Boekhoff Lab: Peripheral taste systems and reproductive pharmacology
Scientific Assistant: Elisabeth Arlt,
Postgraduate Students: Stefanie Resenberger
Our research focusses on the molecular mechanisms of Ca2+-regulated exocytosis and the functional relevance of extra-oral expression of gustatory receptors.
Ca2+ regulated exocytosis
Ca2+-regulated exocytosis is a fundamental process mediating delivery of peptide hormones, neurotransmitters or digestive enzymes to the exact moment-to-moment requirement of a biological system. In all cellular systems, regardless of how many vesicles are harbored, membrane fusion is governed by a conserved family of proteins, called SNAREs, which form a membrane bridging complex composed of a complementary set of SNAREs at the active zone, the fusion site for synaptic vesicles. Importantly, vesicle recruitment, docking and priming of synaptic vesicles as well as physically approximating the SNARE fusion machinery to voltage gated calcium channels and thus precise synchronization of membrane fusion to the increase in intracellular Ca2+ is controlled by a dense protein network, termed cytomatrix of the active zone (CAZ). Recent studies in neurons indicate that the CAZ network -which is formed by a small set of highly conserved multi domain scaffolding proteins- is responsible for the coordination of the interrelated events of vesicle fusion in space and time.
A special form of Ca2+-regulated exocytosis is the sperm acrosome reaction, a mandatory event in the fertilization process, initiating two essential events: release of hydrolyzing enzymes, necessary for sperm to penetrate the egg’s glycoprotein matrix, and exposure of the inner acrosomal membrane, which thereby becomes accessible for the ultimate fusion event with egg’s plasma membrane. Interestingly, remarkable parallels have been outlined between the acrosome reaction in sperm and vesicle fusion in neurons and neuroendocrine or exocrine cells: These basic similarities include the composition of the core fusion apparatus, which consists of the nerve-transmission-related SNARE-proteins. Additionally, we have recently found that a Ca2+-controlled pre-fusion clamp, which locks the fully assembled SNARE-fusion apparatus at a pre-fusion stage in neuroendocrine cells, is also present in spermatozoa.
Moreover, we have found that an “acrosomal synapse“ is formed by membrane raft-derived micro-environments. However, there are some unique features whose underlying molecular mechanisms are largely unknown: Fusion of the outer acrosomal membrane and the overlying head plasma membrane occurs at hundreds of distinct loci. This is not only of particular importance because it ensures complete release of stored hydrolyzing enzymes but it also exposes the inner acrosomal membrane for the ultimate fusion event with the egg. Another unique feature of acrosomal secretion is that the acidic lysosome-related acrosomal vesicle supplies Ca2+ for its own exocytosis by behaving like a lysosome-related Ca2+ reservoir. Due to the parallels between vesicle fusion at the presynapse and exocytosis of the sperm's acrosomal vesicle, we currently investigate whether proteins constituting the CAZ at the presynaptic terminal are expressed in mammalian spermatozoa and whether they are responsible for ensuring synchronized zipper-like fusion pore formation during acrosomal exocytosis (s. Figure 1).
Moreover, since increase of Ca2+ coming from inside the acrosome is absolutely required to trigger multiple fusion pore formation during acrosome reaction we examine whether the recently identified endo-lysosomal cation channels, termed two-pore channels (TPCs) are expressed in mammalian spermatozoa and whether these channel proteins and/or their potential activator NAADP contribute to acrosome reaction.
Taste receptors and reproduction
However, male fertility is a complex process taking place at different organic systems. This includes the production of an adequate number of motile sperm in the testis, the travelling through the different segments of the epididymis, the long journey through the diverse compartments of the female genital tract and the final fusion with the mature oocyte at the ampullary part of the oviduct. Importantly, distinct regions of the male and female reproductive organs are characterized by a unique microenvironment.
Thus, a reliable ability of germ cells to recognize distinct chemical components in the corresponding surrounding milieu, such as hormones, changes in pH and ion compositions, amino acids, sugars and even encountered toxins is not only essential for controlling germ cell differentiation and regulated apoptosis in order to delete damaged germ cells but is also valuable to increase the incredible chance of a single sperm out of about 100 to 300 million sperm per ejaculate to finally fertilize the mature egg. The recent observation that taste receptors and components of the coupled taste transduction cascade are expressed during the different phases of spermatogenesis as well as in mature spermatozoa now allows uncovering the physiological function of taste receptors for male germ cell’s ability and elucidating of how sperm adequately respond to environmental external stimuli.
This knowledge may in the future be helpful to develop new strategies for the treatment of infertile individuals. However, since extra-oral expression of taste receptors has been described for many different tissues, sperm cells may also present a useful cellular system to study and thus better understand the functional role of peripheral taste receptors in biological systems other than the mouth.
Borth, H., Weber, N., Meyer, D., Wartenberg, A., Arlt, E., Zierler, S., Breit, A., Wennemuth, G, Gudermann., T., Boekhoff, I. (2016) The IP3R binding protein released with inositol 1,4,5-trisphosphate, is expressed in rodent reproductive tissue and spermatozoa. J Cell Physiol., 231(5):1114-29.
Weber N, Vieweg L, Henze F, Oprisoreanu AM, Solinski HJ, Breit A, Fecher-Trost C, Schalkowsky P, Wilhelm B, Wennemuth G, Schoch S, Gudermann T, Boekhoff I. (2014) RIM2alpha is a molecular scaffold for Zona pellucida-induced acrosome reaction. J Mol Cell Biol. Oct; 6(5):434-7.
Arndt, L., Castonguay, J., Arlt, E., Meyer, D., Hassan, S., Borth, H., Zierler, S., Wennemuth, G., Breit, A., Biel, M., Wahl-Schott, C., Gudermann, T., Klugbauer, N., and Boekhoff, I. (2014). NAADP and the two-pore channel protein 1 participate in the acrosome reaction in mammalian spermatozoa. Molecular biology of the cell 25, 948-964.
Meyer, D., Voigt, A., Widmayer, P., Borth, H., Huebner, S., Breit, A., Marschall, S., de Angelis, M.H., Boehm, U., Meyerhof, W., Gudermann, T., and Boekhoff, I. (2012). Expression of tas1 taste receptors in Mammalian spermatozoa: functional role of tas1r1 in regulating Basal ca and cAMP concentrations in spermatozoa. PloS one 7, e32354.
Zitranski, N., Borth, H., Ackermann, F., Meyer, D., Viewig, L., Breit, A., Gudermann, T., and Boekhoff, I. (2010). The "acrosomal synapse": Subcellular organization by lipid rafts and scaffolding proteins exhibits high similarities in neurons and mammalian spermatozoa. Communicative & integrative biology 3, 513-521.
Ackermann, F., Zitranski, N., Borth, H., Buech, T., Gudermann, T., and Boekhoff, I. (2009). CaMKIIalpha interacts with multi-PDZ domain protein MUPP1 in spermatozoa and prevents spontaneous acrosomal exocytosis. Journal of cell science 122, 4547-4557.
Ackermann, F., Zitranski, N., Heydecke, D., Wilhelm, B., Gudermann, T., and Boekhoff, I. (2008). The Multi-PDZ domain protein MUPP1 as a lipid raft-associated scaffolding protein controlling the acrosome reaction in mammalian spermatozoa. Journal of cellular physiology 214, 757-768.
Fehr, J., Meyer, D., Widmayer, P., Borth, H.C., Ackermann, F., Wilhelm, B., Gudermann, T., and Boekhoff, I. (2007). Expression of the G-protein alpha-subunit gustducin in mammalian spermatozoa. Journal of comparative physiology 193, 21-34.
Heydecke, D., Meyer, D., Ackermann, F., Wilhelm, B., Gudermann, T., and Boekhoff, I. (2006). The multi PDZ domain protein MUPP1 as a putative scaffolding protein for organizing signaling complexes in the acrosome of mammalian spermatozoa. Journal of andrology 27, 390-404.