Ultrastructural evidence that motility changes caused by variations in ATP, Mg2+, and ADP correlate to conformational changes in reactivated bull sperm axonemes

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Abstract
We report the results of an ultrastructural study of Triton X‐100‐extracted, Mg‐adenosine triphosphate (ATP)‐reactivated bull sperm. We utilized a rapid fixation method to look for differences in the flagellar apparatus that correlate to the state of motility of reactivated sperm models. In a companion article, we examined the motility characteristics induced in bull sperm models by varying the concentration ratio of ATP and Mg2+ and the stabilizing effect of adenosine diphosphate (ADP) on coordinated beating. Based on the results of that report, we selected four dissimilar states that appeared to represent extremes. One reactivation condition produces vigorous motility similar to live sperm, another produces large amplitude, low frequency beating while the remaining two conditions produce small amplitude vibrations of the flagellum with little coordinated beating. Morphometric analysis of transmission electron micrographs of sperm from these four treatment conditions revealed statistically significant differences between the samples in regard to axoneme diameter, inter‐microtubule doublet spacing, and outer dense fiber (ODF) spacing. Our results show that Mg2+ decreases the axoneme diameter and reduces interdoublet spacing, while ATP, uncomplexed with Mg2+, had the opposite effect. We also provide supporting evidence that this may be due to Mg2+ increasing, and ATP decreasing, the interdoublet adhesion of dynein. We also found that 4 mM ADP significantly increases the separation between the ODFs and the space between the ODFs and the central axoneme within the middle piece. We present a hypothetical explanation that is consistent with our results to explain how ATP, ADP, and Mg2+ act to regulate the beat cycle.
Funding Information
  • National Science Foundation (MCB-0918294)

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