Inducibility of doxycycline-regulated gene in neural and neuroendocrine cells strongly depends on the appropriate choice of a tetracycline-responsive promoter

Modifications in the C-terminal tail of TrkC significantly alter neurotrophin-3-promoted outgrowth of neurite-like processes from PC12 cells

TrkB and TrkC are quite common neurotrophin receptors found on the same cells in CNS. In the C-terminal tail, TrkB and TrkC differ only in two amino acid residues at positions immediately preceding the tyrosine residue, which, upon phosphorylation, becomes the docking site for phospholipase Cγ1 (PLCγ1). The question arose whether such a difference near the PLCγ1 docking site might contribute to differential response to neurotrophin. PC12 clones with the following receptors were obtained: wild-type TrkC, TrkC-Y820F with a defective PLCγ1 binding site, TrkC-T817S-I819V with two amino acid residues replaced with those in the TrkB tail. The outgrowth of neurite-like processes from TrkC-Y820F-containing cells appeared to be impaired, while the TrkC-T817S-I819V variant appeared more effective than wild-type TrkC in promoting the outgrowth of neurite-like processes after neurotrophin stimulation, at least in the compared PC12 cell clones. Taken together, both the tyrosine residue at the PLCγ1 docking site and the amino acid residues immediately preceding it appear important for TrkC-supported outgrowth of neurite-like processes.

Expression of A152T human tau causes age-dependent neuronal dysfunction and loss in transgenic mice

A152T-variant human tau (hTau-A152T) increases risk for tauopathies, including Alzheimer's disease. Comparing mice with regulatable expression of hTau-A152T or wild-type hTau (hTau-WT), we find age-dependent neuronal loss, cognitive impairments, and spontaneous nonconvulsive epileptiform activity primarily in hTau-A152T mice. However, overexpression of either hTau species enhances neuronal responses to electrical stimulation of synaptic inputs and to an epileptogenic chemical. hTau-A152T mice have higher hTau protein/mRNA ratios in brain, suggesting that A152T increases production or decreases clearance of hTau protein. Despite their functional abnormalities, aging hTau-A152T mice show no evidence for accumulation of insoluble tau aggregates, suggesting that their dysfunctions are caused by soluble tau. In human amyloid precursor protein (hAPP) transgenic mice, co-expression of hTau-A152T enhances risk of early death and epileptic activity, suggesting copathogenic interactions between hTau-A152T and amyloid-β peptides or other hAPP metabolites. Thus, the A152T substitution may augment risk for neurodegenerative diseases by increasing hTau protein levels, promoting network hyperexcitability, and synergizing with the adverse effects of other pathogenic factors.

The transgenic cloned pig population with integrated and controllable GH expression that has higher feed efficiency and meat production

Sustained expression of the GH gene has been shown to have detrimental effects on the health of animals. In the current study, transgenic founder pigs, with controllable pig growth hormone (pGH) expression, were cloned via the handmade cloning method (HMC), and pGH expression levels were examined at the cellular and organismal levels. The serum pGH levels in 3 founder male pigs were found to be significantly higher after induction with intramuscular injection of doxycycline (DOX) compared to baseline. A daily dose of DOX was administered via feed to these animals for a period of 65 to 155 days. The growth rate, feed efficiency and pGH serum concentration increased in the DOX-induced transgenic group compared with the other groups. 8 numbers of animals were euthanized and the dressing percentage, loin muscle and lean meat percentage were significantly higher in the DOX-induced F1 transgenic group compared with the other groups. In this study a large population of transgenic pigs, with integrated controllable expression of a transgene, was obtained. The transgenic pigs were healthy and normal in terms of reproductive capability. At the same time, feed efficiency was improved, production processes were accelerated and meat yield was increased.

Establishment of a cellular model to study TrkC-dependent neuritogenesis

The rat PC12 cell line has become a widely used research tool for many aspects of neurobiology. Nerve growth factor (NGF)-responsive PC12 cells were engineered to drive expression of doxycycline (Dox)-induced gene of interest in the Tet-On expression system that resulted in obtaining PC12-Tet-On cells. TrkA and TrkC are neurotrophin receptors derived from the tropomyosin-related kinase (Trk) family of receptor tyrosine kinases. TrkA receptor binds and is activated mainly by NGF, while TrkC receptor binds and is activated by neurotrophin 3 (NT3). The purpose of this research was to design and describe PC12-based neuronal cell model to study TrkC-triggered versus TrkA-triggered neurite outgrowth. The second-generation tetracycline-responsive promoter (P tight) was used in order to provide low basal expression in the absence of Dox and high-level Dox-induced expression of TrkC. The main advantage of presented model system is dependence of TrkC level on Dox concentration. It also allows to compare activation of intracellular signaling proteins and neurite outgrowth following activation of TrkA and TrkC receptors by NGF and NT3, respectively, in the context of the same quality and quantity of intracellular adaptor proteins, Ras proteins, protein kinases and phosphatases, and phospholipase Cγ1, as a difference in the activation of intracellular signaling network by these two distinct although related receptor tyrosine kinases is expected. The results of our studies suggest that despite slightly weaker activation of ERK1/2 mitogen-activated protein kinases, NT3-triggered TrkC seems to provide apparently stronger than NGF-triggered TrkA signal for neurite elongation in differentiating PC12 cells.

Tbx2 misexpression impairs deployment of second heart field derived progenitor cells to the arterial pole of the embryonic heart

Tbx2 is a member of the T-box family of transcription factors that play important roles during heart development. In the embryonic heart tube, Tbx2 is expressed in non-chamber myocardium (outflow tract and interventricular canal) and has been shown to block chamber formation. We have developed a genetic system to conditionally misexpress Tbx2 in the embryonic mouse heart at early stages of development. We show that Tbx2 expression throughout the myocardium of the heart tube both represses proliferation and impairs secondary heart field (SHF) progenitor cell deployment into the outflow tract (OFT). Repression of proliferation is accompanied by the upregulation of Ndrg2 and downregulation of Ndrg4 expression, both genes believed to be involved in cell growth and proliferation. Impaired deployment of SHF cells from the pharyngeal mesoderm is accompanied by downregulation of the cell adhesion molecules Alcam and N-cadherin in the anterior part of the embryonic heart. Tbx2 misexpression also results in downregulation of Tbx20 within the OFT, indicating complex and region-specific transcriptional cross-regulation between the two T-box genes.

Promiscuous expression of H2B-GFP transgene in hematopoietic stem cells

Background

The study of adult stem cells relies on the ability to isolate them using complex combinations of markers for flow cytometry. A recent study has used a tetracycline-regulatable H2B-GFP transgenic mouse model analogous to BrdU pulse-chase methods to fluorescently label quiescent skin stem cells in vivo. In this study, we sought to use these mice to fluorescently label hematopoietic stem cells to study niche interactions.

Methods and Findings

We crossed the H2B-GFP mice to mice carrying a tetracycline-regulated transactivator protein. When these mice were administered doxycycline, we observed a gradual decrease in total bone marrow GFP⁺ cells over 12 weeks but the hematopoietic stem cell population remained largely GFP⁺ (>85%). In histological bone sections, the long-term GFP label-retaining cells tended to concentrate at the endosteal surface and competitive transplantation assays showed that the majority of hematopoietic stem cell activity was contained in the GFP⁺ cell fraction. However, in response to stimulation with 5-fluorouracil, the hematopoietic stem cells of the crossed mice still retained a high level of GFP expression when it was anticipated the label should be lost when the cells divide. Upon further review, it was determined that the founder H2B-GFP mice showed spurious expression of the transgene at high levels in the hematopoietic stem cell population, thus the observed response of hematopoietic stem cells in the double transgenic mice to doxycycline was due to aberrant expression of the transgene and not the correct tetracycline-regulatable system.

Conclusions

We observed promiscuous expression of the H2B-GFP transgene in the hematopoietic stem cell compartment of the bone marrow. This leaky expression prohibits the use of this model to study hematopoietic stem cells in vivo and careful characterization for each organ must be done if this transgenic system is to be used to isolate other prospective tissue stem cells.

Imaging cyclic AMP changes in pancreatic islets of transgenic reporter mice

Cyclic AMP (cAMP) and Ca(2+) are two ubiquitous second messengers in transduction pathways downstream of receptors for hormones, neurotransmitters and local signals. The availability of fluorescent Ca(2+) reporter dyes that are easily introduced into cells and tissues has facilitated analysis of the dynamics and spatial patterns for Ca(2+) signaling pathways. A similar dissection of the role of cAMP has lagged because indicator dyes do not exist. Genetically encoded reporters for cAMP are available but they must be introduced by transient transfection in cell culture, which limits their utility. We report here that we have produced a strain of transgenic mice in which an enhanced cAMP reporter is integrated in the genome and can be expressed in any targeted tissue and with tetracycline induction. We have expressed the cAMP reporter in beta-cells of pancreatic islets and conducted an analysis of intracellular cAMP levels in relation to glucose stimulation, Ca(2+) levels, and membrane depolarization. Pancreatic function in transgenic mice was normal. In induced transgenic islets, glucose evoked an increase in cAMP in beta-cells in a dose-dependent manner. The cAMP response is independent of (in fact, precedes) the Ca(2+) influx that results from glucose stimulation of islets. Glucose-evoked cAMP responses are synchronous in cells throughout the islet and occur in 2 phases suggestive of the time course of insulin secretion. Insofar as cAMP in islets is known to potentiate insulin secretion, the novel transgenic mouse model will for the first time permit detailed analyses of cAMP signals in beta-cells within islets, i.e. in their native physiological context. Reporter expression in other tissues (such as the heart) where cAMP plays a critical regulatory role, will permit novel biomedical approaches.