Background: GAG/CXCL12 interactions are critical for chemokine function but co-administration may abrogate their individual cardioprotective effects in a clinical setting. Results: Biophysical studies distinguish CXCL12 residues involved in dimerization from those likely to contact heparin directly. Conclusion: CXCL12 dimerization is required for high affinity heparin binding and protects N-terminal degradation. Significance: Knowledge of the GAG-binding site will enable future development of heparin-insensitive CXCL12 therapeutics.
High-density lipoproteins (HDLs) are athero-protective, primarily because of their ability to promote cholesterol flux from peripheral tissues to the liver by reverse cholesterol transport (RCT). The delivery of HDL-cholesteryl esters (CE) into cells is mediated by the HDL receptor, scavenger receptor class B type I (SR-BI), a promising target for enhancing whole body cholesterol disposal and preventing cardiovascular disease. A detailed understanding of the structural determinants underlying proper SR-BI/HDL alignment that supports the selective uptake of HDL-CE into cells remains lacking. To this end, we exploited CD36, a class B scavenger receptor with a predicted topology similar to that of SR-BI that binds HDL but is unable to mediate efficient selective uptake of HDL-CE. We generated a series of SR-BI/CD36 chimeric receptors that span the extracellular (EC) domain of SR-BI to delineate regions that are essential for SR-BI’s cholesterol transport functions. All 16 SR-BI/CD36 chimeras were transiently expressed in COS-7 cells, and their plasma membrane localization was confirmed. The majority of SR-BI/CD36 chimeric receptors displayed significant reductions in their ability to (i) bind HDL, (ii) deliver HDL-CE to cells, (iii) mediate efflux of free cholesterol (FC) to HDL, and (iv) redistribute plasma membrane domains of FC. We also demonstrated that changes in SR-BI function were independent of receptor oligomerization. Altogether, we have identified discrete subdomains, particularly in the N-terminal and C-terminal regions of the EC domain of SR-BI, that are critical for productive receptor–ligand interactions and the various cholesterol transport functions of SR-BI.
CD36 acts as an important participant in the prothrombotic state associated with chronic inflammatory diseases such as atherosclerosis and diabetes by serving as a signaling relay point for danger-associated molecular patterns (DAMPs), including oxidized low-density lipoproteins (oxLDL) and advanced glycated proteins. Although CD36-mediated signaling pathways have been well characterized in macrophages, less is known about CD36-mediated signaling in platelets. Our group identified important roles for specific Src family kinases (SFK) Lyn and Fyn, and for the guanine nucleotide exchange factors Vav1 and Vav3. Since platelet activation by the collagen receptor GPVI also involves downstream activation of SFKs, we now hypothesize that oxLDL signaling via CD36 primes platelets for hyperactivity by activating components of the GPVI signaling pathway. To test this hypothesis, we treated fluorescently labeled murine platelets with either native LDL (nLDL) or oxLDL and exposed them to immobilized collagen under defined shear flow in a microfluidic flow chamber. A significant increase (~1.2 fold) in platelet accumulation was observed with oxLDL treatment. No increase in accumulation was observed in oxLDL-treated platelets from cd36 null mice, suggesting that this phenomenon was CD36-dependent. Furthermore, addition of tirofiban ablated the oxLDL-induced increase in platelet accumulation, suggesting that the increase was likely due to enhanced platelet-platelet contacts (i.e. enhanced platelet activation). This supports previous data from our laboratory showing that treatment of murine platelets with oxLDL alone resulted in activation of αIIbβ3 integrin. Additionally, oxLDL-treated platelets from vav1/vav3 double null mice phenocopied cd36 null platelets and displayed no increase in platelet accumulation compared to nLDL-treated platelets, strengthening the functional link between CD36 and GPVI signaling pathways. These data suggest a functional link between CD36 and GPVI signaling pathways in platelets, which may contribute to platelet hyperactivity in athero-inflammatory diseases.
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