Fly mutants hold the key to communication in nerve cells

All animal behaviour, from insect flight to human thought, is generated within circuits of nerve cells, or neurons. The unique ability of theses neurons to receive and send signals from one neuron to the next is the domain of neuroscience. This transfer of information occurs at specialized contacts called synapses, at which the first neuron releases a chemical messenger, or neurotransmitter, that acts on the second cell. These neurotransmitters, such as acetylcholine or histamine, are stored within tiny synaptic vesicles of the first or presynaptic neuron. When released, these chemical messenger molecules bind to so-called receptor proteins located in the membrane surface of the second or postsynaptic neuron. In order to understand how circuits of neurons generate sequences of behaviour, we firstly need to understand how this information transfer at the synapses is regulated. What is it that regulates synaptic trans-mission between neurons? To answer this physiological question, we need to determine how synaptic proteins interact, which will essentially tell us how genes function. When an electric signal is present, calcium ions enter the presynaptic neuron and cause synaptic vesicles to attach to vesicle docking sites. There they fuse with the membrane allowing the neurotransmitter to diffuse out and across the short distance in the cleft between the two neurons. Subsequently, the vesicle membrane and the neurotransmitter are recycled for reuse by the first neuron. Many proteins regulate this entire process. Each of these proteins is encoded by a gene, requiring a system which can be used to study genetic function. Currently the most powerful approach to understand gene function is to alter, or mutate this system and to examine the outcome in mutant neurons. This gene alteration can be achieved most easily in the fruit fly Drosophila melanogaster, the model organism that has already told us most about genetics. My proposed research project at the Meinertzhagen lab, Dalhousie University, focuses on the function of Drosophila`s genes. Most of these genes (~70%) are identical to human genes because both flies and humans evolved from a common ancestor. Thus, what we learn about synaptic transmission in Drosophila can readily be compared with and largely imported to our knowledge of synaptic transmission in mammals such as ourselves. In order to understand synaptic function, I propose to study this protein-gene function system in the visual system, particularly the photoreceptors, of normal and mutant Drosophila. While most genes when altered are lethal to the fruit fly, mutations causing blindness are not, and allow the flies to survive and reproduce normally. The genetic term for these flies, with a regular body but a mutant eye, is `mosaic` and powerful genetic methods exist to create such whole-eye mosaics in Drosophila. Using high-magnification electron microscopy (EM) I propose to investigate how proteins in photoreceptor synapses differ between normal and mutant fruit flies. In a recent development of this EM method, extremely thin sections of the specimen can be progressively tilted in the electron beam to produce high-resolution three- dimensional images. In a process called EM tomography, information from this tilt series can then generate 3-D models, much like a CAT scan does for the human brain. These EM tomographic models have sufficient resolution to actually show proteins, which can differ between normal and mutant synapses. Consequently, the presence and roles of proteins can be directly related to specific genes. Synaptic transmission between neurons plays a crucial role in brain function. Applying this new and very direct approach will bring us a step further in our understanding of how information transfer in brains is regulated, both in the fruit fly and humans.