The effects of the antihormones RU486 and tamoxifen on fetoplacental development and placental bed vascularisation in the rat: a model for intrauterine fetal growth retardation

Strategies for site-specific recombination with high efficiency and precise spatiotemporal resolution

Site-specific recombinases (SSRs) are invaluable genome engineering tools that have enormously boosted our understanding of gene functions and cell lineage relationships in developmental biology, stem cell biology, regenerative medicine, and multiple diseases. However, the ever-increasing complexity of biomedical research requires the development of novel site-specific genetic recombination technologies that can manipulate genomic DNA with high efficiency and fine spatiotemporal control. Here, we review the latest innovative strategies of the commonly used Cre-loxP recombination system and its combinatorial strategies with other site-specific recombinase systems. We also highlight recent progress with a focus on the new generation of chemical- and light-inducible genetic systems and discuss the merits and limitations of each new and established system. Finally, we provide the future perspectives of combining various recombination systems or improving well-established site-specific genetic tools to achieve more efficient and precise spatiotemporal genetic manipulation.

Tamoxifen administration in pregnant mice can be deleterious to both mother and embryo

Since it was introduced 20 years ago, tamoxifen-inducible genetic recombination in vivo has become a standard tool in many fields. This technique has great utility, allowing precise temporal and spatial gene recombination mediated by expression of a Cre recombinase-oestrogen receptor hormone binding domain fusion protein. It is frequently used in developmental biology, either for accurate spatio-temporal gene deletion or for lineage-labelling. Administration of high doses of tamoxifen can rapidly induce abortion in pregnant mice but this can be partially overcome by progesterone co-administration. However, administration of tamoxifen to pregnant mice early in pregnancy may have potentially lethal effects on the mother independently of abortion, and can also severely perturb embryonic development. Despite this, only a few published studies mention this fact in passing, and standard parameters for successful or unsuccessful use of tamoxifen in pregnant mice have not been reported. Therefore, in the interests of providing a framework for more humane animal research, we describe our experiences of tamoxifen administration during early gestation in mice. These observations should assist the design of future studies in accordance with the principles of the three Rs (Replacement, Reduction and Refinement of Animals in Research).

Wt1 and epicardial fate mapping

RATIONALE

The embryonic epicardium is a crucial cell source of the cardiac fibrous skeleton as well as of the coronary system. Genetic lineage tracing systems based on Wt1 regulatory sequences provided evidence that epicardium-derived cells also adopt a myocardial fate in the mouse.

OBJECTIVE

To define the adequacy of Wt1-based lineage tracing systems for epicardial fate mapping.

METHODS AND RESULTS

Using in situ hybridization analysis and immunofluorescence on tissue sections, we detected endogenous expression of Wt1 mRNA and Wt1 protein in the proepicardium and epicardium and also in endothelial cells throughout cardiogenesis. Expression analysis of a sensitive GFP reporter showed that recombination mediated by cre recombinase in the Wt1(creEGFP) line occurs randomly and sporadically in all cells of the embryo. Recombination in cardiomyocytes was found in the linear heart tube before establishment of a (pro)epicardium. In contrast, the tamoxifen-inducible Wt1(creERT2) mouse line mediated poor and variable recombination in the epicardium. Recombination in cardiomyocytes was not detected in this case.

CONCLUSIONS

Frequently used Wt1 based cre-mediated lineage tracing systems are not suitable for epicardial fate mapping because of endogenous endothelial expression of Wt1, ectopic recombination (Wt1(creEGFP)), and poor recombination efficiency (Wt1(creERT2)) in the developing heart. We conclude that claims of a cardiomyocyte fate of epicardial cells in the mouse are not substantiated.

The impairment of metrial gland development in tamoxifen exposed rats

We examined the sequential histopathological changes in the placenta from rats exposed to tamoxifen. Tamoxifen was administered intraperitoneally at doses of 0 and 2 mg/kg/day on gestation days (GDs) 8, 9 and 10, and the placentas were sampled on GDs 11, 13, 15, 17, and 21. The fetal mortality rates in the tamoxifen group were increased up to 56%. However, there were no effects on the weights of live embryos/fetuses and their placentas. Histopathologically, the size of metrial gland in the tamoxifen group was reduced on all sampling times. The spiral arteries appeared less well developed in the hypoplastic metrial gland. A decrease in uterine natural killer (uNK) cells and mitotic uNK cells around the spiral arteries in the metrial gland was detected from GD 13 onward and on GDs 11 and 13, respectively. There were no obvious changes in the labyrinth zone or basal zone. We consider that the anti-estrogen effect of tamoxifen inhibits the proliferation of decidualized endometrial stromal cells in the metrial gland and leads to inhibition of the proliferative activity of uNK cells, followed by defective development of spiral arteries, and metrial gland hypoplasia. It is assumed that the metrial gland hypoplasia might be involved in the tamoxifen-induced embryo/fetus-toxicity.

Temporal regulation of Cre-recombinase activity in Scl-positive neurons of the central nervous system

The Cre/LoxP system provides a powerful tool to investigate gene function in vivo. This system requires Cre-recombinase expressing mouse lines that permit control of gene recombination in a tissue-specific and time-dependent manner. To allow spatio-temporal gene deletion in specific central nervous system (CNS) neuronal populations, we generated mice with a tamoxifen-inducible Cre (Cre-ER(T)) transgene under control of the Scl/Tal1 neural promoter/enhancer -0.9E3 (-0.9E3CreER(T) transgenic mice). Using Cre-reporter mice we have shown that tamoxifen-mediated Cre-ER(T) recombination in -0.9E3CreER(T) mice recapitulated the anticipated expression pattern of Scl in the caudal thalamus, midbrain, hindbrain, and spinal cord. Cre-mediated recombination was also effectively induced during embryogenesis and marked the same population of neurons as observed in the adult. Additionally, we identified a tamoxifen-independent constitutively active -0.9E3CreER(T) mouse line that will be useful for gene deletion during early neurogenesis. These -0.9E3CreER(T) mice will provide tools to investigate the role of neuronal genes in the developing and mature CNS. CNS.

Analysis of mouse development with conditional mutagenesis

Explorations into the molecular embryology of the mouse have played a vital role in our understanding of the basic mechanisms of gene regulation that govern development and disease. In the last 15 years, these mechanisms have been analyzed with vastly greater precision and clarity with the advent of systems that allow the conditional control of gene expression. Typically, this control is achieved by silencing or activating the gene of interest with site-specific DNA recombination or transcriptional transactivation. In this review, I discuss the application of these technologies to mouse development, focusing on recent innovations and experimental designs that specifically aid the study of the mouse embryo.

Timed mutation and cell-fate mapping reveal reiterated roles of Tbx1 during embryogenesis, and a crucial function during segmentation of the pharyngeal system via regulation of endoderm expansion

The definition of time-specific requirements for a developmental gene can pinpoint the processes within which the gene is involved and can reveal potential late functions in structures and organs that fail to develop in germline mutants. Here, we show the first systematic time-course deletion, in parallel with timed cell fate mapping, of a developmentally crucial gene, Tbx1, during mouse embryogenesis. Tbx1 mouse mutants model DiGeorge syndrome, a disorder of pharyngeal and cardiovascular development. Results revealed different time requirements for the development of individual structures, as well as multiple and time-distinct roles during the development of the same organ or system. We also show that Tbx1 is required throughout pharyngeal segmentation for the regulation of endoderm expansion, thus this is the first gene implicated directly in this process. A genetic-based blueprint of crucial developmental times for organs and systems should be a valuable asset for our understanding of birth defect pathogenesis.

Recent innovations in tissue-specific gene modifications in the mouse

Annotating the functions of individual genes in in vivo contexts has become the primary task of mouse genetics in the post-genome era. In addition to conventional approaches using transgenic technologies and gene targeting, the recent development of conditional gene modification techniques has opened novel opportunities for elucidating gene function at the level of the whole mouse to individual tissues or cell types. Tissue-specific gene modifications in the mouse have been made possible using site-specific DNA recombinases and conditional alleles. Recent innovations in this basic technology have facilitated new types of experiments, revealing novel insights into mammalian embryology. In this review, we focus on these recent innovations and new technical issues that impact the success of these conditional gene modification approaches.

Genetically modified animal models as tools for studying bone and mineral metabolism

Genetic modification of mice is a powerful tool for the study of bone development and metabolism. This review discusses the advantages and disadvantages of various approaches used in bone-related research and the contributions these studies have made to bone biology. Genetic modification of mice is a powerful tool for the study of bone development and metabolism. This review discusses the advantages and disadvantages of various approaches used in bone-related research and the contributions these studies have made to bone biology. The approaches to genetic modification included in this review are (1) overexpression of genes, (2) global gene knockouts, (3) tissue-specific gene deletion, and (4) gene knock-in models. This review also highlights issues that should be considered when using genetically modified animal models, including the rigorous control of genetic background, use of appropriate control lines, and confirmation of tissue specificity of gene expression where appropriate. This technology provides a unique and powerful way to probe the function of genes and is already revolutionizing our approach to understanding the physiology of bone development and metabolism.

Effects of estradiol administration on feto-placental growth in rat

BACKGROUND

To clarify the effect of estradiol benzoate on placental structure and its consequences for fetal survival and fetoplacental growth.

STUDY DESIGN

Estradiol benzoate (0, 0.1, 1, 10, 100 microg/day) was infused intraperitoneally into pregnant Wistar rats from 12 to 19 days' gestation. Survival rate, weight of pups and placentas at 20 days' gestation, and plasma levels of estrogen and progesterone were measured. Pathological changes in the placenta were also examined.

RESULTS

Estradiol benzoate reduced fetal survival (1 microg/day: 100%, 10 microg/day: 70%, 100 microg/day: 14.6%) and the weights of the pups and placentas in a dose-dependent manner. Maternal estradiol concentration was raised 23-fold with 100microg/day of estradiol benzoate. Trophoblast degeneration, including apoptosis and destruction of placental labyrinth was induced but the structures of the maternal kidney and liver were not affected.

CONCLUSIONS

In pregnant rats, estradiol benzoate causes fetal mortality at a pharmacological dose (more than 10 microg/day) and fetoplacental growth retardation via trophoblastic degeneration and destruction of the placental labyrinth even at a physiological dose (1 microg/day).

Fertility preservation in breast cancer patients: IVF and embryo cryopreservation after ovarian stimulation with tamoxifen

BACKGROUND

Breast cancer chemotherapy commonly causes premature ovarian failure and infertility. Because increased estrogen levels are thought to be potentially risky in breast cancer patients, natural cycle IVF (NCIVF) has been used to preserve fertility and treat infertility in these women.

METHODS

Twelve women with breast cancer received 40-60 mg tamoxifen for 6.9 +/- 0.6 days beginning on days 2-3 of their menstrual cycle (15 cycles), and had IVF (TamIVF) with either fresh embryo transfer (six cycles) or cryopreservation (nine cycles). They were compared to a retrospective control group (n = 5) who had natural cycle IVF (NCIVF, nine cycles).

RESULTS

Cycle cancellation was significantly less frequent in TamIVF, compared with NCIVF (1/15 versus 4/9, P < 0.05). Compared with NCIVF, TamIVF patients had a greater number of mature oocytes (1.6 +/- 0.3 versus 0.7 +/- 0.2, P = 0.03) and embryos (1.6 +/- 0.3 versus 0.6 +/- 0.2, P = 0.02) per initiated cycle. TamIVF resulted in the generation of embryo(s) in every patient (12/12) while only three out of five patients had an embryo following NCIVF. Two out of six patients in TamIVF, and 2/5 in NCIVF conceived. One patient in the TamIVF group delivered a set of twins. After a mean follow up of 15 +/- 3.6 months (range 3-54), none of the patients had a recurrence of cancer.

CONCLUSIONS

Tamoxifen stimulation appears to result in a higher number of embryos and may provide a safe method of IVF and fertility preservation in breast cancer patients.

Efficient recombination in diverse tissues by a tamoxifen-inducible form of Cre: a tool for temporally regulated gene activation/inactivation in the mouse

In recent years, the Cre integrase from bacteriophage P1 has become an essential tool for conditional gene activation and/or inactivation in mouse. In an earlier report, we described a fusion protein between Cre and a mutated form of the ligand binding domain of the estrogen receptor (Cre-ER) that renders Cre activity tamoxifen (TM) inducible, allowing for conditional modification of gene activity in the mammalian neural tube in utero. In the current work, we have generated a transgenic mouse line in which Cre-ER is ubiquitously expressed to permit temporally regulated Cre-mediated recombination in diverse tissues of the mouse at embryonic and adult stages. We demonstrate that a single, intraperitoneal injection of TM into a pregnant mouse at 8.5 days postcoitum leads to detectable recombination in the developing embryo within 6 h of injection and efficient recombination of a reporter gene in derivatives of all three germ layers within 24 h of injection. In addition, by varying the dose of TM injected, the percentage of cells undergoing a recombination event in the embryo can be controlled. Dose-dependent excision induced by TM was also possible in diverse tissues in the adult mouse, including the central nervous system, and in cultured cells derived from the transgenic mouse line. This inducible Cre system will be a broadly useful tool to modulate gene activity in mouse embryos, adults, and culture systems where temporal control is an important consideration.

Temporally regulated and tissue-specific gene manipulations in the adult and embryonic heart using a tamoxifen-inducible Cre protein

The advent of conditional and tissue-specific recombination systems in gene-targeted or transgenic mice has permitted an assessment of single gene function in a temporally regulated and cell-specific manner. Here we generated transgenic mice expressing a tamoxifen-inducible Cre recombinase protein fused to two mutant estrogen-receptor ligand-binding domains (MerCreMer) under the control of the alpha-myosin heavy chain promoter. These transgenic mice were crossed with the ROSA26 lacZ-flox-targeted mice to examine Cre recombinase activity and the fidelity of the system. The data demonstrate essentially no Cre-mediated recombination in the embryonic, neonatal, or adult heart in the absence of inducing agent but >80% recombination after only four tamoxifen injections. Expression of the MerCreMer fusion protein within the adult heart did not affect cardiac performance, cellular architecture, or expression of hypertrophic marker genes, demonstrating that the transgene-encoded protein is relatively innocuous. In summary, MerCreMer transgenic mice represent a tool for temporally regulated inactivation of any loxP-targeted gene within the developing and adult heart or for specifically directing recombination and expression of a loxP-inactivated cardiac transgene in the heart.

Tamoxifen as systemic treatment of advanced breast cancer during pregnancy--case report and literature review

BACKGROUND

When patients with metastatic breast cancer become pregnant, management is complicated by the potential harms of drug treatment to the fetus and by the potential effects of the pregnancy on the cancer. Chemotherapy is considered optimal systemic anti-cancer therapy from the second trimester, while tamoxifen has been considered inappropriate due to concerns over possible teratogenesis and lack of efficacy.

CASE

We report a patient who became pregnant concurrent with the identification of metastatic breast cancer and who elected to continue her pregnancy with tamoxifen as sole systemic anti-cancer therapy. The pregnancy was difficult, but a normal child was delivered and the mother responded to subsequent hormone manipulation. The putative teratogenic effects of tamoxifen and the mechanisms underlying tamoxifen resistance in this setting are discussed.

CONCLUSIONS

The use of tamoxifen in pregnancy is complex, but is not necessarily associated with fetal harm and may be considered a therapeutic option in selected cases.

Expression of mouse 17beta-hydroxysteroid dehydrogenase/17-ketosteroid reductase type 7 in the ovary, uterus, and placenta: localization from implantation to late pregnancy

Rodent 17beta-hydroxysteroid dehydrogenase/17-ketosteroid reductase type 7 (17HSD/KSR7) catalyzes the conversion of estrone (E1) to estradiol (E2) and is abundantly expressed in the ovaries of pregnant animals in particular. In the present work we demonstrate cell-specific expression of 17HSD/KSR7 in the ovaries, uteri, and placentas of pregnant and nonpregnant mice using in situ hybridization. The results show that mouse 17HSD/KSR7 (m17HSD/KSR7) messenger RNA is distinctly and exclusively expressed in a proportion of corpora lutea (CLs). During pregnancy, expression of m17HSD/KSR7 is most abundant around embryonic day 14.5 (E14.5), when the ovaries are filled with CLs expressing 17HSD/KSR7. In the uterus, m17HSD/KSR7 is first detected on E5.5, when expression surrounds the implantation site on the antimesometrial side. As gestation progresses, m17HSD/KSR7 is expressed in the decidua capsularis on E8 and E9.5, disappearing thereafter from the antimesometrial decidua. On E9 onward, m17HSD/KSR7 messenger RNA expression takes place at the junctional zone of the developing placenta. On E12.5 and E14.5, m17HSD/KSR7 is abundantly expressed in the spongiotrophoblasts, where expression gradually declines toward parturition. In conclusion, m17HSD/KSR7 expression in the CL is related to the life span of the CL. Moreover, spatial and temporal expression of m17HSD/KSR7 in the uterus suggests that locally produced E2 plays a role in implantation and/or decidualization. Finally, the results indicate that mouse placenta is capable of converting E1 to E2 in situ, and that the synthesized E2 may be effective in a paracrine, autocrine, and/or intracrine manner and be involved in placentation.

Attenuated hypotensive response to proadrenomedullin N-terminal 20 peptide in pregnant rats: modulation by steroid hormones

The hypotensive effect of proadrenomedullin N-terminal 20 peptide (PAMP) was examined in conscious pregnant (8, 14, and 20 days of pregnancy) and nonpregnant rats. Intravenous administration of PAMP (3-60 nmol/kg) produced a dose-dependent depressor response in both pregnant and nonpregnant rats. However, the maximum decrease in blood pressure was significantly attenuated in pregnant rats in mid- and late-gestation (14 and 20 days), but not in early gestation (8 days), than in nonpregnant rats. In ovariectomized rats, the depressor responses in 17beta-estradiol (E2)-treated, progesterone (P)-treated, and E2+P-treated rats were significantly attenuated compared with the control rats. We also demonstrated that treatment of sex hormones reduces the depressor response to PAMP in 8-day pregnant rats. In addition, we showed that treatment of sex hormone receptor antagonists partially prevents the attenuation of the depressor response to PAMP in 20 day pregnant rats. These findings suggested that the hypotensive response to PAMP was more attenuated in pregnant rats in mid- and late-gestation than in nonpregnant rats, and that the changes in depressor response that occur at term in pregnant rats may be mediated by sex hormones. PAMP may play some important role in cardiovascular regulation during pregnancy.

Embryo survival, and fetal and placental growth following elevation of maternal estradiol blood concentrations in the rat

High doses of estrogens cause embryonic mortality, and fetal and placental growth retardation in rats. This study addresses the physiological relevance of such findings. Estradiol benzoate (EB), by s.c. injection, or estradiol-17beta (E2), delivered by a miniosmotic pump, raised maternal E2 concentrations from only slightly above control values to 5-fold. EB (1 microgram/day) over Days 6-13, 8-13, and 11-13, and continuous infusion of E2 (15 ng/h; Days 10-13) reduced fetal survival to 0%, 0%, 22%, and 75%, respectively. Single injections of EB showed that its lethal effect declined rapidly over Days 9 (44% survival) to 13 (90% survival). Embryos died within 48 h, but death was not due to luteal failure since progesterone levels were maintained and progesterone administered with EB did not reduce mortality. Administration of EB at 1 microgram/day (Days 14-21) or E2 at 40 ng/h (Days 13-16) retarded fetal and placental growth but did not affect survival. The rat embryo is highly sensitive to elevated maternal estradiol concentrations over much of gestation. The early lethal effect implies that endogenous E2 production is carefully regulated to maintain pregnancy; the latter growth-retarding effect suggests that E2 may have a role in the normal control of fetal growth.

Temporally and spatially regulated somatic mutagenesis in mice

In mice transgenesis through oocyte injection or DNA recombination in embryonal stem (ES) cells allows mutations to be introduced into the germline. However, the earliest phenotype of the introduced mutation can eclipse later effects. We show in mice that site-specific genomic recombination can be induced in a selected cell type, B lymphocytes, at a chosen time. This precision of somatic mutagenesis was accomplished by limiting expression of a Cre recombinase-estrogen receptor fusion protein to B lymphocytes by use of tissue-specific elements in the promoter of the transgene employed. The expressed fusion protein remained inactive until derepressed by systemic administration of an exogenous ligand for the estrogen receptor, 4-OH-tamoxifen. Upon derepression the Cre recombinase enzyme deleted specific DNA segments, flanked by loxP sites, in B lymphocytes only. The efficiency of recombination in cells expressing the fusion protein could be varied from low levels to >80%, depending on the dose of ligand administered. Our work presents a paradigm applicable to other uses of site-specific recombination in somatic mutagenesis where both temporal and spatial regulation are desired.