Historical perspective of cell transplantation in Parkinson’s disease

Perinatal Tissue-Derived Stem Cells: An Emerging Therapeutic Strategy for Challenging Neurodegenerative Diseases

Over the past 20 years, stem cell therapy has been considered a promising option for treating numerous disorders, in particular, neurodegenerative disorders. Stem cells exert neuroprotective and neurodegenerative benefits through different mechanisms, such as the secretion of neurotrophic factors, cell replacement, the activation of endogenous stem cells, and decreased neuroinflammation. Several sources of stem cells have been proposed for transplantation and the restoration of damaged tissue. Over recent decades, intensive research has focused on gestational stem cells considered a novel resource for cell transplantation therapy. The present review provides an update on the recent preclinical/clinical applications of gestational stem cells for the treatment of protein-misfolding diseases including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS). However, further studies should be encouraged to translate this promising therapeutic approach into the clinical setting.

Embryoid Body Cells from Human Embryonic Stem Cells Overexpressing Dopaminergic Transcription Factors Survive and Initiate Neurogenesis via Neural Rosettes in the Substantia Nigra

Transplantation of immature dopaminergic neurons or neural precursors derived from embryonic stem cells (ESCs) into the substantia nigra pars compacta (SNpc) is a potential therapeutic approach for functional restitution of the nigrostriatal pathway in Parkinson's disease (PD). However, further studies are needed to understand the effects of the local microenvironment on the transplanted cells to improve survival and specific differentiation in situ. We have previously reported that the adult SNpc sustains a neurogenic microenvironment. Non-neuralized embryoid body cells (EBCs) from mouse ESCs (mESCs) overexpressing the dopaminergic transcription factor Lmx1a gave rise to many tyrosine hydroxylase (Th⁺) cells in the intact and damaged adult SNpc, although only for a short-term period. Here, we extended our study by transplanting EBCs from genetically engineered naive human ESC (hESC), overexpressing the dopaminergic transcription factors LMX1A, FOXA2, and OTX2 (hESC-LFO), in the SNpc. Unexpectedly, no graft survival was observed in wild-type hESC EBCs transplants, whereas hESC-LFO EBCs showed viability in the SNpc. Interestingly, neural rosettes, a developmental hallmark of neuroepithelial tissue, emerged at 7- and 15-days post-transplantation (dpt) from the hESC-LFO EBCs. Neural rosettes expressed specification dopaminergic markers (Lmx1a, Otx2), which gave rise to several Th⁺ cells at 30 dpt. Our results suggest that the SNpc enables the robust initiation of neural differentiation of transplanted human EBCs prompted to differentiate toward the midbrain dopaminergic phenotype.

Stem Cell-based and Advanced Therapeutic Modalities for Parkinson's Disease: A Risk-effectiveness Patient-centered Analysis

Treatment of Parkinson's disease (PD), the second most prevalent neurodegenerative disorder, is currently considered a challenging issue since it causes substantial disability, poor quality of life, and mortality. Despite remarkable progress in advanced conventional therapeutic interventions, the global burden of the disease has nearly doubled, prompting us to assess the riskeffectiveness of different treatment modalities. Each protocol could be considered as the best alternative treatment depending on the patient's situation. Prescription of levodopa, the most effective available medicine for this disorder, has been associated with many complications, i.e., multiple episodes of "off-time" and treatment resistance. Other medications, which are typically used in combination with levodopa, may have several adverse effects as well. As a result, the therapies that are more in line with human physiology and make the least interference with other pathways are worth investigating. On the other hand, remaining and persistent symptoms after therapy and the lack of effective response to the conventional approaches have raised expectations towards innovative alternative approaches, such as stem cell-based therapy. It is critical to not overlook the unexplored side effects of innovative approaches due to the limited number of research. In this review, we aimed to compare the efficacy and risk of advanced therapies with innovative cell-based and stemcell- based modalities in PD patients. This paper recapitulated the underlying factors/conditions, which could lead us to more practical and established therapeutic outcomes with more advantages and few complications. It could be an initial step to reconsider the therapeutic blueprint for patients with Parkinson's disease.

An alginate-based encapsulation system for delivery of therapeutic cells to the CNS

Treatment options for neurodegenerative conditions such as Parkinson's disease have included the delivery of cells which release dopamine or neurotrophic factors to the brain. Here, we report the development of a novel approach for protecting cells after implantation into the central nervous system (CNS), by developing dual-layer alginate beads that encapsulate therapeutic cells and release an immunomodulatory compound in a sustained manner. An optimal alginate formulation was selected with a view to providing a sustained physical barrier between engrafted cells and host tissue, enabling exchange of small molecules while blocking components of the host immune response. In addition, a potent immunosuppressant, FK506, was incorporated into the outer layer of alginate beads using electrosprayed poly-ε-caprolactone core–shell nanoparticles with prolonged release profiles. The stiffness, porosity, stability and ability of the alginate beads to support and protect encapsulated SH-SY5Y cells was demonstrated, and the release profile of FK506 and its effect on T-cell proliferation in vitro was characterized. Collectively, our results indicate this multi-layer encapsulation technology has the potential to be suitable for use in CNS cell delivery, to protect implanted cells from host immune responses whilst providing permeability to nutrients and released therapeutic molecules.

Novel composite cell encapsulation system: dual-layer, micro-scale beads maintain cell survival while releasing immunomodulatory FK506 in a sustained manner. This biotechnology platform could be applicable for treatment of CNS and other disorders.

New Horizons: Novel Adrenal Regenerative Therapies

Adrenal insufficiency requires lifelong corticoid replacement therapies. However, current therapies are not able to replace the physiological circadian pattern of the adrenal cortex and are associated with many metabolic, vascular, neuroendocrine, and mental perturbations. Therefore, regenerative and more curative strategies would be desirable. In the current perspective, we describe emerging new regenerative therapies for the treatment of adrenal insufficiency. In particular, we discuss gene therapy and cell replacement strategies. Furthermore, we discuss how adrenal cells might be used as a source for regenerative therapies of nonadrenal neurodegenerative diseases such as Parkinson disease.

Is the Immunological Response a Bottleneck for Cell Therapy in Neurodegenerative Diseases?

Neurodegenerative disorders such as Parkinson's (PD) and Huntington's disease (HD) are characterized by a selective detrimental impact on neurons in a specific brain area. Currently, these diseases have no cures, although some promising trials of therapies that may be able to slow the loss of brain cells are underway. Cell therapy is distinguished by its potential to replace cells to compensate for those lost to the degenerative process and has shown a great potential to replace degenerated neurons in animal models and in clinical trials in PD and HD patients. Fetal-derived neural progenitor cells, embryonic stem cells or induced pluripotent stem cells are the main cell sources that have been tested in cell therapy approaches. Furthermore, new strategies are emerging, such as the use of adult stem cells, encapsulated cell lines releasing trophic factors or cell-free products, containing an enriched secretome, which have shown beneficial preclinical outcomes. One of the major challenges for these potential new treatments is to overcome the host immune response to the transplanted cells. Immune rejection can cause significant alterations in transplanted and endogenous tissue and requires immunosuppressive drugs that may produce adverse effects. T-, B-lymphocytes and microglia have been recognized as the main effectors in striatal graft rejection. This review aims to summarize the preclinical and clinical studies of cell therapies in PD and HD. In addition, the precautions and strategies to ensure the highest quality of cell grafts, the lowest risk during transplantation and the reduction of a possible immune rejection will be outlined. Altogether, the wide-ranging possibilities of advanced therapy medicinal products (ATMPs) could make therapeutic treatment of these incurable diseases possible in the near future.

Peptide-Based Scaffolds for the Culture and Transplantation of Human Dopaminergic Neurons

Cell replacement therapy is a promising treatment strategy for Parkinson's disease (PD); however, the poor survival rate of transplanted neurons is a critical barrier to functional recovery. In this study, we used self-assembling peptide nanofiber scaffolds (SAPNS) based on the peptide RADA16-I to support the in vitro maturation and in vivo post-transplantation survival of encapsulated human dopaminergic (DA) neurons derived from induced pluripotent stem cells. Neurons encapsulated within the SAPNS expressed mature neuronal and midbrain DA markers and demonstrated in vitro functional activity similar to neurons cultured in two dimensions. A microfluidic droplet generation method was used to encapsulate cells within monodisperse SAPNS microspheres, which were subsequently used to transplant adherent, functional networks of DA neurons into the striatum of a 6-hydroxydopamine-lesioned PD mouse model. SAPNS microspheres significantly increased the in vivo survival of encapsulated neurons compared with neurons transplanted in suspension, and they enabled significant recovery in motor function compared with control lesioned mice using approximately an order of magnitude fewer neurons than have been previously needed to demonstrate behavioral recovery. These results indicate that such biomaterial scaffolds can be used as neuronal transplantation vehicles to successfully improve the outcome of cell replacement therapies for PD. Impact Statement Transplantation of dopaminergic (DA) neurons holds potential as a treatment for Parkinson's disease (PD), but low survival rates of transplanted neurons is a barrier to successfully improving motor function. In this study, we used hydrogel scaffolds to transplant DA neurons into PD model mice. The hydrogel scaffolds enhanced survival of the transplanted neurons compared with neurons that were transplanted in a conventional manner, and they also improved recovery of motor function by using significantly fewer neurons than have typically been transplanted to see functional benefits. This cell transplantation technology has the capability to improve the outcome of neuron transplantation therapies.

Replacing what's lost: a new era of stem cell therapy for Parkinson's disease

Background

Stem cells hold tremendous promise for regenerative medicine because they can be expanded infinitely, giving rise to large numbers of differentiated cells required for transplantation. Stem cells can be derived from fetal sources, embryonic origins (embryonic stem cells or ESCs) or reprogrammed from adult cell types (induced pluripotent stem cells or iPSCs). One unique property of stem cells is their ability to be directed towards specific cell types of clinical interest, and can mature into functional cell types in vivo. While transplantations of fetal or ESC-derived tissues are known to illicit a host immunogenic response, autologous transplantations using cell types derived from one’s own iPSCs eliminate risks of tissue rejection and reduce the need for immunosuppressants. However, even with these benefits, cell therapy comes with significant hurdles that researchers are starting to overcome. In this review, we will discuss the various steps to ensure safety, efficacy and clinical practicality of cell replacement therapy in neurodegenerative diseases, in particular, Parkinson’s disease.

Main body

Parkinson’s disease (PD) results from a loss of dopaminergic neurons from the substantia nigra and is an ideal target for cell replacement therapy. Early trials using fetal midbrain material in the late 1980s have resulted in long term benefit for some patients, but there were multiple shortcomings including the non-standardization and quality control of the transplanted fetal material, and graft-induced dyskinesia that some patients experience as a result. On the other hand, pluripotent stem cells such as ESCs and iPSCs serve as an attractive source of cells because they can be indefinitely cultured and is an unlimited source of cells. Stem cell technologies and our understanding of the developmental potential of ESCs and iPSCs have deepened in recent years and a clinical trial for iPSC-derived dopaminergic cells is currently undergoing for PD patients in Japan. In this focused review, we will first provide a historical aspect of cell therapies in PD, and then discuss the various challenges pertaining to the safety and efficacy of stem cell-based cell transplantations, and how these hurdles were eventually overcome.

Conclusion

With the maturity of the iPSC technology, cell transplantation appears to be a safe and effective therapy. Grafts in non-human primates survive and remain functional for more than 2 years after transplantation, with no signs of tumorigenesis, indicating safety and efficacy of the treatment. However, immunosuppressants are still required because of the lack of “universal stem cells” that would not evoke an immune response. The results of ongoing and upcoming trials by a global consortium known as GForce-PD would be highly anticipated because the success of these trials would open up possibilities for using cell therapy for the treatment of PD and other degenerative diseases.

The Substantia Nigra Is Permissive and Gains Inductive Signals When Lesioned for Dopaminergic Differentiation of Embryonic Stem Cells

Transplantation of dopaminergic (DA) cells into the striatum can rescue from dopamine deficiency in a Parkinson's disease condition, but this is not a suitable procedure for regaining the full control of motor activity. The minimal condition toward recovering the nigrostriatal pathway is the proper innervation of transplanted DA neurons or their precursors from the substancia nigra pars compacta (SNpc) to their target areas. However, functional integration of transplanted cells would require first that the host SNpc is suitable for their survival and/or differentiation. We recently reported that the intact adult SNpc holds a strong neurogenic environment, but primed embryonic stem cells (ie, embryoid body cells, EBCs) could not derive into DA neurons. In this study, we transplanted into the intact or lesioned SNpc, EBCs derived from embryonic stem cells that were prompt to differentiate into DA neurons by the forced expression of Lmx1a in neural precursor cells (R1B5/NesE-Lmx1a). We observed that, 6 days posttransplantation (dpt), R1B5 or R1B5/NesE-Lmx1a EBCs gave rise to Nes⁺ and Dcx⁺ cells within the host SNpc, but a large number of Th⁺ cells derived only from EBCs exogenously expressing Lmx1a. In contrast, when transplantation was carried out into the 6-hydroxidopamine-lesioned SNpc, the emergence of Th⁺ cells from EBCs was independent of exogenous Lmx1a expression, although these cells were not found by 15 dpt. These results suggest that the adult SNpc is not only a permissive niche for initiation of DA differentiation of non-neuralized cells but also releases factors upon damage that promote the acquisition of DA characteristics by transplanted EBCs.

Stem Cell Transplantation and Physical Exercise in Parkinson's Disease, a Literature Review of Human and Animal Studies

The absence of effective and satisfactory treatments that contribute to repairing the dopaminergic damage caused by Parkinson's Disease (PD) and the limited recovery capacity of the nervous system are troubling issues and the focus of many research and clinical domains. Recent advances in the treatment of PD through stem cell (SC) therapy have recognized their promising restorative and neuroprotective effects that are implicated in the potentiation of endogenous mechanisms of repair and contribute to functional locomotor improvement. Physical exercise (PE) has been considered an adjuvant intervention that by itself induces beneficial effects in patients and animal models with Parkinsonism. In this sense, the combination of both therapies could provide synergic or superior effects for motor recovery, in contrast with their individual use. This review aims to provide an update on recent progress and the potential effectiveness of SC transplantation and PE for the treatment of locomotor deficits in PD. It has reviewed the neuropathological pathways involved in the classical motor symptoms of this condition and the mechanisms of action described in experimental studies that are associated with locomotor enhancement through exercise, cellular transplantation, and their union in some neurodegenerative conditions.

Encapsulation of young donor age dopaminergic grafts in a GDNF-loaded collagen hydrogel further increases their survival, reinnervation, and functional efficacy after intrastriatal transplantation in hemi-Parkinsonian rats

Biomaterials have been shown to significantly improve the outcome of cellular reparative approaches for Parkinson's disease in experimental studies because of their ability to provide transplanted cells with a supportive microenvironment and shielding from the host immune system. However, given that the margin for improvement in such reparative therapies is considerable, further studies are required to fully investigate and harness the potential of biomaterials in this context. Given that several recent studies have demonstrated improved brain repair in Parkinsonian models when using dopaminergic grafts derived from younger foetal donors, we hypothesized that encapsulating these cells in a supportive biomaterial would further improve their reparative efficacy. Thus, this study aimed to determine the impact of a GDNF-loaded collagen hydrogel on the survival, reinnervation, and functional efficacy of dopaminergic neurons derived from young donors. To do so, hemi-Parkinsonian (6-hydroxydopamine-lesioned) rats received intrastriatal transplants of embryonic day 12 cells extracted from the rat ventral mesencephalon either alone, in a collagen hydrogel, with GDNF, or in a GDNF-loaded collagen hydrogel. Methamphetamine-induced rotational behaviour was assessed at three weekly intervals for a total of 12 weeks, after which rats were sacrificed for postmortem assessment of graft survival. We found that, following intrastriatal transplantation to the lesioned striatum, the GDNF-loaded collagen hydrogel significantly increased the survival (4-fold), reinnervation (5.4-fold), and functional efficacy of the embryonic day 12 dopaminergic neurons. In conclusion, this study further demonstrates the significant potential of biomaterial hydrogel scaffolds for cellular brain repair approaches in neurodegenerative diseases such as Parkinson's disease.