Probing the light scattering properties of insulin secretory granules in single live cells

β-Cell Pathophysiology: A Review of Advanced Optical Microscopy Applications

β-cells convert glucose (input) resulting in the controlled release of insulin (output), which in turn has the role to maintain glucose homeostasis. β-cell function is regulated by a complex interplay between the metabolic processing of the input, its transformation into second-messenger signals, and final mobilization of insulin-containing granules towards secretion of the output. Failure at any level in this process marks β-cell dysfunction in diabetes, thus making β-cells obvious potential targets for therapeutic purposes. Addressing quantitatively β-cell (dys)function at the molecular level in living samples requires probing simultaneously the spatial and temporal dimensions at the proper resolution. To this aim, an increasing amount of research efforts are exploiting the potentiality of biophysical techniques. In particular, using excitation light in the visible/infrared range, a number of optical-microscopy-based approaches have been tailored to the study of β-cell-(dys)function at the molecular level, either in label-free mode (i.e., exploiting intrinsic autofluorescence of cells) or by the use of organic/genetically-encoded fluorescent probes. Here, relevant examples from the literature are reviewed and discussed. Based on this, new potential lines of development in the field are drawn.

Strategies to improve insulin delivery through oral route: A review

Diabetes mellitus is found to be among the most suffered and lethal diseases for mankind. Diabetes mellitus type-1 is caused by the demolition of pancreatic islets responsible for the secretion of insulin. Insulin is the peptide hormone (anabolic] that regulates the metabolism of carbohydrates, fats, and proteins. Upon the breakdown of the natural process of metabolism, the condition leads to hyperglycemia (increased blood glucose levels]. Hyperglycemia demands outsourcing of insulin. The subcutaneous route was found to be the most stable route of insulin administration but faces patient compliance problems. Oral Insulin delivery systems are the patient-centered and innovative novel drug delivery system, eliminating the pain caused by the subcutaneous route of administration. Insulin comes in contact across various barriers in the gastrointestinal tract, which has been discussed in detail in this review. The review describes about the different bioengineered formulations, including microcarriers, nanocarriers, Self-Microemulsifying drug delivery systems (SMEDDs), Self-Nanoemulsifying drug delivery systems (SNEDDs), polymeric micelles, cochleates, etc. Surface modification of the carriers is also possible by developing ligand anchored bioconjugates. A study on evaluation has shown that the carrier systems facilitate drug encapsulation without tampering the properties of insulin. Carrier-mediated transport by the use of natural, semi-synthetic, and synthetic polymers have shown efficient results in drug delivery by protecting insulin from harmful environment. This makes the formulation readily acceptable for a variety of populations. The present review focuses on the properties, barriers present in the GI tract, overcome the barriers, strategies to formulate oral insulin formulation by enhancing the stability and bioavailability of insulin.

Pro-Inflammatory Cytokines Induce Insulin and Glucagon Double Positive Human Islet Cells That Are Resistant to Apoptosis

The presence of islet cells double positive for insulin and glucagon (Ins⁺/Glu⁺) has been described in the pancreas from both type 2 (T2D) and type 1 (T1D) diabetic subjects. We studied the role of pro-inflammatory cytokines on the occurrence, trajectory, and characteristics of Ins⁺/Glu⁺ cells in human pancreatic islets. Pancreas samples, isolated islets, and dispersed islet cells from 3 T1D and 11 non-diabetic (ND) multi-organ donors were studied by immunofluorescence, confocal microscopy, and/or electron microscopy. ND islet cells were exposed to interleukin-1β and interferon-γ for up to 120 h. In T1D islets, we confirmed an increased prevalence of Ins⁺/Glu⁺ cells. Cytokine-exposed islets showed a progressive increase of Ins⁺/Glu⁺ cells that represented around 50% of endocrine cells after 120h. Concomitantly, cells expressing insulin granules only decreased significantly over time, whereas those containing only glucagon granules remained stable. Interestingly, Ins⁺/Glu⁺ cells were less prone to cytokine-induced apoptosis than cells containing only insulin. Cytokine-exposed islets showed down-regulation of β-cell identity genes. In conclusion, pro-inflammatory cytokines induce Ins⁺/Glu⁺ cells in human islets, possibly due to a switch from a β- to a β-/α-cell phenotype. These Ins⁺/Glu⁺ cells appear to be resistant to cytokine-induced apoptosis.

Metabolic response of Insulinoma 1E cells to glucose stimulation studied by fluorescence lifetime imaging

A cascade of highly regulated biochemical processes connects glucose stimulation to insulin secretion in specialized cells of mammalian pancreas, the β-cells. Given the importance of this process for systemic glucose homeostasis, noninvasive and fast strategies capable to monitor the response to glucose in living cells are highly desirable. Here, we use the phasor-based approach to Fluorescence Lifetime IMaging (FLIM) microscopy to quantify the ratio between protein-bound and free Nicotinamide adenine dinucleotide (phosphate) species in their reduced form (NAD(P)H), and the Insulinoma cell line INS-1E as a β-like cellular model. Phasor-FLIM analysis shows that the bound/free ratio of NAD(P)H species increases upon pulsed glucose stimulation. Such response is impaired by 48-hours preincubation of cells under hyperglycemic conditions. Phasor-FLIM concomitantly monitors the appearance of long-lifetime species (LLS) as characteristic products of hyperglycemia-induced oxidative stress.

Insulin secretory granules labelled with phogrin-fluorescent proteins show alterations in size, mobility and responsiveness to glucose stimulation in living β-cells

The intracellular life of insulin secretory granules (ISGs) from biogenesis to secretion depends on their structural (e.g. size) and dynamic (e.g. diffusivity, mode of motion) properties. Thus, it would be useful to have rapid and robust measurements of such parameters in living β-cells. To provide such measurements, we have developed a fast spatiotemporal fluctuation spectroscopy. We calculate an imaging-derived Mean Squared Displacement (iMSD), which simultaneously provides the size, average diffusivity, and anomalous coefficient of ISGs, without the need to extract individual trajectories. Clustering of structural and dynamic quantities in a multidimensional parametric space defines the ISGs’ properties for different conditions. First, we create a reference using INS-1E cells expressing proinsulin fused to a fluorescent protein (FP) under basal culture conditions and validate our analysis by testing well-established stimuli, such as glucose intake, cytoskeleton disruption, or cholesterol overload. After, we investigate the effect of FP-tagged ISG protein markers on the structural and dynamic properties of the granule. While iMSD analysis produces similar results for most of the lumenal markers, the transmembrane marker phogrin-FP shows a clearly altered result. Phogrin overexpression induces a substantial granule enlargement and higher mobility, together with a partial de-polymerization of the actin cytoskeleton, and reduced cell responsiveness to glucose stimulation. Our data suggest a more careful interpretation of many previous ISG-based reports in living β-cells. The presented data pave the way to high-throughput cell-based screening of ISG structure and dynamics under various physiological and pathological conditions.