Correlative imaging reveals metal dyshomeostasis and altered zinc coordination environments in a pre-clinical Type 2 diabetes model

Zinc ions are highly abundant in pancreatic islet tissue, and multiple lines of evidence link loss of zinc homeostasis to poor glucose regulation in both type 1 and type 2 diabetes. Two major islet zinc-binding proteins, insulin and metallothionein, play crucial roles in beta cell function and glucose regulation. Here we used X-ray fluorescence microscopy (XFM) to map zinc and five additional elements (Cl, K, Ca, Fe, and Cu) to compare the metallome of exocrine, peri-islet and islet regions in young and old, non-diabetic control and diabetic (db/db) mice. We also determined the main forms of zinc found in pancreatic tissue using X-ray absorption near-edge structure (XANES) spectroscopic imaging. This allowed investigation of the relationship between zinc speciation and its protein ligands using correlative immunofluorescent imaging to assess whether zinc coordination may play a role in diabetes pathology. The anticipated depletion of zinc in young diabetic islets was accompanied by a significant decrease in insulin expression and increase in metallothionein expression. A parallel change in the contribution of cysteine vs histidine zinc speciation was also observed. Counter-intuitively, zinc abundance and speciation appeared to normalise in old diabetic animals with more advanced disease, despite large differences in labile zinc-binding protein content. These results are consistent with disrupted zinc coordination, where metallothionein-regulated muffling to minimise ionic activity is overwhelmed and zinc binds to unidentified ligands in histidine-like conformations. This opens future study questions focussed on the complex interplay between labile zinc, metallothionein, and oxidative mechanisms that may interfere with normal zinc homeostasis.