Differential effect on myelination through abolition of activity-dependent synaptic vesicle release or reduction of overall electrical activity of selected cortical projections in the mouse.

Kim V Korrell,Auguste Vadisiute,Maï‐Carmen Requena‐Komuro,Anna Hoerder‐Suabedissen,Zoltán Molnár,Harriet Fodder,Jolande Disser,Kristina Parley,Charlotte Pollart,Graham Knott

doi:10.1111/joa.12974

Abstract

Myelination of axons by oligodendrocytes in the central nervous system is crucial for fast, saltatory conduction of action potentials. As myelination is central for brain development and plasticity, and deficits are implicated in several neural disorders such as multiple sclerosis, major depressive disorder, bipolar disorder and schizophrenia, it is important to elucidate the underlying mechanisms regulating myelination. Numerous mechanisms have been proposed by which the communication between oligodendrocytes and active axons may regulate the onset and maintenance of activity‐dependent myelination. We compared two models of ‘silencing' layer V and/or VI cortical projection neurons from early stages by either decreasing their excitability through Kir2.1 expression, an inward rectifying potassium channel, introduced through in utero electroporation at embryonic day (E)13.5, or inhibiting regulated vesicular release through Cre‐dependent knock‐out of synaptosomal associated protein 25 kDA (SNAP25). SNAP25 is a component of the soluble N‐ethylmaleimide fusion protein attachment protein receptor (SNARE) complex, which, among others, is needed for calcium‐dependent regulated vesicle release from synapses. In layer VI cortical projection neurons in the Ntsr1‐Cre;Ai14;Snap25 fl/fl mouse, we found that inhibiting regulated vesicular release significantly decreased the amount of myelin basic protein (MBP, used as marker for myelination) and the amount of myelinated projections at postnatal day (P)14 without affecting the initial timing of onset of myelination in the brain (at P7/P8). Additionally, overall oligodendrocyte maturation appears to be affected. A strong trend towards reduced node of Ranvier (NoR) length was also observed in Ntsr1‐Cre;Ai14;Snap25 fl/fl corpus callosum. An equally strong trend towards reduced NoR length was observed in Rbp4‐Cre;Ai14;Snap25 fl/fl corpus callosum at P14, and the g‐ratio in the spinal cord dorsal column was reduced at P18. However, no measurable differences in levels of MBP were detected in the striatum when comparing Rbp4‐Cre;Ai14;Snap25 fl/fl and control brains. Conversely, Kir2.1 in utero electroporation at E13.5 did not significantly affect the amount of MBP or number of myelinated callosal axons at P14 but did significantly decrease the NoR length measured in the corpus callosum. It therefore seems likely that the excitability of the neuron can potentially perform a modulating function of myelin characteristics, whereas regulated vesicular release has the potential to have a more pronounced effect on overall myelination, but in a cell‐type specific manner.

Highlights

During development of the central nervous system (CNS) of mammals and other vertebrates, oligodendrocytes extend membrane processes that wrap axons in a lipid-rich myelin membrane
We reduced electrical excitability of a mixed layer V (LV) and layer VI (LVI) population by in utero electroporation of Kir2.1 at E13.5 (Fig. 1A), or ablated calcium-dependent vesicle release in two distinct cortical cell populations (LV in Rbp4-Cre and LVI in Ntsr1Cre, see Fig. 1C,B) by conditional knock-out of Snap25
LVI cells labelled in Ntsr1-Cre;Ai14; Snap25 fl/fl brains have a much slower axonal degeneration, only obvious by 2 months of age (Hoerder-Suabedissen et al 2018b)

Summary

Introduction

During development of the central nervous system (CNS) of mammals and other vertebrates, oligodendrocytes extend membrane processes that wrap axons in a lipid-rich myelin membrane. Within the mouse corpus callosum, myelination is first detectable at postnatal day (P) and the percentage of myelinated axons increases rapidly for several months, but continues to change at least until P240 (Sturrock, 1980). Axons with larger diameter become preferentially myelinated (mean diameter 0.46 lm for myelinated and 0.25 lm for unmyelinated axons in the mouse corpus callosum). There is considerable overlap between the axon diameters of myelinated and unmyelinated axons, and little change in the distribution of axon diameters in the corpus callosum during the time of peak myelination in mice (Sturrock, 1980). Axon diameter is clearly one determinant of whether myelination occurs or not, it cannot be the only factor regulating which axons become myelinated in vivo

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