Background and Significance

Mitochondria are most commonly referred to as the “powerhouse of the cell”. A prominent function of these eukaryotic organelles is the production of ATP, the cellular “energy currency”. In this process, acetyl-CoA generated from the breakdown of sugars, fats or amino acids, is converted, by a biochemical process called the Krebs cycle, into carbon dioxide and reducing equivalents . The latter fuel the respiratory chain to produce the proton gradient across the inner mitochondrial membrane that drives ATP synthase, a fantastic molecular turbine that extracts energy from this gradient during proton passage back to the mitochondrial matrix to form the anhydride bond of ATP from ADP and phosphate. Mitochondria fulfill another, equally essential role in the synthesis of iron sulfur clusters, and are also involved in many other processes like amino acid breakdown, the synthesis of phospholipids, or the breakdown of short and medium chain fatty acids in higher eukaryotes.

A little known, evolutionarily conserved feature of mitochondria is their ability of fatty acid synthesis. Like most prokaryotes, and in contrast to the eukaryotic cytosolic fatty acid synthesis (FAS) machinery, mitochondria carry out dissociated or typ2 II FAS, where the individual reactions are performed by individual, monofunctional enzymes. The first component of the mitochondrial FAS (mtFAS) system found was an acyl carrier protein (ACP), identified in the fungus Neurospora crassa. The past twenty years have witnessed the isolation and characterization of all the enzymes and proteins of mtFAS in the yeast Saccharomyces cerevisiae, and most of the corresponding factors in mammals. I have been involved in the discovery of several of these enzymes during my postdoctoral period with Prof. Kalervo Hiltunen here at the Department of Biochemistry, and Kalervo remains to be a very important collaborator in my work. Several of our students are under shared supervision.

MtFAS is required for respiratory competence in yeast, and overexpression of some of the enzymes of the pathway has been shown to result in dramatically enlarged mitochondria. A well-documented product of this pathway is mitochondrially synthesized octanoic acid. This fatty acid apparently serves as the exclusive precursor for the intramitochondrial production of lipoic acid, a cofactor required for the function of several mitochondrial enzyme complexes. The lack of lipoic acid alone may be sufficient to explain the respiratory deficiency of yeast mtFAS mutants. Our collaborators in Carol Dieckmann’s group at the University of Arizona at Tucson, have reported that inactivation of mtFAS also affects mitochondrial RNA processing in yeast, a connection that cannot be easily explained by a lipoic acid effect. In 2013, we published evidence indicating a role for products of mtFAS other than octanoic/lipoic acid in several aspects of mitochondrial physiology and metabolism. As a matter of fact, our data suggests that lipoic acid may not at all be required for mitochondrial respiratory function. We propose a role for mtFAS in sensing of acetyl-CoA, a substrate that mtFAS shared with the Krebs cycle. According to our model, the fatty acid output of mtFAS is a reflection of the availability of acetyl-CoA in mitochondria. How exactly this fatty acid “signal” exerts its effects on mitochondrial genesis is one of the current key questions of our research. Our data published in 2013 indicated that control of mitochondrial biogenesis is exerted on multiple levels from control of translation to respiratory complex assembly.

Recently, the Rutter group in Utah published evidence strongly suppporting our model, and revealing that acylated ACP serves as a key component in the intermitochondrial signaling circuit. It appears that the role of ACP of mtFAS in mitochondrial and general cellular function has been far underestimated, as this protein does not only act as the moiety to which the growing fatty acid chain is attached via a phosphopantetheine cofactor. It has been recently shown by others that holo ACP is intimately associated with the process of iron sulfur cluster biosynthesis in mitochondria and also features as a structural component of respiratory complex I in mammals and the mitochondrial ribosome.

Proposed mtFAS regulatory circuit in yeast (Kursu et al. Molecular Microbiology 2013). While some features are restricted to the yeast system (e.g. mRNA splicing effects), several aspects of this circuit are bound to be conserved in mammals.

I have established a group of young, enthusiastic scientists to push this work further. Building on our results on mtFAS enzymology and our functional studies in the yeast model, we have started to expand our research to mammals. It was previously shown by our collaborators that transgenic mice overexpressing the Mecr mitochondrial enoyl reductase suffer from cardiomyopathy and develop enlarged mitochondria in the cardiac muscle, an intriguing similarity to the yeast overexpression phenotype. Furthermore, vertebrate mtFAS genes have been demonstrated to be involved in kidney development and kidney disease. The recent description of a patient suffering from lipoic acid synthase deficiency lends even more urgency to our quest of dissecting these pathways. We have no doubt that the powerful new whole exome sequecing technology used by physicians to investigate e.g. mitochondrial diseases with unknown cause will soon also lead to the identification of patients with defects in mtFAS.

We are pursuing several avenues towards the ends of understanding the physiological relevance of mtFAS and lipoic acid synthesis also in higher eukaryotes. With the help of specialists like Aki Manninen and Raija Soininen of the Biocenter Oulu, we have established an RNA knockdown model in mouse cell culture, generated floxed Mecr mice for conditional knockout studies, and are developing several mtFAS disease mouse models. We are also collaborating with physicians in the search of patients suffering from an mtFAS defect. Meanwhile, the yeast work is still going ahead at full steam. Our expertise in fatty acid metabolism has also yielded in collaborations concerning lipid-related research in plants and pathogenic microorganisms.

One of the lipoic acid-dependent mitochondrial enzyme complexes is the glycine cleavage system (GCS). GCS produces one-carbon units (i.e. methyl groups) that are used in the generation of folates, which have an important role in mitochondrial function. One of our subprojects is concerned with the effects of mtFAS deletions on folate metabolism.

The yeast mitochondrial acetyl-CoA transferase Hfa1, generating the malonyl-CoA primer for mtFAS, is the only protein in yeast exclusively translated from a non-AUG translation initiation codon. We have recenly published evidence that the distinct genes encoding Hfa1 and the yeast cytosolic counterpart Acc1are derived by gene duplication from one gene encoding both the cytosolic and the mitochondrial form, the former from a canonical translation initiation codon and the latter from a non-AUG start codon. We found several more likely examples for this type of arrangement in yeast.  These results on cryptic dual localization of mtFAS components in yeast and mammals have hence  roused our interest in cellular protein partitioning events, and we are currently investigating the results of an in silico screen for obscured mitochondrial targeting sequences in S. cerevisiae.

Last updated: 30.10.2018