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This protocol demonstrates the accurate and reproducible measurement of oxygen consumption in non-human primate pancreatic islets. The islet loading techniques and coating of the microplate provide a framework for efficient measurement of respiration in other types of cultured spheroids.
The measurement of oxygen consumption in spheroid clusters of cells, such as ex vivo pancreatic islets, has historically been challenging. We demonstrate the measurement of islet oxygen consumption using a 96-well microplate designed for the measurement of oxygen consumption in spheroids. In this assay, spheroid microplates are coated with a cell and tissue adhesive on the day prior to the assay. We utilize a small volume of adhesive solution to encourage islet adherence to only the bottom of the well. On the day of the assay, 15 islets are loaded directly into the base of each well using a technique that ensures optimal positioning of islets and accurate measurement of oxygen consumption. Various aspects of mitochondrial respiration are probed pharmacologically in non-human primate islets, including ATP-dependent respiration, maximal respiration, and proton leak. This method allows for consistent, reproducible results using only a small number of islets per well. It can theoretically be applied to any cultured spheroids of similar size.
In order to maintain normal blood glucose levels, the pancreatic β cell must sense elevations in glucose and secrete insulin accordingly. The coupling of insulin secretion with glucose levels is directly linked to glucose metabolism and the production of ATP through mitochondrial oxidative phosphorylation. Thus, mitochondria play a critical role in stimulus-secretion coupling1. Assessing β-cell mitochondrial function can reveal defects that lead to impaired insulin secretion. The secretion of glucagon by pancreatic α cells is also closely tied to mitochondrial function2. Although immortalized islet cell lines have proven useful for some types of assays, the physiology of these cells does not accurately recapitulate whole islet function, as illustrated by the potentiation of insulin secretion by glucagon3,4 and the inhibition of glucagon secretion by insulin/somatostatin5,6 in intact islets. This demonstrates the need for measuring oxygen consumption using whole, intact islets.
Techniques for the measurement of islet cell respirometry have evolved over time, from the use of oxygen-sensitive fluorescent dyes7 to solid-state sensors that directly measure oxygen consumption8. Initially designed for monolayer, adherent cells, commonly used cell culture plate systems have proven to be ineffective for pancreatic islets. As islets do not naturally adhere to the wells, they are prone to being pushed to the periphery of the culture well resulting in inaccurate measurement of oxygen consumption9. To combat this problem, specialized 24-well plates with a central depression that could contain islets were developed9. However, the 24-well plate system was limited by the large number of islets required (50-80 per well) and the number of conditions that could be tested simultaneously10. The recent development of 96-well microplates designed specifically for extracellular flux analysis in spheroids has overcome these barriers, enabling the measurement of islet respirometry with 20 or fewer islets per well10.
Here, we demonstrate the use of this system to measure oxygen consumption in islets from the Japanese macaque (Macaca fuscata), an animal model with similar islet biology to humans11,12. In this protocol, 15 macaque islets are analyzed per well. In our hands, 15 islets per well produced higher baseline oxygen consumption than fewer islets, with robust activation and repression of respiration in response to pharmacologic manipulation. We highlight the steps to prepare for the assay, an effective method for consistent loading of islets at the center of each well, and common challenges when performing this assay.
1. Preparation of Microplate and Sensor Cartridge on the Day Prior to Running the Assay
Islets were isolated from three year old Japanese macaques as previously described13. This method is very similar to that used to isolate human islets from cadaver donors, but differs from mice, in which pancreata are often inflated with collagenase solution while the animal is under sedation and prior to organ removal. Islet retrieval was conducted in accordance with the guidelines of the Institutional Animal Care and Use Committee (IACUC) of the Oregon National Primate Research Center (ONPRC) and Oregon Health and Science University and were approved by the ONPRC IACUC. The ONPRC abides by the Animal Welfare Act and Regulations enforced by the United States Department of Agriculture (USDA) and the Public Health Service Policy on Humane Care and Use of Laboratory Animals in accordance with the Guide for the Care and Use of Laboratory Animals published by the National Institutes of Health.
2. Protocol for Media Preparation, Loading of Islets, and Loading of Sensor Cartridge on the Day of Assay
To load islets into microplate, 15 islets should be aspirated in 15 µL of media, as shown in Figure 1A. Islets will naturally settle toward the bottom of the pipet tip within a few seconds. Then, the pipet tip is lowered to the bottom of the well. The tip is very slightly lifted, and a small volume (about 5 µL) is pipetted out along with the islets. This technique results in consistent placement of islet at the bottom of the micropl...
The study of islet oxygen consumption has previously been hampered by the spherical shape of islets, their lack of adherence to culture surfaces, and the number of islets required per well. In this protocol, we highlight the efficacy of the 96-well spheroid microplate for measuring islet oxygen consumption on a small number of islets and demonstrate a technique for handling and loading islets which is technically feasible and produces consistent results.
In order for islets to adhere to the bo...
The authors have nothing to disclose.
The authors would like to acknowledge the Vanderbilt High Throughput Screening Core for the use of their facilities, Agilent Biotechnologies, Dr. Paul Kievit (Oregon Health and Science University) for non-human primate islet isolations, and Eric Donahue (Vanderbilt University) for assistance with Figure 1. J.M.E. was supported by NIGMS of the National Institutes of Health under award number T32GM007347. M.G. was supported by the NIH/NIDDK (R24DK090964-06) and the Department of Veterans Affairs (BX003744).
Name | Company | Catalog Number | Comments |
Cell culture dish, 60 mm X 15 mm style | Corning | 430166 | |
Cell-Tak Cell and Tissue Adhesive | Corning | 354240 | |
Conical tube, 50 mL | Falcon | 352070 | |
Dextrose anhydrous | Fisher Scientific | BP350-1 | For glucose solution, 200 mg/ml, sterile filetered |
Disposable reservoirs (sterile), 25 ML | Vistalab | 3054-1033 | for loading multichannel pipet |
EZFlow Sterile 0.45 μm PES Syringe Filter, 13 mm | Foxx Life Sciences | 371-3115-OEM | |
L-glutamine | Gibco | 25030-081 | 200 mM (100x) |
Multichannel pipette tips | ThermoFisher Scientific | 94410810 | |
Multichannel pipette, 15-1250 μL | ThermoFisher Scientific | 4672100BT | Recommended |
P20, P200, and P1000 pipettes | Eppendorf | 2231000602 | |
pH Probe | Hanna Instruments | HI2210-01 | |
Pipette tips, 20 μL, 200 μL, 1000 μL | Olympus | 24-404, 24-412, 24-430 | |
Seahorse XF Base Media | Agilent | 103334-100 | |
Seahorse XF Cell Mito Stress Test Kit | Agilent | 103015-100 | Includes Oligomycin, FCCP, and Rotenone/Antimycin A |
Seahorse XFe96 Analyzer | Agilent | S7800B | Including prep station with 37 °C non-CO2 incubator |
Seahorse XFe96 Spheroid Fluxpak Mini | Agilent | 102905-100 | Includes sensor cartridge, spheroid microplate, and calibrant |
Sodium bicarbonate | Fisher Scientific | BP328-500 | |
Sodium pyruvate | Gibco | 11360-070 | 100 mM (100x) |
Stereo Microscope | Olympus | SZX9 | |
Syringe (sterile), 5 mL | BD | 309603 | For sterile filtration |
Water (sterile) | Sigma | W3500-500mL |
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