Abstract
Precise timing of neuronal inputs is crucial for brain circuit function and development, where it contributes critically to experience-dependent plasticity. Myelination therefore provides an important adaptation mechanism for vertebrate circuits. Despite its importance to circuit activity, the interplay between neuronal activity and myelination has yet to be fully elucidated. In recent years, significant attention has been devoted to uncovering and explaining the phenomenon of white matter (WM) plasticity. Here, we summarize some of the critical evidence for modulation of the WM by neuronal activity, ranging from human diffusion tensor imaging (DTI) studies to experiments in animal models. These experiments reveal activity-dependent changes in the differentiation and proliferation of the oligodendrocyte lineage, and in the critical properties of the myelin sheaths. We discuss the implications of such changes for synaptic function and plasticity, and present the underlying mechanisms of neuron–glia communication, with a focus on glutamatergic signaling and the axomyelinic synapse. Finally, we examine evidence that myelin plasticity may be subject to critical periods. Taken together, the present review aims to provide insights into myelination in the context of brain circuit formation and function, emphasizing the bidirectional interplay between neurons and myelinating glial cells to better inform future investigations of nervous system plasticity.
Highlights
Reviewed by: Lester Melie-Garcia, Lausanne University Hospital (CHUV), Switzerland Davide Lecca, Università degli Studi di Milano, Italy
Myelin changes may be reflected at the gross level as white matter (WM) changes, which have been extensively observed in the human brain using diffusion tensor imaging (DTI) to measure fractional anisotropy (FA)
Defined synaptic junctions between unmyelinated axons and or progenitor cells (OPC) in the rat corpus callosum were observed by immunofluorescence and electron microscopy (Kukley et al, 2007; Ziskin et al, 2007). They appeared to be the sites of neurotransmitter release from unmyelinated axons onto Ca2+-permeable amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR) on postsynaptic-like densities of OPCs, investigations in DRG and OPC cocultures implicated glutamate and ATP release directly from axon varicosities to nearby OPC processes at axoglial contact sites, which lacked the characteristic features of synapses and likely arose from spontaneous vesicular release (Wake et al, 2015)
Summary
Myelin changes may be reflected at the gross level as WM changes, which have been extensively observed in the human brain using diffusion tensor imaging (DTI) to measure fractional anisotropy (FA) (reviewed by Zatorre et al, 2012; Fields, 2015; Sampaio-Baptista and Johansen-Berg, 2017). Cross-sectional studies have a caveat: even though they may attempt to reduce confounds by matching participants, such as, for culture when examining reading skills (Carreiras et al, 2009) or for years of training and experience in musicians (Steele et al, 2013), by design, they cannot parse out pre-existing from experience-dependent structural changes This disadvantage can be reduced using a longitudinal approach to examine activity-dependent WM changes, since structural images acquired before and after task-specific training can be compared. The link between task demands and FA increases in corresponding brain regions tends to be correlated with the amount of training rather than outcome (final skill level), perhaps pointing to different anatomical correlates of learning versus performance (Scholz et al, 2009; Takeuchi et al, 2010) In these studies, gray versus WM structural changes seem to be relatively independent, both spatially and temporally. NEURONAL ACTIVITY IS ASSOCIATED WITH MYELINATION CHANGES AT THE CELLULAR AND MOLECULAR LEVELS
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