Myelin sheath remodelling in remyelinated rat sciatic nerve

Waning efficacy in a long-term AAV-mediated gene therapy study in the murine model of Krabbe disease

Neonatal AAV9-gene therapy of the lysosomal enzyme galactosylceramidase (GALC) significantly ameliorates central and peripheral neuropathology, prolongs survival, and largely normalizes motor deficits in Twitcher mice. Despite these therapeutic milestones, new observations identified the presence of multiple small focal demyelinating areas in the brain after 6-8 months. These lesions are in stark contrast to the diffuse, global demyelination that affects the brain of naive Twitcher mice. Late-onset lesions exhibited lysosomal alterations with reduced expression of GALC and increased psychosine levels. Furthermore, we found that lesions were closely associated with the extravasation of plasma fibrinogen and activation of the fibrinogen-BMP-SMAD-GFAP gliotic response. Extravasation of fibrinogen correlated with tight junction disruptions of the vasculature within the lesioned areas. The lesions were surrounded by normal appearing white matter. Our study shows that the dysregulation of therapeutic GALC was likely driven by the exhaustion of therapeutic AAV episomal DNA within the lesions, paralleling the presence of proliferating oligodendrocyte progenitors and glia. We believe that this is the first demonstration of diminishing expression in vivo from an AAV gene therapy vector with detrimental effects in the brain of a lysosomal storage disease animal model. The development of this phenotype linking localized loss of GALC activity with relapsing neuropathology in the adult brain of neonatally AAV-gene therapy-treated Twitcher mice identifies and alerts to possible late-onset reductions of AAV efficacy, with implications to other genetic leukodystrophies.

Is There Evidence for Myelin Modeling by Astrocytes in the Normal Adult Brain?

A set of astrocytic process associated with altered myelinated axons is described in the forebrain of normal adult rodents with confocal, electron microscopy, and 3D reconstructions. Each process consists of a protuberance that contains secretory organelles including numerous lysosomes which polarize and open next to disrupted myelinated axons. Because of the distinctive asymmetric organelle distribution and ubiquity throughout the forebrain neuropil, this enlargement is named paraxial process (PAP). The myelin envelope contiguous to the PAP displays focal disruption or disintegration. In routine electron microscopy clusters of large, confluent, lysosomes proved to be an effective landmark for PAP identification. In 3D assemblies lysosomes organize a series of interconnected saccules that open up to the plasmalemma next to the disrupted myelin envelope(s). Activity for acid hydrolases was visualized in lysosomes, and extracellularly at the PAP-myelin interface and/or between the glial and neuronal outer aspects. Organelles in astrocytic processes involved in digesting pyknotic cells and debris resemble those encountered in PAPs supporting a likewise lytic function of the later. Conversely, processes entangling tripartite synapses and glomeruli were devoid of lysosomes. Both oligodendrocytic and microglial processes were not associated with altered myelin envelopes. The possible roles of the PAP in myelin remodeling in the context of the oligodendrocyte-astrocyte interactions and in the astrocyte's secretory pathways are discussed.

Evaluating dermal myelinated nerve fibers in skin biopsy

Although there has been extensive research on small, unmyelinated fibers in the skin, little research has investigated dermal myelinated fibers in comparison. Glabrous, nonhairy skin contains mechanoreceptors that afford a vantage point for observation of myelinated fibers that have previously been seen only with invasively obtained nerve biopsies. This review discusses current morphometric and molecular expression data of normative and pathogenic glabrous skin obtained by various processing and analysis methods for cutaneous myelinated fibers. Recent publications have shed light on the role of glabrous skin biopsy in identifying signs of peripheral neuropathy and as a potential biomarker of distal myelin and mechanoreceptor integrity. The clinical relevance of a better understanding of the role of dermal myelinated nerve terminations in peripheral neuropathy will be addressed in light of recent publications in the growing field of skin biopsy.

Magnetic resonance imaging of functional Schwann cell transplants labelled with magnetic microspheres

There is increasing interest in the use of magnetic resonance imaging (MRI) methods for tracking the fate of labelled cells in vivo post-implantation. The majority of studies have employed cell labels based on nanometer-sized ultrasmall dextran-coated iron oxide particles (USPIO), which are detected through signal hypointensity in T2-weighted images. Although sensitive to MR detection, these labels can be difficult to distinguish from other sources of signal loss in vivo and can be diluted by cell division. Recently, a micron-sized cell label has been described that is much more sensitive to MR detection and which allows detection of single labels in vivo. We show here that glial cells readily take up this label in culture and that the labelled Schwann cells can be detected in vivo by MRI following their implantation into a demyelinated lesion in the rat spinal cord. Signal loss due to the label is sufficiently great that the labelled cells can easily be distinguished from surrounding haemorrhage at the lesion site. Subsequent histological analysis of the lesion area showed that the transplanted cells were remyelinating the demyelinated axons, demonstrating that the labelled cells retained their biological function and that the majority of the label had remained within the transplanted cells.

Clusters of axonal Na+ channels adjacent to remyelinating Schwann cells

Rat sciatic nerve fibres were demyelinated by injection of lysolecithin and examined at several stages as Schwann cells proliferated, adhered, and initiated remyelination. Immunoperoxidase EM has been used to follow the clustering of Na+ channels that represents an early step in the formation of new nodes of Ranvier. At the peak of demyelination, 1 week post-injection, only isolated sites, suggestive of the original nodes, were labelled. As Schwann cells adhered and extended processes along the axons, regions of axonal Na+ channel immunoreactivity were often found just beyond their leading edges. These channel aggregates were associated only with the axolemma and Na+ channels were not detected on glial membranes. Sites with more than one cluster in close proximity and broadly labelled aggregates between Schwann cells suggested that new nodes of Ranvier formed as neighbouring Na+ channel groups merged. Schwann cells thus seem to play a major role in ion channel distributions in the axolemma. In all of these stages Na+ channel label was found primarily just outside the region of close contact between axon and Schwann cell. This suggests that Schwann cell adherence acts in part to exclude Na+ channels, or that diffusible substances are involved and can act some distance from regions of direct contact.

Functional properties and nodal spacing of myelinated fibers in developing rat mental and sural nerves

While the postnatal length growth of the largest internodes in the rat sural nerve (SN) is proportional to nerve elongation, in the developing inferior alveolar nerve (IAN), early postnatal myelin sheath remodelling allows internodal lengthening to exceed the growth rate of the whole nerve. To assess the functional consequences of ongoing myelin sheath remodelling in a developing nerve, we examined the physiological properties of the mental nerve (MN), a cutaneous IAN branch and the SN during maturation. In addition, the nodal spacing and the microscopic anatomy of the nodes in the two nerves were studied. The youngest MNs and SNs (2 weeks) exhibited comparable sensitivities to K(+)-channel blockade with 4-aminopyridine (4-AP), although myelin sheath remodelling was more frequent in the MNs. Subsequently, myelin sheath remodelling ceased in both nerves but the MNs exhibited a greater sensitivity to 4-AP. Large fibers in adult MNs and SNs had a similar nodal anatomy but the former had shorter internodes. Thus, myelin sheath remodeling, per se, does not appear to be a determinant of 4-AP sensitivity in mammalian myelinated fibers. Rather, sensitivity to potassium channel blockade is more likely mediated at the internodal or molecular level.

Aberrant regeneration of motor axons into the pia mater after ventral root neuroma formation

The spinal pia mater receives a rich innervation of small sensory and autonomic axons via the ventral roots. In the present study this pathway was interrupted by the transection of the L7 ventral root in young kittens. The animals were killed 12-18 months postoperatively. It was observed that the pia mater adjacent to the divided ventral root contained large numbers of myelinated axons. We suggest that these axons represent sprouts which had reached the pia mater by retrograde growth from the neuroma on the ventral root. Some of these aberrant pial axons ended blindly in the pia mater. Abnormal terminal-like swellings were observed along pial blood vessels. Fibers with diameters exceeding 11 microns were observed. Many fibers had an internodal spacing below 100 microns and the maximum value was only about 300 microns. Thus, motor axons which are forced to grow into a foreign territory show a maldevelopment which is more obvious with regard to nodal spacing than to fiber diameter.

Morphological and physiological properties of neurons after long-term axonal regeneration: observations on chronic and delayed sequelae of peripheral nerve injury

Axonal regeneration has been the focus of extensive investigation of mechanisms which mediate structural and functional recovery after injury to mammalian peripheral nerves and has proven to be a valuable model for development and plasticity in the nervous system. Although details of the acute morphological and physiological responses to nerve injury are well-described, less information is available to nerve injury are well-described, less information is available about long-term alterations which persist or develop after regenerated axons have established connections with their targets. The present paper briefly discusses the mammalian neuron's initial response to peripheral nerve injury and subsequent events which occur during regeneration. Morphological and physiological alterations observed in neurons after long-term axonal regeneration are described and are considered in the context of their potential implications for clinical recovery after nerve injury, as well as their potential contribution to the appearance of delayed neurological dysfunction. Selective responses to neuronal injury during development and in different fiber populations are discussed.