Role of Insig-Mediated Cholesterol Homeostasis in the Development of the Palate and Hair Follicles in Mice
Proper cholesterol homeostasis is crucial in mammalian development. Currently, there are 8 known human malformation syndromes due to genetic defects in cholesterol biosynthesis. Patients with these syndromes present with a constellation of developmental defects, ranging from skeletal and craniofacial dysmorphologies to skin and cardiovascular anomalies. In each of these diseases, there is a block at a specific enzymatic step leading to a deficiency in cholesterol and a concomitant buildup in sterol intermediates proximal to the block; however, it is unclear whether cholesterol deficiency or sterol intermediate accumulation causes the observed abnormalities. In this work, we generated mice lacking Insig-1 and Insig-2. These proteins regulate cholesterol biosynthesis by both inhibiting the proteolytic activation of SREBPs, transcription factors for cholesterol biosynthetic genes, and enhancing the degradation of HMG-CoA reductase, the rate-limiting enzyme of cholesterol biosynthesis. We first created Insig-double-knockout (Insig-DKO) mice homozygous for null mutations in Insig-1 and Insig-2. Insig-DKO mice exhibited defects in midline facial fusion, ranging from cleft palate to complete cleft face, and overproduced sterols, creating a significant buildup of cholesterol and sterol intermediates. When Insig-DKO mice were treated with lovastatin, an HMG-CoA reductase inhibitor, sterol intermediate levels were significantly reduced and the craniofacial defects were ameliorated. Next, we generated mice lacking Insigs in the epidermis (Epi-Insig-DKO). All Epi-Insig-DKO mice failed to grow body hair, had decreased body weights, and died 6 weeks after birth. Early hair follicle development was normal while later development was disrupted and led to hair cycle arrest in these mice. Skin from these mutant mice showed a significant buildup of both cholesterol and sterol intermediates. Topical treatment of Epi-Insig-DKO mice with the HMG-CoA reductase inhibitor simvastatin during the first two postnatal weeks lowered sterol intermediate levels in the skin and completely corrected the hairless and low body weight phenotypes. We conclude that the accumulation of sterol intermediates, not a deficiency of cholesterol, interferes with both the fusion of facial structures and the proper development of hair follicles in mice. Our findings have implications for the pathogenesis of the palate and skin/hair follicle defects observed in human malformation syndromes due to aberrant cholesterol biosynthesis.