Cloning and characterization of HuR, a ubiquitously expressed Elav-like protein

ELAV protein HuA (HuR) can redistribute between nucleus and cytoplasm and is upregulated during serum stimulation and T cell activation

ELAV proteins are implicated in regulating the stability and translation of cytokine and growth regulatory mRNAs such as GM-CSF, IL-2, c-myc, c-fos and GLUT1 by binding to their AU-rich 3′UTRs. The tissue-specific ELAV protein HuB (aka. Hel-N1) is predominantly cytoplasmic and has been shown to stabilize GLUT1 and c-myc mRNAs and to increase their translation following ectopic expression in 3T3-L1 cells. We report that the most widely expressed mouse ELAV protein, mHuA, is predominately nuclear in cultured NIH-3T3 cells, but is localized in the cytoplasm during early G1 of the cell cycle. Therefore, much like the primarily cytoplasmic HuB, HuA becomes temporally localized in the cytoplasm where it can potentially regulate the stability or translation of bound mRNAs. Moreover, we report that stimulation of mouse spleen cells using either mitogenic or sub-mitogenic levels of anti-CD3/CD28 resulted in a dramatic increase in the level of HuA. Upregulation of HuA corresponds to previously documented increases in cytokine expression which are due to increased mRNA stability following T cell activation. Consistent with these findings, HuA was down regulated in quiescent cells and upregulated in 3T3 cells following serum stimulation. The increase of murine HuA during the cell cycle closely resembles that of cyclin B1 which peaks in G2/M. Together with our earlier studies, these data indicate that mammalian ELAV proteins function during cell growth and differentiation due in part to their effects on posttranscriptional stability and translation of multiple growth regulatory mRNAs. This supports the hypothesis that ELAV proteins can function as transacting factors which affect a default pathway of mRNA degradation involved in the expression of growth regulatory proteins.

The neuronal RNA-binding protein Nova-2 is implicated as the autoantigen targeted in POMA patients with dementia

Paraneoplastic opsoclonus myoclonus ataxia (POMA) is a neurologic disorder thought to be mediated by an autoimmune attack against onconeural disease antigens that are expressed by gynecologic or lung tumors and by neurons. One POMA disease antigen, termed Nova-1, has been identified as a neuron-specific KH-type RNA-binding protein. Nova-1 expression is restricted to specific regions of the central nervous system, primarily the hindbrain and ventral spinal cord, which correlate with the predominantly motor symptoms in POMA. However, POMA antisera recognize antigens that are widely expressed in both caudal and rostral regions of the central nervous system, and some patients develop cognitive symptoms. We have used POMA antisera to clone a cDNA encoding a second POMA disease antigen termed Nova-2. Nova-2 is closely related to Nova-1, and is expressed at high levels in neurons during development and in adulthood, and at lower levels in the adult lung. In the postnatal mouse brain, Nova-2 is expressed in a pattern that is largely reciprocal with Nova-1, including high levels of Nova-2 expression in the neocortex and hippocampus. Functional characterization of Nova-2 in RNA selection and nitrocellulose filter-binding assays reveals that Nova-2 binds RNA with high affinity and with sequence specificity that differs from Nova-1. Our results demonstrate that the immune response in POMA targets a family of highly related sequence-specific neuronal RNA-binding proteins. The expression pattern of the Nova-2 protein is likely to underlie the development of cognitive deficits in some POMA patients.

c-jun N-terminal kinase is involved in AUUUA-mediated interleukin-3 mRNA turnover in mast cells

Whereas signalling pathways involved in transcriptional control have been studied extensively, the pathways regulating mRNA turnover remain poorly understood. We are interested in the role of mRNA stability in cell activation and oncogenesis using PB-3c mast cells as a model system. In these cells the short-lived interleukin-3 (IL-3) mRNA is stabilized by ionomycin treatment and following oncogenesis. To identify the signalling pathways involved in these mechanisms, we analysed the effect of different kinase inhibitors. SB202190 and wortmannin were shown to antagonize ionomycin-induced IL-3 mRNA stabilization in PB-3c cells in the presence of actinomycin D, and this effect coincided with their ability to inhibit c-jun N-terminal kinase (JNK) activation by ionomycin. Moreover, transfection of activated MEKK1 amplified ionomycin-induced IL-3 mRNA expression at the post-transcriptional level, and a dominant-negative mutant of JNK counteracted mRNA stabilization by ionomycin. Taken together, these data indicate that JNK is involved in the regulation of IL-3 mRNA turnover in mast cells. In addition, transfection experiments revealed that the cis-acting AU-rich element in the 3' untranslated region of IL-3 mRNA is necessary and sufficient to confer JNK-dependent mRNA stabilization in response to cell activation.

Evidence for 3' untranslated region-dependent autoregulation of the Drosophila gene encoding the neuronal nuclear RNA-binding protein ELAV

The Drosophila locus embryonic lethal abnormal visual system (elav) encodes a nuclear RNA-binding protein essential for normal neuronal differentiation and maintenance of neurons. ELAV is thought to play its role by binding to RNAs produced by other genes necessary for neuronal differentiation and consequently to affect their metabolism by an as yet unknown mechanism. ELAV structural homologues have been identified in a wide range of organisms, including humans, indicating an important conserved role for the protein. Analysis of elav germline transformants presented here shows that one copy of elav minigenes lacking a complete 3' untranslated region (3' UTR) rescues null mutations at elav, but that two copies are lethal. Additional in vivo experiments demonstrate that elav expression is regulated through the 3' UTR of the gene and indicate that this level of regulation is dependent upon ELAV itself. Because ELAV is an RNA-binding protein, the simplest model to account for these findings is that ELAV binds to the 3' UTR of its own RNA to autoregulate its expression. I discuss the implications of these results for normal elav function.

Translational inhibition in vitro of human papillomavirus type 16 L2 mRNA mediated through interaction with heterogenous ribonucleoprotein K and poly(rC)-binding proteins 1 and 2

Human papillomavirus (HPV) type 16 belongs to the group of "high risk" HPV types that are frequently detected in anogenital cancers. The expression of HPV-16 late genes encoding the virus capsid proteins L1 and L2 is restricted to terminally differentiated epithelial cells in the superficial layers of the squamous epithelium. We have previously identified negative elements in the 3' end of L2 RNA that act in cis to reduce mRNA utilization without substantially affecting mRNA levels. The experiments reported here demonstrate the interaction of cellular proteins with an inhibitory sequence present in the coding region of the L2 mRNA. Using RNA gel shift assays and UV cross-linking, we have detected three cellular proteins interacting specifically with the sense strand of the L2 mRNA, two of which were identified as heterogeneous ribonucleoprotein K (hnRNP K) and the poly(rC) binding- protein (PCBP). Recombinant hnRNP K, PCBP-1, and PCBP-2 that were over expressed in bacteria and partially purified bound to the HPV-16 L2 mRNA in a sequence-specific manner. Interestingly, PCBP-1, PCBP-2, and hnRNP K specifically and efficiently inhibited translation of the HPV-16 L2 mRNA in vitro. Therefore, these proteins may play an important role in the regulation of HPV-16 late gene expression and virus production in vivo.

Expression of mRNA for the elav-like neural-specific RNA binding protein, HuD, during nervous system development

The expression of mRNA for the neuronal antigen HuD (Elavl4) associated with paraneoplastic encephalomyelitis and sensory neuronopathy was evaluated in the developing and adult rat nervous system. Using RNase protection assay and non-radioactive in situ hybridization histochemistry HuD expression was shown to be expressed at high levels at the earliest time point observed (E15), but declined significantly during the first postnatal week to levels which were maintained into adulthood. In the adult, HuD expression became restricted primarily to large pyramidal-like neurons. Exceptions of note were many smaller neurons within a variety of thalamic nuclei. Expression of HuD was observed to be coincident with terminal differentiation of all neuronal structures evaluated regardless of the timing of their development, providing correlative evidence for a role in neuronal differentiation or the maintenance of neuronal phenotype. The marked restriction of HuD mRNA expression with maturity suggests that its functional role in adult neurons varies significantly throughout the CNS.

p21(waf1) mRNA contains a conserved element in its 3'-untranslated region that is bound by the Elav-like mRNA-stabilizing proteins

The Elav-like proteins are specific mRNA-binding proteins that regulate mRNA stability. The neuronal members of this family (HuD, HuC, and Hel-N1) are required for neuronal differentiation. In this report, using purified HuD protein we have localized a high affinity HuD binding site to a 42-nucleotide region within a U-rich tract in the 3'-untranslated region p21(waf1) mRNA. The binding of HuD to this site is readily displaced by an RNA oligonucleotide encoding the HuD binding site of c-fos. The sequence of this binding site is well conserved in human, mouse, and rat p21(waf1) mRNA. p21(waf1) is an inhibitor of cyclin-dependent kinases and proliferating cell nuclear antigen and induces cell cycle arrest at G1/S, a requisite early step in cell differentiation. The identification of an Elav-like protein binding site in the 3'-untranslated region of p21(waf1) provides a novel link between the induction of differentiation, mRNA stability, and the termination of the cell cycle.

Hypoxic Regulation of VEGF mRNA Stability by RNA-binding Proteins

Vascular endothelial growth factor (VEGF), is a potent angiogenic factor whose expression is dramatically induced by hypoxia. We have previously demonstrated that the induction of VEGF by hypoxia is in large part the result of an increased stability of VEGF mRNA. The stabilization of VEGF mRNA by hypoxia is mediated by the binding of sequence-specific RNA-binding proteins. This review focuses on one such protein, HuR, an RNA-binding protein which we have recently shown is critical for the hypoxic stabilization of VEGF mRNA.

Down regulation of extramacrochaetae mRNA by a Drosophila neural RNA binding protein Rbp9 which is homologous to human Hu proteins

Rbp9 is an RNA binding protein expressed mainly in the central nervous system of adult Drosophilamelanogaster. Rbp9 shares a high degree of sequence similarity with human neural proteins referred to as Hu antigens. Hu antigens bind to U-rich mRNA destabilizing elements with a high affinity and, thus, have been implicated as regulators of mRNA stability. Using in vitro RNA binding assays, we found that Rbp9 binds strongly to poly U sequences. We then employed a Selex system to identify a consensus Rbp9 binding site (UUUXUUUU). Information obtained from the Selex results allowed the detection of two repeats of the Rbp9 consensus binding sequence in the 3' untranslated region of extramacrochaetae mRNA. UV crosslinking experiments demonstrated that Rbp9 interactsspecifically with emc mRNA. The requirement of Rbp9 protein in the down regulation of emc mRNA was confirmed by northern (RNA) analysis, which revealed that the level of emc mRNA increased 10-fold in rbp9 mutant flies. Taken together with the in vitro RNA binding results, the genetic evidence obtained strongly supports the hypothesis that Rbp9 functions as a regulator of RNA stability.

Overexpression of HuR, a nuclear-cytoplasmic shuttling protein, increases the in vivo stability of ARE-containing mRNAs

The messenger RNAs of many proto-oncogenes, cytokines and lymphokines are targeted for rapid degradation through AU-rich elements (AREs) located in their 3' untranslated regions (UTRs). HuR, a ubiquitously expressed member of the Elav family of RNA binding proteins, exhibits specific affinities for ARE-containing RNA sequences in vitro which correlate with their in vivo decay rates, thereby implicating HuR in the ARE-mediated degradation pathway. We have transiently transfected HuR into mouse L929 cells and observed that overexpression of HuR enhances the stability of beta-globin reporter mRNAs containing either class I or class II AREs. The increase in mRNA stability parallels the level of HuR overexpression, establishing an in vivo role for HuR in mRNA decay. Furthermore, overexpression of HuR deletion mutants lacking RNA recognition motif 3 (RRM 3) does not exert a stabilizing effect, indicating that RRM 3 is important for HuR function. We have also developed polyclonal anti-HuR antibodies. Immunofluorescent staining of HeLa and L929 cells using affinity-purified anti-HuR antibody shows that both endogenous and overexpressed HuR proteins are localized in the nucleus. By forming HeLa-L929 cell heterokaryons, we demonstrate that HuR shuttles between the nucleus and cytoplasm. Thus, HuR may initially bind to ARE-containing mRNAs in the nucleus and provide protection during and after their export to the cytoplasmic compartment.

RNA stabilization by the AU-rich element binding protein, HuR, an ELAV protein

An important paradigm for post-transcriptional regulation is the control of cytoplasmic mRNA stability mediated by AU-rich elements (AREs) in the 3' untranslated region of transcripts encoding oncoproteins, cytokines and transcription factors. While many RNA-binding proteins have been shown to bind to AREs in vitro, neither the functional consequences nor the physiological significance of their interactions are known. Here we demonstrate a role for the embryonic lethal abnormal visual (ELAV) RNA-binding protein HuR in mRNA turnover in vivo. The ELAV family of RNA-binding proteins is highly conserved in vertebrates. In humans, there are four members; HuR is expressed in all proliferating cells, whereas Hel-N1, HuC and HuD are expressed in terminally differentiated neurons. We show that elevation of cytoplasmic HuR levels inhibits c-fos ARE-mediated RNA decay but has little effect on rapid decay directed by c-jun ARE. It appears that HuR has little effect on deadenylation but delays onset of decay of the RNA body and slows down its subsequent decay. We also show that HuR can be induced to redistribute from the nucleus to the cytoplasm and that this redistribution is associated with an altered function. Modulation of the ARE-mediated decay pathway through controlling distribution of the ELAV proteins between nucleus and cytoplasm may be a mechanism by which cell growth and differentiation is regulated.

Posttranscriptional regulation of urokinase plasminogen activator receptor messenger RNA levels by leukocyte integrin engagement

As an adhesion receptor, the beta2 integrin lymphocyte function-associated antigen-1 (LFA-1) contributes a strong adhesive force to promote T lymphocyte recirculation and interaction with antigen-presenting cells. As a signaling molecule, LFA-1-mediates transmembrane signaling, which leads to the generation of second messengers and costimulation resulting in T cell activation. We recently have demonstrated that, in costimulatory fashion, LFA-1 activation promotes the induction of T cell membrane urokinase plasminogen activator receptor (uPAR) and that this induced uPAR is functional. To investigate the mechanism(s) of this induction, we used the RNA polymerase II inhibitor 5, 6-dichloro-1-beta-D-ribobenzimidazole and determined that uPAR mRNA degradation is delayed by LFA-1 activation. Cloning of the wild-type, deleted and mutated 3'-untranslated region of the uPAR cDNA into a serum-inducible rabbit beta-globin cDNA reporter construct revealed that the AU-rich elements and, in particular the nonameric UUAUUUAUU sequence, are crucial cis-acting elements in uPAR mRNA degradation. Experiments in which Jurkat T cells were transfected with reporter constructs demonstrated that LFA-1 engagement was able to stabilize the unstable reporter mRNA containing the uPAR 3'-untranslated region. Our study reveals a consequence of adhesion receptor-mediated signaling in T cells, which is potentially important in the regulation of T cell activation, including production of cytokines and expression of proto-oncogenes, many of which are controlled through 3' AU-rich elements.

Characterization of the rat Star gene that encodes the predominant 3.5-kilobase pair mRNA. ACTH stimulation of adrenal steroids in vivo precedes elevation of Star mRNA and protein

The steroidogenic acute regulatory protein (STAR) participates in steroidogenesis through the mitochondrial transfer of cholesterol to cytochrome P450scc. The rat adrenal Star gene is transcribed as a 3. 5-kilobase pair (kb) and 1.6-kb mRNA with the larger mRNA predominating ( approximately 85% of total) in vivo. Hypophysectomy (HPX) produced a 3-5-fold decrease in Star mRNA along with a loss of adrenal steroids, whereas P450scc mRNA decreased by less than 2-fold. Adrenocorticotropic hormone (ACTH) treatment of HPX rats maximally stimulated steroidogenesis rates within 5 min with over 10-fold elevation of steady state blood levels occurring within 10 min. For intact rats there was a 5-10-fold larger increase, paralleling previously observed elevations of cholesterol-cytochrome P450scc association and metabolism in subsequently isolated adrenal mitochondria. ACTH did not increase either total STAR protein or a group of modified forms until at least 30 min after completion of acute stimulation, indicating that elevated translation of STAR protein cannot alone mediate this acute stimulation. Parallel slow changes in STAR protein and corticosterone formation after ACTH treatment are consistent with participation of STAR forms as co-regulators of these hormonal responses. ACTH stimulation of HPX rats increased Star mRNA by 2.5-fold within 20 min and by 4.5-fold after 1 h, thus preceding the rise in the STAR protein. A 3.5-kb Star cDNA clone isolated from a rat adrenal cDNA library exhibited a 0.9-kb open reading frame and a 2.5-kb 3'-untranslated region (3'-UTR). The open reading frame sequence differed at only 12 amino acids from that of the mouse Star. The rat Star gene seven exons with exon 7 encoding the entire 2.5 kb of 3'-UTR of the 3.5-kb mRNA. The 3'-UTR sequence suggests that 1.6- and 3.5-kb mRNA are formed by an alternative usage of different polyadenylation signals. Multiple UUAUUUA(U/A)(U/A) motifs also suggest additional regulation through this extended 3'-UTR. Although elevation of STAR protein by ACTH does not cause the acute increase in adrenal cholesterol metabolism, changes in the turnover or distribution of an active STAR subfraction cannot be excluded.

Hypoxic stabilization of vascular endothelial growth factor mRNA by the RNA-binding protein HuR

Vascular endothelial growth factor (VEGF) is a potent angiogenic factor whose expression is dramatically induced by hypoxia due in large part to an increase in the stability of its mRNA. Here we show that HuR binds with high affinity and specificity to the element that regulates VEGF mRNA stability by hypoxia. Inhibition of HuR expression abrogates the hypoxia-mediated increase in VEGF mRNA stability. Overexpression of HuR increases the stability of VEGF mRNA. However, this only occurs efficiently in hypoxic cells. We further show that the stabilization of VEGF mRNA can be recapitulated in vitro. Using an S-100 extract, we show that the addition of recombinant HuR stabilizes VEGF mRNA markedly. These data support the critical role of HuR in mediating the hypoxic stabilization of VEGF mRNA by hypoxia.

Selection of recombinant anti-HuD Fab fragments from a phage display antibody library of a lung cancer patient with paraneoplastic encephalomyelitis

Antibodies against the HuD antigen expressed in small-cell lung cancer (SCLC) cross-react with proteins expressed in neurons of the central and peripheral nervous system and are associated with paraneoplastic encephalomyelitis and sensory neuropathy (PEM/SN). We isolated anti-HuD Fab fragments from an antibody phage display library that was constructed from mRNA of a metastatic lymph node from a patient with SCLC and Pem/SN. Fab GLN495 recognized HuD and other related proteins (HuC and Hel-N1, or Hu antigens) in immunoblots of these recombinant proteins and in immunohistochemical and Western blot analysis of SCLC and neurons. Fab GLN495 inhibited up to 75% of the anti-Hu antibodies of the patient from which it was derived, suggesting that recognizes a dominant epitope in the polyclonal anti-Hu antibody response. Fab GLN495 also competed with anti-Hu sera from most but not all patients with PEM/SN, indicating that the same epitope is recognized by a large subgroup of patients. Human monoclonal anti-HuD antibodies may be useful in diagnosis of HuD expressing tumors and in clarifying the autoimmune etiology of PEM/SN. This study, the first to demonstrate that tumor specific recombinant antibodies can be isolated from metastatic lymph node tissue, shows that this approach may be generally applicable to isolate human antibodies against tumor specific antigens.

p53-dependent elevation of p21Waf1 expression by UV light is mediated through mRNA stabilization and involves a vanadate-sensitive regulatory system

Exposure of mammalian cells to adverse stimuli triggers the expression of numerous stress response genes, many of which are presumed to enhance cell survival. In this study, we examined the mechanisms contributing to the induction of p21Waf1 by stress and its influence on the survival of cells subjected to short-wavelength UVC irradiation. UVC was found to elevate p21Waf1 mRNA expression in mouse embryonal fibroblasts (MEFs) and human colorectal carcinoma (RKO) cells in a p53-dependent manner. The lack of p21Waf1 induction in p53-deficient MEFs and RKO cells correlated with diminished cell survival following UVC irradiation. Unexpectedly, UVC treatment was also found to block the induction of p21Waf1 by various stress-inducing agents such as mimosine in the p53-deficient cells. Additional studies indicated that induction of p21Waf1 by UVC occurs primarily through enhanced mRNA stability rather than increased transcription; in p53-/- MEFs, failure to elevate p21Waf1 after treatment with UVC appears to be due to their inability to stabilize the p21Waf1 transcripts. Treatment of the p53-/- MEFs with the protein tyrosine phosphatase inhibitor vanadate reversed the UVC-induced block on p21Waf1 induction and resulted in their enhanced survival following irradiation. Thus, in cells bearing normal p53, UVC augments p21Waf1 expression by increasing the half-life of p21Waf1 mRNA; without p53, p21Waf1 mRNA remains unstable after UVC, apparently due to a pathway involving tyrosine phosphatase activity.

Gene organization and chromosome location of the neural-specific RNA binding protein Elavl4

We have isolated the gene that encodes the neural-specific RNA binding protein HuD in the mouse (Elavl4), and have mapped its location to the mid-distal region of chromosome 4, close to the neurological mutant clasper. The coding region of the Elavl4 gene covers approximately 44 kb; the first two RNA binding domains (RBDs) that are homologous to the two RBDs found in the Drosophila sex-lethal gene are each encoded in two exons, whereas the third RBD is encoded in a single exon. Elavl4 mRNAs are alternatively spliced in the region between RBDs 2 and 3 due to the variable use of two micro-exons, and RNase protection analysis indicates that two of four possible splice variants are the predominant isoforms expressed in the central nervous system. The high degree of sequence conservation between the Hu proteins suggests that the exon organization of all the Hu protein genes will be similar, if not identical, to the Elavl4 gene.

Messenger RNAs in dendrites: localization, stability, and implications for neuronal function

In the mammalian central nervous system (CNS), each neuron receives signals from other neurons through numerous synapses located on its cell body and dendrites. Molecules involved in the postsynaptic signaling pathways need to be targeted to the appropriate subcellular domains at the right time during both synaptogenesis and the maintenance of synaptic functions. The presence of messenger RNAs (mRNAs) in dendrites offers a mechanism for synthesizing the appropriate molecules at the right place in response to local extracellular stimuli. Several dendritic mRNAs have been identified, and the mechanisms controlling their localization are beginning to be understood. In many cell types, controls on mRNA stability play an important role in the regulation of gene expression, but it is unclear to what extent this type of control operates in dendrites. The regulation of protein synthesis and the control of mRNA stability in dendrites could have important implications for neuronal function.

Microtubule-associated protein 1 light chain 3 is a fibronectin mRNA-binding protein linked to mRNA translation in lamb vascular smooth muscle cells

Intimal cushions form in the fetal ductus arteriosus by fibronectin-dependent smooth muscle cell migration which is associated with greater efficiency of fibronectin mRNA translation. We investigated whether the AU-rich element (ARE), UUAUUUAU, in the 3'-untranslated region (3'UTR) of fibronectin mRNA is involved in this mechanism by transfecting smooth muscle cells with plasmids containing the chloramphenicol acetyltransferase coding region with its 3'UTR replaced by fibronectin 3'UTR bearing intact or mutated ARE. More efficient translation of fusion mRNA with intact versus mutated ARE was observed. This effect was amplified in ductus (10.9-fold) compared with nonmigratory, lower fibronectin-producing aorta cells (6.5-fold). Ductus cells transfected with wild-type but not ARE-mutated plasmid reverted to the stellate phenotype of aorta cells associated with reduced fibronectin production. This suggested that plasmid ARE sequesters RNA-binding factors, thereby reducing endogenous fibronectin mRNA translation. We next purified a 15-kD fibronectin ARE-dependent RNA-binding protein and identified it as microtubule-associated protein 1 light chain 3 (LC3). LC3 is present in greater amounts in ductus compared with aorta cells, and overexpression of LC3 in aortic cells by transfection enhances fibronectin mRNA translation to levels observed in ductus cells.

The 36-kilodalton embryonic-type cytoplasmic polyadenylation element-binding protein in Xenopus laevis is ElrA, a member of the ELAV family of RNA-binding proteins

The translational activation of several maternal mRNAs in Xenopus laevis is dependent on cytoplasmic poly(A) elongation. Messages harboring the UUUUUAU-type cytoplasmic polyadenylation element (CPE) in their 3' untranslated regions (UTRs) undergo polyadenylation and translation during oocyte maturation. This CPE is bound by the protein CPEB, which is essential for polyadenylation. mRNAs that have the poly(U)12-27 embryonic-type CPE (eCPE) in their 3' UTRs undergo polyadenylation and translation during the early cleavage and blastula stages. A 36-kDa eCPE-binding protein in oocytes and embryos has been identified by UV cross-linking. We now report that this 36-kDa protein is ElrA, a member of the ELAV family of RNA-binding proteins. The proteins are identical in size, antibody directed against ElrA immunoprecipitates the 36-kDa protein, and the two proteins have the same RNA binding specificity in vitro. C12 and activin receptor mRNAs, both of which contain eCPEs, are detected in immunoprecipitated ElrA-mRNP complexes from eggs and embryos. In addition, this in vivo interaction requires the eCPE. Although a number of experiments failed to define a role for ElrA in cytoplasmic polyadenylation, the expression of a dominant negative ElrA protein in embryos results in an exogastrulation phenotype. The possible functions of ElrA in gastrulation are discussed.

Mouse chromosomal locations of nine genes encoding homologs of human paraneoplastic neurologic disorder antigens

The paraneoplastic neurologic disorders (PND) are a rare group of neurologic syndromes that arise when an immune response to systemic tumors expressing neuronal proteins ("onconeural antigens") develops into an autoimmune neuronal degeneration. The use of patient antisera to clone the genes encoding PND antigens has led to new insight into the mechanism of these autoimmune disorders. The tumor antigens can now be grouped into three classes: (1) neuron-specific RNA-binding proteins, (2) nerve terminal vesicle-associated proteins, and (3) cytoplasmic signaling proteins. To understand better the evolutionary relatedness of these genes and to evaluate them as candidates for inherited neurological disorders, we have determined the mouse chromosomal locations of nine of these genes-Hua, Hub, Huc, Hud, Nova1, Nova2, Natpb, Cdr2, and Cdr3. These data suggest that the Hua-Hud genes arose from gene duplication and dispersion, while the other genes are dispersed in the genome. We also predict the chromosomal locations of these genes in human and discuss the potential of these genes as candidates for uncloned mouse and human mutations.

AU-rich elements target small nuclear RNAs as well as mRNAs for rapid degradation

AU-rich elements (AREs, usually containing repeated copies of AUUUA), when present in the 3'-untranslated regions (UTRs) of many mammalian mRNAs, confer instability on their host RNA molecules. The viral small nuclear RNA (snRNA) Herpesvirus saimiri U RNA 1 (HSUR 1) also contains an AUUUA-rich sequence. Here, we report that this ARE induces rapid degradation of HSUR 1 itself and of other snRNAs including HSUR 2 and cellular U1. Mutational analyses of the viral ARE establish that sequence requirements for mRNA and snRNA decay are the same, suggesting a similar mechanism. Moreover, the in vivo degradation activity of mutant AREs correlates with their in vitro binding to the HuR protein, implicated previously as a component of the mRNA degradation machinery. Our results suggest that ARE-mediated instability can be uncoupled from both ongoing translation and deadenylation of the target RNA.

Paraneoplastic syndromes

Paraneoplastic syndromes (i.e. organ/tissue disorders associated with cancer) affecting the nervous system are thought to be the result of an autoimmune response triggered by specific cancer antigens. Several of these antigens have recently been identified and include the Hu, Yo and Ri proteins, with the Hu antigens being the best studied. Immunization of animals with HuD has been shown to retard the growth of HuD-positive neuroblastomas. In addition, the presence of anti-HuD antibody in humans with small-cell lung cancer predicts the slow growth of the tumor. The associated neurological disorders, however, limit the use of these and other antigens with similar characteristics in cancer vaccines.

The Elav-like proteins bind to AU-rich elements and to the poly(A) tail of mRNA

The Elav-like proteins are specific mRNA binding proteins which are required for cellular differentiation. They contain three characteristic RNP2/RNP1-type RNA binding motifs. Previously we have shown that the first and second RNA binding domains bind to AU-rich elements in the 3'-UTR of mRNA. In this paper we show that the Elav-like proteins exhibit poly(A) binding activity. This activity is distinct from poly(A) binding activities that have been previously described. The Elav-like proteins specifically bind to long chain poly(A) tails. We have shown that the third RNA binding domain encompasses this poly(A) binding activity. Using poly(A)-Sepharose beads in a 'sandwich' assay we have shown that the Elav-like proteins can bind simultaneously to the AU-rich element and to the poly(A) tail.

A characteristic arrangement of aromatic amino acid residues in the solution structure of the amino-terminal RNA-binding domain of Drosophila sex-lethal

The Sex-lethal (Sxl) protein from Drosophila melanogaster has two RNA-binding domains (RBDs). As the amino-terminal RBD (RBD1) of the Sxl protein exhibits low sequence homology to the typical RBDs, particularly at the putative functional residues, it was difficult to unambiguously locate the RNP1 and RNP2 motifs. Therefore, in the present study, we defined the amino and carboxy-terminal borders of the first RNA-binding domain (RBD1) of the Sxl protein by limited tryptic digestion. By replacement of Phe166 by Tyr, we constructed a highly soluble mutant, which exhibits the same RNA-binding properties as those of the wild-type. Using this mutant protein, we performed NMR measurements, and elucidated the secondary and tertiary structures of the Sxl RBD1 in solution. The betaalphabetabetaalphabeta folding pattern is conserved in the solution structure of the Sxl RBD1, as in other reported RBD structures. This allowed us to identify both the RNP1 and RNP2 motifs of the Sxl RBD1 unambiguously. Intriguingly, the RNP2 motif of the Sxl RBD1 has an Ile residue at the second position, which is generally occupied by an aromatic amino acid residue in RBDs and has been suggested to be involved in their RNA binding. Furthermore, the loop region between beta2 and beta3 of the Sxl RBD1 has an exceptional cluster of aromatic amino acid residues, in place of the normal basic amino acid cluster. In contrast, the second RBD of Sxl does not exhibit these characteristic features.

Sequential expression and role of Hu RNA-binding proteins during neurogenesis

We have identified three avian (chicken) Hu/elav family RNA-binding protein genes. cHuD and cHuC are expressed specifically in neurons of both the central and peripheral nervous systems. Although cHuA is expressed in a wide variety of tissues, including neurogenic precursor cells, it is transiently down-regulated, and is then re-expressed in maturing neurons. Misexpression of cHuD in cultured neural crest cells results in a dramatic increase in the proportion of cells exhibiting neuronal morphology, molecular markers for neurons, and neurotrophin dependence. These data confirm that cHuD protein is involved in regulating neuronal differentiation.

Modulation of the fate of cytoplasmic mRNA by AU-rich elements: key sequence features controlling mRNA deadenylation and decay

Regulation of cytoplasmic deadenylation has a direct impact on the fate of mRNA and, consequently, its expression in the cytoplasm. AU-rich elements (AREs) found in the 3' untranslated regions of many labile mRNAs are the most common RNA-destabilizing elements known in mammalian cells. AREs direct accelerated deadenylation as the first step in mRNA turnover. Recently we have proposed that AREs can be divided into three different classes. mRNAs bearing either the class I AUUUA-containing ARE or the class III non-AUUUA ARE display synchronous poly(A) shortening, whereas class II ARE-containing mRNAs are deadenylated asynchronously, with the formation of poly(A)- intermediates. In this study, we have systematically characterized the deadenylation kinetics displayed by various AREs and their mutant derivatives. We find that a cluster of five or six copies of AUUUA motifs in close proximity forming various degrees of reiteration is the key feature that dictates the choice between processive versus distributive deadenylation. An AU-rich region 20 to 30 nucleotides long immediately 5' to this cluster of AUUUA motifs can greatly enhance the destabilizing ability of the AUUUA cluster and is, therefore, an integral part of the class I and class II AREs. These two features are the defining characteristics of class II AREs. Our results are consistent with the interpretation that the pentanucleotide AUUUA, rather than the nonamer UUAUUUA(U/A)(U/A), is both an essential and the minimal sequence motif of AREs. Our study provides the groundwork for future characterization of ARE-binding proteins identified by in vitro gel shift assays in order to stringently define their potential role in the ARE-mediated decay pathway. Moreover, transformation of deadenylation kinetics from one type to the other by mutations of AREs implies the existence of cross talk between the ARE and 3' poly(A) tail, which dictates the decay kinetics.

A hierarchy of Hu RNA binding proteins in developing and adult neurons

The Hu proteins are a group of antigens targeted in an immune-mediated neurodegenerative disorder associated with cancer. We have cloned and characterized four members of the Hu gene family from mouse. We find that the Hu genes encode a large number of alternatively spliced transcripts to produce a series of related neuron-specific RNA binding proteins. Despite this complexity, we have discerned several ordered features of Hu expression. In the embryo, specific Hu genes are expressed in a hierarchy during early neurogenesis. In the E16 developing cortex, mHuB is induced in very early postmitotic neurons exiting the ventricular zone, mHuD is expressed in migrating neurons of the intermediate zone, and mHuC is expressed in mature cortical plate neurons. Such a hierarchy suggests distinct functional roles for each gene in developing neurons. In the adult, all neurons express some set of Hu mRNA and protein. However, specific patterns are evident such that individual neuronal types in the hippocampus, cerebellum, olfactory cortex, neocortex, and elsewhere express from one to several Hu genes. The complexity of potential protein variants within a gene family and of different Hu family members within a neuron suggests a diverse array of function. Given the strong homologies among the Hu proteins, the Drosophila neurogenic gene elav, and the Drosophila splicing factor sxl, we predict that different combinations of Hu proteins determine different neuron-specific aspects of post-transcriptional RNA regulation. Our findings of specific developmental patterns of expression and the correlation between immune targeting of the Hu proteins and adult neurodegenerative disease suggest that the Hu proteins are critical in both the proper development and function of mature neurons.

Identification of HuR as a protein implicated in AUUUA-mediated mRNA decay

Expression of many proto-oncogenes, cytokines and lymphokines is regulated by targeting their messenger RNAs for rapid degradation. Essential signals for this control are AU-rich elements (AREs) in the 3' untranslated region (UTR) of these messages. The ARE is loosely defined as the five-nucleotide sequence AUUUA embedded in a uracil-rich region. A transacting factor, presumably a protein, binds the ARE and initiates recognition by the destabilization machinery. Numerous candidate ARE-binding proteins have been proposed. We show that a 32 kDa protein in HeLa nuclear extracts characterized previously has RNA-binding specificity that correlates with the activity of an ARE in directing mRNA decay. Purification and subsequent analyses demonstrate that this 32 kDa protein is identical to a recently identified member of the Elav-like gene family (ELG) called HuR. The in vitro binding selectivity of HuR is indicative of an ARE sequence's ability to destabilize a mRNA in vivo, suggesting a critical role for HuR in the regulation of mRNA degradation.

A 20-amino-acid autonomous RNA-binding domain contained in an enoyl-CoA hydratase

A+U-rich elements in the 3' untranslated region of mRNA species coding for lymphokines and early response genes play a pivotal role in the control of their rapid turnover. In a search for corresponding trans-acting factors, we have previously affinity-purified and cloned a human 32-kDa A+U-binding protein, termed AUH. AUH exhibited dual activities, namely A+U-specific RNA-binding and catalytic activity as enoyl-CoA hydratase. In this report we map the RNA-binding site by analysis of a series of deletion and substitution recombinant proteins. Ultraviolet cross-linking experiments demonstrated that the deletion of a 20-amino-acid segment, Lys109-Ile128, abolished more than 80% of the relative RNA-binding activity. This segment conferred RNA-binding activity when fused to maltose binding protein. Binding of this fusion protein to A+U-rich RNA was significantly competed by an AUUUA cluster and poly(U), followed by poly(G), but not by poly(A) nor poly(C). Furthermore, RNA binding of the fusion protein was competed by a synthetic peptide corresponding to Lys109-Ile128. Circular dichroic measurement indicated formation of a specific complex between this peptide and poly(U) but not with poly(A). The identified 20 amino acids therefore constitute an automonous RNA-binding domain, distinct from the RNA-recognition motifs of the family of ribonucleoproteins or NAD/RNA-binding sites in dehydrogenases found in hitherto reported A+U-binding proteins. Replacement of Arg125 in this motif with Glu reduced binding twofold, indicating this residue is integral to the binding function. Deletion of other parts of the protein did not impair RNA binding to any significant extent. By contrast, the hydratase activity of AUH required an intact three-dimensional conformation, as most mutations downstream of Ser68 impaired enzymatic activity.

The Elav-like proteins bind to a conserved regulatory element in the 3'-untranslated region of GAP-43 mRNA

Previous studies have identified three brain proteins (40, 65 and 95 kDa, respectively) that specifically bind to the 3'-untranslated region of GAP-43 mRNA. In this study, using a specific monoclonal antibody, we now show that the 40-kDa proteins are members of the Elav-like protein family. This family of specific RNA-binding proteins comprise three neural specific members called HuD, HuC, and Hel-N1. We have shown that purified recombinant HuD can bind with high affinity to GAP-43 mRNA. In addition, we have mapped the binding site to a highly conserved 26-nucleotide sequence within the regulatory element. The binding of HuD to this site is readily displaced by RNA oligonucleotides encoding other HuD binding sites. We also show that only the first and second RNA binding domains of HuD are required for selective binding to GAP-43 mRNA.

Differential activity of ELAV-like RNA-binding proteins in human neuroblastoma

Many short-lived mRNAs contain AU-rich instability elements within their 3'-untranslated region. Cellular factors that bind to these elements are thought to play a role in the regulation of mRNA degradation. In the accompanying paper (Chagnovich, D., and Cohn, S. L. (1996) J. Biol. Chem. 271, 33580-33586) we characterized the binding activity of a 40-kDa protein (p40) that interacts with high specificity with at least two AU-rich elements located within the 3'-untranslated region of N-myc. p40 activity correlates with N-myc mRNA stability in subclones of the NBL-W neuroblastoma cells line (W-N and W-S). In an effort to determine the identity of p40 we performed immunoblotting studies, immunoprecipitation experiments, and RNA gel mobility shift assays using antibodies that are directed against known RNA-binding proteins. In this paper we demonstrate that in W-N and W-S cells, p40 activity parallels the expression of embryonic letal abnormal vision (ELAV)-like proteins, and that antibodies directed against this family of RNA-binding proteins recognize p40. We also show that purified ELAV-like proteins (HuD and Hel-N1) bind with high specificity to the same N-myc 3'-untranslated region sequences as p40. Our data indicate that p40 is a member of the ELAV-like family, and suggest that this family of RNA-binding proteins may regulate N-myc mRNA turnover.

Two different RNA binding activities for the AU-rich element and the poly(A) sequence of the mouse neuronal protein mHuC

HuC is one of the RNA binding proteins which are suggested to play important roles in neuronal differentiation and maintenance. We cloned and sequenced cDNAs encoding a mouse protein which is homologous to human HuC (hHuC). The longest cDNA encodes a 367 amino acid protein with three RNA recognition motifs (RRMs) and displays 96% identity to hHuC. Northern blot analysis showed that two different mRNAs, of 5.3 and 4.3 kb, for mouse HuC (mHuC) are expressed specifically in brain tissue. Comparison of cDNA sequences with the corresponding genomic sequence revealed that alternative 3' splice site selection generates two closely related mHuC isoforms. Iterative in vitro RNA selection and binding analyses showed that both HuC isoforms can bind with almost identical specificity to sequences similar to the AU-rich element (ARE), which is involved in the regulation of mRNA stability. Functional domain mapping using mHuC deletion mutants showed that the first RRM binds to ARE, that the second RRM has no RNA binding activity by itself, but facilitates ARE binding by the first RRM and that the third RRM has specific binding activity for the poly(A) sequence.

Purification and properties of HuD, a neuronal RNA-binding protein

HuD is a human neuronal specific RNA-binding protein. In this study we have purified HuD and examined its RNA binding properties in detail. HuD binds to mRNAs that contain an AU-rich element with high affinity. In the case of the c-fos AU-rich element, HuD binds to a 27-nucleotide core element comprising AUUUA, AUUUUA, and AUUUUUA motifs. Mutation in any two of these motifs abrogates binding. HuD contains two tandem RNA recognition motifs (RRM), a basic domain, and a third RRM. Deletion analysis has shown that only the first and second RRMs are essential for RNA binding. Thus, these specific RNA binding properties support the idea that the HuD regulates gene expression at the posttranscriptional level.

[Venous pathomorphology in occlusive lesions of the arteries of the lower extremities]

The study was performed of 80 low extremities amputated because of gangrene related to atherosclerosis (28 cases) or obliterating thromboangiitis (52 cases). Two types of vein histological changes were established: 1) changes similar to those in arteries as in the thromboangiitis 2) adaptive-compensatory changes resulting from haemodynamics disturbances as in atherosclerosis. A great number of arteriovenous anastomoses, vein wall hypertrophy with the change of their calibre were observed. Hypertrophy of the muscle layer and dilatation of the vein lumen are found in cases with long duration of the process. The differences in vein morphology in the above diseases, apart from etiology, are due to the fact that in obliterating thromboangiitis the process starts in the peripheral vessels while in atherosclerosis it begins in large arteries and the vein alterations develop at late stages. The vein alterations may serve as the differential diagnostic index in these diseases.