Dysfunctional pancreatic beta-cells are crucial in the pathophysiology of diabetes. Based on light microscopy, diabetic cats have reduced number of beta-cells but whether secretory granules and mitochondria, which are central to cellular function and viability, have abnormal morphology is unknown. Furthermore, a recent investigation questioned the role of islet amyloid in the pathogenesis of diabetes in cats since its amount did not differ between diabetic and age-matched control cats. However, intracellular aggregation of amylin into oligomers, rather than extracellular amyloid, may be the principle of beta-cell toxicity in type 2 diabetes. Therefore, the aims of the study were to characterize ultrastructural lesions of beta-cells in diabetic cats with emphasis on granules, mitochondria and intracellular amylin aggregation.

Pancreases of diabetic and control cats euthanized for any disease were prospectively collected. Samples were harvested within 1 hour from death and glutaraldehyde-fixed. Control cats were selected to be matched for age, sex and body weight. Sections were prepared for electron microscopy and immunogold labelling by using anti-insulin and anti-amylin antibodies. Beta-cell morphology was assessed, including beta-cell granule area, eccentricity (minimum-to-maximum diameter ratio), granule number, as well as beta-cell mitochondrial area, number and inter-cristae distance. Intracellular accumulation of amylin-positive material outside the granules was explored. Non-parametric tests were used for statistical analysis.

Five diabetic cats and 5 controls were included. Diabetic cats had beta-cells with clearer cytoplasm than control cats. The median area of beta-cell granules was higher in diabetic than controls [0.144 µm² (0.047–0.252) vs. 0.035 µm² (range: 0.027–0.046); p<0.05] and their median number lower [0.5/µm² (0.3–2.8) vs. 5.6/µm² (range: 3.9–7.8); p<0.05]; eccentricity did not differ. The median inter-cristae distance of mitochondria was higher in diabetic than controls [0.064 µm (0.052–0.075) vs. 0.034 µm (range: 0.032–0.043); p<0.05]; their area and number did not differ. Mitochondria of diabetic cats appeared swollen. Amylin-positive material outside the granules was observed in the cytoplasm of 3 diabetic cats and in none of the controls.

In conclusion, the reduced number of granules might suggest that the remaining pool of beta-cells in diabetic cats is less functional or exhausted by the excessive workload. The reason behind the larger granule size is uncertain. The increased inter-cristae distance of mitochondria in diabetic cats, as well as their swollen appearance, might suggest increased permeability and loss of function. The appearance of amylin-positive material outside of granules suggests intracellular aggregation of amylin monomers to oligomers, which may possibly contribute to beta-cell death.

Disclosures

Disclosures to report:

Claudia Reusch was consultant for Boehringer Ingelheim and Novartis Animal Health and is currently consultant for Dechra Limited. She has received financial support for her endocrine research from various companies such as Nestlé Purina, Hills, Provet, Antlia SA, Glycemicon and from the clinical studies fund of the ECVIM-CA and from the Society of Comparative Endocrinology.