CD300LG

CD300LG (nepmucin) is a membrane-bound Ig V-like domain receptor of the CD300 family expressed on capillary and microvascular endothelium that bridges lipoprotein metabolism and immune cell trafficking (PMID:16876123). At the apical and basolateral endothelial surface it undergoes bidirectional transcytosis and selectively mediates uptake of IgA2 and IgM but not IgG (PMID:16876123). Its best-defined function is as an endothelial receptor for triglyceride-rich lipoproteins (VLDL and chylomicrons): CD300LG directly binds ApoA4 to drive postprandial TRL clearance, and its loss in mice produces postprandial hypertriglyceridemia independent of VLDL secretion, intestinal absorption, or lipoprotein lipase activity, with reduced human CD300LG protein causally linked to elevated TRL and coronary artery disease risk [PMID:bio_10.1101_2025.08.08.669356]. Consistent with a metabolic role, the Arg82Cys polymorphism and reduced CD300LG expression associate with impaired glucose uptake, increased intramyocellular lipid, and altered HDL/ApoA1 metabolism (PMID:26336608, PMID:35382499), and male Cd300lg knockout mice show impaired glucose tolerance while Mendelian randomization supports a causal relationship between CD300LG levels and fasting glucose, OGTT glucose, and HbA1c (PMID:39190027). In parallel, CD300LG supports lymphocyte binding and transmigration at the endothelium, where its expression is rapidly downregulated by TNF-α during acute inflammation and induced in chronically inflamed HEV-like vessels (PMID:24376728). The mouse ortholog Trem4 associates directly with DAP12 through positively charged transmembrane residues, a coupling required for surface expression on myeloid cells (PMID:27141827), and on dendritic cells and tumor-associated monocytes CD300LG acts as a contact-dependent co-stimulatory molecule that primes lung-homing CD4+ T cells and reprograms CD8+ T cells into central memory-like cells (PMID:31396211, PMID:38386350).

1. 2006 High

   Established CD300LG as an endothelial-restricted Ig-domain membrane protein capable of transcytosis and selective immunoglobulin uptake, defining its baseline molecular identity and trafficking behavior.

   Evidence Immuno-EM, transcytosis assay in polarized MDCK cells, and IgA2/IgM uptake in HeLa cells

   PMID:16876123

   Open questions at the time

   • Endogenous physiological cargo of transcytosis not identified
   • Mechanism and receptor basis of selective IgA2/IgM versus IgG discrimination unresolved
2. 2013 Medium

   Linked CD300LG to lymphocyte trafficking by showing its luminal endothelial expression is dynamically regulated by inflammation, framing it as a context-dependent adhesion/transmigration molecule.

   Evidence Immunohistochemistry, TNF-α treatment, and in vivo inflammatory and NOD mouse models

   PMID:24376728

   Open questions at the time

   • No direct blocking experiment demonstrating requirement in transmigration
   • Lymphocyte-expressed ligand(s) not molecularly identified
3. 2015 Medium

   Connected CD300LG to human metabolic physiology by tying the Arg82Cys variant and reduced muscle/fat expression to lipid accumulation and impaired glucose uptake.

   Evidence Recall-by-genotype human study with euglycemic clamp, MR spectroscopy, and qPCR

   PMID:26336608

   Open questions at the time

   • Associative, not mechanistic — does not show how CD300LG controls IMCL or glucose uptake
   • No causal demonstration in this study
4. 2016 Medium

   Defined a signaling competence for the ortholog Trem4/CD300LG by showing DAP12 coupling via charged transmembrane residues is mandatory for myeloid surface expression.

   Evidence Co-IP of Trem4 with DAP12, surface expression dependency, and polyI:C/IFN stimulation in BMDCs/BMDMs

   PMID:27141827

   Open questions at the time

   • Downstream DAP12 signaling output not characterized
   • Single lab; relationship to endothelial CD300LG function not established
5. 2019 Medium

   Demonstrated a co-stimulatory immune function by showing Trem4 blockade on dendritic cells reduces priming of lung-homing CD4+ T cells.

   Evidence Flow cytometry, DC–T cell co-culture, surface blockade, and in vivo immunization in mice

   PMID:31396211

   Open questions at the time

   • Ligand/receptor on T cells driving co-stimulation unknown
   • Molecular signaling mechanism not defined
6. 2022 Medium

   Extended the metabolic link to lipoprotein biology by associating Arg82Cys with lower large-HDL cholesterol and ApoA1.

   Evidence Large cohort (Oxford BioBank) plus recall-by-genotype HDL subclass quantification

   PMID:35382499

   Open questions at the time

   • Genetic association only; no biochemical mechanism for HDL effect
   • Directionality with respect to TRL handling not resolved here
7. 2024 High

   Provided causal evidence for CD300LG in glucose homeostasis by combining exercise-responsive serum proteomics, Mendelian randomization, and a knockout mouse with impaired glucose tolerance.

   Evidence SomaScan serum proteomics, euglycemic clamp, muscle/fat mRNA-seq, UK Biobank MR, and Cd300lg knockout mice

   PMID:39190027

   Open questions at the time

   • Molecular basis of the sex difference (male-only KO phenotype) unexplained
   • Mechanism connecting serum CD300LG, angiogenesis genes, and insulin sensitivity not dissected
8. 2024 Medium

   Revealed a tumor-immunity function in which CD300LG on tumor-associated monocytes reprograms CD8+ T cells into central memory-like cells through cell-cell contact.

   Evidence Monocyte subset expression profiling, TAMo–CD8 T cell co-culture, and in vivo antitumor experiments

   PMID:38386350

   Open questions at the time

   • T cell-side receptor for CD300LG contact signal unidentified
   • Single lab; signaling pathway driving reprogramming undefined
9. 2025 High

   Identified the core biochemical function of CD300LG as an endothelial TRL receptor that binds ApoA4 to clear postprandial lipoproteins, with human genetics tying its loss to coronary artery disease.

   Evidence Direct CD300LG–ApoA4 binding assay, KO mouse postprandial lipid challenge with VLDL/absorption/LPL controls, and human genetic causal analysis (preprint)

   PMID:bio_10.1101_2025.08.08.669356

   Open questions at the time

   • Preprint; awaits peer review
   • Structural basis of CD300LG–ApoA4 interaction not resolved
   • Mechanistic link between TRL receptor activity and the glucose/HDL phenotypes not directly tested

• It remains unknown how CD300LG's endothelial lipoprotein-receptor activity mechanistically unifies its separate roles in glucose homeostasis, HDL metabolism, and immune cell co-stimulation.
• No single model linking lipid clearance to glucose and immune phenotypes
• Cytoplasmic/DAP12 signaling output not connected to endothelial cargo function
• Structural and binding determinants of CD300LG ligand selectivity undefined