Cell transplantation therapy has long been investigated as a therapeutic intervention for neurodegenerative disorders, including spinal cord injury, Parkinson’s disease, and amyotrophic lateral sclerosis. Indeed, patients have high hopes for a cell-based therapy. However, there are numerous practical challenges for clinical translation. One major problem is that only very low numbers of donor cells survive and achieve functional integration into the host. Glial scar tissue in chronic neurodegenerative disorders strongly inhibits regeneration, and this inhibition must be overcome to accomplish successful cell transplantation. Intraneural cell transplantation is considered to be the best way to deliver cells to the host. We questioned this view with experiments in vivo on a rat glial scar model of the auditory system. Our results show that intraneural transplantation to the auditory nerve, preceded by chondroitinase ABC (ChABC)-treatment, is ineffective. There is no functional recovery, and almost all transplanted cells die within a few weeks. However, when donor cells are placed on the surface of a ChABC-treated gliotic auditory nerve, they autonomously migrate into it and recapitulate glia- and neuron-guided cell migration modes to repair the auditory pathway and recover auditory function. Surface transplantation may thus pave the way for improved functional integration of donor cells into host tissue, providing a less invasive approach to rescue clinically important neural tracts.

A three-dimensional model for the transmembrane domains of human endothelin-A receptor was built using structural information from bacteriorhodopsin and sequence alignment to other guanine-nucleotide-binding regulatory(G) protein-coupled receptors. Based on this model, 18 amino acids located at the inside of the receptor were mutated and analyzed for binding of the natural ligand endothelin-1 and bosentan, a recently described potent orally active endothelin antagonist [Clozel, M., Breu, V., Gray, G., Kalina, B., Löffler, B.-M., Burri, K., Cassal, J.-M., Hirth, G., Müller, M., Neidhart, W. & Ramuz, H. (1994) Pharmacological characterization of bosentan, a new potent orally active nonpeptide endothelin receptor antagonist, J. Pharmacol. Exp. Ther. 270, 228-235]. Mutation of Gly97, Lys140, Lys159, Gln165 and Phe315, located in transmembrane region 1, 2, 3, 3, and 6, respectively, caused reduced specific binding of 125I-labelled endothelin-1, despite an expression level similar to wild-type endothelin-A receptor. Mutation of Tyr263, Arg326 and Asp351 preserved endothelin-1 binding but caused reduced binding of bosentan. These amino acids, located on transmembrane regions 5, 6 and 7, respectively, are conserved among endothelin-A and endothelin-B receptors but not in other G-protein-coupled receptors. These observations demonstrate a dissociation of the binding site for the peptidic natural agonist endothelin-1 and the synthetic non-peptide antagonist bosentan. They provide the molecular basis for bosentan being a specific antagonist for both, endothelin-A as well as endothelin-B receptors and may in combination with studies on structure/activity relationship support the design of novel and more potent endothelin receptor antagonists.

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A three-dimensional model for the transmembrane domains of human endothelin-A receptor was built using structural information from bacteriorhodopsin and sequence alignment to other guanine-nucleotide-binding regulatory(G) protein-coupled receptors. Based on this model, 18 amino acids located at the inside of the receptor were mutated and analyzed for binding of the natural ligand endothelin-1 and bosentan, a recently described potent orally active endothelin antagonist [Clozel, M., Breu, V., Gray, G., Kalina, B., Löffler, B.-M., Burri, K., Cassal, J.-M., Hirth, G., Müller, M., Neidhart, W. & Ramuz, H. (1994) Pharmacological characterization of bosentan, a new potent orally active nonpeptide endothelin receptor antagonist, J. Pharmacol. Exp. Ther. 270, 228-235]. Mutation of Gly97, Lys140, Lys159, Gln165 and Phe315, located in transmembrane region 1, 2, 3, 3, and 6, respectively, caused reduced specific binding of 125I-labelled endothelin-1, despite an expression level similar to wild-type endothelin-A receptor. Mutation of Tyr263, Arg326 and Asp351 preserved endothelin-1 binding but caused reduced binding of bosentan. These amino acids, located on transmembrane regions 5, 6 and 7, respectively, are conserved among endothelin-A and endothelin-B receptors but not in other G-protein-coupled receptors. These observations demonstrate a dissociation of the binding site for the peptidic natural agonist endothelin-1 and the synthetic non-peptide antagonist bosentan. They provide the molecular basis for bosentan being a specific antagonist for both, endothelin-A as well as endothelin-B receptors and may in combination with studies on structure/activity relationship support the design of novel and more potent endothelin receptor antagonists.

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