Use of the tetracycline system for inducible protein synthesis in the kidney

Endothelial thioredoxin interacting protein (TXNIP) modulates endothelium-dependent vasorelaxation in hyperglycemia

Endothelial dysfunction, hallmarked by an imbalance between vasoconstriction and vasorelaxation, is associated with diabetes. Thioredoxin Interacting protein (TXNIP), controlled by an exquisitely glucose sensitive gene, is increasingly recognized for its role in diabetes. However, the role of TXNIP in modulating diabetes-related endothelial dysfunction remains unclear. To elucidate the role of TXNIP, we generated two novel mouse strains; endothelial-specific TXNIP knockout (EKO) and a Tet-O inducible, endothelial-specific TXNIP overexpression (EKI). Hyperglycemia was induced by streptozotocin (STZ) treatment in floxed control (fl/fl) and EKO mice. Doxycycline (DOX) was given to EKI mice to induce endothelial TXNIP overexpression. The ablation of endothelial TXNIP improved glucose tolerance in EKO mice. Acetylcholine-induced, endothelium-dependent vasorelaxation was impaired in STZ-treated fl/fl mice while this STZ impaired vasorelaxation was attenuated in EKO mice. Hyperglycemia induction of NLRP3 and reductions in Akt and eNOS phosphorylation were also mitigated in EKO mice. Overexpression of endothelial TXNIP did not impair glucose tolerance in DOX-treated EKI mice, however induction of endothelial TXNIP led to impaired vasorelaxation in EKI mice. This was associated with increased NLRP3 and reduced Akt and eNOS activation. In conclusion, deletion of endothelial TXNIP is protective against and overexpression of endothelial TXNIP induces endothelial dysfunction; thus, endothelial TXNIP plays a critical role in modulating endothelial dysfunction.

LMX1B is essential for the maintenance of differentiated podocytes in adult kidneys

Mutations of the LMX1B gene cause nail-patella syndrome, a rare autosomal-dominant disorder affecting the development of the limbs, eyes, brain, and kidneys. The characterization of conventional Lmx1b knockout mice has shown that LMX1B regulates the development of podocyte foot processes and slit diaphragms, but studies using podocyte-specific Lmx1b knockout mice have yielded conflicting results regarding the importance of LMX1B for maintaining podocyte structures. In order to address this question, we generated inducible podocyte-specific Lmx1b knockout mice. One week of Lmx1b inactivation in adult mice resulted in proteinuria with only minimal foot process effacement. Notably, expression levels of slit diaphragm and basement membrane proteins remained stable at this time point, and basement membrane charge properties also did not change, suggesting that alternative mechanisms mediate the development of proteinuria in these mice. Cell biological and biophysical experiments with primary podocytes isolated after 1 week of Lmx1b inactivation indicated dysregulation of actin cytoskeleton organization, and time-resolved DNA microarray analysis identified the genes encoding actin cytoskeleton-associated proteins, including Abra and Arl4c, as putative LMX1B targets. Chromatin immunoprecipitation experiments in conditionally immortalized human podocytes and gel shift assays showed that LMX1B recognizes AT-rich binding sites (FLAT elements) in the promoter regions of ABRA and ARL4C, and knockdown experiments in zebrafish support a model in which LMX1B and ABRA act in a common pathway during pronephros development. Our report establishes the importance of LMX1B in fully differentiated podocytes and argues that LMX1B is essential for the maintenance of an appropriately structured actin cytoskeleton in podocytes.

Development of single-vector Tet-on inducible systems with high sensitivity to doxycycline

Single-vector Tet-on systems were developed to enable the tight regulation of transgenes in mammalian cells with a low dosage of doxycycline. Both the regulatory and the responsive units were integrated in a single vector and separated by a short DNA segment (214 bp). In the developed single-vector Tet-on systems, a high level of expression of the transgene can be induced by doxycycline at a concentration of as low as 1 ng/ml, which is 500-1,000 times lower than that usually utilized in other Tet-on systems. The single-vector Tet-on system developed here exhibited 3.5-10.8 times greater inducibility of the transgene in response to doxycycline than did a dual-vector system from a commercial source. Further studies indicate that the basal activity of Tet-on systems depends greatly on the strength of the promoter that controls the transactivator. The basal activity of Tet-on systems was high when the transactivator that was directed by the human cytomegalovirus promoter, and it was almost undetectable when the transactivator was placed under the control of a moderate strength mouse mammary tumor virus promoter. Moreover, the introduction of selectable markers allows the developed single-vector Tet-on systems to facilitate the generation of conditional transgenic cells and animals with high inducibility, low basal activity and detrimental effects of the long-term administration of doxycycline.

Prevention of diabetic neuropathy by regulatable expression of HSV-mediated erythropoietin

Previous studies have demonstrated that gene transfer of genes coding for neurotrophic factors to the dorsal root ganglion (DRG) using nonreplicating herpes simplex virus (HSV)-based vectors injected subcutaneously can prevent the progression of diabetic neuropathy. Because prolonged expression of neurotrophic factors could potentially have unwanted adverse effects, we constructed a nonreplicating HSV vector, vHrtEPO, to express erythropoietin (EPO) under the control of a tetracycline response element (TRE)-minimal cytomegalovirus (CMV) fusion promoter. Primary DRG neurons in culture infected with vHrtEPO express and release EPO in response to exposure to doxycycline (DOX). Animals infected with vHrtEPO by footpad inoculation demonstrated regulated expression of EPO in DRG under the control of DOX administered by gavage. Mice rendered diabetic by injection of streptozotocin (STZ), inoculated with vHrtEPO, and treated with DOX 4 days out of 7 each week for 4 weeks were protected against the development of diabetic neuropathy as assessed by electrophysiologic and behavioral measures. These studies indicate that intermittent expression of EPO in DRG achieved from a regulatable vector is sufficient to protect against the progression of neuropathy in diabetic animals, and provides proof-of-principle preclinical evidence for the development of such vectors for clinical trial.

Development of a BAC vector for integration-independent and tight regulation of transgenes in rodents via the Tet system

The establishment of functional transgenic mouse lines is often limited by problems caused by integration site effects on the expression construct. Similarly, tetracycline (Tet) controlled transcription units most commonly used for conditional transgene expression in mice are strongly influenced by their genomic surrounding. Using bacterial artificial chromosome (BAC) technology in constitutive expression systems, it has been shown that integration site effects resulting in unwanted expression patterns can be largely eliminated. Here we describe a strategy to minimize unfavourable integration effects on conditional expression constructs based on a 75 kb genomic BAC fragment. This fragment was derived from a transgenic mouse line, termed LC-1, which carries the Tet-inducible genes luciferase and cre (Schönig et al. 2002). Animals of this mouse line have previously been shown to exhibit optimal expression properties in terms of tightness in the off state and the absolute level of induction, when mated to appropriate transactivator expressing mice. Here we report the cloning and identification of the transgenic LC-1 integration site which was subsequently inserted into a bacterial artificial chromosome. We demonstrate that this vector facilitates the efficient generation of transgenic mouse and rat lines, where the Tet-controlled expression unit is shielded from perturbations caused by the integration site.

Targeted transgenesis at the HPRT locus: an efficient strategy to achieve tightly controlled in vivo conditional expression with the tet system

The tet-inducible system has been widely used to achieve conditional gene expression in genetically modified mice. To alleviate the frequent difficulties associated with recovery of relevant transgenic founders, we tested whether a controlled strategy of transgenesis would support reliable cell-specific, doxycycline (Dox)-controlled transgene expression in vivo. Taking advantage of the potent hypoxanthine-aminopterin-thymidine selection strategy and an embryonic stem (ES) cell line supporting efficient germ-line transmission, we used hypoxanthine phosphoribosyltransferase ( HPRT) targeting to insert a single copy tet-inducible construct designed to allow both glucocorticoid receptor (GR) and β-galactosidase (β-Gal) expression. Conditional, Dox-dependent GR and β-Gal expression was evidenced in targeted ES cells. Breeding ES-derived single copy transgenic mice with mice bearing appropriate tet transactivators resulted in β-Gal expression both qualitatively and quantitatively similar to that observed in mice with random integration of the same construct. Interestingly, GR expression in mice was dependent on transgene orientation in the HPRT locus while embryonic stem cell expression was not. Thus, a conditional construct inserted in single copy and in predetermined orientation at the HPRT locus demonstrated a Dox-dependent gene expression phenotype in adult mice suggesting that controlled insertion of tet-inducible constructs at the HPRT locus can provide an efficient alternative strategy to reproducibly generate animal models with tetracycline-induced transgene expression.

Tissue-specific expression of ferritin H regulates cellular iron homoeostasis in vivo

Ferritin is a ubiquitously distributed iron-binding protein. Cell culture studies have demonstrated that ferritin plays a role in maintenance of iron homoeostasis and in the protection against cytokine- and oxidant-induced stress. To test whether FerH (ferritin H) can regulate tissue iron homoeostasis in vivo, we prepared transgenic mice that conditionally express FerH and EGFP (enhanced green fluorescent protein) from a bicistronic tetracycline-inducible promoter. Two transgenic models were explored. In the first, the FerH and EGFP transgenes were controlled by the tTA(CMV) (Tet-OFF) (where tTA and CMV are tet transactivator protein and cytomegalovirus respectively). In skeletal muscle of mice bearing the FerH/EGFP and tTA(CMV) transgenes, FerH expression was increased 6.0+/-1.1-fold (mean+/-S.D.) compared with controls. In the second model, the FerH/EGFP transgenes were controlled by an optimized Tet-ON transactivator, rtTA2(S)-S2(LAP) (where rtTA is reverse tTA and LAP is liver activator protein), resulting in expression predominantly in the kidney and liver. In mice expressing these transgenes, doxycycline induced FerH in the kidney by 14.2+/-4.8-fold (mean+/-S.D.). Notably, increases in ferritin in overexpressers versus control littermates were accompanied by an elevation of IRP (iron regulatory protein) activity of 2.3+/-0.9-fold (mean+/-S.D.), concurrent with a 4.5+/-2.1-fold (mean+/-S.D.) increase in transferrin receptor, indicating that overexpression of FerH is sufficient to elicit a phenotype of iron depletion. These results demonstrate that FerH not only responds to changes in tissue iron (its classic role), but can actively regulate overall tissue iron balance.

Liver-specific expression of interferon gamma following adenoviral gene transfer controls hepatitis B virus replication in mice

Interferons control viral replication and the growth of some malignant tumors. Since systemic application may cause severe adverse effects, tissue-specific expression is an attractive alternative. Liver-directed interferon gene therapy offers promising applications such as chronic viral hepatitis B or C or hepatocellular carcinoma and thus needs testing in vivo in suitable animal models. We therefore used the Tet-On system to regulate gene expression in adenoviral vectors, and studied the effect of liver-specific and regulated interferon gamma expression in a mouse model of chronic hepatitis B virus (HBV) infection. In a first generation adenoviral vector, genes encoding for firefly luciferase and interferons alpha, beta or gamma, respectively, were coexpressed under control of the bidirectional tetracycline-regulated promoter P(tet)bi. Liver-specific promoters driving expression of the reverse tetracycline controlled transactivator ensured local expression in the livers of HBV transgenic mice. Following gene transfer, we demonstrated low background, tight regulation and a 1000-fold induction of gene expression by doxycycline. Both genes within the bidirectional transcription unit were expressed simultaneously, and in a liver-specific fashion in cell culture and in living mice. Doxycycline-dependent interferon gamma expression effectively controlled HBV replication in mice, but did not eliminate HBV transcripts. This system will help to study the effects of local cytokine expression in mouse disease models in detail.