Mutation-related apparent myelin, not axon density, drives white matter differences in premanifest Huntington's disease: Evidence from in vivo ultra-strong gradient MRI

White matter (WM) alterations have been observed early in Huntington's disease (HD) progression but their role in the disease-pathophysiology remains unknown. We exploited ultra-strong-gradient MRI to tease apart contributions of myelin (with the magnetization transfer ratio), and axon density (with the restricted volume fraction from the Composite Hindered and Restricted Model of Diffusion) to WM differences between premanifest HD patients and age- and sex-matched controls. Diffusion tensor MRI (DT-MRI) measures were also assessed. We used tractometry to investigate region-specific changes across callosal segments with well-characterized early- and late-myelinating axonal populations, while brain-wise alterations were explored with tract-based cluster analysis (TBCA). Behavioural measures were included to explore disease-associated brain-function relationships. We detected lower myelin in the rostrum of patients (tractometry: p = 0.0343; TBCA: p = 0.030), but higher myelin in their splenium (p = 0.016). Importantly, patients' myelin and mutation size were positively associated (all p-values < 0.01), indicating that increased myelination might be a direct result of the mutation. Finally, myelin was higher than controls in younger patients but lower in older patients (p = 0.003), suggesting detrimental effects of increased myelination later in the course of the disease. Higher FR in patients' left cortico-spinal tract (CST) (p = 0.03) was detected, and was found to be positively associated with MTR in the posterior callosum (p = 0.033), possibly suggesting compensation to myelin alterations. This comprehensive, ultra-strong gradient MRI investigation provides novel evidence of CAG-driven myelin alterations in premanifest HD which may reflect neurodevelopmental, rather than neurodegenerative disease-associated changes.

Abstract P195: No Functional Nerve Recovery To 6 Months Post Renal Denervation With Rf Energy In A Normotensive Pig Model

Background: The safety and efficacy of catheter-based radio frequency (RF) renal denervation (RDN) have been demonstrated in randomized, sham-controlled trials. Long-term durability of blood pressure reduction following RDN has also been demonstrated by all-comer registries, although published pre-clinical reports of functional renal nerve regrowth are not consistent. We quantified the processes that support RDN procedural durability utilizing animal models. Methods: Animal studies were conducted in accordance with published guidelines. RDN was performed (4 lesions in the main renal artery) in normotensive swine using the Symplicity Spyral™ RDN system (Medtronic, Santa Rosa, CA, USA). Two additional groups not undergoing RDN served as control. Serial histological tissue samples were obtained in separate groups at 7 (n=12/group) and 180 (N=16/group) days post-procedure in all animals followed by bioanalytical quantification of cortical norepinephrine (NE) levels and immunohistochemical analysis of renal cortical axon density in matched samples. Results: Renal cortical axon density and NE levels were significantly reduced at 7 days and persisted through 180 days following RDN compared with control ( Figure ). Nerve fibrosis and necrosis were observed in the region of ablation, while nerve body atrophy was apparent distal to ablation location at 180 days. Conclusions: Reductions in both NE and renal cortical axon density were sustained at 7 and 180 days post-RDN procedure using RF renal denervation in a normotensive swine model. These data confirm and extend other pre-clinical and clinical evidence of long-term durability of the RDN procedure using RF energy.

Nerve transfer for restoration of lower motor neuron-lesioned bladder function. Part 3: Correlation between histological changes and nerve evoked contractions

We determined the effect of pelvic organ decentralization and reinnervation one year later on urinary bladder histology and function. Nineteen canines underwent decentralization by bilateral transection of all coccygeal and sacral (S) spinal roots, dorsal roots of lumbar (L)7 and hypogastric nerves. After exclusions, 8 were reinnervated 12 months post-decentralization with obturator-to-pelvic and sciatic-to-pudendal nerve transfers, then euthanized 8-12 months later; four served as long-term decentralized only animals. Before euthanasia, pelvic or transferred nerves and L1-S3 spinal roots were stimulated and maximum detrusor pressure (MDP) recorded. Bladder specimens were collected for histological and ex vivo smooth muscle contractility studies. Both reinnervated and decentralized animals showed less or denuded urothelium, fewer intramural ganglia, and more inflammation and collagen, than controls, although percent muscle was maintained. In reinnervated animals, pgp9.5+ axon density was higher, compared to decentralized animals. Ex vivo smooth muscle contractions in response to KCl correlated positively with submucosal inflammation, detrusor muscle thickness, pgp9.5+ axon density. In vivo, reinnervated animals showed higher MDP after stimulation of L1-L6 roots, compared to their transected L7-S3 roots, and reinnervated and decentralized animals showed lower MDP than controls after stimulation of nerves (due likely to fibrotic nerve encapsulation). MDP correlated negatively with detrusor collagen and inflammation, and positively with pgp9.5+ axon density and intramural ganglia numbers. These results demonstrate that bladder function can be improved by transfer of obturator nerves to pelvic nerves at one year after decentralization, although the fibrosis and inflammation that developed were associated with decreased contractile function.

Parity Attenuates Intraepithelial Corneal Sensory Nerve Loss in Female Mice

Aging impacts the ocular surface and reduces intraepithelial corneal nerve (ICN) density in male and female mice. Many researchers use retired breeders to study naturally aged female mice. Yet, the impact of parity and the length of time since breeders were retired on age-related changes in the intraepithelial corneal nerves is not known. Here we study 2 month (M) nulliparous (NP) females as well as 9M, 10M, and 11M NP and multiparous (MP) female mice to determine whether parity impacts the age-related decline seen in corneal axon density; 9M male mice are also included in these assessments. After showing that parity attenuates age-related loss in axon density, we also assess the impact of parity on corneal epithelial cell proliferation and find that it impacts cell proliferation and axon density normalized by cell proliferation. Stromal nerve arborization is also impacted by aging with parity enhancing stromal nerves in older mice. qPCR was performed on 20 genes implicated in ICN density using corneal epithelial RNA isolated from 10M NP and MP mice and showed that NGF expression was significantly elevated in MP corneal epithelium. Corneal sensitivity was significantly higher in 9M MP mice compared to NP mice and increased sensitivity in MP mice was accompanied by increased nerve terminals in the apical and middle cell layers. Together, these data show that parity in mice attenuates several aspects of the age-related decline seen on the ocular surface by retaining sensory axons and corneal sensitivity as mice age.

Feasibility of axon density metrics for brain asymmetry evaluation in the UK Biobank subsample

Standard diffusion MRI model with intra- and extra-axonal water pools offers a set of microstructural parameters describing brain white matter architecture. However, a non-linearity of the general model and diffusion data contamination by noise and imaging artefacts make estimation of diffusion metrics challenging. In order to develop reproducible and reliable diffusion approaches and to avoid computational model degeneracy, one needs to devise additional theoretical assumptions allowing a stable numerical implementation. As a result, it is possible to estimate intra-axonal water fraction (AWF) representing one of the important structural parameters. AWF can be treated as an indirect measure of axon density and has a strong potential as useful clinical biomarker. A few diffusion approaches such as white matter tract integrity, neurite orientation dispersion and density imaging, and spherical mean technique, allow one to evaluate AWF in the frame of their theoretical assumptions. In the present study, we considered the compatibility of axon density metrics obtained from different diffusion models and the influence of the diffusion metric on a brain asymmetry estimation in UK Biobank sample consisting of 182 subjects. We found AWF derived from a spherical mean technique is the most statistically representative measure. As a result, we revealed that brain asymmetry indecies derived from intra-axonal water fraction weakly decrease along the lifespan, reducing the left-right hemisphere difference within increased age.

The Relationship Between Axon Density, Myelination, and Fractional Anisotropy in the Human Corpus Callosum

The corpus callosum serves the functional integration and interaction between the two hemispheres. Many studies investigate callosal microstructure via diffusion tensor imaging (DTI) fractional anisotropy (FA) in geometrically parcellated segments. However, FA is influenced by several different microstructural properties such as myelination and axon density, hindering a neurobiological interpretation. This study explores the relationship between FA and more specific measures of microstructure within the corpus callosum in a sample of 271 healthy participants. DTI tractography was used to assess 11 callosal segments and gain estimates of FA. We quantified axon density and myelination via neurite orientation dispersion and density imaging (NODDI) to assess intra-neurite volume fraction and a multiecho gradient spin-echo sequence estimating myelin water fraction. The results indicate three common factors in the distribution of FA, myelin content and axon density, indicating potentially shared rules of topographical distribution. Moreover, the relationship between measures varied across the corpus callosum, suggesting that FA should not be interpreted uniformly. More specific magnetic resonance imaging-based quantification techniques, such as NODDI and multiecho myelin water imaging, may thus play a key role in future studies of clinical trials and individual differences.

Precommissural and postcommissural fornix microstructure in healthy aging and cognition

The fornix is a key tract of the hippocampal formation, whose status is presumed to contribute to age-related cognitive decline. The precommissural and postcommissural fornix subdivisions form respective basal forebrain/frontal and diencephalic networks that may differentially affect aging and cognition. We employed multi-parametric magnetic resonance imaging (MRI) including neurite orientation density and dispersion imaging, quantitative magnetization transfer (qMT), and T1-relaxometry MRI to investigate the microstructural properties of these fornix subdivisions and their relationship with aging and cognition in 149 asymptomatic participants (38–71 years). Aging was associated with increased free water signal and reductions in myelin-sensitive R1 and qMT indices but no apparent axon density differences in both precommissural and postcommissural fibers. Precommissural relative to postcommissural fibers showed a distinct microstructural pattern characterised by larger free water signal and axon orientation dispersion, with lower apparent myelin and axon density. Furthermore, differences in postcommissural microstructure were related to performance differences in object-location paired-associate learning. These results provide novel in vivo neuroimaging evidence for distinct microstructural properties of precommissural and postcommissural fibers that are consistent with their anatomy as found in axonal tracer studies, as well as for a contribution of postcommissural fibers to the learning of spatial configurations.