Whitington Lab ~ Axon Guidance

Members of the Whitington Lab.

Principal investigator: Assoc Prof Paul Whitington

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Welcome to the Whitington Lab. Our group is interested in the problem of how, during development, axons navigate along specific pathways to find their appropriate partners. This pathfinding ability is essential for the correct "wiring-up" of neural circuits in the brain and other parts of the nervous system.

The problem is akin to that faced by a commuter seeking to travel from one station to another on the London Underground... The growing axon, like the commuter, faces a series of choices - which line to take? which direction to follow?

The growing end of the axon is a special structure called the growth cone (see image below). Finger-like projections (filopodia) rapidly extend from the growth cone, exploring the environment in its vicinity.

Reproduced from The Journal of Cell Biology, 2002, 157 (5) by copyright permission of The Rockefeller University Press

The growth cone possesses the amazing ability to navigate through the complex environment of the developing embryo, responding, we believe, to a series of molecular sign-posts at each junction along the long journey to its final target. A major goal of Neuroscience is to identify these molecular guidance cues and understand how they steer the axon.

Our Model: The Fruitfly Drosophila melanogaster

We have chosen the embryo of the fruit-fly Drosophila melanogaster as our model organism for studying this problem. Drosophila offers an unmatched array of powerful genetic tools which enable us to isolate mutants in which axon growth has gone awry. The genes that have been knocked-out in these mutants could code for proteins that guide the growth cone along its normal trajectory. This gives us a way of finding "axon guidance" molecules, a strategy that has been highly successful.

Because most of the genes in the Drosophila genome are shared with higher animals, including mammals, these discoveries have also greatly improved our understanding of how genes control the pathfinding of growing axons in humans. Such knowledge will underpin future clinical strategies for reconnecting neural pathways damaged through injury or disease. For example, it may help us find ways to stimulate and guide the regrowth of axons in crushed spinal cords.

Drosophila goes through a series of stages in its life cycle: the fertilized egg develops into an embryo, which hatches as a larva. After further growth the larva constructs a pupal case, within which it first breaks down then reconstructs its body, to emerge as a new adult fly. The initial construction of the nervous system takes place during embryonic development, before hatching of the egg. We have focussed our attention on the embryo, as we wish to understand how axons first make their way to their targets.

The Drosophila embryo has a relatively simple nervous system in comparison to a higher animal, such as a mammal. The central nervous system (CNS) consists of the brain and a segmented ventral nerve cord (VNC). Each segment contains a repeated set of around 600 neurons, made up of motorneurons and interneurons. The ladder-like arrangement of axon pathways in the VNC is clearly seen in the embryo below. The rungs of the ladder are bundles of axons running from one side of a segment to the other; the uprights are axons running between segments.

Ventral view of a whole Drosophila embryo stained with the monoclonal antibody BP104, which binds to axons of all neurons in the CNS. The outlines of the brain and the VNC are shown below. Image and diagram by Paul Whitington.

The sensory nervous system is even simpler than the CNS, comprising just 42 sensory neurons per hemisegment, each of which has been individually identified. These neurons lie in four clusters in the body wall (see image and drawing below). Axons from the sensory neurons bundle together as they head towards their synaptic targets in the ventral nerve cord.

Side view of a whole Drosophila embryo (head to the left, dorsal up), stained with the monoclonal antibody 22c10, which binds to all sensory neurons. The drawing below shows the outline of the CNS and the four clusters of sensory neurons in one hemisegment. Image by Kerri-Lee Harris.

We have been attracted to the elegant simplicity of the sensory system, as it allows us to study axon guidance at the level of single neurons. This gives a high degree of precision to our observations.