Imaging beta cell regeneration and interactions with islet vasculature in transparent adult zebrafish

β-cells regeneration by WL15 of cysteine and glycine-rich protein 2 which reduces alloxan induced β-cell dysfunction and oxidative stress through phosphoenolpyruvate carboxykinase and insulin pathway in zebrafish in-vivo larval model

BACKGROUND

Pancreatic β-cells are susceptible to oxidative stress, leading to β-cell death and dysfunction due to enhanced ROS levels and type 2 diabetes. To inhibit the β-cells damages induced by the oxidative stress, the present study investigates the beneficial effect of various peptides (WL15, RF13, RW20, IW13 and MF18) of immune related proteins (cysteine and glycine-rich protein 2, histone acetyltransferase, vacuolar protein sorting associated protein 26B, serine threonine-protein kinase and CxxC zinc finger protein, respectively). Also, the molecular mechanism of WL15 from cysteine and glycine-rich protein 2 on β-cell regeneration was identified through PEPCK and insulin pathway.

MATERIALS AND METHODS

In this study, a total of five peptides including WL15, RF13, RW20, IW13, and MF18 were derived from immune-related proteins such as cysteine and glycine-rich protein 2, histone acetyltransferase, vacuolar protein sorting associated protein 26B, serine threonine-protein kinase and CxxC zinc finger protein, respectively. These protein sequences were obtained from an earlier constructed transcriptome database of a teleost Channa striatus. The identified peptides were evaluated for their antioxidant as well as antidiabetic activity. Based on the in silico analysis and in-vitro screening experiments, WL15 was predicted to have better antioxidant and antidiabetic activity among the five different peptides. Therefore, WL15 alone was further analyzed for apoptosis, antioxidant capacity, glucose metabolism, and gene expression performance, which was investigated on the alloxan (500 µM) induced zebrafish in vivo larval model.

RESULTS

The results showed alloxan exposure to zebrafish larvae for a day, the ROS was generated in the β-cells. Interestingly, WL15 treatment showed a protective effect by reducing the toxicity of alloxan exposed zebrafish larvae by increasing their survival and heart rate. Moreover, WL15 reduced the intracellular ROS level and apoptosis in alloxan-induced larvae. The superoxide anion and lipid peroxidation levels are also reduced by improving the glutathione content after the WL15 treatment. Besides, WL15 treatment increased the proliferation rate of β-cells and decreased the glucose level. Further, the gene expression studies revealed that WL15 treatment normalized the PEPCK expression while upregulating the insulin expression in alloxan exposed larvae.

CONCLUSION

Overall, the findings indicate that WL15 of cysteine and glycine-rich protein 2 can act as a potential antioxidant for type 2 diabetes patients in respect of improving β-cell regeneration.

Microtubule organization of vertebrate sensory neurons in vivo

Dorsal root ganglion (DRG) neurons are the predominant cell type that innervates the vertebrate skin. They are typically described as pseudounipolar cells that have central and peripheral axons branching from a single root exiting the cell body. The peripheral axon travels within a nerve to the skin, where free sensory endings can emerge and branch into an arbor that receives and integrates information. In some immature vertebrates, DRG neurons are preceded by Rohon-Beard (RB) neurons. While the sensory endings of RB and DRG neurons function like dendrites, we use live imaging in zebrafish to show that they have axonal plus-end-out microtubule polarity at all stages of maturity. Moreover, we show both cell types have central and peripheral axons with plus-end-out polarity. Surprisingly, in DRG neurons these emerge separately from the cell body, and most cells never acquire the signature pseudounipolar morphology. Like another recently characterized cell type that has multiple plus-end-out neurites, ganglion cells in Nematostella, RB and DRG neurons maintain a somatic microtubule organizing center even when mature. In summary, we characterize key cellular and subcellular features of vertebrate sensory neurons as a foundation for understanding their function and maintenance.

Vegfa/vegfr2 signaling is necessary for zebrafish islet vessel development, but is dispensable for beta-cell and alpha-cell formation

The mechanisms underlying zebrafish pancreatic islet vascularization have not been well characterized. We sought to determine the angiogenic factors responsible for islet vascularization and assess whether an absence of endothelial cells affects beta-cell and alpha-cell formation. We used a double transgenic zebrafish Tg(fli1:EGFP; insa:tagRFP) to label endothelial cells and beta-cells, respectively. Beta-cells developed adjacent to endothelial cells and by 72 hours post fertilization (hpf) the zebrafish pancreatic islet was highly vascularized. Zebrafish beta-cells express vascular endothelial growth factors (vegf), vegfaa and vegfab. Double knockdown of vegfaa and vegfab or the primary Vegfa receptors (Vegfr2), kdr and kdrl, resulted in vessel deficient islets. While beta-cell and alpha-cell numbers remained unchanged in vessel deficient islets, insulina expression was downregulated relative to controls. Vegfaa/Vegfab-Vegfr2 signaling is necessary for proper islet vessel development, but not for the initial formation of beta-cells and alpha-cells.

Sweet fish: Fish models for the study of hyperglycemia and diabetes

Fish are good for your health in more ways than you may expect. For one, eating fish is a common dietary recommendation for a healthy diet. However, fish have much more to provide than omega-3 fatty acids to your circulatory system. Some fish species now serve as important and innovative model systems for diabetes research, providing novel and unique advantages compared with classical research models. Not surprisingly, the largest share of diabetes research in fish occurs in the laboratory workhorse among fish, the zebrafish (Danio rerio). Established as a genetic model system to study development, these small cyprinid fish have eventually conquered almost every scientific discipline and, over the past decade, have emerged as an important model system for metabolic diseases, including diabetes mellitus. In this review we highlight the practicability of using zebrafish to study diabetes and hyperglycemia, and summarize some of the recent research and breakthroughs made using this model. Equally exciting is the appearance of another emerging discipline, one that is taking advantage of evolution by studying cases of naturally occurring insulin resistance in fish species. We briefly discuss two such models in this review, namely the rainbow trout (Oncorhynchus mykiss) and the cavefish (Astyanax mexicanus).

In vivo monitoring of intracellular Ca2+ dynamics in the pancreatic β-cells of zebrafish embryos

Assessing the response of pancreatic islet cells to glucose stimulation is important for understanding β-cell function. Zebrafish are a promising model for studies of metabolism in general, including stimulus-secretion coupling in the pancreas. We used transgenic zebrafish embryos expressing a genetically-encoded Ca²⁺ sensor in pancreatic β-cells to monitor a key step in glucose induced insulin secretion; the elevations of intracellular [Ca²⁺]_i. In vivo and ex vivo analyses of [Ca²⁺]_i demonstrate that β-cell responsiveness to glucose is well established in late embryogenesis and that embryonic β-cells also respond to free fatty acid and amino acid challenges. In vivo imaging of whole embryos further shows that indirect glucose administration, for example by yolk injection, results in a slow and asynchronous induction of β-cell [Ca²⁺]_i responses, while intravenous glucose injections cause immediate and islet-wide synchronized [Ca²⁺]_i fluctuations. Finally, we demonstrate that embryos with disrupted mutation of the Ca_V1.2 channel gene cacna1c are hyperglycemic and that this phenotype is associated with glucose-independent [Ca²⁺]_i fluctuation in β-cells. The data reveal a novel central role of cacna1c in β-cell specific stimulus-secretion coupling in zebrafish and demonstrate that the novel approach we propose - to monitor the [Ca²⁺]_i dynamics in embryonic β-cells in vivo - will help to expand the understanding of β-cell physiological functions in healthy and diseased states.

LITTLE FISH, BIG DATA: ZEBRAFISH AS A MODEL FOR CARDIOVASCULAR AND METABOLIC DISEASE

The burden of cardiovascular and metabolic diseases worldwide is staggering. The emergence of systems approaches in biology promises new therapies, faster and cheaper diagnostics, and personalized medicine. However, a profound understanding of pathogenic mechanisms at the cellular and molecular levels remains a fundamental requirement for discovery and therapeutics. Animal models of human disease are cornerstones of drug discovery as they allow identification of novel pharmacological targets by linking gene function with pathogenesis. The zebrafish model has been used for decades to study development and pathophysiology. More than ever, the specific strengths of the zebrafish model make it a prime partner in an age of discovery transformed by big-data approaches to genomics and disease. Zebrafish share a largely conserved physiology and anatomy with mammals. They allow a wide range of genetic manipulations, including the latest genome engineering approaches. They can be bred and studied with remarkable speed, enabling a range of large-scale phenotypic screens. Finally, zebrafish demonstrate an impressive regenerative capacity scientists hope to unlock in humans. Here, we provide a comprehensive guide on applications of zebrafish to investigate cardiovascular and metabolic diseases. We delineate advantages and limitations of zebrafish models of human disease and summarize their most significant contributions to understanding disease progression to date.

Expression and function of ATP-dependent potassium channels in zebrafish islet β-cells

ATP-sensitive potassium channels (KATP channels) are critical nutrient sensors in many mammalian tissues. In the pancreas, KATP channels are essential for coupling glucose metabolism to insulin secretion. While orthologous genes for many components of metabolism-secretion coupling in mammals are present in lower vertebrates, their expression, functionality and ultimate impact on body glucose homeostasis are unclear. In this paper, we demonstrate that zebrafish islet β-cells express functional KATP channels of similar subunit composition, structure and metabolic sensitivity to their mammalian counterparts. We further show that pharmacological activation of native zebrafish KATP using diazoxide, a specific KATP channel opener, is sufficient to disturb glucose tolerance in adult zebrafish. That β-cell KATP channel expression and function are conserved between zebrafish and mammals illustrates the evolutionary conservation of islet metabolic sensing from fish to humans, and lends relevance to the use of zebrafish to model islet glucose sensing and diseases of membrane excitability such as neonatal diabetes.

Assessment of Toxicological Perturbations and Variants of Pancreatic Islet Development in the Zebrafish Model

The pancreatic islets, largely comprised of insulin-producing beta cells, play a critical role in endocrine signaling and glucose homeostasis. Because they have low levels of antioxidant defenses and a high perfusion rate, the endocrine islets may be a highly susceptible target tissue of chemical exposures. However, this endpoint, as well as the integrity of the surrounding exocrine pancreas, is often overlooked in studies of developmental toxicology. Disruption of development by toxicants can alter cell fate and migration, resulting in structural alterations that are difficult to detect in mammalian embryo systems, but that are easily observed in the zebrafish embryo model (Danio rerio). Using endogenously expressed fluorescent protein markers for developing zebrafish beta cells and exocrine pancreas tissue, we documented differences in islet area and incidence rates of islet morphological variants in zebrafish embryos between 48 and 96 h post fertilization (hpf), raised under control conditions commonly used in embryotoxicity assays. We identified critical windows for chemical exposures during which increased incidences of endocrine pancreas abnormalities were observed following exposure to cyclopamine (2–12 hpf), Mono-2-ethylhexyl phthalate (MEHP) (3–48 hpf), and Perfluorooctanesulfonic acid (PFOS) (3–48 hpf). Both islet area and length of the exocrine pancreas were sensitive to oxidative stress from exposure to the oxidant tert-butyl hydroperoxide during a highly proliferative critical window (72 hpf). Finally, pancreatic dysmorphogenesis following developmental exposures is discussed with respect to human disease.