Lateral-flow immunoassay for detecting drug-induced inhibition of mitochondrial DNA replication and mtDNA-encoded protein synthesis

Reductions in Hydrogen Sulfide and Changes in Mitochondrial Quality Control Proteins Are Evident in the Early Phases of the Corneally Kindled Mouse Model of Epilepsy

Epilepsy is a heterogenous neurological disorder characterized by recurrent unprovoked seizures, mitochondrial stress, and neurodegeneration. Hydrogen sulfide (H₂S) is a gasotransmitter that promotes mitochondrial function and biogenesis, elicits neuromodulation and neuroprotection, and may acutely suppress seizures. A major gap in knowledge remains in understanding the role of mitochondrial dysfunction and progressive changes in H₂S levels following acute seizures or during epileptogenesis. We thus sought to quantify changes in H₂S and its methylated metabolite (MeSH) via LC-MS/MS following acute maximal electroshock and 6 Hz 44 mA seizures in mice, as well as in the early phases of the corneally kindled mouse model of chronic seizures. Plasma H₂S was acutely reduced after a maximal electroshock seizure. H₂S or MeSH levels and expressions of related genes in whole brain homogenates from corneally kindled mice were not altered. However, plasma H₂S levels were significantly lower during kindling, but not after established kindling. Moreover, we demonstrated a time-dependent increase in expression of mitochondrial membrane integrity-related proteins, OPA1, MFN2, Drp1, and Mff during kindling, which did not correlate with changes in gene expression. Taken together, short-term reductions in plasma H₂S could be a novel biomarker for seizures. Future studies should further define the role of H₂S and mitochondrial stress in epilepsy.

Toxicology Strategies for Drug Discovery: Present and Future

Attrition due to nonclinical safety represents a major issue for the productivity of pharmaceutical research and development (R&D) organizations, especially during the compound optimization stages of drug discovery and the early stages of clinical development. Focusing on decreasing nonclinical safety-related attrition is not a new concept, and various approaches have been experimented with over the last two decades. Front-loading testing funnels in Discovery with in vitro toxicity assays designed to rapidly identify unfavorable molecules was the approach adopted by most pharmaceutical R&D organizations a few years ago. However, this approach has also a non-negligible opportunity cost. Hence, significant refinements to the "fail early, fail often" paradigm have been proposed recently to reflect the complexity of accurately categorizing compounds with early data points without taking into account other important contextual aspects, in particular efficacious systemic and tissue exposures. This review provides an overview of toxicology approaches and models that can be used in pharmaceutical Discovery at the series/lead identification and lead optimization stages to guide and inform chemistry efforts, as well as a personal view on how to best use them to meet nonclinical safety-related attrition objectives consistent with a sustainable pharmaceutical R&D model. The scope of this review is limited to small molecules, as large molecules are associated with challenges that are quite different. Finally, a perspective on how several emerging technologies may impact toxicity evaluation is also provided.

Effects of ionizing radiation on mitochondria

The current concept of radiobiology posits that damage to the DNA in the cell nucleus is the primary cause for the detrimental effects of radiation. However, emerging experimental evidence suggests that this theoretical framework is insufficient for describing extranuclear radiation effects, particularly the response of the mitochondria, an important site of extranuclear, coding DNA. Here, we discuss experimental observations of the effects of ionizing radiation on the mitochondria at (1) the DNA and (2) functional levels. The roles of mitochondria in (3) oxidative stress and (4) late radiation effects are discussed. In this review, we summarize the current understanding of targets for ionizing radiation outside the cell nucleus. Available experimental data suggest that an increase in the tumoricidal efficacy of radiation therapy might be achievable by targeting mitochondria. Likewise, more specific protection of mitochondria and its coding DNA should reduce damage to healthy cells exposed to ionizing radiation.

Development of monoclonal antibody-based assays for the detection of Vibrio tubiashii zinc-metalloprotease (VtpA)

Vibrio tubiashii has been linked to disease outbreaks in molluscan species, including oysters, geoducks, and clams. In particular, oyster hatcheries in the Pacific Northwest have been plagued by intermittent vibriosis since 2006. Accurate detection of vibrios, including V. tubiashii, is critical to the hatcheries in order to allow for rapid remediation efforts. The current methods for detection of Vibrio spp. are not ideal for use at the hatchery. Plating samples require time and is not sensitive to extracelluar pathogenic products, such as the secreted zinc-metalloprotease, VtpA. Other sensitive methods to detect bacteria, such as qPCR, require a high level of laboratory skills and expensive supplies that are prohibitive for use at hatchery sites. Thus, hatcheries would benefit from a sensitive, simple method to detect V. tubiashii and its secreted toxin. Here, we describe the development of two inexpensive and highly specific tests for the shellfish-toxic zinc-metalloprotease secreted by V. tubiashii: enzyme-linked immunoassays (ELISA) and a lateral flow immunoassay (dipstick assay). Both technologies rely on a set of monoclonal antibodies used in a sandwich format, with the capture antibody recognizing a different epitope than the detection antibody on the mature VtpA protein. Both assays are quantitative and give colorimetric readouts. The sandwich ELISA was sensitive when VtpA was diluted into PBS, but was markedly less sensitive in conditions that correlate with the environment of hatchery-derived samples, such as in the presence of seawater, algae, or oyster larvae. In contrast, the dipstick assay remained very sensitive in the presence of these contaminants, is less work-intensive, and much more rapid, making this format the preferred assay method for detecting VtpA on site in a hatchery or environmental setting.

Mitochondria as a target of environmental toxicants

Enormous strides have recently been made in our understanding of the biology and pathobiology of mitochondria. Many diseases have been identified as caused by mitochondrial dysfunction, and many pharmaceuticals have been identified as previously unrecognized mitochondrial toxicants. A much smaller but growing literature indicates that mitochondria are also targeted by environmental pollutants. We briefly review the importance of mitochondrial function and maintenance for health based on the genetics of mitochondrial diseases and the toxicities resulting from pharmaceutical exposure. We then discuss how the principles of mitochondrial vulnerability illustrated by those fields might apply to environmental contaminants, with particular attention to factors that may modulate vulnerability including genetic differences, epigenetic interactions, tissue characteristics, and developmental stage. Finally, we review the literature related to environmental mitochondrial toxicants, with a particular focus on those toxicants that target mitochondrial DNA. We conclude that the fields of environmental toxicology and environmental health should focus more strongly on mitochondria.

High-Content Screening for Compounds That Affect mtDNA-Encoded Protein Levels in Eukaryotic Cells

Compounds that interfere with the synthesis of either mitochondrial DNA or mtDNA-encoded proteins reduce the levels of 13 proteins essential for oxidative phosphorylation, leading to a decrease in mitochondrial adenosine triphosphate (ATP) production. Toxicity caused by these compounds is seldom identified in 24- to 72-h cytotoxicity assays due to the low turnover rates of both mtDNA and mtDNA-encoded proteins. To address this problem, the authors developed a 96-well format, high-content screening (HCS) assay that measures, in eukaryotic cells, the level of Complex IV–subunit 1, an mtDNA-encoded protein synthesized on mitochondrial ribosomes, and the level of Complex V–α subunit, a nuclear DNA-encoded protein synthesized on cytosolic ribosomes. The effect of several antibiotics and antiretrovirals on these 2 proteins was assessed, in transformed human liver epithelial cells, 6 days after compound treatment. The results confirmed effects of drugs known to reduce mtDNA-encoded protein levels and also revealed novel information showing that several fluoroquinolones and a macrolide, josamycin, impaired expression of mtDNA-encoded proteins. The HCS assay was robust with an average Z′ factor of 0.62. The assay enables large-scale screening of compounds to identify those that potentially affect mtDNA-encoded protein levels and can be implemented within a screening paradigm to minimize compound attrition.

Development of hexadecyloxypropyl tenofovir (CMX157) for treatment of infection caused by wild-type and nucleoside/nucleotide-resistant HIV

CMX157 is a lipid (1-0-hexadecyloxypropyl) conjugate of the acyclic nucleotide analog tenofovir (TFV) with activity against both wild-type and antiretroviral drug-resistant HIV strains, including multidrug nucleoside/nucleotide analog-resistant viruses. CMX157 was consistently >300-fold more active than tenofovir against multiple viruses in several different cell systems. CMX157 was active against all major subtypes of HIV-1 and HIV-2 in fresh human peripheral blood mononuclear cells (PBMCs) and against all HIV-1 strains evaluated in monocyte-derived macrophages, with 50% effective concentrations (EC(50)s) ranging between 0.20 and 7.2 nM. The lower CMX157 EC(50)s can be attributed to better cellular uptake of CMX157, resulting in higher intracellular levels of the active antiviral anabolite, TFV-diphosphate (TFV-PP), inside target cells. CMX157 produced >30-fold higher levels of TFV-PP in human PBMCs exposed to physiologically relevant concentrations of the compounds than did TFV. Unlike conventional prodrugs, including TFV disoproxil fumarate (Viread), CMX157 remains intact in plasma, facilitating uptake by target cells and decreasing relative systemic exposure to TFV. There was no detectable antagonism with CMX157 in combination with any marketed antiretroviral drug, and it possessed an excellent in vitro cytotoxicity profile. CMX157 is a promising clinical candidate to treat wild-type and antiretroviral drug-resistant HIV, including strains that fail to respond to all currently available nucleoside/nucleotide reverse transcriptase inhibitors.

Novel antibody-based strategies for the rapid diagnosis of mitochondrial disease and dysfunction

We are developing rapid immunoassays to measure the protein levels, enzymatic activities and post-translational modifications of mitochondrial proteins. These assays can be arrayed in multi-analyte panels for biomarker discovery and they can also be used individually at point of care where the level or activity of a small number proteins or even a single protein is highly informative. For example, we have characterized OXPHOS deficits associated with lipoatrophy, an adverse metabolic side-effect of anti-retroviral therapy, and have shown that OXPHOS deficits observed in vitro are also exhibited not only in clinically affected tissue (peripheral fat) but also in more easily accessible tissue (peripheral blood mononucleated cells). Similarly, we have shown that a small set of assays can be used to identify almost all patients with genetic deficits in OXPHOS complexes I or IV, the most common cause of inherited mitochondrial disease. Finally, we recently reported that Friedreich's Ataxia (FA) patients and carriers can be identified on the basis of a simple dipstick test to measure levels of a single protein, frataxin, an iron regulatory protein whose disrupted expression is the proximal cause of neurodegeneration in FA. Because each of these tests can be performed in an extremely simple, rapid dipstick format using non-invasive samples such as cheek swabs and fingerprick blood, they have potential for use as point of care diagnostics for mitochondrial disease and as front-line screening tools to help guide drug therapies and minimize adverse off-target drug effects.