Dinh T P, Carpenter D, Leslie F M, Freund T F, Katona I, Sensi S L, Kathuria S, Piomelli D
Department of Pharmacology, University of California, Irvine, CA 92697-4625, USA.
Proc Natl Acad Sci U S A. 2002 Aug 6;99(16):10819-24. doi: 10.1073/pnas.152334899. Epub 2002 Jul 22.
The endogenous cannabinoids (endocannabinoids) are lipid molecules that may mediate retrograde signaling at central synapses and other forms of short-range neuronal communication. The monoglyceride 2-arachidonoylglycerol (2-AG) meets several criteria of an endocannabinoid substance: (i) it activates cannabinoid receptors; (ii) it is produced by neurons in an activity-dependent manner; and (iii) it is rapidly eliminated. 2-AG inactivation is only partially understood, but it may occur by transport into cells and enzymatic hydrolysis. Here we tested the hypothesis that monoglyceride lipase (MGL), a serine hydrolase that converts monoglycerides to fatty acid and glycerol, participates in 2-AG inactivation. We cloned MGL by homology from a rat brain cDNA library. Its cDNA sequence encoded for a 303-aa protein with a calculated molecular weight of 33,367 daltons. Northern blot and in situ hybridization analyses revealed that MGL mRNA is heterogeneously expressed in the rat brain, with highest levels in regions where CB(1) cannabinoid receptors are also present (hippocampus, cortex, anterior thalamus, and cerebellum). Immunohistochemical studies in the hippocampus showed that MGL distribution has striking laminar specificity, suggesting a presynaptic localization of the enzyme. Adenovirus-mediated transfer of MGL cDNA into rat cortical neurons increased MGL expression and attenuated N-methyl-D-aspartate/carbachol-induced 2-AG accumulation in these cells. No such effect was observed on the accumulation of anandamide, another endocannabinoid lipid. The results suggest that hydrolysis by means of MGL is a primary mechanism for 2-AG inactivation in intact neurons.
内源性大麻素是一类脂质分子，可能在中枢突触介导逆行信号传递以及其他形式的短程神经元通讯。单甘油酯2-花生四烯酸甘油酯（2-AG）符合内源性大麻素物质的多项标准：（i）它能激活大麻素受体；（ii）它由神经元以活动依赖的方式产生；（iii）它能被快速清除。2-AG的失活机制仅得到部分了解，但可能通过转运进入细胞和酶促水解发生。在此，我们检验了这样一个假说，即单甘油酯脂肪酶（MGL），一种将单甘油酯转化为脂肪酸和甘油的丝氨酸水解酶，参与2-AG的失活过程。我们通过同源克隆从大鼠脑cDNA文库中克隆出MGL。其cDNA序列编码一个303个氨基酸的蛋白质，计算分子量为33,367道尔顿。Northern印迹和原位杂交分析显示，MGL mRNA在大鼠脑中呈异质性表达，在也存在CB(1)大麻素受体的区域（海马、皮层、前丘脑和小脑）水平最高。海马的免疫组织化学研究表明，MGL的分布具有显著的层特异性，提示该酶位于突触前。腺病毒介导的MGL cDNA转入大鼠皮层神经元增加了MGL的表达，并减弱了这些细胞中N-甲基-D-天冬氨酸/卡巴胆碱诱导的2-AG积累。对另一种内源性大麻素脂质花生四烯乙醇胺的积累未观察到此类效应。结果表明，通过MGL进行水解是完整神经元中2-AG失活的主要机制。
