RNA, whither goest thou?

The Roles of Par3, Par6, and aPKC Polarity Proteins in Normal Neurodevelopment and in Neurodegenerative and Neuropsychiatric Disorders

Normal neural circuits and functions depend on proper neuronal differentiation, migration, synaptic plasticity, and maintenance. Abnormalities in these processes underlie various neurodevelopmental, neuropsychiatric, and neurodegenerative disorders. Neural development and maintenance are regulated by many proteins. Among them are Par3, Par6 (partitioning defective 3 and 6), and aPKC (atypical protein kinase C) families of evolutionarily conserved polarity proteins. These proteins perform versatile functions by forming tripartite or other combinations of protein complexes, which hereafter are collectively referred to as "Par complexes." In this review, we summarize the major findings on their biophysical and biochemical properties in cell polarization and signaling pathways. We next summarize their expression and localization in the nervous system as well as their versatile functions in various aspects of neurodevelopment, including neuroepithelial polarity, neurogenesis, neuronal migration, neurite differentiation, synaptic plasticity, and memory. These versatile functions rely on the fundamental roles of Par complexes in cell polarity in distinct cellular contexts. We also discuss how cell polarization may correlate with subcellular polarization in neurons. Finally, we review the involvement of Par complexes in neuropsychiatric and neurodegenerative disorders, such as schizophrenia and Alzheimer's disease. While emerging evidence indicates that Par complexes are essential for proper neural development and maintenance, many questions on their in vivo functions have yet to be answered. Thus, Par3, Par6, and aPKC continue to be important research topics to advance neuroscience.

The Oxytocin Receptor: From Intracellular Signaling to Behavior

The many facets of the oxytocin (OXT) system of the brain and periphery elicited nearly 25,000 publications since 1930 (see FIGURE 1 , as listed in PubMed), which revealed central roles for OXT and its receptor (OXTR) in reproduction, and social and emotional behaviors in animal and human studies focusing on mental and physical health and disease. In this review, we discuss the mechanisms of OXT expression and release, expression and binding of the OXTR in brain and periphery, OXTR-coupled signaling cascades, and their involvement in behavioral outcomes to assemble a comprehensive picture of the central and peripheral OXT system. Traditionally known for its role in milk let-down and uterine contraction during labor, OXT also has implications in physiological, and also behavioral, aspects of reproduction, such as sexual and maternal behaviors and pair bonding, but also anxiety, trust, sociability, food intake, or even drug abuse. The many facets of OXT are, on a molecular basis, brought about by a single receptor. The OXTR, a 7-transmembrane G protein-coupled receptor capable of binding to either Gαior Gαqproteins, activates a set of signaling cascades, such as the MAPK, PKC, PLC, or CaMK pathways, which converge on transcription factors like CREB or MEF-2. The cellular response to OXT includes regulation of neurite outgrowth, cellular viability, and increased survival. OXTergic projections in the brain represent anxiety and stress-regulating circuits connecting the paraventricular nucleus of the hypothalamus, amygdala, bed nucleus of the stria terminalis, or the medial prefrontal cortex. Which OXT-induced patterns finally alter the behavior of an animal or a human being is still poorly understood, and studying those OXTR-coupled signaling cascades is one initial step toward a better understanding of the molecular background of those behavioral effects.

Lack of Pur-alpha alters postnatal brain development and causes megalencephaly

Pur-alpha (Purα) plays an important role in a variety of cellular processes including transcriptional regulation, cell proliferation and oncogenic transformation. To better understand the role of Purα in the developing and mature brain, we generated Purα-deficient mice, which we were able to raise to the age of six months. Purα(-/-) mice were born with no obvious pathological condition. We obtained convincing evidence that lack of Purα prolongs the postnatal proliferation of neuronal precursor cells both in the hippocampus and in the cerebellum, however, without affecting the overall number of postmitotic neurons. Independent of these findings, we observed alterations in the expression and distribution of the dendritic protein MAP2, the translation of which has been proposed previously to be Purα-dependent. At the age of 2 weeks, Purα(-/-) mice generated a continuous tremor which persisted throughout lifetime. Finally, adult Purα(-/-) mice displayed a megalencephaly and histopathological findings including axonal swellings and hyperphosphorylation of neurofilaments. Our studies underline the importance of Purα in the proliferation of neuronal precursor cells during postnatal brain development and suggest a role for Purα in the regulation of the expression and cellular distribution of dendritic and axonal proteins. Since recent studies implicate a link between Purα and the fragile X tremor/ataxia syndrome, our Purα(-/-) mouse model will provide new opportunities for understanding the mechanisms of neurodegeneration.

Dopaminergic regulation of dopamine D3 and D3nf receptor mRNA expression

Dopamine D3 receptors have the highest dopamine affinity of all dopamine receptors, and may thereby regulate dopamine signaling mediated by volume transmission. Changes in D3 receptor isoform expression may alter D3 receptor function, however, little is known regarding coordination of D3 isoform expression in response to perturbations in dopaminergic stimulation. To determine the effects of dopamine receptor stimulation and blockade on D3 receptor alternative splicing, we determined D3 and D3nf isoform mRNA expression following treatment with the D3 receptor antagonist NGB 2904, and the indirect dopamine agonist amphetamine. Expression of tyrosine hydroxylase (TH) mRNA, the rate-limiting enzyme in dopamine synthesis, was also determined. The D3/D3nf mRNA expression ratio was increased in ventral striatum, prefrontal cortex, and hippocampus 6 h following D3 antagonist NGB 2904 treatment, and remained persistently elevated at 24 h in hippocampus and substantia nigra/ventral tegmentum. D3 mRNA decreased 65% and D3nf mRNA expression decreased 71% in prefrontal cortex 24 h following amphetamine treatment, however, these changes did not reach statistical significance. TH mRNA expression was unaffected by D3 antagonist NGB 2904, but was elevated by amphetamine in ventral striatum, hippocampus, and prefrontal cortex. These findings provide evidence for an adaptive response to altered D3 receptor stimulation involving changes in D3 receptor alternative splicing. Additionally, these data suggest D3 autoreceptor regulation of dopamine synthesis does not involve regulation of TH mRNA expression. Finally, the observation of regulated TH mRNA expression in dopamine terminal fields provides experimental support for the model of local control of mRNA expression in adaptation to synaptic activity.

MicroRNA expression profile in murine central nervous system development

MicroRNAs (miRNAs) regulate gene expression in a post-transcriptional sequence-specific manner. In order to better understand the possible roles of miRNAs in central nervous system (CNS) development, we examined the expression profile of 104 miRNAs during murine brain development. We obtained brain samples from animals at embryonic days (E) E15, E17, and postnatal days (P) P1 and P7. Total RNA was isolated from tissue and used to obtain mature miRNAs by reverse transcription. Our results indicate that there is a group of 12 miRNAs that show a distinct expression profile, with the highest expression during embryonic stages and decreasing significantly during development. This profile suggests key roles in processes occurring during early CNS development.

Nerve growth factor enhances voltage-gated Na+ channel activity and Transwell migration in Mat-LyLu rat prostate cancer cell line

The highly dynamic nature of voltage-gated Na+ channel (VGSC) expression and its controlling mechanism(s) are not well understood. In this study, we investigated the possible involvement of nerve growth factor (NGF) in regulating VGSC activity in the strongly metastatic Mat-LyLu cell model of rat prostate cancer (PCa). NGF increased peak VGSC current density in a time- and dose-dependent manner. NGF also shifted voltage to peak and the half-activation voltage to more positive potentials, and produced currents with faster kinetics of activation; sensitivity to the VGSC blocker tetrodotoxin (TTX) was not affected. The NGF-induced increase in peak VGSC current density was suppressed by both the pan-trk antagonist K252a, and the protein kinase A (PKA) inhibitor KT5720. NGF did not affect the Nav1.7 mRNA level, but the total VGSC alpha-subunit protein level was upregulated. NGF potentiated the cells' migration in Transwell assays, and this was not affected by TTX. We concluded that NGF upregulated functional VGSC expression in Mat-LyLu cells, with PKA as a signaling intermediate, but enhancement of migration by NGF was independent of VGSC activity.

Involvement of protein synthesis and degradation in long-term potentiation of Schaffer collateral CA1 synapses

Expression of synaptic plasticity involves the translation of mRNA into protein and, probably, active protein degradation via the proteasome pathway. Here, we report on the rapid activation of synthesis and degradation of a probe protein with the induction of long-term potentiation (LTP) in the hippocampal Schaffer collateral CA1 pathway. The proteasome inhibitor MG132 significantly reduced the field EPSP slope potentiation and LTP maintenance without acutely affecting basal synaptic transmission. To visualize protein dynamics, CA1 pyramidal cells of hippocampal slices were transfected with Semliki Forest virus particles expressing a recombinant RNA. This RNA contained the coding sequence for a degradable green fluorescence protein with a nuclear localization signal (NLS-d1EGFP) followed by a 3'- untranslated region dendritic targeting sequence. NLS-d1EGFP fluorescence remained stable in the low-frequency test stimulation but increased with LTP induction in the cell body and in most dendritic compartments of CA1 neurons. Applying anisomycin, a protein synthesis inhibitor, caused NLS-d1EGFP levels to decline; a proteasome inhibitor MG132 reversed this effect. In the presence of anisomycin, LTP induction accelerated the degradation of NLS-d1EGFP. When both inhibitors were present, NLS-d1EGFP levels remained unaffected by LTP induction. Moreover, LTP-induced acceleration of NLS-d1EGFP synthesis was blocked by rapamycin, which is consistent with the involvement of dendritic mammalian target of rapamycin in LTP-triggered translational activity. Our results clearly demonstrate that LTP induction not only leads to a rapid increase in the rate of protein synthesis but also accelerates protein degradation via the proteasome system.

What is the biological significance of BDNF mRNA targeting in the dendrites? Clues from epilepsy and cortical development

The neurotrophin brain-derived neurotrophic factor (BDNF) is a regulatory factor of several, partially contrasting, aspects of the biology of neural cells, including survival, growth, differentiation, and cell death. Regulation of the local availability of BDNF at distinct subcellular domains such as the cell soma, dendrites, axons, and spines appears to be the key to conferring spatial and temporal specificity of the different effects elicited by this neurotrophin. This article reviews recent findings in the context of epileptogenesis and visual cortex maturation that showed that different BDNF messenger RNA (mRNA) transcripts are localized at different subcellular locations in hippocampal and cortical neurons. It also reviews findings demonstrating that strong depolarizing stimuli, both in vitro and in vivo, elicit accumulation of BDNF mRNA and protein in the distal dendrites through a signaling pathway involving the activation of the N-methyl-D-aspartate and tyrosine kinase B receptors and an intracellular increase in Ca2+ concentration. Finally, this article proposes that the regulation of the delivery of BDNF mRNA and protein to the different subcellular domains--particularly the dendritic compartment--may represent a fundamental aspect of the processes of cellular and synaptic morphological rearrangements underlying epileptogenesis and postnatal development of the visual cortex.

Activity-dependent regulation of voltage-gated Na+ channel expression in Mat-LyLu rat prostate cancer cell line

We have shown previously that voltage-gated Na(+) channels (VGSCs) are up-regulated in human metastatic disease (prostate, breast and small-cell lung cancers), and that VGSC activity potentiates metastatic cell behaviours. However, the mechanism(s) regulating functional VGSC expression in cancer cells remains unknown. We investigated the possibility of activity-dependent (auto)regulation of VGSC functional expression in the strongly metastatic Mat-LyLu model of rat prostate cancer. Pretreatment with tetrodotoxin (TTX) for 24-72 h subsequently suppressed peak VGSC current density without affecting voltage dependence. The hypothesis was tested that the VGSC auto-regulation occurred via VGSC-mediated Na(+) influx and subsequent activation of protein kinase A (PKA). Indeed, TTX pretreatment reduced the level of phosphorylated PKA, and the PKA inhibitor KT5720 decreased, whilst the adenylate cyclase activator forskolin and the Na(+) ionophore monensin both increased the peak VGSC current density. TTX reduced the mRNA level of Nav1.7, predominant in these cells, and VGSC protein expression at the plasma membrane, although the total VGSC protein level remained unchanged. TTX pretreatment eliminated the VGSC-dependent component of the cells' migration in Transwell assays. We concluded that the VGSC activity in Mat-LyLu rat prostate cancer cells was up-regulated in steady-state via a positive feedback mechanism involving PKA, and this enhanced the cells' migratory potential.

A novel RNA-binding protein in neuronal RNA granules: regulatory machinery for local translation

Local translation in neuronal dendrites is an important basis for long-term synaptic plasticity, and RNA granules in the dendrites are involved in the local translation. Here, we identify RNG105 (RNA granule protein 105), a novel RNA-binding protein, as a component of the RNA granules in dendrites of hippocampal neurons. The RNG105-localizing RNA granules contain mRNAs, the translational products of which play key roles in synaptic plasticity. RNG105 has an ability to repress translation both in vitro and in vivo, consistent with the finding that the RNA granule is translationally arrested in the basal conditions. Dissociation of RNG105 from the RNA granules is induced by BDNF, a growth factor responsible for synaptic plasticity. The RNG105 dissociation is coincident with the induction of local translation near the granules. These findings suggest that RNG105 is a translational repressor in the RNA granules and provide insight into the link between RNG105 dynamics and local translational regulation.

Inhibitory effect of naked neural BC1 RNA or BC200 RNA on eukaryotic in vitro translation systems is reversed by poly(A)-binding protein (PABP)

Regulated protein biosynthesis in dendrites of neurons might be a key mechanism underlying learning and memory. Neuronal dendritic BC1 RNA and BC200 RNA and similar small untranslated RNAs inhibit protein translation in vitro systems, such as rabbit reticulocyte lysate. Likewise, co-transfection of these RNAs with reporter mRNA suppressed translation levels in HeLa cells. The oligo(A)-rich region of all active small RNAs were identified as the RNA domains chiefly responsible for the inhibitory effects. Addition of recombinant human poly(A)-binding protein (PABP) significantly compensated the inhibitory effect of the small oligo(A)-rich RNA. In vivo, all BC1 RNA appears to be complexed with PABP. Nevertheless, in the micro-environment of dendritic spines of neuronal cells, BC1 RNPs or BC200 RNPs might mediate regulatory functions by differential interactions with locally limited PABP and/or directly or indirectly, with other translation initiation factors.

Protein synthesis-dependent LTP in isolated dendrites of CA1 pyramidal cells

Local dendritic protein synthesis provides a mechanism by which the effects of gene expression can be spatially restricted within synaptodendritic compartments of neurons. In the present study, we show that long-term potentiation (LTP), induced by two strong tetanic trains, can be produced in isolated CA1 apical dendrites and that new protein synthesis is critical for this dendritic LTP. LTP in isolated dendrites is also blocked by rapamycin, an inhibitor of growth-related protein synthesis, similar to that observed for LTP in the intact hippocampal slice. These results show that increased dendritic translation of proteins is a critical molecular mechanism for hippocampal LTP.

Local translational control in dendrites and its role in long-term synaptic plasticity

Local protein synthesis in dendrites has emerged as a key mechanism contributing to enduring forms of synaptic plasticity. Although the translational capability of dendrites has been appreciated for over 20 years, it is only recently that significant progress has been made in elucidating mechanisms that contribute to its regulation. It is clear from work over the last few years that the control of translation in dendrites is complex, involving a host of unique (and often surprising) mechanisms that can operate together or in parallel to tightly control gene expression in time and space. Here, we discuss the strategies used by neurons to regulate translation in dendrites and how these are implemented in the service of long-term information storage.

Regulation of miRNA expression during neural cell specification

MicroRNA (miRNA) are a newly recognized class of small, noncoding RNA molecules that participate in the developmental control of gene expression. We have studied the regulation of a set of highly expressed neural miRNA during mouse brain development. Temporal control is a characteristic of miRNA regulation in C. elegans and Drosophila, and is also prominent in the embryonic brain. We observed significant differences in the onset and magnitude of induction for individual miRNAs. Comparing expression in cultures of embryonic neurons and astrocytes we found marked lineage specificity for each of the miRNA in our study. Two of the most highly expressed miRNA in adult brain were preferentially expressed in neurons (mir-124, mir-128). In contrast, mir-23, a miRNA previously implicated in neural specification, was restricted to astrocytes. mir-26 and mir-29 were more strongly expressed in astrocytes than neurons, others were more evenly distributed (mir-9, mir-125). Lineage specificity was further explored using reporter constructs for two miRNA of particular interest (mir-125 and mir-128). miRNA-mediated suppression of both reporters was observed after transfection of the reporters into neurons but not astrocytes. miRNA were strongly induced during neural differentiation of embryonic stem cells, suggesting the validity of the stem cell model for studying miRNA regulation in neural development.

Dendritic BC1 RNA: modulation by kindling-induced afterdischarges

Local protein synthesis in dendrites is thought to provide a mechanism for long-lasting modifications of synapses in response to physiological activity and behavioral experience. New synthesis of dendritic proteins may be triggered by various paradigms, including induction of epileptiform activity. Prerequisite for such modulated synthesis is a mechanism that limits translation of synaptodendritic mRNAs to times of demand. Recently identified as a translational repressor that is localized to dendrites, small untranslated BC1 RNA has been implicated in the regulation of postsynaptic protein synthesis. Here we show that translational repressor BC1 RNA is itself undergoing modulation as a result of neuronal stimulation. Induction of hippocampal epileptiform activity resulted in a significant decrease of BC1 RNA in the CA3 region over several hours after excitation. The observed decrease was cell-wide, thus indicating reduced expression rather than intracellular redistribution. We suggest that a downregulation of the translational repressor BC1 RNA serves to modulate postsynaptic protein complements in response to the induction of epileptiform activity. Such increased protein synthesis in dendrites may be required for the consolidation of enduring epileptogenic mechanisms.

Cellular and synaptic distribution of NR2A and NR2B in macaque monkey and rat hippocampus as visualized with subunit-specific monoclonal antibodies

The functional and pharmacological attributes of the N-methyl-D-aspartate (NMDA) receptor are related to its subunit composition, thus resolving the subunit composition of NMDA receptors in specific classes of synapses is an important step in characterizing excitatory circuits. Toward this end, mouse monoclonal antibodies were raised against fusion protein antigens corresponding to the putative amino acid sequences of human NMDA receptor subunits NR2A and NR2B. The subunit specificity of these monoclonal antibodies was demonstrated with transfected human and rat NMDA receptor cDNAs, and their immunoreactivity was established in rat, macaque monkey, and human brain tissue. At the light microscopic level, both NR2A and NR2B exhibit a distribution in monkey and rat hippocampus very similar to NMDA receptor subunit NR1, and both are highly colocalized with NR1. Electron microscopic immunogold studies demonstrated that both NR2A and NR2B are often present in asymmetric synapses in CA1, commonly colocalized with NR1, and often colocalized with each other in the same asymmetric synapses. Both assembly and synthetic pools are present within spines and spine necks, respectively, particularly for NR2A. The confocal and ultrastructural data suggest that whereas NR1, NR2A, and NR2B are essentially uniformly colocalized in hippocampal projection neurons, there is extensive heterogeneity at the synaptic level that would lead to multiple functional classes of NMDA receptor-mediated synapses, and extensive capacity for plasticity at the synapse. Thus, the subunit profile of a given synapse may be dynamic, with regulation of local synthesis and insertion of different subunits into the synapse leading to a complex, heterogeneous, and shifting set of functional attributes of the NMDA receptor.

Translational control at the synapse

The strength of synaptic connections can undergo long-lasting changes, and such long-term plasticity is thought to underlie higher brain functions such as learning and memory. De novo synthesis of proteins is required for such plastic changes. This model is now supported by several lines of experimental data. Components of translational machinery have been identified in dendrites, including ribosomes, translation-al factors, numerous RNAs, and components of posttranslational secretory pathways. Various RNAs have been shown to be actively and rapidly transported to dendrites. Dendritic RNAs typically contain transport-specifying elements (dendritic targeting elements). Such dendritic targeting elements associate with trans-acting factors to form transport-competent ribonucleoprotein particles. It is assumed that molecular motors mediate transport of such particles along dendritic cytoskeletal elements. Once an mRNA has arrived at its dendritic destination site, appropriate spatiotemporal control of its translation, for example, in response to transsynaptic activity, becomes vital. Such local translational control, recent evidence indicates, is implemented at different levels and through various pathways. In the default state, translation is assumed to be repressed, and several mechanisms, some including small untranslated RNAs, have been proposed to contribute to such repression. Translational control at the synapse thus provides a molecular basis for the long-term, input-specific modulation of synaptic strength.

Dendritic transport and localization of protein kinase Mzeta mRNA: implications for molecular memory consolidation

Protein kinase Mzeta (PKMzeta) is an atypical protein kinase C isoform that has been implicated in the protein synthesis-dependent maintenance of long term potentiation and memory storage in the brain. Synapse-associated kinases are uniquely positioned to promote enduring consolidation of structural and functional modifications at the synapse, provided that kinase mRNA is available on site for local input-specific translation. We now report that the mRNA encoding PKMzeta is rapidly transported and specifically localized to synaptodendritic neuronal domains. Transport of PKMzeta mRNA is specified by two cis-acting dendritic targeting elements (Mzeta DTEs). Mzeta DTE1, located at the interface of the 5'-untranslated region and the open reading frame, directs somato-dendritic export of the mRNA. Mzeta DTE2, in contrast, is located in the 3'-untranslated region and is required for delivery of the mRNA to distal dendritic segments. Colocalization with translational repressor BC1 RNA in hippocampal dendrites suggests that PKMzeta mRNA may be subject to translational control in local domains. Dendritic localization of PKMzeta mRNA provides a molecular basis for the functional integration of synaptic signal transduction and translational control pathways.

Role of a neuronal small non-messenger RNA: behavioural alterations in BC1 RNA-deleted mice

BC1 RNA is a small non-messenger RNA common in dendritic microdomains of neurons in rodents. In order to investigate its possible role in learning and behaviour, we compared controls and knockout mice from three independent founder lines established from separate embryonic stem cells. Mutant mice were healthy with normal brain morphology and appeared to have no neurological deficits. A series of tests for exploration and spatial memory was carried out in three different laboratories. The tests were chosen as to ensure that different aspects of spatial memory and exploration could be separated and that possible effects of confounding variables could be minimised. Exploration was studied in a barrier test, in an open-field test, and in an elevated plus-maze test. Spatial memory was investigated in a Barnes maze and in a Morris water maze (memory for a single location), in a multiple T-maze and in a complex alley maze (route learning), and in a radial maze (working memory). In addition to these laboratory tasks, exploratory behaviour and spatial memory were assessed under semi-naturalistic conditions in a large outdoor pen. The combined results indicate that BC1 RNA-deficient animals show behavioural changes best interpreted in terms of reduced exploration and increased anxiety. In contrast, spatial memory was not affected. In the outdoor pen, the survival rates of BC1-depleted mice were lower than in controls. Thus, we conclude that the neuron-specific non-messenger BC1 RNA contributes to the aptive modulation of behaviour.

An RNA-interacting Protein, SYNCRIP (Heterogeneous Nuclear Ribonuclear Protein Q1/NSAP1) Is a Component of mRNA Granule Transported with Inositol 1,4,5-Trisphosphate Receptor Type 1 mRNA in Neuronal Dendrites

mRNA transport and local translation in the neuronal dendrite is implicated in the induction of synaptic plasticity. Recently, we cloned an RNA-interacting protein, SYNCRIP (heterogeneous nuclear ribonuclear protein Q1/NSAP1), that is suggested to be important for the stabilization of mRNA. We report here that SYNCRIP is a component of mRNA granules in rat hippocampal neurons. SYNCRIP was mainly found at cell bodies, but punctate expression patterns in the proximal dendrite were also seen. Time-lapse analysis in living neurons revealed that the granules labeled with fluorescent protein-tagged SYNCRIP were transported bi-directionally within the dendrite at approximately 0.05 microm/s. Treatment of neurons with nocodazole significantly inhibited the movement of green fluorescent protein-SYNCRIP-positive granules, indicating that the transport of SYNCRIP-containing granules is dependent on microtubules. The distribution of SYNCRIP-containing granules overlapped with that of dendritic RNAs and elongation factor 1alpha. SYNCRIP was also found to be co-transported with green fluorescent protein-tagged human staufen1 and the 3'-untranslated region of inositol 1,4,5-trisphosphate receptor type 1 mRNA. These results suggest that SYNCRIP is transported within the dendrite as a component of mRNA granules and raise the possibility that mRNA turnover in mRNA granules and the regulation of local protein synthesis in neuronal dendrites may involve SYNCRIP.

Neuronal MAP2 mRNA: Species-dependent Differential Dendritic Targeting Competence

Providing the basis for local protein synthesis in dendritic microdomains, RNA transport in dendrites is thought to be underlying long-term neuronal plasticity. Dendritic RNA targeting mechanisms can therefore be expected to confer selective advantages in the evolution of complex neural systems. The question thus arises as to when and how dendritically targeted transcripts first acquired their targeting competence. To address this question, the dendritic targeting competence of MAP2 transcripts was examined in chicken, mouse and rat. In one approach, we established the somato-dendritic distribution of MAP2 transcripts in vivo. We found that in contrast to rodent MAP2 mRNAs, which are highly enriched in dendritic regions of the retina, chicken MAP2 transcripts are virtually absent from such areas and are rather confined to neuronal somata. In an independent line of investigation, we determined that a dendritic targeting element (DTE) corresponding to the mammalian MAP2 DTE is not contained in the 3' untranslated region (UTR) of avian MAP2 mRNA. The combined results indicate that in contrast to mammalian MAP2 transcripts, avian MAP2 mRNA is lacking dendritic targeting competence. The data thus suggest that the acquisition of such competence has likely been a relatively recent event in evolution.

Differential expression and dendritic transcript localization of Shank family members: identification of a dendritic targeting element in the 3' untranslated region of Shank1 mRNA

Shank proteins are scaffolding proteins in the postsynaptic density of excitatory synapses in the mammalian brain. In situ hybridization revealed that Shank1/SSTRIP and Shank2/ProSAP1 mRNAs are widely expressed early in postnatal brain development whereas Shank3/ProSAP2 expression increases during postnatal development especially in the cerebellum and thalamus. Shank1 and Shank3 (but not Shank2) mRNAs are present in the molecular layers of the hippocampus, consistent with a dendritic transcript localization. Shank1 and Shank2 transcripts are detectable in the dendritic fields of Purkinje cells, whereas Shank3 mRNA is restricted to cerebellar granule cells. The appearance of dendritic Shank mRNAs in cerebellar Purkinje cells coincides with the onset of dendrite formation. Expression of reporter transcripts in hippocampal neurons identifies a 200-nucleotide dendritic targeting element (DTE) in the Shank1 mRNA. The widespread presence of Shank mRNAs in dendrites suggests a role for local synthesis of Shanks in response to stimuli that induce alterations in synaptic morphology.

Compartmentalized synthesis and degradation of proteins in neurons

An important aspect of gene expression in neurons involves the delivery of mRNAs to particular subcellular domains, where translation of the mRNAs is locally controlled. Local synthesis of protein within dendrites plays a key role in activity-dependent synaptic modifications. In growing axons, local synthesis in the growth cone is important for extension and guidance. Recent evidence also documents the existence of mechanisms permitting local protein degradation, providing bidirectional control of protein composition in local domains. Here, we summarize what is known about local synthesis and degradation of protein in dendrites and axons, highlighting key unresolved questions.

Neuronal untranslated BC1 RNA: targeted gene elimination in mice

Despite the potentially important roles of untranslated RNAs in cellular form or function, genes encoding such RNAs have until now received surprisingly little attention. One such gene encodes BC1 RNA, a small non-mRNA that is delivered to dendritic microdomains in neurons. We have now eliminated the BC1 RNA gene in mice. Three independent founder lines were established from separate embryonic stem cells. The mutant mice appeared to be healthy and showed no anatomical or neurological abnormalities. The gross brain morphology was unaltered in such mice, as were the subcellular distributions of two prototypical dendritic mRNAs (encoding MAP2 and CaMKIIalpha). Due to the relatively recent evolutionary origin of the gene, we expected molecular and behavioral consequences to be subtle. Behavioral analyses, to be reported separately, indicate that the lack of BC1 RNA appears to reduce exploratory activity.

Molecular determinants and physiological relevance of extrasomatic RNA localization in neurons

Specific sorting of mRNA molecules to subcellular microdomains is an evolutionarily conserved mechanism by which the polarized nature of eukayotic cells may be established and maintained. The molecular composition of the RNA localization machinery is complex. Sequence motifs within RNA molecules to be transported, called cis-acting elements, and proteins, referred to as trans-acting factors, are essential components. Transport of the resulting ribonucleoprotein complexes to distinct cytoplasmic regions occurs along the cytoskeletal network. The pathway is observed in organisms as diverse as yeast and human and it plays a critical role in development and cell differentiation. Moreover, RNA localization takes place in differentiated cell types including neurons. There is ample evidence to suggest that sorting of defined mRNA species to the neurites of nerve cells and on-site translation has an impact on various aspects of nerve cell biology.

Dendritic BC1 RNA: functional role in regulation of translation initiation

In neurons, local protein synthesis in synaptodendritic microdomains has been implicated in the growth and plasticity of synapses. Prerequisites for local translation are the targeted transport of RNAs to distal sites of synthesis in dendrites and translational control mechanisms to limit synthesis to times of demand. Here we identify dendritic BC1 RNA as a specific repressor of translation. Experimental use of internal ribosome entry mechanisms and sucrose density gradient centrifugation showed that BC1-mediated repression targets translation at the level of initiation. Specifically, BC1 RNA inhibited formation of the 48S preinitiation complex, i.e., recruitment of the small ribosomal subunit to the messenger RNA (mRNA). However, 48S complex formation that is independent of the eukaryotic initiation factor 4 (eIF4) family of initiation factors was found to be refractory to inhibition by BC1 RNA, a result that implicates at least one of these factors in the BC1 repression pathway. Biochemical experiments indicated a specific interaction of BC1 RNA with eIF4A, an RNA unwinding factor, and with poly(A)-binding protein. Both proteins were found enriched in synaptodendritic microdomains. Significantly, BC1-mediated repression was shown to be effective not only in cap-dependent translation initiation but also in eIF4-dependent internal initiation. The results suggest a functional role of BC1 RNA as a mediator of translational control in local protein synthesis in nerve cells.

Rat vasopressin mRNA: a model system to characterize cis-acting elements and trans-acting factors involved in dendritic mRNA sorting

The genes encoding the vasopressin (VP) and oxytocin (OT) precursors are expressed in magnocellular neurons of the hypothalamo-neurohypophyseal system. The neuropeptides have a dual function: (1) they are secreted from the nerve terminals into the systemic circulation to act as hormones on various peripheral target organs; and (2) VP and OT are also released from the dendrites into the central nervous system where they presumably play a role as either neurotransmitters or as modulators of the classical transmitters. Substantial amounts of VP and OT mRNAs are sorted to both axons and dendrites. Since the latter are equipped with components of the translation machinery, the peptide hormone precursors are likely to be locally synthesized in dendrites of magnocellular neurons. Evidence for axonal precursor synthesis, on the other hand, has not been obtained. Subcellular mRNA localization is a complex pathway. It is determined by sequences (cis-acting elements) within the RNA and proteins (trans-acting factors) which interact with these elements in order to guide the molecules to their ultimate destination. We have investigated the mechanisms involved in mRNA targeting in neurons by using VP mRNA as a model system. Recombinant eukaryotic expression vectors harboring the VP cDNA have been microinjected into the cell nuclei of cultured superior cervical ganglion (SCG) neurons. The subcellular distribution of the vector-expressed mRNAs was determined by non-radioactive in situ hybridization techniques. This revealed transport of VP mRNA to the dendrites, but not to the axonal compartment of SCG neurons. A complex dendritic localizer sequence (DLS) that spans part of the coding region as well as the 3'-untranslated region was identified by microinjecting constructs encoding partial sequences of the VP mRNA. In order to characterize trans-acting factors interacting with this element, protein/RNA binding experiments with radiolabeled in vitro synthesized VP RNA probes and proteins extracted from rat brain have been carried out. A protein specifically interacts with the DLS of the VP mRNA but not with sequences that obviously lack a role in subcellular RNA transport. Biochemical purification revealed that this protein is the multifunctional poly(A)-binding protein (PABP). It is well known for its ability to bind with high affinity to poly(A) tails of mRNAs, prerequisite for mRNA stabilization and stimulation of translational initiation, respectively. With lower affinities, PABP can also associate with non-poly(A) sequences. The physiological consequences of these PABP/RNA interactions include functions such as translational silencing. The translational state of mRNAs subject to dendritic sorting is most likely influenced by external stimuli. Consequently, PABP could represent one of several components necessary to regulate local synthesis of the VP precursor and possibly of other proteins.

Visualization of translated tau protein in the axons of neuronal P19 cells and characterization of tau RNP granules

Localization of tau mRNA to the axon requires the axonal localization cis signal (ALS), which is located within the 3' untranslated region, and trans-acting binding proteins, which are part of the observed granular structures in neuronal cells. In this study, using both biochemical and morphological methods, we show that the granules contain tau mRNA, HuD RNA-binding protein, which stabilizes mRNA, and KIF3A, a member of the kinesin microtubule-associated motor protein family involved in anterograde transport. The granules are detected along the axon and accumulate in the growth cone. Inhibition of KIF3A expression caused neurite retraction and inhibited tau mRNA axonal targeting. Taken together, these results suggest that HuD and KIF3A proteins are present in the tau mRNA axonal granules and suggest an additional function for the kinesin motor family in the microtubule-dependent translocation of RNA granules. Localized tau-GFP expression was blocked by a protein synthesis inhibitor, and upon release from inhibition, nascent tau-GFP 'hot spots' were directly observed in the axon and growth cones. These observations are consistent with local protein synthesis in the axon resulting from the transported tau mRNA.

Poly(A)-binding protein is associated with neuronal BC1 and BC200 ribonucleoprotein particles

BC1 RNA and BC200 RNA are two non-homologous, small non-messenger RNAs (snmRNAs) that were generated, evolutionarily, quite recently by retroposition. This process endowed the RNA polymerase III transcripts with central adenosine-rich regions. Both RNAs are expressed almost exclusively in neurons, where they are transported into dendritic processes as ribonucleoprotein particles (RNPs). Here, we demonstrate with a variety of experimental approaches that poly(A)-binding protein (PABP1), a regulator of translation initiation, binds to both RNAs in vitro and in vivo. We identified the association of PABP with BC200 RNA in a tri-hybrid screen and confirmed this binding in electrophoretic mobility-shift assays and via anti-PABP immunoprecipitation of BC1 and BC200 RNAs from crude extracts, immunodepleted extracts, partially purified RNPs and cells transfected with naked RNA. Furthermore, PABP immunoreactivity was localized to neuronal dendrites. Competition experiments using variants of BC1 and BC200 RNAs demonstrated that the central adenosine-rich region of both RNAs mediates binding to PABP. These findings lend support to the hypothesis that the BC1 and BC200 RNPs are involved in protein translation in neuronal dendrites.

Transport of Neuronal BC1 RNA in Mauthner Axons

In neurons, localized RNAs have been identified in dendrites and axons; however, RNA transport in axons remains poorly understood. Here we analyzed axonal RNA transport in goldfish Mauthner neurons in vivo. BC1 RNA, a noncoding RNA polymerase III transcript that is targeted to dendrites in neurons of the rodent nervous system, was used as a probe for axonal RNA transport. Somata of Mauthner neurons were microinjected with various RNAs. Full-length BC1 RNA, but not control RNAs of similar length, was targeted to both axons and dendrites of Mauthner neurons. BC1 RNA was transported in the form of a rapidly advancing wave front that progressed along axons, in a microtubule-dependent manner, at a rate of 2 micrometer/sec. Whereas a BC1 5' segment of 65 nucleotides was transported to axons and dendrites in a way indistinguishable from full-length BC1 RNA, a BC1 3' segment of 60 nucleotides did not enter Mauthner cell processes to any significant extent. In the wake of the wave advancing through the axon, BC1 RNA was found localized to discrete, spatially delimited domains at the axonal surface. Such demarcated cortical concentrations of BC1 RNA could not be observed after disruption of F-actin organization in the axon. It is concluded that the specific delivery of BC1 RNA to spatially defined axonal target sites is a two-step process that requires the sequential participation of microtubules for long-range axial transport and of actin filaments for local radial transfer and focal accumulation in cortical domains.

The RNA-binding protein Staufen from rat brain interacts with protein phosphatase-1

In mammalian neurones, homologues of the Drosophila RNA-binding protein Staufen are part of ribonucleoprotein complexes that move bidirectionally along dendritic microtubules and appear to regulate mRNA translocation and translation. In this study, putative components of Staufen granules were identified in a yeast two-hybrid screen of a rat brain cDNA library with a rat Staufen bait. Protein phosphatase-1 was found as an interacting partner. Binding appears to be mediated by a five amino acid residue sequence motif (R-K-V-T-F) in Staufen that is conserved in a number of proteins interacting with the phosphatase. A two amino acid residue mutation within this motif (R-K-V-G-A) disrupted the interaction. A cytoplasmic interaction of both proteins was shown by coimmunoprecipitation of rat Staufen and protein phosphatase-1 from the cytoplasm of transfected cells and rat brain homogenates. In mammalian brain, the phosphatase represents the first described endogenous interaction partner of Staufen. In primary hippocampal neurones, both proteins partially colocalize in somata and neuronal processes. Staufen does not modulate the in vitro protein phosphatase activity. These findings show that protein phosphatase-1 is a native component of Staufen particles. Cellular functions of Staufen may be regulated via phosphorylation or Staufen may recruite the phosphatase into specific ribonucleoprotein complexes.

Target interaction regulates distribution and stability of specific mRNAs

Several factors regulate export of mRNAs from neuronal cell bodies. Using in situ hybridization and RT-PCR, we examined how target interaction influences the distribution of mRNAs expressed in sensory neurons (SNs) of Aplysia maintained in cell culture. Interaction with a synaptic target has two effects on the distribution of mRNA encoding an SN-specific peptide, sensorin: the target affects the accumulation of sensorin mRNA at the axon hillock and the stability of sensorin mRNA exported to distal sites. Synapse formation with motor neuron L7 results in the accumulation of high levels of sensorin mRNA in the axon hillock of the SN and in SN neurites contacting L7. SNs cultured alone or in contact with motor neuron L11, with which no synapses form, show a more uniform distribution of sensorin mRNA in the cytoplasm of the SN cell body, with little expression in neurites. Contact with L7 or L11 had little or no effect on the distribution of two other mRNAs in the cytoplasm of SN cell bodies. Sensorin mRNA exported to SN neurites after 1 d in culture is more stable when the SN contacts L7 compared with SN neurites that contact L11. After removal of the SN cell body, the amounts of sensorin mRNA already exported to the neurites are greater when neurites contact L7 compared with neurites in contact with L11. The results indicate that target interaction and synapse formation regulate both the accumulations of specific mRNAs destined for export and their stability at distant sites.

A small RNA in testis and brain: implications for male germ cell development

BC1 RNA, a small non-coding RNA polymerase III transcript, is selectively targeted to dendritic domains of a subset of neurons in the rodent nervous system. It has been implicated in the regulation of local protein synthesis in postsynaptic microdomains. The gene encoding BC1 RNA has been suggested to be a master gene for repetitive ID elements that are found interspersed throughout rodent genomes. A prerequisite for the generation of repetitive elements through retroposition and subsequent transmission in the germline is expression of the master gene RNA in germ cells. To test this hypothesis, we have investigated expression of BC1 RNA in murine male germ cells. We report that BC1 RNA is expressed at substantial levels in a subset of male germ cells. Results from cell fractionation experiments, developmental analysis,and northern and in situ hybridization showed that the RNA was expressed in pre-meiotic spermatogonia, with particularly high amounts in syncytial ensembles of cells that are primed for synchronous spermatogenic differentiation. BC1 RNA continued to be expressed in spermatocytes, but expression levels decreased during further spermatogenic development, and low or negligible amounts of BC1 RNA were identified in round and elongating spermatids. The combined data indicate that BC1 RNA operates in groups of interconnected germ cells, including spermatogonia, where it may function in the mediation of translational control. At the same time, the identification of BC1 RNA in germ cells provides essential support for the hypothesis that repetitive ID elements in rodent genomes arose from the BC1 RNA gene through retroposition.

NPY Y1 receptors are present in axonal processes of DRG neurons

Using a sensitive immunohistochemical method, the localization of the neuropeptide Y (NPY) Y1 receptor (Y1R) was studied in contralateral and ipsilateral dorsal root ganglion (DRG) neurons of rats subjected to different unilateral manipulations with focus on their axonal processes and projection areas. Y1R-like immunoreactivity (LI) was observed in the contralateral sciatic nerve and dorsal roots of lesioned rats, and double staining revealed colocalization with calcitonin gene-related peptide (CGRP). Y1R-LI was also seen in fibers close to and even within the epidermis. A fairly small number of nerve endings double-labeled for Y1R and CGRP were present in the dorsal horn. After unilateral crush of the sciatic nerve Y1R- and CGRP-LI accumulated in the same axons proximal to the lesion. After dorsal rhizotomy CGRP-LI was strongly reduced in the ipsilateral dorsal horn. No certain change was observed for Y1R- or NPY-LI, but Y1R/CGRP double-labeled nerve endings disappeared after the lesion. These results strongly suggest centrifugal transport of Y1Rs in DRG neurons, mainly to the peripheral sensory branches. To what extent these Y1Rs are functional has not been analyzed here, but a recent study on Y1R null mice provides evidence for involvement of prejunctional Y1Rs in peripheral sensory functions

Messenger RNAs in synaptosomal fractions from rat brain

Synaptosomal fractions from rat brain have been analyzed with semi-quantitative RT-PCR methods to determine their content of mRNAs coding for presynaptic, postsynaptic, glial, and neuronal proteins. Each mRNA was determined with reference to the standard HPRT mRNA. In our analyses, mRNAs were considered to be associated with synaptosomes only if their relative amounts were higher than in microsomes prepared in a polysome stabilizing medium, rich in Mg(++) and K(+) ions, or in the homogenate. According to this stringent criterion, the following synaptosomal mRNAs could not be attributed to microsomal contamination and were assumed to derive from the subcellular structures known to harbor their translation products, i.e. GAT-1 mRNAs from presynaptic terminals and glial processes, MAP2 mRNA from dendrites, GFAP mRNA from glial processes, and TAU mRNA from neuronal fragments. This interpretation is in agreement with the involvement of extrasomatic mRNAs in local translation processes.

Localisation of endothelin-1 mRNA expression and immunoreactivity in the retina and optic nerve from human and porcine eye. Evidence for endothelin-1 expression in astrocytes

We have investigated the localisation of endothelin-1 (ET-1) mRNA and ET-1-like immunoreactivity in retina and anterior portion of optic nerve from human and porcine eyes. In situ hybridisation method revealed expression of ET-1 mRNA mainly in the innermost layers of the retinas, in the retinal pigment epithelium cells as well as in the astrocytes of the optic nerve. Immunohistochemical studies showed that ET-1-like immunoreactivity appeared in the same regions where ET-1 mRNA was expressed as well as in the inner nuclear layer and in the inner segments of photoreceptors. In the nerve fibre and ganglion cell layers, astrocytes expressed both glial fibrillary acidic protein and ET-1 proteins suggesting that these cells may secrete ET-1. Expression of ETA and ETB receptors in human retina were demonstrated by reverse transcription-polymerase chain reaction. Our results demonstrated expression of ET-1 in glial, neural and vascular components of retina and optic nerve from human and porcine eyes.

Neurotrophin-induced transport of a beta-actin mRNP complex increases beta-actin levels and stimulates growth cone motility

Neurotrophin regulation of actin-dependent changes in growth cone motility may depend on the signaling of beta-actin mRNA transport. Formation of an RNP complex between the beta-actin mRNA zipcode sequence and Zipcode Binding Protein 1 (ZBP1) was required for its localization to growth cones. Antisense oligonucleotides to the zipcode inhibited formation of this RNP complex in vitro and the neurotrophin-induced localization of beta-actin mRNA and ZBP1 granules. Live cell imaging of neurons transfected with EGFP-ZBP1 revealed fast, bidirectional movements of granules in neurites that were inhibited by antisense treatment, as visualized by FRAP analysis. NT-3 stimulation of beta-actin protein localization was dependent on the 3'UTR and inhibited by antisense treatment. Growth cones exhibited impaired motility in the presense of antisense. These results suggest a novel mechanism to influence growth cone dynamics involving the regulated transport of mRNA.

Vasopressin mRNA localization in nerve cells: characterization of cis-acting elements and trans-acting factors

mRNA localization is a complex pathway. Besides mRNA sorting per se, this process includes aspects of regulated translation. It requires protein factors that interact with defined sequences (or sequence motifs) of the transcript, and the protein/RNA complexes are finally guided along the cytoskeleton to their ultimate destinations. The mRNA encoding the vasopressin (VP) precursor protein is localized to the nerve cell processes in vivo and in primary cultured nerve cells. Sorting of VP transcripts to dendrites is mediated by the last 395 nucleotides of the mRNA, the dendritic localizer sequence, and it depends on intact microtubules. In vitro interaction studies with cytosolic extracts demonstrated specific binding of a protein, enriched in nerve cell tissues, to the radiolabeled dendritic localizer sequence probe. Biochemical purification revealed that this protein is the multifunctional poly(A)-binding protein (PABP). It is well known for its ability to bind with high affinity to poly(A) tails of mRNAs, prerequisite for mRNA stabilization and stimulation of translational initiation, respectively. With lower affinities, PABP can also associate with non-poly(A) sequences. The physiological consequences of these PABP/RNA interactions are far from clear but may include functions such as translational silencing. Presumably, the translational state of mRNAs subject to dendritic sorting is influenced by external stimuli. PABP thus could be a component required to regulate local synthesis of the VP precursor and possibly of other proteins.

Tracking the estrogen receptor in neurons: implications for estrogen-induced synapse formation

Estrogens (E) and progestins regulate synaptogenesis in the CA1 region of the dorsal hippocampus during the estrous cycle of the female rat, and the functional consequences include changes in neurotransmission and memory. Synapse formation has been demonstrated by using the Golgi technique, dye filling of cells, electron microscopy, and radioimmunocytochemistry. N-methyl-d-aspartate (NMDA) receptor activation is required, and inhibitory interneurons play a pivotal role as they express nuclear estrogen receptor alpha (ERalpha) and show E-induced decreases of GABAergic activity. Although global decreases in inhibitory tone may be important, a more local role for E in CA1 neurons seems likely. The rat hippocampus expresses both ERalpha and ERbeta mRNA. At the light microscopic level, autoradiography shows cell nuclear [3H]estrogen and [125I]estrogen uptake according to a distribution that primarily reflects the localization of ERalpha-immunoreactive interneurons in the hippocampus. However, recent ultrastructural studies have revealed extranuclear ERalpha immunoreactivity (IR) within select dendritic spines on hippocampal principal cells, axon terminals, and glial processes, localizations that would not be detectable by using standard light microscopic methods. Based on recent studies showing that both types of ER are expressed in a form that activates second messenger systems, these findings support a testable model in which local, non-genomic regulation by estrogen participates along with genomic actions of estrogens in the regulation of synapse formation.

Molecular kinesis in cellular function and plasticity

Intracellular transport and localization of cellular components are essential for the functional organization and plasticity of eukaryotic cells. Although the elucidation of protein transport mechanisms has made impressive progress in recent years, intracellular transport of RNA remains less well understood. The National Academy of Sciences Colloquium on Molecular Kinesis in Cellular Function and Plasticity therefore was devised as an interdisciplinary platform for participants to discuss intracellular molecular transport from a variety of different perspectives. Topics covered at the meeting included RNA metabolism and transport, mechanisms of protein synthesis and localization, the formation of complex interactive protein ensembles, and the relevance of such mechanisms for activity-dependent regulation and synaptic plasticity in neurons. It was the overall objective of the colloquium to generate momentum and cohesion for the emerging research field of molecular kinesis.

Neuronal BC1 RNA: co-expression with growth-associated protein-43 messenger RNA

Brain-specific cytoplasmic RNA 1 (BC1-RNA), a non-coding RNA polymerase III transcript, is a neuronal RNA that is specifically targeted to dendritic domains. It is co-localized with components of the dendritic protein synthetic machinery, and it has been suggested to operate in the regulation of local translation-related processes in postsynaptic microdomains, thus subserving long-term synaptic plasticity in neurons. To probe the relevance of BC1 expression in neuronal plasticity, we have analyzed the expression pattern of BC1 RNA in the rat nervous system. We found that BC1 RNA is expressed by a specific subset of neurons (but not by non-neuronal cells) in the central and peripheral nervous system of the adult rat. The BC1 labeling pattern indicates that the subcellular location of the RNA is typically postsynaptic which, depending on cell type, manifests itself in a predominantly somatic, somatodendritic, or dendritic location. Our results further show that BC1-expressing neurons typically co-express the messenger RNA for growth-associated protein-43 (GAP-43). Such co-expression was observed in diverse brain areas, including the olfactory bulb, neocortex, and hippocampus, among others. While BC1 RNA was in many neuronal cell types detectable in distal dendritic domains, GAP-43 messenger RNA was typically more restricted to neuronal perikarya. In the mature nervous system, expression of GAP-43 has been described as an intrinsic determinant of predominantly presynaptic plasticity, while BC1 RNA has been implicated in postsynaptic plasticity. Co-expression of both RNAs, as reported here, thus identifies a distinct subset of neurons in the rat nervous system that exhibits both types of plasticity.

Identification of a cis-acting dendritic targeting element in the mRNA encoding the alpha subunit of Ca2+/calmodulin-dependent protein kinase II

In mammalian neurons a selected group of mRNAs, including the transcript encoding the alpha subunit of Ca2+/calmodulin-dependent protein kinase II, is found in dendrites. The molecular mechanisms underlying extrasomatic RNA trafficking are not well described. It is thought that dendritic transcripts contain cis-acting elements that direct their selective subcellular sorting. Here we report the identification of an extrasomatic targeting element in the 3' untranslated region of the mRNA encoding the alpha subunit of Ca2+/calmodulin-dependent protein kinase II. In primary hippocampal neurons, this 1200-nucleotide-spanning, cis-acting element is sufficient to mediate dendritic localization of chimeric reporter transcripts. The trafficking signal does not share any striking sequence similarity with a previously characterized dendritic targeting element in transcripts encoding the microtubule-associated protein 2. In dendrites of transfected primary neurons, recombinant RNAs form granules with an average diameter of 0.45 microm that may represent preferential RNA docking sites or multimolecular transport units. These findings imply that extrasomatic sorting of individual dendritic mRNAs involves at least partially distinct molecular mechanisms, as well as large trafficking complexes.

VP-RBP, a protein enriched in brain tissue, specifically interacts with the dendritic localizer sequence of rat vasopressin mRNA

The concept of mRNA localization suggests that this process is mediated by sequences residing in the transcript to which proteins specifically bind and ultimately deliver the mRNA along cytoskeletal elements to specific intracellular destinations. The mRNA encoding the vasopressin (VP) precursor protein is localized to the nerve cell processes both in hypothalamic magnocellular neurons and in primary cultured neurons derived from embryonic rat superior cervical ganglia microinjected with a corresponding eukaryotic expression vector. The last 395 nucleotides of the VP mRNA encompassing part of the coding region, as well as the complete 3'-untranslated region, are sufficient to confer dendritic targeting to a normally nonlocalized reporter transcript. Here we report that, by employing in vitro crosslinking analyses with rat brain proteins and radiolabelled VP transcripts, an RNA-binding protein specifically interacts with the dendritic localizer sequence of the VP mRNA. This protein is enriched in nerve cell tissues. Peripheral tissues and various cell lines contain only low amounts of the binding activity. It therefore represents a candidate protein that may be involved in any aspect related to subcellular VP mRNA sorting in nerve cells, including transport and anchoring of the mRNA and possibly its translational control.

Two rat brain staufen isoforms differentially bind RNA

In neurones, a limited number of mRNAs is found in dendrites, including transcripts encoding the microtubule-associated protein 2 (MAP2). Recently, we identified a cis-acting dendritic targeting element (DTE) in MAP2 mRNAs. Here we used the yeast tri-hybrid system to identify potential trans-acting RNA-binding factors of the DTE. A cDNA clone was isolated that encodes a member of a mammalian protein family that is highly homologous to the Drosophila RNA-binding protein Staufen. Mammalian Staufen appears to be expressed in most tissues and brain areas. Two distinct rat brain Staufen isoforms, rStau+I6 and rStau-I6, are encoded by alternatively spliced mRNAs. Both isoforms contain four double-stranded RNA-binding domains (dsRBD). In the larger rStau+I6 isoform, six additional amino acids are inserted in the second dsRBD. Although both isoforms interacted with the MAP2-DTE and various additional RNA fragments in an in vitro north-western assay, rStau-I6 exhibited a stronger signal of bound radioactively labelled RNAs as compared with rStau+I6. Using an antibody directed against mammalian Staufen, the protein was detected in somata and dendrites of neurones of the adult rat hippocampus and cerebral cortex. Ultrastructural studies revealed that in dendrites, rat Staufen accumulates along microtubules. Thus in neurones, rat Staufen may serve to link RNAs to the dendritic microtubular cytoskeleton and may thereby regulate their subcellular localization.