Multiple nuclear factors interact with the promoter of the human neurofilament M gene

Regulation of neural stem cell differentiation by transcription factors HNF4-1 and MAZ-1

Neural stem cells (NSCs) are promising candidates for a variety of neurological diseases due to their ability to differentiate into neurons, astrocytes, and oligodentrocytes. During this process, Rho GTPases are heavily involved in neuritogenesis, axon formation and dendritic development, due to their effects on the cytoskeleton through downstream effectors. The activities of Rho GTPases are controlled by Rho-GDP dissociation inhibitors (Rho-GDIs). As shown in our previous study, these are also involved in the differentiation of NSCs; however, little is known about the underlying regulatory mechanism. Here, we describe how the transcription factors hepatic nuclear factor (HNF4-1) and myc-associated zinc finger protein (MAZ-1) regulate the expression of Rho-GDIγ in the stimulation of NSC differentiation. Using a transfection of cis-element double-stranded oligodeoxynucleotides (ODNs) strategy, referred to as "decoy" ODNs, we examined the effects of HNF4-1 and MAZ-1 on NSC differentiation in the NSC line C17.2. Our results show that HNF4-1 and MAZ-1 decoy ODNs significantly knock down Rho-GDIγ gene transcription, leading to NSC differentiation towards neurons. We observed that HNF4-1 and MAZ-1 decoy ODNs are able enter to the cell nucleolus and specifically bind to their target transcription factors. Furthermore, the expression of Rho-GDIγ-mediated genes was identified, suggesting that the regulatory mechanism for the differentiation of NSCs is triggered by the transcription factors MAZ-1 and HNF4-1. These findings indicate that HNF4-1 and MAZ-1 regulate the expression of Rho-GDIγ and contribute to the differentiation of NSCs. Our findings provide a new perspective within regulatory mechanism research during differentiation of NSCs, especially the clinical application of transcription factor decoys in vivo, suggesting potential therapeutic strategies for neurodegenerative disease.

Nuclear factor I genes regulate neuronal migration

Neuronal migration plays a central role in the formation of the brain, and deficits in this process can lead to aberrant brain function and subsequent disease. Neuronal migration is a complex process that involves the interaction of the neuron with the surrounding environmental milieu, and as such involves both cell-intrinsic and cell-extrinsic mechanisms. Studies performed in rodent models to investigate the formation of brain structures have provided key insights into how neuronal migration is coordinated during development. Within the cerebral cortex, glutamatergic neurons derived from the cortical ventricular zone migrate radially into the cortical plate, whereas interneurons derived within the ventrally located ganglionic eminences migrate tangentially into the cortex. Within the embryonic cerebellum, cerebellar granule neuron progenitors migrate from the rhombic lip over the surface of the cerebellar anlage, before differentiating and migrating radially into the internal granule layer of the cerebellum perinatally. In this review, we focus on one family of proteins, the nuclear factor I transcription factors, and review our understanding of how these molecules contribute to the formation of the hippocampus and the cerebellum via the regulation of neuronal migration.

Absence of the transcription factor Nfib delays the formation of the basilar pontine and other mossy fiber nuclei

Transcription factors of the Nuclear Factor I (Nfi) family are important for the development of specific neuronal and glial populations in the nervous system. One such population, the neurons of the basilar pontine nuclei, expresses high levels of Nfi proteins, and the pontine nuclei are greatly reduced in mice lacking a functional Nfib gene. Pontine neurons, along with other precerebellar neurons that populate the hindbrain, arise from precursors in the lower rhombic lip and migrate anteroventrally to reach their final location. Using immunohistochemistry, we find that NFI-B expression is specific for mossy fiber populations of the precerebellar system. Analysis of the Nfib(-/-) hindbrain indicates that the development of the basilar pontine nuclei is delayed, with pontine neurons migrating 1-2 days later than in control animals, and that significantly fewer pontine neurons are produced. While the mossy fiber nuclei of the caudal medulla do form, they also exhibit a developmental delay. Nfia and Nfix null mice exhibit no apparent pontine phenotype, implying specificity in the action of NFI family members. Collectively, these data demonstrate that Nfib plays an important role in the generation of precerebellar mossy fiber neurons, and may do so at least in part by regulating neurogenesis.

Nuclear factor one transcription factors in CNS development

Transcription factors are key regulators of central nervous system (CNS) development and brain function. Research in this area has now uncovered a new key player-the nuclear factor one (NFI) gene family. It has been almost a decade since the phenotype of the null mouse mutant for the nuclear factor one A transcription factor was reported. Nfia null mice display a striking brain phenotype including agenesis of the corpus callosum and malformation of midline glial populations needed to guide axons of the corpus callosum across the midline of the developing brain. Besides NFIA, there are three other NFI family members in vertebrates: NFIB, NFIC, and NFIX. Since generation of the Nfia knockout (KO) mice, KO mice for all other family members have been generated, and defects in one or more organ systems have been identified for all four NFI family members (collectively referred to as NFI here). Like the Nfia KO mice, the Nfib and Nfix KO mice also display a brain phenotype, with the Nfib KO forebrain phenotype being remarkably similar to that of Nfia. Over the past few years, studies have highlighted NFI as a key payer in a variety of CNS processes including axonal outgrowth and guidance and glial and neuronal cell differentiation. Here, we discuss the importance and role of NFI in these processes in the context of several CNS systems including the neocortex, hippocampus, cerebellum, and spinal cord at both cellular and molecular levels.

Lineage pathway of human brain progenitor cells identified by JC virus susceptibility

Multipotential human central nervous system progenitor cells, isolated from human fetal brain tissue by selective growth conditions, were cultured as undifferentiated, attached cell layers. Selective differentiation yielded highly purified populations of neurons or astrocytes. This report describes the novel use of this cell culture model to study cell type-specific recognition of a human neurotropic virus, JC virus. Infection by either JC virions or a plasmid encoding the JC genome demonstrated susceptibility in astrocytes and, to a lesser degree, progenitor cells, whereas neurons remained nonpermissive. JC virus susceptibility correlated with significantly higher expression of the NFI-X transcription factor in astrocytes than in neurons. Furthermore, transfection of an NFI-X expression vector into progenitor-derived neuronal cells before infection resulted in viral protein production. These results indicate that susceptibility to JC virus infection occurs at the molecular level and also suggest that differential recognition of the viral promoter sequences can predict lineage pathways of multipotential progenitor cells in the human central nervous system.

The Nuclear Factor I (NFI) gene family in mammary gland development and function

Mammary gland development and function require the coordinated spatial and temporal expression of a large fraction of the mammalian genome. A number of site-specific transcription factors are essential to achieve the appropriate growth, branching, expansion, and involution of the mammary gland throughout early postnatal development and the lactation cycle. One family of transcription factors proposed to play a major role in the mammary gland is encoded by the Nuclear Factor I (NFI) genes. The NFI gene family is found only in multicellular animals, with single genes being present in flies and worms and four genes in vertebrates. While the NFI family expanded and diversified prior to the evolution of the mammary gland, it is clear that several mammary-gland specific genes are regulated by NFI proteins. Here we address the structure and evolution of the NFI gene family and examine the role of the NFI transcription factors in the expression of mammary-gland specific proteins, including whey acidic protein and carboxyl ester lipase. We discuss current data showing that unique NFI proteins are expressed during lactation and involution and suggest that the NFI gene family likely has multiple important functions throughout mammary gland development.

Differential interactions of specific nuclear factor I isoforms with the glucocorticoid receptor and STAT5 in the cooperative regulation of WAP gene transcription

The distal region (-830 to -720 bp) of the rat whey acidic protein (WAP) gene contains a composite response element (CoRE), which has been demonstrated previously to confer mammary gland-specific and hormonally regulated WAP gene expression. Point mutations in the binding sites for specific transcription factors present within this CoRE have demonstrated the importance of both nuclear factor I (NFI) and STAT5 as well as cooperative interactions with the glucocorticoid receptor (GR) in the regulation of WAP gene expression in the mammary gland of transgenic mice. This study reports the characterization of NFI gene expression during mammary gland development and the identification and cloning of specific NFI isoforms (NFI-A4, NFI-B2, and NFI-X1) from the mouse mammary gland during lactation. Some but not all of these NFI isoforms synergistically activate WAP gene transcription in cooperation with GR and STAT5, as determined using transient cotransfection assays in JEG-3 cells. On both the WAP CoRE and the mouse mammary tumor virus long terminal repeat promoter, the NFI-B isoform preferentially activated gene transcription in cooperation with STAT5A and GR. In contrast, the NFI-A isoform suppressed GR and STAT cooperativity at the WAP CoRE. Finally, unlike their interaction with the NFI consensus binding site in the adenovirus promoter, the DNA-binding specificities of the three NFI isoforms to the palindromic NFI site in the WAP CoRE were not identical, which may partially explain the failure of the NFI-A isoform to cooperate with GR and STAT5A.

Interaction of Myc-associated zinc finger protein with DCC, the product of a tumor-suppressor gene, during the neural differentiation of P19 EC cells

Expression of the DCC (deleted in colorectal cancer) protein is strongly induced during the neural differentiation of mouse P19 embryonal carcinoma (EC) cells that occurs when these cells are treated with retinoic acid (RA). Myc-associated zinc finger protein (MAZ) is a DNA-binding protein that is widely expressed and functions in human, mouse and hamster cells as an activator, an initiator or a terminator of transcription. However, the biological functions of MAZ remain elusive. We report here that MAZ associates with the cytoplasmic domain of the DCC protein in vivo and in vitro. Yeast two-hybrid assays confirmed this association. An immunofluorescence study demonstrated that DCC protein is expressed at elevated levels in neuron-like P19 EC cells, in particular in axons, in which the MAZ protein is also expressed. We found that MAZ was translocated from the nucleus to the cytoplasm during the RA-induced terminal differentiation of P19 EC cells with resultant loss of the ability of MAZ to bind to the ME1a1 site of the c-myc promoter. Taken together, our observations imply that the DCC protein might play a critical role as a signaling molecule in the regulation of the transcriptional activity of MAZ during the neural differentiation of P19 EC cells.

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.

Roles of the NFI/CTF gene family in transcription and development

The Nuclear Factor I (NFI) family of site-specific DNA-binding proteins (also known as CTF or CAAT box transcription factor) functions both in viral DNA replication and in the regulation of gene expression. The classes of genes whose expression is modulated by NFI include those that are ubiquitously expressed, as well as those that are hormonally, nutritionally, and developmentally regulated. The NFI family is composed of four members in vertebrates (NFI-A, NFI-B, NFI-C and NFI-X), and the four NFI genes are expressed in unique, but overlapping, patterns during mouse embryogenesis and in the adult. Transcripts of each NFI gene are differentially spliced, yielding as many as nine distinct proteins from a single gene. Products of the four NFI genes differ in their abilities to either activate or repress transcription, likely through fundamentally different mechanisms. Here, we will review the properties of the NFI genes and proteins and their known functions in gene expression and development.

Disruption of the murine nuclear factor I-A gene (Nfia) results in perinatal lethality, hydrocephalus, and agenesis of the corpus callosum

The phylogenetically conserved nuclear factor I (NFI) family of transcription/replication proteins is essential both for adenoviral DNA replication and for the transcription of many cellular genes. We showed previously that the four murine NFI genes (Nfia, Nfib, Nfic, and Nfix) are expressed in unique but overlapping patterns during mouse development and in adult tissues. Here we show that disruption of the Nfia gene causes perinatal lethality, with >95% of homozygous Nfia(-/-) animals dying within 2 weeks after birth. Newborn Nfia(-/-) animals lack a corpus callosum and show ventricular dilation indicating early hydrocephalus. Rare surviving homozygous Nfia(-/-) mice lack a corpus callosum, show severe communicating hydrocephalus, a full-axial tremor indicative of neurological defects, male-sterility, low female fertility, but near normal life spans. These findings indicate that while the Nfia gene appears nonessential for cell viability and DNA replication in embryonic stem cells and fibroblasts, loss of Nfia function causes severe developmental defects. This finding of an NFI gene required for a developmental process suggests that the four NFI genes may have distinct roles in vertebrate development.

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.

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.

Developmental analysis of factors binding to the mouse 68-kDa neurofilament promoter

Whole tissue extracts prepared from mouse brain regions at various postnatal ages were characterized for binding of factors to the DNase I hypersensitive site (HSSI) which is located closest to the transcription start site of the 68-kDa mouse neurofilament gene (NF-L). Gel mobility shift assays detected changes in factor binding during postnatal development of the neocortex. Competition experiments suggested that one of the complexes resulted from factor binding to a 9 bp sequence found in both the light and medium neurofilament promoter regions (NF-L/M). Gel mobility shifts performed with an oligonucleotide probe containing the NF-L/M sequence detected two brain-specific DNA-protein complexes, and a third complex in both brain and liver. During cerebellar and neocortical development, one of the NF-L/M complexes was most intense at postnatal day 10 when transcription of the NF-L gene is upregulated.

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.

Long-term expression of the 35,000 mol. wt fos-related antigen in rat brain after kainic acid treatment

Systemic injection of kainic acid, a rigid analogue of glutamate, induces both the short-term and the long-term expression of activator protein-1 transcription factors. The short-term responses of activator protein-1 factors such as c-fos and fos-related antigens have been well studied. However, the long-term expression of activator protein-1 factor(s) induced by kainic acid is poorly understood. The present study was designed to document the long-term expression (up to seven months) of the fos-related antigens and to map their distributions in the rat brain after systemic treatment with kainic acid. A single dose of kainic acid (8 mg/kg) was injected i.p. into Fischer 344 rats and their epileptic seizure behaviour was monitored. The rats with full limbic seizures were chosen for long-term study. By using immunocytochemistry with an antibody that cross-reacts with all known fos-related antigens, western blot analysis and a gel mobility-shift assay, we have now shown that a 35,000 mol. wt fos-related antigen was induced by kainic acid treatment and expressed at high levels for up to five months. This fos-related antigen still maintains the activator protein-1 DNA binding activity in the rat brain seven months after kainic acid treatment. The fos-related antigens and activator protein-1 binding activity were continuously expressed at high levels throughout the experimental period in the dentate granule cells where mossy fibre collateral sprouting occurred after kainic acid treatment. Our results suggested that long-term expression of fos-related antigen may reflect the pathophysiological changes after kainic acid administration.

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.

The 66-kDa neurofilament protein (NF-66): sequence analysis and evolution

A 2.5 kb cDNA clone encoding the mouse 66 kd neurofilament protein (NF-66) was isolated and sequenced. The deduced protein sequence contains 501 amino acid residues. Comparison of the mouse, rat and human NF-66 indicated > 90% homology in protein sequence and 85% homology in coding nucleotide sequence. A high degree of homology was observed between NF-66 and other intermediate filament proteins especially in the alpha-helical domain. Zooblot analyses suggested that the putative ancestral gene for vimentin and NF-66 was detectable in the avian. By comparison, the ancestral sequence for GFAP appeared after that for vimentin.

Sequencing of the rat light neurofilament promoter reveals differences in methylation between expressing and non-expressing cell lines, but not tissues

The DNA methylation pattern of the promoter (pNF-L) region of the rat light-neurofilament-encoding gene (NF-L), a neuron-specific gene, was assessed in NF-L expressing and non-expressing cell lines and tissues by genomic sequencing using PCR amplification of bisulfite-modified DNA. We analysed twenty-five potential CpG methylation sites between nucleotide (nt) positions -311 and +103 of pNF-L, containing Sp1- and AP-2-binding sites, a CGCCCCCGC box and a cAMP-responsive element. Six out of 25 possible CpG methylation sites are within these elements. The pNF-L promoter was unmethylated in NF-L-expressing rat brain, as well as in liver not expressing NF-L. In NF-L-expressing PC12 cells, the promoter was unmethylated, whereas in non-expressing glioma C6 cells intensive methylation occurred. A cluster of methylated CpG dinucleotides spanned the region from nt -176 to -67 bp. Thus, methylation of this promoter region could play a role in silencing NF-L in the glioma cell line in vitro, but not in liver tissue in vivo. In a non-CpG sequence, in the CpApG trinucleotide at nt position -114, cytosine was found to be partially methylated. It is thus possible to describe the methylation state of each cytosine present in the area of genomic DNA of interest.

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.

Neurofilament-deficient axons and perikaryal aggregates in viable transgenic mice expressing a neurofilament-beta-galactosidase fusion protein

Interactions between neurofilament side arms may modulate axon caliber. To investigate this hypothesis, we derived transgenic mice expressing a fusion protein in which the carboxyl terminus of the high molecular weight neurofilament protein (NFH) was replaced by beta-galactosidase. The transgene, regulated by NFH sequences, was expressed in projection neurons. However, the fusion protein remained in perikarya precipitating large filamentous aggregates. Axons were not invested with neurofilaments and developed only small calibers. Perikaryal aggregates, with similar structural features, are associated with neurodegenerative diseases, but these mice showed few ill effects and their neurons rarely degenerated. We conclude that an organized neurofilament cytoskeleton is required by axons to achieve large calibers but is not essential for neuronal function or extended survival.

Molecular biology of neuronal intermediate filaments

In the past few years, several neuronal intermediate filament proteins have been characterized. While ongoing investigations have continued to shed light on their developmental expression, the importance of different domains of the proteins for assembly, the elements in their genes necessary for tissue-specific expression, and the role of phosphorylation of neurofilaments, the function(s) of these structures remain a matter of speculation.

Targeting of Sp1 to a non-Sp1 site in the human neurofilament (H) promoter via an intermediary DNA-binding protein

The human neurofilament (H) promoter contains multiple binding sites for nuclear proteins including a Proximal (Prox) site centered around the sequence GGTTGGACC and an adjacent pyrimidine (Pyr) tract site centered around the sequence CCCTCCTCCCC. Surprisingly binding to a probe containing the Prox/Pyr region of the NF(H) promoter was competed in gel shifts by an oligonucleotide containing only an Sp1 binding site (GGGGCGGGG). Supershift assays with a polyclonal anti-Sp1 antisera confirmed that Sp1 was part of the complex formed with the Prox/Pyr probe. However neither bacterially expressed Sp1 516C or vaccinia virus expressed full-length Sp1 778C bound to the Prox or Pyr sequences in DNase I footprints or gel shift assays. Gel shift competitions and supershift assays with probes containing either Prox or Pyr tract sites alone demonstrated targeting of Sp1 to the Prox binding site and identified a non-Sp1 containing complex which contains a Prox binding protein. Adding exogenous Sp1 to a HeLa nuclear extract enhanced the Sp1-containing complex but had no effect on the Prox complex. These studies show that Sp1 can be targeted to a non-Sp1 site in the human NF(H) promoter through protein/protein interactions with a distinct sequence specific DNA-binding protein.

Novel DNA binding proteins participate in the regulation of human neurofilament H gene expression

By a combination of DNase I footprinting, methylation interference, and gel shift analyses we have identified multiple binding sites for nuclear proteins within the promoter region of the human neurofilament H gene. Two sites likely bind the transcription factor Sp1 while two others may be targets for previously unrecognized DNA binding proteins. One site, PAL, occurs within the 10 bp sequence GGGGAGGAGG. Two copies of the PAL sequence form an interrupted palindrome around one of the Sp1 sites. A second site, PROX, is found within the sequence GGTTGGACC. Nuclear extracts prepared from both neural and non-neural cell lines, mouse brain, and mouse liver contain proteins that recognize and bind to the PROX and PAL sequences indicating that proteins which bind to these target sequences are widespread. The appearance of these target sequences in the 5' upstream region of several neuron specific genes suggests that they play key roles in the transcription of neuron specific genes. The functional activity of these target DNA sequences was demonstrated by transfection assays using a reporter gene fused to nested deletions of the NF(H) promoter region. Interestingly, these assays revealed that maximal transient expression was obtained with DNA fusion genes containing the PAL, PROX and TATA sequences. Inclusion of the Sp1 sites into the fusion genes failed to enhance the expression of the reporter gene. To determine if the NF(H) promoter can be activated in a tissue specific manner during development transgenic mice containing the promoter region linked to a beta-galactosidase reporter gene were generated. In one line sporadic expression of the transgene occurred in the CNS and testis while in four other lines no expression occurred. Collectively these results suggest that the NF(H) gene promoter is active in a tissue specific manner only by interactions with regulatory elements that lie further upstream or downstream of the start site of initiation.