Cell Replacement Therapy for Parkinson's Disease

How Close Are We to the Clinic?

Javier Ganz; Nirit Lev; Eldad Melamed; Daniel Offen

Expert Rev Neurother. 2011;11(9):1325-1339.

Mesenchymal Stem Cells

Adult stem cell populations isolated from various compartments in the mature organism can display an unexpected plasticity, a process previously thought to be an exclusive feature of ESCs. Subpopulations of adult stem cells are capable of differentiating into mature cells not related to their original lineage, a process termed 'transdifferentiation'. The most well-characterized adult stem cell population considered to possess a transdifferentiation capacity is bone marrow MSCs, also known as mesodermal stromal cells. Most studies on neuronal cell replacement involved the use of MSCs directly, while others differentiated them into neural cell population. In contrast to ESCs or NSCs, MSCs are easy to isolate, can be derived from the patient's own bone marrow and they represent a potential source for autologous cell transplantation, avoiding or reducing immunological rejections. MSCs usage for CRT circumvents the ethical problems concerning fetal tissue usage, thus making them attractive for regenerative medicine research. Despite initial skepticism regarding the capacity of MSCs to differentiate into neurons or glial cells, several studies have established the neurogenic predisposition of these cells as a result of the considerable repertoire of neural gene expression.^[78–81] Undifferentiated MSCs transplanted into the striatum of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD improved mice motor functions and showed double-stained BrdU⁺/TH⁺ cells, indicating that the BrdU-labeled MSCs developed into DA cells in vivo.^[82] Moreover, recent publications show that the differentiation process of neural-like cells induces the upregulation of genes involved in synaptic transmission, long-term potentialization^[78] and the differentiated cells exhibit typical neuronal electrophysiological traits.^[83–85] Rodent or human MSCs can be induced to differentiate into TH⁺ dopamine-producing cells that appear to be immature neuroblasts of DA lineage, which show weak electrical activity related to low expression levels of voltage-gated ion channels.^[86,87] Functional maturity of these induced MSCs improves when the expression of the RE-1 (REST) silencing factor is knocked down by siRNA or by exposure to BDNF.^[88,89] Differentiation of MSCs can be induced by exposure to soluble morphogenic proteins (FGFs and sonic hedgehog [SHH]),^[39,90] chemical inducers (retinoic acid [RA], dibutyryl cyclic AMP [dbcAMP], 3-isobutyl-1-methylxanthine [IBMX], ascorbic acid [AA] and butylatedhydroxyanisole [BHA])^[39,90] and also by lentiviral transduction of DA transcription factors (LMX1, NURR1 and PITX3).^[86] Transplantation of induced human MSCs into the striatum of 6-OHDA hemiparkinsonian rats, ameliorated behavioral parameters, showing a reduction of 58–80% in apomorphine-induced rotations measured 85, 99 and 125 days after transplantation.^[39]

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Table 1. Stem cells and differentiation protocols.
Stem cells	Differentiation factors	Graft survival (weeks)	Behavioral improvement	Ref.
NSC
Fetal progenitor cells	EGF, FGF-2	20	Yes	[24]
Fetal VM NSC	FGF-2, BDNF, GDNF, NT4/5, SHH	12	Yes	[25]
Fetal VM NSC	FGF-2, FGF-8b, SHH, tgWnt5a	11	Yes	[26]
Fetal VM NSC	FGF-2, FGF-8b	9	Yes	[27]
SVZ aNSC	FGF-8, SHH	ND	ND	[28]
Cograft VM explant/NSC	VM explant coculture, tgPitx3	ND	Yes	[10]
SVZ aNSC	NT-3, BDNF, tgNurr1, tgMash1	6	Yes	[29]
ESC
D3	In vivo differentiation	16	Yes	[9]
R1	FGF-2, FGF-8b, SHH, tgNurr1	8	Yes	[16]
CCE or EB5	PA6 stromal feeder	2	ND	[33]
R1, E14.1, B5	FGF-2, FGF-8b, SHH	ND	ND	[34]
WA09, WA01	FGF-2, FGF-8, SHH, coculture with telomerase-immortalized human fetal midbrain astrocytes	14	Yes	[35]
H1, H7	EGF, FGF-2, IGF-1, PDGF, NT-3, BDNF	ND	ND	[37]
H1, H9, HES-3, R366, Cyno1	FGF-8, SHH, BDNF, GDNF, TGF-β3, AA, dbcAMP, coculture with MS5 stromal cells (tgWnt1 and nontg)	ND	ND	[41]
SNUhES1, SNUhES2, SNUhES3	FGF-8, SHH, AA	ND	ND	[77]
MSC
BM MSC	FGF-2, EGF, BHA, DHA, dbcAMP, RA, IBMX	18	Yes	[18]
BM MSC	In vivo differentiation	4	Yes	[43]
BM MSC	FGF-2, FGF-8, SHH, tgLMX1a	ND	ND	[51]
BM MSC	SHH, FGF-2, FGF-8, BDNF	ND	ND	[54]
iPSC
O9	(OCT4, SOX2, c-MYC, KLF4); MEF feeder, FGF-2, FGF-8, SHH	4	Yes	[11]
Dermal fibroblasts from PD patients	(OCT4, SOX2, c-MYC, KLF4); MEF Feeder/Ebody, FGF-2, FGF-8, SHH	ND	ND	[93]

Summary of the different stem cells used, their respective differentiation protocols, as well as clinical important properties of dopaminergic stem cell grafts in animal models of PD.
Transgene expression is shown as (tg) followed by the name of the expressed gene.
aNSC: Adult neural stem cell; BM: Bone marrow; ESC: Embryonic stem cell; iPSC: Induced pluripotent cell; MSC: Mesenchymal stem cell; ND: Not demonstrated; NSC: Neural stem cell; SVZ: Subventricular zone; tg: Transgene; VM: Ventral mesencephalon.

Sidebar

Key Issues

As 'proof of principle', it was demonstrated by several clinical studies performed in the 1990s that replacement of lost dopaminergic neurons in the striatum can improve motor symptoms in animal models and Parkinson's disease (PD) patients.
Open-label clinical studies using fetal ventral midbrain tissues demonstrated encouraging results with long-term survival of grafted fetal dopaminergic neurons and local reinnervation and increased fluorodopa uptake.
These studies have suggested that approximately 100,000 dopaminergic neurons need to be present within the grafted striatum to achieve significant clinical benefit, and that those of nigral origin are most competent in innervating the striatum.
Long-term evaluation of grafted cells in post-mortem PD patients have reported that some of the transplanted cells contained α-synuclein, ubiquitin-positive Lewy bodies and neuritis.
Double-blind placebo-controlled trials of fetal ventral midbrain transplantation in PD suggested that transplanted dopaminergic neurons could survive in the brains of PD patients, become functionally integrated and suggested possible clinical improvement. Nevertheless, these clinical trials did not reach successful results.
Two clinical trials involving cell replacement therapy and PD are now being performed. These trials are now in Phase I and III and include autologous transplantation into the striatum of bone marrow mesenchymal stem cells and embryonic doaminergic mesencephalic cells, respectively. A clinical trial format on induced pluripotent stem cell generation from somatic cells of PD patients has been initiated recently.
Owing to new advancements in biology, other cell sources, such as neural, embryonic, mesenchymal and induced pluripotent stem cells, are being investigated.
Currently, most protocols rely on early induction of cell sources through sonic hedgehog, FGF8, WNT1/5A, TGFβ and retinoic acid, which can be combined with the introduction of genes codifying for transcription factors, such as LMX1A and NURR1.
The success of emerging cell replacement therapy for PD will reside among other issues, such as implantation site, environment modification or patient selection, the accurate combination of chosen stem cells type and the methodology or differentiation protocol that will be applied.