Floor Plate - Axon Guidance

Axon Guidance

In the development of the central nervous system, the decision of a neuron to cross or not cross the midline is critical. In vertebrates, this choice is mediated by the floor plate, and enables the embryo to develop successful left and right body halves with respect to nervous tissue. For example, while ipsilateral neurons do not cross the midline, commissural neurons cross the midline forming a single commissure. These particular neurons develop in the dorsal region of the neural tube and travel ventrally toward the floor plate. Upon reaching the floor plate, commissural neurons cross through the structure to emerge on the opposite side of the neural tube, whereupon they project anteriorly or posteriorly within the tube.

• Netrins: Netrins are proteins expressed and secreted by cells of the floor plate. Experiments using floor plate extracts and commissural neurons embedded in a collegen matrix show attraction of neurons towards the floor plate in vitro. Moreover, Isolation and transfection of Netrin-1 and Netrin-2, two secreted proteins, into Cos cells has similar effects. Further research confirmed that Netrins act as attractant proteins in addition to Shh to guide commissural axons toward the floor plate. Netrins are secreted by the floor plate cells and function to bind the axon receptor DCC in a chemotactic manner.

• Slit: Slit is a secreted ligand expressed in the floor plate and functions to inhibit axonal crossing of the neural tube. While netrins attract commissural neurons toward the midline, slit proteins repel and expel neurons from the midline. As axons not destined to cross the midline project through the neural tube they are repelled by the ligand slit which is expressed in the cells of the floor plate. Slit acts through its receptors Roundabout (Robo) 1 and 2. This interaction inhibits the chemotaxis provided by the Netrin/DCC pathway. However, Robo-3 (Rig-1) is upregulated during growth of commissural axons during migration toward the floor plate, which sequesters Robo-1/2 inside the cell within vesicles. Consequently, the Netrin/DCC attraction pathway dominates over the Slit/Robo repulsion pathway and the axon can grow toward the midline and enter the floor plate. Upon entering, through a mechanism not yet fully understood, Robo-3 becomes downregulated and this liberates and upregulates Robo-1/2, effectively repelling the neuron from the floor plate midline. Through this complex cross talk of Slit, Robo-1/2, and Robo-3, commissural axons are guided toward the midline to cross the neural tube and prevented from crossing back.

The signaling molecules guiding the growth and projections of commissural neurons have well studied homologs in invertebrates. In the Netrin/DCC chemoattraction pathway the C. elegans homologs are Unc-6/Unc-40 and Unc-5 while the Drosophila homologs are Netrin-A and Netrin-B/Frazzled and Dunc5. In the Slit/Robo chemoreppelant pathway the C. elegans homologs are Slt-1/Sax-3 whereas the Drosophila homologs are also known as Slit/Robo(1-3).