Adjusting transgene expression levels in lymphocytes with a set of inducible promoters

Designed membrane protein heterodimers and control of their affinity by binding domain and membrane linker properties

Many membrane proteins utilize dimerization to transmit signals across the cell membrane via regulation of the lateral binding affinity. The complexity of natural membrane proteins hampers the understanding of this regulation on a biophysical level. We designed simplified membrane proteins from well-defined soluble dimerization domains with tunable affinities, flexible linkers, and an inert membrane anchor. Live-cell single-molecule imaging demonstrates that their dimerization affinity indeed depends on the strength of their binding domains. We confirm that as predicted, the 2-dimensional affinity increases with the 3-dimensional binding affinity of the binding domains and decreases with linker lengths. Models of extended and coiled linkers delineate an expected range of 2-dimensional affinities, and our observations for proteins with medium binding strength agree well with the models. Our work helps in understanding the function of membrane proteins and has important implications for the design of synthetic receptors.

Kappa but not delta or mu opioid receptors form homodimers at low membrane densities

Opioid receptors (ORs) have been observed as homo- and heterodimers, but it is unclear if the dimers are stable under physiological conditions, and whether monomers or dimers comprise the predominant fraction in a cell. Here, we use three live-cell imaging approaches to assess dimerization of ORs at expression levels that are 10-100 × smaller than in classical biochemical assays. At membrane densities around 25/µm², a split-GFP assay reveals that κOR dimerizes, while µOR and δOR stay monomeric. At receptor densities < 5/µm², single-molecule imaging showed no κOR dimers, supporting the concept that dimer formation depends on receptor membrane density. To directly observe the transition from monomers to dimers, we used a single-molecule assay to assess membrane protein interactions at densities up to 100 × higher than conventional single-molecule imaging. We observe that κOR is monomeric at densities < 10/µm² and forms dimers at densities that are considered physiological. In contrast, µOR and δOR stay monomeric even at the highest densities covered by our approach. The observation of long-lasting co-localization of red and green κOR spots suggests that it is a specific effect based on OR dimerization and not an artefact of coincidental encounters.

Apoptotic stress induces Bax-dependent, caspase-independent redistribution of LINC complex nesprins

The canonical function of Bcl-2 family proteins is to regulate mitochondrial membrane integrity. In response to apoptotic signals the multi-domain pro-apoptotic proteins Bax and Bak are activated and perforate the mitochondrial outer membrane by a mechanism which is inhibited by their interaction with pro-survival members of the family. However, other studies have shown that Bax and Bak may have additional, non-canonical functions, which include stress-induced nuclear envelope rupture and discharge of nuclear proteins into the cytosol. We show here that the apoptotic stimuli cisplatin and staurosporine induce a Bax/Bak-dependent degradation and subcellular redistribution of nesprin-1 and nesprin-2 but not nesprin-3, of the linker of nucleoskeleton and cytoskeleton (LINC) complex. The degradation and redistribution were caspase-independent and did not occur in Bax/Bak double knockout (DKO) mouse embryo fibroblasts (MEFs). Re-expression of Bax in Bax/Bak DKO MEFs restored stress-induced redistribution of nesprin-2 by a mechanism which requires Bax membrane localization and integrity of the α helices 5/6, and the Bcl-2 homology 3 (BH3) domain. We found that nesprin-2 interacts with Bax in close proximity to perinuclear mitochondria in mouse and human cells. This interaction requires the mitochondrial targeting and N-terminal region but not the BH3 domain of Bax. Our results identify nesprin-2 as a Bax binding partner and also a new function of Bax in impairing the integrity of the LINC complex.

NAD+ Depletion Triggers Macrophage Necroptosis, a Cell Death Pathway Exploited by Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) kills infected macrophages by inhibiting apoptosis and promoting necrosis. The tuberculosis necrotizing toxin (TNT) is a secreted nicotinamide adenine dinucleotide (NAD⁺) glycohydrolase that induces necrosis in infected macrophages. Here, we show that NAD⁺ depletion by TNT activates RIPK3 and MLKL, key mediators of necroptosis. Notably, Mtb bypasses the canonical necroptosis pathway since neither TNF-α nor RIPK1 are required for macrophage death. Macrophage necroptosis is associated with depolarized mitochondria and impaired ATP synthesis, known hallmarks of Mtb-induced cell death. These results identify TNT as the main trigger of necroptosis in Mtb-infected macrophages. Surprisingly, NAD⁺ depletion itself was sufficient to trigger necroptosis in a RIPK3- and MLKL-dependent manner by inhibiting the NAD⁺ salvage pathway in THP-1 cells or by TNT expression in Jurkat T cells. These findings suggest avenues for host-directed therapies to treat tuberculosis and other infectious and age-related diseases in which NAD⁺ deficiency is a pathological factor.

Specific role of RhoC in tumor invasion and metastasis

Rho GTPases are regulators of many cellular functions and are often dysregulated in cancer. However, the precise role of Rho proteins for tumor development is not well understood. In breast cancer, overexpression of RhoC is linked with poor prognosis. Here, we aim to compare the function of RhoC and its homolog family member RhoA in breast cancer progression. We established stable breast epithelial cell lines with inducible expression of RhoA and RhoC, respectively. Moreover, we made use of Rho-activating bacterial toxins (Cytotoxic Necrotizing Factors) to stimulate the endogenous pool of Rho GTPases in benign breast epithelial cells and simultaneously knocked down specific Rho proteins. Whereas activation of Rho GTPases was sufficient to induce an invasive phenotype in three-dimensional culture systems, overexpression of RhoA or RhoC were not. However, RhoC but not RhoA was required for invasion, whereas RhoA and RhoC equally regulated proliferation. We further identified downstream target genes of RhoC involved in invasion and identified PTGS2 (COX-2) being preferentially upregulated by RhoC. Consistently, the COX-2 inhibitor Celecoxib blocked the invasive phenotype induced by the Rho-activating toxins.

Ligand-induced type II interleukin-4 receptor dimers are sustained by rapid re-association within plasma membrane microcompartments

The spatiotemporal organization of cytokine receptors in the plasma membrane is still debated with models ranging from ligand-independent receptor pre-dimerization to ligand-induced receptor dimerization occurring only after receptor uptake into endosomes. Here, we explore the molecular and cellular determinants governing the assembly of the type II interleukin-4 receptor, taking advantage of various agonists binding the receptor subunits with different affinities and rate constants. Quantitative kinetic studies using artificial membranes confirm that receptor dimerization is governed by the two-dimensional ligand–receptor interactions and identify a critical role of the transmembrane domain in receptor dimerization. Single molecule localization microscopy at physiological cell surface expression levels, however, reveals efficient ligand-induced receptor dimerization by all ligands, largely independent of receptor binding affinities, in line with the similar STAT6 activation potencies observed for all IL-4 variants. Detailed spatiotemporal analyses suggest that kinetic trapping of receptor dimers in actin-dependent microcompartments sustains robust receptor dimerization and signalling.

The contribution of ligands for cytokine receptor dimerization is still not fully understood. Here, the authors show the efficient ligand-induced dimerization of type II interleukin-4 receptor at the plasma membrane and the kinetic trapping of signalling complexes by actin-dependent membrane microdomains.

Phosphorylation of CDC25A on SER283 in late S/G2 by CDK/cyclin complexes accelerates mitotic entry

The Cdc25A phosphatase is an essential activator of CDK-cyclin complexes at all steps of the eukaryotic cell cycle. The activity of Cdc25A is itself regulated in part by positive and negative feedback regulatory loops performed by its CDK-cyclin substrates that occur in G1 as well as during the G1/S and G2/M transitions. However, the regulation of Cdc25A during G2 phase progression before mitotic entry has not been intensively characterized. Here, we identify by mass spectrometry analysis a new phosphorylation event of Cdc25A on Serine283. Phospho-specific antibodies revealed that the phosphorylation of this residue appears in late S/G2 phase of an unperturbed cell cycle and is performed by CDK-cyclin complexes. Overexpression studies of wild-type and non-phosphorylatable mutant forms of Cdc25A indicated that Ser283 phosphorylation increases the G2/M-promoting activity of the phosphatase without impacting its stability or subcellular localization. Our results therefore identify a new positive regulatory loop between Cdc25A and its CDK-cyclin substrates which contributes to accelerate entry into mitosis through the regulation of Cdc25A activity in G2.

MAP3K11 is a tumor suppressor targeted by the oncomiR miR-125b in early B cells

MicroRNAs (miRNAs) are a class of small, non-coding RNAs that posttranscriptionally regulate gene expression and thereby control most, if not all, biological processes. Aberrant miRNA expression has been linked to a variety of human diseases including cancer, but the underlying molecular mechanism often remains unclear. Here we have screened a miRNA expression library in a growth factor-dependent mouse pre-B-cell system to identify miRNAs with oncogenic activity. We show that miR-125b is sufficient to render pre-B cells growth factor independent and demonstrate that continuous expression of miR-125b is necessary to keep these cells in a transformed state. Mechanistically, we find that the expression of miR-125b protects against apoptosis induced by growth factor withdrawal, and that it blocks the differentiation of pre-B to immature B cells. In consequence, miR-125b-transformed cells maintain expression of their pre-B-cell receptor that provides signals for continuous proliferation and survival even in the absence of growth factor. Employing microarray analysis, we identified numerous targets of miR-125b, but only reconstitution of MAP3K11, a critical regulator of mitogen- and stress-activated kinase signaling, interferes with the cellular fitness of the transformed cells. Together, this indicates that MAP3K11 might function as an important tumor suppressor neutralized by oncomiR-125b in B-cell leukemia.

Nucleotide Excision Repair and Transcription-coupled DNA Repair Abrogate the Impact of DNA Damage on Transcription

DNA adducts derived from carcinogenic polycyclic aromatic hydrocarbons like benzo[a]pyrene (B[a]P) and benzo[c]phenanthrene (B[c]Ph) impede replication and transcription, resulting in aberrant cell division and gene expression. Global nucleotide excision repair (NER) and transcription-coupled DNA repair (TCR) are among the DNA repair pathways that evolved to maintain genome integrity by removing DNA damage. The interplay between global NER and TCR in repairing the polycyclic aromatic hydrocarbon-derived DNA adducts (+)-trans-anti-B[a]P-N(6)-dA, which is subject to NER and blocks transcription in vitro, and (+)-trans-anti-B[c]Ph-N(6)-dA, which is a poor substrate for NER but also blocks transcription in vitro, was tested. The results show that both adducts inhibit transcription in human cells that lack both NER and TCR. The (+)-trans-anti-B[a]P-N(6)-dA lesion exhibited no detectable effect on transcription in cells proficient in NER but lacking TCR, indicating that NER can remove the lesion in the absence of TCR, which is consistent with in vitro data. In primary human cells lacking NER, (+)-trans-anti-B[a]P-N(6)-dA exhibited a deleterious effect on transcription that was less severe than in cells lacking both pathways, suggesting that TCR can repair the adduct but not as effectively as global NER. In contrast, (+)-trans-anti-B[c]Ph-N(6)-dA dramatically reduces transcript production in cells proficient in global NER but lacking TCR, indicating that TCR is necessary for the removal of this adduct, which is consistent with in vitro data showing that it is a poor substrate for NER. Hence, both global NER and TCR enhance the recovery of gene expression following DNA damage, and TCR plays an important role in removing DNA damage that is refractory to NER.

Weaknesses and Pitfalls of Using Mice and Rats in Cancer Chemoprevention Studies

Many studies, using different chemical agents, have shown excellent cancer prevention efficacy in mice and rats. However, equivalent tests of cancer prevention in humans require decades of intake of the agents while the rodents' short lifespans cannot give us information of the long-term safety. Therefore, animals with a much longer lifespan should be used to bridge the lifespan gap between the rodents and humans. There are many transgenic mouse models of carcinogenesis available, in which DNA promoters are used to activate transgenes. One promoter may activate the transgene in multiple cell types while different promoters are activated at different ages of the mice. These spatial and temporal aspects of transgenes are often neglected and may be pitfalls or weaknesses in chemoprevention studies. The variation in the copy number of the transgene may widen data variation and requires use of more animals. Models of chemically-induced carcinogenesis do not have these transgene-related defects, but chemical carcinogens usually damage metabolic organs or tissues, thus affecting the metabolism of the chemopreventive agents. Moreover, many genetically edited and some chemically-induced carcinogenesis models produce tumors that exhibit cancerous histology but are not cancers because the tumor cells are still mortal, inducer-dependent, and unable to metastasize, and thus should be used with caution in chemoprevention studies. Lastly, since mice prefer an ambient temperature of 30-32°C, it should be debated whether future mouse studies should be performed at this temperature, but not at 21-23°C that cold-stresses the animals.

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.

tLivin displays flexibility by promoting alternative cell death mechanisms

Livin is a member of the Inhibitor of Apoptosis (IAP) protein family that inhibits apoptosis triggered by a variety of stimuli. We previously demonstrated that while Livin inhibits caspase activity, caspases can cleave Livin to produce a truncated protein, tLivin and that this newly formed tLivin paradoxically induces cell death. However to date, the mechanism of tLivin-induced cell death is not fully understood. In this study, we set out to characterize the form of cell death mediated by tLivin. Here we demonstrate that, unlike most death-promoting proteins, tLivin is a flexible inducer of cell death capable of promoting necrosis or apoptosis in different cell lines. The unusual flexibility of tLivin is displayed by its ability to activate an alternative form of cell death when apoptosis is inhibited. Thus, tLivin can promote more than one form of cell death in the same cell type. Interestingly, in cells where tLivin induces necrosis, deletion of the caspase binding BIR domain results in tLivin-induced apoptosis, suggesting the BIR domain can potentially hamper the ability of tLivin to induce apoptosis. We further elucidate that tLivin activates the JNK pathway and both tLivin-induced apoptosis and necrosis are partially mediated by JNK activity. Acquired resistance to apoptosis, common in many tumors, impinges on the efficiency of conventional anti-cancer agents that function primarily by inducing apoptosis. The ability of tLivin to induce death of apoptosis-compromised cells makes it an attractive candidate for targeted cancer therapy.

Clinical application of genetically modified T cells in cancer therapy

Immunotherapies are emerging as highly promising approaches for the treatment of cancer. In these approaches, a variety of materials are used to boost immunity against malignant cells. A key component of many of these approaches is functional tumor-specific T cells, but the existence and activity of sufficient T cells in the immune repertoire is not always the case. Recent methods of generating tumor-specific T cells include the genetic modification of patient lymphocytes with receptors to endow them with tumor specificity. These T cells are then expanded in vitro followed by infusion of the patient in adoptive cell transfer protocols. Genes used to modify T cells include those encoding T-cell receptors and chimeric antigen receptors. In this review, we provide an introduction to the field of genetic engineering of T cells followed by details of their use against cancer in the clinic.

An outer membrane channel protein of Mycobacterium tuberculosis with exotoxin activity

The ability to control the timing and mode of host cell death plays a pivotal role in microbial infections. Many bacteria use toxins to kill host cells and evade immune responses. Such toxins are unknown in Mycobacterium tuberculosis. Virulent M. tuberculosis strains induce necrotic cell death in macrophages by an obscure molecular mechanism. Here we show that the M. tuberculosis protein Rv3903c (channel protein with necrosis-inducing toxin, CpnT) consists of an N-terminal channel domain that is used for uptake of nutrients across the outer membrane and a secreted toxic C-terminal domain. Infection experiments revealed that CpnT is required for survival and cytotoxicity of M. tuberculosis in macrophages. Furthermore, we demonstrate that the C-terminal domain of CpnT causes necrotic cell death in eukaryotic cells. Thus, CpnT has a dual function in uptake of nutrients and induction of host cell death by M. tuberculosis.

A next step in adeno-associated virus-mediated gene therapy for neurological diseases: regulation and targeting

Recombinant adeno-associated virus (rAAV) vectors mediating long term transgene expression are excellent gene therapy tools for chronic neurological diseases. While rAAV2 was the first serotype tested in the clinics, more efficient vectors derived from the rh10 serotype are currently being evaluated and other serotypes are likely to be tested in the near future. In addition, aside from the currently used stereotaxy-guided intraparenchymal delivery, new techniques for global brain transduction (by intravenous or intra-cerebrospinal injections) are very promising. Various strategies for therapeutic gene delivery to the central nervous system have been explored in human clinical trials in the past decade. Canavan disease, a genetic disease caused by an enzymatic deficiency, was the first to be approved. Three gene transfer paradigms for Parkinson's disease have been explored: converting L-dopa into dopamine through AADC gene delivery in the putamen; synthesizing GABA through GAD gene delivery in the overactive subthalamic nucleus and providing neurotrophic support through neurturin gene delivery in the nigro-striatal pathway. These pioneer clinical trials demonstrated the safety and tolerability of rAAV delivery in the human brain at moderate doses. Therapeutic effects however, were modest, emphasizing the need for higher doses of the therapeutic transgene product which could be achieved using more efficient vectors or expression cassettes. This will require re-addressing pharmacological aspects, with attention to which cases require either localized and cell-type specific expression or efficient brain-wide transgene expression, and when it is necessary to modulate or terminate the administration of transgene product. The ongoing development of targeted and regulated rAAV vectors is described.

Gene-engineered T cells for cancer therapy

T cells have the capacity to eradicate diseased cells, but tumours present considerable challenges that render T cells ineffectual. Cancer cells often make themselves almost 'invisible' to the immune system, and they sculpt a microenvironment that suppresses T cell activity, survival and migration. Genetic engineering of T cells can be used therapeutically to overcome these challenges. T cells can be taken from the blood of cancer patients and then modified with genes encoding receptors that recognize cancer-specific antigens. Additional genes can be used to enable resistance to immunosuppression, to extend survival and to facilitate the penetration of engineered T cells into tumours. Using genetic modification, highly active, self-propagating 'slayers' of cancer cells can be generated.

Guiding Differentiation of Stem Cells in Vivo by Tetracycline-Controlled Expression of Key Transcription Factors

Transplantation of stem or progenitor cells is an attractive strategy for cell replacement therapy. However, poor long-term survival and insufficiently reproducible differentiation to functionally appropriate cells in vivo still present major obstacles for translation of this methodology to clinical applications. Numerous experimental studies have revealed that the expression of just a few transcription factors can be sufficient to drive stem cell differentiation toward a specific cell type, to transdifferentiate cells from one fate to another, or to dedifferentiate mature cells to pluripotent stem/progenitor cells (iPSCs). We thus propose here to apply the strategy of expressing the relevant key transcription factors to guide the differentiation of transplanted cells to the desired cell fate in vivo. To achieve this requires tools allowing us to control the expression of these genes in the transplant. Here, we describe drug-inducible systems that allow us to sequentially and timely activate gene expression from the outside, with a particular emphasis on the Tet system, which has been widely and successfully used in stem cells. These regulatory systems offer a tool for strictly limiting gene expression to the respective optimal stage after transplantation. This approach will direct the differentiation of the immature stem/progenitor cells in vivo to the desired cell type.

The Coxiella burnetii type IV secretion system substrate CaeB inhibits intrinsic apoptosis at the mitochondrial level

Manipulation of host cell apoptosis is a virulence property shared by many intracellular pathogens to ensure productive replication. For the obligate intracellular pathogen Coxiella burnetii anti-apoptotic activity, which depends on a functional type IV secretion system (T4SS), has been demonstrated. Accordingly, the C. burnetii T4SS effector protein AnkG was identified to inhibit pathogen-induced apoptosis, possibly by binding to the host cell mitochondrial protein p32 (gC1qR). However, it was unknown whether AnkG alone is sufficient for apoptosis inhibition or if additional effector proteins are required. Here, we identified two T4SS effector proteins CaeA and CaeB (C. burnetii anti-apoptotic effector) that inhibit the intrinsic apoptotic pathway. CaeB blocks apoptosis very efficiently, while the anti-apoptotic activity of CaeA is weaker. Our data suggest that CaeB inhibits apoptosis at the mitochondrial level, but does not bind to p32. Taken together, our results demonstrate that C. burnetii harbours several anti-apoptotic effector proteins and suggest that these effector proteins use different mechanism(s) to inhibit apoptosis.

Genetic aspects of cell line development from a synthetic biology perspective

Animal cells can be regarded as factories for the production of relevant proteins. The advances described in this chapter towards the development of cell lines with higher productivity capacities, certain metabolic and proliferation properties, reduced apoptosis and other features must be regarded in an integrative perspective. The systematic application of systems biology approaches in combination with a synthetic arsenal for targeted modification of endogenous networks are proposed to lead towards the achievement of a predictable and technologically advanced cell system with high biotechnological impact.

Strong negative feedback from Erk to Raf confers robustness to MAPK signalling

This study shows that MAPK signalling is robust against protein level changes due to a strong negative feedback from Erk to Raf. Surprisingly, robustness is provided through a fast post-translational mechanism although variation of Erk levels occurs on a timescale of days.

MAPK signalling is robust against variation in protein level.

Robustness is mediated by a negative feedback to Raf.

Loss of negative feedback due to mutation in B-Raf opens the door for targeted intervention.

Protein levels within signal transduction pathways vary strongly from cell to cell. For example, it has been reported that concentrations of the last kinase within the MAPK signalling module, Erk, varies about four-fold between clonal cells under the same conditions. In the present study, we analysed how signalling pathways can still process information quantitatively despite strong heterogeneity in protein levels. Mathematical analysis of isolated phosphorylation–dephosphorylation cycles predicts that phosphorylation of a signalling molecule is proportional to the protein concentration. We systematically perturbed the protein levels of Erk in human cell lines by siRNA. We found that the steady-state phosphorylation of Erk is very robust against perturbations of Erk protein level, suggesting that there are mechanisms that provide robustness to the pathway against protein fluctuations. Using mathematical modelling, we identified three potential mechanisms that may provide robustness against fluctuating protein levels:

1. Kinetic effects (saturation of the activating kinase Mek),

2. Transcriptional negative feedbacks,

3. Negative feedbacks on the post-translational level.

By experimental analysis of the systems, which included analysis of Erk phosphorylation under Mek overexpression, measuring transcript levels of negative feedback regulators, and application of generic inhibitors of transcription and translation, we could exclude kinetic effects and transcriptional negative feedback as mechanisms of robustness.

By analysing a panel of cell lines, we found that cells are robust as long as the signal passes through Raf-1. In contrast, cells where the pathway is activated by a mutation in B-Raf lose robustness. Detailed molecular analysis of the system shows that a single post-translational feedback to Raf mediates robustness. Thus, robustness is provided through a fast post-translational mechanism although variation of Erk levels occurs on a timescale of days.

Protein levels within signal transduction pathways vary strongly from cell to cell. Here, we analysed how signalling pathways can still process information quantitatively despite strong heterogeneity in protein levels. We systematically perturbed the protein levels of Erk, the terminal kinase in the MAPK signalling pathway in a panel of human cell lines. We found that the steady-state phosphorylation of Erk is very robust against perturbations of Erk protein level. Although a multitude of mechanisms exist that may provide robustness against fluctuating protein levels, we found that one single feedback from Erk to Raf-1 accounts for the observed robustness. Surprisingly, robustness is provided through a fast post-translational mechanism although variation of Erk levels occurs on a timescale of days.

Improved Tet-responsive promoters with minimized background expression

Background

The performance of the tetracycline controlled transcriptional activation system (Tet system) depends critically on the choice of minimal promoters. They are indispensable to warrant low expression levels with the system turned "off". On the other hand, they must support high level of gene expression in the "on"-state.

Results

In this study, we systematically modified the widely used Cytomegalovirus (CMV) minimal promoter to further minimize background expression, resulting in an improved dynamic expression range. Using both plasmid-based and retroviral gene delivery, our analysis revealed that especially background expression levels could be significantly reduced when compared to previously established "standard" promoter designs. Our results also demonstrate the possibility to fine-tune expression levels in non-clonal cell populations. They also imply differences regarding the requirements for tight regulation and high level induction between transient and stable gene transfer systems.

Conclusions

Until now, our understanding of mammalian transcriptional regulation including promoter architecture is limited. Nevertheless, the partly empirical modification of cis-elements as shown in this study can lead to the specific improvement of the performance of minimal promoters. The novel composite Ptet promoters introduced here will further expand the utility of the Tet system.