Intestinal deletion of 3-Hydroxy-3-Methylglutaryl-Coenzyme A Reductase promotes expansion of the resident stem cell compartment.

Objective: The intestine occupies the critical interface between cholesterol absorption and excretion. Despite this, surprisingly little is known about the role of de novo cholesterol synthesis in this organ, and its relationship to whole body cholesterol homeostasis. In addition to cholesterol, the mevalonate pathway is responsible for the synthesis of numerous non-sterol isoprenoids. Here we investigate the physiological importance of the mevalonate pathway within the intestine, through genetic deletion of the rate-limiting enzyme. Approach and Results: Mice lacking 3-hydroxy-3-methylglutaryl-coenzyme A reductase (Hmgcr) in intestinal villus and crypt epithelial cells were generated using a Villin-Cre transgene. In contrast to intestine-specific Srebp-2 and Scap knockouts, mice with intestinal-specific loss of Hmgcr are viable through adulthood and fertile. Hmgcr was efficiently deleted based on mRNA levels, as well as quantitative analysis of Hmgcr alleles by droplet digital PCR. Lipidomics revealed substantial reductions in the abundance of numerous non-sterol isoprenoids and sterol intermediates within the epithelial layer, while cholesterol levels were preserved. Although the intestinal knockout mice are born smaller, there is no net defect in feed efficiency or triglyceride absorption due to compensatory changes in bile acid composition and intestinal growth. At the cellular level, loss of Hmgcr is compensated for quickly after weaning through a dramatic expansion of the stem cell compartment within the crypts. Conclusions: Genetic loss of Hmgcr in the intestine is compatible with life, through mechanisms involving compensatory changes in bile acid composition, increased absorptive surface area, and expansion of the resident stem cell compartment. Intestinal epithelial scrapings and liver tissue mRNA profiles from Vil1Cre(+)Hmgcr(+/+) (WT) and Vil1Cre(+)Hmgcr(fl/fl) (i-KO)研究目标：肠道是胆固醇吸收与排泄的关键界面。尽管如此，人们对该器官内从头合成胆固醇（de novo cholesterol synthesis）的作用及其与全身胆固醇稳态（cholesterol homeostasis）的关系却知之甚少。除胆固醇外，甲羟戊酸途径（mevalonate pathway）还负责合成多种非甾体类异戊二烯类化合物。本研究通过基因敲除该途径的限速酶，探究肠道中甲羟戊酸途径的生理学重要性。研究方法与结果：利用Villin-Cre转基因（Villin-Cre transgene）构建了肠绒毛及隐窝上皮细胞缺失3-羟基-3-甲基戊二酰辅酶A还原酶（3-hydroxy-3-methylglutaryl-coenzyme A reductase，简称Hmgcr）的小鼠模型。与肠道特异性Srebp-2及Scap敲除小鼠不同，肠道特异性Hmgcr缺失小鼠可存活至成年且具有生育能力。通过mRNA水平检测以及液滴数字PCR（droplet digital PCR）对Hmgcr等位基因的定量分析，证实Hmgcr被有效敲除。脂质组学分析显示，上皮层内多种非甾体类异戊二烯类化合物及甾醇中间体的丰度显著降低，而胆固醇水平得以维持。尽管肠道敲除小鼠出生时体型更小，但由于胆汁酸组成及肠道生长的代偿性变化，其饲料转化率或甘油三酯吸收并未出现明显缺陷。在细胞层面，Hmgcr缺失可在断奶后通过隐窝内干细胞区室的显著扩增得到快速代偿。研究结论：肠道内Hmgcr的基因缺失可通过胆汁酸组成代偿性改变、吸收表面积增加以及驻留干细胞区室扩增等机制维持机体存活。本研究的样本包括Vil1Cre(+)Hmgcr(+/+)（野生型，WT）及Vil1Cre(+)Hmgcr(fl/fl)（肠道特异性敲除型，i-KO）小鼠的肠上皮刮取物与肝脏组织mRNA表达谱。