Richard Schultz

Molecular mechanisms of egg activation.

Changes in egg calcium homeostasis following sperm-egg fusion initiate events that constitute egg activation, e.g., cortical granule (CG) exocytosis, cell cycle resumption. Fertilization-induced changes Ca2+ are characterized by an initial rise in Ca2+ that lasts for several minutes, followed by a series of Ca2+ oscillations of shorter duration. The initial Ca2+ rise, as well as the subsequent oscillations, is due to the release of Ca2+ from an IP3-sensitive pool followed by Ca2+ sequestration. The ability of eggs to display this Ca2+ oscillatory behavior is acquired following oocyte maturation, occurs only in the 1-cell embryo, and ceases following pronucleus formation. Although Ca2+ oscillations are critical for successful development, little is known about the underlying molecular mechanisms that link these oscillations to successful development. We are pursuing how Ca2+ oscillations are linked to recruitment/degradation of maternal mRNAs, as well as changes in gene expression that occur during pre- and post-implantation development. In addition, we are exploring the role of CaMKII-gamma in integrating the calcium signal. Current studies focus on whether the sole function of CaMKII-gamma is to mediate exit from the arrest at metaphase II.

RNAi in mouse oocytes and preimplantation embryos

Double-strand RNA (dsRNA) mediated post-transcriptional gene silencing, also known as RNA interference (RNAi), is a powerful tool to inhibit gene expression in several experimental model systems including Arabidopsis, C. elegans, and Drosophila. We have shown that RNAi operates in mouse oocytes and preimplantation embryos and studies centered on the role of Dicer in oocyte development have revealed that oocytes deficient in Dicer do not undergo proper maturation and that degradation of many maternal mRNAs does not occur correctly. Recently we noted that in contrast to somatic cells, endogenous miRNAs in fully-grown GV oocytes poorly repressed translation of mRNA reporters whereas their RNAi-like activity was much less affected. In addition, reporter mRNA carrying let-7-binding sites failed to localize to P-body-like structures in oocytes. These findings indicate that miRNA-mediated degradation of maternal mRNAs is not a robust pathway in oocytes and that the failure of Dicer-deficient oocytes to mature properly can be ascribed to failure to generate endo-siRNAs. Current studies are focused on deciphering the molecular mechanisms that why this pathway does not function robustly in oocytes and what pathways are involved in degradation of maternal mRNAs that initiates with the onset of oocyte maturation (see below).

Maternal age-associated increase in incidence of aneuploidy

A hallmark of animal development is an age-related decrease in fertility. In mammals, an increase in aneuploidy is the major underlying factor responsible for the increase in infertility with advancing age in human females. Aneuploidy is a leading cause of pregnancy loss, and when development goes to term, aneuploidy is an aggravating source of developmental disabilities and mental retardation, e.g., trisomy 21. The incidence of aneuploidy in eggs from women in their 20s’ is ~2%, but dramatically increases to 35% around 40 years-of-age. Spindle abnormalities and faulty chromosome congression on the metaphase plate are associated with advanced maternal age and likely contributes to the observed increased incidence of aneuploidy. Remarkably, little is known regarding the underlying molecular basis for the age-associated increase in aneuploidy.

Mice also exhibit an increase in the incidence of aneuploidy with increasing maternal age. Expression profiling global patterns of gene expression of oocytes obtained from young and old mice revealed mis-expression of many genes, including genes involved the Spindle Assembly Checkpoint (SAC), chromosome congression and attachment to kinetocore microtubules, and spindle assembly. In addition, the normal pattern of degradation of maternal mRNAs was not observed following maturation of old oocytes. We are currently imaging individual oocytes obtained from young and old mice to determine the temporal and spatial dynamics of spindle assembly, spindle microtubule-kinetochore attachment, and chromosome congression and separation. Results of these studies have demonstrated that old oocytes possess a highly functional SAC, i.e., a defective SAC is unlikely the primary cause of aneuploidy associated with maternal age. Noteworthy, is our recent finding that ~90% of age-related aneuploidies are best explained by weakened centromere cohesion. Moreover, levels of the meiotic cohesin protein REC8 are severely reduced on chromosomes in oocytes from old mice. Together, these results demonstrate that the maternal age-associated increase in aneuploidy is primarily due to a failure to effectively replace cohesin proteins that are lost from chromosomes during aging. This work is done in collaboration with Michael Lampson.

Regulation of degradation of maternal mRNA

Degradation of maternal mRNA is thought to be essential to undergo the maternal-to-embryonic transition. Messenger RNA is extremely stable during oocyte growth in mouse and MSY2, an abundant germ cell-specific RNA-binding protein, likely serves as a mediator of global mRNA stability. Oocyte maturation, however, triggers an abrupt transition in which most mRNAs are significantly degraded. We noted that CDK1-mediated phosphorylation of MSY2 triggers this transition. In addition, we have found that critical components of the RNA degradation machinery are maternal mRNAs that are recruited during oocyte maturation and are also critical for the transition from mRNA stability to instability.  Current studies are focusing on dissecting out the contributions of the 5'-to-3' and 3'-to-5' degradation pathways and the effect of inhibiting degradation of maternal mRNAs on early development.

Effect of culture on gene expression and behavior

The use of assisted reproductive technologies (ART) to treat human infertility is gaining widespread use, and it is estimated ~5 million children have been conceived using ART. Disconcerting to many researchers is that the clinical procedures used in ART are rapidly outpacing the underlying science, an example of ART before science. We have noted that culture conditions can perturb global patterns of gene expression in preimplantation mouse embryos. In particular, certain culture conditions result in biallelic expression of the imprinted H19 gene in the blastocyst, and this biallelic expression persists in extra-embryonic tissue following implantation. Recent retrospective studies have unmasked an increased incidence of certain syndromes that are the result of loss-of-imprinting, highlighting the concern about the aggressive practice of ART. The long-term developmental and behavioral consequences of ART are unknown. To address this, we have developed a mouse model to study the effects of embryo culture, which is an integral part of every ART-conceived child, on behavior in the offspring. We find that mice derived from cultured embryos exhibit specific behavioral alterations in anxiety and spatial memory, e.g., mice derived from cultured embryos display deficiencies in spatial memory. We are pursuing these studies by (1) examining the effect of different culture conditions on the expression of a battery of imprinted genes, as well as on global patterns of gene expression , (2) altering the culture conditions to minimize or eliminate the behavioral consequences of culture, and (3) mimicking clinical procedures known to produce “low quality eggs” used in ART affect gene expression in the embryos and behavior in the offspring in our mouse model system. The work on imprinted gene expression is done collaboration with Marisa Bartolomei and the work on behavior is done in collaboration with Ted Abel.