Reconstruction of the nigrostriatal pathway by simultaneous intrastriatal and intranigral dopaminergic transplants

Better Outcomes with Intranigral versus Intrastriatal Cell Transplantation: Relevance for Parkinson’s Disease

Intrastriatal embryonic ventral mesencephalon grafts have been shown to integrate, survive, and reinnervate the host striatum in clinical settings and in animal models of Parkinson’s disease. However, this ectopic location does not restore the physiological loops of the nigrostriatal pathway and promotes only moderate behavioral benefits. Here, we performed a direct comparison of the potential benefits of intranigral versus intrastriatal grafts in animal models of Parkinson’s disease. We report that intranigral grafts promoted better survival of dopaminergic neurons and that only intranigral grafts induced recovery of fine motor skills and normalized cortico-striatal responses. The increase in the number of toxic activated glial cells in host tissue surrounding the intrastriatal graft, as well as within the graft, may be one of the causes of the increased cell death observed in the intrastriatal graft. Homotopic localization of the graft and the subsequent physiological cell rewiring of the basal ganglia may be a key factor in successful and beneficial cell transplantation procedures.

A combined cell and gene therapy approach for homotopic reconstruction of midbrain dopamine pathways using human pluripotent stem cells

Midbrain dopamine (mDA) neurons can be replaced in patients with Parkinson's disease (PD) in order to provide long-term improvement in motor functions. The limited capacity for long-distance axonal growth in the adult brain means that cells are transplanted ectopically, into the striatal target. As a consequence, several mDA pathways are not re-instated, which may underlie the incomplete restoration of motor function in patients. Here, we show that viral delivery of GDNF to the striatum, in conjunction with homotopic transplantation of human pluripotent stem-cell-derived mDA neurons, recapitulates brain-wide mDA target innervation. The grafts provided re-instatement of striatal dopamine levels and correction of motor function and also connectivity with additional mDA target nuclei not well innervated by ectopic grafts. These results demonstrate the remarkable capacity for achieving functional and anatomically precise reconstruction of long-distance circuitry in the adult brain by matching appropriate growth-factor signaling to grafting of specific cell types.

Emerging regenerative medicine and tissue engineering strategies for Parkinson's disease

Parkinson's disease (PD) is the second most common progressive neurodegenerative disease, affecting 1-2% of people over 65. The classic motor symptoms of PD result from selective degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc), resulting in a loss of their long axonal projections to the striatum. Current treatment strategies such as dopamine replacement and deep brain stimulation (DBS) can only minimize the symptoms of nigrostriatal degeneration, not directly replace the lost pathway. Regenerative medicine-based solutions are being aggressively pursued with the goal of restoring dopamine levels in the striatum, with several emerging techniques attempting to reconstruct the entire nigrostriatal pathway-a key goal to recreate feedback pathways to ensure proper dopamine regulation. Although many pharmacological, genetic, and optogenetic treatments are being developed, this article focuses on the evolution of transplant therapies for the treatment of PD, including fetal grafts, cell-based implants, and more recent tissue-engineered constructs. Attention is given to cell/tissue sources, efficacy to date, and future challenges that must be overcome to enable robust translation into clinical use. Emerging regenerative medicine therapies are being developed using neurons derived from autologous stem cells, enabling the construction of patient-specific constructs tailored to their particular extent of degeneration. In the upcoming era of restorative neurosurgery, such constructs may directly replace SNpc neurons, restore axon-based dopaminergic inputs to the striatum, and ameliorate motor deficits. These solutions may provide a transformative and scalable solution to permanently replace lost neuroanatomy and improve the lives of millions of people afflicted by PD.

Transplantation in the nonhuman primate MPTP model of Parkinson's disease: update and perspectives

In order to calibrate stem cell exploitation for cellular therapy in neurodegenerative diseases, fundamental and preclinical research in NHP (nonhuman primate) models is crucial. Indeed, it is consensually recognized that it is not possible to directly extrapolate results obtained in rodent models to human patients. A large diversity of neurological pathologies should benefit from cellular therapy based on neural differentiation of stem cells. In the context of this special issue of Primate Biology on NHP stem cells, we describe past and recent advances on cell replacement in the NHP model of Parkinson's disease (PD). From the different grafting procedures to the various cell types transplanted, we review here diverse approaches for cell-replacement therapy and their related therapeutic potential on behavior and function in the NHP model of PD.

A Sequential Intrastriatal Dopaminergic Graft Strategy in the Rodent Model for Parkinson's Disease: Implications for Graft Survival and Targeting

Optimal placement of intrastriatal dopaminergic grafts is likely crucial to optimize clinical recovery in Parkinson's disease (PD). The target sites of dopaminergic grafts vary among clinical trials and may partially explain the variable results in clinical efficacy reported thus far. In this study we hypothesized that a subsequent dopaminergic graft may promote functional recovery following a suboptimal initial graft. To test this hypothesis, rats with unilateral 6-hydroxydopamine lesions of the right nigrostriatal pathway were randomly divided into three groups. The first group received 900,000 fetal nigral cells in the medial striatum only (n = 6). The second group received 900,000 cells in both the medial and lateral striatum simultaneously (1.8 million total; n = 8). The final group received a second graft of 900,000 cells in the lateral striatum 6 weeks following initial transplantation of a medial graft (n = 6). Amphetamine-induced circling behavior was significantly reduced in both simultaneous and sequential graft groups at 9 and 12 weeks following transplantation of the initial graft. However, no recovery was noted in the single medial graft group at those time points. Furthermore, increased survival of dopaminergic cells was observed in the lateral graft of sequentially grafted animals compared with the medial graft. We conclude that a well-positioned subsequent graft can restore function in animals with a suboptimal initial graft and that the initial graft may improve survival of the second graft. These results are further discussed in relation to their important clinical implication for neural transplantation in PD.

The role of nonhuman primate models in the development of cell-based therapies for Parkinson's disease

Through the course of over three decades, nonhuman primate (NHP) studies on cell-based therapies (CBTs) for Parkinson’s disease (PD) have provided insight into the feasibility, safety and efficacy of the approach, methods of cell collection and preparation, cell viability, as well as potential brain targets. Today, NHP research continues to be a vital source of information for improving cell grafts and analyzing how the host affects graft survival, integration and function. Overall, this article aims to discuss the role that NHP models of PD have played in CBT development and highlights specific issues that need to be considered to maximize the value of NHP studies for the successful clinical translation of CBTs.

Semaphorin 3C Released from a Biocompatible Hydrogel Guides and Promotes Axonal Growth of Rodent and Human Dopaminergic Neurons

Cell therapy in experimental models of Parkinson's disease replaces the lost dopamine neurons (DAN), but we still need improved methods to guide dopaminergic axons (DAx) of grafted neurons to make proper connections. The protein Semaphorin 3C (Sema3C) attracts DAN axons and enhances their growth. In this work, we show that the hydrogel PuraMatrix, a self-assembling peptide-based matrix, incorporates Sema3C and releases it steadily during 4 weeks. We also tested if hydrogel-delivered Sema3C attracts DAx using a system of rat midbrain explants embedded in collagen gels. We show that Sema3C released by this hydrogel attracts DAx, in a similar way to pretectum, which is known to attract growing DAN axons. We assessed the effect of Sema3C on the growth of DAx using microfluidic devices. DAN from rat midbrain or those differentiated from human embryonic stem cells showed enhanced axonal extension when exposed to hydrogel-released Sema3C, similar to soluble Sema3C. Notably, DAN of human origin express the cognate Sema3C receptors, Neuropilin1 and Neuropilin2. These results show that PuraMatrix is able to incorporate and release Sema3C, and such delivery guides and promotes the axonal growth of DAN. This biocompatible hydrogel might be useful as a Sema3C carrier for in vivo studies in parkinsonian animal models.

Proceedings: cell therapies for Parkinson's disease from discovery to clinic

In March 2013, the California Institute for Regenerative Medicine, in collaboration with the NIH Center for Regenerative Medicine, held a 2-day workshop on cell therapies for Parkinson's disease (PD), with the goals of reviewing the state of stem cell research for the treatment of PD and discussing and refining the approach and the appropriate patient populations in which to plan and conduct new clinical trials using stem cell-based therapies for PD. Workshop participants identified priorities for research, development, and funding; discussed existing resources and initiatives; and outlined a path to the clinic for a stem cell-based therapy for PD. A consensus emerged among participants that the development of cell replacement therapies for PD using stem cell-derived products could potentially offer substantial benefits to patients. As with all stem cell-based therapeutic approaches, however, there are many issues yet to be resolved regarding the safety, efficacy, and methodology of transplanting cell therapies into patients. Workshop participants agreed that designing an effective stem cell-based therapy for PD will require further research and development in several key areas. This paper summarizes the meeting.

Recovery from experimental parkinsonism by semaphorin-guided axonal growth of grafted dopamine neurons

Cell therapy in animal models of Parkinson's disease (PD) is effective after intrastriatal grafting of dopamine (DA) neurons, whereas intranigral transplantation of dopaminergic cells does not cause consistent behavioral recovery. One strategy to promote axonal growth of dopaminergic neurons from the substantia nigra (SN) to the striatum is degradation of inhibitory components such as chondroitin sulphate proteoglycans (CSPG). An alternative is the guidance of DA axons by chemotropic agents. Semaphorins 3A and 3C enhance axonal growth of embryonic stem (ES) cell-derived dopaminergic neurons in vitro, while Semaphorin 3C also attracts them. We asked whether intranigral transplantation of DA neurons, combined with either degradation of CSPG or with grafts of Semaphorin 3-expressing cells, towards the striatum, is effective in establishing a new nigrostriatal dopaminergic pathway in rats with unilateral depletion of DA neurons. We found depolarization-induced DA release in dorsal striatum, DA axonal projections from SN to striatum, and concomitant behavioral improvement in Semaphorin 3-treated animals. These effects were absent in animals that received intranigral transplants combined with Chondroitinase ABC treatment, although partial degradation of CSPG was observed. These results are evidence that Semaphorin 3-directed long-distance axonal growth of dopaminergic neurons, resulting in behavioral improvement, is possible in adult diseased brains.

Repair of the CNS using endogenous and transplanted neural stem cells

Restoration of the damaged central nervous system is a vast challenge. However, there is a great need for research into this topic, due to the prevalence of central nervous system disorders and the devastating impact they have on people's lives. A number of strategies are being examined to achieve this goal, including cell replacement therapy, enhancement of endogenous plasticity and the recruitment of endogenous neurogenesis. The current chapter reviews this topic within the context of Parkinson's disease, Huntington's disease and stroke. For each disease exogenous cell therapies are discussed including primary (foetal) cell transplants, neural stem cells, induced pluripotent stem cells and marrow stromal cells. This chapter highlights the different mechanistic approaches of cell replacement therapy versus cells that deliver neurotropic factors, or enhance the endogenous production of these factors. Evidence of exogenously transplanted cells functionally integrating into the host brain, replacing cells, and having a behavioural benefit are discussed, along with the ability of some cell sources to stimulate endogenous neuroprotective and restorative events. Alongside exogenous cell therapy, the role of endogenous neurogenesis in each of the three diseases is outlined and methods to enhance this phenomenon are discussed.

Human induced pluripotent stem cells improve stroke outcome and reduce secondary degeneration in the recipient brain

Human induced pluripotent stem cells (hiPSCs) are a most appealing source for cell replacement therapy in acute brain lesions. We evaluated the potential of hiPSC therapy in stroke by transplanting hiPSC-derived neural progenitor cells (NPCs) into the postischemic striatum. Grafts received host tyrosine hydroxylase-positive afferents and contained developing interneurons and homotopic GABAergic medium spiny neurons that, with time, sent axons to the host substantia nigra. Grafting reversed stroke-induced somatosensory and motor deficits. Grafting also protected the host substantia nigra from the atrophy that follows disruption of reciprocal striatonigral connections. Graft innervation by tyrosine hydoxylase fibers, substantia nigra protection, and somatosensory functional recovery were early events, temporally dissociated from the slow maturation of GABAergic neurons in the grafts and innervation of substantia nigra. This suggests that grafted hiPSC-NPCs initially exert trophic effects on host brain structures, which precede integration and potential pathway reconstruction. We believe that transplantation of NPCs derived from hiPSCs can provide useful interventions to limit the functional consequences of stroke through both neuroprotective effects and reconstruction of impaired pathways.

Delivery of chemotropic proteins and improvement of dopaminergic neuron outgrowth through a thixotropic hybrid nano-gel

Chemotropic proteins guide neuronal projections to their final target during embryo development and are useful to guide axons of neurons used in transplantation therapies. Site-specific delivery of the proteins however is needed for their application in the brain to avoid degradation and pleiotropic affects. In the present study we report the use of Poly (ethylene glycol)-Silica (PEG-Si) nanocomposite gel with thixotropic properties that make it injectable and suitable for delivery of the chemotropic protein semaphorin 3A. PEG-Si gel forms a functional gradient of semaphorin that enhances axon outgrowth of dopaminergic neurons from rat embryos or differentiated from stem cells in culture. It is not cytotoxic and its properties allowed its injection into the striatum without inflammatory response in the short term. Long term implantation however led to an increase in macrophages and glial cells. The inflammatory response could have resulted from non-degraded silica particles, as observed in biodegradation assays.

Cell transplantation and gene therapy in Parkinson's disease

Parkinson's disease is a progressive neurodegenerative disorder affecting, in part, dopaminergic motor neurons of the ventral midbrain and their terminal projections that course to the striatum. Symptomatic strategies focused on dopamine replacement have proven effective at remediating some motor symptoms during the course of disease but ultimately fail to deliver long-term disease modification and lose effectiveness due to the emergence of side effects. Several strategies have been experimentally tested as alternatives for Parkinson's disease, including direct cell replacement and gene transfer through viral vectors. Cellular transplantation of dopamine-secreting cells was hypothesized as a substitute for pharmacotherapy to directly provide dopamine, whereas gene therapy has primarily focused on restoration of dopamine synthesis or neuroprotection and restoration of spared host dopaminergic circuitry through trophic factors as a means to enhance sustained controlled dopamine transmission. This seems now to have been verified in numerous studies in rodents and nonhuman primates, which have shown that grafts of fetal dopamine neurons or gene transfer through viral vector delivery can lead to improvements in biochemical and behavioral indices of dopamine deficiency. However, in clinical studies, the improvements in parkinsonism have been rather modest and variable and have been plagued by graft-induced dyskinesias. New developments in stem-cell transplantation and induced patient-derived cells have opened the doors for the advancement of cell-based therapeutics. In addition, viral-vector-derived therapies have been developed preclinically with excellent safety and efficacy profiles, showing promise in clinical trials thus far. Further progress and optimization of these therapies will be necessary to ensure safety and efficacy before widespread clinical use is deemed appropriate.

Axon responses of embryonic stem cell-derived dopaminergic neurons to semaphorins 3A and 3C

Class 3 Semaphorins are a subfamily of chemotropic molecules implicated in the projection of dopaminergic neurons from the ventral mesencephalon and in the formation of the nigrostriatal pathway (NSP) during embryonic development. In humans, loss of mesencephalic dopaminergic neurons leads to Parkinson's disease (PD). Cell replacement therapy with dopaminergic neurons generated from embryonic stem cells (ES-TH(+)) is being actively explored in models of PD. Among several requisites for this approach to work are adequate reconstruction of the NSP and correct innervation of normal striatal targets by dopaminergic axons. In this work, we characterized the response of ES-TH(+) neurons to semaphorins 3A, 3C, and 3F and compared it with that of tyrosine hidroxylase-positive neurons (TH(+)) obtained from embryonic ventral mesencephalon (VM-TH(+)). We observed that similar proportions of ES-TH(+) and VM-TH(+) neurons express semaphorin receptors neuropilins 1 and 2. Furthermore, the axons of both populations responded very similarly to semaphorin exposure: semaphorin 3A increased axon length, and semaphorin 3C attracted axons and increased their length. These effects were mediated by neuropilins, insofar as addition of blocking antibodies against these proteins reduced the effects on axonal growth and attraction, and only TH(+) axons expressing neuropilins responded to the semaphorins analyzed. The observations reported here show phenotypic similarities between VM-TH(+) and ES-TH(+) neurons and suggest that semaphorins 3A and 3C could be employed to guide axons of grafted ES-TH(+) in therapeutic protocols for PD.

Embryonic substantia nigra grafts in the mesencephalon send neurites to the host striatum in non-human primate after overexpression of GDNF

In spite of partial success in treating Parkinson's disease by using ectopically placed grafts of dopamine-producing cells, restoration of the original neuroanatomical circuits, if possible, might work better. Previous evidence of normal anatomic projections from ventral mesencephalic (VM) grafts placed in the substantia nigra (SN) has been limited to neonatal rodents and double grafting or bridging procedures. This study attempted to determine whether injection of a potent growth-promoting factor, glial cell line-derived neurotrophic factor (GDNF), into the target regions or placement of fetal striatal co-grafts in the nigrostriatal pathway might elicit neuritic outgrowth to the caudate nucleus. Four adult St. Kitts green monkeys received embryonic VM grafts into the rostral mesencephalon near the host SN, and injections of adeno-associated virus 2 (AAV2)/GDNF or equine infectious anemia virus (EIAV)/GDNF into the caudate. Three adult monkeys were co-grafted with fetal VM tissue near the SN and fetal striatal grafts (STR) 2.5 mm rostral in the nigrostriatal pathway. Before sacrifice, the striatal target regions were injected with the retrograde tracer Fluoro-Gold (FG). FG label was found in tyrosine hydroxylase-labeled neurons in VM grafts in the SN of only those monkeys that received AAV2/GDNF vector injections into the ipsilateral striatum. All monkeys showed FG labeling in the host SN when FG labeling was injected on the same side. These data show that grafted dopaminergic neurons can extend neurites to a distant target releasing an elevated concentration of GDNF, and suggest that grafted neurons can be placed into appropriate loci for potential tract reconstruction.

Transgenic reporter mice as tools for studies of transplantability and connectivity of dopamine neuron precursors in fetal tissue grafts

Cell therapy for Parkinson's disease (PD) is based on the idea that new midbrain dopamine (mDA) neurons, implanted directly into the brain of the patient, can structurally and functionally replace those lost to the disease. Clinical trials have provided proof-of-principle that the grafted mDA neurons can survive and function after implantation in order to provide sustained improvement in motor function for some patients. Nonetheless, there are a number of issues limiting the application of this approach as mainstream therapy, including: the use of human fetal tissue as the only safe and reliable source of transplantable mDA neurons, and variability in the therapeutic outcome. Here we review recent progress in this area from investigations using rodent models of PD, paying particular attention to the use of transgenic reporter mice as tools for neural transplantation studies. Cell type-specific expression of reporter genes, such as green fluorescent protein, affords valuable technical advantages in transplantation experiments, such as the ability to selectively isolate specific cell fractions from mixed populations prior to grafting, and the unambiguous visualization of graft-derived dopamine neuron fiber patterns after transplantation. The results from these investigations have given new insights into the transplantability of mDA precursors as well as their connectivity after grafting and have interesting implications for the development of stem cell based approaches for the treatment of PD.

Adjunctive use of the non-ionic surfactant Poloxamer 188 improves fetal dopaminergic cell survival and reinnervation in a neural transplantation strategy for Parkinson's disease

Although neural transplantation of fetal dopaminergic cells is a promising therapy for Parkinson's disease, poor transplanted cell survival limits its efficacy. In the present study it was hypothesized that the use of Poloxamer 188 (P188), a non-ionic surfactant, during cell preparation and transplantation may protect cells from associated mechanical injury and thus improve transplanted cell survival in a rat model of Parkinson's disease. Fetal rat dopaminergic tissue was dissociated in media with or without P188 and then cultured for 1 week or transplanted into the striatum of rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal dopaminergic pathway. Fetal dopaminergic cell survival and reinnervation of the host brain were examined using tyrosine hydroxylase immunohistochemistry and stereological quantification. The number of surviving tyrosine hydroxylase-immunoreactive cells in vitro and in vivo was significantly increased by 2.2-fold by incubating fetal dopaminergic cells with P188 during tissue dissociation. Furthermore, the striatal reinnervation in parkinsonian rats that received intrastriatal transplants of P188-exposed dopaminergic cells was significantly enhanced (1.8-fold increase) compared with rats that received non-P188-treated cells. In conclusion, P188 protects fetal dopaminergic cells from mechanical injury by increasing cell survival and enhances dopaminergic fibre outgrowth into the transplanted striatum. Use of P188 may thus be an important adjunct to improve the clinical efficacy of neural transplantation for Parkinson's disease.

Orthotopic transplantation of immortalized mesencephalic progenitors (CSM14.1 cells) into the substantia nigra of hemiparkinsonian rats induces neuronal differentiation and motoric improvement

Neural progenitor cell grafting is a promising therapeutic option in the treatment of Parkinson's disease. In previous experiments we grafted temperature-sensitive immortalized CSM14.1 cells, derived from the ventral mesencephalon of E14-rats, bilaterally in the caudate putamen of adult hemiparkinsonian rats. In these studies we were not able to demonstrate either a therapeutic improvement or neuronal differentiation of transplanted cells. Here we examined whether CSM14.1 cells grafted bilaterally orthotopically in the substantia nigra of hemiparkinsonian rats have the potential to differentiate into dopaminergic neurons. Adult male rats received 6-hydroxydopamine into the right medial forebrain bundle, and successful lesions were evaluated with apomorphine-induced rotations 12 days after surgery. Two weeks after a successful lesion the animals received bilateral intranigral grafts consisting of either about 50 000 PKH26-labelled undifferentiated CSM14.1 cells (n = 16) or a sham-graft (n = 9). Rotations were evaluated 3, 6, 9 and 12 weeks post-grafting. Animals were finally perfused with 4% paraformaldehyde. Cryoprotected brain slices were prepared for immunohistochemistry using the freeze-thaw technique to preserve PKH26-labelling. Slices were immunostained against neuronal epitopes (NeuN, tyrosine hydroxylase) or glial fibrillary acidic protein. The CSM14.1-cell grafts significantly reduced the apomorphine-induced rotations 12 weeks post-grafting compared to the sham-grafts (P < 0.05). There was an extensive mediolateral migration (400-700 microm) of the PKH26-labelled cells within the host substantia nigra. Colocalization with NeuN or glial fibrillary acidic protein in transplanted cells was confirmed with confocal microscopy. No tyrosine hydroxylase-immunoreactive grafted cells were detectable. The therapeutic effect of the CSM14.1 cells could be explained either by their glial cell-derived neurotrophic factor-expression or their neural differentiation with positive effects on the basal ganglia neuronal networks.

Transplantation of bioreactor-produced neural stem cells into the rodent brain

The development of new cell replacement strategies using neural stem cells (NSC) may provide an alternative and unlimited cell source for clinical neural transplantation in neurodegenerative diseases such as Parkinson's and Huntington's disease. The clinical application of neural transplantation using NSC will therefore depend upon the availability of clinical grade NSC that are generated in unlimited quantities in a standardized manner. In order to investigate the utility of NSC in clinical neural transplantation, undifferentiated murine NSC were first expanded for an extended period of time in suspension bioreactors containing a serum-free medium. Following expansion in suspension bioreactors, NSC were still able to differentiate in vitro into both astrocytes and neurons after exposure to brain-derived neurotrophic factor (BDNF), suggesting that bioreactor expansion does not alter cell lineage potentiality. Undifferentiated bioreactor-expanded NSC were then transplanted into the rodent striatum. Immunohistochemical examination revealed undifferentiated bioreactor-expanded NSC survived transplantation for up to 8 weeks and expressed the astrocytic immunohistochemical marker glial fibrillary acidic protein (GFAP), suggesting that the host striatal environment influences NSC cell fate upon transplantation. Moreover, no tumor formation was observed within the graft site, indicating that NSC expanded in suspension bioreactors for an extended period of time are a safe source of tissue for transplantation. Future studies should focus on predifferentiating NSC towards specific neuronal phenotypes prior to transplantation in order to restore behavioral function in rodent models of neurodegenerative disease.

Transplantation of immortalized mesencephalic progenitors (CSM14.1 cells) into the neonatal parkinsonian rat caudate putamen

The present study analyzed whether grafts of the mesencephalic progenitor cell line CSM14.1 into the neonatal rat caudate putamen (CPu) differentiate into neurons and whether this is accompanied by a functional improvement in 6-hydroxydopamine (6-OHDA)-lesioned animals. As in previous studies, a neuronal differentiation of CSM14.1 cells transplanted into the CPu of adult animals could not be observed, so we here used neonatal rats, because graft location and host age seemingly are crucial parameters for neural transplant differentiation and integration. Rats bilaterally lesioned at postnatal day 1 by intraventricular 6-OHDA-injections 2 days later received 100,000 CSM14.1 cells prelabelled with the fluorescent dye PKH26 into the right CPu. Five weeks after grafting, the cylinder test was performed, and the data compared with data from age-matched intact controls and bilaterally lesioned-only animals. Brain slices immunostained for tyrosine hydroxylase (TH) were quantified by optical densitometry. We observed a significant preference of left forelimb use exclusively in transplanted animals. In these rats, TH-containing perikarya were found in the grafted CPu, presumedly leading to the significant increase of TH-immunoreactive fibers in this region. Moreover, confocal laser microscopy revealed a differentiation of transplanted PKH26-labelled CSM14.1 cells into neuronal nuclei antigen or TH-immunoreactive cells. Thus, CSM14.1 cells differentiate into TH-containing neurons, which most probably contribute to the preferred forelimb use, indicating a functional integration of CSM14.1 cells into the host basal ganglia loops during early postnatal development. These findings that are in contrast to observations in adult rats suggest instructive cues for neuronal differentiation and integration given by the neonatal microenvironment.

Erythropoietin and GDNF enhance ventral mesencephalic fiber outgrowth and capillary proliferation following neural transplantation in a rodent model of Parkinson's disease

Low dopaminergic cell survival and suboptimal fiber reinnervation are likely major contributing factors for the limited benefits of neural transplantation in Parkinson's disease (PD) patients. Glial cell lined-derived neurotrophic factor (GDNF) has been shown to enhance dopaminergic cell survival and fiber outgrowth of the graft site as well as promote behavioral recovery in rodent models of PD, while erythropoietin (EPO) can produce dopaminergic neuroprotective effects against 6-hydroxydopamine (6-OHDA) exposure on cultured neurons and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice. The aim of this study was to determine if fetal ventral mesencephalic (FVM) tissue exposed to hibernation media containing a combination of GDNF and EPO could enhance dopaminergic graft survival, striatal reinnervation and functional recovery in a 6-OHDA rodent model of PD. FVM tissue was dissected from 14-day-old rat fetuses and placed for 6 days in hibernation media alone, and in hibernation media that received either a daily administration of GDNF, EPO or a combination of GDNF and EPO. Following hibernation, FVM cells were transplanted as a single cell suspension into the striatum of unilateral 6-OHDA-lesioned rats. Rotational behavioral assessment revealed animals that received FVM tissue exposed to GDNF, EPO or the combination of both drugs had accelerated functional recovery. Immunohistochemical and stereological assessment revealed a significant increase in graft fiber density and angiogenesis into the graft when compared with control. These findings suggest that the hibernation of FVM tissue in a combination of GDNF and EPO can enhance graft efficacy and may have important implications for tissue preparation protocols for clinical neural transplantation in PD.

Effects of intranigral vs intrastriatal fetal mesencephalic neural grafts on motor behavior disorders in a rat Parkinson model

BACKGROUND

Numerous experimental and clinical studies have shown that intrastriatal fetal mesencephalic grafts grow, survive, and reinnervate host brain tissue, resulting in partial recovery of motor deficits. In addition, pharmacological evidence indicates that these grafts increase dopamine secretion in lesioned brain. However, to date, no grafting method has completely restored the nigrostriatal pathway, and there is no consensus on optimal graft numbers or locations. This study compared outcomes with multiple striatal grafts vs a single intranigral graft in a rat model of Parkinson disease.

METHODS

Forty-one female Wistar rats weighing 200 to 250 g were used. First, baseline rotational behavior testing with amphetamine injection was done to identify each animal's dominant nigrostriatal pathway (left vs right hemisphere). Some rats then received a unilateral intranigral injection of 6-hydroxydopamine (4 microL [8 microg]) to produce the Parkinson model lesion, and rotational testing was repeated. One group of the lesioned rats received a single intranigral injection of suspended fetal ventral mesencephalic cells (n = 11), and another received multiple intrastriatal grafts of the same type (n = 11).

RESULTS

Both grafted groups showed significant improvement on rotational testing with amphetamine and apomorphine at 6 weeks "postgrafting" (P < .001 for "postlesioning" vs postgrafting results in each of the 2 groups); however, the animals with multiple intrastriatal grafts showed complete recovery from motor asymmetry, whereas the rats with single intranigral grafts showed only partial improvement.

CONCLUSION

The findings indicate that multiple intrastriatal grafts result in significantly greater functional improvement than single intranigral grafts in this rat Parkinson model.

Dopaminergic reinnervation of the globus pallidus by fetal nigral grafts in the rodent model of Parkinson's disease

The current neural transplantation strategy for Parkinson's disease (PD) involves the dopaminergic reinnervation of the striatum (STR). Although up to 85% reinnervation of the STR has been attained by neural transplantation, functional recovery in animal models and transplanted patients is incomplete. This limitation may be due to an incomplete restoration of the dopaminergic input to other basal ganglia structures such as the external segment of the globus pallidus (GPe, homologue of the rodent GP), which normally receives dopaminergic input from the substantia nigra (SN). As part of our investigation into a multiple grafting strategy for PD, we have explored the effects of dopaminergic grafts in the GP of rodents with unilateral 6-hydroxydopamine (6-OHDA) lesions. In this experiment, lesioned rats received either 300,000 fetal ventral mesencephalic (FVM) cells or a sham injection into the GP. Functional assessment consisted of rotational behavior at 3 and 6 weeks posttransplantation. A fluorogold tracer study was conducted to rule out any behavioral improvement due to striatal outgrowth of the GP graft. Sections were stained for glial fibrillary acidic protein (GFAP) to assess the degree of trauma in the GP by the graft in comparison to the sham injection. Immunohistochemistry for tyrosine hydroxylase (TH) was performed after transplantation to assess graft survival. Animals with GP grafts demonstrated a significant improvement in rotational behavior at 3 and 6 weeks posttransplantation (p < 0.05) while sham control animals did not improve. All animals receiving FVM cells showed TH-immunoreactive grafts in the GP posttransplantation. TH-positive neurons in the GP showed no double labeling with an intrastriatal injection of fluorogold, indicating that behavioral improvement was not due to striatal innervation by the GP graft. These observations suggest that functional recovery was the result of dopaminergic reinnervation of the GP and that this nucleus may be a potential target for neural transplantation in clinical PD.

Long distance selective fiber outgrowth of transplanted hNT neurons in white matter tracts of the adult rat brain

Terminally differentiated neurons derived from a human teratocarcinoma cell line (NT2N or hNT neurons) are promising as a cell source for transplantation, as they have been shown to be safe for transplantation in humans. We have shown previously that hNT neurons can express a catecholaminergic phenotype in a rat Parkinson model. In this study, we investigated the long-term survival and ability of hNT neurons to express tyrosine hydroxylase and reconstruct the dopamine-denervated nigrostriatal pathway. Hemiparkinsonian rats received grafts of 400,000 viable hNT neurons into each of the denervated striatum and substantia nigra. Robust hNT grafts were detected up to 24 weeks posttransplantation, although few cells expressed tyrosine hydroxylase. Many hNT fibers were often associated with ipsilateral and contralateral white matter tracts--corpus callosum, rostral migratory stream, optic tract, and external capsule. Fewer fibers were associated with the superior cerebellar peduncle, medial lemniscus, and nigrostriatal pathway. Axons also projected into the frontal cortex and extended parallel to the surface of the brain in the superficial cortical layers. These pathways were seen in all grafted animals, suggesting that specific guidance cues exist in the adult brain governing hNT fiber outgrowth. Injured adult axons and transplanted embryonic neuronal axons rarely extend for such distances in the adult nervous system. We propose that elucidating the factors promoting and guiding hNT axonal outgrowth could provide important clues to enhancing regeneration and target reinnervation in the adult brain, two factors of critical importance for cell restoration strategies aimed at brain repair.

Neuroprotective properties of cultured neural progenitor cells are associated with the production of sonic hedgehog

Numerous studies have shown that abnormal motor behavior improves when neural progenitor cells (NPCs) are transplanted into animal models of neurodegeneration. The mechanisms responsible for this improvement are not fully understood. Indirect anatomical evidence suggests that attention of abnormal motor behavior is attributed, at least in part, to the secretion of trophic factors from the transplanted NPCs. However, there is little direct evidence supporting this hypothesis. Here we show that NPCs isolated from the subventricular zone (SVZ) of neonatal mice are highly teratogenic when transplanted into the neural tube of developing chick embryos and are neuroprotective for fetal dopaminergic neurons in culture because they release sonic hedgehog (Shh). In addition, the neuroprotective properties of NPCs can be exploited to promote better long-term survival of transplanted fetal neurons in an animal model of Parkinson's disease. Thus, cultured NPCs isolated from the SVZ can secrete at least one potent mitogen (Shh) that dramatically affects the fate of neighboring cells. This trait may account for some of the improvement in motor behavior often reported in animal models of neurodegeneration after transplantation of cultured NPCs that were isolated from the SVZ.

Functional Improvement with Low-dose Dopaminergic Grafts in Hemiparkinsonian Rats

OBJECTIVE

The beneficial functional effects of neural transplantation in Parkinson's disease are often directly attributed to the number of surviving dopaminergic cells within a graft. However, recent clinical trials of fetal neural transplantation suggest that a high number of dopaminergic cells may induce serious side effects. In this study, we explored the ability of low-dose dopaminergic grafts to produce functional benefits in the 6-hydroxydopamine rodent model of Parkinson's disease over a long period of observation.

METHODS

Twelve rats received either 50,000 or 400,000 fetal ventral mesencephalic cells implanted into the striatum. Rotational behavior was assessed after the lesion and at 3, 6, 9, and 12 weeks after transplantation. Twelve weeks after transplantation, animals were perfused, and microtome sections were stained for tyrosine hydroxylase, glial fibrillary acidic protein, heat-shock protein 27, and vimentin.

RESULTS

The low-dose group had a three-fold increase in tyrosine hydroxylase-positive cell survival rate compared with the high-dose group rate. The low-dose group also had a mean cell diameter significantly higher than the high-dose group. There was no significant difference between groups in fiber density; however, a higher percentage of longer fibers was encountered in the low-dose group. The low-dose group had a lower degree of trauma in the striatum, as assessed by optical density scores from glial fibrillary acidic protein, heat-shock protein 27, and vimentin staining. There was significant improvement in rotational behavior in the high-dose group at 3 weeks after transplantation, whereas the rotational behavior normalized in the low-dose group at 6 weeks after grafting. There was no significant difference in rotational behavior scores between groups at 6 weeks after grafting.

CONCLUSION

This study demonstrates that over time, a low-dose dopaminergic graft has the capability of eliciting the same functional effect as a high-dose graft. Furthermore, low-dose grafts may increase graft survival, fiber outgrowth, and dopamine production and decrease trauma to the brain.

Mesencephalic progenitors can improve rotational behavior and reconstruct nigrostriatal pathway in PD rats

The aim of this study was to investigate the possibility of mesencephalic progenitors (MP) in treating Parkinson's disease (PD). MP were prepared from E(11-13) rats and proliferated in serum-free medium with basic fibroblast growth factor (bFGF) for 10 days. Cells were then collected and implanted into the striatum only--single grafts or simultaneously into the substantia nigra (SN) and the striatum--double grafts. Twelve weeks after transplantation, DiI, a fluorescent dye, was microinjected into the ipsilateral striatum. Using this strategy, it was found that MP of double grafts had more potent effects on rotational behavior than that of single grafts. Injection of the retrograde tracer DiI into the striatum resulted in fluorescent-labeled cells within the intranigral grafts in double grafts. These data greatly support that MP transplants could not only improve rotational behavior, but does help to re-establish nigrostriatal connections so that it may become one efficient way in treating PD.

Glial cell line-derived neurotrophic factor-supplemented hibernation of fetal ventral mesencephalic neurons for transplantation in Parkinson disease: long-term storage

OBJECT

Transplantation of fetal dopaminergic tissue is being investigated in animal models and clinical trials for its potential as a treatment for advanced Parkinson disease. At the same time, the availability of fetal tissue is limited, making its storage time prior to transplantation a key practical issue. Although it results in a smaller percentage of surviving cells. a longer storage time enables fetal tissue obtained over several days to be pooled for transplantation in a recipient. Glial cell line-derived neurotrophic factor (GDNF) has been shown to improve survival of human dopaminergic tissue that has been stored prior to transplantation. The objective of this study was to evaluate the effects on fetal dopaminergic tissue of GDNF-supplemented hibernation for extended periods of 6 to 15 days.

METHODS

The ventral mesencephalon (VM) was harvested in a total of 27 14-day-old rat fetuses, and three VMs were cultured immediately (fresh control group). The remaining 24 VMs were divided sagittally along the midline to yield 48 equal pieces of hemimesencephalon. Twenty-four pieces were stored with GDNF-supplemented hibernation medium for 6, 9, 12, or 15 days, and the 24 "partner" hemimesencephalon pieces were stored in control hibernation medium for the same periods of time. Tissue was cultured for 48 hours and processed for tyrosine hydroxylase (TH) immunoreactivity and double-stained with cresyl violet. Cell counts for all cultures and the percentage of TH-immunoreactive cells were obtained. The percentage of TH-immunoreactive cells for the fresh control group was 6.3 +/- 0.5%. The percentage of TH-immunoreactive cells in cultures derived from tissue stored in GDNF-supplemented medium was significantly increased at 6 and 9 days posthibernation compared with the fresh control group and the "partner" groups stored in hibernation medium only. No significant increase in the percentage of TH-immunoreactive cells was observed in the 12- and 15-day groups.

CONCLUSIONS

In this study the authors have demonstrated that fetal dopaminergic tissue can be safely stored for up to 9 days in GDNF-supplemented hibernation medium. Furthermore, the percentage of TH-immunoreactive cells is significantly increased after 6 and 9 days of storage in this medium, improving the yield of TH-immunoreactive cells prior to transplantation. These observations have practical clinical implications for collecting fetal dopaminergic cells and improving their survival after transplantation.

A multiple target neural transplantation strategy for Parkinson's disease

Intracerebral transplantation of embryonic ventral mesencephalic tissue is a potential treatment for patients with Parkinson's disease for whom medical management is unsatisfactory. Neural transplantation for parkinsonism has been studied experimentally in animal models of Parkinson's disease for more than two decades. These animal studies have shown significant graft survival, synapse formation, graft induced-dopamine release, and behavioural recovery in transplanted animals. Encouraged by these results, clinical programs have been initiated over the past 15 years; more than 250 patients worldwide have undergone neural transplantation. Both animal and clinical studies indicate that neural transplantation has the potential to become a valuable treatment option for Parkinson's disease. However, while many transplant recipients obtain clinically useful symptom relief, in all cases functional recovery is incomplete. Certain symptoms do not respond well to transplant therapy, and those symptoms that do typically do not resolve completely. This has spurred efforts to optimize the transplant procedure. One important approach is exploring novel methods such as multiple site transplantation. This transplantation strategy results in a more complete reinnervation of the dopaminergic circuitry that is affected in Parkinson's disease. In principle, multiple site transplantation should provide a more satisfactory resolution of symptoms. Here we review the progress made in multiple site neural transplantation for Parkinson's disease. The effects of intrastriatal, intranigral, intrasubthalamic nucleus, and intrapallidal grafts in animal models of Parkinson's disease are analysed. The current data suggest that intrastriatal grafts alone are inadequate to promote complete functional recovery. A multiple target strategy may restore dopaminergic input to affected basal ganglia nuclei and improve outcomes of neural transplantation in Parkinson's disease.

Glial cell line-derived neurotrophic factor-supplemented hibernation of fetal ventral mesencephalic neurons for transplantation in Parkinson disease: long-term storage

OBJECT

Investigation of fetal dopaminergic tissue transplantation is being conducted in animal models and clinical trials as a potential treatment for advanced Parkinson disease (PD). Because the availability of fetal tissue is limited, however, the duration of its storage prior to transplantation is a key practical issue. Longer storage times may enable fetal tissue obtained over several days to be pooled together for transplantation in a recipient. Glial cell line-derived neurotrophic factor (GDNF) has been shown to improve survival of stored human dopaminergic tissue prior to transplantation. The objective of this study was to evaluate GDNF-supplemented hibernation of fetal dopaminergic tissue for extended periods of 6 to 15 days.

METHODS

A total of 27 rat ventral mesencephalons (VMs) were obtained in gestation Day 14 rat fetuses, and three were cultured immediately (fresh-culture control group). The remaining 24 VMs were divided sagittally along the midline to form 48 equal pieces of hemimesencephalons. Twenty-four pieces were stored with GDNF-supplemented hibernation medium for 6, 9, 12, or 15 days, and the 24 "partner" hemimesencephalons were stored in control hibernation medium for the same periods of time. Tissue was cultured for 48 hours and processed for tyrosine hydroxylase (TH) immunoreactivity and cresyl violet. Cell counts for all cultures and percentage of TH-immunoreactive cells were obtained. The percentage of TH-positive cells for the fresh control group was 6.3 +/- 0.5%; that measured in cultures derived from tissue hibernated in GDNF-supplemented medium was significantly increased at 6 and 9 days posthibernation compared with the fresh-culture control group and the partner groups stored in hibernation medium only. No significant increase in percentage of TH-immunoreactive cells was observed in the 12- and 15-day hibernation groups.

CONCLUSIONS

In summary the authors found that fetal dopaminergic tissue can safely be stored up to 9 days in GDNF-supplemented hibernation medium. Furthermore the percentage of TH-immunoreactive cells is significantly increased after 6 and 9 days of storage in this medium, improving the yield of TH-positive cells prior to transplantation. These observations may have important clinical implications for collecting fetal dopaminergic cells and improving their survival after transplantation.

Cellular replacement therapy for Parkinson's disease--where we are today?

The concept of replacing lost dopamine neurons in Parkinson's disease using mesencephalic brain cells from fetal cadavers has been supported by over 20 years of research in animals and over a decade of clinical studies. The ambitious goal of these studies was no less than a molecular and cellular "cure" for Parkinson's disease, other neurodegenerative diseases, and spinal cord injury. Much research has been done in rodents, and a few studies have been done in nonhuman primate models. Early uncontrolled clinical reports were enthusiastic, but the outcome of the first randomized, double blind, controlled study challenged the idea that dopamine replacement cells can cure Parkinson's disease, although there were some significant positive findings. Were the earlier animal studies and clinical reports wrong? Should we give up on the goal? Some aspects of the trial design and implantation methods may have led to lack of effects and to some side effects such as dyskinesias. But a detailed review of clinical neural transplants published to date still suggests that neural transplantation variably reverses some aspects of Parkinson's disease, although differing methods make exact comparisons difficult. While the randomized clinical studies have been in progress, new methods have shown promise for increasing transplant survival and distribution, reconstructing the circuits to provide dopamine to the appropriate targets and with normal regulation. Selected promising new strategies are reviewed that block apoptosis induced by tissue dissection, promote vascularization of grafts, reduce oxidant stress, provide key growth factors, and counteract adverse effects of increased age. New sources of replacement cells and stem cells may provide additional advantages for the future. Full recovery from parkinsonism appears not only to be possible, but a reliable cell replacement treatment may finally be near.

Simultaneous intrastriatal and intranigral fetal dopaminergic grafts in patients with Parkinson disease: a pilot study. Report of three cases

The main neural transplantation strategy in Parkinson disease (PD) has been focused on reinnervating the striatum. The clinical results reported in patients who receive transplants have been limited and do not justify the use of neural transplantation as a routine therapeutic procedure for PD. Identifying the optimal target for transplantation may be one of the critical factors for optimizing clinical outcomes. Evidence from preclinical studies indicates that simultaneous intrastriatal and intranigral grafts (double grafts) may produce a more complete functional recovery. The authors report the clinical and positron emission tomography (PET) scanning results in three patients enrolled in a safety and feasibility pilot study who received double grafts and who have been followed for up to 13 months posttransplantation. Patients included in the study had idiopathic PD. All patients underwent detailed assessments before and after surgery, in accordance with the Core Assessment Program for Intracerebral Transplantation. The patients received implants of fetal mesencephalic cell suspensions in the putamen and substantia nigra (SN) bilaterally. There were no intraoperative or perioperative complications. Follow-up PET scans demonstrated an increase in the mean fluorodopa uptake constant values in the putamen and SN 12 months postsurgery. Improvements were also noted in the total Unified Parkinson's Disease Rating Scale, Hoehn and Yahr, Schwab and England, and pronation/supination scores after transplantation. The authors demonstrate the feasibility of reinnervating the SN and striatum by using a double transplant strategy in humans.

Embryonic ventral mesencephalic grafts to the substantia nigra of MPTP-treated monkeys: feasibility relevant to multiple-target grafting as a therapy for Parkinson's disease

Transplantation of embryonic dopamine (DA) neurons is being studied as an experimental replacement therapy for the DA-deficiency characteristic of Parkinson's disease. Some studies suggest that one of the limitations of this approach is that intrastriatal placement of implants fails to consistently restore completely normal movement. One potential cause of this suboptimal therapeutic outcome is that changes in the neural activity of several structures in the basal ganglia circuitry resulting from striatal DA depletion is not adequately normalized by graft-derived DA replacement in striatum alone. In the present study, we assessed the feasibility of grafting embryonic DA neurons into the substantia nigra (SN) of adult parkinsonian monkeys as an approach to restoration of the DA modulation of striatal-nigral afferents that is lost after degeneration of SN neurons. Sixteen St. Kitts African green monkeys treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) received implants of embryonic monkey ventral mesencephalon (VM), or sham implants, aimed at the rostral SN. At 6 months after grafting, staining for tyrosine hydroxylase (TH) indicated that grafted DA neurons survived at this site, albeit often in reduced numbers compared with VM grafts to striatum. Grafted neurons extended neurites into the parenchyma of the SN, but there was no evidence of lengthy extension of graft-derived neurites rostrally along the trajectory of the mesostriatal fiber system. A region-specific, modest increase in DA levels and TH-positive fiber density in the ventral-medial putamen was detected, accompanied by modest but significant decreases in parkinsonian behaviors at 5-6 months after grafting. Our findings support the view that grafting embryonic tissue to the SN is a feasible procedure in nonhuman primates that provides a modest but detectable benefit of its own. These results encourage the further development of multiple-target grafting strategies as a means of restoring modulation of anatomically widespread basal ganglia structures relevant to treatment of Parkinson's disease.

Enhanced axonal growth from fetal human bcl-2 transgenic mouse dopamine neurons transplanted to the adult rat striatum

Embryonic neurons transplanted to the adult CNS extend axons only for a developmentally defined period. There are certain intercellular factors that control the axonal extension, one of which may be the expression of the bcl-2 protein. In this study, rats with complete striatal dopamine fiber denervation received embryonic day 14 mouse ventral mesencephalon cells overexpressing human bcl-2 or control wild-type ventral mesencephalon cells. All rats were treated with cyclosporine to prevent rejection and the surviving grafts were analyzed for cell survival and outgrowth of dopaminergic fibers. The results demonstrate that bcl-2 overexpression does not enhance neuronal graft survival. However, the bcl-2 overexpressing neurons had a higher number of dopaminergic fibers that grew longer distances. These results show that overexpression of bcl-2 can result in longer distance axonal growth of transplanted fetal dopaminergic neurons and that genetic modification of embryonic donor cells may enhance their ability to reinnervate a neuronal target territory.

Enhancement of sensorimotor behavioral recovery in hemiparkinsonian rats with intrastriatal, intranigral, and intrasubthalamic nucleus dopaminergic transplants

One of the critical variables that influences the efficacy of clinical neural transplantation for Parkinson's disease (PD) is optimal graft placement. The current transplantation paradigm that focuses on ectopic placement of fetal grafts in the striatum (ST) fails to reconstruct the basal ganglia circuitry or normalize neuronal activity in important basal ganglia structures, such as the substantia nigra (SN) and the subthalamic nucleus (STN). The aim of this study was to investigate a multitarget neural transplantation strategy for PD by assessing whether simultaneous dopaminergic transplants in the ST, SN, and STN induce functional recovery in hemiparkinsonian rats. Forty-six female Wistar rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway were randomly divided into eight groups and received lesions only or injections of 900,000 embryonic rat ventral mesencephalic cells in the (1) ST, (2) SN, (3) STN, (4) ST and SN, (5) ST, SN, and STN, (6) ST and STN, or (7) SN and STN. The number of cells transplanted was equally divided among grafting sites. Animals with two grafts received 450,000 cells in each structure, and animals with three grafts received 300,000 cells per structure. Recovery was assessed by amphetamine-induced rotations and the stepping tests. Graft survival was assessed using tyrosine hydroxylase immunohistochemistry. At 8 weeks after transplantation, simultaneous dopaminergic transplants in the ST, SN, and STN induced significant improvement in rotational behavior and stepping test scores. Intrastriatal transplants were associated with significant recovery of rotational asymmetry, whereas SN and STN transplants were associated with improved forelimb function scores. These results suggest that restoration of dopaminergic activity to multiple basal ganglia targets, such as the ST and SN, or the ST and STN, promotes a more complete functional recovery of complex sensorimotor behaviors. A multitarget transplant strategy aimed at optimizing dopaminergic reinnervation of the basal ganglia may be crucial in improving clinical outcomes in PD patients.

Neural transplantation in spinal cord injury

Although medical advancements have significantly increased the survival of spinal cord injury patients, restoration of function has not yet been achieved. Neural transplantation has been studied over the past decade in animal models as a repair strategy for spinal cord injury. Although spinal cord neural transplantation has yet to reach the point of clinical application and much work remains to be done, reconstructive strategies offer the greatest hope for the treatment of spinal cord injury in the future. This article presents the scientific basis of neural transplantation as a repair strategy and reviews the current status of neural transplantation in spinal cord injury.

Anatomic basis of sequence-dependent predictability exhibited by nigral dopamine neuron firing patterns

Interspike intervals (ISIs) of dopamine (DA) neurons recorded in the substantia nigra are predicted partially by their immediate prior history. This study was designed to assess neuroanatomic origins of these sequential relationships. ISI data recorded from three groups of nigral DA neurons were studied: 1) 16 neurons recorded in unlesioned animals, 2) 14 neurons recorded after forebrain hemisection, 3) 12 neurons recorded after partial forebrain hemisection that reproduced nonspecific effects of the surgical lesion while leaving forebrain connections intact. As predicted, DA neurons recorded after full forebrain hemisection yielded statistically significant reductions in sequential predictability relative to control neurons and neurons recorded following partial hemisection. These data support the hypothesis that the sequence-dependent behavior of DA neurons arise in part from interactions with forebrain structures. ISI sequences recorded from unlesioned rats demonstrated maximum predictability when an average of 3.7 prior events were incorporated into the forecasting algorithm, thereby suggesting a physiological process whose "depth" of history-dependence is approximately 600-800 msec. Additional studies examining the functional significance of sequence-dependent ISI structure exhibited by nigral DA neurons are indicated.

Intranigral transplantation of solid tissue ventral mesencephalon or striatal grafts induces behavioral recovery in 6-OHDA-lesioned rats

Parkinson's disease (PD) is characterized by a degeneration of the dopamine (DA) pathway from the substantia nigra (SN) to the basal forebrain. Prior studies in unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats have primarily concentrated on the implantation of fetal ventral mesencephalon (VM) into the striatum in attempts to restore DA function in the target. We implanted solid blocks of fetal VM or fetal striatal tissue into the SN to investigate whether intra-nigral grafts would restore motor function in unilaterally 6-OHDA-lesioned rats. Intra-nigral fetal striatal and VM grafts elicited a significant and long-lasting reduction in apomorphine-induced rotational behavior. Lesioned animals with ectopic grafts or sham surgery as well as animals that received intra-nigral grafts of fetal cerebellar cortex showed no recovery of motor symmetry. Subsequent immunohistochemical studies demonstrated that VM grafts, but not cerebellar grafted tissue expressed tyrosine hydroxylase (TH)-positive cell bodies and were associated with the innervation by TH-positive fibers into the lesioned SN as well as adjacent brain areas. Striatal grafts were also associated with the expression of TH-positive cell bodies and fibers extending into the lesioned SN and an induction of TH-immunolabeling in endogenous SN cell bodies. This finding suggests that trophic influences of transplanted fetal striatal tissue can stimulate the re-expression of dopaminergic phenotype in SN neurons following a 6-OHDA lesion. Our data support the hypothesis that a dopaminergic re-innervation of the SN and surrounding tissue by a single solid tissue graft is sufficient to improve motor asymmetry in unilateral 6-OHDA-lesioned rats.

Intrastriatal and intranigral grafting of hNT neurons in the 6-OHDA rat model of Parkinson's disease

The clinical findings on neural transplantation for Parkinson's disease (PD) reported thus far are promising but many issues must be addressed before neural transplantation can be considered a routine therapeutic option for PD. The future of neural transplantation for the treatment of neurological disorders may rest in the discovery of a suitable alternative cell type for fetal tissue. One such alternative may be neurons derived from a human teratocarcinoma (hNT). hNT neurons have been shown to survive and integrate within the host brain following transplantation and provide functional recovery in animal models of stroke and Huntington's disease. In this study, we describe the transplantation of hNT neurons in the substantia nigra (SN) and striatum of the rat model for PD. Twenty-seven rats were grafted with one of three hNT neuronal products; hNT neurons, hNT-DA neurons, or lithium chloride (LiCl) pretreated hNT-DA neurons. Robust hNT grafts could be seen with anti-neural cell adhesion molecule and anti-neuron-specific enolase immunostaining. Immunostaining for tyrosine hydroxylase (TH) expression revealed no TH-immunoreactive (THir) neurons in any animals with hNT neuronal grafts. THir cells were observed in 43% of animals with hNT-DA neuronal grafts and all animals with LiCl pretreated hNT-DA neuronal grafts (100%). The number of THir neurons in these animals was low and not sufficient to produce significant functional recovery. In summary, this study has demonstrated that hNT neurons survive transplantation and express TH in the striatum and SN. Although hNT neurons are promising as an alternative to fetal tissue and may have potential clinical applications in the future, further improvements in enhancing TH expression are needed.

Simultaneous intrastriatal and intranigral grafting (double grafts) in the rat model of Parkinson's disease

Experimental and clinical studies of neural transplantation in Parkinson's disease have focused on the placement of fetal dopaminergic grafts not in their ontogenic site (substantia nigra) but in the main nigral target area (striatum). The reason for this is the apparent inability of intranigral nigral grafts to extend axons for long distances reinnervating the ipsilateral striatum. This review presents previous work by our laboratory [I. Mendez, M. Hong, Reconstruction of the striato-nigro-striatal circuitry by simultaneous double dopaminergic grafts: a tracer study using fluorogold and horseradish peroxidase, Brain Res. 778 (1997) 194-205; I. Mendez, D. Sadi, M. Hong., Reconstruction of the nigrostriatal pathway by simultaneous intrastriatal and intranigral dopaminergic transplants, J. Neurosci. 16 (1996) 7216-7227] using a new transplantation strategy aimed at restoring dopaminergic innervation of the nigra and striatum by simultaneous dopaminergic transplants placed in the substantia nigra and ipsilateral striatum (double grafts) in the 6-hydroxydopamine lesioned adult rat brain. These double grafts achieve not only greater striatal reinnervation than the standard intrastriatal grafts but also produce a faster and more complete behavioural recovery six weeks after transplantation. Injection of the retrograde tracer fluorogold into the striatum and nigra resulted in fluorescent labeled cells within the intranigral graft and the intrastriatal graft and surrounding striatum, respectively suggesting that these double grafts promote at least partial reconstruction of the nigrostriatal dopaminergic pathway. This double graft strategy may have potential implications in clinical neural transplantation for Parkinson's disease.