LDL receptor-related protein-1 is a sialic-acid-independent receptor for myelin-associated glycoprotein that functions in neurite outgrowth inhibition by MAG and CNS myelin

Abstract

SummaryIn the injured adult mammalian central nervous system (CNS), products are generated that inhibit neuronal sprouting and regeneration. In recent years, most attention has focused on the myelin-associated inhibitory proteins (MAIs) Nogo-A, OMgp, and myelin-associated glycoprotein (MAG). Binding of MAIs to neuronal cell-surface receptors leads to activation of RhoA, growth cone collapse, and neurite outgrowth inhibition. In the present study, we identify low-density lipoprotein (LDL) receptor-related protein-1 (LRP1) as a high-affinity, endocytic receptor for MAG. In contrast with previously identified MAG receptors, binding of MAG to LRP1 occurs independently of terminal sialic acids. In primary neurons, functional inactivation of LRP1 with receptor-associated protein, depletion by RNA interference (RNAi) knock-down, or LRP1 gene deletion is sufficient to significantly reverse MAG and myelin-mediated inhibition of neurite outgrowth. Similar results are observed when LRP1 is antagonized in PC12 and N2a cells. By contrast, inhibiting LRP1 does not attenuate inhibition of neurite outgrowth caused by chondroitin sulfate proteoglycans. Mechanistic studies in N2a cells showed that LRP1 and p75NTR associate in a MAG-dependent manner and that MAG-mediated activation of RhoA may involve both LRP1 and p75NTR. LRP1 derivatives that include the complement-like repeat clusters CII and CIV bind MAG and other MAIs. When CII and CIV were expressed as Fc-fusion proteins, these proteins, purified full-length LRP1 and shed LRP1 all attenuated the inhibition of neurite outgrowth caused by MAG and CNS myelin in primary neurons. Collectively, our studies identify LRP1 as a novel MAG receptor that functions in neurite outgrowth inhibition.

Identification of the Low Density Lipoprotein (LDL) Receptor-related Protein-1 Interactome in Central Nervous System Myelin Suggests a Role in the Clearance of Necrotic Cell Debris

“…When CCRII and CCRIV were used to identify myelin-specific LRP1 ligands (Table 1 and Fig. 2A), we confirmed the binding of MBP and myelin-associated glycoprotein, two proteins that we have previously reported as ligands for LRP1 (4,14). The myelin proteins, CNP and PLP, also were identified as ligands for LRP1 using our screen.…”

“…We confirmed the interaction of MBP and myelin-associated glycoprotein with LRP1 (Table 1 and supplemental Table 1) (4,14) and also detected PLP1 as LRP1 ligands ( Table 1). The most surprising result we obtained by proteomics screening was the identification of numerous intracellular LRP1 ligands in myelin (Table 1).…”

“…In addition to its function as a phagocytic receptor for myelin, LRP1 also participates in myelin-mediated inhibition of axon formation (14). The goal of this study was to characterize the interactome of LRP1 in myelin.…”

Physiological Exploration of the Long Term Evolutionary Selection against Expression of N-Glycolylneuraminic Acid in the Brain

“…MAG is a sialic acid-binding protein expressed on myelin (innermost layer of myelin) and is known to prefer Neu5Ac to Neu5Gc for its ligand (43,44). MAG is considered to mediate tight interactions between axon and myelin and axonal outgrowth via its binding to glycan ligands and protein-protein interaction (42,45). Magnull mice and B4galnt1 (␤-1,4-N-acetylgalactosaminyltransferase 1)-null mice lacking complex gangliosides, which have been used as MAG ligand-deficient mice, showed dysmyelination especially as they aged (46 -49).…”

The NMDA receptor functions independently and as an LRP1 co-receptor to promote Schwann cell survival and migration

“…Other LRP1 co-receptors in neurons include Trk receptors and p75 NTR (Shi et al, 2009;Yoon et al, 2013;Stiles et al, 2013). Very little is known about the molecular nature of the LRP1 signaling system in other cell types.…”