Aernoud Fiolet

55 Viewing Atherosclerosis through a Crystal Lens structures grow over days or weeks their tubular diameter increases and eventually notched flat plate crystals of various size begin to shed from the ends of their main structure (Fig. 2 upper panels). 16,17 This process of crystal formation is not unique in nature, as a variety of elemental and organic molecules including amyloid crystals 18 undergo the same morphologic transition as CCs, forming filaments and then twisting into helices as they develop into more stable crystalline structures. 19 Metastable forms of CCs, including filaments and helical ribbons have been seen in vivo within atherosclerotic plaque 22 (Fig. 2 lower panel). In contrast to the rate at which they form and transition in vitro, intracellular CCs may develop and transition more slowly in a confined (lysosomal) space in which the physiochemical environment is highly regulated. However, in the extracellular space of a lipid rich core they may develop more rapidly and become much larger because the physiochemical milieu is less regulated, the pool of free cholesterol may be much larger, and growth is not restrained by cellular membranes. 23, 24, 25 The appearance of free flat plate CCs has major pathological consequences. Unlike tubular metastable CCs, flat plate CCs are rigid, and their unfurled structure has a large exposed sharp edge that is more likely to cause direct trauma to cell membranes and tissues. 7 In addition, the flat plate morphology exposes large areas of the rigid repetitive macromolecular structure of the CC surface that can be recognized by a range of free macromolecules and cellular receptors. 26, 27, 28, 29, 30, 31, 32 INTRACELLULAR CHOLESTEROL CRYSTALS PROMOTE ATHEROMA Atherogenesis results from continual excess uptake and accumulation of lipoproteins (predominantly low-density lipoprotein [LDL]) by the vascular endothelium. In animal models, the vascular endothelium becomes laden with cholesterol withinweeks of initiating a cholesterol rich diet. 33,34 As the lipid content of the endothelial cells increases they begin to generate oxidized LDL (ox-LDL), and CCs begin to form in their cellular membrane. 35,36 These altered lipid elements lead to endothelial dysfunction and stimulate endothelial expression of interleukin 1 β (IL-1 β ), which can be enhanced by hypertension, atheroprone flow, and a range of other factors including smoking and diabetes. 36, 37, 38, 39, 40 IL-1 β acts as a potent chemoattractant but may also affect cholesterol hemostasis in the endothelium by activating SREBP2 41 further enhancing intracellular cholesterol accumulation and the formation of intracellular CCs.