Public PhD Defence Kerlijne Moorkens
Maternal Metabolic Health and Fertility: acute and chronic effects on a western-type diet on oviductal and ovarian cells. A multi-omic approach.
It is our pleasure to invite you for the public PhD defence of Kerlijne Moorkens on 21st June 2024 at 5pm.
Location: Building Q, Room D.Q.002 ‘Promotiezaal’ @ Campus Drie Eiken, Universiteitsplein 1, 2610 Wilrijk
Abstract
Maternal metabolic disorders associated with the consumption of an obesogenic (high fat/high sugar, HF/HS) diet are strongly linked with reduced fertility in women. The focus of the studies presented in this thesis was to determine the temporal changes that occur in the ovarian and oviductal microenvironment upon starting HF/HS diet feeding in mice. In Chapter 1, we provide a literature review on obesity and the obesogenic diet-induced metabolic changes and dynamics that lead to systemic metabolic stress and metabolic disease. We describe the involved mechanisms such as dyslipidaemia, oxidative stress and inflammation. The causative link between maternal metabolic health and subfertility is discussed, including the well-documented direct detrimental effects of the metabolic alterations on oocyte quality and ovarian cell functions. These changes appear to be different depending on the used mouse strain. In addition, the function and importance of the oviductal microenvironment, where fertilization and early embryo development take place is described. Furthermore, we discussed the dynamics and the progressive nature of the diet-induced metabolic alterations in the blood, muscles and liver, and defined the gaps in knowledge about the onset and progression of diet-induced effects on the reproductive tissues.
In Chapter 2, the specific aims of the thesis are overviewed. We first investigated the impact of feeding an obesogenic diet for different time periods on cellular stress markers, at the gene expression level, in the oviductal epithelial cells of outbred and inbred mice (CHAPTER 3). Concomitant changes in live body weight, and blood cholesterol and cytokine concentrations were also described. Next, we wanted to study whether feeding an obesogenic diet for different time periods would also influence the lipidomic profile of the oviductal epithelium. This was investigated in situ using MALDI-Imaging Lipidomics in an outbred mouse model (CHAPTER 4). Finally, to check the temporal changes in the ovarian follicle microenvironment, the impact on granulosa cell transcriptomics (RNAseq analysis) and oocyte quality (lipid content and mitochondrial features) were examined after different time periods of HF/HS diet feeding in outbred mice (CHAPTER 5).
The impact of obesity and an obesogenic diet on the oviductal microenvironment is much less characterized compared to the impact on the follicular environment and oocyte quality. Furthermore, the onset and duration of changes after the start of an obesogenic diet were unclear. In addition, in diet-induced obesity studies the inbred C57BL/6N strain is most commonly used, however, an outbred strain (Swiss mice) is more pathophysiologically relevant to the human (outbred) physiology, which might facilitate translation to the human population. The sensitivity to obesogenic diet-induced obesity and its influence on the metabolic profile and oocyte quality have been shown to be dependent on the genetic background of the mouse model. In Chapter 3, we investigated the impact of feeding an obesogenic diet for different time periods on the transcriptome of oviductal epithelial cells (OECs) in Swiss compared to C57BL/6N mice, specifically focusing on genes involved in oxidative and cellular stress levels and inflammatory responses. We could demonstrate, for the first time, that feeding an obesogenic diet for a short period of only three days resulted in acute systemic changes and acute local oxidative stress or mitochondrial stress effects on OECs, evident already before the development of an obese phenotype. The acute effects in the OECs initiated a cascade of transcriptomic changes to control mitochondrial ROS production and endoplasmic reticulum (ER)-stress. However, prolonged, long-term feeding resulted in a persistent upregulation of (mitochondrial) oxidative stress and ER-stress, with ultimate signs of local and systemic inflammation. Furthermore, different responses to the obesogenic diet were observed between Swiss and C57BL/6N mice.
Since fatty acids play an important role as structural membrane components, in cell signalling and cell-to-cell interactions, alterations to the lipid composition of the oviduct may reduce its ability to support early embryo development, which may have long-lasting consequences. Therefore, in Chapter 4, we studied the dynamics and the nature of changes in the lipidomic profile of the oviductal epithelium upon and during obesogenic diet feeding over time in an outbred Swiss mouse model. We demonstrated that feeding the obesogenic diet resulted in acute changes in the lipid profile in the oviductal epithelium already after 3 days on the obesogenic diet. These remarkable effects are in line with the transcriptomic changes described in chapter 3. The changes in the lipid profile progressively increased and became more persistent after long-term obesogenic diet feeding. Functional annotation revealed a differential abundance of phospholipids, sphingomyelins and lysophospholipids in particular.
In the ovary, it is known that the impact of lipotoxicity, caused by an obesogenic diet, on oocyte quality is mediated by oxidative stress, ER stress and mitochondrial dysfunction. However, the nature and magnitude of these changes may be dependent on the duration of feeding (and thus the development of the obesogenic phenotype), which remained to be investigated. Therefore, in Chapter 5, we investigated the impact of feeding an obesogenic diet for different time periods on the ovary and ovarian cells of outbred Swiss mice, with a focus on dynamic changes in granulosa cell transcriptomics and oocyte quality. We reported that feeding an obesogenic diet resulted in acute changes in the lipid content of oocytes and impacted endoplasmic reticulum and mitochondrial functions in granulosa cells already after short-term feeding of only 24 hours till 1 week. The effects progressively increased over time after prolonged obesogenic diet feeding, with increased mitochondrial abnormalities in oocytes and different affected pathways, linked to DNA repair, cell signalling, UPRs and many other GO annotation biological terms in the granulosa cells.
Finally, in Chapter 6, the current findings were summarized and discussed in the context of previous existing literature. We conclude that we are the first to show strong evidence of the sensitivity of the oviduct and oviductal epithelium to very short-term changes in the dietary composition, both at the transcriptomic and lipidomic levels in vivo. The different durations of feeding allowed a clear distinction of acute responses and later changes after long-term feeding and after the development of the obese phenotype, and the associated unhealthy metabolic phenotype. We provided a deep insight into a network of sequential events that could demonstrate how the OECs can quickly sense metabolic stress in a matter of only three days, and how such local metabolic stress can evolve over time. Furthermore, we demonstrated important differences in cellular responses between the inbred C57BL/6N strain and outbred Swiss strain which further highlights the importance of choosing physiologically relevant mouse models in other studies. Our findings in the oviduct illustrate that the mechanism by which the obesogenic diet impacts fertility is not only mediated through reduced oocyte quality but might also directly impact early embryo development in the oviduct, potentially leading to long-term effects on foetal development, pregnancy success and postnatal health through epigenetic alterations. In addition, exposure to an obesogenic diet resulted in cell specific changes in oocytes and granulosa cells, already after 24 hours to 1w, which can directly affect oocyte quality and early embryo development, eventually leading to reduced fertility.
We believe that the deep analysis and novel insights of this PhD thesis can be useful for many researchers and form an important fundament for further studies that focus on protecting and enhancing oocyte and embryo development during the very early stages under diet-induced metabolic stressed conditions. Understanding the magnitude and nature of reproductive cell dysfunction after different exposure periods to HF/HS diet is crucial to develop more efficient preconception care intervention strategies to enhance fertility under diet-induced metabolic stress conditions.