Negative regulatory regions are present upstream in the three mouse neurofilament genes

Expression profiling upon Nex1/MATH-2-mediated neuritogenesis in PC12 cells and its implication in regeneration

The expression of Nex1 peaks during brain development when neurite outgrowth and synaptogenesis are highly active. We previously showed that Nex1 is a critical effector of the nerve growth factor (NGF) pathway and its overexpression results in spontaneous neuritogenesis. Furthermore, the PC12-Nex1 cells exhibit accelerated neurite extension upon NGF exposure, and have the capacity to regenerate neurites in the absence of NGF. In this study, we identify the repertoire of genes targeted by Nex1 to unravel the molecular mechanisms by which Nex1 promotes differentiation and regeneration. Our transcriptional analysis reveals that Nex1 modulates a wide spectrum of genes with diverse functions, many of them being key downstream regulators of the NGF pathway, and critical to neuritogenesis, such as microtubules, microtubule-associated proteins (MAPs) and intermediate filaments. We also provide the first evidence that a basic helix-loop-helix (bHLH) protein stimulates the expression of the cyclin-dependent kinase (CDK) inhibitors belonging to the INK4 family, which plays a role in promoting cell-cycle arrest. Finally, we show a dramatic synergistic effect between Nex1 and cAMP, resulting in an impressive regeneration of an elaborate and dense neurite network. Thus, Nex1 has endowed the PC12-Nex1 cells with a distinct combination of gene products that takes part in the complex regulation of neuritogenesis and regeneration.

Collaboration of JNKs and ERKs in nerve growth factor regulation of the neurofilament light chain promoter in PC12 cells

Nerve growth factor (NGF) induces transcription-dependent neural differentiation of PC12 cells, and the ERK family of MAPKs has been implicated as the dominant signal pathway that mediates this response. We employed a neurofilament light chain (NFLC) promoter-luciferase (NFLC-Luc) reporter to define the role of the ERKs as well as additional MAPK pathways in NGF induction of this neural specific gene. Constitutive active forms of c-Raf-1, MEKK1 and MKK6, proximal regulators of the ERKs, JNKs, and p38 MAPKs, respectively, all stimulated NFLC-Luc activity. NFLC-Luc activity stimulated by NGF, however, was partially (approximately 50%) inhibited by the MEK inhibitor, PD098059, or by co-transfection of kinase-inactive MEK1 but not by the p38 MAPK inhibitor, SB203580, indicating a role for the ERKs, but not the p38 MAPKs, in NGF regulation of the NFLC promoter. Importantly, a gain-of-function MKK7-JNK3 fusion protein stimulated NFLC-Luc and synergized with gain-of-function c-Raf-1 to activate the NFLC promoter. In addition, transfection of kinase-inactive forms of MEK1 and MKK7 produced an additive inhibition of NGF-stimulated NFLC-Luc relative to either inhibitor alone. These findings indicate that the ERK and JNK pathways collaborate downstream of the NGF receptor for regulation of the NFLC promoter. Truncation analysis and electromobility shift assays established the requirement for a cAMP-response element/activating transcription factor-like site in the NFLC promoter that minimally interacts with constitutively expressed cAMP-response element-binding protein and JunD as well as c-Jun which is induced by NGF in an ERK-dependent manner. Cumulatively, these findings indicate that the ERK pathway requires collaboration with the JNK pathway for maximal activation of the NFLC gene in PC12 cells through the integrated control of c-Jun function.

Neurofilament homeostasis and motoneurone degeneration

Neurofilament disorganisation is a hallmark of various neurodegenerative diseases. We review here current knowledge of neurofilament structure, gene expression and function. Neurofilament involvement in motoneurone neurological diseases is discussed in view of recent data from transgenic and spontaneous mouse mutants. In the mammalian neurone, the three neurofilament subunits are assembled into intermediate filaments as obligate heteropolymers. The subunits are expressed differentially during development and adult life according to the cell type and its physiological state. In addition to the well-established role of neurofilaments in the control of axonal calibre, there is increasing evidence that neurofilaments can interact with other cytoskeletal components and can modulate the axoplasmic flow. Although the extent to which neurofilament abnormalities contribute to the pathogenesis in human diseases remains unknown, emerging evidence suggests that disorganised neurofilaments can provoke degeneration and death of neurones. BioEssays 23:24-33, 2001.

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.

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.

Stability determinants are localized to the 3'-untranslated region and 3'-coding region of the neurofilament light subunit mRNA using a tetracycline-inducible promoter

The tetracycline-responsive expression system of Bujard was used to compare rates of decay of wild-type and mutant neurofilament (NF) light subunit (NF-L) mRNAs. Optimal conditions for activation and inactivation of the target transgene were determined using a luciferase reporter gene. Analyses of mRNA stability were thereupon conducted on cells that were doubly transfected with transactivator and inducible target genes and derived from pooled clones of transfected cells. Rates of mRNA decay were compared upon inactivation of the transgenes after high levels of mRNA had been induced. Deletion of the 445-nucleotide (nt) 3'-untranslated region (3'-UTR) (L/++(+)-) or 527 nt of the 3'-coding region (3'-CR) (L/++-+) increased the stability of NF-L mRNA compared with the full-length (L/++(++)) transcript in neuronal (N2a and P19 cells) and non-neuronal (L cells) lines. Deletion of both the 3'-UTR and 3'-CR (L/++--) led to a further stabilization of the transcript. A major stability determinant was then localized to a 68-nt sequence that forms the junction between the 3'-CR and 3'-UTR of NF-L and is the binding site of a unique ribonucleoprotein complex (Cañete-Soler, R., Schwartz, M. L., Hua, Y., and Schlaepfer, W. W. (1998) J. Biol. Chem. 273, 12655-12661). The studies establish a novel system for mapping determinants of mRNA stability and have applied the system to localize determinants that regulate the stability of the NF-L mRNA.

Transgenic analyses reveal developmentally regulated neuron- and muscle-specific elements in the murine neurofilament light chain gene promoter

We report here the developmental activity of regulatory elements that reside within 1.7 kilobases of the murine neurofilament light chain (NF-L) gene promoter. NF-L promoter activity is first detected at embryonic day 8.5 in neuroepithelial cells. Neuron-specific gene expression is maintained in the spinal cord until embryonic day 12.5 and at later developmental stages in the brain and sensory neuroepithelia. After day 14.5, the promoter becomes active in myogenic cells. Transgene expression in both neurons and muscle is consistent with the detection of endogenous NF-L transcript in both neuronal and myogenic tissues of neonates by reverse transcriptase-polymerase chain reaction. Neuron- and muscle-specific activities of the NF-L promoter decrease and are nearly undetectable after birth. Thus, the 1.7-kilobase NF-L promoter contains regulatory elements for initiation but not maintenance of transcription from the NF-L locus. Deletion analyses reveal that independent regulatory elements control the observed tissue-specific activities and implicate a potential MyoD binding site as the muscle-specific enhancer. Our results demonstrate that the NF-L promoter contains distinct regulatory elements for both neuron- and muscle-specific gene expression and that these activities are temporally separated during embryogenesis.

In vitro activation of the mouse mid-sized neurofilament gene by an NF-1-like transcription factor

In vitro transcription using nuclear extracts from rat brain and liver were used to assess the tissue-specific and functional elements of the mouse neurofilament mid-sized gene promoter (pNF-M). Deletion from -2.7 to -103 (relative to the start site of transcription) resulted in a small increase (2-fold) in the activity of the NF-M promoter in both extracts. Promoter strength was slightly higher in brain vs. liver extracts. Deletion to -49 resulted in a 10-fold loss of promoter activity in brain extracts and 6-fold drop in liver. Transcription in both extracts was TATA box-dependent. The region between -65 and -40 was shown to contain sequences responsible for high-level NF-M promoter activity in brain and liver extracts. Within this region are Sp1 and NF-1-like binding sites. Mutation of the NF-1-like site (-53/-39) caused a large drop in the activity of the NF-M promoter while mutation of the Sp1 site (-64/-57) possibly slightly diminished promoter activity in brain and liver extracts. Both the Sp1 and NF-1-like sites were shown by gel shift competition and supershift assays to be able to bind their respective factors. We conclude that the basic mouse NF-M promoter is a promiscuous promoter whose activity is modulated by a NF-1-like transcription factor. The lack of tissue specificity in an in vitro system strongly suggests an important role for chromatin structure in the regulation of the mouse NF-M promoter.

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

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

Transcriptional regulation of neurofilament expression by protein kinase A

RN46A cells, a conditionally immortalized neuronal cell line derived from E12 rat medullary raphe nucleus, upregulate low M(r) (68 kDa, neurofilament [NF]-L) and medium M(r) (160 kDa, NF-M) neurofilament protein expression upon activation of protein kinase A (PKA). To examine possible transcriptional regulation of neurofilament protein expression by PKA, two cell lines were used; RN46A cells and C alpha EV6 cells, a cell line derived from RN46A cells that stably expresses the catalytic subunit of PKA under the control of the metallothionein promoter. Treatment of RN46A cells with dbcAMP resulted in an increase in the steady-state levels of both NF-L and NF-M, but not high M(r) (200 kDa, NF-H) neurofilament mRNA. These increases were both time and dose dependent and were sensitive to treatment with the protein synthesis inhibitor cycloheximide. In C alpha EV6 cells, activation of PKA by 80 microM ZnSO4 upregulated the expression of C alpha mRNA with maximal levels reached 8 hr post-treatment and maintained at 24 hr. Reporter gene assays in C alpha EV6 cells following transfection with increasing lengths of the NF-L promoter demonstrated that both a putative Sp1-like and a cAMP response (CRE), but not a NGFI-A, element were likely involved in PKA-dependent activation of the NF-L promoter. Electrophoretic mobility shift assays confirmed these results but showed that the nuclear proteins induced by PKA which bound to the NF-L promoter Sp1-like sequence were not Sp1. Collectively, these data suggest that constitutively expressed Sp1 may be involved in basal NF-L promoter activity, and newly synthesized, PKA-dependent nuclear proteins may synergistically activate the rat NF-L promoter.

Characterization of DNase I hypersensitive sites in the mouse 68-kDa neurofilament gene

Four brain-specific DNase I hypersensitive sites (HSS) have previously been identified flanking the mouse 68-kDa neurofilament gene within a 1.7 kb upstream sequence which confers neuronal specificity of expression of this gene in transgenic mice. Previously several DNA-binding factors were detected at the HSS closest to the transcription start site (HSS1). However, no major brain-specific factors were identified, suggesting a possible role for the three remaining HSS in conferring tissue-specificity to the NF-L gene. Sequence analysis of the NF-L promoter region demonstrated the presence of an extensive CT repeat and several potential binding sites which are also found in other neurofilament promoters. Gel mobility shift assays revealed a similar but not identical banding pattern with brain and liver nuclear extracts at HSS2, and HSS3, however the banding pattern for HSS4 was predominantly brain-specific. DNase I footprinting revealed several factors binding to the upstream HSS regions in brain and liver nuclear extracts. These include a CCAAT box at HSS2, a novel brain-specific footprint near an adenovirus promoter element E2aE-C beta and a single liver-specific footprint associated with an POU/octamer binding site at HSS4. The presence of brain-specific gel shift bands and tissue-specific footprints associated with HSS4, suggest that this region may play an important role in the regulation of the NF-L gene.

Early upregulation of medium neurofilament gene expression in developing spinal cord of the wobbler mouse mutant

Homozygous wobbler mouse mutants develop a progressive paralysis due to spinal motoneuron degeneration. To understand the molecular aspect underlying the genetic defect we have studied the embryonic (from E13) and postnatal expression of the three neurofilament and choline acetyltransferase genes in each member from several wild-type (wt) and wobbler (wr) progenies. There are no variations among wt littermates at all ages studied. In contrast, analyses of neurofilament mRNA reveals a 3-4-fold increase of medium neurofilament (NFM) mRNA in wobbler mice (wr/wr). The pattern of increased NFM mRNA during development, prior to the appearance of the wobbler phenotype, among littermates (from heterozygous carriers) conforms to a mendelian inheritance of a single gene defect 1:2:1 (wr/wr:wr/+:+/+). Light and heavy neurofilament mRNA levels are also increased later in development exclusively in those individuals with high NFM mRNA values indicating that increase of the latter is associated with increase of the light and heavy subunit expression. Also NF proteins are increased. Expression of choline acetyltransferase gene is instead always comparable to normal control. Our study provides novel insights into the nature of the wobbler defect, strengthening the hypothesis that neurofilament accumulation plays a pivotal role in the etiopathogenesis of motoneuron degeneration.

Deletion of 3'-untranslated region alters the level of mRNA expression of a neurofilament light subunit transgene

High levels of neurofilament (NF) mRNA expression are attained during early postnatal development and are a major determinant of axonal size. High level NF expression is also dependent upon axonal continuity since NF mRNA levels are down-regulated after nerve transection. This study shows that both postnatal up-regulation and axotomy-induced down-regulation are altered by deletion of 3'-UTR from the mouse light NF subunit (NF-L). Transgenes with (NF-L+) or without (NF-L-) 3'-UTR display similar patterns of neuron-specific expression but differ in their respective levels of expression. Whereas changes in the level of NF-L+ mRNA parallel those of the endogenous mouse NF-L mRNA, changes in the level of NF-L- mRNA differ from the pattern of endogenous NF-L expression during postnatal up-regulation and axotomy-induced down-regulation. Specifically, the NF-L- transgene undergoes a 3-fold aberrant up-regulation between embryonic days 15 (E15) and 18 (E18) and has lost its susceptibility to axotomy-induced down-regulation. Studies of transfected P19 cells show that 3'-UTR deletion leads to a severalfold stabilization of NF-L mRNA and an increase in steady-state mRNA level. The findings support the working hypothesis that the 3'-UTR contains determinants that alter stability and that stabilization of NF-L mRNA regulates the levels of NF-L mRNA in neuronal tissues and cells.

The first intron of the mouse neurofilament light gene (NF-L) increases gene expression

Neurofilament expression is developmentally and post-transcriptionally controlled. Using transient transfection assays in mouse L cells, we demonstrate that the expression of the mouse neurofilament light subunit (NF-L) is influenced by intron sequences. NF-L expression was decreased twenty fold upon deletion of the three intron sequences. Elements contained principally within a 350 bp region of intron 1 were responsible for enhanced NF-L expression. Enhancement of expression did not occur when intron I was placed 3' to a heterologous chloramphenicol acetyl transferase (CAT) gene whose expression was driven by NF-L 5' sequences. The intron enhancement of NF-L expression was not promoter-specific and also occurred with the mouse sarcoma virus (MSV) LTR promoter. These data suggest intron sequences may be important in regulating NF gene expression.

Overexpression of the human NFM subunit in transgenic mice modifies the level of endogenous NFL and the phosphorylation state of NFH subunits

Neurofilaments (NFs), the major intermediate filaments of central nervous system (CNS) and peripheral nervous system (PNS) neurons, are heteropolymers formed from the high (NFH), middle (NFM), and low (NFL) molecular weight NF subunits. To gain insights into how the expression of NF subunit proteins is regulated in vivo, two transgenes harboring coding sequences for human NFM (hNFM) with or without the hNFM multiphosphorylation repeat domain were introduced into mice. Expression of both hNFM constructs was driven by the hNFM promoter and resulted in increased levels of hNFM subunits concomitant with an elevation in the levels of mouse NFL (mNFL) proteins in the CNS of both lines of transgenic mice. The increased levels of mNFL appear specific to NFM because previous studies of transgenic mice overexpressing either NFL or NFH did not result in increased expression of either of the other two NF subunits. Further, levels of the most heavily phosphorylated isoforms of mouse NFH (mNFH) were reduced in the brains of these transgenic mice, and electron microscopic studies showed a higher packing density of NFs in large-diameter CNS axons of transgenic versus wild-type mice. Thus, reduced phosphorylation of the mNFH carboxy terminal domain may be a compensatory response of CNS neurons to the increase in NFs, and reduced negative charges on mNFH sidearms may allow axons to accommodate more NFs by increasing their packing density. Taken together, these studies imply that NFM may play a dominant role in the in vivo regulation of the levels of NFL protein, the stoichiometry of NF subunits, and the phosphorylation state of NFH. NFM and NFH proteins may assume similar functions in regulation of NF packing density in vivo.

Transcriptional and post-transcriptional mechanisms regulating neurofilament and tubulin gene expression during normal development of the rat brain

The transcription of the beta II-, beta IV- and alpha 1-tubulin genes as well as that of the three neurofilament genes, NF-L, NF-M and NF-H, was examined during the course of postnatal brain development. Changes in the transcriptional activity of these genes were studied using run-off transcription assays with nuclei isolated from the rat cerebral cortex at postnatal days P2, P5, P10 and adult stages. Northern blotting of total RNA isolated from the cerebral cortex was used to compare changes in steady-state mRNA levels with transcriptional changes that occurred in the cerebral cortex during the postnatal interval. Nuclear run-off assays showed that beta II- and alpha 1-tubulin gene transcription rates were maximal from P2-P5 and declined at later times. Changes in the steady-state mRNA levels for these two genes followed the same general pattern as transcription, but in the case of beta II-tubulin mRNA, were more dramatic. beta IV-tubulin gene transcription dropped between P2 and P5 and then increased progressively to the adult stage, coordinate with an increase in beta IV-tubulin steady-state mRNA levels. NF-L and NF-H genes showed similar patterns of transcriptional change during the postnatal interval, with maximal rates of transcription at P5 followed by a decline at later times. The steady-state levels of NF-L and NF-H mRNAs changed in a manner opposite to that of transcription and increased progressively during the postnatal interval. This suggests that mRNA stabilization is the main factor regulating the steady-state levels of NF-L and NF-H mRNAs in postnatal brain. For the NF-M gene, the developmental transcription pattern was also dissociated from steady-state mRNA level changes, but differed from the transcription patterns of the NF-L and NF-H genes. This suggests the importance of post-transcriptional mechanisms in regulating NF-M mRNA levels in brain and also indicates that some differences exist in the regulatory mechanisms which control NF-M compared to NF-L and NF-H mRNA levels.

Characterization of the rat light neurofilament (NF-L) gene promoter and identification of NGF and cAMP responsive regions

We have isolated a genomic DNA clone covering the coding and 14 kb upstream region of the rat light neurofilament (NF-L) gene and sequenced 2.3 kb of its promoter. DNase I hypersensitive sites have been mapped in PC12 cells. For functional analysis of the NF-L promoter, constructs carrying 38, 97, 407, 564, 650, 1,099, 1,660, 2,003 base pairs (bp) upstream region in front of the chloramphenicol acetyltransferase (CAT) reporter gene were tested for their capability to direct CAT expression after transient transfection into various cell lines. Similar CAT activities were recorded both in rat pheochromocytoma (PC12) and mouse neuroblastoma N115 cells and also in several nonneural cell lines (HeLa, C127, NIH 3T3). Regions responsible for the basic promoter activity were located between -407 and +75 bp from the transcription initiation site. The NGF-responsive element was located between -38 and +75 bp, and sequence -97 to -38 was found to contain a functional cAMP-responsive element. In PC12 cells in which nerve growth factor (NGF) induces neurite outgrowth and NF-L transcription, NF-L promoter-driven CAT expression was stimulated up to 12-fold within three days of NGF treatment, whereas epidermal growth factor (EGF) had no effect. Rat NF-L promoter contained Sp1, AP-2 and CGCCCCCGC elements. In PC12 cells, NGF transiently induced the binding of transcription factors to the deoxyoligonucleotide probes containing the binding sites of these elements. The role of these factors in NF-L gene transcriptional induction by NGF in PC12 cells is discussed.

Neuronal intermediate filaments: new progress on an old subject

Neurofilaments (NFs) are the major intermediate filaments in most mature neurons. Genetic approaches have now proven that NFs are an essential determinant for radial growth of axons. NF phosphorylation most probably plays an important role in this function. Further, forced over-expression of NF subunits in transgenic mice yields NF misaccumulation in motor neurons and, subsequently, causes motor neuron dysfunction. This has important implications for human motor neuron diseases because similar accumulations are nearly universally found in the early stages of many motor neuron disorders.

Both upstream and intragenic sequences of the human neurofilament light gene direct expression of lacZ in neurons of transgenic mouse embryos

Initial expression of the neurofilament light gene coincides with the appearance of postmitotic neurons. To investigate the molecular mechanisms involved in neuron-specific gene expression during embryogenesis, we generated transgenic mice carrying various regions of the human neurofilament light gene (hNF-L) fused to the lacZ reporter gene. We found that 2.3 or 0.3 kb of the hNF-L promoter region directs expression of lacZ in neurons of transgenic embryos. Addition of 1.8 kb hNF-L intragenic sequences (IS) enlarges the neuronal pattern of transgene expression. The 2.3-kb hNF-L promote lacZ-IS construct contains all regulatory elements essential for both spatial and temporal expression of the hNF-L gene during embryogenesis and in the adult. The use of a heterologous promoter demonstrated that the 1.8-kb hNF-L intragenic sequences are sufficient to direct the expression of lacZ in a NF-L-specific manner both temporally and spatially during development and in the adult. We conclude that these hNF-L intragenic sequences contain cis-acting DNA regulatory elements that specify neuronal expression. Taken together, these results show that the neurofilament light gene contains separate upstream and intragenic elements, each of which directs lacZ expression in embryonic neurons.

Negative regulatory elements upstream of a novel exon of the neuronal nicotinic acetylcholine receptor alpha 2 subunit gene

The expression of the nicotinic acetylcholine receptor alpha 2 subunit gene is highly restricted to the Spiriform lateralis nucleus of the Chick diencephalon. As a first step toward understanding the molecular mechanism underlying this regulation, we have investigated the structural and regulatory properties of the 5' sequence of this gene. A strategy based on the ligation of an oligonucleotide to the first strand of the cDNA (SLIC) followed by PCR amplification was used. A new exon was found approximately 3kb upstream from the first coding exon, and multiple transcription start sites of the gene were mapped. Analysis of the flanking region shows many consensus sequences for the binding of nuclear proteins, suggesting that the 1 kb flanking region contains at least a portion of the promoter of the gene. We have analysed the negative regulatory elements present within this region and found that a silencer region located between nucleotide -144 and +76 is active in fibroblasts as well as in neurons. This silencer is composed of six tandem repeat Oct-like motifs (CCCCATGCAAT), but does not bind any member of the Oct family. Moreover these motifs were found to act as a silencer only when they were tandemly repeated. When two, four or five motifs were deleted, the silencer activity of the motifs unexpectedly became an enhancer activity in all cells we have tested.

Methylation and expression of neurofilament genes in tissues and in cell lines of the mouse

The light (NF-L), mid-sized (NF-M) and heavy (NF-H) neurofilament (NF) genes were probed with methylation-sensitive restriction enzymes and patterns of methylation and expression of the NF genes were compared in tissues and cell lines of the mouse. The 5' regions of all three NF genes are identified as CpG islands that remain unmethylated in expressing and non-expressing tissues, although partial methylation occurs at -795 in NF-H and at -525 in NF-M. Methylation of the NF CpG islands is associated with the inactivation of NF genes in L cells and with the selective inactivation of NF-L and NF-M in Neuro 2a cells. We also show that methylation diminishes the ability of the NF promoters to drive transcription of a CAT reporter gene. Hence, the presence of CpG islands may be important in determining patterns of NF transcription in vitro. Moreover, the preservation of CpG islands may be an evolutionary link that bears upon the nature of the NF genes and the mechanisms that have evolved to limit NF expression.

Functional analysis of the human neurofilament light chain gene promoter

We have carried out a structural and functional analysis on the human NF-L (H-NF-L) gene. It contains a methylation-free island, spanning the 5' flanking sequences and the first exon and a number of neuronal-specific DNase I hypersensitive sites have been identified in the upstream region as well as within the body of the gene. Analysis in cell lines and transgenic mice using a combination of these sites has revealed the presence of a conserved element(s) between -300bp and -190bp which is required for neuronal-specific expression.