Abstract
Mitophagy is an emerging paradigm for mitochondrial quality control and cell homeostasis. Dysregulation of mitophagy can lead to human pathologies such as neurodegenerative disorders and contributes to the aging process. Complex protein signaling cascades have been described that regulate mitophagy. We have identified a novel lipid signaling pathway that involves the phospholipid cardiolipin (CL). CL is synthesized and normally confined at the inner mitochondrial membrane. However, upon a mitophagic trigger, ie, collapse of the inner membrane potential, CL is rapidly externalized to the mitochondrial surface with the assistance of the hexameric nucleoside diphosphate kinase D (NME4, NDPK-D, or NM23-H4). In addition to its NDP kinase activity, NME4/NDPK-D shows intermembrane phospholipid transfer activity in vitro and in cellular systems, which relies on NME4/NDPK-D interaction with CL, CL-dependent crosslinking of inner and outer mitochondrial membranes by symmetrical, hexameric NME4/NDPK-D, and a putative NME4/NDPK-D-based CL-transfer pathway. CL exposed at the mitochondrial surface then serves as an ‘eat me' signal for the mitophagic machinery; it is recognized by the LC3 receptor of autophagosomes, targeting the dysfunctional mitochondrion to lysosomal degradation. Similar NME4-supported CL externalization is likely also involved in apoptosis and inflammatory reactions.
Highlights
The selective elimination of dysfunctional mitochondria, mitophagy, is emerging as a key player among cellular quality control mechanisms.[1,2] These mechanisms comprise signaling pathways acting on proteins and entire organelles, and are increasingly recognized as being causative in various pathological dysfunctions as well as for the aging process.[3]
Very much like phosphatidylserine externalized to the cellular surface serves for elimination of damaged cells via apoptosis, CL externalized to the mitochondrial surface acts as a signal for the elimination of damaged mitochondria via mitophagy.[11,12,13,14]
In contrast to the CL that triggers mitophagy through recognition via light chain 3 (LC3), the CL externalized during apoptosis has mostly peroxidized acyl chains, due to the CL oxidase function of cytochrome c,30 and this oxidation step seems to be important for execution of the apoptotic program.[31,32]
Summary
The selective elimination of dysfunctional mitochondria, mitophagy, is emerging as a key player among cellular quality control mechanisms.[1,2] These mechanisms comprise signaling pathways acting on proteins and entire organelles, and are increasingly recognized as being causative in various pathological dysfunctions as well as for the aging process.[3].
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