Abnormal sterols in cholesterol-deficiency diseases cause secretory granule malformation and decreased membrane curvature

Abstract

Cholesterol is an abundant lipid in eukaryotic membranes, implicated in numerous structural and functional capacities. Here, we have investigated the mechanism by which cholesterol affects secretory granule biogenesis in vivo using Dhcr7-/- and Sc5d-/- mouse models of the human diseases, Smith-Lemli-Opitz syndrome (SLOS) and lathosterolosis. These homozygous-recessive multiple-malformation disorders are characterized by the functional absence of one of the last two enzymes in the cholesterol biosynthetic pathway, resulting in the accumulation of precursors. Cholesterol-deficient mice exhibit a significant decrease in the numbers of secretory granules in the pancreas, pituitary and adrenal glands. Moreover, there was an increase in morphologically aberrant granules in the exocrine pancreas of Dhcr7-/- acinar cells. Regulated secretory pathway function was also severely diminished in these cells, but could be restored with exogenous cholesterol. Sterol precursors incorporated in artificial membranes resulted in decreased bending rigidity and intrinsic curvature compared with cholesterol, thus providing a cholesterol-mediated mechanism for normal granule budding, and an explanation for granule malformation in SLOS and lathosterolosis.

A distinct trans-Golgi network subcompartment for sorting of synaptic and granule proteins in neurons and neuroendocrine cells

“…Consistent with the latter, we observed a pool of SYN-GFP and VAChT-GFP, which stayed in the TfnR-positive recycling endosomes, and was not significantly affected by 20°C block. By contrast, LDCV proteins such as the neuropeptide precursor, chromogranin A (CgA), are sorted and packaged at the inner face of the TGN at cholesterol-and sphingolipid-rich membrane microdomains that bud off to form LDCVs (Dhanvantari and Loh, 2000;Gondré-Lewis et al, 2006;Kim et al, 2006). These LDCVs are transported to secretion sites and stored until stimulated release, without recycling at the PM to recycling endosomes.…”

“…Synaptic vesicle proteins such as vesicular acetylcholine transporter (VAChT) and synaptophysin are sorted at the Golgi into specific transport (constitutive) vesicles for delivery to the active zone at the presynaptic or plasma membrane (PM), where they are recycled for the production of synaptic vesicles (De Camilli and Jahn, 1990;Kelly, 1991). Neuropeptides, however, are sorted to large densecore vesicles (LDCVs), which are also delivered to the release site, but are not fused to the presynaptic membrane or PM under resting conditions (Gondré-Lewis et al, 2006;Park and Loh, 2008). Thus far, no clear distinction has been made between sorting compartments and transport routes from the Golgi complex to the presynaptic membrane or PM for synaptic vesicle and LDCV proteins.…”

Malformation syndromes caused by disorders of cholesterol synthesis

“…Alteration of the sterol composition also appears to alter other physiochemical properties of membranes. Gondrè-Lewis et al ( 142 ) found, in comparison to cholesterol, that 7DHC decreases the bending rigidity and intrinsic curvature of artifi cial membranes. This observation may explain abnormal pancreatic secretory granule formation ( 142 ).…”

“…Gondrè-Lewis et al ( 142 ) found, in comparison to cholesterol, that 7DHC decreases the bending rigidity and intrinsic curvature of artifi cial membranes. This observation may explain abnormal pancreatic secretory granule formation ( 142 ). In addition to altered sterol composition, Pappu et al ( 143 ) demonstrated increased levels of dolichol and ubiquinone synthesis in SLOS and postulated that these nonsterol isoprenoids could alter membrane fl uidity, permeability, and function.…”

Cholesterol in islet dysfunction and type 2 diabetes

“…In mouse models of Smith-Lemli-Opitz syndrome and lathosterolosis, in which cholesterol synthesis is disrupted, there is impaired formation of secretory granules in the endocrine pancreas as well as the anterior pituitary and adrenal medulla, suggesting that cholesterol is essential for the formation of these secretory granules (44). ABCA1-deficient islets have impaired insulin secretion in response to both non-glucose secretagogues as well as glucose (45), yet have elevated insulin content, suggestive of a defect distal in the exocytotic process rather than in islet glucose sensing per se.…”