Growth cones are dynamic structures present at the tip of developing and regenerating axons. Axonal growth cones unerringly navigate the embryonic environment to reach their appropriate synaptic partners. In doing so they establish the basic neuro-architecture upon which successful nervous system function depends. By uncovering the molecular mechanisms that enable growth cones to navigate we can learn much about how the structural foundation of a functional nervous system arises, and what mechanisms may need re-activation for successful regeneration to occur.
To begin to elucidate the molecular basis of growth cone navigation we are utilizing a genetically amenable, anatomically simple model system - the embryonic neuromuscular system in the fruit fly, Drosophila melanogaster. By employing a large-scale mutagenesis screen we uncovered mutants with defective motor axon pathfinding and targeting behaviors. Cloning of mutants with the most compelling phenotypes is underway.
The first gene cloned from the screen is sidestep. We found that it encodes a protein (Side) that is a new member of the immunoglobulin superfamily of cell adhesion molecules. The Side protein is essential for instructing motor axon growth cones to move from axonal to muscle substrates. In the absence of Side protein, motor axons remain fasciculated (bundled) with one another rather than moving into their appropriate muscle domains. When Side is expressed at higher than normal levels on surfaces contacted by the motor axon growth cones, their contact is extensive and prolonged. These observations demonstrate that Side is a potent attractant for motor axon growth cones. We are proceeding with genetic and non-genetic approaches to uncover the molecular mechanisms that enable Side to direct motor axon growth cones.
A second line of research involves seeking the molecular genetic factors that contribute to development of somatic muscles. We have identified an extracellular protein, Hibris, that is expressed by muscle cells prior to fusion, and also at the sites where muscles form attachments. We are using an in vitro approach to identify how Hibris interacts with its extracellular binding partner, and genetic approachs to identify its intracellular partners.
