Alternative splicing of the Oct-2 transcription factor RNA is differentially regulated in neuronal cells and B cells and results in protein isoforms with opposite effects on the activity of octamer/TAATGARAT-containing promoters

The Role of the Transcriptional Coactivator BOB.1/OBF.1 in Adaptive Immunity

BOB.1/OBF.1 is a transcriptional coactivator involved in octamer-dependent transcription. Thereby, BOB.1/OBF.1 is involved in the transcriptional regulation of genes important for lymphocyte physiology. BOB.1/OBF.1-deficient mice reveal multiple B- and T-cell developmental defects. The most prominent defect of these mice is the complete absence of germinal centers (GCs) resulting in severely impaired T-cell-dependent immune responses. In humans, BOB.1/OBF.1 is associated with several autoimmune and inflammatory diseases but also linked to liquid and solid tumors. Although its role for B-cell development is relatively well understood, its exact role for the GC reaction and T-cell biology has long been unclear. Here, the contribution of BOB.1/OBF.1 for B-cell maturation is summarized, and recent findings regarding its function in GC B- as well as in various T-cell populations are discussed. Finally, a detailed perspective on how BOB.1/OBF.1 contributes to different pathologies is provided.

OCT2 expression in histiocytoses

Diagnosis of histiocytosis can be difficult and one of the biggest challenges is to distinguish between reactive and neoplastic histiocytes on histology alone. Recently, OCT2 nuclear expression was reported in Rosai-Dorfman disease (RDD). Our purpose was to expand the testing of OCT2 on a broader variety of sporadic or H syndrome-related histiocytoses. Cases of histiocytoses were retrieved from the files of Ambroise Paré Pathology Department. All slides and molecular analyses were reviewed, and staining was completed with immunohistochemistry for OCT2. A total of 156 samples from different localizations were tested. Among sporadic cases, 52 patients had RDD, and 10 patients had mixed histiocytosis combining RDD with Erdheim Chester disease (ECD, n = 8), Langerhans cell histiocytosis (LCH, n = 2) or juvenile xanthogranuloma (JXG, n = 1). All these patients were positive for OCT2 in RDD characteristic histiocytes. Twenty-three patients had ECD and all but two (91% - 21/23) were negative for OCT2. By contrast, OCT2 was positive in 11/27 (41%) LCH and 6/16 (38%) JXG. Among the 10 samples of H syndrome-associated histiocytosis, 3 had typical RDD histology, 6 had unclassified histiocytosis, and one had mixed RDD-LCH; all were positive for OCT2. On 16 samples of granulomatous lymphadenitis, OCT2 was negative in epithelioid histiocytes. Our study shows that OCT2 has a sensitivity of 100% for RDD cases and mixed histiocytoses with an RDD component. It is negative in 92% of ECD but expressed in at least 38% of LCH, JXG, and C group histiocytoses. Finally, OCT2 is positive in all H syndrome-related histiocytoses, independent of their histology.

Arc Regulates Transcription of Genes for Plasticity, Excitability and Alzheimer’s Disease

The immediate early gene Arc is a master regulator of synaptic function and a critical determinant of memory consolidation. Here, we show that Arc interacts with dynamic chromatin and closely associates with histone markers for active enhancers and transcription in cultured rat hippocampal neurons. Both these histone modifications, H3K27Ac and H3K9Ac, have recently been shown to be upregulated in late-onset Alzheimer’s disease (AD). When Arc induction by pharmacological network activation was prevented using a short hairpin RNA, the expression profile was altered for over 1900 genes, which included genes associated with synaptic function, neuronal plasticity, intrinsic excitability, and signalling pathways. Interestingly, about 100 Arc-dependent genes are associated with the pathophysiology of AD. When endogenous Arc expression was induced in HEK293T cells, the transcription of many neuronal genes was increased, suggesting that Arc can control expression in the absence of activated signalling pathways. Taken together, these data establish Arc as a master regulator of neuronal activity-dependent gene expression and suggest that it plays a significant role in the pathophysiology of AD.

Pou2f2 Regulates the Distribution of Dorsal Interneurons in the Mouse Developing Spinal Cord

Spinal dorsal interneurons, which are generated during embryonic development, relay and process sensory inputs from the periphery to the central nervous system. Proper integration of these cells into neuronal circuitry depends on their correct positioning within the spinal parenchyma. Molecular cues that control neuronal migration have been extensively characterized but the genetic programs that regulate their production remain poorly investigated. Onecut (OC) transcription factors have been shown to control the migration of the dorsal interneurons (dINs) during spinal cord development. Here, we report that the OC factors moderate the expression of Pou2f2, a transcription factor essential for B-cell differentiation, in spinal dINs. Overexpression or inactivation of Pou2f2 leads to alterations in the differentiation of dI2, dI3 and Phox2a-positive dI5 populations and to defects in the distribution of dI2-dI6 interneurons. Thus, an OC-Pou2f2 genetic cascade regulates adequate diversification and distribution of dINs during embryonic development.

Onecut Factors and Pou2f2 Regulate the Distribution of V2 Interneurons in the Mouse Developing Spinal Cord

Acquisition of proper neuronal identity and position is critical for the formation of neural circuits. In the embryonic spinal cord, cardinal populations of interneurons diversify into specialized subsets and migrate to defined locations within the spinal parenchyma. However, the factors that control interneuron diversification and migration remain poorly characterized. Here, we show that the Onecut transcription factors are necessary for proper diversification and distribution of the V2 interneurons in the developing spinal cord. Furthermore, we uncover that these proteins restrict and moderate the expression of spinal isoforms of Pou2f2, a transcription factor known to regulate B-cell differentiation. By gain- or loss-of-function experiments, we show that Pou2f2 contribute to regulate the position of V2 populations in the developing spinal cord. Thus, we uncovered a genetic pathway that regulates the diversification and the distribution of V2 interneurons during embryonic development.

RNA Splicing: A New Paradigm in Host-Pathogen Interactions

RNA splicing brings diversity to the eukaryotic proteome. Different spliced variants of a gene may differ in their structure, function, localization, and stability influencing protein stoichiometry and physiological outcomes. Alternate spliced variants of different genes are known to associate with various chronic pathologies including cancer. Emerging evidence suggests precise regulation of splicing as fundamental to normal well-being. In this context, infection-induced alternative splicing has emerged as a new pivot of host function, which pathogenic microbes can alter-directly or indirectly-to tweak the host immune responses against the pathogen. The implications of these findings are vast, and although not explored much in the case of pathogenic infections, we present here examples from splicing mediated regulation of immune responses across a variety of conditions and explore how this fascinating finding brings a new paradigm to host-pathogen interactions.

Nubbin isoform antagonism governs Drosophila intestinal immune homeostasis

Gut immunity is regulated by intricate and dynamic mechanisms to ensure homeostasis despite a constantly changing microbial environment. Several regulatory factors have been described to participate in feedback responses to prevent aberrant immune activity. Little is, however, known about how transcriptional programs are directly tuned to efficiently adapt host gut tissues to the current microbiome. Here we show that the POU/Oct gene nubbin (nub) encodes two transcription factor isoforms, Nub-PB and Nub-PD, which antagonistically regulate immune gene expression in Drosophila. Global transcriptional profiling of adult flies overexpressing Nub-PB in immunocompetent tissues revealed that this form is a strong transcriptional activator of a large set of immune genes. Further genetic analyses showed that Nub-PB is sufficient to drive expression both independently and in conjunction with nuclear factor kappa B (NF-κB), JNK and JAK/STAT pathways. Similar overexpression of Nub-PD did, conversely, repress expression of the same targets. Strikingly, isoform co-overexpression normalized immune gene transcription, suggesting antagonistic activities. RNAi-mediated knockdown of individual nub transcripts in enterocytes confirmed antagonistic regulation by the two isoforms and that both are necessary for normal immune gene transcription in the midgut. Furthermore, enterocyte-specific Nub-PB expression levels had a strong impact on gut bacterial load as well as host lifespan. Overexpression of Nub-PB enhanced bacterial clearance of ingested Erwinia carotovora carotovora 15. Nevertheless, flies quickly succumbed to the infection, suggesting a deleterious immune response. In line with this, prolonged overexpression promoted a proinflammatory signature in the gut with induction of JNK and JAK/STAT pathways, increased apoptosis and stem cell proliferation. These findings highlight a novel regulatory mechanism of host-microbe interactions mediated by antagonistic transcription factor isoforms.

Oct-1, to go or not to go? That is the PolII question

The Oct transcription factors recognise an octamer DNA element from which they regulate transcription of specific target genes. Oct-1 is the only member of the subfamily that is ubiquitously expressed and has a wide role in transcriptional control. Through interaction with various partner proteins, Oct-1 can modulate accessibility to the chromatin to recruit the transcription machinery and form the pre-initiation complex. The recruited PolII is induced to initiate transcription and stalled until elongation is triggered on interaction with signalling transcription factors. In this way, Oct-1 can fulfil general roles in transcription by opening the chromatin as well as transduce extracellular signals by relaying activation through various interacting partners. The emerging picture of Oct-1 is that of a complex and versatile transcription factor with fundamental functions in cell homeostasis and signal response in general as well as cell specific contexts. This article is part of a Special Issue entitled: The Oct Transcription Factor Family, edited by Dr. Dean Tantin.

Alternative splicing can lead to chaos

Alternative splicing is a widespread phenomenon in higher eukaryotes, where it serves as a mechanism to increase the functional diversity of proteins. This phenomenon has been described for different classes of proteins, including transcription regulatory proteins. We demonstrated that in the simplest genetic system model the formation of the alternatively spliced isoforms with opposite functions (activators and repressors) could be a cause of transition to chaotic dynamics. Under the simplest genetic system we understand a system consisting of a single gene encoding the structure of a transcription regulatory protein whose expression is regulated by a feedback mechanism. As demonstrated by numerical analysis of the models, if the synthesized isoforms regulate the expression of their own gene acting through different sites and independently of each other, for the generation of chaotic dynamics it is sufficient that the regulatory proteins have a dimeric structure. If regulatory proteins act through one site, the chaotic dynamics is generated in the system only when the repressor protein is either a tetrameric or a higher-dimensional multimer. In this case the activator can be a dimer. It was also demonstrated that if the transcription factor isoforms exhibit either activating or inhibiting activity and are lower-dimensional multimers (< 4), independently of the regulation type the model demonstrates either cyclic or stationary trajectories.

Oct-2 transcription factor binding activity and expression up-regulation in rat cerebral ischaemia is associated with a diminution of neuronal damage in vitro

Brain plasticity provides a mechanism to compensate for lesions produced as a result of stroke. The present study aims to identify new transcription factors (TFs) following focal cerebral ischaemia in rat as potential therapeutic targets. A transient focal cerebral ischaemia model was used for TF-binding activity and TF-TF interaction profile analysis. A permanent focal cerebral ischaemia model was used for the transcript gene analysis and for the protein study. The identification of TF variants, mRNA analysis, and protein study was performed using conventional polymerase chain reaction (PCR), qPCR, and Western blot and immunofluorescence, respectively. Rat cortical neurons were transfected with small interfering RNA against the TF in order to study its role. The TF-binding analysis revealed a differential binding activity of the octamer family in ischaemic brain in comparison with the control brain samples both in acute and late phases. In this study, we focused on Oct-2 TF. Five of the six putative Oct-2 transcript variants are expressed in both control and ischaemic rat brain, showing a significant increase in the late phase of ischaemia. Oct-2 protein showed neuronal localisation both in control and ischaemic rat brain cortical slices. Functional studies revealed that Oct-2 interacts with TFs involved in important brain processes (neuronal and vascular development) and basic cellular functions and that Oct-2 knockdown promotes neuronal injury. The present study shows that Oct-2 expression and binding activity increase in the late phase of cerebral ischaemia and finds Oct-2 to be involved in reducing ischaemic-mediated neuronal injury.

Different pattern of Galleria mellonella jhbp gene expression in high five and Sf9 cells

Juvenile hormone binding protein (JHBP) is the key element of the system that transmits hormone signals to target tissues. Recently, we found that the core promoter of the jhbp gene is strongly under the control of the TATA box and the transcription start site. In this report, we have shown that the jhbp promoter contains distal regulatory elements whose functionality clearly depends on the particular cell environment and that the scope of research from one cell line is insufficient to generalize the conclusions of the analysis. Cf1/Usp (where Usp is ultraspiracle protein previously known as Cf1, chorion factor 1) elements suppressed transcription of the reporter gene in the High Five cell line but not in the Sf9 cell line. However, upstream from all three Cf1/Usp elements there is a DNA sequence, containing the Zeste element, which activates jhbp in both systems. We found that juvenile hormone strongly inhibited the activity of the jhbp promoter in the Sf9 cell line, whereas it did not have an effect in the High Five cell line. A second key hormone that controls insect development--20-hydroxyecdysone, was also found to suppress the transcription of jhbp. This is the first report describing how these two hormones affect jhbp gene expression in different cell lines.

Expression of the Oct-2 transcription factor in mouse mammary gland and cloning and characterization of a novel Oct-2 isoform

Oct-2 is a member of the POU family of transcription factors, which specifically bind to the octamer DNA motif ATGCAAAT and its closely related sequences. Unlike its ubiquitous counterpart Oct-1, Oct-2 is thought to be expressed only in B lymphocytes and neuronal cells and is mainly involved in immunoglobulin gene expression. We show here that Oct-2 is also expressed in the epithelial cells of mouse mammary gland, and that this expression is developmentally regulated. Rapid amplification of cDNA ends and subsequent cDNA cloning indicate that the mammary gland expresses multiple Oct-2 isoforms, including a novel isoform, named Oct-2.7. Compared with Oct-2 (isoform 2.1), the deduced Oct-2.7 sequence has an additional 22 amino acids close to the N-terminus and a novel 76-amino-acid C-terminus resulting from alternative splicing, with retention of the last intron that is spliced out in all other isoforms. Although Oct-2.7 has intact POU-specific and POU-homeo domains, it is unable to bind to the octamer motif, unlike all other known isoforms. Like Oct-1, both Oct-2.1 and Oct-2.7 can activate basal beta-casein gene promoter activity. However, activation by Oct-2.7, which is independent of DNA binding, is significantly lower than that by Oct-2.1. Moreover, deletion of the first 114 amino acids at the N-terminus of Oct-2.1 has no effect on activation; this does not support previous reports of the presence of an inhibitory domain in this region.

Glutamate-induced octamer DNA binding and transcriptional control in cultured radial glia cells

Glutamate, the main excitatory neurotransmitter in the vertebrate brain, is critically involved in gene expression regulation in neurons and in glia cells. Neuron-glia interactions provide the framework for synaptic plasticity. Retinal and cerebellar radial glia cells surround glutamatergic excitatory synapses and sense synaptic activity through glutamate receptors expressed in their membranes. Several glutamate-dependent membrane to nuclei signaling cascades have been described in these cells. Octamer DNA binding factors, namely Oct-1 and Oct-2 recognize similar DNA sequences on regulatory regions, but their final transcriptional effect depends on several factors. By these means, different responses can be achieved in different cell types. Here, we describe a comparison between the glutamate-induced DNA binding of octamer factors and their functional activities in two important types of radial glia, retinal Müller and cerebellar Bergmann glial cells. While Oct-1 is expressed in both cell types and in both glutamate treatments results in an increase in Oct-1 DNA binding, this complex is capable of transactivating a reporter gene only in Müller glia cells. In contrast, Oct-2 expression is restricted to Bergmann glia cells in which glutamate treatment results in an augmentation of Oct-2 DNA binding complexes and the repression of kainate binding protein gene transcription. Our present findings demonstrate a differential role for Oct-1 and Oct-2 transcription factors in glial glutamate signaling, and further strengthen the notion of an important role for glial cells in glutamatergic transactions in the central nervous system.

Spliced Isoforms of LIM-Domain-Binding Protein (CLIM/NLI/Ldb) Lacking the LIM-Interaction Domain

LIM-domain-binding proteins (CLIM/NLI/Ldb) are nuclear cofactors for LIM homeodomain transcription factors (LIM-HDs) and LIM-only proteins (LMOs). The LIM-interaction domain (LID) of Ldb is located in the carboxy-terminal region and encoded by the last exon (exon 10) of Ldb genes. It is known that the mammalian CLIM1/Ldb2 gene has a splice isoform, named CLIM1b, lacking the LID. However, little is known about the nature of CLIM1b or the evolutionary conservation of this type of alternative splicing in amphibians and teleost fish. Here, we demonstrate that splice isoforms lacking the LID are also present in the Ldb1 genes of mammals, chick, and Xenopus, as well as in fish paralog Ldb4. All these splicing variations occur in intron 9 and exon 10. We observed that Ldb4b (splice isoform lacking LID) is localized in the nucleus when expressed in mammalian culture cells, and binds to Ldb4a (splice isoform containing LID) but not directly to LIM proteins. However, Ldb4b binds to LMO4 via Ldb4a when coexpressed in culture cells. We also found that mouse Ldb1b lacks the ability to activate protein 4.2 promoter, which is stimulated by LMO2 and Ldb1. These findings suggest that splice isoforms of Ldb lacking LID are potential regulators of Ldb function.

Functional interaction of Oct transcription factors with the family of repeats in Epstein-Barr virus oriP

The family of repeats (FR) is a major upstream enhancer of the Epstein-Barr virus (EBV) latent C promoter (Cp) that controls transcription of six different latent nuclear proteins following interaction with the EBV nuclear protein EBNA1. Here, it was shown that Cp could also be activated by octamer-binding factor (Oct) proteins. Physical binding to the FR by the cellular transcription factors Oct-1 and Oct-2 was demonstrated by using an electrophoretic mobility-shift assay. Furthermore, Oct-1 in combination with co-regulator Bob.1, or Oct-2 alone, could drive transcription of a heterologous thymidine kinase promoter linked to the FR in both B cells and epithelial cells. Cp controlled by the FR was also activated by binding of Oct-2 to the FR. This may have direct implications for B cell-specific regulation of Cp.

Function of alternative splicing

Alternative splicing is one of the most important mechanisms to generate a large number of mRNA and protein isoforms from the surprisingly low number of human genes. Unlike promoter activity, which primarily regulates the amount of transcripts, alternative splicing changes the structure of transcripts and their encoded proteins. Together with nonsense-mediated decay (NMD), at least 25% of all alternative exons are predicted to regulate transcript abundance. Molecular analyses during the last decade demonstrate that alternative splicing determines the binding properties, intracellular localization, enzymatic activity, protein stability and posttranslational modifications of a large number of proteins. The magnitude of the effects range from a complete loss of function or acquisition of a new function to very subtle modulations, which are observed in the majority of cases reported. Alternative splicing factors regulate multiple pre-mRNAs and recent identification of physiological targets shows that a specific splicing factor regulates pre-mRNAs with coherent biological functions. Therefore, evidence is now accumulating that alternative splicing coordinates physiologically meaningful changes in protein isoform expression and is a key mechanism to generate the complex proteome of multicellular organisms.

The in vivo form of the murine class VI POU protein Emb is larger than that encoded by previously described transcripts

The class VI POU domain family member known as Emb in the mouse (rat Brn5 or human mPOU/TCFbeta1) is present in vivo as a protein migrating at about 80 kDa on western blots, considerably larger than that predicted (about 42 kDa) from previously cloned coding sequences. By RT-PCR and 5' RACE strategies a full-length Emb sequence, Emb FL, is now identified. Shorter sequences encoding the -COOH terminal, and an -NH(2) terminal isoform, EmbN, were also isolated. Comparisons of Emb coding sequences between species, including the full-length zebra fish, POU(c), are presented, together with a compilation of the multiple transcripts produced by alternative splicing and the presence of different transcriptional start and stop sites, from the Emb gene.

Glutamate regulates Oct-2 DNA-binding activity through α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors in cultured chick Bergmann glia cells

Ionotropic glutamate receptors in cerebellar Bergmann glial cells are linked to transcriptional regulation and, by these means, are thought to play an important role in plasticity, learning and memory and in several neuropathologies. Within the CNS, the transcription factors of the POU family bind their target DNA sequences after a growth factor-dependent phosphorylation-dephosphorylation cascade. Exposure of cultured Bergmann glial cells to glutamate leads to a time- and dose-dependent increase in Oct-2 DNA-binding activity. The use of specific pharmacological tools established the involvement of Ca2+-permeable alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors. Furthermore, the signaling cascade includes phosphatidyl inositol 3-kinase as well as protein kinase C activation. Interestingly, transcriptional as well as translational inhibitors abolish the glutamate effect, suggesting a transcriptional up-regulation of the oct-2 gene. These data demonstrate that Oct-2 expression is not restricted to neurons and further strengthen the notion that the glial glutamate receptors participate in the modulation of glutamatergic cerebellar neurotransmission.

POU domain factors in the neuroendocrine system: lessons from developmental biology provide insights into human disease

POU domain factors are transcriptional regulators characterized by a highly conserved DNA-binding domain referred to as the POU domain. The structure of the POU domain has been solved, facilitating the understanding of how these proteins bind to DNA and regulate transcription via complex protein-protein interactions. Several members of the POU domain family have been implicated in the control of development and function of the neuroendocrine system. Such roles have been most clearly established for Pit-1, which is required for formation of somatotropes, lactotropes, and thyrotropes in the anterior pituitary gland, and for Brn-2, which is critical for formation of magnocellular and parvocellular neurons in the paraventricular and supraoptic nuclei of the hypothalamus. While genetic evidence is lacking, molecular biology experiments have implicated several other POU factors in the regulation of gene expression in the hypothalamus and pituitary gland. Pit-1 mutations in humans cause combined pituitary hormone deficiency similar to that found in mice deleted for the Pit-1 gene, providing a striking example of how basic developmental biology studies have provided important insights into human disease.

Cloning and characterization of two neural-salient serine/arginine-rich (NSSR) proteins involved in the regulation of alternative splicing in neurones

BACKGROUND

In neurones, alternative splicing regulates the functions of many gene products. However, the molecular basis of neural-specific splicing, and how splicing regulation is modulated in different neurones remains to be determined.

RESULTS

We cloned two new SR proteins, Neural-salient SR proteins (NSSR) 1 and 2, which are present at higher levels in brain and testis. During the differentiation, NSSR 1 is detected only in the neuronal stage. Both the purified recombinant NSSR 1 and 2 proteins enhance the in vitro splicing activity of nuclear extract. Moreover, recombinant NSSR 1 protein enhances the assembly of ribonucleoprotein complexes with S100 fraction. Over-expression of NSSR 2 prevents the inclusion of either the Flip or Flop exons in the splicing of the GluR-B gene, resulting in an increase in the abnormal exon-skipping product. In contrast, transient transfection with NSSR 1 promotes the inclusion of the Flip exon so that the abnormal product is spliced to the mature spliced form. This suppression of exon skipping by NSSR 1 is observed even with co-transfection of NSSR 2.

CONCLUSIONS

NSSR 1 and 2 were cloned from mouse cDNA libraries. Results indicate that NSSR 1 may play a crucial role in the regulation of alternative splicing in neurones.

Dominant negative murine serum response factor: alternative splicing within the activation domain inhibits transactivation of serum response factor binding targets

Primary transcripts encoding the MADS box superfamily of proteins, such as MEF2 in animals and ZEMa in plants, are alternatively spliced, producing several isoformic species. We show here that murine serum response factor (SRF) primary RNA transcripts are alternatively spliced at the fifth exon, deleting approximately one-third of the C-terminal activation domain. Among the different muscle types examined, visceral smooth muscles have a very low ratio of SRFDelta5 to SRF. Increased levels of SRFDelta5 correlates well with reduced smooth muscle contractile gene activity within the elastic aortic arch, suggesting important biological roles for differential expression of SRFDelta5 variant relative to wild-type SRF. SRFDelta5 forms DNA binding-competent homodimers and heterodimers. SRFDelta5 acts as a naturally occurring dominant negative regulatory mutant that blocks SRF-dependent skeletal alpha-actin, cardiac alpha-actin, smooth alpha-actin, SM22alpha, and SRF promoter-luciferase reporter activities. Expression of SRFDelta5 interferes with differentiation of myogenic C2C12 cells and the appearance of skeletal alpha-actin and myogenin mRNAs. SRFDelta5 repressed the serum-induced activity of the c-fos serum response element. SRFDelta5 fused to the yeast Gal4 DNA binding domain displayed low transcriptional activity, which was complemented by overexpression of the coactivator ATF6. These results indicate that the absence of exon 5 might be bypassed through recruitment of transcription factors that interact with extra-exon 5 regions in the transcriptional activating domain. The novel alternatively spliced isoform of SRF, SRFDelta5, may play an important regulatory role in modulating SRF-dependent gene expression.

POU family transcription factors in the nervous system

The POU (Pit-Oct-Unc) family of transcription factors was originally defined on the basis of a common DNA binding domain in the mammalian factors Pit-1, Oct-1, and Oct-2 as well as the nematode protein Unc-86. Subsequently, a number of other POU family factors have been identified in both vertebrates and invertebrates. Many of these original and subsequently isolated members of the family have been shown to play critical roles in the development and functioning of the nervous system. To exemplify this, studies are described involving the functional characterisation of the Oct-2 factor, one of the original POU factors, and of the Brn-3 factors, which were isolated subsequently and are the mammalian factors most closely related to Unc-86.

Distinct responses of the herpes simplex virus and varicella zoster virus immediate early promoters to the cellular transcription factors Brn-3a and Brn-3b

The related viruses herpes simplex virus (HSV) and varicella zoster virus (VZV) show distinct but related patterns of latent infection and reactivation in human sensory ganglia. The cellular POU family transcription factors Brn-3a and Brn-3b are expressed in sensory ganglia and bind to the TAATGARAT (R stands for purine) regulatory motifs in the immediate-early gene promoters of these viruses. We show that Brn-3a activates the full length HSV IE1 promoter whereas Brn-3b represses its activity. In contrast both Brn-3a and Brn-3b activate the full length VZV IE promoter. The response of the full length VZV promoter to Brn-3b is not observed with a minimal VZV immediate-early promoter lacking any TAATGARAT elements and cannot be restored by addition of either the upstream TAATGARAT-containing region of the HSV IE promoter or a VZV TAATGARAT-like element to this minimal promoter. The unique effect of Brn-3b on the full length VZV immediate early gene promoter may play a key role in the distinct pattern of latent infection and reactivation observed with this virus in vivo.

Regulation of DNA virus transcription by cellular POU family transcription factors

In the same way as other viral functions, the transcription of viral genes is frequently controlled by cellular regulatory proteins either acting alone or together with virally encoded factors. In this review, I discuss three examples of such regulation in different types of DNA viruses by different members of the POU family of transcription factors, all of which involve viruses which play a role in the aetiology of specific human diseases. These are the glial cell-specific transcription of JC virus which is controlled by the glial cell specific POU factor Tst-1; the regulation of human papillomavirus gene expression in the cervix by positively and negatively acting POU factors and the manner in which the balance between lytic or latent infection with HSV is controlled by positively and negatively acting POU factors which differ in their ability to interact with the virally encoded transactivator VP16. As well as being of interest in themselves, these processes may offer a therapeutic target for controlling the diseases caused by these very different viruses.

Activation of the iNOS gene promoter by Brn-3 POU family transcription factors is dependent upon the octamer motif in the promoter

The promoter of the gene encoding the inducible nitric oxide synthase (iNOS) contains an octamer motif which is of importance for its activation by specific stimuli. We show that in contrast to the promoter of the neuronal nitric oxide synthase gene (nNOS) which is strongly activated by the Oct-2 octamer-binding POU family transcription factor, the iNOS gene is only weakly activated by Oct-2 via its octamer motif. Unlike the nNOS promoter, however, the iNOS promoter is strongly activated by the POU family transcription factors Brn-3a and Brn-3b. This activation is dependent upon the octamer motif in the iNOS promoter and requires the activation domain located within the POU domain of Brn-3a or Brn-3b but not the N-terminal activation domain of Brn-3a. Thus different but related POU proteins play important roles in the regulation of the genes encoding different forms of nitric oxide synthase.

Adjacent proline residues in the inhibitory domain of the Oct-2 transcription factor play distinct functional roles

A 40 amino acid region of Oct-2 from amino acids 142 to 181 functions as an active repressor domain capable of inhibiting both basal activity and activation of promoters containing a TATA box, but not of those that contain an initiator element. Based on our observation that the equivalent region of the closely related Oct-1 factor does not act as an inhibitory domain, we have mutated specific residues in the Oct-2 domain in an attempt to probe their importance in repressor domain function. Although mutations of several residues have no or minimal effect, mutation of proline 175 to arginine abolishes the ability to inhibit both basal and activated transcription. In contrast, mutation of proline 174 to arginine confers upon the domain the ability to repress activation of an initiator-containing promoter by acidic activation domains, and also suppresses the effect of the proline 175 mutation. Hence, adjacent proline residues play key roles in the functioning of the inhibitory domain and in limiting its specificity to TATA-box-containing promoters.

Cell type specific repression of the varicella zoster virus immediate early gene 62 promoter by the cellular Oct-2 transcription factor

The cellular transcription factor Oct-2.1 has previously been shown to repress the transactivation of the varicella zoster virus (VZV) immediate early gene promoter by viral transactivators but not to inhibit its basal activity. In the case of the related virus herpes simplex virus (HSV), the effect of Oct-2 on the IE promoters has been shown to be cell type specific and to differ between the different alternatively spliced forms of Oct-2. Here we show that as well as Oct-2.1, the Oct-2.4 and 2.5 isoforms which are expressed in neuronal cells can inhibit transactivation of the VZV immediate early promoter regardless of the cell type used. In contrast, all the isoforms of Oct-2 can inhibit basal activity of the VZV promoter in neuronal cells but not in other cell types indicating that this effect is cell type specific. These effects are discussed in terms of the differential regulation of latent infections with HSV or VZV in dorsal root ganglia.

The TAATGARAT motif in the herpes simplex virus immediate-early gene promoters can confer both positive and negative responses to cellular octamer-binding proteins when it is located within the viral genome

The TAATGARAT motif in the herpes simplex virus (HSV) immediate-early (IE) gene promoters plays a key role in their activation by the Oct-1-Vmw65 complex, but its role in mediating inhibitory effects of cellular octamer-binding proteins is less clear. We have used indicator viruses containing reporter constructs with different IE promoters driving a reporter beta-galactosidase gene within the viral genome to investigate this. We showed that deletion of the upstream IE promoter region containing the TAATGARAT motifs abolishes the inhibitory effect of the cellular octamer-binding proteins Oct-2.4 and Oct-2.5 on the viral IE promoter. This inhibitory effect can be restored by addition of a single TAATGARAT motif to the minimal promoter within the viral genome. Hence, the TAATGARAT motif can indeed mediate both positive and negative effects of cellular transcription factors when it is located within the viral genome.

POU domain genes are differentially expressed in the early stages after lineage commitment of the PNS-derived stem cell line, RT4-AC

RT4 is a family of cell lines derived from a rat peripheral neurotumor and consists of a multipotential stem cell which spontaneously gives rise to a glial derivative and two neuronal derivatives. To begin to understand the role(s) of transcription factors in neural differentiation, we examined the expression of ten transcription factor genes (MASH1, REST/NRSF, Oct-1, Oct-2, Tst-1/SCIP, Brn-1, Brn-2, Brn-3.0, Brn-4, Brn-5) in the RT4 cell lines. We report here that all of the RT4 cells express REST/NRSF, Oct-1 and Brn-5, but do not express MASH1, Brn-3.0 or Brn-4. Furthermore, Brn-2 and Tst-1/SCIP expression was restricted to the RT4 stem cell line and glial derivative, while Oct-2 was expressed predominantly by the RT4 stem cell line and neuronal derivatives. We propose that the lack of expression of MASH1 (which is expressed relatively early in autonomic neuron differentiation) and Brn-3.0 (which is expressed early in sensory neuron differentiation), in combination with the presence of REST/NRSF (a repressor of neuronal gene expression), in all of the RT4 cell lines, establishes the RT4 system as a unique model for examining very early events in neuronal versus glial cell fate determination.

Differential regulation of genes encoding synaptic proteins by the Oct-2 transcription factor

In order to investigate the effect of the Oct-2 POU family transcription factor on the regulation of genes encoding synaptic proteins, we have used cell lines in which the level of Oct-2 has been greatly reduced using an antisense approach. The reduced Oct-2 level results in enhanced expression of SNAP-25 and synapsin I, indicating that the genes encoding these proteins are normally repressed by Oct-2 in neuronal cells. In contrast, no alteration was observed in the levels of the synaptic proteins, synaptophysin and synaptotagmin. Although the neuronal forms of Oct-2 can repress the synapsin I promoter in co-transfection experiments, indicating that they have a direct effect on the expression of this gene, they have no effect on the activity of the SNAP-25 promoter, indicating that the effect of Oct-2 on this gene is likely to be indirect. These effects are discussed in terms of the differential effect of Oct-2 and the related POU family transcription factor Brn-3a, on the promoters of genes encoding different synaptic proteins.

The different inhibitory domains of the Oct-2 transcription factor have distinct functional activities

The Oct-2 POU family transcription factor contains three distinct regions whose deletion reduces its ability to inhibit transcription via its octamer binding site. Here we show that only one of these inhibitory domains is capable of also inhibiting the activity of activating molecules bound at adjacent sites upstream of a TATA box-containing promoter whereas the other two regions are inactive in this assay. None of the three regions is able to achieve this effect when located upstream of the same promoter containing an initiator motif. The mechanisms of action of these domains and their role in the functioning of the Oct-2 factor are discussed.

Differential regulation of the two neuronal nitric-oxide synthase gene promoters by the Oct-2 transcription factor

The Oct-2 transcription factor has been shown previously to repress both the cellular tyrosine hydroxylase and the herpes simplex virus immediate-early genes in neuronal cells. Here we identify the gene encoding the neuronal nitric-oxide synthase (nNOS) as the first example of a gene activated in neuronal cells by Oct-2. The levels of the nNOS mRNA and protein are greatly reduced in neuronal cell lines in which Oct-2 levels have been reduced by an antisense method, although these cells have enhanced levels of tyrosine hydroxylase. Moreover, the nNOS gene regulatory region is activated by Oct-2 expression vectors upon cotransfection into both neuronal and non-neuronal cells, and this response is dependent upon a 20-amino acid region within the COOH-terminal activation domain of Oct-2. Of the two closely linked promoters that drive nNOS gene expression, only the downstream 5.1 promoter is activated by Oct-2, whereas the 5.2 promoter is unaffected. These effects are discussed in terms of the potential role of Oct-2 in regulating nNOS expression in the nervous system.

The Oct-2 transcription factor

The Oct-2 transcription factor is a member of the POU (Pit-Oct-Unc) family of transcription factors and is expressed only in B lymphocytes and in neuronal cells but not in other cell types. The primary RNA transcript of the gene is subject to alternative splicing to yield different variants which can either activate or repress gene expression. The forms produced in B lymphocytes have a predominantly activating effect on gene expression whereas those produced in neuronal cells have a predominantly inhibitory effect and can repress the expression of both the herpes simplex virus immediate-early genes and the cellular tyrosine hydroxylase gene. Thus Oct-2 plays an important role in the regulation of cellular gene expression in both B cells and neuronal cells as well as in the control of viral latency.

Inhibitory transcription factors

Although the majority of the transcription factors that were initially characterized had a stimulatory effect on gene expression, it is now clear that transcription factors that inhibit gene transcription are at least as important in regulating a wide variety of processes, including development. These factors can act either by interfering with the action of a positively acting factor (indirect repression) or by directly interfering with transcription by interacting with the basal transcriptional complex of RNA polymerase and associated factors (direct repression). Indirect repression often operates by the negative factor preventing the positively acting factor binding to DNA. This can involve reorganization of chromatin structure, blockage of the binding site in the DNA by binding of the inhibitory factor or formation of a non-DNA binding protein-protein complex. Indirect repression can also occur via quenching of the activity of a positive factor that remains bound to DNA. Direct repression can be produced by factors that interact with the basal transcriptional complex to reduce its activity or stability. This can be achieved either by factors that interact with the complex following binding to DNA or by those which bind directly to it. Factors that act by each of these means, and their mechanisms of action, are discussed.

The ability of the inhibitory domain of the POU family transcription factor Oct-2 to interfere with promoter activation by different classes of activation domains is dependent upon the nature of the basal promoter elements

The Oct-2 transcription factor contains an inhibitory domain which is able to repress transcription following DNA binding. Here we show that within the neuronally expressed Oct-2.5 form, the inhibitory domain can strongly inhibit activation by transcription factor activation domains which are either composed predominantly of acidic residues or contain the HOB motif, whereas it has a weaker effect or no effect on proline-rich activation domains and on a glutamine-rich domain. In contrast, the isolated inhibitory domain of Oct-2 can efficiently repress all types of activation domains. This effect is observed however, only on TATA box-containing promoters and not on promoters containing an initiator motif. This widespread inhibition of different activation domains and its dependence on the nature of the basal promoter elements indicate that the inhibitory domain is likely to act by contacting a common downstream target of activation domains within the basal transcriptional complex bound at the TATA box rather than quenching specific activation domains by direct interaction. These effects are discussed in terms of the functional role of the inhibitory domain within Oct-2.5 and the mechanism by which it acts.

Alternative splicing of the Brn-3a and Brn-3b transcription factor RNAs is regulated in neuronal cells

The closely related and functionally antagonistic POU family transcription factors Brn-3a and Brn-3b are encoded by two distinct genes that are expressed primarily in neuronal cells. In addition, however, the primary transcript of each of these genes is alternatively spliced to produce two distinct mRNAs encoding long and short isoforms that differ at the N-terminus of the protein. We show that this process is regulated so that different proportions of the mRNAs encoding the long and short forms of either Brn-3a or Brn-3b are produced in different rat tissues. Similarly, the ratio of each of these forms can be modulated by specific stimuli in both a neuronal cell line and primary neurons. The significance of these effects is discussed in relation to the functional differences between the two forms of each factor.

Nerve growth factor-regulated properties of sensory neurones in Oct-2 null mutant mice

The POU-domain transcription factor Oct-2 is expressed in both B lymphocytes and sensory neurones, where its expression is regulated by nerve growth factor (NGF). In order to define a possible role for Oct-2 in neurotrophin signalling, we examined the expression of an NGF-regulated channel (capsaicin-evoked ion fluxes), neuropeptides (substance P, calcitonin gene-related peptide), structural proteins (neurofilaments and peripherin) and receptors (trks) in dorsal root ganglion neurones derived from perinatal transgenic mice containing a defective Oct-2 structural gene. Northern blots show that central nervous tissue contains a larger than normal (> 10 kb) mRNA transcript corresponding in size to an Oct-2 transcript encoding a defective protein. PCR analysis shows the absence of normal Oct-2 transcripts in dorsal root ganglia. In null mutants, capsaicin sensitivity, and neuropeptide and cytoskeletal protein expression were unaffected by the loss of Oct-2 expression. The number of sensory neurones and the gross morphology of CNS tissues that normally express high levels of Oct-2 were also examined and found to be normal in the null mutant. Heterozygous animals show normal thresholds of sensitivity to noxious heat and normal inflammatory responses. Oct-2 does not therefore play an essential role in the NGF responsiveness of sensory neurones in these animals.

Direct evidence that the POU family transcription factor Oct-2 represses the cellular tyrosine hydroxylase gene in neuronal cells

The POU family transcription factor Oct-2 was originally identified in B lymphocytes but has been shown to be expressed in neuronal cells, although it is absent in most other cell types. Cotransfection of Oct-2 expression vectors into nonneuronal cells with a tyrosine hydroxylase promoter/reporter plasmid suggests that Oct-2 can repress this promoter in this artificial situation. Here we report that reduction of endogenous Oct-2 levels in a neuronal cell line by an antisense approach results in an increase in endogenous tyrosine hydroxylase levels. In contrast, the level of the neuronal marker protein PGP9.5 remains unchanged in the antisense lines whereas that of the neuronal nitric oxide synthase decreases. Hence, the tyrosine hydroxylase gene is a natural target for repression by Oct-2 in neuronal cells. The significance of this effect is discussed in terms of the processes that regulate tyrosine hydroxylase gene expression and the role of Oct-2 in neuronal cells.

Nerve growth factor induces the Oct-2 transcription factor in sensory neurons with the kinetics of an immediate-early gene

The Oct-2 transcription factor has a predominantly inhibitory effect on gene expression in neuronal cell lines. This factor and its corresponding mRNA have previously been shown to be elevated in adult rat dorsal root ganglion (DRG) neurons chronically exposed to nerve growth factor (NGF). Here we show that the Oct-2 mRNA is rapidly induced in DRG cells exposed to NGF and that such induction still occurs to a lesser extent in the presence of the protein synthesis inhibitor cycloheximide. These findings characterize Oct-2 as a novel member of the immediate-early class of NGF-induced transcription factors whose previously defined members have a predominantly stimulatory effect on the expression of other genes. Induction of the Oct-2 mRNA was also observed in DRG neurons treated with acidic fibroblast growth factor or epidermal growth factor but not with a range of other growth factors and neurotrophins. The role of the Oct-2 transcription factor in mediating the response of DRG neurons to NGF is discussed.

Regulated splicing of the Oct-2 transcription factor RNA in neuronal cells

The primary RNA transcript derived from the gene encoding the Oct-2 transcription factor is alternatively spliced to yield a number of different mRNAs which encode different isoforms of this protein. The mRNAs encoding two such isoforms Oct-2c and mini Oct-2 were originally detected in neuronal cells. We show here that the mRNAs encoding these forms also occur in other tissues with the proportion of the mini Oct-2 mRNA being much higher in the spleen than in the brain. However, the levels of the mini Oct-2 mRNA increase in neuronal cell lines in response to differentiation-inducing stimuli and decrease upon exposure to growth factors. Hence the splicing of the Oct-2 transcript can be regulated in both a tissue specific manner and in neuronal cells in response to specific stimuli. The significance of this effect is discussed in terms of the differing ability of different forms of Oct-2 to activate or inhibit gene expression.

The inhibitory domain in the Oct-2 transcription factor represses gene activity in a cell type-specific and promoter-independent manner

The Oct-2 transcription factor contains an N-terminal inhibitory domain which can act to inhibit promoter activity when linked to either its corresponding DNA-binding POU domain or the heterologous DNA binding domain of the yeast transcription factor GAL4. This inhibitory effect is independent of the number of DNA binding sites or their context in the target promoter. In contrast the effect is cell type-specific and can be relieved by over-expression of the isolated inhibitory domain in the absence of a DNA binding domain. These results suggest that the inhibitory domain acts by decreasing the activity of the basal transcriptional complex but that it operates indirectly by recruiting a second cell type-specific factor to the promoter which then interacts with the basal complex decreasing its activity.

Constitutively expressed Oct-2 prevents immunoglobulin gene silencing in myeloma x T cell hybrids

Recent experiments involving disruption of the Oct-2 gene have shown that this largely B cell-restricted transcription factor is not required in the early stages of B cell development. However, B cells that lack Oct-2 may be blocked from differentiation past the surface immunoglobulin-positive stage. To identify a possible function for Oct-2 in the late stage immunoglobulin-secreting cell, we have used the method of somatic cell fusion. When the immunoglobulin-producing myeloma MPC11 is fused to a T lymphoma, Oct-2 production ceases, as does the expression of immunoglobulin, J chain, and several other B cell-specific gene products. In the present study, we show that by preventing the loss of Oct-2 in the hybrid cells, we can preserve expression of all other tested B cell-specific genes. These results establish a central role for Oct-2 in maintaining the genetic program of the immunoglobulin-secreting plasmacyte.

Nerve growth factor induces expression of immediate-early genes NGFI-A (Egr-1) and NGFI-B (nur 77) in adult rat dorsal root ganglion neurons

We have used primary cultures of adult rat dorsal root ganglia (DRG), enriched in sensory neurons, to investigate the induction of immediate-early genes by NGF and a variety of other growth factors. Using the polymerase chain reaction we have quantitatively amplified specific mRNA transcripts induced by NGF, in the presence and absence of the protein synthesis inhibitor cycloheximide. NGFIA (Egr-1) and NGFIB (nur 77) mRNAs were elevated in level within 60 min of NGF treatment and independently of de novo protein synthesis. This was consistent with the behaviour of immediate-early genes. These kinetics were seen at a range of NGF concentrations. NGFIA and NGFIB mRNAs were also found to be induced in DRG cultures by a variety of other growth factors. Different patterns of induction of NGFIA and NGFIB mRNA observed in DRG cultures suggested that transcript-specific pathways of signal transduction were operating within neurons, dependent upon the particular growth factor stimulus. Comparison of data reported from growth factor treatment of other cell types with data from DRG cultures also revealed patterns of NGFIA and NGFIB mRNA induction specific to DRG neurons.

Mice lacking Snrpn expression show normal regulation of neuronal alternative splicing events

The SmN protein is closely related to the constitutively expressed RNA splicing protein SmB but is expressed only in brain and heart tissue. Mice which lack expression of SmN die shortly after birth suggesting a critical role for this protein possibly in the regulation of neuronal-specific alternative splicing events. We show here however that the neuronal-specific alternative splicing of the RNAs encoding several different classes of protein proceeds normally in mice lacking SmN expression. The potential role of SmN and the reasons for the lethal effect observed in non-expressing mice are discussed.

Analysis of Oct2-isoform expression in lipopolysaccharide-stimulated B lymphocytes

Oct2-isoform expression in splenic B cells stimulated with lipopolysaccharide or lipopolysaccharide plus phorbol-di-butyrate was analysed by cDNA cloning. The frequency of Oct2-positive clones was 1/15,000 in both libraries. Two new isoforms were found that generate novel amino- or carboxy-terminal sequences. An isoform lacking exon 11 destroyed the carboxy-terminal leucin-zipper region and introduced a frame shift creating a novel, proline-rich carboxy terminus. A new exon containing a highly basic region (4c) was characterized, between exons 4 and 5. This exon was inserted between glutamine-rich regions 2 and 3, carboxy terminal of a tentative leucine-zipper structure. In addition, a new combination isoform containing Oct2a's amino terminal insert (exon 7a) and Oct2b's carboxy terminal insert (exon 13) was found that created a novel large isoform, Oct2ab. More frequent use of the classical Oct2a and Oct2b isoforms was observed in the lipopolysaccharide-stimulated B cells, while a preference for the Oct2ab and Oct2ba isoforms was observed in lipopolysaccharide plus phorbol-di-butyrate-treated cells.