The multifunctional type 1 receptor sortilin is involved in endocytosis and intracellular transport of ligands. The short intracellular domain of sortilin binds several cytoplasmic adaptor proteins (e.g., the AP-1 complex and GGA1 to -3), most of which target two well-defined motifs: a C-terminal acidic cluster dileucine motif and a YXXΦ motif in the proximal third of the domain. Both motifs contribute to endocytosis as well as Golgi-endosome trafficking of sortilin. The C-terminal acidic cluster harbors a serine residue, which is subject to phosphorylation by casein kinase. Phosphorylation of this serine residue is known to modulate adaptor binding to sortilin. Here, we show that the cytoplasmic domain of sortilin also engages Rac-p21-activated kinases 1 to 3 (PAK1-3) via a binding segment that includes a tyrosine-based motif, also encompassing a serine residue. We further demonstrate that PAK1-3 specifically phosphorylate this serine residue and that this phosphorylation alters the affinity for AP-1 binding and consequently changes the intracellular localization of sortilin as a result of modulated trafficking. Our findings suggest that trafficking of ligands bound to sortilin is in part regulated by group A PAK kinases, which are downstream effectors of Rho GTPases and are known to affect a variety of processes by remodeling the cytoskeleton and by promoting gene transcription and cell survival.

The heterodimeric cytokine Cardiotrophin-like Cytokine:Cytokine-like Factor-1 (CLC:CLF-1) targets the glycosylphosphatidylinositol (GPI)-anchored CNTFRα to form a trimeric complex that subsequently recruits glycoprotein 130/Leukemia Inhibitory Factor Receptor-β (gp130/LIFRβ) for signaling. Both CLC and CNTFRα are necessary for signaling but so far CLF-1 has only been known as a putative facilitator of CLC secretion. However, it has recently been shown that CLF-1 contains three binding sites: one for CLC; one for CNTFRα (that may promote assembly of the trimeric complex); and one for the endocytic receptor sorLA. The latter site provides high affinity binding of CLF-1, CLC:CLF-1, as well as the trimeric (CLC:CLF-1:CNTFRα) complex to sorLA, and in sorLA-expressing cells the soluble ligands CLF-1 and CLC:CLF-1 are rapidly taken up and internalized. In cells co-expressing CNTFRα and sorLA, CNTFRα first binds CLC:CLF-1 to form a membrane-associated trimeric complex, but it also connects to sorLA via the free sorLA-binding site in CLF-1. As a result, CNTFRα, which has no capacity for endocytosis on its own, is tugged along and internalized by the sorLA-mediated endocytosis of CLC:CLF-1. The present protocol describes the experimental procedures used to demonstrate i) the sorLA-mediated and CLC:CLF-1-dependent downregulation of surface-membrane CNTFRα expression; ii) sorLA-mediated endocytosis and lysosomal targeting of CNTFRα; and iii) the lowered cellular response to CLC:CLF-1-stimulation upon sorLA-mediated downregulation of CNTFRα.

Sortilin is a neuronal receptor involved in transmembrane signaling, endocytosis, and intracellular sorting of proteins. It cycles through a number of cellular compartments where it encounters various acidic conditions. The crystal structure of the sortilin ectodomain has previously been determined at neutral pH. Here, we present the 3.5-Å resolution crystal structure of sortilin at pH 5.5, which represents an environment similar to that of late endosomes, where ligands are released. The structure reveals an overall distortion of the 10-bladed β-propeller domain. This distortion and specific conformational changes, caused by protonation of a number of histidine residues, render the currently known binding sites unavailable for ligand binding. Access to the binding sites is furthermore blocked by a reversible and pH-dependent formation of tight sortilin dimers, also confirmed by electron microscopy, size-exclusion chromatography, and mutational studies. This study reveals how sortilin binding sites are disrupted and explains pH-dependent ligand affinity.

Interleukin-6 (IL-6) is a multifunctional cytokine with important functions in various physiologic processes. Mice lacking IL-6 exhibit multiple phenotypic abnormalities, such as an inadequate immune and acute-phase response, and elevated levels of circulating IL-6 have been found to accompany several pathological conditions. IL-6 binds the nonsignaling IL-6 receptor (IL-6R), which is expressed as a transmembrane, as well as a secreted circulating protein, before it engages homodimeric gp130 for signaling. Complex formation between IL-6 and the membrane-bound IL-6 receptor gives rise to classic cis signaling, whereas complex formation between IL-6 and the soluble IL-6R results in trans signaling. Here, we report that the endocytic receptor SorLA targets IL-6 and IL-6R. We present evidence that SorLA mediates efficient cellular uptake of both IL-6 and the circulating IL-6R in astrocytes. We further show that SorLA interacts with the membrane-bound IL-6R at the cell surface and thereby downregulates IL-6 cis signaling. Finally, we find that the SorLA ectodomain, released from the cell membrane upon enzymatic cleavage of full-length SorLA, may act as an IL-6 carrier protein that stabilizes IL-6 and its capacity for trans signaling.

Objectives: Circulating PCSK9 binds to hepatic low-density lipoprotein receptor (LDLR) and triggers its degradation, and hereby has a major impact on clearance of cholesterol from the circulation. We have recently identified sortilin from the sortilin-related receptor family as a high affinity PCSK9 sorting receptor that facilitates PCSK9 secretion from primary hepatocytes and in vivo. The objective of the present study was to investigate if the sortilin-related receptor SorLA also modulates PCSK9 activity.

Cardiotrophin-like cytokine:cytokine-like factor-1 (CLC:CLF-1) is a heterodimeric neurotropic cytokine that plays a crucial role during neuronal development. Mice lacking CLC:CLF-1 die soon after birth due to a suckling defect and show reduced numbers of motor neurons. Humans carrying mutations in CLC:CLF-1 develop similar disorders, known as Sohar-Crisponi or cold-induced sweating syndrome, and have a high risk of early death. It is well known that CLC binds the ciliary neurotrophic factor receptor α (CNTFRα) and is a prerequisite for signaling through the gp130/leukemia inhibitory factor receptor β (LIFRβ) heterodimer, whereas CLF-1 serves to promote the cellular release of CLC. However, the precise role of CLF-1 is unclear. Here, we report that CLF-1, based on its binding site for CLC and on two additional and independent sites for CNTFRα and sorLA, is a key player in CLC and CNTFRα signaling and turnover. The site for CNTFRα enables CLF-1 to promote CLC:CNTFRα complex formation and signaling. The second site establishes a link between the endocytic receptor sorLA and the tripartite CLC:CLF-1:CNTFRα complex and allows sorLA to downregulate the CNTFRα pool in stimulated cells. Finally, sorLA may bind and concentrate the tripartite soluble CLC:CLF-1:CNTFRα complex on cell membranes and thus facilitate its signaling through gp130/LIFRβ.

Neurotrophic factors have been investigated in relation to depression. The aim of the present study was to widen this focus to sortilin, a receptor involved in neurotrophic signalling. The serum sortilin level was investigated in 152 individuals with depression and 216 control individuals, and eight genetic markers located within the SORT1 gene were successfully analysed for association with depression. Genotyping was performed using the Sequenom MassARRAY platform. All the individuals returned a questionnaire and participated in a semi-structured diagnostic interview. Sortilin levels were measured by immunoassay, and potential determinants of the serum sortilin level were assessed by generalized linear models. Serum levels of brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF) were measured in previous studies. We identified a significant increase of serum sortilin levels in depressed individuals compared with controls (P=0.0002) and significant positive correlation between serum sortilin levels and the corresponding levels of BDNF and VEGF. None of the genotyped SNPs were associated with depression. Additional analyses showed that the serum sortilin level was influenced by several other factors. Alcohol intake and body mass index, as well as depression, serum BDNF and serum VEGF were identified as predictors of serum sortilin levels in our final multivariate model. In conclusion, the results suggest a role of circulating sortilin in depression which may relate to altered activity of neurotrophic factors.

Most neonatal cholestatic disorders are rare diseases (RD), but they represent the leading indication for pediatric liver transplantation. From the pediatric surgical point of view, biliary atresia (BA) and choledochal cyst are from particular interest because diagnostic pathways and therapeutic algorithms are diverging as well as outcome parameters of follow-up studies. In addition, screening programs for BA are still at an early stage while transition of those patients with and without liver transplantation is not established yet. The newly created Web site www.bard-online.com focuses on BA and related diseases and includes decentralized online registries, information for patients and health workers, as well as a platform for interdisciplinary communication and service offerings. This initiative was started to compensate for disadvantages for patients with RD and to help unifying the community of clinicians and researchers in this field.

Biliary atresia (BA) is an orphan medical condition of the newborn, resulting in end-stage liver cirrhosis due to obliterative cholangiopathy of the extrahepatic bile duct. Although Kasai’s hepatoportoenterostomy (KPE) is the well-established first-line therapy, little is known about its surgical complications. 153 patients receiving open KPE treated at a single center between 1994 and 2014 were analysed retrospectively regarding short-term complications and survival with the native liver. In brief, 40.5% of patients suffered from 1–3 surgical complications, inguinal hernias (IH) being most prevalent (40.0%). In BA patients, incidence of IH was associated with male gender (p=0.002), the syndromic form of BA (p=0.038), and percutaneous drainage for ascites (p=0.002). No association was found with prematurity (p=0.074) or birth weight (p=0.912) in our study. In conclusion, IH frequently develops after open KPE of BA patients, but this complication does not negatively affect the patient’s outcome. Nevertheless, inspection of the internal inguinal ring and prophylactic closure of inapparent hernias should be discussed in order to prevent secondary surgical procedures.

Objective: The Vps10p family member sortilin is expressed in thyroid epithelial cells where it contributes to recycling of the thyroid hormone precursor thyroglobulin (Tg), a process that is thought to render hormone release more effective. Here we investigated the functional impact of sortilin in the thyroid gland using sortilin-deficient mice.Methods: We measured free T-4, thyroid-stimulating hormone (TSH) and Tg serum levels and studied thyroid morphology in 14 sortilin-deficient (Sort1)(-/-) and 12 wildtype (WT) mice.Results: Serum free T-4 levels did not differ between Sort1(-/-) and WT females but were significantly lower in Sort1(-/-) males compared with WT (P = .0424). Neither serum TSH nor Tg levels differed between Sort1(-/-) and WT mice, regardless of sex. On the same line, no thyroid histology differences were observed.Conclusion: Our findings seem to exclude a role of sortilin in thyroid hormone secretion, although it is possible that the absence of sortilin may result in a thyroid phenotype if combined with other molecular defects of thyroid hormone synthesis and secretion or under iodine deficiency.

Several genome-wide association studies have linked novel loci to a wide range of cardiovascular phenotypes including low-density lipoprotein (LDL)-cholesterol, early onset myocardial infarction, coronary artery calcification, coronary artery stenosis, and abdominal aorta aneurysm. Especially, one locus, namely, 1p13.3, has attracted much attention. This locus harbors four candidate genes, CELSR2 , PSRC1 , MYBPHL , and SORT1. SORT1 encodes sortilin, a type I sorting receptor that has recently been implicated in LDL-cholesterol metabolism, VLDL secretion, PCSK9 secretion, and development of atherosclerotic lesions. Furthermore, sortilin also seems to be involved in the development of atherosclerosis, by mechanisms not directly involving LDL-cholesterol, but possibly resulting from the attenuated secretion of proinflammatory cytokines, such as IL6 and TNFα, which accompanies lack of sortilin in immune cells. Sortilin seems to play an important role in the development of cardiovascular disease and have functions beyond regulating LDL-cholesterol.

We describe altered sortilin sorting in AP-1/σ1B complex deficient adipocytes, which leads to the inhibition of adipogenesis. The AP-1 complex mediates protein sorting between the trans-Golgi network and endosomes. Vertebrates express three σ1-subunit isoforms: σ1A, σ1B and σ1C. σ1B −/− mice have impaired synaptic-vesicle recycling and a lipodystrophy. Sortilin is over-expressed in σ1B −/− adipose tissue and its over-expression in wild-type cells is sufficient to suppress adipogenesis. σ1B-specific binding of sortilin requires its DxxD-x12-DSxxxL motif. σ1B-deficiency does not lead to a block of sortilin transport out of a specific organelle, but the fraction which reaches lysosomes is reduced. Sortilin binds the DLK1 receptor, an inhibitor of adipocyte differentiation, and sortilin overexpression prevents DLK1 down-regulation, which leads to enhanced inhibition of adipogenesis. DLK1 and sortilin expression are not increased in the brain, although it is the tissue of highest σ1B and sortilin expression. Thus adipose tissue specific and σ1B-dependent routes for transport of sortilin exist and take part in the regulation of adipogenesis and adipose-tissue mass.

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.

The development and progression of Alzheimer's disease is linked to excessive production of toxic amyloid-β peptide, initiated by β-secretase cleavage of the amyloid precursor protein (APP). In contrast, soluble APPα (sAPPα) generated by the α-secretase is known to stimulate dendritic branching and enhance synaptic function. Regulation of APP processing, and the shift from neurotrophic to neurotoxic APP metabolism remains poorly understood, but the cellular localization of APP and its interaction with various receptors is considered important. We here identify sortilin as a novel APP interaction partner. Like the related APP receptor SorLA, sortilin is highly expressed in the CNS, but whereas SorLA mainly colocalizes with APP in the soma, sortilin interacts with APP in neurites. The presence of sortilin promotes α-secretase cleavage of APP, unlike SorLA, which inhibits the generation of all soluble products. Also, sortilin and SorLA both bind and mediate internalization of sAPP but to different cellular compartments. The interaction involves the 6A domain of APP, present in both neuronal and non-neuronal APP isoforms. This is important as sAPP receptors described so far only bind the non-neuronal isoforms, leaving SorLA and sortilin as the only receptors for sAPP generated by neurons. Together, our findings establish sortilin, as a novel APP interaction partner that influences both production and cellular uptake of sAPP.

Apolipoprotein E (APOE) is the major risk factor for sporadic Alzheimer's disease. Among other functions, APOE is proposed to sequester neurotoxic amyloid-β (Aβ) peptides in the brain, delivering them to cellular catabolism via neuronal APOE receptors. Still, the receptors involved in this process remain controversial. Here, we identified the pro-neurotrophin receptor sortilin as major endocytic pathway for clearance of APOE/Aβ complexes in neurons. Sortilin binds APOE with high affinity. Lack of receptor expression in mice results in accumulation of APOE and of Aβ in the brain and in aggravated plaque burden. Also, primary neurons lacking sortilin exhibit significantly impaired uptake of APOE/Aβ complexes despite proper expression of other APOE receptors. Despite higher than normal brain APOE levels, sortilin-deficient animals display anomalies in brain lipid metabolism (e.g., accumulation of sulfatides) seen in APOE-deficient mice, indicating functional deficiency in cellular APOE uptake pathways. Together, our findings identified sortilin as an essential neuronal pathway for APOE-containing lipoproteins in vivo and suggest an intriguing link between Aβ catabolism and pro-neurotrophin signaling converging on this receptor.

Injury of glomerular podocytes is prominent in Fabry disease and caused by progressive lysosomal accumulation of glycosphingolipid in the form of globotriaosylceramide (GL-3). Podocyte repair is therefore an important therapeutic target.

The precursor of the neurotrophin (NT) nerve growth factor (NGF) (proNGF) serves physiological functions distinct from its mature counterpart as it induces neuronal apoptosis through activation of a p75 NT receptor (p75NTR) and Sortilin death-signalling complex. The NTs brain-derived nerve growth factor (BDNF) and NT3 provide essential trophic support to auditory neurons. Injury to the NT-secreting cells in the inner ear is followed by irreversible degeneration of spiral ganglion neurons with consequences such as impaired hearing or deafness. Lack of mature NTs may explain the degeneration of spiral ganglion neurons, but another mechanism is possible as unprocessed proNTs released from the injured cells may contribute to the degeneration by induction of apoptosis. Recent studies demonstrate that proBDNF, like proNGF, is a potent inducer of Sortilin:p75NTR-mediated apoptosis. In addition, a coincident upregulation of proBDNF and p75NTR has been observed in degenerating spiral ganglion neurons, but the Sortilin expression in the inner ear is unresolved. Here we demonstrate that Sortilin and p75NTR are coexpressed in neurons of the neonatal inner ear. Furthermore, we establish that proNT3 exhibits high-affinity binding to Sortilin and has the capacity to enhance cell surface Sortilin:p75NTR complex formation as well as to mediate apoptosis in neurons coexpressing p75NTR and Sortilin. Based on the examination of wildtype and Sortilin-deficient mouse embryos, Sortilin does not significantly influence the developmental selection of spiral ganglion neurons. However, our results suggest that proNT3 and proBDNF may play important roles in the response to noise-induced injuries or ototoxic damage via the Sortilin:p75NTR death-signalling complex.

Trafficking of amyloid precursor protein (APP) through the intracellular compartments of neurons is a major regulatory process in the breakdown of this precursor, leading to Abeta production and senile plaque formation. However, the cellular mechanisms that control APP transport and that, when altered, cause Alzheimer's disease (AD) remain incompletely understood. Since quantitative aspects of APP processing are considered critical both to normal neuronal function and to neurodegeneration, a better understand of the tightly regulated kinetics of transport and proteolytic turnover of the precursor protein in neurons is warranted. Recently, we demonstrated that SORLA (aka SORL1 or LR11), a neuronal receptor that controls intracellular transport and processing of APP, acts as retention factor for the precursor protein in the trans-Golgi network, preventing release of the precursor into regular processing pathways. SORLA-mediated APP retention leads to decreased Abeta levels in neuronal cell lines, whereas ablation of receptor expression in knockout mice results in increased levels of Abeta in the brain similar to the situation in patients with late-onset AD who exhibit poor receptor expression. To test there levance of subtle alterations in SORLA levels for risk of amyloidogenic processing, we developed a novel cell system in which the amount of APP and SORLA that are being produced can be varied. These cell lines have been applied to produce quantitative data to estimate reaction constants of the proteolytic processing of APP into sAPPalpha, sAPPbeta, and Abeta in the presence or absence of SORLA. Based on the experimental data, we developed nonlinear ordinary differential equation models faithfully describing the cleavage of APP by alpha-and beta- secretases and the effect of SORLA on these processes. Taken together, our quantitative biochemical data demonstrate that alpha- and beta-secretases are allosteric enzymes that depend on formation of APP oligomers for efficient processing. We also demonstrate that SORLA does not affect the activity of secretases per se. Rather it acts as competitive inhibitor that prevents APP oligomerization, thereby eliminating the preferred form of the secretase substrate and reducing amyloidogenic processing.