Transcription factors in dendrites: dendritic imprinting of the cellular nucleus

A Genome-Wide Screen for Dendritically Localized RNAs Identifies Genes Required for Dendrite Morphogenesis

Localizing messenger RNAs at specific subcellular sites is a conserved mechanism for targeting the synthesis of cytoplasmic proteins to distinct subcellular domains, thereby generating the asymmetric protein distributions necessary for cellular and developmental polarity. However, the full range of transcripts that are asymmetrically distributed in specialized cell types, and the significance of their localization, especially in the nervous system, are not known. We used the EP-MS2 method, which combines EP transposon insertion with the MS2/MCP in vivo fluorescent labeling system, to screen for novel localized transcripts in polarized cells, focusing on the highly branched Drosophila class IV dendritic arborization neurons. Of a total of 541 lines screened, we identified 55 EP-MS2 insertions producing transcripts that were enriched in neuronal processes, particularly in dendrites. The 47 genes identified by these insertions encode molecularly diverse proteins, and are enriched for genes that function in neuronal development and physiology. RNAi-mediated knockdown confirmed roles for many of the candidate genes in dendrite morphogenesis. We propose that the transport of mRNAs encoded by these genes into the dendrites allows their expression to be regulated on a local scale during the dynamic developmental processes of dendrite outgrowth, branching, and/or remodeling.

Brain Corticosteroid Receptor Dynamics and Trafficking: Implications from Live Cell Imaging

Adrenal corticosteroids (cortisol in humans or corticosterone in rodents) exert numerous effects in the central nervous system that regulate the stress response, mood, learning and memory, and various neuroendocrine functions. Corticosterone actions in the brain are mediated by two corticosteroid receptors, glucocorticoid receptor (GR) and mineralocorticoid receptor (MR), and they show a high degree of colocalization in the hippocampal region. These receptors predominantly reside in the cytoplasm without ligand and are translocated into the nucleus upon ligand binding to act as transcriptional factors. Thus, their subcellualr localizations are an important component of their biological activity. Given the differential action of MR and GR in the central nervous system, it is important to elucidate how the trafficking of these receptors between the cytoplasm and the nucleus and their interactions are regulated by ligand or other molecules to exert transcriptional activity. In this review, the authors focus on the nucleocytoplasmic and subnuclear trafficking of GR and MR in neural cells and nonneural cells and discuss various factors affecting the dynamics of these receptors.

Retinoic acid-gated sequence-specific translational control by RARalpha

Retinoic acid (RA) plays important roles in development by modulating gene transcription through nuclear receptor activation. Increasing evidence supports a role for RA and RA receptors (RARs) in synaptic plasticity in the brain. We have recently reported that RA mediates a type of homeostatic synaptic plasticity through activation of dendritic protein synthesis, a process that requires dendritically localized RARalpha and is independent of transcriptional regulation. The molecular basis of this translational regulation by RA/RARalpha signaling, however, is unknown. Here we show that RARalpha is actively exported from the nucleus. Cytoplasmic RARalpha acts as an RNA-binding protein that associates with a subset of mRNAs, including dendritically localized glutamate receptor 1 (GluR1) mRNA. This binding is mediated by the RARalpha carboxyl terminal F-domain and specific sequence motifs in the 5'UTR of the GluR1 mRNA. Moreover, RARalpha association with the GluR1 mRNA directly underlies the translational control of GluR1 by RA: RARalpha represses GluR1 translation, while RA binding to RARalpha reduces its association with the GluR1 mRNA and relieves translational repression. Taken together, our results demonstrate a ligand-gated translational regulation mechanism mediated by a non-genomic function of RA/RARalpha signaling.

Characterization of mRNA species that are associated with postsynaptic density fraction by gene chip microarray analysis

We previously reported the partial identification by random sequencing of mRNA species that are associated with the postsynaptic density (PSD) fraction prepared from the rat forebrain [Tian et al., 1999. Mol. Brain Res. 72, 147-157]. We report here further characterization by gene chip analysis of the PSD fraction-associated mRNAs, which were prepared in the presence of RNase inhibitor. We found that mRNAs encoding various postsynaptic proteins, such as channels, receptors for neurotransmitters and neuromodulators, proteins involved in signaling, scaffold and adaptor proteins and cytoskeletal proteins, were highly concentrated in the PSD fraction, whereas those encoding housekeeping proteins, such as enzymes in the glycolytic pathway, were not. We extracted approximately 1900 mRNA species that were highly concentrated in the PSD fraction. mRNAs related to certain neuronal diseases were also enriched in the PSD fraction. We also constructed a cDNA library using the PSD fraction-associated mRNAs as templates, and identified 1152 randomly selected clones by sequencing. Our data suggested that the PSD fraction-associated mRNAs are a very useful resource, in which a number of as yet uncharacterized mRNAs are concentrated. Identification and functional characterization of them are essential for complete understanding of synaptic function.

Region-directed phototransfection reveals the functional significance of a dendritically synthesized transcription factor

Multiple nuclear transcription factors including E-26-like protein 1 (Elk-1) have been found in neuronal dendrites, yet the functional significance of such localization has not yet been explained. Here we use a focal transfection procedure, 'phototransfection', to introduce Elk1 mRNA into specific regions of live, intact primary rat neurons. Introduction and translation of Elk1 mRNA in dendrites produced cell death, whereas introduction and translation of Elk1 mRNA in cell bodies did not produce cell death. Elk-1 translated in dendrites was transported to the nucleus, and cell death depended upon transcription, supporting the dendritic imprinting hypothesis and highlighting the importance of the dendritic environment on protein function. Our demonstration of the utility of phototransfection for spatially controlled introduction of mRNAs opens the broader opportunity to use this method to introduce selected quantities of small molecules into discrete regions of live cells to assess their biological functions.

Dendritic localization of the transcriptional co-repressor Groucho/TLE1 in cortical and cerebellar neurons

In the present study we show that the transcription factor Groucho/TLE1 (TLE1) is expressed in virtually all major cortical subdivisions, hippocampus, amygdala, and thalamus, as well as in the cerebellum of the adult rat brain. In both neocortex and subcortical structures, TLE1 expression was mostly localized to neurons. In addition to the expected nuclear localization, TLE1 immunoreactivity was also detected in apical dendritic shafts of neocortical layer III and V pyramidal cells and in Purkinje cell dendrites. These results demonstrate that TLE1 expression occurs in the mature nervous system and suggest that this protein may perform new functions outside of the nucleus in selected cortical and cerebellar neurons.

Visualization of glucocorticoid receptor in the brain of green fluorescent protein–glucocorticoid receptor knockin mice

Glucocorticoids exert various neuroendocrinological effects, including stress response, in the central nervous system via glucocorticoid receptor (GR). GRs are transported from the cytoplasm to the nucleus upon ligand binding, and then exert the transcriptional activity. Although it is important for unraveling the actual property of the GR in vivo, subcellular dynamics of the GR are still unclear within the brain tissue in which the neuronal circuitry is maintained. To address this issue, we generated green fluorescent protein (GFP)-GR knockin mice, whose GR has been replaced by a GFP-GR fusion protein that is functionally indistinguishable from endogenous GR. In fixed brain sections of the GFP-GR knockin mice, the distribution of the green fluorescence was similar to that of GR immunoreactivity. By subtracting autofluorescence using fluorescent emission fingerprinting method with confocal laser scanning microscope, nuclear localization of GFP-GR was identifiable in the hippocampal CA3 subregion, where subcellular localization of the GR has been unsolved compared with other areas. To examine the subcellular trafficking of GFP-GR in vivo, we performed adrenalectomy on the GFP-GR knockin mice. GFP-GR was translocated from the nucleus to the cytoplasm and neurites two days after adrenalectomy. Furthermore, laser scanning cytometry by which fluorescence intensity in situ can be quantitatively measured revealed the entire GFP-GR expression level was increased. We then examined the dynamic changes in the subcellular localization of GFP-GR in living hippocampal neurons both in dissociated culture and in tissue slices. GFP-GR was localized in not only the perikarya but also neurites in the absence of ligand, and nuclear translocation following ligand treatment was observed. This is the first report visualizing subcellular trafficking of the GR in the mouse brain in more physiological condition. The present results propose new avenues for the research of the GR dynamics both in vitro and in vivo.

Analysis of subcellularly localized mRNAs using in situ hybridization, mRNA amplification, and expression profiling

Targeting of mRNAs to distinct subcellular regions occurs in all polarized cells. The mechanisms by which RNA transport occurs are poorly understood. With the advent of RNA amplification methodologies and expression profiling it is now possible to catalogue the RNAs that are targeted to particular subcellular regions. In particular, neurons are polarized cells in which dendrites receive signals from presynaptic neurons. Upon stimulation (information receipt) the dendrite processes the information such that an immediate dendritic response is generated as well as a longer-term somatic response. The integrated cellular response results in a signal that can be propagated through the axon to the next post-synaptic neuron. Much previous work has shown that mRNAs can be localized in dendrites and that local translation in dendrites can occur. In this chapter the methods for analysis of RNAs that are localized to dendrites are reviewed and a partial list of dendritically localized RNAs is presented. This information may be useful in identifying RNA regulatory regions that are responsible for specifying rate of RNA transport and the dendritic sites at which targeted RNAs dock so that they can be translated.

The identity and distribution of neural cells expressing the mesodermal determinant spadetail

BACKGROUND

The spadetail (spt) gene of zebrafish is expressed in presomitic mesoderm and in neural cells previously suggested to be Rohon-Beard neurons. The mechanism(s) generating the apparently irregular rostrocaudal distribution of spt-expressing cells in the developing CNS is unknown.

RESULTS

spt-expressing neural cells co-express huC, a marker of neurons. These cells also co-express the genes islet-1, -2 and -3 but not valentino. The islet-1 gene expression, irregular distribution and dorsolateral position of spt-expressing cells in the developing CNS are characteristic of dorsal longitudinal ascending (DoLA) interneurons. Shortly after their birth, these neurons extend processes rostrally into which spt mRNA is transported. At 24 hours post fertilisation(hpf), spt-expressing neurons occur most frequently at rostral levels caudal of the 5th-formed somite pair. There is no apparent bias in the number of spt-expressing cells on the left or right sides of embryos. Extended staining for spt-transcription reveals expression in the dorsocaudal cells of somites at the same dorsoventral level as the spt-expressing neurons. There is frequent juxtaposition of spt-expression in newly formed somites and in neurons. This suggests that both types of spt-expressing cell respond to a common positional cue or that neurons expressing spt are patterned irregularly by flanking somitic mesoderm.

CONCLUSIONS

spt-expressing cells in the developing CNS appear to be DoLA interneurons. The irregular distribution of these cells along the rostrocaudal axis of the spinal cord may be due to "inefficient" patterning of neural spt expression by a signal(s) from flanking, regularly distributed somites also expressing spt.

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.

Local translation of classes of mRNAs that are targeted to neuronal dendrites

The functioning of the neuronal dendrite results from a variety of biological processes including mRNA transport to and protein translation in the dendrite. The complexity of the mRNA population in dendrites suggests that specific biological processes are modulated through the regulation of dendritic biology. There are various classes of mRNAs in dendrites whose translation modulates the ability of the dendrite to receive and integrate presynaptic information. Among these mRNAs are those encoding selective transcription factors that function in the neuronal soma and ionotropic glutamate receptors that function on the neuronal membrane. Conclusive evidence that these mRNAs can be translated is reviewed, and identification of the endogenous sites of translation in living dendrites is presented. These data, as well as those described in the other articles resulting from this colloquium, highlight the complexity of dendritic molecular biology and the exquisitely selective and sensitive modulatory role played by the dendrite in facilitating intracellular and intercellular communication.