Relocalization of the polypyrimidine tract-binding protein during PKA-induced neurite growth

Modulation of D3R Splicing, Signaling, and Expression by D1R through PKA→PTB Phosphorylation

The D1R and D3R receptors functionally and synergistically interact in striatonigral neurons. Dopaminergic denervation turns this interaction antagonistic, which is correlated with a decrement in D3nf isoform and an increment in D3R membranal expression. The mechanisms of such changes in D3R are attributed to the dysregulation of the expression of their isoforms. The cause and mechanism of this phenomenon remain unknown. Dopaminergic denervation produces a decrement in D1R and PKA activity; we propose that the lack of phosphorylation of PTB (regulator of alternative splicing) by PKA produces the dysregulation of D3R splicing and changes D3R functionality. By using in silico analysis, we found that D3R mRNA has motifs for PTB binding and, by RIP, co-precipitates with PTB. Moreover, D1R activation via PKA promotes PTB phosphorylation. Acute and 5-day D1R blockade decreases the expression of D3nf mRNA. The 5-day treatment reduces D3R, D3nf, and PTB protein in the cytoplasm and increases D3R in the membrane and PTB in the nucleus. Finally, the blockade of D1R mimics the effect of dopaminergic denervation in D1R and D3R signaling. Thus, our data indicate that through PKA→PTB, D1R modulates D3R splicing, expression, and signaling, which are altered during D1R blockade or the lack of stimulation in dopaminergic denervation.

Therapeutic Potential of PTB Inhibition Through Converting Glial Cells to Neurons in the Brain

Cell replacement therapy represents a promising approach for treating neurodegenerative diseases. Contrary to the common addition strategy to generate new neurons from glia by overexpressing a lineage-specific transcription factor(s), a recent study introduced a subtraction strategy by depleting a single RNA-binding protein, Ptbp1, to convert astroglia to neurons not only in vitro but also in the brain. Given its simplicity, multiple groups have attempted to validate and extend this attractive approach but have met with difficulty in lineage tracing newly induced neurons from mature astrocytes, raising the possibility of neuronal leakage as an alternative explanation for apparent astrocyte-to-neuron conversion. This review focuses on the debate over this critical issue. Importantly, multiple lines of evidence suggest that Ptbp1 depletion can convert a selective subpopulation of glial cells into neurons and, via this and other mechanisms, reverse deficits in a Parkinson's disease model, emphasizing the importance of future efforts in exploring this therapeutic strategy.

Transcriptome-Wide Detection of Intron/Exon Definition in the Endogenous Pre-mRNA Transcripts of Mammalian Cells and Its Regulation by Depolarization

Pairing of splice sites across an intron or exon is the central point of intron or exon definition in pre-mRNA splicing with the latter mode proposed for most mammalian exons. However, transcriptome-wide pairing within endogenous transcripts has not been examined for the prevalence of each mode in mammalian cells. Here we report such pairings in rat GH3 pituitary cells by measuring the relative abundance of nuclear RNA-Seq reads at the intron start or end (RISE). Interestingly, RISE indexes are positively correlated between 5′ and 3′ splice sites specifically across introns or exons but inversely correlated with the usage of adjacent exons. Moreover, the ratios between the paired indexes were globally modulated by depolarization, which was disruptible by 5-aza-Cytidine. The nucleotide matrices of the RISE-positive splice sites deviate significantly from the rat consensus, and short introns or exons are enriched with the cross-intron or -exon RISE pairs, respectively. Functionally, the RISE-positive genes cluster for basic cellular processes including RNA binding/splicing, or more specifically, hormone production if regulated by depolarization. Together, the RISE analysis identified the transcriptome-wide regulation of either intron or exon definition between weak splice sites of short introns/exons in mammalian cells. The analysis also provides a way to further track the splicing intermediates and intron/exon definition during the dynamic regulation of alternative splicing by extracellular factors.

Molecular mechanism of nur77 gene expression and downstream target genes in the early stage of forskolin-induced differentiation in PC12 cells

Forskolin promotes neuronal differentiation of PC12 cells via the PKA-CREB-dependent signaling pathway. Activation of PKA by forskolin phosphorylates CREB, which then binds to CRE sites in numerous gene promoters. However, it is unclear which gene contains the CRE sites responsible for forskolin-induced neuronal differentiation. In this study, we investigated how an immediate early gene, nur77, which has CRE sites in the promoter region, contributes to the early stage of differentiation of forskolin-treated PC12 cells. After treatment with forskolin, expression of Nur77 was upregulated within 1 hr. In addition, knockdown of nur77 inhibited neurite outgrowth induced by forskolin. We also revealed that the specific four CRE sites near the transcriptional start site (TSS) of nur77 were strongly associated with phosphorylated CREB within 1 hr after treatment with forskolin. To analyze the roles of these four sites, reporter assays using the nur77 promoter region were performed. The results showed that nur77 expression was mediated through three of the CRE sites, -242, -222, and -78, and that -78, the nearest of the three to the TSS of nur77, was particularly important. An analysis of neuronal markers controlled by Nur77 after A-CREB-Nur77-Synapsin1 signaling pathway plays a pivotal role in differentiation of forskolin-induced PC12 cells.

The histone deacetylase inhibitor tubacin mitigates endothelial dysfunction by up-regulating the expression of endothelial nitric oxide synthase

Endothelial nitric oxide (NO) synthase (eNOS) plays a critical role in the maintenance of blood vessel homeostasis. Recent findings suggest that cytoskeletal dynamics play an essential role in regulating eNOS expression and activation. Here, we sought to test whether modulation of cytoskeletal dynamics through pharmacological regulation of histone deacetylase 6 (HDAC6)-mediated tubulin deacetylation affects eNOS expression and endothelial function in vitro and in vivo We found that tubulin acetylation inducer (tubacin), a compound that appears to selectively inhibit HDAC6 activity, dramatically increased eNOS expression in several different endothelial cell lines, as determined by both immunoblotting and NO production assays. Mechanistically, we found that these effects were not mediated by tubacin's inhibitory effect on HDAC6 activity, but rather were due to its ability to stabilize eNOS mRNA transcripts. Consistent with these findings, tubacin also inhibited proinflammatory cytokine-induced degradation of eNOS transcripts and impairment of endothelium-dependent relaxation in the mouse aorta. Furthermore, we found that tubacin-induced up-regulation in eNOS expression in vivo is associated with improved endothelial function in diabetic db/db mice and with a marked attenuation of ischemic brain injury in a murine stroke model. Our findings indicate that tubacin exhibits potent eNOS-inducing effects and suggest that this compound might be useful for the prevention or management of endothelial dysfunction-associated cardiovascular diseases.

Multilevel Differential Control of Hormone Gene Expression Programs by hnRNP L and LL in Pituitary Cells

The pituitary-derived somatolactotrophe GH₃ cells secrete both growth hormone (GH) and prolactin (PRL). We have found that the hnRNP L and L-like (LL) paralogs differentially regulate alternative splicing of genes in these cells. Here, we show that hnRNP L is essential for PRL only, but LL is essential for both PRL and GH production. Transcriptome-wide RNA sequencing (RNA-Seq) analysis indicates that they differentially control groups of hormone or hormone-related genes involved in hormone production/regulation at total transcript and alternative exon levels. Interestingly, hnRNP L also specifically binds and prevents the aberrant usage of a nonconserved CA-rich intron piece of Prl pre-mRNA transcripts, and many others involved in endocrine functions, to prevent mostly cryptic last exons and mRNA truncation. Essential for the full hnRNP L effect on specific exons is a proline-rich region that emerged during evolution in vertebrate hnRNP L only but not LL. Together, our data demonstrate that the hnRNP L and its paralog, LL, differentially control hormone gene expression programs at multiple levels, and hnRNP L in particular is critical for protecting the transcriptome from aberrant usage of intronic sequences. The multilevel differential control by hnRNPs likely tailors the transcriptome to help refine and safeguard the different gene expression programs for different hormones.

Diverse regulation of 3' splice site usage

The regulation of splice site (SS) usage is important for alternative pre-mRNA splicing and thus proper expression of protein isoforms in cells; its disruption causes diseases. In recent years, an increasing number of novel regulatory elements have been found within or nearby the 3'SS in mammalian genes. The diverse elements recruit a repertoire of trans-acting factors or form secondary structures to regulate 3'SS usage, mostly at the early steps of spliceosome assembly. Their mechanisms of action mainly include: (1) competition between the factors for RNA elements, (2) steric hindrance between the factors, (3) direct interaction between the factors, (4) competition between two splice sites, or (5) local RNA secondary structures or longer range loops, according to the mode of protein/RNA interactions. Beyond the 3'SS, chromatin remodeling/transcription, posttranslational modifications of trans-acting factors and upstream signaling provide further layers of regulation. Evolutionarily, some of the 3'SS elements seem to have emerged in mammalian ancestors. Moreover, other possibilities of regulation such as that by non-coding RNA remain to be explored. It is thus likely that there are more diverse elements/factors and mechanisms that influence the choice of an intron end. The diverse regulation likely contributes to a more complex but refined transcriptome and proteome in mammals.

Multiple effects of curcumin on promoting expression of the exon 7-containing SMN2 transcript

Survival of motor neuron 2 (SMN2) is a modifier gene for spinal muscular atrophy (SMA), a neurodegenerative disease caused by insufficient SMN protein mostly due to SMN1 defect. SMN2 is nearly identical to SMN1 but unfortunately only able to produce a small amount of SMN protein due to exon 7 skipping. The exon 7-containing SMN2 transcript (SMN2_E7+) can be increased by a dietary compound, curcumin, but the involved molecular changes are not clear. Here we have found that in fibroblast cells of a SMA type II patient, curcumin enhanced the inclusion of SMN2 exon 7. Examination of the potential splicing factors showed that curcumin specifically increased the protein and transcript levels of SRSF1. The increased SRSF1 protein was mainly nuclear and hyperphosphorylated. Interestingly, the curcumin effects on the SMN2 and SRSF1 transcripts were inhibited by a protein deacetylase inhibitor, trichostatin A. Moreover, in support of its role in the SMN2 splicing, knocking down SRSF1 reduced the inclusion of SMN2 exon 7. Thus, curcumin appears to have multiple effects on the SMN2 transcript and its splicing regulators, including the change of alternative splicing and transcript/protein level as well as phosphorylation. Protein deacetylases and phosphatases are likely involved in these effects. Interestingly, the effects all seem to favor production of the SMN2_E7+ transcript in SMA patient cells.

Post-Transcriptional Regulation of Endothelial Nitric Oxide Synthase Expression by Polypyrimidine Tract-Binding Protein 1

OBJECTIVE

Endothelial nitric oxide synthase (eNOS) is an important regulator of vascular function and its expression is regulated at post-transcriptional levels through a yet unknown mechanism. The purpose of this study is to elucidate the post-transcriptional factors regulating eNOS expression and function in endothelium.

APPROACHES AND RESULTS

To elucidate the molecular basis of tumor necrosis factor (TNF)-α-mediated eNOS mRNA instability, biotinylated eNOS 3'-untranslational region (UTR) was used to purify its associated proteins by RNA affinity chromatography from cytosolic fractions of TNF-α-stimulated human umbilical vein endothelial cells (HUVECs). We identified 2 cytosolic proteins, with molecular weight of 52 and 57 kDa, which specifically bind to eNOS 3'-UTR in response to TNF-α stimulation. Matrix-assisted laser desorption ionization time-of-flight mass spectrometric analysis identified the 57-kDa protein as polypyrimidine tract-binding protein 1 (PTB1). RNA gel mobility shift and UV cross-linking assays demonstrated that PTB1 binds to a UCUU-rich sequence in eNOS 3'-UTR, and the C-terminal half of PTB1 is critical to this interaction. Importantly, PTB1 overexpression leads to decreased activity of luciferase gene fused with eNOS 3'-UTR as well as reduced eNOS expression and activity in human ECs. In HUVECs, we show that TNF-α markedly increased PTB1 expression, whereas adenovirus-mediated PTB1 overexpression decreased eNOS mRNA stability and reduced protein expression and endothelium-dependent relaxation. Furthermore, knockdown of PTB1 substantially attenuated TNF-α-induced destabilization of eNOS transcript and downregulation of eNOS expression.

CONCLUSIONS

These results indicate that PTB1 is essential for regulating eNOS expression at post-transcriptional levels and suggest a novel therapeutic target for treatment of vascular diseases associated with inflammatory endothelial dysfunction.

Differential expression, distinct localization and opposite effect on Golgi structure and cell differentiation by a novel splice variant of human PRMT5

Alternative splicing contributes greatly to the proteomic diversity of metazoans. Protein arginine methyltransferase 5 (PRMT5) methylates arginines of Golgi components and other factors exerting diverse effects on cell growth/differentiation, but the underlying molecular basis for its subcellular distribution and diverse roles has not been fully understood. Here we show the detailed properties of an evolutionarily emerged splice variant of human PRMT5 (PRMT5S) that is distinct from the original isoform (PRMT5L). The isoforms are differentially expressed among mammalian cells and tissues. The PRMT5S is distributed all over the cell but PRMT5L mainly colocalizes with Giantin, a Golgi marker. PRMT5 knockdown led to an enlarged Giantin pattern, which was prevented by the expression of either isoform. Rescuing PRMT5S also increased the percentage of cells with an interphase Giantin pattern compacted at one end of the nucleus, consistent with its cell cycle-arresting effect, while rescuing PRMT5L increased that of the mitotic Giantin patterns of dynamically fragmented structures. Moreover, the isoforms are differentially expressed during neuronal or dendritic cell differentiation, and their ectopic expression showed an opposite effect on dendritic cell differentiation. Furthermore, besides their differential regulation of gene expression, both isoforms also similarly regulate over a thousand genes particularly those involved in apoptosis and differentiation. Taking these properties together, we propose that their differential expression and subcellular localization contribute to spatial and temporal regulation of arginine methylation and gene expression to exert different effects. The novel PRMT5S likely contributes to the observed diverse effects of PRMT5 in cells.

Increased stability of heterogeneous ribonucleoproteins by a deacetylase inhibitor

Splicing factors are often influenced by various signaling pathways, contributing to the dynamic changes of protein isoforms in cells. Heterogeneous ribonucleoproteins (hnRNPs) regulate many steps of RNA metabolism including pre-mRNA splicing but their control by cell signaling particularly through acetylation and ubiquitination pathways remains largely unknown. Here we show that TSA, a deacetylase inhibitor, reduced the ratio of Bcl-x splice variants Bcl-xL/xS in MDA-MB-231 breast cancer cells. This TSA effect was independent of TGFβ1; however, only in the presence of TGFβ1 was TSA able to change the splicing regulators hnRNP F/H by slightly reducing their mRNA transcripts but strongly preventing protein degradation. The latter was also efficiently prevented by lactacystin, a proteasome inhibitor, suggesting their protein stability control by both acetylation and ubiquitination pathways. Three lysines K87, K98 and K224 of hnRNP F are potential targets of the mutually exclusive acetylation or ubiquitination (K(Ac/Ub)) in the protein modification database PhosphoSitePlus. Mutating each of them but not a control non-K(Ac/Ub) (K68) specifically abolished the TSA enhancement of protein stability. Moreover, mutating K98 (K98R) and K224 (K224R) also abolished the TSA regulation of alternative splicing of a Bcl-x mini-gene. Furthermore, about 86% (30 of 35) of the multi-functional hnRNP proteins in the database contain lysines that are potential sites for acetylation/ubiquitination. We demonstrate that the degradation of three of them (A1, I and L) are also prevented by TSA. Thus, the deacetylase inhibitor TSA enhances hnRNP F stability through the K(Ac/Ub) lysines, with some of them essential for its regulation of alternative splicing. Such a regulation of protein stability is perhaps common for a group of hnRNPs and RNA metabolism.

Increase of a group of PTC(+) transcripts by curcumin through inhibition of the NMD pathway

Nonsense-mediated mRNA decay (NMD), an mRNA surveillance mechanism, eliminates premature termination codon-containing (PTC⁺) transcripts. For instance, it maintains the homeostasis of splicing factors and degrades aberrant transcripts of human genetic disease genes. Here we examine the inhibitory effect on the NMD pathway and consequent increase of PTC+ transcripts by the dietary compound curcumin. We have found that several PTC⁺ transcripts including that of serine/arginine-rich splicing factor 1 (SRSF1) were specifically increased in cells by curcumin. We also observed a similar curcumin effect on the PTC⁺ mutant transcript from a Tay-Sachs-causing HEXA allele or from a beta-globin reporter gene. The curcumin effect was accompanied by significantly reduced expression of the NMD factors UPF1, 2, 3A and 3B. Consistently, in chromatin immunoprecipitation assays, curcumin specifically reduced the occupancy of acetyl-histone H3 and RNA polymerase II at the promoter region (-376 to -247nt) of human UPF1, in a time- and dosage-dependent way. Importantly, knocking down UPF1 abolished or substantially reduced the difference of PTC(+) transcript levels between control and curcumin-treated cells. The disrupted curcumin effect was efficiently rescued by expression of exogenous Myc-UPF1 in the knockdown cells. Together, our data demonstrate that a group of PTC⁺ transcripts are stabilized by a dietary compound curcumin through the inhibition of UPF factor expression and the NMD pathway.

Evolutionary emergence of a novel splice variant with an opposite effect on the cell cycle

Alternative splicing contributes greatly to the diversification of mammalian proteomes, but the molecular basis for the evolutionary emergence of splice variants remains poorly understood. We have recently found a novel class of splicing regulatory elements between the polypyrimidine tract (Py) and 3' AG (REPA) at intron ends in many human genes, including the multifunctional PRMT5 (for protein arginine methyltransferase 5) gene. The PRMT5 element is comprised of two G tracts that arise in most mammals and accompany significant exon skipping in human transcripts. The G tracts inhibit splicing by recruiting heterogeneous nuclear ribonucleoprotein (hnRNP) H and F (H/F) to reduce U2AF65 binding to the Py, causing exon skipping. The resulting novel shorter variant PRMT5S exhibits a histone H4R3 methylation effect similar to that seen with the original longer PRMT5L isoform but exhibits a distinct localization and preferential control of critical genes for cell cycle arrest at interphase in comparison to PRMT5L. This report thus provides a molecular mechanism for the evolutionary emergence of a novel splice variant with an opposite function in a fundamental cell process. The presence of REPA elements in a large group of genes implies their wider impact on different cellular processes for increased protein diversity in humans.

Functions of the 5' and 3' ends of calicivirus genomes

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Noroviruses are now recognized as the most common cause of viral gastroenteritis.

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The 5′ and 3′ ends of caliciviruses genome fold into characteristic structures conserved within the family.

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The tirmini of calicivirus genome is involved in recruiting host factors to the replication complex.

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The 5′ and 3′ ends of the MNV genome have been shown to interact with host proteins and further stabilize this interaction.

The Caliciviridae family of small positive sense RNA viruses contains a diverse range of pathogens of both man and animals. The molecular mechanisms of calicivirus genome replication and translation have not been as widely studied as many other RNA viruses. With the relatively recent development of robust cell culture and reverse genetics systems for several members of the Caliciviridae family, a more in-depth analysis of the finer detail of the viral life cycle has now been obtained. As a result, the identification and characterization of the role of RNA structures in the calicivirus life cycle has also been possible. This review aims to summarize the current state of knowledge with respect to the role of RNA structures at the termini of calicivirus genomes.

New insights into functional roles of the polypyrimidine tract-binding protein

Polypyrimidine Tract Binding Protein (PTB) is an intensely studied RNA binding protein involved in several post-transcriptional regulatory events of gene expression. Initially described as a pre-mRNA splicing regulator, PTB is now widely accepted as a multifunctional protein shuttling between nucleus and cytoplasm. Accordingly, PTB can interact with selected RNA targets, structural elements and proteins. There is increasing evidence that PTB and its paralog PTBP2 play a major role as repressors of alternatively spliced exons, whose transcription is tissue-regulated. In addition to alternative splicing, PTB is involved in almost all steps of mRNA metabolism, including polyadenylation, mRNA stability and initiation of protein translation. Furthermore, it is well established that PTB recruitment in internal ribosome entry site (IRES) activates the translation of picornaviral and cellular proteins. Detailed studies of the structural properties of PTB have contributed to our understanding of the mechanism of RNA binding by RNA Recognition Motif (RRM) domains. In the present review, we will describe the structural properties of PTB, its paralogs and co-factors, the role in post-transcriptional regulation and actions in cell differentiation and pathogenesis. Defining the multifunctional roles of PTB will contribute to the understanding of key regulatory events in gene expression.

Regulation of β-cell function by RNA-binding proteins

β-cells of the pancreatic islets are highly specialized and high-throughput units for the production of insulin, the key hormone for maintenance of glucose homeostasis. Elevation of extracellular glucose and/or GLP-1 levels triggers a rapid upregulation of insulin biosynthesis through the activation of post-transcriptional mechanisms. RNA-binding proteins are emerging as key factors in the regulation of these mechanisms as well as in other aspects of β-cell function and glucose homeostasis at large, and thus may be implicated in the pathogenesis of diabetes. Here we review current research in the field, with a major emphasis on RNA-binding proteins that control biosynthesis of insulin and other components of the insulin secretory granules by modulating the stability and translation of their mRNAs.

Drosophila Hephaestus/polypyrimidine tract binding protein is required for dorso-ventral patterning and regulation of signalling between the germline and soma

In the Drosophila oocyte, gurken (grk) mRNA encodes a secreted TGF-α signal that specifies the future embryonic dorso-ventral axes by altering the fate of the surrounding epithelial follicle cells. We previously identified a number of RNA binding proteins that associate specifically with the 64 nucleotide grk localization signal, including the Drosophila orthologue of polypyrimidine tract-binding protein (PTB), Hephaestus (Heph). To test whether Heph is required for correct grk mRNA or protein function, we used immunoprecipitation to validate the association of Heph with grk mRNA and characterized the heph mutant phenotype. We found that Heph is a component of grk mRNP complexes but heph germline clones show that Heph is not required for grk mRNA localization. Instead, we identify a novel function for Heph in the germline and show that it is required for proper Grk protein localization. Furthermore, we show that Heph is required in the oocyte for the correct organization of the actin cytoskeleton and dorsal appendage morphogenesis. Our results highlight a requirement for an mRNA binding protein in the localization of Grk protein, which is independent of mRNA localization, and we propose that Heph is required in the germline for efficient Grk signalling to the somatic follicle cells during dorso-ventral patterning.

Connexins and Cap-independent translation: role of internal ribosome entry sites

Cap-independent translation using an internal ribosome entry site instead of the 5'-Cap structure has been discovered in positive-sense RNA viruses and eukaryotic genomes including a subset of gap junction forming connexins genes. With a growing number of mutations found in human connexin genes and studies on genetically modified mouse models mechanisms highlighting the important role of gap junctional communication in multicellular organism it is obvious that mechanism need to be in place to preserve this critical property even under conditions when Cap-mediated translation is scrutinized. To ensure sustained gap junctional communication, rapid initiation of translation of preexisting connexin mRNAs is one possibility, and the presence of internal ribosome entry sites in gap junction genes comply with such a requirement. In this review, we will summarize past and recent findings to build a case for IRES mediated translation as an alternative regulatory pathway facilitating gap junctional communication. This article is part of a Special Issue entitled Electrical Synapses.

mRNA localization: an orchestration of assembly, traffic and synthesis

Asymmetrical mRNA localization and subsequent local translation provide efficient mechanisms for protein sorting in polarized cells. Defects in mRNA localization have been linked to developmental abnormalities and neurological diseases. Thus, it is critical to understand the machineries mediating and mechanisms underlying the asymmetrical distribution of mRNA and its regulation. The goal of this review is to summarize recent advances in the understanding of mRNA transport and localization, including the assembly and sorting of transport messenger ribonucleic protein (mRNP) granules, molecular mechanisms of active mRNP transport, cytoskeletal interactions and regulation of these events by extracellular signals.

Control of alternative splicing by forskolin through hnRNP K during neuronal differentiation

The molecular basis of cell signal-regulated alternative splicing at the 3′ splice site remains largely unknown. We isolated a protein kinase A-responsive ribonucleic acid (RNA) element from a 3′ splice site of the synaptosomal-associated protein 25 (Snap25) gene for forskolin-inhibited splicing during neuronal differentiation of rat pheochromocytoma PC12 cells. The element binds specifically to heterogeneous nuclear ribonucleo protein (hnRNP) K in a phosphatase-sensitive way, which directly competes with the U2 auxiliary factor U2AF65, an essential component of early spliceosomes. Transcripts with similarly localized hnRNP K target motifs upstream of alternative exons are enriched in genes often associated with neurological diseases. We show that such motifs upstream of the Runx1 exon 6 also bind hnRNP K, and importantly, hnRNP K is required for forskolin-induced repression of the exon. Interestingly, this exon encodes the peptide domain that determines the switch of the transcriptional repressor/activator activity of Runx1, a change known to be critical in specifying neuron lineages. Consistent with an important role of the target genes in neurons, knocking down hnRNP K severely disrupts forskolin-induced neurite growth. Thus, through hnRNP K, the neuronal differentiation stimulus forskolin targets a critical 3′ splice site component of the splicing machinery to control alternative splicing of crucial genes. This also provides a regulated direct competitor of U2AF65 for cell signal control of 3′ splice site usage.

Neuronal regulation of pre-mRNA splicing by polypyrimidine tract binding proteins, PTBP1 and PTBP2

Alternative splicing patterns are regulated by RNA binding proteins that assemble onto each pre-mRNA to form a complex RNP structure. The polypyrimidine tract binding protein, PTB, has served as an informative model for understanding how RNA binding proteins affect spliceosome assembly and how changes in the expression of these proteins can control complex programs of splicing in tissues. In this review, we describe the mechanisms of splicing regulation by PTB and its function, along with its paralog PTBP2, in neuronal development.

A conserved serine of heterogeneous nuclear ribonucleoprotein L (hnRNP L) mediates depolarization-regulated alternative splicing of potassium channels

Molecular mechanisms of gene regulation underlying the activity-dependent long term changes of cellular electrical properties, such as those during memory, are largely unknown. We have shown that alternative splicing can be dynamically regulated in response to membrane depolarization and Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) activation, through special CaM kinase responsive RNA elements. However, proteins that mediate this regulation and how they are affected by CaMKIV are not known. Here we show that the regulation of the stress axis-regulated exon of the Slo1 potassium channel transcripts by membrane depolarization requires a highly conserved CaMKIV target serine (Ser-513) of the heterogeneous ribonucleoprotein L. Ser-513 phosphorylation within the RNA recognition motif 4 enhanced heterogeneous ribonucleoprotein L interaction with the CaMKIV-responsive RNA element 1 of stress axis-regulated exon and inhibited binding of the large subunit of the U2 auxiliary factor U2AF65. Both of these activities were abolished by a S513A mutation. Thus, through Ser-513, membrane depolarization/calcium signaling controls a critical spliceosomal assembly step to regulate the variant subunit composition of potassium channels.

Drosophila Syncrip binds the gurken mRNA localisation signal and regulates localised transcripts during axis specification

In the Drosophila oocyte, mRNA transport and localised translation play a fundamental role in axis determination and germline formation of the future embryo. gurken mRNA encodes a secreted TGF-α signal that specifies dorsal structures, and is localised to the dorso-anterior corner of the oocyte via a cis-acting 64 nucleotide gurken localisation signal. Using GRNA chromatography, we characterised the biochemical composition of the ribonucleoprotein complexes that form around the gurken mRNA localisation signal in the oocyte. We identified a number of the factors already known to be involved in gurken localisation and translational regulation, such as Squid and Imp, in addition to a number of factors with known links to mRNA localisation, such as Me31B and Exu. We also identified previously uncharacterised Drosophila proteins, including the fly homologue of mammalian SYNCRIP/hnRNPQ, a component of RNA transport granules in the dendrites of mammalian hippocampal neurons. We show that Drosophila Syncrip binds specifically to gurken and oskar, but not bicoid transcripts. The loss-of-function and overexpression phenotypes of syncrip in Drosophila egg chambers show that the protein is required for correct grk and osk mRNA localisation and translational regulation. We conclude that Drosophila Syncrip is a new factor required for localisation and translational regulation of oskar and gurken mRNA in the oocyte. We propose that Syncrip/SYNCRIP is part of a conserved complex associated with localised transcripts and required for their correct translational regulation in flies and mammals.

Rhythmic interaction between Period1 mRNA and hnRNP Q leads to circadian time-dependent translation

The mouse PERIOD1 (mPER1) protein, along with other clock proteins, plays a crucial role in the maintenance of circadian rhythms. mPER1 also provides an important link between the circadian system and the cell cycle system. Here we show that the circadian expression of mPER1 is regulated by rhythmic translational control of mPer1 mRNA together with transcriptional modulation. This time-dependent translation was controlled by an internal ribosomal entry site (IRES) element in the 5' untranslated region (5'-UTR) of mPer1 mRNA along with the trans-acting factor mouse heterogeneous nuclear ribonucleoprotein Q (mhnRNP Q). Knockdown of mhnRNP Q caused a decrease in mPER1 levels and a slight delay in mPER1 expression without changing mRNA levels. The rate of IRES-mediated translation exhibits phase-dependent characteristics through rhythmic interactions between mPer1 mRNA and mhnRNP Q. Here, we demonstrate 5'-UTR-mediated rhythmic mPer1 translation and provide evidence for posttranscriptional regulation of the circadian rhythmicity of core clock genes.

Differential effects of PKA-controlled CaMKK2 variants on neuronal differentiation

Regulation between protein kinases is critical for the establishment of signaling pathways/networks to 'orchestrate' cellular processes. Besides posttranslational phosphorylation, alternative pre-mRNA splicing is another way to control kinase properties, but splicing regulation between two kinases and the effect of resulting variants on cells has barely been explored. Here we examined the effect of the protein kinase A (PKA) pathway on the alternative splicing and variant properties of the Ca²⁺/calmodulin-dependent protein kinase kinase 2 (CaMKK2) gene in B35 neuroblastoma cells. Inclusion of the exon 16 of CaMKK2 was significantly reduced by H89, a PKA selective inhibitor. Consistently, overexpressed PKA strongly promoted the exon inclusion in a CaMKK2 sequence-dependent way in splicing reporter assays. In vitro, purified CaMKKβ1 variant proteins were found to be kinase-active. In cells, they were differentially phosphorylated by PKA. In RNA interference assays, CaMKKβ1 was found to be essential for forskolin-induced neurite growth. Interestingly, overexpression of the variant without exon 16 (-E16) promoted neurite elongation while the other one (+E16) promoted neurite branching; in contrast, reduction of the latter one enhanced neurite elongation. Moreover, the variants are differentially expressed and the exon 16-containing transcripts highly enriched in the brain, particularly the cerebellum and hippocampus. Thus, PKA regulates the alternative splicing of CaMKK2 to produce variants that differentially modulate neuronal differentiation. Taken together with the many distinct variants of kinases, alternative splicing regulation likely adds another layer of modulation between protein kinases in cellular signaling networks.

mRNA trafficking and local translation: the Yin and Yang of regulating mRNA localization in neurons

Localized translation and the requisite trafficking of the mRNA template play significant roles in the nervous system including the establishment of dendrites and axons, axon path-finding, and synaptic plasticity. We provide a brief review on the regulation of localizing mRNA in mammalian neurons through critical post-translational modifications of the factors involved. These examples highlight the relationship between mRNA trafficking and the translational regulation of trafficked mRNAs and provide insight into how extracellular signals target these events during signal transduction.

The polypyrimidine tract-binding protein affects coronavirus RNA accumulation levels and relocalizes viral RNAs to novel cytoplasmic domains different from replication-transcription sites

The coronavirus (CoV) discontinuous transcription mechanism is driven by long-distance RNA-RNA interactions between transcription-regulating sequences (TRSs) located at the 5' terminal leader (TRS-L) and also preceding each mRNA-coding sequence (TRS-B). The contribution of host cell proteins to CoV transcription needs additional information. Polypyrimidine tract-binding protein (PTB) was reproducibly identified in association with positive-sense RNAs of transmissible gastroenteritis coronavirus (TGEV) TRS-L and TRS-B by affinity chromatography and mass spectrometry. A temporal regulation of PTB cytoplasmic levels was observed during infection, with a significant increase from 7 to 16 h postinfection being inversely associated with a decrease in viral replication and transcription. Silencing the expression of PTB with small interfering RNA in two cell lines (Huh7 and HEK 293T) led to a significant increase of up to 4-fold in mRNA levels and virus titer, indicating a negative effect of PTB on CoV RNA accumulation. During CoV infection, PTB relocalized from the nucleus to novel cytoplasmic structures different from replication-transcription sites in which stress granule markers T-cell intracellular antigen-1 (TIA-1) and TIA-1-related protein (TIAR) colocalized. PTB was detected in these modified stress granules in TGEV-infected swine testis cells but not in stress granules induced by oxidative stress. Furthermore, viral genomic and subgenomic RNAs were detected in association with PTB and TIAR. These cytoplasmic ribonucleoprotein complexes might be involved in posttranscriptional regulation of virus gene expression.

PTBP1 is required for embryonic development before gastrulation

Polypyrimidine-tract binding protein 1 (PTBP1) is an important cellular regulator of messenger RNAs influencing the alternative splicing profile of a cell as well as its mRNA stability, location and translation. In addition, it is diverted by some viruses to facilitate their replication. Here, we used a novel PTBP1 knockout mouse to analyse the tissue expression pattern of PTBP1 as well as the effect of its complete removal during development. We found evidence of strong PTBP1 expression in embryonic stem cells and throughout embryonic development, especially in the developing brain and spinal cord, the olfactory and auditory systems, the heart, the liver, the kidney, the brown fat and cartilage primordia. This widespread distribution points towards a role of PTBP1 during embryonic development. Homozygous offspring, identified by PCR and immunofluorescence, were able to implant but were arrested or retarded in growth. At day 7.5 of embryonic development (E7.5) the null mutants were about 5x smaller than the control littermates and the gap in body size widened with time. At mid-gestation, all homozygous embryos were resorbed/degraded. No homozygous mice were genotyped at E12 and the age of weaning. Embryos lacking PTBP1 did not display differentiation into the 3 germ layers and cavitation of the epiblast, which are hallmarks of gastrulation. In addition, homozygous mutants displayed malformed ectoplacental cones and yolk sacs, both early supportive structure of the embryo proper. We conclude that PTBP1 is not required for the earliest isovolumetric divisions and differentiation steps of the zygote up to the formation of the blastocyst. However, further post-implantation development requires PTBP1 and stalls in homozygous null animals with a phenotype of dramatically reduced size and aberration in embryonic and extra-embryonic structures.

Polypyrimidine tract-binding protein is critical for the turnover and subcellular distribution of CD40 ligand mRNA in CD4+ T cells

CD40L (CD154) is regulated at the posttranscriptional level by an activation-induced process that results in a highly stable transcript at extended times of T cell activation. Transcript stability is mediated by polypyrimidine tract-binding protein (PTB)-containing complexes (complex I and II) that bind to three adjacent CU-rich sequences within the 3' untranslated region. To assess the role of PTB in the expression and distribution of CD40L mRNA, PTB was targeted using short hairpin RNA in both primary T cells and a T cell line that recapitulates the stability phase of regulated CD40L mRNA decay. PTB knockdown resulted in a marked decrease in the mRNA stability that resulted in lowered CD40L surface expression. PTB was also critical for appropriate distribution of CD40L mRNA between the nucleus and cytoplasm and in the cytoplasm between the cytosol and the translating polysomes. The activation-induced formation of PTB-specific ribonucleoprotein complexes was observed only with cytoplasmic and not nuclear PTB indicating functional differences in the protein defined by cellular localization. Finally, we observed that cytoplasmic and nuclear PTB isoforms were differentially modified relative to each other and that the changes in cytoplasmic PTB were consistent with activation-induced phosphorylation. Together this work suggests that differentially modified PTB regulates CD40L expression at multiple steps by 1) retaining CD40L mRNA in the nucleus, 2) directly regulating mRNA stability at late times of activation, and 3) forming a ribonuclear complex that preferentially associates with translating ribosomes thus leading to an enhanced level of CD40L protein.

Spatially restricting gene expression by local translation at synapses

mRNA localization and regulated translation provide a means of spatially restricting gene expression within each of the thousands of subcellular compartments made by a neuron, thereby vastly increasing the computational capacity of the brain. Recent studies reveal that local translation is regulated by stimuli that trigger neurite outgrowth and/or collapse, axon guidance, synapse formation, pruning, activity-dependent synaptic plasticity, and injury-induced axonal regeneration. Impairments in the local regulation of translation result in aberrant signaling, physiology and morphology of neurons, and are linked to neurological disorders. This review highlights current advances in understanding how mRNA translation is repressed during transport and how local translation is activated by stimuli. We address the function of local translation in the context of fragile X mental retardation.

Next-generation SELEX identifies sequence and structural determinants of splicing factor binding in human pre-mRNA sequence

Many splicing factors interact with both mRNA and pre-mRNA. The identification of these interactions has been greatly improved by the development of in vivo cross-linking immunoprecipitation. However, the output carries a strong sampling bias in favor of RNPs that form on more abundant RNA species like mRNA. We have developed a novel in vitro approach for surveying binding on pre-mRNA, without cross-linking or sampling bias. Briefly, this approach entails specifically designed oligonucleotide pools that tile through a pre-mRNA sequence. The pool is then partitioned into bound and unbound fractions, which are quantified by a two-color microarray. We applied this approach to locating splicing factor binding sites in and around approximately 4000 exons. We also quantified the effect of secondary structure on binding. The method is validated by the finding that U1snRNP binds at the 5' splice site (5'ss) with a specificity that is nearly identical to the splice donor motif. In agreement with prior reports, we also show that U1snRNP appears to have some affinity for intronic G triplets that are proximal to the 5'ss. Both U1snRNP and the polypyrimidine tract binding protein (PTB) avoid exonic binding, and the PTB binding map shows increased enrichment at the polypyrimidine tract. For PTB, we confirm polypyrimidine specificity and are also able to identify structural determinants of PTB binding. We detect multiple binding motifs enriched in the PTB bound fraction of oligonucleotides. These motif combinations augment binding in vitro and are also enriched in the vicinity of exons that have been determined to be in vivo targets of PTB.

Mechanisms of alternative splicing regulation: insights from molecular and genomics approaches

Alternative splicing of mRNA precursors provides an important means of genetic control and is a crucial step in the expression of most genes. Alternative splicing markedly affects human development, and its misregulation underlies many human diseases. Although the mechanisms of alternative splicing have been studied extensively, until the past few years we had not begun to realize fully the diversity and complexity of alternative splicing regulation by an intricate protein-RNA network. Great progress has been made by studying individual transcripts and through genome-wide approaches, which together provide a better picture of the mechanistic regulation of alternative pre-mRNA splicing.

A role for polypyrimidine tract binding protein in the establishment of focal adhesions

Polypyrimidine tract binding protein (PTB) is a widely expressed RNA binding protein. In the nucleus PTB regulates the splicing of alternative exons, while in the cytoplasm it can affect mRNA stability, translation, and localization. Here we demonstrate that PTB transiently localizes to the cytoplasm and to protrusions in the cellular edge of mouse embryo fibroblasts during adhesion to fibronectin and the early stages of cell spreading. This cytoplasmic PTB is associated with transcripts encoding the focal adhesion scaffolding proteins vinculin and alpha-actinin 4. We demonstrate that vinculin mRNA colocalizes with PTB to cytoplasmic protrusions and that PTB depletion reduces vinculin mRNA at the cellular edge and limits the size of focal adhesions. The loss of PTB also alters cell morphology and limits the ability of cells to spread after adhesion. These data indicate that during the initial stages of cell adhesion, PTB shuttles from the nucleus to the cytoplasm and influences focal adhesion formation through coordinated control of scaffolding protein mRNAs.

In vivo selection of kinase-responsive RNA elements controlling alternative splicing

Alternative pre-mRNA splicing is often controlled by cell signals, for example, those activating the cAMP-dependent protein kinase (PKA) or the Ca2+/calmodulin-dependent protein kinase IV (CaMKIV). We have shown that CaMKIV regulates alternative splicing through short CA repeats and hnRNP L. Here we use a splicing reporter that shows PKA/CaMKIV promotion of exon inclusion to select from exons containing random 13-nt sequences for RNA elements responsive to the kinases in cultured cells. This selection not only identified both PKA- and CaMKIV-responsive elements that are similar to the CaMKIV-responsive RNA element 1 (CaRRE1) or CA repeats, but also A-rich elements not previously known to respond to these kinases. Consistently, hnRNP L is identified as a factor binding the CA-rich elements. Analyses of the motifs in the highly responsive elements indicate that they are indeed critical for the kinase effect and are enriched in alternative exons. Interestingly, a CAAAAAA motif is sufficient for the PKA/CaMKIV-regulated splicing of the exon 16 of the CaMK kinase beta1 (CaMKK2) transcripts, implying a role of this motif in signaling cross-talk or feedback regulation between these kinases through alternative splicing. Therefore, these experiments identified a group of RNA elements responsive to PKA and CaMKIV from in vivo selection. This also provides an approach for selecting RNA elements similarly responsive to other cell signals controlling alternative splicing.

Involvement of polypyrimidine tract-binding protein (PTB)-related proteins in pollen germination in Arabidopsis

The pollen grains of most angiosperms contain stores of RNAs and their translation products required for pollen germination and subsequent early elongation of pollen tubes. Polypyrimidine tract-binding protein (PTB), which is involved in the regulation of pre-mRNA alternative splicing, internal ribosomal entry site (IRES)-mediated translation and mRNA localization/sorting, is known to act as a bridging molecule between RNAs and a variety of cellular factors to fulfill cellular functions in both the nucleus and cytoplasm. Moreover, it has been reported that PTB plays roles in the differentiation and development of animal cells and tissues. In the Arabidopsis genome, there are two PTB-related genes, tentatively termed AtPTB1 and AtPTB2. In the present study, the physiological functions of AtPTBs were investigated using genetic and cytological approaches. The AtPTB promoter was highly active in vegetative cells of mature pollen grains, and AtPTB was localized in the nucleus and cytoplasm of these vegetative cells. Mutations in the AtPTB genes resulted in decreased germination efficiency, and this effect was rescued by introduction of the AtPTB2 promoter::AtPTB2-GFP. Taken together, these findings suggest that AtPTB is involved in pollen germination through possible RNA metabolism processes in late-maturing and mature pollen grains.

Drosophila PTB promotes formation of high-order RNP particles and represses oskar translation

Local translation of asymmetrically enriched mRNAs is a powerful mechanism for functional polarization of the cell. In Drosophila, exclusive accumulation of Oskar protein at the posterior pole of the oocyte is essential for development of the future embryo. This is achieved by the formation of a dynamic oskar ribonucleoprotein (RNP) complex regulating the transport of oskar mRNA, its translational repression while unlocalized, and its translational activation upon arrival at the posterior pole. We identified the nucleo-cytoplasmic shuttling protein PTB (polypyrimidine tract-binding protein)/hnRNP I as a new factor associating with the oskar RNP in vivo. While PTB function is largely dispensable for oskar mRNA transport, it is necessary for translational repression of the localizing mRNA. Unexpectedly, a cytoplasmic form of PTB can associate with oskar mRNA and repress its translation, suggesting that nuclear recruitment of PTB to oskar complexes is not required for its regulatory function. Furthermore, PTB binds directly to multiple sites along the oskar 3' untranslated region and mediates assembly of high-order complexes containing multiple oskar RNA molecules in vivo. Thus, PTB is a key structural component of oskar RNP complexes that dually controls formation of high-order RNP particles and translational silencing.

The heterogeneous nuclear ribonucleoprotein L is an essential component in the Ca2+/calmodulin-dependent protein kinase IV-regulated alternative splicing through cytidine-adenosine repeats

The regulation of gene expression through alternative pre-mRNA splicing is common in metazoans and is often controlled by intracellular signaling pathways that are important in cell physiology. We have shown that the alternative splicing of a number of genes is controlled by membrane depolarization and Ca2+/calmodulin-dependent protein kinase IV (CaMKIV) through CaMKIV-responsive RNA elements (CaRRE1 and CaRRE2); however, the trans-acting factors remain unknown. Here we show that the heterogeneous nuclear ribonucleoprotein (hnRNP) L is a CaRRE1 binding factor in nuclear extracts. An hnRNP L high affinity CA (cytidine-adenosine) repeat element is sufficient to mediate CaMKIV and hnRNP L repression of splicing in a location (3'-splice site proximity)-dependent way. Depletion of hnRNP L by RNA interference followed by rescue with coexpressed exogenous hnRNP L demonstrates that hnRNP L mediates the CaMKIV-regulated splicing through CA repeats in heterologous contexts. Depletion of hnRNP L also led to increased inclusion of the stress axis-regulated exon and a CA repeat-harboring exon under depolarization or with activated CaMKIV. Moreover, hnRNP L binding to CaRRE1 was increased by CaMKIV and, conversely, was reduced by pretreatments with protein phosphatases. Therefore, hnRNP L is an essential component of CaMKIV-regulated alternative splicing through CA repeats, with its phosphorylation likely playing a critical role.

Rapid changes of mRNA-binding protein levels following glucose and 3-isobutyl-1-methylxanthine stimulation of insulinoma INS-1 cells

Glucose and cAMP-inducing agents such as 3-isobutyl-1-methylxanthine (IBMX) rapidly change the expression profile of insulin-producing pancreatic beta-cells mostly through post-transcriptional mechanisms. A thorough analysis of these changes, however, has not yet been performed. By combining two-dimensional differential gel electrophoresis and mass spectrometry, we identified 165 spots, corresponding to 78 proteins, whose levels significantly change after stimulation of the beta-cell model INS-1 cells with 25 mM glucose + 1 mM IBMX for 2 h. Changes in the expression of selected proteins were verified by one- and two-dimensional immunoblotting. Most of the identified proteins are novel targets of rapid regulation in beta-cells. The transcription inhibitor actinomycin D failed to block changes in two-thirds of the spots, supporting their post-transcriptional regulation. More spots changed in response to IBMX than to glucose alone conceivably because of phosphorylation. Fourteen mRNA- binding proteins responded to stimulation, thus representing the most prominent class of rapidly regulated proteins. Bioinformatics analysis indicated that the mRNA 5'- and 3'-untranslated regions of 22 regulated proteins contain potential binding sites for polypyrimidine tract-binding protein 1, which promotes mRNA stability and translation in stimulated beta-cells. Overall our findings support the idea that mRNA-binding proteins play a major role in rapid adaptive changes in insulin-producing cells following their stimulation with glucose and cAMP-elevating agents.

Polypyrimidine-tract-binding protein: a multifunctional RNA-binding protein

PTB (polypyrimidine-tract-binding protein) is a ubiquitous RNA-binding protein. It was originally identified as a protein with a role in splicing but it is now known to function in a large number of diverse cellular processes including polyadenylation, mRNA stability and translation initiation. Specificity of PTB function is achieved by a combination of changes in the cellular localization of this protein (its ability to shuttle from the nucleus to the cytoplasm is tightly controlled) and its interaction with additional proteins. These differences in location and trans-acting factor requirements account for the fact that PTB acts both as a suppressor of splicing and an activator of translation. In the latter case, the role of PTB in translation has been studied extensively and it appears that this protein is required for an alternative form of translation initiation that is mediated by a large RNA structural element termed an IRES (internal ribosome entry site) that allows the synthesis of picornaviral proteins and cellular proteins that function to control cell growth and cell death. In the present review, we discuss how PTB regulates these disparate processes.

Control of alternative pre-mRNA splicing by Ca(++) signals

Alternative pre-mRNA splicing is a common way of gene expression regulation in metazoans. The selective use of specific exons can be modulated in response to various manipulations that alter Ca(++) signals, particularly in neurons. A number of splicing factors have also been found to be controlled by Ca(++) signals. Moreover, pre-mRNA elements have been identified that are essential and sufficient to mediate Ca(++)-regulated splicing, providing model systems for dissecting the involved molecular components. In neurons, this regulation likely contributes to the fine-tuning of neuronal properties.

Post-transcriptional control of gene expression through subcellular relocalization of mRNA binding proteins

Eukaryotic cells have developed multiple mechanisms to respond to different physiological cues, such as cellular stress, which allow the cells to adapt themselves to their new environment. The regulation of post-transcriptional gene expression is an important component of the cellular stress response and is mediated by mRNA binding proteins (mRBPs). Recently, several studies have shown that regulated subcellular localization of mRBPs upon diverse stimuli (such as cellular stress) provides an additional level of regulation for gene expression.

IRES-mediated translation of the carboxy-terminal domain of the horizontal cell specific connexin Cx55.5 in vivo and in vitro

BACKGROUND

Changes of the interneuronal coupling mediated by electrical synapse proteins in response to light adaptation and receptive field shaping are a paramount feature in the photoreceptor/horizontal cell/bipolar cell (PRC/HC/BPC) complex of the outer retina. The regulation of these processes is not fully understood at the molecular level but they may require information transfer to the nucleus by locally generated messengers. Electrical synapse proteins may comprise a feasible molecular determinant in such an information-laden signalling pathway.

RESULTS

Connexin55.5 (Cx55.5) is a connexin with horizontal cell-restricted expression in zebrafish accumulating at dendritic sites within the PRC/HC/BPC complex in form of hemichannels where light-dependent plasticity occurs. Here we provide evidence for the generation of a carboxy-terminal domain of Cx55.5. The protein product is translated from the Cx55.5 mRNA by internal translation initiation from an in-frame ATG codon involving a putative internal ribosome entry site (IRES) element localized in the coding region of Cx55.5. This protein product resembling an 11 kDa domain of Cx55.5 is partially located in the nucleus in vivo and in vitro.

CONCLUSION

Our results demonstrate the generation of a second protein from the coding region of Cx55.5 by an IRES mediated process. The nuclear occurrence of a fraction of this protein provides first evidence that this electrical synapse protein may participate in a putative cytoplasmic to nuclear signal transfer. This suggests that Cx55.5 could be involved in gene regulation making structural plasticity at the PRC/HC/BPC complex feasible.

Modulation of F-actin rearrangement by the cyclic AMP/cAMP-dependent protein kinase (PKA) pathway is mediated by MAPK-activated protein kinase 5 and requires PKA-induced nuclear export of MK5

The MAPK-activated protein kinases belong to the Ca2+/calmodulin-dependent protein kinases. Within this group, MK2, MK3, and MK5 constitute three structurally related enzymes with distinct functions. Few genuine substrates for MK5 have been identified, and the only known biological role is in ras-induced senescence and in tumor suppression. Here we demonstrate that activation of cAMP-dependent protein kinase (PKA) or ectopic expression of the catalytic subunit Calpha in PC12 cells results in transient nuclear export of MK5, which requires the kinase activity of both Calpha and MK5 and the ability of Calpha to enter the nucleus. Calpha and MK5, but not MK2, interact in vivo, and Calpha increases the kinase activity of MK5. Moreover, Calpha augments MK5 phosphorylation, but not MK2, whereas MK5 does not seem to phosphorylate Calpha. Activation of PKA can induce actin filament accumulation at the plasma membrane and formation of actin-based filopodia. We demonstrate that small interfering RNA-triggered depletion of MK5 interferes with PKA-induced F-actin rearrangement. Moreover, cytoplasmic expression of an activated MK5 variant is sufficient to mimic PKA-provoked F-actin remodeling. Our results describe a novel interaction between the PKA pathway and MAPK signaling cascades and suggest that MK5, but not MK2, is implicated in PKA-induced microfilament rearrangement.