The beaded form of myelinated nerve fibers

Neuronal Activity in the Sciatic Nerve Is Accompanied by Immediate Cytoskeletal Changes

Mechanical events and alterations in neuronal morphology that accompany neuronal activity have been observed for decades. However, no clear neurophysiological role, nor an agreed molecular mechanism relating these events to the electrochemical process, has been found. Here we hypothesized that intense, yet physiological, electrical activity in neurons triggers cytoskeletal depolymerization. We excited the sciatic nerve of anesthetized mice with repetitive electric pulses (5, 10, and 100 Hz) for 1 and 2 min and immediately fixed the excised nerves. We then scanned the excised nerves with high-resolution transmission electron microscopy, and quantified cytoskeletal changes in the resulting micrographs. We demonstrate that excitation with a stimulation frequency that is within the physiological regime is accompanied by a significant reduction in the density of cytoskeletal proteins relative to the baseline values recorded in control nerves. After 10 Hz stimulation with durations of 1 and 2 min, neurofilaments density dropped to 55.8 and 51.1% of the baseline median values, respectively. In the same experiments, microtubules density dropped to 23.7 and 38.5% of the baseline median values, respectively. These changes were also accompanied by a reduction in the cytoskeleton-to-cytoplasm contrast that we attribute to the presence of depolymerized electron-dense molecules in the lumen. Thus, we demonstrate with an in vivo model a link between electrical activity and immediate cytoskeleton rearrangement at the nano-scale. We suggest that this cytoskeletal plasticity reduces cellular stiffness and allows cellular homeostasis, maintenance of neuronal morphology and that it facilitates in later stages growth of the neuronal projections.

A computational model coupling mechanics and electrophysiology in spinal cord injury

Traumatic brain injury and spinal cord injury have recently been put under the spotlight as major causes of death and disability in the developed world. Despite the important ongoing experimental and modeling campaigns aimed at understanding the mechanics of tissue and cell damage typically observed in such events, the differentiated roles of strain, stress and their corresponding loading rates on the damage level itself remain unclear. More specifically, the direct relations between brain and spinal cord tissue or cell damage, and electrophysiological functions are still to be unraveled. Whereas mechanical modeling efforts are focusing mainly on stress distribution and mechanistic-based damage criteria, simulated function-based damage criteria are still missing. Here, we propose a new multiscale model of myelinated axon associating electrophysiological impairment to structural damage as a function of strain and strain rate. This multiscale approach provides a new framework for damage evaluation directly relating neuron mechanics and electrophysiological properties, thus providing a link between mechanical trauma and subsequent functional deficits.

Mercury in the spinal cord after inhalation of mercury

Amyotrophic lateral sclerosis (ALS) affects anterior horn cells of the spinal cord causing an indolent slow and steady deterioration of muscle strength leading inevitably to death in respiratory failure. ALS is a model condition for neurodegenerative disorders. Exposure to different agents dispersed in the environment has been suggested to cause neurodegeneration but no convincing evidence for such a link has yet been presented. Respiratory exposure to metallic mercury (Hg(0)) from different sources may be suspected. Body distribution of metallic mercury is fast and depends on solubility properties. Routes of transport, metabolism, excretion and biological half-life determine the overall toxic effects. Inhalation experiments were performed in 1984 where small marmoset monkeys (Callithrix jacchus) were exposed to (203) Hg(0 vapour) mixed into the breathing air (4-5 μg/l). After 1 hr of exposure, they were killed and whole body autoradiograms prepared to study the distribution of mercury within organs. Autoradiograms showed that Hg was deposited inside the spinal cord. Areas of enhanced accumulation anatomically corresponding to motor nuclei could be observed. This study describes a reinvestigation, with new emphasis on the spinal cord, of these classical metal exposure data in a primate, focusing on their relevance for the causation of neurodegenerative disorders. A comparison with more recent rodent experiments with similar findings is included. The hypothesis that long-time low-dose respiratory exposure to metals, for example, Hg, contributes to neurodegenerative disorders is forwarded and discussed.

The role of the cytoskeleton in volume regulation and beading transitions in PC12 neurites

We present investigations on volume regulation and beading shape transitions in PC12 neurites, conducted using a flow-chamber technique. By disrupting the cell cytoskeleton with specific drugs, we investigate the role of its individual components in the volume regulation response. We find that microtubule disruption increases both swelling rate and maximum volume attained, but does not affect the ability of the neurite to recover its initial volume. In addition, investigation of axonal beading-also known as pearling instability-provides additional clues on the mechanical state of the neurite. We conclude that volume recovery is driven by passive diffusion of osmolites, and propose that the initial swelling phase is mechanically slowed down by microtubules. Our experiments provide a framework to investigate the role of cytoskeletal mechanics in volume homeostasis.

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.

Simple morphometry of axonal swellings cannot be used in isolation for dating lesions after traumatic brain injury

Disruption of fast axonal transport as a result of traumatic brain injury is characterized by the accumulation of beta-amyloid precursor protein (APP) in axonal swellings. A recent report has suggested a correlation between the size of axonal swellings and survival time up to about 85 h after blunt head injury. The authors of the report concluded that this correlation, in conjunction with other evidence, might be useful in forensic science for timing injuries. To test this hypothesis we have used image analysis software to measure a number of different morphological parameters of axonal swellings. Paraffin sections from 63 cases of fatal head injury were stained with an antibody raised against the N-terminus of APP and counterstained with haematoxylin. Three different measurements were made of the APP-immunoreactive axonal swellings from the corpus callosum: (i) minimum and (ii) maximum Feret diameters, and (iii) area. Linear regression revealed a significant correlation between survival time and the minimum Feret diameter (p < 0.0001) and the area (p < 0.001) of axonal swellings. Our findings are in agreement with the previous study in that there is a significant correlation between axonal swelling size and survival time. However, we would suggest that the large variability in swelling size within individual cases and the heterogeneity of the original trauma seriously compromise the utility of such information in the timing of lesions.

Duration of neuronal stretch correlates with functional loss

Postoperative cranial nerve weakness or paralysis is not uncommon in many otolaryngologic surgical procedures. Our study used a rat model to test the hypothesis that the length of time that a nerve is under tension may be an important variable in the amount of postoperative paresis. Forty Sprague-Dawley rats were divided into 4 groups that underwent either a sham operation or a traction injury for 1, 2, or 5 minutes. The traction injury was performed with a vessel loop placed around the sciatic nerve with 50 g of tension. Traction injury for 1 or 2 minutes did not result in any statistical differences in the motor capabilities of the lower limb. However, those animals with a stretch injury for 5 minutes had a significant loss of function (P < 0.01) when compared with all other groups. Histologic examination of nerves harvested on postoperative day 7 showed no evidence of mechanical injury. This study demonstrates that even minimal tension, if maintained for a significant amount of time, may result in postoperative weakness.

Studies on the mechanisms responsible for the formation of focal swellings on neuronal processes using a novel in vitro model of axonal injury

A novel in vitro model of axonal injury using PC12 cells was designed to introduce traumatic alterations on neuronal processes and to identify mechanisms responsible for the formation of focal swellings by observation with phase-contrast and transmission electron microscopes. The injury on the processes was produced by one-dimensional, horizontal oscillation. The fluid shear stress applied by the oscillation did not exceed 380 dyne/cm2. The injured processes showed two forms. One involved an increase in the terminal diameter of the processes and the other entailed beading along the injured portions. Long-term observation of cellular responses to the mechanical insult disclosed that the terminal swelling coincided with the detachment of growth cones from the culture plate. The finding suggests that the detachment of the growth cone destroys the cytoskeletal network, which determines and maintains the cell shape, resulting in spherical deformation of the processes. When the cytoskeletal destruction occurred at non-terminal sites along the processes, spherical deformations developed slowly, and these appeared as beads. The beading also caused the detachment of the growth cones. As the most proximal bead grew, they absorbed the distal segment and their growth cones were pulled proximally with the spreading cytoskeletal destruction. The processes with terminal swellings as well as the bead segments showed regeneration with time evidence of and growth cone formation.

Pearling in cells: a clue to understanding cell shape

Gradual disruption of the actin cytoskeleton induces a series of structural shape changes in cells leading to a transformation of cylindrical cell extensions into a periodic chain of "pearls." Quantitative measurements of the pearling instability give a square-root behavior for the wavelength as a function of drug concentration. We present a theory that explains these observations in terms of the interplay between rigidity of the submembranous actin shell and tension that is induced by boundary conditions set by adhesion points. The theory allows estimation of the rigidity and thickness of this supporting shell. The same theoretical considerations explain the shape of nonadherent edges in the general case of untreated cells.

Biomechanics of stretch-induced beading

To account for the beading of myelinated fibers, and axons of unmyelinated nerve fibers as well of neurites of cultured dorsal root ganglia caused by mild stretching, a model is presented. In this model, membrane tension and hydrostatic pressure are the basic factors responsible for axonal constriction, which causes the movement of axonal fluid from the constricted regions into the adjoining axon, there giving rise to the beading expansions. Beading ranges from a mild undulation, with the smallest degree of stretch, to more globular expansions and narrow intervening constrictions as stretch is increased: the degree of constriction is physically limited by the compaction of the cytoskeleton within the axons. The model is a general one, encompassing the possibility that the membrane skeleton, composed mainly of spectrin and actin associated with the inner face of the axolemma, could be involved in bringing about the constrictions and beading.

Axonal cytoskeletal changes after nondisruptive axonal injury. II. Intermediate sized axons

Earlier studies of axonal cytoskeletal responses to stretch injury in the guinea pig optic nerve, a model of nondisruptive axonal injury such as occurs in human diffuse axonal injury, have demonstrated different cytoskeletal responses between the smallest and largest axons. But these form only approximately 3% of the total number of axons in the optic nerve. It was then posited that the pathology described in the latter axons may not be representative of the pathology in the majority of axons after stretch injury. In order to test this hypothesis, we carried out a quantitative, morphological analysis of structural changes in the cytoskeleton of intermediate (axonal diameter of 0.5-2.0 mM) sized axons at 4 h after stretch injury. Neurofilaments in axons up to 1.00 microm in diameter increased in number and in axons up to 1.50 microm diameter were compacted. This did not occur in larger axons (diameter of 1.51-2.00 microm) in the present study. However, there was focal compaction of neurofilaments in some of the larger fibers at sites where the integrity of the axolemma was lost. The response by microtubules to stretch injury differed from that of neurofilaments in that there was an increased spacing between microtubules and a loss of their number in axons of >1.51 microm diameter. We provide quantitative, morphological evidence (a) that the neurofilamentous cytoskeleton of different sized axons responds in different ways to stretch and (b) that the response by microtubules differs from that of neurofilaments.

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

Nerve fibers which appear beaded (varicose, spindle-shaped, etc.) are often considered the result of pathology, or a preparation artifact. However, beading can be promptly elicited in fresh normal nerve by a mild stretch and revealed by fast-freezing and freeze-substitution, or by aldehyde fixating at a temperature near 0 degree C (cold-fixation). The key change in beading are the constrictions, wherein the axon is much reduced in diameter. Axoplasmic fluid and soluble components are shifted from the constrictions into the expansions leaving behind compacted microtubules and neurofilaments. Labeled cytoskeletal proteins carried down by slow axonal transport are seen to move with the soluble components and not to have been incorporated into and remain with, the cytoskeletal organelles on beading the fibers. Lipids and other components of the myelin sheath are also shifted from the constrictions into the expansions, with preservation of its fine structure and thickness. Additionally, myelin intrusions into the axons are produced and a localized bulging into the axon termed "leafing". The beading constrictions do not arise from the myelin sheath: beading occurs in the axons of unmyelinated fibers. It does not depend on the axonal cytoskeleton: exposure of nerves in vitro to beta, beta'-iminodipropionitrile (IDPN) disaggregates the cytoskeletal organelles and even augments beading. The hypothesis advanced was that the beading constrictions are due to the membrane skeleton; the subaxolemmal network comprised of spectrin/fodrin, actin, ankyrin, integrins and other transmembrane proteins. The mechanism can be activated directly by neurotoxins, metabolic changes, and by an interruption of axoplasmic transport producing Wallerian degeneration.

Origin of beading constrictions at the axolemma: presence in unmyelinated axons and after beta,beta'-iminodipropionitrile degradation of the cytoskeleton

Myelinated nerve fibres become beaded when nerves are subjected to a mild stretch; the beading is seen as varicosities, a series of alternating constrictions and enlargements, when using freeze-substitution or cold-fixation to hold this labile form change in place during fixation. One possibility for how this form change comes about is that the myelin sheath or its Schwann cell initiates beading. We now report, however, that a similar beading is seen in the axons of unmyelinated fibres. In electron micrographs, longitudinal sections of axons show the series of constrictions and expansions typical of beading. In cross-sections, axons with unusually small diameter, corresponding to the constrictions, are seen to contain closely packed microtubules and neurofilaments while neighbouring swollen axons with widely dispersed microtubules correspond to the beading expansions. Another possibility for the form change is that the cytoskeleton is responsible for beading. We discovered that direct exposure of nerves to beta, beta'-iminodipropionitrile in vitro for 1-6 h causes both axonal microtubules and neurofilaments to become degraded and replaced by an amorphous residue. Nevertheless, beta,beta'-iminodipropionitrile-treated nerves show constrictions in myelinated fibres when stretched. An even greater degree of beading with narrower and longer constrictions appears in some fibres, with the expanded regions having oblate ends giving the appearance of a string of sausages. In cross-sections taken through the constrictions, a greater than usual reduction of axonal area was seen, this was due to the loss of cytoskeletal organelles which would act to limit the degree of constriction. With longer exposure to beta, beta'-iminodipropinitrile more fibres show complete degeneration of the cytoskeleton and form ovoids typical of Wallerian degeneration. Unmyelinated axons of beta, beta'-iminodipropionitrile-treated nerves which showed degeneration of their cytoskeleton with its replacement by amorphous material still demonstrated beading. As neither the myelin sheath nor the intact cytoskeleton within the axon is necessary for beading, by exclusion, we consider beading constrictions to be initiated at the level of the axolemma. In our hypothesis the membrane skeleton is responsible; namely, the spectrin, actin and other molecular species lining the inside of the axolemma and binding to transmembrane proteins. The membrane skeleton may be activated by stretch via transmembrane proteins (e.g. beta 1-integrins). The membrane skeleton mechanism may also be directly engaged in the production of Wallerian degeneration or be induced by neurotoxic agents.

The relation of the beading of myelinated nerve fibers to the bands of Fontana

The bands of Fontana, appearing as spirals or irregular light and dark strips crossing the surface of unstretched nerves, are due to the wavy disposition of nerve fibers within the epineural-perineural sheaths. A mean tension of 2.7 +/- 0.23 (S.E.M.) g applied to segments of rat tibial nerves straightens the fibers and unbands the nerves causing them to lengthen by 9.35 +/- 0.89%. The nerves cold-fixed in situ at that point showed the myelinated fibers to be beaded. On relaxation the nerves rebanded and the fibers were no longer beaded. The tension at which unbanding occurred was better determined when the epineural-perineural sheaths were slit longitudinally. Under these conditions, unbanding occurred at a mean tension of 0.59 +/- 0.08 g and the nerves lengthened by 8.56 +/- 0.58%. The lengthening was not statistically different from that seen in sheathed nerves. In preparations with the epineural-perineural sheaths removed, banding was lost with tensions of 0.20 +/- 0.03 g and the nerves lengthened by 12.1 +/- 1.04%. The tensions needed were significantly lower than that for the sheathed and slit-sheath nerve groups. When cold-fixed, when banding was lost, the fibers were seen to be beaded. Banding of the desheathed nerves returned on relaxation of the nerves. However, after tensions of 8 g they showed plasticity in which the ends of the nerves needed to be pushed together to initiate rebanding in comparison to sheathed or sheath-slit nerves which rebanded spontaneously following relaxation after even higher tensions of 40 g. At the highest tensions the nerves remained extended and could not be forcibly rebanded.(ABSTRACT TRUNCATED AT 250 WORDS)