Analysis of SCG10 gene expression in transgenic mice reveals that neural specificity is achieved through selective derepression

AAV-p40 bioengineering platform for variant selection based on transgene expression

The power of AAV directed evolution for identifying novel vector variants with improved properties is well established, as evidenced by numerous publications reporting novel AAV variants. However, most capsid variants reported to date have been identified using either replication-competent selection platforms or PCR-based capsid DNA recovery methods, which can bias the selection towards efficient replication or unproductive intracellular trafficking, respectively. A central objective of this study was to validate a functional transduction (FT)-based method for rapid identification of novel AAV variants based on AAV capsid mRNA expression in target cells. We performed a comparison of the FT platform to existing replication competent strategies. Based on the selection kinetics and function of novel capsids identified in an in vivo screen in a xenograft model of human hepatocytes, we identified the mRNA-based FT selection as the most optimal AAV selection method. Lastly, to gain insight into the mRNA-based selection mechanism driven by the native AAV-p40 promoter, we studied its activity in a range of in vitro and in vivo targets. We found AAV-p40 to be a ubiquitously active promoter that can be modified for cell type-specific expression by incorporating binding sites for silencing transcription factors, allowing for cell-type-specific library selection.

Neurogenic maturation of human dental pulp stem cells following neurosphere generation induces morphological and electrophysiological characteristics of functional neurons

Cell-based therapies are emerging as an alternative treatment option to promote functional recovery in patients suffering from neurological disorders, which are the major cause of death and permanent disability. The present study aimed to differentiate human dental pulp stem cells (hDPSCs) toward functionally active neuronal cells in vitro. hDPSCs were subjected to a two-step protocol. First, neuronal induction was acquired through the formation of neurospheres, followed by neuronal maturation, based on cAMP and neurotrophin-3 (NT-3) signaling. At the ultrastructural level, it was shown that the intra-spheral microenvironment promoted intercellular communication. hDPSCs grew out of the neurospheres in vitro and established a neurogenic differentiated hDPSC culture (d-hDPSCs) upon cAMP and NT-3 signaling. d-hDPSCs were characterized by the increased expression of neuronal markers such as neuronal nuclei, microtubule-associated protein 2, neural cell adhesion molecule, growth-associated protein 43, synapsin I, and synaptophysin compared with nondifferentiated hDPSCs. Enzyme-linked immunosorbent assay demonstrated that the secretion of brain-derived neurotrophic factor, vascular endothelial growth factor, and nerve growth factor differed between d-hDPSCs and hDPSCs. d-hDPSCs acquired neuronal features, including multiple intercommunicating cytoplasmic extensions and increased vesicular transport, as shown by the electron microscopic observation. Patch clamp analysis demonstrated the functional activity of d-hDPSCs by the presence of tetrodotoxin- and tetraethyl ammonium-sensitive voltage-gated sodium and potassium channels, respectively. A subset of d-hDPSCs was able to fire a single action potential. The results reported in this study demonstrate that hDPSCs are capable of neuronal commitment following neurosphere formation, characterized by distinct morphological and electrophysiological properties of functional neuronal cells.

Silencer-delimited transgenesis: NRSE/RE1 sequences promote neural-specific transgene expression in a NRSF/REST-dependent manner

BACKGROUND

We have investigated a simple strategy for enhancing transgene expression specificity by leveraging genetic silencer elements. The approach serves to restrict transgene expression to a tissue of interest - the nervous system in the example provided here - thereby promoting specific/exclusive targeting of discrete cellular subtypes. Recent innovations are bringing us closer to understanding how the brain is organized, how neural circuits function, and how neurons can be regenerated. Fluorescent proteins enable mapping of the 'connectome', optogenetic tools allow excitable cells to be short-circuited or hyperactivated, and targeted ablation of neuronal subtypes facilitates investigations of circuit function and neuronal regeneration. Optimally, such toolsets need to be expressed solely within the cell types of interest as off-site expression makes establishing causal relationships difficult. To address this, we have exploited a gene 'silencing' system that promotes neuronal specificity by repressing expression in non-neural tissues. This methodology solves non-specific background issues that plague large-scale enhancer trap efforts and may provide a means of leveraging promoters/enhancers that otherwise express too broadly to be of value for in vivo manipulations.

RESULTS

We show that a conserved neuron-restrictive silencer element (NRSE) can function to restrict transgene expression to the nervous system. The neuron-restrictive silencing factor/repressor element 1 silencing transcription factor (NRSF/REST) transcriptional repressor binds NRSE/repressor element 1 (RE1) sites and silences gene expression in non-neuronal cells. Inserting NRSE sites into transgenes strongly biased expression to neural tissues. NRSE sequences were effective in restricting expression of bipartite Gal4-based 'driver' transgenes within the context of an enhancer trap and when associated with a defined promoter and enhancer. However, NRSE sequences did not serve to restrict expression of an upstream activating sequence (UAS)-based reporter/effector transgene when associated solely with the UAS element. Morpholino knockdown assays showed that NRSF/REST expression is required for NRSE-based transgene silencing.

CONCLUSIONS

Our findings demonstrate that the addition of NRSE sequences to transgenes can provide useful new tools for functional studies of the nervous system. However, the general approach may be more broadly applicable; tissue-specific silencer elements are operable in tissues other than the nervous system, suggesting this approach can be similarly applied to other paradigms. Thus, creating synthetic associations between endogenous regulatory elements and tissue-specific silencers may facilitate targeting of cellular subtypes for which defined promoters/enhancers are lacking.

Drosophila stathmins bind tubulin heterodimers with high and variable stoichiometries

In vertebrates, stathmins form a family of proteins possessing two tubulin binding repeats (TBRs), which each binds one soluble tubulin heterodimer. The stathmins thus sequester two tubulins in a phosphorylation-dependent manner, providing a link between signal transduction and microtubule dynamics. In Drosophila, we show here that a single stathmin gene (stai) encodes a family of D-stathmin proteins. Two of the D-stathmins are maternally deposited and then restricted to germ cells, and the other two are detected in the nervous system during embryo development. Like in vertebrates, the nervous system-enriched stathmins contain an N-terminal domain involved in subcellular targeting. All the D-stathmins possess a domain containing three or four predicted TBRs, and we demonstrate here, using complementary biochemical and biophysical methods, that all four predicted TBR domains actually bind tubulin. D-stathmins can indeed bind up to four tubulins, the resulting complex being directly visualized by electron microscopy. Phylogenetic analysis shows that the presence of regulated multiple tubulin sites is a conserved characteristic of stathmins in invertebrates and allows us to predict key residues in stathmin for the binding of tubulin. Altogether, our results reveal that the single Drosophila stathmin gene codes for a stathmin family similar to the multigene vertebrate one, but with particular tubulin binding properties.

A function for the calponin family member NP25 in neurite outgrowth

The neuronal protein 25 (NP25), a member of the calponin (CaP) protein family, has previously been identified as neuron-specific protein in the adult rat brain. Here, we show an early onset of NP25 expression in the chick embryo neural tube. NP25 represents, together with NeuroM, one of the earliest markers for postmitotic neurons. To elucidate its function in the developing nervous system, NP25 was overexpressed in E5 and E9 sensory neurons, E7 sympathetic neurons and PC12 cells that show different endogenous NP25 expression levels. Whereas E5 and E9 sensory neurons and PC12 cells, which express low endogenous levels of NP25, responded by enhanced neurite outgrowth, a reduction of neurite length was observed in sympathetic neurons, which already express high endogenous levels of NP25. Knockdown of NP25 in sensory neurons using NP25 siRNA resulted in shorter neurites, whereas reduction of NP25 expression in sympathetic neurons led to increased neurite length. These results suggest a dynamic function for NP25 in the regulation of neurite growth, with an optimal level of NP25 required for maximal growth.

Phosphotyrosine 1062 is critical for the in vivo activity of the Ret9 receptor tyrosine kinase isoform

The receptor tyrosine kinase Ret plays a critical role in the development of the mammalian excretory and enteric nervous systems. Differential splicing of the primary Ret transcript results in the generation of two main isoforms, Ret9 and Ret51, whose C-terminal amino acid tails diverge after tyrosine (Y) 1062. Monoisoformic mice expressing only Ret9 develop normally and are healthy and fertile. In contrast, animals expressing only Ret51 have aganglionosis of the distal gut and hypoplastic kidneys. By generating monoisoformic mice in which Y1062 of Ret9 has been mutated to phenylalanine, we demonstrate that this amino acid has a critical role in Ret9 signaling that is necessary for the development of the kidneys and the enteric nervous system. These findings argue that the distinct activities of Ret9 and Ret51 result from the differential regulation of Y1062 by C-terminal flanking sequences. However, a mutation which places Y1062 of Ret51 in a Ret9 context improves only marginally the ability of Ret51 to support renal and enteric nervous system development. Finally, monoisoformic mice expressing a variant of Ret9 in which a C-terminal PDZ-binding motif was mutated develop normally and are healthy. Our studies identify Y1062 as a critical regulator of Ret9 signaling and suggest that Ret51-specific motifs are likely to inhibit the activity of this isoform.

A segment of the Mecp2 promoter is sufficient to drive expression in neurons

Rett syndrome (RTT) is caused by mutations in the gene encoding methyl CpG-binding protein 2 (MeCP2). Although MeCP2 shows widespread expression in both neuronal and non-neuronal tissues, the symptoms of RTT are largely neurological. Herein, we have identified the regulatory region of the mouse Mecp2 gene that is sufficient for its restricted expression in neurons. A segment of the Mecp2 gene (-677/+56) exhibited strong promoter activity in neuronal cell lines and cortical neurons, but was inactive in non-neuronal cells and glia. The region necessary for neuronal-specific promoter activity was located within a 19 bp region (-63/-45). Several nuclear factors were found to bind to this region and some of these factors were enriched in nuclear extracts prepared from the brain. To examine the activity of the Mecp2 promoter in vivo, we generated transgenic mice expressing the LacZ reporter driven by the -677/+56 region of the Mecp2 gene. The transgene was expressed in the mesencephalon as early as embryonic day 10 and in the hindbrain and spinal cord by E12. Interestingly, a marked induction of transgene expression was observed postnatally throughout the brain, similar to that of endogenous MeCP2. However, expression of the transgene was absent in non-neuronal tissues that are known to express Mecp2. Taken together, these data indicate that the -677/+56 region of the Mecp2 promoter partially recapitulates the native expression pattern of the Mecp2 gene, which possesses restricted expression in neurons of the central nervous system.

Ectopic Myf5 or MyoD prevents the neuronal differentiation program in addition to inducing skeletal muscle differentiation, in the chick neural tube

Forced expression of the bHLH myogenic factors, Myf5 and MyoD, in various mammalian cell lines induces the full program of myogenic differentiation. However, this property has not been extensively explored in vivo. We have taken advantage of the chick model to investigate the effect of electroporation of the mouse Myf5 and MyoD genes in the embryonic neural tube. We found that misexpression of either mouse Myf5 or MyoD in the chick neural tube leads to ectopic skeletal muscle differentiation, assayed by the expression of the myosin heavy chains in the neural tube and neural crest derivatives. We also showed that the endogenous neuronal differentiation program is inhibited under the influence of either ectopic mouse Myf5 or MyoD. We used this new system to analyse, in vivo, the transcriptional regulation between the myogenic factors. We found that MyoD and Myogenin expression can be activated by ectopic mouse Myf5 or MyoD, while Myf5 expression cannot be activated either by mouse MyoD or by itself. We also analysed the transcriptional regulation between the myogenic factors and the different genes involved in myogenesis, such as Mef2c, Pax3, Paraxis, Six1, Mox1, Mox2 and FgfR4. We established the existence of an unexpected regulatory loop between MyoD and FgfR4. The consequences for myogenesis are discussed.

Analysis of transcriptional regulatory sequences of the N-methyl-D-aspartate receptor 2A subunit gene in cultured cortical neurons and transgenic mice

The postnatal appearance and up-regulation of the NR2A subunit of the N-methyl-d-aspartate receptor contributes to the functional heterogeneity of the receptor during development. To elucidate the molecular mechanisms that regulate the neural and developmental specific expression of NR2A, an upstream approximately 9-kb region of the gene harboring the promoter was isolated and characterized in transgenic mice and transfected cortical neurons. Transgenic mouse lines generated with luciferase reporter constructs driven by either 9 or 1 kb of upstream sequence selectively transcribe the transgene in brain, as compared with other non-neural tissues. Reporter luciferase levels in dissociated cultures made from these mice are over 100-fold greater in neuronal/glial co-cultures than in pure glial cultures. Analysis of NR2A 5'-nested deletions in transfected cultures of cortical neurons and glia indicate that while sequences residing upstream of -1079 bp augment NR2A neuronal expression, sequences between -486 and -447 bp are sufficient to maintain neuronal preference. An RE1/NRSE element is not necessary for NR2A neuron specificity. Furthermore, comparison of the 5'-deletion constructs in cortical neurons grown for 5, 8, 11, or 14 days in vitro indicate that sequences between -1253 and -1180 bp are necessary for maturational up-regulation of NR2A. Thus, different cis-acting sequences control the regional and temporal expression of NR2A, implicating distinct regulatory pathways.

Regulation of neuronal traits by a novel transcriptional complex

The transcriptional repressor, REST, helps restrict neuronal traits to neurons by blocking their expression in nonneuronal cells. To examine the repercussions of REST expression in neurons, we generated a neuronal cell line that expresses REST conditionally. REST expression inhibited differentiation by nerve growth factor, suppressing both sodium current and neurite growth. A novel corepressor complex, CoREST/HDAC2, was shown to be required for REST repression. In the presence of REST, the CoREST/HDAC2 complex occupied the native Nav1.2 sodium channel gene in chromatin. In neuronal cells that lack REST and express sodium channels, the corepressor complex was not present on the gene. Collectively, these studies define a novel HDAC complex that is recruited by the C-terminal repressor domain of REST to actively repress genes essential to the neuronal phenotype.

Gene expression in transgenic mice using neural promoters

In the first part of this unit, major considerations for the analysis of neural promoters in transgenic mice are discussed. Detailed protocols on the production of transgenic mice are not described in this unit. Advantages and disadvantages of the transgenic approach for analysis of neural cis-acting elements are also presented. The concept of transient transgenic mice is then introduced; this method compensates for some disadvantages associated with the conventional transgenic approach. Finally, major factors influencing the efficiency of transgenic mouse production are discussed. The second part of the unit presents detailed information on a variety of neural-specific cis-acting elements that have been characterized by a transgenic approach. This information is useful both as a guide for carrying out the analysis of cis-acting elements and as a reference for selection of promoter/enhancer elements for designing an appropriate transgenic construct.

A binary mechanism for the selective action of a pancreatic beta -cell transcriptional silencer

The pancreatic elastase I gene (ELA1) is selectively transcribed to high levels in pancreatic acinar cells. Pancreatic specificity is imparted by a 100-base pair enhancer that activates transcription in beta-cells of the islets of Langerhans as well as in acinar cells. Adjacent to the enhancer is a silencer that renders transcription specific to acinar cells by selectively suppressing the inherent beta-cell activity of the enhancer. We show that the selective repression of beta-cell transcription is due neither to a beta-cell specific activity of the silencer nor to selective interference with beta-cell-specific transcriptional activators acting on the enhancer. Rather, the silencer is effective in both pancreatic endocrine and acinar cell types against all low and moderate strength enhancers and promoters tested. The silencer appears to act in a binary manner by reducing the probability that a promoter will be active without affecting the rate of transcription from active promoters. We propose that the ELA1 silencer is a weak off switch capable of inactivating enhancer/promoter combinations whose strength is below a threshold level but ineffective against stronger enhancer/promoters. The apparent cell-specific effects on the ELA1 enhancer appear due to the ability of the silencer to inactivate the weak beta-cell activity of the enhancer but not the stronger acinar cell activity.

Transcriptional repression by neuron-restrictive silencer factor is mediated via the Sin3-histone deacetylase complex

A large number of neuron-specific genes characterized to date are under the control of negative transcriptional regulation. Many promoter regions of neuron-specific genes possess the repressor element repressor element 1/neuron-restrictive silencing element (RE1/NRSE). Its cognate binding protein, REST/NRSF, is an essential transcription factor; its null mutations result in embryonic lethality, and its dominant negative mutants produce aberrant expression of neuron-specific genes. REST/NRSF acts as a regulator of neuron-specific gene expression in both nonneuronal tissue and developing neurons. Here, we shown that heterologous expression of REST/NRSF in Saccharomyces cerevisiae is able to repress transcription from yeast promoters engineered to contain RE1/NRSEs. Moreover, we have taken advantage of this observation to show that this repression requires both yeast Sin3p and Rpd3p and that REST/NRSF physically interacts with the product of the yeast SIN3 gene in vivo. Furthermore, we show that REST/NRSF binds mammalian SIN3A and HDAC-2 and requires histone deacetylase activity to repress neuronal gene transcription in both nonneuronal and neuronal cell lines. We show that REST/NRSF binding to RE1/NRSE is accompanied by a decrease in the acetylation of histones around RE1/NRSE and that this decrease requires the N-terminal Sin3p binding domain of REST/NRSF. Taken together, these data suggest that REST/NRSF represses neuronal gene transcription by recruiting the SIN3/HDAC complex.

Differential, regional, and cellular expression of the stathmin family transcripts in the adult rat brain

Stathmin is a ubiquitous cytosolic phosphoprotein, preferentially expressed in the nervous system, and previously described as a relay integrating diverse intracellular signaling pathways. Stathmin is the generic element of a mammalian protein family including SCG10, SCLIP, and RB3 with its splice variants RB3' and RB3". In contrast with stathmin, SCG10, SCLIP, and RB3/RB3'/RB3" are exclusively expressed in the nervous system, stathmin and SCG10 being mostly expressed during cell proliferation and differentiation, and SCLIP and RB3 rather in mature neural cells. To further understand their specific roles in the CNS, we compared the localization of the stathmin, SCG10, SCLIP, and RB3 transcripts in adult rat brain. Northern blot analysis as well as in situ hybridization experiments showed that all stathmin-related mRNAs are expressed in a wide range of adult rat brain areas. At a regional level, SCG10 and SCLIP appear generally distributed similarly except in a few areas. The pattern of expression of the RB3 transcript is very different from that of the three other members of the stathmin family. Furthermore, unlike SCG10 and SCLIP, which were detected only in neurons, but like stathmin, RB3 was detected in neurons and also in glial cells of the white matter. Altogether, our results suggest distinct roles for each member of the stathmin-related phosphoprotein family, in regard to their specific regional and cellular localization in the rat brain.

Genetically modified mice in neuropharmacology

Since the first transgenic mouse was reported in 1980, genetically engineered mice have become an invaluable biological tool for better understanding of physiological and pathological processes in many fields of biomedical research. The transgenic technology allows researchers to carry out specific genetic manipulation in all cells of a laboratory animal, and makes it possible to dissect gene function in a living organism. In the field of neurosciences these animals have contributed greatly to shed light on basic mechanisms of brain function as well as to generate useful animal models for studying human neurological disorders. In this review, the different techniques available for generating specific mutations in the mouse genome will be described, from pronuclear microinjection to gene targeting in embryonic stem cells, and to the second generation of inducible and conditional knockout mice. Then, the impact of transgenic mouse models as an alternative or additional approach to neuropharmacology will be discussed, not only for the study of molecular mechanisms in the central nervous system but also for the identification of new biological targets for innovative pharmacological therapy.

Differential regulation of the growth-associated proteins, GAP-43 and SCG-10, in response to unilateral cortical ablation in adult rats

Synapse replacement after brain injury has been widely documented by anatomical studies in various parts of both the developing and adult nervous system. However, the molecular events that define the specificity of the empirically derived rules of reactive synaptogenesis in different regions of the adult brain remain unclear. In this study we examined the differential regulation of the lesion-induced response of the two growth-associated proteins, superior cervical ganglia-10 and growth-associated protein-43, after unilateral cortex ablation, and determined a hierarchical order for the lesion response from remaining afferent projection neurons originating from the contralateral cortex, ipsilateral thalamus and substantia nigra. We report that in response to unilateral cortex ablation both messenger RNA, by northern blot, and protein, by western blot, for superior cervical ganglia-10 but not growth-associated protein-43 was increased in the homologous area of the contralateral cortex but not the ipsilateral thalamus or substantia nigra. In addition, the specificity of the superior cervical ganglia-10 response, assessed by combined in situ hybridization and retrograde FluoroGold labeling of striatal afferent neurons, found that superior cervical ganglia-10 messenger RNA was increased prominently in layer V pyramidal neurons of the contralateral corticostriatal pathway but was unchanged in afferent projection neurons from the thalamus and substantia nigra. Furthermore, the increase in both superior cervical ganglia-10 messenger RNA and protein seen at three days postlesion in contralateral corticostriatal neurons coincides in time with the initiation of neurite outgrowth in the deafferented striatum by contralateral corticostriatal axons described in our previous ultrastructural study. However, if cortical input to the striatum was removed bilaterally the lesion-induced response for superior cervical ganglia-10 messenger RNA shifted secondarily to thalamostriatal neurons in the ipsilateral thalamus. These data provide evidence that superior cervical ganglia-10 and growth-associated protein-43 are differentially regulated in neurons of the contralateral corticostriatal pathway in response to unilateral cortex ablation and suggests that superior cervical ganglia-10 plays a role in the regulation of neurite outgrowth in the adult striatum after brain injury. However, the specific role that superior cervical ganglia-10 may play in reactive synaptogenesis remains unclear. In addition, our data suggest that a hierarchical order exists for the reinnervation of deafferented striatal neurons after unilateral cortex ablation with preference given to homologous axons from the contralateral cortex.

Contribution of a PS1-like element to the tissue- and cell-specific expression of the human GLP-1 receptor gene

The GLP-1 receptor (GLP-1R) mediates the insulinotropic effects of the incretion hormone glucagon-like peptide 1 (7-36) amide (GLP-1). Recently, we cloned the 5'-flanking region of the human GLP-1R gene. To characterize tissue- and cell-specific cis-regulatory elements, we constructed a series of 5'-deletions of the promoter. The activity of these constructs was tested in different cell lines. An element with high homology to PS1 was found to repress GLP-1R promoter activity in fibroblasts and pancreatic D-cells, but was not active in pancreatic A- and B-cells. PS1 was described to inhibit activation of a D-cell-specific enhancer. Cloning the PS1-like element upstream a heterologous promoter (SV40) revealed that it is functionally active independently from this enhancer. Our data suggest that basal activity of the GLP-1R promoter is silenced in a tissue- and cell-specific manner by negatively acting cis-regulatory elements, including a PS1-like element.

NRSF/REST is required in vivo for repression of multiple neuronal target genes during embryogenesis

The neuron-restrictive silencer factor NRSF (also known as REST and XBR) can silence transcription from neuronal promoters in non-neuronal cell lines, but its function during normal development is unknown. In mice, a targeted mutation of Rest, the gene encoding NRSF, caused derepression of neuron-specific tubulin in a subset of non-neural tissues and embryonic lethality. Mosaic inhibition of NRSF in chicken embryos, using a dominant-negative form of NRSF, also caused derepression of neuronal tubulin, as well as of several other neuronal target genes, in both non-neural tissues and central nervous system neuronal progenitors. These results indicate that NRSF is required to repress neuronal gene expression in vivo, in both extra-neural and undifferentiated neural tissue.

Activation and repression in the nervous system

The mechanisms underlying transcriptional activation and repression have become much clearer. Recent evidence suggests that transcription factors that do not bind DNA directly, the co-activators and co-repressors, mediate a large number of cell signaling events. Their association with histone acetylases, to mediate activation, or deacetylases, to mediate repression, provide a model for explaining how gene expression is regulated.

Manipulation of gene expression in the mammalian nervous system: application in the study of neurite outgrowth and neuroregeneration-related proteins

A fundamental issue in neurobiology entails the study of the formation of neuronal connections and their potential to regenerate following injury. In recent years, an expanding number of gene families has been identified involved in different aspects of neurite outgrowth and regeneration. These include neurotrophic factors, cell-adhesion molecules, growth-associated proteins, cytoskeletal proteins and chemorepulsive proteins. Genetic manipulation technology (transgenic mice, knockout mice, viral vectors and antisense oligonucleotides) has been instrumental in defining the function of these neurite outgrowth-related proteins. The aim of this paper is to provide an overview of the above-mentioned four approaches to manipulate gene expression in vivo and to discuss the progress that has been made using this technology in helping to understand the molecular mechanisms that regulate neurite outgrowth. We will show that work with transgenic mice and knockout mice has contributed significantly to the dissection of the function of several proteins with a key role in neurite outgrowth and neuronal survival. Recently developed viral vectors for gene transfer in postmitotic neurons have opened up new avenues to analyze the function of a protein following local expression in naive adult rodents. The initial results with viral vector-based gene transfer provide a conceptual framework for further studies on genetic therapy of neuroregeneration and neurodegenerative diseases.

Regulation of cell-type specific expression of lacZ by the 5'-flanking region of mouse GAD67 gene in the central nervous system of transgenic mice

The transcriptional regulation of the murine gene encoding the 67-kDa form of glutamic acid decarboxylase (GAD67) was studied by beta-galactosidase histochemistry in transgenic mice carrying fusion genes between progressively longer portions of the 5'-upstream regulatory region of GAD67 and E. coli lacZ. No expression was detected in brains of mice carrying 1.3 kb of upstream sequences including a housekeeping and two conventional promoters, and two negative regulatory elements with homology to known silencers. In mice carrying the same portion of the promoter region plus the first intron, lacZ expression in the adult central nervous system was found in few, exclusively neuronal sites. The number of correctly stained GABAergic centres increased dramatically with increasing the length of the 5'-upstream region included in the construct which suggests that multiple putative spatial enhancers are located in this region. Their action is influenced by epigenetic mechanisms that may be due to site-of-integration and transgene copy-number effects. Additional cis-acting elements are needed to obtain fully correct expression in all GABAergic neurons of the adult central nervous system.

Tissue-specific expression of the L1 cell adhesion molecule is modulated by the neural restrictive silencer element

The cell adhesion molecule L1 mediates neurite outgrowth and fasciculation during embryogenesis and mutations in its gene have been linked to a number of human congenital syndromes. To identify DNA sequences that restrict expression of L1 to the nervous system, we isolated a previously unidentified segment of the mouse L1 gene containing the promoter, the first exon, and the first intron and examined its activity in vitro and in vivo. We found that a neural restrictive silencer element (NRSE) within the second intron prevented expression of L1 gene constructs in nonneural cells. For optimal silencing of L1 gene expression by the NRSE-binding factor RE-1-silencing transcription factor (REST)/NRSF, both the NRSE and sequences in the first intron were required. In transgenic mice, an L1lacZ gene construct with the NRSE generated a neurally restricted expression pattern consistent with the known pattern of L1 expression in postmitotic neurons and peripheral glia. In contrast, a similar construct lacking the NRSE produced precocious expression in the peripheral nervous system and ectopic expression in mesenchymal derivatives of the neural crest and in mesodermal and ectodermal cells. These experiments show that the NRSE and REST/NRSF are important components in restricting L1 expression to the embryonic nervous system.

The molecular biology of preprotachykinin-A gene expression

The expression of neuropeptides is largely tissue-specific and under strictly regulated and complex control. In view of the diversity of neuronal phenotypes, with concomitant plasticity of gene expression within any phenotype, it is obvious that there is coordinated activation and repression of genes. One of the central observations from these studies is that neuropeptide gene expression is dependent upon the combinatorial interaction of multiple transcription factors with the regulatory elements which determine mRNA synthesis. These factors mediate both tissue specific and stimulus inducible gene expression. We will illustrate some of the mechanisms that regulate neuropeptide gene expression utilizing our own studies on the rat preprotachykinin-A gene (rPPT) and, where appropriate, expand on the generality of these findings to other neuropeptide genes.

Neuronal-specific gene expression--the interaction of both positive and negative transcriptional regulators

Gene expression patterns in neurons are complex and are modulated in response to multiple extracellular stimuli. In addition, during development and as neurons differentiate into distinct neuronal phenotypes, there is a co-ordinated activation and repression of a variety of genes. It is becoming increasingly evident that negative regulatory elements are present in neuronal-specific promoters. These elements have been shown, in part, to restrict promoter activity to the correct physiological cell type, both in transient transfection and in transgenic mouse models. Repression can be effected by different mechanisms depending on location within the promoter of silencer complexes and their relationship to other bound transcription factors. This review will discuss the molecular mechanisms regulating promoter function, in particular: (1) the combinatorial interaction between transcription factors which generate regulated promoter function; and (2) the restriction of promoter function to the correct cell type by bound repressor molecules. Determination of the mechanism of regulated gene expression will allow advances in gene therapy and definition of novel targets for pharmaceutical intervention. At the more basic level, functional dissection of the promoters of specific neuronal expressed genes will provide information of importance in two key areas of neurobiology: (1) the mechanism by which extracellular factors, such as neurotrophins and cytokines, regulate gene expression; (2) the events which lead to the tissue-specific expression of genes in subpopulations of neurons, both in the adult and during development.

Identification of cis-acting sequences in the promoter of the herpes simplex virus type 1 latency-associated transcripts required for activation by nerve growth factor and sodium butyrate in PC12 cells

In the absence of detectable viral proteins, expression of the latency-associated transcripts (LATs) is likely regulated by cellular factors during latent infection of neurons with herpes simplex virus type 1. The amounts and activation states of these factors may in turn be regulated by extracellular regulatory factors. Consistent with this hypothesis, we have recently demonstrated that LAT expression is significantly enhanced by nerve growth factor (NGF) and sodium butyrate (NaB) in neurally derived PC12 cells. With the ultimate goal of identifying trans-acting cellular factors involved in regulating LAT expression during latency, we have attempted to identify the cis-acting elements to which these putative cellular factors bind by characterizing the LAT promoter and a series of 5' promoter deletion mutants in PC12 cells following treatment with the LAT-enhancing agents NGF and NaB. Transient expression assays demonstrated that distinct cis-acting sequences mediate basal and induced LAT promoter expression. Basal activity in PC12 cells is mediated by two elements: a negative regulatory element between -435 and -270 and a positive element between -240 and -204. The positive element contains binding sites for the transactivator Sp-1, whereas the negative element bears some resemblance to known neuron-specific silencer elements. In contrast to basal expression, maximum induction of the LAT promoter by NGF and NaB requires sequences between -159 and -81. Using gel mobility shift assays, we have identified three sets of protein-DNA complexes that bind to this 78-bp region and shown by competition analysis that binding is specific. The abundance and mobility of these complexes were altered by treatment with NGF or NaB. The nucleotide sequences to which these complexes bind were fine mapped by competition analysis with oligonucleotide probes containing substitution mutations. The target sequences identified exhibit no homology to binding sites of known transcription factors. These regions were critical for complex formation in vitro and for maximum induction of the LAT promoter by NGF and NaB in transient expression assays. The protein complexes that form with target sequences likely participate in the regulation of LAT expression in response to physiological stimuli in neurons in vivo.

Distinct N-methyl-D-aspartate receptor 2B subunit gene sequences confer neural and developmental specific expression

Expression of the N-methyl--aspartate (NMDA) receptor 2B (NR2B) subunit is neural-specific and differentially regulated. It is expressed in the forebrain and in cerebellar granule cells at early postnatal stages and selectively repressed in the cerebellum after the second postnatal week, where it is replaced by the NR2C subunit. This switch confers distinct properties to the receptor. In order to understand the molecular mechanisms that differentially regulate the NR2B gene in the forebrain and cerebellum during development, we have isolated and characterized the promoter region of the NR2B gene. Two 5' noncoding exons and multiple transcription start sites were identified. Transcriptional analysis in transgenic mice reveals that an upstream 800-base pair region, which includes the first exon, is sufficient to direct neural-specific transcription. Developmental repression of the gene in the cerebellum requires additional regulatory elements residing in the first intron or second exon. Sequence elements that may participate in the regulation of the NR2B gene were identified by comparison to other neural genes. These studies provide insight into the molecular mechanisms regulating the switch of NMDA receptor subunit expression in the cerebellum, which ultimately account for the physiological changes in receptor function during development.

Activity of the distal positive element of the peripherin gene is dependent on proteins binding to an Ets-like recognition site and a novel inverted repeat site

The peripherin gene, encoding a neuron-specific intermediate filament protein, is transcriptionally induced when PC12 cells begin to terminally differentiate into neurons in response to nerve growth factor. Previously we identified two regulatory sequences of the peripherin gene: a proximal negative element (centered at -173), which prevents peripherin expression in undifferentiated PC12 cells, and a distal positive region (-2660 to -2308) necessary for full induction of peripherin in differentiated PC12 cells (Thompson, M., Lee, E. Lawe, D., Gizang-Ginsberg, E., and Ziff, E. (1992) Mol. Cell. Biol. 12,2501-2513). Here we define a distal positive element (DPE, -2445 to -2337) within the distal positive region. Methylation interference footprinting of the DPE identified DNA-protein contact points at a novel inverted repeat sequence (AACCACTGGTT) and an Ets-like recognition sequence (CAGGAG). Functional analysis using site-directed mutagenesis demonstrates that both sites are necessary for the activity of the DPE. In addition, ternary complex formation at the DPE is dependent on both sites. Antibody competition assays confirm that an Ets family member participates in the DNA-protein complex. We have indirect evidence that the inverted repeat binding protein and the Ets-related protein interact directly with each other. Finally, we demonstrate that the DPE is constitutively active and that neuron-specific regulation of peripherin expression may be due to interaction with distal and proximal negative regulatory elements.

The POU homeodomain transcription factor Oct-1 is essential for activity of the gonadotropin-releasing hormone neuron-specific enhancer

The mechanisms of specification of gene expression in a complex tissue such as the brain remain poorly understood. To provide a model system for the study of gene regulation in a specific subpopulation of differentiated neurons, we have derived cell lines from tumors created in transgenic mice by targeting simian virus 40 T antigen expression by using the regulatory regions of the gene for gonadotropin-releasing hormone (GnRH), a decapeptide released from specialized neurons in the hypothalamus. Transfections into the cultured GnRH-secreting hypothalamic neuronal cell line GT1 have identified a neuron-specific enhancer, 1.5 kb upstream of the GnRH gene, which binds multiple GT1 nuclear proteins. In particular, one AT-rich protein-binding region, AT-a, is critical for enhancer activity. In this study, we used electrophoretic mobility shift assays to detect a GT1 nuclear protein complex that binds the AT-a region. Close inspection of the AT-a bottom-strand sequence revealed homology to the octamer motif, a sequence known to bind members of the POU homeodomain transcription factor family. Although we demonstrate expression of a number of POU homeodomain genes in GT1 cells, a supershift assay with Oct-1 antibody demonstrates that Oct-1 is the protein binding the enhancer. Finally, specific mutations in the AT-a region that affected Oct-1 binding were correlated with decreased transcription. Thus, Oct-1 binds to the GnRH enhancer in vitro, and this binding is critical to the transcriptional activity of this neuron-specific enhancer in GT1 cells.

SCG10, a neuron-specific growth-associated protein in Alzheimer's disease

Neuronal growth-associated proteins (nGAPs) are markers of neuronal process outgrowth and are associated with both degenerative and sprouting responses in Alzheimer's disease (AD) brain. To study possible involvement of SCG10, an nGAP, in AD, we cloned human SCG10 cDNA and analyzed SCG-10 at mRNA and protein levels in control and AD brains. The deduced amino acid sequence of human SCG10 was 69% identical to stathmin, another nGAP. By in situ hybridization, both SCG10 and stathmin mRNAs were detected in selected neuronal populations in aged human brains. Quantitative analysis by RNase protection revealed that levels of neither SCG10 nor stathmin mRNAs were significantly altered in AD. Using an SCG10-specific antibody, Western blot analysis did not reveal any quantitative changes of SCG10 in AD. However, when the concentration of SCG10 protein was plotted against the number of tangles, a positive correlation was found. SCG10 levels did not correlate with plaque numbers. Furthermore, immunohistochemical study revealed that neuronal SCG10 protein accumulated in the cell bodies in AD-affected regions. Thus, SCG10 compartmentalization and metabolism may be altered in AD possibly due to mechanisms related to tangle formation in this disease.

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

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

An upstream stimulatory factor (USF) binding motif is critical for rat preprotachykinin-A promoter activity in PC12 cells

We demonstrate the presence of a functional E box motif in the proximal rat preprotachykinin-A (rPPT) promoter. This element (spanning nucleotides -67 to -47) exhibits the sequence 5'-CACGTG-3' which is recognized and bound by the basic helix-loop-helix family of regulatory proteins. We also show that at least one of the factors bound to this rPPT promoter element in both HeLa and PC12 nuclear extract is the ubiquitously expressed transcription factor, the upstream stimulatory factor (USF). Mutation of this element by insertion of a 10 bp linker into the E box motif, in an rPPT promoter fragment spanning -865 to +92, destroys the ability of this promoter fragment to support reporter gene expression in a PC12 cell model of rPPT promoter activity. The data indicate that this rPPT E box element is likely to function as an important cis-regulatory domain in the rPPT promoter.

Repression of somatostatin gene transcription mediated by two promoter silencer elements

We report that expression of the somatostatin gene in pancreatic islets and in non-islet cells is negatively regulated by two proximal silencer elements, PS1 and PS2. Transient transfection assays showed that PS1 decreases somatostatin gene promoter activity stimulated by an upstream enhancer in the islet D-cell line RIN-1027-B2, but not in the islet B-cell line RIN-1046-38, whereas PS2 inhibits gene transcription both B- and D-cell lines. In BHK fibroblasts, both PS1 and PS2 independently inhibit somatostatin gene in non-islet cells. DNA-binding studies revealed that both PS1 and PS2 bind similar nuclear protein complexes in islet and non-islet cells (120 and 130 kDa). PS1 also binds a 100-kDa protein present in islet B- and D-cell lines. In addition, both PS1 and PS2 bind three D-cell-specific proteins (40, 43 and 45 kDa). These observations support a direct involvement of both positive and negative transcriptional control mechanisms in the regulation of the islet cell-specific expression of the somatostatin gene.

Transcriptional control of neuropeptide gene expression in sensory neurons, using the preprotachykinin-A gene as a model

Control of neuropeptide gene expression in sensory neurons is determined in part by a variety of tissue-specific, developmental, and stimulus-induced transcription factors that interact with the promoters of these genes. We have analysed the regulation of the rat preprotachykinin-A (rPPT) gene, which is expressed in a subset of dorsal root ganglia neurons. A region of the promoter encompassing approximately 1300 base pairs spanning the transcriptional start site has been analysed in detail both by functional analysis of promoter activity in clonal cell lines and dorsal root ganglia neurons grown in culture and by in vitro characterisation of transcription factor interaction with this region. Interestingly our analysis indicates an important role in rPPT gene expression for the E box transcription factor family. This class of transcription factor has been demonstrated to be a major determinant of calcitonin gene related peptide (CGRP) expression, which is also expressed in dorsal root ganglia neurons often under similar conditions as rPPT. In addition, multiple regulatory domains have been identified in the rPPT promoter, which act as activators in a variety of cell types. These elements are silenced in the context of the rPPT promoter in many non-neuronal cells. Therefore, tissue-specific expression of reporter genes directed by the rPPT promoter in transient transfection is determined in part by a variety of silencer elements, which act to repress the function of several domains that act as constitutive enhancers of expression in a wide range of cells. Removal or modulation of silencer elements in the rPPT promoter allows activity in a wider variety of cell types. We postulate that control of rPPT gene expression is the results of dynamic interplay of both positive and negative regulatory elements, a phenomenon observed in several other neuronal-specific genes, including that encoding CGRP.

Three immediate early gene response elements in the proximal preprotachykinin-A promoter in two functionally distinct domains

The preprotachykinin-A promoter contains two blocks of DNA sequence, with a high degree of homology to one another, both containing activator protein 1/cAMP response element-like elements which constitute cis-acting regulatory domains. These two domains are differentially regulated in HeLa cells and primary cultures of dorsal root ganglion neurons when they are placed in the context of a reporter gene driven by the c-fos minimum promoter. One of the domains, corresponding to a region of the preprotachykinin promoter spanning nucleotides -345 to -308, contains two activator protein 1 elements adjacent to an E-box binding protein consensus sequence. Both of the activator protein 1 elements can bind a complex containing c-fos/c-fos related antigen proteins and the adjacent E-box element is specifically recognized by proteins present in HeLa nuclear extract. This domain requires the synergistic action of both activator protein 1 elements to drive expression of the reporter gene in both HeLa and dorsal root ganglion cells. The second or proximal domain spans nucleotides -198 to -155 and contains a previously characterized activator protein 1/cAMP response element/ATF enhancer element which, in contrast to the activator protein 1 elements in the distal domain, functions in both HeLa and dorsal root ganglion cells as one copy. This domain is differentially regulated in HeLa and dorsal root ganglia. The previously characterized enhancer activity is repressed in the context of the extended cis-acting domain in HeLa cells but remains active in dorsal root ganglion, although no further enhancement of activity supported by the single enhancer is observed when in the context of the extended sequence. This proximal domain, in addition to binding the enhancer complex, can be bound by at least two other complexes, one of which binds to an E-box consensus sequence. As the elements corresponding to the E-box consensus in both domains cross-compete for binding of specific complex(es) it would appear that repression of the activity of the proximal domain is correlated with a specific protein complex binding adjacent to the characterized enhancer in the region spanning nucleotides -198 to -155. The preprotachykinin-A proximal promoter is therefore bound by multiple activator protein I complexes, which in the context of the cis-acting domains in which they are present can be differentially regulated. In the proximal domain their function may also be regulated in a tissue-specific manner by other proteins which bind to adjacent regulatory elements.

Transcriptional analysis of acetylcholine receptor alpha 3 gene promoter motifs that bind Sp1 and AP2

In this study, we performed an analysis of the neuronal nicotinic acetylcholine receptor alpha 3 subunit gene promoter region, -238/+47, to identify cis and trans elements that are important for basal activity in PC12 cells. Sequence analyses of the alpha 3 promoter and footprint assays revealed an Sp1 binding site between -79 and -57 (termed the alpha 3 GA motif) and an AP2 binding site between -30 and -7. Using mobility shift analysis, we found that PC12 cell extracts contain proteins that specifically bind to the alpha 3 GA motif and are immunologically related to Sp1. Mutation of the alpha 3 GA motif, which prevented binding of Sp1, resulted in a 75% decrease in promoter activity. Mutation of the AP2 site resulted in only a minor loss of promoter activity, which is consistent with the lack of AP2 binding activity in PC12 extracts. In Drosophila Schneider line 2 (S2) cell cotransfection assays, Sp1 activated the alpha 3 promoter in a GA motif-dependent manner. Furthermore, multimerization of the GA motif upstream of the beta-globin TATA box conferred Sp1 responsiveness. Our results indicate that Sp1 can activate transcription through direct interaction with the alpha 3 GA motif and that this motif plays a major role in alpha 3 promoter basal activity in PC12 cells.

Repression of preprotachykinin-A promoter activity is mediated by a proximal promoter element

The rat preprotachykinin-A promoter, which is able to direct reporter gene expression in adult dorsal root ganglia neurons grown in culture, has no detectable activity in HeLa and PC12 cells. DNAase 1 footprinting and electrophoretic mobility shift analyses with HeLa nuclear extract indicated the presence of a protein complex binding to a region of the rat preprotachykinn-A gene promoter between the TATA box and the major transcriptional start site. We demonstrate that the sequence of the preprotachykinin-A promoter spanning nucleotides -47 to +92 functions to repress reporter gene expression in HeLa and PC12 cells but not in adult rat dorsal root ganglia grown in culture, and that this repression is correlated with a protein(s) binding to the element between the TATA box and major transcription initiation site. These results indicate that the tissue-specific expression of the preprotachykinin-A gene could require the interaction of both positive and negative regulatory DNA elements.

NF1-L is the DNA-binding component of the protein complex at the peripherin negative regulatory element

The peripherin gene, which encodes a neuronal-specific intermediate filament protein, is transcriptionally induced with a late time course when nerve growth factor stimulates PC12 cells to differentiate into neurons. We have defined a negative regulatory element (NRE) that has a functional role in repressing peripherin expression in undifferentiate and nonneuronal cells. Nerve growth factor-induced derepression of peripherin gene expression is associated with alterations in proteins binding to a GC-rich DNA sequence in the NRE as detected by the DNA electrophoretic mobility shift assay (EMSA). We have utilized DNA affinity chromatography to purify from rat liver a 33-kDa DNA-binding protein that specifically recognizes the NRE. Microsequencing reveals identity with NF1-L, a member of the CTF/NF-1 transcription factor family. This protein forms a single complex when incubated with the NRE probe using EMSA analysis. The more slowly migrating complexes characteristic of crude undifferentiated PC12 cell extract are reconstituted by mixing the purified protein with the flow-through from the DNA affinity column, thereby demonstrating that protein-protein interactions are involved in complex formation. Supershift experiments incubating anti-CTF-1 antibody with undifferentiated PC12 cell extract prior to EMSA analysis confirm that NF1-L, or a closely related family member, is the DNA-binding protein component of the multiprotein complex at the NRE.

REST: a mammalian silencer protein that restricts sodium channel gene expression to neurons

Expression of the type II voltage-dependent sodium channel gene is restricted to neurons by a silencer element active in nonneuronal cells. We have cloned cDNA coding for a transcription factor (REST) that binds to this silencer element. Expression of a recombinant REST protein confers the ability to silence type II reporter genes in neuronal cell types lacking the native REST protein, whereas expression of a dominant negative form of REST in nonneuronal cells relieves silencing mediated by the native protein. REST transcripts in developing mouse embryos are detected ubiquitously outside of the nervous system. We propose that expression of the type II sodium channel gene in neurons reflects a default pathway that is blocked in nonneuronal cells by the presence of REST.

The neuron-restrictive silencer factor (NRSF): a coordinate repressor of multiple neuron-specific genes

The neuron-restrictive silencer factor (NRSF) binds a DNA sequence element, called the neuron-restrictive silencer element (NRSE), that represses neuronal gene transcription in nonneuronal cells. Consensus NRSEs have been identified in 18 neuron-specific genes. Complementary DNA clones encoding a functional fragment of NRSF were isolated and found to encode a novel protein containing eight noncanonical zinc fingers. Expression of NRSF mRNA was detected in most nonneuronal tissues at several developmental stages. In the nervous system, NRSF mRNA was detected in undifferentiated neuronal progenitors, but not in differentiated neurons. NRSF represents the first example of a vertebrate silencer protein that potentially regulates a large battery of cell type-specific genes, and therefore may function as a master negative regulator of neurogenesis.

The 5' flanking region of the serotonin 2 receptor gene directs brain specific expression in transgenic animals

The neuron is the predominant cell type expressing the serotonin 2 (5-HT2) receptor in the central nervous system. Transcriptional control elements involved in the restriction of 5-HT2 receptor gene expression to neuronal cells and tissues were studied using both transgenic mice and cultured cells. Sequences extending from a site near the translational initiation codon to -5.6 kb in the 5' flanking region of the murine receptor gene were found to be sufficient to target gene expression to the brain in transgenic animals. In transient transfection experiments a basal promoter was identified which was functional in both neuronal and nonneuronal cells. Upstream of the basal promoter two repressor domains were found within the 5' flanking sequence of the receptor gene. These sequences repressed gene activity in all cells except cells of neuronal origin, thus the repressor domains are the primary determinants to generate neuronal cell-specific transcription of the 5-HT2 receptor gene.

SCG10 mRNA localization in the hippocampus: comparison with other mRNAs encoding neuronal growth-associated proteins (nGAPs)

SCG10 is a nerve growth factor (NGF)-inducible, neuron-specific protein whose expression is tightly correlated with axonal and/or dendritic growth. We have recently shown that the mRNA encoding SCG10 is expressed at significant levels in certain subsets of neurons in the adult rat brain, while its expression is undetectable or negligible in other non-neuronal tissues. Here we show that regional SCG10 mRNA expression in the adult mouse brain is comparable to that in the rat, however, in the hippocampus its expression profile is distinct. In the mouse, SCG10 mRNA is expressed at high levels in pyramidal cells of CA3-CA4 sub-fields of Ammon's horn and at low levels in the CA1-CA2 sub-fields, while it is found rather uniformly throughout the pyramidal cell layer of the rat hippocampus. SCG10 mRNA is not detectable in the dentate gyrus of the mouse hippocampus, although it is expressed in the rat dentate gyrus. Comparison with other mRNAs encoding neuronal growth-associated proteins (nGAPs) such as GAP-43, MAP2, alpha 1-tubulin and stathmin suggests that dentate granule cells express a different repertoire of neuronal growth-associated genes in mouse and rat.

5'-flanking sequences of the human HPRT gene direct neuronal expression in the brain of transgenic mice

Total deficiency of hypoxanthine phosphoribosyltransferase (HPRT) in humans causes the neurological disorder Lesch-Nyhan syndrome. The HPRT gene is expressed at basal levels in all tissues but at higher levels in the brain, the relevance and mechanism of which is unknown. To determine if cis-acting DNA elements play a role in the tissue-differential pattern of expression, we generated transgenic mice carrying different sequences of the human HPRT (hHPRT) promoter fused to the bacterial lacZ gene. We show that a 1.6 kb fragment of the hHPRT promoter contains essential information to direct beta-galactosidase expression preferentially to the basal ganglia, cerebral cortex, hippocampus, and several other areas of the forebrain. At least two elements within the 1.6 kb fragment appear to be required for neuronal expression. A 182 bp element (hHPRT-NE) represents one of these sequences and is involved not only in conferring neuronal specificity but also in repressing transgene expression in non-neuronal tissues. These studies provide molecular insight into the mechanism of increased HPRT expression in the brain.