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

Brn3a controls the soma localization and axonal extension patterns of developing spinal dorsal horn neurons

The spinal dorsal horn comprises heterogeneous neuronal populations, that interconnect with one another to form neural circuits modulating various types of sensory information. Decades of evidence has revealed that transcription factors expressed in each neuronal progenitor subclass play pivotal roles in the cell fate specification of spinal dorsal horn neurons. However, the development of subtypes of these neurons is not fully understood in more detail as yet and warrants the investigation of additional transcription factors. In the present study, we examined the involvement of the POU domain-containing transcription factor Brn3a in the development of spinal dorsal horn neurons. Analyses of Brn3a expression in the developing spinal dorsal horn neurons in mice demonstrated that the majority of the Brn3a-lineage neurons ceased Brn3a expression during embryonic stages (Brn3a-transient neurons), whereas a limited population of them continued to express Brn3a at high levels after E18.5 (Brn3a-persistent neurons). Loss of Brn3a disrupted the localization pattern of Brn3a-persistent neurons, indicating a critical role of this transcription factor in the development of these neurons. In contrast, Brn3a overexpression in Brn3a-transient neurons directed their localization in a manner similar to that in Brn3a-persistent neurons. Moreover, Brn3a-overexpressing neurons exhibited increased axonal extension to the ventral and ventrolateral funiculi, where the axonal tracts of Brn3a-persistent neurons reside. These results suggest that Brn3a controls the soma localization and axonal extension patterns of Brn3a-persistent spinal dorsal horn neurons.

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.

Sensory neuron differentiation potential of in utero mesenchymal stem cell transplantation in rat fetuses with spina bifida aperta

BACKGROUND

In previous studies, we found that the deficiency of sensory and motor neurons was a primary defect associated with the spinal malformation. Upon prenatal treatment of spina bifida through in utero stem cell transplantation in a retinoic acid-induced spina bifida rat model, we found that the mesenchymal stem cell (MSCs) survived, migrated, and differentiated into cells of a neural lineage. In the present study, we investigated whether the transplanted MSCs had the potential to differentiate into sensory neurons or to protect sensory neurons in the defective spinal cord.

METHODS

Pregnant rats treated with retinoic acid on embryonic day (E) 10, underwent fetal surgery for MSC transplantation on E16. The fetuses were harvested on E20. Immunofluorescence was used to detect the expression of Brn3a protein in the transplanted MSCs and dorsal root ganglion (DRG) neurons in the defective spinal cords. The expression of the transcription factors Brn3a and Runx1 in spinal cords was analyzed using real-time polymerase chain reaction.

RESULTS

Some of the transplanted MSCs expressed sensory neuron cell specific phenotypes. The expression of Brn3a and Runx1 was upregulated in the defective spinal cords when compared to controls. The percentage of Brn3a-positive neurons in DRG was also increased after transplantation.

CONCLUSION

Our results indicate that the transplantation of MSCs into the spinal cord could promote the transplanted MSCs and the surrounding cells to differentiate toward a sensory neuron cell fate and to play an important role in protecting sensory neurons in DRG. This approach might be of value in the treatment of sensory neuron deficiency in spina bifida aperta.

Co-expression of POU4F2/Brn-3b with p53 may be important for controlling expression of pro-apoptotic genes in cardiomyocytes following ischaemic/hypoxic insults

Cardiomyocyte death following ischaemic/hypoxic injury causes irreversible damage to cardiac function and contributes to chronic diseases such as heart failure. Understanding the mechanisms associated with myocyte loss under these conditions can help to identify strategies to minimise/abrogate such detrimental effects. The p53 protein can induce apoptosis or cell cycle arrest, but effects on cell fate depend on interactions with other regulators such as POU4F2/Brn-3b (Brn-3b), which co-operates with p53 to increase the expression of pro-apoptotic genes. In contrast, the related POU4F1/Brn-3a (Brn-3a) blocks p53-mediated apoptosis but co-operates with p53 to enhance cell cycle arrest. In this study, we showed that permanent coronary artery ligation in mouse hearts, which induced apoptotic markers, activated caspase-3 and -8 and necroptosis markers; RIP-1 and -3 also increased Brn-3b and Brn-3a expression. However, Brn-3a was only detected in uninjured myocardium but not at the site of injury, whereas Brn-3b showed generalised increase, including within the infarct zone. Conversely, p53 was detected in the infarct zone and in some cells adjacent to the site of injury but not in uninjured myocardium. Co-localisation studies showed Brn-3a co-expression with p53 in cardiomyocytes adjacent to the infarct zone, whereas Brn-3b was co-localised with p53 in the infarct zone only. Increased Brn-3b and p53 correlated with elevated expression of pro-apoptotic target genes, Bax, Noxa and PUMA, whereas cleaved caspase-3 confirmed the presence of apoptotic cells within this region of the injured heart. Similarly, simulated ischaemia/reoxygenation (sI/R) injury in neonatal rat ventricular cardiomyocytes (NRVM) and heart derived H9c2 myoblasts increased Brn-3b, p53 as well as apoptotic genes, and this was associated with enhanced apoptosis. Furthermore, targeted reduction of Brn-3b using shRNA caused reduction in pro-apoptotic Bax and Noxa proteins, even though p53 expression remained intact, suggesting that Brn-3b is important for controlling the fate of the myocardium in the injured heart.

A bovine herpesvirus 1 protein expressed in latently infected neurons (ORF2) promotes neurite sprouting in the presence of activated Notch1 or Notch3

Bovine herpesvirus 1 (BHV-1) infection induces clinical symptoms in the upper respiratory tract, inhibits immune responses, and can lead to life-threatening secondary bacterial infections. Following acute infection, BHV-1 establishes latency in sensory neurons within trigeminal ganglia, but stress can induce reactivation from latency. The latency-related (LR) RNA is the only viral transcript abundantly expressed in latently infected sensory neurons. An LR mutant virus with stop codons at the amino terminus of the first open reading frame (ORF) in the LR gene (ORF2) is not reactivated from latency, in part because it induces higher levels of apoptosis in infected neurons. ORF2 inhibits apoptosis in transiently transfected cells, suggesting that it plays a crucial role in the latency-reactivation cycle. ORF2 also interacts with Notch1 or Notch3 and inhibits its ability to trans activate certain viral promoters. Notch3 RNA and protein levels are increased during reactivation from latency, suggesting that Notch may promote reactivation. Activated Notch signaling interferes with neuronal differentiation, in part because neurite and axon generation is blocked. In this study, we demonstrated that ORF2 promotes neurite formation in mouse neuroblastoma cells overexpressing Notch1 or Notch3. ORF2 also interfered with Notch-mediated trans activation of the promoter that regulates the expression of Hairy Enhancer of Split 5, an inhibitor of neurite formation. Additional studies provided evidence that ORF2 promotes the degradation of Notch3, but not that of Notch1, in a proteasome-dependent manner. In summary, these studies suggest that ORF2 promotes a mature neuronal phenotype that enhances the survival of infected neurons and consequently increases the pool of latently infected neurons.

A simple technique for the prediction of interacting proteins reveals a direct Brn-3a-androgen receptor interaction

The formation of multiprotein complexes constitutes a key step in determining the function of any translated gene product. Thus, the elucidation of interacting partners for a protein of interest is of fundamental importance to cell biology. Here we describe a simple methodology for the prediction of novel interactors. We have applied this to the developmental transcription factor Brn-3a to predict and verify a novel interaction between Brn-3a and the androgen receptor (AR). We demonstrate that these transcription factors form complexes within the nucleus of ND7 neuroblastoma cells, while in vitro pull-down assays show direct association. As a functional consequence of the Brn-3a-AR interaction, the factors bind cooperatively to multiple elements within the promoter of the voltage-gated sodium channel, Nav1.7, leading to a synergistic increase in its expression. Thus, these data define AR as a direct Brn-3a interactor and verify a simple interacting protein prediction methodology that is likely to be useful for many other proteins.

Brn-3a deficiency transiently increases expression of calbindin D-28 k and calretinin in the trigeminal ganglion during embryonic development

Immunohistochemistry for neuron-specific nuclear protein (NeuN), caspase-3, calcitonin gene-related peptide (CGRP), and calcium-binding proteins was performed on the trigeminal ganglion (TG) in wild type and Brn-3a knockout mice at embryonic days 12.5-16.5 (E12.5-E16.5). In Brn-3a knockout mice, the number of NeuN-immunoreactive (ir) neuron profiles increased at E14.5 (40.0% increase) and decreased at E16.5 (28.3% reduction) compared to wild type mice. Caspase-3-ir neuron profiles were abundant in the TG of wild type mice at E12.5-E16.5. However, the loss of Brn-3a decreased the number of caspase-3-ir neuron profiles at E12.5 (69.7% reduction) and E14.5 (51.7% reduction). At E16.5, the distribution of caspase-3-ir neuron profiles was barely affected by the deficiency. CGRP-ir neuron profiles were observed in the TG of wild type mice but not knockout mice at E12.5. At E14.5 and E16.5, CGRP-ir neuron profiles were abundant in both wild type and knockout mice. Calbindin D-28 k (CB)-ir neuron profiles decreased in the TG of mutant mice at E12.5 compared to wild type mice (56.4% reduction). At E14.5, however, Brn-3a deficiency transiently increased CB-ir neuron profiles (169.4% increase as compared to wild type mice). Calretinin (CR)-ir neuron profiles could not be detected in the TG of wild type mice at E12.5-16.5. However, numerous CR-ir neuron profiles transiently appeared in the knockout mouse at E14.5. Parvalbumin (PV)-ir neurons appeared in wild type and knockout mice at E14.5. At this stage, the number of large (>50 mum(2)) PV-ir neuron profiles in knockout mice was fewer than that in wild type mice. The number and cell size of PV-ir neuron profiles were barely affected by the deficiency at E16.5. The present study indicates that the loss of Brn-3a causes increase of TG neurons at E14.5 and decrease of TG neurons at E16.5. It is also suggested that Brn-3a deficiency affects the number and cell size of CGRP- and calcium-binding protein-containing neurons at E12.5 and E14.5. Caspase-3-dependent cell death of CB- and CR-ir neurons may be suppressed by the deficiency at E14.5.

Novel Brn3a cis-acting sequences mediate transcription of human trkA in neurons

TrkA, the receptor tropomyosin-related kinase for nerve growth factor, is critical not only for the correct spatial and temporal development of sensory neurons during embryogenesis but also for the survival of sensory neurons, the differentiation and apoptosis of neuronal tumors and suppression of latent herpes simplex virus genomes. While the regulation of the expression of trkA is a complex process, the transcription factor Brn3a is known to play an important role as an enhancer of trkA transcription during development in the mouse. Despite considerable information on the regulation of trkA during embryogenesis, the mechanisms by which the expression of trkA is regulated in differentiated neurons, or the factors that influence its expression in tumor cells, have not been identified. We initiated studies to determine whether Brn3a/trkA promoter interactions may be important in a model of differentiated neurons and in medulloblastoma cells. We constructed a plasmid that contains 1043 base pairs of genomic sequences that extend to 30 nucleotides upstream of trkA coding region. In contrast to previous data, a short 190 bp region that lies proximal to the trkA initiation codon was sufficient for Brn3a responsiveness in Vero cells. This region was also sufficient for Brn3a trans-activation in nerve growth factor-differentiated PC12 cells. At least two portions of the 190 bp fragment bind to Brn3a with an affinity high enough to be detected in electromobility shift assays. In addition, Brn3a increased levels of endogenous trkA transcripts in PC12 cells and initiated trkA expression in medulloblastoma cells, which normally do not express trkA.

Brn-3b enhances the pro-apoptotic effects of p53 but not its induction of cell cycle arrest by cooperating in trans-activation of bax expression

The Brn-3a and Brn-3b transcription factor have opposite and antagonistic effects in neuroblastoma cells since Brn-3a is associated with differentiation whilst Brn-3b enhances proliferation in these cells. In this study, we demonstrate that like Brn-3a, Brn-3b physically interacts with p53. However, whereas Brn-3a repressed p53 mediated Bax expression but cooperated with p53 to increase p21^cip1/waf1, this study demonstrated that co-expression of Brn-3b with p53 increases trans-activation of Bax promoter but not p21^cip1/waf1. Consequently co-expression of Brn-3b with p53 resulted in enhanced apoptosis, which is in contrast to the increased survival and differentiation, when Brn-3a is co-expressed with p53. For Brn-3b to cooperate with p53 on the Bax promoter, it requires binding sites that flank p53 sites on this promoter. Furthermore, neurons from Brn-3b knock-out (KO) mice were resistant to apoptosis and this correlated with reduced Bax expression upon induction of p53 in neurons lacking Brn-3b compared with controls. Thus, the ability of Brn-3b to interact with p53 and modulate Bax expression may demonstrate an important mechanism that helps to determine the fate of cells when p53 is induced.

Neural expression of alpha-internexin promoter in vitro and in vivo

alpha-Internexin is a 66 kDa neuronal intermediate filament protein found most abundantly in the neurons of the nervous systems during early development. To characterize the function of mouse alpha-internexin promoter, we designed two different expression constructs driven by 0.7 kb or 1.3 kb of mouse alpha-internexin 5'-flanking sequences; one was the enhanced green fluorescent protein (EGFP) reporter for monitoring specific expression in vitro, and the other was the cre for studying the functional DNA recombinase in transgenic mice. After introducing DNA constructs into non-neuronal 3T3 fibroblasts and a neuronal Neuro2A cell line by lipofectamine transfection, we observed that the expression of EGFP with 1.3 kb mouse alpha-internexin promoter was in a neuron-dominant manner. To establish a tissue-specific pattern in the nervous system, we generated a transgenic mouse line expressing Cre DNA recombinase under the control of 1.3 kb alpha-Internexin promoter. The activity of the Cre recombinase at postnatal day 1 was examined by mating the cre transgenic mice to ROSA26 reporter (R26R) mice with knock-in Cre-mediated recombination. Analyses of postnatal day 1 (P1) newborns showed that beta-galactosidase activity was detected in the peripheral nervous system (PNS), such as cranial nerves innervating the tongue and the skin as well as spinal nerves to the body trunk. Furthermore, X-gal-labeled dorsal root ganglionic (DRG) neurons showed positive for alpha-Internexin in cell bodies but negative in their spinal nerves. The motor neurons in the spinal cord did not exhibit any beta-galactosidase activity. Therefore, the cre transgene driven by mouse alpha-internexin promoter, described here, provides a useful animal model to specifically manipulate genes in the developing nervous system.

Effects of Brn-3a protein and RNA expression in rat brain following low-level lead exposure during development on spatial learning and memory

The developing nervous system is preferentially vulnerable to lead exposure with alterations in neuronal and glial cells of the brain. Chronic exposure to lead (Pb2+) causes deficits of learning and memory in children and spatial learning deficits in developing rats. Brn-3a is a member of the Pit-Oct-Unc (POU) family of transcription factors that is expressed predominantly in neuronal cells. It exists in two forms, with the long form containing 84 amino acids at the N-terminus that are lacking in the short form. The N-terminal domain unique to the long form induces expression of the Bcl-2 gene and protects neuronal cells against apoptosis whereas the C-terminal POU domain common to both forms is sufficient for activating a number of other neuronally expressed genes and stimulating neuronal process outgrowth. We examined Brn-3a protein and RNA expression in rat brain following low-level lead exposure during development and subsequent effects on spatial learning and memory. Two groups of rats were investigated: a control group and a lead-exposed group (0.2% lead acetate in the drinking water of the dam from gestational day 15 to postnatal day 21). Levels of Brn-3a were measured in rat cortex, hippocampus and cerebellum by immunohistochemistry and in situ hybridization, both protein and mRNA levels were reduced in lead-exposed group (p < 0.05). In Morris water maze, we found spatial learning deficits in rats of lead-exposed group (p < 0.05). These data suggest that the alteration of Brn-3a may play a key role in the mechanisms underlying lead neurotoxicity.

Phosphorylation of the Brn-3a transcription factor is modulated during differentiation and regulates its functional activity

Brn-3a is a transcription factor expressed in a subset of neurons of the peripheral nervous system. Its role encompasses the activation of genes involved in neuronal differentiation and survival. While a lot of data have been produced on Brn-3a target promoters, very little is known about the upstream regulatory signals that mediate its activation in response to differentiation. In this work, we describe for the first time that Brn-3a is phosphorylated in IMR-32 neuroblastoma cells in response to differentiation induced by retinoic acid treatment and that its post-translational modification is potentially mediated by the activation of the MAPK/ERK pathway. Furthermore, we show that the mutation of a putative phosphorylated amino acid strongly reduces the ability of Brn-3a to mediate the differentiation of IMR-32 cells.

Transcriptional regulation by Barhl1 and Brn-3c in organ of corti derived cell lines

Barhl1 and Brn-3c have been identified as transcription factors that are essential for survival and maintenance of hair cells of the inner ear. Little is known about the mechanism of how Brn-3c or Barhl1 may regulate transcription in the inner ear. In this study, the transcriptional function of both Brn-3c and Barhl1 was investigated in the organ-of-Corti-derived cell lines, OC-1 and OC-2. We examined regulatory domains in these transcription factors by linking regions of Barhl1 and Brn-3c to the DNA binding domain of the heterologous transcription factor GAL4 and assayed their effect on a heterologous promoter containing GAL4 DNA binding sites by co-transfection into OC-1 and OC-2 cell lines. Brn-3c was found to contain an independent N-terminal activation domain that is sufficient to activate gene transcription in the organ of corti derived cell lines. Barhl1 on the other hand was found to act as a transcriptional repressor with repressive activity not restricted to a particular domain of Barhl1. In addition, we analyzed the effect of Barhl1 on the promoters of the neurotrophin genes NT-3 and BDNF in OC-1 and OC-2 cell lines. However, Barhl1 was not found to directly regulate neurotrophin promoter constructs in these cells.

Brn-3a is required for the generation of proprioceptors in the mesencephalic trigeminal tract nucleus

The distribution of motor and proprioceptive neurons was investigated in the trigeminal nervous system of wild-type and Brn-3a knockout mice at embryonic day 18.5 and postnatal day 0. We found that the trigeminal motor nucleus (Mo5) contained abundant motoneurons in wild-type (mean number +/- SD per section = 128 +/- 22, range = 93-167) and knockout (mean number +/- SD per section = 121 +/- 23, range = 75-158) mice and that the cell size of Mo5 neurons was similar between these mice (wild-type, mean +/- SD = 165 +/- 59 microm2, range = 65-326 microm2; knockout, mean +/- SD = 167 +/- 59 microm2, range = 71-327 microm2). Mo5 neurons were immunoreactive for calcitonin gene-related peptide and such immunoreactive neurons were abundant in both wild-type and mutant mice. In the mesencephalic tract nucleus (Mes5) of wild-type mice, many proprioceptors (mean number +/- SD per section = 56 +/- 19, range = 27-85) that contained parvalbumin immunoreactivity were also observed. In knockout mice, however, Mes5 neurons could not be detected. The area of brainstems which normally contained the Mes5 was devoid of parvalbumin-immunoreactive proprioceptors. The present study suggests that Brn-3a is required for the development of proprioceptors but not motoneurons in the trigeminal nervous system.

Axotomy-induced early down-regulation of POU-IV class transcription factors Brn-3a and Brn-3b in retinal ganglion cells

It has been proposed that neurons being exposed to proapoptotic stimuli undergo dedifferentiation, a process that can either allow for regeneration and axon regrowth or, if remaining incomplete, can force the cell to activate apoptotic pathways. A pivotal step in the differentiation program from neuronal precursor cells to differentiated, postmitotic neurons is their exit from the cell cycle. The POU domain transcription factors Brn-3b and Brn-3a, which are expressed in retinal ganglion cells (RGCs) directly after the exit of RGC precursors from the cell cycle, can be employed as RGC-specific differentiation markers to study potential dedifferentiation of RGCs after axotomy. Here, we examined mRNAand protein expression of Brn-3a and -3b in rat RGCs following axonal lesion. We observed a rapid down-regulation of Brn-3a and -3b protein expression in axotomized RGCs, clearly preceding apoptosis of RGCs. Using real-time PCR, we show that regulation of Brn-3 expression occurred at the transcriptional level. The small subset of RGCs regenerating into a peripheral nerve graft did not (re-)express Brn-3a or -b. In conclusion, we found further evidence supporting the hypothesis of a dedifferentiation process in severed mature neurons. As Brn-3b expression has been shown to be a prerequisite for developmental survival of most RGCs and Brn-3a activates transcription of anti-apoptotic genes, down-regulation of Brn-3 transcription factors might be causally involved in the secondary death of adult RGCs following axotomy.

Brn-3a deficiency increases tyrosine hydroxylase-immunoreactive neurons in the dorsal root ganglion

Immunohistochemistry for tyrosine hydroxylase (TH) was performed on the dorsal root ganglia (DRG) in wild-type, heterozygous and Brn-3a knockout mice at embryonic day 18.5. TH-immunoreactive (-IR) neurons were detected in the DRG of wild-type and heterozygous mice, but their proportion was greatly increased by the loss of Brn-3a function (wild-type and heterozygot, 8.4%; knockout, 20.9%). IR neurons were of various sizes in wild-type (mean+/-S.D.=118.1+/-55.4 microm2, range=26.6-306.3 microm2) and heterozygous mice. In the knockout mice, however, TH-IR neurons were mostly small (mean+/-S.D.=68.2+/-34.3 microm2, range=11.8-166.8 microm2). The present study suggests that Brn-3a may normally suppress TH expression in many small DRG neurons but activate TH expression in large DRG neurons.

Effect of Brn-3a deficiency on primary nociceptors in the trigeminal ganglion

Immunohistochemistry for substance P, somatostatin and vanilloid receptor subtype 1 as well as receptors for somatostatin and opioids was performed on the trigeminal ganglion in wild-type and Brn-3a knockout mice at postnatal day 0. In wild-type mice, the trigeminal ganglion contained abundant substance P-, vanilloid receptor subtype 1-, sst2A receptor- and delta-opioid receptor-immunoreactive neurons, while the ganglion had only a few mu-opioid receptor-immunoreactive neurons. The Brn-3a deficiency had an effect on the cell size but not the number of substance P-immunoreactive neurons. In knockout mice, the proportion of small immunoreactive neurons markedly increased and that of medium- to large-sized immunoreactive ones correspondingly decreased (mean +/- S.D. = 54.7 +/- 29.1 microm2, range = 10.9-220.8 microm2) compared to wild-type mice (mean +/- S.D. = 116.6 +/- 58.6 microm2, range = 27.3-400.7 microm2). As for vanilloid receptor subtype 1-immunoreactive neurons, the number and cell size was barely affected by the deficiency. On the other hand, the loss of Brn-3a caused a decrease in the number of sst2A receptor- or delta-opioid receptor-immunoreactive neurons (more than 95% reduction) and an increase in the number of mu-opioid receptor-immunoreactive neurons (9.3-fold increase). Somatostatin-immunoreactive neurons were not detected in the trigeminal ganglion of wild-type or mutant mice at postnatal day 0. The present study suggests that Brn-3a deficiency may have effects on the survival of trigeminal nociceptors and their expression of some neurochemical substances.

Brn-3a transcription factor blocks p53-mediated activation of proapoptotic target genes Noxa and Bax in vitro and in vivo to determine cell fate

The Brn-3a POU transcription factor is associated with survival and the differentiation of sensory neuronal cells during development. Brn-3a mediates its effects either by the direct regulation of target genes or indirectly upon interaction with proteins such as p53. Brn-3a differentially regulates p53-mediated gene expression and modifies its effect on cell fate. Here we show that, like Bax, Brn-3a antagonizes p53-mediated transcription of another proapoptotic target, Noxa, significantly reducing transactivation of the Noxa promoter by p53. This effect requires the p53 binding site, and electrophoretic mobility shift assay studies suggest that Brn-3a is associated with p53 when it is bound to its site in the Noxa promoter. The wild type but not the mutant promoter can be immunoprecipitated with Brn-3a in chromatin immunoprecipitation assays. Thus, Brn-3a may act by preventing the recruitment of cofactors required for p53 to transactivate this promoter. The co-expression of Brn-3a and p53 results in decreased endogenous Noxa protein in the neuronal cell line, ND7, suggesting a direct functional effect of this interaction. Moreover, there is a significant elevation of both proapoptotic Bax and Noxa proteins in sensory neuronal tissue taken from Brn-3a-/- embryos during development, compared with wild type controls. Striking changes occurred at embryonic day 14.5, a time that precedes a significant loss of specific neurons in the mutant embryos, but not at embryonic day 16.5 when Brn-3a-expressing cells are already lost by apoptosis. Therefore, the lack of antagonism by Brn-3a on activation of proapoptotic p53 target genes may contribute to the increased apoptosis seen in the Brn-3a-/- embryos. These results support a crucial role for Brn-3a in determining the pathway taken by p53 when co-expressed during development and thus in controlling the fate of these cells.

Islet-1 expression in the developing chicken inner ear

The cell types of the inner ear originate from the otic placode, a thickened layer of ectoderm adjacent to the developing hindbrain. The placode invaginates and forms the otic pit, which pinches off as a small vesicle called the otocyst. Presumptive cochleovestibular neurons delaminate from the anterior ventral part of the otocyst and form the cochleovestibular ganglion of the inner ear. Here we show that the LIM/homeodomain protein islet-1 is expressed in cells of the ventral part of the otic placode and that this ventral expression is maintained at the otic pit and the otocyst stages. Auditory and vestibular neurons originate from this islet-1-positive zone of the otocyst, and these neurons maintain islet-1 expression until adulthood. We also demonstrate that islet-1 becomes up-regulated in the presumptive sensory epithelia of the inner ear in regions that are defined by the expression domains of BMP4. The up-regulation of islet-1 in developing inner ear hair and supporting cells is accompanied by down-regulation of Pax-2 in these cell types. Islet-1 expression in hair and supporting cells persists until early postnatal stages, when the transcriptional regulator is down-regulated in hair cells. Our data is consistent with a role for islet-1 in differentiating inner ear neurons and sensory epithelia cells, perhaps in the specification of cellular subtypes in conjunction with other LIM/homeodomain proteins.

Distinct domains of Brn-3a regulate apoptosis and neurite outgrowth in vivo

The Brn-3a transcription factor is critical for the normal development of the nervous system, promoting both neuronal survival and neurite outgrowth. By manipulating the Brn-3a gene in intact mice, we show that these two functions are separately controlled with an N-terminal domain being essential for neuronal survival, whereas the POU domain is essential for neurite outgrowth. Hence the two naturally occurring forms of Brn-3a, which either contain or lack the N-terminal domain, are likely to play distinct roles in the nervous system.

Coordinated regulation of gene expression by Brn3a in developing sensory ganglia

Mice lacking the POU-domain transcription factor Brn3a exhibit marked defects in sensory axon growth and abnormal sensory apoptosis. We have determined the regulatory targets of Brn3a in the developing trigeminal ganglion using microarray analysis of Brn3a mutant mice. These results show that Brn3 mediates the coordinated expression of neurotransmitter systems, ion channels, structural components of axons and inter- and intracellular signaling systems. Loss of Brn3a also results in the ectopic expression of transcription factors normally detected in earlier developmental stages and in other areas of the nervous system. Target gene expression is normal in heterozygous mice, consistent with prior work showing that autoregulation by Brn3a results in gene dosage compensation. Detailed examination of the expression of several of these downstream genes reveals that the regulatory role of Brn3a in the trigeminal ganglion appears to be conserved in more posterior sensory ganglia but not in the CNS neurons that express this factor.

Effect of Brn-3a deficiency on parvalbumin-immunoreactive primary sensory neurons in the dorsal root ganglion

Immunohistochemistry for parvalbumin, a marker for primary proprioceptors, was performed on the dorsal root ganglion (DRG) of wildtype and knockout mice for Brn-3a at postnatal day 0 and embryonic day 18.5. The DRG contained many parvalbumin-immunoreactive (ir) neurons in wildtype (5.4%) and knockout mice (5.6%). Cell size analysis demonstrated that such neurons were mostly medium-sized to large in these mice. Therefore, it is unlikely that the survival of proprioceptors is dependent upon Brn-3a in the DRG. In the dorsal column and gray matter of the spinal cord of knockout mice, however, parvalbumin-ir nerve fibers were sparse compared to wildtype mice. The number of parvalbumin-ir varicosities around motoneurons decreased in the mutant. Thus, our data suggest that Brn-3a may play an important role in the central projection and terminal formation of DRG proprioceptors in the spinal cord.

The Brn-3b transcription factor regulates the growth, behavior, and invasiveness of human neuroblastoma cells in vitro and in vivo

Neuroblastomas are the second most common solid tumor in children but the molecular mechanisms underlying the initiation and progression of this disease are poorly understood. We previously showed that the Brn-3b transcription factor is highly expressed in actively proliferating neuroblastoma cells but is significantly decreased when these cells are induced to differentiate. In this study, we analyzed the effects of manipulating Brn-3b levels in the human neuroblastoma cell line, IMR-32 and showed that constitutive overexpression of Brn-3b consistently increased cellular growth and proliferation in monolayer as well as in an anchorage-independent manner compared with controls whereas stably decreasing Brn-3b can reduce the rate of growth of these cells. Cells with high Brn-3b also fail to respond to growth inhibitory retinoic acid, as they continue to proliferate. Moreover, Brn-3b levels significantly modified tumor growth in vivo with elevated Brn-3b resulting in faster tumor growth in xenograft models whereas decreasing Brn-3b resulted in slower growth compared with controls. Interestingly, elevated Brn-3b levels also enhances the invasive capacity of these neuroblastoma cells with significantly larger numbers of migrating cells observed in overexpressing clones compared with controls. Because invasion and metastasis influence morbidity and mortality in neuroblastoma and so significantly affect the course and outcome of neuroblastomas, this finding is very important. Our results therefore suggest that Brn-3b transcription factor contributes to proliferation of neuroblastoma cells in vivo and in vitro but may also influence progression and/or invasion during tumorigenesis. It is possible that decreasing Brn-3b levels may reverse some effects on growth and proliferation of these cells.

Ca2+ and CaM kinase regulate neurofilament expression

Cultured cerebellar granule neurons were used to examine the hypothesis that NMDA or KCl-dependent Ca2+ influx activates a signaling cascade mediating transcription, translation and assembly of neurofilaments. The data indicate that 25 mM KCl and 140 microM NMDA induce neurofilament protein, mRNA, and assembly in neurites, and these require Ca2+ entry through voltage-sensitive Ca2+ channels or NMDA receptors, respectively. The CaM kinase and transcriptional inhibitors, KN-62 and actinomycin D, respectively, attenuate the stimulatory effect of Ca2+. Granular neurons are a valuable model to explore the molecular basis for transcriptional regulation of neurofilament gene expression by neuronal activity.

Brn3a regulation of TrkA/NGF receptor expression in developing sensory neurons

The TrkA/NGF receptor is essential for the survival and differentiation of sensory neurons. The molecular mechanisms regulating tissue and stage-specific expression of TrkA are largely unknown. The Brn3a POU-domain transcription factor has been implicated in the development of the PNS and proposed as a transcription regulator for TrkA. The molecular mechanisms underlying the regulation of TrkA by Brn3a is unclear. In this study, we provide genetic, transgenic and biochemical evidence that Brn3a binds to novel, specific sites in the 457 bp enhancer that regulates TrkA expression in embryonic sensory neurons. We employ Bax-knockout mice, in which sensory neurons no longer require neurotrophins for survival, to uncouple TrkA-dependent cell death from downregulation of TrkA expression. In addition, when mutagenized, the novel Brn3a-binding sites identified fail to drive appropriate reporter transgene expression in sensory neurons. Thus, TrkA, a gene that is crucial for the differentiation and survival of sensory nociceptive neurons, requires Brn3a to maintain normal transcriptional activity.

Neurofilaments in diabetic neuropathy

This review discusses the role of abnormal neurofilament (NF) expression, processing, and structure as an etiological factor in diabetic neuropathy. Diabetic sensory and autonomic neuropathy in humans is associated with a spectrum of structural changes in peripheral nerve that includes axonal degeneration, paranodal demyelination, and loss of myelinated fibers-- the latter is probably the result of a dying-back of distal axons. NF filaments are composed of three subunit proteins, NFL, NFM, and NFH, and are major constituents of the axonal cylinder. It is clear that any abnormality in synthesis, delivery, or processing of these critical proteins could lead to severe impairments in axon structure and function. This article describes mechanisms of synthesis, phosphorylation, and delivery of NF and discusses how these processes may be abnormal in diabetics. The pathological alterations in the ganglion and preipheral nerve that occur in sensory and autonomic neuropath will be outlined and related to possible abnormal processing of NF. A major focus is the role or aberrant NF phosphorylation and its possible involvement in the imparied delivery of NF to the distal axon. Identification of stress-activated protein kinases (SAPKs) as NF kinases is discussed in detail and it is proposed that hyperglycemia-induced activation of SAPKs may be a primary etiological event in diabetic neuropathy.

Effect of Brn-3a deficiency on parvalbumin-, calbindin D-28k-, calretinin- and calcitonin gene-related peptide-immunoreactive primary sensory neurons in the trigeminal ganglion

Immunohistochemistry for parvalbumin, calbindin D-28k, calretinin and calcitonin gene-related peptide (CGRP) was performed on the trigeminal ganglion and oro-facial tissues in Brn-3a wildtype and knockout mice at embryonic day 18.5 and postnatal day 0. In wildtype mice, the trigeminal ganglion contained abundant parvalbumin-, calbindin D-28k- and CGRP-immunoreactive neurons while the ganglion was almost devoid of calretinin-immunoreactive neurons. In Brn-3a knockout mice, a 63% decrease of parvalbumin-immunoreactive neurons was detected. In contrast, the absence of Brn-3a dramatically increased the number of calbindin D-28k-immunoreactive (3.5-fold increase) and calretinin-immunoreactive neurons (91-fold increase). The number of CGRP-immunoreactive neurons, however, was not altered by the Brn-3a deficiency. Cell size analysis indicated that loss of Brn-3a increased the proportions of small (<100 microm (2)) parvalbumin-, calbindin D-28k- and CGRP-immunoreactive neurons while it decreased those of large (>200 microm(2)) immunoreactive cells. Calretinin-immunoreactive neurons were either small or medium (100-200 microm (2)) in mutant mice. The oro-facial tissues contained parvalbumin-, calbindin D-28k- and CGRP-immunoreactive fibers, but not calretinin-immunoreactive ones in wildtype mice. In Brn-3a knockout mice, the number of parvalbumin-immunoreactive fibers markedly decreased in the infraorbital nerve and parvalbumin-immunoreactive endings disappeared in the vibrissa. In contrast, the number of calbindin D-28k-immunoreactive fibers increased significantly in the infraorbital and mental nerves. In addition, calbindin D-28k-immunoreactive endings appeared in the vibrissa. As well, some fibers showed calretinin-immunoreactivity in the infraorbital nerve of the mutant. However, no obvious change of CGRP-immunoreactive fibers was observed in the oro-facial region of knockout mice. Taken together, our data suggest that Brn-3a deficiency has effects on the expression of neurochemical substances in the trigeminal ganglion.

Distinct promoter elements mediate the co-operative effect of Brn-3a and p53 on the p21 promoter and their antagonism on the Bax promoter

Although the promoters of both the Bax and p21 genes are activated by p53, they differ in the effect on this activation of the POU family transcription factor Brn-3a. Thus, Brn-3a inhibits activation of the Bax promoter by p53 but enhances the ability of p53 to activate the p21 promoter. We demonstrate that repression of p53-mediated activation of the Bax promoter involves a complex upstream sequence in which two Brn-3a response elements flank the p53 response element. In contrast, a minimal p21 promoter is activated by Brn-3a and such activation cannot be abolished without abolishing basal promoter activity. Moreover, synergistic activation by Brn-3a and p53 continues to be observed when the p53-binding sites in the p21 promoter are substituted by the Bax p53 site or by the region of the Bax promoter essential for Brn-3a-mediated repression, indicating that the p21 core promoter plays a central role in this response. The significance of these effects is discussed in terms of the different responses of the Bax and p21 promoters and the overlapping but distinct roles of Brn-3a and p53 in neuronal growth arrest and apoptosis.

The Brn-3a transcription factor inhibits the pro-apoptotic effect of p53 and enhances cell cycle arrest by differentially regulating the activity of the p53 target genes encoding Bax and p21(CIP1/Waf1)

We have previously shown that the anti-apoptotic transcription factor, Brn-3a and the pro-apoptotic p53 factor have antagonistic effects on the promoter of the gene encoding the anti-apoptotic Bcl-2 protein, with p53 abolishing activation by Brn-3a. Here we demonstrate that this antagonism is also observed on the gene encoding the pro-apoptotic Bax protein with Brn-3a abolishing the ability of p53 to activate the Bax promoter and induce Bax protein expression. In contrast, Brn-3a and p53 co-operative to induce maximal activation of another p53 target gene encoding the cyclin dependent kinase inhibitor, p21(CIP1/Waf1). These differential effects of Brn-3a on p53-inducible genes involved in apoptosis or growth arrest are paralleled by its effects on these processes themselves. Thus, we show that Brn-3a antagonises the anti-apoptotic effect of p53 but co-operates with p53 to induce cell cycle arrest. The potential role of Brn-3a in determining the outcome of enhanced p53 levels is discussed.

Effect of Brn-3a deficiency on nociceptors and low-threshold mechanoreceptors in the trigeminal ganglion

Immunohistochemistry for protein gene product 9.5 (PGP 9.5, a neuron specific protein) and vanilloid receptor 1-like receptor (VRL-1, a marker for medium-sized to large primary nociceptors) were used to assess the effects of Brn-3a deficiency on neuronal innervation of oral tissues and neurons of the trigeminal ganglion (TG). In the knockout mouse, the number of PGP 9.5-immunoreactive (-ir) nerve fibers decreased in the facial cutaneous and oral mucous epithelia, as well as the incisor and molar tooth germs. The reduction of PGP 9.5-ir Merkel endings was also observed in some vibrissae. No obvious change was detected in other tissues. Cell size analysis demonstrated that the proportion of small neurons markedly increased while that of medium-sized and large neurons significantly decreased in the TG of the mutant. Moreover, Brn-3a deficiency caused the disappearance of TG neurons which were immunoreactive for VRL-1. Together, our data suggest that nociceptors and low-threshold mechanoreceptors with medium-sized to large cell bodies may be sensitive to the loss of Brn-3a.

The C. elegans POU-domain transcription factor UNC-86 regulates the tph-1 tryptophan hydroxylase gene and neurite outgrowth in specific serotonergic neurons

A fundamental question in developmental neurobiology is how a common neurotransmitter is specified in different neuronal types?. We describe cell-specific regulation of the serotonergic phenotype by the C. elegans POU-transcription factor UNC-86. We show that unc-86 regulates particular aspects of the terminal neuronal identity in four classes of serotonergic neurons, but that the development of the ADF serotonergic neurons is regulated by an UNC-86-independent program. In the NSM neurons, the role of unc-86 is confined in late differentiation; the neurons are generated but do not express genes necessary for serotonergic neurotransmission. unc-86-null mutations affect the expression in NSM of tph-1, which encodes the serotonin synthetic enzyme tryptophan hydroxylase, and cat-1, which encodes a vesicular transporter that loads serotonin into synaptic vesicles, suggesting that unc-86 coordinately regulates serotonin synthesis and packaging. However, unc-86-null mutations do not impair the ability of NSM to reuptake serotonin released from the ADF serotonergic chemosensory neurons and this serotonin reuptake is sensitive to the serotonin reuptake block drugs imipramine and fluoxetine, demonstrating that serotonin synthesis and reuptake is regulated by distinct factors. The NSM neurons in unc-86-null mutants also display abnormal neurite outgrowth, suggesting a role of unc-86 in regulating this process as well.

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.

Effect of Brn-3a deficiency on CGRP-immunoreactivity in the dorsal root ganglion

Immunohistochemistry for calcitonin gene-related peptide (CGRP) was performed on the dorsal root ganglion (DRG) and spinal cord in wildtype and knockout mice for Brn-3a. CGRP-immunoreactive (-IR) neurons were abundant in the DRG of wildtype, heterozygous and knockout mice. Cell size analysis revealed that CGRP-IR neurons were of various sizes in wildtype and heterozygous mice. In the knockout mice, however, most of CGRP-IR neurons were small. In the spinal cord of knockout mice, the number of CGRP-IR fibers increased in the dorsal column but decreased in the deep part of the dorsal horn. The loss of Brn-3a may have different effects on CGRP-IR expression in small and large DRG neurons.

A minimal Bcl-x promoter is activated by Brn-3a and repressed by p53

The Brn-3a transcription factor stimulates the expression of the anti-apoptotic Bcl-2 and Bcl-x proteins and protects neuronal cells from apoptosis. Here we show that a minimal Bcl-x promoter is activated by Brn-3a and that this stimulation is prevented by the pro-apoptotic p53 protein. Both these effects are mediated via Bcl-x promoter sequences, which are indistinguishable from those required for minimal basal promoter activity. A newly described upstream Bcl-x promoter is also activated by Brn-3a with this activation being prevented by p53. Hence, Brn-3a-mediated activation of two distinct Bcl-x promoters and of the Bcl-2 promoter is blocked by p53 whereas this is not observed for Brn-3a activated promoters derived from genes not involved in inhibiting apoptosis. p53 therefore appears to specifically target the activation by Brn-3a of promoters derived from genes with an anti-apoptotic effect and this may be involved in the pro-apoptotic activity of p53.

The BRN-3A transcription factor protects sensory but not sympathetic neurons from programmed cell death/apoptosis

Inactivation of the gene encoding the POU domain transcription factor BRN-3A results in the absence of specific neurons in knockout mice. Here we demonstrate for the first time a direct effect of BRN-3A on the survival of neuronal cells. Specifically, overexpression of BRN-3A in cultured trigeminal ganglion or dorsal root ganglion sensory neurons enhanced their survival following the withdrawal of nerve growth factor. Moreover, reduction of BRN-3A levels impaired the survival of these neurons. The survival of sympathetic neurons was not affected by either approach. Similarly, overexpression of BRN-3A activated the endogenous Bcl-2 gene in trigeminal neurons, but not in sympathetic neurons. The protective effect of BRN-3A on trigeminal neuron survival following nerve growth factor withdrawal significantly increased during embryonic development. In contrast, overexpression of the related factor BRN-3B enhanced survival of trigeminal neurons only at an early stage of embryonic development. Thus, BRN-3A (and in some circumstances, BRN-3B) can promote the survival of nerve growth factor-dependent sensory but not sympathetic neurons, allowing it to play a direct role in the survival of some (but not all) neuronal populations in the developing and adult nervous systems.

Structure, biological activity of the upstream regulatory sequence, and conserved domains of a middle molecular mass neurofilament gene of Xenopus laevis

During development, the molecular compositions of neurofilaments (NFs) undergo progressive modifications that correlate with successive stages of axonal outgrowth. Because NFs are the most abundant component of the axonal cytoskeleton, understanding how these modifications are regulated is essential for knowing how axons control their structural properties during growth. In vertebrates ranging from lamprey to mammal, orthologs of the middle molecular mass NF protein (NF-M) share similar patterns of expression during axonal outgrowth, which suggests that these NF-M genes may share conserved regulatory elements. These elements might be identified by comparing the sequences and activities of regulatory domains among the vertebrate NF-M genes. The frog, Xenopus laevis, is a good choice for such studies, because its early neural development can be observed readily and because transgenic embryos can be made easily. To begin such studies, we isolated genomic clones of Xenopus NF-M(2), tested the activity of its upstream regulatory sequence (URS) in transgenic embryos, and then compared sequences of regulatory regions among vertebrate NF-M genes to search for conserved elements. Studies with reporter genes in transgenic embryos found that the 1. 5 kb URS lacked the elements sufficient for neuron-specific gene expression but identified conserved regions with basal regulatory activity. These studies further demonstrated that the NF-M 1.5 kb URS was highly susceptible to positional effects, a property that may be relevant to the highly variant, tissue-specific expression that is seen among members of the intermediate filament gene family. Non-coding regions of vertebrate NF-M genes contained several conserved elements. The region of highest conservation fell within the 3' untranslated region, a region that has been shown to regulate expression of another NF gene, NF-L. Transgenic Xenopus may thus prove useful for testing further the activity of conserved elements during axonal development and regeneration.

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.

Lineages and transcription factors in the specification of vertebrate primary sensory neurons

Recent advances have indentified some of the key transcriptional regulators of mammalian genes, the neurogenins. Neurogenins 1 and 2 appear to control distinct sublineages for different classes of sensory neurons, including a 'pioneer' lineage for proprioceptors specified early in neural crest migration. Neurogenins act via a cascade of downstream transcriptional regulators, some of which have been identified.

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.

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.

The Brn-3a transcription factor

Brn-3a is a member of the POU family of transcription factors which is expressed predominantly in neuronal cells. It exists in two forms, with the long form containing 84 amino acids at the N-terminus which are lacking in the short form. The N-terminal domain unique to the long form allows it to induce expression of the Bcl-2 gene and protect neuronal cells from apoptosis whereas the C-terminal POU domain common to both forms is sufficient for activating a number of other neuronally expressed genes and stimulating neuronal process outgrowth. These effects of Brn-3a suggest that manipulation of its expression by pharmacological or gene therapy procedures may be of benefit in human neurological diseases.

The N-terminal domain unique to the long form of the Brn-3a transcription factor is essential to protect neuronal cells from apoptosis and for the activation of Bbcl-2 gene expression

The ability of the POU family transcription factor Brn-3a to stimulate neurite outgrowth and the expression of the genes encoding neuronal proteins such as the neurofilaments and SNAP-25 has previously been shown to be dependent upon the C-terminal POU domain which can mediate both DNA binding and transcriptional activation. We show here, however, that the ability of Brn-3a to activate Bcl-2 expression and protect neuronal cells from apoptosis (programmed cell death) requires a distinct N-terminal activation domain. Bcl-2 gene activation and protection from apoptosis are thus produced only by the long form of Brn-3a which contains this domain and not by a naturally occurring short form lacking this domain or by the isolated POU domain, although all these forms of Brn-3a can stimulate neurite outgrowth. Hence Brn-3a is a multi-functional transcription factor with different regions of the factor mediating its different effects and two distinct forms with different properties being generated by alternative splicing.

Neuronal expression of the 5HT3 serotonin receptor gene requires nuclear factor 1 complexes

The 5HT3 receptor (5HT3R) is a serotonin-gated ion channel whose expression is restricted to a subset of cells within the central and peripheral nervous systems. In vitro analysis shows that a small proximal region of the TATA-less 5HT3R promoter is sufficient to direct neuronal-specific reporter gene expression. Three potential regulatory elements conserved between the mouse and human genes were identified within this proximal promoter, two of which are known sites for the ubiquitously expressed factors Sp1 and nuclear factor 1 (NF1). Surprisingly, mutation of the NF1 binding site abolished all reporter activity in cell transfection studies, suggesting that this element is essential for neuronal-specific transcriptional activity of the 5HT3R. Furthermore, a complex of neuronal proteins that includes a member(s) of the NF1 family binds to this site, as shown by gel mobility super shift and DNaseI footprinting analyses. Although NF1 has been proposed to mediate basal transcription of many ubiquitously expressed genes, our data suggest that a member of the NF1 transcription factor family participates in neuronal-specific gene expression by promoting interactions with other regulatory factors found in sensory ganglia.

Characterization of the mouse neurofilament light (NF-L) gene promoter by in vitro transcription

We have used in vitro transcription to access the basic sequences and factors required for the transcription of the mouse neurofilament light promoter (pNF-L) in the absence of chromatin structure. Deletion from -1.7 to -154 results in little change in NF-L promoter activity using nuclear extracts from either brain (expressing) or liver (non-expressing) tissues. Further deletion to -29 results in a gradual five-fold drop in promoter activity in both extracts. Only replacement of the entire -148 to -29 region results in a drop in NF-L promoter activity to basal levels. Thus, the NF-L promoter differs from the mouse NF heavy (NF-H) and mid-sized (NF-M) promoters in that no specific sequence within the immediate upstream NF-L promoter region (-154 to -29) appears to be responsible for enhancement or brain-specific transcription. We show that the order of strength of the three NF promoters is NF-H>NF-M>NF-L and identify sequences that can increase or reduce transcription when placed in front of heterologous NF promoters. We conclude that the NF-L promoter is a modular, weak and promiscuous promoter whose regulation differs from NF-H or NF-M. Our data suggest that chromatin structure may play an important role in the regulation of the NF-L promoter.