The role of spectraplakins as key integrators of axonal microtubule networks

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The ability of neurons to extend long processes (axons) towards other neurons, muscles or glands, is a key process underlying the formation of the neuronal networks that make up our brain and coordinate our behaviour. For example, failure of axonal growth is often fatal or causes paralysis upon spinal injury. To find cures we have to acquire a thorough understanding of the mechanisms underlying axonal growth. Growing axons follow reproducible paths signposted by chemical cues that direct the growing axon. Axonal elongation is essentially driven by filamentous skeletal elements of cells, called actin and microtubules. Actin and microtubules have to cooperate closely, and their activity must be adaptable to instructions through the guiding chemical cues. Unravelling how these different factors (actin, microtubules and signals) integrate and cross-coordinate each other during axonal growth is the key task that needs to be addressed. To this end, we focus our work on putative integrator molecules, called spectraplakins. They can physically link microtubules to actin and signalling components and, in their absence, axonal growth is severely inhibited. However, how the links of spectraplakins to actin, microtubules, and signalling components help them to perform their growth promoting function remains to be resolved. To facilitate this task, we study the spectraplakin Short stop (Shot) of fruitflies, which represents a most helpful model. Thus, Shot can be studied with powerful and rapid strategies available in fruitflies; the data obtained are relevant and can be translated into biomedical research, since the characteristics and functions of Shot are virtually identical to those of mammalian or human spectraplakins. As the starting point of our project, we have identified three regions/domains of the Shot molecule that can link Shot to microtubules, actin and, most likely, signalling factors. We have shown that all of them are absolutely required for Shot function in axonal growth. Here we will address the detail of their function, using advanced microscopy, structure-function studies, and state-of-the art technology (mass spectrometry) to identify yet unknown binding factors. As an essential further strategy we will make systematic use of combinatorial genetics. We will combine mutations of Shot with mutations in other genes known to contribute to axonal growth. The combined mutant defects will give essential insights into the functional relationships of Shot to other factors, thus mapping Shot function into the systemic context of axonal growth. Our results will provide essential new insights into the function of spectraplakins in health and disease and the regulatory networks underlying axonal growth.