Oxidative stress is specifically defined as a condition that results from an unbalanced cellular redox state. The complex redox reactions that routinely occur inside cells determine the cellular redox-state. Redox reactions consist of all chemical reactions that consist in the transfer of electrons between atoms of biological molecules producing reduction and oxidation of molecules (i.e. redox reactions). These reactions are catalyzed by electronically activated species (i.e. pro-oxidative species), which are characterized by an extreme structural instability and spontaneous activation of unbalanced electrons that exchange with neighboring biomolecules. These irregular reactions result into DNA damage, protein carboxylation, and lipid oxidation, and eventually lead to cell death1. Increased levels of oxidative stress have been associated with aging and the progression of different pathological states2. Oxidative stress has been reported to be responsible for vascular alterations in diabetes and cardiovascular diseases3,4. It also plays a critical role in neuronal degeneration in Alzheimer's disease and Parkinson's disease5. Moreover, oxidative stress has been demonstrated as a critical factor in governing cancer progression and metastatic events6,7. In addition, inflammation and immune responses may elicit and further support oxidative stress8.
In living cells, pro-oxidative species are derived from oxygen (ROS; reactive oxygen species) or nitrogen (RNS; reactive nitrogen species). ROS include the hydroxyl radical (.OH), the superoxide anion (O2-), and the hydrogen peroxide (H2O2). The primary RNS is nitrous oxide (NO.). A series of secondary reactive species can be generated by spontaneous interactions between ROS and RNS or free metals ions9. For example, the superoxide anion reacts with nitrous oxide to form peroxynitrate (ONOO-), while H2O2 reacting with Fe2+ generates hydroxyl radicals. ROS and RNS, due to their ability to react with several biomolecules, are considered a dangerous threat for the maintenance of the physiological redox state10. To maintain the redox state cells are equipped with a series of detoxifying anti-oxidant molecules and enzymes. The superoxide dismutase (SOD), Catalase, Glutathione peroxidase and Peroxiredoxins essentially constitute the anti-oxidant enzymatic-arsenal that provides cellular protection from pro-oxidative species including H2O2 , .OH and OONO- 11. Also anti-oxidant molecules like vitamin C and E, polyphenols and CoenzymeQ10 (CoQ10) are of critical importance to quench ROS and their dangerous derivatives12,13. However, an excessive production of ROS and RNS, or a dysfunction in the anti-oxidant system, shifts the cellular redox-state toward oxidative stress14.
Besides their negative connotation, ROS can play various physiological roles in cells of different origin. Cells normally produce ROS as signaling molecules to mediate normal biological events such as host defense and wound repair15-17. Reactive species are normally produced in cells by intracellular enzymes such as NOX (NADPH Oxidase) and XO (Xantine Oxidase) in response to signaling factors, growth factors, and intracellular fluctuations of calcium levels18,19. It has been reported that ROS may differentially modulate the activity of important nuclear factors such as p53 or cellular components such as the ATM-kinase, a master regulator of the response to DNA damage20. Analogously ROS strongly influence cellular signaling by mediating the oxidation and inactivation of protein tyrosine phosphatases (PTPs), which are established as critical regulators of signal transduction21. Moreover, proteomic based methodologies demonstrate that RNS are also responsible for specific protein modifications and alterations of molecular signaling. RNS react with the cysteine thiol groups modifying them into S-nitrothiols (SNO) and triggering molecular pathways concomitant with pathological states such as inflammatory and autoimmune diseases22,23.
Since cell culture experiments only partially reproduce the multitude of factors acting in vivo, it is of great interest to perform redox studies in animal models24,25. To achieve this, the zebrafish has been considered a suitable vertebrate animal model to study oxidative stress dynamics26. The zebrafish is a new model system that grants several advantages to study cellular and genetic events during vertebrate development and disease. Large clusters of embryos can be generated and available weekly for experimental needs. Moreover the extraordinary optical clarity of zebrafish embryos, as well their small size, enables single cell imaging and dynamic tracking in a whole organisms27. In the last decade, a considerable number of zebrafish mutants have been generated to model human pathological conditions such as cancer and genetic diseases28-31. Most importantly, a multitude of transgenic lines has been produced to allow extensive opportunities of genetic and biological manipulations32. For example, transgenic tissue-specific zebrafish lines are regularly utilized for in vivo studies. These lines express a fluorescent protein under the control of a selected promoter, offering the ability to identify single cells in vivo, as well as the anatomical structure they comprise.
Several toxicological studies have already used the zebrafish to evaluate the in vivo effect of chemicals on redox homeostasis, suggesting the suitability of this vertebrate as an animal model for the field of drug discovery and oxidative stress33-35. Even though some fluorescent probes have been tested to monitor oxidative stress in zebrafish larvae36,37, there are no established assays to detect and measure the levels of oxidative stress in zebrafish tissues and living cells. Here we describe a procedure for in vivo quantification of oxidative stress in living cells of zebrafish embryos. Imaging tools, FACS sorting, fluorescent probes and pro-oxidative conditions are all combined to generate a simple assay for the detection and quantification of oxidative species in zebrafish embryos and tissues.