Soluble sortilin is present in excess and positively correlates with progranulin in CSF of aging individuals

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.

The Membrane Protein Sortilin Is a Potential Biomarker and Target for Glioblastoma

Glioblastoma (GBM) is a devastating brain cancer with no effective treatment, and there is an urgent need for developing innovative biomarkers as well as therapeutic targets for better management of the disease. The membrane protein sortilin has recently been shown to participate in tumor cell invasiveness in several cancers, but its involvement and clinical relevance in GBM is unclear. In the present study, we explored the expression of sortilin and its potential as a clinical biomarker and therapeutic target for GBM. Sortilin expression was investigated by immunohistochemistry and digital quantification in a series of 71 clinical cases of invasive GBM vs. 20 non-invasive gliomas. Sortilin was overexpressed in GBM and, importantly, higher expression levels were associated with worse patient survival, pointing to sortilin tissue expression as a potential prognostic biomarker for GBM. Sortilin was also detectable in the plasma of GBM patients by enzyme-linked immunosorbent assay (ELISA), but no differences were observed between sortilin levels in the blood of GBM vs. glioma patients. In vitro, sortilin was detected in 11 brain-cancer-patient-derived cell lines at the anticipated molecular weight of 100 kDa. Interestingly, targeting sortilin with the orally bioavailable small molecule inhibitor AF38469 resulted in decreased GBM invasiveness, but cancer cell proliferation was not affected, showing that sortilin is targetable in GBM. Together, these data suggest the clinical relevance for sortilin in GBM and support further investigation of GBM as a clinical biomarker and therapeutic target.

The multifaceted role of sortilin/neurotensin receptor 3 in human cancer development

Sortilin (also known as neurotensin receptor 3) is a multitasking protein implicated in numerous pathophysiological processes, including cancer development, cardiovascular impairment, Alzheimer-type dementia, and depression. Although the definitive role of sortilin in human solid and hematological malignancies has been evidenced, few articles reviewed the task. The aim of the current review is to unravel the mechanisms by which sortilin controls oncogenicity and cancer progression; and also to summarize and discuss the original data obtained from international research laboratories on this topic. Questions on how sortilin is involving in the impairment of cell junctions, in exosomes composition and release, as well as in the regulation of epidermal growth factor receptor trafficking are also responded. In addition, we provide a special focus on the regulatory role of sortilin in signal transduction by either neurotrophins or neurotensin in normal and malignant cells. The relevance of sortilin with normal and cancer stem cells is also discussed. The last section provides a general overview of sortilin applications as a diagnostic and prognostic biomarker in the context of cancer detection. Finally, we comment on the future research aspects in which the field of cancer diagnosis, prognosis, and therapy might be developed.

Fluid biomarkers in frontotemporal dementia: past, present and future

The frontotemporal dementia (FTD) spectrum of neurodegenerative disorders includes a heterogeneous group of conditions. However, following on from a series of important molecular studies in the early 2000s, major advances have now been made in the understanding of the pathological and genetic underpinnings of the disease. In turn, alongside the development of novel methodologies for measuring proteins and other molecules in biological fluids, the last 10 years have seen a huge increase in biomarker studies within FTD. This recent past has focused on attempting to develop markers that will help differentiate FTD from other dementias (particularly Alzheimer's disease (AD)), as well as from non-neurodegenerative conditions such as primary psychiatric disorders. While cerebrospinal fluid, and more recently blood, markers of AD have been successfully developed, specific markers identifying primary tauopathies or TDP-43 proteinopathies are still lacking. More focus at the moment has been on non-specific markers of neurodegeneration, and in particular, multiple studies of neurofilament light chain have highlighted its importance as a diagnostic, prognostic and staging marker of FTD. As clinical trials get under way in specific genetic forms of FTD, measures of progranulin and dipeptide repeat proteins in biofluids have become important potential measures of therapeutic response. However, understanding of whether drugs restore cellular function will also be important, and studies of key pathophysiological processes, including neuroinflammation, lysosomal function and synaptic health, are also now becoming more common. There is much still to learn in the fluid biomarker field in FTD, but the creation of large multinational cohorts is facilitating better powered studies and will pave the way for larger omics studies, including proteomics, metabolomics and lipidomics, as well as investigations of multimodal biomarker combinations across fluids, brain imaging and other domains. Here we provide an overview of the past, present and future of fluid biomarkers within the FTD field.

Inter-Species Differences in Regulation of the Progranulin-Sortilin Axis in TDP-43 Cell Models of Neurodegeneration

Cytoplasmic aggregates and nuclear depletion of the ubiquitous RNA-binding protein TDP-43 have been described in the autoptic brain tissues of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTLD) patients and both TDP-43 loss-of-function and gain-of-function mechanisms seem to contribute to the neurodegenerative process. Among the wide array of RNA targets, TDP-43 regulates progranulin (GRN) mRNA stability and sortilin (SORT1) splicing. Progranulin is a secreted neurotrophic and neuro-immunomodulatory factor whose endocytosis and delivery to the lysosomes are regulated by the neuronal receptor sortilin. Moreover, GRN loss-of-function mutations are causative of a subset of FTLD cases showing TDP-43 pathological aggregates. Here we show that TDP-43 loss-of-function differently affects the progranulin–sortilin axis in murine and human neuronal cell models. We demonstrated that although TDP-43 binding to GRN mRNA occurs similarly in human and murine cells, upon TDP-43 depletion, a different control of sortilin splicing and protein content may determine changes in extracellular progranulin uptake that account for increased or unchanged secreted protein in murine and human cells, respectively. As targeting the progranulin–sortilin axis has been proposed as a therapeutic approach for GRN-FTLD patients, the inter-species differences in TDP-43-mediated regulation of this pathway must be considered when translating studies from animal models to patients.

Sortilin gates neurotensin and BDNF signaling to control peripheral neuropathic pain

Neuropathic pain is a major incurable clinical problem resulting from peripheral nerve trauma or disease. A central mechanism is the reduced expression of the potassium chloride cotransporter 2 (KCC2) in dorsal horn neurons induced by brain-derived neurotrophic factor (BDNF), causing neuronal disinhibition within spinal nociceptive pathways. Here, we demonstrate how neurotensin receptor 2 (NTSR2) signaling impairs BDNF-induced spinal KCC2 down-regulation, showing how these two pathways converge to control the abnormal sensory response following peripheral nerve injury. We establish how sortilin regulates this convergence by scavenging neurotensin from binding to NTSR2, thus modulating its inhibitory effect on BDNF-mediated mechanical allodynia. Using sortilin-deficient mice or receptor inhibition by antibodies or a small-molecule antagonist, we lastly demonstrate that we are able to fully block BDNF-induced pain and alleviate injury-induced neuropathic pain, validating sortilin as a clinically relevant target.

Abnormal TDP-43 function impairs activity-dependent BDNF secretion, synaptic plasticity, and cognitive behavior through altered Sortilin splicing

Aberrant function of the RNA-binding protein TDP-43 has been causally linked to multiple neurodegenerative diseases. Due to its large number of targets, the mechanisms through which TDP-43 malfunction cause disease are unclear. Here, we report that knockdown, aggregation, or disease-associated mutation of TDP-43 all impair intracellular sorting and activity-dependent secretion of the neurotrophin brain-derived neurotrophic factor (BDNF) through altered splicing of the trafficking receptor Sortilin. Adult mice lacking TDP-43 specifically in hippocampal CA1 show memory impairment and synaptic plasticity defects that can be rescued by restoring Sortilin splicing or extracellular BDNF. Human neurons derived from patient iPSCs carrying mutated TDP-43 also show altered Sortilin splicing and reduced levels of activity-dependent BDNF secretion, which can be restored by correcting the mutation. We propose that major disease phenotypes caused by aberrant TDP-43 activity may be explained by the abnormal function of a handful of critical proteins, such as BDNF.

CSF progranulin increases in the course of Alzheimer's disease and is associated with sTREM2, neurodegeneration and cognitive decline

Progranulin (PGRN) is predominantly expressed by microglia in the brain, and genetic and experimental evidence suggests a critical role in Alzheimer's disease (AD). We asked whether PGRN expression is changed in a disease severity-specific manner in AD We measured PGRN in cerebrospinal fluid (CSF) in two of the best-characterized AD patient cohorts, namely the Dominant Inherited Alzheimer's Disease Network (DIAN) and the Alzheimer's Disease Neuroimaging Initiative (ADNI). In carriers of AD causing dominant mutations, cross-sectionally assessed CSF PGRN increased over the course of the disease and significantly differed from non-carriers 10 years before the expected symptom onset. In late-onset AD, higher CSF PGRN was associated with more advanced disease stages and cognitive impairment. Higher CSF PGRN was associated with higher CSF soluble TREM2 (triggering receptor expressed on myeloid cells 2) only when there was underlying pathology, but not in controls. In conclusion, we demonstrate that, although CSF PGRN is not a diagnostic biomarker for AD, it may together with sTREM2 reflect microglial activation during the disease.

Exploring the sortilin related receptor, SorLA, in depression

BACKGROUND

Studies of individual biomarkers for depression have shown insufficient sensitivity and specificity for clinical use, and most likely combinations of biomarkers may provide a better signature. The sorting-related receptor with A-type repeats (SorLA) is a well-studied pathogenic factor for Alzheimer's. SorLA belongs to the Vps10p domain receptor family, which also encompasses sortilin and SorCS1-3. All family members have been implicated in neurological and mental disorders. Notably, the SORCS3 gene is genome-wide significantly associated with depression and serum protein levels of sortilin are reduced in depressed individuals. SorLA regulates the activity of neurotrophic factors and cytokines and we hence speculated that SorLA might be implicated in depression.

METHODS

Serum SorLA levels were measured in two well-defined clinical samples using ELISA. Generalized linear models were used in the statistical analyses.

RESULTS

We identified a multivariate model to discriminate depressed individuals from healthy controls. Interestingly, the model consisted of serum SorLA levels and additional four predictors: previous depressive episode, stressful life events, serum levels of sortilin and VEGF. However, as an isolated factor, we observed no significant difference in SorLA levels between 140 depressed individuals and 140 healthy controls. Nevertheless, we observed a significant increase in SorLA levels following 12 weeks of treatment with nortriptyline, but not escitalopram.

LIMITATIONS

The number of biomarkers included in the multivariate model for depression and lack of replication limit our study.

CONCLUSIONS

Our results suggest SorLA as one of five factors that in combination may support the depression diagnosis, but not as an individual biomarker for depression or treatment response.

Highly segregated localization of the functionally related vps10p receptors sortilin and SorCS2 during neurodevelopment

Nervous system development is a precisely orchestrated series of events requiring a multitude of intrinsic and extrinsic cues. Sortilin and SorCS2 are members of the Vps10p receptor family with complementary influence on some of these cues including the neurotrophins (NTs). However, the developmental time points where sortilin and SorCS2 exert their activities in conjunction or independently still remain unclear. In this study we present the characterization of the spatiotemporal expression pattern of sortilin and SorCS2 in the developing murine nervous system. Sortilin is highly expressed in the fetal nervous system with expression localized to distinct cell populations. Expression was high in neurons of the cortical plate and developing allocortex, as well as subpallial structures. Furthermore, the neuroepithelium lining the ventricles and the choroid plexus showed high expression of sortilin, together with the developing retina, spinal ganglia, and sympathetic ganglia. In contrast, SorCS2 was confined in a marked degree to the thalamus and, at E13.5, the floor plate from midbrain rostrally to spinal cord caudally. SorCS2 was also found in the ventricular zones of the ventral hippocampus and nucleus accumbens areas, in the meninges and in Schwann cells. Hence, sortilin and SorCS2 are extensively present in several distinct anatomical areas in the developing nervous system and are rarely co-expressed. Possible functions of sortilin and SorCS2 pertain to NT signaling, axon guidance and beyond. The present data will form the basis for hypotheses and study designs for unravelling the functions of sortilin and SorCS2 during the establishment of neuronal structures and connections.

Sortilin Fragments Deposit at Senile Plaques in Human Cerebrum

Genetic variations in the vacuolar protein sorting 10 protein (Vps10p) family have been linked to Alzheimer’s disease (AD). Here we demonstrate deposition of fragments from the Vps10p member sortilin at senile plaques (SPs) in aged and AD human cerebrum. Sortilin changes were characterized in postmortem brains with antibodies against the extracellular and intracellular C-terminal domains. The two antibodies exhibited identical labeling in normal human cerebrum, occurring in the somata and dendrites of cortical and hippocampal neurons. The C-terminal antibody also marked extracellular lesions in some aged and all AD cases, appearing as isolated fibrils, mini-plaques, dense-packing or circular mature-looking plaques. Sortilin and β-amyloid (Aβ) deposition were correlated overtly in a region/lamina- and case-dependent manner as analyzed in the temporal lobe structures, with co-localized immunofluorescence seen at individual SPs. However, sortilin deposition rarely occurred around the pia, at vascular wall or in areas with typical diffuse Aβ deposition, with the labeling not enhanced by section pretreatment with heating or formic acid. Levels of a major sortilin fragment ~15 kDa, predicted to derive from the C-terminal region, were dramatically elevated in AD relative to control cortical lysates. Thus, sortilin fragments are a prominent constituent of the extracellularly deposited protein products at SPs in human cerebrum.

Focal Adhesion Kinase-Dependent Role of the Soluble Form of Neurotensin Receptor-3/Sortilin in Colorectal Cancer Cell Dissociation

The aim of the present review is to unravel the mechanisms of action of the soluble form of the neurotensin (NT) receptor-3 (NTSR3), also called Sortilin, in numerous physiopathological processes including cancer development, cardiovascular diseases and depression. Sortilin/NTSR3 is a transmembrane protein thought to exert multiple functions both intracellularly and at the level of the plasma membrane. The Sortilin/NTSR3 extracellular domain is released by shedding from all the cells expressing the protein. Although the existence of the soluble form of Sortilin/NTSR3 (sSortilin/NTSR3) has been evidenced for more than 10 years, the studies focusing on the role of this soluble protein at the mechanistic level remain rare. Numerous cancer cells, including colonic cancer cells, express the receptor family of neurotensin (NT), and particularly Sortilin/NTSR3. This review aims to summarize the functional role of sSortilin/NTSR3 characterized in the colonic cancer cell line HT29. This includes mechanisms involving signaling cascades through focal adhesion kinase (FAK), a key pathway leading to the weakening of cell-cell and cell-extracellular matrix adhesions, a series of events which could be responsible for cancer metastasis. Finally, some future approaches targeting the release of sNTSR3 through the inhibition of matrix metalloproteases (MMPs) are suggested.