Pluripotent stem cell-based therapy for Parkinson's disease: Current status and future prospects

Abstract

Parkinson's disease (PD) is one of the most common neurodegenerative disorders, which affects about 0.3% of the general population. As the population in the developed world ages, this creates an escalating burden on society both in economic terms and in quality of life for these patients and for the families that support them. Although currently available pharmacological or surgical treatments may significantly improve the quality of life of many patients with PD, these are symptomatic treatments that do not slow or stop the progressive course of the disease. Because motor impairments in PD largely result from loss of midbrain dopamine neurons in the substantia nigra pars compacta, PD has long been considered to be one of the most promising target diseases for cell-based therapy. Indeed, numerous clinical and preclinical studies using fetal cell transplantation have provided proof of concept that cell replacement therapy may be a viable therapeutic approach for PD. However, the use of human fetal cells as a standardized therapeutic regimen has been fraught with fundamental ethical, practical, and clinical issues, prompting scientists to explore alternative cell sources. Based on groundbreaking establishments of human embryonic stem cells and induced pluripotent stem cells, these human pluripotent stem cells have been the subject of extensive research, leading to tremendous advancement in our understanding of these novel classes of stem cells and promising great potential for regenerative medicine. In this review, we discuss the prospects and challenges of human pluripotent stem cell-based cell therapy for PD.

Keywords: Induced pluripotent stem cell; Midbrain dopamine neuron; Parkinson’s disease; Personalized cell therapy; Pluripotent stem cell-based therapy; Transplantation.

Figure 1

Schematic overview of hiPSC-based personalized cell replacement therapy for PD. PD patient-derived somatic cells (e.g., skin fibroblasts or blood cells) are reprogrammed to autologous iPSCs, which are then differentiated to authentic mDA cell populations (e.g., mDA progenitor cells (PCs), mature mDA neurons, or their mixed populations) and transplanted to the PD patient’s brain.

Figure 2

Comparison of potential cell sources that can be used in cell replacement therapy for PD. The proof of concept of cell replacement therapy for PD was provided by implanting a heterogeneous population of fetal ventral mesencephalon (fVM) cells derived from 6 to 9 weeks old aborted embryos that consist of immature neural progenitors, neuroblasts, and neurons from which approximately 5% have the mDA phenotype. Their allogeneic characteristics makes the use of immune-suppressive regimen necessary. Pluripotent hESCs are derived from fertilized eggs or by somatic cell nuclear transfer (SCNT). In contrast, iPSCs can be derived from patient’s somatic tissues, allowing autologous cell transplantation after guided in vitro differentiation to mDA PCs and mDA neurons. Alternatively, autologous mDA cells can also be generated by direct conversion of somatic cells. These autologous cell grafts may avoid immunosuppression after transplantation. While autologous adult stem cells (see text for details) are potential cell sources, they are not included here because their potential to develop authentic mDA neurons are not clear. Autologous and allogeneic grafts are highlighted by blue and red lines, respectively.

Figure 3

Schematic diagrams of most representative in vitro differentiation protocols to generate mDA cells from hESCs and hiPSCs. Although these protocols show significant differences in specific chemicals, their concentrations, culture media and protocols, they commonly use chemicals/proteins for activation of the two regulatory loops (WNT1-LMX1A and SHH-FOXA2) and dual inhibition of SMADs (targeting BMP and TGFβ signaling). Kriks et al. induced direct differentiation of hESCs into mDA progenitors using dual-SMAD inhibitors (LDN and SB), SHH, FGF8, PMN and CHIR. On day 20, differentiating cells were dissociated by treatment with accutase and re-plated onto maturation media containing BDNF, GDNF, TGF-β3, DAPT, dbcAMP and AA (Kriks et al., 2011), in which cells were harvested at day 25 for transplantation. In contrast, Jun Takahashi and his colleagues used floating conditions, sorted cells at day 12 by Corin, and replated them in low cell adhesion 96 wells until Day28 for transplantation (Kikuchi et al., 2017a; Kikuchi et al., 2017b). Here, differentiation of hESCs/iPSCs was induced from day 1 to 12 using dual-SMAD inhibitors (LDN and A83-01), PMN, FGF8 and CHIR. The floating culture medium contained GDNF, BDNF, dbcAMP and AA. Next, Su-Chun Zhang and colleagues used direct differentiation at the starting point (D0-D8) and then cultured cells as floating aggregates until day 28 (Chen et al., 2016). Cells were cultured in neural induction condition with dual-SMAD inhibitors (DMH1 and SB), SHH and CHIR for induction of floor plate progenitors for 12 days. On day 8, cells were gently detached using a pipette and expanded as floating aggregates for 4 days in suspension condition. The spheres were triturated into smaller aggregates and differentiated in the presence of SAG, SHH and FGF8b until transplantation at day 32, when neurospheres were dissociated into single cells with accutase, and further differentiated in medium containing BDNF, GDNF, TGF-β3, cAMP, AA and compound E for in vitro study. Finally, Malin Parmar and colleagues induced hESCs differentiation using Noggin, SB, SHH and CHIR until day 9. Then, FGF8b was added until day 11 when cells were dissociated into single cells with accutase and replated with BDNF, FGF8b and AA until day 16 for transplantation. For terminal in vitro differentiation of the cells, dbcAMP and DAPT were added to medium from day 16 and onwards (Kirkeby et al., 2017a; Nolbrant et al., 2017). Numbers represent concentrations in ng/ml and those in parentheses represent concentrations in μM. Red dots represent the timing of cell transplantation. Yellow and green bars display adhesion and suspension (floating) culture conditions, respectively. AA, Ascorbic acid; BDNF, Brain-derived neurotrophic factor; CHIR, CHIR99021; dbcAMP, Dibutyryl cyclic adenosine monophosphate; F, Fibronectin; FP, Floor plate; GDNF, Glial cell line-derived neurotrophic factor; KSR, knockout serum replacement; LDN, LDN193189; PLO, Polyornithine; PMN, Purmorphamine; SAG, Smo agonist; SB, SB431542; SHH, Sonic Hedgehog; TGF-β3, transforming growth factor beta 3; Y, Y-27632

Figure 4

Steps of the clinical roadmap towards iPSC-based personalized cell replacement for PD. Detailed information is provided in the text.