The nicotinic acetylcholine receptors (nAChR) are members of the Cys-loop receptor superfamily. Playing important neurophysiological roles they are in the focus of biomedical research. The acetylcholine binding proteins (AChBP) in the hemolymph of gastropods are structural and functional homologues of the ligand binding domain of nAChR. In contrast to the membrane-bound nAChR, AChBP is water soluble. Therefore, AChBP is a very efficient surrogate for indirect investigations of nAChR functions . AChBP is based on 25 kDa polypeptides. Like nAChR (Fig. 1A), AChBP is usually a pentamer (Fig. 1B,C). However, in the hemolymph of the planorbid snail Biomphalaria glabrata (vector snail of the schistosomiasis parasite) it exists as a pentagonal dodecahedron (12 pentamers), 25 nm in diameter (Fig. 1D). This large size and high symmetry makes it well accessible to 3D electron microscopy . Moreover, in B. glabrata two different AChBP polypeptides (designated as Bg-ACHBP1 and Bg-AChBP2) were detected, recombinantly expressed, and reassembled to functional oligomers . Recombinant Bg-AChBP1 readily formed dodecahedra, whereas recombinant Bg-AChBP2 formed only pentamers and dipentamers . The existence of hetero-oligomers could be excluded, but not the possibility that in the hemolymph also Bg-AChBP2 might be present as dodecahedra. Agonist and antagonist binding properties of AChBP and nAChR are comparable . Concerning biological functions of AChBP in snails, several possibilities have been discussed (for references, see [1,2]): (i) suppressor of cholinergic transmission; (ii) regulator of CaCO3 crystallization (shell growth); (iii) binding agent of algal toxins.
Here we show by in situ hybridization that in B. glabrata, AChBP is expressed in the mantle epithelium (Fig. 2A). In this context, it should be remembered that the molluscan shell is secreted by the mantle. Transmission electron microscopy revealed ultrastructural details of this epithelium (Fig. 2B). The applied cDNA probe was inefficient to discriminate between Bg-AChBP1 and Bg-AChBP2, but immunofluorescence microscopy using isoform-specific rabbit antibodies revealed the presence of Bg-AChBP1 and the absence of Bg-AChBP2 in the mantle epithelium (not shown). Therefore, Bg-AChBP1 might be involved in regulating shell growth. Although these data are preliminary, they suggest that the two Bg-AChBP isoforms significantly differ in their biological functions within the snail. We now perform CaCO3 crystallization experiments in the presence of Bg-AChBP1.
We thank Kristiane Rusitzka, Dr. Wolfgang Gebauer, Sandra Braukmann, Dr. Nadine Leidecker, Julia Markl and Martin Haugwitz for experimental advices, Thomas Schubert, Elisabeth Sehn and Gabriele Stern-Schneider for technical support, Mirko Montigny, Prof. Dr. Wolfgang Tremel and Prof. Dr. Dirk Schneider for cooperation and the Max Planck Graduate Center (MPGC) in Mainz for financial support and a grant to DT.
 Shahsavar et al. Basic Clin Pharmacol Toxicol. 2015; doi: 10.1111/bcpt.12528
 Saur et al. PLOS One 2012; 7:e43685
To cite this abstract:Daniela Treiber, Maria Kokkinopoulou, Bernhard Lieb, Jürgen Markl; Tracing acetylcholine binding-proteins microscopically in tissues of the snail Biomphalaria glabrata. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/tracing-acetylcholine-binding-proteins-microscopically-in-tissues-of-the-snail-biomphalaria-glabrata/. Accessed: May 27, 2019
EMC Abstracts - https://emc-proceedings.com/abstract/tracing-acetylcholine-binding-proteins-microscopically-in-tissues-of-the-snail-biomphalaria-glabrata/