Methods for the detection of reactive oxygen species employed in the identification of plant photosensitizers

Extracts of Trichocline sinuata (Asteraceae) as natural sensitizers in the photodynamic inactivation of Candida albicans

Despite significant progress in the development of phototherapy drugs, it is widely recognized that natural products remain the primary source of new photoactive compounds. Exploring uncharted flora in the east-central region of Argentina may offer a vast array of opportunities to isolate new photoactive molecules or plant extracts with high potential for use in antimicrobial photodynamic therapy (aPDT) against Candida albicans. To assess the photofungicidal potential of T. sinuata ("contrayerba") against C. albicans, the extracts underwent spectroscopic and chromatographic analysis, resulting in the identification of furanocoumarin metabolites with similar spectrophotometric properties in all extracts. The extract profiles were created using UHPLC-DAD, and seven furanocoumarins (FCs) were detected. The highest photoinactivation against C. albicans was observed for dicholormethanic extracts (MFC = 62.5 μg/mL), equal to xanthotoxin employed as a positive control. Furthermore, we determine that photochemical mechanisms dependent on oxygen (type I and type II processes) and mechanisms independent of oxygen (photoadduct formation) are involved in the death of these yeasts. These results support the use of native plants of the east-central region of Argentina as potent sensitizers for aPDT and suggest that they can replace xanthotoxin in treating superficial yeast infections of the skin.

Ascorbate peroxidase 1 allows monitoring of cytosolic accumulation of effector-triggered reactive oxygen species using a luminol-based assay

Biphasic production of reactive oxygen species (ROS) has been observed in plants treated with avirulent bacterial strains. The first transient peak corresponds to pattern-triggered immunity (PTI)-ROS, whereas the second long-lasting peak corresponds to effector-triggered immunity (ETI)-ROS. PTI-ROS are produced in the apoplast by plasma membrane-localized NADPH oxidases, and the recognition of an avirulent effector increases the PTI-ROS regulatory module, leading to ETI-ROS accumulation in the apoplast. However, how apoplastic ETI-ROS signaling is relayed to the cytosol is still unknown. Here, we found that in the absence of cytosolic ascorbate peroxidase 1 (APX1), the second phase of ETI-ROS accumulation was undetectable in Arabidopsis (Arabidopsis thaliana) using luminol-based assays. In addition to being a scavenger of cytosolic H2O2, we discovered that APX1 served as a catalyst in this chemiluminescence ROS assay by employing luminol as an electron donor. A horseradish peroxidase (HRP)-mimicking APX1 mutation (APX1W41F) further enhanced its catalytic activity toward luminol, whereas an HRP-dead APX1 mutation (APX1R38H) reduced its luminol oxidation activity. The cytosolic localization of APX1 implies that ETI-ROS might accumulate in the cytosol. When ROS were detected using a fluorescent dye, green fluorescence was observed in the cytosol 6 h after infiltration with an avirulent bacterial strain. Collectively, these results indicate that ETI-ROS eventually accumulate in the cytosol, and cytosolic APX1 catalyzes luminol oxidation and allows monitoring of the kinetics of ETI-ROS in the cytosol. Our study provides important insights into the spatial dynamics of ROS accumulation in plant immunity.

Hydrogen sulfide decreases photodynamic therapy outcome through the modulation of the cellular redox state

Photodynamic therapy (PDT) is a non-surgical treatment that has been approved for its human medical use in many cancers. PDT involves the interaction of a photosensitizer (PS) with light. The amino acid 5- aminolevulinic acid (ALA) can be used as a pro-PS, leading to the synthesis of Protoporphyrin IX. Hydrogen sulfide (H₂S) is an endogenously produced gas that belongs to the gasotransmitter family, which can diffuse through biological membranes and have relevant physiological effects such as cardiovascular functions, vasodilatation, inflammation, cell cycle and neuro-modulation. It was also proposed to have cytoprotective effects. We aimed to study the modulatory effects of H₂S on ALAPDT in the mammary adenocarcinoma cell line LM2. Exposure of the cells to NaHS (donor of H₂S) in concentrations up to 10 mM impaired the response to ALA-PDT in a dose-dependent manner. The addition of 3 doses of NaHS showed the highest effect. This decreased response to the photodynamic treatment was correlated to an increase in the GSH levels, catalase activity, a dose dependent reduction of PpIX and increased intracellular ALA, decreased levels of oxidized proteins and a decrease of PDT-induced ROS. NaHS also reduced the levels of singlet oxygen in an in vitro assay. H₂S also protected other cells of different origins against PDT mediated by ALA and other PSs. These results suggest that H₂S has a role in the modulation of the redox state of the cells, and thus impairs the response to ALA-PDT through multifactor pathways. These findings could contribute to developing new strategies to improve the effectiveness of PDT particularly mediated by ALA or other ROS-related treatments.

Insights Into the Detection Selectivity of Redox and Non-redox Based Probes for the Superoxide Anion Using Coumarin and Chromone as the Fluorophores

In this study, we evaluated the applicability of various superoxide anion sensors which were designed based on either redox or non-redox mechanisms. Firstly, both redox- and non-redox-based superoxide anion probes were designed and synthesized using either coumarin or chromone as the fluorophores, and the photophysical properties of these probes were measured. Subsequently, the sensing preference of both types of probes toward various reactive oxygen species (ROS) was evaluated. We found that non-redox-based O2 •- probes exhibited broad sensing ability toward various ROS. By contrast, redox based O2 •- probes showed a clear reactivity hierarchy which was well correlated to the oxidizing strength of the ROS. Lastly, the detection selectivity of redox-based O2 •- recognizing probes was also observed when balancing various factors, such as reactant ROS concentrations, temperature, and changing reaction transformation rates. Herein, we concluded the selectivity advantage of redox-based O2 •- probes.

Determining Essential Requirements for Fluorophore Selection in Various Fluorescence Applications Taking Advantage of Diverse Structure-Fluorescence Information of Chromone Derivatives

Herein, we report our work exploring the essential requirements for fluorophore selection during the development of various fluorescence applications. We assembled a library of chromone-derived fluorophores with diverse structure-fluorescence properties, which allowed us to choose the fluorophore pairs with similar structures but differing fluorescence properties and compared the performance of the selected fluorophore pairs in three types of commonly used fluorescence applications. We found that the selection standard of a suitable fluorophore is variable depending on the application. (1) In fluorescence imaging, fluorophores with strong and constant fluorescence under various conditions, such as a large pH range, are preferred. Notably, (2) in the detection of bioactive species, fluorophores with relatively lower fluorescence quantum yield favor the detection sensitivity. Furthermore, (3) in enzymatic assays employing fluorescence, the key parameter is the binding affinity between the fluorophore and the enzyme.

The photoactivity of natural products - An overlooked potential of phytomedicines?

BACKGROUND

Photoactivity, though known for centuries, is only recently shifting back into focus as a treatment option against cancer and microbial infections. The external factor light is the ingenious key-component of this therapy: Since light activates the drug locally, a high level of selectivity is reached and side effects are avoided. The first reported photoactive medicines were plant extracts. Synthetic entities (so-called photosensitizers PSs), however, paved the route towards the clinical approval of the so-called photodynamic therapy (PDT), and thus natural PSs took a backseat in the past.

HYPOTHESIS

Many isolated bioactive phytochemicals hold a hidden photoactive potential, which is overlooked due to the reduced common awareness of photoactivity.

METHODS

A systematic review of reported natural PSs and their supposed medicinal application was conducted by employing PubMed, Scifinder, and Web of Science. The identified photoactive natural products were compiled including information about their natural sources, their photoyield, and their pharmacological application. Furthermore, the common chemical scaffolds of natural PS are shown to enable the reader to recognize potentially overlooked natural PSs.

RESULTS

The literature review revealed over 100 natural PS, excluding porphyrins. The PSs were classified according to their scaffold. Thereby it was shown that some PS-scaffolds were analyzed in a detailed way, while other classes were only scarcely investigated, which leaves space for future discoveries. In addition, the literature revealed that many PSs are phytoalexins, thus the selection of the starting material significantly matters in order to find new PSs.

CONCLUSION

Photoactive principles are ubiquitous and can be found in various plant extracts. With the increasing availability of light-irradiation setups for the identification of photoactive natural products, we anticipate the discovery of many new natural PSs in the near future. With the accumulation of chemically diverse PSs, PDT itself might finally reach its clinical breakthrough as a promising alternative treatment against multi-resistant microbes and cancer types.