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

Alexandra Lucas,Elizabeth A Mccullagh,Shani Poleg,Achim Klug

doi:10.3389/fnins.2021.772943

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.

Highlights

Fragile X Syndrome (FXS) is the most common monogenic form of autism spectrum disorder (ASD) and occurs in 1:4,000–1:8,000 people in the United States (Hagerman and Hagerman, 2008)
We used several (EM and coherent anti-stokes Raman spectroscopy (CARS)) microscopy techniques to examine the myelin microstructure of putative globular bushy cell axons in the auditory brainstem that innervate the medial nucleus of the trapezoid body (MNTB) and form the Calyx of Held in Fmr1 KO mice compared to wildtype controls (B6)
CARS microscopy for myelination imaging was used over electron microscopy (EM) due to its speed and compatibility with immunofluorescence (Wang et al, 2005), in this case we used a Nissl stain for simultaneous cell body staining

Summary

Introduction

Fragile X Syndrome (FXS) is the most common monogenic form of autism spectrum disorder (ASD) and occurs in 1:4,000–1:8,000 people in the United States (Hagerman and Hagerman, 2008). Recent work has shown that myelination may underly some the phenotypes common in ASD and FXS (Pacey et al, 2013; Phan et al, 2020). Auditory phenotypes, such as auditory hypersensitivity, are often conserved between mouse and human FXS, and it is possible that myelination deficits are conserved (McCullagh et al, 2020b; Phan et al, 2020). The mechanisms by which binaural hearing is impaired in people with FXS and ASD are unknown, there is evidence the auditory brainstem is involved (Kulesza and Mangunay, 2008; Kulesza et al, 2011; Wang et al, 2014; Rotschafer et al, 2015; Garcia-Pino et al, 2017; McCullagh et al, 2017, 2020a; Zorio et al, 2017; Curry et al, 2018; El-Hassar et al, 2019; Lu, 2019)

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