Subcellular distribution of rRNA and poly(A) RNA in hippocampal neurons in culture

Can ID repetitive elements serve as cis-acting dendritic targeting elements? An in vivo study

Dendritic localization of mRNA/RNA involves interaction of cis-elements and trans-factors. Small, non-protein coding dendritic BC1 RNA is thought to regulate translation in dendritic microdomains. Following microinjections into cultured cells, BC1 RNA fused to larger mRNAs appeared to impart transport competence to these chimeras, and its 5' ID region was proposed as the cis-acting dendritic targeting element. As these ID elements move around rodent genomes and, if transcribed, form a long RNA stem-loop, they might, thereby, lead to new localizations for targeted gene products. To test their targeting ability in vivo we created transgenic mice expressing various ID elements fused to the 3' UTR of reporter mRNA for Enhanced Green Fluorescent Protein. In vivo, neither ID elements nor the BC1 RNA coding region were capable of transporting EGFP RNA to dendrites, although the 3' UTR of alpha-CaMKII mRNA, an established cis-acting element did produce positive results. Other mRNAs containing naturally inserted ID elements are also not found in neuronal dendrites. We conclude that the 5' ID domain from BC1 RNA is not a sufficient dendritic targeting element for mRNAs in vivo.

Noncoding RNAs and RNA Editing in Brain Development, Functional Diversification, and Neurological Disease

The progressive maturation and functional plasticity of the nervous system in health and disease involve a dynamic interplay between the transcriptome and the environment. There is a growing awareness that the previously unexplored molecular and functional interface mediating these complex gene-environmental interactions, particularly in brain, may encompass a sophisticated RNA regulatory network involving the twin processes of RNA editing and multifaceted actions of numerous subclasses of non-protein-coding RNAs. The mature nervous system encompasses a wide range of cell types and interconnections. Long-term changes in the strength of synaptic connections are thought to underlie memory retrieval, formation, stabilization, and effector functions. The evolving nervous system involves numerous developmental transitions, such as neurulation, neural tube patterning, neural stem cell expansion and maintenance, lineage elaboration, differentiation, axonal path finding, and synaptogenesis. Although the molecular bases for these processes are largely unknown, RNA-based epigenetic mechanisms appear to be essential for orchestrating these precise and versatile biological phenomena and in defining the etiology of a spectrum of neurological diseases. The concerted modulation of RNA editing and the selective expression of non-protein-coding RNAs during seminal as well as continuous state transitions may comprise the plastic molecular code needed to couple the intrinsic malleability of neural network connections to evolving environmental influences to establish diverse forms of short- and long-term memory, context-specific behavioral responses, and sophisticated cognitive capacities.

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.

Protein synthetic machinery in the dendrites of the magnocellular neurosecretory neurons of wild-type Long-Evans and homozygous Brattleboro rats

There is growing evidence of local protein synthesis in neuronal dendrites, especially in relation to synaptic activity. The hypothalamic magnocellular system is a robust model for peptidergic neurons, especially for the study of dendrites. Quantitative electron microscopy, immunocytochemistry and non-radioactive in situ hybridization (with tyramide signal amplification) were used to compare dendrites of magnocellular neurons in the supraoptic nucleus of wild-type rats and of homozygous Brattleboro (BB) rats which are subject to long-term hyper-osmotic stimulation because they cannot secrete vasopressin. The dendrites contained free polyribosomes, cisterns of rough endoplasmic reticulum (ER) and small Golgi-like elements. These were clustered in the dendrites, mostly near the plasma membrane. All were increased in amount in the enlarged dendrites of the BB rats. The presence of polyribosomes and cisterns of rER implies that both cytosolic and membrane-inserting proteins are synthesized in the dendrites. The ER marker protein disulfide isomerase extended far into dendrites, but Golgi element markers (mid-Golgi and trans-Golgi network) were distributed mainly in their proximal parts. In BB rats, all the labeling was stronger. 28S rRNA, initiator tRNA(Met), and poly(A) mRNA were revealed extending into proximal and middle parts of dendrites where intensely reactive punctate structures were common. 28S rRNA could be detected in the distal parts of the dendrites. The length of positively stained dendrites was increased significantly for all these RNAs in BB rats. The results provide morphological evidence that magnocellular dendrites have the capacity for local protein syntheses and that this is increased in chronic hyperosmotic stress.

A role for a rat homolog of staufen in the transport of RNA to neuronal dendrites

RNAs are present in dendrites and may be used for local protein synthesis in response to synaptic activity. To begin to understand dendritic RNA targeting, we cloned a rat homolog of staufen, a Drosophila gene that participates in mRNA targeting during development. In hippocampal neurons, rat staufen protein displays a microtubule-dependent somatodendritic distribution pattern that overlaps with dendritic RNAs. To determine whether r-staufen is required for dendritic RNA targeting, we constructed a mutant version containing the RNA binding domains (stau-RBD) but lacking the C-terminal portion potentially involved in dendritic targeting. Stau-RBD expression was restricted to the cell bodies and proximal dendrites. Expression of stau-RBD significantly decreased, while overexpression of wild-type r-staufen increased, the amount of dendritic mRNA. Taken together, these results suggest that the rat staufen protein plays an important role in the delivery of RNA to dendrites.

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.

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.

Identification of a cis-acting dendritic targeting element in MAP2 mRNAs

In neurons, a limited number of mRNAs have been identified in dendritic processes, whereas other transcripts are restricted to the cell soma. Here we have investigated the molecular mechanisms underlying extrasomatic localization of mRNAs encoding microtubule-associated protein 2 (MAP2) in primary neuronal cultures. Vectors expressing recombinant mRNAs were introduced into hippocampal and sympathetic neurons using DNA transfection and microinjection protocols, respectively. Chimeric mRNAs containing the entire 3' untranslated region of MAP2 transcripts fused to a nondendritic reporter mRNA are detected in dendrites. In contrast, RNAs containing MAP2 coding and 5' untranslated regions or tubulin sequences are restricted to the cell soma. Moreover, 640 nucleotides from the MAP2 3' untranslated region (UTR) are both sufficient and essential for extrasomatic localization of chimeric mRNAs in hippocampal and sympathetic neurons. Thus, a cis-acting dendritic targeting element that is effective in two distinct neuronal cell types is contained in the 3' UTR of MAP2 transcripts. The observation of RNA granules in dendrites implies that extrasomatic transcripts seem to assemble into multimolecular complexes that may function as transport units.

Identification of mRNAs localizing in the postsynaptic region

Local protein synthesis using mRNAs readily distributed in the dendrites is believed to play an important role in maintaining the already expressed synaptic plasticity. To find proteins translated in the postsynaptic region, such as neuronal dendrites, we tried to identify the mRNAs associated with the postsynaptic density (PSD) fraction prepared from a rat's forebrain. The PSD-associated mRNAs were amplified by reverse transcriptase-based polymerase chain reaction (RT-PCR), separated by polyacrylamide gel electrophoresis, and sequenced. The database search revealed, among 130 mRNAs sequenced, 17 known and 108 unknown sequences, while five mRNAs were too short for the search. Of the mRNAs with unknown sequences, we selected 33 genes with a length longer than 150 bases, performed in situ hybridization, and found that at least 12 mRNA types were localized in the dendrites. These results suggest that a large number of mRNAs localize around the postsynaptic area of the neuronal cells in the central nervous system. In addition, our method proved efficient in identifying collectively the mRNAs localizing in the dendrites.

Transient expression of GABAA receptor subunit mRNAs in the cellular processes of cultured cortical neurons and glia

In this study, we have studied by in situ hybridisation histochemistry the expression and intracellular distribution of the GABAA receptor subunit mRNAs in cultured neurons obtained from postnatal day 1-3 rats in order to determine how neurotransmitter receptor expression may be regulated during development of the nervous system. In postnatal cortical cells, we found that GABAA receptor subunit mRNAs coding for alpha2, alpha5, beta2, beta3 and gamma2 subunits were transiently expressed in the cellular processes and growth cones after 1-3 days in culture. These observations indicate that GABAA receptor subunit mRNAs are transported (or trafficked) into the cellular processes of early postnatal cortical cells. These selective localisations were rarely observed after 5 days in culture and only in cells which had not made cell-to-cell contact. The localisation of subunit mRNAs in the processes was more effectively maintained up to 5 days or even longer if cell-to-cell contact was avoided by culturing the cells at low density or by inhibiting neurite sprouting pharmacologically with the GABA receptor channel antagonist TBPS. Finally, immunocytochemistry revealed the expression of GABAA receptors in the growth cones of pyramidal neurons in culture. Thus, the expression of mRNA correlates to the expression of protein. These results suggest that the selective trafficking of GABAA receptor subunit mRNAs during synaptogenesis may be regulated by synapse formation and/or glial-neural communication.

Ribosome association contributes to restricting mRNAs to the cell body of hippocampal neurons

In neurons, mRNAs are differentially sorted to axons, dendrites, and the cell body. Recently, regions of certain mRNAs have been identified that target those mRNAs for translocation to the processes. However, the mechanism by which many, if not most mRNAs are retained in the cell body is not understood. Total inhibition of translation, by puromycin or cycloheximide, results in the mislocalization of cell body mRNAs to dendrites. We have examined the effect of translational inhibitors on the localization of ferritin mRNA, the translation of which can also be inhibited specifically by reducing iron levels. Using nonisotopic in situ hybridization, ferritin mRNA is found restricted to the cell body of cultured rat hippocampal neurons. Following treatment with either puromycin or cycloheximide, it migrates into dendrites. Control experiments reveal that the drugs affect neither the viability of the neuronal cultures, nor the steady-state level of ferritin mRNA. When transcription and protein synthesis are inhibited simultaneously, ferritin mRNA is found in the dendrites of puromycin, but not of cycloheximide-treated neurons. However, the localization of ferritin mRNA is unaffected by changes in iron concentration that regulate its translation rate specifically. We propose a model whereby cell body-restricted mRNAs are maintained in that location by association with ribosomes and with another cell component, which traps mRNAs when they are freed of ribosome association. The release of all mRNA species, as happens after total protein synthesis inhibition, floods the system and allows cell body mRNAs to diffuse into dendrites. In contrast, the partial release of the single ferritin mRNA species does not saturate the trapping system and the mRNA is retained in the cell body.

Activity-dependent regulation of dendritic BC1 RNA in hippocampal neurons in culture

Several neuronal RNAs have been identified in dendrites, and it has been suggested that the dendritic location of these RNAs may be relevant to the spatiotemporal regulation of mosaic postsynaptic protein repertoires through transsynaptic activity. Such regulation would require that dendritic RNAs themselves, or at least some of them, be subject to physiological control. We have therefore examined the functional regulation of somatodendritic expression levels of dendritic BC1 RNA in hippocampal neurons in culture. BC1 RNA, an RNA polymerase III transcript that is a component of a ribonucleoprotein particle, became first detectable in somatodendritic domains of developing hippocampal neurons at times of initial synapse formation. BC1 RNA was identified only in such neurons that had established synapses on cell bodies and/or developing dendritic arbors. When synaptic contact formation was initiated later in low-density cultures, BC1 expression was coordinately delayed. Inhibition of neuronal activity in hippocampal neurons resulted in a substantial but reversible reduction of somatodendritic BC1 expression. We conclude that expression of BC1 RNA in somatic and dendritic domains of hippocampal neurons is regulated in development, and is dependent upon neuronal activity. These results establish (for the first time to our knowledge) that an RNA polymerase III transcript can be subject to control through physiological activity in nerve cells.

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

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

Multiple subcellular mRNA distribution patterns in neurons: a nonisotopic in situ hybridization analysis

Previous studies have established that most of the mRNAs that neurons express are localized in the cell body and very proximal dendrites, whereas a small subset of mRNAs is present at relatively high levels in dendrites. It is not clear, however, whether particular mRNAs have the same subcellular distribution in different types of neurons or whether different types of neurons sort mRNAs in different ways. The present study was undertaken to address these questions. Nonisotopic in situ hybridization techniques were used to define the subcellular localization of representative mRNAs including beta-tubulin, low-molecular-weight neurofilament protein (NF-68), high-molecular-weight microtubule-associated protein (MAP2), growth-associated protein 43 (F1/GAP43), the alpha subunit of calcium/calmodulin-dependent protein kinase II (alpha CaMII kinase), and poly (A+) mRNA. The mRNAs for beta-tubulin, neurofilament 68, and F1/GAP43 were restricted to the region of the cell body and very proximal dendrites in most neurons. In some neuron types, however, labeling for NF-68 extended for considerable distances into dendrites. In some neurons that express MAP2, the mRNA was present at the highest levels in the proximal third to half of the dendritic arbor, whereas in other neurons the highest levels of labeling were in the cell body. In most neurons that express alpha CaMII kinase, the highest levels of the mRNA were in the cell body, but labeling was also present throughout dendrites. However, in a few types of neurons, alpha CaMII kinase mRNA was largely restricted to the cell body. The fact that there are no general rules for mRNA localization that apply to all neuron types implies the existence of neuron type-specific mechanisms that regulate mRNA distribution.

RNA transport in dendrites: a cis-acting targeting element is contained within neuronal BC1 RNA

In nerve cells, a select group of RNAs has been localized to dendritic domains. Here we have examined dendritic RNA transport in sympathetic neurons in primary culture, using a microinjection protocol with neuronal BC1 RNA and with BC1-derived sequence segments. After cytoplasmic microinjection, full-length BC1 RNA was selectively transported to dendrites; in contrast, control RNAs such as nuclear RNAs and random-sequence irrelevant RNAs remained restricted to cytoplasmic areas proximal to the injection sites. Chimeric RNAs were constructed that contained the full-length BC1 sequence inserted upstream or downstream of the coding regions of nondendritic mRNAs. After microinjection, such chimeric RNAs were specifically targeted to dendrites; microinjected corresponding nonchimeric mRNAs were not. Dendritic transport of BC1 RNA was rapid: the average dendritic delivery rate within the first hour after microinjection was 242 +/- 25 microm/hr. Whereas a 5'-BC1 segment of 62 nucleotides was transported to dendrites to extents and at levels similar to full-length BC1 RNA, a 3'-BC1 segment of 60 nucleotides did not exit injected somata to any significant degree. A cis-acting dendritic targeting element is thus contained in the 5' part of neuronal BC1 RNA. These results demonstrate that mechanisms exist in neurons for fast and specific transport of selected RNAs to dendrites.

Translational machinery in dendrites of hippocampal neurons in culture

In neurons, several mRNAs are selectively delivered to dendritic domains where they are presumably translated by local protein synthetic machinery. Although electron microscopy has identified polyribosomes in dendrites, in particular in postsynaptic dendritic compartments, the functional composition of the local protein synthetic apparatus and the scope of its translational capacity have not been analyzed. To ascertain the translational competence of dendrites, we have probed hippocampal neurons in primary culture for various integral and associated factors of the translational apparatus. We report here that dendrites of such neurons are equipped with a spectrum of translational machinery components, including ribosomes, tRNAs, initiation and elongation factors, and elements of the cotranslational signal recognition mechanism. These components are differentially and nonuniformly distributed in dendritic arbors. Their dendritic location illustrates the soma-independent potential of dendrites to synthesize selected proteins in local domains.

Protein synthesis within dendrites: glycosylation of newly synthesized proteins in dendrites of hippocampal neurons in culture

There is increasing evidence that certain mRNAs are present in dendrites and can be translated there. The present study uses two strategies to evaluate whether dendrites also possess the machinery for protein glycosylation. First, precursor labeling techniques were used to conjunction with autoradiography to visualize glycosyltransferase activities that are characteristic of the rough endoplasmic reticulum (RER) (mannose) or the Golgi apparatus (GA) (galactose and fucose) in dendrites that had been separated from their cell bodies and in intact neurons treated with brefeldin A or low temperature. Second, immunocytochemical techniques were used to define the subcellular distribution of proteins that are considered markers of the RER (ribophorin I) and GA (p58, alpha-mannosidase II, galactosyltransferase, and TGN38/41). Autoradiographic analysis revealed that isolated dendrites incorporated sugar precursors in a tunicamycin-sensitive and protein synthesis-dependent manner. Moreover, when intact neurons were pulse-labeled with 3H-labeled sugars at low temperature or after treatment with brefeldin A, labeling was distributed over proximal and sometimes distal dendrites. Immunolabeling for RER markers was predominantly localized in cell bodies but extended for a considerable distance into dendrites of all neurons. Immunolabeling for GA markers was confined to the cell body in approximately 70% of the neurons, but in 30% of the neurons, the staining extended into proximal and middle dendrites. These results indicate that the machinery for glycosylation extends well into dendrites in many neurons.

Ultrastructural basis for gene expression at the synapse: synapse-associated polyribosome complexes

This review summarizes what is known about the protein synthetic machinery that is selectively localized beneath postsynaptic sites on the dendrites of CNS neurons. This machinery, made up of polyribosomes and associated membranous cisterns, allows a local synthesis of key proteins at individual postsynaptic sites.

mRNA distribution within dendrites: relationship to afferent innervation

The majority of neuronal mRNAs are confined to cell bodies, but a few mRNAs are present at high levels in dendrites. Here we report an initial analysis of the relationship between afferent innervation and the distribution of mRNA within dendritic fields. In situ hybridization techniques were used to compare the subcellular distribution of dendritic mRNAs in principal neurons of the hippocampal formation in vivo. The mRNA encoding the alpha subunit of calcium/calmodulin dependent protein kinase II (CAMII kinase) was present at high levels throughout the layers that contain the dendrites of hippocampal pyramidal cells and dentate granule cells. In contrast, the mRNA encoding the high molecular weight microtubule-associated protein MAP2 had a more limited distribution. In the dentate gyrus, labeling for MAP2 was present in a discrete band in the lamina containing proximal dendrites and decreased to low levels in laminae containing distal dendrites. This laminar pattern resembles the distinct terminations of the commissural/associational projection (high MAP2 labeling) and the entorhinal projection (lower MAP2 labeling) upon dendrites of granule cells. To determine if the differential distribution of dendritic mRNAs was regulated by either the presence or activity of afferents, we evaluated mRNA distribution in the dentate molecular layer following (1) removal of the entorhinal input by lesions of the entorhinal cortex or (2) prolonged delivery of potentiating stimulation to entorhinal afferents. Denervation led to modest decreases in the levels of mRNAs for both CAMII and MAP2 but did not lead to detectable alterations in mRNA distribution. Also, prolonged stimulation did not lead to detectable alterations in MAP2 or CAMII mRNA distribution although such stimulation clearly elevated the expression of mRNA for glial fibrillary acidic protein (GFAP).

Targeting of mRNAs to subsynaptic microdomains in dendrites

Recent studies have revealed that a heterogeneous population of mRNAs is present in neuronal dendrites (including mRNAs that encode proteins involved in intracellular signaling), that different types of neurons have different assortments of dendritic mRNAs, and that the levels of some dendritic mRNAs are up-regulated by activity. These findings reinforce and extend the hypothesis that the localization of mRNA in dendrites provides a means of synthesizing proteins locally that are important for synaptic function.

The beta 1 subunit mRNA of the rat brain Na+ channel is expressed in glial cells

Although the molecular characteristics of glial Na+ channels are not well understood, recent studies have shown the presence of mRNA for rat brain Na+ channel alpha subunits in astrocytes and Schwann cells. In this study, we asked whether the mRNA for the rat brain Na+ channel beta 1 subunit is expressed in glial cells. We performed in situ hybridization using a complementary RNA probe for the coding regions of the rat brain Na+ channel beta 1 subunit mRNA and detected beta 1 subunit mRNA in cultured rat optic nerve astrocytes and sciatic nerve Schwann cells. The beta 1 subunit was amplified by reverse transcription-polymerase chain reaction in rat optic and sciatic nerves, which lack neuronal somata but contain astrocytes and Schwann cells, respectively. Doublet bands of the beta 1 subunit mRNA were amplified from both optic and sciatic nerves. Through the cloning and sequencing of these bands, we confirmed the amplification of a mRNA highly homologous to the previously cloned rat brain Na+ channel beta 1 subunit (beta 1.1) and a novel form of the beta 1 subunit mRNA (beta 1.2), which is closely homologous to beta 1.1 but contains an additional 86-nucleotide insert in 3' noncoding regions. Two beta 1 subunit mRNAs were also amplified from rat brain and skeletal muscle, but not from rat liver or kidney. These results indicate that rat brain Na+ channel beta 1 subunit mRNAs are expressed in glial cells as well as in neurons.

Rat brain Na+ channel mRNAs in non-excitable Schwann cells

The expression of rat brain voltage-sensitive Na+ channel mRNAs in Schwann cells was examined using in situ hybridization cytochemistry and RT-PCR. The mRNAs of rat brain Na+ channel subtype II and III, but not subtype I, were detected in cultured Schwann cells from sciatic nerve and in intact sciatic nerve, which contains Schwann cells but not neuronal cell bodies. These results indicate that rat brain Na+ channel mRNAs, which have been considered as mainly neuronal-type messages, are also expressed in glial cells in vitro and in vivo.