Imaging Brain Signal Transduction via Arachidonic and Docosahexaenoic Acids

细胞外钙并不通过二十二碳六烯酸启动神经传递。 二十二碳六烯酸(DHA，22：6N-3)是一种营养必需的多不饱和脂肪酸，在突触膜磷脂中含量很高，但其在体内神经传递中的作用尚不清楚。体外研究表明，DHA可通过钙非依赖性磷脂酶A2(IPLA2)从突触后膜磷脂中释放出来，但不能通过钙依赖性胞内cPLA2选择性释放花生四烯酸(AA)。由于谷氨酸能N-甲基-D-天冬氨酸(NMDA)受体的激活允许细胞外钙离子进入细胞内，我们假设如果iPLA2也选择性地调节体内的DHA信号，那么注射NMDA的大鼠脑AA信号而不是DHA信号将增加。我们使用定量放射自显影技术证实了这一假设。在过度刺激NMDA受体的谷氨酸时，AA的释放超过DHA，可能会导致兴奋性毒性脑损伤(Ramadan等人，2010年)。 成像降低了ipla缺陷小鼠的脑内二十二碳六烯酸代谢和信号。 钙非依赖性磷脂酶A2β(iPLA2β)选择性地从膜磷脂中水解二十二碳六烯酸(DHA，22：6N-3)。编码这种酶的PLA2G6基因突变发生在神经疾病患者中，包括特发性神经变性伴脑铁蓄积和无铁蓄积的肌张力障碍-帕金森综合症。缺乏PLA2G6的小鼠在13个月后表现出神经功能障碍和神经病理，我们假设它们可能干扰了DHA的功能。我们用我们的定量成像技术证实了这一点，即在4个月大的iPLA2β缺陷小鼠出现明显的神经病理之前，大脑DHA代谢和胆碱能受体介导的DHA信号减少。因此，iPLA2β对于在完整的生物体中维持正常的脑DHA代谢和信号是至关重要的。正电子发射断层扫描(PET)有望显示PLA2G6突变患者大脑DHA代谢紊乱，饮食中增加DHA的治疗可能对此类患者有所帮助(Basselin等，2010)。 二十二碳六烯酸(DHA)从血浆掺入脑内是脑DHA代谢和神经传递的体内生物标志物。二十二碳六烯酸(DHA)对维持正常的大脑结构和功能至关重要，被认为具有神经保护作用。它的大脑浓度取决于饮食中DHA的含量和其饮食衍生的n-3前体-α-亚麻酸(α-LNA)的肝脏转化率。我们的体内定量放射自显影和静脉注射放射性标记DHA的方法可以用来成像未酯化的血浆DHA进入大脑的净掺入速率，这等于在静止和激活时大脑代谢DHA的消耗速率。我们还将该方法扩展到正电子发射断层扫描(PET)在人类中的应用。在给予急性N-甲基-d-天冬氨酸(NMDA)的未麻醉大鼠中，局部DHA信号不依赖于细胞外钙，并由钙非依赖性磷脂酶A2(IPLA2)介导。在iPLA2-VIA(Beta)被敲除的小鼠中的研究证实，这种酶对涉及DHA的基线和胆碱能信号至关重要。因此，在动物或人类中，DHA从血浆进入大脑的定量成像可用作脑DHA代谢和神经传递的体内生物标记物(Rapoport等人，2011年)。

EXTRACELLULAR-DERIVED CALCIUM DOES NOT INITIATE NEUROTRANSMISSION VIA DOCOSAHEXAENOIC ACID. Docosahexaenoic acid (DHA, 22:6n-3), a nutritionally essential polyunsaturated fatty, is found in high concentrations in synaptic membrane phospholipids, but its role in neurotransmission in vivo is poorly understood. In vitro studies show that DHA is released from postsynaptic membrane phospholipid by calcium-independent phospholipase A2, iPLA2, but not by calcium-dependent cytosolic cPLA2, which selectively releases arachidonic acid (AA). Since glutamatergic N-methyl-D-aspartate (NMDA) receptor activation allows extracellular calcium into cells, we hypothesized that brain AA but not DHA signaling would be increased in rats injected with NMDA if iPLA2 also selectively modulated DHA signaling in vivo. We confirmed this hypothesis using quantitative autoradiography. Greater AA than DHA release during excess glutamate stimulation of NMDA receptors could contribute to excitotoxic brain damage (Ramadan et al., 2010). IMAGING DECREASED BRAIN DOCOSAHEXAENOIC ACID METABOLISM AND SIGNALING IN iPLA-DEFICIENT MICE. Calcium-independent phospholipase A2beta (iPLA2beta) selectively hydrolyzes docosahexaenoic acid (DHA, 22:6n-3) from membrane phospholipid. Mutations in the PLA2G6 gene encoding this enzyme occur in patients with neurological disorders including idiopathic neurodegeneration plus brain iron accumulation and dystonia-parkinsonism without iron accumulation. Mice lacking PLA2G6 show neurological dysfunction and neuropathology after 13 months, and we hypothesized that they would have disturbed DHA function. We confirmed this by showing with our quantitative imaging technique that brain DHA metabolism and cholinergic receptor mediated DHA-signaling were reduced in 4-month-old iPLA2beta-deficient mice, before overt neuropathology had developed. Thus, iPLA2beta is critical for maintaining normal brain DHA metabolism and signaling in the intact organism. Positron emission tomography (PET) would be expected to show disturbed brain DHA metabolism in patients with PLA2G6 mutations, and treatment with elevated dietary DHA may be helpful in such patients (Basselin et al., 2010). DOCOSAHEXAENOIC ACID (DHA) INCORPORATION INTO BRAIN FROM PLASMA IS AN IN VIVO BIOMARKER OF BRAIN DHA METABOLISM AND NEUROTRANSMISSION. Docosahexaenoic acid (DHA) is critical for maintaining normal brain structure and function, and is considered neuroprotective. Its brain concentration depends on dietary DHA content and hepatic conversion from its diet derived n-3 precursor, alpha-linolenic acid (alpha-LNA). Our in vivo method using quantitative autoradiography and intravenously injected radiolabeled DHA can be used to image net rate of incorporation into the brain of unesterified plasma DHA, which equals the rate of brain metabolic DHA consumption at rest and during activation. We also extended the method for use in humans with positron emission tomography (PET). Imaging in unanesthetized rats given acute N-methyl-d-aspartate (NMDA), regional DHA signaling is independent of extracellular calcium, and mediated by calcium-independent phospholipase A2 (iPLA2). Studies in mice in which iPLA2-VIA (beta) was knocked out confirmed that this enzyme is critical for baseline and cholinergic signaling involving DHA. Thus, quantitative imaging of DHA incorporation from plasma into brain, in animals or humans, can be used as an in vivo biomarker of brain DHA metabolism and neurotransmission (Rapoport et al., 2011).