Thesis

Sex-dependent mitochondrial function in the right ventricle in pulmonary arterial hypertension

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Awarding institution
  • University of Strathclyde
Date of award
  • 2024
Thesis identifier
  • T17146
Person Identifier (Local)
  • 202083626
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Abstract
  • Background: Pulmonary arterial hypertension (PAH) has a female predominance with a ~4:1 female-to-male ratio. However, female PAH patients exhibit better right ventricular (RV) compensation in the face of increased pulmonary arterial pressures and thus better survival than the males. The sex differences and the associated mechanism in RV in PAH are not well characterised. Additionally, the mitochondria become dysfunctional in PAH, which involves a glycolytic shift in the mitochondrial metabolism. The mitochondria also undergo excessive fission in PAH, losing their filamentous structure and becoming fragmented. These key mitochondrial dysfunctional features have been shown to contribute to RV dysfunction, and within PAH, the status of the RV serves as the main determinant of survival. Hence, we aim to explore whether there are any sex differences in mitochondrial function and RV remodelling in a Sugen-hypoxia (SuHx) induced PAH rat model. The work in this thesis also describes the impact of a mitochondria-targeted antioxidant, Mitoquinone (MitoQ), in vivo and whether MitoQ is able to exhibit protective activity with regard to RV status in PAH. Methods: Male and female Sprague-Dawley rats were injected with Sugen (25 mg/kg) and exposed to hypoxia for 3 weeks followed by up to 5 weeks in normoxia. Control rats received vehicle (CMC) and were kept in normoxia for up to 8 weeks. RV haemodynamics were measured via pressure-volume (PV) loop measurement using the open chest method. Confocal imaging was performed on fresh RV tissue stained with mitochondrial dyes to examine mitochondrial properties. RV fibrosis, myocyte size, and right coronary artery (RCA) remodelling were assessed by staining the RV tissue with picrosirius red. Mitochondria-linked biogenesis, dynamics and oxidative stress genes were examined in both the RV and lung tissue by quantitative polymerase chain reaction (qPCR). The effect of MitoQ (300 nM) and estradiol (E2; 10 nM) on mitochondrial superoxide levels, cell proliferation and protein expression were examined in vitro in male and female human cardiac fibroblasts (HCF) and primary RV fibroblasts isolated from male monocrotaline (MCT) rats. MitoQ (5 mg/kg) was given by intraperitoneal injection (IP) injection to male and female SuHx rats twice a week from week 5 post Sugen injection. The rats were assessed in vivo using the PV loop method to obtain the hemodynamic parameters. RV from these rats were then examined histologically for collagen deposition and myocyte size. Results: SuHx caused significant increases in RV systolic pressure (RVSP), RV hypertrophy, RV fibroblast proliferation, and RV myocyte size in both sexes. Although SuHx did not cause differences in the RVSP and effective arterial elastance (Ea) between the sexes, it caused a slightly greater increase in RV end-systolic elastance (Ees) in females vs. males, leading to a preserved RV-pulmonary artery coupling (Ees/Ea) in females and a significant decrease in Ees/Ea in males. SuHx caused a significant increase in RV end diastolic pressure (RVEDP) only in males, which was associated with a significantly higher amount of RV interstitial fibrosis and increased expression of fibrotic gene expression in males vs. females. The male SuHx rats had more severe RCA remodelling in their smaller vessels (diameter of 15-50 μm) which was not detected in the females. SuHx caused a significant decrease in mitochondrial membrane potential and fusion in RV from male SuHx rats only. Within the lung, SuHx reduced mitochondrial biogenesis and mitochondrial fusion gene expression in only the male SuHx rats. E2 (10 nM) and MitoQ (300 nM) were able to mediate protective effects by reducing mitochondrial superoxide levels, fibroblast proliferation in male and female HCF cells and male MCT RV fibroblasts. Both MitoQ and E2 were able to reduce markers of fibrosis protein expression and increase mitochondrial biogenesis protein expression. In vivo, MitoQ (5mg/kg) was not able to reduce RVSP, RVEDP, RV hypertrophy, and Ea or increase Ees in the SuHx rats of both sexes. However, MitoQ reduced RV interstitial fibrosis and increased RVpulmonary artery coupling (Ees/Ea) in male SuHx rats. Conclusion: Overall, this project shows that the male SuHx rats have worse RV hemodynamic parameters, which correlate with worse interstitial collagen production. Mitochondrial function in the RV and lung displayed sexual dimorphisms, with the male RV associated with more impaired mitochondrial properties compared to the females. We show that both E2 and MitoQ are protective in vitro by reducing MitoSOX, cell proliferation, and collagen expression. Meanwhile, MitoQ treatment in vivo was not able to reverse the RV hypertrophy in either sex. However, it did improve the RV-pulmonary artery coupling and lower RV collagen deposition in the male SuHx rats. Taken together, this provides preliminary evidence that male SuHx rats are associated with worse PAH characteristics and that MitoQ could potentially offer therapeutic value for RV, but only in the male sex in PAH.
Advisor / supervisor
  • McLean, Margaret R.
  • Tian, Lian
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DOI

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