Peroxidase Mechanism of Lipid-dependent Cross-linking of Synuclein with Cytochrome c

Abstract

Damage of presynaptic mitochondria could result in release of proapoptotic factors that threaten the integrity of the entire neuron. We discovered that ␣-synuclein (Syn) forms a triple complex with anionic lipids (such as cardiolipin) and cytochrome c, which exerts a peroxidase activity. The latter catalyzes covalent hetero-oligomerization of Syn with cytochrome c into high molecular weight aggregates. Syn is a preferred substrate of this reaction and is oxidized more readily than cardiolipin, dopamine, and other phenolic substrates. Lewy bodies (LBs), 3 mitochondrial impairment, and oxidative stress are cardinal features of Parkinson disease (PD) and several related neurodegenerative disorders (1, 2). Aggregation of ␣-synuclein (Syn), an abundant protein in synaptic terminals, plays a major role in the formation of LBs (3, 4). Neither the mechanisms of LB production nor their pathogenic or protective roles in neurodegeneration are well understood.In nigrostriatal dopaminergic synaptic terminals, mitochondria, harboring a host of death-initiating factors, are in peril of damage by reactive oxygen species generated by disrupted electron transport and/or oxidative metabolism of dopamine (DA). Because cytochrome c (cyt c)-dependent formation of apoptosomes and activation of caspases designates a point of no return in the apoptotic program, release of proapoptotic factors from synaptic mitochondria could threaten the integrity of the entire neuron. How neurons protect themselves against inadvertent release of death signals from damaged synaptic mitochondria is not known.The N-terminal fragment of Syn contains six variants of an 11-amino acid consensus motif that include an apolipoprotein-like class A2 helix participating in binding of different lipids, particularly anionic phospholipids (5). This domain is believed to be important for Syn functions in regulation of neuronal lipid metabolism, particularly turnover of a mitochondria-specific phospholipid, cardiolipin (CL) (6). However, the relevance of the Syn lipid binding capacity in regulating neuronal injury (apoptotic) responses has not been established.

Role of cytochrome c in α-synuclein radical formation: implications of α-synuclein in neuronal death in Maneb- and paraquat-induced model of Parkinson’s disease

“…α-Synuclein has a very high affinity for anionic phospholipids such as phosphatidylserine, and these lipids can facilitate its cross linking to cytochrome c [28, 39]. The cross-linking of α-synuclein to cytochrome c in the cytosol delays initiation of apoptosis in neurons [28].…”

“…Damage to presynaptic mitochondria, caused by environmental insults such as pesticides, results in the release of cytochrome c . Once in the cytosol, cytochrome c has one of two fates: It either binds to apaf1 and initiates the apoptotic cascade or can act as a peroxidase [28]. This peroxidase activity of cytochrome c is triggered either through cleavage of methionine-80 from heme iron by action of proteolytic enzymes present in cytosol or through its oxidation [29–32].…”

“…As one of the most abundant proteins in the terminals of dopaminergic neurons, α-synuclein in the presence of anionic phospholipids can easily divert cytochrome c and, hence, prevent binding of cytochrome c to apaf1 [35–38]. In addition, α-synuclein has a very high affinity for anionic phospholipids such as phosphatidylserine, and these lipids can facilitate its crosslinking to cytochrome c [28, 39]. …”

“…Owing to reports showing co-localization of cytochrome c and α-synuclein in the brains of PD patients [28], we hypothesized the role of cytochrome c as a peroxidase in α-synuclein radical formation. Altogether, we aimed to investigate (I) a possible correlation between mitochondrial dysfunction concurrent with cytochrome c release and the role of cytochrome c in α-synuclein radical formation, (II) the role of cytochrome c in α-synuclein oligomerization, and (III) the biological implications of α-synuclein radical formation or the role of α-synuclein per se in terms of its effects on biological pathways and neuronal death.…”

Biosynthesis and roles of phospholipids in mitochondrial fusion, division and mitophagy

“…The elucidation of the functional relationships between these two division mechanisms is of interest. Similar to Drp1, alpha-synuclein also controls apoptosis through interactions with CL and cytochrome c in Lewy bodies [140]. This complex formation prevents cytochrome c -promoted apoptosis.…”