toni Claudio, Ph.D. - US grants

The goals of this research are to elucidate the molecular mechanism of acetylcholine receptor (AChR) clustering on cell surfaces, study the process of subunit assembly and define the acetylcholine (ACh) binding site using the techniques of gene cloning, in vitro mutagenesis and gene transfer. The long-term objectives are to further define the roles the AChR plays in synaptic transmission, in synaptogenesis and in the autoimmune disease, myasthenia gravis. Project I: Clustering Mammalian tissue culture cells will be transformed with the 4 Torpedo AChR subunit cDNA clones engineered into appropriate expression vectors, and cell lines which stably express functional AChR complexes on their surface will be established. Extensive biochemical, kinetic, immunological and electrophysiological analyses will be performed on these cell lines before they will be used to study the process of AChR clustering. The transformed cell lines will be co-cultured with nerve cells or treated with various factors in order to induce cluster formation. New cell lines will next be created that contain mutant AChR subunit genes. Receptors which possess all normal properties except the ability to cluster will allow identification of those regions of the protein required for clustering. Project II: Assembly Cells will be transformed with all possible combinations of 1, 2 and 3 genes. Biochemical and immunological analyses of these lines will reveal whether subunits can stably self-assemble and whether one subunit can substitute for any other subunit in terms of assembly or function. If stable receptor complexes can be formed with fewer than 4 different subunits, then it may be possible to assign certain biological properties to specific subunits. Project III: Ligand Binding Site Biochemical studies have identified specific regions of the AChR as the putative ACh binding sites. In vitro site-specific mutations creating single base-pair changes that lead to single amino acid changes will further define the binding site region, identify amino acids in the site critical for agonist and antagonist binding and ultimately further our understanding of how agonist binding and channel opening are coupled.

Our long-term objectives are to further define the role of the nicotinic acetylcholine receptor (AChR) in synaptic transmission, synaptogenesis, and various motor neurone diseases. Our approach is to molecularly dissect the neuromuscular junction (NMJ) and to establish an artificial system in which individual components of the NMJ can be precisely defined and manipulated. We have recently achieved our goal of establishing such a system (stable expression of cloned Torpedo californica AChRs in mouse fibroblast cells). Not only is this the first stable expression of a receptor/channel, but it is the first stable expression of a protein composed of four different subunits. These subunits are synthesized, assembled into the correct pentameric complex, and the complex is inserted into the plasma membrane and functional. With such a system, three powerful approaches can be employed that will help us to define the different properties of the AChR and to define the role of the AChR in different processs and diseases. 1) By expressing the AChR in a foreign environment, we have isolated it away from other muscle specific gene products, thus allowing the specific properties of the AChR to be determined. 2) By introducing the AChR into foreign cells via cloned cDNAs, the AChR can be manipulated at the DNA level using site-directed mutagenesis techniques. 3) By expressing the AChR in non-muscle and non-neuronal cell types, components of the NMJ can be added to the system in a controlled fashion such that their effect on the AChR can be determined. In this grant application, we are proposing to undertake three major projects. Project #1 will be a thorough physiological characterization of Torpedo AChRs in mouse fibroblast cells. Our system has provided us with the unique opportunity for performing physiological studies of this AChR that have not been possible previously. Once characterized, we will then add to the system (and thereby study the effects of) circulating neurotoxic factors in sera from patients with motor neurone diseases, neurotrophic viruses, antibodies, and toxins. Project #2 will attempt to elucidate the molecular mechanism of AChR clustering on cell surfaces. Agents will be added to our cell lines in order to induce cluster formation. Using site-directed mutagenesis techniques, specific deletions of cytoplasmic domains of each of the receptor subunits will be made to further define the role of each subunit in the clusteing process. If necessary, other proteins thought to play a role in clustering will be added to the system. Project #3 will study the processes of AChR subunit biosynthesis and assembly. Some of the questions we will address include: are there specific subunit-subunit interactions, where are the subunits assembled and degraded, do the subunits interact with heavy chain binding protein, what role do the oligosaccharides play in AChR assembly and AChR function?