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4. apoE-lipoproteins, and influences CSF apoE and apoA-I levels. Keywords:central nervous system, HDL, apoE, apoA-I, ABCA1, mouse, lipoprotein metabolism In plasma, LCAT ML 228 is the sole enzyme capable of esterifying cholesterol in the circulation. LCAT is a 416 amino acid protein that circulates in plasma predominately bound to lipoproteins, where it catalyses the transfer of an unsaturated fatty acid from phosphatidylcholine, or lecithin, to the free -hydroxyl residue of cholesterol to generate cholesterol esters (CE) and lysoPC (lysolecithin) (1). Esterification of lipoprotein cholesterol results in the segregation of CE into the lipoprotein core, an essential step in peripheral HDL maturation. Mutations in the human LCAT gene underlie two distinct metabolic diseases, Familial LCAT Deficiency and Fish Eye Disease, both of which present with low HDL levels (2). The preferred plasma substrate for circulating LCAT is free cholesterol found on HDL, and apolipoprotein A-I (apoA-I), the primary protein constituent of HDL, is considered the major physiological activator of LCAT (3). In vitro experiments show that other plasma apolipoproteins, including apolipoprotein E (apoE), apoC-I, and apoA-IV, are capable of activating LCAT, albeit less efficiently than apoA-I (3). Moreover, apoA-I, and to a lesser extent, apoE appear to be the predominant in vivo activators of LCAT, as ML 228 a recent analysis of apoA-I-, apoE-, and double apoA-I/apoE-deficient mice shows that the percentage of free cholesterol esterified in plasma drops to less than 2% of wild-type (WT) ML 228 values after deletion of apoA-I and apoE (4). LCAT is synthesized mainly in liver, but is also abundant in brain and testes (58). Indeed, brain exhibits the second highest LCAT mRNA level after liver in rats and rhesus monkeys (6,9). Brain LCAT mRNA expression has been demonstrated in cortex, cerebellum, hippocampus, and brain stem. In situ hybridization assays show that LCAT mRNA is found in neurons and glial cells, as determined by nuclear morphology (9), and LCAT activity can be detected in conditioned media of two-thirds of 25 neuronal and gliomal cell ML 228 lines of human and rodent origin (10). LCAT derived from these cell lines responds to the same inhibitor (DTNB) and activator (apoA-I) as plasma LCAT (10). In humans, LCAT protein has been found in cerebrospinal fluid (CSF) at levels that also suggest local synthesis within the central nervous system (CNS) rather than import from the circulation (7). For example, human CSF contains LCAT at levels corresponding to 2.5% that of plasma LCAT (11), roughly comparable to that of apoE, which is present in CSF at approximately 4% of plasma apoE. However, the relevance of LCAT function in brain cholesterol metabolism has not been addressed. As apoB is not found within the CNS, brain lipoprotein metabolism is based entirely on HDL-like particles, which are initially generated as lipid-poor discoidal particles by both astrocytes and microglia (12). CNS HDL particles differ from plasma HDL particles primarily in that apoE, rather than apoA-I, constitutes their main apolipoprotein component. In vitro, the discoidal, lipid-poor, apoE- and apoJ-containing lipoproteins secreted by astrocytes and microglia contain only 0% to 18% of their cholesterol as esters (1316), whereas mature lipoprotein particles isolated from human CSF contain 64% of their cholesterol as esters. In addition, CSF HDL particles are spherical and associated with several other apolipoproteins, including apoA-I, A-IV, C-II, C-III, and apoD (1721), similar to the properties of mature plasma HDL. These observations suggest that brain LCAT may participate in the maturation of nascent glial-derived lipoproteins into the particles found in CSF, much like LCAT functions in plasma HDL metabolism. The most important CSF apolipoproteins to CNS lipid and lipoprotein metabolism are apoE and apoA-I, found at approximately 4% and 0.5% of their plasma concentrations, respectively (17,18). In situ hybridization studies have documented mRNA expression of apoE but not apoA-I within the CNS, although apoA-I is synthesized in cultured brain capillary endothelial cells and ML 228 in porcine choroid plexus extracts (22). These observations suggest that apoA-I is imported into the brain from Rabbit polyclonal to TSP1 the circulation or released into the brain from cerebrovascular endothelial cells. ApoA-I and apoE are the physiological activators of LCAT in mouse plasma as evaluated by ex vivo cholesterol esterification assays.