Transgenic mice as probes into complex systems

TH-MYCN tumors, but not tumor-derived cell lines, are adrenergic lineage, GD2+, and responsive to anti-GD2 antibody therapy

Neuroblastoma is a commonly lethal solid tumor of childhood and intensive chemoradiotherapy treatment cures ~50% of children with high-risk disease. The addition of immunotherapy using dinutuximab, a monoclonal antibody directed against the GD2 disialoganglioside expressed on neuroblasts, improves survival when incorporated into front-line therapy and shows robust activity in regressing relapsed disease when combined with chemotherapy. Still, many children succumb to neuroblastoma progression despite receiving dinutuximab-based immunotherapy, and efforts to counteract the immune suppressive signals responsible are warranted. Animal models of human cancers provide useful platforms to study immunotherapies. TH-MYCN transgenic mice are immunocompetent and develop neuroblastomas at autochthonous sites due to enforced MYCN expression in developing neural crest tissues. However, GD2-directed immunotherapy in this model has been underutilized due to the prevailing notion that TH-MYCN neuroblasts express insufficient GD2 to be targeted. We demonstrate that neuroblasts in TH-MYCN-driven tumors express GD2 at levels comparable to human neuroblastomas but rapidly lose GD2 expression when explanted ex vivo to establish tumor cell lines. This occurs in association with a transition from an adrenergic to mesenchymal differentiation state. Importantly, not only is GD2 expression retained on tumors in situ, treatment with a murine anti-GD2 antibody, 14G2a, markedly extends survival in such mice, including durable complete responses. Tumors in 14G2a-treated mice have fewer macrophage and myeloid-derived suppressor cells in their tumor microenvironment. Our findings support the utility of this model to inform immunotherapy approaches for neuroblastoma and potential opportunities to investigate drivers of adrenergic to mesenchymal fate decisions.

Xenograft models for pediatric cancer therapies

The prognosis for childhood cancer has improved considerably over the past 50 years. This improvement is attributed to well-designed clinical trials which have incorporated chemotherapy, surgery, and radiation. With an increased understanding of cancer biology and genetics, we have entered an era of precision medicine and immunotherapy that provides potential for improved cure rates. However, preclinical evaluation of these therapies is more nuanced, requiring more robust animal models. Evaluation of targeted treatments requires molecularly defined xenograft models that can capture the diversity within pediatric cancer. The development of novel immunotherapies ideally involves the use of animal models that can accurately recapitulate the human immune response. In this review, we provide an overview of xenograft models for childhood cancers, review successful examples of novel therapies translated from xenograft models to the clinic, and describe the modern tools of xenograft biobanks and humanized xenograft models for the study of immunotherapies.

A History of Mouse Genetics: From Fancy Mice to Mutations in Every Gene

The laboratory mouse has become the model organism of choice in numerous areas of biological and biomedical research, including the study of congenital birth defects. The appeal of mice for these experimental studies stems from the similarities between the physiology, anatomy, and reproduction of these small mammals with our own, but it is also based on a number of practical reasons: mice are easy to maintain in a laboratory environment, are incredibly prolific, and have a relatively short reproductive cycle. Another compelling reason for choosing mice as research subjects is the number of tools and resources that have been developed after more than a century of working with these small rodents in laboratory environments. As will become obvious from the reading of the different chapters in this book, research in mice has already helped uncover many of the genes and processes responsible for congenital birth malformations and human diseases. In this chapter, we will provide an overview of the methods, scientific advances, and serendipitous circumstances that have made these discoveries possible, with a special emphasis on how the use of genetics has propelled scientific progress in mouse research and paved the way for future discoveries.

Preclinical SPECT and SPECT-CT in Oncology

Molecular imaging enables both spatial and temporal understanding of the complex biologic systems underlying carcinogenesis and malignant spread. Single-photon emission tomography (SPECT) is a versatile nuclear imaging-based technique with ideal properties to study these processes in vivo in small animal models, as well as to identify potential drug candidates and characterize their antitumor action and potential adverse effects. Small animal SPECT and SPECT-CT (single-photon emission tomography combined with computer tomography) systems continue to evolve, as do the numerous SPECT radiopharmaceutical agents, allowing unprecedented sensitivity and quantitative molecular imaging capabilities. Several of these advances, their specific applications in oncology as well as new areas of exploration are highlighted in this chapter.

A preclinical micro-computed tomography database including 3D whole body organ segmentations

The gold-standard of preclinical micro-computed tomography (μCT) data processing is still manual delineation of complete organs or regions by specialists. However, this method is time-consuming, error-prone, has limited reproducibility, and therefore is not suitable for large-scale data analysis. Unfortunately, robust and accurate automated whole body segmentation algorithms are still missing. In this publication, we introduce a database containing 225 murine 3D whole body μCT scans along with manual organ segmentation of most important organs including heart, liver, lung, trachea, spleen, kidneys, stomach, intestine, bladder, thigh muscle, bone, as well as subcutaneous tumors. The database includes native and contrast-enhanced, regarding spleen and liver, μCT data. All scans along with organ segmentation are freely accessible at the online repository Figshare. We encourage researchers to reuse the provided data to evaluate and improve methods and algorithms for accurate automated organ segmentation which may reduce manual segmentation effort, increase reproducibility, and even reduce the number of required laboratory animals by reducing a source of variability and having access to a reliable reference group.

Culture time of vitrified/warmed zygotes before microinjection affects the production efficiency of CRISPR-Cas9-mediated knock-in mice

Robust reproductive engineering techniques are required for the efficient and rapid production of genetically modified mice. We have reported the efficient production of genome-edited mice using reproductive engineering techniques, such as ultra-superovulation, in vitro fertilization (IVF) and vitrification/warming of zygotes. We usually use vitrified/warmed fertilized oocytes created by IVF for microinjection because of work efficiency and flexible scheduling. Here, we investigated whether the culture time of zygotes before microinjection influences the efficiency of producing knock-in mice. Knock-in mice were generated using clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9) system and single-stranded oligodeoxynucleotide (ssODN) or PITCh (Precise Integration into Target Chromosome) system, a method of integrating a donor vector assisted by microhomology-mediated end-joining. The cryopreserved fertilized oocytes were warmed, cultured for several hours and microinjected at different timings. Microinjection was performed with Cas9 protein, guide RNA(s), and an ssODN or PITCh donor plasmid for the ssODN knock-in and the PITCh knock-in, respectively. Different production efficiencies of knock-in mice were observed by changing the timing of microinjection. Our study provides useful information for the CRISPR-Cas9-based generation of knock-in mice.

Summary: We report variable production efficiencies of CRISPR-Cas9-mediated knock-in mice depending on a series of microinjection timings using vitrified, warmed, and cultured zygotes created via ultra-superovulation and in vitro fertilization.

Monte Carlo-based evaluation of S-values in mouse models for positron-emitting radionuclides

In addition to being a powerful clinical tool, Positron emission tomography (PET) is also used in small laboratory animal research to visualize and track certain molecular processes associated with diseases such as cancer, heart disease and neurological disorders in living small animal models of disease. However, dosimetric characteristics in small animal PET imaging are usually overlooked, though the radiation dose may not be negligible. In this work, we constructed 17 mouse models of different body mass and size based on the realistic four-dimensional MOBY mouse model. Particle (photons, electrons and positrons) transport using the Monte Carlo method was performed to calculate the absorbed fractions and S-values for eight positron-emitting radionuclides (C-11, N-13, O-15, F-18, Cu-64, Ga-68, Y-86 and I-124). Among these radionuclides, O-15 emits positrons with high energy and frequency and produces the highest self-absorbed S-values in each organ, while Y-86 emits γ-rays with high energy and frequency which results in the highest cross-absorbed S-values for non-neighbouring organs. Differences between S-values for self-irradiated organs were between 2% and 3%/g difference in body weight for most organs. For organs irradiating other organs outside the splanchnocoele (i.e. brain, testis and bladder), differences between S-values were lower than 1%/g. These appealing results can be used to assess variations in small animal dosimetry as a function of total-body mass. The generated database of S-values for various radionuclides can be used in the assessment of radiation dose to mice from different radiotracers in small animal PET experiments, thus offering quantitative figures for comparative dosimetry research in small animal models.

Transgenic animal models in toxicology: historical perspectives and future outlook

Transgenic animal models are powerful tools for developing a more detailed understanding on the roles of specific genes in biological pathways and systems. Applications of these models have been made within the field of toxicology, most notably for the screening of mutagenic and carcinogenic potential and for the characterization of toxic mechanisms of action. It has long been a goal of research toxicologists to use the data from these models to refine hazard identification and characterization to better inform human health risk assessments. This review provides an overview on the applications of transgenic animal models in the assessment of mutagenicity and carcinogenicity, their use as reporter systems, and as tools for understanding the roles of xenobiotic-metabolizing enzymes and biological receptors in the etiology of chemical toxicity. Perspectives are also shared on the future outlook for these models in toxicology and risk assessment and how transgenic technologies are likely to be an integral tool for toxicity testing in the 21st century.

Modeling prostate cancer: a perspective on transgenic mouse models

Despite considerable success in treatment of early stage localized prostate cancer (PC), acute inadequacy of late stage PC treatment and its inherent heterogeneity poses a formidable challenge. Clearly, an improved understanding of PC genesis and progression along with the development of new targeted therapies are warranted. Animal models, especially, transgenic immunocompetent mouse models, have proven to be the best ally in this respect. A series of models have been developed by modulation of expression of genes implicated in cancer-genesis and progression; mainly, modulation of expression of oncogenes, steroid hormone receptors, growth factors and their receptors, cell cycle and apoptosis regulators, and tumor suppressor genes have been used. Such models have contributed significantly to our understanding of the molecular and pathological aspects of PC initiation and progression. In particular, the transgenic mouse models based on multiple genetic alterations can more accurately address the inherent complexity of PC, not only in revealing the mechanisms of tumorigenesis and progression but also for clinically relevant evaluation of new therapies. Further, with advances in conditional knockout technologies, otherwise embryonically lethal gene changes can be incorporated leading to the development of new generation transgenics, thus adding significantly to our existing knowledge base. Different models and their relevance to PC research are discussed.

Widespread expression of the Supv3L1 mitochondrial RNA helicase in the mouse

Supv3L1 is an evolutionarily conserved helicase that plays a critical role in the mitochondrial RNA surveillance and degradation machinery. Conditional ablation of Supv3L1 in adult mice leads to premature aging phenotypes including loss of muscle mass and adipose tissue and severe skin abnormalities. To get insights into the spatial and temporal expression of Supv3L1 in the mouse, we generated knock-in and transgenic strains in which an EGFP reporter was placed under control of the Supv3L1 native promoter. During development, expression of Supv3L1 begins at the blastocyst stage, becomes widespread and strong in all fetal tissues and cell types, and continues during postnatal growth. In mature animals reporter expression is only slightly diminished in most tissues and continues to be highly expressed in the brain, peripheral sensory organs, and testis. Together, these data confirm that Supv3L1 is an important developmentally regulated gene, which continues to be expressed in all mature tissues, particularly the rapidly proliferating cells of testes, but also in the brain and sensory organs. The transgenic mice and cell lines derived from them constitute a valuable tool for the examination of the spatial and temporal aspects of Supv3L1 promoter activity, and should facilitate future screens for small molecules that regulate Supv3L1 expression.

Ciliary neurotrophic factor recruitment of glucagon-like peptide-1 mediates neurogenesis, allowing immortalization of adult murine hypothalamic neurons

The distinct lack of cell lines derived from the adult brain is evident. Ciliary neurotrophic factor (CNTF) triggers neurogenesis in primary culture from adult mouse hypothalamus, as detected by bromodeoxyuridine and Ki67 immunostaining. Using SV-40 T-antigen, we immortalized dividing neurons and generated clonal cell lines expressing neuropeptides and receptors involved in neuroendocrine function. We hypothesized that proglucagon-derived peptides may be the mechanistic downstream effectors of CNTF due to documented neuroprotective and proliferative effects. Indeed, proglucagon gene expression was induced by CNTF, and exposure of primary cells to glucagon-like peptide-1 receptor (GLP-1) agonist, exendin-4, induced cell proliferation. Intracerebroventricular injection of CNTF into adult mice caused increased expression of proglucagon peptide in the hypothalamus. Using a specific GLP-1-receptor antagonist, we found that neurogenesis was significantly attenuated and primary culture from GLP-1-receptor-knockout mice lacked CNTF-mediated neuronal proliferation, thus linking the induction of neurogenesis in the hypothalamus to GLP-1-receptor signaling.

A spontaneous acinar cell carcinoma model for monitoring progression of pancreatic lesions and response to treatment through noninvasive bioluminescence imaging

PURPOSE

We have generated an EL1-luc/TAg transgenic mouse model that develops spontaneous and bioluminescent acinar cell carcinomas. We applied this model to noninvasively monitor tumor development and drug response.

EXPERIMENTAL DESIGN

EL1-luc/TAg transgenic mice of 11 weeks of age were treated with rapamycin (5 mg/kg, i.p.) or vehicle for 6 to 12 weeks. Tumor development was monitored through bioluminescence imaging and necropsy at the study end point.

RESULTS

EL1-luc/TAg transgenic mice showed pancreas-specific bioluminescence signal before tumor progression and produced increasing light emission from the onset of the pancreatic acinar cell carcinomas. The latency of tumor development ranged from 10 to >20 weeks of age in these mice. Progression of the primary acinar cell carcinoma was accompanied by emergence of metastatic lesions in the abdominal organs, including liver and gastrointestinal fat tissues. Rapamycin treatment suppressed tumor development.

CONCLUSIONS

The EL1-luc/TAg mouse provides a noninvasive approach for monitoring spontaneous acinar cell carcinoma development and comprises a convenient tool for the evaluation of novel therapeutics against pancreatic cancers. Tumor growth suppression through inhibition of the mammalian target of rapamycin pathway further validates this model as clinically relevant.

Limitations of the cancer stem cell theory

Cancer stem cells (CSCs) can be operationally defined as a subset of neoplastic cells which are responsible for the growth and re-growth of primary and metastatic tumors. Although the existence of perpetually dividing cells is a logical necessity to explain the malignant properties of human tumors, experimental data supporting their existence have only recently been obtained. New knowledge in basic stem cell biology and the availability of several cell surface markers for the definition and isolation of small subsets of immature cells coupled to the use of the classical model of xenotransplantation in immune deficient mice has identified putative CSCs in several solid tumors such as mammary, colon, brain, pancreas, prostate, melanoma and others. However, the theory must be considered as still in its infancy, since tumors grown in mice only partially recapitulate the biology of human cells. In addition, whether the "transformed" cell is the neoplastic counterpart of a normal stem cell or whether complete malignant behaviour can occur in a more differentiated cell has still to be demonstrated. In spite of these difficulties, the CSC hypothesis could be of clinical relevance, especially in the definition of new ways to assay drug sensitivity of primary human tumors.

Immortalized hypothalamic gonadotropin-releasing hormone neurons

The neuroendocrine hypothalamus has been intensively studied using whole animals and tissue slices. However, it has been difficult to approach questions at the molecular and cellular level. By targeting expression of the oncogene product, simian virus 40 T antigen, in transgenic mice using the regulatory domain of the rat gonadotropin-releasing hormone (GnRH) gene, we have produced specific hypothalamic tumours. These tumours have been cultured to produce clonal cell lines (GT-1 cells) that express T antigen, GnRH and many other neuronal markers, but do not express other hypothalamic hormones. These immortal cell lines have a distinctive neuronal phenotype, process the GnRH peptide accurately and secrete GnRH in a pulsatile pattern. Thus, by targeting oncogenesis to a defined population of neurons using the regulatory region of a gene that is expressed late in differentiation of that cell lineage, we have succeeded in immortalizing hypothalamic GnRH neurons. The GT-1 cell lines are an excellent model for future molecular, cell biological, physiological and biochemical investigations into the mechanisms involved in regulation of GnRH and the characteristics of an isolated central nervous system neuron. Their derivation demonstrates the utility of targeting tumorigenesis to specific differentiated neurons of the central nervous system in transgenic mice.

Biomechanics of the cardiovascular system: the aorta as an illustratory example

Biomechanics relates the function of a physiological system to its structure. The objective of biomechanics is to deduce the function of a system from its geometry, material properties and boundary conditions based on the balance laws of mechanics (e.g. conservation of mass, momentum and energy). In the present review, we shall outline the general approach of biomechanics. As this is an enormously broad field, we shall consider a detailed biomechanical analysis of the aorta as an illustration. Specifically, we will consider the geometry and material properties of the aorta in conjunction with appropriate boundary conditions to formulate and solve several well-posed boundary value problems. Among other issues, we shall consider the effect of longitudinal pre-stretch and surrounding tissue on the mechanical status of the vessel wall. The solutions of the boundary value problems predict the presence of mechanical homeostasis in the vessel wall. The implications of mechanical homeostasis on growth, remodelling and postnatal development of the aorta are considered.

Establishment and characterization of new mammary adenocarcinoma cell lines derived from double transgenic mice expressing GFP and neu oncogene

A new murine cell line, named GFPneu, was established from a mammary adenocarcinoma arising in double transgenic MMTVneu x CMV-GFP mice. Breast tumours develop in 100% of females after 2 months latency, as a result of the over-expression of the activated rat neu oncogene in the mammary glands. All tissues, and in particular the breast tumours, express the GFP protein. This cell line was tumorigenic when inoculated into nude mice and the derived tumours showed the same histological features as the primaries from which they were isolated. Their histopathology reproduces many characteristics of human breast adenocarcinomas, in particular their ability to metastasize. The GFP marker allows us to visualize the presence of lung metastases in fresh tissues immediately, to confirm the histopathology. From a lung metastatic fluorescent nodule, we derived a further cell line, named MTP-GFP, which we also characterized. These two cell lines could be useful to study the role played by the neu oncogene in the maintenance of the transformed phenotype, in the metastatic process, to test novel therapeutic strategies to inhibit primary tumour growth and to observe the generation of distant metastases.

Genetically engineered models have advantages over xenografts for preclinical studies

Mouse models of human cancer are valuable tools for cancer research. Although xenografts and genetically engineered models (GEMs) possess limitations as well as advantages, each system plays a significant role in preclinical testing. Tumor xenografts are easy to use, relatively inexpensive, and reproducible. The main drawback of xenografts is that the genetics and histology of the tumors are frequently not representative of the respective human tumor and, thus far, these models have not been as predictive of therapeutic success as one would like. By contrast, GEMs are histologically and genetically accurate models of human cancer but have disadvantages of heterogeneity with regard to frequency, latency, and growth. These disadvantages are reminiscent of the variable behavior of actual human tumors. Recently, these shortcomings have been partly overcome with the development of anatomic and molecular in vivo imaging techniques such as magnetic resonance imaging and bioluminescence imaging. These new technologies will hopefully support the use of GEMs in preclinical trials and help determine if trials in GEMs are more predicative than xenografts of human responses.

Recent developments and future trends in nuclear medicine instrumentation

Molecular imaging using high-resolution single-photon emission computed tomography (SPECT) and positron emission tomography (PET) has advanced elegantly and has steadily gained importance in the clinical and research arenas. Continuous efforts to integrate recent research findings for the design of different geometries and various detector technologies of SPECT and PET cameras have become the goal of both the academic comcameras have become the goal of both the academic community and nuclear medicine industry. As PET has recently become of more interest for clinical practice, several different design trends seem to have developed. Systems are being designed for "low cost" clinical applications, very high-resolution research applications (including small-animal imaging), and just about everywhere in-between. The development of dual-modality imaging systems has revolutionized the practice of nuclear medicine. The major advantage being that SPECT/PET data are intrinsically aligned to anatomical information from the X-ray computed tomography (CT), without the use of external markers or internal landmarks. On the other hand, combining PET with Magnetic Resonance Imaging (MRI) technology is scientifically more challenging owing to the strong magnetic fields. Nevertheless, significant progress has been made resulting in the design of a prototype small animal PET scanner coupled to three multichannel photomultipliers via optical fibers, so that the PET detector can be operated within a conventional MR system. Thus, many different design paths are being pursued--which ones are likely to be the main stream of future commercial systems? It will be interesting, indeed, to see which technologies become the most popular in the future. This paper briefly summarizes state-of-the art developments in nuclear medicine instrumentation. Future prospects will also be discussed.

Epistasis, Not Numbers, Regulates Functions of Clustered Dahl Rat Quantitative Trait Loci Applicable to Human Hypertension

Quantitative trait loci (QTLs) for blood pressure (BP) were found on chromosome 10 of Dahl salt-sensitive rats and are potentially important to human essential hypertension. But their identities and how they influence BP together were not known. Presently, we first fine mapped existing QTLs, C10QTL1, C10QTL2, and C10QTL3, by constructing congenic strains. In the process, a new QTL, C10QTL4, was identified. Because the intervals harboring C10QTL1 and C10QTL4 contain a maximum of 16 and 10 possible genes, respectively, a limited number of specific gene targets has been identified to be QTLs residing in human homologous regions on chromosome 17. Moreover, because none of these candidates encodes a gene known to influence BP, the 2 QTLs will represent novel genes for BP regulations. Second, we used congenic strains with QTL combinations to analyze the interactions between the QTLs. Consequently, a double combination of C10QTL4 and C10QTL1 possessed the same BP as each of the 2 QTLs alone. BP of a triple combination of C10QTL4, C10QTL1, and C10QTL3 was not different from BP of the C10QTL4 and C10QTL1 double combination. These results demonstrate that C10QTL4, C10QTL1, and C10QTL3 are epistatic to one another in their BP effects. In contrast, when adding C10QTL2 into the triple formation of the 3 QTLs above to create a quadruple QTL combination, BP increased proportionately, indicating that C10QTL2 acts independently of C10QTL4, C10QTL1, and C10QTL3. The epistatic and additive interactions uncovered in the animal model will help elucidate similar interactions playing a role in human essential hypertension.

Mice with the enhanced green fluorescent protein gene knocked in to chromosome 11 exhibit normal transmission ratios

Meiotic recombination between homologous chromosomes can be suppressed within a chosen segment by a regional inversion. In mice, this feature can be engineered and conveniently used in genetic screens to maintain chemically induced mutations within the homologous chromosome. The efficiency of an inversion-based mutagenesis screen can be substantially enhanced provided that the inversion chromosome and its wild-type (WT) homologue are both visibly tagged by two different coat color markers. Dual tagging eliminates labor associated with molecular genotyping. Previously, we reported the generation of the In(11)10Brd strain of mice carrying K14-agouti tagging a 30-cM inversion between the Trp53 and Egfr loci on mouse chromosome 11. Since K14-agouti causes yellowing of ears and tails, the In(11)10Brd mice are easily distinguishable from their WT littermates. In this paper, we describe the construction of a second strain of mice that carry the enhanced green fluorescent protein (EGFP) transgene at the Egfr locus. The EGFP carriers are visually recognizable by emitting green fluorescent light upon UV illumination. We found that the EGFP function was transmitted from one generation to another with expected Mendelian frequencies, and no detrimental effects of EGFP expression were detected in hemizygous or homozygous animals. The EGFP mice together with the previously generated In(11)10Brd inversion carriers constitute a complete set of reagents required for initiation of a regional ENU mutagenesis screen to address functionally more than one-third of mouse chromosome 11.

Quantitative in situ cancer proteomics: molecular pathology comes of age with automated tissue microarray analysis

Tissue microarrays provide a high-throughput method for assessing a large number of samples by incorporating small cores of tissue into an array that can fit onto one microscope slide. Analyses of tissue microarrays were previously limited by semiquantitative protein expression analysis using brown stain (chromagen-based) methods. These methods are imperfect for protein expression analyses because of a smaller dynamic range and decreased ability for multiplexing many markers, as compared with objective in situ quantitation of protein expression in tumor samples with fluorescence microscopy by a new technology called Automated Quantitative Analysis (AQUA™). By using AQUA analysis, tissue microarrays can serve a unique role as both a discovery tool and as a validation tool for nucleic-acid expression profiling-based target discoveries with results equivalent to enzyme-linked immunosorbent assay quantitation. The identification of novel prognostic markers can identify subsets of patients at high or low risk upon diagnosis, as well as new targets for potential future therapeutic development or metastatic disease treatment decisions. Thus, AQUA provides an unparalleled opportunity to advance personalized medicine through its ability to multiplex, quantitate and localize in situ protein expression.

Use of regulated secretion in protein production from animal cells: an overview

Traditional industrial cell culture processes require extensive downstream product refining due to low product titer and purity in the spent growth medium. A controlled secretion process incorporating cells derived from endocrine or exocrine organs could potentially alleviate this processing burden by dynamically decoupling product recovery from cell growth and product biosynthesis. In addition, such specialized secretory cells may be uniquely capable of performing desirable post-translational processing of the secretory product. We briefly review the biology of regulated protein secretion as well as the biology and biochemistry of the signal transduction mechanisms by which regulated systems respond to environmental stimuli. Drawing on these and other basic principles from cell biology and bioengineering, we describe the important features of a controlled secretion process. Among other issues we discuss the choice of cell lines, expression systems, cell culture methods, and bioreactor configurations. We extensively analyze the kinetics of regulated secretion in the context of a controlled secretion process. This discussion is illustrated with experimental results from two model cell lines, recombinant AtT-20 and beta TC3, expressing recombinant human endocrine hormones or native murine insulin respectively.

A novel targeted therapy of Leydig and granulosa cell tumors through the luteinizing hormone receptor using a hecate-chorionic gonadotropin beta conjugate in transgenic mice

We investigated the antitumoral efficacy, endocrine consequences, and molecular mechanisms underlying cell death induced by the Hecate-chorionic gonadotropin (CG)beta conjugate, a fusion protein of a 23-amino acid lytic peptide Hecate with a 15-amino acid (81-95) fragment of the human CGbeta chain. Transgenic (TG) mice expressing the inhibin alpha-subunit promoter (inhalpha)/Simian Virus 40 T-antigen (Tag) transgene, developing luteinizing hormone (LH) receptor (R) expressing Leydig and granulosa cell tumors, and wild-type control littermates were treated either with vehicle, Hecate, or Hecate-CGbeta conjugate for 3 weeks. Hecate-CGbeta conjugate treatment reduced the testicular and ovarian tumor burden (P < .05), whereas a concomitant increase (testis; P < .05) or no change (ovary) in tumor volumes occured with Hectate treatment. A drop in serum progesterone, produced by the tumors, and an increase in LH levels occured in Hecate-CGbeta treated mice, in comparison with vehicle and Hecate groups, providing further support for the positive treatment response. Hecate-CGbeta conjugate induced a rapid and cell-specific membrane permeabilization of LHR-expressing cells in vitro, suggesting a necrotic mode of cell death without activation of apoptosis. These results prove the principle that the Hecate-CGbeta conjugate provides a novel specific lead into gonadal somatic cell cancer therapy by targeted destruction of LHR-expressing tumor cells.

Comparison of murine cirrhosis models induced by hepatotoxin administration and common bile duct ligation

AIM: To build up the research models of hepatic fibrosis in mice.

METHODS: Inbred wild-type FVB/N mice were either treated with alpha-naphthyl-isothiocyanate (ANIT), allyl alcohol (AA), carbon tetrachloride (CCl₄), 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC), and silica, or subjected to common bile duct ligation (CBDL) to induce hepatic injury. Liver biopsies were performed every 4 wk to evaluate hepatic fibrosis over a period of 6 mo. Cumulative cirrhosis and survival curves were constructed by life table method and compared with Wilcoxon test.

RESULTS: Under the dosages used, there was neither mortality nor cirrhosis in AA and silica-treated groups. DDC and ANIT caused cirrhosis within 4-12 and 12-24 wk, respectively. Both showed significantly faster cirrhosis induction at high dosages without significant alteration of survival. The duration for cirrhosis induction by CCl₄ ranged from 4 to 20 wk, mainly dependent upon the dosage. However, the increase in CCl₄ dosage significantly worsened survival. Intraperitoneal CCl₄ administration resulted in better survival in comparison with gavage administration at high dosage, but not at medium and low dosages. After CBDL, all the mice developed liver cirrhosis within 4-8 wk and then died by the end of 16 wk.

CONCLUSION: CBDL and administrations of ANIT, CCl₄, and DDC ensured liver cirrhosis. CBDL required the least amount of time in cirrhosis induction, but caused shortened lives of mice. It was followed by DDC and ANIT administration with favorable survival. As for CCl₄, the speed of cirrhosis induction and the mouse survival depended upon the dosages and the administration route.

Development of a Transgenic Mouse Model Immune Tolerant for Human Interferon Beta

PURPOSE

Therapeutic proteins may induce antibodies that inhibit their efficacy or have other serious biological effects. There is a great need for strategies to predict whether a certain formulation will induce an immune response. In principle, conventional animals develop an immune response against all human proteins no matter how they are formulated, which restricts their use. The aim of this study was to develop a mouse model immune tolerant for human interferon beta (hIFNbeta).

METHODS

A transgenic mouse model immune tolerant for hIFNbeta was developed by making C57Bl/6 mice transgenic for the hIFNbeta gene. To evaluate the model, both wild-type and transgenic mice were immunized with recombinant human interferon beta 1a (rhIFNbeta-1a) and recombinant human interferon beta 1b (rhIFNbeta-1b). Serum antibodies against rhIFNbeta were detected by ELISA.

RESULTS

The genetically modified mice were shown to be immune tolerant for mammalian cell-derived rhIFNbeta-1a, which has a relative low immunogenicity in patients. However, Escherichia coli-derived rhIFNbeta-1b, known to have a relatively high immunogenicity in patients, was shown not only to be immunogenic in the wild-type mice but could also break the immune tolerance of the genetically modified mice.

CONCLUSIONS

This animal model offers the possibility to study the many factors influencing the immunogenicity of hIFNbeta and test new formulations before going into clinical trials. The model also provides the first evidence that the rhIFNbetas differ in the immunological mechanisms responsible for the development of antibodies.

An efficient system for tissue-specific overexpression of transgenes in podocytes in vivo

The utility of promoter fragments isolated from the 5'-flanking region of endogenous mammalian genes to drive transgene expression in vivo is often limited by low expression levels. In this study, a bigenic system was established that allows constitutive overexpression of transgenes in a tissue-specific fashion in transgenic mice in a time- and cost-effective fashion. A modified floxed expression vector was constructed [CMVflox-enhanced green fluorescent protein (eGFP)], in which a lacZ cassette (beta-galactosidase) flanked by lox sites was placed between a CMV-promoter and the transgene of interest (eGFP). Before Cre recombination, expression of eGFP was effectively prevented by the interposed floxed lacZ cassette, whereas beta-galactosidase was strongly expressed in transiently transfected cells. Transcription of the gene of interest (eGFP) could be irreversibly activated by cotransfection with Cre recombinase. Mice transgenic for CMVflox-eGFP were generated by pronuclear injection. A rapid assay was developed to identify transgenic founders with active transgene expression by measuring transgene activity (beta-galactosidase) in tail biopsies. Transgene activity in tails correlated with transgene expression in most other tissues tested including podocytes within the kidney. To activate expression of the gene of interest in a tissue-specific fashion, founder mice were mated to the Cre mouse line 2.5P-Cre previously shown to mediate 100% Cre recombination exclusively in podocytes (Moeller MJ, Sanden SK, Soofi A, Wiggins RC, and Holzman LB. Genesis 35: 39-42, 2003). In doubly transgenic offspring, high-level eGFP expression resulting from Cre excision of the interposed lacZ cassette was detected in four of seven CMVflox-eGFP founder lines. This approach should also circumvent common limitations arising from lethality or transgene silencing as a consequence of transgene overexpression.

Testicular cell lines

The range of in vivo or in vitro immortalized cell lines currently available provides a variety of model systems for studies of normal and pathological cell functions. The cell lines have been derived from spontaneous or experimentally induced tumors, or through in vitro immortalization. The transgenic (TG) techniques provide a powerful approach, allowing the production of in vivo animal models for a variety of diseases, including malignant tumors, through tissue-specific expression of oncogenes or other tumor-promoting genes. The TG techniques also enable the production of cell lines with specific characteristics, through insertion of desired genes into specific cell types, which can then be immortalized upon cell culture. The use of temperature-sensitive immortalizing genes offers an additional advantage of controlling gene expression, including the proliferation and differentiation of the cells to be immortalized. As regards the male reproductive system, a number of cell lines of testicular somatic cells are currently available. This review covers mainly the immortalized cell lines of testicular Leydig and Sertoli cells, with special reference to murine cell lines for the study of testicular endocrine function and tumorigenesis. These cell lines also provide useful tools to investigate the molecular basis of hormone actions and testicular cell interactions.

Analyzing corticosterone metabolites in fecal samples of mice: a noninvasive technique to monitor stress hormones

In small animals like mice, the monitoring of endocrine functions over time is constrained seriously by the adverse effects of blood sampling. Therefore, noninvasive techniques to monitor, for example, stress hormones in these animals are highly demanded in laboratory as well as in field research. The aim of our study was to evaluate the biological relevance of a recently developed technique to monitor stress hormone metabolites in fecal samples of laboratory mice. In total, six experiments were performed using six male and six female mice each. Two adrenocorticotropic hormone (ACTH) challenge tests, two dexamethasone (Dex) suppression tests and two control experiments [investigating effects of the injection procedure itself and the diurnal variation (DV) of glucocorticoids (GCs), respectively] were conducted. The experiments clearly demonstrated that pharmacological stimulation and suppression of adrenocortical activity was reflected accurately by means of corticosterone metabolite (CM) measurements in the feces of males and females. Furthermore, the technique proved sensitive enough to detect dosage-dependent effects of the ACTH/Dex treatment and facilitated to reveal profound effects of the injection procedure itself. Even the naturally occurring DV of GCs could be monitored reliably. Thus, our results confirm that measurement of fecal CM with the recently established 5alpha-pregnane-3beta,11beta,21-triol-20-one enzyme immunoassay is a very powerful tool to monitor adrenocortical activity in laboratory mice. Since mice represent the vast majority of all rodents used for research worldwide and the number of transgenic and knockout mice utilized as animal models is still increasing, this noninvasive technique can open new perspectives in biomedical and behavioral science.

Angiogenesis in transgenic models of multistep angiogenesis

The histopathology and the epidemiology of human cancers, as well as studies of animal models of tumorigenesis, have led to a widely accepted notion that multiple genetic and epigenetic changes have to accumulate for progression to malignancy. Formation of new blood vessels (tumor angiogenesis) has been recognized, in addition to proliferative capabilities and ability to down-modulate cell death (apoptosis), as essential for the progressive growth and expansion of solid tumors. Mice overexpressing activated forms of oncogenes or carrying targeted mutations in tumor suppressor genes have proven extremely useful for linking the function of these genes with specific tumor features such as continuous proliferation, escape from apoptosis, invasion and neo-angiogenesis. The interbreeding of these mice allows for studying the extent of cooperativity between different genetic lesions in disease progression, leading to a greater understanding of multi-stage nature of tumorigenesis.

Functional genomics of sleep

Functional genomics is a systematic and high-throughput effort to analyze the functions of genes and gene products. Functional genomics is divided into gene- and phenotype-driven approaches. Gene-driven approaches to the functional genomics of sleep have demonstrated that transcripts of many genes change as a function of behavioral state. A phenotype-driven approach includes identification and characterization of gene function through the analyses of natural polygenic traits, creation of transgenic animals or high-throughput mutagenesis. Identification of a gene for narcolepsy through QTL analyses and concomitantly using a transgenic approach is one example of the phenotype-driven approach to the functional genomics of sleep. Though the majority of functional genomics is currently performed in mice, the rat is emerging as an important model for genomic research. Since rest in Drosophila shares many features with mammalian sleep, this allows a comparative functional genomics approach to the study of rest and sleep. The concepts outlined here for the functional genomics of sleep are applicable to respiration research.

Dual-modality imaging of function and physiology

Dual-modality imaging is a technique in which computed tomography (CT) or magnetic resonance imaging is combined with positron emission tomography or single-photon emission CT to acquire structural and functional images with an integated system. The data are acquired in a single procedure; the patient remains on the scanner table while undergoing both x-ray and radionuclide studies to facilitate correlation between the structural and functional images. The resulting data can aid in localization, enabling more specific diagnosis than can be obtained with a conventional imaging study. In addition, the anatomic information can be used to compensate the correlated radionuclide data for physical perturbations such as photon attenuation, scatter radiation, and partial volume errors. Thus, dual-modality imaging provides a priori information that can improve both the visual quality and the quantitative accuracy of the radionuclide images. Dual-modality imaging systems are also being developed for biologic research involving small animals. Small-animal dual-modality systems offer advantages for measurements that currently are performed invasively with autoradiography and tissue sampling. By acquiring data noninvasively, dual-modality imaging permits serial studies in a single animal, enables measurements to be performed with fewer animals, and improves the statistical quality of the data.

Morphometry and strain distribution of the C57BL/6 mouse aorta

The goal of the present study was to obtain a systematic set of data along the length of the mouse aorta to study variations of morphometry (diameter, wall thickness, and curvature), strain, and stress of the mouse aorta. Five mice were imaged with a 13-MHz ultrasound probe to determine the in vivo diameter along the aorta. A cast was made of these aortas to validate the ultrasonic diameter measurements. The root mean squared and systematic errors for these measurements were 12.6% and 6.4% of the mean ultrasound diameter, respectively. The longitudinal variations of geometry, stress, and strain from the aortic valve to the common iliac bifurcation were documented. Our results show that the residual circumferential strain leads to a uniformity of transmural strain of the aorta in the loaded state along the entire length of the aorta. Furthermore, we validated the incompressibility condition along the length of the aorta. These data of normal mice will serve as a reference state for the study of disease in future knockout models.

Mouse transgenic for murine oviduct-specific glycoprotein promoter-driven simian virus 40 large T-antigen: tumor formation and its hormonal regulation

Transgenic mice were generated in which a 2.2-kb segment of the 5'-flanking sequence of the mouse oviduct-specific glycoprotein (OGP) gene was used to drive expression of the simian virus 40 large T antigen (Tag). These mice spontaneously developed tumors in the female reproductive tract. Analysis using reverse transcriptase-polymerase chain reaction showed that the 2.2-kb OGP 5'-flanking region drove Tag mRNA expression in the oviduct, uterus, vagina, and ovary, but not in other tissues. Histological and immunohistochemical analyses revealed that the tumor cells were distributed in the oviduct, endometrium, myometrium, and vagina; and had atypical features, abnormal mitosis, and Tag expression. Ovariectomy suppressed Tag expression, and thereby, blocked tumorigenesis in the transgenic mice. Estradiol administration to ovariectomized transgenic mice led to dramatic hyperplasia of the reproductive tract tissues in association with enhanced Tag expression, both in intensity and distribution. These results demonstrated that a 2.2-kb fragment of the 5'-flanking sequence of the mouse OGP gene was capable of directing the expression of Tag and inducing tumorigenesis in female reproductive tract tissues in an estrogen-dependent manner. Estrogen response elements present in the promoter region were functional in vivo. These transgenic mice represent a unique model, since they develop tumors in the oviducts as well as in other tissues derived from the Mullerian duct.

Targeting oncogenesis by introduction of a 5.2-kbp segment of the 5' regulatory region of the human thyrotropin beta-subunit gene

We produced transgenic mice carrying a fusion gene (TTP-5) consisting of a 5.2-kbp segment of the 5' flanking sequence of the human thyrotropin beta-subunit (TSH beta) gene linked to the simian virus 40 large T antigen (SVT) gene. These mice developed pituitary tumors 6 months after birth and wasted away. With the 5.2-kbp TSH beta 5' flanking region governing SVT expression, SVT mRNA was present in the pituitary and testis but not in other tissues, as detected by the reverse transcriptase-polymerase chain reaction. Histological and immunohistochemical analyses showed that the pituitary tumors of the transgenic mice were composed of moderately differentiated pituitary cells that expressed TSH, growth hormone, and prolactin. These results indicate that the 5.2-kbp segment of the human TSH beta 5' regulatory region is sufficient to drive expression of SVT and induce tumorigenesis of hormone-producing pituitary cells in transgenic mice.

In vitro response of v-Ha-ras transgenic mouse lymphocytes after in vivo treatment with cocaine

Oncomouse is a transgenic mouse carrying an activated v-Ha-ras oncogene under the control of the mouse mammary tumor virus promoter. The objective of this paper was to learn if the in vitro secretion of IL-2 and IFN-gamma and the release of sIL-2R by Oncomice spleen and thymus cells depended on the presence of the oncogene product, on the in vivo pretreatment with cocaine, or on the in vitro treatment with cocaine or morphine. Oncomice thymocytes from different experimental groups released less sIL-2R than FVB thymocytes. Oncomice thymocytes secreted more IFN-gamma than FVB thymocytes. Oncomice thymocytes cultured in the presence of Con A and cocaine showed a diminished release of sIL-2R and a lower secretion of IFN-gamma, a phenomenon not observed in FVB thymocytes. IFN-gamma secretion was lower in Oncomice splenocytes. In general, Oncomice thymocytes and splenocytes responded in a nearly opposite fashion to their FVB counterparts. In this study, the in vitro response to mitogens, cocaine or morphine depended on genetic background and not on the in vivo pretreatment with cocaine. Our results emphasize the role of the v-Ha-ras oncogene in defining the host immune response.

Genetic engineering of neural function in transgenic rodents: towards a comprehensive strategy?

As mammalian genome projects move towards completion, the attention of molecular neuroscientists is currently moving away from gene identification towards both cell-specific gene expression patterns (neuronal transcriptions) and protein expression/interactions (neuronal proteomics). In the long term, attention will increasingly be directed towards experimental interventions which are able to question neuronal function in a sophisticated manner that is cognisant of both transcriptomic and proteomic organization. Central to this effort will be the application of a new generation of transgenic approaches which are now evolving towards an appropriate level of molecular, temporal and spatial resolution. In this review, we summarize recent developments in transgenesis, and show how they have been applied in the principal model species for neuroscience, namely rats and mice. Current concepts of transgene design are also considered together with an overview of new genetically-encoded tools including both cellular indicators such as fluorescent activity reporters, and cellular regulators such as dominant negative signalling factors. Application of these tools in a whole animal context can be used to question both basic concepts of brain function, and also current concepts of underlying dysfuction in neurological diseases.

Ovarian tumorigenesis in mice transgenic for murine inhibin alpha subunit promoter-driven Simian Virus 40 T-antigen: ontogeny, functional characteristics, and endocrine effects

We previously reported formation of ovarian granulosa cell tumors with 100% penetration in a transgenic mouse model with murine inhibin alpha subunit promoter-driven (inhalpha)/Simian Virus 40 T-antigen (Tag). The tumor-bearing inhalpha/Tag mice showed highly elevated serum levels of immunoreactive inhibin. To investigate the onset of tumorigenesis and related endocrine consequences, 6-8 female mice of two inhalpha/Tag lines and their mating control littermates were killed monthly between 1 and 6 mo of age. We also investigated tumorigenesis-related fertility aspects of these two mouse lines. The ontogeny and progression of tumors could be monitored in both inhalpha/Tag lines by alterations of ovarian weights and serum hormone levels. Serum progesterone levels increased in both inhalpha/Tag lines in an age-dependent manner as ovarian tumorigenesis progressed, and a reciprocal decrease occurred in serum LH and FSH. Neither serum estradiol (E(2)) nor uterine weights were significantly altered during tumorigenesis, suggesting that the ovarian tumors represented late stages of granulosa cell differentiation. In conclusion, the present findings show in the inhalpha/Tag TG mice a relation between endocrine consequences of granulosa cell tumorigenesis, and a connection of onset of tumor formation with aberrant steroidogenesis and gonadotropin secretion. These findings indicate that tumors are endocrinologically active and able to exert enhanced negative feedback effects on pituitary function. The tumors provide a good model for endocrinologically active hormone-dependent tumors.

Immortalization of murine male germ cells at a discrete stage of differentiation by a novel directed promoter-based selection strategy

We developed a novel promoter-based selection strategy that could be used to produce cell lines representing sequential stages of spermatogenesis. The method is based on immortalization and subsequent targeted selection by using differentiation-specific promoter regions. As an example for this approach, a new murine germ cell line (GC-4spc) was established using a vector construct that contains the SV40 large T antigen and the neomycin phosphotransferase II gene under the control of the SV40 early promoter and a spermatocyte-specific promoter for human phosphoglycerate kinase 2, respectively. The GC-4spc was characterized as a cell line at the stage between preleptotene and early pachytene spermatocytes. Transcription of three germ cell-specific expressed genes, Pgk2, proacrosin, and the A-myb proto-oncogene, were detected in the GC-4spc cell line using reverse transcription-polymerase chain reaction. Furthermore, TSPY (human testis-specific protein, Y-encoded) and PGK2 (human phosphoglycerate kinase 2) promoter regions showed different transcriptional activities in the GC-4spc cell line compared with the spermatogonia-derived cell line GC-1spg. Thus, our strategy could be used for immortalization of cells at specific stages of differentiation, allowing production of a series of cultured cell lines representing sequential stages of differentiation in given cell lineages.

Angiogenesis in transgenic models of multistep carcinogenesis

The histopathology and the epidemiology of human cancers, as well as studies of animal models of tumorigenesis, have led to a widely accepted notion that multiple genetic and epigenetic changes have to accumulate for progression to malignancy. Formation of new blood vessels (tumor angiogenesis) has been recognized, in addition to proliferative capabilities and to the ability to down-modulate cell death (apoptosis), as essential for the progressive growth and expansion of solid tumors beyond microscopic sizes of about 1-2 mm in diameter. Mice overexpressing activated forms of oncogenes or carrying targeted mutations in tumor suppressor genes have proven extremely useful for to linking the function of these genes with specific tumor processes; the interbreeding of these mice let us study the extent of cooperativity between different genetic lesions in disease progression, leading to a greater understanding of multi-stage nature of tumorigenesis.

Development of endothelial cell lines from embryonic stem cells: A tool for studying genetically manipulated endothelial cells in vitro

Totipotent embryonic stem cells can be induced to differentiate to endothelium in vitro. This may be a useful tool for obtaining cultures of genetically manipulated endothelial cells because embryonic stem cells are relatively easy to transfect and are commonly used for gene inactivation experiments in mice. However, embryonic stem cell-derived endothelial cells could not be easily separated from embryoid bodies and maintained in culture. In this study, we describe the isolation and characterization of immortalized endothelial cell lines obtained from embryonic stem cells differentiated in vitro. The cell lines were analyzed for expression of endothelial cell markers, including growth factor receptors and adhesion molecules, and compared with endothelial cells obtained from the yolk sac, the embryo proper, or the heart microcirculation of the adult. We propose that this approach may be useful for obtaining endothelial cells carrying gene mutations that are lethal at very early stages of development.

Behavioral and immune changes in v-Ha-ras transgenic mice

Transgenic mice (Oncomice) with an activated v-Ha-ras oncogene under the control of the mouse mammary tumor virus promoter develop mammary tumors. We wondered if the expression of the v-Ha-ras oncogene product would induce changes in mice behavioral activity, that could be associated with alterations in their immune system. Behavior was evaluated in an open field study considering line crossings and rears. Oncomice consistently showed less activity than FVB mice. Lieber-DeCarli diet decreased both types of activity in both strains. Cocaine treatment increased line crossings in both strains. Oncomice spleen and thymus cell supernatants contained higher levels of IL-2. Oncomice serum had higher levels of IL-1alpha. Our results suggest a direct association between higher levels of IL-1alpha and lower open field activity. Therefore, we can infer that the increased level of IL-1alpha found in Oncomice, could have a key role in oncogene induced immune and behavioral changes, and could be a requirement to facilitate its transforming activity.

Use of transgenic animals for carcinogenicity testing: considerations and implications for risk assessment

Advances in genetic engineering have created opportunities for improved understanding of the molecular basis of carcinogenesis. Through selective introduction, activation, and inactivation of specific genes, investigators can produce mice of unique genotypes and phenotypes that afford insights into the events and mechanisms responsible for tumor formation. It has been suggested that such animals might be used for routine testing of chemicals to determine their carcinogenic potential because the animals may be mechanistically relevant for understanding and predicting the human response to exposure to the chemical being tested. Before transgenic and knockout mice can be used as an adjunct or alternative to the conventional 2-year rodent bioassay, information related to the animal line to be used, study design, and data analysis and interpretation must be carefully considered. Here, we identify and review such information relative to Tg.AC and rasH2 transgenic mice and p53+/- and XPA-/- knockout mice, all of which have been proposed for use in chemical carcinogenicity testing. In addition, the implications of findings of tumors in transgenic and knockout animals when exposed to chemicals is discussed in the context of human health risk assessment.

Transgenic animal models for neuropharmacology

The establishment of novel animal models using gene targeting and transgenic technology has opened a new area of neuropharmacological research. For the first time, it became possible to alter the expression of a gene in a specific cell type of an intact animal by either overexpression, inhibition or ablation. This review describes the technology and lists the relevant tools, such as reporter genes, suicide genes, immortalizing genes, and promoters, necessary for the targeted expression of these and other genes in specific cells of the central nervous system. In addition, the problem is discussed that the mouse is the species in which this technology is by far the most developed, while the rat has been used as the model species for neuropharmacology during the last century.

Openings between defective endothelial cells explain tumor vessel leakiness

Leakiness of blood vessels in tumors may contribute to disease progression and is key to certain forms of cancer therapy, but the structural basis of the leakiness is unclear. We sought to determine whether endothelial gaps or transcellular holes, similar to those found in leaky vessels in inflammation, could explain the leakiness of tumor vessels. Blood vessels in MCa-IV mouse mammary carcinomas, which are known to be unusually leaky (functional pore size 1.2-2 microm), were compared to vessels in three less leaky tumors and normal mammary glands. Vessels were identified by their binding of intravascularly injected fluorescent cationic liposomes and Lycopersicon esculentum lectin and by CD31 (PECAM) immunoreactivity. The luminal surface of vessels in all four tumors had a defective endothelial monolayer as revealed by scanning electron microscopy. In MCa-IV tumors, 14% of the vessel surface was lined by poorly connected, overlapping cells. The most superficial lining cells, like endothelial cells, had CD31 immunoreactivity and fenestrae with diaphragms, but they had a branched phenotype with cytoplasmic projections as long as 50 microm. Some branched cells were separated by intercellular openings (mean diameter 1.7 microm; range, 0.3-4.7 microm). Transcellular holes (mean diameter 0.6 microm) were also present but were only 8% as numerous as intercellular openings. Some CD31-positive cells protruded into the vessel lumen; others sprouted into perivascular tumor tissue. Tumors in RIP-Tag2 mice had, in addition, tumor cell-lined lakes of extravasated erythrocytes. We conclude that some tumor vessels have a defective cellular lining composed of disorganized, loosely connected, branched, overlapping or sprouting endothelial cells. Openings between these cells contribute to tumor vessel leakiness and may permit access of macromolecular therapeutic agents to tumor cells.