MRI identification of the rostral-caudal pattern of pathology within the corpus callosum in the cuprizone mouse model

Neural stem cells and oligodendrocyte progenitor cells compete for remyelination in the corpus callosum

A major therapeutic goal in demyelinating diseases, such as Multiple Sclerosis, is to improve remyelination, thereby restoring effective axon conduction and preventing neurodegeneration. In the adult central nervous system (CNS), parenchymal oligodendrocyte progenitor cells (pOPCs) and, to a lesser extent, pre-existing oligodendrocytes (OLs) and oligodendrocytes generated from neural stem cells (NSCs) in the sub-ventricular zone (SVZ) are capable of forming new myelin sheaths. Due to their self-renewal capabilities and the ability of their progeny to migrate widely within the CNS, NSCs represent an additional source of remyelinating cells that may be targeted to supplement repair by pOPCs. However, in demyelinating disorders and disease models, the NSC contribution to myelin repair is modest and most evident in regions close to the SVZ. We hypothesized that NSC-derived cells may compete with OPCs to remyelinate the same axons, with pOPCs serving as the primary remyelinating cells due to their widespread distribution within the adult CNS, thereby limiting the contribution of NSC-progeny. Here, we have used a dual reporter, genetic fate mapping strategy, to characterize the contribution of pOPCs and NSC-derived OLs to remyelination after cuprizone-induced demyelination. We confirmed that, while pOPCs are the main remyelinating cells in the corpus callosum, NSC-derived cells are also activated and recruited to demyelinating lesions. Blocking pOPC differentiation genetically, resulted in a significant increase in the recruitment NSC-derived cells into the demyelinated corpus callosum and their differentiation into OLs. These results strongly suggest that pOPCs and NSC-progeny compete to repair white matter lesions. They underscore the potential significance of targeting NSCs to improve repair when the contribution of pOPCs is insufficient to affect full remyelination.

Inhomogeneous Magnetization Transfer (ihMT) imaging in the acute cuprizone mouse model of demyelination/remyelination

BACKGROUND

To investigate the association of ihMT (inhom signals with the demyelination and remyelination phases of the acute cuprizone mouse model in comparison with histology, and to assess the extent of tissue damage and repair from MRI data.

METHODS

Acute demyelination by feeding 0.2% cuprizone for five weeks, followed by a four-week remyelination period was applied on genetically modified plp-GFP mice. Animals were scanned at different time points of the demyelination and remyelination phases of the cuprizone model using a multimodal MRI protocol, including ihMT T_1D-filters, MPF (Macromolecular Proton Fraction) and R₁ (longitudinal relaxation rate). For histology, plp-GFP (proteolipid protein - Green Fluorescent Protein) microscopy and LFB (Luxol Fast Blue) staining were employed as references for the myelin content. Comparison of MRI with histology was performed in the medial corpus callosum (mCC) and cerebral cortex (CTX) at two brain levels whereas ROI-wise and voxel-based analyses of the MRI metrics allowed investigating in vivo the spatial extent of myelin alterations.

RESULTS

IhMT high-pass T_1D-filters, targeted toward long T_1D components, showed significant temporal variations in the mCC consistent with the effects induced by the cuprizone toxin. In addition, the corresponding signals correlated strongly and significantly with the myelin content assessed by GFP fluorescence and LFB staining over the demyelination and the remyelination phases. The signal of the band-pass T_1D-filter, which isolates short T_1D components, showed changes over time that were poorly correlated with histology, hence suggesting a sensitivity to pathological processes possibly not related to myelin. Although MPF was also highly correlated to histology, ihMT high-pass T_1D-filters showed better capability to characterize the spatial-temporal patterns during the demyelination and remyelination phases of the acute cuprizone model (e.g., rostro-caudal gradient of demyelination in the mCC previously described in the literature).

CONCLUSIONS

IhMT sequences selective for long T_1D components are specific and sensitive in vivo markers of demyelination and remyelination and have successfully captured the spatially heterogeneous pattern of the demyelination and remyelination mechanisms in the cuprizone model. Interestingly, differences in signal variations between the ihMT high-pass and band-pass T_1D-filter, suggest a sensitivity of the ihMT sequences targeted to short T_1Ds to alterations other than those of myelin. Future studies will need to further address these differences by examining more closely the origin of the short T_1D components and the variation of each T_1D component in pathology.

Cuprizone feed formulation influences the extent of demyelinating disease pathology

Cuprizone is a copper-chelating agent that induces pathology similar to that within some multiple sclerosis (MS) lesions. The reliability and reproducibility of cuprizone for inducing demyelinating disease pathology depends on the animals ingesting consistent doses of cuprizone. Cuprizone-containing pelleted feed is a convenient way of delivering cuprizone, but the efficacy of these pellets at inducing demyelination has been questioned. This study compared the degree of demyelinating disease pathology between mice fed cuprizone delivered in pellets to mice fed a powdered cuprizone formulation at an early 3 week demyelinating timepoint. Within rostral corpus callosum, cuprizone pellets were more effective than cuprizone powder at increasing astrogliosis, microglial activation, DNA damage, and decreasing the density of mature oligodendrocytes. However, cuprizone powder demonstrated greater protein nitration relative to controls. Furthermore, mice fed control powder had significantly fewer mature oligodendrocytes than those fed control pellets. In caudal corpus callosum, cuprizone pellets performed better than cuprizone powder relative to controls at increasing astrogliosis, microglial activation, protein nitration, DNA damage, tissue swelling, and reducing the density of mature oligodendrocytes. Importantly, only cuprizone pellets induced detectable demyelination compared to controls. The two feeds had similar effects on oligodendrocyte precursor cell (OPC) dynamics. Taken together, these data suggest that demyelinating disease pathology is modelled more effectively with cuprizone pellets than powder at 3 weeks. Combined with the added convenience, cuprizone pellets are a suitable choice for inducing early demyelinating disease pathology.

Demyelination and remyelination detected in an alternative cuprizone mouse model of multiple sclerosis with 7.0 T multiparameter magnetic resonance imaging

The aim of this study was to investigate the mechanisms underlying demyelination and remyelination with 7.0 T multiparameter magnetic resonance imaging (MRI) in an alternative cuprizone (CPZ) mouse model of multiple sclerosis (MS). Sixty mice were divided into six groups (n = 10, each), and these groups were imaged with 7.0 T multiparameter MRI and treated with an alternative CPZ administration schedule. T₂-weighted imaging (T₂WI), susceptibility-weighted imaging (SWI), and diffusion tensor imaging (DTI) were used to compare the splenium of the corpus callosum (sCC) among the groups. Prussian blue and Luxol fast blue staining were performed to assess pathology. The correlations of the mean grayscale value (mGSV) of the pathology results and the MRI metrics were analyzed to evaluate the multiparameter MRI results. One-way ANOVA and post hoc comparison showed that the normalized T₂WI (T₂-nor), fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD), and axial diffusivity (AD) values were significantly different among the six groups, while the mean phase (Φ) value of SWI was not significantly different among the groups. Correlation analysis showed that the correlation between the T₂-nor and mGSV was higher than that among the other values. The correlations among the FA, RD, MD, and mGSV remained instructive. In conclusion, ultrahigh-field multiparameter MRI can reflect the pathological changes associated with and the underlying mechanisms of demyelination and remyelination in MS after the successful establishment of an acute CPZ-induced model.

An interactive meta-analysis of MRI biomarkers of myelin

Several MRI measures have been proposed as in vivo biomarkers of myelin, each with applications ranging from plasticity to pathology. Despite the availability of these myelin-sensitive modalities, specificity and sensitivity have been a matter of discussion. Debate about which MRI measure is the most suitable for quantifying myelin is still ongoing. In this study, we performed a systematic review of published quantitative validation studies to clarify how different these measures are when compared to the underlying histology. We analyzed the results from 43 studies applying meta-analysis tools, controlling for study sample size and using interactive visualization (https://neurolibre.github.io/myelin-meta-analysis). We report the overall estimates and the prediction intervals for the coefficient of determination and find that MT and relaxometry-based measures exhibit the highest correlations with myelin content. We also show which measures are, and which measures are not statistically different regarding their relationship with histology.

Machine learning based white matter models with permeability: An experimental study in cuprizone treated in-vivo mouse model of axonal demyelination

The intra-axonal water exchange time (τ_i), a parameter associated with axonal permeability, could be an important biomarker for understanding and treating demyelinating pathologies such as Multiple Sclerosis. Diffusion-Weighted MRI (DW-MRI) is sensitive to changes in permeability; however, the parameter has so far remained elusive due to the lack of general biophysical models that incorporate it. Machine learning based computational models can potentially be used to estimate such parameters. Recently, for the first time, a theoretical framework using a random forest (RF) regressor suggests that this is a promising new approach for permeability estimation. In this study, we adopt such an approach and for the first time experimentally investigate it for demyelinating pathologies through direct comparison with histology. We construct a computational model using Monte Carlo simulations and an RF regressor in order to learn a mapping between features derived from DW-MRI signals and ground truth microstructure parameters. We test our model in simulations, and find strong correlations between the predicted and ground truth parameters (intra-axonal volume fraction f: R² =0.99, τ_i: R² =0.84, intrinsic diffusivity d: R² =0.99). We then apply the model in-vivo, on a controlled cuprizone (CPZ) mouse model of demyelination, comparing the results from two cohorts of mice, CPZ (N=8) and healthy age-matched wild-type (WT, N=8). We find that the RF model estimates sensible microstructure parameters for both groups, matching values found in literature. Furthermore, we perform histology for both groups using electron microscopy (EM), measuring the thickness of the myelin sheath as a surrogate for exchange time. Histology results show that our RF model estimates are very strongly correlated with the EM measurements (ρ = 0.98 for f, ρ = 0.82 for τ_i). Finally, we find a statistically significant decrease in τ_i in all three regions of the corpus callosum (splenium/genu/body) of the CPZ cohort (<τ_i>=310ms/330ms/350ms) compared to the WT group (<τ_i>=370ms/370ms/380ms). This is in line with our expectations that τ_i is lower in regions where the myelin sheath is damaged, as axonal membranes become more permeable. Overall, these results demonstrate, for the first time experimentally and in vivo, that a computational model learned from simulations can reliably estimate microstructure parameters, including the axonal permeability .

Increased blood-brain barrier hyperpermeability coincides with mast cell activation early under cuprizone administration

The cuprizone induced animal model of demyelination is characterized by demyelination in many regions of the brain with high levels of demyelination in the corpus callosum as well as changes in neuronal function by 4–6 weeks of exposure. The model is used as a tool to study demyelination and subsequent degeneration as well as therapeutic interventions on these effects. Historically, the cuprizone model has been shown to contain no alterations to blood-brain barrier integrity, a key feature in many diseases that affect the central nervous system. Cuprizone is generally administered for 4–6 weeks to obtain maximal demyelination and degeneration. However, emerging evidence has shown that the effects of cuprizone on the brain may occur earlier than measurable gross demyelination. This study sought to investigate changes to blood-brain barrier permeability early in cuprizone administration. Results showed an increase in blood-brain barrier permeability and changes in tight junction protein expression as early as 3 days after beginning cuprizone treatment. These changes preceded glial morphological activation and demyelination known to occur during cuprizone administration. Increases in mast cell presence and activity were measured alongside the increased permeability implicating mast cells as a potential source for the blood-brain barrier disruption. These results provide further evidence of blood-brain barrier alterations in the cuprizone model and a target of therapeutic intervention in the prevention of cuprizone-induced pathology. Understanding how mast cells become activated under cuprizone and if they contribute to blood-brain barrier alterations may give further insight into how and when the blood-brain barrier is affected in CNS diseases. In summary, cuprizone administration causes an increase in blood-brain barrier permeability and this permeability coincides with mast cell activation.

alpha-Synuclein: a Modulator During Inflammatory CNS Demyelination

Neuroinflammation and demyelination are hallmarks of several neurological disorders such as multiple sclerosis and multiple system atrophy. To better understand the underlying mechanisms of de- and regeneration in respective diseases, it is critical to identify factors modulating these processes. One candidate factor is alpha-Synuclein (aSyn), which is known to be involved in the pathology of various neurodegenerative diseases. Recently, we have shown that aSyn is involved in the modulation of peripheral immune responses during acute neuroinflammatory processes. In the present study, the effect of aSyn deficiency on de- and regenerative events in the CNS was analyzed by using two different demyelinating animal models: chronic MOG_35–55-induced experimental autoimmune encephalomyelitis (EAE) and the cuprizone model. Histopathological analysis of spinal cord cross sections 8 weeks after EAE induction revealed a significant reduction of CNS inflammation accompanied by decreased myelin loss during late-stage inflammatory demyelination in aSyn-deficient mice. In contrast, after cuprizone-induced demyelination or remyelination following withdrawal of cuprizone, myelination and neuroinflammatory patterns were not affected by aSyn deficiency. These data provide further evidence for aSyn as regulator of peripheral immune responses under neuroinflammatory conditions, thereby also modulating degenerative events in late-stage demyelinating disease.

Longitudinal evaluation of demyelinated lesions in a multiple sclerosis model using ultrashort echo time magnetization transfer (UTE-MT) imaging

Alterations in myelin integrity are involved in many neurological disorders and demyelinating diseases, such as multiple sclerosis (MS). Although magnetic resonance imaging (MRI) is the gold standard method to diagnose and monitor MS patients, clinically available MRI protocols show limited specificity for myelin detection, notably in cerebral grey matter areas. Ultrashort echo time (UTE) MRI has shown great promise for direct imaging of lipids and myelin sheaths, and thus holds potential to improve lesion detection. In this study, we used a sequence combining magnetization transfer (MT) with UTE ("UTE-MT", TE ​= ​76 ​μs) and with short TE ("STE-MT", TE ​= ​3000 ​μs) to evaluate spatial and temporal changes in brain myelin content in the cuprizone mouse model for MS on a clinical 7 ​T scanner. During demyelination, UTE-MT ratio (UTE-MTR) and STE-MT ratio (STE-MTR) values were significantly decreased in most white matter and grey matter regions. However, only UTE-MTR detected cortical changes. After remyelination in subcortical and cortical areas, UTE-MTR values remained lower than baseline values, indicating that UTE-MT, but not STE-MT, imaging detected long-lasting changes following a demyelinating event. Next, we evaluated the potential correlations between imaging values and underlying histopathological markers. The strongest correlation was observed between UTE-MTR and percent coverage of myelin basic protein (MBP) immunostaining (r² ​= ​0.71). A significant, although lower, correlation was observed between STE-MTR and MBP (r² ​= ​0.48), and no correlation was found between UTE-MTR or STE-MTR and gliosis immunostaining. Interestingly, correlations varied across brain substructures. Altogether, our results demonstrate that UTE-MTR values significantly correlate with myelin content as measured by histopathology, not only in white matter, but also in subcortical and cortical grey matter regions in the cuprizone mouse model for MS. Readily implemented on a clinical 7 ​T system, this approach thus holds great potential for detecting demyelinating/remyelinating events in both white and grey matter areas in humans. When applied to patients with neurological disorders, including MS patient populations, UTE-MT methods may improve the non-invasive longitudinal monitoring of brain lesions, not only during disease progression but also in response to next generation remyelinating therapies.

Conditional Deletion of LRP1 Leads to Progressive Loss of Recombined NG2-Expressing Oligodendrocyte Precursor Cells in a Novel Mouse Model

The low-density lipoprotein receptor-related protein 1 (LRP1) is a transmembrane receptor, mediating endocytosis and activating intracellular signaling cascades. LRP1 is highly expressed in the central nervous system (CNS), especially in oligodendrocyte precursor cells (OPCs). Previous studies have suggested LRP1 as a regulator in early oligodendrocyte development, repair of chemically induced white matter lesions, and cholesterol homeostasis. To circumvent embryonic lethality observed in the case of global LRP1 deletion, we generated a new inducible conditional knockout (KO) mouse model, which enabled an NG2-restricted LRP1 deficiency (NG2-CreERT2^ct2/wtxR26eGFP^flox/floxxLRP1^flox/flox). When characterizing our triple transgenic mouse model, we noticed a substantial and progressive loss of recombined LRP1-deficient cells in the oligodendrocyte lineage. On the other hand, we found comparable distributions and fractions of oligodendroglia within the Corpus callosum of the KO and control animals, indicating a compensation of these deficits. An initial study on experimental autoimmune encephalomyelitis (EAE) was performed in triple transgenic and control mice and the cell biology of oligodendrocytes obtained from the animals was studied in an in vitro myelination assay. Differences could be observed in these assays, which, however, did not achieve statistical significance, presumably because the majority of recombined LRP1-deficient cells has been replaced by non-recombined cells. Thus, the analysis of the role of LRP1 in EAE will require the induction of acute recombination in the context of the disease process. As LRP1 is necessary for the survival of OPCs in vivo, we assume that it will play an important role in myelin repair.

The Spatial and Temporal Characters of Demyelination and Remyelination in the Cuprizone Animal Model

Multiple sclerosis (MS) is the most common central nervous system disease due to demyelination in young adults, and currently, there is no cure. Some experimental animal models were generated to mimic specific aspects of MS pathological characteristics. Among them, the cuprizone (CPZ)-induced mouse demyelination model presents heterogeneous pathologies with both focal and diffuse lesions. Considering that MS is a progressive disease, it is important to study the spatial and temporal characters of de- and remyelination in MS animal models. However, such data especially in some brain regions such as lateral septal area, fimbria of hippocampus, and hippocampus are still lacking. In this study, we investigated the alterations of myelin in these areas in parallel to the changes in corpus callosum using coronal sections. We found that the progression of demyelinating varied in different brain regions in C57BL/6J mice treated with CPZ for 1 to 5 weeks. This result suggests that each brain region has a distinct sensitivity to CPZ intoxication. Interestingly, activated microglia appeared not only in the active demyelinating areas but also in the non-myelinolysis regions. After CPZ withdrawal, significant remyelination was started in corpus callosum as early as 3 days. The completion of remyelination in the entire brain regions took 3 weeks. Our study detailed characterized the dynamics of myelin alterations and microglial status in the brain of the CPZ model. This information is valuable to facilitate further MS studies utilizing the CPZ model. Anat Rec, 302:2020-2029, 2019. © 2019 American Association for Anatomy.

Stereological Investigation of Regional Brain Volumes after Acute and Chronic Cuprizone-Induced Demyelination

Brain volume measurement is one of the most frequently used biomarkers to establish neuroprotective effects during pre-clinical multiple sclerosis (MS) studies. Furthermore, whole-brain atrophy estimates in MS correlate more robustly with clinical disability than traditional, lesion-based metrics. However, the underlying mechanisms leading to brain atrophy are poorly understood, partly due to the lack of appropriate animal models to study this aspect of the disease. The purpose of this study was to assess brain volumes and neuro-axonal degeneration after acute and chronic cuprizone-induced demyelination. C57BL/6 male mice were intoxicated with cuprizone for up to 12 weeks. Brain volume, as well as total numbers and densities of neurons, were determined using design-based stereology. After five weeks of cuprizone intoxication, despite severe demyelination, brain volumes were not altered at this time point. After 12 weeks of cuprizone intoxication, a significant volume reduction was found in the corpus callosum and diverse subcortical areas, particularly the internal capsule and the thalamus. Thalamic volume loss was accompanied by glucose hypermetabolism, analyzed by [¹⁸F]-fluoro-2-deoxy-d-glucose (18F-FDG) positron-emission tomography. This study demonstrates region-specific brain atrophy of different subcortical brain regions after chronic cuprizone-induced demyelination. The chronic cuprizone demyelination model in male mice is, thus, a useful tool to study the underlying mechanisms of subcortical brain atrophy and to investigate the effectiveness of therapeutic interventions.

Diffusion tensor imaging identifies aspects of therapeutic estrogen receptor β ligand-induced remyelination in a mouse model of multiple sclerosis

Diffusion tensor imaging (DTI) has been shown to detect white matter degeneration in multiple sclerosis (MS), a neurodegenerative autoimmune disease that presents with diffuse demyelination of the central nervous system. However, the utility of DTI in evaluating therapeutic remyelination has not yet been well-established. Here, we assessed the ability of DTI to distinguish between remyelination and neuroprotection following estrogen receptor β ligand (Indazole chloride, IndCl) treatment, which has been previously shown to stimulate functional remyelination, in the cuprizone (CPZ) diet mouse model of MS. Adult C57BL/6 J male and female mice received a normal diet (control), demyelination-inducing CPZ diet (9wkDM), or CPZ diet followed by two weeks of a normal diet (i.e., remyelination period) with either IndCl (RM + IndCl) or vehicle (RM + Veh) injections. We evaluated tissue microstructure of the corpus callosum utilizing in vivo and ex vivo DTI and immunohistochemistry (IHC) for validation. Compared to control mice, the 9wkDM group showed decreased fractional anisotropy (FA), increased radial diffusivity (RD), and no changes in axial diffusivity (AD) both in vivo and ex vivo. Meanwhile, RM + IndCl groups showed increased FA and decreased RD ex vivo compared to the RM + Veh group, in accordance with the evidence of remyelination by IHC. In conclusion, the DTI technology used in the present study can identify some changes in myelination and is a valuable translational tool for evaluating MS pathophysiology and therapeutic efficacy.

Neural Stem Cells of the Subventricular Zone Contribute to Neuroprotection of the Corpus Callosum after Cuprizone-Induced Demyelination

Myelin loss occurring in demyelinating diseases, including multiple sclerosis, is the leading cause of long-lasting neurological disability in adults. While endogenous remyelination, driven by resident oligodendrocyte precursor cells (OPCs), might partially compensate myelin loss in the early phases of demyelinating disorders, this spontaneous reparative potential fails at later stages. To investigate the cellular mechanisms sustaining endogenous remyelination in demyelinating disorders, we focused our attention on endogenous neural precursor cells (eNPCs) located within the subventricular zone (SVZ) since this latter area is considered one of the primary sources of new OPCs in the adult forebrain. First, we fate mapped SVZ-eNPCs in cuprizone-induced demyelination and found that SVZ endogenous neural stem/precursor cells are recruited during the remyelination phase to the corpus callosum (CC) and are capable of forming new oligodendrocytes. When we ablated SVZ-derived eNPCs during cuprizone-induced demyelination in female mice, the animals displayed reduced numbers of oligodendrocytes within the lesioned CC. Although this reduction in oligodendrocytes did not impact the ensuing remyelination, eNPC-ablated mice experienced increased axonal loss. Our results indicate that, in toxic models of demyelination, SVZ-derived eNPCs contribute to support axonal survival.SIGNIFICANCE STATEMENT One of the significant challenges in MS research is to understand the detrimental mechanisms leading to the failure of CNS tissue regeneration during disease progression. One possible explanation is the inability of recruited oligodendrocyte precursor cells (OPCs) to complete remyelination and to sustain axonal survival. The contribution of endogenous neural precursor cells (eNPCs) located in the subventricular zone (SVZ) to generate new OPCs in the lesion site has been debated. Using transgenic mice to fate map and to selectively kill SVZ-derived eNPCs in the cuprizone demyelination model, we observed migration of SVZ-eNPCs after injury and their contribution to oligodendrogenesis and axonal survival. We found that eNPCs are dispensable for remyelination but protect partially from increased axonal loss.

Ultrahigh field imaging of myelin disease models: Toward specific markers of myelin integrity?

Specific magnetic resonance imaging (MRI) markers of myelin are critical for the evaluation and development of regenerative therapies for demyelinating diseases. Several MRI methods have been developed for myelin imaging, based either on acquisition schemes or on mathematical modeling of the signal. They generally showed good sensitivity but validation for specificity toward myelin is still warranted to allow a reliable interpretation in an in vivo complex pathological environment. Experimental models of dys-/demyelination are characterized by various levels of myelin disorders, axonal damage, gliosis and inflammation, and offer the opportunity for powerful correlative studies between imaging metrics and histology. Here, we review how ultrahigh field MRI markers have been correlated with histology in these models and provide insights into the trends for future developments of MRI tools in human myelin diseases. To this end, we present the biophysical basis of the main MRI methods for myelin imaging based on T₁ , T₂ , water diffusion, and magnetization transfer signal, the characteristics of animal models used and the outcomes of histological validations. To date such studies are limited, and demonstrate partial correlations with immunohistochemical and electron microscopy measures of myelin. These MRI metrics also often correlate with axons, glial, or inflammatory cells in models where axonal degeneration or inflammation occur as potential confounding factors. Therefore, the MRI markers' specificity for myelin is still perfectible and future developments should improve mathematical modeling of the MR signal based on more complex systems or provide multimodal approaches to better disentangle the biological processes underlying the MRI metrics.

mCSF-Induced Microglial Activation Prevents Myelin Loss and Promotes Its Repair in a Mouse Model of Multiple Sclerosis

A pathological hallmark of multiple sclerosis (MS) is myelin loss in brain white matter accompanied by compromised remyelination. Demyelinated lesions are deeply associated with oligodendrocyte apoptosis and a robust inflammatory response. Although various studies point towards a noxious role of inflammation in MS, others emphasize a positive role for the innate immune cells in disease progression. A cytokine well-known to stimulate cell survival, proliferation and differentiation of myeloid cells, macrophage colony-stimulating factor (mCSF), was administered to mice during a 5 week-long cuprizone diet. Treated mice exhibited reduced myelin loss during the demyelination phase, together with an increased number of microglia and oligodendrocyte precursor cells in lesion sites. Tamoxifen-induced conditional deletion of the mCSF receptor in microglia from cuprizone-fed mice caused aberrant myelin debris accumulation in the corpus callosum and reduced microglial phagocytic response. mCSF therefore plays a key role in stimulating myelin clearance by the brain innate immune cells, which is a prerequisite for proper remyelination and myelin repair processes.

Protective potential of dimethyl fumarate in a mouse model of thalamocortical demyelination

Alterations in cortical cellular organization, network functionality, as well as cognitive and locomotor deficits were recently suggested to be pathological hallmarks in multiple sclerosis and corresponding animal models as they might occur following demyelination. To investigate functional changes following demyelination in a well-defined, topographically organized neuronal network, in vitro and in vivo, we focused on the primary auditory cortex (A1) of mice in the cuprizone model of general de- and remyelination. Following myelin loss in this model system, the spatiotemporal propagation of incoming stimuli in A1 was altered and the hierarchical activation of supra- and infragranular cortical layers was lost suggesting a profound effect exerted on neuronal network level. In addition, the response latency in field potential recordings and voltage-sensitive dye imaging was increased following demyelination. These alterations were accompanied by a loss of auditory discrimination abilities in freely behaving animals, a reduction of the nuclear factor-erythroid 2-related factor-2 (Nrf-2) protein in the nucleus in histological staining and persisted during remyelination. To find new strategies to restore demyelination-induced network alteration in addition to the ongoing remyelination, we tested the cytoprotective potential of dimethyl fumarate (DMF). Therapeutic treatment with DMF during remyelination significantly modified spatiotemporal stimulus propagation in the cortex, reduced the cognitive impairment, and prevented the demyelination-induced decrease in nuclear Nrf-2. These results indicate the involvement of anti-oxidative mechanisms in regulating spatiotemporal cortical response pattern following changes in myelination and point to DMF as therapeutic compound for intervention.

Alzheimer's Disease Mutant Mice Exhibit Reduced Brain Tissue Stiffness Compared to Wild-type Mice in both Normoxia and following Intermittent Hypoxia Mimicking Sleep Apnea

Background

Evidence from patients and animal models suggests that obstructive sleep apnea (OSA) may increase the risk of Alzheimer's disease (AD) and that AD is associated with reduced brain tissue stiffness.

Aim

To investigate whether intermittent hypoxia (IH) alters brain cortex tissue stiffness in AD mutant mice exposed to IH mimicking OSA.

Methods

Six-eight month old (B6C3-Tg(APPswe,PSEN1dE9)85Dbo/J) AD mutant mice and wild-type (WT) littermates were subjected to IH (21% O₂ 40 s to 5% O₂ 20 s; 6 h/day) or normoxia for 8 weeks. After euthanasia, the stiffness (E) of 200-μm brain cortex slices was measured by atomic force microscopy.

Results

Two-way ANOVA indicated significant cortical softening and weight increase in AD mice compared to WT littermates, but no significant effects of IH on cortical stiffness and weight were detected. In addition, reduced myelin was apparent in AD (vs. WT), but no significant differences emerged in the cortex extracellular matrix components laminin and glycosaminoglycans when comparing baseline AD and WT mice.

Conclusion

AD mutant mice exhibit reduced brain tissue stiffness following both normoxia and IH mimicking sleep apnea, and such differences are commensurate with increased edema and demyelination in AD.

Quantitative temporal changes in DTI values coupled with histological properties in cuprizone-induced demyelination and remyelination

Diffusion tensor imaging (DTI) is widely used to evaluate microstructural variations in brain tissue. In particular, fractional anisotropy (FA), reflecting the magnitude and orientation of anisotropic water diffusion, allows us to detect pathological events in white matter. An ex vivo DTI study coupled with histological assessment is an efficient strategy to evaluate the myelination process, i.e. demyelination and remyelination. The relationship between DTI values and myelin content or the individual cellular components such as oligodendrocytes, microglia, and astrocytes during both processes of demyelination and remyelination are not well-understood. To address this issue, we employed a cuprizone-inducible demyelination mouse model. Demyelination can be induced in this model during cuprizone exposure and termination of cuprizone exposure induces remyelination. We fed the mice cuprizone-containing chow for 4 weeks and then normal chow for an additional 4 weeks. The ex vivo DTI was performed to evaluate the white matter profiles observed by FA, mean diffusivity (MD), and radial diffusivity (RD) at both demyelinating and remyelinating time points, and then we evaluated histological properties at the same time points. The results indicated a gradual FA decrease during the cuprizone treatment (0, 2, 3, 4 weeks). A lower peak was seen at 1 week after the normal chow was resumed, with recovery to baseline at 2 and 4 weeks. MD and RD showed an opposing pattern to that of FA. These DTI values were positively or negatively correlated with myelin content regardless of the status of the white matter. The RD value was more sensitive to myelination status than FA and MD. We have clarified the temporal changes in the DTI values coupled with histological properties over both the demyelination and remyelination processes.

Sex matters: repetitive mild traumatic brain injury in adolescent rats

Objective

Whether sex differences contribute to the heterogeneity of mild traumatic brain injury (mTBI) and repeated mTBI (RmTBI) outcomes in adolescents is unknown. Therefore, this study examined changes in, and differences between, male and female rats following single mTBI and RmTBI.

Methods

Rats were given a single mTBI, RmTBI (i.e., 3x), or sham injuries. Injuries were administered using a lateral impact model that mimics forces common in human mTBI. After the final injury, rats underwent extensive behavioral testing to examine cognition, motor function, and anxiety‐ and depressive‐like behavior. Postmortem analyses investigated gene expression and structural changes in the brain.

Results

Many of the outcomes exhibited a sex‐dependent response to RmTBI. While all rats given RmTBI had deficits in balance, motor coordination, locomotion, and anxiety‐like behavior, only male rats given RmTBI had short‐term working memory deficits, whereas only females given RmTBI had increased depressive‐like behavior. Volumetric and diffusion weighted MRI analyses found that while RmTBI‐induced atrophy of the prefrontal cortex was greater in female rats, only the male rats exhibited worse white matter integrity in the corpus callosum following RmTBI. Sex‐dependent changes in brain expression of mRNA for glial fibrillary acidic protein, myelin basic protein, and tau protein were also observed following injury.

Interpretation

These findings suggest that in adolescent mTBI, sex matters; and future studies incorporating both male and females are warranted to provide a greater understanding of injury prognosis and better inform clinical practice.

Histological validation of fast macromolecular proton fraction mapping as a quantitative myelin imaging method in the cuprizone demyelination model

Cuprizone-induced demyelination in mice is a frequently used model in preclinical multiple sclerosis research. A recent quantitative clinically-targeted MRI method, fast macromolecular proton fraction (MPF) mapping demonstrated a promise as a myelin biomarker in human and animal studies with a particular advantage of sensitivity to both white matter (WM) and gray matter (GM) demyelination. This study aimed to histologically validate the capability of MPF mapping to quantify myelin loss in brain tissues using the cuprizone demyelination model. Whole-brain MPF maps were obtained in vivo on an 11.7T animal MRI scanner from 7 cuprizone-treated and 7 control С57BL/6 mice using the fast single-point synthetic-reference method. Brain sections were histologically stained with Luxol Fast Blue (LFB) for myelin quantification. Significant (p < 0.05) demyelination in cuprizone-treated animals was found according to both LFB staining and MPF in all anatomical structures (corpus callosum, anterior commissure, internal capsule, thalamus, caudoputamen, and cortex). MPF strongly correlated with quantitative histology in all animals (r = 0.95, p < 0.001) as well as in treatment and control groups taken separately (r = 0.96, p = 0.002 and r = 0.93, p = 0.007, respectively). Close agreement between histological myelin staining and MPF suggests that fast MPF mapping enables robust and accurate quantitative assessment of demyelination in both WM and GM.

Whole-brain ex-vivo quantitative MRI of the cuprizone mouse model

Myelin is a critical component of the nervous system and a major contributor to contrast in Magnetic Resonance (MR) images. However, the precise contribution of myelination to multiple MR modalities is still under debate. The cuprizone mouse is a well-established model of demyelination that has been used in several MR studies, but these have often imaged only a single slice and analysed a small region of interest in the corpus callosum. We imaged and analyzed the whole brain of the cuprizone mouse ex-vivo using high-resolution quantitative MR methods (multi-component relaxometry, Diffusion Tensor Imaging (DTI) and morphometry) and found changes in multiple regions, including the corpus callosum, cerebellum, thalamus and hippocampus. The presence of inflammation, confirmed with histology, presents difficulties in isolating the sensitivity and specificity of these MR methods to demyelination using this model.

Gray and White Matter Demyelination and Remyelination Detected with Multimodal Quantitative MRI Analysis at 11.7T in a Chronic Mouse Model of Multiple Sclerosis

Myelin is a component of the nervous system that is disrupted in multiple sclerosis, resulting in neuro-axonal degeneration. The longitudinal effect of chronic cuprizone-induced demyelination was investigated in the cerebral gray and white matter of treated mice and the spontaneous remyelination upon treatment interruption. Multimodal Magnetic Resonance Imaging and a Cryoprobe were used at 11.7T to measure signal intensity ratios, T₂ values and diffusion metrics. The results showed significant and reversible modifications in white matter and gray matter regions such as in the rostral and caudal corpus callosum, the external capsule, the cerebellar peduncles, the caudate putamen, the thalamus, and the somatosensory cortex of treated mice. T₂ and radial diffusivity metrics appeared to be more sensitive than fractional anisotropy, axial diffusivity or mean diffusivity to detect those cuprizone-induced changes. In the gray matter, only signal and T₂ metrics and not diffusion metrics were sensitive to detect any changes. Immunohistochemical qualitative assessments in the same regions confirmed demyelination and remyelination processes. These multimodal data will provide better understanding of the dynamics of cuprizone-induced de- and remyelination in white and gray matter structures, and will be the basis to test therapies in experimental models.

Inherited and acquired disorders of myelin: The underlying myelin pathology

Remyelination is a major therapeutic goal in human myelin disorders, serving to restore function to demyelinated axons and providing neuroprotection. The target disorders that might be amenable to the promotion of this repair process are diverse and increasing in number. They range primarily from those of genetic, inflammatory to toxic origin. In order to apply remyelinating strategies to these disorders, it is essential to know whether the myelin damage results from a primary attack on myelin or the oligodendrocyte or both, and whether indeed these lead to myelin breakdown and demyelination. In some disorders, myelin sheath abnormalities are prominent but demyelination does not occur. This review explores the range of human and animal disorders where myelin pathology exists and focusses on defining the myelin changes in each and their cause, to help define whether they are targets for myelin repair therapy.

•

We reviewed myelin disorders of the CNS in humans and animals.

•

Myelin damage results from primary attack on the oligodendrocyte or myelin sheath.

•

All major categories of disease can affect CNS myelin.

•

Myelin vacuolation is common, yet does not always result in demyelination.

Spatio-Temporal Patterns of Demyelination and Remyelination in the Cuprizone Mouse Model

Cuprizone administration in mice provides a reproducible model of demyelination and spontaneous remyelination, and has been useful in understanding important aspects of human disease, including multiple sclerosis. In this study, we apply high spatial resolution quantitative MRI techniques to establish the spatio-temporal patterns of acute demyelination in C57BL/6 mice after 6 weeks of cuprizone administration, and subsequent remyelination after 6 weeks of post-cuprizone recovery. MRI measurements were complemented with Black Gold II stain for myelin and immunohistochemical stains for associated tissue changes. Gene expression was evaluated using the Allen Gene Expression Atlas. Twenty-five C57BL/6 male mice were split into control and cuprizone groups; MRI data were obtained at baseline, after 6 weeks of cuprizone, and 6 weeks post-cuprizone. High-resolution (100 μm isotropic) whole-brain coverage magnetization transfer ratio (MTR) parametric maps demonstrated concurrent caudal-to-rostral and medial-to-lateral gradients of MTR decrease within corpus callosum (CC) that correlated well with demyelination assessed histologically. Our results show that demyelination was not limited to the midsagittal line of the corpus callosum, and also that opposing gradients of demyelination occur in the lateral and medial CC. T2-weighted MRI gray/white matter contrast was strong at baseline, weak after 6 weeks of cuprizone treatment, and returned to a limited extent after recovery. MTR decreases during demyelination were observed throughout the brain, most clearly in callosal white matter. Myelin damage and repair appear to be influenced by proximity to oligodendrocyte progenitor cell populations and exhibit an inverse correlation with myelin basic protein gene expression. These findings suggest that susceptibility to injury and ability to repair vary across the brain, and whole-brain analysis is necessary to accurately characterize this model. Whole-brain parametric mapping across time is essential for gaining a real understanding of disease processes in-vivo. MTR increases in healthy mice throughout adolescence and adulthood were observed, illustrating the need for appropriate age-matched controls. Elucidating the unique and site-specific demyelination in the cuprizone model may offer new insights into in mechanisms of both damage and repair in human demyelinating diseases.

In vivo quantification of demyelination and recovery using compartment-specific diffusion MRI metrics validated by electron microscopy

There is a need for accurate quantitative non-invasive biomarkers to monitor myelin pathology in vivo and distinguish myelin changes from other pathological features including inflammation and axonal loss. Conventional MRI metrics such as T2, magnetization transfer ratio and radial diffusivity have proven sensitivity but not specificity. In highly coherent white matter bundles, compartment-specific white matter tract integrity (WMTI) metrics can be directly derived from the diffusion and kurtosis tensors: axonal water fraction, intra-axonal diffusivity, and extra-axonal radial and axial diffusivities. We evaluate the potential of WMTI to quantify demyelination by monitoring the effects of both acute (6weeks) and chronic (12weeks) cuprizone intoxication and subsequent recovery in the mouse corpus callosum, and compare its performance with that of conventional metrics (T2, magnetization transfer, and DTI parameters). The changes observed in vivo correlated with those obtained from quantitative electron microscopy image analysis. A 6-week intoxication produced a significant decrease in axonal water fraction (p<0.001), with only mild changes in extra-axonal radial diffusivity, consistent with patchy demyelination, while a 12-week intoxication caused a more marked decrease in extra-axonal radial diffusivity (p=0.0135), consistent with more severe demyelination and clearance of the extra-axonal space. Results thus revealed increased specificity of the axonal water fraction and extra-axonal radial diffusivity parameters to different degrees and patterns of demyelination. The specificities of these parameters were corroborated by their respective correlations with microstructural features: the axonal water fraction correlated significantly with the electron microscopy derived total axonal water fraction (ρ=0.66; p=0.0014) but not with the g-ratio, while the extra-axonal radial diffusivity correlated with the g-ratio (ρ=0.48; p=0.0342) but not with the electron microscopy derived axonal water fraction. These parameters represent promising candidates as clinically feasible biomarkers of demyelination and remyelination in the white matter.

Diffusion kurtosis imaging probes cortical alterations and white matter pathology following cuprizone induced demyelination and spontaneous remyelination

Although MRI is the gold standard for the diagnosis and monitoring of multiple sclerosis (MS), current conventional MRI techniques often fail to detect cortical alterations and provide little information about gliosis, axonal damage and myelin status of lesioned areas. Diffusion tensor imaging (DTI) and diffusion kurtosis imaging (DKI) provide sensitive and complementary measures of the neural tissue microstructure. Additionally, specific white matter tract integrity (WMTI) metrics modelling the diffusion in white matter were recently derived. In the current study we used the well-characterized cuprizone mouse model of central nervous system demyelination to assess the temporal evolution of diffusion tensor (DT), diffusion kurtosis tensor (DK) and WMTI-derived metrics following acute inflammatory demyelination and spontaneous remyelination. While DT-derived metrics were unable to detect cuprizone induced cortical alterations, the mean kurtosis (MK) and radial kurtosis (RK) were found decreased under cuprizone administration, as compared to age-matched controls, in both the motor and somatosensory cortices. The MK remained decreased in the motor cortices at the end of the recovery period, reflecting long lasting impairment of myelination. In white matter, DT, DK and WMTI-derived metrics enabled the detection of cuprizone induced changes differentially according to the stage and the severity of the lesion. More specifically, the MK, the RK and the axonal water fraction (AWF) were the most sensitive for the detection of cuprizone induced changes in the genu of the corpus callosum, a region less affected by cuprizone administration. Additionally, microgliosis was associated with an increase of MK and RK during the acute inflammatory demyelination phase. In regions undergoing severe demyelination, namely the body and splenium of the corpus callosum, DT-derived metrics, notably the mean diffusion (MD) and radial diffusion (RD), were among the best discriminators between cuprizone and control groups, hence highlighting their ability to detect both acute and long lasting changes. Interestingly, WMTI-derived metrics showed the aptitude to distinguish between the different stages of the disease. Both the intra-axonal diffusivity (Da) and the AWF were found to be decreased in the cuprizone treated group, Da specifically decreased during the acute inflammatory demyelinating phase whereas the AWF decrease was associated to the spontaneous remyelination and the recovery period. Altogether our results demonstrate that DKI is sensitive to alterations of cortical areas and provides, along with WMTI metrics, information that is complementary to DT-derived metrics for the characterization of demyelination in both white and grey matter and subsequent inflammatory processes associated with a demyelinating event.

Infection as an Environmental Trigger of Multiple Sclerosis Disease Exacerbation

Over the past several decades, significant advances have been made in identifying factors that contribute to the pathogenesis of multiple sclerosis (MS) and have culminated in the approval of some effective therapeutic strategies for disease intervention. However, the mechanisms by which environmental factors, such as infection, contribute to the pathogenesis and/or symptom exacerbation remain to be fully elucidated. Relapse frequency in MS patients contributes to neurological impairment and, in the initial phases of disease, serves as a predictor of poor disease prognosis. The purpose of this review is to examine the evidence that supports a role for peripheral infection in modulating the natural history of this disease. Evidence supporting a role for infection in promoting exacerbation in animal models of MS is also reviewed. Finally, a few mechanisms by which infection may exacerbate symptoms of MS and other neurological diseases are discussed. Those who comprise the majority of MS patients acquire approximately two upper-respiratory infections per year; furthermore, this type of infection doubles the risk for MS relapse, underscoring the contribution of this relationship as being potentially important and particularly detrimental.

Cellular and molecular neuropathology of the cuprizone mouse model: clinical relevance for multiple sclerosis

The cuprizone mouse model allows the investigation of the complex molecular mechanisms behind nonautoimmune-mediated demyelination and spontaneous remyelination. While it is generally accepted that oligodendrocytes are specifically vulnerable to cuprizone intoxication due to their high metabolic demands, a comprehensive overview of the etiology of cuprizone-induced pathology is still missing to date. In this review we extensively describe the physico-chemical mode of action of cuprizone and discuss the molecular and enzymatic mechanisms by which cuprizone induces metabolic stress, oligodendrocyte apoptosis, myelin degeneration and eventually axonal and neuronal pathology. In addition, we describe the dual effector function of the immune system which tightly controls demyelination by effective induction of oligodendrocyte apoptosis, but in contrast also paves the way for fast and efficient remyelination by the secretion of neurotrophic factors and the clearance of cellular and myelinic debris. Finally, we discuss the many clinical symptoms that can be observed following cuprizone treatment, and how these strengthened the cuprizone model as a useful tool to study human multiple sclerosis, schizophrenia and epilepsy.

Region and dynamic specificities of adult neural stem cells and oligodendrocyte precursors in myelin regeneration in the mouse brain

Myelin regeneration can occur in the brain following demyelination. Parenchymal oligodendrocyte progenitors (pOPC) are known to play a crucial role in this process. Neural stem cells (NSC) residing in the ventricular-subventricular zone (V-SVZ) also have the ability to generate oligodendrocytes but their contribution to endogenous myelin repair was so far considered to be negligible. Here, we addressed the relative contribution of pOPC and V-SVZ-derived neural progenitors (SVZdNP) to remyelination in cuprizone mouse models of acute or chronic corpus callosum (CC) demyelination. Using genetic tracing, we uncover an unexpected massive and precocious recruitment of SVZdNP in the anterior CC after acute demyelination. These cells very quickly adopt an oligodendrocytic fate and robustly generate myelinating cells as efficiently as pOPC do. In more posterior areas of the CC, SVZdNP recruitment is less important whereas pOPC contribute more, underlining a regionalization in the mobilization of these two cell populations. Strikingly, in a chronic model when demyelination insult is sustained in time, SVZdNP minimally contribute to myelin repair, a failure associated with a depletion of NSC and a drastic drop of progenitor cell proliferation in V-SVZ. In this context, pOPC remain reactive, and become the main contributors to myelin regeneration. Altogether our results highlight a region and context-dependent contribution of SVZdNP to myelin repair that can equal pOPC. They also raise the question of a possible exhaustion of V-SVZ proliferation potential in chronic pathologies.

Adult neural precursor cells from the subventricular zone contribute significantly to oligodendrocyte regeneration and remyelination

Parenchymal oligodendrocyte progenitor cells (pOPCs) are considered the principal cell type responsible for oligodendrogenesis and remyelinaton in demyelinating diseases. Recent studies have demonstrated that neural precursor cells (NPCs) from the adult subventricular zone (SVZ) can also generate new oligodendrocytes after demyelination. However, the relative contribution of NPCs versus pOPCs to remyelination is unknown. We used in vivo genetic fate mapping to assess the behavior of each progenitor type within the corpus callosi (CCs) of mice subjected to cuprizone-induced demyelination. Nestin-CreER(T2) and Pdgfra-CreER(T2) transgenic mice were crossed with fluorescent Cre reporter strains to map the fate of NPCs and pOPCs respectively. In cuprizone-challenged mice, substantial numbers of NPCs migrated into the demyelinated CC and contributed to oligodendrogenesis. This capacity was most prominent in rostral regions adjacent to the SVZ where NPC-derived oligodendrocytes significantly outnumbered those generated from pOPCs. Sixty-two percent of all nodes of Ranvier in this region were flanked by at least one paranode generated from an NPC-derived oligodendrocyte. Remarkably, g-ratios (ratio of the axon diameter to the diameter of the axon plus myelin sheath) of myelinated axons in regions subject to significant NPC-derived remyelination were equivalent to those of unchallenged controls, and immunoelectron microscopy revealed that NPC-derived myelin was significantly thicker than that generated by pOPCs, regardless of axonal caliber. We also demonstrate that a reduced efficiency of remyelination in the caudal CC was associated with long-term impairment in the maturation of oligodendrogenic NPCs but only transient delay in pOPC differentiation. Collectively, our data define a major distinct role for NPCs in remyelination, identifying them as a key target for enhancing myelin repair in demyelinating diseases.

Chemokines Referee Inflammation within the Central Nervous System during Infection and Disease

The discovery that chemokines and their receptors are expressed by a variety of cell types within the normal adult central nervous system (CNS) has led to an expansion of their repertoire as molecular interfaces between the immune and nervous systems. Thus, CNS chemokines are now divided into those molecules that regulate inflammatory cell migration into the CNS and those that initiate CNS repair from inflammation-mediated tissue damage. Work in our laboratory throughout the past decade has sought to elucidate how chemokines coordinate leukocyte entry and interactions at CNS endothelial barriers, under both homeostatic and inflammatory conditions, and how they promote repair within the CNS parenchyma. These studies have identified several chemokines, including CXCL12 and CXCL10, as critical regulators of leukocyte migration from perivascular locations. CXCL12 additionally plays an essential role in promoting remyelination of injured white matter. In both scenarios we have shown that chemokines serve as molecular links between inflammatory mediators and other effector molecules involved in neuroprotective processes.

Histological correlation of diffusional kurtosis and white matter modeling metrics in cuprizone-induced corpus callosum demyelination

The cuprizone mouse model is well established for studying the processes of both demyelination and remyelination in the corpus callosum, and it has been utilized together with diffusion tensor imaging (DTI) to investigate myelin and axonal pathology. Although some underlying morphological mechanisms contributing to the changes in diffusion tensor (DT) metrics have been identified, the understanding of specific associations between histology and diffusion measures remains limited. Diffusional kurtosis imaging (DKI) is an extension of DTI that provides metrics of diffusional non-Gaussianity, for which an associated white matter modeling (WMM) method has been developed. The main goal of the present study was to quantitatively assess the relationships between diffusion measures and histological measures in the mouse model of cuprizone-induced corpus callosum demyelination. The diffusional kurtosis (DK) and WMM metrics were found to provide additional information that enhances the sensitivity to detect the morphological heterogeneity in the chronic phase of the disease process in the rostral segment of the corpus callosum. Specifically, in the rostral segment, axonal water fraction (d = 2.6; p < 0.0001), radial kurtosis (d = 2.0; p = 0.001) and mean kurtosis (d = 1.5; p = 0.005) showed the most sensitivity between groups with respect to yielding statistically significant p values and high Cohen's d values. These results demonstrate the ability of DK and WMM metrics to detect white mater changes and inflammatory processes associated with cuprizone-induced demyelination. They also validate, in part, the application of these new WMM metrics for studying neurological diseases, as well as helping to elucidate their biophysical meaning.

Current developments in MRI for assessing rodent models of multiple sclerosis

ABSTRACT: 

MRI is a key radiological imaging technique that plays an important role in the diagnosis and characterization of heterogeneous multiple sclerosis (MS) lesions. Various MRI methodologies such as conventional T 1/T 2 contrast, contrast agent enhancement, diffusion-weighted imaging, magnetization transfer imaging and susceptibility weighted imaging have been developed to determine the severity of MS pathology, including demyelination/remyelination and brain connectivity impairment from axonal loss. The broad spectrum of MS pathology manifests in diverse patient MRI presentations and affects the accuracy of patient diagnosis. To study specific pathological aspects of the disease, rodent models such as experimental autoimmune encephalomyelitis, virus-induced and toxin-induced demyelination have been developed. This review aims to present key developments in MRI methodology for better characterization of rodent models of MS.

Understanding disease processes in multiple sclerosis through magnetic resonance imaging studies in animal models

There are exciting new advances in multiple sclerosis (MS) resulting in a growing understanding of both the complexity of the disorder and the relative involvement of grey matter, white matter and inflammation. Increasing need for preclinical imaging is anticipated, as animal models provide insights into the pathophysiology of the disease. Magnetic resonance (MR) is the key imaging tool used to diagnose and to monitor disease progression in MS, and thus will be a cornerstone for future research. Although gadolinium-enhancing and T₂ lesions on MRI have been useful for detecting MS pathology, they are not correlative of disability. Therefore, new MRI methods are needed. Such methods require validation in animal models. The increasing necessity for MRI of animal models makes it critical and timely to understand what research has been conducted in this area and what potential there is for use of MRI in preclinical models of MS. Here, we provide a review of MRI and magnetic resonance spectroscopy (MRS) studies that have been carried out in animal models of MS that focus on pathology. We compare the MRI phenotypes of animals and patients and provide advice on how best to use animal MR studies to increase our understanding of the linkages between MR and pathology in patients. This review describes how MRI studies of animal models have been, and will continue to be, used in the ongoing effort to understand MS.

•

MRI studies of pathology in various animal models of MS are reviewed.

•

MRI phenotypes in animal models of MS and MS patients are compared.

•

Animal MRI can increase understanding of links between MR and pathology in patients.

Quantitative MRI and ultrastructural examination of the cuprizone mouse model of demyelination

The cuprizone mouse model of demyelination was used to investigate the influence that white matter changes have on different magnetic resonance imaging results. In vivo T2 -weighted and magnetization transfer images (MTIs) were acquired weekly in control (n = 5) and cuprizone-fed (n = 5) mice, with significant increases in signal intensity in T2 -weighted images (p < 0.001) and lower magnetization transfer ratio (p < 0.001) in the corpus callosum of the cuprizone-fed mice starting at 3 weeks and peaking at 4 and 5 weeks, respectively. Diffusion tensor imaging (DTI), quantitative MTI (qMTI), and T1/T2 measurements were used to analyze freshly excised tissue after 6 weeks of cuprizone administration. In multicomponent T2 analysis with 10 ms echo spacing, there was no visible myelin water component associated with the short T2 value. Quantitative MTI metrics showed significant differences in the corpus callosum and external capsule of the cuprizone-fed mice, similar to previous studies of multiple sclerosis in humans and animal models of demyelination. Fractional anisotropy was significantly lower and mean, axial, and radial diffusivity were significantly higher in the cuprizone-fed mice. Cellular distributions measured in electron micrographs of the corpus callosum correlated strongly to several different quantitative MRI metrics. The largest Spearman correlation coefficient varied depending on cellular type: T1 versus the myelinated axon fraction (ρ = -0.90), the bound pool fraction (ƒ) versus the myelin sheath fraction (ρ = 0.93), and axial diffusivity versus the non-myelinated cell fraction (ρ = 0.92). Using Pearson's correlation coefficient, ƒ was strongly correlated to the myelin sheath fraction (r = 0.98) with a linear equation predicting myelin content (5.37ƒ - 0.25). Of the calculated MRI metrics, ƒ was the strongest indicator of myelin content, while longitudinal relaxation rates and diffusivity measurements were the strongest indicators of changes in tissue structure.

Microvascular damage is involved in the pathogenesis of heroin induced spongiform leukoencephalopathy

OBJECTIVE

To investigate whether microvascular damage is involved in the pathogenesis of heroin induced spongiform leukoencephalopathy (HSLE).

METHODS

The brain tissues were collected from 4 HSLE patients and 5 controls and then fixed in 4% paraformaldehyde. The frontal lobe, corpus callosum and cerebellum were separated. The expressions of myelin base protein (MBP) and CD34 were detected by immunohistochemistry. TUNEL staining was applied to detect cell apoptosis. The correlation between microvascular changes and pathological vacuoles was evaluated.

RESULTS

No obvious abnormalities were found in the brain of controls. Immunohistochemistry for MBP showed the collapse and fracture of myelin sheath and vacuole formation in the subcortical white matter, corpus callosum, and cerebellar white matter of HSLE patients. TUNEL staining showed the number of apoptotic cells in the cerebellar white matter and corpus callosum of HSLE patients was significantly higher than that in controls (F = 389.451, P < 0.001). Masson's trichrome staining revealed vacuolar degeneration in the cerebral white matter of HSLE patients, and the vacuoles were distributed around the microvessels. Immunohistochemistry revealed CD34 positive cells were seldom found besides the vessels in the cerebellar white matter and corpus callosum of HSLE patients, but a variety of CD34 positive cells was found in the vascular wall of controls (F = 838.500, P < 0.001).

CONCLUSION

Apoptosis of oligodendrocytes may be related to the HSLE. Cerebral vascular injury and microcirculation dysfunction are involved in the pathogenesis of HSLE. The interrelation between apoptosis of oligodendrocytes and the microvascular damage are required to be studied in future investigations.

Unmyelinated axons show selective rostrocaudal pathology in the corpus callosum after traumatic brain injury

Axonal injury is consistently observed after traumatic brain injury (TBI). Prior research has extensively characterized the post-TBI response in myelinated axons. Despite evidence that unmyelinated axons comprise a numerical majority of cerebral axons, pathologic changes in unmyelinated axons after TBI have not been systematically studied. To identify morphologic correlates of functional impairment of unmyelinated fibers after TBI, we assessed ultrastructural changes in corpus callosum axons. Adult rats received moderate fluid percussion TBI, which produced diffuse injury with no contusion. Cross-sectional areas of 13,797 unmyelinated and 3,278 intact myelinated axons were stereologically measured at survival intervals from 3 hours to 15 days after injury. The mean caliber of unmyelinated axons was significantly reduced at 3 to 7 days and recovered by 15 days, but the time course of this shrinkage varied among the genu, mid callosum, and splenium. Relatively large unmyelinated axons seemed to be particularly vulnerable. Injury-induced decreases in unmyelinated fiber density were also observed, but they were more variable than caliber reductions. By contrast, no significant morphometric changes were observed in myelinated axons. The finding of a preferential vulnerability in unmyelinated axons has implications for current concepts of axonal responses after TBI and for development of specifically targeted therapies.

Cuprizone-induced demyelination in the rat cerebral cortex and thyroid hormone effects on cortical remyelination

Multiple Sclerosis (MS) is an inflammatory demyelinating disease of the Central Nervous System which is characterized by multifocal demyelinated lesions dispersed throughout the brain. Although white matter lesions have been the most extensively studied, cortical demyelinaton lesions are also detected in MS brains. Cuprizone (CPZ)-induced demyelination in rodents has been widely used as a model for MS. Most of these studies focus on oligodendrocyte-rich structures, such as the corpus callosum (CC) and the cerebellar peduncles. However, it has been recently described that CPZ administration in mice also produces cortical demyelination, resembling some of the lesions found in MS patients. In this work we used CPZ-demyelinating model in Wistar rats to study demyelination in cortical forebrain areas. At the ultrastructural level, demyelination in the cortex was observed before detectable myelin loss in the subcortical white matter. During the course of CPZ intoxication Myelin Basic Protein immunodetection was decreased in cortical layers I-III due to a reduction in the number of cortical oligodendrocytes (OL). Oligodendroglial loss in CPZ-intoxicated rats correlated with an increase in the number of Glial Fibrillary Acidic Protein positive astrocytes and a shift in the location of Carbonic Anhydrase II from OL to astrocytes. After removal of CPZ from the diet, we evaluate intranasal Thyroid hormone (TH) effects on the progression of cortical lesions. As previously reported in the CC, TH treatment also accelerates remyelination rate in the cortex compared to rats undergoing spontaneous remyelination. Our results suggest that manipulation of TH levels could be considered as a strategy to promote remyelination process in the cortex and to prevent neuronal irreversible damage in patients suffering from MS.

Quantification of increased cellularity during inflammatory demyelination

Multiple sclerosis is characterized by inflammatory demyelination and irreversible axonal injury leading to permanent neurological disabilities. Diffusion tensor imaging demonstrates an improved capability over standard magnetic resonance imaging to differentiate axon from myelin pathologies. However, the increased cellularity and vasogenic oedema associated with inflammation cannot be detected or separated from axon/myelin injury by diffusion tensor imaging, limiting its clinical applications. A novel diffusion basis spectrum imaging, capable of characterizing water diffusion properties associated with axon/myelin injury and inflammation, was developed to quantitatively reveal white matter pathologies in central nervous system disorders. Tissue phantoms made of normal fixed mouse trigeminal nerves juxtaposed with and without gel were employed to demonstrate the feasibility of diffusion basis spectrum imaging to quantify baseline cellularity in the absence and presence of vasogenic oedema. Following the phantom studies, in vivo diffusion basis spectrum imaging and diffusion tensor imaging with immunohistochemistry validation were performed on the corpus callosum of cuprizone treated mice. Results demonstrate that in vivo diffusion basis spectrum imaging can effectively separate the confounding effects of increased cellularity and/or grey matter contamination, allowing successful detection of immunohistochemistry confirmed axonal injury and/or demyelination in middle and rostral corpus callosum that were missed by diffusion tensor imaging. In addition, diffusion basis spectrum imaging-derived cellularity strongly correlated with numbers of cell nuclei determined using immunohistochemistry. Our findings suggest that diffusion basis spectrum imaging has great potential to provide non-invasive biomarkers for neuroinflammation, axonal injury and demyelination coexisting in multiple sclerosis.

The contribution of myelin to magnetic susceptibility-weighted contrasts in high-field MRI of the brain

T(2)*-weighted gradient-echo MRI images at high field (≥ 7T) have shown rich image contrast within and between brain regions. The source for these contrast variations has been primarily attributed to tissue magnetic susceptibility differences. In this study, the contribution of myelin to both T(2)* and frequency contrasts is investigated using a mouse model of demyelination based on a cuprizone diet. The demyelinated brains showed significantly increased T(2)* in white matter and a substantial reduction in gray-white matter frequency contrast, suggesting that myelin is a primary source for these contrasts. Comparison of in-vivo and in-vitro data showed that, although tissue T(2)* values were reduced by formalin fixation, gray-white matter frequency contrast was relatively unaffected and fixation had a negligible effect on cuprizone-induced changes in T(2)* and frequency contrasts.

Demyelination and remyelination in anatomically distinct regions of the corpus callosum following cuprizone intoxication

Multiple sclerosis is a chronic demyelinating disease of the central nervous system. Spontaneous remyelination during early disease stages is thought to preserve and partially restore function. However, this process ceases in later stages despite the presence of pre-oligodendrocytes. Cuprizone-induced demyelination is a useful model with which to study the remyelination process. Previous studies have demonstrated heterogeneities in demyelination in individual animals. Here we investigated regional differences in demyelination and remyelination within the corpus callosum. C57BL/6 mice were fed 0.2% cuprizone for 5 weeks to induce demyelination. Remyelination was examined 2-5 weeks after cuprizone withdrawal. Immunohistochemistry and electron microscopy were used to quantify regional differences in demyelination, gliosis, and remyelination. We found that, while demyelination was limited in the rostral region of corpus callosum, nearly complete demyelination occurred in the caudal callosum, beginning at approximately -0.5mm from bregma. Astrogliosis and microgliosis were correlated with demyelination and differed between the rostral and caudal callosal structures. Remyelination upon cessation of cuprizone ensued at different rates with splenium remyelinating faster than dorsal hippocampal commissure. Our data show anatomical differences of cuprizone-induced demyelination and remyelination in the corpus callosum and the importance of examining specific callosal regions in myelin repair studies using this model.

Diffusion tensor imaging of the optic radiations after optic neuritis

Trans-synaptic degeneration could exacerbate neurodegeneration in multiple sclerosis (MS). We aimed to assess whether anterograde trans-synaptic degeneration could be identified in the primary visual pathway in vivo. Diffusion tensor imaging (DTI) was used to assess the optic radiations in 15 patients with previous optic nerve inflammation and 9 healthy volunteers. A probabilistic atlas of the optic radiations was created from healthy diffusion tractography data. Lengthwise profiles for DTI parameters (axial [λ(||) ], radial [λ(⟂) ] and mean diffusivity [MD], fractional anisotropy [FA] and the angle of deviation of the principal eigenvector [α]) were analyzed for patients and controls. Patients also underwent multifocal visual evoked potential (mfVEP) assessments to characterize the latency and amplitude of cortical potentials. Correlations were performed between mfVEP latency and amplitude in the left and right visual hemi-fields and DTI parameters in the contra-lateral optic radiations. Patients displayed a significant decrease in λ(||) within the body of both optic radiations, which significantly correlated with loss of mfVEP amplitude. Abnormal λ(⟂) and FA were detected bilaterally throughout the optic radiations in patients but the abnormality was not associated with amplitude reduction or latency prolongation of the mfVEP. An abnormal α value was observed in the left optic radiations of patients, and the α value in the body of the optic radiations also correlated with mfVEP amplitude loss. The assocation between bilateral DTI abnormalities within the optic radiations and loss of afferent electrical activity could indicate anterograde trans-synaptic degeneration occurs following optic neuritis.

Fibroblast growth factor 1 (FGFR1) modulation regulates repair capacity of oligodendrocyte progenitor cells following chronic demyelination

The adult mammalian brain contains multiple populations of endogenous progenitor cell types. However, following CNS trauma or disease, the regenerative capacity of progenitor populations is typically insufficient and may actually be limited by non-permissive or inhibitory signals in the damaged parenchyma. Remyelination is the most effective and simplest regenerative process in the adult CNS yet is still insufficient following repeated or chronic demyelination. Our previous in vitro studies demonstrated that fibroblast growth factor receptor 1 (FGFR1) signaling inhibited oligodendrocyte progenitor (OP) differentiation into mature oligodendrocytes. Therefore, we questioned whether FGFR1 signaling may inhibit the capacity of OP cells to generate oligodendrocytes in a demyelinating disease model and whether genetically reducing FGFR1 signaling in oligodendrocyte lineage cells could enhance the capacity for remyelination. FGFR1 was found to be upregulated in the corpus callosum during cuprizone mediated demyelination and expressed on OP cells just prior to remyelination. Plp/CreER(T):Fgfr1(fl/fl) mice were administered tamoxifen to induce conditional Fgfr1 deletion in oligodendrocyte lineage cells. Tamoxifen administration during chronic demyelination resulted in reduced FGFR1 expression in OP cells. OP proliferation and population size were not altered one week after tamoxifen treatment. Tamoxifen was then administered during chronic demyelination and mice were given a six week recovery period without cuprizone in the chow. After the recovery period, OP numbers were reduced and the number of mature oligodendrocytes was increased, indicating an effect of FGFR1 reduction on OP differentiation. Importantly, tamoxifen administration in Plp/CreER(T):Fgfr1(fl/fl) mice significantly promoted remyelination and axon integrity. These results demonstrate a direct effect of FGFR1 signaling in oligodendrocyte lineage cells as inhibiting the repair capacity of OP cells following chronic demyelination in the adult CNS.