Sinitski, D. | Search bioRxiv

bioRxiv2023

Structures of Arabidopsis thaliana MDL Proteins and Synergistic Effects with the Cytokine MIF on Human Receptors

Vertebrates have developed effective immune mechanisms to fight microbial attacks, relying on a sophisticated network of innate and adaptive responses, a circulatory system, and numerous orchestrating soluble mediators such as cytokines. Mammalian macrophage migration inhibitory factor (MIF) and its paralog D-dopachrome tautomerase (D-DT/MIF-2) are multifunctional inflammatory cytokines with chemokine-like properties that modulate immunity. Plants possess orthologous MIF/D-DT-like (MDL) proteins, whose function is largely unexplored. Driven by the previous discovery of cross-kingdom mimicry of plant (Arabidopsis thaliana) MDL proteins and human MIF receptor signaling, we here characterized the structures of the three A. thaliana MDLs by X-ray crystallography and explored the mechanism underlying the interplay between plant MDLs, human MIF, and its receptors. We obtained high-resolution structures at 1.56 [A], 1.40 [A], and 2.00 [A] resolution for MDL1, MDL2, and MDL3, respectively, revealing a typical trimeric assembly and a high three-dimensional similarity to human MIF. Although residues at the catalytic site of the three MDLs show high identity to human MIF, the proteins showed low tautomerase activity for the substrate 4-hydroxyphenylpyruvate (HPP). Structural differences likely explain the enzymatic inactivity of plant MDLs for HPP. Strikingly, employing in vitro, in vivo, and in planta test systems, we found that MIF and MDL proteins interact with each other and have the capacity to form hetero-oligomeric complexes. The functional consequences of this interaction were demonstrated applying a yeast-based reporter system specific for the MIF chemokine receptors CXCR2 and CXCR4. MDLs not only triggered receptor signaling on their own, but exhibited pronounced synergism regarding the activation of the CXCR2- and CXCR4-dependent signaling pathways, when co-applied with MIF. These findings were substantiated by the co-administration of pharmacological inhibitors that either disrupt MIF receptor binding or block the catalytic cavity. Moreover, biochemical and biophysical experiments using an allosteric oligomer-specific MIF inhibitor established hexa-oligomer formation between MIF and MDLs as the putative basis for the synergistic effect. Our results are the starting point for a mechanistic understanding of the immunomodulatory activity of a family of highly conserved plant proteins. One Sentence SummaryA. thaliana MDLs and human macrophage inhibitory factor interact with each other and induce synergistic effects in activating CXCR2 and CXCR4.

immunology↗

bioRxiv2021

MIF-2/D-DT is an atypical atherogenic chemokine that promotes advanced atherosclerosis and hepatic lipogenesis

Atherosclerosis is the underlying cause of cardiovascular diseases (CVDs) such as myocardial infarction and ischemic stroke. It is a lipid-triggered chronic inflammatory condition of the arterial vascular wall that is driven by various inflammatory pathways including atherogenic cytokines and chemokines. D-dopachrome tautomerase (D-DT), also known as macrophage migration inhibitory factor-2 (MIF-2), belongs to the MIF protein family, which is best known for its pathogenic role in a variety of inflammatory and immune conditions including CVDs. While MIF is well known as a promoter of atherogenic processes, MIF-2 has not been studied in atherosclerosis. Here, we investigated atherosclerosis in hyperlipidemic Mif-2-/-Apoe-/- mice and studied the role of MIF-2 in various atherogenic assays in vitro. We found that global Mif-2 deficiency as well as its pharmacological blockade by 4-CPPC protected against atherosclerotic lesion formation and vascular inflammation in models of early and advanced atherogenesis. On cellular level, MIF-2 promoted monocyte migration in 2D and 3D and monocyte arrest on aortic endothelial monolayers, promoted B-cell chemotaxis in vitro and B-cell homing in vivo, and increased macrophage foam cell formation. Dose curves and direct comparison in a 3D migration set-up suggest that MIF-2 may be a more potent chemokine than MIF for monocytes and B cells. We identify CXCR4 as a novel receptor for MIF-2. The evidence relies on a CXCR4 inhibitor, CXCR4 internalization experiments, MIF-2/CXCR4 binding studies by yeast-CXCR4 transformants, and fluorescence spectroscopic titrations with a soluble CXCR4 surrogate. Of note, Mif-2-/-Apoe-/- mice exhibited decreased plasma cholesterol and triglyceride levels, lower body weights, smaller livers, and profoundly reduced hepatic lipid accumulation compared to Apoe-/- mice. Mechanistic experiments in Huh-7 hepatocytes suggest that MIF-2 regulates the expression and activation of sterol-regulatory element binding protein-1 and -2 (SREBP-1, SREBP-2) to induce lipogenic downstream genes such as FASN and LDLR, while it attenuated the activation of the SREBP inhibiting AMPK pathway. Studies using receptor Inhibitors showed that SREBP activation and hepatic lipoprotein uptake by MIF-2 is mediated by both CXCR4 and CD74. Lastly and in line with a combined role of MIF-2 in vascular inflammation and hepatic lipid accumulation, MIF-2 was found to be profoundly upregulated in unstable human carotid plaques, underscoring a critical role for MIF-2 in advanced stages of atherosclerosis. Together, these data identify MIF-2 as a novel atherogenic chemokine and CXCR4 ligand that not only promotes lesion formation and vascular inflammation but also strongly affects hepatic lipogenesis in an SREBP-mediated manner, possibly linking atherosclerosis and hepatic steatosis.

immunology↗