Neuroprotectin D1 (NPD1): A DHA-Derived Mediator that Protects Brain and Retina Against Cell Injury-Induced Oxidative Stress

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Abstract
The biosynthesis of oxygenated arachidonic acid messengers triggered by cerebral ischemia‐reperfusion is preceded by an early and rapid phospholipase A2 activation reflected in free arachidonic and docosahexaenoic acid (DHA) accumulation. These fatty acids are released from membrane phospholipids. Both fatty acids are derived from dietary essential fatty acids; however, only DHA, the omega‐3 polyunsaturated fatty acyl chain, is concentrated in phospholipids of various cells of brain and retina. Synaptic membranes and photoreceptors share the highest content of DHA of all cell membranes. DHA is involved in memory formation, excitable membrane function, photoreceptor cell biogenesis and function, and neuronal signaling, and has been implicated in neuroprotection. In addition, this fatty acid is required for retinal pigment epithelium cell (RPE) functional integrity. Here we provide an overview of the recent elucidation of a specific mediator generated from DHA that contributes at least in part to its biological significance. In oxidative stress‐challenged human RPE cells and rat brain undergoing ischemia‐reperfusion, 10,17S‐docosatriene (neuroprotectin D1, NPD1) synthesis evolves. In addition, calcium ionophore A23187, IL‐1β, or the supply of DHA enhances NPD1 synthesis. A time‐dependent release of endogenous free DHA followed by NPD1 formation occurs, suggesting that a phospholipase A2 releases the mediator's precursor. When NPD1 is infused during ischemia‐reperfusion or added to RPE cells during oxidative stress, apoptotic DNA damage is down‐regulated. NPD1 also up‐regulates the anti‐apoptotic Bcl‐2 proteins Bcl‐2 and BclxL and decreases pro‐apoptotic Bax and Bad expression. Moreover, NPD1 inhibits oxidative stress‐induced caspase‐3 activation. NPD1 also inhibits IL‐Iβ‐stimulated expression of COX‐2. Overall, NPD1 protects cells from oxidative stress‐induced apoptosis. Because photoreceptors are progressively impaired after RPE cell damage in retinal degenerative diseases, understanding of how these signals contribute to retinal cell survival may lead to the development of new therapeutic strategies. Moreover, NPD1 bioactivity demonstrates that DHA is not only a target of lipid peroxidation, but rather is the precursor to a neuroprotective signaling response to ischemia‐reperfusion, thus opening newer avenues of therapeutic exploration in stroke, neurotrauma, spinal cord injury, and neurodegenerative diseases, such as Alzheimer disease, aiming to up‐regulate this novel cell‐survival signaling.

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