Morita Masaya, Takemura Akihiro, Okuno Makoto
Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan.
J Exp Biol. 2004 Jan;207(Pt 2):337-45. doi: 10.1242/jeb.00748.
Euryhaline tilapia Oreochromis mossambicus can reproduce in freshwater and in seawater. Regulation of sperm motility appears to be modulated during acclimation of the fish from freshwater to seawater, being independent of extracellular Ca2+ in freshwater and dependent on extracellular Ca2+ in seawater. In the presence of extracellular Ca2+, sperm of seawater-acclimated tilapia (SWT) showed motility even in a hypertonic environment, whereas sperm of freshwater-acclimated tilapia (FWT) were not motile. The Ca2+ indicator, fluo-3, revealed that intracellular Ca2+ concentration, [Ca2+]i, of SWT sperm increased only in the presence of extracellular Ca(2+) in hypotonic or hypertonic conditions. Since the increased [Ca2+]i in FWT sperm occurred under hypotonic conditions via intracellular Ca2+ stores, it is likely that tilapia modulate their source of increasing [Ca2+]i from intracellular stores (in FWT sperm) to extracellular stores (in SWT sperm). Experiments using demembranated sperm revealed that Ca2+ is necessary for activation of motility, suggesting that Ca2+ plays a key role in motility regulation in SWT sperm. We detected three phosphoproteins associated with the activation of sperm motility. Serine and threonine residues of two proteins of 15 kDa and 18 kDa became dephosphorylated in hypotonic conditions but remained phosphorylated in hypertonic conditions, suggesting that these protein phosphorylations were not only related to motility activation under hypertonic conditions but also resistant to osmotic pressure. The threonine residue(s) of a 41 kDa protein was also phosphorylated in dry sperm, even in FWT sperm in motility-feasible hypotonic conditions. It is likely that acclimation of the motility apparatus is associated with modulation of the flow of Ca2+ to increase [Ca2+]i and protein phosphorylation.
广盐性罗非鱼莫桑比克罗非鱼能够在淡水和海水中繁殖。在罗非鱼从淡水适应到海水的过程中，精子活力的调节似乎受到调控，在淡水中精子活力与细胞外Ca2+无关，而在海水中则依赖于细胞外Ca2+。在存在细胞外Ca2+的情况下，适应海水的罗非鱼（SWT）精子即使在高渗环境中也能表现出活力，而适应淡水的罗非鱼（FWT）精子则不具有活力。Ca2+指示剂fluo-3显示，SWT精子的细胞内Ca2+浓度[Ca2+]i仅在低渗或高渗条件下存在细胞外Ca(2+)时才会增加。由于FWT精子中[Ca2+]i的增加是在低渗条件下通过细胞内Ca2+储存发生的，因此罗非鱼很可能将其增加[Ca2+]i的来源从细胞内储存（FWT精子中）调节为细胞外储存（SWT精子中）。使用去膜精子进行的实验表明，Ca2+是激活精子活力所必需的，这表明Ca2+在SWT精子的活力调节中起关键作用。我们检测到三种与精子活力激活相关的磷蛋白。在低渗条件下，15 kDa和18 kDa两种蛋白的丝氨酸和苏氨酸残基发生去磷酸化，但在高渗条件下仍保持磷酸化，这表明这些蛋白磷酸化不仅与高渗条件下的活力激活有关，而且对渗透压具有抗性。即使在活力可行的低渗条件下的FWT精子中，41 kDa蛋白的苏氨酸残基在干燥精子中也会发生磷酸化。精子活力装置的适应很可能与Ca2+流量的调节有关，以增加[Ca2+]i和蛋白磷酸化。
