title: Mitochondrial Alterations Through Gestation and in Placental Pathologies
Creators: name: Bartho, Lucy A
Publisher: name: Griffith University
Year: 2021
Subjects: subject: Placenta
subject: placental aging
subject: mitochondrial physiology
subject: healthy pregnancy

description: The placenta is a transient organ, essential for the growth and development of the fetus throughout pregnancy. This temporary organ brings maternal and fetal blood circulation into close proximity, which allows for the exchange of oxygen, carbon dioxide, waste, and other essential nutrients. Despite constant influences by internal and external factors, the human placenta has a defined life span of approximately 280 days. From conception, through to term, the placenta undergoes chronological aging, which is regulated by a range of cellular processes. Advanced placental aging and cellular senescence have been known to contribute to the pathophysiology of preterm birth, fetal growth restriction and may cause an increased risk of stillbirth. However, the molecular mechanisms behind placental aging are still poorly understood. As a key regulator of cell homeostasis, mitochondria have been recognised as an important mediator of age-related disease processes through the production of reactive oxygen species which activate mechanisms that induce cellular senescence. Currently, we do not understand the molecular link between cellular aging processes and the role of mitochondria in chronological and pathological placental aging. Therefore, this research aimed to, 1) identify key areas of mitochondrial physiology that change with placental development, 2) characterise a set of markers that define aging in the human placenta, 3) assess the role of the mitochondria in the placenta as it develops throughout a healthy pregnancy, 4) to measure the chosen markers of aging in placentas complicated by pregnancy pathologies. Chapter 1 presents a comprehensive review of the literature to date and highlights key gaps in our current knowledge. It sets the scene for the experimental chapters to follow. Chapter 2 provides the details of the methods and materials that have been used in the laboratory to generate the data presented in the results chapters. Chapter 3 explored the molecular changes within healthy and pathological placentas, through analysing large publicly available datasets. This chapter aimed to establish an understanding of placental aging and mitochondrial physiology through measuring mitochondrial biogenesis, dynamics, mitophagy, apoptosis and senescence transcripts. Furthermore, this chapter measured altered transcripts in an additional validation cohort consisting of placentas affected by term, preterm, post-term and FGR pregnancies. This study was a large-scale investigation across multiple datasets that identified altered TOMM2020, MFN1, and MFN2 expression throughout preterm, post-term and FGR pregnancies, which may be a contributing factor to placental insufficiency. It established key markers that influence mitochondrial physiology in placental aging, which informed future studies in this thesis. Chapter 4 focused on understanding healthy aging by measuring mitochondrial and senescent changes in term and post-term placenta. Post-term placentae from healthy pregnancies selectively retain highly functioning mitochondria through increases in mitochondrial dynamics proteins MFN1, MFN2 and mitochondrial complex specific proteins. This study directly associated mitochondrial adaptions with increases in cellular senescence in the placenta and may be the reason why some post-term pregnancies are healthy, whilst others turn pathological. These findings have helped to expand our knowledge of the role of mitochondria and healthy aging in the placenta. The current literature on placental aging has focused on comparing the differences between two timepoints in pregnancy, or healthy and complicated pregnancies. The reason being that it is nearly impossible to ethically collect healthy placental tissue from early in pregnancy. Even when these samples are collected, they are inherently impacted by factors which lead to early termination. Therefore, Chapter 5 used placental samples from an established rodent ontogeny model that were collected between mid- and term gestation, without pathologies. These placentas were used to understand the role of mitochondrial biology, senescence, and the ER and in the developing placenta. Markers associated with mitochondrial biogenesis, dynamics and senescence were differentially altered in healthy placentas collected throughout gestation, which was different to what was identified in previous chapters. Therefore, throughout different stages of pregnancy, mitochondria function differently compared to placentas from post-term and growth restricted pregnancies. Lastly, Chapter 6 measured the most differentially expressed genes from previous chapters in placentas complicated by preterm, term, post-term, fetal growth restriction (FGR), preterm preeclampsia (PE), FGR/ PE pregnancies. The aim of this chapter was to utilise MetaboAnalyst data software to identify relationships between genes related to mitochondria, ER and cellular senescence. This study revealed that placentas complicated by pathologies, PE and FGR have tremendously different transcription patterns, compared to the healthy controls. Although this study only investigated a small number of genes in a relatively small sample size, it revealed that the TOMM20/PARK2 ratio is a promising marker to discriminate between healthy placenta and placenta that have been affected by pregnancy pathologies. Overall, the findings in this thesis highlight the importance of mitochondrial alterations and cellular senescence within chronological and pathological aging of the placenta. Whilst the exact mechanisms of mitochondrial aging in the placenta still requires further investigation, MFN1, MFN2, TOMM20 and PARK2 are promising markers of placental aging and should be investigated in further models of placental insufficiency. This work provides the foundation for future work in mitochondrial aging within the human placenta.