The origin and nature of beading: a reversible transformation of the shape of nerve fibers

Investigating microstructural changes between in vivo and perfused ex vivo marmoset brains using oscillating gradient and b-tensor encoded diffusion MRI at 9.4 T

PURPOSE

To investigate microstructural alterations induced by perfusion fixation in brain tissues using advanced diffusion MRI techniques and estimate their potential impact on the application of ex vivo models to in vivo microstructure.

METHODS

We used oscillating gradient spin echo (OGSE) and b-tensor encoding diffusion MRI to examine in vivo and ex vivo microstructural differences in the marmoset brain. OGSE was used to shorten effective diffusion times, whereas b-tensor encoding allowed for the differentiation of isotropic and anisotropic kurtosis. Additionally, we performed Monte Carlo simulations to estimate the potential microstructural changes in the tissues.

RESULTS

We report large changes (˜50%-60%) in kurtosis frequency dispersion (OGSE) and in both anisotropic and isotropic kurtosis (b-tensor encoding) after perfusion fixation. Structural MRI showed an average volume reduction of about 10%. Monte Carlo simulations indicated that these alterations could likely be attributed to extracellular fluid loss possibly combined with axon beading and increased dot compartment signal fraction. Little evidence was observed for reductions in axonal caliber.

CONCLUSION

Our findings shed light on advanced MRI parameter changes that are induced by perfusion fixation and potential microstructural sources for these changes. This work also suggests that caution should be exercised when applying ex vivo models to infer in vivo tissue microstructure, as significant differences may arise.

Corneal neuroepithelial compartmentalized microfluidic chip model for evaluation of toxicity-induced dry eye

Part of the lacrimal functional unit, the cornea protects the ocular surface from numerous environmental aggressions and xenobiotics. Toxicological evaluation of compounds remains a challenge due to complex interactions between corneal nerve endings and epithelial cells. To this day, models do not integrate the physiological specificity of corneal nerve endings and are insufficient for the detection of low toxic effects essential to anticipate Toxicity-Induced Dry Eye (TIDE). Using high-content imaging tool, we here characterize toxicity-induced cellular alterations using primary cultures of mouse trigeminal sensory neurons and corneal epithelial cells in a compartmentalized microfluidic chip. We validate this model through the analysis of benzalkonium chloride (BAC) toxicity, a well-known preservative in eyedrops, after a single (6h) or repeated (twice a day for 15 min over 5 days) topical 5.10-4% BAC applications on the corneal epithelial cells and nerve terminals. In combination with high-content image analysis, this advanced microfluidic protocol reveal specific and tiny changes in the epithelial cells and axonal network as well as in trigeminal cells, not directly exposed to BAC, with ATF3/6 stress markers and phospho-p44/42 cell activation marker. Altogether, this corneal neuroepithelial chip enables the evaluation of toxic effects of ocular xenobiotics, distinguishing the impact on corneal sensory innervation and epithelial cells. The combination of compartmentalized co-culture/high-content imaging/multiparameter analysis opens the way for the systematic analysis of toxicants but also neuroprotective compounds.

The role of mechanics in axonal stability and development

Much of the focus of neuronal cell biology has been devoted to growth cone guidance, synaptogenesis, synaptic activity, plasticity, etc. The axonal shaft too has received much attention, mainly for its astounding ability to transmit action potentials and the transport of material over long distances. For these functions, the axonal cytoskeleton and membrane have been often assumed to play static structural roles. Recent experiments have changed this view by revealing an ultrastructure much richer in features than previously perceived and one that seems to be maintained at a dynamic steady state. The role of mechanics in this is only beginning to be broadly appreciated and appears to involve passive and active modes of coupling different biopolymer filaments, filament turnover dynamics and membrane biophysics. Axons, being unique cellular processes in terms of high aspect ratios and often extreme lengths, also exhibit unique passive mechanical properties that might have evolved to stabilize them under mechanical stress. In this review, we summarize the experiments that have exposed some of these features. It is our view that axonal mechanics deserves much more attention not only due to its significance in the development and maintenance of the nervous system but also due to the susceptibility of axons to injury and neurodegeneration.

A new form of axonal pathology in a spinal model of neuromyelitis optica

Neuromyelitis optica is a chronic neuroinflammatory disease, which primarily targets astrocytes and often results in severe axon injury of unknown mechanism. Neuromyelitis optica patients harbour autoantibodies against the astrocytic water channel protein, aquaporin-4 (AQP4-IgG), which induce complement-mediated astrocyte lysis and subsequent axon damage. Using spinal in vivo imaging in a mouse model of such astrocytopathic lesions, we explored the mechanism underlying neuromyelitis optica-related axon injury. Many axons showed a swift and morphologically distinct 'pearls-on-string' transformation also readily detectable in human neuromyelitis optica lesions, which especially affected small calibre axons independently of myelination. Functional imaging revealed that calcium homeostasis was initially preserved in this 'acute axonal beading' state, ruling out disruption of the axonal membrane, which sets this form of axon injury apart from previously described forms of traumatic and inflammatory axon damage. Morphological, pharmacological and genetic analyses showed that AQP4-IgG-induced axon injury involved osmotic stress and ionic overload, but does not appear to use canonical pathways of Wallerian-like degeneration. Subcellular analysis demonstrated remodelling of the axonal cytoskeleton in beaded axons, especially local loss of microtubules. Treatment with the microtubule stabilizer epothilone, a putative therapy approach for traumatic and degenerative axonopathies, prevented axonal beading, while destabilizing microtubules sensitized axons for beading. Our results reveal a distinct form of immune-mediated axon pathology in neuromyelitis optica that mechanistically differs from known cascades of post-traumatic and inflammatory axon loss, and suggest a new strategy for neuroprotection in neuromyelitis optica and related diseases.

Axonal spheroids in neurodegeneration

Axonal spheroids are bubble-like biological features that form on most degenerating axons, yet little is known about their influence on degenerative processes. Their formation and growth has been observed in response to various degenerative triggers such as injury, oxidative stress, inflammatory factors, and neurotoxic molecules. They often contain cytoskeletal elements and organelles, and, depending on the pathological insult, can colocalize with disease-related proteins such as amyloid precursor protein (APP), ubiquitin, and motor proteins. Initial formation of axonal spheroids depends on the disruption of axonal and membrane tension governed by cytoskeleton structure and calcium levels. Shortly after spheroid formation, the engulfment signal phosphatidylserine (PS) is exposed on the outer leaflet of spheroid plasma membrane, suggesting an important role for axonal spheroids in phagocytosis and debris clearance during degeneration. Spheroids can grow until they rupture, allowing pro-degenerative factors to exit the axon into extracellular space and accelerating neurodegeneration. Though much remains to be discovered in this area, axonal spheroid research promises to lend insight into the etiologies of neurodegenerative disease, and may be an important target for therapeutic intervention. This review summarizes over 100 years of work, describing what is known about axonal spheroid structure, regulation and function.

The cytoskeleton as a modulator of tension driven axon elongation

Throughout development, neurons are capable of integrating external and internal signals leading to the morphological changes required for neuronal polarization and axon growth. The first phase of axon elongation occurs during neuronal polarization. At this stage, membrane remodeling and cytoskeleton dynamics are crucial for the growth cone to advance and guide axon elongation. When a target is recognized, the growth cone collapses to form the presynaptic terminal. Once a synapse is established, the growth of the organism results in an increased distance between the neuronal cell bodies and their targets. In this second phase of axon elongation, growth cone-independent molecular mechanisms and cytoskeleton changes must occur to enable axon growth to accompany the increase in body size. While the field has mainly focused on growth-cone mediated axon elongation during development, tension driven axon growth remains largely unexplored. In this review, we will discuss in a critical perspective the current knowledge on the mechanisms guiding axon growth following synaptogenesis, with a particular focus on the putative role played by the axonal cytoskeleton.

The Roles of Microtubules and Membrane Tension in Axonal Beading, Retraction, and Atrophy

Axonal beading, or the formation of a series of swellings along the axon, and retraction are commonly observed shape transformations that precede axonal atrophy in Alzheimer's disease, Parkinson's disease, and other neurodegenerative conditions. The mechanisms driving these morphological transformations are poorly understood. Here, we report controlled experiments that can induce either beading or retraction and follow the time evolution of these responses. By making quantitative analysis of the shape modes under different conditions, measurement of membrane tension, and using theoretical considerations, we argue that membrane tension is the main driving force that pushes cytosol out of the axon when microtubules are degraded, causing axonal thinning. Under pharmacological perturbation, atrophy is always retrograde, and this is set by a gradient in the microtubule stability. The nature of microtubule depolymerization dictates the type of shape transformation, vis-à-vis beading or retraction. Elucidating the mechanisms of these shape transformations may facilitate development of strategies to prevent or arrest axonal atrophy due to neurodegenerative conditions.

The Science and Art of Nerve Fiber Teasing for Myelinated Nerves: Methodology and Interpretation

Nerve fiber teasing is a sensitive technique utilized in diagnostic neuropathology practice, laboratory research, and animal toxicity studies for characterizing changes in single myelinated nerve fibers over extended distances. In animal toxicity studies, a nerve portion (approximately 10 mm in length) is stained with Sudan black for 24 to 48 hours and then transferred into a drop of viscous medium (eg, glycerin) mounted on an adhesive-coated glass slide, positioning it such that the proximodistal orientation is known. Individual fibers are removed using fine forceps while the sample is viewed under a stereomicroscope. In general, lesions can be identified during teasing, but more detailed characterization and photodocumentation is undertaken once nerve fibers have been dried and coverslipped. Nerve fiber teasing is particularly useful for distinguishing early stages of axonal degeneration (which presents as ovoid fiber fragments in the midinternodal region) from segmental demyelination (which presents as loss of original myelin segments and their replacement by thinner, shorter segments in the absence of axonal damage). The slow, laborious nature of nerve fiber teasing dictates that the technique will be employed on a few samples as an auxiliary method to better define the pathogenesis of nerve lesions first identified by conventional histopathologic assessment.

White Matter Changes in Cervical Dystonia Relate to Clinical Effectiveness of Botulinum Toxin Treatment

In a previous report showing white matter microstructural hemispheric asymmetries medial to the pallidum in focal dystonias, we showed preliminary evidence that this abnormality was reduced 4 weeks after botulinum toxin (BTX) injections. In the current study we report the completed treatment study in a full-size cohort of CD patients (n = 14). In addition to showing a shift toward normalization of the hemispheric asymmetry, we evaluated clinical relevance of these findings by relating white matter changes to degree of symptom improvement. We also evaluated whether the magnitude of the white matter asymmetry before treatment was related to severity, laterality, duration of dystonia, and/or number of previous BTX injections. Our results confirm the findings of our preliminary report: we observed significant fractional anisotropy (FA) changes medial to the pallidum 4 weeks after BTX in CD participants that were not observed in controls scanned at the same interval. There was a significant relationship between magnitude of hemispheric asymmetry and dystonia symptom improvement, as measured by percent reduction in dystonia scale scores. There was also a trend toward a relationship between magnitude of pre-injection white matter asymmetry and symptom severity, but not symptom laterality, disorder duration, or number of previous BTX injections. Post-hoc analyses suggested the FA changes at least partially reflected changes in pathophysiology, but a dissociation between patient perception of benefit from injections and FA changes suggested the changes did not reflect changes to the primary "driver" of the dystonia. In contrast, there were no changes or group differences in DTI diffusivity measures, suggesting the hemispheric asymmetry in CD does not reflect irreversible white matter tissue loss. These findings support the hypothesis that central nervous system white matter changes are involved in the mechanism by which BTX exerts clinical benefit.

White matter microstructure from nonparametric axon diameter distribution mapping

We report the development of a double diffusion encoding (DDE) MRI method to estimate and map the axon diameter distribution (ADD) within an imaging volume. A variety of biological processes, ranging from development to disease and trauma, may lead to changes in the ADD in the central and peripheral nervous systems. Unlike previously proposed methods, this ADD experimental design and estimation framework employs a more general, nonparametric approach, without a priori assumptions about the underlying form of the ADD, making it suitable to analyze abnormal tissue. In the current study, this framework was used on an ex vivo ferret spinal cord, while emphasizing the way in which the ADD can be weighted by either the number or the volume of the axons. The different weightings, which result in different spatial contrasts, were considered throughout this work. DDE data were analyzed to derive spatially resolved maps of average axon diameter, ADD variance, and extra-axonal volume fraction, along with a novel sub-micron restricted structures map. The morphological information contained in these maps was then used to segment white matter into distinct domains by using a proposed k-means clustering algorithm with spatial contiguity and left-right symmetry constraints, resulting in identifiable white matter tracks. The method was validated by comparing histological measures to the estimated ADDs using a quantitative similarity metric, resulting in good agreement. With further acquisition acceleration and experimental parameters adjustments, this ADD estimation framework could be first used preclinically, and eventually clinically, enabling a wide range of neuroimaging applications for improved understanding of neurodegenerative pathologies and assessing microstructural changes resulting from trauma.

In vivo imaging reveals rapid astrocyte depletion and axon damage in a model of neuromyelitis optica-related pathology

Objective

Neuromyelitis optica (NMO) is an autoimmune disease of the central nervous system, which resembles multiple sclerosis (MS). NMO differs from MS, however, in the distribution and histology of neuroinflammatory lesions and shows a more aggressive clinical course. Moreover, the majority of NMO patients carry immunoglobulin G autoantibodies against aquaporin‐4 (AQP4), an astrocytic water channel. Antibodies against AQP4 can damage astrocytes by complement, but NMO histopathology also shows demyelination, and — importantly—axon injury, which may determine permanent deficits following NMO relapses. The dynamics of astrocyte injury in NMO and the mechanisms by which toxicity spreads to axons are not understood.

Methods

Here, we establish in vivo imaging of the spinal cord, one of the main sites of NMO pathology, as a powerful tool to study the formation of experimental NMO‐related lesions caused by human AQP4 antibodies in mice.

Results

We found that human AQP4 antibodies caused acute astrocyte depletion with initial oligodendrocyte survival. Within 2 hours of antibody application, we observed secondary axon injury in the form of progressive swellings. Astrocyte toxicity and axon damage were dependent on AQP4 antibody titer and complement, specifically C1q.

Interpretation

In vivo imaging of the spinal cord reveals the swift development of NMO‐related acute axon injury after AQP4 antibody‐mediated astrocyte depletion. This approach will be useful in studying the mechanisms underlying the spread of NMO pathology beyond astrocytes, as well as in evaluating potential neuroprotective interventions. Ann Neurol 2016;79:794–805

Joint radius-length distribution as a measure of anisotropic pore eccentricity: an experimental and analytical framework

In this work, we present an experimental design and analytical framework to measure the nonparametric joint radius-length (R-L) distribution of an ensemble of parallel, finite cylindrical pores, and more generally, the eccentricity distribution of anisotropic pores. Employing a novel 3D double pulsed-field gradient acquisition scheme, we first obtain both the marginal radius and length distributions of a population of cylindrical pores and then use these to constrain and stabilize the estimate of the joint radius-length distribution. Using the marginal distributions as constraints allows the joint R-L distribution to be reconstructed from an underdetermined system (i.e., more variables than equations), which requires a relatively small and feasible number of MR acquisitions. Three simulated representative joint R-L distribution phantoms corrupted by different noise levels were reconstructed to demonstrate the process, using this new framework. As expected, the broader the peaks in the joint distribution, the less stable and more sensitive to noise the estimation of the marginal distributions. Nevertheless, the reconstruction of the joint distribution is remarkably robust to increases in noise level; we attribute this characteristic to the use of the marginal distributions as constraints. Axons are known to exhibit local compartment eccentricity variations upon injury; the extent of the variations depends on the severity of the injury. Nonparametric estimation of the eccentricity distribution of injured axonal tissue is of particular interest since generally one cannot assume a parametric distribution a priori. Reconstructing the eccentricity distribution may provide vital information about changes resulting from injury or that occurred during development.

Reduction of Diffusion-Weighted Imaging Contrast of Acute Ischemic Stroke at Short Diffusion Times

Background and Purpose—

Diffusion-weighted imaging (DWI) of tissue water is a sensitive and specific indicator of acute brain ischemia, where reductions of the diffusion of tissue water are observed acutely in the stroke lesion core. Although these diffusion changes have been long attributed to cell swelling, the precise nature of the biophysical mechanisms remains uncertain.

Methods—

The potential cause of diffusion reductions after stroke was investigated using an advanced DWI technique, oscillating gradient spin-echo DWI, that enables much shorter diffusion times and can improve specificity for alterations of structure at the micron level.

Results—

Diffusion measurements in the white matter lesions of patients with acute ischemic stroke were reduced by only 8% using oscillating gradient spin-echo DWI, in contrast to a 37% decrease using standard DWI. Neurite beading has recently been proposed as a mechanism for the diffusion changes after ischemic stroke with some ex vivo evidence. To explore whether beading could cause such differential results, simulations of beaded cylinders and axonal swelling were performed, yielding good agreement with experiment.

Conclusions—

Short diffusion times result in dramatically reduced diffusion contrast of human stroke. Simulations implicate a combination of neuronal beading and axonal swelling as the key structural changes leading to the reduced apparent diffusion coefficient after stroke.

Elastocapillarity can control the formation and the morphology of beads-on-string structures in solid fibers

Beads-on-string patterns have been experimentally observed in solid cylinders for a wide range of material properties and structural lengths, from millimetric soft gels to nanometric hard fibers. In this work, we combine theoretical analysis and numerical tools to investigate the formation and nonlinear dynamics of such beaded structures. We show that this morphological transition is driven by elastocapillarity, i.e., a complex interplay between the effects of surface tension and bulk elasticity. Unlike buckling or wrinkling, the presence of an axial elongation is found here to favor the onset of fiber beading, in agreement with existing experimental results on electrospun fibers, hydrogels, and nerves. Our results also prove that the applied stretch can be used in fabrication techniques to control the morphology of the emerging beads-on-string patterns. Such quantitative predictions open the way for several applications, from tissue engineering to the design of stretchable electronics and the microfabrication of functionalized surfaces.

Time-dependent structural changes of the dentatothalamic pathway in children treated for posterior fossa tumor

BACKGROUND AND PURPOSE

Injury to the dentatothalamic pathway that originates in the cerebellum has been suggested as a mechanism for neurologic complications in children treated for posterior fossa tumors. We hypothesized that time-dependent changes occur in the dentatothalamic pathway.

MATERIALS AND METHODS

Diffusion tensor evaluation was performed in 14 children (median age, 4.1 years; age range, 1-20 years) who underwent serial MR imaging at 3T as part of routine follow-up after posterior fossa tumor resection with or without adjuvant therapy. Tensor metrics were obtained in the acute (≤1 week), subacute (1 to <6 months), and chronic (≥6 months) periods after surgery. We evaluated the following dentatothalamic constituents: bilateral dentate nuclei, cerebellar white matter, and superior cerebellar peduncles. Serial dentate nuclei volumes were also obtained and compared with the patient's baseline.

RESULTS

The most significant tensor changes to the superior cerebellar peduncles and cerebellar white matter occurred in the subacute period, regardless of the tumor pathology or therapy regimen, with signs of recovery in the chronic period. However, chronic volume loss and reduced mean diffusivity were observed in the dentate nuclei and did not reverse. This atrophy was associated with radiation therapy and symptoms of ataxia.

CONCLUSIONS

Longitudinal diffusion MR imaging in children treated for posterior fossa tumors showed time-dependent tensor changes in components of the dentatothalamic pathway that suggest evolution of structural damage with inflammation and recovery of tissue directionality. However, the dentate nuclei did not show tensor or volumetric recovery, suggesting that the injury may be chronic.

Immunohistochemical study of mouse sciatic nerves under various stretching conditions with "in vivo cryotechnique"

BACKGROUND

In living animal bodies, some morphological changes of nerve fibers will probably occur when peripheral nerves are stretched or not stretched during various joint exercises. We aimed to capture the dynamic structures of nerves under various stretching conditions and to keep soluble serum proteins in their tissue sections.

NEW METHOD

Morphological changes of stretched or non-stretched sciatic nerve fibers were examined with "in vivo cryotechnique" (IVCT). Fibers were directly frozen with liquid isopentane-propane cryogen (-193°C). Immunolocalizations of protein 4.1G and albumin were also examined in the fibers.

RESULTS

The structures of IVCT-prepared sciatic nerves under the stretched condition showed a beaded appearance. By immunostaining for membrane skeletal protein 4.1G, Schmidt-Lanterman incisures (SLIs) were clearly identified, and the heights of their circular truncated cones were increased at narrow sites of the nerve fibers under the stretched condition, compared to those of non-stretched nerve fibers. Albumin was immunolocalized in blood vessels and also along endoneurium including regions near the node of Ranvier.

COMPARISON WITH EXISTING METHODS

With the conventional perfusion-fixation method (PF), it was difficult to keep stable postures of living mouse limbs for tissue preparation. In nerve fibers after PF, the structures of SLI were easily modified, and albumin was heterogeneously immunolocalized due to diffusion artifacts.

CONCLUSIONS

IVCT revealed (1) the structures of peripheral nerve fibers under dynamically different conditions, indicating that the morphological changes of SLIs play a functional role as a bumper structure against mechanical forces, and (2) accurate immunolocalization of serum albumin in the sciatic nerve fibers.

C. elegans model of neuronal aging

Aging of the nervous system underlies the behavioral and cognitive decline associated with senescence. Understanding the molecular and cellular basis of neuronal aging will therefore contribute to the development of effective treatments for aging and age-associated neurodegenerative disorders. Despite this pressing need, there are surprisingly few animal models that aim at recapitulating neuronal aging in a physiological context. We recently developed a C. elegans model of neuronal aging, and showed that age-dependent neuronal defects are regulated by insulin signaling. We identified electrical activity and epithelial attachment as two critical factors in the maintenance of structural integrity of C. elegans touch receptor neurons. These findings open a new avenue for elucidating the molecular mechanisms that maintain neuronal structures during the course of aging.

Evidence for altered basal ganglia-brainstem connections in cervical dystonia

Background

There has been increasing interest in the interaction of the basal ganglia with the cerebellum and the brainstem in motor control and movement disorders. In addition, it has been suggested that these subcortical connections with the basal ganglia may help to coordinate a network of regions involved in mediating posture and stabilization. While studies in animal models support a role for this circuitry in the pathophysiology of the movement disorder dystonia, thus far, there is only indirect evidence for this in humans with dystonia.

Methodology/Principal Findings

In the current study we investigated probabilistic diffusion tractography in DYT1-negative patients with cervical dystonia and matched healthy control subjects, with the goal of showing that patients exhibit altered microstructure in the connectivity between the pallidum and brainstem. The brainstem regions investigated included nuclei that are known to exhibit strong connections with the cerebellum. We observed large clusters of tractography differences in patients relative to healthy controls, between the pallidum and the brainstem. Tractography was decreased in the left hemisphere and increased in the right hemisphere in patients, suggesting a potential basis for the left/right white matter asymmetry we previously observed in focal dystonia patients.

Conclusions/Significance

These findings support the hypothesis that connections between the basal ganglia and brainstem play a role in the pathophysiology of dystonia.

Lesion growth and degeneration patterns measured using diffusion tensor 9.4-T magnetic resonance imaging in rat spinal cord injury

OBJECT

Using diffusion tensor MR imaging, the authors conducted a study to explore lesion growth and degeneration patterns, from the acute through chronic stages of spinal cord injury (SCI), in an experimental animal model.

METHODS

In vivo and ex vivo diffusion tensor imaging was performed using a 9.4-T MR imaging system in rats allowed to recover from traumatic contusion SCI from 2 weeks through 25 weeks postinjury, mimicking progression of human SCI from the acute through chronic stages.

RESULTS

Results showed significant growth of the traumatic lesion up to 15 weeks postinjury, where both the size and mean diffusivity (MD) reached a maximum that was maintained through the remainder of recovery. Mean diffusivity was sensitive to overall spinal cord integrity, whereas fractional anisotropy showed specificity to sites of cavity formation. The use of an MD contour map for in vivo data and a 3D surface map for ex vivo data, showing MD as a function of rostral-caudal distance and recovery time, allowed documentation of rostral and caudal spreading of the lesion.

CONCLUSIONS

Results from this study demonstrate changes in both lesion morphology and diffusivity beyond previously reported time points and provide a unique perspective on the process of cavity formation and degeneration following traumatic SCI. Additionally, results suggest that MD more accurately defines regions of histological damage than do regions of T2 signal hyperintensity. This could have significant clinical implications in the detection and potential treatment of posttraumatic syringes in SCI.

q-space and conventional diffusion imaging of axon and myelin damage in the rat spinal cord after axotomy

Parallel and perpendicular diffusion properties of water in the rat spinal cord were investigated 3 and 30 days after dorsal root axotomy, a specific insult resulting in early axonal degeneration followed by later myelin damage in the dorsal column white matter. Results from q-space analysis (i.e., the diffusion probability density function) obtained with strong diffusion weighting were compared to conventional anisotropy and diffusivity measurements at low b-values, as well as to histology for axon and myelin damage. q-Space contrasts included the height (return to zero displacement probability), full width at half maximum, root mean square displacement, and kurtosis excess of the probability density function, which quantifies the deviation from gaussian diffusion. Following axotomy, a significant increase in perpendicular diffusion (with decreased kurtosis excess) and decrease in parallel diffusion (with increased kurtosis excess) were found in lesions relative to uninjured white matter. Notably, a significant change in abnormal parallel diffusion was detected from 3 to 30 days with full width at half maximum, but not with conventional diffusivity. Also, directional full width at half maximum and root mean square displacement measurements exhibited different sensitivities to white matter damage. When compared to histology, the increase in perpendicular diffusion was not specific to demyelination, whereas combined reduced parallel diffusion and increased perpendicular diffusion was associated with axon damage.

Neurite beading is sufficient to decrease the apparent diffusion coefficient after ischemic stroke

Diffusion-weighted MRI (DWI) is a sensitive and reliable marker of cerebral ischemia. Within minutes of an ischemic event in the brain, the microscopic motion of water molecules measured with DWI, termed the apparent diffusion coefficient (ADC), decreases within the infarcted region. However, although the change is related to cell swelling, the precise pathological mechanism remains elusive. We show that focal enlargement and constriction, or beading, in axons and dendrites are sufficient to substantially decrease ADC. We first derived a biophysical model of neurite beading, and we show that the beaded morphology allows a larger volume to be encompassed within an equivalent surface area and is, therefore, a consequence of osmotic imbalance after ischemia. The DWI experiment simulated within the model revealed that intracellular ADC decreased by 79% in beaded neurites compared with the unbeaded form. To validate the model experimentally, excised rat sciatic nerves were subjected to stretching, which induced beading but did not cause a bulk shift of water into the axon (i.e., swelling). Beading-induced changes in cell-membrane morphology were sufficient to significantly hinder water mobility and thereby decrease ADC, and the experimental measurements were in excellent agreement with the simulated values. This is a demonstration that neurite beading accurately captures the diffusion changes measured in vivo. The results significantly advance the specificity of DWI in ischemia and other acute neurological injuries and will greatly aid the development of treatment strategies to monitor and repair damaged brain in both clinical and experimental settings.

Interactive image analysis programs for quantifying injury-induced axonal beading and microtubule disruption

Focal axonal beading and focal disruption of microtubule structure are characteristic to traumatic axonal injury. We have recently reproduced these morphological and structural changes in our in vitro model system [D. Kilinc, G. Gallo, K.A. Barbee, Mechanically induced membrane poration causes axonal beading and localized cytoskeletal damage, Exp. Neurol. 212 (2008) 422-430]. In order to measure bead formation objectively, an observer-independent quantification of beading was necessary. In addition, a quantitative measure for the extent of co-localization of axonal beads and microtubule disruptions was required to establish a causal relationship between focal cytoskeletal damage and bead formation. In this paper we describe Matlab-based, interactive image analysis programs for axonal beading quantification and co-localization analysis. Injury-induced increases in the axonal beading could be successfully detected using the bead analysis program.

Physical model for the width distribution of axons

The distribution of widths of axons was recently investigated, and was found to have a distinct peak at an optimized value. The optimized axon width at the peak may arise from the conflicting demands of minimizing energy consumption and assuring signal transmission reliability. The distribution around this optimized value is found to have a distinct non-Gaussian shape, with an exponential "tail". We propose here a mechanical model whereby this distribution arises from the interplay between the elastic energy of the membrane surrounding the axon core, the osmotic pressure induced by the neurofilaments inside the axon bulk, and active processes that remodel the microtubules and neurofilaments inside the axon. The axon's radius of curvature can be determined by the cell's control of the osmotic pressure difference across the membrane, the membrane tension or by changing the composition of the different components of the membrane. We find that the osmotic pressure, determined by the neurofilaments, seems to be the dominant control parameter.

Transgenic mice expressing the Tyr437His mutant of human myocilin protein develop glaucoma

PURPOSE

To developed a genetic mouse model of primary open-angle glaucoma induced by expression of mutated human myocilin in transgenic mice and to test whether expression of mutated human myocilin in the eye angle structures produces more significant damage to the eye than does mutated mouse myocilin.

METHODS

Recombineering in Escherichia coli was used to introduce the Tyr437His point mutation into a BAC carrying the full-length human MYOCILIN (MYOC) gene and long flanking regions. This BAC was used to produce transgenic mice. The expression of myocilin in the iridocorneal angle tissues and aqueous humor was studied by immunohistochemistry and Western blot analysis. Intraocular pressure was measured noninvasively with a fiber optic transducer. Retinal ganglion cells were retrograde labeled with fluorescent gold, and counted 5 days after labeling.

RESULTS

BAC transgenic mice expressed elevated levels of myocilin in tissues of the iridocorneal angle. Expression of mutated myocilin induced its intracellular accumulation and prevented secretion of both mutated and wild-type myocilin into the aqueous humor. Transgenic mice demonstrated a moderate elevation of intraocular pressure, which was more pronounced at night than in daytime. In the peripheral retina, transgenic mice lost 20% of the retinal ganglion cells and 55% of large retinal ganglion cells. Axonal degeneration was observed at the periphery of the optic nerve.

CONCLUSIONS

Expression of equivalent levels of mutated human or mouse myocilin in the eyes of transgenic mice produce comparable pathologic changes that are similar to those observed in patients with glaucoma.

Artifactual dendritic beading in rat spinal cord induced by perfusion with cold saline and paraformaldehyde

Extensive dendritic beading of MAP2 (microtubule-associated protein 2) immunoreactivity has previously been observed in the contused rat spinal cord. However, we have also observed dendritic beading in occasional uninjured animals. The purpose of this study was to examine the possibility that perfusion conditions contributed to the dendritic beading. Under deep anesthesia, uninjured rats (adult female Long-Evans, 200-225 g) were transcardially perfused with 0.9% saline solution followed by 4% paraformaldehyde at cold (4 degrees C) or warm (20 degrees C) temperature, and at a low (20 ml/min) or high (50 ml/min) flow rate. Dendrites were visualized by MAP2 immunoreactivity. The results demonstrate that perfusion with cold solutions at a high flow rate induces pronounced dendritic beading, and when perfused at a low flow rate, results in moderate dendritic beading. Warm perfusates did not induce dendritic beading when administered at a low flow rate, but occasional beading was observed with a high flow rate. Western blots revealed spectrin breakdown, but not MAP2 loss, in rats perfused with cold saline solution at a high flow rate, conditions that also resulted in dendritic beading. These findings demonstrate that dendritic morphology is sensitive to both temperature and flow rate of the perfusate. Warm fixative and a low perfusion flow rate minimized the perfusion-induced dendritic beading.

White matter abnormalities in dystonia normalize after botulinum toxin treatment

The pathophysiology of dystonia is still poorly understood. We used diffusion tensor imaging to screen for white matter abnormalities in regions between the basal ganglia and the thalamus in cervical and hand dystonia patients. All patients exhibited an abnormal hemispheric asymmetry in a focal region between the pallidum and the thalamus. This asymmetry was absent 4 weeks after the same patients were treated with intramuscular botulinum toxin injections. These findings represent a new systems-level abnormality in dystonia, which may lead to new insights about the pathophysiology of movement disorders. More generally, these findings demonstrate central nervous system changes following peripheral reductions in muscle activity. This raises the possibility that we have observed activity-dependent white matter plasticity in the adult human brain.

Osmotically driven shape transformations in axons

We report a cylindrical-peristaltic shape transformation in axons exposed to a controlled osmotic perturbation. The peristaltic shape relaxes and the axon recovers its original geometry within minutes. We show that the shape instability depends critically on the swelling rate and that volume and membrane area regulation are responsible for the shape relaxation. We propose that volume regulation occurs via leakage of ions driven by elastic pressure, and analyze the peristaltic shape dynamics taking into account the internal structure of the axon. The results obtained provide a framework for understanding peristaltic shape dynamics in nerve fibers occurring in vivo.

γ-Diketone neuropathy: axon atrophy and the role of cytoskeletal protein adduction

Multifocal giant neurofilamentous axonal swellings and secondary distal degeneration have been historically considered the hallmark features of gamma-diketone neuropathy. Accordingly, research conducted over the past 25 years has been directed toward discerning mechanisms of axonal swelling. However, this neuropathological convention has been challenged by recent observations that swollen axons were an exclusive product of long-term 2.5-hexanedione (HD) intoxication at lower daily dose-rates (e.g., 175 mg/kg/day); that is, higher HD dose-rates (e.g., 400 mg/kg/day) produced neurological deficits in the absence of axonal swellings. The observation that neurological toxicity can be expressed without axonal swelling suggests that this lesion is not an important pathophysiological event. Instead, several research groups have now shown that axon atrophy is prevalent in nervous tissues of laboratory animals intoxicated over a wide range of HD dose-rates. The well-documented nerve conduction defects associated with axon atrophy, in conjunction with the temporal correspondence between this lesion and the onset of neurological deficits, strongly suggest that atrophy has pathophysiological significance. In this commentary, we present evidence that supports a pathognomonic role for axon atrophy in gamma-diketone neuropathy and suggests that the functional consequences of this lesion mediate the corresponding neurological toxicity. Previous research has demonstrated that HD interacts with proteins via formation of pyrrole adducts. We therefore discuss the possibility that this chemical process is essential to the mechanism of atrophy. Evidence presented in this review suggests that "distal axonopathy" is an inaccurate classification and future nosological schemes should be based on the apparent primacy of axon atrophy.

Cellular damage and prevention in childhood hydrocephalus

The literature concerning brain damage due to hydrocephalus, especially in children and animal models, is reviewed. The following conclusions are reached: 1. Hydrocephalus has a deleterious effect on brain that is dependent on magnitude and duration of ventriculomegaly and modified by the age of onset. 2. Animal models have many histopathological similarities to humans and can be used to understand the pathogenesis of brain damage. 3. Periventricular axons and myelin are the primary targets of injury. The pathogenesis has similarities to traumatic and ischemic white matter injury. Secondary changes in neurons reflect compensation to the stress or ultimately the disconnection. 4. Altered efflux of extracellular fluid could result in accumulation of waste products that might interfere with neuron function. Further research is needed in this as well as the blood-brain barrier in hydrocephalus. 5. Some, but not all, of the changes are preventable by shunting CSF. However, axon loss cannot be reversed, therefore shunting in a given case must be considered carefully. 6. Experimental work has so far failed to show any benefit in reducing CSF production. Pharmacologic protection of the brain, at least as a temporary measure, holds some promise but more pre-clinical research is required.

gamma-diketone central neuropathy: quantitative morphometric analysis of axons in rat spinal cord white matter regions and nerve roots

A quantitative analytical method was used to measure myelinated axon morphometric parameters (e.g., axon area, ratio of axon area/fiber area, and index of circularity) in rat nervous tissue during intoxication with 2,5-hexanedione (HD). Parameters were assessed in nerve roots (dorsal and ventral) and in ascending (gracile fasciculus and spinocerebellar tract) and descending (corticospinal and rubrospinal tracts) spinal cord white matter tracts (L4-L5) of rats intoxicated with HD at two different daily dose-rates (175 or 400 mg HD/kg/day, gavage). For each dose-rate, tissue was sampled at four neurological endpoints: unaffected, slight, moderate, and severe toxicity, as determined by gait analysis and measurements of grip strength. Results indicate that, regardless of the HD dose-rate, axon atrophy (reduced axon area) was a widespread, abundant effect that developed in concert with neurological deficits. The atrophy response occurred contemporaneously in both ascending and descending spinal tracts, which suggests that loss of caliber developed simultaneously along the proximodistal axon axis. In contrast, swollen axons were a numerically small component and were present in nerve roots and spinal tracts only during subchronic intoxication at the lower HD dose-rate (i.e., 175 mg/kg/day). Intoxication at the higher dose-rate (400 mg/kg/day) produced neurological deficits in the absence of axonal swellings. These observations in conjunction with our previous studies of HD-induced peripheral neuropathy (Toxicol. Appl. Pharmacol. 135 (1995) 58; and Toxicol. Appl. Pharmacol. 165 (2000) 127) indicate that axon atrophy, and not axonal swelling, is a primary neuropathic phenomenon.

Activity regulates programmed cell death of zebrafish Rohon-Beard neurons

Programmed cell death is a normal aspect of neuronal development. Typically, twice as many neurons are generated than survive. In extreme cases, all neurons within a population disappear during embryogenesis or by early stages of postnatal development. Examples of transient neuronal populations include Cajal-Retzius cells of the cerebral cortex and Rohon-Beard cells of the spinal cord. The novel mechanisms that lead to such massive cell death have not yet been identified.

We provide evidence that electrical activity regulates the cell death program of zebrafish Rohon-Beard cells. Activity was inhibited by reducing Na+ current in Rohon-Beard cells either genetically (the macho mutation) or pharmacologically (tricaine). We examined the effects of activity block on three different reporters of cell death: DNA fragmentation, cytoskeletal rearrangements and cell body loss. Both the mao mutation and pharmacological blockade of Na+ current reduced these signatures of the cell death program. Moreover, the mao mutation and pharmacological blockade of Na+ current produced similar reductions in Rohon-Beard cell death. The results indicate that electrical activity provides signals that are required for the normal elimination of Rohon-Beard cells.

Membrane bistability in olfactory bulb mitral cells

Whole-cell patch-clamp recordings were used to investigate the electrophysiological properties of mitral cells in rat main olfactory bulb brain slice preparations. The majority of mitral cells are bistable. These cells spontaneously alternate between two membrane potentials, separated by approximately 10 mV: a relatively depolarized potential (upstate), which is perithreshold for spike generation, and a relatively hyperpolarized potential (downstate), in which spikes do not occur. Bistability occurs spontaneously in the absence of ionotropic excitatory or inhibitory synaptic inputs. Bistability is voltage dependent; transition from the downstate to the upstate is a regenerative event activated by brief depolarization. A brief hyperpolarization can switch the membrane potential from the upstate to the downstate. In response to olfactory nerve (ON) stimulation, mitral cells in the upstate are more likely to fire an action potential than are those in the downstate. ON stimulation can switch the membrane potential from the downstate to the upstate, producing a prolonged and amplified depolarization in response to a brief synaptic input. We conclude that bistability is an intrinsic property of mitral cells that is a major determinant of their responses to ON input.

Functional immunohistochemistry of neuropeptides and nitric oxide synthase in the nerve fibers of the supratentorial dura mater in an experimental migraine model

The supratentorial cerebral dura of the albino rat is equipped with a rich sensory innervation both in the connective tissue and around blood vessels, which includes nociceptive axons and their terminals; these display intense calcitonin gene-related peptide (CGRP) immunoreactivity. Stereotactic electrical stimulation of the trigeminal (Gasserian) ganglion, regarded as an experimental migraine model, caused marked increase and disintegration of club-like perivascular CGRP-immunopositive nerve endings in the dura mater and induced an apparent increase in the lengths of CGRP-immunoreactive axons. Intravenous administration of sumatriptan or eletriptan, prior to electrical stimulation, prevented disintegration of perivascular terminals and induced accumulation of CGRP in terminal and preterminal portions of peripheral sensory axons. Consequently, immunopositive terminals and varicosities increased in size; accumulation of axoplasmic organelles resulted in the "hollow" appearence of numerous varicosities. Since triptans exert their anti-migraine effect by virtue of agonist action on 5-HT(1D/B) receptors, we suggest that these drugs prevent the release of CGRP from perivascular nerve terminals in the dura mater by an action at 5-HT(1D/B) receptors. Nitroglycerine (NitroPOHL), given subcutaneously to rats, induces increased beading of nitric oxide synthase (NOS)-immunoreactive nerve fibers in the supratentorial cerebral dura mater, and an apparent increase in the number of NOS-immunoreactive nerve fibers in the dural areas supplied by the anterior and middle meningeal arteries, and the sinus sagittalis superior. Structural alterations of nitroxidergic axons innervating blood vessels of the dura mater support the idea that nitric oxide (NO) is involved in the induction of headache, a well-known side effect of coronary dilator agents.

Effects of eletriptan on the peptidergic innervation of the cerebral dura mater and trigeminal ganglion, and on the expression of c-fos and c-jun in the trigeminal complex of the rat in an experimental migraine model

Nociceptive axons and terminals in the supratentorial cerebral dura mater display an intense calcitonin gene-related peptide (CGRP) immunoreactivity. In an experimental migraine model, it has been shown that electrical stimulation of the rat trigeminal ganglion induced an increase in the lengths of CGRP-immunoreactive axons, increased size and number of pleomorphic axonal varicosities in the dura mater, and an increased number of c-jun and c-fos protein-expressing nerve cells in the trigeminal complex. We demonstrate the effect of the highly specific and moderately lipophilic serotonin agonist eletriptan (Pfizer) which prevents the effects of electrical stimulation in the dura mater. Eletriptan also affected the caudal trigeminal complex; it markedly reduced the numbers of the oncoprotein-expressing cells, mainly after stimulation and to some extent also in nonstimulated animals. Eletriptan also affected expression of CGRP in perikarya of trigeminal ganglion cells, insofar as the number of small nerve cells exhibiting a compact CGRP immunoreaction was decreased to one quarter of the original value. In all these respects, eletriptan acted in a similar way to sumatriptan, with the notable exception that eletriptan also blocked the stimulation-induced effects in the nucleus caudalis trigemini and the upper cervical spinal cord (trigeminal complex), whereas sumatriptan did not. It is concluded that eletriptan, acting on perikarya and both the peripheral and the central axon terminals of primary sensory neurons, exerts its antimigraine effect by an agonist action on 5-HT1B/1D receptors throughout the entire trigeminal system, probably by passing the blood-brain-barrier because of its lipophilic character.

Rate of neurotoxicant exposure determines morphologic manifestations of distal axonopathy

Exposure to a variety of agricultural, industrial, and pharmaceutical chemicals produces nerve damage classified as a central-peripheral distal axonopathy. Morphologically, this axonopathy is characterized by distal axon swellings and secondary degeneration. Over the past 25 years substantial research efforts have been devoted toward deciphering the molecular mechanisms of these presumed hallmark neuropathic features. However, recent studies suggest that axon swelling and degeneration are related to subchronic low-dose neurotoxicant exposure rates (i.e., mg toxicant/kg/day) and not to the development of neurophysiological deficits or behavioral toxicity. This suggests these phenomena are nonspecific and of uncertain pathophysiologic relevance. This possibility has significant implications for research investigating mechanisms of neurotoxicity, development of exposure biomarkers, design of risk assessment models, neurotoxicant classification schemes, and clinical diagnosis and treatment of toxic neuropathies. In this commentary we will review the evidence for the dose-related dependency of distal axonopathies and discuss how this concept might influence our current understanding of chemical-induced neurotoxicities.

gamma-diketone peripheral neuropathy. I. Quality morphometric analyses of axonal atrophy and swelling

Quantitative morphometric analysis was used to characterize expression of myelinated axon swelling and atrophy in rat peripheral nerve during 2,5-hexanedione (HD) intoxication. HD was administered by gavage according to different daily dosing regiments (100, 175, 250, or 400 mg/kg/day) and four proximodistal nerve regions (5th lumbar spinal nerve, proximal and distal sciatic nerve, and tibial nerve) were examined morphometrically. Morphometric determinations were made at four behavioral endpoints (unaffected, slight, moderate, and severe toxicity) and were correlated to electrophysiologic measurements of peripheral nerve function. Results show that, for all HD dose rates, onsets of behavioral neurotoxicity and nerve dysfunction were generally related to development of abundant axon atrophy. The proximodistal manifestation of atrophy was dependent upon the dosing rate; i.e., the atrophy response produced by subacute intoxication with higher daily dosing rates (250 and 400 mg/kg/day) was restricted to distal nerve regions whereas subchronic induction with lower dosing rates (100 and 175 mg/kg/day) produced abundant fiber atrophy in all proximodistal areas. In contrast to atrophy, axonal swellings constituted an inconsistent minor morphologic response, the expression of which was dependent upon subchronic dosing rates (100-250 mg/kg/day). Subacute HD administration (400 mg/kg/day) produced significant changes in neurobehavior and nerve electrophysiologic parameters in the absence of peripheral axon swelling. Thus, conditional expression of swellings suggests they are an epiphenomenon related to low-dose induction rates. Fiber atrophy, however, was numerically dominant, correlated with nerve dysfunction, and occurred at all dosing levels. These characteristics suggest atrophy is a neurotoxicologically significant feature of gamma-diketone peripheral neuropathy.

Teased-fiber technique for peripheral myelinated nerves: methodology and interpretation

Teased-fiber technique is the best approach for studying peripheral myelinated nerve fibers in their continuity. It enables the assessment of size of myelin segments formed by Schwann cells and characterization of pathologic changes affecting the internodia, the paranodal regions, and the invested axons. Fiber teasing is performed on prestained proximodistally oriented portions of peripheral nerves. Specimens about 10 mm long are stained for 24-48 hours in Sudan black and then transferred to glycerin, where, using a pair of fine forceps and a stereomicroscope, they are separated into smaller fiber bundles from which single fibers are isolated. The work is performed on a glass slide with an adhesive surface (albuminized or "superfrost"), on which the fibers are placed in strict proximodistal orientation. Following drying in an oven, the slides are mounted with glycerin-gelatine (same as used for frozen sections). The changes, when present, can usually be recognized during the preparation, but fibers are reexamined and changes confirmed in mounted slides. Photographic reconstruction of the fibers facilitates their assessment and enables the documentation of findings. The teased-fiber technique is auxiliary to histopathology, and to limit the workload and save costs, it can be performed on only a few specimens selected for better characterization of changes recognized or suspected in tissue sections. In particular, segmental demyelination and early stages of Wallerian or secondary axonal degeneration can be recognized in teased fibers. Segmental demyelination is characterized by loss of fully myelinated segments and their replacement by newly formed short and thin segments, remyelinating the preserved axon. The early stage of secondary axonal degeneration is recognized by formation of ovoidal fiber fragments in the midinternodal region.

Experimental spinal cord injury: spatiotemporal characterization of elemental concentrations and water contents in axons and neuroglia

To examine the role of axonal ion deregulation in acute spinal cord injury (SCI), white matter strips from guinea pig spinal cord were incubated in vitro and were subjected to graded focal compression injury. At several postinjury times, spinal segments were removed from incubation and rapidly frozen. X-ray microanalysis was used to measure percent water and dry weight elemental concentrations (mmol/kg) of Na, P, Cl, K, Ca, and Mg in selected morphological compartments of myelinated axons and neuroglia from spinal cord cryosections. As an index of axon function, compound action potentials (CAP) were measured before compression and at several times thereafter. Axons and mitochondria in epicenter of severely compressed spinal segments exhibited early (5 min) increases in mean Na and decreases in K and Mg concentrations. These elemental changes were correlated to a significant reduction in CAP amplitude. At later postcompression times (15 and 60 min), elemental changes progressed and were accompanied by alterations in compartmental water content and increases in mean Ca. Swollen axons were evident at all postinjury times and were characterized by marked element and water deregulation. Neuroglia and myelin in severely injured epicenter also exhibited significant disruptions. In shoulder areas (adjacent to epicenter) of severely injured spinal strips, axons and mitochondria exhibited modest increases in mean Na in conjunction with decreases in K, Mg, and water content. Following moderate compression injury to spinal strips, epicenter axons exhibited early (10 min postinjury) element and water deregulation that eventually recovered to near control values (60 min postinjury). Na(+) channel blockade by tetrodotoxin (TTX, 1 microM) perfusion initiated 5 min after severe crush diminished both K loss and the accumulation of Na, Cl, and Ca in epicenter axons and neuroglia, whereas in shoulder regions TTX perfusion completely prevented subcellular elemental deregulation. TTX perfusion also reduced Na entry in swollen axons but did not affect K loss or Ca gain. Thus graded compression injury of spinal cord produced subcellular elemental deregulation in axons and neuroglia that correlated with the onset of impaired electrophysiological function and neuropathological alterations. This suggests that the mechanism of acute SCI-induced structural and functional deficits are mediated by disruption of subcellular ion distribution. The ability of TTX to reduce elemental deregulation in compression-injured axons and neuroglia implicates a significant pathophysiological role for Na(+) influx in SCI and suggests Na(+) channel blockade as a pharmacotherapeutic strategy.