Poly(A) tail length-dependent stabilization of GAP-43 mRNA by the RNA-binding protein HuD

Novel recognition motifs and biological functions of the RNA-binding protein HuD revealed by genome-wide identification of its targets

HuD is a neuronal ELAV-like RNA-binding protein (RBP) involved in nervous system development, regeneration, and learning and memory. This protein stabilizes mRNAs by binding to AU-rich instability elements (AREs) in their 3′ unstranslated regions (3′ UTR). To isolate its in vivo targets, messenger ribonucleoprotein (mRNP) complexes containing HuD were first immunoprecipitated from brain extracts and directly bound mRNAs identified by subsequent GST-HuD pull downs and microarray assays. Using the 3′ UTR sequences of the most enriched targets and the known sequence restrictions of the HuD ARE-binding site, we discovered three novel recognition motifs. Motifs 2 and 3 are U-rich whereas motif 1 is C-rich. In vitro binding assays indicated that HuD binds motif 3 with the highest affinity, followed by motifs 2 and 1, with less affinity. These motifs were found to be over-represented in brain mRNAs that are upregulated in HuD overexpressor mice, supporting the biological function of these sequences. Gene ontology analyses revealed that HuD targets are enriched in signaling pathways involved in neuronal differentiation and that many of these mRNAs encode other RBPs, translation factors and actin-binding proteins. These findings provide further insights into the post-transcriptional mechanisms by which HuD promotes neural development and synaptic plasticity.

Dynamics of axonal mRNA transport and implications for peripheral nerve regeneration

Locally generating new proteins in subcellular regions provide means to spatially and temporally modify protein content in polarized cells. Recent years have seen resurgence of the concept that axonal processes of neurons can locally synthesize proteins. Experiments from a number of groups have now shown that axonal protein synthesis helps to initiate growth, provides a means to respond to guidance cues, and generates retrograde signaling complexes. Additionally, there is increasing evidence that locally synthesized proteins provide functions beyond injury responses and growth in the mature peripheral nervous system. A key regulatory event in this translational regulation is moving the mRNA templates into the axonal compartment. Transport of mRNAs into axons is a highly regulated and specific process that requires interaction of RNA binding proteins with specific cis-elements or structures within the mRNAs. mRNAs are transported in ribonucleoprotein particles that interact with microtubule motor proteins for long-range axonal transport and likely use microfilaments for short-range movement in the axons. The mature axon is able to recruit mRNAs into translation with injury and possibly other stimuli, suggesting that mRNAs can be stored in a dormant state in the distal axon until needed. Axotomy triggers a shift in the populations of mRNAs localized to axons, indicating a dynamic regulation of the specificity of the axonal transport machinery. In this review, we discuss how axonal mRNA transport and localization are regulated to achieve specific changes in axonal RNA content in response to axonal stimuli.

A p53-CBP/p300 transcription module is required for GAP-43 expression, axon outgrowth, and regeneration

Transcription regulates axon outgrowth and regeneration. However, to date, no transcription complexes have been shown to control axon outgrowth and regeneration by regulating axon growth genes. Here, we report that the tumor suppressor p53 and its acetyltransferases CBP/p300 form a transcriptional complex that regulates the axonal growth-associated protein 43, a well-characterized pro-axon outgrowth and regeneration protein. Acetylated p53 at K372-3-82 drives axon outgrowth, GAP-43 expression, and binds specific elements on the neuronal GAP-43 promoter in a chromatin environment through CBP/p300 signaling. Importantly, in an axon regeneration model, both CBP and p53 K372-3-82 are induced following axotomy in facial motor neurons, where p53 K372-3-82 occupancy of GAP-43 promoter is enhanced as shown by in vivo chromatin immunoprecipitation. Finally, by comparing wild-type and p53 null mice, we demonstrate that the p53/GAP-43 transcriptional module is specifically switched on during axon regeneration in vivo. These data contribute to the understanding of gene regulation in axon outgrowth and may suggest new molecular targets for axon regeneration.

Diverse molecular functions of Hu proteins

Hu proteins are RNA-binding proteins involved in diverse biological processes. The neuronal members of the Hu family, HuB, HuC, and HuD play important roles in neuronal differentiation and plasticity, while the ubiquitously expressed family member, HuR, has numerous functions mostly related to cellular stress response. The pivotal roles of Hu proteins are dictated by their molecular functions affecting a large number of target genes. Hu proteins affect many post-transcriptional aspects of RNA metabolism, from splicing to translation. In this communication, we will focus on these molecular events and review our current understanding of how Hu proteins mediate them. In particular, emphasis will be put on the nuclear functions of these proteins, which were recently discovered. Three examples including calcitonin/calcitonin gene-related peptide, neurofibromatosis type 1, and Ikaros will be discussed in detail. In addition, an intriguing theme of antagonism between Hu proteins and other AU-rich sequence binding proteins will be discussed.

RNA–protein interactions and control of mRNA stability in neurons

In addition to transcription, posttranscriptional mechanisms play a vital role in the control of gene expression. There are multiple levels of posttranscriptional regulation, including mRNA processing, splicing, editing, transport, stability, and translation. Among these, mRNA stability is estimated to control about 5-10% of all human genes. The rate of mRNA decay is regulated by the interaction of cis-acting elements in the transcripts and sequence-specific RNA-binding proteins. One of the most studied cis-acting elements is the AU-rich element (ARE) present in the 3' untranslated region (3'UTR) of several unstable mRNAs. These sequences are targets of many ARE-binding proteins; some of which induce degradation whereas others promote stabilization of the mRNA. Recently, these mechanisms were uncovered in neurons, where they have been associated with different physiological phenomena, from early development and nerve regeneration to learning and memory processes. In this Mini-Review, we briefly discuss the general mechanisms of control of mRNA turnover and present evidence supporting the importance of these mechanisms in the expression of an increasing number of neuronal genes.

Regulation of heme oxygenase-1 mRNA deadenylation and turnover in NIH3T3 cells by nitrosative or alkylation stress

Background

Heme oxygenase-1 (HO-1) catalizes heme degradation, and is considered one of the most sensitive indicators of cellular stress. Previous work in human fibroblasts has shown that HO-1 expression is induced by NO, and that transcriptional induction is only partially responsible; instead, the HO-1 mRNA half-life is substantially increased in response to NO. The mechanism of this stabilization remains unknown.

Results

In NIH3T3 murine fibroblasts, NO exposure increased the half-life of the HO-1 transcript from ~1.6 h to 11 h, while treatments with CdCl₂, NaAsO₂ or H₂O₂ increased the half-life only up to 5 h. Although poly(A) tail shortening can be rate-limiting in mRNA degradation, the HO-1 mRNA deadenylation rate in NO-treated cells was ~65% of that in untreated controls. In untreated cells, HO-1 poly(A) removal proceeded until 30–50 nt remained, followed by rapid mRNA decay. In NO-treated cells, HO-1 deadenylation stopped with the mRNA retaining poly(A) tails 30–50 nt long. We hypothesize that NO treatment stops poly(A) tail shortening at the critical 30- to 50-nt length. This is not a general mechanism for the post-transcriptional regulation of HO-1 mRNA. Methyl methane sulfonate also stabilized HO-1 mRNA, but that was associated with an 8-fold decrease in the deadenylation rate compared to that of untreated cells. Another HO-1 inducer, CdCl₂, caused a strong increase in the mRNA level without affecting the HO-1 mRNA half-life.

Conclusion

The regulation of HO-1 mRNA levels in response to cellular stress can be induced by transcriptional and different post-transcriptional events that act independently, and vary depending on the stress inducer. While NO appears to stabilize HO-1 mRNA by preventing the final steps of deadenylation, methyl methane sulfonate achieves stabilization through the regulation of earlier stages of deadenylation.

Expression of PKC substrate proteins, GAP-43 and neurogranin, is downregulated by cAMP signaling and alterations in synaptic activity

Growth-associated protein 43 (GAP-43) and neurogranin are protein kinase C substrate proteins that are thought to play an important role in synaptic plasticity, but little is currently known about the mechanisms that may regulate their function at the synapse. In this study, we show that long-term elevation of intracellular cAMP levels in rat primary cortical cultures results in a persistent downregulation of GAP-43 and neurogranin, most likely at the transcriptional level. This effect may be at least partially mediated by protein kinase A, but is independent of protein kinase C activation. Moreover, it is mimicked and occluded by manipulations that alter the levels of spontaneous synaptic activity in primary cultures, such as bicuculline and tetrodotoxin. These data suggest that levels of GAP-43 and neurogranin are regulated by factors known to modulate synaptic strength, thus providing a potential mechanism by which protein kinase C signaling pathways and their substrates might contribute to synaptic function and/or plasticity.

RNA transport and localized protein synthesis in neurological disorders and neural repair

Neural cells are able to finely tune gene expression through post-transcriptional mechanisms. Localization of mRNAs to subcellular regions has been detected in neurons, oligodendrocytes, and astrocytes providing these domains with a locally renewable source of proteins. Protein synthesis in dendrites has most frequently been associated with synaptic plasticity, while axonally synthesized proteins appear to facilitate pathfinding and injury responses. For oligodendrocytes, mRNAs encoding several proteins for myelin formation are locally generated suggesting that this mechanism assists in myelination. Astrocytic processes have not been well studied but localization of GFAP mRNA has been demonstrated. Both RNA transport and localized translation are regulated processes. RNA transport appears to be highly selective and, at least in part, the destiny of individual mRNAs is determined in the nucleus. RNA-protein and protein-protein interactions determine which mRNAs are targeted to subcellular regions. Several RNA binding proteins that drive mRNA localization have also been shown to repress translation during transport. Activity of the translational machinery is also regulated in distal neural cell processes. Clinically, disruption of mRNA localization and/or localized mRNA translation may contribute to pathophysiology of fragile X mental retardation and spinal muscular atrophy. Axonal injury has been shown to activate localized protein synthesis, providing both a means to initiate regeneration and retrogradely signal injury to the cell body. Decreased capacity to transport mRNAs and translational machinery into distal processes could jeopardize the ability to respond to injury or local stimuli within axons and dendrites.

Coordinated expression of HuD and GAP-43 in hippocampal dentate granule cells during developmental and adult plasticity

Previous work from our laboratory demonstrated that the RNA-binding protein HuD binds to and stabilizes the GAP-43 mRNA. In this study, we characterized the expression of HuD and GAP-43 mRNA in the hippocampus during two forms of neuronal plasticity. During post-natal development, maximal expression of both molecules was found at P5 and their levels steadily decreased thereafter. At P5, HuD was also present in the subventricular zone, where it co-localized with doublecortin. In the adult hippocampus, the basal levels of HuD and GAP-43 were lower than during development but were significantly increased in the dentate gyrus after seizures. The function of HuD in GAP-43 gene expression was confirmed using HuD-KO mice, in which the GAP-43 mRNA was significantly less stable than in wild type mice. Altogether, these results demonstrate that HuD plays a role in the post-transcriptional control of GAP-43 mRNA in dentate granule cells during developmental and adult plasticity.

The RNA-binding protein HuD binds acetylcholinesterase mRNA in neurons and regulates its expression after axotomy

After axotomy, expression of acetylcholinesterase (AChE) is greatly reduced in the superior cervical ganglion (SCG); however, the molecular events involved in this response remain unknown. Here, we first examined AChE mRNA levels in the brain of transgenic mice that overexpress human HuD. Both in situ hybridization and reverse transcription-PCR demonstrated that AChE transcript levels were increased by more than twofold in the hippocampus of HuD transgenic mice. Additionally, direct interaction between the HuD transgene product and AChE mRNA was observed. Next, we examined the role of HuD in regulating AChE expression in intact and axotomized rat SCG neurons. After axotomy of the adult rat SCG neurons, AChE transcript levels decreased by 50 and 85% by the first and fourth day, respectively. In vitro mRNA decay assays indicated that the decrease in AChE mRNA levels resulted from changes in the stability of presynthesized transcripts. A combination of approaches performed using the region that directly encompasses an adenylate and uridylate (AU)-rich element within the AChE 3'-untranslated region demonstrated a decrease in RNA-protein complexes in response to axotomy of the SCG and, specifically, a decrease in HuD binding. After axotomy, HuD transcript and protein levels also decreased. Using a herpes simplex virus construct containing the human HuD sequence to infect SCG neurons in vivo, we found that AChE and GAP-43 mRNA levels were maintained in the SCG after axotomy. Together, the results of this study demonstrate that AChE expression in neurons of the rat SCG is regulated via post-transcriptional mechanisms that involve the AU-rich element and HuD.

Associative and spatial learning and memory deficits in transgenic mice overexpressing the RNA-binding protein HuD

HuD is a neuronal specific RNA-binding protein associated with the stabilization of short-lived mRNAs during brain development, nerve regeneration and synaptic plasticity. To investigate the functional significance of this protein in the mature brain, we generated transgenic mice overexpressing HuD in forebrain neurons under the control of the alphaCaMKinII promoter. We have previously shown that one of the targets of HuD, GAP-43 mRNA, was stabilized in neurons in the hippocampus, amygdala and cortex of transgenic mice. Animals from two independent lines expressing different levels of the transgene were subjected to a battery of behavioral tests including contextual fear conditioning and the Morris water maze. Our results show that although HuD is increased after learning and memory, constitutive HuD overexpression impaired the acquisition and retention of both cued and contextual fear and the ability to remember the position of a hidden platform in the Morris water maze. No motor-sensory abnormalities were observed in HuD transgenic mice, suggesting that the poor performance of the mice in these tests reflect a true cognitive impairment. We conclude that posttranscriptional regulation of gene expression by stabilization of specific mRNAs may have to be restricted temporally and spatially for proper acquisition and storage of memories.

The RNA-binding protein HuD: a regulator of neuronal differentiation, maintenance and plasticity

mRNA stability is increasingly recognized as being essential for controlling the expression of a wide variety of transcripts during neuronal development and synaptic plasticity. In this context, the role of AU-rich elements (ARE) contained within the 3' untranslated region (UTR) of transcripts has now emerged as key because of their high incidence in a large number of cellular mRNAs. This important regulatory element is known to significantly modulate the longevity of mRNAs by interacting with available stabilizing or destabilizing RNA-binding proteins (RBP). Thus, in parallel with the emergence of ARE, RBP are also gaining recognition for their pivotal role in regulating expression of a variety of mRNAs. In the nervous system, the member of the Hu family of ARE-binding proteins known as HuD, has recently been implicated in multiple aspects of neuronal function including the commitment and differentiation of neuronal precursors as well as synaptic remodeling in mature neurons. Through its ability to interact with ARE and stabilize multiple transcripts, HuD has now emerged as an important regulator of mRNA expression in neurons. The present review is designed to provide a comprehensive and updated view of HuD as an RBP in the nervous system. Additionally, we highlight the role of HuD in multiple aspects of a neuron's life from early differentiation to changes in mature neurons during learning paradigms and in response to injury and regeneration. Finally, we describe the current state of knowledge concerning the molecular and cellular events regulating the expression and activity of HuD in neurons.

CARM1 regulates proliferation of PC12 cells by methylating HuD

HuD is an RNA-binding protein that has been shown to induce neuronal differentiation by stabilizing labile mRNAs carrying AU-rich instability elements. Here, we show a novel mechanism of arginine methylation of HuD by coactivator-associated arginine methyltransferase 1 (CARM1) that affected mRNA turnover of p21cip1/waf1 mRNA in PC12 cells. CARM1 specifically methylated HuD in vitro and in vivo and colocalized with HuD in the cytoplasm. Inhibition of HuD methylation by CARM1 knockdown elongated the p21cip1/waf1 mRNA half-life and resulted in a slow growth rate and robust neuritogenesis in response to nerve growth factor (NGF). Methylation-resistant HuD bound more p21cip1/waf1 mRNA than did the wild type, and its overexpression upregulated p21cip1/waf1 protein expression. These results suggested that CARM1-methylated HuD maintains PC12 cells in the proliferative state by committing p21cip1/waf1 mRNA to its decay system. Since the methylated population of HuD was reduced in NGF-treated PC12 cells, downregulation of HuD methylation is a possible pathway through which NGF induces differentiation of PC12 cells.

In vivo post-transcriptional regulation of GAP-43 mRNA by overexpression of the RNA-binding protein HuD

HuD is a neuronal-specific RNA-binding protein that binds to and stabilizes the mRNAs of growth-associated protein-43 (GAP-43) and other neuronal proteins. HuD expression increases during brain development, nerve regeneration, and learning and memory, suggesting that this protein is important for controlling gene expression during developmental and adult plasticity. To examine the function of HuD in vivo, we generated transgenic mice overexpressing human HuD under the control of the calcium-calmodulin-dependent protein kinase IIalpha promoter. The transgene was expressed at high levels throughout the forebrain, including the hippocampal formation, amygdala and cerebral cortex. Using quantitative in situ hybridization, we found that HuD overexpression led to selective increases in GAP-43 mRNA in hippocampal dentate granule cells and neurons in the lateral amygdala and layer V of the neorcortex. In contrast, GAP-43 pre-mRNA levels were unchanged or decreased in the same neuronal populations. Comparison of the levels of mature GAP-43 mRNA and pre-mRNA in the same neurons of transgenic mice suggested that HuD increased the stability of the transcript. Confirming this, mRNA decay assays revealed that the GAP-43 mRNA was more stable in brain extracts from HuD transgenic mice than non-transgenic littermates. In conclusion, our results demonstrate that HuD overexpression is sufficient to increase GAP-43 mRNA stability in vivo.

AU-rich elements and associated factors: are there unifying principles?

The control of mRNA stability is an important process that allows cells to not only limit, but also rapidly adjust, the expression of regulatory factors whose over expression may be detrimental to the host organism. Sequence elements rich in A and U nucleotides or AU-rich elements (AREs) have been known for many years to target mRNAs for rapid degradation. In this survey, after briefly summarizing the data on the sequence characteristics of AREs, we present an analysis of the known ARE-binding proteins (ARE-BP) with respect to their mRNA targets and the consequences of their binding to the mRNA. In this analysis, both the changes in mRNA stability and the lesser studied effects on translation are considered. This analysis highlights the multitude of mRNAs bound by one ARE-BP and conversely the large number of ARE-BP that associate with any particular ARE-containing mRNA. This situation is discussed with respect to functional redundancies or antagonisms. The potential relationship between mRNA stability and translation is also discussed. Finally, we present several hypotheses that could unify the published data and suggest avenues for future research.

Role of ELAV-like RNA-binding proteins HuD and HuR in the post-transcriptional regulation of acetylcholinesterase in neurons and skeletal muscle cells

Over the last few years, several laboratories have focused their attention on elucidating the molecular events that control the expression and localization of acetylcholinesterase (AChE) in neurons and skeletal muscle cells. In this context, results from a number of studies have clearly shown the important contribution of transcriptional events in regulating AChE expression. Specifically, these studies have highlighted the roles of several cis- and trans-acting factors that control transcription of the AChE gene in these excitable cells. However, it has also become apparent that changes in the transcriptional activity of the AChE gene cannot fully account for the alterations seen in the overall abundance of AChE transcripts in neurons and muscle cells placed under a variety of experimental conditions. This indicates, therefore, that post-transcriptional mechanisms also play a significant role in controlling AChE mRNA expression. With this in mind, we have recently begun to address this issue in greater detail. Here, we provide a summary of our most recent findings dealing with the post-transcriptional regulation of AChE. Together, our studies have shown so far the important contribution of an AU-rich element located in the 3'UTR of AChE transcripts and of the stabilizing RNA-binding proteins of the ELAV-like family in regulating AChE expression in differentiating neuronal and muscle cells.

The RNA-binding protein HuR binds to acetylcholinesterase transcripts and regulates their expression in differentiating skeletal muscle cells

During myogenic differentiation, acetylcholinesterase (AChE) transcript levels are known to increase dramatically. Although this increase can be attributed in part to increased transcriptional activity, posttranscriptional mechanisms have also been implicated in the high levels of AChE mRNA in myotubes. In this study, we observed that transfection of a luciferase reporter construct containing the full-length AChE 3'-untranslated region (UTR) resulted in significantly higher (5-fold) luciferase activity in differentiated myotubes versus myoblasts. RNA-electrophoretic mobility shift assays (REMSAs) performed with a full-length AChE 3'-UTR probe and the AU-rich element revealed that the intensity of RNA-binding protein complexes increased as myogenic differentiation proceeded. Using several complementary approaches including supershift REMSA, mRNA-binding protein pull-down assays, and immunoprecipitation followed by reverse transcription-PCR, we found that the mRNA-stabilizing protein HuR interacts directly with AChE transcripts. Stable overexpression of HuR in C2C12 cells increased the expression of endogenous AChE transcripts as well as that of the luciferase reporter construct containing the AChE 3'-UTR. In vitro stability assays performed with protein extracts from these cells versus controls resulted in a slower rate of AChE mRNA decay. The down-regulation of HuR expression mediated through small interfering RNA further confirmed the role of HuR in the regulation of AChE mRNA levels. Taken together, these studies demonstrate that HuR interacts with the AChE 3'-UTR to regulate posttranscriptionally the expression of AChE mRNA during myogenic differentiation.

GAP-43 mRNA in growth cones is associated with HuD and ribosomes

The neuron-specific ELAV/Hu family member, HuD, interacts with and stabilizes GAP-43 mRNA in developing neurons, and leads to increased levels of GAP-43 protein. As GAP-43 protein is enriched in growth cones, it is of interest to determine if HuD and GAP-43 mRNA are associated in developing growth cones. HuD granules in growth cones are found in the central domain that is rich in microtubules and ribosomes, in the peripheral domain with its actin network, and in filopodia. This distribution of HuD granules in growth cones is dependent on actin filaments but not on microtubules. GAP-43 mRNA is localized in granules found in both the central and peripheral domains, but not in filopodia. Ribosomes were extensively colocalized with HuD and GAP-43 mRNA granules in the central domain, consistent with a role in the control of GAP-43 mRNA stability in the growth cone. Together, these results demonstrate that many of the components necessary for GAP-43 mRNA translation/stabilization are present within growth cones.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

TAP/NXF1, the primary mRNA export receptor, specifically interacts with a neuronal RNA-binding protein HuD

Hu proteins are RNA-binding proteins that are implicated in the control of stabilization, nuclear export, and/or translation of specific mRNAs with AU-rich elements (AREs) in the 3'-untranslated region. Three neuron-specific Hu proteins (HuD, HuB, and HuC), but not a ubiquitously expressed Hu protein HuR, have an activity to induce neurite outgrowth when they are overexpressed in a rat neuronal cell line PC12. Here we show that TAP/NXF1, the primary export receptor for the bulk mRNA, is a specific binding partner for HuD. In vitro binding experiments using recombinant proteins revealed that the interaction between TAP and HuD is direct and that HuD can form a ternary complex together with both TAP and RNA. Interestingly, HuR does not interact with TAP. These results suggest that HuD acts as a novel adaptor protein to recruit TAP for efficient export of ARE-containing mRNAs in neuronal cells.

Sarcoplasmic reticulum Ca2+ pump mRNA stability in cardiac and smooth muscle: role of poly A+ tail length

Transcripts of the sarco/endoplasmic reticulum Ca2+ pump SERCA2 are alternatively spliced to produce SERCA2a (expressed in left ventricular muscle (LVM)) and SERCA2b (in stomach smooth muscle (SSM)) mRNA that use different polyadenylation sequences. SERCA2 mRNA in LVM is more stable than in SSM. Here, we report that the SERCA2 poly A+ tail length in LVM (32 +/- 2 bases) is longer than in SSM (20 +/- 1). In the in vitro decay assays, the 3'-region mRNA of SERCA2a or SERCA2b is more stable when the poly A+ tail is longer. However, when the poly A+ tail length is similar for SERCA2a and SERCA2b, the SERCA2a RNA is more stable. Thus, the longer poly A+ tail may contribute to, but does not appear to be the sole determinant of, greater stability of SERCA2 mRNA in LVM than in SSM.

Increase of the RNA-binding protein HuD and posttranscriptional up-regulation of the GAP-43 gene during spatial memory

Neuronal ELAV-like proteins (HuB, HuC, and HuD) are highly conserved RNA-binding proteins able to selectively associate with the 3' UTR of a subset of target mRNAs and increase their cytoplasmic stability and rate of translation. We previously demonstrated the involvement of these proteins in learning, reporting that they undergo a sustained up-regulation in the hippocampus of mice trained in a spatial discrimination task. Here, we extend this finding, showing that a similar up-regulation occurs in the hippocampus of rats trained in another spatial learning paradigm, the Morris water maze. HuD, a strictly neuron-specific ELAV-like protein, is shown to increase after learning, with a preferential binding to the cytoskeletal fraction. HuD up-regulation is associated with an enhancement of GAP-43 mRNA and protein levels, with an apparently increased HuD colocalization with the GAP-43 mRNA and an increased association of neuronal ELAV-like proteins with the GAP-43 mRNA. These learning-dependent biochemical events appear to be spatiotemporally controlled, because they do not occur in another brain region involved in learning, the retrosplenial cortex, and at the level of protein expression they show extinction 1 month after training despite memory retention. By contrast, HuD mRNA levels still remain increased after 1 month in the CA1 region. This persistence may have implications for long-term memory recall.

XSEB4R, a novel RNA-binding protein involved in retinal cell differentiation downstream of bHLH proneural genes

RNA-binding proteins play key roles in the post-transcriptional regulation of gene expression but so far they have not been studied extensively in the context of developmental processes. We report on the molecular cloning and spatio-temporal expression of a novel RNA-binding protein, XSEB4R, which is strongly expressed in the nervous system. This study is focused on the analysis of Xseb4R in the context of primary neurogenesis and retinogenesis. To study Xseb4R function during eye development, we set up a new protocol allowing in vivo lipofection of antisense morpholino oligonucleotides into the retina. The resulting XSEB4R knockdown causes an impairment of neuronal differentiation, with an increase in the number of glial cells. By contrast, our gain-of-function analysis demonstrates that Xseb4R strongly promotes neural differentiation. We also showed a similar function during primary neurogenesis. Consistent with this proneural effect, we found that in the open neural plate Xseb4R expression is upregulated by the proneural gene XNgnr1, as well as by the differentiation gene XNeuroD, but is inhibited by the Notch/Delta pathway. Altogether, our results suggest for the first time a proneural effect for a RNA-binding protein involved in the genetic network of retinogenesis.

Characterization of the interaction between neuronal RNA-binding protein HuD and AU-rich RNA

Hu proteins have been shown to bind to AU-rich elements (AREs) in the 3'-untranslated region of unstable mRNAs. They can thereby inhibit the decay of labile transcripts by antagonizing destabilizing proteins that target these AU-rich sequences. Here we examine the sequence preferences of HuD to elucidate its possible role in counteracting mRNA decay. Using repeats of the prototype destabilizing sequence UU(AUUU)nAUU, we show that all three HuD RNA-binding domains participate in binding to AU-tracts that can be as short as 13 residues, depending on the position of the remaining As. Removal of the A residues, resulting in a poly(U)-tract, increased the affinity of HuD for RNA, suggesting that the presence of As in destabilizing elements might favor the recruitment of other proteins and/or prevent HuD from binding too tightly to AREs. In vitro selection experiments with randomized RNAs confirmed the preference of HuD for poly(U). RNA binding analysis of the related protein HuB showed a similar preference for poly(U). In contrast, tristetraprolin, an mRNA destabilizing protein, strongly prefers AU-rich RNA. Many labile mRNAs contain U-tracts in or near their AREs. Individual AREs may thus differentially affect mRNA half-life by recruiting a unique complement of stabilizing and destabilizing factors.

Role of HuR in skeletal myogenesis through coordinate regulation of muscle differentiation genes

In this report, we investigate the role of the RNA-binding protein HuR during skeletal myogenesis. At the onset of myogenesis in differentiating C2C12 myocytes and in vivo in regenerating mouse muscle, HuR cytoplasmic abundance increased dramatically, returning to a predominantly nuclear presence upon completion of myogenesis. mRNAs encoding key regulators of myogenesis-specific transcription (myogenin and MyoD) and cell cycle withdrawal (p21), bearing AU-rich regions, were found to be targets of HuR in a differentiation-dependent manner. Accordingly, mRNA half-lives were highest during differentiation, declining when differentiation was completed. Importantly, HuR-overexpressing C2C12 cells displayed increased target mRNA expression and half-life and underwent precocious differentiation. Our findings underscore a critical function for HuR during skeletal myogenesis linked to HuR's coordinate regulation of muscle differentiation genes.

Differential function of RNCAM isoforms in precise target selection of olfactory sensory neurons

Olfactory sensory neurons (OSNs) are individually specified to express one odorant receptor (OR) gene among approximately 1000 different and project with precision to topographically defined convergence sites, the glomeruli, in the olfactory bulb. Although ORs partially determine the location of convergence sites, the mechanism ensuring that axons with different OR identities do not co-converge is unknown. RNCAM (OCAM, NCAM2) is assumed to regulate a broad zonal segregation of projections by virtue of being a homophilic cell adhesion molecule that is selectively expressed on axons terminating in a defined olfactory bulb region. We have identified NADPH diaphorase activity as being an independent marker for RNCAM-negative axons. Analyses of transgenic mice that ectopically express RNCAM in NADPH diaphorase-positive OSNs show that the postulated function of RNCAM in mediating zone-specific segregation of axons is unlikely. Instead, analyses of one OR-specific OSN subpopulation (P2) reveal that elevated RNCAM levels result in an increased number of P2 axons that incorrectly co-converge with axons of other OR identities. Both Gpi-anchored and transmembrane-bound RNCAM isoforms are localized on axons in the nerve layer, while the transmembrane-bound RNCAM is the predominant isoform on axon terminals within glomeruli. Overexpressing transmembrane-bound RNCAM results in co-convergence events close to the correct target glomeruli. By contrast, overexpression of Gpi-anchored RNCAM results in axons that can bypass the correct target before co-converging on glomeruli located at a distance. The phenotype specific for Gpi-anchored RNCAM is suppressed in mice overexpressing both isoforms, which suggests that two distinct RNCAM isoform-dependent activities influence segregation of OR-defined axon subclasses.