Investigating the mechanisms of glutamatergic axon-myelin communication in zebrafish

Abstract

The proper functioning of our nervous system relies not only on neurons but also on the critical support provided by glial cells. Among these glial cells, oligodendrocytes play a vital role by wrapping myelin around axons, a process essential for efficient action potential conduction. Disruption or breakdown of myelin has been associated with various diseases, from neurodevelopmental to neurodegenerative, highlighting the significance of neuron-oligodendrocyte interactions in maintaining axonal health.

In a healthy organism, myelin plays a crucial role in influencing axonal conduction velocity. Increased myelin thickness and sheath length accelerate conduction velocity, making it essential to synchronise the signal integration into the wider network. This synchrony and acceleration of the action potential is actively mediated by alterations of the myelin sheath morphology during learning. Myelin adapts to the level of neuronal activity by changing its sheath length, number, and thickness.

Therefore, myelin is assumed to dynamically fine-tune network function by altering the conduction velocity of the action potential.

Remarkably, this process is pivotal during various learning events. However, the mechanistic link between changes in neuronal activity and the regulation of myelin sheath length remains largely unknown.

In this study, I utilized the zebrafish model to investigate the molecular mechanisms governing axon-myelin communication, specifically focusing on activity-induced sheath elongation. The neurotransmitter glutamate has long been thought likely to facilitate this activity-dependent axon-myelin communication. However, previous studies on ionotropic glutamate receptors (iGluRs) at the axon-myelin interface did not report iGluRs as regulators of in vivo myelination.

I investigated the role of mGluRs (metabotropic glutamate receptors) in activitydependent myelination. Using pharmacology to manipulate mGluR5 activity, I observed that mGluR5 activation led to the elongation of myelin sheaths and stimulated high-amplitude Ca²⁺ events in myelin, indicating its physiological impact on myelination in zebrafish. Ca²⁺ transients have been characterised to play an important role in myelin sheath growth, oligodendrocyte lineage progression and canonical mGluR5 signalling.

To further understand the role of mGluR5 in myelination, I generated mGluR5 mutant fish. The average myelin sheath length in mGluR5 mutants was significantly reduced. Transgenic overexpression of mGluR5 specifically in myelinating oligodendrocytes rescued the short myelin sheath length of mGluR5 mutants. Additionally, mGluR5 mutants failed to induce the characteristic high-amplitude Ca²⁺ releases upon pharmacological mGluR5 stimulation.

To assess the role of mGluR5 as a receptor in activity-driven myelination, I developed an opto-stimulation platform to drive neuronal activity. Notably, optical stimulation of chx10 motor-circuit interneurons led to increased frequency and amplitude of myelin Ca²⁺ releases and longer myelin sheaths. Strikingly, the myelin of mGluR5-deficient animals did not respond to this neuronal stimulation. mGluR5-deficient animals did not show activity-induced high-amplitude myelin Ca²⁺ releases and their myelin sheath length was not increased following stimulation of neuronal activity.

Through these in vivo investigations, I demonstrate that mGluR5 plays a crucial role as a receptor in myelin sheath elongation in response to heightened neuronal activity. My findings shed light on the intricate mechanisms underlying adaptive myelination, providing a deeper understanding of the dynamic interplay between neuronal activity and glial cell function.