Impact of ageing on mitochondrial complex expression and islet cell composition in mtDNA mutator mice and human pancreas

Abstract

Age-related cumulative mitochondrial mutations can cause mitochondrial dysfunction. The mitochondrial DNA mutator mouse (PolgAmut/mut) is a model of premature ageing and has been previously shown to develop impaired insulin secretion with age. Normal human ageing has been associated with islet β-cell dysfunction and the change of islet cell composition. The overall aim was to explore the impact of ageing on mitochondrial complex expression and islet cell composition in PolgAmut/mut mice and human pancreas. Immunofluorescence was used to study mitochondrial respiratory chain protein expression (complex I and IV) by normalizing to mitochondrial mass marker (Tomm20). This technique was also applied to the study of islet cell composition with proliferation marker Ki67. Experiments were conducted on pancreas tissue from PolgAmut/mut mice and age-matched wild type (WT) mice at two ages: 3 months (young) and 11 months(old). Some of the PolgAmut/mut mice underwent exercise training from 4 months of age. Non-diabetic normal human pancreas tissue from two age groups: 20~40 years (young) and 60~80 years (old) were also studied for mitochondrial complex expression and islet cell composition. The findings of the first three result chapters described the impact of ageing in mtDNA mutator mice model. Complex I expression was decreased in islets from young PolgAmut/mut mice, and it persisted with age in comparison with the WT mice. Investigation of islet cell composition identified an increase in absolute α-cell number with age in the PolgAmut/mut mice that contributed to a decreased β:α ratio in islets from the old PolgAmut/mut mice versus the age-matched WT mice. Ki67 examination showed that increased α-cell proliferation contributed to the increase in α-cell mass. Islet cell subtype analysis in old PolgAmut/mut mice revealed that the complex I deficiency was greater in the α-cells compared with the β-cells, suggesting that the increased α-cell proliferation observed in old PolgAmut/mut mouse islets maybe a direct response to complex I deficiency. Although none of these mice developed diabetes with age, decreased insulin expression was discovered in the pancreatic islets of the old PolgAmut/mut mice. Endurance exercise did not have significant impact on mitochondrial complex expression nor islet cell composition in old PolgAmut/mut mice. The last result chapter explored the impact of ageing in human pancreatic tissues from non-diabetic donors. Tomm20 expression was higher in pancreatic islets from old versus young donors, but there was no difference in complex I and IV expression. Whole islet cell number and absolute β-cell number were lower in the old versus young islets, but there was no significant difference in the β:α ratio. Taken together, my findings show that

mitochondrial dysfunction alters pancreatic islet cell composition in response to ageing in the PolgAmut/mut mice. In non-diabetic human islets, there is no evidence of altered mitochondrial subunit expression but Tomm20 expression was increased in the islets from old donors appears to reflect a compensatory increase in mitochondrial mass.