Selective expression of an aP2/Fatty Acid Binding Protein4-Cre transgene in non-adipogenic tissues during embryonic development

Mouse strains expressing the site-specific Cre recombinase facilitate conditional ablation or activation of genomic sequences when one or several exons of a gene of interest are flanked by loxP sites. Recently, several strains targeting Cre expression to adipocytes have been developed using promoter sequences from the aP2 (Fatty Acid Binding Protein 4, FABP4) gene for adipose tissue-specific gene expression studies. aP2/FABP4 is predominantly expressed in adipose tissue, and while this promoter provides adipocyte-restricted expression postnatally, its expression throughout embryonic development had not been previously characterized. In this report, we demonstrate that the aP2-Cre transgene is expressed and consistently localized within the embryo from mid-gestation stage 9.5 dpc. By 15.5 dpc, β-gal activity was detected primarily in the brown adipose tissue, trigeminal ganglia, dorsal root ganglia, cartilage primordia and vertebrae. Immunofluorescence staining for Cre recombinase and FABP4 protein showed a corresponding staining pattern similar to that of β-gal, confirming that Cre recombinase was produced in the transgenic line at late stages of development, and overlapped with endogenous aP2/FABP4 production. Further, fat-specific oil red O staining of tissue sections validated the presence of lipids in the stained tissues indicating that adipocytes and/or adipocyte-like cells were indeed present in these tissues. This is the first report to our knowledge to describe and confirm aP2/FABP4 promoter expression in this transgenic line during development in the mouse embryo and indicates that aP2/FABP4 expression occurs not only in mature adipocytes, but has a wider embryonic expression pattern than previously appreciated.Lucy Liaw and Deena Small contributed equally to this work     

Dkk3-Cre BAC transgenic mouse line: a tool for highly efficient gene deletion in retinal progenitor cells

To establish the genetic tools for conditional gene deletion in mouse retinal progenitors, we generated a Dkk3-Cre transgenic mouse line using bacterial artificial chromosome (BAC) transgenesis. Cre recombination efficiency in vivo was assayed by crossing this transgenic line, termed BAC-Dkk3-Cre, with the CAG-CAT-Z reporter line. This BAC-Dkk3-Cre line showed Cre recombinase activity in most retinal progenitors. Cre activity was detectable from embryonic day 10.5 (E10.5) and generally restricted to the retina during embryogenesis. To verify that BAC-Dkk3-Cre mice successfully circumvented lethality, we generated Otx2flox/flox/BAC-Dkk3-Cre+ mice as Otx2 conditional knockout mice. The Otx2flox/flox/BAC-Dkk3-Cre+ mice were viable, and their retina showed loss of mature cell-type markers of photoreceptor cells, bipolar cells, and horizontal cells, in contrast, amacrine-like cells noticeably increased. Thus, the BAC-Dkk3-Cre transgenic mouse line provides a powerful tool for generating conditional knockout mouse lines for studying loss of gene functions in the developing retina.     

Efficient Cre-mediated deletion in cardiac progenitor cells conferred by a 3'UTR-ires-Cre allele of the homeobox gene Nkx2-5

Conditional gene targeting and transgenic strategies utilizing Cre recombinase have been successfully applied to the analysis of development in mouse embryos. To create a conditional system applicable to heart progenitor cells, a Cre recombinase gene linked at its 5' end to an internal ribosome entry site (IRES) was inserted into the 3' untranslated region of the cardiac homeobox gene Nkx2-5 using gene targeting. Nkx2-5IRESCre mice were fully viable as homozygotes. We evaluated the efficacy of Cre-mediated deletion by crossing Nkx2-5IRESCre mice with the Cre-dependent R26R and Z/AP reporter strains. Efficient deletion was observed in the cardiac crescent and heart tube in both strains. However, the Z/AP locus showed transient resistance to deletion in caudal heart progenitors. Such resistance was not evident at the R26R locus, suggesting that Cre-mediated deletion in myocardium may be locus-dependent. From cardiac crescent stages, deletion was seen not only in myocardium, but also endocardium, dorsal mesocardium and pericardial mesoderm. The Cre domain apparently includes cells dorsal to the heart that have been shown to constitute a secondary heart field, contributing myocardium to the outflow tract. Other sites of Nkx2-5 expression, including pharyngeal endoderm and its derivatives, branchial arch epithelium, stomach, spleen, pancreas and liver, also showed efficient deletion. Our data suggest that the Nkx2-5IRESCre strain will be useful for genetic dissection of the multiple tiers of lineage allocation to the forming heart as well as of molecular interactions within the heart fields and heart tube.     

The right ventricle, outflow tract, and ventricular septum comprise a restricted expression domain within the secondary/anterior heart field

The vertebrate heart arises from the fusion of bilateral regions of anterior mesoderm to form a linear heart tube. Recent studies in mouse and chick have demonstrated that a second cardiac progenitor population, known as the anterior or secondary heart field, is progressively added to the heart at the time of cardiac looping. While it is clear that this second field contributes to the myocardium, its precise boundaries, other lineages derived from this population, and its contributions to the postnatal heart remain unclear. In this study, we used regulatory elements from the mouse mef2c gene to direct the expression of Cre recombinase exclusively in the anterior heart field and its derivatives in transgenic mice. By crossing these mice, termed mef2c-AHF-Cre, to Cre-dependent lacZ reporter mice, we generated a fate map of the embryonic, fetal, and postnatal heart. These studies show that the endothelial and myocardial components of the outflow tract, right ventricle, and ventricular septum are derivatives of mef2c-AHF-Cre expressing cells within the anterior heart field and its derivatives. These studies also show that the atria, epicardium, coronary vessels, and the majority of outflow tract smooth muscle are not derived from this anterior heart field population. Furthermore, a transgene marker specific for the anterior heart field is expressed in the common ventricular chamber in mef2c mutant mice, suggesting that the cardiac looping defect in these mice is not due to a failure in anterior heart field addition to the heart. Finally, the Cre transgenic mice described here will be a crucial tool for conditional gene inactivation exclusively in the anterior heart field and its derivatives.     

Efficient gene-specific expression of cre recombinase in the mouse embryo by targeted insertion of a novel IRES-Cre cassette into endogenous loci.

Site specific recombinases have provided the experimental strategy necessary to modulate the expression of gene products in the mouse embryo. In this study we have exploited Cre recombinase to develop a widely applicable cell marking system which functions efficiently even at early post-implantation embryonic stages. Importantly, the techniques and reagents derived in this study are generally applicable to any recombinase driven approach, including strategies to temporally and spatially modulate endogenous or ectopic gene expression in the embryo. The cell marking scheme has two essential components which were derived as separate mouse lines. The first line carries a universal conditional lacZ reporter (UCR) locus which was prepared by using gene targeting in a novel approach to modify a ubiquitously expressed retroviral lacZ promoter trap insertion. The UCR locus is silent until it undergoes a Cre mediated DNA rearrangement to restore lacZ expression. To generate the Cre expressing allele, we outline a flexible strategy which requires the introduction of a novel IRES-Cre cassette into exon sequence of an endogenous locus by gene targeting. We successfully demonstrate this approach by generating a Cre expressing allele of the EphA2 gene, an Eph receptor protein tyrosine kinase expressed early in development. Analysis of double heterozygote embryos clearly demonstrates that Cre recombinase is expressed in vivo from the EphA2 IRES-Cre allele, and that the conditional reporter locus is efficiently restored in EphA2-expressing cells as early as 7.5 dpc. This cell marking experiment establishes the feasibility of expressing Cre recombinase from a single copy allele in the embryo and demonstrates the utility of the conditional reporter mouse which can be used in the analysis of any Cre expressing allele.     

Inducible cardiomyocyte‐specific gene disruption directed by the rat Tnnt2 promoter in the mouse

We developed a conditional and inducible gene knockout methodology that allows effective gene deletion in mouse cardiomyocytes. This transgenic mouse line was generated by coinjection of two transgenes, a “reverse” tetracycline‐controlled transactivator (rtTA) directed by a rat cardiac troponin T (Tnnt2) promoter and a Cre recombinase driven by a tetracycline‐responsive promoter (TetO). Here, Tnnt2‐rtTA activated TetO‐Cre expression takes place in cardiomyocytes following doxycycline treatment. Using two different mouse Cre reporter lines, we demonstrated that expression of Cre recombinase was specifically and robustly induced in the cardiomyocytes of embryonic or adult hearts following doxycycline induction, thus, allowing cardiomyocyte‐specific gene disruption and lineage tracing. We also showed that rtTA expression and doxycycline treatment did not compromise cardiac function. These features make the Tnnt2‐rtTA;TetO‐Cre transgenic line a valuable genetic tool for analysis of spatiotemporal gene function and cardiomyocyte lineage tracing during developmental and postnatal periods. genesis 48:63–72, 2010. © 2009 Wiley‐Liss, Inc.     

Z/EG, a double reporter mouse line that expresses enhanced green fluorescent protein upon Cre-mediated excision

The Cre/loxP system has become an important tool in designing postintegrational switch mechanisms for transgenes in mice. The power and spectrum of application of this system depends on transgenic mouse lines that provide Cre recombinase activity with a defined cell type-, tissue-, or developmental stage-specificity. We have developed a novel mouse line that acts as a Cre reporter. The mice, designated Z/EG (lacZ/EGFP), express lacZ throughout embryonic development and adult stages. Cre excision, however, removes the lacZ gene, which activates expression of the second reporter, enhanced green fluorescent protein. We have found that the double-reporter Z/EG line is able to indicate the occurrence of Cre excision from early embryonic to adult lineages. The advantage of the Z/EG line is that Cre-mediated excision can be monitored in live samples and that live cells with Cre-mediated excision can be isolated using a single-step FACS. It will be a valuable reagent for the increasing number of investigators taking advantage of the powerful tools provided by the Cre/loxP site-specific recombinase system.     

Transgenic mice that express Cre recombinase in hypertrophic chondrocytes

In order to investigate the physiological control of hypertrophic chondrocytes which present the terminally differentiated form of chondrocytes, we generated a mouse line expressing the Cre recombinase under the control of the mouse type X collagen (Col10a1) promoter. In situ hybridization analysis demonstrated the expression of Col10a1-Cre transgene in hypertrophic chondrocytes of femur at postnatal day 2 (P2). In order to test the excision activity of the Cre recombinase, the Col10a1-Cre transgenic line was crossed with the mouse strain carrying the Smad4 conditional alleles (Smad4co/co) and the reporter line ROSA26. Multiple tissue PCR of Col10a1-Cre;Smad4co/+ mice revealed the restricted Cre activity in tissues containing hypertrophic chondrocytes. LacZ staining revealed that the Cre activity was observed in the cartilage primordia of ribs at E14.5 and only detected in the lower hypertrophic region of ribs at P1. These data suggest that the Col10a1-Cre mouse line described here could be used to achieve conditional gene targeting in hypertrophic chondrocytes.     

Z/AP, a double reporter for cre-mediated recombination

The Cre/loxP site-specific recombination system combined with embryonic stem cell-mediated technologies has greatly expanded our capability to address normal and disease development in mammals using genetic approaches. The success of this emerging technology hinges on the production of Cre-expressing transgenic lines that provide cell type-, tissue-, or developmental stage-specific recombination between loxP sites placed in the genome. Here we describe and characterize the production of a double-reporter mouse line that provides a convenient and reliable readout of Cre recombinase activity. Throughout all embryonic and adult stages, the transgenic animal expresses the lacZ reporter gene before Cre-mediated excision occurs. Cre excision, however, removes the lacZ gene, allowing expression of the second reporter, the human alkaline phosphatase gene. This double-reporter transgenic line is able to indicate the occurrence of Cre excision in an extremely widespread manner from early embryonic to adult lineages. It will be a valuable reagent for the increasing number of investigators taking advantage of the powerful tools provided by the Cre/loxP site-specific recombinase system.     

Tie2-Cre transgenic mice: a new model for endothelial cell-lineage analysis in vivo

Endocardial cells are thought to contribute at least in part to the formation of the endocardial cushion mesenchyme. Here, we created Tie2-Cre transgenic mice, in which expression of Cre recombinase is driven by an endothelial-specific promoter/enhancer. To analyze the lineage of Cre expressing cells, we used CAG-CAT-Z transgenic mice, in which expression of lacZ is activated only after Cre-mediated recombination. We detected pan-endothelial expression of the Cre transgene in Tie2-Cre;CAG-CAT-Z double-transgenic mice. This expression pattern is almost identical to Tie2-lacZ transgenic mice. However, interestingly, we observed strong and uniform lacZ expression in mesenchymal cells of the atrioventricular canal of Tie2-Cre;CAG-CAT-Z double-transgenic mice. We also detected lacZ expression in the mesenchymal cells in part of the proximal cardiac outflow tract, but not in the mesenchymal cells of the distal outflow tract and branchial arch arteries. LacZ staining in Tie2-Cre;CAG-CAT-Z embryos is consistent with endocardial-mesenchymal transformation in the atrioventricular canal and outflow tract regions. Our observations are consistent with previously reported results from Cx43-lacZ, Wnt1-Cre;R26R, and Pax3-Cre;R26R transgenic mice, in which lacZ expression in the cardiac outflow tract identified contributions in part from the cardiac neural crest. Tie2-Cre transgenic mice are a new genetic tool for the analyses of endothelial cell-lineage and endothelial cell-specific gene targeting.     

肥大软骨细胞特异性表达Cre重组酶转基因小鼠的建立

肥大软骨细胞是软骨细胞的终末分化形式,在软骨内成骨过程中发挥十分关键的作用。为了研究肥大软骨细胞在骨骼发育过程中的功能,我们构建了在8.2 kb小鼠X型胶原基因（Col10a1）启动子控制下表达Cre重组酶的转基因小鼠品系（Col10a1-8.2-Cre）。采用显微注射法将11.5 kb的转基因片段引入小鼠基因组,共注射受精卵328枚,获得子代鼠51只,经PCR基因型鉴定有3只在基因组上整合有Cre重组酶基因。PCR检测发现Col10a1-8.2-Cre转基因在含有肥大软骨细胞的组织中表达。为了检测Cre重组酶表达的强度和组织特异性,转基因小鼠与ROSA26报告小鼠交配。子代ROSA26;Col10a1-8.2-Cre双转基因小鼠LacZ染色检测的结果显示,Cre重组酶在所有的肥大软骨细胞中表达。原位杂交的结果验证Col10a1-8.2-Cre转基因表达在肥大区的上端。以上结果表明,我们建立的肥大软骨细胞特异性表达Cre重组酶的转基因小鼠品系可以作为一种遗传学工具,介导目的基因在肥大软骨细胞中的敲除。     

Differential oocyte-specific expression of Cre recombinase activity in GDF-9-iCre, Zp3cre, and Msx2Cre transgenic mice

Oocyte-specific deletion of ovarian genes using Cre/loxP technology provides an excellent tool to understand their physiological roles during folliculogenesis, oogenesis, and preimplantation embryonic development. We have generated a transgenic mouse line expressing improved Cre recombinase (iCre) driven by the mouse growth differentiation factor-9 (GDF-9) promoter. The resulting transgenic mouse line was named GDF-9-iCre mice. Using the floxed ROSA reporter mice, we found that Cre recombinase was expressed in postnatal ovaries, but not in heart, liver, spleen, kidney, and brain. Within the ovary, the Cre recombinase was exclusively expressed in the oocytes of primordial follicles and follicles at later developmental stages. The expression of iCre of GDF-9-iCre mice was shown to be earlier than the Cre expression of Zp3Cre and Msx2Cre mice, in which the Cre gene is driven by zona pellucida protein 3 (Zp3) promoter and a homeobox gene Msx2 promoter, respectively, in the postnatal ovary. Breeding wild-type males with heterozygous floxed germ cell nuclear factor (GCNF) females carrying the GDF-9-iCre transgene did not produce any progeny having the floxed GCNF allele, indicating that complete deletion of the floxed GCNF allele can be achieved in the female germline by GDF-9-iCre mice. These results suggest that GDF-9-iCre mouse line provides an excellent genetic tool for understanding functions of oocyte-expressing genes involved in folliculogenesis, oogenesis, and early embryonic development. Comparison of the ontogeny of the Cre activities of GDF-9-iCre, Zp3Cre, and Msx2Cre transgenic mice shows there is sequential Cre activity of the three transgenes that will allow inactivation of a target gene at different points in folliculogenesis.     

Conditional expression of SV40 T-antigen in mouse cardiomyocytes facilitates an inducible switch from proliferation to differentiation

Studies of cardiac muscle gene expression and signaling have been hampered by the lack of immortalized cardiomyocyte cell lines capable of proliferation and irreversible withdrawal from the cell cycle. With the goal of creating such cell lines, we generated transgenic mice using cardiac-specific cis-regulatory elements from the mouse Nkx2.5 gene to drive the expression of a simian virus 40 large T-antigen (TAg) gene flanked by sites for recombination by Cre recombinase. These transgenic mice developed tumors within the ventricular myocardium. Cells isolated from these tumors expressed cardiac markers and proliferated rapidly during serial passage in culture, without apparent senescence. However, they were unable to exit the cell cycle and failed to exhibit morphological features of terminal differentiation. Introduction of Cre recombinase to these cardiac cell lines by adenoviral delivery resulted in the elimination of TAg expression, accompanied by rapid cessation of cell division, and increase in cell size without an apparent induction of cellular differentiation. Incubation of cells lacking TAg in serum-deficient media with various pharmacological agents (norepinephrine, phenylephrine, or bone morphogenetic protein-2/4) or constitutively active calcium/calmodulin-dependent protein kinase I and/or calcineurin led to the formation of sarcomeres and up-regulation of cardiac genes involved in excitation-contraction coupling. The combination of TAg expression under the control of an early cardiac promoter and Cre-mediated recombination allowed us to derive an immortal cell line from the ventricular myocardium that could be controllably withdrawn from the cell cycle. The conditional expression of TAg in this manner permits propagation and regulated growth termination of cell types that are otherwise unable to be maintained in cell culture and may have applications for cardiac repair technologies.     

Tamoxifen-inducible NaV1.8-CreERT2 recombinase activity in nociceptive neurons of dorsal root ganglia

To explore the function of genes expressed in adult mouse nociceptive neurons, we generated heterozygous knock-in mice expressing the tamoxifen-inducible Cre recombinase construct CreERT2 downstream of the Na(V)1.8 promoter. CreERT2 encodes a Cre recombinase (Cre) fused to a mutant estrogen ligand-binding domain (ERT2) that requires the presence of tamoxifen for activity. We have previously shown that heterozygous Na(V)1.8-Cre mice will delete loxP flanked genes specifically in nociceptive sensory neurons from embryonic day 14. We therefore used the same strategy of homologous recombination and mouse generation, substituting the Cre cassette with CreERT2. No functional Cre recombinase activity was found in CreERT2 mice crossed with reporter mice in the absence of tamoxifen. We found that, as with Na(V)1.8-Cre mice, functional Cre recombinase was present in nociceptive sensory neurons after tamoxifen induction in vivo. However, the percentage of dorsal root ganglion (DRG) neurons expressing functional Cre activity was much reduced (<10% of the number found in the Na(V)1.8-Cre mouse). We also examined Cre recombinase activity in sensory neurons in culture. After treatment with 1 muM tamoxifen for 48 h, 15% of DRG neurons showed Cre activity. Na(V)1.8-CreERT2 animals may thus be useful for single cell studies of the functional consequences of gene ablation in culture, but are unlikely to be useful for behavioral studies.     

Foxl1-Cre BAC transgenic mice: a new tool for gene ablation in the gastrointestinal mesenchyme

The use of Cre-loxP technology for the purpose of cell type-specific gene ablation has revolutionized developmental biology and biomedicine. Several transgenic mouse lines have been developed for the analysis of gene function in the gastrointestinal tract, but in all of these the expression of Cre is limited to the epithelial cell layer. No Cre- expressing transgenic mouse lines ("Cre lines") exist for the deletion of loxP-flanked genes specifically in gut mesoderm. To address this deficiency, we have derived a bacterial artificial chromosome based transgenic mouse line in which the Cre gene is controlled by the Foxl1 promoter and enhancer elements. X-Gal staining of Foxl1-Cre; Rosa26R bi-transgenic lines confirm that Foxl1-Cre results in recombination specifically in the gastrointestinal mesenchyme. The Foxl1-Cre line will facilitate the dissection of mesenchymal to epithelial signaling that is known to play a major role in the patterning and function of the gastrointestinal tract.     

血管内皮细胞特异表达Cre重组酶转基因小鼠的建立

血管内皮细胞参与血管形成、血管稳态维持、血栓形成、炎症和血管重建等生理和病理过程。为了便于通过Cre-LoxP系统研究相关基因在血管内皮细胞中的功能,创建了Tie2-Cre转基因小鼠,利用Tie2基因的启动子驱动Cre重组酶基因在血管内皮细胞中表达。经基因组PCR和Southern Blot鉴定有6只小鼠在基因组上整合有Cre基因,整合率为11％。为了验证Cre重组酶的剪切活性和表达组织分布,我们将Tie2-Cre转基因小鼠分别与Smad4条件基因打靶小鼠和报告小鼠ROSA26交配。Tie2-Cre;Smad4^co/＋小鼠的多个组织的基因组DNA的PCR结果显示,Cre重组酶在所有包含血管内皮细胞的组织中表达并能介导LoxP间的重组。Tie2-Cre;ROSA26双转基因胚胎LacZ染色结果显示,Cre重组酶在所有被检测组织的血管内皮细胞中特异性表达。因此．Tie2-Cre转基因小鼠可作血管内皮细胞谱系分析和在血管内皮细胞进行条件基因打靶的理想工具小鼠。