Structure, chromosome location, and expression of the mouse zinc finger gene Krox-20: multiple gene products and coregulation with the proto-oncogene c-fos

Haplotype-specific modulation of a SOX10/CREB response element at the Charcot-Marie-Tooth disease type 4C locus SH3TC2

Loss-of-function mutations in the Src homology 3 (SH3) domain and tetratricopeptide repeats 2 (SH3TC2) gene cause autosomal recessive demyelinating Charcot-Marie-Tooth neuropathy. The SH3TC2 protein has been implicated in promyelination signaling through axonal neuregulin-1 and the ERBB2 Schwann cell receptor. However, little is known about the transcriptional regulation of the SH3TC2 gene. We performed computational and functional analyses that revealed two cis-acting regulatory elements at SH3TC2-one at the promoter and one ∼150 kb downstream of the transcription start site. Both elements direct reporter gene expression in Schwann cells and are responsive to the transcription factor SOX10, which is essential for peripheral nervous system myelination. The downstream enhancer harbors a single-nucleotide polymorphism (SNP) that causes an ∼80% reduction in enhancer activity. The SNP resides directly within a predicted binding site for the transcription factor cAMP response element binding protein (CREB), and we demonstrate that this regulatory element binds to CREB and is activated by CREB expression. Finally, forskolin induces Sh3tc2 expression in rat primary Schwann cells, indicating that SH3TC2 is a CREB target gene. These findings prompted us to determine if SNP genotypes at SH3TC2 are associated with differential phenotypes in the most common demyelinating peripheral neuropathy, CMT1A. Interestingly, this revealed several associations between SNP alleles and disease severity. In summary, our data indicate that SH3TC2 is regulated by the transcription factors CREB and SOX10, define a regulatory SNP at this disease-associated locus and reveal SH3TC2 as a candidate modifier locus of CMT disease phenotypes.

Gene expression changes in a transgenic mouse model overexpressing human wildtype and mutant torsinA

Primary torsion dystonia is an autosomal-dominantly inherited, neurodevelopmental movement disorder caused by a GAG deletion (ΔGAG) in the DYT1 gene, encoding torsinA. This mutation is responsible for approximately 70% of cases of early-onset primary torsion dystonia. The function of wildtype torsinA is still unknown, and it is unsolved how the deletion in the DYT1 gene contributes to the development of the disease. To better understand the molecular processes involved in torsinA pathology, we used genome-wide oligonucleotide microarrays to characterize gene expression patterns in the striatum of mouse models overexpressing the human wildtype and mutant torsinA. By this approach we were able to detect gene expression changes that seem to be specific for torsinA pathology. We found an impact of torsinA, independent from genotype, on vesicle trafficking, exocytosis, and neurotransmitter release in our mouse model. In addition, we were able to identify several new pathways and processes involved in the development of the nervous system that are affected by wildtype and mutant torsinA. Furthermore, we have striking evidence from our gene expression data that glutamate receptor mediated synaptic plasticity in the striatum is the affected underlying cellular process for impaired motor learning in human ΔGAG torsinA transgenic mice.

Molecular genetics of autosomal-dominant demyelinating Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is a clinically and genetically heterogeneous group of disorders and is the most common inherited neuromuscular disorder, with an estimated overall prevalence of 17-40/10,000. Although there has been major advances in the understanding of the genetic basis of CMT in recent years, the most useful classification is still a neurophysiological classification that divides CMT into type 1 (demyelinating; median motor conduction velocity < 38 m/s) and type 2 (axonal; median motor conduction velocity > 38 m/s). An intermediate type is also increasingly being described. Inheritance can be autosomal-dominant (AD), X-linked, or autosomal-recessive (AR). AD CMT1 is the most common type of CMT and was the first form of CMT in which a causative gene was described. This review provides an up-to-date overview of AD CMT1 concentrating on the molecular genetics as the clinical, neurophysiological, and pathological features have been covered elsewhere. Four genes (PMP22, MPZ, LITAF, and EGR2) have been described in the last 15 yr associated with AD CMTI and a further gene (NEFL), originally described as causing AD CMT2 can also cause AD CMT1 (by neurophysiological criteria). Studies have shown many of these genes, when mutated, can cause a wide range of CMT phenotypes from the relatively mild CMT1 to the more severe Dejerine-Sottas disease and congenital hypomyelinating neuropathy, and even in some cases axonal CMT2. This review discusses what is known about these genes and in particular how they cause a peripheral neuropathy, when mutated.

Krox-20 gene expression: influencing hindbrain-craniofacial developmental interactions

Krox-20 is a C(2)H(2)-type zinc-finger transcription factor that plays an essential role in hindbrain development. The Krox-20 null mutation results in hindbrain anomalies that result in neonatal death due to respiratory and feeding deficits. Here we review our studies of how the Krox- 20 null mutation impacts the development of motor and sensory systems critical for the production of consummatory behaviors (suckling/chewing). First, we demonstrated that Krox-20 null mutants suffer a selective loss of primary jaw-opening muscles during prenatal development. In vivo and in vitro studies are reviewed that highlight intrinsic defects in mutant jaw-opener muscles that contribute to muscle degeneration. Next we focus on the impact of the mutation on proprioceptive neurons activated during consummatory behaviors. Mesencephalic trigeminal (Me5) neurons are primary sensory neurons that relay jaw proprioception to the central nervous system. These cells are unique because their cell bodies are located in the central as opposed to the peripheral nervous system. Data are reviewed that demonstrate the impact of the mutation on Me5 neurons, a cell group traditionally thought to emerge from the mesencephalon. We show that Krox-20 null mutants have twice as many Me5 neurons relative to wildtypes at E15, but by birth have half the number of Me5 cells as wildtypes. TUNEL assays performed in each set of studies reveal that Krox-20 expression acts to protect both muscle and mesencephalic trigeminal neurons against apoptosis, suggesting that Krox-20, in addition to its role in hindbrain patterning, has a broader, long-lasting role in development.

Rostral hindbrain patterning involves the direct activation of a Krox20 transcriptional enhancer by Hox/Pbx and Meis factors

The morphogenesis of the vertebrate hindbrain involves the generation of metameric units called rhombomeres (r), and Krox20 encodes a transcription factor that is expressed in r3 and r5 and plays a major role in this segmentation process. Our knowledge of the basis of Krox20 regulation in r3 is rather confusing, especially concerning the involvement of Hox factors. To investigate this issue, we studied one of the Krox20 hindbrain cis-regulatory sequences, element C, which is active in r3-r5 and which is the only initiator element in r3. We show that element C contains multiple binding sites for Meis and Hox/Pbx factors and that these proteins synergize to activate the enhancer. Mutation of these binding sites allowed us to establish that Krox20 is under the direct transcriptional control of both Meis (presumably Meis2) and Hox/Pbx factors in r3. Furthermore, our data indicate that element C functions according to multiple modes, in Meis-independent or -dependent manners and with different Hox proteins, in r3 and r5. Finally, we show that the Hoxb1 and Krox20 expression domains transiently overlap in prospective r3, and that Hoxb1 binds to element C in vivo, supporting a cell-autonomous involvement of Hox paralogous group 1 proteins in Krox20 regulation. Altogether, our data clarify the molecular mechanisms of an essential step in hindbrain patterning. We propose a model for the complex regulation of Krox20, involving a novel mode of initiation, positive and negative controls by Hox proteins, and multiple direct and indirect autoregulatory loops.

Heroin self-administration: II. CNS gene expression following withdrawal and cue-induced drug-seeking behavior

In the accompanying paper, we described incubation of heroin-seeking behavior in rats following 14 days of abstinence. To gain an understanding of genomic changes that accompany this behavioral observation, we measured the expression of genes previously reported to respond to drugs of abuse. Specifically, after 1 or 14 days of abstinence, mRNA expression was measured for 11 genes in the medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) immediately following a single 90 min extinction session. Additionally, the role of contingency was examined in control rats that received yoked, response-independent heroin administration. Gene expression was quantified by real-time quantitative PCR. Expression of five genes (Arc, EGR1, EGR2, Fos, and Homer1b/c) was changed in the mPFC. EGR1 and EGR2 expression was increased following the 90 min extinction session in a contingency-specific manner and this increase persisted through the 14 days of abstinence. Fos expression was also increased after 1 and 14 days of abstinence, but at 14 days this increase was response-independent (i.e., it occurred in both the rats with a history of heroin self-administration and in the yoked controls). Arc expression increased following the extinction session only in rats with a history of heroin self-administration and only when tested following 1, but not 14, days of abstinence. Homer 1 b/c decreased after 14 days of enforced abstinence in rats that received non-contingent heroin. Expression of only a single gene (EGR2) was increased in the NAc. These data demonstrate that behavioral incubation is coincident with altered levels of specific transcripts and that this response is contingently-specific. Moreover, EGR1 and EGR2 are specifically upregulated in self-administering rats following extinction and this finding persists through 14 days of abstinence, suggesting that these genes are particularly associated with the incubation phenomenon. These latter observations of persistent changes in gene expression following abstinence may reflect molecular correlates of relapse liability.

Egr1 and Gas6 facilitate the adaptation of HEK-293 cells to serum-free media by conferring enhanced viability and higher growth rates

Animal-derived serum is an essential media supplement for mammalian cells in cell culture. For a number of reasons including cost, regulatory concerns, lot inconsistency, potential contamination with adventitious agents, and down-stream processing it is desirable to eliminate the use of serum. Existing protocols designed to adapt cells to serum-free media (SFM) are time-consuming and provide little insight into how the cells adapt. To better understand the physiological responses associated with serum withdrawal and to expedite the adaptation process, a Human Embryonic Kidney-293 (HEK-293) cell line was propagated in 10% fetal bovine serum (FBS) and was progressively adapted to SFM and analyzed at specific serum levels by oligonucleotide microarrays. Of the differentially expressed genes two, early growth response 1 (egr1) and growth arrest specific 6 (gas6), were selected for further analysis based on their level of differential expression, overall expression patterns, and proposed functionalities. HEK-293 cells, propagated in 10% FBS were transfected with egr1 or gas6 and then adapted to SFM. Results indicated that higher expression of either gene moderately enhanced the ability of both cell lines to adapt to SFM. Egr1 appeared to have a greater impact on adaptability than gas6. Results also indicated that specific protein production was unaltered when the expression of egr1 was increased. Flow cytometric analysis revealed increased expression of egr1 was associated with an increase in the percentage of cells in the G2/M phases. These results indicate that enhanced expression of egr1 or gas6 facilitate adaptation to SFM by improving growth and viability.

Genetic effects of anorganic bovine bone (Bio-Oss®) on osteoblast-like MG63 cells

Bio-Oss (Geistlich, Wolhusen, Switzerland) is composed by anorganic bovine bone and is widely used in several bone regeneration procedures in oral surgery. How this biomaterial alters osteoblast gene expression to promote bone formation is poorly understood. We therefore attempted to address this question by using microarray techniques to identify genes that are differentially regulated in osteoblasts exposed to Bio-Oss. By using DNA microarrays containing 20,000 genes, we identified in osteoblast-like cells line (MG-63) cultured with Bio-Oss several genes which expression was significantly up- and down-regulated. The differentially expressed genes cover a broad range of functional activities: (a) signaling transduction, (b) transcription, (c) cell cycle regulation, (d) vesicular transport, (e) apoptosis, and (f) immunity. These results could explain the reported bioaffinity of Bio-Oss to host animals, its biological affinity to osteogenic cells and its capability to stimulate osteoblastic differentiation. The data reported are, to our knowledge, the first genetic portrait of Bio-Oss effects. They can be relevant to our improved understanding of the molecular mechanism underlying bone regenerative procedures and as a model for comparing other materials with similar clinical effects.

Overexpression of connexin43 alters the mutant phenotype of midgestational wnt-1 null mice resulting in recovery of the midbrain and cerebellum

The midbrain-hindbrain (MHB) junction plays a key role in the patterning of the embryonic neural tube and the formation of brain structures such as the cerebellum. The mitogen wnt-1 is critical for cerebellar development, as evidenced by the lack of MHB region and cerebellar formation in the wnt-1 null embryo. We have generated wnt-1 null embryos overexpressing the gap junction gene connexin43 by crossing wnt-1 null heterozygotes into the CMV43 mouse line. We have confirmed that these mice show an increase in gap junctional communication by dye coupling analysis. Two-thirds of wnt-1 null CMV43(+) mouse embryos at E18.5 have a cerebellum. In addition, changes in the wnt-1 null phenotype in mouse embryos overexpressing connexin43 are observed as early as E9.5. At this stage, one-quarter of wnt-1 null CMV43(+) embryos display extra or expanded tissue present at the MHB boundary (a wnt-1 null enlarged phenotype). In situ hybridization studies conducted on these embryos have indicated no changes in the expression of embryonic brain positional markers in this region. We conclude from these studies that overexpression of the connexin43 gap junction restores cerebellar formation by compensating for the loss of wnt-1.

Mesencephalic Trigeminal Nucleus Development Is Dependent on Krox-20 Expression

Krox-20, a C2H2-type zinc-finger transcription factor, plays an important role in rhombomere development. This study reveals that the Krox-20 null mutation impacts the development of mesencephalic trigeminal (Me5) neurons, a cell group traditionally thought to emerge from the mesencephalon. Based on cell counting studies, we show that Krox-20 null mutants have twice as many Me5 neurons relative to wildtypes at E15, but by birth have half the number of Me5 cells as wildtypes. TUNEL studies reveal a period of increased apoptosis from E17-P0 in mutants. The mutation does not result in differences in Me5 cell size, morphology, gene expression or peripheral projection patterns between genotypes, as demonstrated by retrograde tracing and Brn3a immunohistochemistry. The data suggest that Krox-20 regulates the period and extent of Me5 apoptosis, impacting the final number of Me5 neurons. The loss of Me5 in Krox-20-/- mice may highlight species-specific differences in the origin of these cells.

Epidermal growth factor-induced hepatocellular carcinoma: gene expression profiles in precursor lesions, early stage and solitary tumours

Epidermal growth factor is an important mitogen for hepatocytes. Its overexpression promotes hepatocellular carcinogenesis. To identify the network of genes regulated through EGF, we investigated the liver transcriptome during various stages of hepatocarcinogenesis in EGF2B transgenic mice. Targeted overexpression of IgEGF induced distinct hepatocellular lesions and eventually solid tumours at the age of 6-8 months, as evidenced by histopathology. We used the murine MG U74Av2 oligonucleotide microarrays to identify transcript signatures in 12 tumours of small (n=5, pooled), medium (n=4) and large sizes (n=3), and compared the findings with three nontumorous transgenic livers and four control livers. Global gene expression analysis at successive stages of carcinogenesis revealed hallmarks linked to tumour size. A comparison of gene expression profiles of nontumorous transgenic liver versus control liver provided insight into the initial events predisposing liver cells to malignant transformation, and we found overexpression of c-fos, eps-15, TGIF, IGFBP1, Alcam, ets-2 and repression of Gas-1 as distinct events. Further, when gene expression profiles of small manifested tumours were compared with nontumorous transgenic liver, additional changes were obvious and included overexpression of junB, Id-1, minopontin, villin, claudin-7, RR M2, p34cdc2, cyclinD1 and cyclinB1 among others. These genes are therefore strongly associated with tumour formation. Our study provided new information on the tumour stage-dependent network of EGF-regulated genes, and we identified candidate genes linked to tumorigenes and progression of disease.

Functional role of a novel ternary complex comprising SRF and CREB in expression of Krox-20 in early embryos of Xenopus laevis

Krox-20, originally identified as a member of "immediate-early" genes, plays a crucial role in the formation of two specific segments in the hindbrain during early development of the vertebrate nervous system. Here we cloned a genomic sequence of Xenopus Krox-20 (XKrox-20) and studied functions of a promoter element in the flanking sequence and associated transcription factors, which function in early Xenopus embryos. Using the luciferase reporter assay system, we showed that the 5' flanking sequence was sufficient to induce luciferase activities when the reporter construct was injected into embryos at the eight-cell stage. Deletion and mutagenesis analyses of the 5' flanking sequence revealed a minimal promoter element that included two known subelements, a CArG-box and cAMP response element (CRE) within a stretch of 22 bp nucleotide sequence (-72 to -51 from the transcription initiation site), both of which were essential for the promoter activity. The gel mobility shift assay indicated that these two subelements bound to some components in whole cell extracts prepared from stage 20 Xenopus embryos. Antibody supershift and competition experiments revealed that these components in cell extracts were serum response factor (SRF) and a member of CRE binding protein (CREB) family proteins that bound the CArG-box and CRE, respectively. They appeared to assemble on the minimal promoter element to produce a novel ternary complex. When we injected mRNA of a dominant-negative version of Xenopus SRF (XSRFDeltaC) into animal pole blastomeres at the eight-cell stage, expression of XKrox-20 in the nervous system as well as the minimal promoter activity was strongly suppressed. Suppression by XSRFDeltaC was counteracted by coexpressed wild-type XSRF. These results indicate that XSRF functions as an endogenous activator of XKrox-20 by forming a ternary complex with CREB on the minimal promoter element.

Identification of transcription factor binding sites upstream of human genes regulated by the phosphatidylinositol 3-kinase and MEK/ERK signaling pathways

We have taken an integrated approach in which expression profiling has been combined with the use of small molecule inhibitors and computational analysis of transcription factor binding sites to characterize regulatory sequences of genes that are targets of specific signaling pathways in growth factor-stimulated human cells. T98G cells were stimulated with platelet-derived growth factor (PDGF) and analyzed by DNA microarrays, which identified 74 immediate-early gene transcripts. Cells were then treated with inhibitors to identify subsets of genes that are targets of the phosphatidylinositol 3-kinase (PI3K) and MEK/ERK signaling pathways. Four groups of PDGF-induced genes were defined: independent of PI3K and MEK/ERK signaling, dependent on PI3K signaling, dependent on MEK/ERK signaling, and dependent on both pathways. The upstream regions of all genes in the four groups were scanned using TRANSFAC for putative cis-elements as compared with a background set of non-induced genes. Binding sites for 18 computationally predicted transcription factors were over-represented in the four groups of co-expressed genes compared with the background sequences (p < 0.01). Many of the cis-elements identified were conserved in orthologous mouse genes, and many of the predicted elements and their cognate transcription factors were consistent with previous experimental data. In addition, chromatin immunoprecipitation assays experimentally verified nine predicted SRF binding sites in T98G cells, including a previously unknown SRF site upstream of DUSP5. These results indicate that groups of human genes regulated by discrete intracellular signaling pathways share common cis-regulatory elements.

Comparative analysis of transcriptional profiles between two apoptotic pathways of light-induced retinal degeneration

Light exposure can exacerbate the condition of a variety of human retinal diseases by increasing the rate of photoreceptor cell death. How light negatively affects photoreceptor cell survival is not yet fully understood. Previous studies involving light damage models have revealed two independent apoptotic pathways: low levels of light induce retinal degeneration in the arrestin -/- mouse via constitutive activation of the phototransduction cascade, whereas strong light exposure to the retina, such as in an albino eye, elicits photoreceptor cell death via activator protein (AP-1) induction. In order to better understand the initial gene expression changes underlying light damage, dark-reared arrestin -/- and albino BALB/c mice were exposed to constant white light (2000 lux), and their retinal morphology was assessed as a function of time. The expression profiles of retinal transcripts were then compared between dark-adapted and light-exposed arrestin -/-, pigmented wild-type and BALB/c mice at a time point when morphological changes were minimal. As expected, the dark-adapted samples showed little difference in expression pattern between the three genotypes. Among the genes differentially regulated by light in BALB/c, but not arrestin -/- retinas, were c-fos and other stress-induced early response genes. In both mouse models, a marked increase in expression of the bZIP family of transcription factors was observed. Our results show a select group of unique and overlapping sets of genes induced by light in the two mouse models. These expression changes may constitute the underlying initiating events leading to the two distinct mechanisms of light damage.

Analysis of CREM-dependent gene expression during mouse spermatogenesis

The transcription factors CREM, CREB, and ATF-1 constitute a subfamily of beta-Zip transcription factors. Several different kinase cascades regulate the activity of these proteins. The activator splice-isoform CREMtau is specifically and highly expressed in post-meiotic germ cells during mouse spermatogenesis. Male mice lacking CREMtau expression are sterile because of stage-specific arrest of sperm maturation as the spermatids undergo apoptosis. In order to characterize the genes that are controlled by CREM during post-meiotic differentiation of round spermatids, we compared the expression levels of mRNA prepared from testes of wild-type and CREM-deficient mice by suppression subtractive hybridization (SSH) and affymetrix oligonucleotide arrays. A set of 956 unique sequences found in the CREM SSH library was further characterized by generating stage-specific expression profiles during spermatogenesis by hybridization with cDNA from pre-pubertal mice at defined stages of spermatogenesis using nylon DNA arrays. The resulting expression profiles were arranged in a linear order according to similarity in their profile shapes to find co-regulation of functionally related genes. Our data shows that a large number of genes are transcriptionally activated in round spermatids when CREM activity is maximal, including functional groups like transcription factors, proteins involved in signal transduction, and metabolic enzymes, therefore providing novel information of post-meiotic expression of many known as well as novel genes that are either directly or indirectly influenced by CREM expression.

Neural crest patterning: autoregulatory and crest-specific elements co-operate for Krox20 transcriptional control

Neural crest patterning constitutes an important element in the control of the morphogenesis of craniofacial structures. Krox20, a transcription factor gene that plays a critical role in the development of the segmented hindbrain, is expressed in rhombomeres (r) 3 and 5 and in a stream of neural crest cells migrating from r5 toward the third branchial arch. We have investigated the basis of the specific neural crest expression of Krox20 and identified a cis-acting enhancer element (NCE) located 26 kb upstream of the gene that is conserved between mouse, man and chick and can recapitulate the Krox20 neural crest pattern in transgenic mice. Functional dissection of the enhancer revealed the presence of two conserved Krox20 binding sites mediating direct Krox20 autoregulation in the neural crest. In addition, the enhancer included another essential element containing conserved binding sites for high mobility group (HMG) box proteins and which responded to factors expressed throughout the neural crest. Consistent with this the NCE was strongly activated in vitro by Sox10, a crest-specific HMG box protein, in synergism with Krox20, and the inactivation of Sox10 prevented the maintenance of Krox20 expression in the migrating neural crest. These results suggest that the dependency of the enhancer on both crest- (Sox10) and r5- (Krox20) specific factors limits its activity to the r5-derived neural crest. This organisation also suggests a mechanism for the transfer and maintenance of rhombomere-specific gene expression from the hindbrain neuroepithelium to the emerging neural crest and may be of more general significance for neural crest patterning.

Exposure to lead elevates induction of zif268 and Arc mRNA in rats after electroconvulsive shock: the involvement of protein kinase C

Exposure to lead is well known to impair cognitive function in young children. Because of the importance of gene regulation for neurodevelopment, we examined the effect of lead on the induction of the mRNA of the immediate early genes zif268 and Arc. The time course for the induction of zif268 mRNA and Arc mRNA by electroconvulsant shock (ECS) was altered in the area of the dentate gyrus of the hippocampus in rats exposed to lead from postnatal days (PND) 1 to 28. Other areas of the hippocampus were not affected by lead. The effects on the induction of zif268 mRNA were observed at blood lead levels as low as 12 microg/dl. No change in the induction of zif268 mRNA was observed in the hippocampus of rats exposed to lead from PND 28 to PND 56. Because of the possible involvement of protein kinase C (PKC) in the effect of lead, activation of different isoforms of PKC was investigated. An increase in the amount of PKC epsilon and PKC gamma was observed at 60 min after ECS in the membrane fraction from hippocampus, indicating activation of these isoforms. The amount of PKC epsilon in membranes was higher in rats exposed to lead than in rats not exposed to lead after ECS. Taken together, the data suggest that lead may disturb regulation of specific immediate early genes by activating PKC epsilon.

Characterisation of cis-acting sequences reveals a biphasic, axon-dependent regulation of Krox20 during Schwann cell development

In Schwann cells (SC), myelination is controlled by the transcription factor gene Krox20/Egr2. Analysis of cis-acting elements governing Krox20 expression in SC revealed the existence of two separate elements. The first, designated immature Schwann cell element (ISE), was active in immature but not myelinating SC, whereas the second, designated myelinating Schwann cell element (MSE), was active from the onset of myelination to adulthood in myelinating SC. In vivo sciatic nerve regeneration experiments demonstrated that both elements were activated during this process, in an axon-dependent manner. Together the activity of these elements reproduced the profile of Krox20 expression during development and regeneration. Genetic studies showed that both elements were active in a Krox20 mutant background, while the activity of the MSE, but likely not of the ISE, required the POU domain transcription factor Oct6 at the time of myelination. The MSE was localised to a 1.3 kb fragment, 35 kb downstream of Krox20. The identification of multiple Oct6 binding sites within this fragment suggested that Oct6 directly controls Krox20 transcription. Taken together, these data indicate that, although Krox20 is expressed continuously from 15.5 dpc in SC, the regulation of its expression is a biphasic, axon-dependent phenomenon involving two cis-acting elements that act in succession during development. In addition, they provide insight into the complexity of the transcription factor regulatory network controlling myelination.

Progesterone stimulates Krox-20 gene expression in Schwann cells

The gene of the zinc finger transcription factor Krox-20 (Egr-2) is expressed in Schwann cells and plays an important role in myelination of peripheral nerves. We have shown that progesterone promotes myelination in the regenerating sciatic nerve and in cocultures of Schwann cells and sensory neurones. To determine whether progesterone regulates Krox-20 expression, we measured its effects on Krox-20 mRNA levels in the MSC80 mouse Schwann cell line by semi-quantitative RT-PCR. Although low levels of Krox-20 mRNA are detectable in MSC80 cells cultured in defined medium, treatment with 10(-6) M progesterone induces a rapid (15 min) and transient increase in the levels of Krox-20 mRNA. Lower doses of progesterone (10(-9), 10(-8) and 10(-7) M) are also effective in increasing Krox-20 mRNA. Other steroids including testosterone, dexamethasone, and estradiol are ineffective when added to the culture medium at 10(-6) M for 1 h. The induction of Krox-20 mRNA was also observed with the selective progesterone agonist Organon 2058 and was abolished by treating the MSC80 Schwann cells with the progesterone antagonist RU486, indicating that progesterone induces Krox-20 mRNA expression by binding to its intracellular receptor. The induction of Krox-20 by progesterone was also demonstrated in primary cultures of Schwann cells isolated from neonatal rat sciatic nerves, at the mRNA level by RT-PCR and at the protein level by immunohistochemistry. As Krox-20 is a necessary step for the initiation of myelin formation in peripheral nerves, its stimulation by progesterone suggests an important signalling function for this steroid in myelination.

Differential expression of Fas ligand in Th1 and Th2 cells is regulated by early growth response gene and NF-AT family members

Inducible expression of Fas ligand (CD95 ligand) by activated T cells and the resulting apoptosis of CD95-bearing cells is a critical component of peripheral T cell homeostasis and cytotoxic effector mechanisms. Transcriptional control of the expression of Fas ligand has been attributed to a number of factors, including early growth response gene 2 (Egr2), Egr3, Sp1, and NF-AT, although a direct contribution of NF-AT is controversial. The present study confirms a role for Egr factors and indicates that NF-AT is essential for optimal expression of murine Fas ligand through a direct interaction with an NF-AT consensus element. The role of these factors was further defined by studying the differential expression of Fas ligand in Th1 and Th2 lines derived from DO11.10 TCR transgenic mice. EMSA analyses of a composite Egr/NF-AT site showed recruitment of Sp1 to this site in Th2 cells, but not in Th1 cells. Furthermore, gel-shift analyses demonstrated the binding of Egr1, 2, and 3 in Th2 cells and Egr1 and 2, but not Egr3 in Th1 cells at a known Egr site. Northern analysis corroborated the lack of Egr3 in Th1 cells. Differential usage of these transcription factors by Th1 and Th2 cells suggests a potential mechanism underlying the differential expression of Fas ligand by distinct T cell lineages.

Expression of the SRF gene occurs through a Ras/Sp/SRF-mediated-mechanism in response to serum growth signals

Serum Response Factor (SRF) plays a central role in the transcriptional response of mammalian cells to a variety of extracellular signals. It is a key regulator of many cellular early response genes which are believed to be involved in cell growth, differentiation, and development. The mechanism by which SRF activates transcription in response to mitogenic agents has been extensively studied, however, less is known about regulation of the SRF gene itself. Previously, we identified distinct regulatory elements in the SRF promoter that play a role in activation, including an ETS domain binding site, an overlapping Sp1/Egr-1 binding site, and two SRF binding sites. We further showed that serum induces the SRF gene by a mechanism that requires an intact SRF binding site, also termed a CArG box. In the present study we demonstrate that in response to stimulation by cells by lysophosphatidic acid (LPA) or whole serum, the SRF promoter is upregulated by a bipartite pathway that requires both an Sp1 factor binding site and the CArG motifs for maximal stimulation. The CArG box-dependent component of this pathway is targeted by Rho mediated signals, and the Sp1 binding site dependent component is targeted by Ras mediated signals.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

ZNF76 and ZNF143 are two human homologs of the transcriptional activator Staf

The transcriptional activator Staf, originally identified in Xenopus laevis, is implicated in the enhanced transcription of small nuclear RNA (snRNA) and snRNA-type genes by RNA polymerases II (Pol II) and III (Pol III). This zinc finger protein also possesses the capacity to stimulate expression from a Pol II mRNA promoter. Here, we report a study on two human proteins, ZNF76 and ZNF143, that are 64 and 84% identical to their Xenopus counterpart, respectively. Northern blot analysis revealed that ZNF76 and ZNF143 mRNAs were expressed in all normal adult tissues examined. By using in vivo and in vitro assays, we have analyzed the DNA binding capacities and transcriptional properties of ZNF76 and ZNF143. The binding affinities of ZNF76 and ZNF143 for Staf divergent responsive elements were determined by gel shift assays, which revealed that the two proteins bound a same DNA motif with similar affinities. Also, polypeptide sequences containing the seven zinc fingers of ZNF76 and ZNF143 could efficiently repress in vivo the activated transcription from an snRNA-type promoter. Transfection experiments in Drosophila cells showed that ZNF76 and ZNF143 can activate transcription from an mRNA promoter through the Staf binding site. Finally, chimeric ZNF76 and ZNF143 proteins, carrying a heterologous DNA binding domain, are able to activate a Pol II mRNA promoter and snRNA Pol II and Pol III promoters in Xenopus oocytes, through the heterologous DNA binding site. Taken together, these findings demonstrate that ZNF76 and ZNF143 are two members of a same family of transactivator proteins. ZNF143 constitutes the human ortholog of the Xenopus Staf, and ZNF76 is a novel DNA binding protein related to Staf and ZNF143.

Substitution of the degenerate smooth muscle (SM) alpha-actin CC(A/T-rich)6GG elements with c-fos serum response elements results in increased basal expression but relaxed SM cell specificity and reduced angiotensin II inducibility

We have previously demonstrated that both CC(A/T-rich)6GG (CArG) elements A and B of the smooth muscle (SM) alpha-actin promoter are required for smooth muscle cell (SMC)-specific expression and angiotensin II (AII)-induced stimulation. Moreover, results provided evidence that AII responsiveness of SM alpha-actin was at least partially dependent on modulation of serum response factor (SRF) binding to the SM alpha-actin CArGs by the homeodomain containing protein, MHox. The goal of the present study was to investigate whether the degeneracy of the SM alpha-actin CArGs (both contain a Gua or Cyt substitution in their A/T-rich center) and their reduced SRF binding activity as compared with c-fos serum response element (SRE) is important for conferring cell type-specific expression and AII responsiveness. Transient transfection assays using SM alpha-actin reporter gene constructs in which the endogenous SM alpha-actin CArGs were replaced by c-fos SREs demonstrated the following: 1) relaxation of cell-specific expression, 2) a 50% reduction in AII responsiveness, and 3) reduced ability to be transactivated by MHox. In addition, we also showed that the position of the SM alpha-actin CArGs was important in that interchanging them abolished both basal and AII-induced activities. Taken together, these results suggest that the reduced SRF binding activities of the SM alpha-actin CArGs and CArG positional context contribute to SMC-specific expression of SM alpha-actin as well as maximal AII responsiveness.

Mutations in the early growth response 2 (EGR2) gene are associated with hereditary myelinopathies

The early growth response 2 gene (EGR2) is part of a multigene family encoding Cys2His2 type zinc-finger proteins and may play a role in the regulation of cellular proliferation. Egr2, (also known as Krox20) is the mouse orthologue of human EGR2 and was first identified as an immediate-early response gene, encoding a protein that binds DNA in a sequence-specific manner and acts as a transcription factor. Stable expression of Egr2 is specifically associated with the onset of myelination in the peripheral nervous system (PNS). Egr2(-/-) mice display disrupted hindbrain segmentation and development, and a block of Schwann-cell differentiation at an early stage. We hypothesized that Egr2 may be a transcription factor affecting late myelin genes and that human myelinopathies of the PNS may result from mutations in EGR2. In support of this hypothesis, we have identified one recessive and two dominant missense mutations in EGR2 (within regions encoding conserved functional domains) in patients with congenital hypomyelinating neuropathy (CHN) and a family with Charcot-Marie-Tooth type 1 (CMT1).

Expression of Fos, Jun, and Krox family proteins in Alzheimer's disease

Apoptosis is an active process of cell death characterized by distinct morphological features and is often the end result of a genetic program of events, i.e., programmed cell death (PCD). There is growing evidence supporting a role for apoptosis and/or PCD in Alzheimer's disease (AD), based on DNA fragmentation studies and recent findings of increased levels of inducible transcription factors (ITFs) such as c-Jun in AD brains. We have characterized the expression of a large range of ITFs (c-Fos, Fos B, Fos-related antigens, c-Jun, Jun B, Jun D, Krox20, and Krox24) using multiple antisera in AD postmortem hippocampi and compared this with human control hippocampi as well as Huntington's disease hippocampi and human epilepsy biopsy tissue. We found little evidence of nuclear expression of any ITF except c-Jun in the human postmortem tissue, compared with nuclear staining in biopsy tissue. We found some evidence for increased levels of c-Jun and Krox24 protein and krox24 mRNA in the CA1 region of AD hippocampi, suggesting that PCD may be involved in the pathogenesis of AD. In general, staining characteristics of ITFs varied with different antisera directed against the same protein, indicating the need for caution when interpreting results.

The expression pattern of the mafB/kr gene in birds and mice reveals that the kreisler phenotype does not represent a null mutant

The recessive mouse mutation kreisler affects hindbrain segmentation and inner ear development in homozygous mice. The mouse gene affected by the mutation was found to encode a basic domain leucine-zipper (bZIP)-type transcription factor of the Maf-family named kr (Cordes, S.P. and Barsh, G.S. (1994) Cell 79, 1025-1034). The avian bZIP transcription factor mafB, which shows high homology to kr, has been identified as an interaction partner of c-Ets 1 (Sieweke, M.H., Tekotte, M.H., Frampton, J. and Graf, T. (1996) Cell 85, 49-60). Here we demonstrate by Southern blot analysis that mafB is the avian homologue of kr, and present a detailed pattern of its expression during avian and murine embryonic development. Consistent with the kreisler phenotype, mafB is expressed in avians in the tissues which are affected by the mouse mutation: rhombomeres 5 and 6 (r5 and r6) and the neural crest derived from these rhombomeres. However, our analysis reveals a variety of additional expression sites: mafB/kr expression persists in vestibular and acoustic nuclei and is also observed in differentiating neurons of the spinal cord and brain stem. Restricted expression sites are found in the mesonephros, the perichondrium, and in the hemopoietic system. Since these expression sites are conserved between mouse and chicken we reexamined homozygous kreisler mice for unrevealed phenotypes in the hemopoietic system. However, peritoneal macrophages from homozygous kreisler mice were found to be functionally normal and still expressed mafB/kr. Other adult tissues examined from homozygous kreisler mice had also not lost mafB/kr expression. Our results thus indicate that the kreisler mutation involves a tissue specific gene inactivation and suggest additional roles for mafB/kr in later developmental and differentiation processes that are not revealed by the mutation.

Apoptosis, neurotrophic factors and neurodegeneration

Apoptosis is an active process of cell death characterized by distinct morphological features, and is often the end result of a genetic programme of events, i.e. programmed cell death (PCD). There is growing evidence supporting a role for apoptosis in some neurodegenerative diseases. This conclusion is based on DNA fragmentation studies and findings of increased levels of pro-apoptotic genes in human brain and in in vivo and in vitro model systems. Additionally, there is some evidence for a loss of neurotrophin support in neurodegenerative diseases. In Alzheimer's disease, in particular, there is strong evidence from human brain studies, transgenic models and in vitro models to suggest that the mode of nerve cell death is apoptotic. In this review we describe the evidence implicating apoptosis in neurodegenerative diseases with a particular emphasis on Alzheimer's disease.

Expression of zinc finger immediate early genes in rat brain after permanent middle cerebral artery occlusion

The prolonged expression of the leucine zipper fos/jun immediate early genes (IEG) has been correlated with neuronal death after cerebral ischemia. In this study, the expression of six zinc finger IEG was examined using in situ hybridization in adult rats after middle cerebral artery occlusion (MCAO) with the suture model. NGFI-A, NGFI-B, NGFI-C, egr-2, egr-3, and Nurr1 mRNA were all induced throughout the ipsilateral cortex at 1 hour to 12 hours after MCAO. The cortical induction for most of the genes was greatest in the anterior cingulate and the anterior cerebral artery (ACA) and middle cerebral artery (MCA) transition zone. All of the zinc finger IEG were induced at 1 hour in all regions of hippocampus. NGFI-A and NGFI-B were induced in ipsilateral thalamus. Within areas of infarction, the basal IEG mRNA expression, and expression of the housekeeping gene cyclophilin A mRNA, decreased below control levels by 12 hours after the ischemia. Immediate early gene expression outside areas of infarction returned to control levels in most brain regions by 24 hours except for egr-3, which continued to be induced in the MCA/ ACA transition zone for 24 hours, and NGFI-A, which continued to be expressed in specific regions of the thalamus for 72 hours. The induction of these IEG in the cortex is likely caused by ischemia-induced cortical spreading depression, with the hippocampal and thalamic IEG induction being caused by activation of efferent cortical pathways to these regions. The prominent induction of NGFI-B, NGFI-C, egr-2, and egr-3 in the anterior cingulate cortex, the ACA/MCA transition zone, and medial striatum could reflect the ischemic regions around MCA infarcts. The prolonged NGFI-A expression observed in thalamus in this study, and in CA1 of hippocampus after global ischemia in the gerbil in a previous study, suggests that the prolonged NGFI-A, expression could be the result of or the cause of the delayed cell death. Prolonged NGFI-A expression, like c-fos and c-jun, seems to provide a marker for slowly dying neurons.

Organization, sequence, chromosomal localization, and promoter identification of the mouse orphan nuclear receptor Nurr1 gene

We have cloned and characterized the organization of the mouse orphan nuclear receptor Nurr1 gene. The Nurr1 gene is approximately 7 kb long, contains eight exons and seven introns, and mapped to mouse chromosome 2. Although the exon/intron structure of Nurr1 is nearly identical to that of Nur77, Nurr1 possesses an additional untranslated exon. Primer extension was used to identify two major transcription initiation sites mapped 37 nucleotides apart in the first untranslated exon. Functional studies of chimeric Nurr1-luciferase reporter genes delineated the promoter region and underscored the importance of the +1 transcription start site. Sequence analysis of the 5' flanking region surrounding +1 revealed several possible response elements such as a hexanucleotide glucocorticoid binding site, a cAMP-response element, a CArG box, and two c-Jun-binding sites. These data help to explain the different response characteristics of two closely related early response genes, Nurr1 and Nur77.

Induction of the early growth response (Egr) family of transcription factors during thymic selection

There is little known about the regulation of gene expression during TCR-mediated differentiation of immature CD4+8+ (double positive) thymocytes into mature T cells. Using the DPK CD4+8+ thymocyte precursor cell line, we demonstrate that the early growth response-1 gene (Erg-1), encoding a zinc finger transcription factor, is rapidly upregulated after TCR stimulation. We also report that Egr-1 is expressed by a subset of normal double positive thymocytes in the thymic cortex, as well by a majority of medullary single positive thymocytes. Expression of Egr-1 is dramatically reduced in the thymus of major histocompatibility complex knockout mice, but can be induced by anti-CD3 antibody stimulation of isolated thymocytes from these animals. These and other data suggest that high level expression of Egr-1 in the thymus is a consequence of selection. A similar pattern of expression is found for family members Egr-2 and Egr-3. Using the DPK cell line, we also demonstrate that expression of Egr-1, 2, and 3 is dependent upon ras activation, as is the initiation of differentiation to a single positive cell. In contrast, the calcineurin inhibitor cyclosporin A, which inhibits DPK cell differentiation as well as positive selection, inhibits expression of Egr-2 and Egr-3, but not Egr-1. The identification of the Egr family in this context represents the first report of a link between the two known signaling pathways involved in positive selection and downstream transcriptional regulators.

Oct-6 (SCIP/Tst-1) is expressed in Schwann cell precursors, embryonic Schwann cells, and postnatal myelinating Schwann cells: comparison with Oct-1, Krox-20, and Pax-3

The POU domain transcription factor Oct-6 (SCIP/Tst-1) is likely to control important stages of Schwann cell development, including the initiation of myelination around birth. Here, we use immunocytochemical and reverse transcriptase-polymerase chain reaction techniques to examine Oct-6 earlier in nerve development, to test the idea that Oct-6 has an additional role in Schwann cell precursors or early embryonic Schwann cells, a possibility raised by previous studies on transgenic mice. Consistent with this, we find low but unambiguous levels of Oct-6 mRNA and protein in Schwann cell precursors of mouse and rat (nerves from 12- and 14-day-old embryos, respectively), with expression levels gradually increasing during early Schwann cell development and towards birth. Unexpectedly, Oct-6 immunoreactivity is clearly present in nuclei of most myelinating cells at least as late as postnatal day 12. Furthermore, many nonmyelinating Schwann cells express Oct-6 in adult life. A comparison of Oct-6 mRNA with other Schwann cell transcription factors-namely, Oct-1, Krox-20, and Pax-3-reveals that each factor exhibits strong developmental regulation and a unique expression pattern in embryonic nerves. Therefore, they are likely to play distinct regulatory roles in early development of the Schwann cell lineage.

Calcium influx via the NMDA receptor induces immediate early gene transcription by a MAP kinase/ERK-dependent mechanism

The regulation of gene expression by neurotransmitters is likely to play a key role in neuroplasticity both during development and in the adult animal. Therefore, it is important to determine the mechanisms of neuronal gene regulation to understand fully the mechanisms of learning, memory, and other long-term adaptive changes in neurons. The neurotransmitter glutamate stimulates rapid and transient induction of many genes, including the c-fos proto-oncogene. The c-fos promoter contains several critical regulatory elements, including the serum response element (SRE), that mediate glutamate-induced transcription in neurons; however, the mechanism by which the SRE functions in neurons has not been defined. In this study, we sought to identify transcription factors that mediate glutamate induction of transcription through the SRE in cortical neurons and to elucidate the mechanism(s) of transcriptional activation by these factors. To facilitate this analysis, we developed an improved calcium phosphate coprecipitation procedure to transiently introduce DNA into primary neurons, both efficiently and consistently. Using this protocol, we demonstrate that the transcription factors serum response factor (SRF) and Elk-1 can mediate glutamate induction of transcription through the SRE in cortical neurons. There are at least two distinct pathways by which glutamate signals through the SRE: an SRF-dependent pathway that can operate in the absence of Elk and an Elk-dependent pathway. Activation of the Elk-dependent pathway of transcription seems to require phosphorylation of Elk-1 by extracellular signal-regulated kinases (ERKs), providing evidence for a physiological function of ERKs in glutamate signaling in neurons. Taken together, these findings suggest that SRF, Elk, and ERKs may have important roles in neuroplasticity.

Expression of the serum response factor gene is regulated by serum response factor binding sites

The serum response factor (SRF) is a ubiquitous transcription factor that plays a central role in the transcriptional response of mammalian cells to a variety of extracellular signals. Notably, SRF has been found to be a key regulator of members of a class of cellular response genes termed immediate-early genes (IEGs), many of which are believed to be involved in regulating cell growth and differentiation. The mechanism by which SRF activates transcription of IEGs in response to mitogenic agents has been extensively studied. Significantly less is known about how expression of the SRF gene itself is mediated. We and others have previously shown that the SRF gene is itself transiently induced by a variety of mitogenic agents and belongs to a class of "delayed" early response genes. We have cloned the SRF promoter and in the present study have analyzed the upstream regulatory sequences involved in mediating serum responsiveness of the SRF gene. Our analysis indicates that inducible SRF expression requires both SRF binding sites located within the first 63 nucleotides upstream from the start site of transcriptional initiation and an Sp1 site located 83 nucleotides upstream from the start site. Maximal transcriptional activity of the promoter also requires two CCAATT box sites located 90 and 123 nucleotides upstream of the start site.

Elk-1 can recruit SRF to form a ternary complex upon the serum response element

The initial genomic response to serum growth factors is the transcriptional activation of a set of immediate-early genes. Serum-induced transcriptional activation of several of these genes involves the formation of a ternary complex that includes the serum response factor (SRF), a 62 kDa ternary complex factor (TCF) and a serum response element (SRE). TCF alone does not bind the SRE of the protooncogene c-fos, but requires the prior assembly of the SRF-SRE binary complex for it to be recruited into a ternary complex. Here we show that this SRF-SRE binary complex is not an obligatory prerequisite for the formation of a serum responsive ternary complex. We demonstrate that Elk-1, which has properties of TCF can recruit SRF into a ternary complex on elements that do not support formation of the SRF-DNA binary complex. We also show that for two immediate-early genes, pip92 and nur77, formation of the ternary complex may occur without the prior assembly of SRF-DNA binary complex. Finally, we show that the ability of different sequences to support formation of Elk-l-SRF-DNA ternary complex in vitro correlates with their ability to respond to serum growth factors in vivo. Our results suggest that a much broader range of DNA sequences than the consensus SRF and TCF binding sites can support ternary complex formation, and by inference, serum induction. Possible implications of these results are discussed.

Human and mouse Krüppel-like (MOK2) orthologue genes encode two different zinc finger proteins

We have isolated the human homologue of Mok2 gene encoding a Krüppel-like protein. The identification of three cDNAs and genomic clones reveals that the human protein shows substantial structural differences with the mouse MOK2 protein. The mouse MOK2 protein is composed of seven tandem zinc-finger motifs with five additional amino acids at the COOH-terminal. This structural feature is also present at the end of the human MOK2 protein. The seven zinc-finger motifs show 94% identity between the two proteins. In addition, the human protein contains three additional zinc-finger motifs in tandem with the others and a nonfinger acidic domain of 173 amino acids at the NH2-terminal. The Southern analysis indicates that a single copy of these two genes is present in the genome. The human gene has been localized on chromosome 19 on band q13.2-q13.3. The comparison of human and mouse cDNA sequences reveals a strong identity in the sequences localized outside the seven highly conserved zinc-finger motifs. The divergence from their common ancestor results in the loss of a potential transcription activator domain in mouse MOK2 protein.

Forebrain and midbrain regions are deleted in Otx2−/− mutants due to a defective anterior neuroectoderm specification during gastrulation

We have replaced part of the mouse homeogene Otx2 coding region with the E. coli lacZ coding sequence, thus creating a null allele of Otx2. By 9.5 dpc, homozygous mutant embryos are characterized by the absence of forebrain and midbrain regions. From the early to midstreak stages, endomesodermal cells expressing lacZ fail to be properly localized anteriorly. In the ectodermal layer, lacZ transcription is progressively extinguished, being barely detectable by the late streak stage. These data suggest that Otx2 expression in endomesoderm and ectoderm is required for anterior neuroectoderm specification. In gastrulating heterozygous embryos, a post-transcriptional repression acts on lacZ transcripts in the ectoderm, but not in the external layer, suggesting that different post-transcriptional mechanisms control Otx2 expression in both layers.

Identification of a new enhancer in the promoter region of human TR3 orphan receptor gene. A member of steroid receptor superfamily

Human TR3 orphan receptor is a member of the steroid/thyroid hormone receptor superfamily and is the human homologue of the proteins encoded by the rat NGFI-B and mouse nur77 genes. These genes are induced rapidly by androgens/growth factors and may have functions related to cell proliferation, differentiation, and apoptosis. To investigate the TR3 orphan receptor gene transcriptional regulation, a 2.3-kilobase genomic DNA fragment containing the TR3 orphan receptor gene promoter region was isolated, sequenced, and characterized. Sequence homology search within this promoter region revealed some potential cis-acting elements such as cAMP response element, interleukin-6 response element, estrogen response element, and GC box. Deletion analysis and chloramphenicol acetyltransferase assay also showed a novel cis-acting element of TR3 orphan receptor gene (NCAE-TR3), 200-181 base pairs upstream of the transcriptional start site. Gel retardation assay further demonstrated that some nuclear factors can bind to this NCAE-TR3. Together, our data suggest that NCAE-TR3 could be a new enhancer element associated with the transcription of an early response gene for mitogenesis and apoptosis.

Angiotensin II induces a complex activation of transcription factors in the rat brain: expression of Fos, Jun and Krox proteins

We investigated the effects of intracerebroventricular injection of angiotensin II on neuronal immediate early gene-encoded protein synthesis in the brain of conscious rats. The expression of seven immediate early gene-encoded transcription factors (c-Fos, FosB, c-Jun, JunB, JunD, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif/268) was assessed simultaneously. Angiotensin II (1, 10, 100 ng) induced a dose-dependent expression of c-Fos and Krox-24 in the subfornical organ, the median preoptic area and in the paraventricular nucleus and supraoptic nucleus of the hypothalamus, regions known to be involved in the central osmoregulatory and neuroendocrine actions of angiotensin II. FosB expression was induced four hours after icv injection of the highest dose of angiotensin II in the median preoptic area and paraventricular nucleus, c-Jun expression was restricted to the median preoptic area, subfornical organ and paraventricular nucleus, and JunB was only induced in the median preoptic area and subfornical organ. In these above mentioned regions, JunD exhibited a high basal staining, which was not visibly altered by angiotensin II. Krox-20 was not induced by angiotensin II. Intracerebroventricular injections of isotonic saline did not induce immediate early gene expression in any of the above brain areas. The angiotensin II-AT1 receptor antagonist, losartan, applied intracerebroventricular five minutes prior to angiotensin II, prevented the angiotensin II-induced immediate early gene protein expression. Losartan alone had no effects on immediate early gene expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of cDNA clones for 42 different Krüppel-related zinc finger proteins expressed in the human monoblast cell line U-937

To study the complexity and structural characteristics of zinc finger proteins expressed during human hematopoiesis and to isolate novel regulators of blood cell development, a degenerate oligonucleotide probe specific for a consensus zinc finger peptide domain was used to isolate 63 cDNA clones for Krüppel-related zinc finger genes from the human monoblast cell line U-937. By extensive nucleotide sequence and Northern blot analysis, these cDNA clones were found to originate from approximately 42 different genes (HZF 1-42) of which only 8 have previously been described. Northern blot analysis showed that a majority of these genes were expressed at comparable levels in U-937 and HeLa cells. The large number of individual genes represented among the 63 clones and their apparent non-cell-type-specific expression suggest that the majority of the Krüppel-related zinc finger genes are likely to be expressed in most human tissues. In contrast, some of the genes displayed a restricted expression pattern, indicating that they represent potential regulators of monocyte differentiation or proliferation. Detailed structural analysis of the first 12 cDNAs (HZF 1-10) and a partial characterization of HZF 11-42 revealed that a common feature of human Krüppel-related zinc finger proteins is the presence of tandem arrays of zinc fingers ranging in number from 3 to over 20 that are preferentially located in the carboxy-terminal regions of the proteins. In addition, several novel KRAB-containing zinc finger genes and a novel conserved sequence element were identified.

Krox20 may play a key role in the stabilization of long-term potentiation

Long-term potentiation-inducing stimulation of the perforant path was followed in dentate gyrus granule cells by a dramatic increase of mRNA and protein for Krox20, a zinc-finger-containing transcription factor. Induction of Krox20 required stimulation sufficient to induce LTP and was prevented by NMDA antagonists CPP and MK-801, which block LTP induction. Krox20 protein increased within 20 min of tetanization, was maximal between 1 and 8 h, and was still significantly elevated at 24 h after LTP induction. This prolonged appearance is in striking contrast with the more transient induction of the related molecule, Krox24. The elevation in the mRNA for Krox20 and Krox24 was of similar duration, suggesting that the Krox20 protein has a greater stability and may play a key role in the stabilization of long-term potentiation.

Disruption of Krox-20 results in alteration of rhombomeres 3 and 5 in the developing hindbrain

The zinc finger gene Krox-20 is transcribed in two alternate segments (rhombomeres) of the developing hindbrain. To investigate its function, we have used homologous recombination to generate mice carrying an in-frame insertion of the E. coli lacZ gene within Krox-20. Analysis of the beta-galactosidase pattern in heterozygous embryos confirmed the known profile with expression restricted to rhombomeres (r) 3 and 5. Mice homozygous for the mutation die during the first two weeks after birth. Anatomical analysis of the hindbrain and of the cranial nerves during embryogenesis, combined with the determination of the expression patterns of rhombomere-specific genes, demonstrated that Krox-20 inactivation results in a marked reduction or elimination of r3 and r5. We conclude that Krox-20, although not required for the initial delimitation of r3 and r5, plays an important role in the process of segmentation governing hindbrain development.

The KROX-20 transcription factor in the rat central and peripheral nervous systems: novel expression pattern of an immediate early gene-encoded protein

The KROX-20 protein (also termed EGR-2) is encoded by an immediate early gene cloned by cross-hybridization to the Drosophila melanogaster Krüppel gene. It belongs to a class of transcription factors with zinc finger motifs and binding activity to a transcriptional regulatory DNA element termed the early growth response consensus sequence. In the present study the temporospatial expression of KROX-20 was investigated in the central and peripheral nervous systems of normal rats and after various stimuli known to induce immediate early genes, including epileptic seizures, axotomy, pharmacological treatment with glutamate and alpha-adrenergic receptor antagonists, and peripheral noxious stimulation. Immunocytochemistry was performed with a specific polyclonal antiserum generated against a fusion protein containing KROX-20 sequences. In the central nervous system, KROX-20 protein demonstrated distinct constitutive nuclear expression in specific neuronal subpopulations of the cortex, septum, amygdala, olfactory bulb and hypothalamus. In addition, distinct cytoplasmic immunoreactivity was present in spinal and medullary motoneurons, dorsal root ganglion neurons and a few neuronal cell populations of midbrain and forebrain. In the CNS, KROX-20 was only induced by bicuculline-induced epileptic seizures. Topographically, the postictal increase of KROX-20 levels was restricted to areas with constitutive expression, such as cerebral cortex, fornix and amygdala. Induction of KROX-20 peaked at 4-8 h after onset of seizure activity. No increase in immunoreactivity was observed in the hippocampus, the brain region most severely affected by bicuculline-induced seizures. Transection of central and peripheral nerve fibers did not result in KROX-20 induction in axotomized neurons. However, KROX-20 was induced in Schwann-like cells after transection of the sciatic nerve. In contrast to KROX-20, KROX-24, a related transcription factor of the zinc finger family, was markedly induced in hippocampal and spinal neurons following seizures and peripheral noxious stimulation, respectively, as well as in CNS neurons following axotomy. Our data indicate that KROX-20 represents an immediately early gene product with basal expression in selected neuronal populations of the nervous system and a restricted inducibility after intentional stimuli.

Expression of immediate early gene proteins following axotomy and inhibition of axonal transport in the rat central nervous system

The expression of the immediate early gene-encoded proteins c-Jun, Jun B, Jun D, c-Fos, Fos B and Krox-24 in central neurons following transection of, or inhibition of, axonal transport in their axons was investigated in the rat using immunocytochemistry. Transection of the medial forebrain bundle, which produces an essentially complete axotomy of neurons in the ipsilateral mammillary nucleus, substantia nigra pars compacta, ventral tegmental area and parafascicularis, induced the expression of c-Jun, Jun D and, to a lesser extent, Krox-24, in these nuclei. Microinjection of colchicine into the medial forebrain bundle to chemically inhibit axonal transport similarly induced the expression of these proteins in these areas. The expression of the proteins was first evident 24 h after transection, reached a maximum at 48 h and was still present after 10 days. However, after 30 days the proteins were absent from the substantia nigra, ventral tegmentum and parafascicularis, and were still present only in the mammillary nuclei. The other immediate early genes, Jun B, c-Fos and Fos B, were never expressed above the basal levels seen in untreated rats. Transection of the corpus callosum and the hippocampal commissure, which produces only a partial axotomy of neurons in the cerebral cortex and hippocampus, respectively, did not induce the expression of any of the genes in these neurons. Microinjection of colchicine or vinblastine to produce a localized inhibition of axonal transport in the cerebral cortex, hippocampus, thalamus and cerebellum also induced the expression of c-Jun, Jun D and, again to a lesser extent, Krox-24, in neurons surrounding the injection site. In contrast to this selective expression, administration of the neuronal excitant metrazole induced the expression of all six immediate early gene proteins in central nervous system neurons. These results demonstrate that transection of, or inhibition of, transport in the axons of central neurons induces a particular pattern of expression of transcriptionally operating immediate early genes that may be related to the regenerative competency of the neurons.

Regulation of transcription factor mRNA accumulation during 3T3-L1 preadipocyte differentiation by antagonists of adipogenesis

3T3-L1 preadipocytes differentiate into cells having the biochemical properties of adipocytes; tumor necrosis factor-alpha (TNF), retinoic acid (RA), and transforming growth factor-beta (TGF-beta), attenuate this process. Inhibition of differentiation by these agents, thought to be at the level of transcription, has been investigated by examining the accumulation of mRNA for six transcription factors and the autocrine growth factor interleukin 6 (IL-6). Upon induction of differentiation, a rapid and major accumulation of c-fos and jun-B mRNA was observed that returned to near basal levels within 4-6 h. In contrast, c-jun mRNA, although rapidly expressed following induction of differentiation, remained at relatively constant levels throughout the time course. Exposure of the cells to 5 nM TNF potentiated the accumulation of all 3 mRNAs but most significantly c-jun (12-fold), which remained elevated for at least 24 h after treatment. In control differentiating cells, krox-20 and fox-B were expressed transiently, 30 min to 2 h, while fra-1 mRNA accumulated over an extended period, 1 to 8 h. Again, TNF enhanced the accumulation of these mRNAs. Accumulation of mRNA for C/EBP, a transcription factor proposed to control expression of genes involved in the terminally differentiated state was attenuated after exposure of the cells to TNF. C/EBP expression was also inhibited in cells exposed to RA or TGF-beta. IL-6 mRNA was expressed briefly (30 min to 2 h) and again transiently (at 8 h after induction of differentiation). TNF treatment markedly enhanced accumulation of IL-6 message.(ABSTRACT TRUNCATED AT 250 WORDS)

The structure and expression of the Xenopus Krox-20 gene: conserved and divergent patterns of expression in rhombomeres and neural crest

Recent studies in the chick have indicated that rhombomeres (r) are segments that underlie the patterning of hindbrain nerves. These segments may also be important for the specification of branchial arch structures since alternating rhombomeres, r2, r4 and r6, each contribute crest to a specific arch. Krox-20 has been implicated in the segmental patterning of the hindbrain in the mouse by its expression prior to segment formation in alternating domains, which later correspond to r3 and r5. Here, we describe the sequence and developmental expression of the Xenopus Krox-20 gene, XKrox-20. Alternating domains of XKrox-20 expression appear in the early neurula, later correspond to r3 and r5, and persist until late tadpole stages. In contrast to this conserved spatial expression in rhombomeres, we find a pattern in the neural crest of Xenopus that appears different from that found in the mouse: expression occurs in crest that migrates from r5 into the third visceral arch. We speculate that this may reflect a distinct route of neural crest migration due to anatomical differences between these systems, rather than a difference in the site of origin of Krox-20-expressing crest.