LRP1 as a Regulator of the Vasculature and Extracellular Matrix

Abstract

The arterial wall and extracellular matrix (ECM) constantly remodel in response to physiological and pathophysiological signals including aging, atherosclerosis, and aneurysm formation. Low-density lipoprotein receptor-related protein 1 (LRP1) is an endocytic and signaling receptor capable of binding and internalizing over 100 structurally unrelated ligands. LRP1 has an important role in vascular maintenance and homeostasis. Global knock-out of LRP1 in mice results in embryonic lethality due to failure to recruit mural cells to developing vessels. Smooth muscle cell (SMC)-specific knock-out of LRP1 (smLRP1-/-) causes fully penetrant aortic aneurysm formation in mice. Collaborators at University of Texas Health Science Center and Johns Hopkins University have identified multiple rare human LRP1 variants associated with aortic disease. We hypothesize that genetic variants of LRP1 associated with aneurysm formation will impact ligand binding, trafficking, or signaling of LRP1. We used biochemical, cell-based, and mouse studies to determine the functional impact of rare LRP1 variants. We used surface plasmon resonance to investigate the binding relationship between LRP1 and matrix metalloprotease-1 (MMP-1), an ECM protease associated with aneurysm formation. We found that MMP-1 was a novel LRP1 ligand and the endogenous inhibitory MMP-1 complex was the preferred MMP-1 form for LRP1 binding. Complex formation of MMP-1 with tissue inhibitor of metalloproteases 1 (TIMP-1) resulted in a 30- to 40-fold increase in the binding affinity for LRP1 compared to other forms of MMP-1. Next, we introduced rare LRP1 variants by site-directed mutagenesis into truncated forms of LRP1 called mini-receptors to examine LRP1 trafficking. Specific LRP1 mutations impacted LRP1 turnover and/or maturation. We also found that LRP1 mini-receptors have a trafficking impairment that results in targeted, proteasomal degradation and limited LRP1 cell surface delivery. We suggest future caution when using mini-receptors in assays dependent on LRP1 trafficking. Finally, our mouse-based studies examined the impact of loss of LRP1 in SMCs (smLRP1-/-) on the cerebrovasculature and found that the vascular branching pattern and geometry was unaffected. In summary, our results demonstrate that LRP1 rare variants associated with aneurysm formation have functional deficits in LRP1 and that LRP1 is an important regulator of the vasculature and ECM.