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

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.

Extracellular Secretagogin is internalized into the cells through endocytosis

Secretagogin (SCGN) is a calcium-sensor protein with a regulatory role in glucose metabolism and the secretion of several peptide hormones. Many, but not all, functions of SCGN can be explained by its intracellular manifestation. Despite early data on SCGN secretion, the secretory mechanism, biological fate, physiological implications, and trans-cellular signaling of extracellular SCGN remain unknown. We here report that extracellular SCGN is readily internalized into the C2C12 cells in an energy-dependent manner. Using endocytosis inhibitors, we demonstrate that SCGN internalizes via clathrin-mediated endocytosis, following which, SCGN localizes largely in the cytosol. Exogenous SCGN treatment induces a global proteomic reprogramming in C2C12 cells. Gene ontology search suggests that SCGN-induced proteomic reprogramming favors protein synthesis and cellular growth. We thus validated the cell proliferative action of SCGN using C2C12, HepG2, and NIH-3T3 cell lines. Based on the data, we propose that circulatory SCGN is internalized into the target cells and modulates the expression of cell growth-related proteins. The work suggests that extracellular SCGN is a functional protein, which communicates with specific cell types and directly modulates cell proliferation.

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