A little worm as a model system to investigate lipoic acid metabolism

Lipoic acid is a sulfur-containing cofactor, obtained from octanoic acid, that’s essential for the functioning of several multienzyme complexes involved in one-carbon and oxidative metabolism. Lipoic acid is also a powerful antioxidant. This eight-carbon fatty acid derives its oxidative and reducing properties from the structural attachment of sulfur to each of the six and eight carbon atoms.

Lipoic acid is found among eukaryotes and prokaryotes. Although previous studies have been able to characterize lipoic acid metabolism in bacteria, there are still open investigations on the mechanisms of protein lipoylation in animals. This is partly because disorders in the mitochondrial protein lipoylation lead to adverse metabolic disorders and premature death.

Current studies have been able to describe only six enzymatic complexes modified with lipoic acid. However, they become lipoylated in diverse ways. Some species depend on its synthesis, other scavenge it from the medium, while other use both pathways. Also, in some species, lipoylation pathways are segregated by organelles, like in apicomplexans.

It’s synthesis in eukaryotes Saccharomyces cerevisiae and Homo sapiens adheres to the lipoyl-relay model because it depends on four protein activities. Unfortunately, these organisms lack lipoic acid salvage mechanisms. As a result, humans suffering from impaired protein lipoylation often record incredibly high levels of lactate and suffer from grave respiratory deficiency and severe muscle weakness.

Patients with mutations in the LIPT1 gene, coding for the amidotransferase, report normal glycine levels, in coincidence with lipoylation of H subunit of the glycine cleavage system, but undetectable oxoglutarate dehydrogenase and pyruvate dehydrogenase activities. On the contrary, patients deficient in the octanoyltransferase show high glycine levels, and consequently develop neurological disorders such as encephalopathy and neonatal-onset epilepsy. As at the moment lipoic acid deficiency treatments are confined to alleviating symptoms, there is an urgent need for further studies to help develop possible therapies.

In light of this, researchers Dr. Antonela Lavatelli, Dr. Diego de Mendoza, and Dr. María Cecilia Mansilla from the National University of Rosario in Argentina, used Caenorhabditis elegans as a model system to characterize protein lipoylation mechanisms. Their choice of C. elegans was based on the fact that there is a strong conservation in molecular reactions between the worms and mammals and most human disease pathways are present in the worms. Their original research article is now published in the Journal of Biological Chemistry.

The research team hypothesized that the development of an organismal model system could open new opportunities to investigate lipoic acid biology, biochemistry, and physiology. To uncover if C. elegans synthesizes lipoic acid or scavenges it from the E. coli bacterial diet, the authors fed the worms with E. coli K12 strains TM131 or TM136, which can neither synthesize lipoic acid nor obtain it from the environment. When the authors cultured synchronized N2 worms with these E. coli mutant strains in a medium without lipoic acid, they observed them grow for many generations, indicating that animal can synthesize lipoic acid de novo.

The authors discovered that M01F1.3 was a lipoyl synthase, C45G3.3 an amidotransferase, and ZC410.7 an octanoyltransferase. They observed that the worms they subjected to RNAi against M01F1.3 and ZC410.7 manifested larval arrest in the second generation. Lipoic acid supplementation didn’t rescue the arrest showing that endogenous lipoic acid synthesis is fundamental in C. elegans development. Expression of the three enzymes, M01F1.3, ZC410.7, and C45G3.3, rescued yeast mutants or bacteria affected in different stages of the lipoylation pathway, showing a functional overlap.

The findings of close analogy of the human lipoate metabolism defects to those of worms with protein lipoylation deficiency could provide an important discovery tool for elaborating and understanding the diverse roles of lipoic acid in a multicellular organism under various conditions, and therefore, provide new therapy options.