Abstract
ABSTRACTNeurons are highly polarized cells that consist of three main structural and functional domains: a cell body or soma, an axon, and dendrites. These domains contain smaller compartments with essential roles for proper neuronal function, such as the axonal presynaptic boutons and the dendritic postsynaptic spines. The structure and function of these compartments have now been characterized in great detail. Intriguingly, however, in the last decade additional levels of compartmentalization within the axon and the dendrites have been identified, revealing that these structures are much more complex than previously thought. Herein we examine several types of structural and functional sub‐compartmentalization found in neurons of both vertebrates and invertebrates. For example, in mammalian neurons the axonal initial segment functions as a sub‐compartment to initiate the action potential, to select molecules passing into the axon, and to maintain neuronal polarization. Moreover, work in Drosophila melanogaster has shown that two distinct axonal guidance receptors are precisely clustered in adjacent segments of the commissural axons both in vivo and in vitro, suggesting a cell‐intrinsic mechanism underlying the compartmentalized receptor localization. In Caenorhabditis elegans, a subset of interneurons exhibits calcium dynamics that are localized to specific sections of the axon and control the gait of navigation, demonstrating a regulatory role of compartmentalized neuronal activity in behaviour. These findings have led to a number of new questions, which are important for our understanding of neuronal development and function. How are these sub‐compartments established and maintained? What molecular machinery and cellular events are involved? What is their functional significance for the neuron? Here, we reflect on these and other key questions that remain to be addressed in this expanding field of biology.
Highlights
SUB-COMPARTMENTS IN NEURONSNeurons are specialized cells with a high level of polarization defined by the presence of three major compartments: the dendrites, a cell body and an axon (Fig. 1A)
It does not consider the unipolar neurons in which the unique neurite has a mixed axon/dendrite identity, and second, it does not take into account those functional and molecular sub-compartments that have been identified and characterized by a growing body of studies (Fig. 1)
Dendritic spines were first observed as protrusions on the dendrites through gold staining by Ramon y Cajal, and represent the dendritic sub-compartments that receive inputs from a single axon (Gray, 1959; Guillery, 2000)
Summary
Neurons are specialized cells with a high level of polarization defined by the presence of three major compartments: the dendrites, a cell body (or soma) and an axon (Fig. 1A). Dendrites are specialized to receive electrochemical signals, which are processed and transferred through the cell body and along the axon to be transmitted to the target cell/s This broad definition of neurons containing just three major compartments is simplistic for two reasons. We highlight reports of functional and molecular sub-compartmentalization, referred to as intra-neurite patterning, within the major neuronal compartments that do not have distinct morphology Both in vitro and in vivo models provide evidence of neurites with shared axonal and dendritic properties, axonal compartmentalization of membrane proteins, such as axon guidance receptors and ion channels, and clustering of intracellular components within neurites, which include protein aggregates, organelles and ions (Fig. 1). These findings raise intriguing questions on the development, maintenance, and function of these sub-compartments (Katsuki et al, 2011)
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