Postnatal development of microtubules and neurofilaments in the rat optic nerve: a quantitative study

Life-long arsenic exposure damages the microstructure of the rat hippocampus

Epidemiological studies demonstrate that arsenic exposure is associated with cognitive dysfunction. Experimental arsenic exposure models showed learning and memory deficits and molecular changes resembling the functional and pathologic neurodegeneration features. The present work focuses on hippocampal pathological changes in Wistar rats induced by continuous arsenic exposure from in utero up to 12 months of age, evaluated by magnetic resonance imaging along with immunohistochemistry. Diffusion-weighted images revealed age-related lower fractional anisotropy and higher radial-axial and mean diffusivity at 6 and 12 months, indicating that arsenic exposure leads to hippocampal demyelination. These structural alterations were paralleled by immunohistochemical changes that showed a significant loss of myelin basic protein in CA1 and CA3 regions accompanied by increased glial fibrillary acidic protein expression at all time-points studied. Concomitantly, arsenic exposure induced an altered morphology of astrocytes at all studied ages, whereas increased synaptogenesis was only observed at two months of age. These results suggest that environmental arsenic exposure is linked to impaired hippocampal connectivity and perhaps early glial senescence, which together might resemble a premature aging phenomenon leading to cognitive deficits.

Deuterium double quantum-filtered NMR studies of peripheral and optic nerves

OBJECTIVE

Characterization of the nerve components by deuterium double quantum-filtered magnetization transfer (DQF-MT) NMR.

METHODS

Nerves were equilibrated in deuterated saline and ²H single-pulse and ²H DQF-MT NMR spectra were measured, enabling the separation of the different water compartments, according to their quadrupolar splittings.

RESULTS

Rat sciatic and brachial nerves and porcine optic nerve immersed in deuterated saline yielded ²H DQF spectra composed of three pairs of quadrupolar-split signals assigned to the water in the collagenous compartments and the myelin bilayer and one narrow signal assigned to the axonal water. Stretching of the nerves, application of osmotic stress and incubation in collagenase did not affect the quadrupolar splitting of the myelin water. The signals of myelin and axonal water were shown to decay during Wallerian degeneration and to rise during maturation. The chemical exchange between the myelin and the intra-axonal water was measured for optic nerve during maturation. The quadrupolar splitting of the signal of myelin water was not sensitive to its orientation relative to the magnetic field. This resembles liquid crystalline behavior, but leaves its mechanism open for interpretation.

CONCLUSIONS

²H DQF-MT NMR characterizes the different components of nerves, the water exchange between them and their changes during processes such as nerve maturation and Wallerian degeneration.

The third wave: Intermediate filaments in the maturing nervous system

Intermediate filaments are critical for the extreme structural specialisations of neurons, providing integrity in dynamic environments and efficient communication along axons a metre or more in length. As neurons mature, an initial expression of nestin and vimentin gives way to the neurofilament triplet proteins and α-internexin, substituted by peripherin in axons outside the CNS, which physically consolidate axons as they elongate and find their targets. Once connection is established, these proteins are transported, assembled, stabilised and modified, structurally transforming axons and dendrites as they acquire their full function. The interaction between these neurons and myelinating glial cells optimises the structure of axons for peak functional efficiency, a property retained across their lifespan. This finely calibrated structural regulation allows the nervous system to maintain timing precision and efficient control across large distances throughout somatic growth and, in maturity, as a plasticity mechanism allowing functional adaptation.

Temporal and spatial development of axonal maturation and myelination of white matter in the developing brain

BACKGROUND AND PURPOSE

Diffusion tensor imaging (DTI) has been widely used to investigate the development of white matter (WM). However, information about this development in healthy children younger than 2 years of age is lacking, and most previous studies have only measured fractional anisotropy (FA). This study used FA and radial and axonal diffusivities in children younger than 2 years of age, aiming to determine the temporal and spatial development of axonal maturation and myelination of WM in healthy children.

MATERIALS AND METHODS

A total of 60 healthy pediatric subjects were imaged by using a 3T MR imaging scanner. They were divided into 3 groups: 20 at 3 weeks, 20 at 1 year of age, and 20 at 2 years of age. All subjects were imaged asleep without sedation. FA and axial and radial diffusivities were obtained. Eight regions of interest were defined, including both central and peripheral WM for measuring diffusion parameters.

RESULTS

A significant elevation in FA (P < .0001) and a reduction in axial and radial diffusivities (P < .0001) were observed from 22 days to 1 year of age, whereas only radial diffusivity showed significant changes (P = .0014) from 1 to 2 years of age. The region-of-interest analysis revealed that FA alone may not depict the underlying biologic underpinnings of WM development, whereas directional diffusivities provide more insights into the development of WM. Finally, the spatial development of WM begins from the central to the peripheral WM and from the occipital to the frontal lobes.

CONCLUSIONS

With both FA and directional diffusivities, our results demonstrate the temporal and spatial development of WM in healthy children younger than 2 years of age.

Absolute eigenvalue diffusion tensor analysis for human brain maturation

The absolute eigenvalues of the diffusion tensor of white matter in sixteen normal subjects in two groups representing the early developmental stage (ages 1-10 years, n=8) and young adult stage (ages 18-34 years, n=8) were assessed using a high-field (3.0 T) magnetic resonance (MR) system. All three eigenvalues, including the largest eigenvalue, decreased significantly with brain maturation. The rate of the decline in the two small eigenvalues was, however, much higher than that of the largest eigenvalue, resulting in an actual increase in fractional anisotropy, a commonly measured relative index. The data demonstrate that an increase in anisotropy associated with brain maturation represents a significant decline in the small eigenvalue components, rather than an increase in the largest eigenvalue. The observed pattern of eigenvalue changes is best explained by the simultaneous occurrence of two of several independent phenomena within the axonal microenvironment during the myelination process, namely, (1) decline in unrestricted water content in extra-axonal space, and (2) increase in apparent diffusivity within the axon.

The Microtubular Pattern Changes at the Spinal Cord-Root Junction and Reverts at the Root-Peripheral Nerve Junction in Sensory and Motor Fibres of the Rat

In the rat, we studied the microtubular content of central nervous system (CNS) axons (pyramidal tract, dorsal funiculus, and intracord domain of motor axons), of radicular axons (ventral and dorsal roots), and of peripheral axons (sural and lateral gastrocnemius nerves). The microtubular density had an inverse relationship with the size of the axon. Within the CNS, values ranged from over 120 microtubules/microm2 for axons smaller than 0.1 microm2 of the pyramidal tract and dorsal funiculus to 24 for 3-microm motor axons (area, 7 microm2) in their spinal cord domain. Peripheral nerve and CNS axons of the same size had comparable microtubular densities. In contrast, the microtubular density of dorsal and ventral root axons was one half that of CNS or peripheral nerve axons of equal calibre. Considered along the axon, the microtubular density of motor and sensory fibres is high in the CNS domain, low in the root, and high again in the peripheral nerve domain. These observations are inconsistent with the notion that the cytoskeleton moves coherently away from the perikaryon. We conclude that the axonal microtubular content accords with the calibre of the fibre and with the anatomical region where it courses. We propose that axonal microtubules are regulated by local cues.

The return of phosphorylated and nonphosphorylated epitopes of neurofilament proteins to the regenerating optic nerve of Xenopus laevis

Neurofilament proteins of mammalian axotomized peripheral axons, which regenerate effectively, resemble those of embryonic axons. However, injured centrally projecting mammalian axons, which fail to regenerate, have very different neurofilament compositions than during development. If changes in neurofilament composition after injury reflect the ability of axotomized neurons to regenerate effectively, then the neurofilaments of centrally projecting axons that can regenerate should more closely resemble those of developing axons. In this study, the neurofilament compositions of injured optic axons of the frog, Xenopus laevis, were examined, since these axons can regenerate a fully functional projection. Antibodies to phosphorylated and nonphosphorylated forms of neurofilament proteins that had been used previously to study the neurofilament composition of newly developing X. laevis optic axons were used in immunocytochemical studies to examine the return of neurofilaments to the optic nerve after an intraorbital nerve crush. Intraocularly injected wheat germ agglutinin conjugated to horseradish peroxidase was used to label the regenerating axons independently of their neurofilaments. Neurofilament immunoreactivities disappeared rapidly from crushed axons during the first week after surgery. By nine days after surgery, antibodies to nonphosphorylated forms of middle (NF-M) and low molecular weight (NF-L) neurofilament proteins and the Xenopus neuronal intermediate filament protein (XNIF) began to stain the nerve just beyond the lesion. By this time, however, growing axonal terminals had reached the optic chiasm. Antibodies to phosphorylated epitopes of NF-M began to stain axons at 15 days, just as growing axons began to arrive at the optic tectum. Nonphosphorylated high molecular weight neurofilament protein (NF-H) began to appear in axons between 18 and 21 days after surgery. Thus, the reappearance of neurofilaments during optic axon regeneration resembled the general pattern seen during development. The chief difference between development and regeneration was that neurofilament epitopes took longer to emerge during regeneration. One possibility is that cues encountered along the optic pathway influence the neurofilament composition of retinal ganglion cell axons. Then, the greater distances travelled by regenerating axons could account for the longer time taken for their neurofilament compositions to mature.

Developmental study of the expression of B50/GAP-43 in rat retina

B50/GAP-43 has been implicated in neural plasticity, development, and regeneration. Several studies of axonally transported proteins in the optic nerve have shown that this protein is synthesized by developing and regenerating retinal ganglion cells in mammals, amphibians, and fish. However, previous studies using immunohistochemistry to localize B50/GAP-43 in retina have shown that this protein is found in the inner plexiform layer in adults. Since the inner plexiform layer contains the processes of amacrine cells, ganglion cells, and bipolar cells to determine which cells in the retina express B50/GAP-43, we have now used in situ hybridization to localize the mRNA that codes for this protein in the developing rat retina. We have found that B50/GAP-43 is expressed primarily by cells in the retinal ganglion cell layer as early as embryonic day 15, and until 3 weeks postnatal. Some cells in the inner nuclear layer, possibly a subclass of amacrine cells, also express B50/GAP-43 protein and mRNA; however, the other retinal neurons-bipolar cells, photoreceptors, and horizontal cells express little, if any, B50/GAP-43 at any stage in their development. Early in development, the protein appears in the somata and axons of ganglion cells, while later in development, B50/GAP-43 becomes concentrated in the inner plexiform layer, where it continues to be expressed in adult animals. These results are discussed in terms of previous proposals as to the functions of this molecule.

Axon types classified by morphometric and multivariate analysis in the rat optic nerve

Calibers of the rat optic nerve axons distribute unimodally and it is difficult to distinguish groups among them. However, these fibers arose from 3 types of ganglion cells and showed 3 conduction velocities. Performing a cluster analysis over several ultrastructural parameters we found 3 main groups of fibers. These groups are present in a very similar proportion to the ganglion cells groups described in the rat retina.

Cytoskeletal components and calibers in developing fish Mauthner axon (Salmo gairdneri Rich.)

In developing axons of many vertebrates, microtubular density is inversely correlated with fiber caliber. It is suggested that microtubules are causally related to axonal caliber. For this reason, cytoskeletal analysis during development of the fish Mauthner axon, which displays a giant caliber, is of particular interest. The Mauthner axon originates from the Mauthner cell in the medulla and runs in the fasciculus longitudinalis medialis in the spinal cord. At embryonic, larval and postlarval stages in trout (Salmo gairdneri Rich.), the following parameters were measured on conventional electron micrographs of Mauthner axon cross sections; axonal caliber, number of microtubules per axons, and microtubular and neurofilament densities. Results at each stage point to an inverse correlation between axonal caliber (x) and microtubular density (y) expressed by the equation y = axb (R = 0.932). Furthermore, three periods of Mauthner axon development are identified on the basis of the cytoskeletal content: (1) embryonic; the Mauthner axon has small caliber with a high microtubule density, (2) elongation period (larval stages); the axon enlarges and a transient peak of microtubules, corresponding to the caliber increment, is observed, and (3) postlarval; the axon enlarges still further (greater than 500 microns 2) but has the lowest microtubular content. During this period neurofilaments are the main axonal component.

A useful programme in BASIC for axonal morphometry with introduction of new cytoskeletal parameters

Interest in the structure of axons and quantification of their components has been growing over the last years. However, the existing literature contains few reports of available computer programmes to facilitate such studies. This paper presents a fully comprehensive BASIC programme for the morphometric analysis of electron micrographs of cross-sectional nerve fibres. From drawings of fibre and axonal contours and dots of the microtubules and neurofilaments, the programme calculates the following parameters: area, diameter and form factor of the fibres and axons, number and density of microtubules and neurofilaments, proportion between microtubules and neurofilaments (R-proportion), myelin thickness and the diameter of the axon relative to its sheath (g-ratio). The programme also introduces three new parameters to analyse the degree of uniformity of microtubule and neurofilament distribution: distances between microtubules and between neurofilaments, equilateral index and cytoskeletal intermingling index. The programme is written in Microsoft BASIC Interpreter for Apple Macintosh (Microsoft Corporation) but can be used on other computers. Although the programme has been tested on adult rat optic nerve fibres, it can be used for different projects concerning axonal morphometry.

Calibre and Microtubule Content of the Non-Medullated and Myelinated Domains of Optic Nerve Axons of Rats

Calibres and microtubule contents of the non-medullated and myelinated domains of optic nerve axons of adult rats were studied with the electron microscope. The cross-sectional areas of the non-medullated domain was 0.25 microm2, and that of the myelinated domain 0.40 microm2, that is, greater by 59%. The increase in size was uneven across the axonal population; it was marked in fine and medium sized axons, and modest in the largest axons. The number of microtubules increased with axonal size; the density, however, decreased from 85 mirotubules/microm2 in 0.1 microm2 axons to about 20 in 1.2 microm2 axons. In axons of equal cross sectional area, the microtubular density of the myelinated and non-medullated domains was the same. Microtubular density values of optic axons resemble those of dorsal roots more than those of peripheral nerve axons of equal calibre. The facts that optic axons increase in size and gain microtubules behind the eyeball while the microtubular packing decreases suggest a local regulation of the axonal cytoskeleton.