Mass spectrometric characterization and activity of zinc-activated proinsulin C-peptide and C-peptide mutants

Characterizing metal-biomolecule interactions by mass spectrometry

Metal micronutrients are essential for life and exist in a delicate balance to maintain an organism's health. The labile nature of metal-biomolecule interactions clouds the understanding of metal binders and metal-mediated conformational changes that are influential to health and disease. Mass spectrometry (MS)-based methods and technologies have been developed to better understand metal micronutrient dynamics in the intra- and extracellular environment. In this review, we describe the challenges associated with studying labile metals in human biology and highlight MS-based methods for the discovery and study of metal-biomolecule interactions.

Elucidation of a Copper Binding Site in Proinsulin C-peptide and Its Implications for Metal-Modulated Activity

The connecting peptide (C-peptide) is a hormone with promising health benefits in ameliorating diabetes-related complications, yet mechanisms remain elusive. Emerging studies point to a possible dependence of peptide activity on bioavailable metals, particularly Cu(II) and Zn(II). However, little is known about the chemical nature of the interactions, hindering advances in its therapeutic applications. This work uncovers the Cu(II)-binding site in C-peptide that may be key to understanding its metal-dependent function. A combination of spectroscopic studies reveal that Cu(II) and Zn(II) bind to C-peptide at specific residues in the N-terminal region of the peptide and that Cu(II) is able to displace Zn(II) for C-peptide binding. The data point to a Cu(II)-binding site consisting of 1N3O square-planar coordination that is entropically driven. Furthermore, the entire random coil peptide sequence is needed for specific metal binding as mutations and truncations reshuffle the coordinating residues. These results expand our understanding of how metals influence hormone activity and facilitate the discovery and validation of both new and established paradigms in peptide biology.

Analysis of Metal Effects on C-Peptide Structure and Internalization

The connecting peptide (C-peptide) has received increased attention for its potential therapeutic effects in ameliorating illnesses such as kidney disease and diabetes. Although the mechanism of C-peptide signaling remains elusive, evidence supports its internalization and intracellular function. Emerging research is uncovering the diverse biological roles metals play in controlling and affecting the function of bioactive peptides. The work presented herein investigates interactions between C-peptide and first-row d-block transition metals, as well as their effects on C-peptide internalization into cells. Through spectroscopic techniques, it is demonstrated that Cr^III , Cu^II , and Zn^II bind to C-peptide with differing stoichiometries and biologically relevant affinities. In addition, metal binding elicits both subtle changes in secondary structure and inhibits adoption of an α-helical character in environments where the dielectric constants are reduced. This study shows how metal ions can modulate peptide hormone activity through subtle structural changes to disrupt cellular uptake.

Metal-dependent hormone function: the emerging interdisciplinary field of metalloendocrinology

For over 100 years, there has been an incredible amount of knowledge amassed concerning hormones in the endocrine system and their central role in human health. Hormones represent a diverse group of biomolecules that are released by glands, communicate signals to their target tissue, and are regulated by feedback loops to maintain organism health. Many disease states, such as diabetes and reproductive disorders, stem from misregulation or dysfunction of hormones. Increasing research is illuminating the intricate roles of metal ions in the endocrine system where they may act advantageously in concert with hormones or deleteriously catalyze hormone-associated disease states. As the critical role of metal ions in the endocrine system becomes more apparent, it is increasingly important to untangle the complex mechanisms underlying the connections between inorganic biochemistry and hormone function to understand and control endocrinological phenomena. This tutorial review harmonizes the interdisciplinary fields of endocrinology and inorganic chemistry in the newly-termed field of "metalloendocrinology". We describe examples linking metals to both normal and aberrant hormone function with a focus on highlighting insight to molecular mechanisms. Hormone activities related to both essential metal micronutrients, such as copper, iron, zinc, and calcium, and disruptive nonessential metals, such as lead and cadmium are discussed.

Native electrospray mass spectrometry approaches to probe the interaction between zinc and an anti-angiogenic peptide from histidine-rich glycoprotein

Zinc modulates the biological function of histidine-rich glycoprotein (HRG) through binding to its His-rich region (HRR). The Zn2+-binding properties of a 35 amino-acid biologically-active peptide mimic of the HRR, HRGP330, were investigated using dissociative mass spectrometry approaches in addition to travelling-wave ion mobility mass spectrometry (TWIM-MS). Native mass spectrometry confirmed zinc binding to HRGP330; however, broadening of the 1H NMR resonances upon addition of Zn2+ ions precluded the attainment of structural information. A complementary approach employing TWIM-MS indicated that HRGP330 has a more compact structure in the presence of Zn2+ ions. Top-down MS/MS data supported a metal-binding-induced conformational change, as fewer fragments were observed for Zn2+-bound HRGP330. Zn2+-bound fragments of both N-terminal and C-terminal ends of the peptide were identified from collision-induced dissociation (CID) and electron transfer dissociation/proton transfer reaction (ETD/PTR) experiments, suggesting that multiple binding sites exist within this region of HRG. The combination of mass spectrometry and NMR approaches provides new insight into the highly dynamic interaction between zinc and this His-rich peptide.

Zinc-coordination and C-peptide complexation: a potential mechanism for the endogenous inhibition of IAPP aggregation

Aggregation of the highly amyloidogenic IAPP is endogenously inhibited inside beta-cell granules at millimolar concentrations. Combining in vitro experiments and computer simulations, we demonstrated that the stabilization of IAPP upon the formation of zinc-coordinated ion molecular complex with C-peptide might be important for the endogenous inhibition of IAPP aggregation.

Implications of peptide assemblies in amyloid diseases

Neurodegenerative disorders and type 2 diabetes are global epidemics compromising the quality of life of millions worldwide, with profound social and economic implications. Despite the significant differences in pathology - much of which are poorly understood - these diseases are commonly characterized by the presence of cross-β amyloid fibrils as well as the loss of neuronal or pancreatic β-cells. In this review, we document research progress on the molecular and mesoscopic self-assembly of amyloid-beta, alpha synuclein, human islet amyloid polypeptide and prions, the peptides and proteins associated with Alzheimer's, Parkinson's, type 2 diabetes and prion diseases. In addition, we discuss the toxicities of these amyloid proteins based on their self-assembly as well as their interactions with membranes, metal ions, small molecules and engineered nanoparticles. Through this presentation we show the remarkable similarities and differences in the structural transitions of the amyloid proteins through primary and secondary nucleation, the common evolution from disordered monomers to alpha-helices and then to β-sheets when the proteins encounter the cell membrane, and, the consensus (with a few exceptions) that off-pathway oligomers, rather than amyloid fibrils, are the toxic species regardless of the pathogenic protein sequence or physicochemical properties. In addition, we highlight the crucial role of molecular self-assembly in eliciting the biological and pathological consequences of the amyloid proteins within the context of their cellular environments and their spreading between cells and organs. Exploiting such structure-function-toxicity relationship may prove pivotal for the detection and mitigation of amyloid diseases.

C-Peptide replacement therapy in type 1 diabetes: are we in the trough of disillusionment?

Type 1 diabetes is associated with such complications as blindness, kidney failure, and nerve damage. Replacing C-peptide, a hormone normally co-secreted with insulin, has been shown to reduce diabetes-related complications. Interestingly, after nearly 30 years of positive research results, C-peptide is still not being co-administered with insulin to diabetic patients. The following review discusses the potential of C-peptide as an auxilliary replacement therapy and why it's not currently being used as a therapeutic.

Zinc in Pancreatic Islet Biology, Insulin Sensitivity, and Diabetes

About 20 chemical elements are nutritionally essential for humans with defined molecular functions. Several essential and nonessential biometals are either functional nutrients with antidiabetic actions or can be diabetogenic. A key question remains whether changes in the metabolism of biometals and biominerals are a consequence of diabetes or are involved in its etiology. Exploration of the roles of zinc (Zn) in this regard is most revealing because 80 years of scientific discoveries link zinc and diabetes. In pancreatic β- and α-cells, zinc has specific functions in the biochemistry of insulin and glucagon. When zinc ions are secreted during vesicular exocytosis, they have autocrine, paracrine, and endocrine roles. The membrane protein ZnT8 transports zinc ions into the insulin and glucagon granules. ZnT8 has a risk allele that predisposes the majority of humans to developing diabetes. In target tissues, increased availability of zinc enhances the insulin response by inhibiting protein tyrosine phosphatase 1B, which controls the phosphorylation state of the insulin receptor and hence downstream signalling. Inherited diseases of zinc metabolism, environmental exposures that interfere with the control of cellular zinc homeostasis, and nutritional or conditioned zinc deficiency influence the patho-biochemistry of diabetes. Accepting the view that zinc is one of the many factors in multiple gene-environment interactions that cause the functional demise of β-cells generates an immense potential for treating and perhaps preventing diabetes. Personalized nutrition, bioactive food, and pharmaceuticals targeting the control of cellular zinc in precision medicine are among the possible interventions.

Combined zinc supplementation with proinsulin C-peptide treatment decreases the inflammatory response and mortality in murine polymicrobial sepsis

Zinc is a trace element vital for immune function during host response to infection. The proinsulin C-peptide has been shown to exert beneficial effects through activation of the anti-inflammatory peroxisome proliferator-activated receptor γ (PPARγ) in experimental endotoxemia. Some in vitro activities of C-peptide appear dependent on the presence of zinc. We investigated the effect of zinc supplementation before onset of sepsis on the anti-inflammatory properties of C-peptide. Male C57BL/6 mice were subjected to polymicrobial sepsis by cecal ligation and puncture (CLP). Mice received zinc gluconate (1.3 mg/kg) intraperitoneally (i.p.) for 3 days before CLP. One hour after CLP, animals received C-peptide (280 nmol/kg i.p.) or the antimicrobial agent imipenem (25 mg/kg i.p.). Cecal ligation and puncture was associated with an 11% survival rate, pulmonary leukosequestration, and liver injury. Molecular analysis in lungs of septic mice showed increased nuclear activation of the proinflammatory extracellular signal-regulated kinases 1 and 2 and nuclear factor κB, but decreased PPARγ expression, when compared with sham animals. Combination of zinc supplementation with C-peptide posttreatment significantly improved survival rate (61%) similarly to antibiotic treatment (60%), ameliorated lung architecture and liver function, reduced tissue neutrophil infiltration, and increased bacterial clearance when compared with vehicle, C-peptide, or zinc treatment alone. These beneficial effects were associated with restored lung nuclear expression of PPARγ and reduction of phosphorylated extracellular signal-regulated kinases 1 and 2 and nuclear factor κB activities in comparison to vehicle or single treatment protocols. Our data demonstrate that short-term zinc prophylaxis before the infectious insult is a requisite for the anti-inflammatory properties of C-peptide by facilitating modulation of inflammatory pathways.

Synergistic effects of C-peptide and insulin on low O2-induced ATP release from human erythrocytes

Erythrocytes participate in the matching of oxygen (O2) delivery with local need in skeletal muscle via the release of O2 and the vasodilator, ATP. It was reported that a concentration of insulin found in humans with insulin resistance inhibits low O2-induced ATP release. However, in vivo, insulin is coreleased with connecting peptide (C-peptide) at equimolar concentrations, but because of the shorter insulin half-life, the peptides circulate at ratios of C-peptide to insulin ranging from 1:1 to 6:1. Here, we investigate the hypothesis that C-peptide and insulin work synergistically to maintain low O2-induced ATP release from human erythrocytes. Using a thin-film tonometer to alter O2 tension, we determined that either C-peptide or insulin alone inhibits low O2-induced ATP release in a concentration-dependent manner; however, coadministration of the peptides at a 1:1 ratio does not (n = 5; P < 0.05). Because this ratio of C-peptide to insulin is not present in vivo for extended periods, we also investigated the effect of additional physiological ratios on ATP release. In the presence of insulin concentrations that would be found in fasting humans (0.05 nM), C-peptide to insulin ratios of 4:1 and 6:1 did not adversely affect low O2-induced ATP release. However, at a concentration of insulin found in the peripheral circulation of humans under postprandial conditions (0.5 nM), a ratio of C-peptide to insulin of 6:1 inhibited low O2-induced ATP release (n = 5). These findings demonstrate a heretofore unrecognized synergism between C-peptide and insulin that could have physiological importance in the regulation of perfusion distribution in skeletal muscle.

Zinc transporter 8 autoantibodies and their association with SLC30A8 and HLA-DQ genes differ between immigrant and Swedish patients with newly diagnosed type 1 diabetes in the Better Diabetes Diagnosis study

We examined whether zinc transporter 8 autoantibodies (ZnT8A; arginine ZnT8-RA, tryptophan ZnT8-WA, and glutamine ZnT8-QA variants) differed between immigrant and Swedish patients due to different polymorphisms of SLC30A8, HLA-DQ, or both. Newly diagnosed autoimmune (≥1 islet autoantibody) type 1 diabetic patients (n = 2,964, <18 years, 55% male) were ascertained in the Better Diabetes Diagnosis study. Two subgroups were identified: Swedes (n = 2,160, 73%) and immigrants (non-Swedes; n = 212, 7%). Non-Swedes had less frequent ZnT8-WA (38%) than Swedes (50%), consistent with a lower frequency in the non-Swedes (37%) of SLC30A8 CT+TT (RW+WW) genotypes than in the Swedes (54%). ZnT8-RA (57 and 58%, respectively) did not differ despite a higher frequency of CC (RR) genotypes in non-Swedes (63%) than Swedes (46%). We tested whether this inconsistency was due to HLA-DQ as 2/X (2/2; 2/y; y is anything but 2 or 8), which was a major genotype in non-Swedes (40%) compared with Swedes (14%). In the non-Swedes only, 2/X (2/2; 2/y) was negatively associated with ZnT8-WA and ZnT8-QA but not ZnT8-RA. Molecular simulation showed nonbinding of the relevant ZnT8-R peptide to DQ2, explaining in part a possible lack of tolerance to ZnT8-R. At diagnosis in non-Swedes, the presence of ZnT8-RA rather than ZnT8-WA was likely due to effects of HLA-DQ2 and the SLC30A8 CC (RR) genotypes.

Early transcriptional regulation by C-peptide in freshly isolated rat proximal tubular cells

BACKGROUND

Clinical studies have shown that proinsulin C-peptide exerts renoprotective effects in type 1 diabetes, although the underlying mechanisms are poorly understood. As C-peptide has been shown to induce several intracellular events and to localize to nuclei, we aimed to determine whether gene transcription is affected in proximal tubular kidney cells, and if so, whether the genes with altered transcription include those related to protective mechanisms.

METHODS

The effect of C-peptide incubation (2 h) on gene expression was investigated in freshly isolated proximal tubular cells from streptozotocin-diabetic Sprague-Dawley rats using global gene expression profiling and real-time quantitative polymerase chain reaction. Protein expression was assayed using western blotting. Different bioinformatic strategies were employed.

RESULTS

Gene transcription profiling demonstrated differential transcription of 492 genes (p < 0.01) after 2 h of C-peptide exposure, with the majority of these genes repressed (83%). Real-time quantitative polymerase chain reaction validation supported a trend of several G protein-coupled receptors being activated, and certain transcription factors being repressed. Also, C-peptide repressed the transcription of genes associated with the pathways of circulatory and inflammatory diseases.

CONCLUSION

This study shows that C-peptide exerts early effects on gene transcription in proximal tubular cells. The findings also bring further knowledge to the renoprotective mechanisms of C-peptide in type 1 diabetes, and support a transcriptional activity for C-peptide. It is suggested that C-peptide may play a regulatory role in the gene expression of proximal tubular cells.