Dkk1 Stabilizes Wnt Co-Receptor LRP6: Implication for Wnt Ligand-Induced LRP6 Down-Regulation

The low density lipoprotein receptor-related protein-6 (LRP6) is an essential co-receptor for canonical Wnt signaling. Dickkopf 1 (Dkk1), a major secreted Wnt signaling antagonist, binds to LRP6 with high affinity and prevents the Frizzled-Wnt-LRP6 complex formation in response to Wnts. Previous studies have demonstrated that Dkk1 promotes LRP6 internalization and degradation when it forms a ternary complex with the cell surface receptor Kremen.In the present study, we found that transfected Dkk1 induces LRP6 accumulation while inhibiting Wnt/LRP6 signaling. Treatment with Dkk1-conditioned medium or recombinant Dkk1 protein stabilized LRP6 with a prolonged half-life and induces LRP6 accumulation both at the cell surface and in endosomes. We also demonstrated that Kremen2 co-expression abrogated the effect of Dkk1 on LRP6 accumulation, indicating that the effect of Kremen2 is dominant over Dkk1 regulation of LRP6. Furthermore, we found that Wnt3A treatment induces LRP6 down-regulation, an effect paralleled with a Wnt/LRP6 signaling decay, and that Dkk1 treatment blocked Wnt3A-induced LRP6 down-regulation. Finally, we found that LRP6 turnover was blocked by an inhibitor of caveolae-mediated endocytosis.Our results reveal a novel role for Dkk1 in preventing Wnt ligand-induced LRP6 down-regulation and contribute significantly to our understanding of Dkk1 function in Wnt/LRP6 signaling

Low-Density Lipoprotein Receptor-Related Protein 1 (LRP1) Mediates Neuronal Aβ42 Uptake and Lysosomal Trafficking

Alzheimer's disease (AD) is characterized by the presence of early intraneuronal deposits of amyloid-beta 42 (Abeta42) that precede extracellular amyloid deposition in vulnerable brain regions. It has been hypothesized that endosomal/lysosomal dysfunction might be associated with the pathological accumulation of intracellular Abeta42 in the brain. Our previous findings suggest that the LDL receptor-related protein 1 (LRP1), a major receptor for apolipoprotein E, facilitates intraneuronal Abeta42 accumulation in mouse brain. However, direct evidence of neuronal endocytosis of Abeta42 through LRP1 is lacking.Here we show that LRP1 endocytic function is required for neuronal Abeta42 uptake. Overexpression of a functional LRP1 minireceptor, mLRP4, increases Abeta42 uptake and accumulation in neuronal lysosomes. Conversely, knockdown of LRP1 expression significantly decreases neuronal Abeta42 uptake. Disruptions of LRP1 endocytic function by either clathrin knockdown or by removal of its cytoplasmic tail decreased both uptake and accumulation of Abeta42 in neurons. Finally, we show that LRP1-mediated neuronal accumulation of Abeta42 is associated with increased cellular toxicity.These results demonstrate that LRP1 endocytic function plays an important role in the uptake and accumulation of Abeta42 in neuronal lysosomes. These findings emphasize the central function of LRP1 in neuronal Abeta metabolism

The role of CXCR3/LRP1 cross-talk in the invasion of primary brain tumors

CXCR3 plays important roles in angiogenesis, inflammation, and cancer. However, the precise mechanism of regulation and activity in tumors is not well known. We focused on CXCR3-A conformation and on the mechanisms controlling its activity and trafficking and investigated the role of CXCR3/LRP1 cross talk in tumor cell invasion. Here we report that agonist stimulation induces an anisotropic response with conformational changes of CXCR3-A along its longitudinal axis. CXCR3-A is internalized via clathrin-coated vesicles and recycled by retrograde trafficking. We demonstrate that CXCR3-A interacts with LRP1. Silencing of LRP1 leads to an increase in the magnitude of ligand-induced conformational change with CXCR3A focalized at the cell membrane, leading to a sustained receptor activity and an increase in tumor cell migration. This was validated in patient-derived glioma cells and patient samples. Our study defines LRP1 as a regulator of CXCR3, which may have important consequences for tumor biology