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

Epigenetics as "conductor" in "orchestra" of pluripotent states

Pluripotent character is described as the potency of cells to differentiate into all three germ layers. The best example to reinstate the term lies in the context of embryonic stem cells (ESCs). Pluripotent ESC describes the in vitro status of those cells that originate during the complex process of embryogenesis. Pre-implantation to post-implantation development of embryo embrace cells with different levels of stemness. Currently, four states of pluripotency have been recognized, in the progressing order of "naïve," "poised," "formative," and "primed." Epigenetics act as the "conductor" in this "orchestra" of transition in pluripotent states. With a distinguishable gene expression profile, these four states associate with different epigenetic signatures, sometimes distinct while otherwise overlapping. The present review focuses on how epigenetic factors, including DNA methylation, bivalent chromatin, chromatin remodelers, chromatin/nuclear architecture, and microRNA, could dictate pluripotent states and their transition among themselves.

The role of synaptic microRNAs in Alzheimer's disease

Structurally and functionally active synapses are essential for neurotransmission and for maintaining normal synaptic and cognitive functions. Researchers have found that synaptic dysfunction is associated with the onset and progression of neurodegenerative diseases, such as Alzheimer's disease (AD), and synaptic dysfunction is even one of the main physiological hallmarks of AD. MiRNAs are present in small, subcellular compartments of the neuron such as neural dendrites, synaptic vesicles, and synaptosomes are known as synaptic miRNAs. Synaptic miRNAs involved in governing multiple synaptic functions that lead to healthy brain functioning and synaptic activity. However, the precise role of synaptic miRNAs has not been determined in AD progression. This review emphasizes the presence of miRNAs at the synapse, synaptic compartments and roles of miRNAs in multiple synaptic functions. We focused on synaptic miRNAs alteration in AD, and how the modulation of miRNAs effect the synaptic functions in AD. We also discussed the impact of synaptic miRNAs in AD progression concerning the synaptic ATP production, mitochondrial function, and synaptic activity.

The nuclear factor interleukin 3-regulated (NFIL3) transcription factor involved in innate immunity by activating NF-κB pathway in mud crab Scylla paramamosain

NFIL3 is a transcriptional activator of the IL-3 promoter in T cells. In vertebrates, it has been characterized as an essential regulator of several cellular processes such as immunity response, apoptosis and NK cells maturation. However, the identification and functional characterization of NFIL3 still remains unclear in arthropods. In this study, the NFIL3 homologue was firstly cloned and characterized in mud crab Scylla paramamosain. The full-length of SpNFIL3 was 2, 041 bp in length with an open reading frame of 1, 509 bp, containing a conserved basic region of leucin zipper domain. The qRT-PCR analysis indicated that SpNFIL3 was significantly highly expressed in hepatopancreas and in hemocytes. Moreover, the SpNFIL3 transcription could be up-regulated after the challenge of Vibrio alginolyticus or virus-analog Poly (I:C). The dual-luciferase reporter assays revealed that SpNFIL3 could activate NF-κB pathway. The immunofluorescence assay indicated SpNFIL3 was located in nucleus. After NFIL3 was interfered in vivo and in vitro, the expressions of two NF-κB members (SpRelish and SpDorsal), six antimicrobial peptide genes (SpCrustin and SpALF2-6) and pro-inflammatory cytokine SpIL-16 were suppressed, and the bacteria clearance capacity of crabs was also markedly impaired in NFIL3 silenced crabs. These results indicated that SpNFIL3 played crucial role in the innate immunity of S. paramamosain and it also brought new insight into the origin and evolution of NFIL3 in arthropods and even in invertebrates.

The corticotropin releasing factor binding protein: A strange case of Dr. Jekyll and Mr. Hyde in the stress system?

The corticotropin releasing factor (CRF) exerts its effects by acting on its receptors and on the binding protein (CRFBP). Extensive literature suggests a role of CRF in alcohol use disorder (AUD). Less is known about the specific role, if any, of CRFBP in AUD. In this review, we summarize recent interdisciplinary efforts toward identifying the contribution of CRFBP in mediating CRF activation. The role of CRFBP in alcohol-related behaviors has been evaluated with the ultimate goal of designing effective novel therapeutic strategies for AUD. A series of in vitro, in vivo, ex vivo, and genetic studies presented here provides initial evidence that CRFBP may possess both inhibitory and excitatory roles, and supports the original hypothesis that it represents a novel pharmacological target for the treatment of AUD. This report summarizes the proceedings of one of the talks at the Young Investigator Award symposium at the Alcoholism and Stress: A Framework for Future Treatment Strategies Conference, Volterra, Italy.

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.

The first invertebrate NFIL3 transcription factor with role in immune defense identified from the Hong Kong oyster, Crassostrea hongkongensis

NFIL3 (nuclear factor interleukin 3-regulated) is a basic leucine zipper type transcription factor that mediates a variety of immune responses in vertebrates. However, the sequence information and function of NFIL3 homologs in invertebrates, especially mollusks, remains unknown. In the present study, the first NFIL3 homolog was identified in a marine mollusk, Crassostrea hongkongensis (designated as ChNFIL3), followed by its functional characterization. The full-length cDNA of ChNFIL3 is 2221 bp and consists of an open reading frame (ORF) of 1536 bp that encodes a polypeptide of 551 amino acids. Simple Modular Architecture Research Tool (SMART) analysis indicated that ChNFIL3 has two basic leucin zipper domains, similar to the other known NFIL3 family proteins. Tissue distribution analysis of NFIL3 in this mollusk revealed high expression in digestive glands and hemocytes. A significant induction in the mRNA level of ChNFIL3 was observed following bacterial stimulation. ChNFIL3 was found to be localized in the nucleus and over expression of ChNIFL3 led to upregulation of transcriptional activity of an NF-κB reporter gene in HEK 293T cells, indicating its role in innate immunity. Furthermore, addition of exogenous recombinant ChNFIL3 proteins resulted in enhanced mRNA level of hemocyte interleukin 17 in vitro. In conclusion, our findings revealed that NFIL3 in molluscs, plays a conserved role in host defense, similar to its mammalian homolog.

Identification in the rat brain of a set of nuclear proteins interacting with H1° mRNA

Synthesis of H1° histone, in the developing rat brain, is also regulated at post-transcriptional level. Regulation of RNA metabolism depends on a series of RNA-binding proteins (RBPs); therefore, we searched for H1° mRNA-interacting proteins. With this aim, we used in vitro transcribed, biotinylated H1° RNA as bait to isolate, by a chromatographic approach, proteins which interact with this mRNA, in the nuclei of brain cells. Abundant RBPs, such as heterogeneous nuclear ribonucleoprotein (hnRNP) K and hnRNP A1, and molecular chaperones (heat shock cognate 70, Hsc70) were identified by mass spectrometry. Western blot analysis also revealed the presence of cold shock domain-containing protein 2 (CSD-C2, also known as PIPPin), a brain-enriched RBP previously described in our laboratory. Co-immunoprecipitation assays were performed to investigate the possibility that identified proteins interact with each other and with other nuclear proteins. We found that hnRNP K interacts with both hnRNP A1 and Hsc70 whereas there is no interaction between hnRNP A1 and Hsc70. Moreover, CSD-C2 interacts with hnRNP A1, Y box-binding protein 1 (YB-1), and hnRNP K. We also have indications that CSD-C2 interacts with Hsc70. Overall, we have contributed to the molecular characterization of a ribonucleoprotein particle possibly controlling H1° histone expression in the brain.

A genetic in vivo system to detect asymmetrically distributed RNA

Many RNAs show polarized or otherwise non-random subcellular distributions. To create a method for genome-wide genetic screens for RNAs with asymmetric subcellular distributions, we have combined methods for gene tagging and live imaging of messenger RNA (mRNA). A pilot screen in a highly polarized, differentiated cell in the Drosophila larva, the branched terminal cell of the tracheal system, demonstrates the feasibility of the method for identifying new asymmetrically localized mRNAs in vivo.

The 3' untranslated region of human Cyclin-Dependent Kinase 5 Regulatory subunit 1 contains regulatory elements affecting transcript stability

BACKGROUND

CDK5R1 plays a central role in neuronal migration and differentiation during central nervous system development. CDK5R1 has been implicated in neurodegenerative disorders and proposed as a candidate gene for mental retardation. The remarkable size of CDK5R1 3'-untranslated region (3'-UTR) suggests a role in post-transcriptional regulation of CDK5R1 expression.

RESULTS

The bioinformatic study shows a high conservation degree in mammals and predicts several AU-Rich Elements (AREs). The insertion of CDK5R1 3'-UTR into luciferase 3'-UTR causes a decreased luciferase activity in four transfected cell lines. We identified 3'-UTR subregions which tend to reduce the reporter gene expression, sometimes in a cell line-dependent manner. In most cases the quantitative analysis of luciferase mRNA suggests that CDK5R1 3'-UTR affects mRNA stability. A region, leading to a very strong mRNA destabilization, showed a significantly low half-life, indicating an accelerated mRNA degradation. The 3' end of the transcript, containing a class I ARE, specifically displays a stabilizing effect in neuroblastoma cell lines. We also observed the interaction of the stabilizing neuronal RNA-binding proteins ELAV with the CDK5R1 transcript in SH-SY5Y cells and identified three 3'-UTR sub-regions showing affinity for ELAV proteins.

CONCLUSION

Our findings evince the presence of both destabilizing and stabilizing regulatory elements in CDK5R1 3'-UTR and support the hypothesis that CDK5R1 gene expression is post-transcriptionally controlled in neurons by ELAV-mediated mechanisms. This is the first evidence of the involvement of 3'-UTR in the modulation of CDK5R1 expression. The fine tuning of CDK5R1 expression by 3'-UTR may have a role in central nervous system development and functioning, with potential implications in neurodegenerative and cognitive disorders.

Multiple calmodulin mRNAs are selectively transported to functionally different neuronal and glial compartments in the rat hippocampus. An electron microscopic in situ hybridization study

The ultrastructural distribution of the calmodulin (CaM) mRNAs transcribed from the three CaM genes was studied in the CA1 region of the adult rat hippocampus by means of electron microscopic in situ hybridization. Digoxigenin-labeled CaM gene-specific riboprobes were detected with nanogold-anti-digoxigenin antibody conjugate. The CaM mRNAs were differentially distributed in both the neuronal and glial cell compartments. The greatest difference in neuronal distribution of the CaM mRNAs was found in the dendrites, where the mRNAs transcribed from the CaM I and III genes were much more abundant than the CaM II mRNA. The neuronal perikarya were heavy labeled for all the CaM mRNAs. Interestingly, the myelinated axons and axon terminals also contained small amounts of nanogold particles for all the CaM mRNAs, which diminished with increasing distance from the soma. Most of the synaptic profiles, however, contained labeling only in the postsynaptic region. The CaM mRNAs were differentially distributed in the glial cells. While the glial cell somata were only lightly labeled, surprisingly concentrated labeling was present in the perisynaptic and perivascular astrocytic processes. In general, the CaM II mRNA was the least represented in the glial processes. Only a very low CaM gene expression was observed in the endothelial and resting microglial cells. These results provide ultrastructural evidence for differential targeting of the multiple CaM mRNA transcripts to the intracellular compartments and suggest their microdomain-specific regulation.

A paracrine signaling role for serotonin in rat taste buds: expression and localization of serotonin receptor subtypes

Recent advances in peripheral taste physiology now suggest that the classic linear view of information processing within the taste bud is inadequate and that paracrine processing, although undemonstrated, may be an essential feature of peripheral gustatory transduction. Taste receptor cells (TRCs) express multiple neurotransmitters of unknown function that could potentially participate in a paracrine role. Serotonin is expressed in a subset of TRCs with afferent synapses; additionally, TRCs respond physiologically to serotonin. This study explored the expression and cellular localization of serotonin receptor subtypes in TRCs as a possible route of paracrine communication. RT-PCR was performed on RNA extracted from rat posterior taste buds with 14 primer sets representing 5-HT1 through 5-HT7 receptor subtype families. Data suggest that 5-HT1A and 5-HT3 receptors are expressed in taste buds. Immunocytochemistry with a 5-HT1A-specific antibody demonstrated that subsets of TRCs were immunopositive for 5-HT1A. With the use of double-labeling, serotonin- and 5-HT1A-immunopositive cells were observed exclusively in nonoverlapping populations. On the other hand, 5-HT3-immunopositive taste receptor cells were not observed. This observation, combined with other data, suggests 5-HT3 is expressed in postsynaptic neural elements within the bud. We hypothesize that 5-HT release from TRCs activates postsynaptic 5-HT3 receptors on afferent nerve fibers and, via a paracrine route, inhibits neighboring TRCs via 5-HT1A receptors. The role of the 5-HT1A-expressing TRC within the taste bud remains to be explored.

Somatodendritic localization of the mRNA for EFA6A, a guanine nucleotide exchange protein for ARF6, in rat hippocampus and its involvement in dendritic formation

EFA6A is a guanine nucleotide exchange protein (GEP) that can specifically activate ADP-ribosylation factor 6 (ARF6) in vitro. A recent study has demonstrated that ARF6 is involved in the dendritic formation of developing hippocampal neurons [Hernandez-Deviez et al. (2002) Nature Neurosci., 5, 623-624]. This study examined a potential role for EFA6A in hippocampal development in Wistar rats. Our results provided definitive evidence for somatodendritic localization of EFA6A mRNA in both cultured and in vivo hippocampal neurons by nonradioactive in situ hybridization. During postnatal development, EFA6A mRNA was dramatically increased and its dendritic localization was most evident between P7 and P14. In contrast, ARF6 mRNA was confined to the neuronal layers of the hippocampus throughout development. In addition, the overexpression of a GEP-defective mutant of EFA6A enhanced the dendritic formation of the primary hippocampal neurons. The present findings suggest that EFA6A is intimately involved in the regulation of the dendritic development of hippocampal neurons.

Protein synthesis is necessary for dendritic spine proliferation in adult brain slices

Dendritic spines, small protrusions from dendritic shafts, receive most of the excitatory synapses in cortical regions. Spines are highly plastic structures that can be rapidly produced or lost in response to a wide array of internal and external stimuli, and they proliferate in acute slice preparations [J. Neurosci. 19 (1999) 2876]. The goal of the present study was to determine if protein synthesis is necessary for this spine proliferation. We found that the addition of protein synthesis inhibitors to acute slices (in which spines otherwise proliferate) blocked new spine growth. Furthermore, a population of longer spines was observed after 2 h but these did not develop during protein synthesis blockade. These data suggest that protein synthesis is necessary for new spine growth in acute brain slice preparations and support literature suggesting that newly produced spines develop from filopodia-like protrusions.

Estrogen stimulates postsynaptic density-95 rapid protein synthesis via the Akt/protein kinase B pathway

Estrogens induce synaptogenesis in the CA1 region of the dorsal hippocampus during the estrous cycle of the female rat. Functional consequences of such estrogen-mediated synaptogenesis include cyclic changes in neurotransmission and memory. At the molecular level, estrogen stimulates the rapid activation of specific signal transduction pathways, and of particular interest is the activation of Akt (protein kinase B), a key signal transduction intermediate that initiates protein translation by alleviating the downstream translational repression of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1). Using a well established in vitro model system of differentiated NG108-15 neurons to investigate such rapid signaling effects of estrogen, we show that estrogen stimulates the phosphorylation of Akt, an indication of kinase activation, as well as the phosphorylation of 4E-BP1. In turn, the activation of these signaling intermediates suggests a non-genomic mechanism by which estrogen might likewise lead to protein translation of dendrite-localized mRNA transcripts in the hippocampus in vivo. We therefore considered the translation of the dendritic spine scaffolding protein postsynaptic density-95 (PSD-95). Although estrogen does not stimulate a rapid increase in PSD-95 mRNA levels in NG108-15 neurons, we show here that estrogen does however stimulate a rapid increase in PSD-95 new protein synthesis in vitro and that this new protein synthesis is Akt dependent. These results demonstrate an essential role for Akt in estrogen-stimulated dendritic spine protein expression, describe for the first time a signal transduction pathway in PSD-95 expression, and delineate a novel, molecular mechanism by which ovarian hormones might translationally regulate synaptogenesis via activating protein synthesis for dendritic function.

Localizing synaptic mRNAs at the neuromuscular junction: it takes more than transcription

The neuromuscular junction has been used for several decades as an excellent model system to examine the cellular and molecular events involved in the formation and maintenance of a differentiated chemical synapse. In this context, several laboratories have focused their efforts over the last 15 years on the important contribution of transcriptional mechanisms to the regulation of the development and plasticity of the postsynaptic apparatus in muscle fibers. Converging lines of evidence now indicate that post-transcriptional events, operating at the level of mRNA stability and targeting, are likely to also play key roles at the neuromuscular junction. Here, we present the recent findings highlighting the role of these additional molecular events and extend our review to include data showing that post-transcriptional events are also important in the control of the expression of genes encoding synaptic proteins in muscle cells placed under different conditions. Finally, we discuss the possibility that mis-regulation of post-transcriptional events can occur in certain neuromuscular diseases and cause abnormalities of the neuromuscular junction.

Cloning and characterization of a novel synaptosome-enriched mRNA that encodes 31 kDa protein

Although a subpopulation of mRNAs has been identified as translocated to the dendrites or the synaptic regions of neurons, the translocational mechanism has not been elucidated. To find mRNAs enriched in synapses, we compared the synaptosomal mRNAs with those from whole forebrain using differential display (DD). We cloned one of these mRNAs, which encoded a novel 31 kDa protein (PMES-2). PMES-2 mRNA was specifically transcribed in the brain and was present in the dendrites of the hippocampal neurons. PMES-2 protein was partly localized in the postsynaptic density. Although this protein is very similar to human NABC1 protein, its function is still unknown.

Ontogeny of calmodulin gene expression in rat brain

Calmodulin (CaM), a multifunctional intracellular calcium receptor, is a key element in signaling mechanisms. It is encoded in vertebrates by multiple apparently redundant genes (CaM I, II, III). To investigate whether differential expression takes place in the developing rat brain, a quantitative in situ hybridization analysis was carried out involving 15 brain areas at six ages between embryonic day 19 and postnatal day 20 (PD20) with gene-specific [(35)S]cRNA probes. A widespread, developmental stage-specific and differential expression of the three CaM genes was observed. The characteristic changes in the CaM mRNA levels in the examined time frame allowed the brain regions to be classified into three categories. For the majority of the areas (e.g. the piriform cortex for CaM III), the signal intensities peaked at around PD10 and the expression profile was symmetric (type 1). Other regions (e.g. the cerebral cortex, layer 1 for CaM II) displayed their highest signal intensities at the earliest age measured, followed by a gradual decrease (type 2). The signal intensities in the regions in the third group (e.g. the hypothalamus for CaM III) fluctuated from age to age (type 3). Marked CaM mRNA levels were measured for each transcript corresponding to the three CaM genes in the molecular layers of the cerebral and cerebellar cortici and hippocampus, suggesting their dendritic translocation. The highest signal intensity was measured for CaM II mRNA, followed by those for CaM III and CaM I mRNAs on PD1. However, the CaM II and CaM III mRNAs subsequently decreased steeply, while the CaM I mRNAs were readily detected even on PD20. Our results suggest that during development (1) the transcription of the CaM genes is under differential, area-specific control, and (2) a large population of CaM mRNAs is targeted to the dendritic compartment in a gene-specific manner.

The ribosome filter hypothesis

A variety of posttranscriptional mechanisms affects the processing, subcellular localization, and translation of messenger RNAs (mRNAs). Translational control appears to occur primarily at the initiation rather than the elongation stage. It has been suggested that translation is mediated largely by means of a cap-binding/scanning mechanism. On the basis of recent findings, we propose here that differential binding of particular mRNAs to eukaryotic 40S ribosomal subunits before translation may also selectively affect rates of polypeptide chain production. In this view, ribosomal subunits themselves are considered to be regulatory elements or filters that mediate interactions between particular mRNAs and components of the translation machinery. Differences in these interactions affect how efficiently individual mRNAs compete for ribosomal subunits. These competitive interactions would depend in part on the complementarity between sequences in mRNA and rRNA, as well as on structural differences among ribosomes in different cell types. By these means, translation may either be enhanced through increased recruitment of ribosomes or inhibited through strong interactions that sequester mRNAs. We propose that ribosomal filters may be important in cell differentiation and describe experimental tests for the filter hypothesis.

Targeted trafficking of neurotransmitter receptors to synaptic sites

Emerging data are sheding light on the critical task for synapses to locally control the production of neurotransmitter receptors ultimately leading to receptor accumulation and modulation at postsynaptic sites. By analogy with the epithelial-cell paradigm, the postsynaptic compartment may be regarded as a polarized domain favoring the selective recruitment and retention of newly delivered receptors at synaptic sites. Targeted delivery of receptors to synaptic sites is facilitated by a local organization of the exocytic pathway, likely resulting from spatial cues triggered by the nerve. This review focuses on the various mechanisms responsible for regulation of receptor assembly and trafficking. A particular emphasis is given to the role of synaptic anchoring and scaffolding proteins in the sorting and routing of their receptor companion along the exocytic pathway. Other cellular components such as lipidic microdomains, the docking and fusion machinery, and the cytoskeleton also contribute to the dynamics of receptor trafficking at the synapse.

Differential calmodulin gene expression in the rodent brain

Apparently redundant members of the calmodulin (CaM) gene family encode for the same amino acid sequence. CaM, a ubiquitous cytoplasmic calcium ion receptor, regulates the function of a variety of target molecules even in a single cell. Maintenance of the fidelity of the active CaM-target interactions in different compartments of the cell requires a rather complex control of the total cellular CaM pool comprising multiple levels of regulatory circuits. Among these mechanisms, it has long been proposed that a multigene family maximizes the regulatory potentials at the level of the gene expression. CaM genes are expressed at a particularly profound level in the mammalian central nervous system (CNS), especially in the highly polarized neurons. Thus, in the search for clear evidence of the suggested differential expression of the CaM genes, much of the research has been focused on the elements of the CNS. This review aims to give a comprehensive survey on the current understanding of this field at the level of the regulation of CaM mRNA transcription and distribution in the rodent brain. The results indicate that the CaM genes are indeed expressed in a gene-specific manner in the developing and adult brain under physiological conditions. To establish local CaM pools in distant intracellular compartments (dendrites and glial processes), local protein synthesis from differentially targeted mRNAs is also employed. Moreover, the CaM genes are controlled in a unique, gene-specific fashion when responding to certain external stimuli. Additionally, putative regulatory elements have been identified on the CaM genes and mRNAs.

Glycoprotein synthesis at the synapse: fractionation of polypeptides synthesized within isolated dendritic fragments by concanavalin A affinity chromatography

The synthesis of glycosylated proteins at postsynaptic sites was evaluated by combining metabolic labeling of isolated pinched-off dendritic fragments (synaptodendrosomes) with glycoprotein isolation by Con A affinity chromatography. Three major labeled proteins were detected (apparent molecular weights of 128, 42 and 19 kDa) along with seven minor polypeptides. Treatment of the glycoprotein fraction with N-glycosidase F led to shift in the apparent molecular weight of the bands. Also, label incorporation into glycoprotein species was blocked by tunicamycin. Thus, the three prominent polypeptides and most of the minor components of this fraction corresponded to bona fide N-glycoproteins. Incubation of synaptodendrosomes with cycloheximide also inhibited label incorporation into the isolated glycoproteins, indicating that the labeling resulted from local de novo synthesis. Subcellular fractionation revealed that the labeled glycoproteins were present in soluble and particulate fractions, mainly microsomes and synaptic membranes, and one of the species (42 kDa) appeared in the incubation medium, indicating secretion. In addition, these glycoproteins were dissimilarly distributed in several brain regions, and were expressed differentially during development, reaching their highest level of synthesis during the period of synaptogenesis. These results provide evidence for local dendritic synthesis of particular glycoprotein components of the synapse.

Induction of long-term depression in cerebellar Purkinje cells requires a rapidly turned over protein

Evidence is presented indicating that the induction of long-term depression (LTD) in Purkinje cells (PCs) requires a rapidly turned over protein(s) during a critical time period within 15 min after the onset of LTD-inducing stimulation and that synthesis of this protein is maintained by mRNAs supplied via transcription. LTD was induced in granule cell axon (GA)-to-PC synapses by stimulation of these synapses at 1 Hz for 5 min in conjunction with the climbing fibers (CFs) forming synapses on the same PCs and represented by a persistent reduction in the GA-induced excitatory postsynaptic potentials (EPSPs). Not only a prolonged but also a brief (5 min) pulse application of translational inhibitors (anisomycin, puromycin, or cycloheximide) effectively blocked the LTD induction. Pulses applied during the period from 30 min before to 10 min after the onset of conjunctive stimulation blocked the LTD induction, but those applied 15 min after were ineffective. The three translational inhibitors blocked the LTD induction similarly, suggesting that the effect is due to their common action of inhibiting protein synthesis. Infusion of a mRNA cap analogue (7-methyl GTP) into PCs also blocked LTD induction, ensuring that the postsynaptic protein synthesis within PCs is required for LTD induction. Transcriptional inhibitors, actinomycin D and 5,6-dichloro-l-beta-D-ribofuranosyl-benzimidazole, also blocked the LTD induction, but this effect was apparent when 5-min pulses of the transcriptional inhibitors preceded the conjunctive stimulation by 30 min or more. This time lag of 30 min is presumed to be required for depletion of the protein(s) required for LTD induction. The presently observed effects of translational and transcriptional inhibitors on the LTD induction are of temporal characteristics corresponding to their depressant effects on the type-1 metabotropic glutamate-receptor (mGluR1)-mediated slow EPSPs in PCs as we have reported recently. An antagonist of mGluR1s [(RS)-1-aminoindan-1,5-dicarboxylic acid], however, did not block LTD induction when it was applied during the 10-min period following conjunctive stimulation, where translational inhibitors effectively blocked LTD induction. This discrepancy in time course suggests that the rapidly turned over protein(s) required for LTD induction is involved in a process occurring downstream of the activation of mGluR1s.

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.

Mice deficient for spermatid perinuclear RNA-binding protein show neurologic, spermatogenic, and sperm morphological abnormalities

Spermatid perinuclear RNA-binding protein (SPNR) is a microtubule-associated RNA-binding protein that localizes to the manchette in developing spermatids. The Spnr mRNA is expressed at high levels in testis, ovary, and brain and is present in these tissues in multiple forms. We have generated a gene trap allele of the murine Spnr, named Spnr(+/GT). Spnr(GT/GT) mutants show a high rate of mortality, reduced weight, and an abnormal clutching reflex. In addition to minor anatomical abnormalities in the brain, males exhibit defects in spermatogenesis that include a thin seminiferous epithelium and disorganization of spermatogenesis. Most of the sperm from mutant males display defects in the flagellum and consequently show decreased motility and transport within the oviducts. Furthermore, sperm from mutant males achieve in vitro fertilization less frequently. Our findings suggest that SPNR plays an important role in normal spermatogenesis and sperm function. Thus, the Spnr(GT/GT) mutant male mouse provides a unique model for some human male infertility cases.

Odors regulate Arc expression in neuronal ensembles engaged in odor processing

Synaptic activity is critical to developmental and plastic processes that produce long-term changes in neuronal connectivity and function. Genes expressed by neurons in an activity-dependent fashion are of particular interest since the proteins they encode may mediate neuronal plasticity. One such gene encodes the activity-regulated cytoskeleton-associated protein, Arc. The present study evaluated the effects of odor stimulation on Arc expression in rat olfactory bulb. Arc mRNA was rapidly increased in functionally linked cohorts of neurons topographically activated by odor stimuli. These included neurons surrounding individual glomeruli, mitral cells and transynaptically activated granule cells. Dendritic Arc immunoreactivity was also increased in odor-activated glomeruli. Our results suggest that odor regulation of Arc expression may contribute to activity-dependent structural changes associated with olfactory experience.

Dendritic translocation of the rat ferritin H chain mRNA

To elucidate the mechanism regulating the selective transport of mRNAs to synaptic sites, we compared the synaptosomal mRNAs with those from the forebrain using the differential display method. The ferritin H chain mRNA was found to be highly enriched in the synaptosomes. In situ hybridization for the ferritin H chain mRNA in the cultured dissociated neurons and in the hippocampal brain slices demonstrated its existence in the dendritic region. These data clearly indicate the selective translocation of the ferritin H chain mRNA into the dendrites and suggested the local expression of ferritin at the synapse.

The 3' untranslated region of messenger RNA: A molecular 'hotspot' for pathology?

The role of the 3' untranslated region in posttranscriptional regulation of mRNA expression is being elucidated. Here we describe diseases arising from anomalies in this region, that affect the expression of one or more genes.

Selective transport of SC1 mRNA, encoding a putative extracellular matrix glycoprotein, during postnatal development of the rat cerebellum and retina

The selective transport of mRNA species into peripheral processes of cells is an important aspect of gene expression in the nervous system. In this study, we report the transport of SC1 mRNA into the distal processes of Bergmann glial (BG) cells at particular stages of development. SC1 is a putative anti-adhesive extracellular matrix (ECM) glycoprotein that is expressed not only in the developing central nervous system (CNS) but also in the adult brain. The intracellular distribution of SC1 mRNA was examined in two highly laminated neural structures, the cerebellum and retina, during postnatal development and in the adult rat. Our results indicate that SC1 mRNA expression is both spatially and temporally regulated. SC1 message was localized to BG cell bodies at postnatal day 5 (P5) and P10. However, by P15 through to the adult, SC1 mRNA was transported to distal processes of BG cells in the synapse-rich molecular layer (ML) of the cerebellum. In the developing rat retina, SC1 mRNA was expressed in specific neuronal populations by P10, however, transport of SC1 message to the dendrites of these retinal neurons was not detected during development or in the adult. These results indicate neural mechanisms which control the timing and cell type in which selective transport of SC1 mRNA is observed. The localization of SC1 mRNA to the distal processes of BG cells in the synapse-rich ML of the cerebellum could facilitate local control of SC1 protein synthesis which may play roles in synapse formation during development and in synaptic plasticity in the adult.

Mouse testis brain ribonucleic acid-binding protein/translin colocalizes with microtubules and is immunoprecipitated with messenger ribonucleic acids encoding myelin basic protein, alpha calmodulin kinase II, and protamines 1 and 2

Testis brain RNA-binding protein (TB-RBP) is a sequence-dependent RNA-binding protein that binds to conserved Y and H sequence elements present in many brain and testis mRNAs. Using recombinant TB-RBP and a highly enriched tubulin fraction, we demonstrate here that recombinant TB-RBP binds to microtubules assembled in vitro. The interaction between recombinant TB-RBP and microtubules was inhibited by high salt and by the microtubule disassembling agents colcemid and calcium, but not by the microfilament-disassembling agent cytochalasin D. Confocal microscopy confirmed colocalization of TB-RBP and tubulin in the cytoplasm of male germ cells. An affinity-purified antibody prepared against recombinant TB-RBP specifically precipitated mRNAs encoding myelin basic protein and alpha calmodulin-dependent kinase II-two transported mRNAs, and protamines 1 and 2-two translationally regulated testicular mRNAs. These data indicate an intracellular association between TB-RBP and specific target mRNAs and suggest an involvement of TB-RBP in microtubule-dependent mRNA transport in the cytoplasm of cells.

Dendritic secretion of peptides from hypothalamic magnocellular neurosecretory neurones: a local dynamic control system and its functions

The role of the dendrites of magnocellular neurones in the release of neurosecretory peptides and the synthesis of many proteins locally is reviewed. Oxytocin and vasopressin contained in dense-cored neurosecretory vesicles are released from magnocellular dendrites not only by excitatory transmitters such as glutamate acting through well-established receptors, but also by a rapid action of oestradiol acting by a mechanism which appears to involve NMDA receptors. Magnocellular dendrites also contain substantial amounts of the synthetic machinery which could synthesise proteins for local use. The presence in dendrites of polysomes and of mRNAs encoding microtubule-associated protein 2, calcium calmodulin kinase II, alpha-synapsin-associated protein, and components of the GABA(A) and NMDA receptors strongly suggests that these proteins can be translated in the dendrites, close to the sites at which they function. Mechanism(s) which control the translation of these dendritic mRNAs and the insertion into the dendritic membranes of proteins translated by dendritic ribosomes remain to be determined. However, an overall picture emerges of magnocellular dendrites as active secretory and synthetic components of the neurosecretory neurones.

Metabotropic glutamate receptors trigger homosynaptic protein synthesis to prolong long-term potentiation

We investigated the mechanisms by which previous "priming" activation of group I metabotropic glutamate receptors (mGluRs) facilitates the persistence of long-term potentiation (LTP) in area CA1 of rat hippocampal slices. Priming of LTP was elicited by either pharmacological or synaptic activation of mGluRs before a weak tetanic stimulus that normally produced only a rapidly decaying phase of LTP that did not involve protein synthesis or mGluRs. Pharmacological priming of LTP persistence by a selective group I mGluR agonist was blocked by an inhibitor of group I mGluRs and by inhibitors of translation, but not by a transcriptional inhibitor. The same mGluR agonist increased (35)S-methionine incorporation into slice proteins. LTP could also be facilitated using a synaptic stimulation priming protocol, and this effect was similarly blocked by group I mGluR and protein synthesis inhibitors. Furthermore, using a two-pathway protocol, the synaptic priming of LTP was found to be input-specific. To test for the contribution of group I mGluRs and protein synthesis to LTP in nonprimed slices, a longer duration control tetanization protocol was used to elicit a more slowly decaying form of LTP than did the weak tetanus used in the previous experiments. The persistence of the LTP induced by this stronger tetanus was dependent on mGluR activation and protein synthesis but not on transcription. Together, these results suggest that mGluRs couple to nearby protein synthesis machinery to homosynaptically regulate an intermediate phase of LTP dependent on new proteins made from pre-existing mRNA.

Molecular mechanisms for activity-regulated protein synthesis in the synapto-dendritic compartment

The creation of enduring modifications in synaptic efficacy requires new protein synthesis. Neurons face the formidable challenge of directing these newly made proteins to the appropriate subset of synapses. One attractive solution to this problem is the local translation of mRNAs that are targeted to dendrites and perhaps to synapses themselves. The molecular mechanisms mediating such local protein synthesis, notably CPEB-mediated cytoplasmic polyadenylation, are now being elucidated.

Visinin-like protein (VILIP) is a neuron-specific calcium-dependent double-stranded RNA-binding protein

Double-stranded RNA-binding proteins function in regulating the stability, translation, and localization of specific mRNAs. In this study, we have demonstrated that the neuron-specific, calcium-binding protein, visinin-like protein (VILIP) contains one double-stranded RNA-binding domain, a protein motif conserved among many double-stranded RNA-binding proteins. We showed that VILIP can specifically bind double-stranded RNA, and this interaction specifically requires the presence of calcium. Mobility shift studies indicated that VILIP binds double-stranded RNA as a single protein-RNA complex with an apparent equilibrium dissociation constant of 9.0 x 10(-6) M. To our knowledge, VILIP is the first double-stranded RNA-binding protein shown to be calcium-dependent. Furthermore, VILIP specifically binds the 3'-untranslated region of the neurotrophin receptor, trkB, an mRNA localized to hippocampal dendrites in an activity-dependent manner. Given that VILIP is also expressed in the hippocampus, these data suggest that VILIP may employ a novel, calcium-dependent mechanism to regulate its binding to important localized mRNAs in the central nervous system.

Calcium channel activation stabilizes a neuronal calcium channel mRNA

We have identified a calcium-dependent pathway in neurons that regulates expression levels of the alpha1B subunit and N channel current. When neurons are depolarized and voltage-gated calcium channels activated, the half-life of cellular N channel alpha1B mRNA is prolonged. This stabilizing effect of depolarization is mediated through the 3' untranslated region of a long form of the alpha1B mRNA and may represent a form of modulation of N-channel levels that does not require changes in gene transcription. Increases in N channel expression would affect several key neuronal functions controlled by calcium, including transmitter release and neurite outgrowth.

Embryonic lethal abnormal vision-like RNA-binding proteins regulate neurite outgrowth and tau expression in PC12 cells

The embryonic lethal abnormal vision (ELAV)-like proteins are mRNA-binding proteins that regulate mRNA stability. The neuronal members of this family are required for neuronal differentiation. We identified the binding region of purified HuD protein to a target neuronal mRNA encoding for the tau microtubule-associated protein and demonstrated an in vivo interaction between the ELAV-like protein and its target tau mRNA. We show that treatment of neuronal cells with antisense oligodeoxynucleotides directed against HuD blocks the induction of neurite outgrowth and decreases the levels of tau mRNAs, indicating that the ELAV-like proteins are required for neuronal differentiation.

Spinal muscular atrophy

Spinal muscular atrophy is a common cause of disability in childhood and is characterized by weakness and wasting of voluntary muscle. It is frequently fatal. The gene for this disorder has been identified as the SMN gene and is part of a highly complex duplicated region of chromosome 5 that is subject to a high rate of gene deletion and gene conversion. The severity of muscle weakness correlates with the amount of full-length SMN protein produced. Molecular genetic studies support a model in which patients are compound heterozygotes of deleted and converted alleles that predicts a progressively decreasing amount of protein product with severity of muscle weakness. The function of SMN is beginning to be understood and it appears to be involved in ribonucleoprotein biogenesis and thus indirectly in post-transcriptional processing of mRNA. There are theoretical grounds for motor neurons having a cell-specific vulnerability to disturbances of mRNA processing and transport and these are briefly reviewed.

Association of the class V myosin Myo4p with a localised messenger RNA in budding yeast depends on She proteins

Asymmetric distribution of messenger RNAs is a widespread mechanism to localize synthesis of specific protein to distinct sites in the cell. Although not proven yet there is considerable evidence that mRNA localisation is an active process that depends on the activity of cytoskeletal motor proteins. To date, the only motor protein with a specific role in mRNA localisation is the budding yeast type V myosin Myo4p. Myo4p is required for the localisation of ASH1 mRNA, encoding a transcriptional repressor that is essential for differential expression of the HO gene and mating type switching in budding yeast. Mutations in Myo4p, in proteins of the actin cytoskeleton, and in four other specific genes, SHE2-SHE5 disrupt the daughter-specific localisation of ASH1 mRNA. In order to understand if Myo4p is directly participating in mRNA transport, we used in situ colocalisation and coprecipitation of Myo4p and ASH1 mRNA to test for their interaction. Our results indicate an association of Myo4p and ASH1 mRNA that depends on the activity of two other genes involved in ASH1 mRNA localisation, SHE2 and SHE3. This strongly suggests a direct role of Myo4p myosin as a transporter of localised mRNAs, convincingly supporting the concept of motor-protein based mRNA localisation.

Interaction of RAFT1 with gephyrin required for rapamycin-sensitive signaling

RAFT1 (rapamycin and FKBP12 target 1; also called FRAP or mTOR) is a member of the ATM (ataxia telangiectasia mutated)-related family of proteins and functions as the in vivo mediator of the effects of the immunosuppressant rapamycin and as an important regulator of messenger RNA translation. In mammalian cells RAFT1 interacted with gephyrin, a widely expressed protein necessary for the clustering of glycine receptors at the cell membrane of neurons. RAFT1 mutants that could not associate with gephyrin failed to signal to downstream molecules, including the p70 ribosomal S6 kinase and the eIF-4E binding protein, 4E-BP1. The interaction with gephyrin ascribes a function to the large amino-terminal region of an ATM-related protein and reveals a role in signal transduction for the clustering protein gephyrin.

Differential distribution and intracellular targeting of mRNAs corresponding to the three calmodulin genes in rat brain. A quantitative in situ hybridization study

To investigate the pattern of expression of the three calmodulin (CaM) genes by in situ hybridization, gene-specific [35S]-cRNA probes complementary to the multiple CaM mRNAs were hybridized in rat brain sections and subsequently detected by quantitative film or high-resolution nuclear emulsion autoradiography. A widespread and differential area-specific distribution of the CaM mRNAs was detected. The expression patterns corresponding to the three CaM genes differed most considerably in the olfactory bulb, the cerebral and cerebellar cortices, the diagonal band, the suprachiasmatic and medial habenular nuclei, and the hippocampus. Moreover, the significantly higher CaM I and CaM III mRNA copy numbers than that of CaM II in the molecular layers of certain brain areas revealed a differential dendritic targeting of these mRNAs. The results indicate a differential pattern of distribution of the multiple CaM mRNAs at two levels of cellular organization in the brain: (a) region-specific expression and (b) specific intracellular targeting. A precise and gene-specific regulation of synthesis and distribution of CaM mRNAs therefore exists under physiological conditions in the rat brain.

Development of the vertebrate neuromuscular junction

We describe the formation, maturation, elimination, maintenance, and regeneration of vertebrate neuromuscular junctions (NMJs), the best studied of all synapses. The NMJ forms in a series of steps that involve the exchange of signals among its three cellular components--nerve terminal, muscle fiber, and Schwann cell. Although essentially any motor axon can form NMJs with any muscle fiber, an additional set of cues biases synapse formation in favor of appropriate partners. The NMJ is functional at birth but undergoes numerous alterations postnatally. One step in maturation is the elimination of excess inputs, a competitive process in which the muscle is an intermediary. Once elimination is complete, the NMJ is maintained stably in a dynamic equilibrium that can be perturbed to initiate remodeling. NMJs regenerate following damage to nerve or muscle, but this process differs in fundamental ways from embryonic synaptogenesis. Finally, we consider the extent to which the NMJ is a suitable model for development of neuron-neuron synapses.

Identification of 3'UTR region implicated in tau mRNA stabilization in neuronal cells

Tau, a neuronal microtubule-associated protein (MAP) plays an important role in the formation and maintenance of neuronal polarity. Tau mRNA is a stable message and exhibits a relatively long half-life in neuronal cells. The regulation of mRNA stability is a crucial determinant in controlling mRNA steady-state levels in neuronal cells and thereby influences gene expression. The half-lives of specific mRNAs may be dependent on specific sequences located at their 3'untranslated region (UTR), which in turn, may be recognized by tissue-specific proteins. To identify the sequence elements involved in tau mRNA stabilization, selected regions of the 3'UTR were subcloned downstream to c-fos reporter mRNA or to the coding region of the tau mRNA. Using stably transfected neuronal cells, we have demonstrated that a fragment of 240 bp (H fragment) located in the 3'UTR can stabilize c-fos and tau mRNAs. Analysis of stably transfected cells indicated that the transfected tau mRNAs are associated with the microtubules of neuronal cells, suggesting that this association may play a role in tau mRNA stabilization. This step may be a prerequisite in the multistep process leading to the subcellular localization of tau mRNA in neuronal cells.

Ultrastructural localization of beta-actin and amphoterin mRNA in cultured cells: application of tyramide signal amplification and comparison of detection methods

We describe a nonradioactive preembedding in situ hybridization protocol using digoxigenin-labeled RNA probes and tyramide signal amplification to increase the sensitivity of detection. The protocol is sensitive enough for electron microscopic localization of endogenous messenger RNAs encoding beta-actin and amphoterin. Three visualization methods were compared: diaminobenzidine enhanced by nickel, Nanogold enhanced by silver and gold toning, and fluorescently labeled tyramides. Diaminobenzidine and Nanogold can be used in both light and electron microscopy. The nickel-enhanced diaminobenzidine was the most sensitive visualization method. It is easy to accomplish but a drawback is poor spatial resolution, which restricts its use at high magnifications. Nanogold visualization has considerably better spatial resolution and is therefore recommended for electron microscopy. Fluorescent tyramides, especially TRITC-tyramide, offer a good detection method for fluorescence and confocal microscopy. The methods were used to localize amphoterin and beta-actin mRNAs in motile cells. Both mRNAs were found in the soma and cell processes. In double labeling experiments, beta-actin mRNA localized to filamentous structures that also contained ribosomal proteins. Especially in the cortical cytoplasm, beta-actin mRNA was associated with actin filaments. Direct localization to microtubules was only rarely seen. (J Histochem Cytochem 47:99-112, 1999)

Characterization of transcript processing of the gene encoding precerebellin-1

Precerebellin-1 (Cbln1) is a cerebellum-specific protein that shares significant sequence identity with the globular domains of the complement components C1qA, B and C, suggesting some common aspects of function and/or structure. As the C1q complex is composed of heterotrimers of C1qA, B and C it was hypothesized that multiple precerebellins may exist in a ternary complex. Northern blotting for cbln1 revealed multiple bands that could represent further family members or alternatively spliced variants. To discriminate these alternatives, probes derived from different regions of the cbln1 gene were used to identify and clone the transcripts detected on Northern blots. Four independent transcripts were repeatedly cloned from an adult mouse cerebellum cDNA library. Upon sequencing, all of these clones were found to be derived from the cbln1 gene and no additional precerebellin-related genes were isolated. Moreover, these clones accounted for the four cbln1-hybridizing bands (1.9, 2. 2, 3.2 and 5.5 kb) detected on Northern blots of adult cerebellum RNA. With one possible exception, these clones were all derived through alterations in the 3'-untranslated region (3'-UTR) of cbln1 that did not affect the coding sequence. This was achieved by the use of two polyadenylation sites and alternative (non-canonical) splicing in the 3'-UTR. Some additional variation in mRNA structure is provided by the use of alternative transcription start sites in cbln1. The possible significance of this level of diversity in the 3'-UTR is discussed.