The acrosome reaction in sperm: A special type of regulated exocytosis

顶体反应的一个特殊特征是精子头部与溶酶体相关的顶体小泡为其自身的胞吐作用提供钙离子。研究最近描述的两孔通道(TPC)家族的溶酶体内钙释放通道是否在哺乳动物精子中表达，以及它们的潜在激活剂NAADP是否有助于顶体分泌，我们发现两条独立的NAADP依赖的途径引起顶体胞吐，其中TPC1被发现负责激活低亲和力级联。由于我们的结果也为TPC2的表达提供了证据，我们现在将检查TPC2是否负责激活高亲和力的NAADP信号通路，以及TPC1和TPC2的同时基因缺失是否影响透明带诱导的顶体融合和动物的繁殖成功。然而，由于我们之前的研究也发现了顶体IP3R和TPC之间可能存在的功能性串扰，我们还将调查TPC的共同刺激是否以协同或相加的方式促进IP3引起的钙增加，以及我们最近在哺乳动物精子中发现的IP3R的抑制剂Irbit是否对顶体分泌有任何影响。在精子从射出的精子到输卵管中的成熟卵子的漫长旅程中，精子暴露在逐渐变化的不同化合物的浓度下，这些化合物的识别能力直接与精子与卵母细胞周围的透明带相互作用的能力有关。然而，我们目前对识别pH变化的适当化学感觉受体蛋白的分子同一性以及不同糖、氨基酸或激素的梯度的了解仍处于初级阶段。在这方面，最近发现的味觉受体的异位表达值得一提的是，众所周知，在舌头的味蕾中，它可以检测到具有不同化学性质的广泛化合物。研究味觉受体在雄性生殖细胞中的表达，我们发现形成功能性umami受体的两个结合伙伴(Tas1R1/Tas1R3，L谷氨酸单钠)，以及耦合的味觉转导级联的组件，如G蛋白α亚基Gustducin和阳离子通道Trpm5，在从小鼠到人类的精子中都有表达。此外，使用Tas1R1缺陷小鼠品系的精子，可以显著提高自发性顶体反应率。然而，TAS1R3、Gustducin和Trpm5是否也影响顶体分泌尚不清楚，尽管已经证明TAS1R3和Gustducin同时缺失会导致敲除男性不育。因此，我们将在小鼠和人类精子中系统地研究顶体反应效率与Tas1Rs、Gustducin和Trpm5之间的功能关系。

One special feature of the acrosome reaction is that the lysosome-related acrosomal vesicle in the sperm head supplies Ca2+ for its own exocytosis. Investigating whether the recently described endo-lysosomal Ca2+ release channels of the two pore channel (TPC) family are expressed in mammalian spermatozoa, and whether their potential activator NAADP contributes to acrosomal secretion we found that two independent NAADP-dependent pathways cause acrosomal exocytosis, where TPC1 was found to be responsible for activating the low affinity cascade. Since our results also provide evidence for an expression of TPC2, we will now examine whether TPC2 is responsible to activate the high affinity NAADP signaling pathway, and whether a simultaneous genetic deletion of TPC1 and TPC2 affects Zona pellucida induced fusion of the acrosome and breeding successful of the animals. However, since our previous study also uncovered a possible functional cross talk between the acrosomal IP3R and TPCs, we will also investigate whether a co-stimulation of TPCs enhances an IP3 evoked Ca2+ increase in a synergistic or in an additive manner, and whether IRBIT, an inhibitor of the IP3R, which we recently identified in mammalian spermatozoa, has any impact on acrosomal secretion.During their long distant journey of ejaculated sperm to the mature egg in the oviduct, sperm are exposed to progressively changing concentrations of different chemical compounds whose recognition capacity is directly linked to the sperm`s ability to interact with the Zona pellucida surrounding the oocyte. However, our current knowledge about the molecular identity of appropriate chemosensory receptor proteins which recognize changes in pH, but also gradients of different sugars, amino acids or hormones is still rudimental. In this context, the recent discovery of an ectopic expression of taste receptors is worth mentioning, which in taste buds on the tongue are known to detect a broad spectrum of compounds with different chemical properties. Examining the expression of taste receptors in male germ cells we found that the two binding partners, which form the functional umami receptor (Tas1R1/Tas1R3, Monosodium L-Glutamat), and components of the coupled taste transduction cascade, like the G protein alpha subunit Gustducin and the cation channel TRPM5 are expressed in sperm from mice to humans. In addition, using sperm of a Tas1R1 deficient mouse strain, a significant elevation of the spontaneous acrosome reaction rate was detected. However, whether Tas1R3, Gustducin and TRPM5 also affect acrosomal secretion, is not yet clear, although it has been demonstrated that a simultaneous genetic absence of both, Tas1R3 and Gustducin renders knock-out males sterile. Therefore, we will systematically examine the functional relationship between the efficiency of acrosome reaction and Tas1Rs, Gustducin and TRPM5 in mouse as well as in human sperm.