Sortilin is the major 110-kDa protein in GLUT4 vesicles from adipocytes

Network Proteins of Human Sortilin1, Its Expression and Targetability Using Lycopene

BACKGROUND

Sortilin1 (SORT1) is a ubiquitously expressed transporter involved in sorting or clearing proteins and is pathologically linked to tissue fibrosis and calcification. Targeting SORT1 may have potential clinical efficacy in controlling or reversing cardiovascular fibrosis and/or calcification. Hence, this study assessed the protein-protein network of human SORT1 and its targetability using known nutra-/pharmaceuticals.

MATERIAL AND METHODS

Network proteins of human SORT1 were identified using the String database, and the affinity of the protein-protein interaction of this network was analysed using Chimera software (Chimera-1.17.3-mac64). The tissue-specific expression profile of SORT1 was evaluated and assessed for enrichment in different cell types, including immune cells. A library of in-house small molecules and currently used therapeutics for cardiovascular diseases were screened using AutoDock Vina to assess the targetability of human SORT1. The concentration affinity (CA) ratio of the small molecules was estimated to assess the clinical feasibility of targeting SORT1.

RESULTS

IGF2R, NTRK2, GRN and GGA1 were identified as high-affinity interaction networks of SORT1. Of these high-affinity interactions, IGF2R and GRN can be considered relevant networks in regulating tissue fibrosis or the microcalcification process due to their influence on T-cell activation, inflammation, wound repair, and the tissue remodelling process. The tissue cell-type enrichment indicated major expression of SORT1 in adipocytes, specialised epithelial cells, monocytes, cardiomyocytes, and thyroid glandular cells. The binding pocket analysis of human SORT1 showed twelve potential drug interaction sites with varying binding scores (0.86 to 5.83) and probability of interaction (0.004 to 0.304). Five of the drug interaction sites were observed to be targetable at the therapeutically feasible concentration of the small molecules evaluated. Empagliflozin, sitagliptin and lycopene showed a superior affinity and CA ratio compared to established inhibitors of SORT1.

CONCLUSION

IGF2R and GRN are relevant networks of SORT1, regulating tissue fibrosis or the microcalcification process. SORT1 can be targeted using currently approved small-molecule therapeutics (empagliflozin and sitagliptin) or widely used nutraceuticals (lycopene), which should be evaluated in a randomised clinical trial to assess their efficacy in reducing the cardiac/vascular microcalcification process.

Regulation of Human Sortilin Alternative Splicing by Glucagon-like Peptide-1 (GLP1) in Adipocytes

Type 2 diabetes mellitus is a chronic metabolic disease with no cure. Adipose tissue is a major site of systemic insulin resistance. Sortilin is a central component of the glucose transporter -Glut4 storage vesicles (GSV) which translocate to the plasma membrane to uptake glucose from circulation. Here, using human adipocytes we demonstrate the presence of the alternatively spliced, truncated sortilin variant (Sort_T) whose expression is significantly increased in diabetic adipose tissue. Artificial-intelligence-based modeling, molecular dynamics, intrinsically disordered region analysis, and co-immunoprecipitation demonstrated association of Sort_T with Glut4 and decreased glucose uptake in adipocytes. The results show that glucagon-like peptide-1 (GLP1) hormone decreases Sort_T. We deciphered the molecular mechanism underlying GLP1 regulation of alternative splicing of human sortilin. Using splicing minigenes and RNA-immunoprecipitation assays, the results show that GLP1 regulates Sort_T alternative splicing via the splice factor, TRA2B. We demonstrate that targeted antisense oligonucleotide morpholinos reduces Sort_T levels and improves glucose uptake in diabetic adipocytes. Thus, we demonstrate that GLP1 regulates alternative splicing of sortilin in human diabetic adipocytes.

Inductive effect of SORT1 on odontoblastic differentiation of human dental pulp-derived stem cells

This study investigated the expression of sortilin 1 (SORT1) in cultured human dental pulp-derived stem cells (hDPSCs) and its role in their odontoblastic differentiation. Permanent teeth were extracted from five patients, and the dental pulp was harvested for explant culture. Fluorescence-activated cell sorting was used to analyze the outgrowth of adherent cells and cells that had migrated from the tissue margin. SORT1 expression was detected in hDPSCs simultaneously expressing the mesenchymal stem cell markers CD44 and CD90. The odontoblastic differentiation potential of SORT1-positive hDPSCs was examined via staining for alkaline phosphatase (ALP), an early odontoblastic differentiation marker. ALP staining was more intense in SORT1-positive than in SORT1-negative hDPSCs. Consistently, the expression of mRNA encoding SORT1 and p75NTR, a binding partner of SORT1, increased in SORT1-positive hDPSCs during odontoblastic differentiation. In addition, pro-nerve growth factor (NGF), a ligand for SORT1-p75NTR co-receptor, promoted ALP expression in SORT1-positive hDPSCs, and the interaction between SORT1 and p75NTR was detected using a coimmunoprecipitation assay. The function of SORT1 in odontoblastic differentiation was examined via RNA interference using shRNA targeting SORT1. ALP staining intensity in SORT1/shRNA-transfected cells was markedly lower than in control/shRNA-transfected cells. SORT1 knockdown decreased JUN phosphorylation and recruitment of phosphorylated JUN to the ALP promoter. Collectively, these results indicate that SORT1 is involved in the odontoblastic differentiation of hDPSCs through the JUN N-terminal kinases (JNK)/JUN signaling pathway and that the binding of SORT1 and p75NTR plays an important role in this process.

Roles of Matrix Metalloproteinases and Their Natural Inhibitors in Metabolism: Insights into Health and Disease

Matrix metalloproteinases (MMPs) are a family of zinc-activated peptidases that can be classified into six major classes, including gelatinases, collagenases, stromelysins, matrilysins, membrane type metalloproteinases, and other unclassified MMPs. The activity of MMPs is regulated by natural inhibitors called tissue inhibitors of metalloproteinases (TIMPs). MMPs are involved in a wide range of biological processes, both in normal physiological conditions and pathological states. While some of these functions occur during development, others occur in postnatal life. Although the roles of several MMPs have been extensively studied in cancer and inflammation, their function in metabolism and metabolic diseases have only recently begun to be uncovered, particularly over the last two decades. This review aims to summarize the current knowledge regarding the metabolic roles of metalloproteinases in physiology, with a strong emphasis on adipose tissue homeostasis, and to highlight the consequences of impaired or exacerbated MMP actions in the development of metabolic disorders such as obesity, fatty liver disease, and type 2 diabetes.

Finding memo: versatile interactions of the VPS10p-Domain receptors in Alzheimer’s disease

The family of VPS10p-Domain (D) receptors comprises five members named SorLA, Sortilin, SorCS1, SorCS2 and SorCS3. While their physiological roles remain incompletely resolved, they have been recognized for their signaling engagements and trafficking abilities, navigating a number of molecules between endosome, Golgi compartments, and the cell surface. Strikingly, recent studies connected all the VPS10p-D receptors to Alzheimer’s disease (AD) development. In addition, they have been also associated with diseases comorbid with AD such as diabetes mellitus and major depressive disorder. This systematic review elaborates on genetic, functional, and mechanistic insights into how dysfunction in VPS10p-D receptors may contribute to AD etiology, AD onset diversity, and AD comorbidities. Starting with their functions in controlling cellular trafficking of amyloid precursor protein and the metabolism of the amyloid beta peptide, we present and exemplify how these receptors, despite being structurally similar, regulate various and distinct cellular events involved in AD. This includes a plethora of signaling crosstalks that impact on neuronal survival, neuronal wiring, neuronal polarity, and synaptic plasticity. Signaling activities of the VPS10p-D receptors are especially linked, but not limited to, the regulation of neuronal fitness and apoptosis via their physical interaction with pro- and mature neurotrophins and their receptors. By compiling the functional versatility of VPS10p-D receptors and their interactions with AD-related pathways, we aim to further propel the AD research towards VPS10p-D receptor family, knowledge that may lead to new diagnostic markers and therapeutic strategies for AD patients.

Sorting through the extensive and confusing roles of sortilin in metabolic disease

Sortilin is a post-Golgi trafficking receptor homologous to the yeast vacuolar protein sorting receptor 10 (VPS10). The VPS10 motif on sortilin is a 10-bladed β-propeller structure capable of binding more than 50 proteins, covering a wide range of biological functions including lipid and lipoprotein metabolism, neuronal growth and death, inflammation, and lysosomal degradation. Sortilin has a complex cellular trafficking itinerary, where it functions as a receptor in the trans-Golgi network, endosomes, secretory vesicles, multivesicular bodies, and at the cell surface. In addition, sortilin is associated with hypercholesterolemia, Alzheimer's disease, prion diseases, Parkinson's disease, and inflammation syndromes. The 1p13.3 locus containing SORT1, the gene encoding sortilin, carries the strongest association with LDL-C of all loci in human genome-wide association studies. However, the mechanism by which sortilin influences LDL-C is unclear. Here, we review the role sortilin plays in cardiovascular and metabolic diseases and describe in detail the large and often contradictory literature on the role of sortilin in the regulation of LDL-C levels.

Sortilin enhances secretion of apolipoprotein(a) through effects on apolipoprotein B secretion and promotes uptake of lipoprotein(a)

Elevated plasma lipoprotein(a) (Lp(a)) is an independent, causal risk factor for atherosclerotic cardiovascular disease and calcific aortic valve stenosis. Lp(a) is formed in or on hepatocytes from successive noncovalent and covalent interactions between apo(a) and apoB, although the subcellular location of these interactions and the nature of the apoB-containing particle involved remain unclear. Sortilin, encoded by the SORT1 gene, modulates apoB secretion and LDL clearance. We used a HepG2 cell model to study the secretion kinetics of apo(a) and apoB. Overexpression of sortilin increased apo(a) secretion, while siRNA-mediated knockdown of sortilin expression correspondingly decreased apo(a) secretion. Sortilin binds LDL but not apo(a) or Lp(a), indicating that its effect on apo(a) secretion is likely indirect. Indeed, the effect was dependent on the ability of apo(a) to interact noncovalently with apoB. Overexpression of sortilin enhanced internalization of Lp(a), but not apo(a), by HepG2 cells, although neither sortilin knockdown in these cells or Sort1 deficiency in mice impacted Lp(a) uptake. We found several missense mutations in SORT1 in patients with extremely high Lp(a) levels; sortilin containing some of these mutations was more effective at promoting apo(a) secretion than WT sortilin, though no differences were found with respect to Lp(a) internalization. Our observations suggest that sortilin could play a role in determining plasma Lp(a) levels and corroborate in vivo human kinetic studies which imply that secretion of apo(a) and apoB are coupled, likely within the hepatocyte.

New Insights in the Control of Fat Homeostasis: The Role of Neurotensin

Neurotensin (NT) is a small peptide with pleiotropic functions, exerting its primary actions by controlling food intake and energy balance. The first evidence of an involvement of NT in metabolism came from studies on the central nervous system and brain circuits, where NT acts as a neurotransmitter, producing different effects in relation to the specific region involved. Moreover, newer interesting chapters on peripheral NT and metabolism have emerged since the first studies on the NT-mediated regulation of gut lipid absorption and fat homeostasis. Intriguingly, NT enhances fat absorption from the gut lumen in the presence of food with a high fat content, and this action may explain the strong association between high circulating levels of pro-NT, the NT stable precursor, and the increased incidence of metabolic disorders, cardiovascular diseases, and cancer observed in large population studies. This review aims to provide a synthetic overview of the main regulatory effects of NT on several biological pathways, particularly those involving energy balance, and will focus on new evidence on the role of NT in controlling fat homeostasis, thus influencing the risk of unfavorable cardio–metabolic outcomes and overall mortality in humans.

GLUT4 On the move

Insulin rapidly stimulates GLUT4 translocation and glucose transport in fat and muscle cells. Signals from the occupied insulin receptor are translated into downstream signalling changes in serine/threonine kinases within timescales of seconds, and this is followed by delivery and accumulation of the glucose transporter GLUT4 at the plasma membrane. Kinetic studies have led to realisation that there are distinct phases of this stimulation by insulin. There is a rapid initial burst of GLUT4 delivered to the cell surface from a subcellular reservoir compartment and this is followed by a steady-state level of continuing stimulation in which GLUT4 recycles through a large itinerary of subcellular locations. Here, we provide an overview of the phases of insulin stimulation of GLUT4 translocation and the molecules that are currently considered to activate these trafficking steps. Furthermore, we suggest how use of new experimental approaches together with phospho-proteomic data may help to further identify mechanisms for activation of these trafficking processes.

High pro-neurotensin levels in individuals with type 1 diabetes associate with the development of cardiovascular risk factors at follow-up

AIMS

Neurotensin (NT) is a gut hormone that promotes lipids absorption and controls appetite. Elevated circulating pro-NT, the stable precursor of NT, is associated with cardiovascular (CV) disease, metabolic syndrome (MS) and type 2 diabetes (T2D). Features of MS and insulin resistance are reported also in type 1 diabetes (T1D), with detrimental impact on the overall CV risk profile. Aims of the study were to evaluate plasma pro-NT in T1D patients and to test whether its levels are associated with and/or predictive of CV risk factors and overall risk profile.

METHODS

For this longitudinal retrospective study, we analyzed clinical data from 41 T1D individuals referring to the diabetes outpatient clinics at Sapienza University of Rome, Italy, collected at the baseline and after 10 years. Fasting plasma pro-NT levels were measured in T1D subjects at the baseline and in 34 age-, sex-, BMI-comparable healthy individuals recruited in the same period.

RESULTS

Pro-NT did not differ significantly between patients and controls (median[range] pro-NT: 156.3 [96.6-198.2] vs. 179.4 [139.7-230.7] pmol/L, p = 0.26). In T1D, greater fasting pro-NT associated with poor glycemic control at baseline and predicted increased waist circumference, reduced insulin sensitivity, dyslipidemia and hypertension at 10-year follow-up. High pro-NT predicted 10-year very-high CV risk with adjusted OR = 11 (95%C.I.: 1.4-94.5; p = 0.029).

CONCLUSIONS

In T1D individuals, elevated pro-NT levels predict the development of adverse metabolic profile, which translates in higher CV risk profile at 10-year follow-up. Pro-NT represents a novel predictor/marker of CV risk factors in adults with T1D.

Pro-Neurotensin/Neuromedin N and Risk of Incident Metabolic Syndrome and Diabetes Mellitus in the REGARDS Cohort

CONTEXT

The peptide neurotensin is implicated in insulin resistance, diabetes mellitus (DM), and cardiovascular disease.

OBJECTIVE

We studied the association of neurotensin's stable precursor, pro-neurotensin/neuromedin N (pro-NT/NMN) with incident metabolic syndrome (MetS) and DM.

METHODS

We included 3772 participants from the REasons for Geographic and Racial Differences in Stroke (REGARDS) study who completed the baseline exam (2003-2007), the follow-up exam (2013-2016), and had pro-NT/NMN measured by immunoassay. Weighted logistic regression models were fitted to incident DM, incident MetS, and each MetS component, separately, incorporating demographics, metabolic risk factors, homeostasis model of insulin resistance (HOMA-IR), and diet scores. Incident MetS was defined by 3 or more harmonized criteria at follow-up in those with fewer than 3 at baseline. Incident DM was defined by use of hypoglycemic drugs/insulin, fasting glucose 126 mg/dL or greater, or random glucose 200 mg/dL or greater in those without these at baseline.

RESULTS

Median (IQR) plasma pro-NT/NMN was 160 pmol/L (118-218 pmol/L). A total of 564 (of 2770 without baseline MetS) participants developed MetS, and 407 (of 3030 without baseline DM) developed DM. Per SD higher log-pro-NT/NMN, the demographic-adjusted odds ratio (OR) and 95% CI of incident MetS was 1.22 (1.11-1.35), 1.16 (1.00-1.35) for incident low high-density lipoprotein (HDL), and 1.25 (1.11-1.40) for incident dysglycemia. The association of pro-NT/NMN with MetS was attenuated in the model adding HOMA-IR (OR per SD log-pro-NT/NMN 1.14; 95% CI, 1.00-1.30). There was no association with incident DM (OR per SD log-pro-NT/NMN 1.06; 95% CI, 0.94-1.19).

CONCLUSION

Pro-NT/NMN was associated with MetS and 2 components, dysglycemia and low HDL, likely explained by insulin resistance.

Neurotensin is an anti-thermogenic peptide produced by lymphatic endothelial cells

The lymphatic vasculature plays important roles in the physiology of the organs in which it resides, though a clear mechanistic understanding of how this crosstalk is mediated is lacking. Here, we performed single-cell transcriptional profiling of human and mouse adipose tissue and found that lymphatic endothelial cells highly express neurotensin (NTS/Nts). Nts expression is reduced by cold and norepinephrine in an α-adrenergic-dependent manner, suggesting a role in adipose thermogenesis. Indeed, NTS treatment of brown adipose tissue explants reduced expression of thermogenic genes. Furthermore, adenoviral-mediated overexpression and knockdown or knockout of NTS in vivo reduced and enhanced cold tolerance, respectively, an effect that is mediated by NTSR2 and ERK signaling. Inhibition of NTSR2 promoted energy expenditure and improved metabolic function in obese mice. These data establish a link between adipose tissue lymphatics and adipocytes with potential therapeutic implications.

Sortilin as a Biomarker for Cardiovascular Disease Revisited

Genetic variants in the genomic region containing SORT1 (encoding the protein sortilin) are strongly associated with cholesterol levels and the risk of coronary artery disease (CAD). Circulating sortilin has therefore been proposed as a potential biomarker for cardiovascular disease. Multiple studies have reported association between plasma sortilin levels and cardiovascular outcomes. However, the findings are not consistent across studies, and most studies have small sample sizes. The aim of this study was to evaluate sortilin as a biomarker for CAD in a well-characterized cohort with symptoms suggestive of CAD. In total, we enrolled 1,173 patients with suspected stable CAD referred to coronary computed tomography angiography. Sortilin was measured in plasma using two different technologies for quantifying circulating sortilin: a custom-made enzyme-linked immunosorbent assay (ELISA) and OLINK Cardiovascular Panel II. We found a relative poor correlation between the two methods (correlation coefficient = 0.21). In addition, genotyping and whole-genome sequencing were performed on all patients. By whole-genome regression analysis of sortilin levels measured with ELISA and OLINK, two independent cis protein quantitative trait loci (pQTL) on chromosome 1p13.3 were identified, with one of them being a well-established risk locus for CAD. Incorporating rare genetic variants from whole-genome sequence data did not identify any additional pQTLs for plasma sortilin. None of the traditional CAD risk factors, such as sex, age, smoking, and statin use, were associated with plasma sortilin levels. Furthermore, there was no association between circulating sortilin levels and coronary artery calcium score (CACS) or disease severity. Sortilin did not improve discrimination of obstructive CAD, when added to a clinical pretest probability (PTP) model for CAD. Overall, our results indicate that studies using different methodologies for measuring circulating sortilin should be compared with caution. In conclusion, the well-known SORT1 risk locus for CAD is linked to lower sortilin levels in circulation, measured with ELISA; however, the effect sizes are too small for sortilin to be a useful biomarker for CAD in a clinical setting of low- to intermediate-risk chest-pain patients.

Circulating pro-neurotensin levels predict bodyweight gain and metabolic alterations in children

BACKGROUND AND AIMS

Neurotensin (NT) is an intestinal peptide released after fat ingestion, which regulates appetite and facilitates lipid absorption. Elevated plasma levels of its stable precursor pro-neurotensin (pro-NT) are associated with type 2 diabetes, obesity and cardiovascular mortality in adult populations; no data on pro-NT and metabolic disease are available in children. Aim of the study was to evaluate plasma pro-NT in relation to the presence of obesity in children, and to test if high pro-NT associates with the development of metabolic impairment later in life.

METHODS AND RESULTS

For this longitudinal retrospective study, we studied 151 overweight/obese children undergoing metabolic evaluations at University of Cagliari, Italy. Pro-NT was also assessed in 46 normal-weight, age-, sex-comparable normal-weight children, selected as a reference group. At the baseline, pro-NT was comparable between overweight/obese and normal-weight children and correlated positively with age (p < 0.001), triglycerides (p < 0.001) and inversely with HDL levels (p = 0.008). Plasma pro-NT associated with high triglycerides with OR = 5.9 (95%CI: 1.24-28.1; p = 0.026) after adjustment for multiple confounders. At the 6.5-year follow-up, high basal pro-NT associated with impaired β-cell function to compensate for insulin-resistance (disposition index: r = -0.19, p = 0.035) and predicted bodyweight increase, as indicated by percentage change of standard deviation score BMI (median(95%CI) = +20.8(+4.9-+27.5)% in the highest tertile), independently from age, sex, triglycerides and insulin-resistance (standardized β = 0.24; p = 0.036).

CONCLUSIONS

Elevated pro-NT levels in children are significantly associated with weight gain later in life and may represent a marker of susceptibility to metabolic impairment in presence of obesity.

Identification of Sortilin Alternatively Spliced Variants in Mouse 3T3L1 Adipocytes

Type 2 diabetes mellitus is a metabolic disorder defined by systemic insulin resistance. Insulin resistance in adipocytes, an important regulator of glucose metabolism, results in impaired glucose uptake. The trafficking protein, sortilin, regulates major glucose transporter 4 (Glut4) movement, thereby promoting glucose uptake in adipocytes. Here, we demonstrate the presence of an alternatively spliced sortilin variant (Sort^17b), whose levels increase with insulin resistance in mouse 3T3L1 adipocytes. Using a splicing minigene, we show that inclusion of alternative exon 17b results in the expression of Sort^17b splice variant. Bioinformatic analysis indicated a novel intrinsic disorder region (IDR) encoded by exon 17b of Sort^17b. Root mean square deviation (RMSD) and root mean square fluctuation (RMSF) measurements using molecular dynamics demonstrated increased flexibility of the protein backbone within the IDR. Using protein–protein docking and co-immunoprecipitation assays, we show robust binding of Glut4 to Sort^17b. Further, results demonstrate that over-expression of Sort^17b correlates with reduced Glut4 translocation and decreased glucose uptake in adipocytes. The study demonstrates that insulin resistance in 3T3L1 adipocytes promotes expression of a novel sortilin splice variant with thus far unknown implications in glucose metabolism. This knowledge may be used to develop therapeutics targeting sortilin variants in the management of type 2 diabetes and metabolic syndrome.

Insulin-promoted mobilization of GLUT4 from a perinuclear storage site requires RAB10

Insulin controls glucose uptake into muscle and fat cells by inducing a net redistribution of glucose transporter 4 (GLUT4) from intracellular storage to the plasma membrane (PM). The TBC1D4-RAB10 signaling module is required for insulin-stimulated GLUT4 translocation to the PM, although where it intersects GLUT4 traffic was unknown. Here we demonstrate that TBC1D4-RAB10 functions to control GLUT4 mobilization from a trans-Golgi network (TGN) storage compartment, establishing that insulin, in addition to regulating the PM proximal effects of GLUT4-containing vesicles docking to and fusion with the PM, also directly regulates the behavior of GLUT4 deeper within the cell. We also show that GLUT4 is retained in an element/domain of the TGN from which newly synthesized lysosomal proteins are targeted to the late endosomes and the ATP7A copper transporter is translocated to the PM by elevated copper. Insulin does not mobilize ATP7A nor does copper mobilize GLUT4, and RAB10 is not required for copper-elicited ATP7A mobilization. Consequently, GLUT4 intracellular sequestration and mobilization by insulin is achieved, in part, through utilizing a region of the TGN devoted to specialized cargo transport in general rather than being specific for GLUT4. Our results define the GLUT4-containing region of the TGN as a sorting and storage site from which different cargo are mobilized by distinct signals through unique molecular machinery.

Neurotensin pathway in digestive cancers and clinical applications: an overview

Initially, NEUROTENSIN (NTS) has been shown to play physiological and biological functions as a neuro-transmitter/modulator in the central nervous system and as an endocrine factor in the periphery, through its binding to two kinds of receptors: NTSR1 and 2 (G protein-coupled receptors) and NTSR3/sortilin (a vacuolar protein-sorting 10-domain receptor). NTS also plays oncogenic roles in many types of cancer, including digestive cancers. In tumor tissues, NTS and NTSR1 expression is higher than in healthy ones and is associated with poor prognosis. NTS and NTRS1 promote cancer progression and play key functions in metastatic processes; they modulate several signaling pathways and they contribute to changes in the tumor microenvironment. Conversely, NTRS2 involvement in digestive cancers is poorly understood. Discovered for mediating NTS biological effects, sortilin recently emerged as a promising target as its expression was found to be increased in various types of cancers. Because it can be secreted, a soluble form of sortilin (sSortilin) appears as a new serum biomarker which, on the basis of recent studies, promises to be useful in both the diagnosis and tumor progression monitoring. More precisely, it appears that soluble sortilin can be associated with other receptors like TRKB. These associations occur in exosomes and trigger the aggressiveness of cancers like glioblastoma, leading to the concept of a possible composite theranostic biomarker. This review summarizes the oncogenic roles of the NTS signaling pathways in digestive cancers and discusses their emergence as promising early diagnostic and/or prognostic biomarkers and therapeutic targets.

An Allosteric Binding Site on Sortilin Regulates the Trafficking of VLDL, PCSK9, and LDLR in Hepatocytes

ApoB lipoproteins (apo B-Lp) are produced in hepatocytes, and their secretion requires the cargo receptor sortilin. We examined the secretion of apo B-Lp-containing very low-density lipoprotein (VLDL), an LDL progenitor. Sortilin also regulates the trafficking of the subtilase PCSK9, which when secreted binds the LDL receptor (LDLR), resulting in its endocytosis and destruction at the lysosome. We show that the site 2 binding compound (cpd984) has multiple effects in hepatocytes, including (1) enhanced Apo-Lp secretion, (2) increased cellular PCSK9 retention, and (3) augmented levels of LDLR at the plasma membrane. We postulate that cpd984 enhances apo B-Lp secretion in part through binding the lipid phosphatidylinositol 3,4,5-trisphosphate (PIP₃), which is present at higher levels on circulating VLDL form fed rats relative to after fasting. We attribute the enhanced VLDL secretion to its increased binding affinity for sortilin site 1 induced by cpd984 binding site 2. This hinders PCSK9 binding and secretion, which would subsequently prevent its binding to LDLR leading to its degradation. This suggests that site 2 is an allosteric regulator of site 1 binding. This effect is not limited to VLDL, as cpd984 augments binding of the neuropeptide neurotensin (NT) to sortilin site 1. Molecular dynamics simulations demonstrate that the C-terminus of NT (Ct-NT) stably binds site 1 through an electrostatic interaction. This was bolstered by the ability of Ct-NT to disrupt lower-affinity interactions between sortilin and the site 1 ligand PIP₃. Together, these data show that binding cargo at sortilin site 1 is allosterically regulated through site 2 binding, with important ramifications for cellular lipid homeostasis involving proteins such as PCSK9 and LDLR.

Neurotensin up-regulation is associated with advanced fibrosis and hepatocellular carcinoma in patients with MAFLD

BACKGROUND & AIMS

Neurotensin (NTS), a 13-aminoacid peptide localized in central nervous system and gastrointestinal tract, is involved in lipid metabolism and promotes various cancers onset mainly by binding to neurotensin receptor 1 (NTSR1). Increased plasma levels of pro-NTS, the stable NTS precursor, have been associated with type 2 diabetes (T2D), cardiovascular diseases and metabolic associated fatty liver disease (MAFLD). We aimed to evaluate 1) the impact of NTS rs1800832 and NTSR1 rs6090453 genetic variants on liver damage in 1166 MAFLD European individuals, 2) the relation between NTS variant and circulating pro-NTS and 3) the hepatic NTS expression by RNAseq transcriptomic analysis in 125 bariatric patients.

RESULTS

The NTS rs1800832 G allele was associated with hepatic fibrosis (OR 1.27, 95% confidence interval (CI). 1.02-1.58; p = 0.03), even more in carriers of both NTS and NTSR1 G risk alleles (OR 1.17, 95% CI. 1.03-1.34; p = 0.01), with cirrhosis (OR 1.58, 95% CI. 1.07-2.34; p = 0.02) and HCC (OR 1.98, 95% CI. 1.24-3.2; p = 0.004). Pro-NTS circulating levels were correlated with T2D (p = 0.005), BMI, (p = 0.04), age (p = 0.0016), lobular inflammation (p = 0.0025), fibrosis>2 (p < 0.0001), cirrhosis (p = 0.0009) and HCC (p < 0.0001) and more so after stratification for the NTS G allele. Transcriptomic data showed that hepatic NTS expression correlated with that of fibrogenic genes (p < 0.05).

CONCLUSIONS

NTS rs1800832 variant is associated with advanced fibrosis and HCC in MAFLD patients likely affecting NTS protein activity. The rs6090453 NTSR1 gene variant synergizes with NTS rs1800832 mutation to promote liver damage. Prospective studies are necessary to confirm NTS role in liver disease progression.

Neurotensin Broadly Recruits Inhibition via White Matter Neurons in the Mouse Cerebral Cortex: Synaptic Mechanisms for Decorrelation

The neuropeptide, neurotensin (NT), inhibits UP state generation in the cerebral cortex and temporally restricts the response to thalamic input, likely by a generalized increase in inhibition. To investigate the cellular and circuit substrate(s) for how a neuropeptide can shift the balance between cortical excitation and inhibition, we performed whole-cell recordings on slice preparations from mice expressing enhanced green fluorescent protein under control of the promoter for the homeobox gene, lhx6 (lhx6-EGFP mice). These mice identify the 2 largest classes of cortical interneurons; FS and low-threshold-spiking inhibitory neurons. In the presence of NT, both types of lhx6-EGFP neurons were excited through a direct, Na+-dependent depolarization, and through an increase in synaptic excitation. Paired recordings identified cortical white matter (WM) neurons as a source of this excitatory input, which was strengthened in the presence of NT. NT-driven increased synaptic input caused a functional decorrelation of gap junction transmission between lhx6-EGFP neuron pairs. Finally, the synaptic transmission between pyramidal cells and lhx6-EGFP neurons was modulated by addition of NT in favor of stronger inhibition and weaker excitation. These findings demonstrate the existence and functional consequences of an intracortical WM neuron projection, and suggest mechanisms underlying NT-induced promotion of wakefulness.

The association between plasma proneurotensin and glucose regulation is modified by country of birth

The prevalence of type 2 diabetes (T2D) has increased dramatically in Middle Eastern populations that represent the largest non-European immigrant group in Sweden today. As proneurotensin predicts T2D, the aim of this study was to investigate differences in proneurotensin levels across populations of Middle Eastern and Caucasian origin and to study its associations with indices of glucose regulation. Participants in the age 30 to 75 years, living in Malmö, Sweden, and born in Iraq or Sweden, were recruited from the census register. Anthropometrics and fasting samples were collected and oral glucose tolerance tests conducted assessing insulin secretion (DIo) as well as insulin sensitivity (ISI). A total of 2155 individuals participated in the study, 1398 were Iraqi-born and 757 were Swedish-born participants. Higher fasting proneurotensin levels were observed in Iraqi- compared to Swedish-born participants (137.5 vs. 119.8 pmol/L; p < 0.001) data adjusted for age, sex and body mass index. In Iraqi participants only, plasma proneurotensin was associated with impaired glucose regulation assessed as ISI, DIo and HbA1c, and significant interactions between country of birth and proneurotensin were observed (Pinteraction ISI = 0.048; Pinteraction DIo = 0.014; PinteractionHbA1c = 0.029). We report higher levels of proneurotensin in the general Middle Eastern population. The finding that Middle Eastern origin modifies the relationship of proneurotensin with indices of glucose regulation suggests that proneurotensin may be a stronger determinant of T2D in Middle Eastern as compared to Caucasian populations. These findings may explain part of the excess T2D risk in the Middle Eastern population but needs to be explored further.

Sortilin 1 Loss-of-Function Protects Against Cholestatic Liver Injury by Attenuating Hepatic Bile Acid Accumulation in Bile Duct Ligated Mice

Sortilin 1 (Sort1) is an intracellular trafficking receptor that mediates protein sorting in the endocytic or secretory pathways. Recent studies revealed a role of Sort1 in the regulation of cholesterol and bile acid (BA) metabolism. This study further investigated the role of Sort1 in modulating BA detoxification and cholestatic liver injury in bile duct ligated mice. We found that Sort1 knockout (KO) mice had attenuated liver injury 24 h after bile duct ligation (BDL), which was mainly attributed to less bile infarct formation. Sham-operated Sort1 KO mice had about 20% larger BA pool size than sham-operated wildtype (WT) mice, but 24 h after BDL Sort1 KO mice had significantly attenuated hepatic BA accumulation and smaller BA pool size. After 14 days BDL, Sort1 KO mice showed significantly lower hepatic BA concentration and reduced expression of inflammatory and fibrotic marker genes, but similar degree of liver fibrosis compared with WT mice. Unbiased quantitative proteomics revealed that Sort1 KO mice had increased hepatic BA sulfotransferase 2A1, but unaltered phase-I BA metabolizing cytochrome P450s or phase-III BA efflux transporters. Consistently, Sort1 KO mice showed elevated plasma sulfated taurocholate after BDL. Finally, we found that liver Sort1 was repressed after BDL, which may be due to BA activation of farnesoid x receptor. In conclusion, we report a role of Sort1 in the regulation of hepatic BA detoxification and cholestatic liver injury in mice. The mechanisms underlying increased hepatic BA elimination in Sort1 KO mice after BDL require further investigation.

Sortilin in Glucose Homeostasis: From Accessory Protein to Key Player?

The pharmacological properties and physiological roles of the type I receptor sortilin, also called neurotensin receptor-3, are various and complex. Sortilin is involved in important biological functions from neurotensin and pro-Nerve Growth Factor signaling in the central nervous system to regulation of glucose and lipid homeostasis in the periphery. The peripheral functions of sortilin being less extensively addressed, the focus of the current review is to discuss recent works describing sortilin-induced molecular mechanisms regulating blood glucose homeostasis and insulin signaling. Thus, an overview of several roles ascribed to sortilin in diabetes and other metabolic diseases are presented. Investigations on crucial cellular pathways involved in the protective effect of sortilin receptor on beta cells, including recent discoveries about regulation of cell fate, are also detailed. In addition, we provide a special focus on insulin secretion regulation involving complexes between sortilin and neurotensin receptors. The last section comments on the future research areas which should be developed to address the function of new effectors of the sortilin system in the endocrine apparatus.

The Involvement of Sortilin/NTSR3 in Depression as the Progenitor of Spadin and Its Role in the Membrane Expression of TREK-1

The molecular identification of sortilin, also called neurotensin receptor-3, from three different biochemical approaches already predicted the involvement of the protein in numerous biological and cellular functions. The first important observation was that sortilin is synthesized as a precursor that is converted to a mature protein after cleavage by the protein convertase furin in late Golgi compartments. This maturation leads to the formation of a 44 amino acid peptide, the propeptide (PE). The release of this peptide when matured sortilin reached the plasma membrane remained to be demonstrated. Sortilin has been also shown to be shedded by matrix metalloproteases releasing a large extracellular fragment identified as soluble sortilin. Therefore, sortilin has been shown to interact with several proteins and receptors confirming its role in the sorting of cellular components to the plasma membrane and/or to the lysosomal pathway. Interestingly, sortilin physically interacts with the two pore domain potassium channel TREK-1 and the PE as well as its synthetic analog spadin is able to block the activation of TREK-1 highlighting their role in the depression pathology. The present review describes the advance of research that led to these results and how both the soluble form of sortilin and the sortilin-derived PE have been detected in human serum and whose levels are affected in patients with major depressive disorder (MDD). The use of spadin as an antidepressant and the further role of soluble sortilin and of sortilin-derived PE as potential biomarkers during depression statement and/or remission of the pathology are considered and discussed in this review.

Tankyrase modulates insulin sensitivity in skeletal muscle cells by regulating the stability of GLUT4 vesicle proteins

Tankyrase 1 and 2, members of the poly(ADP-ribose) polymerase family, have previously been shown to play a role in insulin-mediated glucose uptake in adipocytes. However, their precise mechanism of action, and their role in insulin action in other cell types, such as myocytes, remains elusive. Treatment of differentiated L6 myotubes with the small molecule tankyrase inhibitor XAV939 resulted in insulin resistance as determined by impaired insulin-stimulated glucose uptake. Proteomic analysis of XAV939-treated myotubes identified down-regulation of several glucose transporter GLUT4 storage vesicle (GSV) proteins including RAB10, VAMP8, SORT1, and GLUT4. A similar effect was observed following knockdown of tankyrase 1 in L6 myotubes. Inhibition of the proteasome using MG132 rescued GSV protein levels as well as insulin-stimulated glucose uptake in XAV939-treated L6 myotubes. These studies reveal an important role for tankyrase in maintaining the stability of key GLUT4 regulatory proteins that in turn plays a role in regulating cellular insulin sensitivity.

Inhibition of insulin/PI3K/AKT signaling decreases adipose Sortilin 1 in mice and 3T3-L1 adipocytes

Sortilin 1(Sort1) is a vesicle trafficking receptor that mediates protein sorting in the endocytic and exocytic pathways. Sort1 is a component of the GLUT4 storage vesicles in adipocytes and is also involved in the regulation of adipogenesis. Sort1 protein is reduced in adipose of obese mice and humans, but the underlying cause is not fully understood. Here we report that insulin/PI3K/AKT signaling cascade critically regulates adipose Sort1 protein abundance. Administration of a PI3K inhibitor rapidly decreased Sort1 protein but not mRNA in adipose of chow-fed mice. In 3T3-L1 adipocytes, serum-starvation or inhibition of the PI3K/AKT signaling also decreased Sort1 protein without affecting Sort1 mRNA expression. Sort1 protein downregulation upon PI3K inhibition was blocked by pretreatment of MG132 but not Bafilomycin A1, suggesting that PI3K inhibition caused Sort1 degradation via the proteasome pathway. Using a phospho-specific Sort1 antibody, we showed that endogenous Sort1 was phosphorylated at S825 adjacent to the DXXLL sorting motif on the cytoplasmic tail. We demonstrated that mutagenesis that abolished Sort1 S825 phosphorylation decreased insulin-stimulated Sort1 localization on the plasma membrane and Sort1 protein stability in 3T3-L1 adipocytes. However, endogenous Sort1 phosphorylation at S825 was not affected by insulin stimulation or by inhibition of PI3K. In conclusion, this study revealed an important role of insulin signaling in regulating adipose Sort1 protein stability, and further suggests that impaired insulin signaling may underlie reduced adipose Sort1 in obesity. The cellular events downstream of insulin/PI3K/AKT signaling that mediates insulin regulation of Sort1 stability requires further investigation.

Circulating sortilin level as a potential biomarker for coronary atherosclerosis and diabetes mellitus

Context

A previous genome-wide association study showed that a genetic variant of sortilin was associated with the risk of coronary artery disease (CAD). However, the role of circulating sortilin is still unknown. We investigated the potential role of plasma sortilin as a biomarker for CAD and diabetes mellitus.

Methods

We enrolled statin-naïve subjects with CAD (n = 31) who underwent coronary artery bypass surgery and control subjects (n = 116) who were free from CAD as evaluated by coronary CT angiography. The presence of diabetes mellitus was evaluated and plasma sortilin levels were measured with a commercial ELISA kit.

Results

Plasma sortilin levels were higher in subjects with CAD and subjects with diabetes mellitus than in those without CAD or diabetes mellitus. Subjects in the highest sortilin tertile group were older and had higher glucose and HbA1c levels, but lipid profiles in the three tertile groups were comparable. Multivariable logistic regression analysis revealed that sortilin levels were independently associated with CAD. In addition, the receiver operating characteristic curve analysis showed that plasma sortilin levels could identify the presence of CAD or diabetes mellitus.

Conclusions

Elevated circulating sortilin levels are associated with CAD and diabetes mellitus and can be used as a biomarker of both diseases in statin-naïve subjects.

Desynchronization of the Rat Cortical Network and Excitation of White Matter Neurons by Neurotensin

Cortical network activity correlates with vigilance state: Deep sleep is characterized by slow, synchronized oscillations, whereas desynchronized, stochastic discharge is typical of the waking state. Neuropeptides, such as orexin and substance P but also neurotensin (NT), promote arousal. Relatively little is known about if NT can directly affect the cortical network, and if so, through which mechanisms and cellular targets. Here, we addressed these issues using rat in vitro cortex preparations. Following NT application specifically to deeper layers, slow oscillation activity was attenuated with a significant reduction in UP state frequency. The cortical response to thalamic stimulation exhibited enhanced temporal precision in the presence of NT, consistent with the transition in vivo from sleep to wakefulness. These changes were associated with a relative shift toward inhibition in the excitation/inhibition balance. Whole-cell recordings from layer 6 revealed presynaptically driven NT-induced inhibition of pyramidal neurons and excitation of fast-spiking interneurons. Deeper in the cortex, neurons within the white matter (WM) were strongly depolarized by NT application. The colocalization of NT and tyrosine hydroxylase immunoreactivities in deep layer fibers throughout the cortical mantle indicates mediation via dopaminergic systems. These data suggest a cortical mechanism for NT-induced wakefulness and support a role for WM neurons in state control.

Sortilin 1 knockout alters basal adipose glucose metabolism but not diet-induced obesity in mice

Sortilin 1 (Sort1) is a trafficking receptor that has been implicated in the regulation of plasma cholesterol in humans and mice. Here, we use metabolomics and hyperinsulinemic-euglycemic clamp approaches to obtain further understanding of the in vivo effects of Sort1 deletion on diet-induced obesity as well as on adipose lipid and glucose metabolism. Results show that Sort1 knockout (KO) does not affect Western diet-induced obesity nor adipose fatty acid and ceramide concentrations. Under the basal fasting state, chow-fed Sort1 KO mice have decreased adipose glycolytic metabolites, but Sort1 deletion does not affect insulin-stimulated tissue glucose uptake during the insulin clamp. These results suggest that Sort1 loss-of-function in vivo does not affect obesity development, but differentially modulates adipose glucose metabolism under fasting and insulin-stimulated states.

Autophagy induction in the skeletal myogenic differentiation of human tonsil-derived mesenchymal stem cells

Mesenchymal stem cells (MSCs) are capable of self-renewal and differentiation and are thus a valuable source for the replacement of diseased or damaged organs. Previously, we reported that the tonsils can be an excellent reservoir of MSCs for the regeneration of skeletal muscle (SKM) damage. However, the mechanisms involved in the differentiation from tonsil-derived MSCs (T-MSCs) to myocytes via myoblasts remain unclear. To clarify these mechanisms, we analyzed gene expression profiles of T-MSCs during differentiation into myocytes compared with human skeletal muscle cells (hSKMCs). Total RNA was extracted from T-MSCs, T-MSC-derived myoblasts and myocytes, and hSKMCs and was subjected to analysis using a microarray. Microarray analysis of the three phases of myogenic differentiation identified candidate genes associated with myogenic differentiation. The expression pattern of undifferentiated T-MSCs was distinguishable from the myogenic differentiated T-MSCs and hSKMCs. In particular, we selected FNBP1L, which among the upregulated genes is essential for antibacterial autophagy, since autophagy is related to SKM metabolism and myogenesis. T-MSCs differentiated toward myoblasts and skeletal myocytes sequentially, as evidenced by increased expression of autophagy-related markers (including Beclin-1, LC3B and Atg5) and decreased expression of Bcl-2. Furthermore, we reconfirmed that autophagy has an effect on the mechanism of skeletal myogenic differentiation derived from T-MSCs by treatment with 5-azacytidine and bafilomycin A1. These data suggest that the transcriptome of the T-MSC-derived myocytes is similar to that of hSKMCs, and that autophagy has an important role in the mechanism of myogenic differentiation of T-MSCs.

Development of Precision Small-Molecule Proneurotrophic Therapies for Neurodegenerative Diseases

Age-related neurodegenerative diseases, such as Alzheimer's disease, will represent one of the largest future burdens on worldwide healthcare systems due to the increasing proportion of elderly in our society. As deficiencies in neurotrophins are implicated in the pathogenesis of many age-related neurodegenerative disorders, it is reasonable to consider that global neurotrophin resistance may also become a major healthcare threat. Central nervous system networks are effectively maintained through aging by neuroprotective and neuroplasticity signaling mechanisms which are predominantly controlled by neurotrophin receptor signaling. Neurotrophin receptors are single pass receptor tyrosine kinases that form dimeric structures upon ligand binding to initiate cellular signaling events that control many protective and plasticity-related pathways. Declining functionality of the neurotrophin ligand-receptor system is considered one of the hallmarks of neuropathological aging. Therefore, it is imperative to develop effective therapeutic strategies to contend with this significant issue. While the therapeutic applications of cognate ligands for neurotrophin receptors are limited, the development of nonpeptidergic, small-molecule ligands can overcome these limitations, and productively regulate this important receptor system with beneficial effects. Using our advanced knowledge of the high-dimensionality complexity of receptor systems, the future generation of precision medicines targeting these systems will be an attainable goal.

Sortilin derived propeptide regulation during adipocyte differentiation and inflammation

In this work, we aimed to correlate the expression of sortilin with the production of sortilin-derived propeptide (PE) during adipocyte differentiation, insulin resistance and inflammation. We also investigated the effect of spadin, a shorter analogue of PE that exerts a potent antidepressant in mice, on adipocyte functions. During adipogenesis, insulin resistance and inflammation, we measured the mRNA and protein expression of sortilin, by quantitative PCR and Western-blot, and quantified the expression of PE by a specific dosing method. We observed that the production of PE was correlated with the sortilin expression during adipogenesis. Immunostaining experiments allowed to visualize the co-localization of sortilin, PE and VAMP2 in 3T3-L1 adipocytes. TNFα treatment induced insulin resistance and a decrease of sortilin expression (mRNA and protein), correlated with the decrease of the PE production. By contrast, treatment with dexamethasone, which also induced insulin resistance, was without effect on sortilin expression and PE production. As a putative bioactive peptide, we have evaluated its autocrine effect by the use of spadin on 3T3-L1 adipocytes by performing glucose uptake and signalling experiments. Any effect was measured on adipocytes indicating that the use of spadin as an antidepressant would have no side effects on adipocyte physiology.

Functional characterization of retromer in GLUT4 storage vesicle formation and adipocyte differentiation

Insulin-stimulated translocation of glucose transporter 4 (GLUT4) storage vesicles (GSVs), the specialized intracellular compartments within mature adipocytes, to the plasma membrane (PM) is a fundamental cellular process for maintaining glucose homeostasis. Using 2 independent adipocyte cell line models, human primary Simpson-Golabi-Behmel syndrome and mouse 3T3-L1 fibroblast cell lines, we demonstrate that the endosome-associated protein-sorting complex retromer colocalizes with GLUT4 on the GSVs by confocal microscopy in mature adipocytes. By use of both confocal microscopy and differential ultracentrifugation techniques, retromer is redistributed to the PM of mature adipocytes upon insulin stimulation. Furthermore, stable knockdown of the retromer subunit-vacuolar protein-sorting 35, or the retromer-associated protein sorting nexin 27, by lentivirus-delivered small hairpin RNA impaired the adipogenesis process when compared to nonsilence control. The knockdown of retromer decreased peroxisome proliferator activated receptor γ expression during differentiation, generating adipocytes with decreased levels of GSVs, lipid droplet accumulation, and insulin-stimulated glucose uptake. In conclusion, our study demonstrates a role for retromer in the GSV formation and adipogenesis.

Coordinated Regulation of Vasopressin Inactivation and Glucose Uptake by Action of TUG Protein in Muscle

In adipose and muscle cells, insulin stimulates the exocytic translocation of vesicles containing GLUT4, a glucose transporter, and insulin-regulated aminopeptidase (IRAP), a transmembrane aminopeptidase. A substrate of IRAP is vasopressin, which controls water homeostasis. The physiological importance of IRAP translocation to inactivate vasopressin remains uncertain. We previously showed that in skeletal muscle, insulin stimulates proteolytic processing of the GLUT4 retention protein, TUG, to promote GLUT4 translocation and glucose uptake. Here we show that TUG proteolysis also controls IRAP targeting and regulates vasopressin action in vivo. Transgenic mice with constitutive TUG proteolysis in muscle consumed much more water than wild-type control mice. The transgenic mice lost more body weight during water restriction, and the abundance of renal AQP2 water channels was reduced, implying that vasopressin activity is decreased. To compensate for accelerated vasopressin degradation, vasopressin secretion was increased, as assessed by the cosecreted protein copeptin. IRAP abundance was increased in T-tubule fractions of fasting transgenic mice, when compared with controls. Recombinant IRAP bound to TUG, and this interaction was mapped to a short peptide in IRAP that was previously shown to be critical for GLUT4 intracellular retention. In cultured 3T3-L1 adipocytes, IRAP was present in TUG-bound membranes and was released by insulin stimulation. Together with previous results, these data support a model in which TUG controls vesicle translocation by interacting with IRAP as well as GLUT4. Furthermore, the effect of IRAP to reduce vasopressin activity is a physiologically important consequence of vesicle translocation, which is coordinated with the stimulation of glucose uptake.

Sortilin and lipoprotein metabolism: making sense out of complexity

PURPOSE OF REVIEW

Genome-wide association studies have been used as an unbiased tool to identify novel genes that contribute to variations in LDL cholesterol levels in the hopes of uncovering new biology and new therapeutic targets for the treatment of atherosclerotic cardiovascular disease. The locus identified by genome-wide association studies with the strongest association with LDL cholesterol and atherosclerotic cardiovascular disease is the 1p13 sortilin-1 (SORT1) locus. Here, we review the identification and characterization of this locus, the initial physiological studies describing the role of SORT1 in lipoprotein metabolism, and recent work that has begun to sort out the complexity of this role.

RECENT FINDINGS

Studies by several groups support an important role for sortilin in lipoprotein metabolism; however, the directionality of the effect of sortilin on plasma cholesterol and its role in the secretion of hepatic lipoproteins remains controversial. Studies by several groups support a role for sortilin in inhibiting lipoprotein export, whereas other studies suggest that sortilin facilitates lipoprotein export.

SUMMARY

Understanding the mechanism by which sortilin affects LDL cholesterol will increase our understanding of the regulation of lipoprotein metabolism and hepatic lipoprotein export and may also allow us to harness the power of the 1p13 SORT1 locus for the treatment of atherosclerotic cardiovascular disease.

σ1B adaptin regulates adipogenesis by mediating the sorting of sortilin in adipose tissue

Here, we describe altered sorting of sortilin in adipocytes deficient for the σ1B-containing AP-1 complex, leading to the inhibition of adipogenesis. The AP-1 complex mediates protein sorting between the trans-Golgi network and endosomes. Vertebrates express three AP1 σ1 subunit isoforms - σ1A, σ1B and σ1C (also known as AP1S1, AP1S2 and AP1S3, respectively). σ1B-deficient mice display impaired recycling of synaptic vesicles and lipodystrophy. Here, we show that sortilin is overexpressed in adipose tissue from σ1B(-/-) mice, and that its overexpression in wild-type cells is sufficient to suppress adipogenesis. σ1B-specific binding of sortilin requires the sortilin DxxD-x12-DSxxxL motif. σ1B deficiency does not lead to a block of sortilin transport out of a specific organelle, but the fraction that reaches lysosomes is reduced. Sortilin binds to the receptor DLK1, an inhibitor of adipocyte differentiation, and the overexpression of sortilin prevents DLK1 downregulation, leading to enhanced inhibition of adipogenesis. DLK1 and sortilin expression are not increased in the brain tissue of σ1B(-/-) mice, although this is the tissue with the highest expression of σ1B and sortilin. Thus, adipose-tissue-specific and σ1B-dependent routes for the transport of sortilin exist and are involved in the regulation of adipogenesis and adipose-tissue mass.

Sorting receptor sortilin-a culprit in cardiovascular and neurological diseases

Sortilin is a sorting receptor that directs target proteins, such as growth factors, signaling receptors, and enzymes, to their destined location in secretory or endocytic compartments of cells. The activity of sortilin is essential for proper function of not only neurons but also non-neuronal cell types, and receptor (dys)function emerges as a major cause of malignancies, including hypercholesterolemia, retinal degeneration, neuronal cell loss in stroke and spinal cord injury, or Alzheimer's disease and other neurodegenerative disorders. In this article, we describe the molecular mechanisms of sortilin action in protein sorting and signaling and how modulation of receptor function may offer novel therapeutic strategies for treatment of common diseases of the cardiovascular and nervous systems.

A proteolytic pathway that controls glucose uptake in fat and muscle

Insulin regulates glucose uptake by controlling the subcellular location of GLUT4 glucose transporters. GLUT4 is sequestered within fat and muscle cells during low-insulin states, and is translocated to the cell surface upon insulin stimulation. The TUG protein is a functional tether that sequesters GLUT4 at the Golgi matrix. To stimulate glucose uptake, insulin triggers TUG endoproteolytic cleavage. Cleavage accounts for a large proportion of the acute effect of insulin to mobilize GLUT4 to the cell surface. During ongoing insulin exposure, endocytosed GLUT4 recycles to the plasma membrane directly from endosomes, and bypasses a TUG-regulated trafficking step. Insulin acts through the TC10α GTPase and its effector protein, PIST, to stimulate TUG cleavage. This action is coordinated with insulin signals through AS160/Tbc1D4 and Tbc1D1 to modulate Rab GTPases, and with other signals to direct overall GLUT4 targeting. Data support the idea that the N-terminal TUG cleavage product, TUGUL, functions as a novel ubiquitin-like protein modifier to facilitate GLUT4 movement to the cell surface. The C-terminal TUG cleavage product is extracted from the Golgi matrix, which vacates an "anchoring" site to permit subsequent cycles of GLUT4 retention and release. Together, GLUT4 vesicle translocation and TUG cleavage may coordinate glucose uptake with physiologic effects of other proteins present in the GLUT4-containing vesicles, and with potential additional effects of the TUG C-terminal product. Understanding this TUG pathway for GLUT4 retention and release will shed light on the regulation of glucose uptake and the pathogenesis of type 2 diabetes.

Sortilins in neurotrophic factor signaling

The sortilin family of Vps10p-domain receptors includes sortilin, SorLA, and SorCS1-3. These type-I transmembrane receptors predominate in distinct neuronal tissues, but expression is also present in certain specialized non-neuronal cell populations including hepatocytes and cells of the immune system. The biology of sortilins is complex as they participate in both cell signaling and in intracellular protein sorting. Sortilins function physiologically in signaling by pro- and mature neurotrophins in neuronal viability and functionality. Recent genome-wide association studies have linked members to neurological disorders such as Alzheimer's disease and bipolar disorder and outside the nervous system to development of coronary artery disease and type-2 diabetes. Particularly well described are the receptor functions in neuronal signaling by pro- (proNT) and mature (NT) neurotrophins and in the processing/metabolism of amyloid precursor protein (APP).

Cellular regulation of glucose uptake by glucose transporter GLUT4

GLUT4 is regulated by its intracellular localization. In the absence of insulin, GLUT4 is efficiently retained intracellularly within storage compartments in muscle and fat cells. Upon insulin stimulation (and contraction in muscle), GLUT4 translocates from these compartments to the cell surface where it transports glucose from the extracellular milieu into the cell. Its implication in insulin-regulated glucose uptake makes GLUT4 not only a key player in normal glucose homeostasis but also an important element in insulin resistance and type 2 diabetes. Nevertheless, how GLUT4 is retained intracellularly and how insulin acts on this retention mechanism is largely unclear. In this review, the current knowledge regarding the various molecular processes that govern GLUT4 physiology is discussed as well as the questions that remain.

Prolonged insulin stimulation down-regulates GLUT4 through oxidative stress-mediated retromer inhibition by a protein kinase CK2-dependent mechanism in 3T3-L1 adipocytes

Although insulin acutely stimulates glucose uptake by promotion of GLUT4 translocation from intracellular compartments to the plasma membrane in adipocytes and muscles, long term insulin stimulation causes GLUT4 depletion that is particularly prominent in the insulin-responsive GLUT4 storage compartment. This effect is caused mainly by accelerated lysosomal degradation of GLUT4, although the mechanism is not fully defined. Here we show that insulin acutely induced dissociation of retromer components from the low density microsomal membranes of 3T3-L1 adipocytes that was accompanied by disruption of the interaction of Vps35 with sortilin. This insulin effect was dependent on the activity of protein kinase CK2 but not phosphatidylinositol 3-kinase or extracellular signal-regulated kinase 1/2. Knockdown of Vps26 decreased GLUT4 to a level comparable with that with insulin stimulation for 4 h. Vps35 with a mutation in the CK2 phosphorylation motif (Vps35-S7A) was resistant to insulin-induced dissociation from the low density microsomal membrane, and its overexpression attenuated GLUT4 down-regulation with insulin. Furthermore, insulin-generated hydrogen peroxide was an upstream mediator of the insulin action on retromer and GLUT4. These results suggested that insulin-generated oxidative stress switches the GLUT4 sorting direction to lysosomes through inhibition of the retromer function in a CK2-dependent manner.

The pro-neurotrophin receptor sortilin is a major neuronal apolipoprotein E receptor for catabolism of amyloid-β peptide in the brain

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.

Novel method to differentiate 3T3 L1 cells in vitro to produce highly sensitive adipocytes for a GLUT4 mediated glucose uptake using fluorescent glucose analog

Adipocytes play a vital role in glucose metabolism. 3T3 L1 pre adipocytes after differentiation to adipocytes serve as excellent in vitro models and are useful tools in understanding the glucose metabolism. The traditional approaches adopted in pre adipocyte differentiation are lengthy exercises involving the usage of IBMX and Dexamethasone. Any effort to shorten the time of differentiation and quality expression of functional differentiation in 3T3 L1 cells in terms of enhanced Insulin sensitivity has an advantage in the drug discovery process. Thus, there is a need to develop a new effective method of differentiating the pre adipocytes to adipocytes and to use such methods for developing efficacious therapeutic molecules. We observed that a combination of Dexamethasone and Troglitazone generated differentiated adipocytes over fewer days as compared to the combination of IBMX and Dexamethasone which constitutes the standard protocol followed in our laboratory. The experiments conducted to compare the quality of differentiation yielded by various differentiating agents indicated that the lipid droplet accumulation increased by 112 % and the GLUT4 mediated glucose uptake by 137 % in cells differentiated with Troglitazone and Dexamethasone than in cells differentiated traditionally. The comparative studies conducted for evaluating efficient measurable glucose uptake by GOPOD assay, radioactive (3)H-2-deoxy-D-glucose assay and by non-radioactive 6-NBDG (fluorescent analog of glucose) indicated that the non-radioactive method using 6-NBDG showed a higher signal to noise ratio than the conventional indirect glucose uptake method (GOPOD assay) and the radioactive (3)H-2-deoxy-D-glucose uptake method. Differentiated 3T3 L1 cells when triggered with 2.5 ng/mL of Insulin showed 3.3 fold more glucose uptake in non-radioactive method over the radioactive (3)H-2-deoxy-D-glucose uptake method. The results of this study have suggested that a combination of Dexamethasone and Troglitazone for 3T3 L1 cell differentiation helps in better quality differentiation over a short period of time with increased sensitivity to Insulin. The application of these findings for developing new methods of screening novel Insulin mimetics and for evaluating the immunological responses has been discussed.

The first luminal loop confers insulin responsiveness to glucose transporter 4

Glucose transporter isoform 4 (GLUT4) is the sole glucose transporter responsible for the effect of insulin on postprandial blood glucose clearance. It is translocated to the plasma membrane by specialized insulin-responsive vesicles. Targeting of GLUT4 to these vesicles is mediated by sortilin, which interacts with the first luminal loop of the transporter.

Glucose transporter isoform 4 (GLUT4), is the sole glucose transporter responsible for the effect of insulin on postprandial blood glucose clearance. However, the nature of the insulin sensitivity of GLUT4 remains unknown. In this study, we replaced the first luminal loop of cellugyrin, a 4-transmembrane protein that does not respond to insulin, with that of GLUT4. The chimera protein is targeted to the intracellular insulin-responsive vesicles and is translocated to the plasma membrane upon insulin stimulation. The faithful targeting of the chimera depends on the expression of the sorting receptor sortilin, which interacts with the unique amino acid residues in the first luminal loop of GLUT4. Thus the first luminal loop may confer insulin responsiveness to the GLUT4 molecule.

Molecular basis of insulin-responsive GLUT4 trafficking systems revealed by single molecule imaging

Development of a 'static retention' property of GLUT4, the insulin-responsive glucose transporter, has emerged as being essential for achieving its maximal insulin-induced surface exposure. Herein, employing quantum-dot-based nanometrology of intracellular GLUT4 behavior, we reveal the molecular basis of its systematization endowed upon adipogenic differentiation of 3T3L1 cells. Specifically, (i) the endosomes-to-trans-Golgi network (TGN) retrieval system specialized for GLUT4 develops in response to sortilin expression, which requires an intricately balanced interplay among retromers, golgin-97 and syntaxin-6, the housekeeping vesicle trafficking machinery. (ii) The Golgin-97-localizing subdomain of the differentiated TGN apparently serves as an intermediate transit route by which GLUT4 can further proceed to the stationary GLUT4 storage compartment. (iii) AS160/Tbc1d4 then renders the 'static retention' property insulin responsive, i.e. insulin liberates GLUT4 from the static state only in the presence of functional AS160/Tbc1d4. (iv) Moreover, sortilin malfunction and the resulting GLUT4 sorting defects along with retarded TGN function might be etiologically related to insulin resistance. Together, these observations provide a conceptual framework for understanding maturation/retardation of the insulin-responsive GLUT4 trafficking system that relies on the specialized subdomain of differentiated TGN.