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Author Spotlight: Advancing Male Infertility Research by Unraveling Sperm Metabolism and Mitochondrial Function
Published on: June 23, 2023
Causative mechanisms and functional correlates of MTT reduction in stallion spermatozoa
Ashlee Jade Medica1, Zamira Gibb1, Alecia Sheridan1
1Priority Research Centre for Reproductive Science, College of Engineering, Science and Environmental, and Faculty of Health and Medicine, University of Newcastle, Callaghan, New South Wales, Australia.
Abstract:
MTT is a commonly used cell vitality probe, due to its ability to form insoluble formazan deposits at cellular locations of intense oxidoreductase activity. Although this response is considered a reflection of mitochondrial redox activity, extra-mitochondrial sites of MTT reduction have been recognized within the spermatozoa of several mammalian species. Therefore, the aim of this study was to determine the major sites and causative mechanisms of MTT reduction in stallion spermatozoa. Our results show that stallion spermatozoa displayed substantial mitochondrial formazan deposition, as well as a single extra-mitochondrial formazan deposit in various locations on the sperm head in approximately 20% of cells. The quality and capacitation status of stallion spermatozoa were positively correlated with the presence of an extra-mitochondrial formazan granule. Additionally, extra-mitochondrial formazan deposition was suppressed by the presence of an NADPH oxidase (NOX) inhibitor (VAS2870; active against NOX2, NOX4 and NOX5), MnTMPyP (SOD mimetic) and zinc (NOX5 inhibitor) suggesting that extra-mitochondrial MTT reduction may be facilitated by NOX-mediated ROS generating activity, conceivably NOX5 or NOX2. When comparing MTT to resazurin, another well-known probe used to detect metabolically active cells, MTT reduction had a higher correlation with sperm concentration and motility parameters (R2= 0.91), than resazurin reduction (R2 = 0.76). We conclude that MTT reduction in stallion spermatozoa follows a species-specific pattern due to a high dependence on oxidative phosphorylation and a degree of NOX activity. As such, MTT reduction is a useful diagnostic tool to assess extra-mitochondrial redox activity, and therefore, the functional qualities of stallion spermatozoa.
Insights
MTT reduction in stallion sperm occurs in mitochondria and uniquely on the sperm head, correlating with sperm quality. This extra-mitochondrial activity, influenced by NADPH oxidase, makes MTT a valuable diagnostic tool.
Area of Science:
- Reproductive Biology
- Cellular Physiology
- Biochemistry
Background:
- MTT assay is a standard for measuring cell viability via formazan deposit formation, reflecting oxidoreductase activity.
- While typically linked to mitochondria, extra-mitochondrial MTT reduction occurs in mammalian spermatozoa.
- Understanding MTT reduction sites in stallion sperm is crucial for interpreting results and assessing sperm function.
Purpose of the Study:
- To identify the primary sites of MTT reduction in stallion spermatozoa.
- To elucidate the mechanisms driving extra-mitochondrial MTT reduction in these cells.
- To compare the diagnostic utility of MTT reduction with resazurin for stallion sperm quality assessment.
Main Methods:
- Microscopic analysis of formazan deposition in stallion spermatozoa after MTT assay.
- Treatment with NADPH oxidase (NOX) inhibitors (VAS2870, zinc) and a SOD mimetic (MnTMPyP) to investigate extra-mitochondrial reduction mechanisms.
- Comparative analysis of MTT and resazurin reduction correlations with sperm concentration and motility parameters.
Main Results:
- Stallion spermatozoa showed significant mitochondrial formazan deposits and extra-mitochondrial deposits on the sperm head in ~20% of cells.
- Extra-mitochondrial formazan deposition positively correlated with stallion sperm quality and capacitation status.
- NOX inhibitors and a SOD mimetic suppressed extra-mitochondrial MTT reduction, implicating NOX-mediated reactive oxygen species (ROS) activity.
- MTT reduction showed a stronger correlation with sperm concentration and motility (R²=0.91) than resazurin reduction (R²=0.76).
Conclusions:
- MTT reduction in stallion spermatozoa exhibits a species-specific pattern, influenced by oxidative phosphorylation and NOX activity.
- Extra-mitochondrial MTT reduction is linked to sperm functional quality and potentially mediated by NOX enzymes (e.g., NOX2, NOX5).
- MTT assay is a valuable diagnostic tool for assessing stallion sperm functional parameters, including extra-mitochondrial redox activity.
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