Effect of exogenous hormones on the ovulation of primary and secondary oocytes in LT/Sv strain mice

The role of estrogen in Alzheimer's disease pathogenesis and therapeutic potential in women

Estrogens are pivotal regulators of brain function throughout the lifespan, exerting profound effects from early embryonic development to aging. Extensive experimental evidence underscores the multifaceted protective roles of estrogens on neurons and neurotransmitter systems, particularly in the context of Alzheimer's disease (AD) pathogenesis. Studies have consistently revealed a greater risk of AD development in women compared to men, with postmenopausal women exhibiting heightened susceptibility. This connection between sex factors and long-term estrogen deprivation highlights the significance of estrogen signaling in AD progression. Estrogen's influence extends to key processes implicated in AD, including amyloid precursor protein (APP) processing and neuronal health maintenance mediated by brain-derived neurotrophic factor (BDNF). Reduced BDNF expression, often observed in AD, underscores estrogen's role in preserving neuronal integrity. Notably, hormone replacement therapy (HRT) has emerged as a sex-specific and time-dependent strategy for primary cardiovascular disease (CVD) prevention, offering an excellent risk profile against aging-related disorders like AD. Evidence suggests that HRT may mitigate AD onset and progression in postmenopausal women, further emphasizing the importance of estrogen signaling in AD pathophysiology. This review comprehensively examines the physiological and pathological changes associated with estrogen in AD, elucidating the therapeutic potential of estrogen-based interventions such as HRT. By synthesizing current knowledge, it aims to provide insights into the intricate interplay between estrogen signaling and AD pathogenesis, thereby informing future research directions and therapeutic strategies for this debilitating neurodegenerative disorder.

Oocyte Spontaneous Activation: An Overlooked Cellular Event That Impairs Female Fertility in Mammals

In mammals, including humans, mature oocytes are ovulated into the oviduct for fertilization. Normally, these oocytes are arrested at metaphase of the second meiosis (MII), and this arrest can be maintained for a certain period, which is essential for fertilization in vivo and oocyte manipulations in vitro, such as assisted reproduction in clinics and nuclear/spindle transfer in laboratories. However, in some species and under certain circumstances, exit from MII occurs spontaneously without any obvious stimulation or morphological signs, which is so-called oocyte spontaneous activation (OSA). This mini-review summarizes two types of OSA. In the first type (e.g., most rat strains), oocytes can maintain MII arrest in vivo, but once removed out, oocytes undergo OSA with sister chromatids separated and eventually scattered in the cytoplasm. Because the stimulation is minimal (oocyte collection itself), this OSA is incomplete and cannot force oocytes into interphase. Notably, once re-activated by sperm or chemicals, those scattered chromatids will form multiple pronuclei (MPN), which may recapitulate certain MPN and aneuploidy cases observed in fertility clinics. The second type of OSA occurs in ovarian oocytes (e.g., certain mouse strains and dromedary camel). Without ovulation or fertilization, these OSA-oocytes can initiate intrafollicular development, but these parthenotes cannot develop to term due to aberrant genomic imprinting. Instead, they either degrade or give rise to ovarian teratomas, which have also been reported in female patients. Last but not the least, genetic models displaying OSA phenotypes and the lessons we can learn from animal OSA for human reproduction are also discussed.

Spindle assembly checkpoint-related failure perturbs early embryonic divisions and reduces reproductive performance of LT/Sv mice

The phenotype of the LT/Sv strain of mice is manifested by abnormalities in oocyte meiotic cell-cycle, spontaneous parthenogenetic activation, teratomas formation, and frequent occurrence of embryonic triploidy. These abnormalities lead to the low rate of reproductive success. Recently, metaphase I arrest of LT/Sv oocytes has been attributed to the inability to timely inactivate the spindle assembly checkpoint (SAC). As differences in meiotic and mitotic SAC functioning were described, it remains obscure whether this abnormality is limited to the meiosis or also impinges on the mitotic divisions of LT/Sv embryos. Here, we show that a failure to inactivate SAC affects mitoses during preimplantation development of LT/Sv embryos. This is manifested by the prolonged localization of MAD2L1 on kinetochores of mitotic chromosomes and abnormally lengthened early embryonic M-phases. Moreover, LT/Sv embryos exhibit elevated frequency of abnormal chromosome separation during the first mitotic division. These abnormalities participate in severe impairment of preimplantation development and significantly decrease the reproductive success of this strain of mice. Thus, the common meiosis and mitosis SAC-related failure participates in a complex LT/Sv phenotype.

Evaluation of triploid<-->diploid and trisomy-3<-->diploid mouse chimeras as models for investigating how lineage restriction occurs in confined placental mosaicism

Human confined placental mosaicism (CPM), where the placental trophoblast is mosaic for a chromosome abnormality but the fetus is chromosomally normal, can cause problems for prenatal diagnosis, but its causes are poorly understood. Tetraploid↔diploid chimeras provide a model for the development of one type of CPM, but animal models for other types of restricted mosaicism are needed. The objective of the present study was to evaluate triploid↔diploid and trisomy-3↔diploid chimeric mouse conceptuses as new models for investigating the development of restricted mosaicism. Novel stocks of mice were generated to produce triploid and trisomy-3 embryos that could be identified by DNA in situ hybridisation to a chromosome 3 transgenic marker. Triploid↔diploid and trisomy-3↔diploid mouse chimeras were produced by embryo aggregation, and the contribution of triploid or trisomy-3 cells was analysed in the fetus and extraembryonic tissues. Only two trisomy-3↔diploid chimeras were analysed but trisomy-3 cells contributed well to all lineages, so these chimeras did not show restricted mosaicism. In contrast, triploid cells usually contributed poorly to all lineages in the ten 3n↔2n chimeras analysed. They contributed more to the primitive endoderm derivatives than other lineages and were present in the primitive endoderm derivatives of all ten chimeras, but excluded from fetuses and trophectoderm derivatives in some cases. This pattern of restricted mosaicism differs from that reported for tetraploid cells in tetraploid↔diploid chimeras, and triploid↔diploid chimeras may provide a useful model for the development of some types of restricted mosaicism in human conceptuses.

Cytostatic activity develops during meiosis I in oocytes of LT/Sv mice

Oocytes of wild-type mice are ovulated as the secondary oocytes arrested at metaphase of the second meiotic division. Their fertilization or parthenogenetic activation triggers the completion of the second meiotic division followed by the first embryonic interphase. Oocytes of the LT/Sv strain of mice are ovulated either at the first meiotic metaphase (M I) as primary oocytes or in the second meiotic metaphase (M II) as secondary oocytes. We show here that during in vitro maturation a high proportion of LT/Sv oocytes progresses normally only until metaphase I. In these oocytes MAP kinase activates shortly after histone H1 kinase (MPF) activation and germinal vesicle breakdown. However, MAP kinase activation is slightly earlier than in oocytes from wild-type F1 (CBA/H x C57Bl/10) mice. The first meiotic spindle of these oocytes forms similarly to wild-type oocytes. During aging, however, it increases in size and finally degenerates. In those oocytes which do not remain in metaphase I the extrusion of first polar bodies is highly delayed and starts about 15 h after germinal vesicle breakdown. Most of the oocytes enter interphase directly after first polar body extrusion. Fusion between metaphase I LT/Sv oocytes and wild-type mitotic one-cell embryos results in prolonged M-phase arrest of hybrids in a proportion similar to control LT/Sv oocytes and control hybrids made by fusion of two M I LT/Sv oocytes. This indicates that LT/Sv oocytes develop cytostatic factor during metaphase I. Eventually, anaphase occurs spontaneously and the hybrids extrude the polar body and form pronuclei in a proportion similar as in controls. In hybrids between LT/Sv metaphase I oocytes and wild-type metaphase II oocytes (which contain cytostatic factor) anaphase I proceeds at the time observed in control LT/Sv oocytes and hybrids between two M I LT/Sv oocytes, and is followed by the parthenogenetic activation and formation of interphase nuclei. Also the great majority of hybrids between M I and M II wild-type oocytes undergoes the anaphase but further arrests in a subsequent M-phase. These observations suggest that an internally triggered anaphase I occurs despite the presence of the cytostatic activity both in LT/Sv and wild-type M I oocytes. Anaphase I triggering mechanism must therefore either inactivate or override the CSF activity. The comparison between spontaneous and induced activation of metaphase I LT/Sv oocytes shows that mechanisms involved in anaphase I triggering are altered in these oocytes. Thus, the prolongation of metaphase I in LT/Sv oocytes seems to be determined by delayed anaphase I triggering and not provoked directly by the cytostatic activity.

The Mos/mitogen-activated protein kinase (MAPK) pathway regulates the size and degradation of the first polar body in maturing mouse oocytes

Mos is an upstream activator of mitogen-activated protein kinase (MAPK) and, in mouse oocytes, is responsible for metaphase II arrest. This activity has been likened to its function in Xenopus oocytes as a component of cytostatic factor. Thus, Mos-deficient female mice (MOS-/-) are less fertile and oocytes derived from these animals fail to arrest at metaphase II and undergo parthenogenetic activation [Colledge, W. H., Carlton, M. B. L., Udy, C. B. & Evans, M. J. (1994) Nature (London) 370, 65-68 and Hashimoto, N., Watanabe, N., Furuta. Y., Tamemoto, B., Sagata, N., Yokoyama, M., Okazaki, K., Nagayoshi, M., Takeda, N., Ikawa, Y. & Aizawa, S. (1994) Nature (London) 370, 68-71]. Here we show that maturing MOS-/- oocytes fail to activate MAPK throughout meiosis, while p34cdc2 kinase activity is normal until late in metaphase II when it decreases prematurely. Phenotypically, the first meiotic division of MOS-/- oocytes frequently resembles mitotic cleavage or produces an abnormally large polar body. In these oocytes, the spindle shape is altered and the spindle fails to translocate to the cortex, leading to the establishment of an altered cleavage plane. Moreover, the first polar body persists instead of degrading and sometimes undergoes an additional cleavage, thereby providing conditions for parthenogenesis. These studies identify meiotic spindle formation and programmed degradation of the first polar body as new and important roles for the Mos/MAPK pathway.

Unusual cytoskeletal and chromatin configurations in mouse oocytes that are atypical in meiotic progression

Meiotic maturation progresses atypically in oocytes of strain LT/Sv and I/LnJ mice. LT/Sv occytes show a high frequency of metaphase I-arrest and parthenogenetic activation. I/LnJ oocytes display retarded kinetics of meiotic maturation and a high frequency of metaphase I-arrest. Some I/LnJ oocytes fail to resume meiosis. Changes in the configuration of chromatin, microtubules, and centrosomes are associated with specific stages of meiotic progression. In this study, the configuration of these subcellular components was examined in LT/Sv, I/LnJ, and C57BL/6J (control) oocytes either freshly isolated from large antral follicles or after culture for 15 hr to allow progression of spontaneous meiotic maturation. Differences were found in the organization of chromatin, microtubules, and centrosomes in LT/Sv and I/LnJ oocytes compared to control oocytes. For example, rather than exhibiting multiple cytoplasmic and nuclear centrosomes as in the normal germinal vesicle-stage oocytes, LT/Sv oocytes typically contain a single large centrosome. In contrast, I/LnJ oocytes displayed many small centrosomes. The microtubules of normal germinal vesicle-stage oocytes were organized as arrays or asters, but microtubules were shorter in LT/Sv oocytes and absent from I/LnJ oocytes. After a 15-hr culture, centrosomal material of normal metaphase II oocytes was organized at both spindle poles. In contrast, metaphase I-arrested LT/Sv oocytes exhibited an elongated spindle with centrosomal material appearing more organized at one pole of the spindle. Both control and LT/Sv oocytes displayed cytoplasmic centrosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Inheritance of a meiotic abnormality that causes the ovulation of primary oocytes and the production of digynic triploid mice

Previous studies have demonstrated that the LT/SvKau strain of mice ovulates a high proportion of oocytes as diploid primary oocytes rather than secondary oocytes. These ovulated primary oocytes are arrested at meiotic metaphase I but may be fertilized to produce digynic triploid embryos. In the present study, 40.4% of eggs analysed from LT/SvKau females were ovulated as primary oocytes, compared to 1.2% from control C57BL/Ws strain mothers. These two inbred strains were intercrossed to produce eight sets of F1, F2 and backcross females and the frequency of triploidy was investigated. The results are compatible with segregation of a co-dominant, autosomal gene that has a major influence on the incidence of triploidy. We suggest that the provisional gene symbol Poo (primary oocyte ovulation) be assigned to this gene, with alleles Poo(l) (the 'mutant' allele present in the LT/SvKau strain) and Poo(b) (the normal allele present in C57BL/Ws mice). Poo is incompletely penetrant and has variable expressivity because the proportion of oocytes ovulated as primary oocytes by LT/SvKau mice was variable and, in some cases, nil. In putative Poo(l)/Poo(b) heterozygotes the frequency of ovulated primary oocytes was increased only marginally (from 1.2% to 6.6%) by the presence of one copy of the Poo(l) allele, but this increase was found consistently (in two reciprocal F1 crosses) and was statistically significant. No evidence was found for tight genetic linkage between Poo and two Mendelian loci (brown on chromosome 4 and glucose phosphate isomerase on chromosome 7), that were segregating in the crosses. The Poo(l) mutant in the LT/SvKau strain of mice provides a valuable resource to study the cell and molecular biology of mammalian oocyte maturation and the control of meiosis.

The cleavage rate of digynic triploid mouse embryos during the preimplantation period

Triploidy is a lethal condition in mammals, with most dying at some stage between implantation and term. In humans, however, a very small proportion of triploids are liveborn but display a wide range of congenital abnormalities. In particular, the placentas of human diandric triploid embryos consistently display "partial" hydatidiform molar degeneration, while those of digynic triploids generally do not show these histopathological features. In mice, the postimplantation development of diandric and digynic triploid embryos also differs. While both classes are capable of developing to the forelimb bud stage, no specific degenerative features of their placentas have been reported. Diandric triploid mouse embryos are morphologically normal while digynic triploid mouse embryos consistently display neural tube and occasionally cardiac abnormalities. Previously it was shown that the preimplantation development of micromanipulated diandric triploid mouse embryos was similar to developmentally matched diploid control embryos. In this study, the preimplantation development of micromanipulated digynic triploid mouse embryos is analysed and compared with that of diandric triploid mouse embryos in order to determine whether there is any difference in cleavage rate between these two classes of triploids. Standard micromanipulatory procedures were used to insert a female or a male pronucleus into a recipient diploid 1-cell stage embryo. The karyoplast was fused to the cytoplasm of the embryo by electrofusion. These tripronucleate 1-cell stage embryos were then transferred to pseudopregnant recipients and, at specific times after the HCG injection to induce ovulation, the embryos were recovered and total cell counts made. These results were plotted and regression lines drawn.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of maternal age on the incidence of digynic triploidy in LT/Sv strain mice: implications for the ovulation of primary and secondary oocytes in this strain

LT/Sv strain mice ovulate both primary and secondary oocytes, which are fertilisable, giving rise to digynic triploid and normal diploid fertilised conceptuses, respectively. Since the proportion of primary and secondary oocytes ovulated in our earlier studies varied widely among females, we investigated whether the proportion of primary oocytes ovulated in LT/Sv strain mice was influenced by maternal age. Females 6, 12, 18, 24, and 30 weeks old were mated with F1 hybrid males. The females were autopsied on the 10th day of gestation, and the intact conceptuses or extraembryonic membranes analysed cytogenetically. Since no selective loss of the triploids occurs up to the 10th day of gestation, analysis at this time also provides indirect information on the proportion of primary and secondary oocytes ovulated. We observed 1) that the overall incidence of triploidy decreased from 55% in the 6-week-old females to 6% in the 30-week-old group; 2) that the number of females from which both triploid and diploid embryos were recovered decreased with increased maternal age; 3) that a substantial decrease in the proportion of triploid embryos was observed in those females in which both triploid and diploid embryos were recovered, in relation to increased maternal age; 4) that there was no overall decrease in the total number of implants with increasing age; and 5) that there was no increase in the incidence of resorptions with increasing maternal age. We believe that no comparable relationship between the ovulation of primary and secondary oocytes and maternal age has previously been reported.

Cytochalasin D-induced triploidy in the mouse

Previous attempts to obtain digynic triploid mouse development in vivo have either been entirely or only marginally successful, generally with the production of heteroploid rather than triploid conceptuses. We report that when a single intraperitoneal injection of 15 micrograms of cytochalasin D is given to recently mated female mice during a restricted period following ovulation induced by exogenous gonadotrophins, between 14 and 18% of conceptuses isolated on the 10th day of gestation had a triploid chromosome constitution. Triploidy was only induced in those eggs that were exposed to cytochalasin D when they were passing through a critical phase of the second meiotic division corresponding to the time when the second polar body was about to be extruded. Exposure to this agent either before or after this critical period only results in the development of normal diploid conceptuses. When females were mated to males carrying an easily recognisable paternally derived 'marker' chromosome, convincing cytogenetic evidence was obtained that only digynic triploidy was induced. No examples of diandric triploidy were recognised when conceptuses were analysed on the 10th day of gestation. The technique described therefore represents a simple and direct means of inducing digynic triploid mouse conceptuses whose development potential may be compared directly with that of their normal diploid littermates.