Targeting of an inducible toxic phenotype in animal cells

Modeling specific aneuploidies: from karyotype manipulations to biological insights

An abnormal chromosome number, or aneuploidy, underlies developmental disorders and is a common feature of cancer, with different cancer types exhibiting distinct patterns of chromosomal gains and losses. To understand how specific aneuploidies emerge in certain tissues and how they contribute to disease development, various methods have been developed to alter the karyotype of mammalian cells and mice. In this review, we provide an overview of both classic and novel strategies for inducing or selecting specific chromosomal gains and losses in human and murine cell systems. We highlight how these customized aneuploidy models helped expanding our knowledge of the consequences of specific aneuploidies to (cancer) cell physiology.

A genetic method specifically delineates Th1-type Treg cells and their roles in tumor immunity

Regulatory T (Treg) cells expressing the transcription factor (TF) Foxp3 also express other TFs shared by T helper (Th) subsets under certain conditions. Here, to determine the roles of T-bet-expressing Treg cells, we generate a mouse strain, called VeDTR, in which T-bet/Foxp3 double-positive cells are engineered to be specifically labeled and depleted by a combination of Cre- and Flp-recombinase-dependent gene expression control. Characterization of T-bet⁺Foxp3⁺ cells using VeDTR mice reveals high resistance under oxidative stress, which is involved in accumulation of T-bet⁺Foxp3⁺ cells in tumor tissues. Moreover, short-term depletion of T-bet⁺Foxp3⁺ cells leads to anti-tumor immunity but not autoimmunity, whereas that of whole Treg cells does both. Although ablation of T-bet⁺Foxp3⁺ cells during Toxoplasma infection slightly enhances Th1 immune responses, it does not affect the course of the infection. Collectively, the intersectional genetic method reveals the specific roles of T-bet⁺Foxp3⁺ cells in suppressing tumor immunity.

Loss-of-function manipulations to identify roles of diverse glia and stromal cells during CNS scar formation

Scar formation is the replacement of parenchymal cells by stromal cells and fibrotic extracellular matrix. Until as recently as 25 years ago, little was known about the major functional contributions of different neural and non-neural cell types in the formation of scar tissue and tissue fibrosis in the CNS. Concepts about CNS scar formation are evolving rapidly with the availability of different types of loss-of-function technologies that allow mechanistic probing of cellular and molecular functions in models of CNS disorders in vivo. Such loss-of-function studies are beginning to reveal that scar formation and tissue fibrosis in the CNS involves complex interactions amongst multiple types of CNS glia and non-neural stromal cells. For example, attenuating functions of the CNS resident glial cells, astrocytes or microglia, can disrupt the formation of limitans borders that form around stromal cell scars, which leads to increased spread of inflammation, increased loss of neural tissue, and increased fibrosis. Insights are being gained into specific neuropathological mechanisms whereby specific dysfunctions of different types of CNS glia could cause or contribute to disorder-related tissue pathology and dysfunction. CNS glia, as well as fibrosis-producing stromal cells, are emerging as potential major contributors to diverse CNS disorders either through loss- or gain-of-functions, and are thereby emerging as important potential targets for interventions. In this article, we will review and discuss the effects on CNS scar formation and tissue repair of loss-of-function studies targeted at different specific cell types in various disorder models in vivo.

Stem-like cells drive NF1-associated MPNST functional heterogeneity and tumor progression

NF1-associated malignant peripheral nerve sheath tumors (MPNSTs) are the major cause of mortality in neurofibromatosis. MPNSTs arise from benign peripheral nerve plexiform neurofibromas that originate in the embryonic neural crest cell lineage. Using reporter transgenes that label early neural crest lineage cells in multiple NF1 MPNST mouse models, we discover and characterize a rare MPNST cell population with stem-cell-like properties, including quiescence, that is essential for tumor initiation and relapse. Following isolation of these cells, we derive a cancer-stem-cell-specific gene expression signature that includes consensus embryonic neural crest genes and identify Nestin as a marker for the MPNST cell of origin. Combined targeting of cancer stem cells along with antimitotic chemotherapy yields effective tumor inhibition and prolongs survival. Enrichment of the cancer stem cell signature in cognate human tumors supports the generality and relevance of cancer stem cells to MPNST therapy development.

Pituitary Remodeling Throughout Life: Are Resident Stem Cells Involved?

The pituitary gland has the primordial ability to dynamically adapt its cell composition to changing hormonal needs of the organism throughout life. During the first weeks after birth, an impressive growth and maturation phase is occurring in the gland during which the distinct hormonal cell populations expand. During pubertal growth and development, growth hormone (GH) levels need to peak which requires an adaptive enterprise in the GH-producing somatotrope population. At aging, pituitary function wanes which is associated with organismal decay including the somatopause in which GH levels drop. In addition to these key time points of life, the pituitary's endocrine cell landscape plastically adapts during specific (patho-)physiological conditions such as lactation (need for PRL) and stress (engagement of ACTH). Particular resilience is witnessed after physical injury in the (murine) gland, culminating in regeneration of destroyed cell populations. In many other tissues, adaptive and regenerative processes involve the local stem cells. Over the last 15 years, evidence has accumulated that the pituitary gland houses a resident stem cell compartment. Recent studies propose their involvement in at least some of the cell remodeling processes that occur in the postnatal pituitary but support is still fragmentary and not unequivocal. Many questions remain unsolved such as whether the stem cells are key players in the vivid neonatal growth phase and whether the decline in pituitary function at old age is associated with decreased stem cell fitness. Furthermore, the underlying molecular mechanisms of pituitary plasticity, in particular the stem cell-linked ones, are still largely unknown. Pituitary research heavily relies on transgenic in vivo mouse models. While having proven their value, answers to pituitary stem cell-focused questions may more diligently come from a novel powerful in vitro research model, termed organoids, which grow from pituitary stem cells and recapitulate stem cell phenotype and activation status. In this review, we describe pituitary plasticity conditions and summarize what is known on the involvement and phenotype of pituitary stem cells during these pituitary remodeling events.

Proliferating osteoblasts are necessary for maximal bone anabolic response to loading in mice

Following mechanical loading, osteoblasts may arise via activation, differentiation, or proliferation to form bone. Our objective was to ablate proliferating osteoblast lineage cells in order to investigate the importance of these cells as a source for loading-induced bone formation. We utilized 3.6Col1a1-tk mice in which replicating osteoblast lineage cells can be ablated in an inducible manner using ganciclovir (GCV). Male and female mice were aged to 5- and 12-months and subjected to 5 days of tibial compression. "Experimental" mice were tk-positive, treated with GCV; "control" mice were either tk-negative treated with GCV, or tk-positive treated with PBS. We confirmed that experimental mice had a decrease in tk-positive cells that arose from proliferation. Next, we assessed bone formation after loading to low (7N) and high (11N) forces and observed that periosteal bone formation rate in experimental mice was reduced by approximately 70% for both forces. Remarkably, woven bone formation induced by high-force loading was blocked in experimental mice. Loading-induced lamellar bone formation was diminished but not prevented in experimental mice. We conclude that osteoblast proliferation induced by mechanical loading is a critical source of bone forming osteoblasts for maximal lamellar formation and is essential for woven bone formation.

Multiplex Genetic Engineering Exploiting Pyrimidine Salvage Pathway-Based Endogenous Counterselectable Markers

Selectable markers are indispensable for genetic engineering, yet their number and variety are limited. Most selection procedures for prototrophic cells rely on the introduction of antibiotic resistance genes. New minimally invasive tools are needed to facilitate sophisticated genetic manipulations. Here, we characterized three endogenous genes in the human fungal pathogen Aspergillus fumigatus for their potential as markers for targeted genomic insertions of DNAs of interest (DOIs). Since these genes are involved in uptake and metabolization of pyrimidines, resistance to the toxic effects of prodrugs 5-fluorocytosine and 5-fluorouracil can be used to select successfully integrated DOIs. We show that DOI integration, resulting in the inactivation of these genes, caused no adverse effects with respect to nutrient requirements, stress resistance, or virulence. Beside the individual use of markers for site-directed integration of reporter cassettes, including the 17-kb penicillin biosynthetic cluster, we demonstrate their sequential use by inserting three genes encoding fluorescent proteins into a single strain for simultaneous multicolor localization microscopy. In addition to A. fumigatus, we validated the applicability of this novel toolbox in Penicillium chrysogenum and Fusarium oxysporum Enabling multiple targeted insertions of DOIs without the necessity for exogenous markers, this technology has the potential to significantly advance genetic engineering.IMPORTANCE This work reports the discovery of a novel genetic toolbox comprising multiple, endogenous selectable markers for targeted genomic insertions of DNAs of interest (DOIs). Marker genes encode proteins involved in 5-fluorocytosine uptake and pyrimidine salvage activities mediating 5-fluorocytosine deamination as well as 5-fluorouracil phosphoribosylation. The requirement for their genomic replacement by DOIs to confer 5-fluorocytosine or 5-fluorouracil resistance for transformation selection enforces site-specific integrations. Due to the fact that the described markers are endogenously encoded, there is no necessity for the exogenous introduction of commonly employed markers such as auxotrophy-complementing genes or antibiotic resistance cassettes. Importantly, inactivation of the described marker genes had no adverse effects on nutrient requirements, growth, or virulence of the human pathogen Aspergillus fumigatus Given the limited number and distinct types of selectable markers available for the genetic manipulation of prototrophic strains such as wild-type strains, we anticipate that the proposed methodology will significantly advance genetic as well as metabolic engineering of fungal species.

Depletion of Bone Marrow-Derived Fibrocytes Attenuates TAA-Induced Liver Fibrosis in Mice

Bone marrow-derived fibrocytes (FC) represent a unique cell type, sharing features of both mesenchymal and hematopoietic cells. FC were shown to specifically infiltrate the injured liver and participate in fibrogenesis. Moreover, FC exert a variety of paracrine functions, thus possibly influencing the disease progression. However, the overall contribution of FC to liver fibrosis remains unclear. We aimed to study the effect of a specific FC depletion, utilizing a herpes simplex virus thymidine kinase (HSV-TK)/Valganciclovir suicide gene strategy. Fibrosis was induced by oral thioacetamide (TAA) administration in C57BL/6J mice. Hepatic hydroxyproline content was assessed for the primary readout. The HSV-TK model enabled the specific depletion of fibrocytes. Hepatic hydroxyproline content was significantly reduced as a result of the fibrocyte ablation (−7.8%; 95% CI: 0.7–14.8%; p = 0.033), denoting a reduced deposition of fibrillar collagens. Lower serum alanine transaminase levels (−20.9%; 95% CI: 0.4–36.9%; p = 0.049) indicate a mitigation of liver-specific cellular damage. A detailed mode of action, however, remains yet to be identified. The present study demonstrates a relevant functional contribution of fibrocytes to chronic toxic liver fibrosis, contradicting recent reports. Our results emphasize the need to thoroughly study the biology of fibrocytes in order to understand their importance for hepatic fibrogenesis.

Cellular mechanisms underlying cardiac engraftment of stem cells

INTRODUCTION

Over the past decade, it has become clear that long-term engraftment of any ex vivo expanded cell product transplanted into injured myocardium is modest and all therapeutic regeneration is mediated by stimulation of endogenous repair rather than differentiation of transplanted cells into working myocardium. Given that increasing the retention of transplanted cells boosts myocardial function, focus on the fundamental mechanisms limiting retention and survival of transplanted cells may enable strategies to help to restore normal cardiac function. Areas covered: This review outlines the challenges confronting cardiac engraftment of ex vivo expanded cells and explores means of enhancing cell-mediated repair of injured myocardium. Expert opinion: Stem cell therapy has already come a long way in terms of regenerating damaged hearts though the poor retention of transplanted cells limits the full potential of truly cardiotrophic cell products. Multifaceted strategies directed towards fundamental mechanisms limiting the long-term survival of transplanted cells will be needed to enhance transplanted cell retention and cell-mediated repair of damaged myocardium for cardiac cell therapy to reach its full potential.

Gene and virotherapy for hematological malignancies

Recent years have seen a transformation in the treatment of hematological malignancies. Advances in gene therapy and molecular techniques and significant gains in computational abilities have supported the rapid development of safer and better tolerated therapies for many patients with hematologic cancers. In this review, we discuss novel applications of gene therapy, including immunomodulation and gene silencing, and report on the rise of oncolytic viruses for use in the treatment of malignancies arising in cells of the blood, lymph, and marrow. We discuss the relationship of the tropism of wildtype viruses and their oncolytic behavior as well as the tumoricidal and immunostimulatory properties of a number of attenuated and recombinant viruses currently in clinical development in countries around the world. While we have focused on promising virotherapy applications for future development, we also present a historical perspective and identify areas of potential clinical and regulatory practice change. We outline several of the virus systems being developed for applications in hematology, and summarize efficacy data in the context of ongoing or future human clinical testing. We also present the advantages and limitations of gene and virus therapy, including challenges and opportunities for improved treatment tolerability and outcomes for patients with hematologic malignancies.

A Transgenic Rat for Specifically Inhibiting Adult Neurogenesis

The growth of research on adult neurogenesis and the development of new models and tools have greatly advanced our understanding of the function of newborn neurons in recent years. However, there are still significant limitations in the ability to identify the functions of adult neurogenesis in available models. Here we report a transgenic rat (TK rat) that expresses herpes simplex virus thymidine kinase in GFAP+ cells. Upon treating TK rats with the antiviral drug valganciclovir, granule cell neurogenesis can be completely inhibited in adulthood, in both the hippocampus and olfactory bulb. Interestingly, neurogenesis in the glomerular and external plexiform layers of the olfactory bulb was only partially inhibited, suggesting that some adult-born neurons in these regions derive from a distinct precursor population that does not express GFAP. Within the hippocampus, blockade of neurogenesis was rapid and nearly complete within 1 week of starting treatment. Preliminary behavioral analyses indicate that general anxiety levels and patterns of exploration are generally unaffected in neurogenesis-deficient rats. However, neurogenesis-deficient TK rats showed reduced sucrose preference, suggesting deficits in reward-related behaviors. We expect that TK rats will facilitate structural, physiological, and behavioral studies that complement those possible in existing models, broadly enhancing understanding of the function of adult neurogenesis.

Genetic cell ablation

Targeted cell ablation has proven to be a valuable approach to study in vivo cell functions during organogenesis, tissue homeostasis, and regeneration. Over the last two decades, various approaches have been developed to refine the control of cell ablation. In this review, we give an overview of the distinct genetic tools available for targeted cell ablation, with a particular emphasis on their respective specificity.

Arginine transcriptional response does not require inositol phosphate synthesis

Inositol phosphates are key signaling molecules affecting a large variety of cellular processes. Inositol-polyphosphate multikinase (IPMK) is a central component of the inositol phosphate biosynthetic routes, playing essential roles during development. IPMK phosphorylates inositol 1,4,5-trisphosphate to inositol tetrakisphosphate and subsequently to inositol pentakisphosphate and has also been described to function as a lipid kinase. Recently, a catalytically inactive mammalian IPMK was reported to be involved in nutrient signaling by way of mammalian target of rapamycin and AMP-activated protein kinase. In yeast, the IPMK homologue, Arg82, is the sole inositol-trisphosphate kinase. Arg82 has been extensively studied as part of the transcriptional complex regulating nitrogen sensing, in particular arginine metabolism. Whether this role requires Arg82 catalytic activity has long been a matter of contention. In this study, we developed a novel method for the real time study of promoter strength in vivo and used it to demonstrate that catalytically inactive Arg82 fully restored the arginine-dependent transcriptional response. We also showed that expression in yeast of catalytically active, but structurally very different, mammalian or plant IPMK homologue failed to restore arginine regulation. Our work indicates that inositol phosphates do not regulate arginine-dependent gene expression.

Transgenic techniques for cell ablation or molecular deletion to investigate functions of astrocytes and other GFAP-expressing cell types

Genetic tools are enabling the molecular dissection of the functions and mechanisms of many biological processes. Transgenic manipulations provide powerful tools with which to test hypotheses regarding functions of specific cell types and molecules in vivo in combination with different types of experimental models. Various techniques are available that can target genetic manipulations specifically to astrocytes and that are enabling the molecular dissection of astrocyte biology in vivo. This article summarizes procedures and experience from our laboratory using transgenic strategies that enable either the ablation of proliferating astrocytes and related cells, or the deletion of specific molecules selectively from astrocytes, to study the functions of astrocytes and related cell types in health and disease in vivo using different experimental mouse models.

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.

Oligodendrocytes as regulators of neuronal networks during early postnatal development

Oligodendrocytes are the glial cells responsible for myelin formation. Myelination occurs during the first postnatal weeks and, in rodents, is completed during the third week after birth. Myelin ensures the fast conduction of the nerve impulse; in the adult, myelin proteins have an inhibitory role on axon growth and regeneration after injury. During brain development, oligodendrocytes precursors originating in multiple locations along the antero-posterior axis actively proliferate and migrate to colonize the whole brain. Whether the initial interactions between oligodendrocytes and neurons might play a functional role before the onset of myelination is still not completely elucidated. In this article, we addressed this question by transgenically targeted ablation of proliferating oligodendrocytes during cerebellum development. Interestingly, we show that depletion of oligodendrocytes at postnatal day 1 (P1) profoundly affects the establishment of cerebellar circuitries. We observed an impressive deregulation in the expression of molecules involved in axon growth, guidance and synaptic plasticity. These effects were accompanied by an outstanding increase of neurofilament staining observed 4 hours after the beginning of the ablation protocol, likely dependent from sprouting of cerebellar fibers. Oligodendrocyte ablation modifies localization and function of ionotropic glutamate receptors in Purkinje neurons. These results show a novel oligodendrocyte function expressed during early postnatal brain development, where these cells participate in the formation of cerebellar circuitries, and influence its development.

Identification and mapping of ten new potential insulators in the FXYD5-COX7A1 region of human chromosome 19q13.12

A positive-negative selection system revealed 10 potential insulators able to block enhancer interaction with promoter in the 10(6) bp human chromosome 19 region between genes FXYD5 and COX7A1. Relative positions of insulators and genes are in accord with the hypothesis that insulators subdivide genomic DNA into independently regulated loop domains.

Schwann cell influence on motor neuron regeneration accuracy

Extensive peripheral nerve injuries can result in the effective paralysis of the entire limb or distal portions of the limb. The major determinant of functional recovery after lesions in the peripheral nervous system is the accurate regeneration of axons to their original target end-organs. We used the mouse femoral nerve as a model to study motor neuron regeneration accuracy in terms of regenerating motor neurons projecting to their original terminal pathway to quadriceps muscle vs. the inappropriate pathway to skin. Using a variety of surgical manipulations and the selective removal of Schwann cells in the distal nerve via molecular targeting, we have examined the respective roles of end-organ influence (muscle) vs. Schwann cells in this model system. We found evidence of a hierarchy of trophic support that regulates motor neuron regeneration accuracy with muscle contact being the most potent, followed by the number or density of Schwann cells in the distal nerve branches. Manipulating the relative levels of these sources of influence resulted in predictable projection patterns of motor neurons into the terminal pathway either to skin or to muscle.

Analysis of beta cell proliferation dynamics in zebrafish

Among the different mechanisms invoked to explain the beta cell mass expansion during postnatal stages and adulthood, self-replication is being considered the major cellular event occurring both under physiological conditions and in regenerating pancreas after partial pancreactomy. Neogenesis, i.e. differentiation from pancreatic progenitors, has been demonstrated to act concurrently with beta cell replication during pancreatic regeneration. Both phenomena have been largely elucidated in higher vertebrates (mouse, rat and guinea pig), but an extensive description of beta cell dynamics in other animal models is currently lacking. We, therefore, explored in zebrafish the cellular origins of new beta cells in both adult and larval stages. By integrating the results from in vivo time lapse analysis and immunostaining, we provide a detailed reconstruction of the major processes involved in fish beta cell genesis and maintenance. Moreover, by establishing the selective ablation of proliferating beta cells, through the ganciclovir-HSVTK system, we could show that in larval stages self-replication is the main mechanism of beta cells expansion. Since the same mechanism of proliferation has been observed to occur during early and late life stages, we suggest that zebrafish larvae can be used as an alternative tool for an in vivo exploration and screening of new potential mitogens specifically targeting beta cells.

Intermittent PTH stimulates periosteal bone formation by actions on post-mitotic preosteoblasts

Intermittent administration of parathyroid hormone (PTH) stimulates bone formation on the surface of cancellous and periosteal bone by increasing the number of osteoblasts. Previous studies of ours in mice demonstrated that intermittent PTH increases cancellous osteoblast number at least in part by attenuating osteoblast apoptosis, but the mechanism responsible for the anabolic effect of the hormone on periosteal bone is unknown. We report that daily injections of 100 ng/g of PTH(1-34) to 4-6 month old mice increased the number of osteoblasts on the periosteum of lumbar vertebrae by 2-3 fold as early as after 2 days. However, the prevalence of apoptotic periosteal osteoblasts was only 0.2% in vehicle treated animals, which is approximately 20-fold lower than is the case for cancellous osteoblasts. Moreover, PTH did not have a discernable effect on periosteal osteoblast apoptosis. Administration of BrdU for 4 days failed to label periosteal osteoblasts under either basal conditions or following administration of PTH. Cancellous osteoblasts, on the other hand, were labeled under basal conditions, but PTH did not increase the percentage of BrdU-positive cells. Thus, intermittent PTH does not increase cancellous or periosteal osteoblast number by stimulating the proliferation of osteoblast progenitors. Consistent with high turnover of cancellous osteoblasts as compared to that of periosteal osteoblasts, ganciclovir-induced ablation of replicating osteoblast progenitors in mice expressing thymidine kinase under the control of the 3.6 kb rat Col1A1 promoter resulted in disappearance of osteoblasts from cancellous bone over a 7-14 day period, whereas periosteal osteoblasts were unaffected. However, 14 days of pre-treatment with ganciclovir prevented PTH anabolism on periosteal bone. We conclude that in cancellous bone, attenuation of osteoblast apoptosis by PTH increases osteoblast number because their rate of apoptosis is high, making this effect of the hormone profound. However, in periosteal bone where the rate of osteoblast apoptosis is low, PTH must exert pro-differentiating and/or pro-survival effects on post-mitotic pre-osteoblasts. Targeting the latter cells is an effective mechanism for increasing osteoblast number in periosteal bone where the production of osteoblasts from replicating progenitors is slow.

Nitroreductase-mediated cell ablation in transgenic zebrafish embryos

Prodrug dependent cell ablation is a method that allows inducible and spatially restricted cell destruction. We describe transgenic methods to express the Escherichia coli nfsB in a tissue restricted manner in the zebrafish. This bacterial gene encodes a nitroreductase (NTR) enzyme that can render prodrugs such as metronidazole (Met) cytotoxic. Using the expression of NTR fused to a fluorescent protein, one can simultaneously make cells susceptible to prodrug treatment and visualize cell ablation as it occurs.

Probing in vivo dendritic cell functions by conditional cell ablation

Dendritic cells (DCs) play a central role in T-cell activation and the control of the inherent autoreactivity of the T-cell compartment. Pleiotropic DC functions are likely associated with discrete DC subsets. However, the latter remain largely defined by phenotype and unique anatomic location, rather than function. The investigation of DC involvement in complex phenomena that rely on multicellular interactions, such as immuno-stimulation and tolerization calls for an assessment of DC functions within physiological context. Given the highly dynamic DC compartment, the method of choice to study in vivo DC functions is their conditional ablation in the intact organism. Here, we summarize the recent progress in this field highlighting pitfalls and prospects of the approach.

Conditional transgenic mouse models: from the basics to genome-wide sets of knockouts and current studies of tissue regeneration

Many mouse models are currently available, providing avenues to elucidate gene function and to recapitulate specific pathological conditions. To a large extent, successful translation of clinical evidence or analytical data into appropriate mouse models is possible through progress in transgenic or gene-targeting technology. Beginning with a review of standard mouse transgenics and conventional gene targeting, this article will move on to discussing the basics of conditional gene expression: the tetracycline (tet)-off and tet-on systems based on the transactivators tet-controlled transactivator (Tta) and reverse tet-on transactivator (rtTA) that allow downregulation or induction of gene expression; Cre or Flp recombinase-mediated modifications, including excision, inversion, insertion and interchromosomal translocation; combination of the tet and Cre systems, permitting inducible knockout, reporter gene activation or activation of point mutations; the avian retroviral system based on delivery of rtTA specifically into cells expressing the avian retroviral receptor, which enables cell type-specific, inducible gene expression; the tamoxifen system, one of the most frequently applied steroid receptor-based systems, allows rapid activation of a fusion protein between the gene of interest and a mutant domain of the estrogen receptor, whereby activation does not depend on transcription; and techniques for cell type-specific ablation. The diphtheria toxin receptor system offers the advantage that it can be combined with the ‘zoo’ of Cre recombinase driver mice. Having described the basics we move on to the cutting edge: generation of genome-wide sets of conditional knockout mice. To this end, large ongoing projects apply two strategies: gene trapping based on random integration of trapping vectors into introns leading to truncation of the transcript, and gene targeting, representing the directed approach using homologous recombination. It can be expected that in the near future genome-wide sets of such mice will be available. Finally, the possibilities of conditional expression systems for investigating gene function in tissue regeneration will be illustrated by examples for neurodegenerative disease, liver regeneration and wound healing of the skin.

Ablation of central nervous system progenitor cells in transgenic rats using bacterial nitroreductase system

Specific ablation of central nervous system (CNS) progenitor cells in the brain of live animals is a powerful method to determine the functions of these cells and to reveal novel avenues for the treatment of several CNS-related disorders. To achieve this goal, we generated a line of transgenic rats expressing a bacterial enzyme, Escherichia coli nitroreductase gene (NTR), under control of the nestin promoter. In this system, NTR(+) cells are selectively eliminated upon application of prodrug CB1954, through activation of programmed cell death machineries. At 5 days of age, which is a time when cerebellar development is occurring, transgenic rats bearing the nestin-NTR/green fluorescent protein (GFP) gene are overtly normal and express NTR/GFP in neuronal stem cells, without any toxicity in these cells. The functional consequence of progenitor cell ablation was demonstrated by administering prodrug CB1954 into the cerebellum at this 5-day time point. Stem cell ablation in these neonates resulted in sensorimotor abnormalities, cerebellar degeneration, overall reduction in cerebellar seize, and manifestation of ataxia. In adult rats, GFP expression was not seen in the hippocampal progenitor cells and seen only at very low levels in the lateral ventricles, indicating a different NTR/GFP expression pattern between neonates and adults. In addition, application of CB1954 by intraventricular delivery reduced the number of 5-bromo-2'-deoxyuridine-labeled proliferating cells in the lateral ventricle but not hippocampus of NTR/GFP rats. These findings shows that targeted expression of NTR under a specific promoter might be of significant value in addressing the function of distinct cell population in vivo.

Targeted cell killing by reconstituted caspases

We have developed a two-component system involving reconstituted caspase (recCaspase) for selective and/or conditional ablation of targeted cells. Caspases, the executioners of programmed cell death, are normally synthesized as inactive zymogens and are activated by proteolytic processing of their subunits. We show here, using two different caspases, Caenorhabditis elegans CED-3 and human Caspase-3, that coexpression of the subunits generates constitutively active caspase activity that leads to cell death. This recCaspase activity, however, occurred only when the subunits associated through binding of linked antiparallel leucine-zipper domains. We exploited the dual-component nature of recCaspases by expressing the individual subunits from combinations of promoters either to target selectively the subset of cells for apoptosis or induce cell death in specific cells at specific times during development. The high degree of target specificity and tight regulation of induction of recCaspase would be advantageous in creating animal models that are ablated for specific cells and in other targeted cell killings.

Mouse models to elucidate the functional roles of adenosine-to-inosine editing

The conversion of adenosine to inosine (A-to-I) by RNA editing is a widespread RNA processing event by which genomically encoded sequences are altered through site-specific deamination of adenosine residue(s) in RNA transcripts through the actions of a family of double-stranded RNA-specific adenosine deaminases (ADARs). While significant advances have been made regarding the functional consequences of A-to-I editing using heterologous expression systems, the physiological relevance of such RNA modifications in mammals has been addressed effectively using gene-targeting strategies in mice via homologous recombination in embryonic stem (ES) cells. These gene-targeting approaches have allowed the generation of mutant mouse strains in which site-specific editing events can be fixed in the fully edited or nonedited state for individual ADAR targets, expression of ADAR proteins can be selectively ablated, or a combination of ADAR elimination and ADAR target modification can be used for a more in-depth understanding of the biological consequences of A-to-I editing dysregulation.

Identification, genome mapping, and CTCF binding of potential insulators within the FXYD5-COX7A1 locus of human chromosome 19q13.12

Identification of insulators is one of the most difficult problems in functional mapping of genomes. For this reason, up to now only a few insulators have been described. In this article we suggest an approach that allows direct isolation of insulators by a simple positive-negative selection based on blocking enhancer effects by insulators. The approach allows selection of fragments capable of blocking enhancers from mixtures of genomic fragments prepared from up to 1-Mb genomic regions. Using this approach, a 1-Mb human genome locus was analyzed and eight potential insulators were selected. Five of the eight sequences were positioned in intergenic regions and two were within introns. The genes of the alpha-polypeptide H+/K+ exchanging ATPase (ATP4A) and amyloid beta (A4) precursor-like protein 1 (APLP1) within the locus studied were found to be flanked by insulators on both sides. Both genes are characterized by distinct tissue-specific expression that differs from the tissue specificity of the surrounding genes. The data obtained are consistent with the conception that insulators subdivide genomic DNA into loop domains that comprise genes characterized by similar expression profiles. Using chromatin immunoprecipitation assay, we demonstrated also that at least six of the putative insulators revealed in this work could bind the CTCF transcription factor in vivo. We believe that the proposed approach could be a useful instrument for functional analysis of genomes.

StpC-based gene therapy targeting latent reservoirs of HIV-1

The ability of HIV-1 to form latent reservoirs presents a major obstacle to eradication. One approach to elimination of the latent reservoir is induction therapy, whereby cells harboring latent virus are activated and therefore initiate virus replication. We have constructed a lentiviral vector encoding Herpesvirus saimiri subgroup C saimiri transformation-associated protein (StpC), which has been shown to modulate HIV-1 replication, under the control of a cytomegalovirus promoter in order to determine the ability of StpC to upregulate latent HIV-1. We have included a suicide gene, herpes simplex virus thymidine kinase (TK), under the control of the HIV-1 long terminal repeat (LTR) promoter. We hypothesized that upon StpC expression in latently infected cells induction of virus replication and subsequent production of viral transactivators of the LTR will activate expression of the tk gene, sensitizing the cells to the nucleoside analogue ganciclovir (GCV). Transduction of the latently infected cell line J1.1 resulted in increased virus replication. In the presence of GCV transduced cells exhibited decreased HIV-1 replication, inhibition of cell proliferation, and increased apoptosis. This prototype vector serves as a proof of concept of the utility of gene-based induction agents and suicide genes as a new method for targeting reservoirs of latent HIV-1.

Stem cells in the hair follicle bulge contribute to wound repair but not to homeostasis of the epidermis

The discovery of long-lived epithelial stem cells in the bulge region of the hair follicle led to the hypothesis that epidermal renewal and epidermal repair after wounding both depend on these cells. To determine whether bulge cells are necessary for epidermal renewal, here we have ablated these cells by targeting them with a suicide gene encoding herpes simplex virus thymidine kinase (HSV-TK) using a Keratin 1-15 (Krt1-15) promoter. We show that ablation leads to complete loss of hair follicles but survival of the epidermis. Through fate-mapping experiments, we find that stem cells in the hair follicle bulge do not normally contribute cells to the epidermis which is organized into epidermal proliferative units, as previously predicted. After epidermal injury, however, cells from the bulge are recruited into the epidermis and migrate in a linear manner toward the center of the wound, ultimately forming a marked radial pattern. Notably, although the bulge-derived cells acquire an epidermal phenotype, most are eliminated from the epidermis over several weeks, indicating that bulge stem cells respond rapidly to epidermal wounding by generating short-lived 'transient amplifying' cells responsible for acute wound repair. Our findings have implications for both gene therapy and developing treatments for wounds because it will be necessary to consider epidermal and hair follicle stem cells as distinct populations.

Production of CETD transgenic mouse line allowing ablation of any type of specific cell population

Diphtheria toxin A-chain (DT-A) is a potent inhibitor of protein synthesis. As little as a single molecule of DT-A can result in cell death. DT-A gene driven by a tissue-specific promoter is used to achieve genetic ablation of a particular cell lineage. However, this transgenic approach often results in aberrant depletion of unrelated cells. To avoid this, we established a method for specific depletion of a cell population by controlled expression of the DT-A gene via the Cre-loxP system. We produced five transgenic mice carrying CETD construct containing loxP-flanked enhanced green fluorescent protein (EGFP) cDNA and the DT-A gene. Transfection of primary cultured cells derived from CETD transgenic fetus with Cre expression plasmid resulted in extensive cell loss, as expected. Bigenic (double transgenic) offspring obtained by crossbreeding between CETD and MNCE transgenic mice in which Cre expression is controlled by the myelin basic protein (MBP) promoter exhibited embryonic lethality, suggesting expression of Cre at embryonic stages. Intravenous injection of Cre expression vector to CETD mice led to generation of glomerular lesions, probably due to predominant depletion of glomerular epithelial cells. This Cre-loxP-based cell ablation technology is powerful and convenient method of generating mice lacking any chosen cell population.

A Cre-inducible diphtheria toxin receptor mediates cell lineage ablation after toxin administration

A new system for lineage ablation is based on transgenic expression of a diphtheria toxin receptor (DTR) in mouse cells and application of diphtheria toxin (DT). To streamline this approach, we generated Cre-inducible DTR transgenic mice (iDTR) in which Cre-mediated excision of a STOP cassette renders cells sensitive to DT. We tested the iDTR strain by crossing to the T cell- and B cell-specific CD4-Cre and CD19-Cre strains, respectively, and observed efficient ablation of T and B cells after exposure to DT. In MOGi-Cre/iDTR double transgenic mice expressing Cre recombinase in oligodendrocytes, we observed myelin loss after intraperitoneal DT injections. Thus, DT crosses the blood-brain barrier and promotes cell ablation in the central nervous system. Notably, we show that the developing DT-specific antibody response is weak and not neutralizing, and thus does not impede the efficacy of DT. Our results validate the use of iDTR mice as a tool for cell ablation in vivo.

Gene-directed enzyme prodrug therapy: a review of enzyme/prodrug combinations

Gene-directed enzyme prodrug therapy (GDEPT) is a promising, new, two-step treatment for cancer chemotherapy. In the first step, the gene for a foreign enzyme is administered and is directed to the tumour, where it is expressed by the use of specific promoters. In the second step, injected prodrugs are activated by the foreign enzyme. The design and synthesis of prodrugs able to undergo enzymatic activation in such systems is an essential component. This review focuses on the requirements which must be fulfilled by the components of GDEPT systems in order for this therapy to be considered a realistic possibility. A special emphasis is placed on the description of the prodrugs used in GDEPT protocols and the requirement for a bystander effect is also discussed.

Adenoviral vectors: development and application

One of the prerequisites for the successful application of gene vaccination and therapy is the development of efficient gene delivery vectors. The rate-limiting nature of vectors was clearly manifested during the first wave of gene therapy testing, resulting in the demand for more effective and suitable vector systems. Adenoviral (Ad) vectors have recently played a central role in the development of gene-vector technology due to their practical advantages and potential applications. A large number of preclinical and clinical studies both have generated an overwhelming amount of data and literature on this vector system. It is the intention of this article to provide a systematic and broad spectrum review of this system, outlining the principle, potential, and limitations, and evaluating the rational development of this delivery approach. Recombinant adenoviruses (Ad), helper cell lines, and related technologies have been developed and applied to many indications owing to progress in virological research, molecular and cellular biology, eukaryotic protein expression, recombinant vaccines, and gene therapy. The technical depth this article covers should be useful to both the experienced researcher and to beginners in this field.

Targeting enzymes to cancers - new developments

Two methods of using tumour located enzymes have been described. These are antibody directed enzyme prodrug therapy (ADEPT) and macromolecule directed enzyme prodrug therapy (MDEPT), where the tumour located enzyme converts a non-toxic prodrug into a cytotoxic drug at tumour sites. The alternative use of tumour located enzymes is to inactivate rescue agents that protect cells from antimetabolite action, and is described as 'Antimetabolite with inactivation of rescue agent at cancer sites' (AMIRACS). The leakiness of tumour blood vessels and poor lymphatic drainage allows enzymes to be targeted to many cancers by attachment to polymeric macromolecules (MDEPT), as well as to antibodies and antibody fragments (ADEPT). To avoid systemic toxicity, enzyme activity in blood and normal tissues must be very low before giving a prodrug or rescue agent. Antibodies directed against the enzyme component of macromolecular conjugates have proved to be very efficient at clearing normal tissues. Human enzymes which are over expressed by cancer cells can be exploited particularly if they require co-factors or co-substrates, either in situ or targeted to extracellular sites. Bacterial enzymes have advantages in specificity but require some form of immunological control in view of their immunogenicity. Prodrugs which generate drugs with very short half lives are desirable, and have been developed, including one which has a differential toxicity between prodrug and the active drug of 1000 to 10,000 fold. The range of antimetabolites available for AMIRACS was initially restricted to inhibitors of dihydrofolate reductase but has been greatly extended by the introduction of inhibitors of other enzymes. The limitations of these systems are discussed.

Bigenic Cre/loxP, puDeltatk conditional genetic ablation

Genetic ablation experiments are used to resolve problems regarding cell lineages and the in vivo function of certain groups of cells. We describe a two-component conditional ablation technology using a mouse carrying an X-linked puDeltatk transgene, which is only activated in cells expressing Cre. Ablation of the Cre-expressing cells can be temporally regulated by the time of ganciclovir (GCV) administration. This strategy was demonstrated using a Col2Cre transgenic line. Differentiating chondrocytes in bigenic animals could be ablated at different developmental stages resulting in disorganized growth plates and dwarfism. Macrocephaly, macroglossia and umbilical hernia were also observed in ablated 18.5 dpc embryos. Crosses between the puDeltatk selector transgenic line and existing cre lines will facilitate numerous temporally regulated tissue-specific ablation experiments.

Suicide gene therapy and the control of graft-vs-host disease

Allogeneic bone marrow transplantation as a cure for leukaemia and lymphoma is limited by the development of graft-vs-host disease (GVHD), an immunological reaction of the donor's T lymphocytes against the host's normal tissues. One therapeutic option to treat GVHD is the transfer of 'suicide' genes into the donor's T lymphocytes to render them susceptible to prodrug administration. This procedure should permit the elimination of unwanted T lymphocytes in GVHD. The main genes proposed for such a strategy will be described in this chapter, together with the advantages and limitations found during preclinical and clinical studies to date.

Induction of cell death in activated hepatic stellate cells by targeted gene expression of the thymidine kinase/ganciclovir system

Liver fibrosis is the result from a relative imbalance between synthesis and degradation of matrix proteins. Following liver injury of any etiology, hepatic stellate cells undergo a response known as activation, which is the transition of quiescent cells into proliferative, fibrogenic, and contractile myofibroblasts. Upon this cellular transdifferentiation the effector cell becomes the major source of fibrillar and non-fibrillar matrix proteins resulting in excessive scar formation and cirrhosis, the end stage of fibrosis. Concomitant with progressive liver fibrosis, the tissue inhibitor of metalloproteinases-1 (TIMP-1) is strongly activated in hepatic stellate cells. We have developed a recombinant replication-defective adenovirus in which the TIMP-1 promoter is coupled to the herpes simplex virus thymidine kinase gene rendering activated hepatic stellate cells susceptible to ganciclovir. This novel targeted suicide gene approach was validated in a culture model considered to reflect an accelerated time course of the cellular and molecular events that occur during liver fibrosis. We demonstrate that transfer of the suicide gene to culture-activated hepatic stellate cells results in a strong expression of the respective transgene as assessed by Northern blot and Western blot analyses. The enzyme catalyzed the proper conversion of its prodrug subsequently initiating programmed cell death as estimated by caspase-3 assay and Annexin V-Fluos staining. Altogether, these results indicate that induction of programmed cell death is a promising approach to eliminate fibrogenic HSC.

Lack of enantioselectivity of herpes virus thymidine kinase allows safer imaging of gene delivery

Herpes simplex virus thymidine kinase (HSV-TK) is widely used in gene therapy. The enzymatic activity of HSV-TK may be traced in vivo by specific radiopharmaceuticals in order to image transgene expression. However, most of these radiopharmaceuticals are toxic per se or after activation by HSV-TK, and therefore do not represent ideal molecules for clinical applications and repeated imaging. Unlike human cytosolic TK, HSV-TK is not enantioselective and can efficiently phosphorylate both D and L enantiomers of beta-thymidine. Here we show that, after phosphorylation by HSV-TK, tritiated L-beta-thymidine (LT) is selectively retained inside the cells in vitro and in vivo. We used the in vivo accumulation of radioactive phosphorylated LT to image the HSV-TK-positive cells inside a transplantable murine brain tumour after inoculation of cells producing retroviruses carrying HSV-TK. Owing to their unnatural enantiomeric conformation, phosphorylated LT metabolites are very poorly processed by mammalian enzymes, thus leading to increased cellular retention and minimal toxicity. The ability to image cells expressing the HSV-TK gene by using radiolabelled LT, without damaging the cells accumulating the phosphorylated L-nucleoside, will be important to monitor the levels and spatial distribution of therapeutic vectors carrying HSV-TK.

Motor training compensates for cerebellar dysfunctions caused by oligodendrocyte ablation

The role played by oligodendrocytes (OLs), the myelinating cells of the CNS, during brain development has not been fully explored. We have addressed this question by inducing a temporal and reversible ablation of OLs on postnatal CNS development. OL ablation in newborn mice leads to a profound alteration in the structure of the cerebellar cortex, which can be progressively rescued by newly generated cells, leading to a delayed myelination. Nevertheless, the temporal shift of the OL proliferation and myelinating program cannot completely compensate for developmental defects, resulting in impaired motor functions in the adult. Strikingly, we show that, despite these abnormalities, epigenetic factors, such as motor training, are able to fully rescue cerebellar-directed motor skills.

Inducible liver injury in the transgenic rat by expressing liver-specific suicide gene

Suicide gene expression in specific tissue of transgenic animals has been used for cell-specific ablation. To examine the influence of hepatocyte removal, we produced the herpes simplex virus thymidine kinase (HSVtk) transgenic rat, whose gene was regulated by an albumin enhancer promoter. The liver presence of HSVtk was demonstrated in one line of the transgenic rats. We injected ganciclovir (GCV, 50mg/kg) into the rat on alternate days. After 28 days of GCV administration, liver tissues, and blood of the rats were collected. The histological investigation revealed infiltration of T cells, macrophages, granulocytes/neutrophils, and hepatocyte cell death. The biochemistry analysis demonstrated elevated levels of AST, ALT, and total bilirubin in transgenic rat. In conclusion, the transgenic rat with expressed albumin-specific HSVtk developed experimental hepatitis with administration of GCV, and will be a useful model to facilitate the evaluation of drug effects for clinical control of liver disease.

Oligodendrocyte ablation impairs cerebellum development

Oligodendrocytes (OLs) are the glial cells of the central nervous system and are classically known to form myelin sheaths around most axons of higher vertebrates. Whether these cells might have other roles, in particular during development, has not been studied. Taking advantage of a transgenic mouse model in which OLs can be selectively killed in a desired time-frame, we have investigated the impact of OL ablation on cerebellar development. OL ablation was induced during the first 3 postnatal weeks, a time at which cerebellum development is ongoing. Strikingly, OL ablation triggers a profound perturbation of the known cerebellum developmental program, characterized by the disorganization of the cortical layers, abnormal foliation and a complete alteration of Purkinje cell dendritic arborization and axonal fasciculation. This phenotype is accompanied by decreased granule cell density, a disorganized Bergmann glia network and impaired migration of interneurons in the molecular layer. These results demonstrate a previously ignored role of OLs in the formation of the cerebellar cytoarchitecture.

An HSV-TK transgenic mouse model to evaluate elimination of fibroblasts for fibrosis therapy

Pathological fibroproliferation after tissue injury is harmful and may lead to organ dysfunction. Unfortunately, fibroproliferative diseases remain intractable to current therapeutic strategies. Thus, new therapeutic approaches are needed. One possible approach is to promote resolution of physiological fibroproliferation that follows injury before it becomes pathological by activating apoptosis selectively in fibrotic lesions. However, it is not known whether selective elimination of fibroblasts will prevent fibrosis or impede repair or worsen injury by eliminating topographic signals essential to organ reconstitution. To address this question, a tractable in vivo model system is needed in which fibroblasts can be targeted to undergo apoptosis at a chosen time and place. We developed transgenic mice expressing HSV-TK from the type I collagen promoter to determine whether selective elimination of fibroblasts actively forming fibrotic lesions is an effective therapeutic strategy for fibroproliferative disorders. The transgene renders fibroblasts actively forming fibrotic tissue susceptible to ganciclovir. To validate the transgenic model we examined whether administration of ganciclovir prevents the development of fibrosis in sponges implanted subcutaneously in the backs of the transgenic mice. We demonstrate that fibroblasts/myofibroblasts isolated from sponges express HSV-TK protein and are selectively ablated by ganciclovir in vitro. In adult transgenic mice, ganciclovir treatment attenuated the development of fibrotic tissue in the sponges both biochemically and histologically. We conclude that this transgenic model system is an ideal approach to determine whether targeted ablation of fibroblasts is an effective therapeutic strategy for fibrotic diseases.

Of mice and models: improved animal models for biomedical research

The ability to engineer the mouse genome has profoundly transformed biomedical research. During the last decade, conventional transgenic and gene knockout technologies have become invaluable experimental tools for modeling genetic disorders, assigning functions to genes, evaluating drugs and toxins, and by and large helping to answer fundamental questions in basic and applied research. In addition, the growing demand for more sophisticated murine models has also become increasingly evident. Good state-of-principle knowledge about the enormous potential of second-generation conditional mouse technology will be beneficial for any researcher interested in using these experimental tools. In this review we will focus on practice, pivotal principles, and progress in the rapidly expanding area of conditional mouse technology. The review will also present an internet compilation of available tetracycline-inducible mouse models as tools for biomedical research (http://www.zmg.uni-mainz.de/tetmouse/).

Viruses in the treatment of brain tumors

The grave outlook for malignant glioma patients in spite of improvements to current modalities has ushered in new approaches to therapy. Viruses have emerged on the scene and gained attention for their ability to play essentially two roles: first, as vectors for therapeutic gene delivery and second, as engineered infectious agents capable of selectively lysing tumor cells. To date, clinical brain tumor trials using viruses for gene delivery have employed retroviral or adenoviral vectors to introduce ganciclovir susceptibility to tumors in the form of the HSV1-TK gene. Clinical oncolytic studies, on the other hand, have evaluated a conditionally replicating HSV as an antineoplastic agent. Despite some promise afforded by these trials, further studies are warranted; the investigation of additional viruses to play these roles is inevitable and is now precedented.

Conditional ablation of T-cell development by a novel viral ion channel transgene

A novel conditional-lethal transgene system is defined in which a mutated influenza A virus ion-channel protein, which is permeable to monovalent cations, is lethal to cells on heterotypic expression and whose activity can be blocked by an antiviral drug (amantadine), is used to reversibly disrupt T-cell development. In vivo expression of the M2 ion channel, as a transgene under control of the T-cell specific p56(Lck) proximal promoter, resulted in total ablation of T-cell development with the accumulation of three distinct populations of early progenitor cells (CD44(+) CD25(-); CD44(+) CD25(+); CD44(+) CD25(hi)) in the thymic rudiment. In vitro development of transgenic fetal thymic progenitors to single-positive T cells could be rescued by antiviral drug treatment. Moreover, there was a radical reduction in B-cell lymphopoiesis, evident at the pre-B-cell stage, with a twofold increase of lymphoid cells 'in cycle' in transgenic bone marrow, indicative of major changes in haematopoietic homeostasis. This system may provide a generic protocol for conditional, lineage-specific cell ablation with available tissue-specific promoters for any eukaryotic developmental system, and provide a window on early T-cell development.

PRL modulates cell cycle regulators in mammary tumor epithelial cells

PRL is essential for normal lobulo-alveolar growth of the mammary gland and may contribute to mammary cancer development or progression. However, analysis of the mechanism of action of PRL in these processes is complicated by the production of PRL within mammary epithelia. To examine PRL actions in a mammary cell-specific context, we selected MCF-7 cells that lacked endogenous PRL synthesis, using PRL stimulation of interferon-gamma-activated sequence-related PRL response elements. Derived clones exhibited a greater proliferative response to PRL than control cells. To understand the mechanism, we examined, by Western analysis, levels of proteins essential for cell cycle progression as well as phosphorylation of retinoblastoma protein. The expression of cyclin D1, a critical regulator of the G1/S transition, was significantly increased by PRL and was associated with hyperphosphorylation of retinoblastoma protein at Ser(780). Cyclin B1 was also increased by PRL. In contrast, PRL decreased the Cip/Kip family inhibitor, p21, but not p16 or p27. These studies demonstrate that PRL can stimulate the cell cycle in mammary epithelia and identify specific targets in this process. This model system will enable further molecular dissection of the pathways involved in PRL-induced proliferation, increasing our understanding of this hormone and its interactions with other factors in normal and pathogenic processes.

Essential role of oligodendrocytes in the formation and maintenance of central nervous system nodal regions

The membrane of myelinated axons is divided into functionally distinct domains characterized by the enrichment of specific proteins. The mechanisms responsible for this organization have not been fully identified. To further address the role of oligodendrocytes in the functional segmentation of the axolemma in vivo, the distribution of nodal (Na+ channels, ankyrin G), paranodal (paranodin/contactin-associated-protein) and juxtaparanodal (Kv1.1 K+ channels) axonal markers, was studied in the brain of MBP-TK and jimpy mice. In MBP-TK transgenic mice, oligodendrocyte ablation was selectively induced by FIAU treatment before and during the onset of myelination. In jimpy mice, oligodendrocytes degenerate spontaneously within the first postnatal weeks after the onset of myelination. Interestingly, in MBP-TK mice treated for 1-20 days with FIAU, despite the ablation of more than 95% of oligodendrocytes, the protein levels of all tested nodal markers was unaltered. Nevertheless, these proteins failed to cluster in the nodal regions. By contrast, in jimpy mice, despite a diffused localization of paranodin, the formation of nodal clusters of Na+ channels and ankyrin G was observed. Furthermore, K+ channels clusters were transiently visible, but were in direct contact with nodal markers. These results demonstrate that the organization of functional domains in myelinated axons is oligodendrocyte dependent. They also show that the presence of these cells is a requirement for the maintenance of nodal and paranodal regions.

Imaging Techniques for Gene Therapy: SPECT, PET, and MRI

Gene therapy by the transfer and expression of suicide genes is performed using genes coding for nonmammalian enzymes that transform nontoxic prodrugs into toxic metabolites. Employing radiolabeled specific substrates and scintigraphic procedures to determine the functional activity of the recombinant enzyme in vivo, a therapeutic window of maximal gene expression and consecutive drug administration may be defined. If the gene therapy approach is based on the transduction of receptor genes, the recombinant gene expression in tumor cells can be monitored with radiolabeled ligands. Transfer of transporter genes as the sodium iodide transporter may also lead to the visualization of transduction via accumulation of iodide or pertechnetate. Furthermore, imaging based on transchelation of oxotechnetate to a polypeptide motif from a biocompatible complex with a higher dissociation constant than that of a diglycilcysteine complex or tyrosinase gene transfer for metal ion scavenging have been described. In addition, therapy effects may be assessed by the evaluation of the morphological changes of the tumor using magnetic resonance imaging or, more effectively, by the measurement of changes in metabolism with positron emission tomography employing tracers of tumor metabolism and proliferation. Finally, enzyme or receptor genes may serve as noninvasive reporter genes, if applied in the context of bicistronic vectors leading to coexpression of the therapeutic gene and the noninvasive reporter gene.