An upstream region of the mouse ZP3 gene directs expression of firefly luciferase specifically to growing oocytes in transgenic mice

Genetic variation in zona pellucida-3 (ZP3) gene and its association with litter size variation in Kari sheep

Variation in litter size (LS) in sheep is linked to genetic factors, including the Zona pellucida-3 (ZP3) gene, which plays a role in ovine reproductive processes. This study examined the association between ZP3 gene variations and LS in Kari sheep. Two groups of 160 Kari ewes were analysed: one consistently producing singletons and another producing twins, with occasional triplets. Additionally, Madakhlasht sheep, which sometimes produce twins, and Balkhi sheep, which produce only singletons, were used as references. The entire ZP3 gene was amplified using PCR and sequenced at 30× with Next Generation Sequencing. Bioinformatics analysis identified 70 variants across the three breeds, located in upstream regions, introns, and exons. Notably, two point mutations and a six-nucleotide insertion were found upstream of the initiation codon in twin-producing Kari ewes, potentially affecting ZP3 expression and LS. Two missense mutations (I101L in exon 1 and R408H in exon 8) were heterozygous in twin-producing Kari ewes but homozygous in other groups, correlating with LS. Protein modelling suggested that the I101L mutation alters the binding site, potentially impacting protein function. These findings indicate that ZP3 gene variations influence reproductive efficiency and LS in sheep, with specific variants serving as potential markers for selective breeding to enhance LS.

Generation of Tailored Extracellular Matrix Hydrogels for the Study of In Vitro Folliculogenesis in Response to Matrisome-Dependent Biochemical Cues

While ovarian tissue cryopreservation (OTC) is an important fertility preservation option, it has its limitations. Improving OTC and ovarian tissue transplantation (OTT) must include extending the function of reimplanted tissue by reducing the extensive activation of primordial follicles (PMFs) and eliminating the risk of reimplanting malignant cells. To develop a more effective OTT, we must understand the effects of the ovarian microenvironment on folliculogenesis. Here, we describe a method for producing decellularized extracellular matrix (dECM) hydrogels that reflect the protein composition of the ovary. These ovarian dECM hydrogels were engineered to assess the effects of ECM on in vitro follicle growth, and we developed a novel method for selectively removing proteins of interest from dECM hydrogels. Finally, we validated the depletion of these proteins and successfully cultured murine follicles encapsulated in the compartment-specific ovarian dECM hydrogels and these same hydrogels depleted of EMILIN1. These are the first, optically clear, tailored tissue-specific hydrogels that support follicle survival and growth comparable to the "gold standard" alginate hydrogels. Furthermore, depleted hydrogels can serve as a novel tool for many tissue types to evaluate the impact of specific ECM proteins on cellular and molecular behavior.

Zona Pellucida Genes and Proteins: Essential Players in Mammalian Oogenesis and Fertility

All mammalian oocytes and eggs are surrounded by a relatively thick extracellular matrix (ECM), the zona pellucida (ZP), that plays vital roles during oogenesis, fertilization, and preimplantation development. Unlike ECM surrounding somatic cells, the ZP is composed of only a few glycosylated proteins, ZP1–4, that are unique to oocytes and eggs. ZP1–4 have a large region of polypeptide, the ZP domain (ZPD), consisting of two subdomains, ZP-N and ZP-C, separated by a short linker region, that plays an essential role in polymerization of nascent ZP proteins into crosslinked fibrils. Both subdomains adopt immunoglobulin (Ig)-like folds for their 3-dimensional structure. Mouse and human ZP genes are encoded by single-copy genes located on different chromosomes and are highly expressed in the ovary by growing oocytes during late stages of oogenesis. Genes encoding ZP proteins are conserved among mammals, and their expression is regulated by cis-acting sequences located close to the transcription start-site and by the same/similar trans-acting factors. Nascent ZP proteins are synthesized, packaged into vesicles, secreted into the extracellular space, and assembled into long, crosslinked fibrils that have a structural repeat, a ZP2-ZP3 dimer, and constitute the ZP matrix. Fibrils are oriented differently with respect to the oolemma in the inner and outer layers of the ZP. Sequence elements in the ZPD and the carboxy-terminal propeptide of ZP1–4 regulate secretion and assembly of nascent ZP proteins. The presence of both ZP2 and ZP3 is required to assemble ZP fibrils and ZP1 and ZP4 are used to crosslink the fibrils. Inactivation of mouse ZP genes by gene targeting has a detrimental effect on ZP formation around growing oocytes and female fertility. Gene sequence variations in human ZP genes due to point, missense, or frameshift mutations also have a detrimental effect on ZP formation and female fertility. The latter mutations provide additional support for the role of ZPD subdomains and other regions of ZP polypeptide in polymerization of human ZP proteins into fibrils and matrix.

Zona pellucida genes and proteins and human fertility

The zona pellucida (ZP) is an extracellular matrix (ECM) that surrounds all mammalian oocytes, eggs, and embryos and plays vital roles during oogenesis, fertilization, and preimplantation development. The mouse and human ZP is composed of three or four unique proteins, respectively, called ZP1-4, that are synthesized, processed, and secreted by oocytes during their growth phase. All ZP proteins have a zona pellucida domain (ZPD) that consists of ≈270 amino acids and has 8 conserved Cys residues present as four intramolecular disulfides. Secreted ZP proteins assemble into long fibrils around growing oocytes with ZP2-ZP3 dimers located periodically along the fibrils. The fibrils are cross-linked by ZP1 to form a thick, transparent ECM to which sperm must first bind and then penetrate during fertilization of eggs. Inactivation of mouse ZP1, ZP2, or ZP3 by gene targeting affects both ZP formation around oocytes and fertility. Female mice with eggs that lack a ZP due to inactivation of either ZP2 or ZP3 are completely infertile, whereas inactivation of ZP1 results in construction of an abnormal ZP and reduced fertility. Results of a large number of studies of infertile female patients strongly suggest that gene sequence variations (GSV) in human ZP1, ZP2, or ZP3 due to point, missense, or frameshift mutations have similar deleterious effects on ZP formation and female fertility. These findings are discussed in light of our current knowledge of ZP protein synthesis, processing, secretion, and assembly.

The Mouse Egg's Zona Pellucida

All mammalian eggs are surrounded by a highly specialized extracellular matrix (ECM), called the zona pellucida (ZP), that functions before, during, and after fertilization. Unlike somatic cell ECM the mouse ZP is composed of three different proteins, ZP1-3, that are synthesized and secreted by growing oocytes and assembled into long interconnected fibrils. ECM or vitelline envelope (VE) that surrounds fish, reptilian, amphibian, and avian eggs also consists of a limited number of proteins all closely related to ZP1-3. Messenger RNAs encoding ZP1-3 are expressed only by growing oocytes at very high levels from single-copy genes present on different chromosomes. Processing at the amino- and carboxy-termini of nascent ZP1-3 permits secretion of mature proteins into the extracellular space and assembly into fibrils and matrix. Structural features of nascent ZP proteins prevent assembly within secretory vesicles of growing oocytes. Homozygous knockout female mice that fail to synthesize either ZP2 or ZP3 are unable to construct a ZP, ovulate few if any eggs, and are infertile. ZP1-3 have a common structural feature, the ZP domain (ZPD), that has been conserved through 600 million years of evolution and is essential for ZP protein assembly into fibrils. The ZPD consists of two subdomains, each with four conserved cysteine residues present as two intramolecular disulfides, and resembles an immunoglobulin (Ig) domain found in a wide variety of proteins that have diverse functions, from receptors to mechanical transducers. ZP2 and ZP3 function as receptors for acrosome-reacted and acrosome-intact sperm, respectively, during fertilization of ovulated eggs, but are inactivated as sperm receptors as a result of fertilization.

Generation of an Oocyte-Specific Cas9 Transgenic Mouse for Genome Editing

The CRISPR/Cas9 system has been developed as an easy-handle and multiplexable approach for engineering eukaryotic genomes by zygote microinjection of Cas9 and sgRNA, while preparing Cas9 for microinjection is laborious and introducing inconsistency into the experiment. Here, we describe a modified strategy for gene targeting through using oocyte-specific Cas9 transgenic mouse. With this mouse line, we successfully achieve precise gene targeting by injection of sgRNAs only into one-cell-stage embryos. Through comprehensive analysis, we also show allele complexity and off-target mutagenesis induced by this strategy is obviously lower than Cas9 mRNA/sgRNA injection. Thus, injection of sgRNAs into oocyte-specific Cas9 transgenic mouse embryo provides a convenient, efficient and reliable approach for mouse genome editing.

Differentially-Expressed Genes Associated with Faster Growth of the Pacific Abalone, Haliotis discus hannai

The Pacific abalone Haliotis discus hannai is used for commercial aquaculture in Korea. We examined the transcriptome of Pacific abalone Haliotis discus hannai siblings using NGS technology to identify genes associated with high growth rates. Pacific abalones grown for 200 days post-fertilization were divided into small-, medium-, and large-size groups with mean weights of 0.26 ± 0.09 g, 1.43 ± 0.405 g, and 5.24 ± 1.09 g, respectively. RNA isolated from the soft tissues of each group was subjected to RNA sequencing. Approximately 1%–3% of the transcripts were differentially expressed in abalones, depending on the growth rate. RT-PCR was carried out on thirty four genes selected to confirm the relative differences in expression detected by RNA sequencing. Six differentially-expressed genes were identified as associated with faster growth of the Pacific abalone. These include five up-regulated genes (including one specific to females) encoding transcripts homologous to incilarin A, perlucin, transforming growth factor-beta-induced protein immunoglobulin-heavy chain 3 (ig-h3), vitelline envelope zona pellucida domain 4, and defensin, and one down-regulated gene encoding tomoregulin in large abalones. Most of the transcripts were expressed predominantly in the hepatopancreas. The genes identified in this study will lead to development of markers for identification of high-growth-rate abalones and female abalones.

Essential Role for endogenous siRNAs during meiosis in mouse oocytes

The RNase III enzyme DICER generates both microRNAs (miRNAs) and endogenous short interfering RNAs (endo-siRNAs). Both small RNA species silence gene expression post-transcriptionally in association with the ARGONAUTE (AGO) family of proteins. In mammals, there are four AGO proteins (AGO1-4), of which only AGO2 possesses endonucleolytic activity. siRNAs trigger endonucleolytic cleavage of target mRNAs, mediated by AGO2, whereas miRNAs cause translational repression and mRNA decay through association with any of the four AGO proteins. Dicer deletion in mouse oocytes leads to female infertility due to defects during meiosis I. Because mouse oocytes express both miRNAs and endo-siRNAs, this phenotype could be due to the absence of either class of small RNA, or both. However, we and others demonstrated that miRNA function is suppressed in mouse oocytes, which suggested that endo-siRNAs, not miRNAs, are essential for female meiosis. To determine if this was the case we generated mice that express a catalytically inactive knock-in allele of Ago2 (Ago2ADH) exclusively in oocytes and thereby disrupted the function of siRNAs. Oogenesis and hormonal response are normal in Ago2ADH oocytes, but meiotic maturation is impaired, with severe defects in spindle formation and chromosome alignment that lead to meiotic catastrophe. The transcriptome of these oocytes is widely perturbed and shows a highly significant correlation with the transcriptome of Dicer null and Ago2 null oocytes. Expression of the mouse transcript (MT), the most abundant transposable element in mouse oocytes, is increased. This study reveals that endo-siRNAs are essential during meiosis I in mouse females, demonstrating a role for endo-siRNAs in mammals.

Murine follicular development requires oocyte DICER, but not DROSHA

Both DICER and DROSHA are RNase III enzymes involved in the biogenesis of small noncoding RNAs. DROSHA cleaves the stem-loop portion of the primary miRNAs and produces precursor miRNAs in the nucleus, whereas DICER processes double-stranded RNA precursors into mature miRNAs and endogenous small interference RNAs in the cytoplasm. Selective inactivation of Dicer in growing oocytes of primary follicles leads to female infertility due to oocyte spindle defects. However, it remains unknown if oocyte Dicer expression in the fetal ovary is required for proper follicular development in the postnatal ovary. Moreover, the role of Drosha in folliculogenesis has never been investigated. Here, we report that conditional knockout of Dicer in prophase I oocytes of the fetal ovary led to compromised folliculogenesis, premature ovarian failure, and female infertility in the adult ovary, whereas selective inactivation of Drosha in oocytes of either the fetal or the developing ovary had no effects on normal folliculogenesis and female fertility in adulthood. Our data indicate that oocyte DICER expression in the fetal ovary is required, and oocyte DROSHA is dispensable, for postnatal follicular development and female fertility in adulthood.

Biogenesis of the mouse egg's extracellular coat, the zona pellucida

Biogenesis of the zona pellucida (ZP), the extracellular coat that surrounds all mammalian eggs, is a universal and essential feature of mammalian oogenesis and reproduction. The mouse egg's ZP consists of only three glycoproteins, called ZP1-3, that are synthesized, secreted, and assembled into an extracellular coat exclusively by growing oocytes during late stages of oogenesis while oocytes are arrested in meiosis. Expression of ZP genes and synthesis of ZP1-3 are gender-specific. Nascent ZP1-3 are synthesized by oocytes as precursor polypeptides that possess several elements necessary for their secretion and assembly into a matrix of long fibrils outside of growing oocytes. Failure to synthesize either ZP2 or ZP3 by homozygous null female mice precludes formation of a ZP during oocyte growth and, due to faulty folliculogenesis and a paucity of ovulated eggs, results in infertility. High-resolution structural analyses suggest that ZP glycoproteins consist largely of immunoglobulin (Ig)-like folds and that the glycoproteins probably arose by duplication of a common Ig-like domain. Mouse ZP1-3 share many features, particularly a ZP domain, with extracellular coat glycoproteins of eggs from other vertebrate and invertebrate animals whose origins date back more than 600 million years. These and other aspects of ZP biogenesis are discussed in this review.

Compensatory functions of histone deacetylase 1 (HDAC1) and HDAC2 regulate transcription and apoptosis during mouse oocyte development

Dramatic changes in chromatin structure and histone modification occur during oocyte growth, as well as a global cessation of transcription. The role of histone modifications in these processes is poorly understood. We report the effect of conditionally deleting Hdac1 and Hdac2 on oocyte development. Deleting either gene has little or no effect on oocyte development, whereas deleting both genes results in follicle development arrest at the secondary follicle stage. This developmental arrest is accompanied by substantial perturbation of the transcriptome and a global reduction in transcription even though histone acetylation is markedly increased. There is no apparent change in histone repressive marks, but there is a pronounced decrease in histone H3K4 methylation, an activating mark. The decrease in H3K4 methylation is likely a result of increased expression of Kdm5b because RNAi-mediated targeting of Kdm5b in double-mutant oocytes results in an increase in H3K4 methylation. An increase in TRP53 acetylation also occurs in mutant oocytes and may contribute to the observed increased incidence of apoptosis. Taken together, these results suggest seminal roles of acetylation of histone and nonhistone proteins in oocyte development.

Stem cell support of oogenesis in the human

The possibility that women produce new oocytes post-natally as part of the normal physiological function of the ovary is currently under investigation. Post-natal production of oocyte-like cells has been detected under experimental conditions in the mouse. Although these cells have many characteristics of oocytes, their potential to mature to fertilization-competence was unproven. Zou et al. (Production of offspring from a germline stem cell line derived from neonatal ovaries. Nat Cell Biol 2009;11:631-636) made use of a striking cell isolation and culture strategy to establish cultures of proliferative germ cells from both newborn and adult ovaries. Their cells, referred to as female germline stem cells (FGSCs), proliferate long-term in culture and accept and maintain expression of a transgenic marker, green fluorescent protein. When delivered to the ovaries of conditioned mice, transgene-bearing FGSC engrafted, were enclosed within follicles, and when host females were mated, transgenic offspring were produced. That proliferative female germ cells capable of giving rise to offspring were detected in adult ovaries poses the question of whether they have a physiological role. Here, we discuss Zou et al.'s data in terms of our current understanding of mouse ovarian physiology, and how this may relate to human reproductive biology and the treatment of ovarian dysfunction.

Zona pellucida glycoproteins

All mammalian eggs are surrounded by a relatively thick extracellular coat, the zona pellucida, that plays vital roles during oogenesis, fertilization, and preimplantation development. The mouse zona pellucida consists of three glycoproteins that are synthesized solely by growing oocytes and assemble into long fibrils that constitute a matrix. Zona pellucida glycoproteins are responsible for species-restricted binding of sperm to unfertilized eggs, inducing sperm to undergo acrosomal exocytosis, and preventing sperm from binding to fertilized eggs. Many features of mammalian and non-mammalian egg coat polypeptides have been conserved during several hundred million years of evolution.

Preimplantation expression of the somatic form of Dnmt1 suggests a role in the inheritance of genomic imprints

Background

Identical DNA methylation differences between maternal and paternal alleles in gametes and adults suggest that the inheritance of genomic imprints is strictly due to the embryonic maintenance of DNA methylation. Such maintenance would occur in association with every cycle of DNA replication, including those of preimplantation embryos.

Results

The expression of the somatic form of the Dnmt1 cytosine methyltransferase (Dnmt1s) was examined in cleavage-stage preimplantation mouse embryos. Low concentrations of Dnmt1s are found in 1-, 2-, 4-, and 8-cell embryos, as well as in morulae and blastocysts. Dnmt1s is present in the cytoplasm at all stages, and in the nuclei of all stages except the 1-cell, pronuclear-stage embryo. The related oocyte-derived Dnmt1o protein is also present in nuclei of 8-cell embryos, along with embryo-synthesized Dnmt1s. Dnmt1s protein expressed in 1-cell and 2-cell embryos is derived from the oocyte, whereas the embryo synthesizes its own Dnmt1s from the 2-cell stage onward.

Conclusion

These observations suggest that Dnmt1s provides maintenance methyltransferase activity for the inheritance of methylation imprints in the early mouse embryo. Moreover, the ability of Dnmt1o and Dnmt1s proteins synthesized at the same time to substitute for one another's maintenance function, but the lack of functional interchange between oocyte- and embryo-synthesized Dnmt1 proteins, suggests that the developmental source is the critical determinant of Dnmt1 function during preimplantation development.

Critical roles for Dicer in the female germline

Dicer is an essential component of RNA interference (RNAi) pathways, which have broad functions in gene regulation and genome organization. Probing the consequences of tissue-restricted Dicer loss in mice indicates a critical role for Dicer during meiosis in the female germline. Mouse oocytes lacking Dicer arrest in meiosis I with multiple disorganized spindles and severe chromosome congression defects. Oogenesis and early development are times of significant post-transcriptional regulation, with controlled mRNA storage, translation, and degradation. Our results suggest that Dicer is essential for turnover of a substantial subset of maternal transcripts that are normally lost during oocyte maturation. Furthermore, we find evidence that transposon-derived sequence elements may contribute to the metabolism of maternal transcripts through a Dicer-dependent pathway. Our studies identify Dicer as central to a regulatory network that controls oocyte gene expression programs and that promotes genomic integrity in a cell type notoriously susceptible to aneuploidy.

Recent aspects of mammalian fertilization research

Mammalian fertilization has been the subject of intensified research in recent times. Application of recombinant DNA, transgenic and gene targeting technology, in particular, to issues in mammalian fertilization has revolutionized the field. Here, we present some of the latest results coming from application of these and other technologies to four aspects of mammalian fertilization: 1. formation of the egg zona pellucida (ZP) during oocyte growth; 2. species-specific binding of sperm to the egg zona pellucida; 3. induction of the sperm acrosome reaction (AR) by the egg zona pellucida 4. binding of sperm to and fusion with egg plasma membrane. In virtually every instance, new information and new insights have come from relatively recent investigations.

A novel model for lymphocytic infiltration of the thyroid gland generated by transgenic expression of the CC chemokine CCL21

Lymphocytic infiltrates and lymphoid follicles with germinal centers are often detected in autoimmune thyroid disease (AITD), but the mechanisms underlying lymphocyte entry and organization in the thyroid remain unknown. We tested the hypothesis that CCL21, a chemokine that regulates homeostatic lymphocyte trafficking, and whose expression has been detected in AITD, is involved in the migration of lymphocytes to the thyroid. We show that transgenic mice expressing CCL21 from the thyroglobulin promoter (TGCCL21 mice) have significant lymphocytic infiltrates, which are topologically segregated into B and T cell areas. Although high endothelial venules expressing peripheral lymph node addressin were frequently observed in the thyroid tissue, lymphocyte recruitment was independent of L-selectin or lymphotoxin-alpha but required CCR7 expression. Taken together, these results indicate that CCL21 is sufficient to drive lymphocyte recruitment to the thyroid, suggest that CCL21 is involved in AITD pathogenesis, and establish TGCCL21 transgenic mice as a novel model to study the formation and function of lymphoid follicles in the thyroid.

Egg-sperm interactions at fertilization in mammals

Mammalian eggs are surrounded by a zona pellucida (ZP) that regulates egg-sperm teractions during fertilization. The ZP consists of long filaments composed of two glycoproteins, ZP2 and ZP3, that are crosslinked by a third glycoprotein, ZP1. The presence of both ZP2 and ZP3 is essential for assembling a ZP around growing oocytes, as well as for fertility of females. Acrosome-intact sperm recognize and bind to O-linked oligosaccharides linked to Ser residues at the sperm combining-site of ZP3. Structural differences in oligosaccharides on ZP3 from different species may account for whether or not sperm are able to bind to the ZP. Bound sperm undergo the acrosome reaction, penetrate the ZP, and can then fuse with egg plasma membrane. Following fertilization, sperm are unable to bind to either ZP3 or the ZP of one-cell embryos.

Lack of homologous sequence-specific DNA methylation in response to stable dsRNA expression in mouse oocytes

Double-stranded RNA (dsRNA) induces sequence-specific mRNA degradation in most eukaryotic organisms via a conserved pathway known as RNA interference (RNAi). Post-transcriptional gene silencing by RNAi is also connected with transcriptional silencing of cognate sequences. In plants, this transcriptional silencing is associated with sequence-specific DNA methylation. To address whether this mechanism operates in mammalian cells, we used bisulfite sequencing to analyze DNA in mouse oocytes constitutively expressing long dsRNA against the Mos gene. Our data show that long dsRNA induces efficient Mos mRNA knockdown but not CpG and non-CpG DNA methylation of the endogenous Mos sequence in oocytes and early embryos. These data demonstrate that dsRNA does not directly induce DNA methylation in the trans form of this sequence in these mammalian cells.

Expression of a novel murine type I IFN in the pancreatic islets induces diabetes in mice

IFN-kappa belongs to a recently identified subclass of type I IFNs. In this study, we report the cloning and preliminary characterization of the murine homologue of IFN-kappa. The gene encodes a 200-aa protein which is 38.5% homologous to human IFN-kappa. Murine IFN-kappa contains four cysteines in analogous positions to those observed in the IFN-alpha and an additional fifth unique cysteine, C174. The murine gene is located on chromosome 4, where other type I murine IFN genes, IFN-alpha and IFN-beta, are clustered. This region is syntenic with human chromosome 9 where the gene encoding IFN-kappa and the type I IFN gene cluster are found. Mouse IFN-kappa is expressed at low levels in peritoneal macrophages and its expression is up-regulated by dsRNA and IFN-gamma. Similar to previously reported transgenic mice carrying type I and type II IFNs, transgenic mice overexpressing murine IFN-kappa in the beta cells of the pancreas develop overt diabetes with hyperglycemia. Histological characterization of pancreatic islets from these transgenic mice showed inflammatory infiltrates with corresponding destruction of beta cells.

Transposition of the Drosophila hydei Minos transposon in the mouse germ line

We tested the suitability of the fly transposon Minos, a member of the Tc1/mariner superfamily, for insertional mutagenesis in the mouse germ line. We generated a transgenic mouse line expressing Minos transposase in growing oocytes and another carrying a tandem array of nonautonomous transposons. The frequency of transposition in the progeny derived from oocytes carrying both transgenes is 8.2%. Analysis of the new integration sites shows a high frequency of transpositions to a different chromosome. Thus Minos transposition could be an effective system for insertional mutagenesis and functional genomic analysis in the mouse.

Ectopic expression of the murine chemokines CCL21a and CCL21b induces the formation of lymph node-like structures in pancreas, but not skin, of transgenic mice

The CC chemokine CCL21 is a potent chemoattractant for lymphocytes and dendritic cells in vitro. In the murine genome there are multiple copies of CCL21 encoding two CCL21 proteins that differ from each other by one amino acid at position 65 (either a serine or leucine residue). In this report, we examine the expression pattern and biological activities of both forms of CCL21. We found that although both serine and leucine forms are expressed in most tissues examined, the former was the predominant form in lymphoid organs while the latter was predominantly expressed in nonlymphoid organs. When expressed in transgenic pancreas, both forms of CCL21 were capable of inducing the formation of lymph node-like structures composed primarily of T and B cells and a few dendritic cells. Induction of lymph node-like structures by these CCL21 proteins, however, could not be reproduced in every tissue. For instance, no lymphocyte recruitment or accumulation was observed when CCL21 was overexpressed in the skin. We conclude that both forms of CCL21 protein are biologically equivalent in promoting lymphocyte recruitment to the pancreas, and that their ability to induce the formation of lymph node-like structures is dependent on the tissues in which they are expressed.

Central nervous system inflammation and neurological disease in transgenic mice expressing the CC chemokine CCL21 in oligodendrocytes

To study the biological role of the chemokine ligands CCL19 and CCL21, we generated transgenic mice expressing either gene in oligodendrocytes of the CNS. While all transgenic mice expressing CCL19 in the CNS developed normally, most (18 of 26) of the CCL21 founder mice developed a neurological disease that was characterized by loss of landing reflex, tremor, and ataxia. These neurological signs were observed as early as postnatal day 9 and were associated with weight loss and death during the first 4 wk of life. Microscopic examination of the brain and spinal cord of CCL21 transgenic mice revealed scattered leukocytic infiltrates that consisted primarily of neutrophils and eosinophils. Additional findings included hypomyelination, spongiform myelinopathy with evidence of myelin breakdown, and reactive gliosis. Thus, ectopic expression of the CC chemokine CCL21, but not CCL19, induced a significant inflammatory response in the CNS. However, neither chemokine was sufficient to recruit lymphocytes into the CNS. These observations are in striking contrast to the reported activities of these molecules in vitro and may indicate specific requirements for their biological activity in vivo.

Disruption of neutrophil migration in a conditional transgenic model: evidence for CXCR2 desensitization in vivo

We developed transgenic mice conditionally expressing the neutrophil chemoattracting chemokine KC and the beta-galactosidase gene in multiple tissues. In these transgenic mice, doxycycline treatment induced a strong up-regulation in the expression of KC in several tissues, including heart, liver, kidney, skin, and skeletal muscle. Expression of KC within these tissues led to a rapid and substantial increase in the serum levels of KC (serum KC levels were higher than 200 ng/ml 24 h after treatment). Accordingly, beta-galactosidase expression was also detected after injection of doxycycline and was highest in skeletal muscle, pancreas, and liver. Surprisingly, despite expression of KC in multiple tissues, no neutrophil infiltration was observed in any of the tissues examined, including skin. Doxycycline treatment of nontransgenic mice grafted with transgenic skin caused dense neutrophilic infiltration of the grafts, but not the surrounding host skin, indicating that the KC produced in transgenic tissues was biologically active. In separate experiments, neutrophil migration toward a localized source of recombinant KC was impaired in animals overexpressing KC but was normal in response to other neutrophil chemoattractants. Analysis of transgenic neutrophils revealed that high concentrations of KC in transgenic blood had no influence on L-selectin cell surface expression but caused desensitization of the receptor for KC, CXCR2. These results confirm the neutrophil chemoattractant properties of KC and provide a mechanistic explanation for the paradoxical lack of leukocyte infiltration observed in the presence of elevated concentrations of this chemokine.

Reconstitution of enhancer function in paternal pronuclei of one-cell mouse embryos

How chromatin-mediated transcription regulates the beginning of mammalian development is currently unknown. Factors responsible for promoter repression and enhancer-mediated relief of this repression are not present in the paternal pronuclei of one-cell mouse embryos but are present in the zygotic nuclei of two-cell embryos. Here we show that coinjection of purified histones and a plasmid-encoded reporter gene into the paternal pronuclei of one-cell embryos at a specific histone-DNA concentration could recreate the behavior observed in two-cell embryos: acquisition of promoter repression and subsequent relief of this repression either by functional enhancers or by histone deacetylase inhibitors. Furthermore, the extent of enhancer-mediated stimulation in one-cell embryos depended on the acetylation status of the injected histones, on the treatment of embryos with a histone deacetylase inhibitor, and on the developmentally regulated appearance of enhancer-specific coactivator activity. The coinjected plasmids in one-cell embryos also exhibited chromatin assembly, as determined by a supercoiling assay. Thus, injection of histones into one-cell embryos faithfully reproduced the chromatin-mediated transcription observed in two-cell embryos. These results suggest that the need for enhancers to stimulate promoters through relief of chromatin-mediated repression occurs once the parental genomes are organized into chromatin. Furthermore, we present a model mammalian system in which the role of individual histones, and particular domains within the histones that are targeted in enhancer function, can be examined using purified mutant histones.

Persistence of physiological self antigen is required for the regulation of self tolerance

Endogenous Ag requirement for induction and maintenance of T cell tolerance has been extensively investigated in mice that express a transgenic Ag and/or its cognate transgenic TCR. In contrast, studies on tolerance for physiologically expressed self Ag and normal T cells are limited. Herein, we showed that the murine ovarian-specific ZP3 Ag is detectable from birth. Tolerance to ZP3 is detected in female relative to male mice. In comparison to males, 100-fold more ovarian peptide (pZP3) is required to elicit a comparable pathogenic response in females. Female tolerance to pZP3 was dependent on the presence of endogenous ovarian Ag, because neonatal ovariectomy converted the female response to that of males. Moreover, in female mice that were ovariectomized from the ages of 1-6 wk, the pZP3 responses were enhanced to the male level if ovaries were removed up to 7 days, but not 3 days, before adult challenge with pZP3. Thus, the physiologically expressed ZP3 Ag induces tolerance to pZP3, and the maintenance of tolerance is critically dependent on the continuous presence of the endogenous ovarian Ag. In contrast, exposure to endogenous ovarian Ag confined to the neonatal period is insufficient for the induction and maintenance of tolerance to ZP3.

Expression analysis of Escherichia coli lacZ reporter gene in transgenic mice

To define a gene expression mechanism, it is often advantageous to use a reporter gene and transgenic mouse. The lacZ reporter gene is particularly useful for studies of the cis-regulatory element for tissue-specific expression in transgenic mice because of the ease of the enzyme assay and visualization on sections. In this report, we describe our method for examining the cis-regulatory element in transgenic mice, including choice of the lacZ gene, generation of transgenic mice, and analysis of beta-galactosidase activity.

Spatial and temporal changes in natural and target deprivation-induced cell death in the mouse inferior olive

The survival of inferior olive neurons is dependent on contact with cerebellar Purkinje cells. There is evidence that this dependence changes with time. Because inferior olivary axons, called climbing fibers, already show significant topographical ordering in cerebellar target zones during late embryogenesis in mice, the question arises as to whether olive neurons are dependent on target Purkinje cells for their survival at this early age. To better characterize this issue, inferior olive development was studied in two transgenic mouse mutants, wnt-1 and L7ADT, with embryonic and early postnatal loss of cerebellar target cells, respectively, and compared to that in the well-studied mutant, Lurcher. Morphological criteria as well as quantitative measures of apoptosis were considered in this developmental analysis. Survival of inferior olive neurons is observed to be independent of Purkinje cells throughout embryogenesis, but dependence begins immediately at birth in both wild types and mutants. Thereafter, wild types and mutants show a rapid increase in olive cell apoptosis, with a peak at postnatal day 4, followed by a period of low-level, but significant, apoptosis that continues to at least postnatal day 11; the main difference is that apoptosis is quantitatively enhanced in the mutants compared to wild types. The multiphasic course of these effects roughly parallels the known phases of climbing fiber synaptogenesis. In addition, despite significant temporal differences among the mutants with respect to absolute numbers of dying cells, there are common spatial features suggestive of distinct intrinsic programs linking different olivary subnuclei to their targets.

Transgenic expression of the chemokine receptor encoded by human herpesvirus 8 induces an angioproliferative disease resembling Kaposi's sarcoma

Human herpesvirus 8 (HHV8, also known as Kaposi's sarcoma [KS]-associated herpesvirus) has been implicated as an etiologic agent for KS, an angiogenic tumor composed of endothelial, inflammatory, and spindle cells. Here, we report that transgenic mice expressing the HHV8-encoded chemokine receptor (viral G protein-coupled receptor) within hematopoietic cells develop angioproliferative lesions in multiple organs that morphologically resemble KS lesions. These lesions are characterized by a spectrum of changes ranging from erythematous maculae to vascular tumors, by the presence of spindle and inflammatory cells, and by expression of vGPCR, CD34, and vascular endothelial growth factor. We conclude that vGPCR contributes to the development of the angioproliferative lesions observed in these mice and suggest that this chemokine receptor may play a role in the pathogenesis of KS in humans.

Secretion of zona pellucida glycoprotein mZP2 by growing oocytes from mZP3(+/+) and mZP3(-/-) mice

The mouse egg extracellular coat, or zona pellucida (ZP), is composed of three glycoproteins, called mZP1-3, which are synthesized and secreted concomitantly by growing oocytes. Disruption of the mZP3 gene by targeted mutagenesis yields mice that are homozygous nulls (mZP3(-/-)). Growing oocytes from mZP3(-/-) mice do not synthesize mZP3 mRNA or protein and, as a result, do not assemble a ZP. Here, we examined secretion of mZP2 by growing oocytes and eggs from mZP3(-/-) mice, as well as incorporation of mZP2 into the ZP of oocytes from mZP3(+/+) mice. Laser scanning confocal microscopy (LSCM) of antibody-labeled samples showed that, indeed, mZP2 was synthesized and secreted by oocytes isolated from mZP3(-/-) mice and cultured in vitro. Nascent mZP2 was found in the culture medium, associated with the surface of the plasma membrane of growing oocytes, and in the oocyte cytoplasm. By contrast, mZP2 was barely detectable at any of these sites when ovulated eggs from mZP3(-/-) mice were examined. Examination of oocytes from wild-type (mZP3(+/+)) mice showed that, while a portion of nascent mZP2 was assembled into the ZP (approximately 40%), here too a significant fraction was secreted into the culture medium (approximately 60%). Similar results also were obtained when intact pre-antral follicles were isolated from mZP3(+/+) mice and cultured in vitro. Several of these observations are consistent with previous results obtained with oocytes from heterozygous null mice (mZP3(+/-)). Furthermore, the results suggest that ZP assembly from nascent glycoproteins may be a stochastic process that requires the presence of both mZP2 and mZP3 and occurs completely outside the growing oocyte.

Targeted expression of Bcl-2 in mouse oocytes inhibits ovarian follicle atresia and prevents spontaneous and chemotherapy-induced oocyte apoptosis in vitro

Members of the Bcl-2 family serve as central checkpoints for cell death regulation, and overexpression of Bcl-2 is known to inhibit apoptosis in many cell types. To determine whether targeted expression of Bcl-2 could be used to protect female germ cells from apoptosis, we generated transgenic mice expressing fully functional human Bcl-2 protein only in oocytes. Transgenic mice were produced using a previously characterized 480-bp fragment of the mouse zona pellucida protein-3 (ZP3) gene 5'-flanking region to direct oocyte-specific expression of a human bcl-2 complementary DNA. Immunohistochemical analyses using a human Bcl-2-specific antibody showed that transgene expression was restricted to growing oocytes and was not observed in the surrounding ovarian somatic cells or in any other nonovarian tissues. Histomorphometric analyses revealed that ovaries collected from transgenic female mice possessed significantly fewer atretic small preantral follicles compared with wild-type sisters, resulting in a larger population of healthy maturing follicles per ovary. However, the number of oocytes ovulated in response to exogenous gonadotropin priming and the number of pups per litter were not significantly different among wild-type vs. transgenic female mice. Nonetheless, oocytes obtained from transgenic mice and cultured in vitro were found to be resistant to spontaneous and anticancer drug-induced apoptosis. We conclude that targeted expression of Bcl-2 only in oocytes can be achieved as a means to convey resistance of the female germ line to naturally occurring and chemotherapy-induced apoptosis.

Gene expression and chromatin organization during mouse oocyte growth

Mouse oocytes can be classified according to their chromatin organization and the presence [surrounded nucleolus (SN) oocytes] or absence [nonsurrounded nucleolus (NSN) oocytes] of a ring of Hoechst-positive chromatin around the nucleolus. Following fertilization only SN oocytes are able to develop beyond the two-cell stage. These studies indicate a correlation between SN and NSN chromatin organization and the developmental competence of the female gamete, which may depend on gene expression. In the present study, we have used the HSP70.1Luc transgene (murine HSP70.1 promoter + reporter gene firefly luciferase) to analyze gene expression in oocytes isolated from ovaries of 2-day- to 13-week-old females. Luciferase was assayed on oocytes after classification as SN or NSN type. Our data show that SN oocytes always exhibit a higher level of luciferase activity, demonstrating a higher gene expression in this category. Only after meiotic resumption, metaphase II oocytes derived from NSN or SN oocytes acquire the same level of transgene expression. We suggest that the limited availability of transcripts and corresponding proteins, excluded from the cytoplasm until GVBD in NSN oocytes, could explain why these oocytes have a lower ability to sustain embryonic development beyond the two-cell stage at which major zygotic transcription occurs. With this study we have furthered our knowledge of epigenetic regulation of gene expression in oogenesis.

Species specificity of human and murine anti-ZP3 synthetic peptide antisera and use of the antibodies for localization and identification of ZP3 or ZPC domains of functional significance

The mammalian zona pellucida has an important function in the fertilization process. The zona pellucida protein 3 (ZP3 or ZPC) is the ligand for primary sperm binding and induces the acrosome reaction. In various species, ZP3 primary structures are highly conserved as revealed by cDNA cloning. The objective of these studies was to localize ZP3 protein using antisera generated against defined synthetic peptides that are specific for mouse or for human ZP3. Immunohistochemistry and transmission electron microscopy were applied to murine and human ovary sections. Immunochemical studies were performed in hemizonae pellucidae from microbisected human oocytes. Using the competitive hemizona assay and various anti-ZP3 antibodies, we further intended to identify human ZP3 epitopes of functional significance. Our results showed that antiserum AS ZP3-9 (mouse specific) detected mouse ZP3 protein in mouse oocytes and in immunoblots, whereas AS ZP3-14 (human specific) detected human ZP3 protein in human ovary sections, native hemizonae pellucidae and in immunoblots. ZP3 material was also detected in cumulus cells by immunohistochemistry. Ultrastructural studies showed an equal distribution of ZP3 throughout the zona pellucida. The human competitive hemizona assay revealed that none of the anti-ZP3 synthetic peptide antisera affected sperm binding suggesting that those epitopes are not involved in primary sperm binding. Anti-porcine ZP3 beta protein antibodies (polyclonal) blocked human sperm-zona pellucida binding. In summary, these anti-ZP3 synthetic peptide antibodies specifically reacted with intact ZP3 protein (murine and human) but did not inhibit human sperm-zona pellucida binding; anti-ZP3 antibodies can therefore be used as biomarkers for ZP3 localization and function.

Two testicular cDNA clones suppressed by gonadotropin stimulation exhibit ZP2- and ZP3-like structures in Japanese eel

A single injection of human chorionic gonadotropin (HCG) can induce complete spermatogenesis in immature eel testes consisting of premitotic spermatogonia. To understand the regulatory mechanisms of spermatogenesis, we have applied a subtractive hybridization method to identify genes in which changes in expression occur after HCG treatment in vivo. The subtraction was carried out 24 hours after HCG injection. Two up-regulated and six down-regulated cDNA clones by HCG stimulation were isolated, and named eel spermatogenesis-related substance (eSRS) 1 to 8. In this paper, down-regulated cDNA clones of eSRS3 and eSRS4 were sequenced. A homology search showed that eSRS3 and eSRS4 have amino acid sequences similar to those of the ZP-domains of zona pellucida sperm-binding protein (ZP)-2 and 3, respectively. Transcripts of eSRS3 and eSRS4 have been detected only in immature testes and ovaries. Both transcripts disappeared immediately after HCG injection and were not detected in testes throughout the experimental period. To determine whether HCG action on down-regulation of eSRS3 and eSRS4 transcription is direct or mediated through 11-ketotestosterone (11-KT), a spermatogenesis-inducing steroid in eel, we investigated the effect of HCG and 11-KT on testicular eSRS3 and eSRS4 mRNA transcription in vitro. Northern blot analysis using poly(A)+ RNA extracted from cultured testis showed that both HCG and 11-KT suppressed the mRNA transcription of both eSRS3 and eSRS4. We speculate that eSRS3 and eSRS4 may play important roles in the prevention of spermatogenesis in the eel.

Expression of an Xist promoter-luciferase construct during spermatogenesis and in preimplantation embryos: regulation by DNA methylation

Dosage compensation for X-linked genes in mammals is accomplished by inactivating one of the two X chromosomes in females, a process involving a regulatory gene, Xist (X-inactive specific transcript). Xist maps to the X-inactivation centre and is expressed from the inactive X chromosome in female somatic cells and at the time of X inactivation during spermatogenesis in the male. In female preimplantation embryos, Xist demonstrates imprinting in that the paternal allele inherited from the sperm is preferentially expressed. This preferential paternal Xist expression is correlated with paternal X inactivation in the extraembryonic lineages at the blastocyst stage. We have analysed a 233-bp Xist promoter fragment (nt -220 to +13) for its ability to direct appropriate expression and its regulation by DNA methylation. This minimal promoter sequence directs expression of the luciferase reporter gene following injection of the construct into one-cell embryos. In vitro methylation of the construct before injection represses transcription. In six different transgenic lines, expression of the Xist promoter-luciferase transgene occurs only in the testis of the males (as for the endogenous Xist gene). The testis-specific expression is correlated with hypomethylation of the transgene, although to different extents in different lines. Following paternal transmission, expression of the Xist promoter-luciferase construct in preimplantation embryos is correlated with degree of hypomethylation in the testis and the degree of hypomethylation of the transgene in embryos at the morula stage. It is concluded that the patterns of methylation of the transgene in sperm (and in microinjected transgenes) can regulate the activity of the Xist promoter in the preimplantation embryo and thus support the hypothesis that gametic methylation patterns govern imprinted expression of the endogenous Xist gene in development.

DNA bending is induced by binding of the peroxisome proliferator-activated receptor gamma 2 heterodimer to its response element in the murine lipoprotein lipase promoter

The peroxisome proliferator activated receptor gamma 2 (PPAR gamma 2) is a critical transcriptional regulator of adipogenesis. Lipoprotein lipase is one of the earliest genes induced following exposure of pre-adipocytes to PPAR gamma 2 ligands such as the thiazolidinediones. A unique PPAR gamma 2 DNA recognition element was mapped to the region between -171 to -149 bp of the murine LPL promoter, based on transfection analysis of deletion constructs and gel retention assays using bacterially expressed, affinity purified recombinant proteins. Circular permutation analysis determined that binding of the PPAR gamma 2/retinoic acid X receptor (RXR) heterodimer to its LPL promoter recognition element induced DNA bending at an angle of approximately 46 degrees. Parallel studies using an optimal PPAR recognition element obtained a comparable bending angle of 56 degrees. This is the first demonstration that binding of a PPAR protein to its recognition element causes a distortion of the DNA configuration. It indicates that PPAR gamma 2 utilizes a common mechanism shared by other nuclear hormone receptor proteins reported to induce bending at their DNA binding sites.

Hypoxia stimulates human preproendothelin-1 promoter activity in transgenic mice

Significant elevations in endothelin (ET)-1 levels accompany many diseases, but the underlying regulatory mechanisms are unclear. To investigate the in vivo regulation of human preproendothelin-1 (PPET-1), we examined the activity of the PPET-1 promoter in transgenic mice exposed to hypoxia. Mice expressing one of three PPET-1 promoter-luciferase (PPET-1/LUC) reporter transgenes (approximately 2.5 kb, 138 bp, or none of the 5'-flanking sequences of the PPET-1 gene) were generated. LUC expression was reduced in mice with a truncated 138-bp PPET-1 promoter. Exposure of mice bearing the 2.5-kb PPET-1/LUC transgene to hypoxia (10% O2 for 24 h) increased LUC expression sixfold in pulmonary tissue but only twofold in other tissues. In situ hybridization revealed the strongest transgene expression in the pulmonary vasculature and bronchiolar epithelium. These data are consistent with the hypothesis that hypoxic induction of the PPET-1 gene leads to increased pulmonary production of ET-1 in diseases associated with low O2 tension.

The myeloperoxidase gene proximal enhancer directs hematopoietic-specific expression in transgenic mice

The myeloperoxidase (MPO) gene is expressed specifically in immature myeloid cells. The MPO gene includes a promoter proximal enhancer which is coincident with DNaseI hypersensitive chromatin sites and is specifically active in myeloid cell lines. We developed transgenic murine lines in which 1.3 kb of murine MPO proximal 5' flanking region DNA was linked to a TATAA homology and RNA initiation site derived from the HSV-TK promoter and to a luciferase reporter (MPOTKLUC). In each of six founder lines, high-level luciferase activity was evident in marrow, thymus and spleen. Modest- to high-level luciferase expression was also evident in brain and in the heart in several of the lines, and luciferase activity was at or near background levels in lung, liver, kidney, stomach, colon, bladder, skeletal muscle, skin and small intestine in all of the MPOTKLUC transgenic mice. Within marrow cells, luciferase activity was evident in myeloid (GR-1+), B lymphoid (B220+) and T-lymphoid (CD4+) cells. Additional regulatory regions, thus, may be required to further restrict MPO gene expression to immature myeloid cells.

Zp3-cre, a transgenic mouse line for the activation or inactivation of loxP-flanked target genes specifically in the female germ line

The site-specific DNA recombinase Cre is being used to develop a new generation of tools for controlling gene expression in mice [1]. Cre mediates the recombination of two directly repeated target (loxP) sites to a single loxP site, with concomitant excision of the DNA segment flanked by the loxP sites (the 'floxed' DNA). Such recombination can function to activate a gene by excising a floxed DNA segment that blocks expression because it either separates the regulatory and coding sequences of the gene [2] or interrupts the gene's open reading frame. Conversely, DNA excision can inactivate a gene if an essential fragment of the gene is floxed [3]. Gene activation or inactivation in vivo can be achieved by mating two different animals, one carrying a 'target gene' with appropriately placed loxP sites and one carrying a cre transgene. In most cases, the specificity of the system is dependent upon stringent regulation of cre expression. We describe here a mouse line in which cre expression is controlled by regulatory sequences from the mouse zona pellucida 3 (Zp3) gene, which is normally expressed exclusively in the growing oocyte prior to the completion of the first meiotic division [4]. We show that in target-bearing Zp3-cre mice, Cre-mediated recombination of the target gene apparently occurs in 100 % of oocytes. Moreover, Cre activity is not detected in the somatic tissues of most target-bearing Zp3-cre mice. Potential uses for this mouse line are discussed.

Molecular cloning, structural analysis, and expression of carp ZP3 gene

Two types of cDNAs coding for a major component of carp egg membrane were clones from a carp ovarian cDNA library. They encode polypeptides of 422-424 amino acid residues whose sequences are homologous to those of medaka and mammalian ZP3. Similar to the mammalian ZP3 genes, carp ZP3 gene also consists of eight exons and seven introns. Carp ZP3 genes are 2.9 kb in length and present in multiple forms. Carp ZP3 is a glycoprotein of 45 kDa. It was transcribed and translated exclusively in oocytes, in contrast with medaka ZP3, which was synthesized in liver. The transcription of carp ZP3 starts very early in oogenesis, but translation occurs during vitellogenesis, as it is present in vitellogenic but not in previtellogenic oocytes. ZP3 content in oocytes increases as vitellogenesis proceeds.

A unique role for enhancers is revealed during early mouse development

Transcription and replication of genes in mammalian cells always requires a promoter or replication origin, respectively, but the ability of enhancers to stimulate these regulatory elements and the interactions that mediate this stimulation are developmentally acquired. The primary function of enhancers is to prevent repression, which appears to result from particular components of chromatin structure. Factors responsible for this repression are present in the maternal nucleus of oocytes and its descendant, the maternal pronucleus of mouse 1-cell embryos and in mouse 2-cell embryos, but are absent in the paternal pronucleus. Thus, enhancers are not needed to achieve efficient transcription and replication in paternal pronuclei. However, enhancers, even in the presence of their specific activation protein, are inactive prior to formation of a 2-cell embryo, suggesting that a coactivator essential for enhancer function is not available until zygotic gene expression begins. Furthermore, enhancer stimulation of transcription appears to be mediated through a promoter transcription factor, but this interaction can change as cells undergo differentiation, switching from a TATA-box independent to a TATA-box dependent mode.

Regulation of gene expression at the beginning of mammalian development

The maternal to zygotic transition can be viewed as a cascade of events that begins when fertilization triggers the zygotic clock that delays early ZGA until formation of a 2-cell embryo. Early ZGA, in turn, appears to be required for expression of late ZGA, and late ZGA is required to form a 4-cell embryo. ZGA in mammals is a time-dependent mechanism rather than a cell cycle-dependent mechanism that delays both transcription and translation of nascent transcripts. Thus, zygotic gene transcripts appear to be handled differently than maternal mRNA, a phenomenon also observed in Xenopus (55). The length of this delay is species-dependent, occurring at the 2-cell stage in mice, the 4-8-cell stage in cows and humans, and the 8-16-cell stage in sheep and rabbits (4). However, concurrent with formation of a 2-cell embryo in the mouse and rabbit (47,56), perhaps in all mammals, a general chromatin-mediated repression of promoter activity appears. Repression factors are inherited by the maternal pronucleus from the oocyte but are absent in the paternal pronucleus and not available until sometime during the transition from a late 1-cell to a 2-cell embryo. This means that paternally inherited genes are exposed to a different environment in fertilized eggs than are maternally inherited genes, a situation that could contribute to genomic imprinting. Chromatin-mediated repression of promoter activity prior to ZGA is similar to what is observed during Xenopus embryogenesis (31,32) and ensures that genes are not expressed until the appropriate time in development when positive acting factors, such as enhancers, can relieve this repression. The ability to use enhancers appears to depend on the acquisition of specific co-activators at the 2-cell stage in mice and perhaps later in other mammals (47,56), concurrent with ZGA. Even then, the mechanism by which enhancers communicate with promoters changes during development (Fig. 2), providing an opportunity for enhancer-mediated stimulating of TATA-less promoters (e.g. housekeeping genes) early during development while eliminating this mechanism later during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Transgenic mice with reduced numbers of functional sperm receptors on their eggs reproduce normally

To initiate fertilization in mice, free-swimming sperm bind to mZP3, an approximately 83-kDa glycoprotein present in the ovulated egg zona pellucida (ZP). mZP3 is located periodically along the filaments that constitute the ZP. Sperm recognize and bind to specific oligosaccharides linked to one or more of five Ser residues clustered in the carboxy-terminal one-third of the mZP3 polypeptide. When all five Ser residues are converted to nonhydroxy amino acids by site-directed mutagenesis of the mZP3 gene, an inactive form of mZP3, called mZP3[ser], is secreted by embryonal carcinoma cells stably transfected with the mutated gene. Here, seven independent transgenic mouse lines were established that harbor the mutated mZP3 gene. In all lines, the mutant gene is expressed by growing oocytes and mZP3[ser] is synthesized, secreted, and incorporated into the ZP. Purified mZP3[ser] prepared from ovaries of transgenic mice, like mZP3[ser] from transfected embryonal carcinoma cells, is inactive in sperm binding assays in vitro. On the other hand, the presence of mZP3[ser] in the ZP does not significantly affect either the binding of sperm to ovulated eggs in vitro or the reproduction of the mice, i.e., the transgenic mice are fertile, breed at normal intervals, and produce litters of normal sizes. These results indicate that the number of functional sperm receptors in the ZP can be reduced by more than 50% without adversely affecting fertilization of eggs in vivo.

Targeting gene expression to the vascular wall in transgenic mice using the murine preproendothelin-1 promoter

To develop a system for overexpressing genes in the vascular wall, we created transgenic mice using the reporter gene luciferase and the murine preproendothelin-1 promoter. In vitro analysis suggested that the murine 5'-flanking region contained endothelial-specific elements in a 5.9-kb fragment. Five transgenic mice colonies established from independent founders all exhibited the highest level of luciferase activity in the aorta with up to 8,540 light units per microgram of protein. Immunohistochemistry with anti-luciferase antisera revealed high levels of expression in the endothelial cells of both large and small arteries and lower levels of expression in veins and capillaries. Significant expression was also seen in arterial smooth muscle cells and in select epithelial surfaces which is consistent with the known distribution of endothelin-1 in mammals. The further demonstrate the targeting capability of this system, we overexpressed the lipid-peroxidating enzyme, human 15-lipoxygenase, in the vessel wall of transgenic mice. As with luciferase, expression of active enzyme and immunohistochemical localization in vascular cells were documented in transgenic animals. Hence, this new system can be used to direct expression of molecules to the vascular wall for the purpose of examining the biological significance of either overexpression or inhibition of select proteins.

In vitro and in vivo analysis of murine lipoprotein lipase gene promoter: tissue-specific expression

Lipoprotein lipase, an enzyme of central importance to lipid metabolism, is most abundant in adipose tissues, cardiac and skeletal muscle, and portions of the brain. The current work examined the murine lipoprotein lipase promoter using transient transfection, gel-retention analyses, and transgenic mice. Maximum expression of the luciferase reporter gene in transfected cells was observed with -101 bp of the promoter. Nuclear extracts from tissues expressing lipoprotein lipase contained DNA binding proteins that recognize the CCAAT box (-64 bp) and an octamer motif (-46 bp); this combination of factors was absent in nonexpressing tissues. Transgenic mice from three of five founders prepared with -1,824-bp promoter constructs expressed the luciferase reporter gene at highest levels in brown adipose tissue and brain. These findings suggest that the -1,824-bp promoter region contains sequence elements responsible for the tissue-specific transcription of lipoprotein lipase in vivo.