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

Human iPSC-derived midbrain organoids functionally integrate into striatum circuits and restore motor function in a mouse model of Parkinson's disease

Rationale: Parkinson's disease (PD) is a prevalent neurodegenerative disorder that is characterized by degeneration of dopaminergic neurons (DA) at the substantia nigra pas compacta (SNpc). Cell therapy has been proposed as a potential treatment option for PD, with the aim of replenishing the lost DA neurons and restoring motor function. Fetal ventral mesencephalon tissues (fVM) and stem cell-derived DA precursors cultured in 2-dimentional (2-D) culture conditions have shown promising therapeutic outcomes in animal models and clinical trials. Recently, human induced pluripotent stem cells (hiPSC)-derived human midbrain organoids (hMOs) cultured in 3-dimentional (3-D) culture conditions have emerged as a novel source of graft that combines the strengths of fVM tissues and 2-D DA cells. Methods: 3-D hMOs were induced from three distinct hiPSC lines. hMOs at various stages of differentiation were transplanted as tissue pieces into the striatum of naïve immunodeficient mouse brains, with the aim of identifying the most suitable stage of hMOs for cellular therapy. The hMOs at Day 15 were determined to be the most appropriate stage and were transplanted into a PD mouse model to assess cell survival, differentiation, and axonal innervation in vivo. Behavioral tests were conducted to evaluate functional restoration following hMO treatment and to compare the therapeutic effects between 2-D and 3-D cultures. Rabies virus were introduced to identify the host presynaptic input onto the transplanted cells. Results: hMOs showed a relatively homogeneous cell composition, mostly consisting of dopaminergic cells of midbrain lineage. Analysis conducted 12 weeks post-transplantation of day 15 hMOs revealed that 14.11% of the engrafted cells expressed TH+ and over 90% of these cells were co-labeled with GIRK2+, indicating the survival and maturation of A9 mDA neurons in the striatum of PD mice. Transplantation of hMOs led to a reversal of motor function and establishment of bidirectional connections with natural brain target regions, without any incidence of tumor formation or graft overgrowth. Conclusion: The findings of this study highlight the potential of hMOs as safe and efficacious donor graft sources for cell therapy to treat PD.

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.

Restoring lost nigrostriatal fibers in Parkinson's disease based on clinically-inspired design criteria

Parkinson's disease is a neurodegenerative disease affecting around 10 million people worldwide. The death of dopaminergic neurons in the substantia nigra and the axonal fibers that constitute the nigrostriatal pathway leads to a loss of dopamine in the striatum that causes the motor symptoms of this disease. Traditional treatments have focused on reducing symptoms, while therapies with human fetal or stem cell-derived neurons have centered on implanting these cells in the striatum to restore its innervation. An alternative approach is pathway reconstruction, which aims to rebuild the entire structure of neurons and axonal fibers of the nigrostriatal pathway in a way that matches its anatomy and physiology. This type of repair could be more capable of reestablishing the signaling mechanisms that ensure proper dopamine release in the striatum and regulation of other motor circuit regions in the brain. In this manuscript, we conduct a review of the literature related to pathway reconstruction as a treatment for Parkinson's disease, delve into the limitations of these studies, and propose the requisite design criteria to achieve this goal at a human scale. We then present our tissue engineering-based platform to fabricate hydrogel-encased dopaminergic axon tracts in vitro for later implantation into the brain to replace and reconstruct the pathway. These tissue-engineered nigrostriatal pathways (TE-NSPs) can be characterized and optimized for cell number and phenotype, axon growth lengths and rates, and the capacity for synaptic connectivity and dopamine release. We then show original data of advances in creating these constructs matching clinical design criteria using human iPSC-derived dopaminergic neurons and a hyaluronic acid hydrogel. We conclude with a discussion of future steps that are needed to further optimize human-scale TE-NSPs and translate them into clinical products.

VGluT2 Expression in Dopamine Neurons Contributes to Postlesional Striatal Reinnervation

A subset of adult ventral tegmental area dopamine (DA) neurons expresses vesicular glutamate transporter 2 (VGluT2) and releases glutamate as a second neurotransmitter in the striatum, while only few adult substantia nigra DA neurons have this capacity. Recent work showed that cellular stress created by neurotoxins such as MPTP and 6-hydroxydopamine can upregulate VGluT2 in surviving DA neurons, suggesting the possibility of a role in cell survival, although a high level of overexpression could be toxic to DA neurons. Here we examined the level of VGluT2 upregulation in response to neurotoxins and its impact on postlesional plasticity. We first took advantage of an in vitro neurotoxin model of Parkinson's disease and found that this caused an average 2.5-fold enhancement of Vglut2 mRNA in DA neurons. This could represent a reactivation of a developmental phenotype because using an intersectional genetic lineage-mapping approach, we find that >98% of DA neurons have a VGluT2+ lineage. Expression of VGluT2 was detectable in most DA neurons at embryonic day 11.5 and was localized in developing axons. Finally, compatible with the possibility that enhanced VGluT2 expression in DA neurons promotes axonal outgrowth and reinnervation in the postlesional brain, we observed that DA neurons in female and male mice in which VGluT2 was conditionally removed established fewer striatal connections 7 weeks after a neurotoxin lesion. Thus, we propose here that the developmental expression of VGluT2 in DA neurons can be reactivated at postnatal stages, contributing to postlesional plasticity of dopaminergic axons.SIGNIFICANCE STATEMENT A small subset of dopamine neurons in the adult, healthy brain expresses vesicular glutamate transporter 2 (VGluT2) and thus releases glutamate as a second neurotransmitter in the striatum. This neurochemical phenotype appears to be plastic as exposure to neurotoxins, such as 6-OHDA or MPTP, that model certain aspects of Parkinson's disease pathophysiology, boosts VGluT2 expression in surviving dopamine neurons. Here we show that this enhanced VGluT2 expression in dopamine neurons drives axonal outgrowth and contributes to dopamine neuron axonal plasticity in the postlesional brain. A better understanding of the neurochemical changes that occur during the progression of Parkinson's disease pathology will aid the development of novel therapeutic strategies for this disease.

Pluripotent Stem Cell Therapies for Parkinson Disease: Present Challenges and Future Opportunities

In Parkinson's disease (PD), there are currently no effective therapies to prevent or slow down disease progression. Cell replacement therapy using human pluripotent stem cell (hPSC)-derived dopamine neurons holds considerable promise. It presents a novel, regenerative strategy, building on the extensive history of fetal tissue grafts and capturing the potential of hPSCs to serve as a scalable and standardized cell source. Progress in establishing protocols for the direct differentiation to midbrain dopamine (mDA) neurons from hPSC have catalyzed the development of cell-based therapies for PD. Consequently, several groups have derived clinical-grade mDA neuron precursors under clinical good manufacture practice condition, which are progressing toward clinical testing in PD patients. Here we will review the current status of the field, discuss the remaining key challenges, and highlight future areas for further improvements of hPSC-based technologies in the clinical translation to PD.

Viral Delivery of GDNF Promotes Functional Integration of Human Stem Cell Grafts in Parkinson's Disease

Dopaminergic neurons (DAns), generated from human pluripotent stem cells (hPSCs), are capable of functionally integrating following transplantation and have recently advanced to clinical trials for Parkinson's disease (PD). However, pre-clinical studies have highlighted the low proportion of DAns within hPSC-derived grafts and their inferior plasticity compared to fetal tissue. Here, we examined whether delivery of a developmentally critical protein, glial cell line-derived neurotrophic factor (GDNF), could improve graft outcomes. We tracked the response of DAns implanted into either a GDNF-rich environment or after a delay in exposure. Early GDNF promoted survival and plasticity of non-DAns, leading to enhanced motor recovery in PD rats. Delayed exposure to GDNF promoted functional recovery through increases in DAn specification, DAn plasticity, and DA metabolism. Transcriptional profiling revealed a role for mitogen-activated protein kinase (MAPK)-signaling downstream of GDNF. Collectively, these results demonstrate the potential of neurotrophic gene therapy strategies to improve hPSC graft outcomes.

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.

Partial Reconstruction of the Nigrostriatal Circuit along a Preformed Molecular Guidance Pathway

The overall goal of our research is to establish a preformed molecular guidance pathway to direct the growth of dopaminergic axons from embryonic ventral mesencephalon (VM), tissue placed within the substantia nigra (SN), into the striatum to reconstruct the nigrostriatal pathway in a hemi-Parkinson's disease rat model. Guidance pathways were prepared by injecting lentivirus encoding either GFP or a combination of glial-cell-line-derived neurotrophic factor (GDNF) with either GDNF family receptor α1 (GFRα1) or netrin1. In another cohort of animals, adeno-associated virus (AAV) encoding brain-derived neurotrophic factor (BDNF) was injected within the striatum after guidance pathway formation. GDNF combined with either GFRα1 or netrin significantly increased growth of dopaminergic axons out of transplants and along the pathway, resulting in a significant reduction in the number of amphetamine-induced rotations. Retrograde tract tracing showed that the dopaminergic axons innervating the striatum were from A9 neurons within the transplant. Increased dopaminergic innervation of the striatum and improved behavioral recovery were observed with the addition of BDNF. Preformed guidance pathways using a combination of GDNF and netrin1 can be used to reconstruct the nigrostriatal pathway and improve motor recovery.

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.

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.

Vesicular expression and release of ATP from dopaminergic neurons of the mouse retina and midbrain

Vesicular nucleotide transporter (VNUT) is required for active accumulation of adenosine tri-phosphate (ATP) into vesicles for purinergic neurotransmission, however, the cell types that express VNUT in the central nervous system remain unknown. This study characterized VNUT expression within the mammalian retina and brain and assessed a possible functional role in purinergic signaling. Two native isoforms of VNUT were detected in mouse retina and brain based on RNA transcript and protein analysis. Using immunohistochemistry, VNUT was found to co-localize with tyrosine hydroxylase (TH) positive, dopaminergic (DA) neurons of the substantia nigra and ventral tegmental area, however, VNUT expression in extranigral non-DA neurons was also observed. In the retina, VNUT labeling was found to co-localize solely with TH-positive DA-cells. In the outer retina, VNUT-positive interplexiform cell processes were in close contact with horizontal cells and cone photoreceptor terminals, which are known to express P2 purinergic-receptors. In order to assess function, dissociated retinal neurons were loaded with fluorescent ATP markers (Quinacrine or Mant-ATP) and the DA marker FFN102, co-labeled with a VNUT antibody and imaged in real time. Fluorescent ATP markers and FFN102 puncta were found to co-localize in VNUT positive neurons and upon stimulation with high potassium, ATP marker fluorescence at the cell membrane was reduced. This response was blocked in the presence of cadmium. These data suggest DA neurons co-release ATP via calcium dependent exocytosis and in the retina this may modulate the visual response by activating purine receptors on closely associated neurons.

Human neural stem cells survive long term in the midbrain of dopamine-depleted monkeys after GDNF overexpression and project neurites toward an appropriate target

Transplanted multipotent human fetal neural stem cells (hfNSCs) significantly improved the function of parkinsonian monkeys in a prior study primarily by neuroprotection, with only 3%-5% of cells expressing a dopamine (DA) phenotype. In this paper, we sought to determine whether further manipulation of the neural microenvironment by overexpression of a developmentally critical molecule, glial cell-derived neurotrophic factor (GDNF), in the host striatum could enhance DA differentiation of hfNSCs injected into the substantia nigra and elicit growth of their axons to the GDNF-expressing target. hfNSCs were transplanted into the midbrain of 10 green monkeys exposed to 1-methyl-4-phenyl-1,2,3,6-tetrahydro-pyridine. GDNF was delivered concomitantly to the striatum via an adeno-associated virus serotype 5 vector, and the fate of grafted cells was assessed after 11 months. Donor cells remained predominantly within the midbrain at the injection site and sprouted numerous neurofilament-immunoreactive fibers that appeared to course rostrally toward the striatum in parallel with tyrosine hydroxylase-immunoreactive fibers from the host substantia nigra but did not mature into DA neurons. This work suggests that hfNSCs can generate neurons that project long fibers in the adult primate brain. However, in the absence of region-specific signals and despite GDNF overexpression, hfNSCs did not differentiate into mature DA neurons in large numbers. It is encouraging, however, that the adult primate brain appeared to retain axonal guidance cues. We believe that transplantation of stem cells, specifically instructed ex vivo to yield DA neurons, could lead to reconstruction of some portion of the nigrostriatal pathway and prove beneficial for the parkinsonian condition.

Characterization of the stability and bio-functionality of tethered proteins on bioengineered scaffolds: implications for stem cell biology and tissue repair

Various engineering applications have been utilized to deliver molecules and compounds in both innate and biological settings. In the context of biological applications, the timely delivery of molecules can be critical for cellular and organ function. As such, previous studies have demonstrated the superiority of long-term protein delivery, by way of protein tethering onto bioengineered scaffolds, compared with conventional delivery of soluble protein in vitro and in vivo. Despite such benefits little knowledge exists regarding the stability, release kinetics, longevity, activation of intracellular pathway, and functionality of these proteins over time. By way of example, here we examined the stability, degradation and functionality of a protein, glial-derived neurotrophic factor (GDNF), which is known to influence neuronal survival, differentiation, and neurite morphogenesis. Enzyme-linked immunosorbent assays (ELISA) revealed that GDNF, covalently tethered onto polycaprolactone (PCL) electrospun nanofibrous scaffolds, remained present on the scaffold surface for 120 days, with no evidence of protein leaching or degradation. The tethered GDNF protein remained functional and capable of activating downstream signaling cascades, as revealed by its capacity to phosphorylate intracellular Erk in a neural cell line. Furthermore, immobilization of GDNF protein promoted cell survival and differentiation in culture at both 3 and 7 days, further validating prolonged functionality of the protein, well beyond the minutes to hours timeframe observed for soluble proteins under the same culture conditions. This study provides important evidence of the stability and functionality kinetics of tethered molecules.

Development of a stereotaxic device for low impact implantation of neural constructs or pieces of neural tissues into the mammalian brain

Implanting pieces of tissue or scaffolding material into the mammalian central nervous system (CNS) is wrought with difficulties surrounding the size of tools needed to conduct such implants and the ability to maintain the orientation and integrity of the constructs during and after their transplantation. Here, novel technology has been developed that allows for the implantation of neural constructs or intact pieces of neural tissue into the CNS with low trauma. By “laying out” (instead of forcibly expelling) the implantable material from a thin walled glass capillary, this technology has the potential to enhance neural transplantation procedures by reducing trauma to the host brain during implantation and allowing for the implantation of engineered/dissected tissues or constructs in such a way that their orientation and integrity are maintained in the host. Such technology may be useful for treating various CNS disorders which require the reestablishment of point-to-point contacts (e.g., Parkinson's disease) across the adult CNS, an environment which is not normally permissive to axonal growth.

Enhancement of polysialic acid expression improves function of embryonic stem-derived dopamine neuron grafts in Parkinsonian mice

There has been considerable progress in obtaining engraftable embryonic stem (ES) cell-derived midbrain dopamine neurons for cell replacement therapy in models of Parkinson's disease; however, limited integration and striatal reinnervation of ES-derived grafts remain a major challenge for future clinical translation. In this paper, we show that enhanced expression of polysialic acid results in improved graft efficiency in correcting behavioral deficits in Parkinsonian mice. This result is accompanied by two potentially relevant cellular changes: greater survival of transplanted ES-derived dopamine neurons and robust sprouting of tyrosine hydroxylase-positive processes into host tissue. Because the procedures used to enhance polysialic acid are easily translated to other cell types and species, this approach may represent a general strategy to improve graft integration in cell-based therapies.

Long distance directional growth of dopaminergic axons along pathways of netrin-1 and GDNF

Different experimental and clinical strategies have been used to promote survival of transplanted embryonic ventral mesencephalic (VM) neurons. However, few studies have focused on the long-distance growth of dopaminergic axons from VM transplants. The aim of this study is to identify some of the growth and guidance factors that support directed long-distance growth of dopaminergic axons from VM transplants. Lentivirus encoding either glial cell line-derived neurotrophic factor (GDNF) or netrin-1, or a combination of lenti-GDNF with either lenti-GDNF family receptor α1 (GFRα-1) or lenti-netrin-1 was injected to form a gradient along the corpus callosum. Two weeks later, a piece of embryonic day 14 VM tissue was transplanted into the corpus callosum adjacent to the low end of the gradient. Results showed that tyrosine hydroxylase (TH(+)) axons grew a very short distance from the VM transplants in control groups, with few axons reaching the midline. In GDNF or netrin-1 expressing groups, more TH(+) axons grew out of transplants and reached the midline. Pathways co-expressing GDNF with either GFRα-1 or netrin-1 showed significantly increased axonal outgrowth. Interestingly, only the GDNF/netrin-1 combination resulted in the majority of axons reaching the distal target (80%), whereas along the GDNF/GFRα-1 pathway only 20% of the axons leaving the transplant reached the distal target. This technique of long-distance axon guidance may prove to be a useful strategy in reconstructing damaged neuronal circuits, such as the nigrostriatal pathway in Parkinson's disease.

Comparison of fetal mesencephalic grafts, AAV-delivered GDNF, and both combined in an MPTP-induced nonhuman primate Parkinson's model

We combined viral vector delivery of human glial-derived neurotrophic factor (GDNF) with the grafting of dopamine (DA) precursor cells from fetal ventral mesencephalon (VM) to determine whether these strategies would improve the anti-Parkinson's effects in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys, an animal model for Parkinson's disease (PD). Both strategies have been reported as individually beneficial in animal models of PD, leading to clinical studies. GDNF delivery has also been reported to augment VM tissue implants, but no combined studies have been done in monkeys. Monkeys were treated with MPTP and placed into four balanced treatment groups receiving only recombinant adeno-associated virus serotype 5 (rAAV5)/hu-GDNF, only fetal DA precursor cells, both together, or a buffered saline solution (control). The combination of fetal precursors with rAAV5/hu-GDNF showed significantly higher striatal DA concentrations compared with the other treatments, but did not lead to greater functional improvement in this study. For the first time under identical conditions in primates, we show that all three treatments lead to improvement compared with control animals.

Midbrain dopaminergic neurons: a review of the molecular circuitry that regulates their development

Dopaminergic (DA) neurons of the ventral midbrain (VM) play vital roles in the regulation of voluntary movement, emotion and reward. They are divided into the A8, A9 and A10 subgroups. The development of the A9 group of DA neurons is an area of intense investigation to aid the generation of these neurons from stem cell sources for cell transplantation approaches to Parkinson's disease (PD). This review discusses the molecular processes that are involved in the identity, specification, maturation, target innervation and survival of VM DA neurons during development. The complex molecular interactions of a number of genetic pathways are outlined, as well as recent advances in the mechanisms that regulate subset identity within the VM DA neuronal pool. A thorough understanding of the cellular and molecular mechanisms involved in the development of VM DA neurons will greatly facilitate the use of cell replacement therapy for the treatment of PD.

Cell intrinsic and extrinsic factors contribute to enhance neural circuit reconstruction following transplantation in Parkinsonian mice

Cell replacement therapy for Parkinson's disease has predominantly focused on ectopic transplantation of fetal dopamine (DA) neurons into the striatum as a means to restore neurotransmission, rather than homotopic grafts into the site of cell loss, which would require extensive axonal growth. However, ectopic grafts fail to restore important aspects of DA circuitry necessary for controlled basal ganglia output, and this may underlie the suboptimal and variable functional outcomes in patients. We recently showed that DA neurons in homotopic allografts of embryonic ventral mesencephalon (VM) can send long axonal projections along the nigrostriatal pathway in order to innervate forebrain targets, although the extent of striatal reinnervation remains substantially less than can be achieved with ectopic placement directly into the striatal target. Here, we examined the possible benefits of using younger VM donor tissue and over-expression of glial cell-derived neurotrophic factor (GDNF) in the striatal target to improve the degree of striatal innervation from homotopic grafts. Younger donor tissue, collected on embryonic day (E)10, generated 4-fold larger grafts with greater striatal targeting, compared to grafts generated from more conventional E12 donor VM. Over-expression of GDNF in the host brain also significantly increased DA axonal growth and striatal innervation. Furthermore, a notable increase in the number and proportion of A9 DA neurons, essential for functional recovery, was observed in younger donor grafts treated with GDNF. Behavioural testing confirmed functional integration of younger donor tissue and demonstrated that improved motor function could be attributed to both local midbrain and striatal innervation. Together, these findings suggest there is significant scope for further development of intra-nigral grafting as a restorative approach for Parkinson's disease.

Survival and functional restoration of human fetal ventral mesencephalon following transplantation in a rat model of Parkinson's disease

Cell replacement therapy by intracerebral transplantation of fetal dopaminergic neurons has become a promising therapeutic option for patients suffering from Parkinson's disease during the last decades. However, limited availability of human fetal tissue as well as ethical issues, lack of alternative nonfetal donor cells, and the absence of standardized transplantation protocols have prevented neurorestorative therapies from becoming a routine procedure in patients suffering from neurodegenerative diseases. Improvement of graft survival, surgery techniques, and identification of the optimal target area are imperative for further optimization of this novel treatment. In the present study, human primary fetal ventral mesencephalon-derived tissue from 7- to 9-week-old human fetuses was transplanted into 6-hydroxydopamine-lesioned adult Sprague-Dawley rats. Graft survival, fiber outgrowth, and drug-induced rotational behavior up to 14 weeks posttransplantation were compared between different intrastriatal transplantation techniques (full single cell suspension vs. partial tissue pieces suspension injected by glass capillary or metal cannula) and the intranigral glass capillary injection of a full (single cell) suspension. The results demonstrate a higher survival rate of dopamine neurons, a greater reduction in amphetamine-induced rotations (overcompensation), and more extensive fiber outgrowth for the intrastriatally transplanted partial (tissue pieces) suspension compared to all other groups. Apomorphine-induced rotational bias was significantly reduced in all groups including the intranigral group. The data confirm that human ventral mesencephalon-derived cells serve as a viable cell source, survive in a xenografting paradigm, and functionally integrate into the host tissue. In contrast to rat donor cells, keeping the original (fetal) neuronal network by preparing only a partial suspension containing tissue pieces seems to be beneficial for human cells, although a metal cannula that causes greater tissue trauma to the host is required for injection. In addition, homotopic intranigral grafts may represent a complimentary grafting approach to the "classical" ectopic intrastriatal target site in PD.

Functional multipotency of stem cells: a conceptual review of neurotrophic factor-based evidence and its role in translational research

We here propose an updated concept of stem cells (SCs), with an emphasis on neural stem cells (NSCs). The conventional view, which has touched principally on the essential property of lineage multipotency (e.g., the ability of NSCs to differentiate into all neural cells), should be broadened to include the emerging recognition of biofunctional multipotency of SCs to mediate systemic homeostasis, evidenced in NSCs in particular by the secretion of neurotrophic factors. Under this new conceptual context and taking the NSC as a leading example, one may begin to appreciate and seek the “logic” behind the wide range of molecular tactics the NSC appears to serve at successive developmental stages as it integrates into and prepares, modifies, and guides the surrounding CNS micro- and macro-environment towards the formation and self-maintenance of a functioning adult nervous system. We suggest that embracing this view of the “multipotency” of the SCs is pivotal for correctly, efficiently, and optimally exploiting stem cell biology for therapeutic applications, including reconstitution of a dysfunctional CNS.

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.

Medical Terminations of Pregnancy: A Viable Source of Tissue for Cell Replacement Therapy for Neurodegenerative Disorders

“Proof-of-principle” that cell replacement therapy works for neurodegeneration has been reported, but only using donor cells collected from fetal brain tissue obtained from surgical terminations of pregnancy. Surgical terminations of pregnancy represent an increasingly limited supply of donor cells due to the tendency towards performing medical termination in much of Europe. This imposes a severe constraint on further experimental and clinical cell transplantation research. Therefore, we explore here the feasibility of using medical termination tissue as a donor source. Products of conception were retrieved from surgical terminations over the last 7 years and from medical terminations over the last 2.5 years. The number of collections that yielded fetal tissue, viable brain tissue, and identifiable brain regions (ganglionic eminence, ventral mesencephalon, and neocortex) were recorded. We studied cell viability, cell physiological properties, and differentiation potential both in vitro and following transplantation into the central nervous system of rodent models of neurodegenerative disease. Within equivalent periods, we were able to collect substantially greater numbers of fetal remains from medical than from surgical terminations of pregnancy, and the medical terminations yielded a much higher proportion of identifiable and dissectible brain tissue. Furthermore, we demonstrate that harvested cells retain the capacity to differentiate into neurons with characteristics appropriate to the region from which they are dissected. We show that, contrary to widespread assumption, medical termination of pregnancy-derived fetal brain cells represent a feasible and more readily available source of human fetal tissue for experimental cell transplantation with the potential for use in future clinical trials in human neurodegenerative disease.

The administration of BDNF and GDNF to the brain via PLGA microparticles patterned within a degradable PEG-based hydrogel: Protein distribution and the glial response

Tailored delivery of neurotrophic factors (NFs) is a critical challenge that continues to inhibit strategies for guidance of axonal growth in vivo. Of particular importance is the ability to recreate innervation of distant brain regions by transplant tissue, for instance rebuilding the nigrostriatal track, one focus in Parkinson's disease research. Many strategies have utilized polymer drug delivery to target NF release in space and time, but combinatorial approaches are needed to deliver multiple NFs at relevant therapeutic times and locations without toxic side effects. Here we engineered a paradigm of PLGA microparticles entrapped within a degradable PEG-based hydrogel device to locally release two different types of NFs with two different release profiles. Hydrogel/microparticle devices were developed and analyzed for their ability to release GDNF in the caudal area of the brain, near the substantia nigra, or BDNF in the rostral area, near the striatum. The devices delivered their respective NFs in a region localized to within 100 μm of the bridge, but not exclusively to the targeted rostral or caudal ends. BDNF was slowly released over a 56-day period, whereas a bolus of GDNF was released around 28 days. The timed delivery of NFs from implanted devices significantly reduced the microglial response relative to sham surgeries. Given the coordinated drug delivery ability and reduced localized inflammatory response, this multifaceted PEG hydrogel/PLGA microparticle strategy may be a useful tool for further development in combining tissue engineering and drug delivery, and recreating the nigrostriatal track.

Cellular repair in the parkinsonian nonhuman primate brain

Parkinson disease (PD) is a neurodegenerative disorder that provides a useful model for testing cell replacement strategies to rejuvenate the affected dopaminergic neural systems, which have been destroyed by aging and the disease. We first showed that grafts of fetal dopaminergic neurons can reverse parkinsonian motor deficits induced by the toxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), validating the feasibility of cellular repair in a primate nervous system. Subsequent clinical trials in Parkinson patients showed encouraging results, including long-term improvement of neurological signs and reduction of medications in some patients. However, many experienced little therapeutic benefit, and some recipients experienced dyskinesias, suggesting a lack of regulated control of the grafts. We have since attempted to improve cell replacements by placing grafts in their correct anatomical location in the substantia nigra and using strategies such as co-grafting fetal striatal tissue or growth factors into the physiologic striatal targets. Moreover, the use of fetal cells depends on a variable supply of donor material, making it difficult to standardize cell quality and quantity. Therefore, we have also explored possibilities of using human neural stem cells (hNSCs) to ameliorate parkinsonism in nonhuman primates with encouraging results. hNSCs implanted into the striatum showed a remarkable migratory ability and were found in the substantia nigra, where a small number appeared to differentiate into dopamine neurons. The majority became growth factor-producing glia that could provide beneficial effects on host dopamine neurons. Studies to determine the optimum stage of differentiation from embryonic stem cells and to derive useful cells from somatic cell sources are in progress.

Reconstruction and quantitative evaluation of dopaminergic innervation of striatal neurons in dissociated primary cultures

Repairing the nigrostriatal pathway is expected to become a future treatment strategy for Parkinson disease. Our aim is to establish an in vitro model for the quantitative analysis of the nigrostriatal projections of dopaminergic neurons using primary dissociated neruons. To form the mesencephalic cell region, mesencephalic cells derived from rat embryos were plated within an isolation wall, which was removed after cell adhesion to the coverslip. After incubation for 11 days, the dopaminergic neurites extending to the outside of the mesencephalic cell region were mainly axons. Treatment with glial cell line-derived neurotrophic factor for 11 days significantly promoted the outgrowth of dopaminergic axons from the mesencephalic cell region in a concentration-dependent manner. When striatal cells were plated outside the mesencephalic cell region, dopaminergic neurites were remarkably extended to the striatal cell region. Moreover, immunocytochemistry for tyrosine hydroxylase and synaptophysin revealed that dopaminergic axons formed synapses with striatal neurons. By contrast, spinal cells did not increase dopaminergic neurite outgrowth. These results indicate that the present method is valuable for evaluating nigrostriatal projections in vitro.

Cell Therapy From Bench to Bedside Translation in CNS Neurorestoratology Era

Recent advances in cell biology, neural injury and repair, and the progress towards development of neurorestorative interventions are the basis for increased optimism. Based on the complexity of the processes of demyelination and remyelination, degeneration and regeneration, damage and repair, functional loss and recovery, it would be expected that effective therapeutic approaches will require a combination of strategies encompassing neuroplasticity, immunomodulation, neuroprotection, neurorepair, neuroreplacement, and neuromodulation. Cell-based restorative treatment has become a new trend, and increasing data worldwide have strongly proven that it has a pivotal therapeutic value in CNS disease. Moreover, functional neurorestoration has been achieved to a certain extent in the CNS clinically. Up to now, the cells successfully used in preclinical experiments and/or clinical trial/treatment include fetal/embryonic brain and spinal cord tissue, stem cells (embryonic stem cells, neural stem/progenitor cells, hematopoietic stem cells, adipose-derived adult stem/precursor cells, skin-derived precursor, induced pluripotent stem cells), glial cells (Schwann cells, oligodendrocyte, olfactory ensheathing cells, astrocytes, microglia, tanycytes), neuronal cells (various phenotypic neurons and Purkinje cells), mesenchymal stromal cells originating from bone marrow, umbilical cord, and umbilical cord blood, epithelial cells derived from the layer of retina and amnion, menstrual blood-derived stem cells, Sertoli cells, and active macrophages, etc. Proof-of-concept indicates that we have now entered a new era in neurorestoratology.