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

Genetics of human prefrontal function

Evolution of the prefrontal cortex was an essential precursor to civilization. During the past decade, it became increasingly obvious that human prefrontal function is under substantial genetic control. In particular, heritability studies of frontal lobe-related neuropsychological function, electrophysiology and neuroimaging have greatly improved our insight. Moreover, the first genes that are relevant for prefrontal function such as catechol-O-methyltransferase (COMT) are currently discovered. In this review, we summarize the present knowledge on the genetics of human prefrontal function. For historical reasons, we discuss the genetics of prefrontal function within the broader concept of general cognitive ability (intelligence). Special emphasis is also given to methodological concerns that need to be addressed when conducting research on the genetics of prefrontal function in humans.

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.

Regulated expression of focal adhesion kinase-related nonkinase, the autonomously expressed C-terminal domain of focal adhesion kinase

Focal adhesion kinase (FAK) has been implicated in cellular processes that control cell adhesion, migration, cell cycle progression, and apoptosis. FRNK (FAK-related nonkinase) is the autonomously expressed, noncatalytic C-terminal portion of FAK. When ectopically expressed in cells, FRNK has been shown to act as a negative regulator of FAK activity, inhibiting cell spreading, migration, and cell cycle progression. The mechanisms that regulate FRNK expression during embryonic development and the functional role of FRNK in normal cell homeostasis remain poorly understood. Herein we show that FRNK expression in chicken cells is directed by an alternative promoter residing within an intron of FAK, positioned 3' of the exon encoding sequences for the catalytic domain and 5' of the exon encoding sequences for the C-terminal domain of FAK (e.g., FRNK). Using probes specific for FRNK, we show that FRNK expression occurs early in chicken embryogenesis, being readily detected at day 3, 6, or 9. Late in embryogenesis, at day 18, FRNK is expressed in a tissue-specific manner, predominately in lung and intestine cells. Western blot analysis of mouse tissues with a FAK-specific antibody revealed the expression of FRNK in the mouse lung. Reverse transcriptase PCR analysis of mouse lung RNA revealed the presence of spliced FRNK mRNAs containing 5' untranslated sequences derived from a positionally conserved exon present in the mouse genome. FAK is the first example of a tyrosine kinase regulated by a domain under the control of an alternative intronic promoter. It is also the first example of a focal adhesion-associated protein regulated by such a mechanism and thus represents a novel means for the modulation of cell adhesion signaling.

Overexpression of a Shaker-type potassium channel in mammalian central nervous system dysregulates native potassium channel gene expression

The nervous system maintains a delicate balance between excitation and inhibition, partly through the complex interplay between voltage-gated sodium and potassium ion channels. Because K+ channel blockade or gene deletion causes hyperexcitability, it is generally assumed that increases in K+ channel gene expression should reduce neuronal network excitability. We have tested this hypothesis by creating a transgenic mouse that expresses a Shaker-type K+ channel gene. Paradoxically, we find that addition of the extra K+ channel gene results in a hyperexcitable rather than a hypoexcitable phenotype. The presence of the transgene leads to a complex deregulation of endogenous Shaker genes in the adult central nervous system as well as an increase in network excitability that includes spontaneous cortical spike and wave discharges and a lower threshold for epileptiform bursting in isolated hippocampal slices. These data suggest that an increase in K+ channel gene dosage leads to dysregulation of normal K+ channel gene expression, and it may underlie a mechanism contributing to the pathogenesis of human aneuploidies such as Down syndrome.

Visualization of beta-galactosidase by enzyme and immunohistochemistry in the olfactory bulb of transgenic mice carrying the LacZ transgene

In the olfactory bulb (OB) of a transgenic mouse line that carries the bacterial LacZ gene under the control of the 5'-regulatory region of the GAD67 gene, expression of the beta-galactosidase was confined almost exclusively to the non-GABAergic mitral and tufted cells. By light microscopy, enzyme histochemistry showed strong staining in the cell bodies and faint diffuse staining in the axons and dendrites. With immunohistochemistry for beta-galactosidase the entire cytoplasm, including the axons and dendrites, was strongly stained. By electron microscopy, beta-galactosidase enzyme histochemistry resulted in a submicroscopic reaction product that was diffusely distributed in the cytoplasm of neurons. In addition, large deposits of the reaction product were also seen attached to the cytoplasmic side of the membranes. In contrast, when the intracellular localization of beta-galactosidase was determined by immunohistochemistry, homogeneous cytoplasmic staining was obtained that filled the entire cytoplasm including the terminal dendrites and fine axons. Therefore, synaptic contacts of the beta-galactosidase-positive output neurons with other beta-galactosidase-negative neuronal cells were readily recognized in the OB. As we demonstrated, transgenic mouse lines expressing the LacZ reporter gene in a well-defined neuronal subpopulation can be used to follow beta-galactosidase-positive neurons and to directly identify their synaptic connections.

Expression of myc-family, myc-interacting, and myc-target genes during preimplantation mouse development

Previous studies indicated that members of the myc gene family may be essential for preimplantation development. Other studies revealed that preimplantation embryos lacking c-myc, N-myc, or L-myc are viable, indicating that these genes are either not essential for preimplantation development or can be substituted for functionally by other myc gene family members. To investigate the possible role of these genes during preimplantation development, we determined the temporal patterns of expression of four members of the myc gene family, genes encoding myc-associated proteins, and four putative MYC target genes. We observed a sequential pattern of myc gene expression, with the L-myc mRNA expressed as a maternal transcript, the c-myc mRNA expressed during the 4-cell through morula stages, and the B-myc mRNA expressed highly at the morula and blastocysts stages. B-myc was the predominant family member expressed during preimplantation development. The mxi mRNA was not detectable and the mad mRNA was detectable only as a maternal transcript. The max mRNA, however, was expressed both as a maternal mRNA and as an embryonic message throughout most of preimplantation development. Three putative MYC target genes (Odc, cyclin E, and prothymosin-alpha) were transcriptionally induced during the 2-cell stage, and their mRNAs increased sharply in abundance during development to the morula and blastocyst stages. Another putative MYC target gene, cyclin A, was expressed both as a maternal mRNA and as an embryonic transcript. These data support the view that the expression of myc target genes may be supported initially through the expression of maternally inherited MYC proteins and corresponding mRNAs and that subsequent stage-specific changes in expression of myc genes, myc-associated genes, and myc target genes may control early differentiative events around the time of implantation.

Huntington disease: new insights into the relationship between CAG expansion and disease

The mutation underlying Huntington disease (HD) is CAG expansion beyond 35 repeats within a novel gene. Recently, new insights into the role of the HD protein (huntingtin) in the pathogenesis of HD have emerged. The CAG is translated and expression of mutant huntingtin is essential for neuronal death. Huntingtin is crucial for normal development and may be regarded as a cell survival gene. Huntingtin is specifically cleaved during apoptosis by a key cysteine protease, apopain, known to play a pivotal role in apoptotic cell death. The rate of cleavage is enhanced by longer polyglutamine tracts, suggesting that inappropriate apoptosis underlies HD. Recently, three proteins have been identified and have been shown specifically to interact with huntingtin, two of these interactions being influenced by CAG length. Several different approaches to develop an animal model for HD include cDNA and YAC transgenics, as well as 'knock-in' strategies. Such a model will be critical for the understanding of the natural history of HD and for the testing of new therapeutic modalities.

Ubiquitous and neuronal DNA-binding proteins interact with a negative regulatory element of the human hypoxanthine phosphoribosyltransferase gene

The hypoxanthine phosphoribosyltransferase (HPRT) gene is constitutively expressed at low levels in all tissues but at higher levels in the brain; the significance and mechanism of this differential expression are unknown. We previously identified a 182-bp element (hHPRT-NE) within the 5'-flanking region of the human HPRT (hHPRT) gene, which is involved not only in conferring neuronal specificity but also in repressing gene expression in nonneuronal tissues. Here we report that this element interacts with different nuclear proteins, some of which are present specifically in neuronal cells (complex I) and others of which are present in cells showing constitutive expression of the gene (complex II). In addition, we found that complex I factors are expressed in human NT2/D1 cells following induction of neuronal differentiation by retinoic acid. This finding correlates with an increase of HPRT gene transcription following neuronal differentiation. We also mapped the binding sites for both complexes to a 60-bp region (Ff; positions -510 to -451) which, when analyzed in transfection assays, functioned as a repressor element analogous to the full-length hHPRT-NE sequence. Methylation interference footprintings revealed a minimal unique DNA motif, 5'-GGAAGCC-3', as the binding site for nuclear proteins from both neuronal and nonneuronal sources. However, site-directed mutagenesis of the footprinted region indicated that different nucleotides are essential for the associations of these two complexes. Moreover, UV cross-linking experiments showed that both complexes are formed by the association of several different proteins. Taken together, these data suggest that differential interaction of DNA-binding factors with this regulatory element plays a crucial role in the brain-preferential expression of the gene, and they should lead to the isolation of transcriptional regulators important in neuronal expression of the HPRT gene.

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.

Model of forebrain regionalization based on spatiotemporal patterns of POU-III homeobox gene expression, birthdates, and morphological features

In situ hybridization was used to map spatiotemporal expression patterns of the four known intronless POU-III transcription factor genes Brn-1, Brn-2, Brn-4, and Tst-1 in the developing rat forebrain vesicle, beginning on embryonic day 10. The results indicate that the proliferation layers (ventricular and subventricular) and mantle layer of the forebrain neural tube each display a strikingly unique pattern of regionalized POU-III expression. Within a particular region, or layer within a region, none to all four of the mRNAs may be detected, and during development a particular mRNA in a particular region displays one of five expression patterns, or a combination of these patterns, which may be described as conserved, lost, transient, acquired, or redeployed expression. In the developing brain as a whole, Brn-1 and Brn-2 early on display somewhat different spatial expression patterns that converge to essential identity in the adult, whereas Brn-4 expression is initially broad and becomes much more restricted in the adult, and Tst-1 expression expands greatly through development. Usually, though not always, expression patterns tend to correlate with major histological features in the forebrain (often internal or external sulci associated with proliferation zones), and little evidence for waves of expression moving through the whole forebrain over time was obtained. Thus, clear differences in hybridization intensity often are observed between the cerebral cortex, basal telencephalic nuclei, hypothalamus, ventral thalamus, dorsal thalamus, and pretectal region. In contrast, transverse bands of hybridization extending from the roof to the floor of the forebrain, corresponding to proposed neuromeres, were not observed with these probes. The results suggest that POU-III transcription factors help define specific regions in the early neuroepithelium as well as different cellular phenotypes in the ventricular, subventricular, and mantle layers of specific regions later in development. Thus, the functions of these regulatory proteins may be different in proliferating neuroepithelial cells, young neurons, and mature neurons and appear to be region-specific.