The transmembrane domain and PXXP motifs of ApoE receptor 2 exclude it from carrying out clathrin-mediated endocytosis

Exploring the diversity of AVPR2 in Primates and its evolutionary implications

The current study focuses on the investigation of AVPR2 (VTR2C) protein-coupled receptor variants specific to different primate taxa. AVPR2 is activated by the neurohormone AVP, which modulates physiological processes, including water homeostasis. Our findings reveal positive selection at three AVPR2 sites at positions 190, 250, and 346. Variation at position 250 is associated with human Congenital Nephrogenic Diabetes Insipidus (cNDI), a condition characterized by excessive water loss. Other 13 functional sites with potential adaptive relevance include positions 185, 202, 204, and 252 associated with cNDI. We identified SH3-binding motifs in AVPR2's ICL3 and N-terminus domains, with some losses observed in clades of Cercopithecidae, Callitrichinae, and Atelidae. SH3-binding motifs are crucial in regulating cellular physiology, indicating that the differences may be adaptive. Co-evolution was found between AVPR2 residues and those in the AVP signal peptide/Neurophysin-2 and AQP2, other molecules in the same signaling cascade. No significant correlation was found between these Primates' taxon-specific variants and the bioclimatic variables of the areas where they live. Distinct co-evolving amino acid sequences in functional sites were found in Platyrrhini and Catarrhini, which may have adaptive implications involving glucocorticoid hormones, suggesting varied selective pressures. Further studies are required to confirm these results.

Canonical and Non-canonical Reelin Signaling

Reelin is a large secreted glycoprotein that is essential for correct neuronal positioning during neurodevelopment and is important for synaptic plasticity in the mature brain. Moreover, Reelin is expressed in many extraneuronal tissues; yet the roles of peripheral Reelin are largely unknown. In the brain, many of Reelin's functions are mediated by a molecular signaling cascade that involves two lipoprotein receptors, apolipoprotein E receptor-2 (Apoer2) and very low density-lipoprotein receptor (Vldlr), the neuronal phosphoprotein Disabled-1 (Dab1), and members of the Src family of protein tyrosine kinases as crucial elements. This core signaling pathway in turn modulates the activity of adaptor proteins and downstream protein kinase cascades, many of which target the neuronal cytoskeleton. However, additional Reelin-binding receptors have been postulated or described, either as coreceptors that are essential for the activation of the "canonical" Reelin signaling cascade involving Apoer2/Vldlr and Dab1, or as receptors that activate alternative or additional signaling pathways. Here we will give an overview of canonical and alternative Reelin signaling pathways, molecular mechanisms involved, and their potential physiological roles in the context of different biological settings.

Reelin and apolipoprotein E receptor 2 in the embryonic and mature brain: effects of an evolutionary change in the apoER2 gene

In the mature cerebral cortex of higher vertebrates, neurons are arranged in layers, each layer containing neurons of the same functional class. The cortical layering pattern is laid down during development by migration of young post-mitotic neurons along glial fibres to their correct positions in the cortical plate. The mechanics of whole-cell movement are well understood, but there is still uncertainty as to how a migrating neuron is instructed to leave its glial support when it reaches its destination. An intraneuronal signalling pathway initiated by reelin and containing apolipoprotein E receptor 2 (apoER2) is essential for normal cortical layering, and there is strong evidence that detachment of a migrating neuron is brought about by reelin-dependent downregulation of alpha3 integrin. But there remains the problem of how the reelin signal is switched on at a position in the cortex appropriate for each class of neuron. ApoER2 of placental mammals contains an amino acid sequence that is encoded in a separate exon in the apoER2 gene and is required for normal memory and spatial learning. The separate exon is not present in marsupials, birds or reptiles. The addition of this exon to the evolving apoER2 gene may have contributed to the success of placental mammals.

Modulation of lipoprotein receptor functions by intracellular adaptor proteins

Members of the low density lipoprotein (LDL) receptor gene family are critically involved in a wide range of physiological processes including lipid and vitamin homeostasis, cellular migration, neurodevelopment, and synaptic plasticity, to name a few. Lipoprotein receptors exert these diverse biological functions by acting as cellular uptake receptors or by inducing intracellular signaling cascades. It was discovered that a short sequence in the intracellular region of all lipoprotein receptors, Asn-Pro-X-Tyr (NPXY) is important for mediating either endocytosis or signal transduction events, and that this motif serves as a binding site for phosphotyrosine-binding (PTB) domain containing scaffold proteins. These molecular adaptors connect the transmembrane receptors with the endocytosis machinery and regulate cellular trafficking, or function as assembly sites for dynamic multi-protein signaling complexes. Whereas the LDL receptor represents the archetype of an endocytic lipoprotein receptor, the structurally closely related apolipoprotein E receptor 2 (apoER2) and very low density lipoprotein (VLDL) receptor activate a kinase-dependent intracellular signaling cascade after binding to the neuronal signaling molecule Reelin. This review focuses on two related PTB domain containing adaptor proteins that mediate these divergent lipoprotein receptor responses, ARH (autosomal recessive hypercholesterolemia protein) and Dab1 (disabled-1), and discusses the structural and molecular basis of this different behaviour.

Reconstitution of the Reelin signaling pathway in fibroblasts demonstrates that Dab1 phosphorylation is independent of receptor localization in lipid rafts

The Reelin signaling pathway operates in migrating neurons and is indispensable for their correct positioning during embryonic brain development. Many biochemical and cell biological studies to dissect the Reelin pathway at the molecular level are hampered by the lack of a cell line harboring a functional Reelin signaling pathway. Here we present fibroblast cell lines in which all required functional components of the pathway have been reconstituted. These cells react upon Reelin treatment in the same way as primary neurons. We have subsequently used these cell lines to study the subcellular localization of ApoER2 and the VLDL receptor and could demonstrate that receptor-mediated Dab1 phosphorylation does not depend on lipid rafts and that phosphorylated Dab1 remains bound to the receptor tail when the pathway is activated by Reelin.

The low-density lipoprotein receptor-related protein-1 associates transiently with lipid rafts

The low-density lipoprotein receptor-related protein-1 (LRP-1) is a multifunctional receptor that undergoes constitutive endocytosis and recycling. To identify LRP-1 in lipid rafts, we biotin-labeled cells using a membrane-impermeable reagent and prepared Triton X-100 fractions. Raft-associated proteins were identified in streptavidin affinity-precipitates of the Triton X-100-insoluble fraction. PDGF beta-receptor was identified exclusively in lipid rafts, whereas transferrin receptor was excluded. LRP-1 distributed partially into rafts in murine embryonic fibroblasts (MEFs) and HT 1080 cells, but not in smooth muscle cells and CHO cells. LRP-1 partitioning into rafts was not altered by ligands, including alpha2-macroglobulin, platelet-derived growth factor-BB, and receptor-associated protein (RAP). To examine LRP-1 trafficking between membrane microdomains, we developed a novel method based on biotinylation and detergent fractionation. Association of LRP-1 with rafts was transient; by 15 min, nearly all of the LRP-1 that was initially raft-associated exited this compartment. LRP-1 in the Triton X-100-soluble fraction, which excludes lipid rafts, demonstrated complex kinetics, with phases reflecting import from rafts, endocytosis, and recycling. Potassium depletion blocked LRP-1 endocytosis but did not inhibit trafficking of LRP-1 from rafts into detergent-soluble microdomains. Our data support a model in which LRP-1 transiently associates with rafts but does not form a stable pool. Fluid movement of LRP-1 between microdomains may facilitate its function in promoting the endocytosis of other plasma membrane proteins, such as the urokinase receptor, which localizes in lipid rafts.

ApoER2 is endocytosed by a clathrin-mediated process involving the adaptor protein Dab2 independent of its Rafts' association

The apolipoprotein E receptor 2 (apoER2) is a member of the low-density lipoprotein receptor family which binds ligands such as reelin, apolipoprotein E and apolipoprotein J/clusterin and has been shown to play roles in neuronal migration during development and in male fertility. The function of apoER2 mainly depends on cellular signaling triggered by ligand binding. Although the receptor is internalized, the mechanism and functional significance of its endocytic trafficking remain unclear. Apolipoprotein E receptor 2 partitions into lipid rafts and interacts with caveolin-1, a feature that could modulate its endocytic behavior. Recent evidence also suggested that apoER2 might be endocytosed by a pathway independent of clathrin. Here, we show that despite a raft association, apoER2 internalization depends on its cytoplasmic FxNPXY motif that is similar to canonical motifs for clathrin-mediated endocytosis. This motif mediates receptor binding to the adaptor protein Dab2, which can interact directly with clathrin. Several inhibitory conditions of clathrin-mediated endocytosis, including expression of the dominant negative forms of eps15 and Dab2, decreased apoER2 internalization. In contrast, treatment with the drug nystatin, which blocks the caveolar/raft internalization pathway, has no effect on the receptor's endocytosis. Neither the transmembrane nor the proline-rich insert of the cytoplasmic domain, which has been previously reported to exclude the receptor from the clathrin-mediated pathway, altered apoER2 endocytic activity. These studies indicate that apoER2 internalizes through a clathrin-mediated pathway and that its association with caveolar and noncaveolar rafts does not determine its endocytosis.