Imaging analysis of insulin secretion with two-photon microscopy

The changing view of insulin granule mobility: From conveyor belt to signaling hub

Before the advent of TIRF microscopy the fate of the insulin granule prior to secretion was deduced from biochemical investigations, electron microscopy and electrophysiological measurements. Since Calcium-triggered granule fusion is indisputably necessary to release insulin into the extracellular space, much effort was directed to the measure this event at the single granule level. This has also been the major application of the TIRF microscopy of the pancreatic beta cell when it became available about 20 years ago. To better understand the metabolic modulation of secretion, we were interested to characterize the entirety of the insulin granules which are localized in the vicinity of the plasma membrane to identify the characteristics which predispose to fusion. In this review we concentrate on how the description of granule mobility in the submembrane space has evolved as a result of progress in methodology. The granules are in a state of constant turnover with widely different periods of residence in this space. While granule fusion is associated +with prolonged residence and decreased lateral mobility, these characteristics may not only result from binding to the plasma membrane but also from binding to the cortical actin web, which is present in the immediate submembrane space. While granule age as such affects granule mobility and fusion probability, the preceding functional states of the beta cell leave their mark on these parameters, too. In summary, the submembrane granules form a highly dynamic heterogeneous population and contribute to the metabolic memory of the beta cells.

In situ graphene liquid cell-transmission electron microscopy study of insulin secretion in pancreatic islet cells

BACKGROUND

Islet cell transplantation is one of the key treatments for type 1 diabetes. Understanding the mechanisms of insulin fusion and exocytosis are of utmost importance for the improvement of the current islet cell transplantation and treatment of diabetes. These phenomena have not been fully evaluated due either to the lack of proper dynamic imaging, or the lack of proper cell preservation during imaging at nanoscales.

METHODS

By maintaining the native environment of pancreatic β-cells between two graphene monolayer sheets, we were able to monitor the subcellular events using in situ graphene liquid cell (GLC)-transmission electron microscopy (TEM) with both high temporal and high spatial resolution.

RESULTS

For the first time, the nucleation and growth of insulin particles until the later stages of fusion were imaged at nanometer scales. The release of insulin from plasma membrane involves the degradation of plasma membrane and drastic reductions in the shorter axis of the insulin particles. Sequential exocytosis results indicated the nucleation, growth and attachment of the new insulin particles to the already anchored ones, which is thermodynamically favorable due to the reduction in total surface, further reducing the Gibbs free energy. The retraction of the already anchored insulin toward the cell is also monitored for the first time live at nanoscale resolution.

CONCLUSION

Investigation of insulin granule dynamics in β-cells can be investigated via GLC-TEM. Our findings with this technology open new realms for the development of novel drugs on pathological pancreatic β-cells, because this approach facilitates observing the effects of the stimuli on the live cells and insulin granules.

Monitoring real-time hormone release kinetics via high-content 3-D imaging of compensatory endocytosis

High-content real-time imaging of hormone secretion in tissues or cell populations is a challenging task, which is unlikely to be resolved directly, despite immense translational value. We approach this problem indirectly, using compensatory endocytosis, a process that closely follows exocytosis in the cell, as a surrogate read-out for secretion. The tissue is immobilized in an open-air perifusion chamber and imaged using a two-photon microscope. A fluorescent polar tracer, perifused through the experimental circuit, gets trapped into the cells via endocytosis, and is quantified using a feature-detection algorithm. The signal of the tracer that accumulates into the endocytotic system reliably reflects stimulated exocytosis, which is demonstrated via co-imaging of the latter using existing reporters. A high signal-to-noise ratio and compatibility with multisensor imaging affords the real-time quantification of the secretion at the tissue/population level, whereas the cumulative nature of the signal allows imprinting of the "secretory history" within each cell. The technology works for several cell types, reflects disease progression and can be used for human tissue.

Video-Rate Bioluminescence Imaging of Degranulation of Mast Cells Attached to the Extracellular Matrix

Degranulation refers to the secretion of inflammatory mediators, such as histamine, serotonin, and proteases, that are stored within the granules of mast cells and that trigger allergic reactions. The amount of these released mediators has been measured biochemically using cell mass. To investigate degranulation in living single cells, fluorescence microscopy has traditionally been used to observe the disappearance of granules and the appearance of these discharged granules within the plasma membrane by membrane fusion and the movement of granules inside the cells. Here, we developed a method of video-rate bioluminescence imaging to directly detect degranulation from a single mast cell by measuring luminescence activity derived from the enzymatic reaction between Gaussia luciferase (GLase) and its substrate coelenterazine. The neuropeptide Y (NPY), which was reported to colocalize with serotonin in the secretory granules, fused to GLase (NPY-GLase) was efficiently expressed in rat basophilic leukemia (RBL-2H3) cells, a mast-cell line, using a preferred human codon-optimized gene. Bioluminescence imaging analysis of RBL-2H3 cells expressing NPY-GLase and adhered on a glass-bottomed dish showed that the luminescence signals from the resting cells were negligible, while the luminescence signals of the secreted NPY-GLase were repeatedly detected after the addition of an antigen. In addition, this imaging method was applicable for observing degranulation in RBL-2H3 cells that adhered to the extracellular matrix (ECM). These results indicated that video-rate bioluminescence imaging using GLase will be a useful tool for detecting degranulation in single mast cells adhered to a variety of ECM proteins.

Pulse-Chase Biosynthetic Radiolabeling of Pancreatic Islets to Measure Beta Cell Function

Pulse-chase radiolabeling of cells with radioactive amino acids is a common method for studying the biosynthesis of proteins. The labeled proteins can then be immunoprecipitated and analyzed by electrophoresis and gel imaging techniques. This chapter presents a protocol for the biosynthetic labeling and immunoprecipitation of pancreatic islet proteins which are known to be affected in disorders such as diabetes, obesity, and metabolic syndrome.

Novel A–(π–D–π–A)1–3 branched fluorophores displaying high two-photon absorption

Novel A–(π–D–π–A)₃ compounds bearing pyridine end groups are apparently effective in achieving large two-photon responses owing to strong charge transfer.

A Two-Photon Ratiometric Fluorescent Probe for Imaging Carboxylesterase 2 in Living Cells and Tissues

In this study, a two-photon ratiometric fluorescent probe NCEN has been designed and developed for highly selective and sensitive sensing of human carboxylesterase 2 (hCE2) based on the catalytic properties and substrate preference of hCE2. Upon addition of hCE2, the probe could be readily hydrolyzed to release 4-amino-1,8-naphthalimide (NAH), which brings remarkable red-shift in fluorescence (90 nm) spectrum. The newly developed probe exhibits good specificity, ultrahigh sensitivity, and has been successfully applied to determine the real activities of hCE2 in complex biological samples such as cell and tissue preparations. NCEN has also been used for two-photon imaging of intracellular hCE2 in living cells as well as in deep-tissues for the first time, and the results showed that the probe exhibited high ratiometric imaging resolution and deep-tissue imaging depth. All these findings suggested that this probe holds great promise for applications in bioimaging of endogenous hCE2 in living cells and in exploring the biological functions of hCE2 in complex biological systems.