Extensive Axonal and Glial Fiber Growth from Fetal Porcine Cortical Xenografts in the Adult Rat Cortex

Grafted Miniature-Swine Neural Stem Cells of Early Embryonic Mesencephalic Neuroepithelial Origin can Repair the Damaged Neural Circuitry of Parkinson's Disease Model Rats

Although recent progress in the use of human iPS cell-derived midbrain dopaminergic progenitors is remarkable, alternatives are essential in the strategies of treatment of basal-ganglia-related diseases. Attention has been focused on neural stem cells (NSCs) as one of the possible candidates of donor material for neural transplantation, because of their multipotency and self-renewal characteristics. In the present study, miniature-swine (mini-swine) mesencephalic neuroepithelial stem cells (M-NESCs) of embryonic 17 and 18 days grafted in the parkinsonian rat striatum were assessed immunohistochemically, behaviorally and electrophysiologically to confirm their feasibility for the neural xenografting as a donor material. Grafted mini-swine M-NESCs survived in parkinsonian rat striatum at 8 weeks after transplantation and many of them differentiated into tyrosine hydroxylase (TH)-positive cells. The parkinsonian model rats grafted with mini-swine M-NESCs exhibited a functional recovery from their parkinsonian behavioral defects. The majority of donor-derived TH-positive cells exhibited a matured morphology at 8 weeks. Whole-cell recordings from donor-derived neurons in the host rat brain slices incorporating the graft revealed the presence of multiple types of neurons including dopaminergic. Glutamatergic and GABAergic post-synaptic currents were evoked in the donor-derived cells by stimulation of the host site, suggesting they receive both excitatory and inhibitory synaptic inputs from host area. The present study shows that non-rodent mammalian M-NESCs can differentiate into functionally active neurons in the diseased xenogeneic environment and could improve the parkinsonian behavioral defects over the species. Neuroepithelial stem cells could be an attractive candidate as a source of donor material for neural transplantation.

Proliferative Response of Human T Lymphocytes to Porcine Fetal Brain Cells

Intracerebral grafting of porcine fetal brain cells is a potential treatment of neurodegenerative disorders in humans. Although it is well known that the survival of fetal pig brain cells in the rat brain requires immunosuppression, the response of human T lymphocytes to fetal pig brain cells is unknown. Here we report on the proliferative response of human T lymphocytes to freshly isolated cells and 14-18 days cultured cells from 28- or 35-day-old porcine fetal brains. After 5 days of mixed lymphocyte-brain cell culture, we observed no or only minor T cell responses to the freshly isolated brain cells, while both CD4 cells and CD8 cells proliferated in response to the cultured brain cells. Pretreatment of the cultured brain cells with heat-inactivated human serum significantly reduced the proliferative T cell response. The data suggest that the porcine fetal brain contains cells that can stimulate the human cellular immune system, and that this stimulation may be reduced by pretreatment of the fetal pig brain cells with human serum.

Map2 Expression in the Developing Human Fetal Spinal Cord and following Xenotransplantation

Human fetal spinal cord (FSC) tissue was obtained from elective abortions at 6-14 wk gestational age (GA). The specimens were then either immediately processed for immunohistochemical analysis or xenotransplantation. In the latter case, donor tissue was prepared as a dissociated cell suspension and then introduced either sub-pially or intraspinally into contusion lesions of the adult rat midthoracic spinal cord. The xenografts were subsequently examined by conventional histological and immunohistochemical methods at 2-3 mo postgrafting. Immunostaining showed that MAP2 was expressed heavily in cells residing in the mantle layer of the human fetal spinal cord in situ as early as 6 wk GA. Subpial and intraparenchymal xenografts also were intensely immunoreactive for MAP2, but no staining of surrounding host neural tissue was detected. We conclude that the differential expression of MAP2 can be used to distinguish human graft tissue from the surrounding rat spinal cord in this xenograft paradigm. Under appropriate staining conditions, MAP2 can thus serve to facilitate analyses of host-graft integration, donor cell migration, and neuritic outgrowth.

Transplantation of Embryonic Porcine Neocortical Tissue into Newborn Rats

Several previous studies, suggesting the potential use of embryonic xenografts in the treatment of neurological disorders, indicate that neural growth and axonal guidance factors may function across species. In this light, blocks of fetal porcine neocortex were grafted into small cortical lesion cavities made in newborn rats. Sacrifice at 3–12.5 weeks posttransplantation revealed healthy looking grafts in several animals. Apparent graft rejection evidenced by areas of necrosis and OX1 reactivity was observed in some of the older transplants. Treatment of nursing mothers or of postweaning newborns with cyclosporin A did not appear to promote graft survival. Some transplants grew to extremely large proportions and were characterized by bands of cells and bundles of axons as observed using immunohistochemical staining for pig neurofilament. Neurofilament-positive axons projected from several of the grafts to course through the corpus callosum to the contralateral cortex or to course ipsilaterally within the subcortical white matter, where labeled fibers could be traced to the midbrain crus cerebri in older transplants. Bundles of axons were also observed coursing within the ipsilateral caudate putamen where terminal branching was apparent. The normal course of transplant efferents within the host brain indicates that growing pig axons can respond to rodent axonal guidance factors.

Transgenic expression of CTLA4-Ig by fetal pig neurons for xenotransplantation

The transplantation of fetal porcine neurons is a potential therapeutic strategy for the treatment of human neurodegenerative disorders. A major obstacle to xenotransplantation, however, is the immune-mediated rejection that is resistant to conventional immunosuppression. To determine whether genetically modified donor pig neurons could be used to deliver immunosuppressive proteins locally in the brain, transgenic pigs were developed that express the human T cell inhibitory molecule hCTLA4-Ig under the control of the neuron-specific enolase promoter. Expression was found in various areas of the brain of transgenic pigs, including the mesencephalon, hippocampus and cortex. Neurons from 28-day old embryos secreted hCTLA4-Ig in vitro and this resulted in a 50% reduction of the proliferative response of human T lymphocytes in xenogenic proliferation assays. Transgenic embryonic neurons also secreted hCTLA4-Ig and had developed normally in vivo several weeks after transplantation into the striatum of immunosuppressed rats that were used here to study the engraftment in the absence of immunity. In conclusion, these data show that neurons from our transgenic pigs express hCTLA4-Ig in situ and support the use of this material in future pre-clinical trials in neuron xenotransplantation.

Is there a future for neural transplantation?

Traditionally neural transplantation has had as its central tenet the replacement of missing neurons that have been lost because of neurodegenerative processes, as exemplified by diseases such as Parkinson disease (PD). However, the effectiveness and widespread application of this approach clinically has been limited, primarily because of the poor donor supply of human fetal neural tissue and the incomplete neurobiological understanding of the circuit reconstruction required to normalize function in these diseases. So, in PD the progress from promising neural transplantation in animal models to proof-of-principle, open-labeled clinical transplants, to randomized, placebo-controlled studies of neural transplantation has not been straightforward. The emergence of previously undescribed adverse effects and lack of significant functional advantage in recent clinical studies has been disappointing and has served to cast a new, and perhaps more realistic, perspective on this treatment approach. In fact, there have been calls by some involved in neural transplantation to return to the drawing board before pressing on with further clinical trials, and the return to basic experimentation. This therefore precipitates the question - is there a future for neural transplantation? It is important to remember that there are a number of possible explanations for the disappointing results from the recent clinical trials in PD, ranging from the mode of transplantation to patient selection. Nevertheless, almost irrespective of these reasons for the current trial results, there have always been significant practical and ethical problems with using human fetal tissue, and so a number of alternative cell sources have been investigated. These alternative sources include stem cells, which are attractive for cell-based therapies because of their potential ease of isolation, propagation and manipulation, and their ability in some cases to migrate to areas of pathology and differentiate into specific and appropriate cell types. Furthermore, the availability of stem cells derived from non-embryonic sources (e.g. adult stem cells derived from the sub-ventricular zone) has removed some of the ethical limitations associated with the use of embryonic human tissue. These potentially beneficial aspects of stem cells means that there is a future for neural transplantation as a means of treating patients with a range of neurological disorders, although whether this will ever translate into a truly effective, widely available therapy remains unknown.

Survival, neuronal differentiation, and fiber outgrowth of propagated human neural precursor grafts in an animal model of Huntington's disease

Expanded neural precursor cells provide an attractive alternative to primary fetal tissue for cell replacement therapies in neurodegenerative diseases. In this study we transplanted epigenetically propagated human neural precursor cells into a rat model of Huntington's disease. Neural precursors survived transplantation and large numbers differentiated to express neuronal antigens, including some that expressed DARPP-32, indicating a mature striatal phenotype had been adopted. Neuronal fibers from the grafts projected diffusely throughout the host brain, although there was no evidence that outgrowth was specifically target directed. This study supports the contention that propagated human neural precursors may ultimately be of use in therapeutic neural transplantation paradigms for diseases such as Huntington's disease.

Pig fetal septal neurons implanted into the hippocampus of aged or cholinergic deafferented rats grow axons and form cross-species synapses in appropriate target regions

The anatomical specificity of axon growth from fetal pig septal xenografts was studied by transplanting septal cells from E30-35 pig fetuses into cholinergic deafferented (192-IgG-saporin-infused) rats or into aged rats (> 18 months). Cell suspensions (100,000 cells/microl) were injected bilaterally into the dorsal and ventral hippocampus of immunosuppressed rats (10 mg/kg/day cyclosporine A). To assess axonal growth and synapse formation, acetylcholinesterase histochemistry, an antibody to choline acetyltransferase (ChAT), and three pig-positive/rat-negative antibodies: bovine 70kD neurofilament (NF70), human low-affinity NGF receptor (hNGFr), and human synaptobrevin (hSB) were used. In rats with surviving grafts at 6 months, NF70 axonal labeling was more extensive than either ChAT or hNGFr labeling. All three markers demonstrated graft axons extending selectively through the hippocampal CA fields and the molecular layer of the dentate gyrus. Graft axons did not extend into adjacent entorhinal cortex or neocortex. The distribution of pig hSB-positive synapses correlated with AChE-positive fiber outgrowth in to the host. Electron microscopic analysis of hSB-immunostained hippocampal sections revealed pig presynaptic terminals in contact with normal rat postsynaptic structures in the CA fields and the dentate gyrus. These data demonstrate target-appropriate growth of pig cholinergic axons and the formation of cross-species synapses in the deafferented or aged rat hippocampus.