Secretagogin Is a Redox-Responsive Ca2+ Sensor

Dynamic changes of SCGN expression imply different phases of clear cell renal cell carcinoma progression

The secretagogin (SCGN) was originally identified as a secreted calcium-binding protein present in the cytoplasm. Recent studies have found that SCGN has a close relationship with cancer. However, its role in the occurrence, progression, and prognosis of clear cell renal cell carcinoma (ccRCC) remains unclear. In this study, we utilized a mutual authentication method based on public databases and clinical samples to determine the role of SCGN in the progression and prognosis of ccRCC. Firstly, we comprehensively analyzed the expression characteristics of SCGN in ccRCC in several public databases. Subsequently, we systematically evaluated SCGN expression on 252 microarrays of ccRCC tissues from different grades. It was found that SCGN was absent in all the normal kidney tissues and significantly overexpressed in ccRCC tumor tissues. In addition, the expression level of SCGN gradually decreased with an increase in tumor grade, and the percentage of SCGN staining positivity over 50% was 86.7% (13/15) and 73.4% (58/79) in Grade1 and Grade2, respectively, while it was only 8.3% (12/144) in Grade3, and the expression of SCGN was completely absent in Grade4 (0/14) and distant metastasis group (0/4). Additionally, the expression of SCGN was strongly correlated with the patient's prognosis, with the higher the expression levels of SCGN being associated with longer overall survival and disease-free survival of patients. In conclusion, our results suggest that reduced expression of SCGN in cancer cells is correlated with the progression and prognosis of ccRCC.

Disrupted and Elevated Circadian Secretion of Glucagon-Like Peptide-1 in a Murine Model of Type 2 Diabetes

Metabolism and circadian rhythms are intimately linked, with circadian glucagon-like peptide-1 (GLP-1) secretion by the intestinal L-cell entraining rhythmic insulin release. GLP-1 secretion has been explored in the context of obesogenic diets, but never in a rodent model of type 2 diabetes (T2D). There is also considerable disagreement regarding GLP-1 levels in human T2D. Furthermore, recent evidence has demonstrated decreased expression of the β-cell exocytotic protein secretagogin (SCGN) in T2D. To extend these findings to the L-cell, we administered oral glucose tolerance tests at 6 time points in 4-hour intervals to the high-fat diet/streptozotocin (HFD-STZ) mouse model of T2D. This revealed a 10-fold increase in peak GLP-1 secretion with a phase shift of the peak from the normal feeding period into the fasting-phase. This was accompanied by impairments in the rhythms of glucose, glucagon, mucosal clock genes (Arntl and Cry2), and Scgn. Immunostaining revealed that L-cell GLP-1 intensity was increased in the HFD-STZ model, as was the proportion of L-cells that expressed SCGN; however, this was not found in L-cells from humans with T2D, which exhibited decreased GLP-1 staining but maintained their SCGN expression. Gcg expression in isolated L-cells was increased along with pathways relating to GLP-1 secretion and electron transport chain activity in the HFD-STZ condition. Further investigation into the mechanisms responsible for this increase in GLP-1 secretion may give insights into therapies directed toward upregulating endogenous GLP-1 secretion.

Secretagogin is a Ca2+-dependent stress-responsive chaperone that may also play a role in aggregation-based proteinopathies

Secretagogin (SCGN) is a three-domain hexa-EF-hand Ca²⁺-binding protein that plays a regulatory role in the release of several hormones. SCGN is expressed largely in pancreatic β-cells, certain parts of the brain, and also in neuroendocrine tissues. The expression of SCGN is altered in several diseases, such as diabetes, cancers, and neurodegenerative disorders; however, the precise associations that closely link SCGN expression to such pathophysiologies are not known. In this work, we report that SCGN is an early responder to cellular stress, and SCGN expression is temporally upregulated by oxidative stress and heat shock. We show the overexpression of SCGN efficiently prevents cells from heat shock and oxidative damage. We further demonstrate that in the presence of Ca²⁺, SCGN efficiently prevents the aggregation of a broad range of model proteins in vitro. Small-angle X-ray scattering (BioSAXS) studies further reveal that Ca²⁺ induces the conversion of a closed compact apo-SCGN conformation into an open extended holo-SCGN conformation via multistate intermediates, consistent with the augmentation of chaperone activity of SCGN. Furthermore, isothermal titration calorimetry establishes that Ca²⁺ enables SCGN to bind α-synuclein and insulin, two target proteins of SCGN. Altogether, our data not only demonstrate that SCGN is a Ca²⁺-dependent generic molecular chaperone involved in protein homeostasis with broad substrate specificity but also elucidate the origin of its altered expression in several cancers. We describe a plausible mechanism of how perturbations in Ca²⁺ homeostasis and/or deregulated SCGN expression would hasten the process of protein misfolding, which is a feature of many aggregation-based proteinopathies.

Conformational scanning of individual EF-hand motifs of calcium sensor protein centrin-1

Centrin-1, a Ca²⁺ sensor protein of the centrin family is a crucial player for cell division in eukaryotes and plays a key role in the microtubule organising centre. Despite being regarded as a calcium sensor with a matched structure to calmodulin/troponin C, the protein undergoes mild changes in conformation and binds Ca²⁺ with moderate affinity. We present an in-depth analysis of the Ca²⁺ sensing by individual EF-hand motifs of centrin-1 and address unsolved questions of the rationales for moderate affinity and conformational transitions of the protein. Employing the more sensitive approach of Trp scanning of individual EF-hand motif, we have undertaken an exhaustive investigation of Ca²⁺ binding to individual EF-hand motifs, named EF1 to EF4. All four EF-hand motifs of centrin-1 are structural as all of them bind both Ca²⁺ and Mg²⁺. EF1 and EF4 are the most flexible sites as they undergo drastic conformational changes following Ca²⁺ binding, whereas EF3 responds to Ca²⁺ minimally. On the other hand, EF2 moves towards the protein surface upon binding Ca²⁺. The independent filling mode of Ca²⁺ to EF-hand motifs and lack of intermotif communication explain the lack of cooperativity of binding, thus constraining centrin-1 to a moderate affinity binding protein. Thus, centrin-1 is distinct from other calcium sensors such as calmodulin.

A novel secretagogin/ATF4 pathway is involved in oxidized LDL-induced endoplasmic reticulum stress and islet β-cell apoptosis

Excessive accumulation of cholesterol in β cells initiates endoplasmic reticulum (ER) stress and associated apoptosis. We have reported that excessive uptake of cholesterol by MIN6 cells decreases the expression of secretagogin (SCGN) and then attenuates insulin secretion. Here, we aimed to determine whether cholesterol-induced SCGN decrease is involved in the modulation of ER stress and apoptosis in pancreatic β cells. In this study, MIN6 cells were treated with oxidized low-density lipoprotein (ox-LDL) for 24 h, and then intracellular lipid droplets and cell apoptosis were quantified, and SCGN and ER stress markers were identified by western blot analysis. Furthermore, small interfer RNA (siRNA)-mediated SCGN knockdown and recombinant plasmid-mediated SCGN restoration experiments were performed to confirm the role of SCGN in ER stress and associated cell apoptosis. Finally, the interaction of SCGN with ATF4 was computationally predicted and then validated by a co-immunoprecipitation assay. We found that ox-LDL treatment increased the levels of ER stress markers, such as phosphorylated protein kinase-like endoplasmic reticulum kinase, phosphorylated eukaryotic initiation factor 2 alpha, activating transcription factor 4 (ATF4), and transcription factor CCAAT-enhancer-binding protein homologous protein, and promoted MIN6 cell apoptosis; in addition, the expression of SCGN was downregulated. siRNA-mediated SCGN knockdown exacerbated β-cell ER stress by increasing ATF4 expression. Pretreatment of MIN6 cells with the recombinant SCGN partly antagonized ox-LDL-induced ER stress and apoptosis. Furthermore, a co-immunoprecipitation assay revealed an interaction between SCGN and ATF4 in MIN6 cells. Taken together, these results demonstrated that pancreatic β-cell apoptosis induced by ox-LDL treatment can be attributed, in part, to an SCGN/ATF4-dependent ER stress response.

Multiple Origins of Secretagogin Expressing Cortical GABAergic Neuron Precursors in the Early Human Fetal Telencephalon

Secretagogin (SCGN) which acts as a calcium signaling sensor, has previously been shown to be expressed by a substantial population of cortical GABAergic neurons at mid-gestation in humans but not in mice. The present study traced SCGN expression in cortical GABAergic neurons in human fetal forebrain from earlier stages than previously studied. Multiple potential origins of SCGN-expressing neurons were identified in the caudal ganglionic eminence (CGE) lateral ganglionic eminence (LGE) septum and preoptic area; these cells largely co-expressed SP8 but not the medial ganglionic eminence marker LHX6. They followed various migration routes to reach their target regions in the neocortex, insular and olfactory cortex (OC) and olfactory bulbs. A robust increase in the number of SCGN-expressing GABAergic cortical neurons was observed in the midgestational period; 58% of DLX2+ neurons expressed SCGN in the cortical wall at 19 post-conceptional weeks (PCW), a higher proportion than expressed calretinin, a marker for GABAergic neurons of LGE/CGE origin. Furthermore, although most SCGN+ neurons co-expressed calretinin in the cortical plate (CP) and deeper layers, in the marginal zone (MZ) SCGN+ and calretinin+ cells formed separate populations. In the adult mouse, it has previously been shown that in the rostral migratory stream (RMS), SCGN, annexin V (ANXA5), and matrix metalloprotease 2 (MMP2) are co-expressed forming a functioning complex that exocytoses MMP2 in response to calcium. In the present study, ANXA5 showed widespread expression throughout the cortical wall, although MMP2 expression was very largely limited to the CP. We found co-expression of these proteins in some SCGN+ neurons in the subventricular zones (SVZ) suggesting a limited role for these cells in remodeling the extracellular matrix, perhaps during cell migration.

Secretagogin Regulates Insulin Signaling by Direct Insulin Binding

Secretagogin (SCGN) is a β-cell enriched, secretory/cytosolic Ca²⁺-binding protein with unknown secretory regulation and functions. Recent findings suggest that SCGN deficiency correlates with compromised insulin response and diabetes. However, the (patho)physiological SCGN-insulin nexus remains unexplored. We here report that SCGN is an insulin-interacting protein. The protein-protein interaction between SCGN and insulin regulates insulin stability and increases insulin potency in vitro and in vivo. Mutagenesis studies suggest an indispensable role for N-terminal domain of SCGN in modulating insulin stability and function. SCGN supplementation in diabetogenic-diet-fed mice preserves physiological insulin responsiveness while relieving obesity and cardiovascular risk. SCGN-insulin interaction mediated alleviation of hyperinsulinemia by increased insulin internalization, which translates to reduced body fat and hepatic lipid accumulation, emerges as a plausible mechanism for the preservation of insulin responsiveness. These findings establish SCGN as a functional insulin-binding protein (InsBP) with therapeutic potential against diabetes.

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SCGN is an insulin-interacting calcium sensor protein

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SCGN binding protects insulin from aggregation

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SCGN potentiates insulin action in vivo

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SCGN administration into HFD-fed animals impedes insulin resistance

Plasma Secretagogin is Increased in Individuals with Glucose Dysregulation

OBJECTIVE

Secretagogin, a Ca²⁺ binding protein, is one of the most abundant proteins in pancreatic β-cells and is critical for maintaining the structural integrity and signaling competence of β-cells. This study seeks to assess the concentrations of plasma secretagogin in participants with prediabetes (pre-DM) and newly diagnosed type 2 diabetes (T2DM) and to explore its relationship to parameters of glucose and lipid metabolism, first-phase insulin secretion, insulin resistance and pancreatic β-cell function.

MATERIALS AND METHODS

A total of 126 eligible subjects were divided into three groups: a normal glucose tolerance (NGT, n=45), a pre-DM (n=30), and a T2DM (n=51) group. An intravenous glucose tolerance test (IVGTT) was performed, and clinical and biochemical parameters were measured for all subjects.

RESULTS

Plasma secretagogin levels were significantly higher in both pre-DM and T2DM patients compared with NGT subjects and were highest in the T2DM group. Correlation analysis showed that plasma secretagogin levels were positively correlated with fasting plasma glucose, postchallenge plasma glucose (2hPG), HbA1c and body mass index (BMI) but were not correlated with waist-hip ratio, blood pressure, lipid profiles, fasting serum insulin, homeostasis model assessment for insulin resistance, homeostasis model assessment for β-cell function and first-phase insulin secretion indicators. Multiple logistic regression analysis revealed that 2hPG and BMI were independent predictors for elevation of plasma secretagogin concentrations.

CONCLUSIONS

Increased circulating secretagogin might be a molecular predictor for early diagnosis of diabetes. Further studies are needed to confirm this finding and explore the role of secretagogin in obesity.

Secretagogin Purification and Quality Control Strategies for Biophysical and Cell Biological Studies

Secretagogin (SCGN) has recently gained attention due to its modulatory effect on insulin/CRH secretion and function. However, a large pool of speculated SCGN functions remains unexplored. A major deficiency is the lack of knowledge about the biological functions of extracellular SCGN. We here describe convenient methods for the scalable production of His-tagged and untagged mouse SCGN. The protocol is optimized to remove endotoxins, and thus the protein is suited for biological applications such as cell culture treatment or animal injections. We also outline expedient methods to check the purity of SCGN preparation for biological applications.

Thalamocortical Afferents Innervate the Cortical Subplate much Earlier in Development in Primate than in Rodent

The current model, based on rodent data, proposes that thalamocortical afferents (TCA) innervate the subplate towards the end of cortical neurogenesis. This implies that the laminar identity of cortical neurons is specified by intrinsic instructions rather than information of thalamic origin. In order to determine whether this mechanism is conserved in the primates, we examined the growth of thalamocortical (TCA) and corticofugal afferents in early human and monkey fetal development. In the human, TCA, identified by secretagogin, calbindin, and ROBO1 immunoreactivity, were observed in the internal capsule of the ventral telencephalon as early as 7-7.5 PCW, crossing the pallial/subpallial boundary (PSB) by 8 PCW before the calretinin immunoreactive corticofugal fibers do. Furthermore, TCA were observed to be passing through the intermediate zone and innervating the presubplate of the dorsolateral cortex, and already by 10-12 PCW TCAs were occupying much of the cortex. Observations at equivalent stages in the marmoset confirmed that this pattern is conserved across primates. Therefore, our results demonstrate that in primates, TCAs innervate the cortical presubplate at earlier stages than previously demonstrated by acetylcholinesterase histochemistry, suggesting that pioneer thalamic afferents may contribute to early cortical circuitry that can participate in defining cortical neuron phenotypes.

Veiled Potential of Secretagogin in Diabetes: Correlation or Coincidence?

Secretagogin (SCGN) is a calcium sensor protein enriched in neuroendocrine cells in general and pancreatic β-cells in particular. SCGN regulates insulin secretion through several Ca²⁺-dependent interactions. Recent studies implicate SCGN in the β-cell physiology and extracellular insulin function, making it an intriguing candidate in diabetes research. Here, we propose a conjoining theme of diversified SCGN function in diabetes pathology. In our opinion, SCGN is an attractive therapeutic candidate ascribed by its role in β-cell maintenance and neuronal functions and in the efficacy of insulin. To scrutinize the therapeutic prospects of SCGN, we abridge putative diabetes-related properties of SCGN and put forth strategies to determine the precise role of SCGN in the pathogenesis/preclusion of diabetes.

20 Years of Secretagogin: Exocytosis and Beyond

Calcium is one of the most important signaling factors in mammalian cells. Specific temporal and spatial calcium signals underlie fundamental processes such as cell growth, development, circadian rhythms, neurotransmission, hormonal actions and apoptosis. In order to translate calcium signals into cellular processes a vast number of proteins bind this ion with affinities from the nanomolar to millimolar range. Using classical biochemical methods an impressing number of calcium binding proteins (CBPs) have been discovered since the late 1960s, some of which are expressed ubiquitously, others are more restricted to specific cell types. In the nervous system expression patterns of different CBPs have been used to discern different neuronal cell populations, especially before advanced methods like single-cell transcriptomics and activity recording were available to define neuronal identity. However, understanding CBPs and their interacting proteins is still of central interest. The post-genomic era has coined the term “calciomics,” to describe a whole new research field, that engages in the identification and characterization of CBPs and their interactome. Secretagogin is a CBP, that was discovered 20 years ago in the pancreas. Consecutively it was found also in other organs including the nervous system, with characteristic expression patterns mostly forming cell clusters. Its regional expression and subcellular location together with the identification of protein interaction partners implicated, that secretagogin has a central role in hormone secretion. Meanwhile, with the help of modern proteomics a large number of actual and putative interacting proteins has been identified, that allow to anticipate a much more complex role of secretagogin in developing and adult neuronal cells. Here, we review recent findings that appear like puzzle stones of a greater picture.

Secretagogin is increased in plasma from type 2 diabetes patients and potentially reflects stress and islet dysfunction

Beta cell dysfunction accompanies and drives the progression of type 2 diabetes mellitus (T2D), but there are few clinical biomarkers available to assess islet cell stress in humans. Secretagogin, a protein enriched in pancreatic islets, demonstrates protective effects on beta cell function in animals. However, its potential as a circulating biomarker released from human beta cells and islets has not been studied. In this study primary human islets, beta cells and plasma samples were used to explore secretion and expression of secretagogin in relation to the T2D pathology. Secretagogin was abundantly and specifically expressed and secreted from human islets. Furthermore, T2D patients had an elevated plasma level of secretagogin compared with matched healthy controls, which was confirmed in plasma of diabetic mice transplanted with human islets. Additionally, the plasma secretagogin level of the human cohort had an inverse correlation to clinical assessments of beta cell function. To explore the mechanism of secretagogin release in vitro, human beta cells (EndoC-βH1) were exposed to elevated glucose or cellular stress-inducing agents. Secretagogin was not released in parallel with glucose stimulated insulin release, but was markedly elevated in response to endoplasmic reticulum stressors and cytokines. These findings indicate that secretagogin is a potential novel biomarker, reflecting stress and islet cell dysfunction in T2D patients.

Developmental and adult characterization of secretagogin expressing amacrine cells in zebrafish retina

Calcium binding proteins show stereotypical expression patterns within diverse neuron types across the central nervous system. Here, we provide a characterization of developmental and adult secretagogin-immunolabelled neurons in the zebrafish retina with an emphasis on co-expression of multiple calcium binding proteins. Secretagogin is a recently identified and cloned member of the F-hand family of calcium binding proteins, which labels distinct neuron populations in the retinas of mammalian vertebrates. Both the adult distribution of secretagogin labeled retinal neurons as well as the developmental expression indicative of the stage of neurogenesis during which this calcium binding protein is expressed was quantified. Secretagogin expression was confined to an amacrine interneuron population in the inner nuclear layer, with monostratified neurites in the center of the inner plexiform layer and a relatively regular soma distribution (regularity index > 2.5 across central–peripheral areas). However, only a subpopulation (~60%) co-labeled with gamma-aminobutyric acid as their neurotransmitter, suggesting that possibly two amacrine subtypes are secretagogin immunoreactive. Quantitative co-labeling analysis with other known amacrine subtype markers including the three main calcium binding proteins parvalbumin, calbindin and calretinin identifies secretagogin immunoreactive neurons as a distinct neuron population. The highest density of secretagogin cells of ~1800 cells / mm² remained relatively evenly along the horizontal meridian, whilst the density dropped of to 125 cells / mm² towards the dorsal and ventral periphery. Thus, secretagogin represents a new amacrine label within the zebrafish retina. The developmental expression suggests a possible role in late stage differentiation. This characterization forms the basis of functional studies assessing how the expression of distinct calcium binding proteins might be regulated to compensate for the loss of one of the others.

Calcium Ion Induced Structural Changes Promote Dimerization of Secretagogin, Which Is Required for Its Insulin Secretory Function

Secretagogin (SCGN), a hexa EF-hand calcium binding protein, plays key roles in insulin secretion in pancreatic β-cells. It is not yet understood how the binding of Ca²⁺ to human SCGN (hSCGN) promotes secretion. Here we have addressed this question, using mass spectrometry combined with a disulfide searching algorithm DBond. We found that the binding of Ca²⁺ to hSCGN promotes the dimerization of hSCGN via the formation of a Cys193-Cys193 disulfide bond. Hydrogen/deuterium exchange mass spectrometry (HDX-MS) and molecular dynamics studies revealed that Ca²⁺ binding to the EF-hands of hSCGN induces significant structural changes that affect the solvent exposure of N-terminal region, and hence the redox sensitivity of the Cys193 residue. These redox sensitivity changes were confirmed using biotinylated methyl-3-nitro-4-(piperidin-1-ylsulfonyl) benzoate (NPSB-B), a chemical probe that specifically labels reactive cysteine sulfhydryls. Furthermore, we found that wild type hSCGN overexpression promotes insulin secretion in pancreatic β cells, while C193S-hSCGN inhibits it. These findings suggest that insulin secretion in pancreatic cells is regulated by Ca²⁺ and ROS signaling through Ca²⁺-induced structural changes promoting dimerization of hSCGN.