Myelination Deficits in the Auditory Brainstem of a Mouse Model of Fragile X Syndrome

Abstract

Auditory symptoms are one of the most frequent sensory issues described in people with Fragile X Syndrome (FXS), the most common genetic form of intellectual disability. However, the mechanisms that lead to these symptoms are under explored. In this study, we examined whether there are defects in myelination in the auditory brainstem circuitry. Specifically, we studied myelinated fibers that terminate in the Calyx of Held, which encode temporally precise sound arrival time, and are some of the most heavily myelinated axons in the brain. We measured anatomical myelination characteristics using coherent anti-stokes Raman spectroscopy (CARS) and electron microscopy (EM) in a FXS mouse model in the medial nucleus of the trapezoid body (MNTB) where the Calyx of Held synapses. We measured number of mature oligodendrocytes (OL) and oligodendrocyte precursor cells (OPCs) to determine if changes in myelination were due to changes in the number of myelinating or immature glial cells. The two microscopy techniques (EM and CARS) showed a decrease in fiber diameter in FXS mice. Additionally, EM results indicated reductions in myelin thickness and axon diameter, and an increase in g-ratio, a measure of structural and functional myelination. Lastly, we showed an increase in both OL and OPCs in MNTB sections of FXS mice suggesting that the myelination phenotype is not due to an overall decrease in number of myelinating OLs. This is the first study to show that a myelination defects in the auditory brainstem that may underly auditory phenotypes in FXS.

Predicting the Influence of Axon Myelination on Sound Localization Precision Using a Spiking Neural Network Model of Auditory Brainstem

“…Moreover, myelination patterns differ between axons responding to low vs. high frequency sound ( Ford et al, 2015 ) and are altered when the animal’s sound experience is experimentally altered ( Sinclair et al, 2017 ), suggesting that sound activity is involved in the establishment, and perhaps the maintenance of myelination. On the other hand, alterations in myelination as they occur in some conditions such as Fragile X ( Lucas et al, 2021 ) or multiple sclerosis ( Levine et al, 1993 ) or in animals with myelination deficits ( Kim E. K. et al, 2013 ; Kim J. H. et al, 2013 ) result in temporally less well-timed activity in the sound localization pathway and/or impaired sound localization abilities, highlighting the functional consequences of these alterations. Taken together, these findings highlight the need for precisely controlled myelination patterns and suggest a possible mechanism to exert this control.…”

Age-related myelin deficits in the auditory brain stem contribute to cocktail-party deficits

“…After establishing the electrophysiological abnormalities in the aging gerbil auditory brainstem, we hypothesized that these abnormalities could be attributed to changes in axonal conductance because previous work has shown the potential for myelination plasticity throughout life 40,41 . Using CARS microscopy 38,39 , we first measured the axonal diameter of putative globular bushy cell axons in the auditory brainstem that innervate the MNTB and form the calyx of Held. In both young and old gerbils, the MNTB was sampled with multiple z-stack images, which were subsequently analyzed for axon thickness (Figure 4).…”

“…Using EM and CARS microscopy, we identified an age-dependent decrease in myelination. Oligodendrocytes (OLs) are specialized glia cells responsible for producing myelin which insulates axons in the central nervous system, and their loss can lead to changes in myelination in MNTB 39 . To investigate whether structural changes are linked to alterations in the quantity of mature (OL) and precursor oligodendrocytes (OPC), we employed an immunohistochemical method.…”

“…In our study, we employ a combination of electrophysiological assessments of auditory circuits, immunohistochemistry, electron microscopy (EM) and coherent anti-stokes Raman spectroscopy (CARS) to investigate the contribution of the myelin microstructure in aging gerbils compared to young adult controls 38,39 . We found significantly attenuated wave-III and earlier onset wave-IV in the click ABRs from old gerbils, suggesting degradation of inhibitory inputs and MNTB activity in the superior olivary complex.…”

Current Best Practices for Analysis of Dendritic Spine Morphology and Number in Neurodevelopmental Disorder Research

“…Many of the same considerations for dendritic spine analysis can be used when determining other types of anatomical questions related to developmental disorders. For example, measurement of myelin, which has recently been shown to be altered in autism spectrum disorders, − can be performed using electron microscopy (EM) or immunofluorescent compatible techniques such as coherent anti-Stokes Raman scattering (CARS). , Similar to considerations for dendritic spines, the technique used will likely depend on cost (EM being more expensive), with trade-offs for resolution on fine myelin microstructure (CARS being limited to the confocal microscope it is paired with), compatibility with dyes (CARS generally compatible with immunofluorescence depending on the experimental setup), or issues related to fixation or preparation requirements, in addition to feasibility such as availability of equipment (EM being more widely available across institutions). Therefore, similar considerations should be made when determining anatomical measurements across types of experiment, and the considerations presented here are widely applicable.…”