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. 2020 Jan 31;16:32. doi: 10.1186/s12917-020-2252-7

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© The Author(s). 2020

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

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Fig. 5 — Overview of genes involved in lipid digestion and absorption in the intestine of Atlantic salmon and studied in the present study. Arrows indicates steps in the pathways. Studied genes are italicized. Green color indicates genes which were significantly down-regulated and red color indicate up-regulated genes. No color represents genes not significantly affected. Dietary choline (CL) is synthesized by choline kinase (chk) to phosphocholine (P-CL) and after an intermediate step not studied here, choline-phosphate cytidylyltransferase (pcyt1a) to phosphatidylcholine (PC). PC could also be synthesized from endogenous phosphatidylethanolamine (PE) by phosphatidylethanolamine N-methyltransferase (pemt). PC is an important element in the membrane portion of lipoproteins preventing triacylglycerol (TAG) from leaking out. Cholesterol (CH) is transported from the lumen and over the membrane by Niemann-Pick C1-Like1 (npc1l1). Acyl-CoA cholesterol acyltransferase (acat) located in ER, facilitates the esterification of CH to cholesterol esters (CE). ATP-binding cassette G5 (abcg5) returns some of the free cholesterol back to the gut for reuse. Some of the free cholesterol is also shuttled to the basolateral membrane for biogenesis of high-density lipoprotein (HDL) mediated by ATP-binding cassette A1 (abca1). Fatty acids (FA) are transported from the gut lumen over the brush border membrane and into the epithelial cell by cd36 (cluster of differentiation 36). The fatty acid-binding protein 2 (fabp2) shuttles the fatty acids within the epithelial cell and the fatty acid transport protein (fatp) further to the smooth endoplasmic reticulum (ER). Monoacylglycerol (MAG) is esterified by monoacylglycerol acyltransferase (mgat2a), located in ER, to diacylglycerol (DAG) which is further transformed into triacylglycerol (TAG), a step not studied here. Microsomal triglyceride transfer protein (mtp) further facilitates the transport of TAG by assisting in the assembly of the lipoprotein. The three apolipoproteins apoB48, apoAI and apoAIV are important elements for successful production and secretion of the lipoprotein. The formation of lipoproteins is again an essential step for export of lipid to the general circulation and to other organs such as the liver. Excess lipid is stored as lipid droplets in the enterocytes. The lipid droplet structure and formation are regulated by the amphiphilic structural protein, adipophilin/perilipin 2 (plin2)