF proteins, which are believed to play roles in normal heart and skeletal muscle development and in the pathology of myotonic dystrophy; the Nova autoimmune antigens, which are neuron-specific proteins involved in the pathogenesis of the neurodegenerative syndrome paraneoplastic opsoclonus-myoclonus ataxia; and the alphaCP proteins, which were originally discovered by virtue of their connection to alpha thalassemia. These proteins are representative of a potentially large repertoire of cell-specific RBPs that, together, help to distinguish among the various cell types. Structure/function studies of these RBPs have begun to yield important insights into how they help to shape the protein expression programs unique to heart, skeletal muscle, brain, and other tissues.

Isolation and expression of Napor/CUG-BP2 in embryo development

The human neuroblastoma apoptosis-related RNA-binding protein NAPOR is an ELAV-like RNA-binding protein with three characteristic RNA recognition motifs (RRMs). We report here the cloning and characterization of a zebrafish Napor that has a high sequence homology to human NAPOR protein. Whole-mount in situ hybridization analysis revealed that zebrafish napor is dynamically expressed in early development. In addition to its maternal expression, napor transcripts were detected in adaxial mesoderm cells and lateral neural plate cells at early somite stages. By 10-somite stage, napor expression was restricted to the central nervous system, having a specific expression domain of rhombomere 5 in the hindbrain. In 24 hpf embryo, napor was expressed in subsets of differentiating neural cells in the forebrain and hindbrain as well as somitic muscle cells. The number of napor-expressing neural cells was greatly increased in the mind bomb mutant that has neurogenic phenotype resulting from deficits in the Notch signaling pathway. Furthermore, overexpression of napor by RNA microinjection resulted in severe defects in nervous system and gastrulation, suggesting the need for tight control of napor gene regulation during embryo development.

Antagonistic regulation of alpha-actinin alternative splicing by CELF proteins and polypyrimidine tract binding protein

The alpha-actinin gene has a pair of alternatively spliced exons. The smooth muscle (SM) exon is repressed in most cell types by polypyrimidine tract binding protein (PTB). CELF (CUG-BP and ETR3-like factors) family proteins, splicing regulators whose activities are altered in myotonic dystrophy, were found to coordinately regulate selection of the two alpha-actinin exons. CUG-BP and ETR3 activated the SM exon, and along with CELF4 they were also able to repress splicing of the NM (nonmuscle) exon both in vivo and in vitro. Activation of SM exon splicing was associated with displacement of PTB from the polypyrimidine tract by binding of CUG-BP at adjacent sites. Our data provides direct evidence for the activity of CELF proteins as both activators and repressors of splicing within a single-model system of alternative splicing, and suggests a model whereby alpha-actinin alternative splicing is regulated by synergistic and antagonistic interactions between members of the CELF and PTB families.

Identification and characterization of murine Brunol4, a new member of the elav/bruno family

RNA-binding proteins are involved in post-transcriptional processes like mRNA stabilization, post-transcriptional modification, and transport and have been suggested to play an important role in developmental gene regulation. We report here the cloning and characterization of Brunol4, a novel mouse cDNA closely related to the elav-type family of genes encoding for RNA-binding proteins and a subfamily recently named after the bruno gene of Drosophila. Murine Brunol4 is localized near the centromere of chromosome 18. The cDNA sequence of Brunol4 is separated by 12 introns and the size of Brunol4 may be around 250 kb due to the large size of several introns. Brunol4 expression is detectable in the developing embryo and, later on becomes mainly restricted to cerebral structures, in particular the cerebellum where it persists in the adult organism. We predict a role of Brunol4 and the respective human homologue in differentiation and maintenance of neuronal structures.

Coupled mRNA stabilization and translational silencing of cyclooxygenase-2 by a novel RNA binding protein, CUGBP2

Cyclooxygenase-2 (COX-2) expression is translationally silenced in epithelial cells undergoing radiation-induced apoptosis. CUGBP2, a predominantly nuclear protein, is also rapidly induced in response to radiation and translocates to the cytoplasm. Antisense-mediated suppression of CUGBP2 renders radioprotection through a COX-2-dependent prostaglandin pathway, providing an in vivo demonstration of translation inhibition activity for CUGBP2. CUGBP2 binds to two sets of AU-rich sequences (AREs) located within the first sixty nucleotides of the COX-2 3' untranslated region (3'UTR). Upon binding, CUGBP2 stabilizes a chimeric luciferase-COX-2 3'UTR mRNA but inhibits its translation. These findings identify a novel paradigm for RNA binding proteins in facilitating opposing functions of mRNA stability and translation inhibition and reveal a mechanism for inhibiting COX-2 expression in cancer cells.

Expression and mutation analysis of BRUNOL3, a candidate gene for heart and thymus developmental defects associated with partial monosomy 10p

Partial monosomy 10p is a rare chromosomal aberration. Patients often show symptoms of the DiGeorge/velocardiofacial syndrome spectrum. The phenotype is the result of haploinsufficiency of at least two regions on 10p, the HDR1 region associated with hypoparathyroidism, sensorineural deafness, and renal defects (HDR syndrome) and the more proximal region DGCR2 responsible for heart defects and thymus hypoplasia/aplasia. While GATA3 was identified as the disease causing gene for HDR syndrome, no genes have been identified thus far for the symptoms associated with DGCR2 haploinsufficiency. We constructed a deletion map of partial monosomy 10p patients and narrowed the critical region DGCR2 to about 300 kb. The genomic draft sequence of this region contains only one known gene, BRUNOL3 ( NAPOR, CUGBP2, ETR3). In situ hybridization of human embryos and fetuses revealed as well as in other tissues a strong expression of BRUNOL3 in thymus during different developmental stages. BRUNOL3 appears to be an important factor for thymus development and is therefore a candidate gene for the thymus hypoplasia/aplasia seen in partial monosomy 10p patients. We did not find BRUNOL3 mutations in 92 DiGeorge syndrome-like patients without chromosomal deletions and in 8 parents with congenital heart defect children.

Region-specific alternative splicing in the nervous system: implications for regulation by the RNA-binding protein NAPOR

Alternative RNA splicing generates extensive proteomic diversity in the nervous system, yet few neural-specific RNA binding proteins have been implicated in splicing control. Here we show that the biochemical properties and spatial expression of mouse neuroblastoma apoptosis-related RNA-binding protein (NAPOR; also called NAPOR-1) are consistent with its roles in the regulation of the exon 5 and exon 21 splicing events of the N-methyl-D-aspartate (NMDA) receptor R1 transcript. NAPOR, which is closely related to CUG binding protein 2 (CUG-BP2), promotes exon 21 and represses exon 5 splicing in functional coexpression assays. These NMDA mRNA isoforms are distributed, in vivo, in a region-specific manner in rat brain, such that high levels of exon 21 selection and exon 5 skipping coincide with high NAPOR mRNA expression in the forebrain. Within the forebrain, this spatial correspondence is most striking in the visual cortex. In contrast, low NAPOR expression coincides with the reciprocal pattern of alternative splicing in the hindbrain. Complementary experiments demonstrate a tissue-specific distribution of NAPOR, CUG-BP, and other highly related proteins within the nervous system as assayed by probing forebrain and hindbrain nuclear extracts with monoclonal antibody, mAb 3B1. Thus, NAPOR may be one of a group of closely related proteins involved in splicing regulation within the brain. An intronic RNA element responsible for the silencing of exon 21 splicing is identified by mutational analysis and shown to bind directly to recombinant NAPOR protein, suggesting a model in which exon 21 selection is positively regulated by an antirepression mechanism of action.

Etr-r3/mNapor, encoding an ELAV-type RNA binding protein, is expressed in differentiating cells in the developing rodent forebrain

The gene encoding human neuroblastoma apoptosis-related RNA binding protein (NAPOR) has rat and mouse homologues, Etr-r3 and mNapor, cloned recently by ourselves and others. Etr-r3/mNapor is transcribed in embryonic brain but its detailed expression patterns are unknown. It has been hypothesised that its expression coincides with the occurrence of programmed cell death, but existing expression data are insufficient to evaluate this possibility. We used northern blots and in situ hybridisation to show that Etr-r3/mNapor is expressed by most postmitotic differentiating cells throughout the forebrain during embryogenesis and the first weeks of postnatal life. Expression is particularly high in the developing cerebral cortex, in nuclei of the ventral telencephalon, in dorsal thalamus and in hypothalamus. In contrast, Etr-r3/mNapor is expressed by very few cells in proliferative regions, where cell death rates are highest.

Dynamic antagonism between ETR-3 and PTB regulates cell type-specific alternative splicing

Inclusion of cardiac troponin T (cTNT) exon 5 in embryonic muscle requires conserved flanking intronic elements (MSEs). ETR-3, a member of the CELF family, binds U/G motifs in two MSEs and directly activates exon inclusion in vitro. Binding and activation by ETR-3 are directly antagonized by polypyrimidine tract binding protein (PTB). We use dominant-negative mutants to demonstrate that endogenous CELF and PTB activities are required for MSE-dependent activation and repression in muscle and nonmuscle cells, respectively. Combined use of CELF and PTB dominant-negative mutants provides an in vivo demonstration that antagonistic splicing activities exist within the same cells. We conclude that cell-specific regulation results from the dominance of one among actively competing regulatory states rather than modulation of a nonregulated default state.

Discovery of a novel compound: insight into mechanisms for acrylamide-induced axonopathy and colchicine-induced apoptotic neuronal cell death

The exposure of humans and experimental animals to certain industrial toxins such as acrylamide is known to cause nerve damage classified as axonopathy, but the mechanisms involved are poorly understood. Here we show that acrylamide induces morphological changes and tyrosine phosphorylation of focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (Pyk2), a member of the FAK subfamily, in human differentiating neuroblastoma SH-SY5Y cells. Furthermore, we identified a novel molecule designated 'compound-1' that inhibits the morphological and biochemical events. Daily oral administrations of the compound also effectively alleviated behavioral deficits in animals elicited by acrylamide in inclined plane testing, landing foot spread testing and rota-rod performance testing. The compound also effectively inhibited the biological and biochemical responses caused by another axonopathy inducer, colchicine, including tyrosine phosphorylation of Pyk2, formation of an 85-kDa poly(ADP-ribose)polymerase (PARP) fragment and apoptosis-associated induction of the NAPOR gene as well as neuronal cell death. Our findings not only provide insight into FAK and Pyk2 functions in neuronal cells, but may also be important in the development of therapeutic agents for peripheral neuropathy and neurodegeneration.

Genomic organization and isoform-specific tissue expression of human NAPOR (CUGBP2) as a candidate gene for familial arrhythmogenic right ventricular dysplasia

Neuroblastoma apoptosis-related RNA-binding protein (NAPOR; HGMW-approved symbol CUGBP2) is a newly discovered gene prominently induced during apoptosis, suggesting that it plays a role during apoptosis. We have found that it is encoded by a gene located on chromosome 10p13-p14 between Généthon markers D10S547 and D10S223, a region to which we have recently localized a gene responsible for arrhythmogenic right ventricular dysplasia (ARVD). To examine its possible role in the pathogenesis of ARVD, we determined the genomic organization of the human NAPOR gene including its exon-intron boundaries and the putative promoter sequence, which provide a plausible mechanism for its alternative mRNA splicing. We also demonstrated that three isoforms of the NAPOR transcript were differently expressed, with NAPOR-3 being nearly neuron specific while the other two forms were ubiquitously expressed. The expression of NAPOR is differentially regulated during development. Finally, we screened the members of the ARVD family for mutations and identified two DNA sequence variants in the protein-coding exons of NAPOR, neither of which was responsible for ARVD. While the function of NAPOR remains to be elucidated, our current characterization of the NAPOR gene will be valuable for further clinical and functional study.

The CELF family of RNA binding proteins is implicated in cell-specific and developmentally regulated alternative splicing

Alternative splicing of cardiac troponin T (cTNT) exon 5 undergoes a developmentally regulated switch such that exon inclusion predominates in embryonic, but not adult, striated muscle. We previously described four muscle-specific splicing enhancers (MSEs) within introns flanking exon 5 in chicken cTNT that are both necessary and sufficient for exon inclusion in embryonic muscle. We also demonstrated that CUG-binding protein (CUG-BP) binds a conserved CUG motif within a human cTNT MSE and positively regulates MSE-dependent exon inclusion. Here we report that CUG-BP is one of a novel family of developmentally regulated RNA binding proteins that includes embryonically lethal abnormal vision-type RNA binding protein 3 (ETR-3). This family, which we call CELF proteins for CUG-BP- and ETR-3-like factors, specifically bound MSE-containing RNAs in vitro and activated MSE-dependent exon inclusion of cTNT minigenes in vivo. The expression of two CELF proteins is highly restricted to brain. CUG-BP, ETR-3, and CELF4 are more broadly expressed, and expression is developmentally regulated in striated muscle and brain. Changes in the level of expression and isoforms of ETR-3 in two different developmental systems correlated with regulated changes in cTNT splicing. A switch from cTNT exon skipping to inclusion tightly correlated with induction of ETR-3 protein expression during differentiation of C2C12 myoblasts. During heart development, the switch in cTNT splicing correlated with a transition in ETR-3 protein isoforms. We propose that ETR-3 is a major regulator of cTNT alternative splicing and that the CELF family plays an important regulatory role in cell-specific alternative splicing during normal development and disease.

Developmentally-regulated expression of mNapor encoding an apoptosis-induced ELAV-type RNA binding protein

Proteins with RNA recognition motifs (RRMs) participate in many aspects of RNA metabolism, and some of them are required for the accomplishment of normal development. The neuroblastoma apoptosis-related RNA binding protein (NAPOR) is an ELAV-type RNA-binding protein with three characteristic RNP2/RNP1-type RRMs, which we identified as a gene induced during apoptosis of neuroblastoma cells. Here we isolated and characterized the cDNA for mNapor, the mouse homolog of NAPOR. The mNapor encodes mRNA sharing striking homology with that of NAPOR, not only in its open reading frame (98.5%) but also in the 3'-untranslated region (80.1%), and is mapped to chromosome 2 A2-A3, a region syntenic to the human NAPOR locus. In situ hybridization analysis revealed that the expression pattern of mNapor is spatially and temporally coincident with the occurrence of programmed cell death, suggesting its involvement in the development of the central nervous system in which apoptosis plays a crucial role.

Cardiac elav-type RNA-binding protein (ETR-3) binds to RNA CUG repeats expanded in myotonic dystrophy

Myotonic dystrophy (DM) is a neuromuscular disorder associated with CTG triplet repeat expansion in the myotonin protein kinase gene ( DMPK ). We previously proposed a hypothesis suggesting that the expanded CUG repeats sequester specific RNA-binding proteins and that such a sequestration results in abnormal RNA processing of several RNAs containing CUG repeats in multiple tissues affected in patients with DM. One of the members of the CUG-binding proteins, CUG-BP, has been identified previously. Here we describe the second member of this family, elav -type ribonucleoprotein (ETR-3), which is highly expressed in heart and is able to interact with CUG repeats. Screening of a mouse liver cDNA library with a CUG-BP probe identified two mETR-3 cDNAs. Two additional cDNAs from mouse heart were amplified by RT-PCR. These cDNAs differ by several insertions/deletions and might be generated via alternative splicing. Mouse ETR-3 has a mol. wt of 50 kDa and displays a high level of homology to CUG-BP protein. The organization of the RNA-binding domains (RBDs) within the ETR-3 molecule is similar to one within CUG-BP. A study of mETR-3 RNA-binding activity showed that the mETR-3 binds to (CUG)8repeats. Sequence analysis of mETR-3 indicates the presence of several CUG repeats within the mETR-3 mRNA. Both CUG-BP and mETR-3 bind to mETR-3 mRNA via CUG repeats, suggesting the possible involvement of CUG-BP-like proteins in the regulation of mETR-3 processing. Analysis of the tissue distribution of ETR-3 showed that in human cells, ETR-3 mRNA is highly expressed in heart, but is undetectable in other tissues examined. Our results suggest the existence of a family of proteins that bind to CUG repeats and might be affected in DM by expansion of CUG repeats.

Fluorescent differential display analysis of gene expression in apoptotic neuroblastoma cells

Identification of differentially expressed genes will provide leads in the elucidation of the molecular mechanisms underlying neuronal cell death associated with neurodegenerative disorders. Using a high-throughput fluorescent differential display (FDD) system based on an automated DNA sequencer, we analyzed global patterns of gene expression during the apoptosis of neuroblastoma SH-SY5Y cells induced by a neurotoxin, colchicine. Initial screening of approximately 24000 cDNA bands displayed with 320 primer combinations has revealed 263 fragments showing differential expression patterns, suggesting that approximately 1% of transcripts are modulated in their expression level. Of these differentially displayed bands, we cloned 18 fragments composed of 17 distinct species and confirmed differential expression of each species by reverse transcription-PCR or Northern blot hybridization, thereby proving the reliability of the approach. These include eight derived from seven known genes, five homologous to expressed sequence tags (ESTs), and five totally lacking any homology to those deposited in the database. Among these, a novel transcript SAI1 induced prominently was characterized further and revealed to encode a putative RNA-binding protein NAPOR (neuroblastoma apoptosis-related RNA-binding protein), containing three copies of evolutionarily conserved RNA recognition motif. Since several RNA-binding proteins have been known to play crucial roles in other apoptosis systems, it is conceivable that NAPOR is also involved in the process of neuronal cell death.