Neuronal Polarity: Establishment and Maintenance

The term polarity in a biological context is used to describe an asymmetry in morphology and distribution of molecules. In neurons, their complex shape with typically one axon and several dendrites reflects this asymmetry. Although neurons assume many different shapes and sizes they always maintain these two domains, which are essential for neuronal function. In a most simple view, neurons use their axon to transmit signals over long distances due to its capacity to extend to enormous lengths. Dendrites, on the other hand, are shorter and receive and integrate signals from different locations. The selection of the site where the axon and dendrites initially emerge during embryonic development is a tightly regulated event, eventually important for the correct formation of neuronal circuits, and disturbances of these processes can have pathological consequences due to circuit malformation. In general, the first sign of asymmetry in pyramidal neurons from the hippocampus or the cortex, developing in culture or in situ, is the formation of a single fast growing axon. Neurons initially extend several equal protrusions referred to as minor neurites, until one of grows more than the others; thus developing into an axon. Later, the other neurites differentiate into dendrites. In vivo excitatory pyramidal neurons form the axon in the process of migration. Thus the axon-dendritic polarity coincides with the establishment of the polarity of a migrating neuron with a leading and trailing edge. An important question is which mechanisms neurons utilize to specify the sites where axonal and dendrite outgrowth occurs and how their identities are maintained during and after development. The formation of these functionally diverse domains is the result of polarized differences of membrane and protein delivery, mitochondria transport, actin dynamics and microtubule stability. However how and in which temporal order all those events which coordinate the selection and maintenance of axons and dendrites is still under investigation. This selection of articles shall highlight new findings, which help to unravel all molecular and cellular events important for neuronal polarity establishment and maintenance.