Nitric oxide and microtubule-actin crosstalk in axon biology

Axon extension, axon branching, and axon retraction are major morphological changes that neurons have to execute to accomplish correct wiring of the nervous system during development and regeneration after injury. These transformations are guided by extracellular signals which are recognized by appropriate receptors and ultimately need to be translated into the rearrangement of the neuronal cytoskeleton, most importantly microtubules and actin filaments. A major focus has been to identify and understand signalling mechanisms which are involved in the concerted regulation of microtubule and actin reorganization (crosstalk) in response to extracellular signals. Results recently obtained in my laboratory suggest that nitric oxide and nitric oxide synthases, apart from their previously established role in synaptic efficacy, have a function in axon guidance and retraction through S- nitrosylation of cytoskeletal proteins such as the microtubule-associated protein MAP1B. Moreover, MAP1B might be one of the elusive components mediating microtubule-actin cross talk and hence the orchestrated reconfiguration of the growth cone cytoskeleton during migration. Based on these findings, the goals of the proposal are to understand the contribution of nitric oxide/MAP1B signalling to axon guidance and retraction and to elucidate the role of MAP1B in microtubule-actin crosstalk on a molecular level. In the course of these studies we will explore the role of the nitric oxide/MAP1B pathway in semaphorin signalling and how the novel pathway is integrated with other signalling networks in the growth cone. We also want to analyse the role of tubulin nitrosylation and of microtubules and dynein as potentially instructive players in growth cone steering, aspects that have received little attention so far. Understanding the impact of nitric oxide on the neuronal cytoskeleton might open new perspectives on the problems neurons face in situations of excessive nitric oxide production during inflammation of the central nervous system and might help to protect neurons and support regeneration under such conditions. The project is designed to be carried out by a postdoc and a PhD student who will benefit for their careers from the know-how already in the laboratory and from working on complementing aspects of this integrated project.

In this project we obtained new insight into the molecular machinery of nerve cells needed to connect to other cells to form or restore a functioning nervous system. Our results contribute to a more detailed understanding of molecular events participating in network formation and could one day be helpful to design therapeutic strategies to restore damaged neuronal connections.The nervous system of humans and animals consists of billions of neurons that are connected to each other through thin extensions to form an extensive network. This network receives information from sensory organs, combines this information, compares it to previously received input (memory), computes an appropriate response and initiates and coordinates the response of the organism, for example by sending the appropriate signals to muscles. In addition, the nervous system regulates the activities of many organs in the body.To fulfill these functions the neurons of the nervous system need to be connected in the correct way. To establish the correct connections during development or re-establish them after the nervous system has been injured by trauma or stroke, the thin extensions emanating from each neuron need to be guided to their appropriate connection target. To achieve this, signposts in form of specific molecules are deposited in the extracellular environment surrounding the neurons. The neuronal extensions can follow these signposts to find the right target.We are interested in finding out how neuronal extensions can respond to these signposts. In principle, signpost molecules trigger rearrangements within the extensions which allow them to continue to migrate on their path or make a turn and change direction where appropriate. Thus, extracellular signpost molecules trigger a series of modifications of intracellular proteins. This in turn leads to an orchestrated rearrangement of intracellular structures which is needed to direct migration. In this project we characterized some of these molecular changes and molecular activities as part of a long standing effort towards a detailed understanding of biochemical processes underlying neuronal network formation.