Specific dynamic and noninvasive labeling of pancreatic beta cells in reporter mice

Noninvasive detection of differentiated cells is increasingly demanded for accurate and reliable assessments of both in vitro and in vivo experimental systems. Here we present an efficient, innovative approach for imaging the beta cells of the pancreatic islets of Langerhans. The main physiologic function of beta cells is glucose-stimulated insulin secretion. This function is facilitated through the synthesis and storage of insulin in secretory vesicles of beta cells, which then release their contents when beta cells are exposed to hyperglycemic conditions. To visualize beta cells in vivo in the mouse, we used targeted mutagenesis techniques to construct a modified insulin II (InsII) gene allele, InsII(EGFP), that expresses a proinsulin-EGFP (enhanced green fluorescent protein) fusion peptide. The EGFP portion of this fusion is entirely within the C-peptide portion of the proinsulin peptide. This fusion protein is processed in beta cells to insulin and EGFP-tagged C peptide, which are stored together in cytoplasmic secretory vesicles. The large amount of vesicular EGFP-tagged C peptide is evident as a characteristic robust and specific fluorescence pattern in the beta cells of InsII(EGFP) mice. This innovative method of visualizing beta cells will be a useful tool in the study of both beta cell physiology and the development of the endocrine cells of the pancreas.