Browsing by Author "Timonen, Martina"

Exercise-induced hyperinsulinism (EIHI) is a pathological condition characterized by aberrant insulin secretion triggered by physical exercise or pyruvate exposure. The monocarboxylate transporter protein (MCT1), encoded by SLC16A1, is ubiquitously expressed in almost all cell types except pancreatic islet cells. In patients with EIHI, mutations in the regulatory regions of the SLC16A1 gene are thought to lead to the unwanted expression of MCT1 on the beta cell membrane, allowing the influx of elevated lactate and pyruvate blood levels during exercise. These substrates feed into the Krebs cycle, increasing insulin release. This excessive insulin secretion can lead to hypoglycemia during exercise, causing weakness, syncope, and confusion. Since EIHI has never been studied using a human stem cell-derived islets, this thesis aims to establish a robust model in which to investigate the disease mechanism in detail. To achieve this, we reprogrammed EIHI patients’ fibroblasts into a stable pluripotent state and further differentiated them into functional pancreatic stem cell-derived islets (SC-islets) in vitro. These SC-islets were then matured further in vivo (in immunocompromised mice) and compared to healthy SC-islet controls. Rigorous quality control measures were implemented throughout the differentiation process to ensure its efficacy and the expression regulation of SLC16A1 was studied during SC-islet development. Extensive phenotypic characterization was conducted using immunohistochemistry, quantitative gene expression level analysis, and insulin secretion assays with glucose and pyruvate. Contrary to expectations, the results of this study demonstrated that despite the SLC16A1 promoter mutation, the expression of SLC16A1 was downregulated similarly to the control cell line during development in vitro, resulting in similar pyruvate-stimulated insulin secretion to the control cells. Interestingly, immunohistochemical analysis of in vivo implanted SC-islets showed a clear phenotype with an increased number of MCT1-positive cells only in the mutant grafts, some of which were endocrine cells. In conclusion, the phenotypic manifestations of EIHI were not visible in the setting of in vitro modeling, which was attributed to the similar expression levels of MCT1 in both the mutant and control cell lines. However, following in vivo implantation, there was a noticeable increase in MCT1 expression exclusively in the mutant cells. This finding suggests a distinct regulatory mechanism of MCT1 expression, which might be impacted by the in vivo surroundings and the maturation state of human islets.