转运蛋白ABCG1/4 和ABCA1对脑胆固醇的调节作用

脑是富含胆固醇的器官,机体大约有25%的胆固醇集中在脑组织中.ATP结合盒超家族转运蛋白对脑组织中胆固醇的膜外转运和动态平衡起着重要的调节作用.研究发现,ATP结合盒超家族转运蛋白亚体ABCG1、ABCG4和ABCA1在成体脑组织中存在不同程度的表达,一种或多种亚体的缺失可以导致神经退行性病变.然而,ATP结合盒超家族转运蛋白亚体对脑发育过程中脑胆固醇动态变化的调节缺乏相关性的报道.在本研究中,从低胆固醇饮食喂养的C57BL/6J小鼠中获取出生后不同发育时期的脑组织,对ABCG1、ABCG4和ABCA1的mRNA与蛋白质表达水平进行测定,并对脑组织和血清中ATP结合盒超家族转运蛋白的表达水平与胆固醇水平的相关性进行研究.同时,使用ABCG1、ABCG4单一基因敲除鼠和ABCG1、ABCG4双基因敲除鼠,研究ATP结合盒超家族转运蛋白对与胆固醇合成的相关基因表达的影响以及对脑组织胆固醇代谢的调节作用.结果发现,ABCG1、ABCG4和ABCA1在机体多个器官中均有表达,但ABCG1和ABCG4在小鼠脑组织中表达量最高.在脑组织发育过程中,ABCG1和ABCG4mRNA水平呈现明显的表达时效性,小鼠于出生后42天达到峰值,而ABCA1 mRNA的表达水平无明显变化.血清和脑组织中中酯化型胆固醇水平呈双高峰分布,也于出生后42天达到最高.基因敲除鼠模型显示,单一敲除ABCG1或者ABCG4基因对脑组织胆固醇水平无明显影响,而ABCG1和ABCG4基因的同时缺失导致脑胆固醇水平显著升高,并明显降低胆固醇合成相关基因的表达水平.本研究表明,在脑发育成熟过程中,ATP结合盒超家族转运蛋白亚体ABCG1和ABCG4,而非ABCA1,以调节脑胆固醇的膜外转运;ABCG1和ABCG4互补调控脑胆固醇的动态平衡.

The Role of Transporters ABCG1/4 and ABCA1 in Brain Cholesterol Metabolism

The brain is the most cholesterol-rich organ in the body and consists of 25% of total cholesterol. ATP-binding cassette (ABC) transporters play essential roles in cellular cholesterol efflux and homeostasis in the brain. Recently, ABCG1, ABCG4 and ABCA1 expression in the adult brain has been described. The absence of one or more of these transporters has been implicated in the development of neurodegenerative diseases. In this study, we characterized the mRNA and protein expression levels of ABCG1, ABCG4 and ABCA1 in the developing postnatal brain of normal C57BL/6J mice fed a chow diet. We studied the correlation between ABC transporters expression and cholesterol levels (free cholesterol, esterified cholesterol) in the brain and serum, to elucidate a potential role of these transporters in cholesterol metabolism in the brain and body during postnatal stages. We further investigated the changes of expression levels of ABCG1, ABCG4, ABCA1, cholesterol related genes and brain cholesterol levels in ABCG1-/-, ABCG4-/- and ABCG1-/- ABCG4-/- double knockout mice. ABCA1 mRNA expression were detectable in multiple tissues, and ABCG1, ABCG4 were highly expressed in adult brain. ABCG1 and ABCG4 mRNA levels peaked at 42 days of age, while ABCA1 mRNA levels were near baseline. ABCG1 protein levels peaked at day 28 then decreased, while ABCG4 levels peaked at day 42. ABCA1 levels remained near baseline. Interestingly, circulating plasma and brain esterified cholesterol levels exhibited a biphasic distribution, which peaked at day 42. Loss of ABCG1 is compensated by increased ABCG4 and vice versa. Loss of both ABCG1 and ABCG4 results in altered expression of cholesterol synthesis related genes and cholesterol accumulation in the brain. The data suggest that ABCG1 and ABCG4, but not ABCA1 are important for transporter function in the developing brain. ABCG1 and ABCG4 play complementary roles in maintaining brain cholesterol homeostasis.