High expression of ?-synuclein in damaged mitochondria with PLA2G6 dysfunction
It is well known that ?-synuclein (?Syn) is a pathological marker of Parkinson disease (PD), because ?Syn/phosphorylated ?Syn (P?Syn) is a main component of Lewy bodies (LBs). The physiological function of ?Syn has recently become known. As a pre-synaptic chaperone, ?Syn promotes soluble NSF-attachment protein receptor (SNARE)-complex assembly [9, 22]. ?Syn also localizes to nuclei and subcellular organelles, including mitochondria and mitochondrion-associated endoplasmic reticulum (ER) membranes [5, 16, 37]. ?Syn binds to lipid membranes [42, 43], in particular, to membranes with high curvature, such as synaptic vesicles and mitochondrial inner membranes [5, 25, 52]. The N-terminus of ?Syn lies along the surface of the membrane [5, 43], where it senses lipid-packing defects and leads to membrane remodeling and stabilization [7, 29].
Mitochondria comprise an inner and an outer membrane that separate the intermembrane space and the matrix. Mitochondria have various functions, including oxidative phosphorylation, lipid metabolism, endocytosis [24], apoptosis, and calcium and iron homeostasis [32]. A mitochondrial inner membrane-specific phospholipid, cardiolipin, is crucial for the integrity and function of mitochondria [25, 32]. In the brains of mice lacking ?Syn, the mitochondrial lipid composition changes, and complex I/III activity is reduced [12]. Moreover, the N-terminus of ?Syn regulates mitochondrial membrane permeability [40]. Together, these findings suggest that ?Syn is integral to maintaining mitochondrial function.
Calcium-independent phospholipase A2?, encoded by PLA2G6, has diverse functions, such as releasing lipid mediators, inflammation, vascular relaxation, and secretion, by hydrolyzing the sn–2 ester bond in phospholipids [8, 10, 27, 28, 51]. In PLA2G6-associated neurodegeneration (PLAN) [19, 26], formerly called Seitelberger disease, a variety of neurological deficits are present from infancy, suggesting the importance of PLA2G6 in the brain. PLAN encompasses a number of phenotypes, such as infantile neuroaxonal dystrophy and adult-onset dystonia-parkinsonism (Park14). Although these phenotypes differ in the degree and distribution of neurodegeneration, ?Syn/Lewy-related pathology is commonly observed [15, 31, 36].
The pathogenesis of PLA2G6 deficiency is thought to involve dysfunction of mitochondria and membrane remodeling [13, 20]. Pla2g6-knockout (KO) mice show slow progression of motor deficits, and there is a progressive formation of spheroids and tubulovesicular structures [23, 41], similar to that seen in PLAN [11, 17]. Recently, we reported that the spinal cord neurons in Pla2g6-KO mice have ultra-microscopically abnormal mitochondria, with degenerated inner membranes, which are periodic acid Schiff (PAS)-positive, negative for an inner membrane protein (cytochrome c oxidase, CCO), and positive for an outer membrane protein (translocase of the outer mitochondrial membrane, TOM20) on immunohistochemistry [4, 45]. In mass spectrometry, the content of mitochondria-specific phospholipid, cardiolipin, is high in Pla2g6-KO mice. These findings suggest dysregulation of phospholipid metabolism in the mitochondrial inner membrane. We also previously demonstrated low expression of CCO and low production of ATP in PLA2G6-knockdown cells, suggesting mitochondrial dysfunction [3].
Genetic mutation of SNCA [34] and multiple copies of SNCA [38] cause PD, suggesting the toxic function due to a genetic abnormality of ?Syn [5, 48]. On the other hand, in the brains of elderly people without neuropsychiatric symptoms, where widespread ?Syn/Lewy-related pathology is seen [39], the severity of ?Syn/Lewy-related pathology is not associated with the clinical course in PD [14, 18]. In spite of many intensive researches [1, 5, 14, 18, 35, 48], the biological significance of LBs in sporadic PD and other familial PD is not yet fully understood. In this study, we aimed to clarify the reason for ?Syn accumulation in neurons, and pathologically analyzed the relationship between ?Syn and mitochondrial membranes in PLAN and in Pla2g6-KO mice.