Paranodal myelin retraction in relapsing experimental autoimmune encephalomyelitis visualized by coherent anti-Stokes Raman scattering microscopy

Interruptible demyelination in avian riboflavin deficient neuropathy

BACKGROUND AND AIMS

The evolution of demyelination in individual internodes remains unclear although it has been noticed the paranodal demyelination precedes internodal demyelination in neuropathies with diverse aetiologies. For therapeutic purpose, it is fundamental to know whether the demyelinating procedure in affected internodes can be interrupted. This study aimed to delineate the development of demyelination in individual internodes in avian riboflavin deficient neuropathy.

METHODS

Newborn broiler meat chickens were maintained either on a routine diet containing 5.0 mg/kg riboflavin, a riboflavin deficient diet containing 1.8 mg/kg riboflavin, or initially a riboflavin deficient diet for 11 days and then routine diet plus riboflavin repletion from day 12. Evolution of demyelination in individual internodes was analyzed by teased nerve fibre studies from day 11 to 21.

RESULTS

In riboflavin deficient chickens, demyelination was the predominant feature: it was mainly confined to the paranodal region at day 11; extended into internodal region, but less than half of the internodal length in most affected internodes at day 16; involved more than half or whole internode at day 21. In the internode undergoing demyelination, myelin degeneration of varying degrees was noticed in the cytoplasm of the Schwann cell wrapping the internode. Two days after riboflavin repletion, co-existence of remyelination and active demyelination within individual internodes was noticed. Remyelination together with preserved short original internodes was the characteristic feature 4 and 9 days after riboflavin repletion.

CONCLUSION

Riboflavin repletion interrupts the progression from paranodal to internodal demyelination in riboflavin deficient chickens and promotes remyelination before complete internodal demyelination.

Vibrational spectroscopy and multiphoton microscopy for label-free visualization of nervous system degeneration and regeneration

Neurological disorders, including spinal cord injury, peripheral nerve injury, traumatic brain injury, and neurodegenerative diseases, pose significant challenges in terms of diagnosis, treatment, and understanding the underlying pathophysiological processes. Label-free multiphoton microscopy techniques, such as coherent Raman scattering, two-photon excited autofluorescence, and second and third harmonic generation microscopy, have emerged as powerful tools for visualizing nervous tissue with high resolution and without the need for exogenous labels. Coherent Raman scattering processes as well as third harmonic generation enable label-free visualization of myelin sheaths, while their combination with two-photon excited autofluorescence and second harmonic generation allows for a more comprehensive tissue visualization. They have shown promise in assessing the efficacy of therapeutic interventions and may have future applications in clinical diagnostics. In addition to multiphoton microscopy, vibrational spectroscopy methods such as infrared and Raman spectroscopy offer insights into the molecular signatures of injured nervous tissues and hold potential as diagnostic markers. This review summarizes the application of these label-free optical techniques in preclinical models and illustrates their potential in the diagnosis and treatment of neurological disorders with a special focus on injury, degeneration, and regeneration. Furthermore, it addresses current advancements and challenges for bridging the gap between research findings and their practical applications in a clinical setting.

Window into the mind: Advanced handheld spectroscopic eye-safe technology for point-of-care neurodiagnostic

Traumatic brain injury (TBI), a major cause of morbidity and mortality worldwide, is hard to diagnose at the point of care with patients often exhibiting no clinical symptoms. There is an urgent need for rapid point-of-care diagnostics to enable timely intervention. We have developed a technology for rapid acquisition of molecular fingerprints of TBI biochemistry to safely measure proxies for cerebral injury through the eye, providing a path toward noninvasive point-of-care neurodiagnostics using simultaneous Raman spectroscopy and fundus imaging of the neuroretina. Detection of endogenous neuromarkers in porcine eyes' posterior revealed enhancement of high-wave number bands, clearly distinguishing TBI and healthy cohorts, classified via artificial neural network algorithm for automated data interpretation. Clinically, translating into reduced specialist support, this markedly improves the speed of diagnosis. Designed as a hand-held cost-effective technology, it can allow clinicians to rapidly assess TBI at the point of care and identify long-term changes in brain biochemistry in acute or chronic neurodiseases.

Novel therapeutic for multiple sclerosis protects white matter function in EAE mouse model

Multiple sclerosis (MS) is a chronic demyelinating disease with prominent axon dysfunction. Our previous studies in an MS mouse model, experimental autoimmune encephalomyelitis (EAE), demonstrated that major histocompatibility complex Class II constructs can reverse clinical signs of EAE. These constructs block binding and downstream signaling of macrophage migration inhibitory factors (MIF-1/2) through CD74, thereby inhibiting phosphorylation of extracellular signal-regulated kinase (ERK) activation and tissue inflammation and promoting remyelination. To directly assess the effects of a novel third generation construct, DRhQ, on axon integrity in EAE, we compared axon conduction properties using electrophysiology on corpus callosum slices and optic nerves. By using two distinct white matter (WM) tracts, we aimed to assess the impact of the EAE and the benefit of DRhQ on myelinated and unmyelinated axons as well as to test the clinical value of DRhQ on demyelinating lesions in CC and optic myelitis. Our study found that EAE altered axon excitability, delayed axon conduction and slowed spatiotemporal summation correlated with diffuse astrocyte and microglia activation. Because MS predisposes patients to stroke, we also investigated and showed that vulnerability to WM ischemia is increased in the EAE MS mouse model. Treatment with DRhQ after the onset of EAE drastically inhibited microglial and astrocyte activation, improved functional integrity of the myelinated axons and enhanced recovery after ischemia. These results demonstrate that DRhQ administered after the onset of EAE promotes WM integrity and function, and reduces subsequent vulnerability to ischemic injury, suggesting important therapeutic potential for treatment of progressive MS.

The Node of Ranvier as an Interface for Axo-Glial Interactions: Perturbation of Axo-Glial Interactions in Various Neurological Disorders

The action potential conduction along the axon is highly dependent on the healthy interactions between the axon and myelin-producing glial cells. Myelin, which facilitates action potential, is the protective insulation around the axon formed by Schwann cells and oligodendrocytes in the peripheral (PNS) and central nervous system (CNS), respectively. Myelin is a continuous structure with intermittent gaps called nodes of Ranvier, which are the sites enriched with ion channels, transmembrane, scaffolding, and cytoskeletal proteins. Decades-long extensive research has identified a comprehensive proteome with strictly regularized localization at the node of Ranvier. Concurrently, axon-glia interactions at the node of Ranvier have gathered significant attention as the pathophysiological targets for various neurodegenerative disorders. Numerous studies have shown the alterations in the axon-glia interactions culminating in neurological diseases. In this review, we have provided an update on the molecular composition of the node of Ranvier. Further, we have discussed in detail the consequences of disruption of axon-glia interactions during the pathogenesis of various CNS and PNS disorders.

Review: Emerging Eye-Based Diagnostic Technologies for Traumatic Brain Injury

The study of ocular manifestations of neurodegenerative disorders, Oculomics, is a growing field of investigation for early diagnostics, enabling structural and chemical biomarkers to be monitored overtime to predict prognosis. Traumatic brain injury (TBI) triggers a cascade of events harmful to the brain, which can lead to neurodegeneration. TBI, termed the "silent epidemic" is becoming a leading cause of death and disability worldwide. There is currently no effective diagnostic tool for TBI, and yet, early-intervention is known to considerably shorten hospital stays, improve outcomes, fasten neurological recovery and lower mortality rates, highlighting the unmet need for techniques capable of rapid and accurate point-of-care diagnostics, implemented in the earliest stages. This review focuses on the latest advances in the main neuropathophysiological responses and the achievements and shortfalls of TBI diagnostic methods. Validated and emerging TBI-indicative biomarkers are outlined and linked to ocular neuro-disorders. Methods detecting structural and chemical ocular responses to TBI are categorised along with prospective chemical and physical sensing techniques. Particular attention is drawn to the potential of Raman spectroscopy as a non-invasive sensing of neurological molecular signatures in the ocular projections of the brain, laying the platform for the first tangible path towards alternative point-of-care diagnostic technologies for TBI.

Contribution of Intravital Neuroimaging to Study Animal Models of Multiple Sclerosis

Multiple sclerosis (MS) is a complex and long-lasting neurodegenerative disease of the central nervous system (CNS), characterized by the loss of myelin within the white matter and cortical fibers, axonopathy, and inflammatory responses leading to consequent sensory-motor and cognitive deficits of patients. While complete resolution of the disease is not yet a reality, partial tissue repair has been observed in patients which offers hope for therapeutic strategies. To address the molecular and cellular events of the pathomechanisms, a variety of animal models have been developed to investigate distinct aspects of MS disease. Recent advances of multiscale intravital imaging facilitated the direct in vivo analysis of MS in the animal models with perspective of clinical transfer to patients. This review gives an overview of MS animal models, focusing on the current imaging modalities at the microscopic and macroscopic levels and emphasizing the importance of multimodal approaches to improve our understanding of the disease and minimize the use of animals.

H-ABC tubulinopathy revealed by label-free second harmonic generation microscopy

Hypomyelination with atrophy of the basal ganglia and cerebellum is a recently described tubulinopathy caused by a mutation in the tubulin beta 4a isoform, expressed in oligodendrocytes. The taiep rat is the only spontaneous tubulin beta 4a mutant available for the study of this pathology. We aimed to identify the effects of the tubulin mutation on freshly collected, unstained samples of the central white matter of taiep rats using second harmonic generation microscopy. Cytoskeletal differences between the central white matter of taiep rats and control animals were found. Nonlinear emissions from the processes and somata of oligodendrocytes in tubulin beta 4a mutant rats were consistently detected, in the shape of elongated structures and cell-like bodies, which were never detected in the controls. This signal represents the second harmonic trademark of the disease. The tissue was also fluorescently labeled and analyzed to corroborate the origin of the nonlinear signal. Besides enabling the description of structural and molecular aspects of H-ABC, our data open the door to the diagnostic use of nonlinear optics in the study of neurodegenerative diseases, with the additional advantage of a label-free approach that preserves tissue morphology and vitality.

Transcranial direct current stimulation as a preventive treatment in multiple sclerosis? Preclinical evidence

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system, presenting with optic neuritis in about 20-30% of cases. Optic nerve demyelination, associated with delay of visual evoked potentials (VEPs), is also observed prior to motor signs in the preclinical MS model Experimental Autoimmune Encephalomyelitis (EAE). Transcranial direct current stimulation (tDCS), inducing polarity-dependent changes in neuronal excitability, is widely used to promote neuroplasticity in several neurological disorders. However, its potential effects on inflammation and demyelination are largely unknown. We tested the effectiveness of a preventive, 5-day tDCS treatment started 3 days post-immunization, in reducing the severity of VEP delays observed in early EAE. In mice undergoing cathodal tDCS (n = 6/26 eyes) VEPs were significantly less delayed compared with eyes from EAE-Sham (n = 24/32 eyes) and EAE-Anodal (n = 22/32 eyes). Optic nerve immunohistochemistry revealed a significantly lower cell density of microglia/macrophages, and less axonal loss in EAE-Cathodal vs EAE-Sham and EAE-Anodal, while the percent demyelination with Luxol-fast blue staining was comparable among EAE groups. Considering the latter result, immunofluorescence paranodal staining was performed, revealing a significantly higher number of complete paranode domains in EAE-Cathodal, closer to healthy mice, compared with EAE-Sham and EAE-Anodal groups. These results were reflected by the negative correlation between the number of complete paranode domains and VEP latency increase with respect to pre-immunization. Finally, cathodal tDCS was associated with a lower number, closer to healthy, of single paranodes in contrast to EAE-Sham. The effects of cathodal stimulation in preventing VEPs delays and optic nerve myelin damage were already observed in the pre-motor onset EAE stage, and were associated with a lower density of inflammatory cells. These findings suggest that tDCS may exert an anti-inflammatory effect with potential therapeutic application to be further explored in autoimmune demyelinating diseases.

White matter integrity in mice requires continuous myelin synthesis at the inner tongue

Myelin, the electrically insulating sheath on axons, undergoes dynamic changes over time. However, it is composed of proteins with long lifetimes. This raises the question how such a stable structure is renewed. Here, we study the integrity of myelinated tracts after experimentally preventing the formation of new myelin in the CNS of adult mice, using an inducible Mbp null allele. Oligodendrocytes survive recombination, continue to express myelin genes, but they fail to maintain compacted myelin sheaths. Using 3D electron microscopy and mass spectrometry imaging we visualize myelin-like membranes failing to incorporate adaxonally, most prominently at juxta-paranodes. Myelinoid body formation indicates degradation of existing myelin at the abaxonal side and the inner tongue of the sheath. Thinning of compact myelin and shortening of internodes result in the loss of about 50% of myelin and axonal pathology within 20 weeks post recombination. In summary, our data suggest that functional axon-myelin units require the continuous incorporation of new myelin membranes.

Myelin is formed of proteins of long half-lives. The mechanisms of renewal of such a stable structure are unclear. Here, the authors show that myelin integrity requires continuous myelin synthesis at the inner tongue, contributing to the maintenance of a functional axon-myelin unit.

Cuprizone Intoxication Results in Myelin Vacuole Formation

Myelin damage is a histopathological hallmark of multiple sclerosis lesions. Results of post mortem studies suggest that impaired myelin-axon interaction characterized by focal myelin detachments is an early event during lesion genesis. In this study, we investigated the ultrastructural changes of the axon-myelin interface in the cuprizone model using serial block face scanning electron microscopy and immunohistochemistry. We show that non-inflammatory injury of oligodendrocytes by cuprizone intoxication results in myelin vacuole formation and axonal swellings, paralleled by early alterations of the node of Ranvier cytoarchitecture. This remarkable resemblance of ultrastructural myelin characteristics in multiple sclerosis and the cuprizone animal model suggests that the cuprizone model is a valuable tool to study early pathologies during lesion formation.

Specific Blockade of Bone Morphogenetic Protein-2/4 Induces Oligodendrogenesis and Remyelination in Demyelinating Disorders

Oligodendrocyte precursor cells (OPCs) are present in demyelinated lesions of multiple sclerosis (MS) patients. However, their differentiation into functional oligodendrocytes is insufficient, and most lesions evolve into nonfunctional astroglial scars. Blockade of bone morphogenetic protein (BMP) signaling induces differentiation of OPCs into myelin-producing oligodendrocytes. We studied the effect of specific blockade of BMP-2/4 signaling, by intravenous (IV) treatment with anti-BMP-2/4 neutralizing mAb in both the inflammatory model of relapsing experimental autoimmune encephalomyelitis (R-EAE) and the cuprizone-toxic model of demyelination in mice. Administration of anti-BMP-2/4 to R-EAE-induced mice, on day 9 post-immunization (p.i.), ameliorated R-EAE signs, diminished the expression of phospho-SMAD1/5/8, primarily within the astrocytic lineage, increased the numbers of de novo immature and mature oligodendrocytes, and reduced the numbers of newly generated astrocytes within the spinal cord as early as day 18 p.i. This effect was accompanied with elevated remyelination, manifested by increased density of remyelinating axons (0.8 < g-ratios < 1), and reduced fully demyelinated and demyelinating axons, in the anti-BMP-2/4-treated R-EAE mice, studied by electron microscopy. No significant immunosuppressive effect was observed in the CNS and in the periphery, during the peak of the first attack, or at the end of the experiment. Moreover, IV treatment with anti-BMP-2/4 mAb in the cuprizone-challenged mice augmented the numbers of mature oligodendrocytes and remyelination in the corpus callosum during the recovery phase of the disease. Based on our findings, the specific blockade of BMP-2/4 has a therapeutic potential in demyelinating disorders such as MS, by inducing early oligodendrogenesis-mediated remyelination in the affected tissue.

Neuron-Oligodendrocyte Interactions in the Structure and Integrity of Axons

The myelination of axons by oligodendrocytes is a highly complex cell-to-cell interaction. Oligodendrocytes and axons have a reciprocal signaling relationship in which oligodendrocytes receive cues from axons that direct their myelination, and oligodendrocytes subsequently shape axonal structure and conduction. Oligodendrocytes are necessary for the maturation of excitatory domains on the axon including nodes of Ranvier, help buffer potassium, and support neuronal energy metabolism. Disruption of the oligodendrocyte-axon unit in traumatic injuries, Alzheimer's disease and demyelinating diseases such as multiple sclerosis results in axonal dysfunction and can culminate in neurodegeneration. In this review, we discuss the mechanisms by which demyelination and loss of oligodendrocytes compromise axons. We highlight the intra-axonal cascades initiated by demyelination that can result in irreversible axonal damage. Both the restoration of oligodendrocyte myelination or neuroprotective therapies targeting these intra-axonal cascades are likely to have therapeutic potential in disorders in which oligodendrocyte support of axons is disrupted.

Neuroinflammation in the normal-appearing white matter (NAWM) of the multiple sclerosis brain causes abnormalities at the nodes of Ranvier

Changes to the structure of nodes of Ranvier in the normal-appearing white matter (NAWM) of multiple sclerosis (MS) brains are associated with chronic inflammation. We show that the paranodal domains in MS NAWM are longer on average than control, with Kv1.2 channels dislocated into the paranode. These pathological features are reproduced in a model of chronic meningeal inflammation generated by the injection of lentiviral vectors for the lymphotoxin-α (LTα) and interferon-γ (IFNγ) genes. We show that tumour necrosis factor (TNF), IFNγ, and glutamate can provoke paranodal elongation in cerebellar slice cultures, which could be reversed by an N-methyl-D-aspartate (NMDA) receptor blocker. When these changes were inserted into a computational model to simulate axonal conduction, a rapid decrease in velocity was observed, reaching conduction failure in small diameter axons. We suggest that glial cells activated by pro-inflammatory cytokines can produce high levels of glutamate, which triggers paranodal pathology, contributing to axonal damage and conduction deficits.

Current thinking on the mechanisms by which multiple sclerosis gives rise to cumulative neurological disability revolves largely around focal lesions of inflammation and demyelination. However, some of the debilitating symptoms, such as severe fatigue, might be better explained by a more diffuse pathology. This study shows that paranodes in the white matter become abnormal as a result of neuroinflammation, which may be the result of the action of cytokines that cause glia to release glutamate.

CD36-mediated uptake of myelin debris by macrophages and microglia reduces neuroinflammation

Background

The presence of foamy macrophages and microglia containing intracellular myelin remnants is a pathological hallmark of neurodegenerative disorders such as multiple sclerosis (MS). Despite the importance of myelin internalization in affecting both central nervous system repair and neuroinflammation, the receptors involved in myelin clearance and their impact on the phagocyte phenotype and lesion progression remain to be clarified.

Methods

Flow cytometry, quantitative PCR, and immunohistochemistry were used to define the mRNA and protein abundance of CD36 in myelin-containing phagocytes. The impact of CD36 and nuclear factor erythroid 2–related factor 2 (NRF2) on the phagocytic and inflammatory features of macrophages and microglia was assessed using a pharmacological CD36 inhibitor (sulfo-N-succinimidyl oleate) and Nrf2^−/− bone marrow-derived macrophages. Finally, the experimental autoimmune encephalomyelitis (EAE) model was used to establish the impact of CD36 inhibition on neuroinflammation and myelin phagocytosis in vivo.

Results

Here, we show that the fatty acid translocase CD36 is required for the uptake of myelin debris by macrophages and microglia, and that myelin internalization increased CD36 expression through NRF2. Pharmacological inhibition of CD36 promoted the inflammatory properties of myelin-containing macrophages and microglia in vitro, which was paralleled by a reduced activity of the anti-inflammatory lipid-sensing liver X receptors and peroxisome proliferator-activated receptors. By using the EAE model, we provide evidence that CD36 is essential for myelin debris clearance in vivo. Importantly, CD36 inhibition markedly increased the neuroinflammatory burden and disease severity in the EAE model.

Conclusion

Altogether, we show for the first time that CD36 is crucial for clearing myelin debris and suppressing neuroinflammation in demyelinating disorders such as MS.

Label-free histomorphometry of peripheral nerve by stimulated Raman spectroscopy

BACKGROUND

Conventional processing of nerve for histomorphometry is resource-intensive, precluding use in intraoperative assessment of nerve quality during nerve transfer procedures. Stimulated Raman scattering (SRS) microscopy is a label-free technique that enables rapid and high-resolution histology.

METHODS

Segments of healthy murine sciatic nerve, healthy human obturator nerve, and human cross-facial nerve autografts were imaged on a custom SRS microscope. Myelinated axon quantification was performed through segmentation using a random forest machine learning algorithm in commercial software.

RESULTS

High contrast, high-resolution imaging of nerve morphology was obtained with SRS imaging. Automated myelinated axon quantification from cross-sections of healthy human nerve imaged using SRS was achieved.

CONCLUSIONS

Herein, we demonstrate the use of a label-free technique for rapid imaging of murine and human peripheral nerve cryosections. We illustrate the potential of this technique to inform intraoperative decision-making through rapid automated quantification of myelinated axons using a machine learning algorithm.

Label-free Imaging of Tissue Architecture during Axolotl Peripheral Nerve Regeneration in Comparison to Functional Recovery

Human peripheral nerves hold the potential to regenerate after injuries; however, whether a successful axonal regrowth was achieved can be elucidated only months after injury by assessing function. The axolotl salamander is a regenerative model where nerves always regenerate quickly and fully after all types of injury. Here, de- and regeneration of the axolotl sciatic nerve were investigated in a single and double injury model by label-free multiphoton imaging in comparison to functional recovery. We used coherent anti-Stokes Raman scattering to visualize myelin fragmentation and axonal regeneration. The presence of axons at the lesion site corresponded to onset of functional recovery in both lesion models. In addition, we detected axonal regrowth later in the double injury model in agreement with a higher severity of injury. Moreover, endogenous two-photon excited fluorescence visualized macrophages and revealed a similar timecourse of inflammation in both injury models, which did not correlate with functional recovery. Finally, using the same techniques, axonal structure and status of myelin were visualized in vivo after sciatic nerve injury. Label-free imaging is a new experimental approach that provides mechanistic insights in animal models, with the potential to be used in the future for investigation of regeneration after nerve injuries in humans.

Nodes of Ranvier in Glaucoma

Retinal ganglion cell axons of the DBA/2J mouse model of glaucoma, a model characterized by extensive neuroinflammation, preserve synaptic contacts with their subcortical targets for a time after onset of anterograde axonal transport deficits, axon terminal hypertrophy, and cytoskeletal alterations. Though retrograde axonal transport is still evident in these axons, it is unknown if they retain their ability to transmit visual information to the brain. Using a combination of in vivo multiunit electrophysiology, neuronal tract tracing, multichannel immunofluorescence, and transmission electron microscopy, we report that eye-brain signaling deficits precede transport loss and axonal degeneration in the DBA/2J retinal projection. These deficits are accompanied by node of Ranvier pathology - consisting of increased node length and redistribution of the voltage-gated sodium channel Na_v1.6 that parallel changes seen early in multiple sclerosis (MS) axonopathy. Further, with age, axon caliber and neurofilament density increase without corresponding changes in myelin thickness. In contrast to these findings in DBA/2J mice, node pathologies were not observed in the induced microbead occlusion model of glaucoma - a model that lacks pre-existing inflammation. After one week of systemic treatment with fingolimod, an immunosuppressant therapy for relapsing-remitting MS, DBA/2J mice showed a substantial reduction in node pathology and mild effects on axon morphology. These data suggest that neurophysiological deficits in the DBA/2J may be due to defects in intact axons and targeting node pathology may be a promising intervention for some types of glaucoma.

Label-free multiphoton microscopy reveals relevant tissue changes induced by alginate hydrogel implantation in rat spinal cord injury

The development of therapies promoting recovery after spinal cord injury is a challenge. Alginate hydrogels offer the possibility to develop biocompatible implants with mechanical properties tailored to the nervous tissue, which could provide a permissive environment for tissue repair. Here, the effects of non-functionalized soft calcium alginate hydrogel were investigated in a rat model of thoracic spinal cord hemisection and compared to lesioned untreated controls. Open field locomotion tests were employed to evaluate functional recovery. Tissue analysis was performed with label-free multiphoton microscopy using a multimodal approach that combines coherent anti-Stokes Raman scattering to visualize axonal structures, two-photon fluorescence to visualize inflammation, second harmonic generation to visualize collagenous scarring. Treated animals recovered hindlimb function significantly better than controls. Multiphoton microscopy revealed that the implant influenced the injury-induced tissue response, leading to decreased inflammation, reduced scarring with different morphology and increased presence of axons. Demyelination of contralateral white matter near the lesion was prevented. Reduced chronic inflammation and increased amount of axons in the lesion correlated with improved hindlimb functions, being thus relevant for locomotion recovery. In conclusion, non-functionalized hydrogel improved functional outcome after spinal cord injury in rats. Furthermore, label-free multiphoton microscopy qualified as suitable technique for regeneration studies.

Quantitative analysis of lipid debris accumulation caused by cuprizone induced myelin degradation in different CNS areas

Degradation of myelin sheath is thought to be the cause of neurodegenerative diseases, such as multiple sclerosis (MS), but definitive agreement on the mechanism of how myelin is lost is currently lacking. Autoimmune initiation of MS has been recently questioned by proposing that the immune response is a consequence of oligodendrocyte degeneration. To study the process of myelin breakdown, we induced demyelination with cuprizone and applied coherent anti-Stokes Raman scattering (CARS) microscopy, a non-destructive label-free method to image lipid structures in living tissue. We confirmed earlier results showing a brain region dependent myelin destructive effect of cuprizone. In addition, high resolution in situ CARS imaging revealed myelin debris forming lipid droplets alongwith myelinated axon fibers. Quantification of lipid debris with custom-made software for segmentation and three dimensional reconstruction revealed brain region dependent accumulation of lipid drops inversely correlated with the thickness of myelin sheaths. Finally, we confirmed that in situ CARS imaging is applicable to living human brain tissue in brain slices derived from a patient. Thus, CARS microscopy is potent tool for quantitative monitoring of myelin degradation in unprecedented spatiotemporal resolution during oligodendrocyte damage. We think that the accumulation of lipid drops around degrading myelin might be instrumental in triggering subsequent inflammatory processes.

Lipid biochemical changes detected in normal appearing white matter of chronic multiple sclerosis by spectral coherent Raman imaging

Multiple sclerosis (MS) exhibits demyelination, inflammatory infiltration, axonal degeneration, and gliosis, affecting widespread regions of the central nervous system (CNS). While white matter MS lesions have been well characterized pathologically, evidence indicates that the MS brain may be globally altered, with subtle abnormalities found in grossly normal appearing white matter (NAWM). These subtle changes are difficult to investigate by common methods such as histochemical stains and conventional magnetic resonance imaging. Thus, the prototypical inflammatory lesion likely represents the most obvious manifestation of a more widespread involvement of the CNS. We describe the application of spectral coherent anti-Stokes Raman Scattering (sCARS) microscopy to study such changes in chronic MS tissue particularly in NAWM. Subtle changes in myelin lipid biochemical signatures and intra-molecular disorder of fatty acid acyl chains of otherwise normal-appearing myelin were detected, supporting the notion that the biochemical involvement of the MS brain is far more extensive than conventional methods would suggest.

Lipid Order Degradation in Autoimmune Demyelination Probed by Polarized Coherent Raman Microscopy

Myelin around axons is currently widely studied by structural analyses and large-scale imaging techniques, with the goal to decipher its critical role in neuronal protection. Although there is strong evidence that in myelin, lipid composition, and lipid membrane morphology are affected during the progression of neurodegenerative diseases, there is no quantitative method yet to report its ultrastructure in tissues at both molecular and macroscopic levels, in conditions potentially compatible with in vivo observations. In this work, we study and quantify the molecular order of lipids in myelin at subdiffraction scales, using label-free polarization-resolved coherent anti-Stokes Raman, which exploits coherent anti-Stokes Raman sensitivity to coupling between light polarization and oriented molecular vibrational bonds. Importantly, the method does not use any a priori parameters in the sample such as lipid type, orientational organization, and composition. We show that lipid molecular order of myelin in the mouse spinal cord is significantly reduced throughout the progression of experimental autoimmune encephalomyelitis, a model for multiple sclerosis, even in myelin regions that appear morphologically unaffected. This technique permits us to unravel molecular-scale perturbations of lipid layers at an early stage of the demyelination progression, whereas the membrane architecture at the mesoscopic scale (here ∼100 nm) seems much less affected. Such information cannot be brought by pure morphological observation and, to our knowledge, brings a new perspective to molecular-scale understanding of neurodegenerative diseases.

Effect of scattering on coherent anti-Stokes Raman scattering (CARS) signals

We develop a computational framework to examine the factors responsible for scattering-induced distortions of coherent anti-Stokes Raman scattering (CARS) signals in turbid samples. We apply the Huygens-Fresnel wave-based electric field superposition (HF-WEFS) method combined with the radiating dipole approximation to compute the effects of scattering-induced distortions of focal excitation fields on the far-field CARS signal. We analyze the effect of spherical scatterers, placed in the vicinity of the focal volume, on the CARS signal emitted by different objects (2μm diameter solid sphere, 2μm diameter myelin cylinder and 2μm diameter myelin tube). We find that distortions in the CARS signals arise not only from attenuation of the focal field but also from scattering-induced changes in the spatial phase that modifies the angular distribution of the CARS emission. Our simulations further show that CARS signal attenuation can be minimized by using a high numerical aperture condenser. Moreover, unlike the CARS intensity image, CARS images formed by taking the ratio of CARS signals obtained using x- and y-polarized input fields is relatively insensitive to the effects of spherical scatterers. Our computational framework provide a mechanistic approach to characterizing scattering-induced distortions in coherent imaging of turbid media and may inspire bottom-up approaches for adaptive optical methods for image correction.

Assessing the efficacy of coherent anti-Stokes Raman scattering microscopy for the detection of infiltrating glioblastoma in fresh brain samples

Coherent anti-Stokes Raman scattering (CARS) microscopy is an emerging technique for identification of brain tumors. However, tumor identification by CARS microscopy on bulk samples and in vivo has been so far verified retrospectively on histological sections, which only provide a gross reference for the interpretation of CARS images without matching at cellular level. Therefore, fluorescent labels were exploited for direct assessment of the interpretation of CARS images of solid and infiltrative tumors. Glioblastoma cells expressing green fluorescent protein (GFP) were used for induction of tumors in mice (n = 7). The neoplastic nature of cells imaged by CARS microscopy was unequivocally verified by addressing two-photon fluorescence of GFP on fresh brain slices and in vivo. In fresh unfixed biopsies of human glioblastoma (n = 10), the fluorescence of 5-aminolevulinic acid-induced protoporphyrin IX was used for identification of tumorous tissue. Distinctive morphological features of glioblastoma cells, i.e. larger nuclei, evident nuclear membrane and nucleolus, were identified in the CARS images of both mouse and human brain tumors. This approach demonstrates that the chemical contrast provided by CARS allows the localization of infiltrating tumor cells in fresh tissue and that the cell morphology in CARS images is useful for tumor recognition. Experimental glioblastoma expressing green fluorescent protein.

Advances in Intravital Non-Linear Optical Imaging of the Central Nervous System in Rodents

Purpose of review: Highly coordinated cellular interactions occur in the healthy or pathologic adult rodent central nervous system (CNS). Until recently, technical challenges have restricted the analysis of these events to largely static modes of study such as immuno-fluorescence and electron microscopy on fixed tissues. The development of intravital imaging with subcellular resolution is required to probe the dynamics of these events in their natural context, the living brain. Recent findings: This review focuses on the recently developed live non-linear optical imaging modalities, the core principles involved, the identified technical challenges that limit their use and the scope of their applications. We highlight some practical applications for these modalities with a specific attention given to Experimental Autoimmune Encephalomyelitis (EAE), a rodent model of a chronic inflammatory disease of the CNS characterized by the formation of disseminated demyelinating lesions accompanied by axonal degeneration. Summary: We conclude that label-free nonlinear optical imaging combined to two photon imaging will continue to contribute richly to comprehend brain function and pathogenesis and to develop effective therapeutic strategies.

Imaging Myelination In Vivo Using Transparent Animal Models

Myelination by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system is essential for nervous system function and health. Despite its importance, we have a relatively poor understanding of the molecular and cellular mechanisms that regulate myelination in the living animal, particularly in the CNS. This is partly due to the fact that myelination commences around birth in mammals, by which time the CNS is complex and largely inaccessible, and thus very difficult to image live in its intact form. As a consequence, in recent years much effort has been invested in the use of smaller, simpler, transparent model organisms to investigate mechanisms of myelination in vivo. Although the majority of such studies have employed zebrafish, the Xenopus tadpole also represents an important complementary system with advantages for investigating myelin biology in vivo. Here we review how the natural features of zebrafish embryos and larvae and Xenopus tadpoles make them ideal systems for experimentally interrogating myelination by live imaging. We outline common transgenic technologies used to generate zebrafish and Xenopus that express fluorescent reporters, which can be used to image myelination. We also provide an extensive overview of the imaging modalities most commonly employed to date to image the nervous system in these transparent systems, and also emerging technologies that we anticipate will become widely used in studies of zebrafish and Xenopus myelination in the near future.

Endogenous Two-Photon Excited Fluorescence Provides Label-Free Visualization of the Inflammatory Response in the Rodent Spinal Cord

Activation of CNS resident microglia and invasion of external macrophages plays a central role in spinal cord injuries and diseases. Multiphoton microscopy based on intrinsic tissue properties offers the possibility of label-free imaging and has the potential to be applied in vivo. In this work, we analyzed cellular structures displaying endogenous two-photon excited fluorescence (TPEF) in the pathologic spinal cord. It was compared qualitatively and quantitatively to Iba1 and CD68 immunohistochemical staining in two models: rat spinal cord injury and mouse encephalomyelitis. The extent of tissue damage was retrieved by coherent anti-Stokes Raman scattering (CARS) and second harmonic generation imaging. The pattern of CD68-positive cells representing postinjury activated microglia/macrophages was colocalized to the TPEF signal. Iba1-positive microglia were found in areas lacking any TPEF signal. In peripheral areas of inflammation, we found similar numbers of CD68-positive microglia/macrophages and TPEF-positive structures while the number of Iba1-positive cells was significantly higher. Therefore, we conclude that multiphoton imaging of unstained spinal cord tissue enables retrieving the extent of microglia activation by acquisition of endogenous TPEF. Future application of this technique in vivo will enable monitoring inflammatory responses of the nervous system allowing new insights into degenerative and regenerative processes.

Beyond the borders--Biomedical applications of non-linear Raman microscopy

Raman spectroscopy offers great promise for label free imaging in biomedical applications. Its use, however, is hampered by the long integration times required and the presence of autofluorescence in many samples which outshines the Raman signals. In order to overcome these limitations, a variety of different non-linear Raman imaging techniques have been developed over the last decade. This review describes biomedical applications of these novel but already mature imaging techniques.

Applications of coherent Raman scattering microscopies to clinical and biological studies

Coherent anti-Stokes Raman scattering (CARS) microscopy and stimulated Raman scattering (SRS) microscopy are two nonlinear optical imaging modalities that are at the frontier of label-free and chemical specific biological and clinical diagnostics. The applications of coherent Raman scattering (CRS) microscopies are multifold, ranging from investigation of basic aspects of cell biology to the label-free detection of pathologies. This review summarizes recent progress of biological and clinical applications of CRS between 2008 and 2014, covering applications such as lipid droplet research, single cell analysis, tissue imaging and multiphoton histopathology of atherosclerosis, myelin sheaths, skin, hair, pharmaceutics, and cancer and surgical margin detection.

Automated method for the segmentation and morphometry of nerve fibers in large-scale CARS images of spinal cord tissue

A fully automated method for large-scale segmentation of nerve fibers from coherent anti-Stokes Raman scattering (CARS) microscopy images is presented. The method is specifically designed for CARS images of transverse cross sections of nervous tissue but is also suitable for use with standard light microscopy images. After a detailed description of the two-part segmentation algorithm, its accuracy is quantified by comparing the resulting binary images to manually segmented images. We then demonstrate the ability of our method to retrieve morphological data from CARS images of nerve tissue. Finally, we present the segmentation of a large mosaic of CARS images covering more than half the area of a mouse spinal cord cross section and show evidence of clusters of neurons with similar g-ratios throughout the spinal cord.

Label-free real-time imaging of myelination in the Xenopus laevis tadpole by in vivo stimulated Raman scattering microscopy

The myelin sheath plays an important role as the axon in the functioning of the neural system, and myelin degradation is a hallmark pathology of multiple sclerosis and spinal cord injury. Electron microscopy, fluorescent microscopy, and magnetic resonance imaging are three major techniques used for myelin visualization. However, microscopic observation of myelin in living organisms remains a challenge. Using a newly developed stimulated Raman scattering microscopy approach, we report noninvasive, label-free, real-time in vivo imaging of myelination by a single-Schwann cell, maturation of a single node of Ranvier, and myelin degradation in the transparent body of the Xenopus laevis tadpole.

The node of Ranvier in CNS pathology

Healthy nodes of Ranvier are crucial for action potential propagation along myelinated axons, both in the central and in the peripheral nervous system. Surprisingly, the node of Ranvier has often been neglected when describing CNS disorders, with most pathologies classified simply as being due to neuronal defects in the grey matter or due to oligodendrocyte damage in the white matter. However, recent studies have highlighted changes that occur in pathological conditions at the node of Ranvier, and at the associated paranodal and juxtaparanodal regions where neurons and myelinating glial cells interact. Lengthening of the node of Ranvier, failure of the electrically resistive seal between the myelin and the axon at the paranode, and retraction of myelin to expose voltage-gated K⁺ channels in the juxtaparanode, may contribute to altering the function of myelinated axons in a wide range of diseases, including stroke, spinal cord injury and multiple sclerosis. Here, we review the principles by which the node of Ranvier operates and its molecular structure, and thus explain how defects at the node and paranode contribute to neurological disorders.

Differential roles of microglia and monocytes in the inflamed central nervous system

Phagocytic monocyte-derived macrophages associate with the nodes of Ranvier and initiate demyelination while microglia clear debris and display a suppressed metabolic gene signature in EAE.

In the human disorder multiple sclerosis (MS) and in the model experimental autoimmune encephalomyelitis (EAE), macrophages predominate in demyelinated areas and their numbers correlate to tissue damage. Macrophages may be derived from infiltrating monocytes or resident microglia, yet are indistinguishable by light microscopy and surface phenotype. It is axiomatic that T cell–mediated macrophage activation is critical for inflammatory demyelination in EAE, yet the precise details by which tissue injury takes place remain poorly understood. In the present study, we addressed the cellular basis of autoimmune demyelination by discriminating microglial versus monocyte origins of effector macrophages. Using serial block-face scanning electron microscopy (SBF-SEM), we show that monocyte-derived macrophages associate with nodes of Ranvier and initiate demyelination, whereas microglia appear to clear debris. Gene expression profiles confirm that monocyte-derived macrophages are highly phagocytic and inflammatory, whereas those arising from microglia demonstrate an unexpected signature of globally suppressed cellular metabolism at disease onset. Distinguishing tissue-resident macrophages from infiltrating monocytes will point toward new strategies to treat disease and promote repair in diverse inflammatory pathologies in varied organs.

RP-CARS: label-free optical readout of the myelin intrinsic healthiness

Here we present a method based on Rotating-Polarization Coherent Anti-Stokes Raman Scattering (RP-CARS) imaging to assess the myelin health status in mouse sciatic nerves. Differently from the existing techniques, our method is based on the readout of intrinsic molecular architecture rather than on the image analysis, relying on the fact that healthy myelin is characterized by a high degree of molecular order. We exploit RP-CARS imaging to demonstrate that the degree of spatial anisotropy of the CARS signal displays a strong correlation with the g-ratio (a well-known image-based index of myelin damage) in a chemical-damage model and therefore that the former is a good indicator for the local myelin health status.

Optical tomographic imaging of near infrared imaging agents quantifies disease severity and immunomodulation of experimental autoimmune encephalomyelitis in vivo

Background

Experimental autoimmune encephalomyelitis (EAE) is an animal model that captures many of the hallmarks of human multiple sclerosis (MS), including blood–brain barrier (BBB) breakdown, inflammation, demyelination and axonal destruction. The standard clinical score measurement of disease severity and progression assesses functional changes in animal mobility; however, it does not offer information regarding the underlying pathophysiology of the disease in real time. The purpose of this study was to apply a novel optical imaging technique that offers the advantage of rapid imaging of relevant biomarkers in live animals.

Methods

Advances in non-invasive fluorescence molecular tomographic (FMT) imaging, in combination with a variety of biological imaging agents, offer a unique, sensitive and quantifiable approach to assessing disease biology in living animals. Using vascular (AngioSense 750EX) and protease-activatable cathepsin B (Cat B 680 FAST) near infrared (NIR) fluorescence imaging agents to detect BBB breakdown and inflammation, respectively, we quantified brain and spinal cord changes in mice with relapsing-remitting PLP_139-151-induced EAE and in response to tolerogenic therapy.

Results

FMT imaging and analysis techniques were carefully characterized and non-invasive imaging results corroborated by both ex vivo tissue imaging and comparison to clinical score results and histopathological analysis of CNS tissue. FMT imaging showed clear differences between control and diseased mice, and immune tolerance induction by antigen-coupled PLGA nanoparticles effectively blocked both disease induction and accumulation of imaging agents in the brain and spinal cord.

Conclusions

Cat B 680 FAST and AngioSense 750EX offered the combination best able to detect disease in both the brain and spinal cord, as well as the downregulation of disease by antigen-specific tolerance. Non-invasive optical tomographic imaging thus offers a unique approach to monitoring neuroinflammatory disease and therapeutic intervention in living mice with EAE.

Node of Ranvier disruption as a cause of neurological diseases

Dysfunction and/or disruption of nodes of Ranvier are now recognized as key contributors to the pathophysiology of various neurological diseases. One reason is that the excitable nodal axolemma contains a high density of Nav (voltage-gated Na⁺ channels) that are required for the rapid and efficient saltatory conduction of action potentials. Nodal physiology is disturbed by altered function, localization, and expression of voltage-gated ion channels clustered at nodes and juxtaparanodes, and by disrupted axon–glial interactions at paranodes. This paper reviews recent discoveries in molecular/cellular neuroscience, genetics, immunology, and neurology that highlight the critical roles of nodes of Ranvier in health and disease.

Local assessment of myelin health in a multiple sclerosis mouse model using a 2D Fourier transform approach

We present an automated two-dimensional Fourier transform (2D-FT) approach to analyze the local organization of myelinated axons in the spinal cord. Coherent anti-Stokes Raman scattering (CARS) microscopy was used to observe lesions in a commonly used animal model of multiple sclerosis (MS), experimental autoimmune encephalomyelitis (EAE). A 2D-FT was applied on the CARS images to find the average orientation and directional anisotropy of the fibers within contiguous image domains. We introduce the corrected correlation parameter (CCP), a measure of the correlation between orientations of adjacent domains. We show that in the EAE animal model of MS, the CCP can be used to quantify the degree of organization/disorganization in the myelin structure. This analysis was applied to a large image dataset from animals at different clinical scores and we show that some descriptors of the CCP probability density function are strongly correlated with the clinical scores. This procedure, compatible with live animal imaging, has been developed to perform local in situ evaluation of myelinated axons afflicted by EAE.