Migration patterns of donor astrocytes after reciprocal striatum-cerebellum transplantation into newborn hosts

Neural cells from primary human striatal xenografts migrate extensively in the adult rat CNS

Primary neural cells do not appear to migrate significantly following transplantation into the adult rodent CNS, which is in contrast to expanded neural precursor cells where migration is well-documented. However, most transplant studies of primary neural tissue have been performed in an allograft situation in which it is difficult to identify graft-derived cells. We have, therefore, used a xenograft paradigm to investigate the potential for cells derived from grafts of primary human fetal striatal tissue (gestational age of 66-72 days) to migrate following intrastriatal transplantation in an athymic adult rat model of Huntington's disease. The use of an antibody specific to human nuclear antigen enabled clear identification of graft-derived cells within the host brain, and specific neural phenotypes were determined using human-specific tau for neurons, glial fibrillary acidic protein for mature astrocytes and Ki67 for proliferative cells. At 6 weeks, the graft mass was very dense with a high proliferative index, few cells had migrated away from the graft, and the cells that had differentiated both within and away from the graft were mainly neurons. In contrast, at 6 months, the graft core was dispersed significantly more and a large number of graft-derived cells had migrated throughout the brain as far rostral as the olfactory bulb and as caudal as the substantia nigra. Cells had differentiated into both neurons and astrocytes and the level of proliferation was significantly lower within the graft. These results demonstrate that primary neural xenografts contain proliferative cells that possess the ability to migrate and differentiate into both neurons and astrocytes, and suggest that these cells could contribute to normal graft function. This property may be a consequence of the xenograft situation and could potentially be exploited to provide the opportunity to target regions of distant pathology in neurodegenerative diseases using xenotransplantation of embryonic neural tissue.

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.

Glial and endothelial cell response to a fetal transplant of purified neurons

Astrocytes, microglia and endothelial cells display very specific phenotypic characteristics in the intact adult CNS, which appear quite versatile when grown in culture without neurons. Indirect evidence from in vitro co-culture studies and analysis of the effects of specific neuronal removal in vivo, does accordingly favour a role of neurons for the phenotypic repression of these cells in the intact brain. In order to provide more direct evidence for such neuronal influence, we attempted to induce, in the rat brain, a reversal of the post-lesional activation of astrocytes, microglia and endothelial cells by transplantation of fetal neurons purified by immunopanning. Host microglial cells which have been activated by the lesion process, penetrated the neuronal graft during the few days after the transplantation. Reactive astrocytes began to appear in the lesioned parenchyma and gathered around the transplant. Thereafter they first sent their processes in the direction of the neuronal graft, before they migrated into the graft a few days later. At this time, which was at the end of the first week post-transplantation, the host endothelial cells sprouted "streamers" of basal lamina within the graft forming small capillaries. During the second week post-transplantation, numerous astrocytes and microglial cells, both displaying a reactive hypertrophied morphology, were observed throughout the grafts. Finally, by the end of the first month, the activated cells differentiated towards a quiescent, resting morphology. At this time the grafts contained a vascular network with morphological characteristics comparable to those observed in the intact brain parenchyma. The results indicate that the interaction of activated astroglia and microglia and endothelial cells with neurons causes the cells to re-differentiate and regain phenotypic features characteristic of intact brain parenchyma, strongly suggesting that neurons play an essential role in the phenotypic restriction of glial and endothelial cells in the adult central nervous system.

Extensive migration and target innervation by striatal precursors after grafting into the neonatal striatum

Embryonic striatal precursors grafted into the lesioned adult host striatum show limited integration with little migration and restricted efferent projections. In the present study, the influence of an immature striatal environment on the integrative capacity of grafted neuroblasts was examined after transplantation of striatal progenitors into the striatum at different stages of postnatal development. Mouse progenitors, derived from embryonic day 13.5-14 lateral or medial ganglionic eminence or the cerebellar primordium, were transplanted as a single cell suspension into the developing postnatal day 1, 7 and 21 rat striatum. The grafted cells and their axonal projections were visualized using antibodies raised against the mouse-specific neural markers, M6 and M2. Cells from the lateral (but not the medial) ganglionic eminence showed a remarkable capacity to innervate selectively the striatal target structures, globus pallidus, entopeduncular nucleus and substantia nigra, reminiscent of endogenous striatal neurons, which is not observed after grafting into adult hosts. M6 and M2-immunopositive cellular profiles from both the lateral and medial ganglionic eminences were observed to have migrated extensively away from the injection site, in contrast to the cerebellar precursors which remained clustered at the implantation site. Cells from the lateral ganglionic eminence were largely confined within the striatal complex where they developed striatal characteristics, displaying expression of DARPP-32, the 32,000 mol. wt dopamine- and cyclic AMP-regulated phosphoprotein, whereas cells from the medial ganglionic eminence had migrated caudally along the internal capsule and were observed predominantly in the globus pallidus and thalamus, in addition to the striatum. The cells located outside the striatum were all DARPP-32 negative. The improved integration and increased projection capacity of the lateral ganglionic eminence precursors grafted into postnatal day 1 hosts gradually declined as the host advanced into later stages of development (postnatal day 7), and in postnatal day 21 hosts the grafted striatal precursors behaved similarly to grafts implanted into adult recipients. These results demonstrate the specific capacity of embryonic striatal progenitors to integrate into the developing basal ganglia circuitry during early postnatal development, and that the extent of neuronal and glial integration and graft host connectivity declines when the host has developed beyond the first postnatal week.

The cerebellar model of neural grafting: structural integration and functional recovery

A synopsis is presented of the recent history of cerebellar tissue transplantation over the past 25 years. The properties of growth and differentiation of cerebellar grafts placed intraocularly or intracranially are reviewed, as well as the interaction of heterotopic and orthotopic grafts with the host brain. Particular emphasis is placed on the use of ataxic mouse mutants as recipients of donor cerebellar tissue for the correction of their structural deficits and the functional recovery of behavioural responses.

Migration of A7 immortalized astrocytic cells grafted into the adult rat striatum

The A7 cell line is an SV40 large T antigen-immortalized astrocyte cell line produced from the neonatal rat optic nerve. Previous studies have demonstrated that A7 cells provide a favorable environment for the survival and growth of cultured neurons and can also stimulate axonal growth after grafting into the rat striatum. The current study was designed to investigate whether A7 cells grafted into adult rat striatum can migrate away from the implantation site. A7 cells were labelled in culture by incorporation of bromodeoxyuridine (BrdU) or by expression of an alkaline phosphatase transgene. The labelled cells were then transplanted into the left striatum of normal adult rats by introducing a blunt-end 22 gauge needle through a trephine hole. The rats were euthanized at periods of up to 30 days after grafting. The A7 cells did not appear to alter the cytoarchitecture of the surrounding brain parenchyma. Labelled A7 cells were observed in both gray and white matter areas, and many were located in areas free of damage due to the implantation procedure. The migration of the BrdU-labelled A7 cells with respect to the implantation needle track was determined on coronal sections. The radial migration distance from the needle tract was similarly determined on horizontal sections. A7 cells migrated progressively longer distances with increasing survival time of the animals: The largest migration distance (1,125 +/- 52 microns) occurred at 30 days after grafting with an estimated migration rate of 31 microns per day. There was no significant directional polarity in the migration of these cells within the striatum. Some of the labelled A7 nuclear profiles were associated with blood vessels, some appeared to be associated with fiber bundles within the striatum, and some were found within the gray matter without apparent association with any anatomical structure. These results demonstrate that A7 immortalized astrocytic cells migrate away from a single implantation site following grafting into the adult rat striatum to populate a large area of the striatum.

The fate of human glial cells following transplantation in normal rodents and rodent models of neurodegenerative disease

Investigations on xenografting in the brain have previously focused on the anatomical and functional integration of the transplanted neurons. More recently, astrocytes are being implicated as having complex functions following transplantation, and are being investigated to determine their role(s) in transplantation. The present study was undertaken to investigate the migration of human astrocytes following transplantation of thalamic, striatal, and mesencephalic tissue into the rodent striatum. Human donor fetuses (9-16 weeks in gestation) obtained through elective and spontaneous abortions were utilized in this study. Following transplantation, donor astrocytes were labeled with an antiserum directed against human glial fibrillary acidic protein. Our results demonstrate that astrocytic elements from all three tissue types are capable of incorporating into the host brain, and have a tendency to follow white matter tracts (such as the corpus callosum, internal capsule, and fiber bundles in the striatum). Human astrocytes, originating from the striatum and thalamus exhibited extensive migration, while migration was more limited in animals with ventral mesencephalon transplants. Ventral mesencephalon transplanted animal demonstrated positive astrocytes within the transplant, with processes (very few cell bodies) extending into white matter of adjacent host striatum. Astrocytes demonstrating immature morphology were observed with all transplant types, but were most prevalent in the striatal transplanted animals. The extent of astrocyte migration and the morphologies observed in this study reflect regional differences of the developing human brain. These results confirm and extend previous investigations on glial cell migration following transplantation in the brain.

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.

Migratory patterns of lac-z transfected human glioma cells in the rat brain

Malignant brain tumors are characterized by extensive tumor-cell infiltration into the normal brain tissue. The present work describes the migratory behavior of human glioma cells transplanted into the adult rat brain with the aim of exploiting the extent of active cell migration and passive cell displacement within the central nervous system. To detect every transplanted tumor cell, a stably bacterial beta-galactosidase (lac-z) transfected human glioma cell line was used. To distinguish between an active cell migration process and passive cell displacement, rat brains were also implanted with inert fluorescent polystyrene microspheres and the distribution of tumor cells and microspheres was studied 1 hr and 3 days after implantation. One hour after implantation the tumor cells were strictly localized at the implantation site. However, 3 days after implantation, both tumor cells and microspheres showed an extensive distribution within the brain. Confirming earlier neuropathological and experimental studies, it is shown that the lac-z-transfected glioma cells had the capacity to move within the Virchow-Robin and subarachnoid spaces. However, since fluorescent microspheres were also found in these areas, this spread of tumor cells may be primarily mediated by the extensive cerebrospinal fluid flow that exists within the brain. Three days after implantation, the glioma cells also showed an active migration over the corpus callosum. In comparison, the fluorescent microspheres showed only limited spread along the callosal body. It is concluded that the bacterial lac-z gene can be stably transfected into human glioma cells and, since every tumor cell can be visualized within the brain, this model provides a tool for studying the mechanisms behind tumor-cell invasion of the brain.

Interactions between fetal rat brain cells and mature brain tissue in vivo and in vitro

Fetal as well as mature neural cells were homografted into the right cerebral hemisphere of adult BD-IX rats. The animals were sacrificed 7 d after implantation, and the localization of implanted cells was visualized by fluorescence and light microscopy. The cell implants were prestained with the fluorescent vital dye 1,1'-Dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate (DiI) to discriminate between implanted cells and host brain tissue. At the implantation site, the fetal brain cells as well as the cells from immature brain cell aggregates showed diffuse infiltration into the surrounding host brain tissue of up to 0.5 mm. Extensive cell migration along the corpus callosum for up to 5 mm in the coronal and to a lesser extent in the sagittal plane was also observed. In addition, fetal cells were distributed in the subarachnoid space of both cerebral hemispheres and showed a distinct association with larger blood vessels. Cells from mature brain aggregates did not migrate as far as fetal cells and showed only a local infiltration into the host neuropil. Fluorescent microspheres as well as fixed fetal brain cells were implanted, either alone or in combination with vital cells to distinguish between active cell migration and passive cell displacement. The microspheres and the fixed cells were found either localized to the implantation pathway or distributed in the corpus callosum for up to 2 mm in the coronal plane without any dispersion in the sagittal plane. The microspheres also showed an extensive displacement in the subarachnoid space. In vitro co-culture experiments between two immature aggregates showed a complete fusion of the two aggregates during a 96 h culture period. In co-cultures between two mature aggregates complete fusion was not prominent, although the confrontation zone appeared diffuse. Confrontations between a mature and an immature aggregate showed the same pattern of interaction as seen for the two mature aggregates. It is concluded that carbocyanine dyes may be used as a tracer for transplanted cells. Cells from fetal rat brain cell aggregates, opposed to those from mature aggregates, showed extensive migration along well defined anatomical structures in the mature along well defined anatomical structures in the mature brain. Some of the spread of cells following implantation is probably due to passive movement since inert microspheres will spread into certain areas of the CNS.

Morphology and migration of cultured Schwann cells transplanted into the fimbria and hippocampus in adult rats

Schwann cells cultured from neonatal rat peripheral nerve were injected into the fimbria and hippocampus of syngeneic adult rats by a microtransplantation technique which causes minimal disturbance to the host brain structure at the site of implantation, and thus allows the grafted cells to come into immediate contact with intact host tissue. Numerous Schwann cells could be identified for up to 6 weeks (and with decreasing frequency for up to 3 months) by intense immunoreactivity for low affinity nerve growth factor receptor. The transplanted cells adopted a distinctive elongated form, with a central, ovoid nucleus flanked by processes which were up to 300 microns long, and which ranged from swollen segments with a diameter as large as 12 microns down to thread-like fibres of 1 micron or less. This morphology is different from that of any of the host cells. The transplanted Schwann cells migrated freely into the host tissue along blood vessels and according to the position of the grafts, they either entered the hippocampal neuropil, or migrated (for distances of up to 2 mm) along the longitudinal axis of the fimbria, where they were interspersed in parallel with the interfascicular glial rows and axons. The host astrocytes did not appear to impede the migration of the donor Schwann cells. Although the host astrocytic processes became hypertrophic, with increased glial fibrillary acidic protein and vimentin expression, the predominant longitudinal orientation of the astrocytic tract processes was maintained. The transplanted Schwann cells did not form peripheral myelin (as detected by P0 immunoreactivity), and it is not clear whether they survive beyond the period at which we detect them.

Directed migration of transplanted glial cells toward a spinal cord demyelinating lesion

To investigate the migration of transplanted glial cells in normal adult mice with a focal demyelinating lesion, we have used A2G mice which have the autosomal dominant Mx-1 allele as donor. Mx-1 protein expression is inducible by interferon and is detected in a dotted pattern in the nucleus of A2G cells. A/J mice were used as recipient animals as they express the same major histocompatibility antigens as A2G but cannot express the Mx-1 protein. An acute demyelinating lesion was produced in the A/J spinal cord by intraspinal injection of lysolecithin. Mixed glial cultures derived from newborn A2G brain were treated with alpha/beta interferon for 24 hr. These Mx-1 expressing glial cells were then transplanted two intervertebral spaces away from the demyelinating lesion. The fate of the grafted cells was followed over the next 13 days, during which the induced Mx-1 protein can still be detected by immunocytochemistry. Grafted cells were found between the transplantation site and the lesion at 24 hr and some of the Mx-1+ cells reached the lesion at 4 days. The majority of the Mx-1+ migrating cells expressed GFAP and were located in the myelinated white matter and around the blood vessels. Scattered MBP+, Mx-1+ cells were detected in the lesion indicating that some of the transplanted cells may participate in the repair process. The Mx-1 is a useful marker to follow the migration events in the days after grafting and to determined what factors may attract transplanted cells toward a demyelinating lesion.

Migration pathways, differentiation and survival of macroglial cells from a xenograft implanted into the thalamus of newborn mice

Embryonic rabbit corpus callosum transplants were grafted into thalamus of newborn shiverer mice in order to compare the fates of oligodendroglial and astroglial cells derived from the transplants. Our model allowed the identification of the two populations of macroglial cells. The thalamus was chosen as site of implantation because of its situation at a crossroad of numerous neuronal fascicles. Previous studies, where the dorsal striatum was used as site of implantation, had shown that corpus callosum was one of the favorite routes of migration for both populations of macroglial cells. In the present study special attention was given to the comparison of the migration pathways and areas of settlement of implanted astroglia and oligodendroglia. The internal capsule, the medial lemniscus, the crus cerebri and the thalamic radiations were used by both populations of transplant derived macroglial cells for their migrations through the host parenchyma. They integrated into the host tissue on these routes or further away in areas such as the putamen, the mesencephalon or the colliculi. Signs of degeneration of the implanted astroglia were often observed after 1 month post-implantation.

Migration of human malignant astrocytoma cells in the mammalian brain: Scherer revisited

Fresh suspensions of human glioblastoma multiforme were preincubated in the plant lectin Phaseolus vulgaris leucoagglutinin (PHAL) and implanted into cortical pockets in adult rat brain. Brains were investigated periodically over 30 postoperative days and the migration of the human glioblastoma cells was traced with anti-PHAL immunofluorescence or the overexpression of human specific p185c-neu a specific marker of a class of human malignant astrocytoma cells. The principal pathway of migration of the implanted human cells in the rat brain was ventrally through cortical gray matter and into the corpus callosum, with rapid lateral distribution in this and other parallel and intersecting white matter fascicles. Human glioblastoma cells also migrated on basement membrane lined blood vessels, pia-glia membrane and spaces of Virchow-Robin, as well as the subependymal space of the ventricles. These paths of migration of human glioblastoma cells in the rat brain are consistent with the pathways of spread of glioblastoma in the human brain as described by Scherer over 50 years ago, indicating that multifocal malignant astrocytomas have common migratory pathways in mature mammalian brain.

In situ transformation of striatal glia into cerebellar-like glia after brain transplantation

Transplants of striatum from rabbit embryo were implanted into the colliculus posterior of newborn mice. After 4 weeks, astroglial cells derived from the transplant had migrated into the cerebellum of the host. Whenever they had settled in the cerebellum they presented forms similar to local glia. Some migrated glial cells were found to transform into forms of glia, such as radial-like glia, which are not present in the striatum. This observation confirms that glial precursor cells are highly plastic. It is an in vivo demonstration that local conditions alone define the morphology of glial cells. After grafting in an heterotopic location they take on forms that they were not destined to express in the region of origin.

Migration of xenogenic astrocytes in myelinated tracts: a novel probe for immune responses in white matter

Experimental brain transplantation allows the study of the development of the immune response against brain antigens within the brain itself. This laboratory has developed a transplantation model in which rabbit embryo brain fragments are placed in the brains of newborn mice. The migration of xenogenic astrocytes is traced by a monoclonal antibody which combines with donor but not host glial fibrillary acidic protein. In the first 4 weeks after transplantation, the donor astrocytes successfully migrate, often within myelinated tracts. Following this period, T cells make their appearance and xenogenic astrocytes disappear by 10 weeks. The propensity for clearly identified foreign astrocytes to migrate in myelinated tracts coupled with a well-defined time course of host-vs-graft interaction suggested that the model could be used to study the immune response in white matter. The studies reported here provide sequential examples of the relationship between migration by foreign astrocytes in myelinated tracts and the development of the host immune response. Extensive migration in white matter tracts was first observed in the absence of any T cell response. Subsequently T cells were found at the transplantation site. Finally Ia was found to be expressed on blood vessels and microglia were strongly reactive in white matter that contained T cells but no foreign astrocytes. These observations support the suggestion that the model can be used to more precisely define cellular immune events that occur within white matter.