Mapping the β-Cell in 3D at the Nanoscale Using Novel Cellular Electron Tomography and Computational Approaches

The three-dimensional (3D) clusters of highly specialized cells known as the “islets of Langerhans” (“islets”) which are distributed throughout the pancreas collectively serve as the coordinately regulated tissue that is commonly referred to as the “endocrine pancreas”. In mammals, the so-called β-cells that predominate within each islet are solely responsible for the regulated biosynthesis and release of the hormone insulin into the bloodstream to maintain blood glucose homeostasis and whole body metabolism at normal/healthy levels. Despite extensive imaging -based investigations of the β-cell at both the light microscope (LM) and electron microscope (EM) levels over the past four decades – and, more recently, using state-of-the-art magnetic resonance (MR) and positron emission tomography (PET) techniques to image pancreatic islets both in vivo and in vitro – many fundamental questions still remain unanswered regarding how structure–function relationships change in the β-cell under different physiological conditions and how such changes contribute to the onset/progression of β-cell dysfunction and/or death. To address these key questions, we have pioneered a number of novel approaches based on the imaging method known as “electron tomography” (ET) to computationally reconstruct and analyze large cellular volumes (“tomograms” ) for immortalized insulin-secreting cell lines as well as β-cells imaged in situ in fast-frozen pancreatic islets. High (∼5 nm) resolution 3D reconstructions of the Golgi region and whole cell tomograms of islet β-cells reconstructed at intermediate (∼10–20 nm) resolution have allowed us to quantitatively map the organization and morphometry of key organelles/compartments of the insulin pathway as well as the microtubule cytoskeleton in 3D at the ultrastructural level. New computational tools developed in parallel that afford significant improvements in terms of both the speed and the accuracy with which islet β-cells can be reconstructed and annotated in toto in 3D now provide a powerful “whole cell mapping” approach for undertaking comparative structure–function studies of islet β-cells at the nanoscale and stand poised to provide unique insights into the β-cell as a model for the study of complex systems biology in situ in 3D in an appropriate physiological and spatial context.