The opposite and antagonistic effects of the closely related POU family transcription factors Brn-3a and Brn-3b on the activity of a target promoter are dependent on differences in the POU domain

Essential but partially redundant roles for POU4F1/Brn-3a and POU4F2/Brn-3b transcription factors in the developing heart

Congenital heart defects contribute to embryonic or neonatal lethality but due to the complexity of cardiac development, the molecular changes associated with such defects are not fully understood. Here, we report that transcription factors (TFs) Brn-3a (POU4F1) and Brn-3b (POU4F2) are important for normal cardiac development. Brn-3a directly represses Brn-3b promoter in cardiomyocytes and consequently Brn-3a knockout (KO) mutant hearts express increased Brn-3b mRNA during mid-gestation, which is linked to hyperplastic growth associated with elevated cyclin D1, a known Brn-3b target gene. However, during late gestation, Brn-3b can cooperate with p53 to enhance transcription of pro-apoptotic genes e.g. Bax, thereby increasing apoptosis and contribute to morphological defects such as non-compaction, ventricular wall/septal thinning and increased crypts/fissures, which may cause lethality of Brn-3a KO mutants soon after birth. Despite this, early embryonic lethality in e9.5 double KO (Brn-3a^-/- : Brn-3b^-/-) mutants indicate essential functions with partial redundancy during early embryogenesis. High conservation between mammals and zebrafish (ZF) Brn-3b (87%) or Brn-3a (76%) facilitated use of ZF embryos to study potential roles in developing heart. Double morphant embryos targeted with morpholino oligonucleotides to both TFs develop significant cardiac defects (looping abnormalities and valve defects) suggesting essential roles for Brn-3a and Brn-3b in developing hearts.

The cellular transcription factor Brn-3a and the smoking-related substance nicotine interact to regulate the activity of the HPV URR in the cervix

The cellular transcription factor Brn-3a differentially regulates different human papilloma virus (HPV)-16 variants that are associated with different risks of progression to cervical carcinoma in infected humans. The upstream regulatory regions (URRs) of high- and intermediate-risk HPV-16 variants are activated by the cellular transcription factor Brn-3a, whereas the URR of a low-risk HPV-16 variant is not. In this study, we show in transfection assays that Brn-3a and the smoking-related substance nicotine produce stronger responsiveness of the URR of the low- and high-risk variants than with either factor alone, but not the intermediate-risk variant. We determined that this synergistic activity of Brn-3a/nicotine is due to two nucleotide differences in the URR, crucial for oncogenic E6/E7 transactivation. Mutant constructs in which the nucleotide residues were substituted alter Brn-3a/nicotine responsiveness. Importantly, women smokers with high levels of Brn-3a infected with low- or high-risk HPV-16 variants have augmented E6 levels, and were more frequently diagnosed with higher grades of cervical intraepithelial neoplasia (CIN) and cancer, as compared with non-smokers who were infected with similar variants and expressed similar levels of Brn-3a. Therefore, this study defines the specific interplay between the cellular transactivator Brn-3a, the environmental smoking-related substance nicotine and specific HPV variants in cervical carcinogenesis, and thus helps to explain why some women are susceptible to rapid CIN progression and cancer and others are not.

The HPV Cellular Transactivator Brn-3a Can Be Used to Predict Cervical Adenocarcinoma and Squamous Carcinoma Precancer Lesions in the Developed and Developing Worlds

The cellular transactivator Brn-3a has previously been shown to be expressed at elevated levels in the cervix of women with squamous cell carcinoma of the cervix (SCC) and to activate the expression of HPV E6 mRNA. In this study, we show that common and rare cervical precancer lesions, including those of adenocarcinoma (AC), which are usually difficult to diagnose using classical procedures, also expressed high levels of Brn-3a and can be diagnosed by measuring the levels of Brn-3a and E6 mRNAs.

Towards an understanding of the herpes simplex virus type 1 latency-reactivation cycle

Infection by herpes simplex virus type 1 (HSV-1) can cause clinical symptoms in the peripheral and central nervous system. Recurrent ocular shedding can lead to corneal scarring and vision loss making HSV-1 a leading cause of corneal blindness due to an infectious agent. The primary site of HSV-1 latency is sensory neurons within trigeminal ganglia. Periodically, reactivation from latency occurs resulting in virus transmission and recurrent disease. During latency, the latency-associated transcript (LAT) is abundantly expressed. LAT expression is important for the latency-reactivation cycle in animal models, in part, because it inhibits apoptosis, viral gene expression, and productive infection. A novel transcript within LAT coding sequences (AL3) and small nonprotein coding RNAs are also expressed in trigeminal ganglia of latently infected mice. In this review, an update of viral factors that are expressed during latency and their potential roles in regulating the latency-reactivation cycle is discussed.

Brn-3a/POU4F1 interacts with and differentially affects p73-mediated transcription

The Brn-3a/POU4F1 POU transcription factor is critical for the survival and differentiation of specific sensory neurons during development or upon injury; by regulating expression of target genes, either directly or indirectly upon interaction with other proteins. In this study, we demonstrated the physical interaction of Brn-3a with different p73 isoforms and showed co-localization in sensory neurons arising from the neural crest. The biological effects of p73/ Brn-3a interaction depend on the particular p73 isoform, because co-expression of Brn-3a with TAp73 enhanced cell cycle arrest, whereas Brn-3a and DeltaNp73 cooperated to increase protection from apoptosis. Brn-3a antagonized TAp73 transactivation of pro-apoptotic Bax, but co-operated to increase transcription of the cell cycle regulator p21 CIP1/Waf1. The region 425-494 amino acids within the TAp73 C terminus were critical for Brn-3a to repress Bax transactivation, but not for cooperation on the p21 CIP1/Waf1 promoter. Our results suggest that co-factors binding to the p73 C terminus facilitate maximal activation on the Bax but not p21 CIP1/Waf1 promoter and that Brn-3a modulates this interaction. Thus, the physical interaction of Brn-3a with specific p73 isoforms will be critical for determining cell fate during neuronal development or in injured neurons expressing both factors.

Brn3 transcription factors control terminal osteoclastogenesis

Osteoclastic bone resorption is a central mechanism in skeletal development, remodeling and pathology. RANKL is a mandatory factor controlling osteoclastogenesis; however, the underlying signaling pathways are only partially characterized. Using a screening array for the investigation of differential transcription factor activation, we identified activation of the Brn3 transcription factor family as a downstream event of RANKL signaling during terminal osteoclastogenesis. RANKL stimulation induces expression of Brn3a and b and maximal transcriptional activity of Brn3 family members concurrent with osteoclastic giant cell formation. Immunohistochemical analysis revealed both nuclear and cytoplasmic localization of Brn3a and b in mature osteoclasts. Functional inhibition of Brn3 transcription factors resulted in inhibition of pre-osteoclast fusion and reduction in bone resorbing activity of mature osteoclasts. Furthermore, we identified synaptotagmin-1, a regulator of membrane and vesicular fusion, as downstream target of Brn3 with a role in osteoclast function. We conclude that Brn-3 represents a novel molecular differentiation factor that controls osteoclast maturation and function, suggesting an important role in bone metabolism.

Tip60 protein isoforms and altered function in skin and tumors that overexpress ornithine decarboxylase

Elevated expression of ornithine decarboxylase (ODC) and increased synthesis of polyamines are hallmarks of epithelial tumorigenesis. The skin and tumors of K6/ODC and ODC/Ras transgenic mice, in which overexpression of ODC has been targeted to hair follicles, were found to exhibit intrinsically high histone acetyltransferase (HAT) activity. We identified Tip60 as a candidate enzyme for contributing significantly to this abnormally high HAT activity. Compared with normal littermate controls, the levels of Tip60 protein and an alternative splice variant Tip53 were found to be greater in K6/ODC mouse skin. Furthermore, skin tumors that spontaneously develop in ODC/Ras bigenic mice typically have substantially more Tip60 protein than adjacent non-tumor-bearing skin and exhibit a unique pattern of Tip60 size variants and chemically modified protein isoforms. Steady-state Tip60 and Tip53 mRNA levels were not affected in ODC-overexpressing skin and tumors, implying novel posttranscriptional regulation by polyamines. Given the diverse roles of Tip60, the overabundance of Tip60 protein is predicted to have biological consequences. Compared with normal littermate skin, we detected altered association of Tip60 with E2F1 and a subset of newly identified Tip60-interacting transcription factors in ODC transgenic mouse skin and tumors. E2F1 was shown to be bound in greater amounts to up-regulated target genes in ODC-overexpressing skin. Thus, up-regulation of Tip60 protein, influencing the expression of Tip60-regulated genes, could play a contributing role in polyamine-mediated tumor promotion. (

Cellular homeoproteins, SATB1 and CDP, bind to the unique region between the human cytomegalovirus UL127 and major immediate-early genes

An AT-rich region of the human cytomegalovirus (CMV) genome between the UL127 open reading frame and the major immediate-early (MIE) enhancer is referred to as the unique region (UR). It has been shown that the UR represses activation of transcription from the UL127 promoter and functions as a boundary between the divergent UL127 and MIE genes during human CMV infection [Angulo, A., Kerry, D., Huang, H., Borst, E.M., Razinsky, A., Wu, J., Hobom, U., Messerle, M., Ghazal, P., 2000. Identification of a boundary domain adjacent to the potent human cytomegalovirus enhancer that represses transcription of the divergent UL127 promoter. J. Virol. 74 (6), 2826-2839; Lundquist, C.A., Meier, J.L., Stinski, M.F., 1999. A strong negative transcriptional regulatory region between the human cytomegalovirus UL127 gene and the major immediate-early enhancer. J. Virol. 73 (11), 9039-9052]. A putative forkhead box-like (FOX-like) site, AAATCAATATT, was identified in the UR and found to play a key role in repression of the UL127 promoter in recombinant virus-infected cells [Lashmit, P.E., Lundquist, C.A., Meier, J.L., Stinski, M.F., 2004. Cellular repressor inhibits human cytomegalovirus transcription from the UL127 promoter. J. Virol. 78 (10), 5113-5123]. However, the cellular factors which associate with the UR and FOX-like region remain to be determined. We reported previously that pancreatic-duodenal homeobox factor-1 (PDX1) bound to a 45-bp element located within the UR [Chao, S.H., Harada, J.N., Hyndman, F., Gao, X., Nelson, C.G., Chanda, S.K., Caldwell, J.S., 2004. PDX1, a Cellular Homeoprotein, Binds to and Regulates the Activity of Human Cytomegalovirus Immediate Early Promoter. J. Biol. Chem. 279 (16), 16111-16120]. Here we demonstrate that two additional cellular homeoproteins, special AT-rich sequence binding protein 1 (SATB1) and CCAAT displacement protein (CDP), bind to the human CMV UR in vitro and in vivo. Furthermore, CDP is identified as a FOX-like binding protein and a repressor of the UL127 promoter, while SATB1 has no effect on UL127 expression. Since CDP is known as a transcription repressor and a nuclear matrix-associated region binding protein, CDP may have a role in the regulation of human CMV transcription.

Detection of cervical abnormalities in a developing country using measurement of Brn-3a in cervical smears

OBJECTIVE

We have previously shown that Brn-3a is elevated in biopsies from women in the United Kingdom with high grade cervical neoplasia, and that it specifically trans-activates the HPV URR in vitro and in vivo. The aim of this study is to establish the relationship of Brn-3a, HPV E6 and pathological diagnosis in cervical smear from women in a developing country with a high prevalence of cervical disease. This is a follow-up of our previous work in which for the first time Brn-3a and E6 levels in cervical smears from women in United Kingdom were shown to correlate with the histological diagnosis of cervical neoplasia and were even better in predicting underlying pre-malignant disease than conventional procedures.

METHOD

Cervical smears from 295 women with cervical abnormalities attending gynecological clinics in Brazil were used to make RNA. The mRNA levels of Brn-3a and HPV E6 were measured and the data obtained were used to establish the relationship between Brn-3a and the histological diagnosis.

RESULTS

The cellular transcription factor Brn-3a was readily measured in cervical smears from the Brazilian population. Its presence was shown to be frequently associated with the expression of HPV E6. The measured level of Brn-3a parallels the severity of the cervical ailment and predicts the pathological categories.

CONCLUSIONS

The ability of Brn-3a to predict for cervical ailments is independent to the geographical characteristics of the population, and hence it could be used routinely as an adjunct to colposcopy and pathological diagnosis in developing and developed countries.

Differential regulation of different human papilloma virus variants by the POU family transcription factor Brn-3a

The Brn-3a POU family transcription factor is overexpressed in human cervical carcinoma biopsies and is able to activate expression of the human papilloma virus type 16 (HPV-16) upstream regulatory region (URR), which drives the expression of the E6 and E7 oncoproteins. Inhibition of Brn-3a expression in human cervical cancer cells inhibits HPV gene expression and reduces cellular growth and anchorage independence in vitro as well as the ability to form tumours in vivo. Here, we show that Brn-3a differentially regulates different HPV-16 variants that have previously been shown to be associated with different risks of progression to cervical carcinoma. In human cervical material, Brn-3a levels correlate directly with HPV E6 levels in individuals infected with a high risk variant of HPV-16, whereas this is not the case for a low-risk variant. Moreover, the URRs of high- and intermediate-risk variants are activated by Brn-3a in transfection assays, whereas the URR of a low-risk variant is not. The change of one or two bases in a low-risk variant URR to their equivalent in a higher-risk URR can render the URR responsive to Brn-3a and vice versa. These results help explain why the specific interplay between viral and cellular factors necessary for the progression to cervical carcinoma only occurs in a minority of those infected with HPV-16.

Elevated expression of the Brn-3a and Brn-3b transcription factors in systemic lupus erythematosus correlates with antibodies to Brn-3 and overexpression of Hsp90

OBJECTIVE

Important developmental and antiapoptotic roles have been described for the Brn-3 family of transcription factors in mammalian cells. Following a report of pathogenic autoantibody-inducing T cell reactivity to the Brn-3 transcription factors in murine lupus, we undertook this study to investigate serum levels of antibodies to Brn-3 and levels of expression of Brn-3 in peripheral blood mononuclear cells (PBMCs) of patients with systemic lupus erythematosus (SLE).

METHODS

Serum and PBMC samples were obtained from 87 SLE patients and 30 normal control subjects. Serum antibodies to the Brn-3a and Brn-3b transcription factors were measured by enzyme-linked immunosorbent assay. Levels of Brn-3a and Brn-3b messenger RNA (mRNA) in PBMCs were measured by reverse transcription and real-time quantitative polymerase chain reaction.

RESULTS

Elevated serum levels of antibodies to Brn-3a and Brn-3b were found in 43% and 32%, respectively, of SLE patients. This elevation paralleled enhanced expression of Brn-3a and Brn-3b in PBMCs of 44% and 31%, respectively, of SLE patients. Furthermore, we observed a significant correlation (P = 0.002) between elevated levels of anti-Brn-3b antibodies and elevated levels of Brn-3b mRNA in individual patients. A preliminary analysis of possible target genes for Brn-3a and Brn-3b revealed a significant correlation (P = 0.01) between the level of Brn-3a mRNA and the level of Hsp90 protein (90-kd heat-shock protein, which is overexpressed in SLE) in PBMCs of SLE patients. In addition, we observed that overexpression of Brn-3a and Brn-3b in cultured cells enhanced expression of Hsp90 protein and transcription of Hsp90 promoter-reporter constructs. Finally, we observed an association between elevated levels of Brn-3a mRNA and active SLE (P = 0.002).

CONCLUSION

Expression of both Brn-3a and Brn-3b was found to be enhanced in SLE, and this correlated with enhanced levels of autoantibodies to these proteins and with the previously reported overexpression of Hsp90, which was shown to be a novel gene regulated by Brn-3a and Brn-3b. The overexpression of Brn-3a correlated with active disease, suggesting that it may play a role in the disease process via its targeting by the immune system and its ability to induce the expression of specific genes.

The effects of Brn-3a on neuronal differentiation and apoptosis are differentially modulated by EWS and its oncogenic derivative EWS/Fli-1

The Brn-3 family of POU (Pit-Oct-Unc) homeodomain transcription factors regulate differentiation of neuronal cell types. The transcriptional activator Brn-3a is expressed in Ewing's sarcomas, which also express characteristic chimaeric proteins as a consequence of fusion of the TET family gene EWS to one of several ETS genes. We have previously demonstrated a physical interaction between Brn-3a and EWS proteins, and show here that the C-terminal POU domain but not N-terminal activation domain of Brn-3a can interact in vitro with the RNA-binding domain of EWS. Likely due to POU domain homology, the related factor Brn-3b can also interact with EWS, but to a lesser extent than Brn-3a. Importantly, Brn-3a but not Brn-3b interacts in vitro with chimaeric EWS/Fli-1, EWS/ATF-1 and EWS/ERG proteins. Furthermore, overexpression of EWS/Fli-1 but not EWS or Fli-1 inhibits Brn-3a-associated growth arrest and neurite outgrowth in neuronal cells, and specifically inhibits Brn-3a-dependent activation of p21 and SNAP-25 transcription. In contrast, upregulation of Bcl-2 expression and inhibition of apoptosis by Brn-3a is antagonized more by EWS than by EWS/Fli-1. These data demonstrate that oncogenic rearrangement of EWS to produce EWS/Fli-1 may enhance the antiapoptotic effect of Brn-3a and inhibit its ability to promote neuronal differentiation.

PDX1, a Cellular Homeoprotein, Binds to and Regulates the Activity of Human Cytomegalovirus Immediate Early Promoter

Cellular homeoproteins have been shown to regulate the transcription of several viruses, including herpes simplex viruses, human papillomaviruses, and mouse mammary tumor viruses. Previous studies investigating the anti-viral mechanisms of several cyclin-dependent kinase inhibitors showed that the homeoproteins, pre B-cell leukemia transcription factor 1 (PBX1) and PBX-regulating protein-1 (PREP1), function as transcriptional activators of Moloney murine leukemia virus. Here, we examined the involvement of cellular homeoproteins in regulating the activity of the human cytomegalovirus immediate early (CMV IE) promoter. We identified a 45-bp element located at position -593 to -549 upstream of the transcription start site of the CMV IE gene, which contains multiple putative homeoprotein binding motifs. Gel shift assays demonstrated the physical association between a homeodomain protein, pancreatic-duodenal homeobox factor-1 (PDX1) and the 45-bp cytomegalovirus (CMV) region. We further determined that PDX1 represses the CMV IE promoter activity in 293 cells. Overexpression of PDX1 resulted in a decrease in transcription of the CMV IE gene. Conversely, blocking PDX1 protein synthesis and mutating the PDX1 binding sites enhanced CMV IE-dependent transcription. Collectively, our results represent the first work demonstrating that a cellular homeoprotein, PDX1, may be a repressor involved in regulation of human CMV gene expression.

Measurement of Brn-3a levels in Pap smears provides a novel diagnostic marker for the detection of cervical neoplasia

OBJECTIVES

We have previously demonstrated that Brn-3a cellular transcription factor activates transcription of the human papillomavirus (HPV) E6 and E7 oncogenes in human cervical cancer cells and that Brn-3a levels are dramatically elevated in biopsies from women with high-grade cervical neoplasia. The aim of this study was to establish the relationship between Brn-3a levels in Pap smears and the histological diagnoses. We also analysed whether Brn-3a levels can be used in combination with Pap smear to predict the presence of cervical intraepithelial lesion.

METHODS

Two hundred thirty-eight women who were referred with abnormal Pap smear underwent a diagnostic colposcopy, repeat in-study Pap smear, colposcopically directed biopsy, and assessment of Brn-3a and HPV-16 E6 m-RNA levels. Data were analysed to assess the association between Brn-3a levels and the histological diagnosis.

RESULTS

Brn-3a was readily measured in smears and showed a statistically significant correlation with the grade of cervical abnormality. Positive Brn-3a is associated with increased relative risk of higher-grade lesion. Moreover, measurement of Brn-3a levels in smears can be used to detect a significant proportion of cervical lesions that were missed by Pap smear.

CONCLUSION

Measurement of Brn-3a levels in routinely taken Pap smears is a feasible technique that correlates with the severity of the epithelial abnormality and is a useful adjunct to cytology. Brn-3a appears to have great promise since it detects activation of oncogenic HPVs rather than simply detecting their presence, as is currently being done.

Functional interaction between Brn-3a and Src-1 co-activates Brn-3a-mediated transactivation

The Brn-3a POU domain transcription factor is able to regulate the transcription of promoters containing a Brn-3 response element via its POU domain. In addition, the POU domain of Brn-3a has been shown to functionally interact with the estrogen receptor and regulate transcription from estrogen responsive promoters. The steroid receptor coactivator, Src-1, enhances transcription with a variety of steroid receptors. Here we describe a functional interaction between Brn-3a and Src-1. In glutathione S-transferase pull-down assays Src-1 was shown to specifically interact with Brn-3 proteins. Moreover, Src-1 co-immunoprecipitated from intact cells with Brn-3a. The transactivation potential of the Brn-3a/Src-1 complex was tested on both the Brn-3 responsive SNAP-25 promoter and the estrogen responsive vitellogenin promoter, in each of two different cell lines, the neuronal ND7 cell line, and the kidney BHK21 cell line. Src-1 consistently and strongly potentiated the activation of Brn-3a on the SNAP promoter construct in both the ND7 and BHK21 cell lines. The vitellogenin promoter construct, however, was only weakly activated by the Brn-3/Src-1 complex in the ND7 cells and there was even less effect on this promoter in the BHK21 cells. These results suggest a functional role for Src-1 in enhancing Brn-3a mediated transactivation, seemingly independent of nuclear hormone receptors, thus broadening the transcriptional repertoire of both Brn-3a and Src-1.

The Brn-3b POU family transcription factor regulates the cellular growth, proliferation, and anchorage dependence of MCF7 human breast cancer cells

The Brn-3b POU domain containing transcription factor is expressed in the developing sensory nervous system as well as in epithelial cells of the breast, cervix, and testes. Brn-3b functionally interacts with the estrogen receptor (ER) and in association with the ER, regulates transcription from estrogen responsive genes. In addition, Brn-3b expression is elevated in breast tumours compared to levels in normal mammary cells. To explore the role of Brn-3b in breast cancer, we established stable cell lines derived from the MCF7 human breast cancer cell line which had been transfected with Brn-3b sense or anti-sense constructs. The Brn-3b over-expressing cell lines exhibited increased growth rate, reached confluence at a higher saturation density, had higher proliferative activity, and an enhanced ability to form colonies in soft agar when compared to the control empty vector transfected cells. Likewise, the Brn-3b anti-sense cell lines showed reduced cellular growth and proliferation, reached confluence at a lower density, and exhibited a decreased ability to form colonies in soft agar when compared to the vector controls. Five to ten per cent of the Brn-3b over-expressing cells exhibited a severely altered morphology characterized by reduced adherence to tissue culture plastic, increased cell size, and a vacuolar cell shape. These results thus further indicate a role for the Brn-3b transcription factor in regulating mammary cell growth and suggest that its elevation in breast cancer is of functional significance.

The Brn-3a transcription factor plays a key role in regulating the growth of cervical cancer cells in vivo

The cellular Brn-3a transcription factor is known to activate transcription of the genes encoding the human papilloma virus E6 and E7 proteins and is over-expressed in women with cervical neoplasia. We show that cervical cell lines with reduced Brn-3a expression show a greatly reduced ability to form tumours in nude mice compared to control cells and also show reduced expression of the HPV E6 and cellular Bcl-2 oncogenes. These effects are also observed in cervical cells over-expressing the related Brn-3b factor, which is known to antagonize activation of HPV gene expression by Brn-3a. These results demonstrate, for the first time, that inhibition of Brn-3a expression or enhanced Brn-3b expression can inhibit cervical cell-derived tumour growth in vivo as well as in vitro. Hence they establish Brn-3a as a key factor in cervical tumorigenesis and as a potential therapeutic target in human cervical neoplasia.

A role for Octamer binding protein motifs in the regulation of the proximal preprotachykinin-A promoter

We have recently developed cell line models that express the endogenous rat preprotachykinin-A (rPPT) gene and support reporter gene expression directed by the rPPT promoter. These are the neuronal derived cell line NF2C and the pancreatic cell lines RINm5F and a derivative RIN-1027-B2. Reporter gene activity in these cell lines is similar to that observed in primary cultures of rat dorsal root ganglion neurons. Analysis of reporter gene expression supported by various fragments of the rPPT promoter demonstrated that although -865 to +92 supported expression, addition of fragments between +92 and +500 led to repression of expression. Two previously defined octamer binding motifs, present both 5' and 3' of the major transcriptional start site, have been postulated to be potential enhancers of rPPT activity and we have now used these cell lines to determine the role of these regions in the rPPT promoter. Site specific mutagenesis of these elements has shown not only that both sites are potential enhancers of gene expression but also that the 3' element binds multiple factors, of which at least one has repressor function. Binding of this repressor protein over or adjacent to the 3' octamer binding protein site leads to the observed repression of promoter activity in the -865 to +500 construct relative to the to -865 to +92 the fragment.

All Brn3 genes can promote retinal ganglion cell differentiation in the chick

Targeted gene disruption studies in the mouse have demonstrated crucial roles for the Brn3 POU domain transcription factor genes, Brn3a, Brn3b, Brn3c (now called Pou4f1, Pou4f2, Pou4f3, respectively) in sensorineural development and survival. During mouse retinogenesis, the Brn3b gene is expressed in a large set of postmitotic ganglion cell precursors and is required for their early and terminal differentiation. In contrast, the Brn3a and Brn3c genes, which are expressed later in ganglion cells, appear to be dispensable for ganglion cell development. To understand the mechanism that causes the functional differences of Brn3 genes in retinal development, we employed a gain-of-function approach in the chick embryo. We find that Brn3b(l) and Brn3b(s), the two isoforms encoded by the Brn3b gene, as well as Brn3a and Brn3c all have similar DNA-binding and transactivating activities. We further find that the POU domain is minimally required for these activities. Consequently, we show that all these Brn3 proteins have a similar ability to promote development of ganglion cells when ectopically expressed in retinal progenitors. During chick retinogenesis, cBrn3c instead of cBrn3b exhibits a spatial and temporal expression pattern characteristic of ganglion cell genesis and its misexpression can also increase ganglion cell production. Based on these data, we propose that all Brn3 factors are capable of promoting retinal ganglion cell development, and that this potential may be limited by the order of expression in vivo.

Regulation of NGFI-A (Egr-1) gene expression by the POU domain transcription factor Brn-3a

NGFI-A is an immediate early gene (IEG) that is transcriptionally induced by nerve growth factor (NGF) in PC12 cells and has been implicated in a number of cellular responses. Studies have shown that elements within the first 106 base pairs of the NGFI-A promoter contribute to its induction by NGF in PC12 cells. One element, within the serum response element (SRE) bridge region, bears strong homology to a motif previously identified in promoters regulated by the Brn-3a POU domain transcription factor. We report here that Brn-3a activates the NGFI-A promoter in neurons (both primary and cell lines). Analysis revealed that this response requires sequences between positions -49 and -106. Whilst DNA-protein interaction studies failed to identify a site bound directly by Brn-3a, the data presented here suggest that Brn-3a may cooperate in the regulation of NGFI-A gene expression in neurons, possibly during the developmental switch between neurotrophin dependency that occurs during neurogenesis.

The Brn-3b POU family transcription factor represses expression of the BRCA-1 anti-oncogene in breast cancer cells

The BRCA-1 tumour supressor gene was identified on the basis of mutations which occur in familial breast cancer indicating that its inactivation can cause this disease. Although BRCA-1 does not appear to be mutated in sporadic breast cancer, its expression has been shown to be reduced in tumour material from such cases. We show here that mammary tumours which have reduced levels of BRCA-1 expression show enhanced expression of the Brn-3b POU family transcription factor at both the mRNA and protein levels. This elevated expression of Brn-3b is not found in normal mammary cells, benign tumours or in malignant tumour samples which do not exhibit reduced levels of BRCA-1. In contrast, no correlation was noted between BRCA-1 and expression of the related factor Brn-3a. Moreover, Brn-3b but not Brn-3a can strongly repress the BRCA-1 promoter approximately 20-fold in mammary tumour cells. To our knowledge, this is the first report of a transcription factor which regulates BRCA-1 expression. Thus, Brn-3b may play an important role in regulating expression of BRCA-1 in mammary tumours with enhanced expression of Brn-3b resulting in reduced BRCA-1 expression and thereby being potentially important in tumour development.

The Brn-3a Transcription Factor Plays a Critical Role in Regulating Human Papilloma Virus Gene Expression and Determining the Growth Characteristics of Cervical Cancer Cells

The Brn-3a POU family transcription factor has previously been shown to activate the human papilloma virus type 16 (HPV-16) promoter driving the expression of the E6- and E7-transforming proteins. Moreover, Brn-3a is overexpressed approximately 300-fold in cervical biopsies from women with cervical intra-epithelial neoplasia type 3 (CIN3) compared with normal cervical material. To test the role of Brn-3a in cervical neoplasia we have manipulated its expression in cervical carcinoma-derived cell lines with or without endogenous HPV genes. In HPV-expressing cells, reduction in Brn-3a expression specifically reduces HPV gene expression, growth rate, saturation density and anchorage-independent growth, whereas these effects are not observed when Brn-3a expression is reduced in cervical cells lacking HPV genomes. Together with our previous observations, these findings indicate a critical role for Brn-3a in regulating HPV gene expression and thereby in controlling the growth/transformation of cervical cells.

Autoregulatory sequences are revealed by complex stability screening of the mouse brn-3.0 locus

The POU-IV or Brn-3 class of transcription factors exhibit conserved structure, DNA-binding properties, and expression in specific subclasses of neurons across widely diverged species. In the mouse CNS, Brn-3.0 expression characterizes specific neurons from neurogenesis through the life of the cell. This irreversible activation of expression suggests positive autoregulation. To search for cis-acting elements that could mediate autoregulation we used a novel method, complex stability screening, which we applied to rapidly identify functional Brn-3.0 recognition sites within a large genomic region encompassing the mouse brn-3.0 locus. This method is based on the observation that the kinetic stability of Brn-3.0 complexes with specific DNA sequences, as measured by their dissociation half-lives, is highly correlated with the ability of those sequences to mediate transcriptional activation by Brn-3.0. The principal Brn-3.0 autoregulatory region lies approximately 5 kb upstream from the Brn-3.0 transcription start site and contains multiple Brn-3.0-binding sites that strongly resemble the optimal binding site for this protein class. This region also mediates transactivation by the closely related protein Brn-3.2, suggesting a regulatory cascade of POU proteins in specific neurons in which Brn-3.2 expression precedes Brn-3.0.

Analysis of functional domains of the host cell factor involved in VP16 complex formation

We present biochemical analyses of the regions of the host cell factor (HCF) involved in VP16 complex formation and in the association between the N- and C-terminal domains of HCF itself. We show that the kelch repeat region of HCF (residues 1-380) is sufficient for VP16 complex formation, but that residues C-terminal to the repeats (positions 381-450) interfere with this activity. However, these latter residues are required for the interaction between the N- and C-terminal regions of HCF. The extreme C-terminal region of HCF, corresponding to an area of strong conservation with a Caenorhabditis elegans homologue, is sufficient for interaction with the N-terminal region. These results are discussed with respect to possible differences in the roles of HCF in VP16 activity versus its normal cellular function.

p53 suppresses the activation of the Bcl-2 promoter by the Brn-3a POU family transcription factor

The Brn-3a POU family transcription factor has been shown to strongly activate expression of the Bcl-2 proto-oncogene and thereby protect neuronal cells from programmed cell death (apoptosis). This activation of the Bcl-2 promoter by Brn-3a is strongly inhibited by the p53 anti-oncogene protein. This inhibitory effect of p53 on Brn-3a-mediated transactivation is observed with nonoverlapping gene fragments containing either the Bcl-2 p1 or p2 promoters but is not observed with other Brn-3a-activated promoters such as in the gene encoding alpha-internexin or with an isolated Brn-3a binding site from the Bcl-2 promoter linked to a heterologous promoter. In contrast, p53 mutants, which are incapable of binding to DNA, do not affect Brn-3a-mediated activation of the Bcl-2 p1 and p2 promoters. Moreover, Brn-3a and p53 have been shown to bind to adjacent sites in the p2 promoter and to directly interact with one another, both in vitro and in vivo, with this interaction being mediated by the POU domain of Brn-3a and the DNA binding domain of p53. The significance of these effects is discussed in terms of the antagonistic effects of Bcl-2 and p53 on the rate of apoptosis and the overexpression of Brn-3a in specific tumor cell types.

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.

Winged helix hepatocyte nuclear factor 3 and POU-domain protein brn-2/N-oct-3 bind overlapping sites on the neuronal promoter of human aromatic L-amino acid decarboxylase gene

The neuronal promoter of human aromatic l-amino acid decarboxylase gene has been analysed to elucidate the mechanisms of neuron type-specific expression. The (-560/+92) promoter segment was sufficient to direct luciferase expression at a higher level in SK-N-BE neuroblastoma cells, than in CHP126 neuroepithelia, HepG2 hepatoma or SK-Hep1 epithelioma cells. Deletions experiments showed that this segment contained a neuronal-specific (element T1) and a SK-N-BE-specific (element N1) cis-activating sequences. Element T1 (-72/-36) bound Sp1 and NF-Y proteins, and unidentified neuronal-specific factors. Element N1 (-102/-72) bound cell-specific factors, identified as HNF-3, N-Oct-3/Brn-2 and N-Oct-2. HNF-3 proteins recognized the sequence TCAGTAAATA that matches the consensus motif. Oct-1, N-Oct-2 and N-Oct-3 bound the AAATAATGC sequence that overlaps the HNF-3 binding site. In addition, we show that the HNF-3 binding sites from aldolase C and HNF-3beta gene promoters also bind N-Oct-2 and N-Oct-3 proteins. These data suggest a functional interplay of winged helix/forkhead and POU-domain transcription factors on a variety of neuronal gene promoters.

NT-3 regulates expression of Brn3a but not Brn3b in developing mouse trigeminal sensory neurons

We have used a quantitative RT-PCR approach to determine the levels of Brn3a and Brn3b POU domain transcription factor mRNAs in the developing mouse trigeminal ganglion from E10 to E18. Using low density neuronal cultures, we have shown that NT-3 can regulate the expression of Brn3a mRNA in trigeminal neurons during the periods that they are differentiating and innervating their peripheral and central targets. In contrast to Brn3a, Brn3b mRNA is expressed at extremely low levels in the early trigeminal ganglion. Trigeminal neurons from early ganglia express low levels of Brn3b mRNA in culture and do not up-regulate Brn3b mRNA in response to a number of growth factors and experimental conditions. However, at later ages, when in vivo levels of Brn3b mRNA are high, FGF2, TGFbeta1 and retinoic acid all up-regulate Brn3b mRNA expression in cultured trigeminal neurons. Since NT-3 regulates the developmental expression of Brn3a, Brn3a may mediate some of the effects that NT-3 exerts on sensory neurons and their progenitors. Similarly, Brn3b may mediate some of the effects that FGF2, TGFbeta1 and retinoic acid have on neurons.

A role for the octamer-binding protein in preprotachykinin-A gene expression

A rat Preprotachykinin-A promoter fragment has been previously identified which supports reporter gene activity in primary cultures of adult dorsal root ganglion neurons. That study demonstrated that two promoter domains which exhibit enhancer activity in these neurons are bound by the same classes of transcription factors. Further, the two domains exhibit similarities with respect to the relationship of bound transcription factors within each domain. This suggests that these domains may function in an identical manner or may act synergistically to regulate gene expression. These domains contain recognition motifs for at least three classes of transcription factors: octamer-binding proteins, Sp1-related proteins and an as yet unidentified but distinct factor. The definition of an octamer-binding protein site within these domains is of interest, as this class of factor has recently been suggested as mediating the effect of nerve growth factor in sensory neurons. Nerve growth factor is a well-characterized regulator of preprotachykinin-A gene expression. Definition of these sites within the promoter allows for the design of rational experiments to address the mechanism of transcriptional regulation of the rat preprotachykinin-A gene.

Identification of a novel repressive element that contributes to neuron-specific gene expression

Multiple signaling pathways are thought to control the selective expression of genes over the course of neuronal differentiation. One approach to elucidating these pathways is to identify specific cis-acting elements that serve as the final targets for these signaling pathways in neural-specific genes. We now identify a novel repressive element from the growth-associated protein 43 (GAP-43) gene that can contribute to neuron-specific gene expression by inhibiting transcription in a wide range of non-neuronal cell types. This repressive element is located downstream of the GAP-43 TATA box and is highly position-dependent. When transferred to viral promoters this element preferentially inhibits transcription in non-neuronal cells. Electrophoretic mobility shift assays show that the repressive element comprises at least two protein recognition sites. One of these is a novel sequence motif that we designate the SNOG element, because it occurs downstream of the TATA boxes of the synaptosomal-associated protein of 25 kDa and neuronal nitric oxide synthase genes, as well as the GAP-43 gene. The GAP-43 repressive element is distinct in sequence and position dependence from the repressor element 1/neuron-restrictive silencer element previously described in other neural genes and therefore is a likely target for a distinct set of signaling pathways involved in the control of neuronal differentiation.

Coordinate induction of the three neurofilament genes by the Brn-3a transcription factor

The POU domain transcription factor Brn-3a is able to stimulate neurite outgrowth when overexpressed in the neuronal ND7 cell line, whereas the closely related Brn-3b factor does not have this effect. We show that Brn-3a overexpression also enhances the expression of the three neurofilament genes at both the mRNA and protein levels, whereas Brn-3b overexpression has no effect. In addition Brn-3a activates the three neurofilament gene promoters in co-transfection assays in both neuronal and non-neuronal cells. As observed for enhanced neurite outgrowth, the stimulation of neurofilament gene expression and activation of the neurofilament gene promoters is observed with the isolated POU domain of Brn-3a. A single amino acid change in the POU homeodomain of Brn-3a to the equivalent amino acid in Brn-3b abolishes its ability to activate the neurofilament promoters, whereas the reciprocal change converts Brn-3b to an activator of these promoters.

Identification of a chicken homologue in the Brn-3 subfamily of POU-transcription factors

Among the many transcription factors thus far identified several are found to be expressed almost exclusively in the nervous system. The Brn-3 subfamily of POU-transcription factors constitutes a highly conserved group of such factors showing expression predominantly in sensory neurons. We now describe the nucleotide sequence and proposed amino acid sequence of a chicken homologue to the murine and human Brn-3 genes. Furthermore we characterise the early embryonic expression pattern of this chicken Brn-3 gene and show it to be expressed in peripheral sensory ganglia as well as in retinal ganglion cells. Based on these findings we conclude that the chicken homologue to the murine and human Brn-3a genes has been cloned. We have begun to examine possible regulatory pathways of Brn-3a by stimulating chick embryonic peripheral ganglia with trophic factors and assaying resulting levels of Brn-3a with a quantitative PCR approach. Trigeminal and dorsal root ganglia stimulated in culture by NGF and NT-3 embryonic day 9 (E9) produce neurites without raising the Brn-3a mRNA levels.

Inhibition of neuronal process outgrowth and neuronal specific gene activation by the Brn-3b transcription factor

The differentiation of the ND7 neuronal cell line to a nondividing phenotype bearing numerous neurite processes is accompanied by a dramatic increase in the levels of the activating POU family transcription factor Brn-3a and a corresponding fall in the levels of the closely related inhibitory factor Brn-3b. We have previously shown that the artificial overexpression of Brn-3a in these cells can induce neurite outgrowth and the activation of genes encoding synaptic vesicle proteins in the absence of a differentiation-inducing stimulus. Here we show that overexpression of Brn-3b can reduce process outgrowth and synaptic vesicle gene expression following exposure to a stimulus which would normally induce differentiation. These inhibitory effects are abolished by altering a single amino acid in the POU homeodomain of Brn-3b to its equivalent in Brn-3a. The converse mutation in Brn-3a allows it to inhibit process outgrowth in response to a differentiation-inducing stimulus. Hence a single amino acid difference results in these closely related factors having opposite effects and allows the balance between them to regulate differentiation.

Differential regulation of genes encoding synaptic proteins by members of the Brn-3 subfamily of POU transcription factors

The three members of the Brn-3 subfamily of POU transcription factors have distinct effects on target gene expression. We show that the promoter of the gene encoding the presynaptic nerve terminal protein SNAP-25 resembles previously characterised target genes in being activated by Brn-3a and Brn-3c, but being repressed by Brn-3b. Unlike other target genes, however, the SNAP-25 promoter can be activated by either the N- or C-terminal activation domains of Brn-3a. In contrast to the SNAP-25 gene, the gene encoding the synaptic vesicle protein synapsin 1 is activated by all the Brn-3 factors, the first gene for which this activation pattern has been reported Interestingly, however, similar activation by all three Brn-3 factors can be observed if the SNAP-25 promoter is truncated by removal of sequences from -2200 to -288 relative to the transcriptional start site. Moreover, a region of the SNAP-25 promoter from -283 to -126 can render a heterologous promoter responsive to activation by all three Brn-3 factors. Differences in promoter structure may thus result in differences in the response to different Brn-3 factors, thus allowing these factors to produce diverse activation patterns of neuronally expressed genes, such as those encoding different synaptic proteins.

Herpes simplex: evolving concepts

A large body of molecular biologic research has begun to clarify some basic aspects of viral latency and reactivation. The clinical definition of herpes simplex virus infection is expanding, with the recognition that the disease is largely asymptomatic and that most transmission occurs during periods of asymptomatic viral shedding. With this awareness, serologic diagnosis has become increasingly important. New treatment modalities are now available, and other promising treatments are in development.

The functionally antagonistic POU family transcription factors Brn-3a and Brn-3b show opposite changes in expression during the growth arrest and differentiation of human neuroblastoma cells

The Brn-3a and Brn-3b POU family transcription factors have previously been shown to have functionally antagonistic effects, with Brn-3a activating specific gene promoters which are repressed by Brn-3b. We show here that proliferating cell lines of human neuroblastoma origin have a high ratio of Brn-3b to Brn-3a compared with other cell lines derived from related tumours. Moreover, the level of Brn-3a rises and that of Brn-3b falls when neuroblastoma cell lines are exposed to treatments which induce a cessation of proliferation. Such treatments also result in the activation of a test promoter which is normally stimulated by Brn-3a and repressed by Brn-3b. Our findings suggest that these antagonistic factors may play a key role in regulation of gene expression during human neuroblastoma differentiation and could thus represent a potential therapeutic target for treatments designed to induce such differentiation.

A single amino acid change converts an inhibitory transcription factor into an activator

The closely related POU family transcription factors Brn-3a and Brn-3b differ in their functional activity with Brn-3a activating several target promoters, which are repressed by Brn-3b. Brn-3b also prevents promoter activation by Brn-3a. Here we have altered a single isoleucine residue in the POU homeodomain of Brn-3b to the valine residue found at the equivalent position in Brn-3a. This change not only abolishes the ability of Brn-3b to repress basal and Brn-3a-stimulated promoter activity but also converts it to an activator of similar potency to Brn-3a. Hence a single amino acid difference determines the difference between an activator and a repressor in the Brn-3 family.

Promoter sequences for the establishment of mec-3 expression in the nematode Caenorhabditis elegans

In certain stereotyped lineages of Caenorhabditis elegans, the mec-3 gene is transcribed in neurons that are anterior daughters of cells containing the UNC-86 protein. UNC-86 binds to the mec-3 promoter and is necessary for transcription activation, but this protein is present in many cells that do not transcribe mec-3, including the posterior sister and parent cells of the mec-3 expressing neurons. To understand how the mec-3 promoter directs transcription in only a subset of cells that contain UNC-86, we have compared sequences within the promoter that are bound by UNC-86 in vitro with sequences that are necessary for early transcription of mec-3 in vivo. We find that upstream of the mec-3 start codon are two blocks of sequence that are each sufficient to generate the cellular pattern of mec-3 transcription. The proximal sequence includes three previously identified short regions that have been conserved in nematode evolution and each contains one high-affinity UNC-86 binding site. The recognition consensus sequence for UNC-86 is CATnnnT/AAAT, which is identical to the recognition sequence for the UNC-86-related mammalian transcription factor Brn-3. Adjacent to the UNC-86 recognition site is an additional sequence that is important for establishment of mec-3 expression and is presumably recognized by an unidentified transcription factor.

Activation and repression of gene expression by POU family transcription factors

The POU family transcription factors Oct-2 and Brn-3 utilize different mechanisms to produce a variety of effects on gene expression particularly in the nervous system. In the case of Oct-2, a single gene produces a primary RNA transcript. This transcript then undergoes alternative splicing to yield a variety of different mRNAs encoding Oct-2 isoforms which either activate or repress gene expression. In contrast, three distinct genes encode the closely related Brn-3 factors, Brn-3b, Brn-3b and Brn-3c. Although the proteins encoded by the Brn-3a and Brn-3c genes activate their target genes Brn-3b represses these genes and can also interfere with activation by Brn-3a or c. These finding indicate that diverse mechanisms are used to generate activating or repressing forms of POU family transcription factors.

The different activities of the two activation domains of the Brn-3a transcription factor are dependent on the context of the binding site

The POU (Pit-Oct-Unc) family transcription factor Brn-3a contains two distinct activation domains, one at the N terminus of the molecule and one at the C terminus coincident with the DNA binding domain. These different activation domains have been shown previously to differ in their ability to activate an artificial test promoter containing a Brn-3a binding site and the naturally occurring alpha-internexin gene promoter. Here we identify the target site for Brn-3a in the alpha-internexin gene promoter and show that it can confer responsiveness to Brn-3a on a heterologous promoter. One of the single-stranded DNA sequences derived from either this novel Brn-3a binding site or from the previously characterized site in the test promoter are shown to bind Brn-3a preferentially compared with the complementary single strand or the corresponding double-stranded sequence. The pattern of responsiveness of these two sequences when cloned upstream of the same test promoter and co-transfected with constructs encoding various portions of Brn-3a indicates that the activity of the two Brn-3a activation domains is dependent upon differences in the context of the target sequence in each promoter rather than on differences in the target sequence itself.

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.

Short isoform of POU factor Brn-3b can form a heterodimer with Brn-3a that is inactive for octamer motif binding

The POU proteins Brn-3a and Brn-3b belong to a family of DNA binding transcription factors that share stretches of extensive homology. Both Brn-3a and Brn-3b are expressed as shorter and longer isoforms. The long form of Brn-3a is able to oncogenically transform primary fibroblasts. By contrast, the short form of Brn-3b (Brn-3b(s)) cannot transform fibroblasts but is able to specifically inhibit the transforming activity of Brn-3a(1). Moreover, Brn-3a(1) can act as a transcriptional transactivator, while Brn-3b(s) is not only unable to do so but in addition specifically inhibits the tranactivating activity of Brn-3a(1). Here, we show that the opposite and antagonistic activities of Brn-3a(1) and Brn-3b(s) proteins are due to their different DNA binding properties; Brn-3a(1) but not Brn-3b(s) can form stable complexes with several octamer-related target DNA sequences. The presence of Brn-3b(s) completely inhibits the binding of Brn-3a(1) to DNA by preventing the formation of Brn-3a(1)-DNA complexes as well as by disrupting preformed complexes. Experiments with GST fusion proteins and in vitro binding studies suggest that the inhibition of Brn-3a(1) activity by Brn-3b(s) occurs via direct interaction of the two transcription factors in solution. Therefore, we hypothesize that Brn-3b(s) can act as a direct antagonist of Brn-3a(1) by inhibiting its DNA binding through the formation of an inactive hetero-oligomeric complex.

Silencing is golden: negative regulation in the control of neuronal gene transcription

Recent work has identified negative-acting DNA regulatory elements that function to prevent the expression of neuronal genes in non-neuronal cell types or in inappropriate neuronal subtypes. In some cases, the protein factors that interact with these silencer elements have been isolated and characterized. For example, the recently cloned silencer-binding factor NRSF/REST is a novel zinc-finger protein that interacts with silencer elements in a number of neuron-specific genes. These data suggest that negative regulation plays a major role in determining the diverse patterns of gene expression within the nervous system.

Regulation of neurite outgrowth and SNAP-25 gene expression by the Brn-3a transcription factor

SNAP-25 is a presynaptic nerve terminal protein which is also essential for the process of neurite outgrowth in vivo and in vitro. However the processes regulating its expression have not been characterized previously. We show that the gene encoding this protein, SNAP, is strongly activated by the Brn-3a POU (Pit-Oct-Unc) family transcription factor. Expression of both Brn-3a and SNAP-25 increases when ND7 neuronal cells are induced to extend neurite processes by serum removal. Inhibition of Brn-3a expression in these cells inhibits SNAP-25 expression and abolishes the neurite outgrowth that normally occurs in response to serum removal. These results identify Brn-3a as the first transcription factor having a role in process outgrowth in neuronal cells acting, at least in part, via the activation of SNAP-25 gene expression.

The neuronal nicotinic acetylcholine receptor alpha 2 subunit gene promoter is activated by the Brn-3b POU family transcription factor and not by Brn-3a or Brn-3c

The regulatory region of the neuronal nicotinic acetylcholine receptor alpha 2 subunit gene, which contains six copies of the octamer-related sequence CCCCATGCAAT, is activated by the Brn-3b POU family transcription factor but not by the closely related factors Brn-3a and Brn-3c. This effect is in contrast to the previously documented inhibitory effect of Brn-3b on octamer-containing promoters that are activated by Brn-3a and Brn-3c. Activation of the alpha 2 gene by Brn-3b requires that both the POU domain and other N-terminal sequences are derived from Brn-3b and is dependent on the intactness of the alpha 2 gene regulatory region, being lost in truncated derivatives containing one, two, or four copies of the octamer-related sequence. Surprisingly, however, these truncated derivatives are activated by Brn-3c. These effects are discussed in terms of both the influence of the target sequence and its context in the promoter on activation by the various forms of Brn-3 as well as of the processes that restrict expression of the alpha 2 subunit gene to a few cells in the nervous system.