Strategies of Conditional Gene Expression in Myocardium

Cardiac-Specific Expression of Cre Recombinase Leads to Age-Related Cardiac Dysfunction Associated with Tumor-like Growth of Atrial Cardiomyocyte and Ventricular Fibrosis and Ferroptosis

Transgenic expression of Cre recombinase driven by a specific promoter is normally used to conditionally knockout a gene in a tissue- or cell-type-specific manner. In αMHC-Cre transgenic mouse model, expression of Cre recombinase is controlled by the myocardial-specific α-myosin heavy chain (αMHC) promoter, which is commonly used to edit myocardial-specific genes. Toxic effects of Cre expression have been reported, including intro-chromosome rearrangements, micronuclei formation and other forms of DNA damage, and cardiomyopathy was observed in cardiac-specific Cre transgenic mice. However, mechanisms associated with Cardiotoxicity of Cre remain poorly understood. In our study, our data unveiled that αMHC-Cre mice developed arrhythmias and died after six months progressively, and none of them survived more than one year. Histopathological examination showed that αMHC-Cre mice had aberrant proliferation of tumor-like tissue in the atrial chamber extended from and vacuolation of ventricular myocytes. Furthermore, the αMHC-Cre mice developed severe cardiac interstitial and perivascular fibrosis, accompanied by significant increase of expression levels of MMP-2 and MMP-9 in the cardiac atrium and ventricular. Moreover, cardiac-specific expression of Cre led to disintegration of the intercalated disc, along with altered proteins expression of the disc and calcium-handling abnormality. Comprehensively, we identified that the ferroptosis signaling pathway is involved in heart failure caused by cardiac-specific expression of Cre, on which oxidative stress results in cytoplasmic vacuole accumulation of lipid peroxidation on the myocardial cell membrane. Taken together, these results revealed that cardiac-specific expression of Cre recombinase can lead to atrial mesenchymal tumor-like growth in the mice, which causes cardiac dysfunction, including cardiac fibrosis, reduction of the intercalated disc and cardiomyocytes ferroptosis at the age older than six months in mice. Our study suggests that αMHC-Cre mouse models are effective in young mice, but not in old mice. Researchers need to be particularly careful when using αMHC-Cre mouse model to interpret those phenotypic impacts of gene responses. As the Cre-associated cardiac pathology matched mostly to that of the patients, the model could also be employed for investigating age-related cardiac dysfunction.

Calreticulin and the Heart

Calreticulin is an endoplasmic Ca²⁺ binding protein and molecular chaperone. As a cardiac embryonic gene, calreticulin is essential for heart development. The protein supports Ca²⁺-dependent signaling events that are critical to cardiomyocyte differentiation and cardiogenesis. The increased expression of calreticulin and endoplasmic reticulum/sarcoplasmic reticulum Ca²⁺ capacity produces cardiomyocytes with enhanced efficiency, and detrimental mechanical stretching of cardiac fibroblasts, leading to cardiac pathology. Deletion of the calreticulin gene in adult cardiomyocytes results in left ventricle dilation, an impaired electrocardiogram, and heart failure. These observations indicate that a well-adjusted endoplasmic reticulum and calreticulin-dependent Ca²⁺ pool in cardiomyocytes are critical for the maintenance of proper cardiac function.

Repression of cyclin D1 expression is necessary for the maintenance of cell cycle exit in adult mammalian cardiomyocytes

The hearts of neonatal mice and adult zebrafish can regenerate after injury through proliferation of preexisting cardiomyocytes. However, adult mammals are not capable of cardiac regeneration because almost all cardiomyocytes exit their cell cycle. Exactly how the cell cycle exit is maintained and how many adult cardiomyocytes have the potential to reenter the cell cycle are unknown. The expression and activation levels of main cyclin-cyclin-dependent kinase (CDK) complexes are extremely low or undetectable at adult stages. The nuclear DNA content of almost all cardiomyocytes is 2C, indicating the cell cycle exit from G1-phase. Here, we induced expression of cyclin D1, which regulates the progression of G1-phase, only in differentiated cardiomyocytes of adult mice. In these cardiomyocytes, S-phase marker-positive cardiomyocytes and the expression of main cyclins and CDKs increased remarkably, although cyclin B1-CDK1 activation was inhibited in an ATM/ATR-independent manner. The phosphorylation pattern of CDK1 and expression pattern of Cdc25 subtypes suggested that a deficiency in the increase in Cdc25 (a and -b), which is required for M-phase entry, inhibited the cyclin B1-CDK1 activation. Finally, analysis of cell cycle distribution patterns showed that >40% of adult mouse cardiomyocytes reentered the cell cycle by the induction of cyclin D1. The cell cycle of these binucleated cardiomyocytes was arrested before M-phase, and many mononucleated cardiomyocytes entered endoreplication. These data indicate that silencing the cyclin D1 expression is necessary for the maintenance of the cell cycle exit and suggest a mechanism that involves inhibition of M-phase entry.

Fibroblast-mediated pathways in cardiac hypertrophy

Under normal physiological conditions, cardiac fibroblasts are the primary producers of extracellular matrix and supply a mechanical scaffold for efficacious heart contractions induced by cardiomyocytes. In the hypertrophic heart, cardiac fibroblasts provide a pivotal contribution to cardiac remodeling. Many growth factors and extracellular matrix components secreted by cardiac fibroblasts induce and modify cardiomyocyte hypertrophy. Recent evidence revealed that cardiomyocyte-cardiac fibroblast communications are complex and multifactorial. Many growth factors and molecules contribute to cardiac hypertrophy via different roles that include induction of hypertrophy and the feedback hypertrophic response, fine-tuning of adaptive hypertrophy, limitation of left ventricular dilation, and modification of interstitial changes. This review focuses on recent work and topics and provides a mechanistic insight into cardiomyocyte-cardiac fibroblast communication in cardiac hypertrophy. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium ".

Cardiac overexpression of Mammalian enabled (Mena) exacerbates heart failure in mice

Mammalian enabled (Mena) is a key regulator of cytoskeletal actin dynamics, which has been implicated in heart failure (HF). We have previously demonstrated that cardiac Mena deletion produced cardiac dysfunction with conduction abnormalities and hypertrophy. Moreover, elevated Mena expression correlates with HF in human and animal models, yet the precise role of Mena in cardiac pathophysiology is unclear. In these studies, we evaluated mice with cardiac myocyte-specific Mena overexpression (TTA/TgTetMena) comparable to that observed in cardiac pathology. We found that the hearts of TTA/TgTetMena mice were functionally and morphologically comparable to wild-type littermates, except for mildly increased heart mass in the transgenic mice. Interestingly, TTA/TgTetMena mice were particularly susceptible to cardiac injury, as these animals experienced pronounced decreases in ejection fraction and fractional shortening as well as heart dilatation and hypertrophy after transverse aortic constriction (TAC). By "turning off" Mena overexpression in TTA/TgTetMena mice either immediately prior to or immediately after TAC surgery, we discovered that normalizing Mena levels eliminated cardiac hypertrophy in TTA/TgTetMena animals but did not preclude post-TAC cardiac functional deterioration. These findings indicate that hearts with increased levels of Mena fare worse when subjected to cardiac injury and suggest that Mena contributes to HF pathophysiology.

Animal models of skin disease for drug discovery

INTRODUCTION

Discovery of novel drugs, treatments, and testing of consumer products in the field of dermatology is a multi-billion dollar business. Due to the distressing nature of many dermatological diseases, and the enormous consumer demand for products to reverse the effects of skin photodamage, aging, and hair loss, this is a very active field.

AREAS COVERED

In this paper, we will cover the use of animal models that have been reported to recapitulate to a greater or lesser extent the features of human dermatological disease. There has been a remarkable increase in the number and variety of transgenic mouse models in recent years, and the basic strategy for constructing them is outlined.

EXPERT OPINION

Inflammatory and autoimmune skin diseases are all represented by a range of mouse models both transgenic and normal. Skin cancer is mainly studied in mice and fish. Wound healing is studied in a wider range of animal species, and skin infections such as acne and leprosy also have been studied in animal models. Moving to the more consumer-oriented area of dermatology, there are models for studying the harmful effect of sunlight on the skin, and testing of sunscreens, and several different animal models of hair loss or alopecia.

S100A1 gene therapy in small and large animals

Myocardial in vivo gene delivery is a valuable technique to investigate the relevance of a protein of interest on cardiac contractile function, hypertrophy, and energy state in healthy animals as well as in a variety of models of cardiovascular disease. Rodent models are used to screen effects and to investigate molecular mechanisms, while large animal models, more closely reflecting human anatomy, physiology, and function, are inevitable for translational therapeutic approaches. The gene of interest, whose expression is driven by a non-cardioselective or cardioselective promotor is cloned into a viral vector. This vehicle is then delivered using an appropriate administration route to target the heart and to achieve efficient protein expression in myocardium. Here we describe myocardial gene therapy in small and large animal models of postischemic heart failure used to reveal the positive inotrope, antihypertrophic, and pro-energetic action of the small calcium sensor protein S100A1.

Gene therapy for heart failure

Congestive heart failure accounts for half a million deaths per year in the United States. Despite its place among the leading causes of morbidity, pharmacological and mechanic remedies have only been able to slow the progression of the disease. Today's science has yet to provide a cure, and there are few therapeutic modalities available for patients with advanced heart failure. There is a critical need to explore new therapeutic approaches in heart failure, and gene therapy has emerged as a viable alternative. Recent advances in understanding of the molecular basis of myocardial dysfunction, together with the evolution of increasingly efficient gene transfer technology, have placed heart failure within reach of gene-based therapy. The recent successful and safe completion of a phase 2 trial targeting the sarcoplasmic reticulum calcium ATPase pump (SERCA2a), along with the start of more recent phase 1 trials, opens a new era for gene therapy for the treatment of heart failure.

Identification of the heart as the critical site of adenosine mediated embryo protection

BACKGROUND

Our understanding of the mechanisms that protect the developing embryo from intrauterine stress is limited. Recently, adenosine has been demonstrated to play a critical role in protecting the embryo against hypoxia via adenosine A1 receptors (A1ARs), which are expressed in the heart, nervous system, and other sites during development. However, the sites of A1AR action that mediate embryo protection are not known. To determine if the heart is a key site of adenosine-mediated embryo protection, A1ARs were selectively deleted in the embryonic heart using a Cre-LoxP system in which the alpha-myosin heavy chain promoter drives Cre-recombinase expression and excision of the A1AR gene from cardiomyocytes.

RESULTS

With increasing exposure of maternal hypoxia (10% O2) from 48-96 hours beginning at embryonic day (E) 8.5, embryo viability decreased in the cardiac-A1AR deleted embryos. 48 hours of hypoxia reduced embryonic viability by 49% in embryos exposed from E10.5-12.5 but no effect on viability was observed in younger embryos exposed to hypoxia from E8.5-10.5. After 72 hours of hypoxia, 57.8% of the cardiac-A1AR deleted embryos were either dead or re-absorbed compared to 13.7% of control littermates and after 96 hours 81.6% of cardiac-A1AR deleted embryos were dead or re-absorbed. After 72 hours of hypoxia, cardiac size was reduced significantly more in the cardiac-A1AR deleted hearts compared to controls. Gene expression analysis revealed clusters of genes that are regulated by both hypoxia and A1AR expression.

CONCLUSIONS

These data identify the embryonic heart as the critical site where adenosine acts to protect the embryo against hypoxia. As such these studies identify a previously unrecognized mechanism of embryo protection.

Inpp5f is a polyphosphoinositide phosphatase that regulates cardiac hypertrophic responsiveness

RATIONALE

Cardiac hypertrophy occurs in response to a variety of extrinsic and intrinsic stimuli that impose increased biomechanical stress. The phosphatidylinositol 3-kinase (PI3K)/Akt pathway has previously been strongly associated with hypertrophic signaling in the heart, and with the control of cell size in multiple contexts. This pathway is tightly regulated by many factors, including a host of kinases and phosphatases that function at multiple steps in the signaling cascade. For example, the PTEN (phosphatase and tensin homolog) tumor suppressor protein is a phosphoinositide 3-phosphatase that, by metabolizing phosphatidylinositol 3,4,5-trisphosphate (PtdIns[3,4,5]P(3), PIP3), acts in direct antagonism to growth factor-stimulated PI3K. Inhibition of PTEN leads to cardiomyocyte hypertrophy. Another polyphoinositide phosphatase, inositol polyphosphate-5-phosphatase F (Inpp5f) has recently been implicated in regulation of cardiac hypertrophy. Like PTEN, this phosphatase can degrade PtdIns(3,4,5)P(3) and thus modulates the PI3K/Akt pathway.

OBJECTIVE

To characterize the role of Inpp5f in regulating cardiac hypertrophy.

METHODS AND RESULTS

We generated homozygous Inpp5f knockout mice and cardiac specific Inpp5f overexpression transgenic mice. We evaluated their hearts for biochemical, structural and functional changes. Inpp5f knockout mice have augmented hypertrophy and reactivation of the fetal gene program in response to stress when compared to wild-type littermates. Furthermore, cardiac overexpression of Inpp5f in transgenic mice reduces hypertrophic responsiveness.

CONCLUSIONS

Our results suggest that Inpp5f is a functionally important endogenous modulator of cardiac myocyte size and of the cardiac response to stress.

Cardiac fibroblasts: at the heart of myocardial remodeling

Cardiac fibroblasts are the most prevalent cell type in the heart and play a key role in regulating normal myocardial function and in the adverse myocardial remodeling that occurs with hypertension, myocardial infarction and heart failure. Many of the functional effects of cardiac fibroblasts are mediated through differentiation to a myofibroblast phenotype that expresses contractile proteins and exhibits increased migratory, proliferative and secretory properties. Cardiac myofibroblasts respond to proinflammatory cytokines (e.g. TNFalpha, IL-1, IL-6, TGF-beta), vasoactive peptides (e.g. angiotensin II, endothelin-1, natriuretic peptides) and hormones (e.g. noradrenaline), the levels of which are increased in the remodeling heart. Their function is also modulated by mechanical stretch and changes in oxygen availability (e.g. ischaemia-reperfusion). Myofibroblast responses to such stimuli include changes in cell proliferation, cell migration, extracellular matrix metabolism and secretion of various bioactive molecules including cytokines, vasoactive peptides and growth factors. Several classes of commonly prescribed therapeutic agents for cardiovascular disease also exert pleiotropic effects on cardiac fibroblasts that may explain some of their beneficial outcomes on the remodeling heart. These include drugs for reducing hypertension (ACE inhibitors, angiotensin receptor blockers, beta-blockers), cholesterol levels (statins, fibrates) and insulin resistance (thiazolidinediones). In this review, we provide insight into the properties of cardiac fibroblasts that underscores their importance in the remodeling heart, including their origin, electrophysiological properties, role in matrix metabolism, functional responses to environmental stimuli and ability to secrete bioactive molecules. We also review the evidence suggesting that certain cardiovascular drugs can reduce myocardial remodeling specifically via modulatory effects on cardiac fibroblasts.

Conditional protein stabilization via the small molecules Shld-1 and rapamycin increases the signal-to-noise ratio with tet-inducible gene expression

Cellular mechanisms control one or more of the three basic levels of regulation (transcription, translation, and protein activity/locality), allowing for finely tuned spatial and temporal regulation of protein expression patterns. Gene regulation constructs in wide use today often employ a constitutively expressed transcription factor whose activity is determined by the presence or absence of a small molecule. A case in point is the tet transcription system, wherein transcription is regulated by doxycycline (Dox), allowing the researcher to turn protein expression on or off depending on the presence/absence of Dox. However in many applications of that system, there is basal transcription from the promoter element that is independent of Dox. Moreover, in vivo, heterogeneous distribution of Dox leads to concurrent differences in gene expression. We addressed these limitations by introducing conditional destabilizing elements to the system. First, we created a transactivator protein fusion regulated at the additional level of protein stability. This modification enabled a system that demonstrated an off state that is less sensitive to variations in Dox concentrations. We also regulated the stability of the protein expressed from the tet operator cassette, observing greatly improved signal-to-noise ratios. The results underscore how investigator-defined regulation at multiple levels of protein expression can attain afiner degree of control over the final expression of introduced genes.

Optical imaging of vascular pathophysiology

Pathophysiological processes in the vascular system are the major cause of mortality and disease. Atherosclerosis, an inflammatory process in arterial walls, can lead to formation of plaques, whose rupture can lead to thrombus formation, obstruction of vessels (thrombosis), reduction of the blood flow (ischemia), cell death in the tissue fed by the occluded vessel, and depending on the affected vessel, to myocardial infarction or stroke. Imaging techniques enabling visualization of the biological processes involved in this scenario are therefore highly desirable. In recent years, a number of reporter agents and reporter systems have been developed to visualize these processes using different imaging modalities including nuclear imaging techniques, such as positron emission tomography or single photon emission computed tomography, magnetic resonance imaging, and ultrasound. This article comprises a brief overview of optical imaging techniques, such as fluorescence imaging and bioluminescence imaging for the visualization of vascular pathophysiology.

Noninvasive radionuclide imaging of cardiac gene therapy: progress and potential

Over the past decade, several clinical trials have evaluated the efficacy of cardiac-specific gene therapy. Despite encouraging results in basic research and preclinical studies, most of the recent large, randomized, placebo-controlled cardiac gene therapy trials have failed to provide convincing evidence of improvements in clinical outcomes. Because many of these problems are due to the lack of appropriate monitoring techniques, there is a critical need to develop noninvasive imaging techniques that can verify vector delivery and gene expression in target and nontarget tissues. The field of molecular imaging of cardiac gene expression is rapidly advancing because it offers distinct advantages over conventional methods, including the ability to noninvasively measure the location, time course, and magnitude of gene expression. We aim to give readers a clear understanding of how molecular imaging can enable noninvasive tracking of cardiac gene transfer and expression. We discuss limitations of current methods for analyzing gene transfer and describe how reporter gene imaging works.

Discordant on/off switching of gene expression in myocytes during cardiac hypertrophy in vivo

To determine whether the expression of cardiac genes changes in a graded manner or by on/off switching when cardiac myocytes change genetic programs in living animals, we have studied two indicator genes that change their expression oppositely in mouse binucleate ventricular cardiomyocytes during development and in response to cardiac hypertrophy. One is a single-copy transgene controlled by an alpha-myosin heavy chain (aMHC) promoter and coding for CFP. The other is the endogenous beta-myosin heavy chain (bMHC) gene modified to code for a YFP-bMHC fusion protein. Using high-resolution confocal microscopy, we determined the expression of the two indicator genes in individual cardiomyocytes perinatally and after inducing cardiac hypertrophy by transverse aortic constriction. Our results provide strong evidence that the cardiac genes respond by switching their expression in an on/off rather than graded manner, and that responding genes within a single cell and within the two nuclei of cardiomyocytes do not necessarily switch concordantly.

Temporal and spatial regulation of CRE recombinase expression in gonadotrophin-releasing hormone neurones in the mouse

Gonadotrophin-releasing hormone (GnRH) neurones located within the brain are the final neuroendocrine output regulating the reproductive hormone axis. Their small number and scattered distribution in the hypothalamus make them particularly difficult to study in vivo. The Cre/loxP system is a valuable tool to delete genes in specific cells and tissues. We report the production of two mouse lines that express the CRE bacteriophage recombinase in a GnRH-specific manner. The first line, the GnRH-CRE mouse, contains a transgene in which CRE is under the control of the murine GnRH promoter and targets CRE expression specifically to GnRH neurones in the hypothalamus. The second line, the GnRH-CRETeR mouse, uses the same murine GnRH promoter to target CRE expression to GnRH neurones, but is modified to be constitutively repressed by a tetracycline repressor (TetR) expressed from a downstream tetracycline repressor gene engineered within the transgene. GnRH neurone-specific CRE expression can therefore be induced by treatment with doxycycline which relieves repression by TetR. These GnRH-CRE and GnRH-CRETeR mice can be used to study the function of genes expressed specifically in GnRH neurones. The GnRH-CRETeR mouse can be used to study genes that may have distinct roles in reproductive physiology during the various developmental stages.

Genetic approaches for changing the heart and dissecting complex syndromes

The genetic, biochemical and molecular bases of human cardiac disease have been the focus of extensive research efforts for many years. Early animal models of cardiovascular disease used pharmacologic or surgical interventions, or took advantage of naturally occurring genetic abnormalities and the data obtained were largely correlative. The inability to directly alter an organism's genetic makeup and cellular protein content and accurately measure the results of that manipulation precluded rigorous examination of true cause-effect and structure-function relationships. Directed genetic manipulation in the mouse gave researchers the ability to modify and control the mammalian heart's protein content, resulting in the rational design of models that could provide critical links between the mutated or absent protein and disease. Two techniques that have proven particularly useful are transgenesis, which involves the random insertion of ectopic genetic material of interest into a "host" genome, and gene targeting, which utilizes homologous recombination at a pre-selected locus. Initially, transgenesis and gene targeting were used to examine systemic loss-of-function and gain-of-function, respectively, but further refinements in both techniques have allowed for investigations of organ-specific, cell type-specific, developmental stage-sensitive and dose-dependent effects. Genetically engineered animal models of pediatric and adult cardiac disease have proven that, when used appropriately, these tools have the power to extend mere observation to the establishment of true causative proof. We illustrate the power of the general approach by showing how genetically engineered mouse models can define the precise signaling pathways that are affected by the gain-of-function mutation that underlies Noonan syndrome. Increasingly precise and modifiable animal models of human cardiac disease will allow researchers to determine not only pathogenesis, but also guide treatment and the development of novel therapies.

Gene Therapy for Cardiac Arrhythmias: A Dream Soon to Come True?

Alternatively, excitement and frustration have been generated from the literature reports of gene therapy for treatment and potential cure of cardiac diseases. The time since the first literature report of in vivo myocardial gene transfer is more than 15 years, and the time since the first report of gene therapy for a cardiac arrhythmia is six years. Clinical trials, let alone clinical usage, of these promising therapies have not yet started. This article reviews the current state of the art for arrhythmia gene therapy, including the literature reports of antiarrhythmic studies and of problems within the field. Gene transfer continues to be a promising technology, but patience is required as these problems are solved and the therapies make their way through the preclinical and clinical testing process.

Positron emission tomography imaging of conditional gene activation in the heart

The Cre-loxP system has been routinely used for conditional activation and deletion of gene expression. However, the spatiotemporal manner of these events in the heart has not yet been defined by in vivo imaging. Adenovirus (1 x 10(9 )pfu) carrying the silent positron emission tomography (PET) reporter gene, herpes simplex virus type 1 thymidine kinase (HSV1-tk), was injected into the left ventricular wall of male transgenic mice (n=15) or FVB controls (n=8). Transgenic mice expressed Cre recombinase driven by a cardiac-specific alpha-myosin heavy chain (alpha-MHC) promoter. Following injection of the 9-[4-fluoro-3-(hydroxymethyl)butyl]guanine ([18F]-FHBG; 137+/-25 microCi) reporter probe, microPET imaging was used to assess the expression of HSV1-tk reporter gene in the myocardium. Two days following adenoviral injection, cardiac HSV1-tk gene activation resulted in tracer uptake of 3.20+/-0.51% ID/g for alpha-MHC-Cre and 0.05+/-0.02%ID/g for control mice (P<0.01). The in vivo results were confirmed by RT-PCR and Western blot analysis. Similar transfections were evaluated in both cardiac-specific and non-cardiac-specific cell lines. Enzyme activity showed a robust correlation (r2=0.82) between in vivo molecular imaging technique and traditional in vitro enzyme assays. With further development and validation, PET imaging will likely play an important role in the noninvasive, repetitive, and quantitative measurement of conditional gene activation in the future.

Animal models in cardiovascular diseases: new insights from conditional models

Conditional systems have proven to be efficient and powerful to delineate several aspects of cardiac pathophysiology and diseases. The possibility of addressing a particular time point in animal life is certainly an important breakthrough allowed by conditional strategies with temporal control of either transgene expression or gene modifications. The purpose of this review is to present various mouse models for cardiovascular diseases based on conditional approaches.

Nanoparticles for bioimaging

The emergence of synthesis strategies for the fabrication of nanosized contrast agents is anticipated to lead to advancements in understanding biological processes at the molecular level in addition to progress in the development of diagnostic tools and innovative therapies. Imaging agents such as fluorescent dye-doped silica nanoparticles, quantum dots and gold nanoparticles have overcome many of the limitations of conventional contrast agents (organic dyes) such as poor photostability, low quantum yield, insufficient in vitro and in vivo stability, etc. Such particulates are now being developed for absorbance and emission in the near infrared region, which is expected to allow for real time and deep tissue imaging via optical routes. Other efforts to facilitate deep tissue imaging with pre-existing technologies have lead to the development of multimodal nanoparticles which are both optical and MRI active. The main focus of this article is to provide an overview of properties and design of contrast agents such as dye-doped silica nanoparticles, quantum dots and gold nanoparticles for non-invasive bioimaging.

Gene therapy to create biological pacemakers

Old age and a variety of cardiovascular disorders may disrupt normal sinus node function. Currently, this is successfully treated with electronic pacemakers, which, however, leave room for improvement. During the past decade, different strategies to initiate pacemaker function by gene therapy were developed. In the search for a biological pacemaker, various approaches were explored, including beta(2)-adrenergic receptor overexpression, down regulation of the inward rectifier current, and overexpression of the pacemaker current. The most recent advances include overexpression of bioengineered ion channels and genetically modified stem cells. This review considers the strengths and the weaknesses of the different approaches and discusses some of the different viral vectors currently used.

Conditional gene expression in the mouse using a Sleeping Beauty gene-trap transposon

Background

Insertional mutagenesis techniques with transposable elements have been popular among geneticists studying model organisms from E. coli to Drosophila and, more recently, the mouse. One such element is the Sleeping Beauty (SB) transposon that has been shown in several studies to be an effective insertional mutagen in the mouse germline. SB transposon vector studies have employed different functional elements and reporter molecules to disrupt and report the expression of endogenous mouse genes. We sought to generate a transposon system that would be capable of reporting the expression pattern of a mouse gene while allowing for conditional expression of a gene of interest in a tissue- or temporal-specific pattern.

Results

Here we report the systematic development and testing of a transposon-based gene-trap system incorporating the doxycycline-repressible Tet-Off (tTA) system that is capable of activating the expression of genes under control of a Tet response element (TRE) promoter. We demonstrate that the gene trap system is fully functional in vitro by introducing the "gene-trap tTA" vector into human cells by transposition and identifying clones that activate expression of a TRE-luciferase transgene in a doxycycline-dependent manner. In transgenic mice, we mobilize gene-trap tTA vectors, discover parameters that can affect germline mobilization rates, and identify candidate gene insertions to demonstrate the in vivo functionality of the vector system. We further demonstrate that the gene-trap can act as a reporter of endogenous gene expression and it can be coupled with bioluminescent imaging to identify genes with tissue-specific expression patterns.

Conclusion

Akin to the GAL4/UAS system used in the fly, we have made progress developing a tool for mutating and revealing the expression of mouse genes by generating the tTA transactivator in the presence of a secondary TRE-regulated reporter molecule. A vector like the gene-trap tTA could provide a means for both annotating mouse genes and creating a resource of mice that express a regulable transcription factor in temporally- and tissue-specific patterns for conditional gene expression studies. These mice would be a valuable resource to the mouse genetics community for purpose of dissecting mammalian gene function.

Overview of recent advances in molecular cardiology

Molecular cardiology is a new and fast-growing area of cardiovascular medicine that aims to apply molecular biology techniques for the mechanistic investigation, diagnosis, prevention and treatment of cardiovascular disease. As an emerging discipline, it has changed conceptual thinking of cardiovascular development, disease etiology and pathophysiology. Although molecular cardiology is still at a very early stage, it has opened a promising avenue for understanding and controlling cardiovascular disease. With the rapid development and application of molecular biology techniques, scientists and clinicians are closer to curing heart diseases that were thought to be incurable 20 years ago. There clearly is a need for a more thorough understanding of the molecular mechanisms of cardiovascular diseases to promote the advancement of stem cell therapy and gene therapy for heart diseases. The present paper briefly reviews the state-of-the-art techniques in the following areas of molecular cardiology: gene analysis in the diseased heart; transgenic techniques in cardiac research; gene transfer and gene therapy for cardiovascular disease; and stem cell therapy for cardiovascular disease.

Conditional and inducible gene recombineering in the mouse inner ear

Genetically engineered mice have greatly improved our understanding of gene functions and disease mechanisms. Nevertheless, the traditional knock-out approach has limitations in the overall viability of mutants. The application of the Cre/loxP system in the inner ear can help bypass this difficulty by generation of conditional gene recombineering. However, to do so requires an expression system that allows ear-specific temporally inducible, gene abrogation of one or more of the increasingly available floxed genes. To date, three approaches have been successfully used to create murine inner ear-specific Cre lines: conventional transgenesis, BAC transgenesis, and gene knock-in. Unfortunately, timing of conditional Cre activity does not extend beyond the regulatory range of the gene controlling Cre expression. Rectification of this problem requires the generation of tamoxifen or tetracycline inducible systems in the inner ear. Examination of integrase expression at different loci will facilitate studies on the expression of exogenous transgenes. These genetic applications for the mouse genome will dramatically advance in vivo gene function studies.

Pharmacologic transgene control systems for gene therapy

Pharmacologic transgene-expression dosing is considered essential for future gene therapy scenarios. Genetic interventions require precise transcription or translation fine-tuning of therapeutic transgenes to enable their titration into the therapeutic window, to adapt them to daily changing dosing regimes of the patient, to integrate them seamlessly into the patient's transcriptome orchestra, and to terminate their expression after successful therapy. In recent years, decisive progress has been achieved in designing high-precision trigger-inducible mammalian transgene control modalities responsive to clinically licensed and inert heterologous molecules or to endogenous physiologic signals. Availability of a portfolio of compatible transcription control systems has enabled assembly of higher-order control circuitries providing simultaneous or independent control of several transgenes and the design of (semi-)synthetic gene networks, which emulate digital expression switches, regulatory transcription cascades, epigenetic expression imprinting, and cellular transcription memories. This review provides an overview of cutting-edge developments in transgene control systems, of the design of synthetic gene networks, and of the delivery of such systems for the prototype treatment of prominent human disease phenotypes.