MDL-1 Ligand
Background story of MDL-1 and its possible ligand.
MDL-1 or CLEC5A is famously known as Dengue virus receptor (Hsieh, 2008). Its binding to Dengue virus receptor is thought to be inducer of cytokine production. Few scientist stated that they could measure the binding of Dengue envelope protein with MDL-1 protein. In the fact, this binding is very weak and no normal measurement technology could catch up the binding affinity. The same case also happen with Japanese Encephalitis Virus.
In the 2018, Hsieh group publish a paper stated a relationship of MDL-1 with Listeria monocytogenes. In the paper, they also found disaccharide (GluNAc-MurNAc) as a binding partner of MDL-1 while the other paper failed to find any binding to MDL-1 from glycan library screening.
A patent application for a binding partner of MDL-1 is being applied(WO2013043516A1). Using immunoprecipitation method, the group found galectin 9 is able to bind to MDL-1 protein. Galectin 9 is especially expressed in T cells and cleaved to make soluble protein. Galectin 9 known to indicate malignancy of a tumor. The binding mode of Galectin 9 to MDL-1 is never known but it is really helpful to understand which part of Galectin 9 and MDL-1 is involved in the binding.
Other possible ligand was found in bone marrow-derived dendritic cells (BMDCs). Our data suggest that lysate or live BMDCs produce some ligands of MDL-1. BMDCs also produce some level of Galectin 9 soluble or membrane bound one. This could be what we found in BMDCs or could be other molecule.
Finding the exact molecule will be very challenging. Lectin known to have very low affinity to its ligand so we need to increase the binding by increasing the avidity. This can be achieved by polymerizing MDL-1 by making fusion protein to other polymer-making protein such as COS7 which make pentamer.
MDL-1 pentamer is then mixed by BMDCs lysate to catch the ligand and applied to mass spectrometry (MS) for further ligand elucidation. Some possible molecule will be detected by MS and we can check them by direct binding to pentamer MDL-1 and through ELISA assay. Further check will be to cells that expressing MDL-1 such as macrophages or neutrophils. MDL-1 expressing cells will produce several inflammatory cytokine upon ligand stimulation.
Binding mode of MDL-1 and the ligand can be analyzed by using X-ray crystallography or cryo-EM. The exact binding mode is very useful to determine the ligand binding site of MDL-1 and further will be helpful to develop some inhibitor that can bind to this site.
