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Intercellular communication is necessary for higher organisms to develop and function. Much has been learned in recent decades about cell-cell signaling via extra-cellular molecules and their receptors. However, the more direct mechanism of communication via gap junctions has been little studied. Gap junctions are present in essentially all tissues of all animal species. They are present from the earliest embryonic stages and are necessary for development to proceed.
We have recently defined a new gene family, the Innexins, whose members code for the invertebrate gap junction channel. The family includes thirty Drosophila and C. elegans genes sharing high sequence homology. This discovery allows us to use the methods of experimental manipulation available in Drosophila to study the role of gap junction communication in develop-mental mechanisms, channel function and neural systems.
Different members of the family are expressed in different tissues. Taken together, the family members are expressed in most, if not all, cell types. Mutations eliminate the gap junction connections in these tissues. We are identifying other members of the gene family and defining their time and place of expression.
For functional studies, we express the genes in specific cells in Drosophila by using the GAL4-UAS system or we express them in heterologous systems such as Xenopus oocytes (by injection of RNA) or in tissue culture cells by transfection.
We know that, functionally, the different genes of the family are not interchangeable. Genes which are chimeras of different innexin genes have been made and are being used to determine whether the distinct aspects of function can be mapped to different molecular domains.
In development, gap junctions often precede the formation of chemical synapses: signals specifying the synapse may pass through the gap junction. To study this possibility, we analyze the development and functioning of the escape response of Drosophila. This simple behavior is mediated by the 16 identified neurons of the Giant Fiber System. We can stimulate, record from and fill each cell with dye. We know the role of each neuron in the generation of the behavior. Mutation of the gap junction gene Passover eliminates the synapse between the Giant Fiber and its postsynaptic partner, the jump motorneuron. The gene is expressed only in the pre- and post-synaptic cells whose connection is disrupted by the mutation. Genes with this degree of specificity are unprecedented. Gap junctions made by this gene family may be involved in the cell recognition responsible for the specificity of synaptic connections.
Curtin, KD., Z Zhang and RJ Wyman. 2002 Gap junction proteins are not interchangeable for development of neural function in the Drosophila visual system. Journal of Cell Science. 115:3379-3388.
Curtin, K. D., Z. Zhang and R. J. Wyman. 2002 Gap junctions proteins expressed during development are required for adult neural function in the Drosophila optic lamina. Journal of Neuroscience. 22:7088-7096.
Wyman, R. J. 2003 The Projection Problem. Population and Environment 24:329-337.