Signaling Over Distances

Abstract

Neurons are extremely polarized cells. Axon lengths often exceed the dimension of the neuronal cell body by several orders of magnitude. These extreme axonal lengths imply that neurons have mastered efficient mechanisms for long distance signaling between soma and synaptic terminal. These elaborate mechanisms are required for neuronal development and maintenance of the nervous system. Neurons can fine-tune long distance signaling through calcium wave propagation and bidirectional transport of proteins, vesicles, and mRNAs along microtubules. The signal transmission over extreme lengths also ensures that information about axon injury is communicated to the soma and allows for repair mechanisms to be engaged. This review focuses on the different mechanisms employed by neurons to signal over long axonal distances and how signals are interpreted in the soma, with an emphasis on proteomic studies. We also discuss how proteomic approaches could help further deciphering the signaling mechanisms operating over long distance in axons. Neurons have unique and highly polarized morphologies. The axon allows intracellular communication between the cell soma and the distantly located synaptic terminal. The extreme length of axons relative to the cell soma diameter poses great challenges for neuronal development, maintenance, and repair. Anterograde and retrograde axonal transport of proteins vesicles and RNA along the microtubule tracks in the axon is crucial to sustain the required signaling events underlying development and maintenance of the nervous system. The retrograde transport system is also used following axon injury to communicate information from the site of injury back to the soma. Communication between distant axon locations and the cell soma is also ensured by a more rapid signal encoded in calcium waves. Incoming signals from distant axon locations are interpreted by the soma to regulate gene expression for survival or repair. The neuronal epigenome is also influenced by incoming signals to appropriately coordinate transcriptional responses. In this review, we discuss the state of the field regarding the different mechanisms employed by neurons to signal intracellularly over long distances in axons and how signals are interpreted in the cell soma, with an emphasis on proteomic studies. We also discuss how the use of proteomics could further help in understanding long distance signaling system in axons. The communication employed by neurons to signal along dendrites has been reviewed in detail elsewhere (1) and will not be discussed here.Calcium Influx, Back Propagation, and Long Distance Effects-Alterations in axonal calcium levels affect multiple and diverse signaling events, including local protein synthesis and degradation. These local signals can have long distance effects. Axon injury provides a powerful system to study the role of calcium in axonal signaling, because injury-induced calcium influx in the axon triggers important downstream events at the injury site and the soma (Fig.

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hnRNPs Interacting with mRNA Localization Motifs Define AxoNAl RNA Regulons

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