Human sorCS1 binds sortilin and hampers its cellular functions

Abstract

Sortilin and sorCS1 [sortilin-related Vps10p (vacuolar protein sorting/targeting protein 10) domain-containing receptor 1], both members of the Vps10p-D (Vps10p-domain) receptor family, are synthesized as precursor proteins and are converted into their mature form by enzymatic cleavage of a short N-terminal propeptide. SorCS1 does not bind its propeptide, but sortilin is able to bind not just its own propeptide, but also that of sorCS1. In the present study we show that the propeptide region of sorCS1 contains two separate sites for binding to sortilin and that only one of these sites is removed from human (as opposed to mouse) sorCS1 during processing. This leaves mature human sorCS1 with a sortilin-binding N-terminus, which allows formation of a complex between the two receptors in solution and on cell membranes. Furthermore, we find that the interaction with sorCS1 has a pronounced effect on sortilin's ability to mediate the cellular uptake of alternative ligands, and to hamper its facilitation of CNTF (ciliary neutrophic factor) signalling and the induction of phosphorylated STAT3 (signal transducer and activator of transcription 3). Thus the present study reveals a novel regulatory mechanism and suggest an entirely new role for sorCS1 as a modulator of sortilin function.

SorLA in Interleukin-6 Signaling and Turnover

“…Finally, our present and previous studies (33, 50, 60, 61) demonstrate that Vps10p-D receptors, notably sortilin and SorLA, are new and significant participants in the functional regulation of cytokines and receptors of the IL-6 cytokine family.…”

“…Metabolic labeling of HEK293 cells using [ 35 S]cysteine and [ 35 S]methionine (Pro-Mix; Amersham) was performed as described previously (61). …”

Cardiotrophin-like cytokine factor 1 (CLCF1) and neuropoietin (NP) signalling and their roles in development, adulthood, cancer and degenerative disorders

“…This is achieved by sortilin binding directly to LIFR and promoting assembly of the gp130:LIFR heterodimer [22]. This interaction may be inhibited by sortilin-related VPS10 domain containing receptor 1 (sorCS1), which binds sortilin directly [26].…”

SorLA and CLC:CLF-1-dependent Downregulation of CNTFRα as Demonstrated by Western Blotting, Inhibition of Lysosomal Enzymes, and Immunocytochemistry

“…Thus, in receptors showing a high level of new-synthesis, ligand-induced downregulation may be compensated for by synthesis of new receptors. Under these circumstances, the downregulation of the receptor pool can instead be determined by means of metabolic labeling and pulse-chase experiments as described in 21 . Alternatively, iodinated antibodies (preferably Fc-fragments) that do not interfere with receptor binding can be used to "tag" the ectodomain of the receptor, and the expected downregulation can then be determined by radioactive counting of cell pellet and medium.…”

“…To clarify this, it was initially tried to determine the turnover of CNTFRα (in the absence and presence of sorLA) by means of metabolic labeling and pulse-chase experiments as described in 21 . However, attempts to label CNTFRα using a mixture of [S 35 ] cysteine and [S 35 ] methionine showed poor incorporation of radioactivity.…”