The basics of Axon guidance: Role of Netrin, SHH, BMP and Ephrin

Abstract

Neurons navigate through tissues, over both long and short distances, to locate their target synaptic partners, forming the neural circuit. Specific guidance cues belonging to highly conserved families direct the neurons through their axons along specific pathways by attractive and repulsive actions. These guidance systems create a surprisingly diverse set of neuronal circuits, in the: spinal cord, forebrain, and retinotopic organization. The exact dynamics of the growth cones while navigating through the signaling pathways are not thoroughly understood, only the major mechanisms and proteins commonly found are recognized. The role of several extracellular receptors and ligands in the process has also been identified. Having bifunctional molecules acting as both growth-inductive and inhibitory cues, axon guidance remains a complex process with immediate as well as long-term visible consequences in the form of neural disorders leading to many diseases. The overall mechanism with four of the key molecules that guide this pathfinding in three separate commissures are the topics of this review.