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Updated: Jul 13, 2026
Tracing de novo Lipids using Stable Isotope Labeling LC-TIMS-TOF MS/MS
Published on: August 23, 2024
Identification of a novel isoform of microsomal triglyceride transfer protein
Peter J Mohler1, Mei-Ying Zhu2, Anna M Blade3
1Department of Internal Medicine, University of Iowa School of Medicine, Iowa City, Iowa 52242.
Abstract:
Microsomal triglyceride transfer protein (MTP) has been studied extensively, primarily because of its role in the assembly of very low density lipoproteins by the liver and chylomicrons by the intestine. Recent studies have suggested that MTP may also play key roles in other cellular processes. In this paper we report the identification of a novel splice variant of MTP in mice. This isoform, MTP-B, has a unique first exon located approximately 2.7 kilobases upstream of canonical MTP (MTP-A) exon 1. The alternative exon encodes 35 amino acids compared with 20 amino acids encoded by exon 1 of MTP-A. MTP-B represents approximately 90% of total MTP mRNA in mouse adipocytes and 3T3-L1 cells and <5% in mouse liver and intestine. Expression of the alternate isoform in mouse liver was confirmed by mass spectrometry. Co-transfection of COS cells with truncated forms of apoB and either MTP-A or MTP-B demonstrated that both isoforms are effective in the assembly and secretion of nascent apoB-containing lipoproteins. Confocal microscopy of 3T3-L1 cells transfected with enhanced green fluorescent protein or DsRed fusions of the two proteins revealed that MTP-A is localized to the endoplasmic reticulum, whereas MTP-B localizes primarily to the Golgi complex in these cells. We conclude that MTP-B functions similarly to MTP-A in lipoprotein assembly. However, in nonlipoprotein-secreting cells, such as the adipocyte, MTP-B may have different localization properties, perhaps reflecting a distinct role in lipid storage and mobilization.
Insights
Researchers discovered a new MTP variant, MTP-B, in mice. This MTP isoform is abundant in adipocytes and functions similarly to MTP-A in lipoprotein assembly, but may have distinct roles in lipid metabolism.
Area of Science:
- Lipid metabolism
- Molecular biology
- Cellular biology
Background:
- Microsomal triglyceride transfer protein (MTP) is crucial for lipoprotein assembly in the liver and intestine.
- Emerging evidence suggests MTP's involvement in additional cellular functions beyond lipid transport.
Purpose of the Study:
- To identify and characterize novel splice variants of MTP.
- To investigate the functional and localization differences between MTP isoforms.
Main Methods:
- Identification of a novel MTP splice variant (MTP-B) with a unique first exon.
- Quantitative analysis of MTP isoform mRNA expression in various mouse tissues and cell lines.
- Mass spectrometry to confirm MTP-B expression in mouse liver.
- Co-transfection assays in COS cells to assess lipoprotein assembly and secretion.
- Confocal microscopy to determine subcellular localization of MTP-A and MTP-B in 3T3-L1 cells.
Main Results:
- A novel MTP splice variant, MTP-B, was identified, featuring an alternative first exon encoding 35 amino acids.
- MTP-B constitutes the majority of MTP mRNA in adipocytes and 3T3-L1 cells, but is found at low levels in liver and intestine.
- Both MTP-A and MTP-B effectively mediate the assembly and secretion of apoB-containing lipoproteins.
- MTP-A localizes to the endoplasmic reticulum, while MTP-B is primarily found in the Golgi complex in 3T3-L1 cells.
Conclusions:
- MTP-B exhibits functional similarity to MTP-A in lipoprotein assembly.
- Distinct subcellular localization of MTP-B in adipocytes suggests a potential unique role in lipid storage and mobilization in non-lipoprotein-secreting cells.
