Identification and characterization of SorCS, a third member of a novel receptor family

The sorLA cytoplasmic domain interacts with GGA1 and -2 and defines minimum requirements for GGA binding

We report that the Vps10p domain receptor sorLA binds the adaptor proteins GGA1 and -2, which take part in Golgi-endosome sorting. The GGAs bind with differential requirements via three critical residues in the C-terminal segment of the sorLA cytoplasmic tail. Unlike in sortilin and the mannose 6-phosphate receptors, the GGA-binding segment in sorLA contains neither an acidic cluster nor a dileucine. Our results support the concept of sorLA as a potential sorting receptor and suggest that key residues in sorLA and sortilin conform to a new type of motif (psi-psi-X-X-phi) defining minimum requirements for GGA binding to cytoplasmic receptor domains.

SorCS1, a member of the novel sorting receptor family, is localized in somata and dendrites of neurons throughout the murine brain

The expression of the sorCS1 protein in the central nervous system of adult mice was studied by immunohistochemistry. A detailed mapping revealed a distribution of sorCS1 immunoreactivity in a widespread population of neurons throughout the brain. Two different types of cellular localization were observed. Many neurons exhibited a punctate cytoplasmic staining which extended into the dendrites, in other neurons sorCS1 immunoreactivity was associated with the plasma membrane. This suggests variable functions for sorCS1 in the neurons of the brain.

Activation and functional characterization of the mosaic receptor SorLA/LR11

We previously isolated and sequenced the approximately 250-kDa type 1 receptor sorLA/LR11, a mosaic protein with elements characterizing the Vps10p domain receptor family as well as the low density lipoprotein receptor family. The N terminus of the Vps10p domain comprises a consensus sequence for cleavage by furin ((50)RRKR(53)) that precedes a truncation found in sorLA isolated from human brain. Here we show that sorLA, like sortilin-1/neurotensin receptor-3, whose lumenal domain consists of a Vps10p domain only, is synthesized as a proreceptor that is cleaved by furin in late Golgi compartments. We show that the truncation conditions the Vps10p domain for propeptide inhibitable binding of neuropeptides and the receptor-associated protein. We further demonstrate that avid binding of the receptor-associated protein, apolipoprotein E, and lipoprotein lipase not inhibited by propeptide occurs to sites located in other lumenal domains. In transfected cells, about 10% of full-length sorLA were expressed on the cell surface capable of mediating endocytosis. However, the major pool of receptors was found in late Golgi compartments, suggesting possible interaction with newly synthesized ligands. The results show that sorLA, following activation by truncation, binds multiple ligands and may mediate both endocytosis and sorting.

The sortilin cytoplasmic tail conveys Golgi-endosome transport and binds the VHS domain of the GGA2 sorting protein

Sortilin belongs to a growing family of multiligand type-1 receptors with homology to the yeast receptor Vps10p. Based on structural features and sortilin's intracellular predominance, we have proposed it to be a sorting receptor for ligands in the synthetic pathway as well as on the cell membrane. To test this hypothesis we examine here the cellular trafficking of chimeric receptors containing constructs of the sortilin tail. We report that sorting signals conforming to YXX and dileucine motifs mediate rapid endocytosis of sortilin chimeras, which subsequently travel to the trans-Golgi network, showing little or no recycling. Furthermore, we found that cation-independent mannose 6-phosphate receptor (MPR300)-sortilin chimeras, expressed in mannose 6-phosphate receptor knockout cells, were almost as efficient as MPR300 itself for transport of newly synthesized beta-hexosaminidase and beta-glucuronidase to lysosomes, and established that the sortilin tail contains potent signals for Golgi-endosome sorting. Finally, we provide evidence suggesting that sortilin is the first example of a mammalian receptor targeted by the recently described GGA family of cytosolic sorting proteins, which condition the Vps10p-mediated sorting of yeast carboxypeptidase Y.

Identification of SorCS2, a novel member of the VPS10 domain containing receptor family, prominently expressed in the developing mouse brain

We report the identification of a fourth member of the VPS10 domain containing receptor family, SorCS2, highly expressed in the developing and mature murine central nervous system. During early central nervous system development its main site of expression is the floor plate. In addition, high transcript levels were detected transiently in a variety of brain regions including the dopaminergic midbrain nuclei and the dorsal thalamus. Outside the nervous system expression is detected in lung and heart and transiently in a variety of mesodermally derived tissues.

Transient expression of SorCS in developing telencephalic and mesencephalic structures of the mouse

Here we describe the expression of a third member of the VPS10 domain containing receptor family, SorCS, during mouse embryonal and early postnatal nervous system development. SorCS is expressed in a unique transient and dynamic pattern in regions where cells proliferate, as well as in areas where already differentiated cells reside, including the cerebral cortex, the ventral tegmental area, and the globus pallidus. Transcripts were absent from fiber tracts hinting at a neuronal expression. The only exception was hybridization signals on the developing optic nerve correlating with the appearance of astrocytes migrating into the retina.

Sortilin/neurotensin receptor-3: a new tool to investigate neurotensin signaling and cellular trafficking?

The identification of gp95sortilin, a sorting protein, as being the 100 kDa neurotensin (NT) receptor, a non-G-protein coupled receptor, constitutes a new and interesting but intriguing step in the neuropeptide signaling as well as in cellular trafficking. The isolation of the same protein by three different experimental approaches sum up the complexity for researchers involved in the functional significance of the so-called sortilin/neurotensin receptor 3 (NTR3). This review will concentrate on the putative physiological and cellular roles of sortilin/NTR3 as most results so far have proposed hypothetical conclusions rather than concrete evidence.

Ectodomain shedding, translocation and synthesis of SorLA are stimulated by its ligand head activator

The single transmembrane receptor SorLA is the mammalian orthologue of the head activator-binding protein, HAB, from hydra. The human neuronal precursor cell line NT2 and the neuroendocrine cell line BON produce head activator (HA) and respond to HA by entry into mitosis and cell proliferation. They express SorLA, and bind HA with nanomolar affinity. HA coupled to Sepharose is able to precipitate SorLA specifically proving that SorLA binds HA. Using antisera directed against extra- and intracellular epitopes we find SorLA as membrane receptor and as soluble protein released from cells into the culture medium. Cell lines differ strongly in processing of SorLA, with NT2 cells expressing SorLA mainly as membrane receptor, whereas release predominates in BON cells. Soluble SorLA lacks the intracellular domain and is shed from the transmembrane protein by a metalloprotease. Release from cells and brain slices is stimulated by HA and by phorbol ester, and it is blocked by a metalloprotease inhibitor and by lowering the temperature to 20 degrees C. Blockade of SorLA shedding and treatment of cells with SorLA antisense oligonucleotides lead to a decrease in the rate of cell proliferation. From this we conclude that SorLA is necessary to mediate the mitogenic effect of endogenous HA. HA enhances the translocation of SorLA from internal membranes to the cell surface and its internalization. In addition, HA stimulates SorLA synthesis hinting at an autocatalytic feedback loop in which the ligand activates production, processing, and translocation of its receptor.

Human homologues of yeast vacuolar protein sorting 29 and 35

In the yeast Saccharomyces cerevisiae, a membrane coat complex is required for endosome to Golgi retrograde transport. The vacuolar protein sorting proteins Vps29p, Vps35p, and Vps26p are required for pre-vacuolar/late endosome to Golgi retrieval of the vacuolar hydrolase receptor Vps10p. They form a cargo recognition and concentration subcomplex, termed the inner shell of the retromer coat, prior to vesicle formation by the addition of the membrane-deforming outer shell. We have cloned the human and murine homologues of yeast Vps29p and the human homologue of Vps35p. They encode 182 and 796 residue proteins, with 43 and 29% identity to their respective yeast. The 10.5 kb, 5 exon, VPS29 gene is located on chromosome 12q24 and the 29.6 kb, 17 exon, VPS35 gene is on chromosome 16. In humans, Vps29p, Vps35p, and Hbeta58, the homologue of Vps26p, may form an inner shell of the retromer coat similar to that found in yeast.

Alternative splicing of murine SorCS leads to two forms of the receptor that differ completely in their cytoplasmic tails

We report the identification of a splice variant of SorCS, a member of the family of VPS10 domain containing receptors. These type I transmembrane proteins share the presence of internalization signals in their cytoplasmic tail as one common characteristic. We show that an alternatively spliced transcript of SorCS is generated by differential processing of a composite internal/terminal exon. This splice variant encodes a protein with an N-terminal VPS10 domain followed by a leucine-rich module and a transmembrane domain identical with the already described SorCS protein, but a divergent cytoplasmic tail. In contrast to the known intracellular regions of the related receptors, this splice variant contains no internalization or sorting signals.