Optic atrophy 1 (OPA1) is essential for NET formation and antibacterial functions in neutrophils

Amini, Poorya (2016). Optic atrophy 1 (OPA1) is essential for NET formation and antibacterial functions in neutrophils (Submitted). (Dissertation, Graduate School for Cellular and Biomedical Sciences, Faculty of Medicine, University of Bern)

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The precise nature of anti-bacterial and anti-fungal inflammatory responses has not yet been completely defined. Besides intracellular killing, neutrophils are able to exert an antibacterial effect in the extracellular space by forming so-called neutrophil extracellular traps (NETs) containing released DNA and granule proteins. We have data indicating that mitochondrial dynamics, namely mitochondrial fusion, is essential for mtDNA release, underlining the active role of mitochondrial dynamics during NET formation. Mitochondrial functions, including energy production, are linked to mitochondrial morphology and dynamics; a process so-called mitochondrial fusion and fission, controlled by a family of GTP-dependent dynamin related proteins. Optic Atrophy 1 (OPA1) is one of the three fusion proteins that mediates fusion of inner membrane of mitochondria and keeps mitochondria cristae junction tight. In this thesis, we present evidence showing that lack of OPA1 reduces mitochondrial electron transport complex I activity, causes reduction of glycolysis metabolism in neutrophils, which then results in lower adenosine-triphosphate (ATP) production, leading to tubulin network disruption. In absence of proper tubulin network formation, activated neutrophils exhibit disorientation of mitochondria localization and defect in mitochondrial DNA (mtDNA) release and neutrophil extracellular trap (NET) formation. NETs are formed by neutrophils as a part of innate immune response against microorganisms. Neutrophils isolated from the autosomal dominant optic atrophy (ADOA) patient genetically deficient in functional Opa1 protein exhibited disrupted microtubule (MT) network formation, and had defect in NET formation upon activation. Conditional knockout mouse lacking Opa1 in myelocyte population (Opa1N∆), exhibited also defect in mitochondrial complex I activity, glycolysis metabolism and lower adenosine-triphosphate (ATP) production by neutrophils. Similar to ADOA patient’ neutrophils, Opa1-deficient mouse neutrophils displayed disruption of tubulin network formation and lacked ability to form NETs. Moreover, less bacterial clearance was detected in lung of Opa1N∆ mice, despite higher neutrophil infiltration upon Pseudomonas aeruginosa intranasal infection compared to control mice. Lack of bacterial clearance was mainly due to deficient NET formation. In addition, using genetically deficient mice and pharmacological approaches, we could clearly show that NET formation by mouse and human neutrophils occurs independently of both RIPK3 and MLKL signalling, and hence, is independent of necroptosis. These findings

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extend the importance of mitochondria to a function in innate immunity beyond their role in energy production.

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