Osteopontin is an autoantigen of the somatostatin cells in human islets: identification by screening random peptide libraries with sera of patients with insulin-dependent diabetes mellitus

Investigation of type 1 diabetes in NOD mice knockout for the osteopontin gene

OBJECTIVE

Type 1 diabetes onset is preceded by a pre-inflammatory stage leading to insulitis and followed by targeted destruction of the insulin-producing beta cells of the pancreas. Osteopontin (OPN) is a secreted phosphoprotein with cytokine properties, implicated in many physiological and pathological processes, including infection and autoimmunity. We have previously identified up-regulated osteopontin transcripts in the pancreatic lymph nodes of the NOD (Non-Obese Diabetic) mouse at the pre-diabetic stages. Investigating the underlined disease initiating mechanisms may well contribute to the development of novel preventive therapies. Our aim was to construct opn null mice in a NOD autoimmune-prone genetic background and address the pathogenic or protective role of the osteopontin molecule in the early stages of type 1 diabetes.

METHODS

We generated opn null mutant mice in a NOD genetic background by serial backcrossing to the existing C57BL/6 opn knockout strain. The presence of opn wild type or null alleles in the congenic lines was evaluated by PCR amplification. We used NOD opn-null mice to assess the phenotypic evolution of type 1 diabetes. The presence of OPN in the serum was evaluated by ELISA and by immunostaining on the mouse tissues. The primary gene structure of the NOD opn encoding gene and protein sequences were compared to the known alleles of other mouse strains. Evaluation of Single Nucleotide Polymorphisms (SNPs) variation between opn alleles of the opn gene is reported.

RESULTS

In the absence of OPN, type 1 diabetes is accelerated, suggesting a protective role of this cytokine on the insulin-producing cells of the pancreatic islets. Conversely, in the presence of the opn gene, an increase of the OPN protein in the serum of young NOD mice indicates that this molecule might be involved in the immune regulatory events taking place at early stages, prior to disease onset. Our data support that OPN acts as a positive regulator of the early islet autoimmune damage, possibly by a shift of the steady-state of T1D pathogenesis. We report that the OPN protein structure of the NOD/ShiLtJ strain corresponds to the a-type allele of the osteopontin gene. Comparative analysis of the single nucleotide polymorphisms between the a-type and b-type alleles indicates that the majority of variations are within the non-coding regions of the gene.

CONCLUSIONS

The construction of opn null mice in an autoimmune genetic background (NOD.B6.Cg-spp1^-/-) provides important tools for the study of the implication of the OPN in type 1 diabetes, offering the possibility to address the significance of this molecule as an early marker of the disease and as a therapeutic agent in preclinical studies.

Embracing the complexity of matricellular proteins: the functional and clinical significance of splice variation

Matricellular proteins influence wide-ranging fundamental cellular processes including cell adhesion, migration, growth and differentiation. They achieve this both through interactions with cell surface receptors and regulation of the matrix environment. Many matricellular proteins are also associated with diverse clinical disorders including cancer and diabetes. Alternative splicing is a precisely regulated process that can produce multiple isoforms with variable functions from a single gene. To date, the expression of alternate transcripts for the matricellular family has been reported for only a handful of genes. Here we analyse the evidence for alternative splicing across the matricellular family including the secreted protein acidic and rich in cysteine (SPARC), thrombospondin, tenascin and CCN families. We find that matricellular proteins have double the average number of splice variants per gene, and discuss the types of domain affected by splicing in matricellular proteins. We also review the clinical significance of alternative splicing for three specific matricellular proteins that have been relatively well characterised: osteopontin (OPN), tenascin-C (TNC) and periostin. Embracing the complexity of matricellular splice variants will be important for understanding the sometimes contradictory function of these powerful regulatory proteins, and for their effective clinical application as biomarkers and therapeutic targets.

Osteopontin Bridging Innate and Adaptive Immunity in Autoimmune Diseases

Osteopontin (OPN) regulates the immune response at multiple levels. Physiologically, it regulates the host response to infections by driving T helper (Th) polarization and acting on both innate and adaptive immunity; pathologically, it contributes to the development of immune-mediated and inflammatory diseases. In some cases, the mechanisms of these effects have been described, but many aspects of the OPN function remain elusive. This is in part ascribable to the fact that OPN is a complex molecule with several posttranslational modifications and it may act as either an immobilized protein of the extracellular matrix or a soluble cytokine or an intracytoplasmic molecule by binding to a wide variety of molecules including crystals of calcium phosphate, several cell surface receptors, and intracytoplasmic molecules. This review describes the OPN structure, isoforms, and functions and its role in regulating the crosstalk between innate and adaptive immunity in autoimmune diseases.

The Role of Osteopontin in the Pathogenesis and Complications of Type 1 Diabetes Mellitus in Children

OBJECTIVE

Type 1 diabetes mellitus (T1DM) is the most common chronic metabolic disorder of childhood and adolescence. Osteopontin plays a significant role in the development and progression of several autoimmune diseases. Moreover, osteopontin promotes adipose tissue inflammation, dysfunction, and insulin resistance. To investigate the levels of serum osteopontin in pediatric patients with T1DM and to explore if these levels have a role in the prediction of diabetes complications.

METHODS

This was a case-control study conducted at the Endocrinology unit of the Children's Hospital of Zagazig University in Egypt, from October 2014 to December 2015. Sixty patients with T1DM and 60 healthy subjects were enrolled. A detailed medical history was taken from all patients/parents. A full clinical examination including ophthalmoscopy was performed on all patients. Fasting blood glucose, hemoglobin A1c (HbA1c), urine albumin/creatinine ratio, and serum osteopontin levels were also determined in all subjects.

RESULTS

Patients with T1DM had significantly higher serum osteopontin levels compared with controls (mean ± standard deviation: 13.7±3.4 μg/L vs. 8.9±2.9 μg/L, p<0.001). Also, serum osteopontin concentrations were higher in patients with microalbuminuria than in patients with normal albumin excretion rate and in the control group. Similarly, those who had retinal disease had higher osteopontin concentrations than those without (16.8±2 vs. 12.4±3 mg/L; p=0.005). Serum osteopontin levels correlated with a diagnosis of T1DM, and in diabetic patients, correlated with higher systolic and diastolic blood pressure, body mass index values and with lower high density lipoprotein values, diagnosis of retinopathy, and microalbuminuria. No correlation was found between osteopontin levels and HbA1c, insulin dose, co-medications, and diabetes duration in T1DM patients. The association between high osteopontin levels and T1DM was independent from all confounders.

CONCLUSION

This study shows that increased osteopontin levels are independently associated with T1DM in pediatric patients and supports the hypothesis that osteopontin may have a role in the prediction of microvascular diabetes complications.

Investigation of osteopontin levels and genomic variation of osteopontin and its receptors in Type 1 diabetes mellitus

BACKGROUND

Failure in self-tolerance towards β-cells in diabetes mellitus (DM) pathogenesis involves a series of complex events that are governed by environmental and genetic factors. Considering the importance of osteopontin (OPN) in T-helper-1 (Th1) cells development, the aim of this study was to evaluate the serum level and gene polymorphism of OPN in Iranian Type 1 diabetic (T1DM) children.

METHODS

In this case-control study, 87 T1DM children and 86 healthy ones were enrolled. Blood samples of both groups were checked for OPN level. The single nucleotide polymorphisms (SNP) were genotyped by RFLP analysis for OPN rs1126772, its receptor integrin α4 (ITGA4) rs 1449263, and CD44 rs8193.

RESULTS

Serum levels of OPN in diabetic children were significantly higher in cases compared to the control group (p=0.023), but there was no significant relationship between OPN rs1126772 (p=0.79), its receptor integrin α4 (p=0.31), and CD44 rs8193 (p=0.45), and T1DM.

CONCLUSION

Higher amounts of OPN were seen in T1DM children. It is assumed that OPN might have inducing effects on T1DM development, particularly when genetically susceptible individuals are predisposed by an environmental insult. However, the 3 SNPs of OPN and its receptors did not show noticeable association with T1DM. The power of our study (~19%) was insufficient to observe any significant statistical difference between the groups; moreover, this study does not exclude the possibility of association of other SNPs of OPN and its receptors with this disease, and more studies are needed to clarify the issue.

An update on the use of NOD mice to study autoimmune (Type 1) diabetes

The widely used nonobese diabetic (NOD) mouse model of autoimmune (Type 1) diabetes mellitus shares multiple characteristics with the human disease, and studies employing this model continue to yield clinically relevant and important information. Here, we review some of the recent key findings obtained from NOD mouse investigations that have both advanced our understanding of disease pathogenesis and suggested new therapeutic targets and approaches. Areas discussed include antigen discovery, identification of genes and pathways contributing to disease susceptibility, development of strategies to image islet inflammation and the testing of therapeutics. We also review recent technical advances that, combined with an improved understanding of the NOD mouse model's limitations, should work to ensure its popularity, utility and relevance in the years ahead.

The immunohistochemical expression profile of osteopontin in normal human tissues using two site-specific antibodies reveals a wide distribution of positive cells and extensive expression in the central and peripheral nervous systems

To elucidate the cellular distribution of osteopontin (OPN) in normal human tissues, we undertook immunohistochemistry using two site-specific OPN antibodies. The 10A16 monoclonal antibody was raised against the amino acid sequence just downstream of the thrombin cleavage site, while the O-17 polyclonal antibody was raised against the N-terminal peptide. Each antibody has been confirmed previously to react with both whole OPN and its relevant fragments. The expression pattern for these two antibodies was similar in distribution. In addition, we also identified expression in Ebner's gland, type II pneumocytes, Kupffer cells, cells of the endocrine organs, anterior lens capsule and ciliary body, synovial type A cells, mesothelia, adipocytes, and mast cells. Neurons and glia in the central nervous system and spinal cord, cranial and peripheral nerve sheaths, ganglion cells in the sympathetic ganglion, intestinal plexuses, retina, and choroid plexus also regularly exhibited OPN positivity. Testicular germ cells, pancreatic exocrine cells, and follicular dendritic cells reacted with 10A16 only, whereas lutein cells and taste bud cells exhibited O-17 reactivity alone. These minor differences were hypothesized to reflect the state of OPN in the cells; that is, whether OPN was in its whole molecule or fragmented form. In conclusion, we demonstrate that OPN is widely distributed in normal human cells, particularly those comprising the central and peripheral nervous systems.

Unravelling autoimmune pathogenesis by screening random peptide libraries with human sera

The incidence of autoimmunity is increasing worldwide. The long preclinical period of autoimmune disorders is characterised by an enhanced exposure over time of autoreactive T cells to an increased number of autoantigenic determinants and autoantibodies production. The discovery of novel autoimmune-disease related epitopes is a task that remains extremely challenging in order to establish predictive and preventive strategies of the disease onset. In this Opinion article we highlight the contribution of screening combinatorial random peptide libraries with patients sera in unravelling the etiopathogenesis of autoimmunity.

Association of alleles at polymorphic sites in the Osteopontin encoding gene in young type 1 diabetic patients

The Osteopontin (OPN) encoding gene, SPP1, can be considered as a candidate for genetic susceptibility to type 1 diabetes (T1D) because of its known function in immune response and inflammation. This work aimed to evaluate the role of SPP1 gene in susceptibility to T1D. Patients (238: 130 male, 108 female) and unaffected adult control individuals (137: 68 males and 69 females) have been genotyped for three variants in the SPP1 gene: -156 (G/GG) and -66 (T/G) in the promoter and a biallelic ins/del variant (TG/TGTG) at +245 in the first intron. The G allele at the -66 SNP had significantly higher frequency in controls than T1D patients. Interestingly, case-control comparison in males showed no significant association, whereas the association was confirmed in females. These results suggest that SPP1 can play a role as susceptibility gene, possibly by a sex-specific mechanism acting in the autoimmune process.

Osteopontin is a novel marker of pancreatic ductal tissues and of undifferentiated pancreatic precursors in mice

Matricellular proteins mediate both tissue morphogenesis and tissue homeostasis in important ways because they modulate cell-matrix and cell-cell interactions. In this study, we found that the matricellular protein osteopontin (Opn) is a novel marker of undifferentiated pancreatic precursors and pancreatic ductal tissues in mice. Our analysis also underscored a specific, dynamic profile of Opn expression in embryonic pancreatic tissues that suggests the participation of this protein's function in processes involving cell migration, cell-cell interactions, or both. Surprisingly, our analysis of Opn-deficient pancreata did not reveal obvious alterations in the morphology or differentiation of these tissues. Therefore, in embryonic pancreatic tissues, it is possible that other proteins act redundantly to Opn or that this protein's function is dispensable for pancreas development. Finally, the maintenance of Opn expression in pancreatic tissues of adults argues for a possible function of this protein in injury and pathologic responses.

Importin beta: a novel autoantigen in human autoimmunity identified by screening random peptide libraries on phage

By screening random peptide libraries (RPLs) with sera of Type 1 diabetes (T1D) patients, we previously identified 5 disease-specific 'mimotopes' displayed on phages (phagotopes). We already characterised 1 phagotope (CH1p), as an epitope of human osteopontin, an autoantigen expressed within the somatostatin cells of human islets. In this paper, we report the characterization of the second phagotope, 195Dyn, by immunohistochemistry, Western Blotting and screening of a human islet cDNA library using rabbit anti-195Dyn antibodies. The 195Dyn mimotope was detected in human islets. The screening of a lambdagt11 cDNA library from human islets has identified a clone, which corresponded to human importin beta. ELISA detected autoantibodies against this protein in sera of around 60% of TD1 patients and in 30% of patients affected by other autoimmune diseases. In summary, RPLs technology proved again successful in identifying another novel autoantigen (importin beta), whose significance in the autoimmune process remains to be fully elucidated.

Alphav-integrin utilization in human beta-cell adhesion, spreading, and motility

The role of individual integrins in human beta-cell development and function is largely unknown. This study describes the contribution of alpha(v)-integrins to human beta-cell adhesion, spreading, and motility. Developmental differences in alpha(v)-integrin utilization are addressed by comparing the responses of adult and fetal beta-cells, and vitronectin is used as a substrate based on its unique pattern of expression in the developing pancreas. Fetal and adult beta-cells attached equally to vitronectin and integrin alpha(v)beta(5) was found to support the adhesion of both mature and immature beta-cell populations. Fetal beta-cells were also observed to spread and migrate on vitronectin, and integrin alpha(v)beta(1) was found to be essential for these responses. In contrast to their fetal counterparts, adult beta-cells failed to either spread or migrate and this deficit was associated with a marked down-regulation of alpha(v)beta(1) expression in adult islet preparations. The integrin alpha(v)beta(3) was not found to support significant beta-cell attachment or migration. Based on our findings, we conclude that integrins alpha(v)beta(5) and alpha(v)beta(1) are important mediators of human beta-cell adhesion and motility, respectively. By supporting fetal beta-cell migration, alpha(v)beta(1) could play an important role in early motile processes required for islet neogenesis.

A mimotope of pre-S2 region of surface antigen of viral hepatitis B screened by phage display

To acquire the phage-displayed mimotopes which mimic the specificity of hepatitis B virus surface antigen (HBsAg), a random peptide library expressing linear peptide with 12 amino acids in length were used to screen with the serum from a hepatitis B virus infected patient in the recovery phase. After 3 rounds of biopanning, the positive phages were confirmed by competitive ELISA using HBsAg/P33. Two phagotopes were identified and one of them was confirmed as mimotope by competition experiment. Based on the mimotpe, a multiple antigenic peptide with four branches was synthesized by solid phase peptide synthesis. The antiginicity and specificity of the synthesized antigen was tested in BALB/c mice compared with the native epitope-based antigen. The results showed that the mimotope-based antigen could evoke higher titer of antibodies with the same specificity of the epitope-based antigen. Those findings indicate mimotopes can be used in antigen and vaccine design.

Random-peptide libraries and antigen-fragment libraries for epitope mapping and the development of vaccines and diagnostics

Random peptide libraries and antigen-fragment libraries (also known as gene-fragment libraries) have been used to identify epitopes on protein antigens. These technologies promise to make significant contributions to diagnostic and vaccine development. Researchers in a number of labs have shown that phage selected from libraries with protective antibodies, raised against whole antigen, can be used as immunogens to stimulate antibody responses that bind native antigen and provide protection in vivo. Others have used the sera of patients with idiopathic diseases to screen libraries, and by this approach have identified candidate antigens involved in immune disease. These may prove useful for diagnosis and, possibly, in determining disease etiology.