Temporally controlled targeted somatic mutagenesis in skeletal muscles of the mouse

Temporal and tissue-specific disruption of LINC complexes in vivo

Migration and anchorage of nuclei within developing and adult tissues rely on Linkers of the Nucleoskeleton to the Cytoskeleton (LINC complexes). These macromolecular assemblies span the nuclear envelope and physically couple chromatin and nuclear lamina to cytoplasmic cytoskeletal networks. LINC complexes assemble within the perinuclear space through direct interactions between the respective evolutionary-conserved SUN and KASH domains of Sun proteins, which reside within the inner nuclear membrane, and Nesprins, which reside within the outer nuclear membrane. Here, we describe and validate a dominant-negative transgenic strategy allowing for the disruption of endogenous SUN/KASH interactions through the inducible expression of a recombinant KASH domain. Our approach, which is based on the Cre/Lox system, allows for the targeted disruption of LINC complexes in a wide array of mouse tissues or specific cell types thereof and bypasses the perinatal lethality and potential cell nonautonomous effects of current mouse models based on germline inactivation of genes encoding Sun proteins and Nesprins. For these reasons, this mouse model provides a useful tool to evaluate the physiological relevance of LINC complexes integrity during development and homeostasis in a wide array of mammalian tissues.

Reducing dynamin 2 expression rescues X-linked centronuclear myopathy

Centronuclear myopathies (CNM) are congenital disorders associated with muscle weakness and abnormally located nuclei in skeletal muscle. An autosomal dominant form of CNM results from mutations in the gene encoding dynamin 2 (DNM2), and loss-of-function mutations in the gene encoding myotubularin (MTM1) result in X-linked CNM (XLCNM, also called myotubular myopathy), which promotes severe neonatal hypotonia and early death. Currently, no effective treatments exist for XLCNM. Here, we found increased DNM2 levels in XLCNM patients and a mouse model of XLCNM (Mtm1(-/y)). Generation of Mtm1(-/y) mice that were heterozygous for Dnm2 revealed that reduction of DNM2 in XLCNM mice restored life span, whole-body strength, and diaphragm function and increased muscle strength. Additionally, classic CNM-associated histological features, including fiber atrophy and nuclei mispositioning, were absent or reduced. Ultrastructural analysis revealed improvement of sarcomere organization and triad structures. Skeletal muscle-specific decrease of Dnm2 during embryogenesis or in young mice after disease onset revealed that the rescue associated with downregulation of Dnm2 is cell autonomous and is able to stop and potentially revert XLCNM progression. These data indicate that MTM1 and DNM2 regulate muscle organization and force through a common pathway. Furthermore, despite DNM2 being a key mechanoenzyme, its reduction is beneficial for XLCNM and represents a potential therapeutic approach for patients.

Skeletal myofiber VEGF is essential for the exercise training response in adult mice

Vascular endothelial growth factor (VEGF) is exercise responsive, pro-angiogenic, and expressed in several muscle cell types. We hypothesized that in adult mice, VEGF generated within skeletal myofibers (and not other cells within muscle) is necessary for the angiogenic response to exercise training. This was tested in adult conditional, skeletal myofiber-specific VEGF gene-deleted mice (skmVEGF-/-), with VEGF levels reduced by >80%. After 8 wk of daily treadmill training, speed and endurance were unaltered in skmVEGF-/- mice, but increased by 18% and 99% (P < 0.01), respectively, in controls trained at identical absolute speed, incline, and duration. In vitro, isolated soleus and extensor digitorum longus contractile function was not impaired in skmVEGF-/- mice. However, training-induced angiogenesis was inhibited in plantaris (wild type, 38%, skmVEGF-/- 18%, P < 0.01), and gastrocnemius (wild type, 43%, P < 0.01; skmVEGF-/-, 7%, not significant). Capillarity was maintained (different from VEGF gene deletion targeted to multiple cell types) in untrained skmVEGF-/- mice. Arteriogenesis (smooth muscle actin+, artery number, and diameter) and remodeling [vimentin+, 5'-bromodeoxycytidine (BrdU)+, and F4/80+ cells] occurred in skmVEGF-/- mice, even in the absence of training. skmVEGF-/- mice also displayed a limited oxidative enzyme [citrate synthase and β-hydroxyacyl CoA dehydrogenase (β-HAD)] training response; β-HAD activity levels were elevated in the untrained state. These data suggest that myofiber expressed VEGF is necessary for training responses in capillarity and oxidative capacity and for improved running speed and endurance.

Muscle insulin sensitivity and glucose metabolism are controlled by the intrinsic muscle clock

Circadian rhythms control metabolism and energy homeostasis, but the role of the skeletal muscle clock has never been explored. We generated conditional and inducible mouse lines with muscle-specific ablation of the core clock gene Bmal1. Skeletal muscles from these mice showed impaired insulin-stimulated glucose uptake with reduced protein levels of GLUT4, the insulin-dependent glucose transporter, and TBC1D1, a Rab-GTPase involved in GLUT4 translocation. Pyruvate dehydrogenase (PDH) activity was also reduced due to altered expression of circadian genes Pdk4 and Pdp1, coding for PDH kinase and phosphatase, respectively. PDH inhibition leads to reduced glucose oxidation and diversion of glycolytic intermediates to alternative metabolic pathways, as revealed by metabolome analysis. The impaired glucose metabolism induced by muscle-specific Bmal1 knockout suggests that a major physiological role of the muscle clock is to prepare for the transition from the rest/fasting phase to the active/feeding phase, when glucose becomes the predominant fuel for skeletal muscle.

Gli3 regulation of myogenesis is necessary for ischemia-induced angiogenesis

RATIONALE

A better understanding of the mechanism underlying skeletal muscle repair is required to develop therapies that promote tissue regeneration in adults. Hedgehog signaling has been shown previously to be involved in myogenesis and angiogenesis: 2 crucial processes for muscle development and regeneration.

OBJECTIVE

The objective of this study was to identify the role of the hedgehog transcription factor Gli3 in the cross-talk between angiogenesis and myogenesis in adults.

METHODS AND RESULTS

Using conditional knockout mice, we found that Gli3 deficiency in endothelial cells did not affect ischemic muscle repair, whereas in myocytes, Gli3 deficiency resulted in severely delayed ischemia-induced myogenesis. Moreover, angiogenesis was also significantly impaired in HSA-Cre(ERT2); Gli3(Flox/Flox) mice, demonstrating that impaired myogenesis indirectly affects ischemia-induced angiogenesis. The role of Gli3 in myocytes was then further investigated. We found that Gli3 promotes myoblast differentiation through myogenic factor 5 regulation. In addition, we found that Gli3 regulates several proangiogenic factors, including thymidine phosphorylase and angiopoietin-1 both in vitro and in vivo, which indirectly promote endothelial cell proliferation and arteriole formation. In addition, we found that Gli3 is upregulated in proliferating myoblasts by the cell cycle-associated transcription factor E2F1.

CONCLUSIONS

This study shows for the first time that Gli3-regulated postnatal myogenesis is necessary for muscle repair-associated angiogenesis. Most importantly, it implies that myogenesis drives angiogenesis in the setting of skeletal muscle repair and identifies Gli3 as a potential target for regenerative medicine.

Androgen receptor (AR) pathophysiological roles in androgen-related diseases in skin, bone/muscle, metabolic syndrome and neuron/immune systems: lessons learned from mice lacking AR in specific cells

The androgen receptor (AR) is expressed ubiquitously and plays a variety of roles in a vast number of physiological and pathophysiological processes. Recent studies of AR knockout (ARKO) mouse models, particularly the cell type- or tissue-specific ARKO models, have uncovered many AR cell type- or tissue-specific pathophysiological roles in mice, which otherwise would not be delineated from conventional castration and androgen insensitivity syndrome studies. Thus, the AR in various specific cell types plays pivotal roles in production and maturation of immune cells, bone mineralization, and muscle growth. In metabolism, the ARs in brain, particularly in the hypothalamus, and the liver appear to participate in regulation of insulin sensitivity and glucose homeostasis. The AR also plays key roles in cutaneous wound healing and cardiovascular diseases, including atherosclerosis and abdominal aortic aneurysm. This article will discuss the results obtained from the total, cell type-, or tissue-specific ARKO models. The understanding of AR cell type- or tissue-specific physiological and pathophysiological roles using these in vivo mouse models will provide useful information in uncovering AR roles in humans and eventually help us to develop better therapies via targeting the AR or its downstream signaling molecules to combat androgen/AR-related diseases.

Inducible Cre transgenic mouse strain for skeletal muscle-specific gene targeting

Background

The use of the Cre/loxP system for gene targeting has been proven to be a powerful tool for understanding gene function. The purpose of this study was to create and characterize an inducible, skeletal muscle-specific Cre transgenic mouse strain.

Methods

To achieve skeletal muscle-specific expression, the human α-skeletal actin promoter was used to drive expression of a chimeric Cre recombinase containing two mutated estrogen receptor ligand-binding domains.

Results

Western blot analysis, PCR and β-galactosidase staining confirmed that Cre-mediated recombination was restricted to limb and craniofacial skeletal muscles only after tamoxifen administration.

Conclusions

A transgenic mouse was created that allows inducible, gene targeting of floxed genes in adult skeletal muscle of different developmental origins. This new mouse will be of great utility to the skeletal muscle community.

RNase H-mediated degradation of toxic RNA in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is an RNA-dominant disease caused by abnormal transcripts containing expanded CUG repeats. The CUG transcripts aggregate in the nucleus to form RNA foci and lead to nuclear depletion of Muscleblind-like 1 (MBNL1) and stabilized expression of CUGBP Elav like family 1 (CELF1), both of which are splicing regulatory proteins. The imbalance of these proteins results in misregulation of alternative splicing and neuromuscular abnormalities. Here, we report the use of antisense oligonucleotides (ASOs) as a therapeutic approach to target the pathogenic RNA in DM1. We designed chimeric ASOs, termed gapmers, containing modified nucleic acid residues to induce RNase H-mediated degradation of CUG-repeat transcripts. The gapmers selectively knockdown expanded CUG transcripts and are sufficient to disrupt RNA foci both in cell culture and mouse models for DM1. Furthermore, combination of gapmers with morpholino ASOs that help release binding of MBNL1 to the toxic RNA can potentially enhance the knockdown effect. Additional optimization will be required for systemic delivery; however, our study provides an alternative strategy for the use of ASOs in DM1 therapy.

Alternative splicing dysregulation secondary to skeletal muscle regeneration

OBJECTIVE

Dysregulation of alternative splicing has become a molecular hallmark of myotonic dystrophy type 1 (DM1), in which neonatal splice variants are expressed in adult skeletal muscle. Splicing dysregulation is induced by RNA containing expanded CUG repeats expressed from the expanded mutant allele by sequestration of muscleblindlike 1 (MBNL1) protein within nuclear RNA foci and increased CUGBP, ELAV-like family member 1 (CELF1) protein levels. Dysregulated splicing has also been identified in other neuromuscular disorders, suggesting either that diseases with different molecular causes share a common pathogenic mechanism or that dysregulated splicing can also be a common secondary consequence of muscle degeneration and regeneration.

METHODS

In this study, we examined regulation of alternative splicing in 4 different mouse models of muscular dystrophy, including DM1, limb-girdle muscular dystrophy, congenital merosin-deficient muscular dystrophy, and Duchenne muscular dystrophy, and 2 myotoxin (cardiotoxin and notexin) muscle injury models.

RESULTS

We show that DM1-like alternative splicing dysregulation and altered expression of MBNL1 and CELF1 occur in non-DM1 mouse models of muscular dystrophy and muscle injury, most likely due to recapitulation of neonatal splicing patterns in regenerating fibers. In contrast, CELF1 was elevated in nuclei of mature myofibers of the DM1 model, consistent with a primary effect of pathogenic RNA expression.

INTERPRETATION

Splicing dysregulation in DM1 is a primary effect of RNA containing expanded CUG repeats. However, we conclude that splicing changes can also be observed secondary to muscle regeneration, and this possibility must be taken into account when evaluating cause-effect relationships between dysregulated splicing and disease processes.

Generation of Spatio-Temporally Controlled Targeted Somatic Mutations in the Mouse

The generation of ligand-activated site-specific Cre recombinases has led to the development of cell type-specific temporally controlled targeted somatic mutagenesis in the mouse. We illustrate this technique using K14-Cre-ER(T2) transgenic mice that express the tamoxifen (tam)-activatable Cre-ER(T2) recombinase in epidermal basal keratinocytes to induce mutations in epidermal keratinocytes of adult mice. Our highly reproducible technique, based on induction of Cre-ER(T2) recombinase activity by tamoxifen administration at low doses (once daily 100-µg intraperitoneal injection for 5 days), has allowed the generation of site-directed somatic mutations of numerous genes in mouse epidermal keratinocytes, and several mouse models of human diseases. The present step-by-step protocol describes how to introduce temporally controlled targeted mutations in epidermal keratinocytes of adult mice. Curr. Protoc. Mouse Biol. 1:55-70. © 2011 by John Wiley & Sons, Inc.

Generating conditional knockout mice

Gene targeting in ES cells is extensively used to generate designed mouse mutants and to study gene function in vivo. Knockout mice that harbor a null allele in their germline provide appropriate genetic models of inherited diseases and often exhibit embryonic or early postnatal lethality. To study gene function in adult mice and in selected cell types, a refined strategy for conditional gene inactivation has been developed that relies on the DNA recombinase Cre and its recognition (loxP) sites. For conditional mutagenesis, a target gene is modified by the insertion of two loxP sites that enable to excise the flanked (floxed) gene segment through Cre-mediated recombination. Conditional mutant mice are obtained by crossing the floxed strain with a Cre transgenic line such that the target gene becomes inactivated in vivo within the expression domain of Cre. A large collection of Cre transgenic lines has been generated over time and can be used in a combinatorial manner to achieve gene inactivation in many different cell types. A growing number of CreER(T2) transgenic mice further allows for inducible inactivation of floxed alleles in adult mice upon administration of tamoxifen. This chapter covers the design and construction of loxP flanked alleles and refers to the vectors, ES cells, and mice generated by the European conditional mouse mutagenesis (EUCOMM) project. We further describe the design and use of Cre and CreER(T2) transgenic mice and a convenient breeding strategy to raise conditional mutants and controls for phenotype analysis.

Myocytic androgen receptor controls the strength but not the mass of limb muscles

The anabolic effects of androgens on skeletal muscles are thought to be mediated predominantly through the androgen receptor (AR), a member of the ligand-dependent nuclear receptor superfamily. However, despite numerous studies performed in men and in rodents, these effects remain poorly understood. To characterize androgen signaling in skeletal muscles, we generated mice in which the AR is selectively ablated in myofibers. We show that myocytic AR controls androgen-induced insulin-like growth factor IEa (IGF-IEa) expression in the highly androgen-sensitive perineal muscles and that it mediates androgen-stimulated postnatal hypertrophy of these muscles. In contrast, androgen-dependent postnatal hypertrophy of limb muscle fibers is independent of myocytic AR. Thus, androgens control perineal and limb muscle mass in male mice through myocytic AR-dependent and -independent pathways, respectively. Importantly, we also show that AR deficiency in limb myocytes impairs myofibrillar organization of sarcomeres and decreases muscle strength, thus demonstrating that myocytic AR controls key pathways required for maximum force production. These distinct androgen signaling pathways in perineal and limb muscles may allow the design of screens to identify selective androgen modulators of muscle strength.

Efficient temporally-controlled targeted mutagenesis in smooth muscle cells of the adult mouse

To generate temporally-controlled targeted somatic mutations selectively and efficiently in smooth muscles, we have established a transgenic SMA-Cre-ER(T2) mouse line in which the expression of the Tamoxifen-dependent Cre-ER(T2) recombinase is under the control of a large genomic DNA segment of the mouse smooth muscle alpha actin (SMA) gene, contained in a Bacterial artificial chromosome (Bac). In this transgenic mouse line, Cre-ER(T2)-mediated recombination of LoxP-flanked target DNA is strictly Tamoxifen-dependent, and efficient in both vascular and visceral smooth muscle cells. Moreover, with the exception of few cardiomyocytes, LoxP-flanked DNA excision is restricted to smooth muscle cells. Thus, SMA-Cre-ER(T2) mice should be of great value to analyze gene function in smooth muscles, and to establish new animal models of human smooth muscle disorders.

A tetracycline-inducible and skeletal muscle-specific Cre recombinase transgenic mouse

We have generated a transgenic mouse that expresses Cre recombinase only in skeletal muscle and only following tetracycline treatment. This spatiotemporal specificity is achieved using two transgenes. The first transgene uses the human skeletal actin (HSA) promoter to drive expression of the reverse tetracycline-controlled transactivator (rtTA). The second transgene uses a tetracycline responsive promoter to drive the expression of Cre recombinase. We monitored transgene expression in these mice by crossing them with ROSA26 loxP-LacZ reporter mice, which express beta-galactosidase when activated by Cre. We find that the expression of this transgene is only detectable within skeletal muscle and that Cre expression in the absence of tetracycline is negligible. Cre is readily induced in this model with tetracycline analogs at a range of embryonic and postnatal ages and in a pattern consistent with other HSA transgenic mice. This mouse improves upon existing transgenic mice in which skeletal muscle Cre is expressed throughout development by allowing Cre expression to begin at later developmental stages. This temporal control of transgene expression has several applications, including overcoming embryonic or perinatal lethality due to transgene expression. This mouse is especially suited for studies of steroid hormone action, as it uses tetracycline, rather than tamoxifen, to activate Cre expression. In summary, we find that this transgenic induction system is suitable for studies of gene function in the context of hormonal regulation of skeletal muscle or interactions between muscle and motoneurons in mice.

Inducible Cre mice

The Cre/lox site-specific recombination system has emerged as an important tool for the generation of conditional somatic mouse mutants. This method allows one to control gene activity in space and time in almost any tissue of the mouse, thus opening new avenues for studying gene function and for establishing sophisticated animal models of human diseases. A major technical advance in terms of in vivo inducibility was the development of ligand-dependent Cre recombinases that can be activated by administration of tamoxifen to the animal. Here we describe how tamoxifen-dependent Cre recombinases, so-called CreER recombinases, work and how they can be used to generate time- and tissue-specific mouse mutants. The focus will be on the CreER(T2) recombinase, which is currently the most successful CreER version. We will give an overview of available CreER(T2) transgenic mouse lines and present protocols that detail the generation of experimental mice for inducible gene knockout studies, the induction of recombination by tamoxifen treatment, and the analysis of the quality and quantity of recombination by reporter gene and target gene studies. Most of the protocols can also be used as general guidelines for the generation and characterization of Cre/lox-mediated genome modifications in mice.

Premature aging in skeletal muscle lacking serum response factor

Aging is associated with a progressive loss of muscle mass, increased adiposity and fibrosis that leads to sarcopenia. At the molecular level, muscle aging is known to alter the expression of a variety of genes but very little is known about the molecular effectors involved. SRF (Serum Response Factor) is a crucial transcription factor for muscle-specific gene expression and for post-natal skeletal muscle growth. To assess its role in adult skeletal muscle physiology, we developed a post-mitotic myofiber-specific and tamoxifen-inducible SRF knockout model. Five months after SRF loss, no obvious muscle phenotype was observed suggesting that SRF is not crucial for myofiber maintenance. However, mutant mice progressively developed IIB myofiber-specific atrophy accompanied by a metabolic switch towards a more oxidative phenotype, muscular lipid accumulation, sarcomere disorganization and fibrosis. After injury, mutant muscles exhibited an altered regeneration process, showing smaller regenerated fibers and persistent fibrosis. All of these features are strongly reminiscent of abnormalities encountered in aging skeletal muscle. Interestingly, we also observed an important age associated decrease in SRF expression in mice and human muscles. Altogether, these results suggest that a naturally occurring SRF down-regulation precedes and contributes to the muscle aging process. Indeed, triggering SRF loss in the muscles of mutant mice results in an accelerated aging process.

Temporally controlled prostate epithelium-specific gene alterations

Employing the Hprt locus as the site for targeted transgenesis we have developed mice expressing the tamoxifen-inducible Cre-ER(T2) fusion protein under the control of the ARR2-rat probasin promoter. This system enables external control over the timing of prostate epithelial cell-specific gene alterations. Using both the ROSA26-lacZ and ROSA26-EYFP reporter strains to monitor recombinase activity, Cre-ER(T2) was found to be specifically expressed in the prostatic epithelium and was strictly tamoxifen dependent. This strain thus allows precise control over the timing of gene alterations in the mouse prostate, enabling analyses of the phenotypic consequences of gene alterations in mice of any age. It also provides an ideal platform to study the impact of environmental, hormonal, and age-related factors on prostate tumorigenesis. This latter feature will be of particular value given the paucity of murine models that accurately mimic the late onset and prolonged natural history of human prostate cancer.

Expanded CTG repeats within the DMPK 3' UTR causes severe skeletal muscle wasting in an inducible mouse model for myotonic dystrophy

Severe skeletal muscle wasting is the most debilitating symptom experienced by individuals with myotonic dystrophy type 1 (DM1). We present a DM1 mouse model with inducible and skeletal muscle-specific expression of large tracts of CTG repeats in the context of DMPK exon 15. These mice recapitulate many findings associated with DM1 skeletal muscle, such as CUG RNA foci with Muscleblind-like 1 (MBNL1) protein colocalization, misregulation of developmentally regulated alternative splicing events, myotonia, characteristic histological abnormalities, and increased CUGBP1 protein levels. Importantly, this DM1 mouse model recapitulates severe muscle wasting, which has not been reported in models in which depletion of MBNL1 is the main feature. Using these mice, we discovered previously undescribed alternative splicing events that are responsive to CUGBP1 and not MBNL, and these events were found to be misregulated in individuals with DM1. Our results indicate that increased CUGBP1 protein levels are associated with DMPK-CUG RNA expression, suggesting a role for CUGBP1-specific splicing or cytoplasmic functions in muscle wasting.

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.

Increased bone mass in male and female mice following tamoxifen administration

Tamoxifen is capable of preserving bone mass in gonadectomized rodents as well as intact female mice; however, a detailed 3D quantitative analysis of the structural changes produced in the growing skeleton of intact mice of both genders by this agent is lacking. Employing quantitative microcomputed tomography (muCT), we assessed the effects of 4-hydroxytamoxifen (OHT) on the femora of C57BL/6J mice administered this agent either for 12 (males and females) or 2 (females) weeks. In mice of either gender, but especially in females, 12 weeks of OHT exposure led to dramatic increases in both cortical and trabecular bone. Females exposed to OHT for either 2 or 12 weeks demonstrated significantly increased cortical wall thickness, trabecular bone volume, connectivity, and number, as well as decreased trabecular separation. Significant increases in several of these parameters were also evident in males after 12 weeks of OHT administration. In view of the expanding use of OHT to induce Cre-mediated recombination events, our findings suggest that care should be exercised when interpreting the skeletal phenotypes of mice exposed this agent, particularly in situations where the effects of OHT might synergize with the phenotypic outcome of a specific genetic alteration.

Conditional Gene Expression in Human Embryonic Stem Cells

Human embryonic stem cells (hESCs) possess unique properties for studying mechanisms controlling cell fate commitment during early mammalian development. Gain of function is a common strategy to study the function of specific genes involved in these mechanisms. However, transgene toxicity can be a major limitation, especially with factors influencing proliferation or differentiation. Here, we describe an efficient method based on the inducible recombinase Cre-ERT2 for conditional gene expression in hESCs and their differentiated derivatives. Using this approach, we have established several hESC sublines inducible for the expression of the enhanced green fluorescent protein and the transforming growth factor beta family member Nodal. Together, these results demonstrate that Cre-ERT2 can be used to control gene expression in undifferentiated and differentiated cells, thereby providing the first conditional transgene expression system that works effectively in hESCs. Disclosure of potential conflicts of interest is found at the end of this article.

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.

Conditional mouse models to study developmental and pathophysiological gene function in muscle

This chapter will review conditional mouse model systems that have been developed to study gene function in skeletal, cardiac, and vascular smooth muscle cells in vivo with an emphasis on the utility of these models for investigating developmental and pathophysiological gene function in muscle. In general, these systems have utilized muscle-specific/selective promoter-enhancers in conjunction with site-specific DNA recombinases, e.g., Cre-loxP, and fusion proteins with these recombinases that confer temporal control, such as tamoxifen-inducible CreER systems. A major focus of this chapter will be to discuss unique challenges of studying Cre-mediated mutagenesis/gene targeting in these muscle types during development and in the adult animal, some of which are inherent of the muscle cell type being studied. For example, unlike cardiac and skeletal muscle cells, the vascular SMC is extremely plastic and able to undergo rapid phenotypic modulation to various environmental cues in vivo. Thus, employing SMC marker gene promoter enhancers for conditional gene targeting in SMCs must take into account the possibility and/or certainty that the particular SMC promoter enhancers used may or may not be transcriptionally active in SMCs of a vessel wall under normal and some pathophysiological conditions. Moreover, individual floxed loci within the same muscle cell type and tissue have different degrees of sensitivity to Cre, most likely dependent on the chromatin state of that particular gene, i.e., closed/condensed state or open/active state. Thus, Cre recombination may be ineffective for specific floxed gene DNA. Lastly, rigorous efforts must be made to confirm the degree of recombination in a tissue, taking into full account the multicellularity of the tissue, to understand the extent of the physiological effect in that organ.

Improved embryonic stem cell technologies

Murine embryonic stem (ES) cells have become an indispensable tool for investigating genetic function both in vitro and, importantly, in vivo. Recent advances, including tetraploid aggregation, new site-specific recombinases and RNAi, have enabled more sophisticated manipulation of the ES cell genome. For instance, it is now possible to control gene expression in both a temporally and spatially restricted manner. Such new technologies are answering complex questions surrounding the function and interaction of an increasing number of genes. This chapter will review both the history and recent technological progress that has been made in mouse ES cell derivation, genetic manipulation and the generation of ES cell-derived chimaeric animals.

PGC1alpha expression is controlled in skeletal muscles by PPARbeta, whose ablation results in fiber-type switching, obesity, and type 2 diabetes

Mice in which peroxisome proliferator-activated receptor beta (PPARbeta) is selectively ablated in skeletal muscle myocytes were generated to elucidate the role played by PPARbeta signaling in these myocytes. These somatic mutant mice exhibited a muscle fiber-type switching toward lower oxidative capacity that preceded the development of obesity and diabetes, thus demonstrating that PPARbeta is instrumental in myocytes to the maintenance of oxidative fibers and that fiber-type switching is likely to be the cause and not the consequence of these metabolic disorders. We also show that PPARbeta stimulates in myocytes the expression of PGC1alpha, a coactivator of various transcription factors, known to play an important role in slow muscle fiber formation. Moreover, as the PGC1alpha promoter contains a PPAR response element, the effect of PPARbeta on the formation and/or maintenance of slow muscle fibers can be ascribed, at least in part, to a stimulation of PGC1alpha expression at the transcriptional level.