Expression of Cre recombinase in dopaminoceptive neurons
© Lemberger et al; licensee BioMed Central Ltd. 2007
Received: 19 June 2006
Accepted: 03 January 2007
Published: 03 January 2007
Dopamine-activated signaling regulates locomotor and emotional responses and alterations in dopamine-signaling are responsible of several psychomotor disorders. In order to identify specific functions of these pathways, the Cre/loxP system has been used. Here, we describe the generation and the characterization of a transgenic mouse line expressing the Cre recombinase in dopaminoceptive neurons. To this purpose, we used as expression vector a 140 kb yeast artificial chromosome (YAC) containing the dopamine D1 receptor gene (Drd1a).
In the chosen line, D1Cre, the spatio-temporal pattern of Cre expression closely recapitulated that of the endogenous Drd1a gene, as assessed by immunohistological approaches in embryonic and adult stages. Efficiency of recombination was confirmed by crossing D1Cre with three different loxP lines (Creb1loxP, CaMKIVloxP and GRloxP) and with the R26R reporter. In the three loxP lines studied, recombination was restricted to the area of Cre expression.
In view of the patterns of recombination restricted to the major dopaminoceptive regions as seen in the context of the CREB, CaMKIV and GR mutations, the D1Cre line will be a useful tool to dissect the contributions of specific genes to biological processes involving dopamine signaling.
Although dopaminergic neurons are few (e.g. 10–20,000 in the rat brain), they regulate a number of physiological, behavioral and cognitive functions, including regulation of locomotor activity, incentive behaviors, short-term and stimulus-dependent memory systems [1–3]. Dysfunction of the dopaminergic system and its targets underlie major human disorders such as Parkinson's disease, Huntington's disease, schizophrenia and addiction to drugs of abuse [4–6]. Five dopamine receptors, termed D1, D2,D3, D4 and D5, encoded by five different genes (Drd1a, Drd2, Drd3, Drd4 and Drd5 respectively) have been identified [7, 8]. The elucidation of the signaling properties of dopaminoceptive neurons is of central importance for the development of therapeutic strategies to treat dysfunctions of the dopaminergic system.
In the past years, use of the Cre/loxP system has enabled generation of cell-specific mutations In the central nervous system, efforts have generally been devoted to the generation of region-specific mutations using promoters to drive the expression of Cre recombinase in specific brain areas [9–11]. In order to target the dopaminoceptive neurons, we have generated a transgenic line that expresses Cre recombinase in dopaminoceptive neurons under the control of the dopamine D1 receptor gene (D1Cre), by using the genomic locus of a dopamine D1 receptor gene cloned in a YAC vector.
Here we examine the cellular expression of Cre recombinase in D1Cre transgenic mice by immunohistochemistry in combination with in situ hybridization, to compare it with the expression of D1 receptor mRNA and other dopaminoceptive molecular markers. The activity of the Cre was confirmed by loss of CREB, CaMKIV and GR proteins in the respective conditional mutants and by induction of beta-galactosidase activity in the R26RD1Cre reporter mice.
A 140 kb yeast artificial chromosome (YAC) containing the entire dopamine D1 receptor gene (Drd1a) was isolated by screening a mouse genomic YAC library . The YAC encompasses the entire Drd1a gene including 40 kb of upstream and 100 kb of downstream sequences. The coding sequence for the nuclear localized nlsCre  was introduced into the Drd1a gene by homologous recombination in yeast. To minimize the chances of altering possible regulatory elements, we avoided the use of a heterologous polyadenylation sequence and minimized the deletion of the Drd1a coding sequence. Specifically, the homology regions were chosen so that the ATG of the nlsCre would replace the ATG of the Drd1a gene and the STOP codon of the nlsCre would interrupt the Drd1a open reading frame after its last possible internal ATG.
To further assess the subpopulation specificity of the D1Cre transgene, we checked whether also D2R-positive cells express Cre protein in the striatum. Therefore, we performed in situ hybridization by using a D2R-specific riboprobe followed by immunohistochemistry with Cre-specific antibody (Fig. 7c). This experiment shows that D1Cre is expressed also in about 40% of D2R-expressing cells, and that only about 20% of D2R positive cells do not express Cre at detectable levels (Fig. 7d) while about 40% of the neurons express D1Cre but not D2R mRNA. Therefore, we conclude that at least 80% of the striatal neurons express D1Cre. It has to be mentioned that some studies indicate that there is only very little overlap in the number of neurons expressing both receptors [19, 21]. According to these studies we were not expecting expression of D1Cre in D2R positive cells to such an extent. However, depending on the sensitivity and resolution of the adopted technical approach, others studies reveal that there is a higher degree of overlap  and that possibly all dopamine receptor-containing neurons, in the striatum, express both receptor types [23, 24]. Our study does not solve this debate about the degree of overlap between D1R and D2R, however, the finding that Cre expression is detectable in both neuronal subpopulations, D1R- and/or D2R-expressing neurons, suggests that these receptors are at least partially coexpressed in a certain proportion of striatal neurons (about 40%).
The pattern of expression obtained with the D1Cre YAC transgene was very reproducible between transgenic lines. The expression level was essentially only dependent on the copy number of the transgene. These observations illustrate well the properties of transgenes based on large DNA segments: position-independent and copy-number dependent expression of the transgene. This correlation between expression pattern and copy number usually does not apply to plasmid-based constructs . The pattern of Cre expression as seen in the D1Cre transgenic animals is very close to the pattern of expression of the endogenous Drd1a gene reported in previous in situ hybridization studies . Moreover, the developmental and postnatal patterns of expression match the patterns described for the endogenous Drd1a gene [15–17]. We have also performed in situ hybridization with D1R riboprobe in combination with Cre immunohistochemistry in adult cortex and striatum and found high levels of overlap (Fig. 3a, b).
Several publications have suggested that D1 and D2 receptors are segregated specifically on distinct neuronal populations of the rat striatum that projects directly to substantia nigra pars reticulata and to globus pallidus respectively [20, 30]. Some other publications have however demonstrated that D1 and D2 might actually be coexpressed [22–24]. These discordant studies clearly indicate that a fine modulation of expression levels of D1R and D2R takes place in the dopaminoceptive neurons. In our transgenic line, we observe that Cre is expressed in a majority of striatal neurons, but it remains open whether our vector, even though it contains large segments of 5'- and 3'- flanking sequences, carries all necessary elements required to achieve this fine modulation of expression levels within striatum.
A potential application of D1Cre transgene for isolation of mutated dopaminoceptive cells is represented by the generation of double transgenic mice with BAC-D1GFP and BAC-D2GFP obtained by the GENSAT BAC Transgenic project . The D1Cre expression pattern matches quite closely the BAC-D1GFP pattern in the medium spiny neurons of the striatum, therefore one would expect to isolate recombined GFP positive cells. However deeper analysis by costaining is required to compare the two transgenic lines.
When recombination was verified in the context of three loxP-modified alleles, loss of the respective proteins were largely confined to the main areas of Cre expression. Thus loss of CREB, CaMKIV and GR were extensive in striatum, layer VI of the cortex and CA2 of the hippocampus (Figure 8). However, induction of beta-galactosidase activity in R26RD1Cre reporter mice showed a more widespread pattern of recombination in layer IV of the cortex and in the CA1 of the hippocampus (Figure 6). One possible explanation for this could be that scattered recombination, as caused by weak and transient Cre expression occurring at early postnatal stages (Figure 5), is easily underestimated when followed by loss of the respective protein but very visible when revealed by beta-galactosidase staining. This may be also related to a tissue-specific difference in the stability of CREB, CaMKIV and GR proteins in CA1 and layer IV.
In view of the patterns of recombination restricted to the major dopaminoceptive regions as seen in the context of the CREB, CaMKIV and GR mutations, the D1Cre line should be useful to evaluate the contribution of these, and others, signaling molecules to the dopaminergic system. The D1Cre line, combined with specific pharmacological treatment may provide a powerful tool do genetically dissect signalling pathways underlying dopaminergic neurotransmission.
We present the generation and characterization of a transgenic mouse line expressing the Cre recombinase under the control of the D1R regulatory elements. We show that this transgenic line can be used successfully for the conditional ablation of specific genes important for biological processes involving dopamine signaling.
D1Cre YAC construction
The small insert Princeton mouse YAC library  was screened by PCR (primers: 5'-TTT CAT CCT CCC TCA TAA GC-3' and 5'-TTCGACAGGGTTTCCATTAC-3'). To allow for subsequent modification, the YAC was transferred into the yeast strain YPH925 . To insert the Cre recombinase into the Drd1a gene, a targeting vector was constructed by cloning a 800 bp 5' homology region (primers:5'-GGG GCG GCC GCG GTC CTG CCC TAA GAA CGA G-3'and 5'-ACC AAG CTT AGC CAG ACT TCC CCC-3'), the nlsCre open reading frame (HindIII-EcoRI fragment from pHD2 nlsCre, ) and a 500 bp 3' homology region (primers: 5'-TTT GGG TGG GCG AAT TCT TCC CTG AAC CCC ATT ATT TAT-3' and 5'-GAT AAT ACT CCC AAA CTG GAT TTC AGA GCC GAA GTC ATT T-3') bearing a unique XbaI restirction site into the NotI and BamHI sites of the pRS306 vector . The XbaI linearized targeting vector was then used to modify the Drd1a containing YAC by classical "pop-in pop-out" modification .
Generation of transgenic animals
The YAC DNA from one of the positive colonies was purified as described  and microinjected into the pronucleus of FVB/N oocytes. Founders were identified by Southern blot analysis of BamHI digested genomic DNA using the 5' homology region as a probe. Since this probe hybridizes identically to the endogenous Drd1a gene and to the modified YAC, it was used to determine the copy number of the transgene in the various founders. For subsequent routine genotyping, dot blot analysis was performed with a probe for the Cre open reading frame. The transgenic line was maintained by backcrossing to C57Bl/6. To generate the respective mutant animals, the D1Cre transgenic mice were first crossed to mice homozygous for the respective loxP-modified allele. Offspring hemizygous for the transgene and heterozygous for the loxP-modified allele were in turn crossed to mice homozygous for the loxP-modified allele to generate mutant animals with an expected frequency of 25%. We did not observe cases of germline recombination with this transgenic line.
Paraffin sections (7 μm) from mouse embryos at different developmental stages and vibratome sections (50 μm) prepared from brains of perfused mice (0.1 M phosphate-buffered 4% paraformaldehyde) were used for immunohistochemical labeling. The following antibodies were used: a rabbit polyclonal anti-Cre antibody (1:3,000, ), a rabbit polyclonal anti-CREB antibody (1:3000), a goat polyclonal anti-CaMKIV antibody (1:200, Santa Cruz, cat. Sc-1546), a rabbit polyclonal GR antibody (1:2000, Santa Cruz, cat. Sc-1004), a mouse monoclonal anti class III beta-tubulin antibody (1:400, clone TujI, Covance, cat. MMS-435P). Immunohistochemistry was performed using the avidin-biotin system (Vectastain, Vector Labs). Immunofluorescence was achieved using Alexa488 coupled anti-rabbit antibody and an Alexa594-coupled anti-mouse antibody (Molecular Probes) as secondary antibodies. Beta-galactosidase activity was revealed by incubating embryos or vibratome sections at 37°C overnight in X-Gal staining solution (5 mM potassium hexacyanoferrate (III), 5 mM potassium hexacyanoferrate (II), 2 mM MgCl2, 0.01% NP-40, 0.02% sodium deoxycholate, 1 mg/ml X-gal, 20 mM Tris/HCl, pH7.5).
Non-radioactive in situ hybridization was performed on vibratome sections (20 μm) prehybridized, after a step with Proteinase K, at 70°C for 1 hour. Hybridization with specific riboprobe occurs overnight at 70°C in the hybridization solution. After two washing steps the sections were incubated with the anti-digoxygenin antibody (1:10000) in 20% normal swine serum. The development of the reaction was done with NBT/BCIP (Roche). After washing in PBS and PBST, the sections were processed for immunohistochemistry as described above.
To estimate the % of neurons expressing D2R and Cre, cells positive for both signals were counted in nine nonoverlapping microscopic field at 400× magnification. Neurons expressing either Cre protein or D2R mRNA were also counted. The data are expressed as mean ± SEM.
Retrograde tracing of striatonigral neurons
Mice were injected stereotaxically into the right and left substantia nigra pars reticulata (AP = -3.28 mm, DV = -4.15 mm, L = -1.3 mm and +1.3 mm) with 2 μl of a carboxylate-modified red latex FluoroSpheres solution (Molecular Probes, cat. L-2783,). Three days later, the animals were perfused through the ascending aorta with 0.1 M phosphate-buffered 4% paraformaldehyde and the brains were removed to be postfixed for 4 hr at 4°C. Cre immunofluorescence was performed on cryostat-cut free-floating 30 μm-thick sections. Sections were examined on a confocal microscope (MRC 1024, Bio-Rad Laboratories, Hemel Hempstead, Hertfordshire, U.K.) fitted on an inverted microscope (Axiovert 100, Zeiss, Oberkochen, Germany).
List of abbreviations
yeast artificial chromosome
cAMP-responsive element binding protein
Ca2+/Calmodulin-dependent protein kinase IV
dopamine D1 receptor
dopamine D2 receptor
We thank Heidrun Kern, Heike Glaser, and Katrin Anlag for expert technical assitance. We thank Dr. Brenda Stride and Dr. Jan Rodriguez for critically reading the manuscript. This work was supported by the DFG (Germany), by the "Fonds der Chemischen Industrie", the European Community, the BMBF (Germany) and by the Alexander von Humboldt-Stiftung to GS, by HFSPO and SNF (Switzerland) to TL and by FMRE (Belgium), FNRS (Belgium) and "Action de Recherche Concerté" to SNS.
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