Development of a novel animal model to investigate the role of cholesterol metabolism in the development and differentiation

角鲨烯合成酶(SS)催化两个法尼基二磷酸(FPP)分子的头对头缩合生成角鲨烯，角鲨烯是胆固醇生物合成途径中的第一个特定中间体。我们使用基因打靶技术敲除了小鼠的SS基因。突变杂合子(SS+/-)的小鼠明显正常。与SS+/+小鼠相比，SS+/-小鼠肝脏SS的mRNA水平降低了60%。在肝脏和睾丸中，SS酶活性一直下降50%。然而，SS+/-和SS+/+小鼠的肝脏胆固醇合成没有差异，血浆脂蛋白谱也没有差异，表明SS不是胆固醇生物合成途径中的限速酶。SS基因突变纯合子(SS-/-)的小鼠在怀孕中期是胚胎致死的。E9.5-10.5 SS-/-胚胎表现出严重的生长迟缓和神经管闭合缺陷。通过补充饲料中的角鲨烯或胆固醇并不能挽救这种致命的表型。我们推测，胆固醇是发育所必需的，尤其是神经系统，而且绒毛尿囊循环系统还不够成熟，不足以在这个发育阶段为快速生长的胚胎提供母体胆固醇。

Squalene synthase (SS) catalyzes the reductive head-to-head condensation of two molecules of farnesyl diphosphate (FPP) to form squalene, the first specific intermediate in the cholesterol biosynthetic pathway. We used gene targeting to knock out the mouse SS gene. The mice heterozygous for the mutation (SS+/-) were apparently normal. SS+/- mice showed 60% reduction in the hepatic mRNA levels of SS compared to SS+/+mice. Consistently, the SS enzymatic activities were reduced by 50% in the liver and testis. Nevertheless, the hepatic cholesterol synthesis was not different between SS+/- and SS+/+ mice and plasma lipoprotein profiles were not different irrespective of the presence of the LDL receptor, indicating that SS is not a rate-limiting enzyme in the cholesterol biosynthetic pathway. The mice homozygous for the disrupted SS gene (SS-/-) were embryonic lethal around midgestation. E9.5-10.5 SS-/- embryos exhibited severe growth retardation and defective neural tube closure. The lethal phenotype was not rescued by supplementing the dams either with dietary squalene or cholesterol. We speculate that cholesterol is required for the development, particularly, of nervous system, and that the chorioallantoic circulatory system is not mature enough to supply the rapidly growing embryos with maternal cholesterol at this developmental stage.