Using exomarkers to assess mitochondrial reactive species in vivo

Background: The ability to measure the concentrations of small damaging and signalling molecules such as reactive oxygen species (ROS) in vivo is essential to understanding their biological roles. While a range of methods can be applied to in vitro systems, measuring the levels and relative changes in reactive species in vivo is challenging. Scope of review: One approach towards achieving this goal is the use of exomarkers. In this, exogenous probe compounds are administered to the intact organism and are then transformed by the reactive molecules in vivo to produce a diagnostic exomarker. The exomarker and the precursor probe can be analysed ex vivo to infer the identity and amounts of the reactive species present in vivo. This is akin to the measurement of biomarkers produced by the interaction of reactive species with endogenous biomolecules. Major conclusions and general significance: Our laboratories have developed mitochondria-targeted probes that generate exomarkers that can be analysed ex vivo by mass spectrometry to assess levels of reactive species within mitochondria in vivo. We have used one of these compounds, MitoB, to infer the levels of mitochondrial hydrogen peroxide within flies and mice. Here we describe the development of MitoB and expand on this example to discuss how better probes and exomarkers can be developed. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn. Abbreviations: EPR, electron paramagnetic resonance; GFP, green fluorescent protein; 4-HNE, 4-hydroxynonenal; MitoB, 3-(dihydroxyboronyl)benzyltriphenylphosphonium bromide; MitoP, (3-hydroxybenzyl)triphenylphosphonium bromide; ROS, reactive oxygen species; SOD, superoxide dismutase; TPMP, methyltriphenylphosphonium; TPP, triphenylphosphonium catio

Measurement of H₂O₂ within living drosophila during aging using a ratiometric mass spectrometry probe targeted to the mitochondrial matrix

Hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) is central to mitochondrial oxidative damage and redox signaling, but its roles are poorly understood due to the difficulty of measuring mitochondrial H<sub>2</sub>O<sub>2</sub> in vivo. Here we report a ratiometric mass spectrometry probe approach to assess mitochondrial matrix H<sub>2</sub>O<sub>2</sub> levels in vivo. The probe, MitoB, comprises a triphenylphosphonium (TPP) cation driving its accumulation within mitochondria, conjugated to an arylboronic acid that reacts with H<sub>2</sub>O<sub>2</sub> to form a phenol, MitoP. Quantifying the MitoP/MitoB ratio by liquid chromatography-tandem mass spectrometry enabled measurement of a weighted average of mitochondrial H<sub>2</sub>O<sub>2</sub> that predominantly reports on thoracic muscle mitochondria within living flies. There was an increase in mitochondrial H<sub>2</sub>O<sub>2</sub> with age in flies, which was not coordinately altered by interventions that modulated life span. Our findings provide approaches to investigate mitochondrial ROS in vivo and suggest that while an increase in overall mitochondrial H<sub>2</sub>O<sub>2</sub> correlates with aging, it may not be causative

Insights on targeting small molecules to the mitochondrial matrix and the preparation of MitoB and MitoP as exomarkers of mitochondrial hydrogen peroxide

Small molecules can be physicochemically targeted to the mitochondrial matrix using the lipophilic alkyltriphenylphosphonium (TPP) group. Once in the mitochondria the TPP conjugate can detect or influence processes within the mitochondrial matrix directly. Alternatively, the conjugate can behave as a prodrug, which is activated by release from the TPP group either using an internal or external instruction. Small molecules can be designed that can be used in any cell line, tissue, or whole organism, allow for temporal control, and can be applied in a reversible dose-dependent fashion. An example is the detection and quantification of hydrogen peroxide in mitochondria of whole living organisms by MitoB. Hydrogen peroxide produced within the mitochondrial matrix is involved in signaling and implicated in the oxidative damage associated with aging and a wide range of conditions including cardiovascular disease, neurodegeneration, and cancer. MitoB accumulates in mitochondria and is converted into the exomarker, MitoP, by hydrogen peroxide in the mitochondrial matrix. The hydrogen peroxide concentration is determined from the ratio of MitoP to MitoB after a period of incubation, and this ratio is determined by mass spectrometry using d15-MitoP and d15-MitoB as internal standards. Here we discuss the targeting of small molecules to the mitochondrial matrix using TPP, and describe the synthesis of MitoB and MitoP and the deuterated standards necessary for quantification of hydrogen peroxide in the mitochondrial matrix of whole living organisms

Caged mitochondrial uncouplers that are released in response to hydrogen peroxide

Caged versions of the most common mitochondrial uncouplers (proton translocators) have been prepared that sense the reactive oxygen species (ROS) hydrogen peroxide to release the uncouplers 2,4-dinitrophenol (DNP) and carbonylcyanide p-trifluoromethoxyphenylhydrazone (FCCP) from caged states with second order rate constants of 10 (±0.8) M−1 s−1 and 64.8 (±0.6) M−1 s−1, respectively. The trigger mechanism involves conversion of an arylboronate into a phenol followed by fragmentation. Hydrogen peroxide-activated uncouplers may be useful for studying the biological process of ageing

Presence of donor-encoded centromeric KIR B content increases the risk of infectious mortality in recipients of myeloablative, T-cell deplete, HLA-matched HCT to treat AML.

The reported influence of donor Killer-cell Immunoglobulin-like Receptor (KIR) genes on the outcomes of haematopoietic cell transplantation (HCT) are contradictory, in part due to diversity of disease, donor sources, era and conditioning regimens within and between different studies. Here, we describe the results of a retrospective clinical analysis establishing the effect of donor KIR motifs on the outcomes of 119 HLA-matched, unrelated donor HCT for adult acute myeloid leukaemia (AML) using myeloablative conditioning (MAC) in a predominantly T-cell deplete (TCD) cohort. We observed that HCT involving donors with at least one KIR B haplotype were more likely to result in non-relapse mortality (NRM) than HCT involving donors with two KIR A haplotypes (p = 0.019). Upon separation of KIR haplotypes into their centromeric (Cen) and telomeric (Tel) motif structures, we demonstrated that the Cen-B motif was largely responsible for this effect (p = 0.001). When the cause of NRM was investigated further, infection was the dominant cause of death (p = 0.006). No evidence correlating donor KIR B haplotype with relapse risk was observed. The results from this analysis confirm previous findings in the unrelated, TCD, MAC transplant setting and imply a protective role for donor-encoded Cen-A motifs against infection in allogeneic HCT recipients

Nitrones for understanding and ameliorating the oxidative stress associated with aging

Oxidative damage from reactive oxygen species (ROS) and the carbon-centred radicals arising from them is important to the process of aging, and age-related diseases are generally caused, exacerbated or mediated by oxidative stress. Nitrones can act as spin traps to detect, identify, quantify and locate the radicals responsible using electron paramagnetic resonance (EPR or ESR) spectroscopy, and a new carnitine-derived nitrone, CarnDOD-7C, designed to accumulate in mitochondria is reported. Nitrones also have potential as therapeutic antioxidants, e.g. for slowing cellular aging, and as tools for chemical biology. Two low-molecular weight nitrones, DIPEGN-2 and DIPEGN-3, are reported, which combine high water-solubility with high lipophilicity and obey Lipinski's rule of five

Using the mitochondria-targeted ratiometric mass spectrometry probe MitoB to measure H₂O₂ in living <i>Drosophila</i>

The role of hydrogen peroxide (H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;) in mitochondrial oxidative damage and redox signaling is poorly understood, because it is difficult to measure H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt; in vivo. Here we describe a method for assessing changes in H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt; within the mitochondrial matrix of living &lt;i&gt;Drosophila&lt;/i&gt;. We use a ratiometric mass spectrometry probe, MitoB ((3-hydroxybenzyl)triphenylphosphonium bromide), which contains a triphenylphosphonium cation component that drives its accumulation within mitochondria. The arylboronic moiety of MitoB reacts with H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt; to form a phenol product, MitoP. On injection into the fly, MitoB is rapidly taken up by mitochondria and the extent of its conversion to MitoP enables the quantification of H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;. To assess MitoB conversion to MitoP, the compounds are extracted and the MitoP/MitoB ratio is quantified by liquid chromatography–tandem mass spectrometry relative to deuterated internal standards. This method facilitates the investigation of mitochondrial H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt; in fly models of pathology and metabolic alteration, and it can also be extended to assess mitochondrial H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt; production in mouse and cell culture studies