DOI: 10.1097/01.mol.0000236374.80872.23

,  

,  

PMID: 16832172

Issn Print: 0957-9672

Publication Date: 2006/08/01


Print

Genetics and molecular biology: Motive studies in mice – two NPXY motifs in the intracellular domain of lipoprotein receptor-related protein 1 exhibit differential importance during fetal development

Jane M Knisely; Guojun Bu

    loading  Checking for direct PDF access through Ovid

Excerpt

The low-density lipoprotein receptor-related protein 1 (LRP1) is an endocytic receptor responsible for the clearance of many ligands with diverse functions including lipoprotein metabolism [1,2]. LRP1 is synthesized as a 600 kDa precursor that is cleaved by furin in the trans-Golgi network. The noncovalently associated 515 and 85 kDa subunits then traffic together to the cell surface, where LRP1 undergoes multiple cycles of ligand binding, endocytosis, and recycling. The 100 amino acid intracellular domain (ICD) of LRP1 contains several protein-binding motifs, including two NPXY motifs, which bind intracellular adaptor proteins. Conventional gene inactivation of the mouse LRP gene leads to embryonic lethality between days E11.5 and E13.5 [3], making the essential functions of LRP1 difficult to decipher. Conditional inactivation of LRP1 in the liver [4], brain [5], and aortic smooth muscle cells [6] has revealed essential roles of the receptor in cholesterol homeostasis, synaptic transmission, and protection from atherosclerosis, respectively.
Although LRP1 undergoes processing by furin as part of its maturation, it is not clear whether this event is required for the proper trafficking and function of the receptor. In addition, both NPXY motifs have been shown to bind cytoplasmic adaptor proteins involved in different signaling and trafficking events [7], but the significance of each NPXY motif for LRP1 function is also unknown. To address these outstanding questions, Roebroek et al.[8••] utilized a recombinase-mediated cassette exchange strategy to generate multiple LRP knock-in mice containing mutations in the essential residues for furin cleavage and the plasma membrane-proximal and distal NPXY motifs. Interestingly, mice that harbored mutations in the proximal NPXY motif died perinatally (E18.5) of hepatomegaly and cell loss in the liver. By contrast, mice bearing mutation in the distal NPXY motif showed no overt phenotype. Finally, mice which lacked a functional furin processing site showed only a mild phenotype, with vacuolization of liver resident macrophages. Mouse embryonic fibroblasts isolated from each of these three mutant mice were sensitive to the LRP1 ligand, Pseudomonas exotoxin A, demonstrating the ability of the receptors to undergo endocytosis.
This study provides valuable data about the relative importance of the two NPXY motifs within the LRP1 ICD. Surprisingly, mutation of the distal NPXY motif, which binds numerous adaptor proteins involved in signaling events [7], and also overlaps with a YXXØ motif, required for the rapid endocytosis of LRP1 [9], produced no phenotype. It is possible that there are subtle defects in these mice that will require further analysis to be revealed. Alternatively, the other adaptor binding and endocytosis motifs in the LRP1 ICD may compensate for the loss of this motif. From the embryonic lethal phenotype, it is clear that the proximal NPXY motif has unique functions. The adaptor protein sorting nexin 17 (SNX17) binds exclusively to this motif and is involved in LRP1 recycling [10••]. Further studies will be needed to determine whether defective LRP recycling accounts for this phenotype.
    loading  Loading Related Articles