Identification of New Activators of Mitochondrial Fusion Reveals a Link between Mitochondrial Morphology and Pyrimidine Metabolism

http://www.sciencedirect.com/science/article/pii/S2451945617304282?via%3Dihub

Laia Miret-Casals, David Sebastián, José Brea, Eva M.Rico-Leo, Manuel Palacín, Pedro M.Fernández-Salguero, M. Isabel Loza, Fernando Albericio, Antonio Zorzano

• The authors develop a high-throughput drug screen on HeLa cells to identify FDA-approved drugs which modulate the activity of the mitochondrial fusion protein MFN2, allowing the authors to find compounds which are able to upregulate MFN2 expression and induce mitochondrial fusion.
• The authors identify leflunomide (a drug used for the treatment of arthritis) as the most potent modulator of MFN2 expression, inducing a 67% increase in MFN2 mRNA levels. The compound was also found to increase both MFN1 and MFN2 protein levels by a factor of ~x2. HeLa cells morphologically appeared to have higher fusion and mitochondrial membrane potential.
• Leflunomide inhibits de novo synthesis of pyrimidines by inhibiting the mitochondrial inner membrane enzyme dihydroorotate dehydrogenase (DHODH).
• As a consequence, leflunomide had anti-proliferative effects upon cells.
• Uridine may be added to cells as an external source of pyrimidines. The authors found that addition of uridine to leflunomide-treated cells abolished the ability of leflunomide to induce MFN2 expression.
• Another drug, brequinar sodium (BRQ), which is an inhibitor of DHODH also has similar properties to leflunomide.
• Hence, inhibition of pyrimidine nucleotide synthesis may induce mitochondrial elongation via MFN induction.
• DHODH uses ubiquinone as a substrate, which is converted to ubiquinol. Ubiquinol is substrate of complex III of the respiratory chain.
• Inhibiting DHODH therefore inhibits the cycling of ubiquinone -> ubiquinol -> ubiquinone, and therefore inhibits the activity of complex III. 
• The authors found that direct inhibition of complex III via the drug myxothiazol inhibited DHODH activity, reflecting the coupling between pyrimidine synthesis via DHODH and complex III activity.
• Complex III inhibition via myxothiazol induced MFN induction and also elongation of mitochondria, even in MFN knockout cells. 

Overall, depletion of pyrimidine pools by complex III inhibition causes cell-cycle arrest and promotes mitochondrial elongation as an adaptive response to energetic stress.