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

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.

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.

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.

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.