The role of myelination in measures of white matter integrity: Combination of diffusion tensor imaging and two-photon microscopy of CLARITY intact brains

Abstract

Diffusion tensor imaging (DTI) is used extensively in neuroscience to noninvasively estimate white matter (WM) microarchitecture. However, the diffusion signal is inherently ambiguous because it infers WM structure from the orientation of water diffusion and cannot identify the biological sources of diffusion changes. To compare inferred WM estimates to directly labeled axonal elements, we performed a novel within-subjects combination of high-resolution ex vivo DTI with two-photon laser microscopy of intact mouse brains rendered optically transparent by Clear Lipid-exchanged, Anatomically Rigid, Imaging/immunostaining compatible, Tissue hYdrogel (CLARITY). We found that myelin basic protein (MBP) immunofluorescence significantly correlated with fractional anisotropy (FA), especially in WM regions with coherent fiber orientations and low fiber dispersion. Our results provide evidence that FA is particularly sensitive to myelination in WM regions with these characteristics. Furthermore, we found that radial diffusivity (RD) was only sensitive to myelination in a subset of WM tracts, suggesting that the association of RD with myelin should be used cautiously. This combined DTI-CLARITY approach illustrates, for the first time, a framework for using brain-wide immuno-labeling of WM targets to elucidate the relationship between the diffusion signal and its biological underpinnings. This study also demonstrates the feasibility of a within-subject combination of noninvasive neuroimaging and tissue clearing techniques that has broader implications for neuroscience research.

Diffusion tensor imaging measures of white matter compared to myelin basic protein immunofluorescence in tissue cleared intact brains

“…Subsequently the CLARITY hydrogels were polymerized, tissue cleared, MBP immunolabeled, and finally imaged intact using a two-photon laser scanning microscope. Details of each step can be found below and also in the research article [1].…”

Design and Validation of Diffusion MRI Models of White Matter

“…Modalities such as two-photon microscopy (Oh et al, 2014) or optical coherence tomography (Magnain et al, 2014; Srinivasan et al, 2012) also image up to tens of microns below the surface and can provide contiguous 3D images of large tissue. Many of the whole-specimen imaging techniques developed for local neuron connectomic studies (Denk and Horstmann, 2004; Dodt et al, 2007; Mikula et al, 2012) have demonstrated utility when coupled with diffusion MRI (Chang et al, 2017). …”

All Wrapped Up: Environmental Effects on Myelination

“…Since water diffusivity is sensitive to subtle differences in the composition of cellular membranes, it allows for three-dimensional characterization of changes in microstructural architecture, i.e. changes in WM integrity [61]. A common metric of DTI used to describe the direction of water diffusion is fractional anisotropy (FA), which increases with myelination, and decreases with hypomyelination or demyelination.…”