Temporal control of gene recombination in astrocytes by transgenic expression of the tamoxifen-inducible DNA recombinase variant CreERT2

Split-cre complementation indicates coincident activity of different genes in vivo

Cre/LoxP recombination is the gold standard for conditional gene regulation in mice in vivo. However, promoters driving the expression of Cre recombinase are often active in a wide range of cell types and therefore unsuited to target more specific subsets of cells. To overcome this limitation, we designed inactive "split-Cre" fragments that regain Cre activity when overlapping co-expression is controlled by two different promoters. Using transgenic mice and virus-mediated expression of split-Cre, we show that efficient reporter gene activation is achieved in vivo. In the brain of transgenic mice, we genetically defined a subgroup of glial progenitor cells in which the Plp1- and the Gfap-promoter are simultaneously active, giving rise to both astrocytes and NG2-positive glia. Similarly, a subset of interneurons was labelled after viral transfection using Gad67- and Cck1 promoters to express split-Cre. Thus, split-Cre mediated genomic recombination constitutes a powerful spatial and temporal coincidence detector for in vivo targeting.

Genetic models of the endocannabinoid system

The endocannabinoid (ECB) system comprises cannabinoid receptors, ECBs and the whole machinery for the synthesis and degradation of ECBs. It has emerged as an important signalling system in the nervous system, controlling numerous physiological processes, including synaptic transmission, learning and memory, reward, feeding, neuroprotection, neuroinflammation, and neural development. This system is also implicated in various diseases of the nervous system, and thus has become a promising therapeutic target. The use of genetically modified mice has contributed crucially to our rapidly expanding knowledge of the ECB system. In this chapter, the existing mouse mutants targeting the ECB system will be discussed in detail. The use of conditional mutants has given an additional dimension to the analysis of the system, and, it is hoped, will finally enable us to understand this widespread and complex system in the context of intricate networks where different brain regions and neurotransmitter systems interact tightly with each other.

Chromosome engineering in ES cells

Chromosomal rearrangements, such as deletions, duplications, inversions and translocations, occur frequently in humans and can be disease-associated or phenotypically neutral. To understand the genetic consequences of such genomic changes, these mutations need to be modelled in experimentally tractable systems. The mouse is an excellent organism for this analysis because of its biological and genetic similarity to humans, the ease with which its genome can be manipulated and the similarity of observed affects. Through chromosome engineering, defined rearrangements can be introduced into the mouse genome. The resulting mouse models are leading to a better understanding of the molecular and cellular basis of dosage alterations in human disease phenotypes, in turn opening new diagnostic and therapeutic opportunities.

Regulated gene insertion by steroid-induced PhiC31 integrase

Nonviral integration systems are widely used genetic tools in transgenesis and play increasingly important roles in strategies for therapeutic gene transfer. Methods to efficiently regulate the activity of transposases and site-specific recombinases have important implications for their spatiotemporal regulation in live transgenic animals as well as for studies of their applicability as safe vectors for genetic therapy. In this report, strategies for posttranslational induction of a variety of gene-inserting proteins are investigated. An engineered hormone-binding domain, derived from the human progesterone receptor, hPR891, and specifically recognized by the synthetic steroid mifepristone, is fused to the Sleeping Beauty, Frog Prince, piggyBac and Tol2 transposases as well as to the Flp and ΦC31 recombinases. By analyzing mifepristone-directed inducibility of gene insertion in cultured human cells, efficient posttranslational regulation of the Flp recombinase and the ΦC31 integrase is documented. In addition, fusion of the ΦC31 integrase with the ER^T2 modified estrogen receptor hormone-binding domain results in a protein, which is inducible by a factor of 22-fold and retains 75% of the activity of the wild-type protein. These inducible ΦC31 integrase systems are important new tools in transgenesis and in safety studies of the ΦC31 integrase for gene therapy applications.

Inducible Cre recombinase activity in mouse mature astrocytes and adult neural precursor cells

Two transgenic mouse lines expressing an inducible form of the Cre recombinase (CreER) under the control of the human GFAP promoter have been generated and characterized. In adult mice, expression of the fusion protein is largely confined to astrocytes in all regions of the central nervous system. Minimal spontaneous Cre activity was detected and recombination was efficiently induced by intraperitoneal administration of tamoxifen in adult mice. The pattern of recombination closely mirrored that of transgene expression. The percentage of astrocytes undergoing recombination varied from region to region ranging from 35% to 70% while a much smaller portion (<1%) of oligodendrocytes and neural precursor cells showed evidence of Cre activity. These mouse lines will provide important tools to dissect gene function in glial cells and in gliomagenesis.

Developmental profile and properties of sulforhodamine 101--Labeled glial cells in acute brain slices of rat hippocampus

The reliable identification of astrocytes for physiological measurements was always time-consuming and difficult. Recently, the fluorescent dye sulforhodamine 101 (SR101) was reported to label cortical glial cells in vivo [Nimmerjahn A, Kirchhoff F, Kerr JN, Helmchen F. Sulforhodamine 101 as a specific marker of astroglia in the neocortex in vivo. Nat Methods 2004;1:31-7]. We adapted this technique for use in acute rat hippocampal slices at early postnatal stages (P3, 7, 15) and in young adults (P24-27) and describe a procedure for double-labeling of SR101 and ion-selective dyes. Using whole-cell patch-clamp, imaging, and immunohistochemistry, we characterized the properties of SR101-positive versus SR101-negative cells in the stratum radiatum. Our data show that SR101, in contrast to Fura-2 or SBFI, only stains a subset of glial cells. Throughout development, SR101-positive and SR101-negative cells differ in their basic membrane properties. Furthermore, SR101-positive cells undergo a developmental switch from variably rectifying to passive between P3 and P15 and lack voltage-gated Na+ currents. At P15, the majority of SR101-positive cells is positive for GFAP. Thus, our data demonstrate that SR101 selectively labels a subpopulation of glial cells in early juvenile hippocampi that shows the typical developmental changes and characteristics of classical astrocytes. Owing to its reliability and uncomplicated handling, we expect that this technique will be helpful in future investigations studying astrocytes in the developing brain.

Transgenic mice for conditional gene manipulation in astroglial cells

Astrocytes are thought to exert diverse functions in the brain, but it has been difficult to prove this in vivo because of a scarcity of tools to manipulate these cells. Here, we report the generation of new transgenic mouse lines that allow for conditional gene ablation in astrocytes using the tamoxifen- (TAM-) inducible CreER(T2)/loxP system and bacterial artificial chromosome (BAC)-based transgenesis. In adult transgenic mice, where CreER(T2) expression is driven by the promoter of the sodium-dependent glutamate/aspartate transporter (Glast/Slc1a3) or of connexin 30 (Cx30/Gjb6), intraperitoneal TAM-injection induced Cre-mediated recombination in astroglial cells throughout the brain. Targeting efficacies varied in a region-specific manner from 20 to 90% as indicated by enzyme-based reporter lines and immunohistochemical staining. In addition, the Glast-line allowed to target retinal Müller cells and adult neural stem/progenitor cells in neurogenic regions of the adult brain. Transgenic mice expressing CreER(T2) under the control of the apolipoprotein e (ApoE) or aquaporin 4 (Aqp4) promoter showed inducible recombination in different areas of the central nervous system (CNS) albeit at low levels. Transgenic lines showed TAM-induced recombination in specific peripheral organs. These new mouse lines should help to further explore the relevance of astrocytes for brain function, as well as their contribution to pathological conditions because of aging, disease or injury.

Inducible gene inactivation in neurons of the adult mouse forebrain

Background

The analysis of the role of genes in important brain functions like learning, memory and synaptic plasticity requires gene inactivation at the adult stage to exclude developmental effects, adaptive changes or even lethality. In order to achieve temporally controlled somatic mutagenesis, the Cre/loxP-recombination system has been complemented with the tamoxifen-inducible fusion protein consisting of Cre recombinase and the mutated ligand binding domain of the human estrogen receptor (CreER^T2). To induce recombination of conditional alleles in neurons of the adult forebrain, we generated a bacterial artificial chromosome-derived transgene expressing the CreER^T2 fusion protein under control of the regulatory elements of the CaMKIIα gene (CaMKCreER^T2 transgene).

Results

We established three mouse lines harboring one, two and four copies of the CaMKCreER^T2 transgene. The CaMKCreER^T2 transgene displayed reliable and copy number-dependent expression of Cre recombinase specifically in neurons of the adult forebrain. Using Cre reporter mice we show very low background activity of the transgene in absence of the ligand and efficient induction of recombination upon tamoxifen treatment in all three lines. In addition, we demonstrate in mice harboring two conditional glucocorticoid receptor (GR) alleles and the CaMKCreER^T2 transgene spatially restricted loss of GR protein expression in neurons of the adult forebrain upon tamoxifen treatment.

Conclusion

This is to our knowledge the first approach allowing highly efficient inducible gene inactivation in neurons of the adult mouse forebrain. This new approach will be a useful tool to dissect the function of specific genes in the adult forebrain. Effects of gene inactivation on pre- and postnatal brain development and compensatory mechanisms elicited by an early onset of gene inactivation can now be excluded.

Characterization of astrocyte-specific conditional knockouts

Conditional gene knockouts are a very powerful tool for elucidating gene function in animal physiology and behavior. To obtain cell-specific knockouts, a promoter is utilized that drives expression of Cre recombinase specifically to the cell population of interest. We describe several transgenic lines of mice that were created in an attempt to obtain astrocyte-specific gene recombination. A 2 kb fragment from the human glial fibrillary acidic protein promoter is utilized to drive expression of inducible Cre recombinase, with both the Tet-Off and tamoxifen responsive systems. We show data obtained from crosses with two Cre reporter lines, ROSA26R and an astrocyte Cre reporter created in our laboratory, to assess the cell specificity of gene recombination. Additionally, our system is shown to successfully recombine a floxed Connexin43 locus, although recombination is not as extensive as seen in crosses with reporter lines.

Glutamate-mediated neuronal-glial transmission

The brain is the most complex organ of the human body. It is composed of several highly specialized and heterogeneous populations of cells, represented by neurones (e.g. motoneurons, projection neurons or interneurons), and glia represented by astrocytes, oligodendrocytes and microglia. In recent years there have been numerous studies demonstrating close bidirectional communication of neurons and glia at structural and functional levels. In particular, the excitatory transmitter glutamate has been shown to evoke a variety of responses in astrocytes and oligodendrocytes in the healthy as well as the diseased brain. Here we overview the multitude of glutamate sensing molecules expressed in glia and describe some general experiments which have been performed to identify the glutamate-responsive molecules, i.e. the ionotropic and metabotropic glutamate receptors as well as the glutamate transporters. We also discuss a transgenic mouse model that permits detailed and specific investigations of the role of glial glutamate receptors.

Molecular mechanisms of astrogliosis: new approaches with mouse genetics

Astrocytes are increasingly being recognized as dynamic participants in many aspects of normal central nervous system function. In disease states, reactive astrocytes undergo complex phenotypic changes, generically referred to as astrogliosis. Unraveling the functions of reactive astrocytes and underlying molecular mechanisms is a difficult problem. The use of genetically modified mice is beginning to yield some answers to long-standing questions in the field. What are the functions of reactive astrocytes? What extracellular factors and intracellular signaling mechanisms are responsible for astrocyte activation in various forms of neural injury? In this review we will highlight studies using astrocyte reporter lines for cellular imaging and lineage tracing, as well as gain- and loss-of-function mutations that have begun to shed light on mechanisms of astrogliosis.

Genetic targeting of principal neurons in neocortex and hippocampus of NEX-Cre mice

Conditional mutagenesis permits the cell type-specific analysis of gene functions in vivo. Here, we describe a mouse line that expresses Cre recombinase under control of regulatory sequences of NEX, a gene that encodes a neuronal basic helix-loop-helix (bHLH) protein. To mimic endogenous NEX expression in the dorsal telencephalon, the Cre recombinase gene was targeted into the NEX locus by homologous recombination in ES cells. The Cre expression pattern was analyzed following breeding into different lines of lacZ-indicator mice. Most prominent Cre activity was observed in neocortex and hippocampus, starting from around embryonic day 11.5. Within the dorsal telencephalon, Cre-mediated recombination marked pyramidal neurons and dentate gyrus mossy and granule cells, but was absent from proliferating neural precursors of the ventricular zone, interneurons, oligodendrocytes, and astrocytes. Additionally, we identified formerly unknown domains of NEX promoter activity in mid- and hindbrain. The NEX-Cre mouse will be a valuable tool for behavioral research and the conditional inactivation of target genes in pyramidal neurons of the dorsal telencephalon.

Conditional somatic mutagenesis in the mouse using site-specific recombinases

In the last decade, site-specific recombinases (SSRs), such as Cre and Flp, have emerged as indispensable tools for the precise in vivo manipulation of the mouse genome. It is now feasible to control, in space and time, the onset of gene knockouts in almost any tissue of the mouse, thus greatly facilitating the creation of sophisticated animal models for human disease and drug development. This review describes the basic principles and current status of the SSR technology, with a focus on strategies for conditional somatic mutagenesis using the Cre/lox system and ligand-activated Cre recombinases. Practical hints for generating and analysing conditional mouse mutants will be given and exciting novel applications of the SSR technology will be discussed, such as cell fate mapping and the combined use of Cre, Flp and other biotechnological tools. It will be shown how genetic manipulation of the mouse by site-specific recombination can provide new solutions to old problems in the analysis of human physiology and pathophysiology and how it can be employed for drug discovery and development.