The use of xenografting to evaluate the remyelinating potential of glial cell cultures

Transplantation of human olfactory ensheathing cells elicits remyelination of demyelinated rat spinal cord

Human olfactory ensheathing cells (OECs) were prepared from adult human olfactory nerves, which were removed during surgery for frontal base tumors, and were transplanted into the demyelinated spinal cord of immunosuppressed adult rats. Extensive remyelination was observed in the lesion site: In situ hybridization using a human DNA probe (COT-1) indicated a similar number of COT-1-positive cells and OEC nuclei within the repaired lesion. The myelination was of a peripheral type with large nuclei and cytoplasmic regions surrounding the axons, characteristic of Schwann cell and OEC remyelination. These results provide evidence that adult human OECs are able to produce Schwann cell-like myelin sheaths around demyelinated axons in the adult mammalian CNS in vivo.

Remodeling of lesioned kitten visual cortex after xenotransplantation of fetal mouse neopallium

Remodeling of the mechanically injured cerebral cortex of kittens was studied in the presence of a neural xenograft taken from mouse fetuses. Solid neural tissue from the neopallium of a 14-day-old fetus was transferred into a cavity prepared in visual cortical area 18 of 33-day-old kittens. Injections of bromodeoxyuridine (BrdU) were used to monitor postoperative cell proliferation. Two months after transplantation, the presence of graft tissue in the recipient brain was assessed by Thy-1 immunohistochemistry. Antibodies specific for neurons, astrocytes, and oligodendrocytes and hematoxylin staining for endothelial cells were used for the characterization of proliferating (BrdU+) cells. The following were the major observations: 1) Of ten transplanted kittens, four had the cavity completely filled with neural tissue that resembled the intact cerebral cortex in its cytoarchitecture, whereas, in four other kittens, the cavity was partially closed. In two kittens, the cavity remained or became larger, which was also the case with all four sham-operated (lesioned, without graft) animals. 2) A substantial part of the remodeled tissue was of host origin. Only a few donor cells survived and dispersed widely in the host parenchyme. 3) In the remodeled region of transplanted animals, the densities of nerve, glial, and endothelial cells were similar to those in intact animals. 4) Cell proliferation increased after transplantation but only within a limited time, because, 2 months after the operation, the number of mitotic cells in the grafted cerebral cortex did not differ from that in intact controls. Our data suggest that the xenograft evokes repair processes in the kitten visual cortex that lead to structural recovery from a mechanical insult. The regeneration seems to rely on a complex interplay of many different mechanisms, including attenuation of necrosis, cell proliferation, and immigration of host cells into the wounded area.

Reconstruction of the glial environment of a photochemically induced lesion in the rat spinal cord by transplantation of mixed glial cells

It is becoming increasingly apparent that the astrocytic environment is critical to the normal development and functioning of the CNS, and that acute injury to the spinal cord causes destruction of glial cells in addition to neurones and axons. The aims of this study were to assess the viability of reconstructing the astrocytic environment of a cystic spinal cord lesion by transplantation of glial cells and to examine the effect of the transplanted cells on meningeal cell invasion and revascularisation of the lesion and on axonal regeneration. Neonatal rat and kitten mixed glial cells and the CG-4 rat O-2A progenitor cell line were transplanted into a lesion produced in the dorsal funiculus of the rat spinal cord by photochemical infarction. The animals were killed 4 weeks after injury, their cords examined with light and electron microscopy and compared with control animals that were injected with medium alone. Transplantation of all three preparations resulted in increased numbers of astrocytes in the area of Wallerian degeneration cranial to the lesion and within the cyst. Mixed glial cell cultures prepared from neonatal rat forebrain contained cells with in vitro characteristics of type-1 astrocytes, and produced dense clusters of astrocytes that were surrounded by meningeal cells, resulting in a fragmented environment in the cyst. In contrast, glial cell cultures prepared from kitten forebrain and the CG-4 cell line produced cells that filled the cyst with a loose network of fine processes and reduced meningeal cell infiltration of the lesion. The CG-4 cell line significantly increased the density of blood vessels in the centre of the lesion and the number of spared axons present dorsal to the lesion, but none of the preparations significantly increased the number of axons regenerating at the caudal end of the lesion. We conclude that O-2A progenitor-derived astrocytes are more suitable for reconstruction of the glial environment of a cystic lesion in the rat spinal cord than 'type-1 like' astrocytes and would therefore be the cell of choice to engineer to produce factors that promote axonal regeneration.

Glial cell transplantation and remyelination of the central nervous system

Glial cell transplantation has proved to be a powerful tool in the study of glial cell biology. The extent of myelination achieved by transplanting myelin-producing cells into the CNS of myelin mutants, or into focal demyelinating lesions has raised hope that such a strategy may have therapeutic applications. Oligodendrocytes or Schwann cells could be used for repair. It is likely that the immature stages of the oligodendrocyte lineage have the best phenotypic characteristics for remyelination when transplanted, either as primary cells or as immortalized cells or cell lines. Prior culturing and growth factor treatment provides opportunities to expand cell populations before transplantation as dissociated cell preparations. Cell lines are attractive candidates for transplantation, but the risk of transformation must be monitored. The application of this technique to human myelin disorders may require proof that migration, division and stable remyelination of axons by the transplanted cells can occur in the presence of gliosis and inflammation.

The use of transplanted glial cells to reconstruct glial environments in the CNS

Transplantation studies have demonstrated that glia-depleted areas of the CNS can be reconstituted by the introduction of cultured cells. Thus, the influx of Schwann cells into glia-free areas of demyelination in the spinal cord can be prevented by the combined introduction of astrocytes and cells of the O-2A lineage. Although Schwann cell invasion of areas of demyelination is associated with destruction of astrocytes, the transplantation of rat tissue culture astrocytes ("type-1") alone cannot suppress this invasion, indicating a role for cells of the O-2A lineage in reconstruction of glial environments. By transplanting different glial cell preparations and manipulating lesions so as to prevent meningeal cell and Schwann cell proliferation it is possible to demonstrate that the behaviour of tissue culture astrocytes ("type-1") and astrocytes derived from O-2A progenitor cells ("type-2") is different. In the presence of meningeal cells, tissue culture astrocytes clump together to form cords of cells. In contrast, type-2 astrocytes spread throughout glia-free areas in a manner unaffected by the presence of meningeal cells or Schwann cells. Thus, progenitor-derived astrocytes show a greater ability to fill glia-free areas than tissue culture astrocytes. Similarly, when introduced into infarcted white matter in the spinal cord, progenitor-derived astrocytes fill the malacic area more effectively than tissue culture astrocytes, although axons do not regenerate into the reconstituted area.

Repair of demyelinated lesions by glial cell transplantation

Transplantation of oligodendrocyte lineage cells results in myelination of naked axons. The most extensive remyelination is achieved using A2B5 positive progenitor cells and these observations encourage the view that glial transplantation may be a useful treatment in human demyelinating diseases. However, several issues need to be resolved before glial cell transplantation can be applied clinically; this review focuses on the choice of cells, their source, whether chronically demyelinated axons can be repaired by transplantation, and whether the principles of glial transplantation established in small rodents are applicable to other mammalian species and to man.

Conduction properties of central demyelinated and remyelinated axons, and their relation to symptom production in demyelinating disorders

The conduction properties of central demyelinated and remyelinated axons are discussed, and related to the expression of symptoms in central demyelinating disease. The mechanisms underlying the block and restoration of conduction in segmentally demyelinated axons are described, together with the range of deficits expressed by the conducting axons. These abnormalities are related to clinical relapses and remissions, and to the phenomena of weakness, fatigue, the temperature sensitivity of symptoms, and the generation of 'positive' symptoms (e.g. Uhthoff's and Lhermitte's symptoms). The potential role of circulating 'blocking factors' in the symptomatology of central demyelinating disease is examined, and some approaches are advanced for the symptomatic therapy of such diseases.