Decreased mitochondrial respiration in aneurysmal aortas of Fibulin-4 mutant mice is linked to PGC1A regulation

Abstract

Aim Thoracic aortic aneurysms are a life-threatening condition often diagnosed too late. To discover novel robust bio-markers, we aimed to better understand the molecular mechanisms underlying aneurysm formation. Methods In Fibulin-4R/Rmice, the extracellular matrix protein Fibulin-4 is 4-fold reduced, resulting in progressive ascending aneu- and results rysm formation and early death around 3 months of age. We performed proteomics and genomics studies on Fibulin-4R/Rmouse aortas. Intriguingly, we observed alterations in mitochondrial protein composition in Fibulin-4R/Raortas. Consistently, functional studies in Fibulin-4R/Rvascular smooth muscle cells (VSMCs) revealed lower oxygen consumption rates, but increased acidification rates. Yet, mitochondria in Fibulin-4R/RVSMCs showed no aberrant cytoplasmic localization. We found similar reduced mitochondrial respiration in Tgfbr-1M318R/þVSMCs, a mouse model for Loeys-Dietz syndrome (LDS). Interestingly, also human fibroblasts from Marfan (FBN1) and LDS (TGFBR2 and SMAD3) patients showed lower oxygen consumption. While individual mitochondrial Complexes I–V activities were unaltered in Fibulin-4R/Rheart and muscle, these tissues showed similar decreased oxygen consumption. Furthermore, aortas of aneurysmal Fibulin-4R/Rmice displayed increased reactive oxygen species (ROS) levels. Consistent with these findings, gene expression analyses revealed dysregulation of metabolic pathways. Accordingly, blood ketone levels of Fibulin-4R/Rmice were reduced and liver fatty acids were decreased, while liver glycogen was increased, indicating dysregulated metabolism at the organismal level. As predicted by gene expression analysis, the activity of PGC1a, a key regulator between mitochondrial function and organismal metabolism, was downregulated in Fibulin-4R/RVSMCs. Increased TGFb reduced PGC1a levels, indicating involvement of TGFb signalling in PGC1a regulation. Activation of PGC1a restored the decreased oxygen consumption in Fibulin-4R/RVSMCs and improved their reduced growth potential, emphasizing the importance of this key regulator. Conclusion Our data indicate altered mitochondrial function and metabolic dysregulation, leading to increased ROS levels and altered energy production, as a novel mechanism, which may contribute to thoracic aortic aneurysm formation.