Oxidized phospholipid signaling in traumatic brain injury

Abstract

Oxidative stress is a major contributor to secondary injury signaling cascades following traumatic brain injury (TBI). The role of lipid peroxidation in the pathophysiology of a traumatic insult to neural tissue is increasingly recognized. As the methods to quantify lipid peroxidation have gradually improved, so has the understanding of mechanistic details of lipid peroxidation and related signaling events in the injury pathogenesis. While free-radical mediated, non-enzymatic lipid peroxidation has long been studied, recent advances in redox lipidomics have demonstrated the significant contribution of enzymatic lipid peroxidation to TBI pathogenesis. Complex interactions between inflammation, phospholipid peroxidation, and hydrolysis define the engagement of different cell death programs and the severity of injury and outcome. This review focuses on enzymatic phospholipid peroxidation after TBI, including the mechanism of production, signaling roles in secondary injury pathology, and temporal course of production with respect to inflammatory response. In light of the newly identified phospholipid oxidation mechanisms, we also discuss possible therapeutic targets to improve neurocognitive outcome after TBI. Finally, we discuss current limitations in identifying oxidized phospholipids and possible methodologic improvements that can offer a deeper insight into the region-specific distribution and subcellular localization of phospholipid oxidation after TBI.

Keywords: Apoptosis; Efferocytosis; Ferroptosis; Inflammation; Lipid mediator; Redox lipidomics.

Fig 1.. Production and signaling of oxidized free fatty acids (FFA-ox) after traumatic brain injury (TBI).

Schema showing the mechanism of production of FFA-ox signals after TBI. Early after injury, phospholipase A₂ (PLA₂) hydrolyzes phospholipid (PL) to generate lyso-PL and free fatty acid (FFA) which can subsequently be oxidized by lipoxygenases (LOX) and cyclooxygenases (COX). At early stages after injury, FFA-ox such as hydroxyeicosatetraenoic acids (HETEs), hydroxyoctadecadienoic acids (HODEs), and prostaglandins act as important inflammatory mediators initiating microglial activation and neutrophil infiltration. At later stages after injury, FFA-ox such as lipoxins, resolvins, and neuroprotectins contribute to the resolution of inflammation. Structures of representative oxidized fatty acids are depicted

Fig 2.. Ferroptosis in injured Brain.

Ferroptosis in injured (Control cortical impact) rat brain is shown using various indicators of ferroptosis. (A) Co-localization of PEBP1 and 15LO2 in brain tissue. Stitched image showing high resolution large area confocal scanning of 3 × 5 image fields. Left: the overlaid emissions for the immunolocalization of PEBP1 (red), 15LO2 (green), and nuclei (blue). Right: co-localization analysis for 15LO2 and PEBP1, with the number of spots having both proteins appearing yellow. Scale bar, 200 μm. (B) Number of co-localized 15LO2 and PEBP1 in brain tissue 4 hr after injury. (C) Changes in GPX4 activity in rat brain cortex at 4 hr after injury. (D) Volcano plot demonstrates changes in the content of PEox at 1 hr post injury. The figure has been reproduced from Wenzel et. al., [48] with permission from Elsevier.

Fig 3.. Production and signaling of phosphatidylethanolamine (PE) oxidation after traumatic brain injury (TBI).

After TBI, the expression of 15-LOX and its formation of a complex with PE binding protein 1 (PEBP1) increase. This complex oxidizes arachidonyl (AA)-PE to 15-hydroperoxyeicosatetraenoic acid-PE (15-HpETE-PE). The glutathione (GSH) and glutathione peroxidase 4 (GPX4) system responsible for the reduction of 15-HpETE-PE to 15-hydroxyeicosatetraenoic acid-PE (15-HETE-PE) is ineffective post-TBI, resulting in 15-HpETE-PE accumulation and ferroptosis. Structure of AA-PE and 15-HpETE-PE are shown in inset.

Fig 4.. Production and signaling of oxidized cardiolipin (CL-ox) after traumatic brain injury (TBI).

After TBI, cardiolipin (CL) translocate from the inner leaflet to the outer leaflet of the inner mitochondrial membrane (IMM). Once in the outer leaflet, CL binds cytochrome (cyt) c, an electron shuttle of the mitochondrial electron transport chain, to form a complex with peroxidase activity. Using hydrogen peroxide (H₂O₂) as an oxidizing equivalent, this enzyme oxidizes CL. This CL-specific oxidation leads to the release of cyt c from the intermembrane space (IMS) into the cytosol triggering apoptosis. Oxidized CL can be hydrolyzed by calcium-independent phospholipase A₂ (iPLA₂γ) to generate monolyso-CL and oxidized free fatty acids (FFA-ox) involved in inflammatory signaling. Inset showing the structure of cardiolipin and oxidized cardiolipin.

Fig 5.. Production and signaling of oxidized phosphatidylserine (PS-οx) after traumatic brain injury (TBI).

The cytochrome (cyt) c released from mitochondria into the cytosol during apoptosis can bind to phosphatidylserine (PS) in the inner leaflet of the plasma membrane to form a peroxidase. This PS-specific oxidation leads to oxidation of PS. Externalization of PS and PS-οχ to the outer leaflet of the plasma membrane results in efferocytosis by serving as “eat me” signals for phagocytic cells. Structures of PS and PS-ox also shown in the figure.