Effects of mouse strain, position of integration and tetracycline analogue on the tetracycline conditional system in transgenic mice

Sterilization of Channel Catfish (Ictalurus punctatus) via Overexpression of bax Gene Regulated by a Tet-off System in the Primordial Germ Cells

Transgenic technologies have been used for genetic improvement of catfish performance with notable success. However, these developments are useless from a commercialization standpoint without extremely efficient confinement. Transgenic sterilization has the potential to accomplish 100% reproductive confinement and avoid genetic exchange between transgenic or domestic genotypes and wild populations. The present study reports a novel sterilization method for channel catfish by overexpressing the pro-apoptosis gene bax, specifically in the primordial germ cells, to inhibit their proliferation. Three transgenic constructs were electroporated into channel catfish one-cell embryos, including Nanos-nanos, Nanos-dnd, and Dazl-vasa. Transgene integration, gonad development, and sex ratio were evaluated in P1 and F1 generations. The transgene was successfully integrated into the channel catfish genome, with variable rates depending on each construct. Mosaicism of transgene integration was widely evident in the P1 fish, as expected. All three constructs showed similar efficacy for sterilizing P1 male channel catfish, with approximately half of all males showing little to no gonadal development, resulting in a significantly lower (p < 0.05) gonadosomatic index (GSI) when compared to the control at four years of age. The same trend occurred but with lower efficacy in P1 females, with approximately one-third showing little gonadal development at four years of age. This technology is potentially useful for generating sterile male fish, where the overexpression of the bax gene can lead to reduced or no gonadal development, presumably due to the death of primordial germ cells.

Squalenoyl siRNA PMP22 nanoparticles are effective in treating mouse models of Charcot-Marie-Tooth disease type 1 A

Charcot-Marie-Tooth disease type 1 A (CMT1A) lacks an effective treatment. We provide a therapy for CMT1A, based on siRNA conjugated to squalene nanoparticles (siRNA PMP22-SQ NPs). Their administration resulted in normalization of Pmp22 protein levels, restored locomotor activity and electrophysiological parameters in two transgenic CMT1A mouse models with different severity of the disease. Pathological studies demonstrated the regeneration of myelinated axons and myelin compaction, one major step in restoring function of myelin sheaths. The normalization of sciatic nerve Krox20, Sox10 and neurofilament levels reflected the regeneration of both myelin and axons. Importantly, the positive effects of siRNA PMP22-SQ NPs lasted for three weeks, and their renewed administration resulted in full functional recovery. Beyond CMT1A, our findings can be considered as a potent therapeutic strategy for inherited peripheral neuropathies. They provide the proof of concept for a new precision medicine based on the normalization of disease gene expression by siRNA.

Treating PMP22 gene duplication-related Charcot-Marie-Tooth disease: the past, the present and the future

Charcot-Marie-Tooth (CMT) disease is the most frequent inherited neuropathy, affecting 1/1500 to 1/10000. CMT1A represents 60%-70% of all CMT and is caused by a duplication on chromosome 17p11.2 leading to an overexpression of the Peripheral Myelin Protein 22 (PMP22). PMP22 gene is under tight regulation and small changes in its expression influences myelination and affect motor and sensory functions. To date, CMT1A treatment is symptomatic and classic pharmacological options have been disappointing. Here, we review the past, present, and future treatment options for CMT1A, with a special emphasis on the highly promising potential of PMP22-targeted small interfering RNA and antisense oligonucleotides.

Low-density lipoprotein receptor-related protein 1 deficiency in cardiomyocytes reduces susceptibility to insulin resistance and obesity

BACKGROUND

Low-density lipoprotein receptor-related protein 1 (LRP1) plays a key role in fatty acid metabolism and glucose homeostasis. In the context of dyslipemia, LRP1 is upregulated in the heart. Our aim was to evaluate the impact of cardiomyocyte LRP1 deficiency on high fat diet (HFD)-induced cardiac and metabolic alterations, and to explore the potential mechanisms involved.

METHODS

We used TnT-iCre transgenic mice with thoroughly tested suitability to delete genes exclusively in cardiomyocytes to generate an experimental mouse model with conditional Lrp1 deficiency in cardiomyocytes (TNT-iCre⁺-LRP1^flox/flox).

FINDINGS

Mice with Lrp1-deficient cardiomyocytes (cm-Lrp1^-/-) have a normal cardiac function combined with a favorable metabolic phenotype against HFD-induced glucose intolerance and obesity. Glucose intolerance protection was linked to higher hepatic fatty acid oxidation (FAO), lower liver steatosis and increased whole-body energy expenditure. Proteomic studies of the heart revealed decreased levels of cardiac pro-atrial natriuretic peptide (pro-ANP), which was parallel to higher ANP circulating levels. cm-Lrp1^-/- mice showed ANP signaling activation that was linked to increased fatty acid (FA) uptake and increased AMPK/ ACC phosphorylation in the liver. Natriuretic peptide receptor A (NPR-A) antagonist completely abolished ANP signaling and metabolic protection in cm-Lrp1^-/- mice.

CONCLUSIONS

These results indicate that an ANP-dependent axis controlled by cardiac LRP1 levels modulates AMPK activity in the liver, energy homeostasis and whole-body metabolism.

Partial Rescue of Ocular Pigment Cells and Structure by Inducible Ectopic Expression of Mitf-M in MITF-Deficient Mice

Purpose

Complete deficiency of microphthalmia transcription factor (MITF) in Mitfmi-vga9/mi-vga9 mice is associated with microphthalmia, retinal dysplasia, and albinism. We investigated the ability of dopachrome tautomerase (DCT) promoter-mediated inducible ectopic expression of Mitf-M to rescue these phenotypic abnormalities.

Methods

A new mouse line was created with doxycycline-inducible ectopic Mitf-M expression on an Mitf-deficient Mitfmi-vga9 background (DMV mouse). Adult DMV mice were phenotypically characterized and tissues were collected for histology, immunohistochemistry, and evaluation of Mitf, pigmentary genes, and retinal pigment epithelium (RPE) gene expression.

Results

Ectopic Mitf-M expression was specifically induced in the eyes, but was not detected in the skin of DMV mice. Inducible expression of Mitf-M partially rescued the microphthalmia, RPE structure, and pigmentation as well as a subset of the choroidal and iris melanocytes but not cutaneous melanocytes. RPE function and vision were not restored in the DMV mice.

Conclusions

Ectopic expression of Mitf-M during development of Mitf-deficient mice is capable of partially rescuing ocular and retinal structures and uveal melanocytes. These findings provide novel information about the roles of Mitf isoforms in the development of mouse eyes.

Differential Neurotoxicity Related to Tetracycline Transactivator and TDP-43 Expression in Conditional TDP-43 Mouse Model of Frontotemporal Lobar Degeneration

Frontotemporal lobar degeneration (FTLD) is among the most prevalent dementias of early-onset. Pathologically, FTLD presents with tauopathy or TAR DNA-binding protein 43 (TDP-43) proteinopathy. A biallelic mouse model of FTLD was produced on a mix FVB/129SVE background overexpressing wild-type human TDP-43 (hTDP-43) using tetracycline transactivator (tTA), a system widely used in mouse models of neurological disorders. tTA activates hTDP-43, which is placed downstream of the tetracycline response element. The original study on this transgenic mouse found hippocampal degeneration following hTDP-43 expression, but did not account for independent effects of tTA protein. Here, we initially analyzed the neurotoxic effects of tTA in postweaning age mice of either sex using immunostaining and area measurements of select brain regions. We observed tTA-dependent toxicity selectively in the hippocampus affecting the dentate gyrus significantly more than CA fields, whereas hTDP-43-dependent toxicity in bigenic mice occurred in most other cortical regions. Atrophy was associated with inflammation, activation of caspase-3, and loss of neurons. The atrophy associated with tTA expression was rescuable by the tetracycline analog, doxycycline, in the diet. MRI studies corroborated the patterns of atrophy. tTA-induced degeneration was strain-dependent and was rescued by moving the transgene onto a congenic C57BL/6 background. Despite significant hippocampal atrophy, behavioral tests in bigenic mice revealed no hippocampally mediated memory impairment. Significant atrophy in most cortical areas due solely to TDP-43 expression indicates that this mouse model remains useful for providing critical insight into co-occurrence of TDP-43 pathology, neurodegeneration, and behavioral deficits in FTLD.SIGNIFICANCE STATEMENT The tTA expression system has been widely used in mice to model neurological disorders. The technique allows investigators to reversibly turn on or off disease causing genes. Here, we report on a mouse model that overexpresses human TDP-43 using tTA and attempt to recapitulate features of TDP-43 pathology present in human FTLD. The tTA expression system is problematic, resulting in dramatic degeneration of the hippocampus. Thus, our study adds a note of caution for the use of the tTA system. However, because FTLD is primarily characterized by cortical degeneration and our mouse model shows significant atrophy in most cortical areas due to human TDP-43 overexpression, our animal model remains useful for providing critical insight on this human disease.

Fluorescent Calcium Indicator Protein Expression in the Brain Using Tetracycline-Responsive Transgenic Mice

To achieve robust long-term fluorescent calcium indicator protein (FCIP) expression in mammalian neurons in vivo, classical mouse transgenesis by pronuclear DNA injection using tetracycline (Tet)-controlled genetic switches can be deployed. This protocol describes methods for regulated expression of FCIP using Tet-responsive transgenic mice. The Tet-inducible system requires three components for inducible and reversible control of gene expression: (1) a potent transcriptional activator protein, either Tet transactivator (tTA) or reverse tTA (rtTA); (2) a minimal Tet-promoter (P(tet)) or a bidirectional Tet-promoter (P(tet)bi) to express one or more responder genes; and (3) Tet or one of its derivatives such as doxycycline (Dox) as an inducer. To ensure a high level of FCIP expression in neurons, transgenic founder mice are screened using an ear fibroblast culture method to identify those that are responsive to Dox treatment before use in experiments. The protocol describes the use of Dox to regulate gene expression and provides a short description of in vivo recording of luciferase activity.

Applying an Inducible Expression System to Study Interference of Bacterial Virulence Factors with Intracellular Signaling

The technique presented here allows one to analyze at which step a target protein, or alternatively a small molecule, interacts with the components of a signaling pathway. The method is based, on the one hand, on the inducible expression of a specific protein to initiate a signaling event at a defined and predetermined step in the selected signaling cascade. Concomitant expression, on the other hand, of the gene of interest then allows the investigator to evaluate if the activity of the expressed target protein is located upstream or downstream of the initiated signaling event, depending on the readout of the signaling pathway that is obtained. Here, the apoptotic cascade was selected as a defined signaling pathway to demonstrate protocol functionality. Pathogenic bacteria, such as Coxiella burnetii, translocate effector proteins that interfere with host cell death induction in the host cell to ensure bacterial survival in the cell and to promote their dissemination in the organism. The C. burnetii effector protein CaeB effectively inhibits host cell death after induction of apoptosis with UV-light or with staurosporine. To narrow down at which step CaeB interferes with the propagation of the apoptotic signal, selected proteins with well-characterized pro-apoptotic activity were expressed transiently in a doxycycline-inducible manner. If CaeB acts upstream of these proteins, apoptosis will proceed unhindered. If CaeB acts downstream, cell death will be inhibited. The test proteins selected were Bax, which acts at the level of the mitochondria, and caspase 3, which is the major executioner protease. CaeB interferes with cell death induced by Bax expression, but not by caspase 3 expression. CaeB, thus, interacts with the apoptotic cascade between these two proteins.

Transgene expression in the Nop-tTA driver line is not inherently restricted to the entorhinal cortex

The entorhinal cortex (EC) plays a central role in episodic memory and is among the earliest sites of neurodegeneration and neurofibrillary tangle formation in Alzheimer's disease. Given its importance in memory and dementia, the ability to selectively modulate gene expression or neuronal function in the EC is of widespread interest. To this end, several recent studies have taken advantage of a transgenic line in which the tetracycline transactivator (tTA) was placed under control of the neuropsin (Nop) promoter to limit transgene expression within the medial EC and pre-/parasubiculum. Although the utility of this driver is contingent on its spatial specificity, no detailed neuroanatomical analysis of its expression has yet been conducted. We therefore undertook a systematic analysis of Nop-tTA expression using a lacZ reporter and have made the complete set of histological sections available through the Rodent Brain Workbench tTA atlas, www.rbwb.org . Our findings confirm that the highest density of tTA expression is found in the EC and pre-/parasubiculum, but also reveal considerable expression in several other cortical areas. Promiscuous transgene expression may account for the appearance of pathological protein aggregates outside of the EC in mouse models of Alzheimer's disease using this driver, as we find considerable overlap between sites of delayed amyloid deposition and regions with sparse β-galactosidase reporter labeling. While different tet-responsive lines can display individual expression characteristics, our results suggest caution when designing experiments that depend on precise localization of gene products controlled by the Nop-tTA or other spatially restrictive transgenic drivers.

A general approach for controlling transcription and probing epigenetic mechanisms: application to the CD4 locus

Synthetic regulatory proteins such as tetracycline (tet)-controlled transcription factors are potentially useful for repression as well as ectopic activation of endogenous genes and also for probing their regulatory mechanisms, which would offer a versatile genetic tool advantageous over conventional gene targeting methods. In this study, we provide evidence supporting this concept using Cd4 as a model. CD4 is expressed in double-positive and CD4 cells but irreversibly silenced in CD8 cells. The silencing is mediated by heterochromatin established during CD8 lineage development via transient action of the Cd4 silencer; once established, the heterochromatin becomes self-perpetuating independently of the Cd4 silencer. Using a tet-sensitive Cd4 allele harboring a removable Cd4 silencer, we found that a tet-controlled repressor recapitulated the phenotype of Cd4-deficient mice, inhibited Cd4 expression in a reversible and dose-dependent manner, and could surprisingly replace the Cd4 silencer to induce irreversible Cd4 silencing in CD8 cells, thus suggesting the Cd4 silencer is not the (only) determinant of heterochromatin formation. In contrast, a tet-controlled activator reversibly disrupted Cd4 silencing in CD8 cells. The Cd4 silencer impeded this disruption but was not essential for its reversal, which revealed a continuous role of the silencer in mature CD8 cells while exposing a remarkable intrinsic self-regenerative ability of heterochromatin after forced disruption. These data demonstrate an effective approach for gene manipulation and provide insights into the epigenetic Cd4 regulatory mechanisms that are otherwise difficult to obtain.

Strain background influences neurotoxicity and behavioral abnormalities in mice expressing the tetracycline transactivator

The tet-off system has been widely used to create transgenic models of neurological disorders including Alzheimer's, Parkinson's, Huntington's, and prion disease. The utility of this system lies in the assumption that the tetracycline transactivator (TTA) acts as an inert control element and does not contribute to phenotypes under study. Here we report that neuronal expression of TTA can affect hippocampal cytoarchitecture and behavior in a strain-dependent manner. While studying neurodegeneration in two tet-off Alzheimer's disease models, we unexpectedly discovered neuronal loss within the dentate gyrus of single transgenic TTA controls. Granule neurons appeared most sensitive to TTA exposure during postnatal development, and doxycycline treatment during this period was neuroprotective. TTA-induced degeneration could be rescued by moving the transgene onto a congenic C57BL/6J background and recurred on reintroduction of either CBA or C3H/He backgrounds. Quantitative trait analysis of B6C3 F2 TTA mice identified a region on Chromosome 14 that contains a major modifier of the neurodegenerative phenotype. Although B6 mice were resistant to degeneration, they were not ideal for cognitive testing. F1 offspring of TTA C57BL/6J and 129X1/SvJ, FVB/NJ, or DBA/1J showed improved spatial learning, but TTA expression caused subtle differences in contextual fear conditioning on two of these backgrounds, indicating that strain and genotype can interact independently under different behavioral settings. All model systems have limitations that should be recognized and mitigated where possible; our findings stress the importance of mapping the effects caused by TTA alone when working with tet-off models.

Tetracycline inducible gene manipulation in serotonergic neurons

The serotonergic (5-HT) neuronal system has important and diverse physiological functions throughout development and adulthood. Its dysregulation during development or later in adulthood has been implicated in many neuropsychiatric disorders. Transgenic animal models designed to study the contribution of serotonergic susceptibility genes to a pathological phenotype should ideally allow to study candidate gene overexpression or gene knockout selectively in serotonergic neurons at any desired time during life. For this purpose, conditional expression systems such as the tet-system are preferable. Here, we generated a transactivator (tTA) mouse line (TPH2-tTA) that allows temporal and spatial control of tetracycline (Ptet) controlled transgene expression as well as gene deletion in 5-HT neurons. The tTA cDNA was inserted into a 196 kb PAC containing a genomic mouse Tph2 fragment (177 kb) by homologous recombination in E. coli. For functional analysis of Ptet-controlled transgene expression, TPH2-tTA mice were crossed to a Ptet-regulated lacZ reporter line (Ptet-nLacZ). In adult double-transgenic TPH2-tTA/Ptet-nLacZ mice, TPH2-tTA founder line L62-20 showed strong serotonergic β-galactosidase expression which could be completely suppressed with doxycycline (Dox). Furthermore, Ptet-regulated gene expression could be reversibly activated or inactivated when Dox was either withdrawn or added to the system. For functional analysis of Ptet-controlled, Cre-mediated gene deletion, TPH2-tTA mice (L62-20) were crossed to double transgenic Ptet-Cre/R26R reporter mice to generate TPH2-tTA/Ptet-Cre/R26R mice. Without Dox, 5-HT specific recombination started at E12.5. With permanent Dox administration, Ptet-controlled Cre-mediated recombination was absent. Dox withdrawal either postnatally or during adulthood induced efficient recombination in serotonergic neurons of all raphe nuclei, respectively. In the enteric nervous system, recombination could not be detected. We generated a transgenic mouse tTA line (TPH2-tTA) which allows both inducible and reversible transgene expression and inducible Cre-mediated gene deletion selectively in 5-HT neurons throughout life. This will allow precise delineation of serotonergic gene functions during development and adulthood.

Genetic approaches to study glial cells in the rodent brain

The development, function, and pathology of the brain depend on interactions of neurons and different types of glial cells, namely astrocytes, oligodendrocytes, microglia, and ependymal cells. Understanding neuron-glia interactions in vivo requires dedicated experimental approaches to manipulate each cell type independently. In this review, we first summarize techniques that allow for cell-specific gene modification including targeted mutagenesis and viral transduction. In the second part, we describe the genetic models that allow to target the main glial cell types in the central nervous system. The existing arsenal of approaches to study glial cells in vivo and its expansion in the future are key to understand neuron-glia interactions under normal and pathologic conditions.

A previously functional tetracycline-regulated transactivator fails to target gene expression to the bone

Background

The tetracycline-controlled transactivator system is a powerful tool to control gene expression in vitro and to generate consistent and conditional transgenic in vivo model organisms. It has been widely used to study gene function and to explore pathological mechanisms involved in human diseases. The system permits the regulation of the expression of a target gene, both temporally and quantitatively, by the application of tetracycline or its derivative, doxycycline. In addition, it offers the possibility to restrict gene expression in a spatial fashion by utilizing tissue-specific promoters to drive the transactivator.

Findings

In this study, we report our problems using a reverse tetracycline-regulated transactivator (rtTA) in a transgenic mouse model system for the bone-specific expression of the Hutchinson-Gilford progeria syndrome mutation. Even though prior studies have been successful utilizing the same rtTA, expression analysis of the transactivator revealed insufficient activity for regulating the transgene expression in our system. The absence of transactivator could not be ascribed to differences in genetic background because mice in a mixed genetic background and in congenic mouse lines showed similar results.

Conclusions

The purpose of this study is to report our negative experience with previously functional transactivator mice, to raise caution in the use of tet-based transgenic mouse lines and to reinforce the need for controls to ensure the stable functionality of generated tetracycline-controlled transactivators over time.

Cardiac-specific NRAP overexpression causes right ventricular dysfunction in mice

The muscle-specific protein NRAP is concentrated at cardiac intercalated disks, plays a role in myofibril assembly, and is upregulated early in mouse models of dilated cardiomyopathy. Using a tet-off system, we developed novel transgenic lines exhibiting cardiac-specific NRAP overexpression ~2.5 times greater than normal. At 40-50 weeks, NRAP overexpression resulted in dilation and decreased ejection fraction in the right ventricle, with little effect on the left ventricle. Expression of transcripts encoding brain natriuretic peptide and skeletal α-actin was increased by cardiac-specific NRAP overexpression, indicative of a cardiomyopathic response. NRAP overexpression did not alter the levels or organization of N-cadherin and connexin-43. The results show that chronic NRAP overexpression in the mouse leads to right ventricular cardiomyopathy by 10 months, but that the early NRAP upregulation previously observed in some mouse models of dilated cardiomyopathy is unlikely to account for the remodeling of intercalated disks and left ventricular dysfunction observed in those cases.

Generation and evaluation of an IPTG-regulated version of Vav-gene promoter for mouse transgenesis

Different bacteria-derived systems for regulatable gene expression have been developed for the use in mammalian cells and some were also successfully adopted for in vivo use in vertebrate model organisms. However, certain limitations apply to most of these systems, including leakiness of transgene expression, inefficient transgene silencing or activation, as well as limited tissue accessibility of transgene-inducers or their unfavourable pharmacokinetics. In this study, we evaluated the suitability of the lac-operon/lac-repressor (lacO/lacI) system for the regulation of the well-established Vav-gene promoter that allows inducible transgene expression in different haematopoietic lineages in mice. Using the fluorescence marker protein Venus as a reporter, we observed that the lacO/lacI system could be amended to modulate transgene-expression in haematopoietic cells. However, reporter expression was not uniform and the lacO elements introduced into the Vav-gene promoter only conferred limited repression and reversion of lacI-mediated gene silencing after administration of IPTG. Although further optimization of the system is required, the lacO-modified version of the Vav-gene promoter may be adopted as a tool where low basal gene-expression and limited transient induction of protein expression are desired, e.g. for the activation of oncogenes or transgenes that act in a dominant-negative manner.

Strict control of transgene expression in a mouse model for sensitive biological applications based on RMCE compatible ES cells

Recombinant mouse strains that harbor tightly controlled transgene expression proved to be indispensible tools to elucidate gene function. Different strategies have been employed to achieve controlled induction of the transgene. However, many models are accompanied by a considerable level of basal expression in the non-induced state. Thereby, applications that request tight control of transgene expression, such as the expression of toxic genes and the investigation of immune response to neo antigens are excluded. We developed a new Cre/loxP-based strategy to achieve strict control of transgene expression. This strategy was combined with RMCE (recombinase mediated cassette exchange) that facilitates the targeting of genes into a tagged site in ES cells. The tightness of regulation was confirmed using luciferase as a reporter. The transgene was induced upon breeding these mice to effector animals harboring either the ubiquitous (ROSA26) or liver-specific (Albumin) expression of CreER^T2, and subsequent feeding with Tamoxifen. Making use of RMCE, luciferase was replaced by Ovalbumin antigen. Mice generated from these ES cells were mated with mice expressing liver-specific CreER^T2. The transgenic mice were examined for the establishment of an immune response. They were fully competent to establish an immune response upon hepatocyte specific OVA antigen expression as indicated by a massive liver damage upon Tamoxifen treatment and did not show OVA tolerance. Together, this proves that this strategy supports strict control of transgenes that is even compatible with highly sensitive biological readouts.

Standing genetic variation and compensatory evolution in transgenic organisms: a growth-enhanced salmon simulation

Genetically modified strains usually are generated within defined genetic backgrounds to minimize variation for the engineered characteristic in order to facilitate basic research investigations or for commercial application. However, interactions between transgenes and genetic background have been documented in both model and commercial agricultural species, indicating that allelic variation at transgene-modifying loci are not uncommon in genomes. Engineered organisms that have the potential to allow entry of transgenes into natural populations may cause changes to ecosystems via the interaction of their specific phenotypes with ecosystem components and services. A transgene introgressing through natural populations is likely to encounter a range of natural genetic variation (among individuals or sub-populations) that could result in changes in phenotype, concomitant with effects on fitness and ecosystem consequences that differ from that seen in the progenitor transgenic strain. In the present study, using a growth hormone transgenic salmon example, we have modeled selection of modifier loci (single and multiple) in the presence of a transgene and have found that accounting for genetic background can significantly affect the persistence of transgenes in populations, potentially reducing or reversing a "Trojan gene" effect. Influences from altered life history characteristics (e.g., developmental timing, age of maturation) and compensatory demographic/ecosystem controls (e.g., density dependence) also were found to have a strong influence on transgene effects. Further, with the presence of a transgene in a population, genetic backgrounds were found to shift in non-transgenic individuals as well, an effect expected to direct phenotypes away from naturally selected optima. The present model has revealed the importance of understanding effects of selection for background genetics on the evolution of phenotypes in populations harbouring transgenes.

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.

Identification of the variant Ala335Val of MED25 as responsible for CMT2B2: molecular data, functional studies of the SH3 recognition motif and correlation between wild-type MED25 and PMP22 RNA levels in CMT1A animal models

Charcot-Marie-Tooth (CMT) disease is a clinically and genetically heterogeneous disorder. All mendelian patterns of inheritance have been described. We identified a homozygous p.A335V mutation in the MED25 gene in an extended Costa Rican family with autosomal recessively inherited Charcot-Marie-Tooth neuropathy linked to the CMT2B2 locus in chromosome 19q13.3. MED25, also known as ARC92 and ACID1, is a subunit of the human activator-recruited cofactor (ARC), a family of large transcriptional coactivator complexes related to the yeast Mediator. MED25 was identified by virtue of functional association with the activator domains of multiple cellular and viral transcriptional activators. Its exact physiological function in transcriptional regulation remains obscure. The CMT2B2-associated missense amino acid substitution p.A335V is located in a proline-rich region with high affinity for SH3 domains of the Abelson type. The mutation causes a decrease in binding specificity leading to the recognition of a broader range of SH3 domain proteins. Furthermore, Med25 is coordinately expressed with Pmp22 gene dosage and expression in transgenic mice and rats. These results suggest a potential role of this protein in the molecular etiology of CMT2B2 and suggest a potential, more general role of MED25 in gene dosage sensitive peripheral neuropathy pathogenesis.

Widespread delta-like-1 expression in normal adult mouse tissue and injured endothelium is reflected by expression of the Dll1LacZ locus

BACKGROUND

Our study characterizes Delta-like 1 (Dll1) in the adult mouse, particularly in normal versus injured vasculature, with the aid of the transgenic Dll1(LacZ) line.

METHODS

Normal mouse adult tissues or those from the Dll1(LacZ) reporter line were analyzed for Dll1 expression and promoter activity. Vascular tissue was analyzed before and after carotid artery ligation.

RESULTS

In wild-type mice, Dll1 transcript expression was widespread. Similarly, the Dll1(LacZ) reporter had beta-galactosidase activity detectable in the cerebellum, cerebrum, spinal cord, liver, lung and cornea, although the normal adult vasculature had no reporter expression. Following arterial ligation, there was acute induction of Dll1(LacZ) reporter expression, both in the ligated left carotid artery, and the uninjured right contralateral artery. Expression returned to low/undetectable levels 4-10 days after arterial ligation.

CONCLUSION

The expression of Dll1 in the adult mouse is more widespread than previously realized, although not in resting large arteries in the adult mouse. Following arterial injury, Dll1 promoter activity is induced selectively in the endothelial cells of both the injured artery and the contralateral uninjured artery. Our results show that while overall expression in the adult mouse is widespread, Dll1 may be selectively expressed in the endothelium of injured vasculature, similar to the endothelial-restricted expression of Dll4.

Inducible and reversible Clock gene expression in brain using the tTA system for the study of circadian behavior

The mechanism of circadian oscillations in mammals is cell autonomous and is generated by a set of genes that form a transcriptional autoregulatory feedback loop. While these "clock genes" are well conserved among animals, their specific functions remain to be fully understood and their roles in central versus peripheral circadian oscillators remain to be defined. We utilized the in vivo inducible tetracycline-controlled transactivator (tTA) system to regulate Clock gene expression conditionally in a tissue-specific and temporally controlled manner. Through the use of Secretogranin II to drive tTA expression, suprachiasmatic nucleus- and brain-directed expression of a tetO::Clock(Delta19) dominant-negative transgene lengthened the period of circadian locomotor rhythms in mice, whereas overexpression of a tetO::Clock(wt) wild-type transgene shortened the period. Low doses (10 mug/ml) of doxycycline (Dox) in the drinking water efficiently inactivated the tTA protein to silence the tetO transgenes and caused the circadian periodicity to return to a wild-type state. Importantly, low, but not high, doses of Dox were completely reversible and led to a rapid reactivation of the tetO transgenes. The rapid time course of tTA-regulated transgene expression demonstrates that the CLOCK protein is an excellent indicator for the kinetics of Dox-dependent induction/repression in the brain. Interestingly, the daily readout of circadian period in this system provides a real-time readout of the tTA transactivation state in vivo. In summary, the tTA system can manipulate circadian clock gene expression in a tissue-specific, conditional, and reversible manner in the central nervous system. The specific methods developed here should have general applicability for the study of brain and behavior in the mouse.

Tetracycline-controlled genetic switches

Unlike recombinase-mediated gene manipulations, tetracycline (Tet)-controlled genetic switches permit reversible control of gene expression in the mouse. Trancriptional activation can be induced by activators termed tTA (Tet-Off) or rtTA (Tet-On) in the absence and presence of Tet, respectively. The Tet-Off and Tet-On systems are complementary, and the decision to choose one over the other depends on the particular experimental strategy. Both systems were optimized over the years and can now be used to develop mouse models.

A conditional model reveals that induction of hepatocyte nuclear factor-1alpha in Hnf1alpha-null mutant beta-cells can activate silenced genes postnatally, whereas overexpression is deleterious

Humans with heterozygous loss-of-function mutations in the hepatocyte nuclear factor-1alpha (HNF1alpha) gene develop beta-cell-deficient diabetes (maturity-onset diabetes of the young type 3), indicating that HNF1alpha gene dosage is critical in beta-cells. However, whether increased HNF1alpha expression might be beneficial or deleterious for beta-cells is unknown. Furthermore, although it is clear that HNF1alpha is required for beta-cell function, it is not known whether this role is cell autonomous or whether there is a restricted developmental time frame for HNF1alpha to elicit gene activation in beta-cells. To address this, we generated a tetracycline-inducible mouse model that transcribes HNF1alpha selectively in beta-cells in either wild-type or Hnf1alpha-null backgrounds. Short-term induction of HNF1alpha in islets from adult Hnf1alpha(-/-) mice that did not express HNF1alpha throughout development resulted in the activation of target genes, indicating that HNF1alpha has beta-cell-autonomous functions that can be rescued postnatally. However, transgenic induction throughout development, which inevitably resulted in supraphysiological levels of HNF1alpha, strikingly caused a severe reduction of cellular proliferation, increased apoptosis, and consequently beta-cell depletion and diabetes. Thus, HNF1alpha is sensitive to both reduced and excessive concentrations in beta-cells. This finding illustrates the paramount importance of using the correct concentration of a beta-cell transcription factor in both gene therapy and artificial differentiation strategies.

Epigenetic silencing and tissue independent expression of a novel tetracycline inducible system in double‐transgenic pigs

The applicability of tightly regulated transgenesis in domesticated animals is severely hampered by the present lack of knowledge of regulatory mechanisms and the long generation intervals. To capitalize on the tightly controlled expression of mammalian genes made possible by using prokaryotic control elements, we have used a single-step transduction to introduce an autoregulative tetracycline-responsive bicistronic expression cassette (NTA) into transgenic pigs. Transgenic pigs carrying one NTA cassette showed a mosaic transgene expression restricted to single muscle fibers. In contrast, crossbred animals carrying two NTA cassettes with different transgenes, revealed a broad tissue-independent and tightly regulated expression of one cassette, but not of the other one. The expression pattern correlated inversely with the methylation status of the NTA transcription start sites indicating epigenetic silencing of one NTA cassette. This first approach on tetracycline regulated transgene expression in farm animals will be valuable for developing precisely controlled expression systems for transgenes in large animals relevant for biomedical and agricultural biotechnology.

Neuronal vulnerability in transgenic mice expressing an inducible dominant-negative FGF receptor

Fibroblast Growth Factors (FGFs) and their receptors (FGFRs) are widely expressed in the mature nervous system and are thought to mediate plasticity and repair. We report the generation of transgenic mice that can be induced to express a dominant-negative FGFR (dnFGFR) in select neuronal populations. We show that a modified Thy1 promoter [Vidal, M., Morris, R., Grosveld, F., and Spanopoulou, E. 1990. Tissue-specific control elements of the Thy-1 gene. EMBO J 9 833-840] can be used to drive widespread neuronal expression of the reverse tetracycline transactivator M2 (rtTA-M2 [Urlinger, S., Baron, U., Thellmann, M., Hasan, M.T., Bujard, H., and Hillen, W., 2000. Exploring the sequence space for tetracycline-dependent transcriptional activators: novel mutations yield expanded range and sensitivity. Proc. Natl. Acad. Sci. U. S. A. 97, 7963-7968]), which after stimulation with doxycycline induces co-expression of dnFGFR in mosaic subpopulations of rtTA-M2-positive forebrain neurons, but not in hindbrain and spinal cord rtTA-M2-positive neurons. Expression of dnFGFR did not cause overt neurodegeneration, but led to increased neuronal vulnerability: four days after a stab injury, cell death was marked in the hippocampus of dnFGFR-expressing animals when compared to controls. The nuclear morphology of dying CA1 pyramidal cells suggested an apoptotic mechanism of cell death. These observations demonstrate the importance of endogenous FGFs in the maintenance of the nervous system.

Differential gene dosage effects of Ad4BP/SF-1 on target tissue development

Ad4BP/SF-1 (NR5A1) was identified as a key regulator of the hypothalamus-pituitary-gonadal and -adrenal axes. Loss-of-function studies revealed that Ad4BP/SF-1 is essential for the development of these tissues and spleen. Here, we generated transgenic mouse with BAC recombinants carrying a dual promoter and Tet-off system. These recombinants have a potential to express lacZ and Ad4BP/SF-1 in the tissues where endogenous Ad4BP/SF-1 is expressed. However, protein level of Ad4BP/SF-1 varied among the tissues of the transgenic mice and probably thereby the target tissues are affected differentially. The BAC-transgenic mice were applied to rescue Ad4BP/SF-1 KO mouse. Interestingly, the mice successfully rescued the gonad and spleen but failed to rescue the adrenal gland. This variation might be dependent on in part the protein expression levels among the tissues and in part on differential sensitivities to the gene dosage.

Development of a single-cassette system for spatiotemporal gene regulation in mice

The tetracycline-regulated gene expression system has been widely used in mice to turn a transgene on and off in a target organ, but with only limited success. We developed an advanced system in which a Tet-off regulation unit was integrated into the ROSA26 locus and became active after Cre-mediated excision of the neo(r) gene. We examined the utility of this system through regulable expression of the homeodomain transcription factor pdx-1 and enhanced green fluorescent protein. The resulting mice showed strict tetracycline-regulable gene expression in all the organs where the neo(r) gene had been removed. When combined with organ-specific Cre recombinase transgenic mice, our system allows us to manipulate the gene expression in an organ-specific and temporal manner. This Tet-off system should serve as an efficient tool to analyze the roles of genes in complex biological systems, such as embryogenesis, metabolism, immune system, etc.

Interferon-gamma induced medulloblastoma in the developing cerebellum

We have generated a mouse model system with a high incidence of medulloblastoma, a malignant neoplasm believed to arise from immature precursors of cerebellar granule neurons. These animals ectopically express interferon-gamma (IFN-gamma) in astrocytes in the CNS in a controlled manner, exploiting the tetracycline-controllable system. More than 80% of these mice display severe ataxia and develop cerebellar tumors that express synaptophysin, the mouse atonal homolog MATH1, sonic hedgehog (SHH), and Gli1. IFN-gamma-induced tumorigenesis in these mice is associated with increased expression of SHH, and SHH induction and tumorigenesis are dependent on signal transducer and activator of transcription 1 (STAT1). When IFN-gamma expression is shut down with doxycycline at postnatal day 12 (P12), the clinical symptoms dissipate and the mice do not develop tumors, whereas if transgene expression is shut down at P16, the clinical symptoms and tumors progress to lethality, indicating that IFN-gamma is required for tumor induction but not progression. The tumors that occur in the continued presence of IFN-gamma display extensive necrosis and apoptosis as well as macrophage and lymphocytic infiltration, whereas the tumors that develop in mice in which IFN-gamma expression is shut down at P16 do not. Thus, IFN-gamma expression in the perinatal period can induce SHH expression and medulloblastoma in the cerebellum by a STAT1-dependent mechanism, and its continued presence appears to promote a host response to the tumor.

Reversible blockade of experience-dependent plasticity by calcineurin in mouse visual cortex

Numerous protein kinases have been implicated in visual cortex plasticity, but the role of serine/threonine protein phosphatases has not yet been established. Calcineurin, the only known Ca2+/calmodulin-activated protein phosphatase in the brain, has been identified as a molecular constraint on synaptic plasticity in the hippocampus and on memory. Using transgenic mice overexpressing calcineurin inducibly in forebrain neurons, we now provide evidence that calcineurin is also involved in ocular dominance plasticity. A transient increase in calcineurin activity is found to prevent the shift of responsiveness in the visual cortex following monocular deprivation, and this effect is reversible. These results imply that the balance between protein kinases and phosphatases is critical for visual cortex plasticity.

Inducible and neuron-specific gene expression in the adult mouse brain with the rtTA2S-M2 system

To achieve inducible and reversible gene expression in the adult mouse brain, we exploited an improved version of the tetracycline-controlled transactivator-based system (rtTA2(S)-M2, rtTA2 hereafter) and combined it with the forebrain-specific CaMKIIalpha promoter. Several independent lines of transgenic mice carrying the CaMKIIalpha promoter-rtTA2 gene were generated and examined for anatomical profile, doxycycline (dox)-dependence, time course, and reversibility of gene expression using several lacZ reporter lines. In two independent rtTA2-expressing lines, dox-treatment in the diet induced lacZ reporter expression in neurons of several forebrain structures including cortex, striatum, hippocampus, amygdala, and olfactory bulb. Gene expression was dose-dependent and was fully reversible. Further, a similar pattern of expression was obtained in three independent reporter lines, indicating the consistency of gene expression. Transgene expression could also be activated in the developing brain (P0) by dox-treatment of gestating females. These new rtTA2-expressing mice allowing inducible and reversible gene expression in the adult or developing forebrain represent useful models for future genetic studies of brain functions.

Rat tyrosine hydroxylase promoter directs tetracycline-inducible foreign gene expression in dopaminergic cell types

A prerequisite for creating animal models in which gene expression is spatially and temporally controlled is the development of promoters to target genetic switches to specific populations of cells. Here we used the dopaminergic biosynthetic enzyme, tyrosine hydroxylase (TH) to test various combinations of tetracycline (Tet) system elements to determine the optimal configuration for inducible, tissue-specific expression. The present study shows that the degree of expression and level of leakiness associated with the Tet transactivators rtTA, rtTA2S-M2, tTS/rtTA or tTS/rtTA2S-M2 was dependent upon both the promoter and cell type utilized. Specifically, CMV-driven tTS/rtTA2S-M2 exhibited the highest level of inducibility in HEK cells (approximately 1000-fold) versus the dopaminergic cell line, MN9D (approximately 70-fold). In contrast, TH-driven rtTA2S-M2 yielded the highest level of expression with the least background in dopaminergic cell types versus HEK cells. Moreover, the TH promoter could be combined with the bi-directional Tet response system, BiTetO, allowing for the co-expression and regulation of two genes in the same cell. To further test the feasibility of this system we replaced the reporter gene with human Bcl-2. Consistent with previous studies, induction of Bcl-2 expression in dopaminergic cell types attenuated cell death due to the neurotoxin, MPP+. Taken together, these data suggest that targeted, inducible gene expression can be achieved in dopaminergic cell types.

Tetracycline-inducible system for regulation of skeletal muscle-specific gene expression in transgenic mice

Tightly regulated control of over-expression is often necessary to study one aspect or time point of gene function and, in transgenesis, may help to avoid lethal effects and complications caused by ubiquitous over-expression. We have utilized the benefits of an optimized tet-on system and a modified muscle creatine kinase (MCK) promoter to generate a skeletal muscle-specific, doxycycline (Dox) controlled over-expression system in transgenic mice. A DNA construct was generated in which the codon optimized reverse tetracycline transactivator (rtTA) was placed under control of a skeletal muscle-specific version of the mouse MCK promoter. Transgenic mice containing this construct expressed rtTA almost exclusively in skeletal muscles. These mice were crossed to a second transgenic line containing a bi-directional promoter centered on a tet responder element driving both a luciferase reporter gene and a tagged gene of interest; in this case the calpain inhibitor calpastatin. Compound hemizygous mice showed high level, Dox dependent muscle-specific luciferase activity often exceeding 10,000-fold over non-muscle tissues of the same mouse. Western and immunocytochemical analysis demonstrated similar Dox dependent muscle-specific induction of the tagged calpastatin protein. These findings demonstrate the effectiveness and flexibility of the tet-on system to provide a tightly regulated over-expression system in adult skeletal muscle. The MCKrtTA transgenic lines can be combined with other transgenic responder lines for skeletal muscle-specific over-expression of any target gene of interest.

Latest developments and in vivo use of the Tet system: ex vivo and in vivo delivery of tetracycline-regulated genes

In June this year, the tetracycline-regulated gene expression system (tet system) celebrated its tenth "birthday". In the past ten years a continuous stream of changes made to the tet system's basic components has led to a remarkable improvement in its overall performance. It was not until this year, however, that the full benefits of these improvements became apparent. In particular, usage of the tet system is no longer limited to immortalized cell lines and transgenic animals. In this review, we will describe the obstacles encountered in delivering the tet system's components to primary cells and tissues as well as the methods now used to overcome them. We will also focus on a novel system that is conceptually similar but based on different antibiotic/transcription factor pairs.