Development of a molecular approach for the monitoring and treatment of diabetic patients

Diabetes is one of the major causes of premature illness and death worldwide due to the severe long-term complications, such as cardiovascular disease, visual disability, limb amputation, renal failure, and neuropathy. The main pathological hallmark of diabetes mellitus consists in a progressive decline of the pancreatic beta cell mass (PBCM), which is responsible of the insufficient insulin secretion and hyperglycemia1. No reliable method is available nowadays to measure non-invasively the PBCM in diabetic patients. On the other hand, adipokines play an important role in the pathophysiology of obesity-related disorders, including diabetes and metabolic syndrome, due to their ability to regulate inflammatory and metabolic processes. Adiponectin is an adipokine, a protein hormone secreted in the blood circulation by the adipose tissue. It interacts with two main receptors: AdipoR1, mostly expressed in skeletal muscles, and AdipoR2, mostly expressed in the liver.2. 3 In diabetic and obese patients, adiponectin blood levels are decreased, while its replenishment has an anti-diabetic effect by improving insulin sensitivity and cell survival, including that of PBC.3, 4
The first research strategy is thus focused on the development of a PBC-targeted imaging probe, which will allow the quantification of PBCM by molecular imaging, the monitoring of its decrease during the pathological process and to adopt novel therapeutic strategies. In this last context, the second part of the present work is dedicated to the identification of a therapeutic peptide able to modulate the activity of adiponectin receptors. Our therapeutic agent would act as an adiponectin agonist, replacing its diminished endogen production in diabetic patients. The improved PBC survival produced by this therapeutic agent could be evaluated by the PBC-targeted imaging probe.
The first part of our work aimed to validate in vitro an imaging probe (USPIO-P88) able to target the PBC5 by interaction with a specific biomarker, the protein FXYD2-gamma-a6. The imaging probe was developed by coupling to ultrasmall particles of iron oxide (USPIO) a peptide (P88) that is specific to FXYD2-gamma-a, previously discovered by phage display in our laboratory. Aiming to develop a therapeutic agent, the AdipoR agonist has been searched within a combinatorial library of random peptides that was screened against a C-terminal fragment of AdipoR (AdipoR-9C) that is homologous in both receptors in humans and mice.
P88 does not bind to the exocrine pancreas and is able to detect by MRI down to ~156 human pancreatic islets/mm3 after conjugation to USPIO. On human histological sections, USPIO-P88 is specific to PBC, but not to duodenum, stomach or kidney tissues.
Two peptides have been identified as promising potential agonists of both AdipoR receptors. They have shown a good affinity, efficacy and specificity for AdipoR-9C, as well as for human AdipoR1. The histological tests performed on control mouse liver and muscle are promising by corroborating the affinity for the tissue-expressed AdipoR. These peptides are being evaluated in vitro on hepatic cells, seeking to confirm their expected pharmacological effects. Our preliminary data have shown that both peptides are able to stimulate up to twice the AMPK phosphorylation, a key enzyme in the cellular energy homeostasis.
To conclude, USPIO-P88 and AdipoR-targeted peptide agonists represent novel and promising tools both for treatment and monitoring of PBCM in diabetic patients.

1. Cnop Met al. Diabetes 2005; 54 (Suppl. 2): S97-S107.
2. Maury E et al. Mol Cell Endocrinol 2010;314(1): 1-16.
3. Kadowaki T & Yamauchi T. Endocrine Reviews 2005; 26: 439-451.
4. Lee YH, Magkos F, Mantzoros CS, Kang ES. Metabolism. 2011; 60(12): 1664-1672.
5. Burtea C, Laurent S, Muller RN, Sermeus C, Eizirik D, EP2537859A1/26-12-2012.
6. Flamez D et al. Diabetologia 2010;53:1372-1383.