Neuron specificity of the neurofilament light promoter in transgenic mice requires the presence of DNA unwinding elements

WEB-THERMODYN: Sequence analysis software for profiling DNA helical stability

WEB-THERMODYN analyzes DNA sequences and computes the DNA helical stability, i.e. the free energy required to unwind and separate the strands of the double helix. A helical stability profile across a selected DNA region or the entire sequence is generated by sliding-window analysis. WEB-THERMODYN can predict sites of low helical stability present at regulatory regions for transcription and replication and can be used to test the influence of mutations. The program can be accessed at: http://wings.buffalo.edu/gsa/dna/dk/WEBTHERMODYN/.

Sequential actions of BMP receptors control neural precursor cell production and fate

Bone morphogenetic proteins (BMPs) have diverse and sometimes paradoxical effects during embryonic development. To determine the mechanisms underlying BMP actions, we analyzed the expression and function of two BMP receptors, BMPR-IA and BMPR-IB, in neural precursor cells in vitro and in vivo. Neural precursor cells always express Bmpr-1a, but Bmpr-1b is not expressed until embryonic day 9 and is restricted to the dorsal neural tube surrounding the source of BMP ligands. BMPR-IA activation induces (and Sonic hedgehog prevents) expression of Bmpr-1b along with dorsal identity genes in precursor cells and promotes their proliferation. When BMPR-IB is activated, it limits precursor cell numbers by causing mitotic arrest. This results in apoptosis in early gestation embryos and terminal differentiation in mid-gestation embryos. Thus, BMP actions are first inducing (through BMPR-IA) and then terminating (through BMPR-IB), based on the accumulation of BMPR-IB relative to BMPR-IA. We describe a feed-forward mechanism to explain how the sequential actions of these receptors control the production and fate of dorsal precursor cells from neural stem cells.

Genetic engineering of neural function in transgenic rodents: towards a comprehensive strategy?

As mammalian genome projects move towards completion, the attention of molecular neuroscientists is currently moving away from gene identification towards both cell-specific gene expression patterns (neuronal transcriptions) and protein expression/interactions (neuronal proteomics). In the long term, attention will increasingly be directed towards experimental interventions which are able to question neuronal function in a sophisticated manner that is cognisant of both transcriptomic and proteomic organization. Central to this effort will be the application of a new generation of transgenic approaches which are now evolving towards an appropriate level of molecular, temporal and spatial resolution. In this review, we summarize recent developments in transgenesis, and show how they have been applied in the principal model species for neuroscience, namely rats and mice. Current concepts of transgene design are also considered together with an overview of new genetically-encoded tools including both cellular indicators such as fluorescent activity reporters, and cellular regulators such as dominant negative signalling factors. Application of these tools in a whole animal context can be used to question both basic concepts of brain function, and also current concepts of underlying dysfuction in neurological diseases.

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.

Transgenic animal models for neuropharmacology

The establishment of novel animal models using gene targeting and transgenic technology has opened a new area of neuropharmacological research. For the first time, it became possible to alter the expression of a gene in a specific cell type of an intact animal by either overexpression, inhibition or ablation. This review describes the technology and lists the relevant tools, such as reporter genes, suicide genes, immortalizing genes, and promoters, necessary for the targeted expression of these and other genes in specific cells of the central nervous system. In addition, the problem is discussed that the mouse is the species in which this technology is by far the most developed, while the rat has been used as the model species for neuropharmacology during the last century.

Codon optimization, genetic insulation, and an rtTA reporter improve performance of the tetracycline switch

The objective of this work was to further develop a tetracycline repressor (TetR) protein system that allows control of transgene expression. First, to circumvent the need for a binary approach, a single plasmid design was constructed and tested in tissue culture. To indirectly assay integrations that express the synthetic transcription factor (rtTA), a bicistronic gene was built which included an internal ribosome entry site (IRES) and a green fluorescent protein coding region (GFP) on the same expression cassette as the coding region of rtTA (pTetGREEN). This construct did not produce fluorescent colonies when stably integrated and provided minimal expression of GFP in the face of adequate expression of rtTA. The coding region for TetR was then altered by introducing 156 silent point mutations to simulate mammalian genes. Replacement of wild-type TetR gene (tetR) in pTetGREEN with 'mammalianized' tetR provided GFP expression. Adjustment of codon usage in the tetR region of rtTA nearly doubled the expression level of functional rtTA. To increase the number of rtTA expressing lines, the chicken egg-white lysozyme matrix attachment region (MAR) was introduced into the single plasmid design just upstream of the tetracycline operators (tetO). Inclusion of the MAR doubled the number of colonies that expressed rtTA (44% vs 88%). With the modifications described here, the number of lines that express rtTA and provide induction from a single plasmid design can be increased by the inclusion of a MAR and the level of rtTA expression can be further increased by adjusting the base composition of the TetR coding region. The MAR also insulates the inducible gene from the promoter driving rtTA.

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.

CRE and TRE sequences of the rat tyrosine hydroxylase promoter are required for TH basal expression in adult mice but not in the embryo

Tyrosine hydroxylase (TH), the rate-limiting enzyme in the biosynthesis of catecholamine neurotransmitters, is expressed in a restricted number of areas, and subject to numerous regulations during development and in adulthood. Two transcription factor binding sites present in the proximal region of the TH gene, the TPA-responsive element (TRE) and the c-AMP responsive element (CRE), have been shown to play important roles in TH gene regulation in vitro. In order to elucidate in vivo the role of these two sites, we produced transgenic mice bearing a 5.3-kb fragment from the 5' flanking sequence of the TH gene with mutations in either the CRE-or TRE-sites. Using the intact 5.3-kb fragment fused to two different reporter genes (HSV1-tk and lacZ), we show that this promoter fragment is able to specifically direct expression in catecholaminergic tissues both in adult mice and embryos. Interestingly, the CRE- and TRE-mutated transgenes were not expressed in adult mice, contrary to the situation in embryos where they were specifically expressed in catecholaminergic regions. These results demonstrate that the CRE and TRE play an essential role in basal TH expression in adult tissues in vivo. Moreover, they suggest that distinct transcription factors are involved in TH regulation in developing and adult tissues. In support of this, gel mobility shift experiments revealed a complex present only in embryonic tissues. Taken together, these data highlight the diversity of the mechanisms underlying the establishment and maintenance of the catecholaminergic phenotype.

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.

Multiple neuron-specific enhancers in the gene coding for the human neurofilament light chain

To define DNA regions involved in the neuron-specific expression of the neurofilament light (NF-L) gene, we generated transgenic mice bearing different NF-L constructs. A 4.9-kilobase human NF-L fragment including -292 base pairs of 5'-flanking sequences contained sufficient elements for nervous system expression in transgenic mice. Deletion of introns 1 and 2 from this 4.9-kilobase DNA fragment resulted in reduced levels of transgene expression in the cortex, while deletion of intron 3 had little effect. Both introns 1 and 2 could act independently as enhancers to confer neuronal expression of the basal heat shock promoter (hsp68) fused to lacZ in transgenic mice. The hNF-L basal promoter (-292 base pairs) was found to contain elements for directing neuronal expression of either the lacZ reporter gene or an intronless hNF-L construct. Sequence comparison revealed that intron 1, intron 2, and the basal human NF-L promoter all contain an ETS-like motif, CAGGA, present in a variety of genes expressed in the nervous system.