Cell-based therapy for Parkinson’s disease: past, present and future.

Parkinson’s disease (PD) is a chronic neurodegenerative condition effecting dopaminergic neurons of the midbrain. PD manifests itself around age 50 with mainly motor symptoms, such as tremor (shaking), slowness of movement, rigidity and postural instability. Number of pharmaceutical agents (e.g., L-Dopa and MAO-B inhibitors) has been used for symptomatic relief in PD patients, but the ultimate therapy target is the replacement of degenerating dopaminergic neurons with new, healthy neurons.

describe the imageCell replacement therapy for PD dates back to mid 80s with the transplantation of adrenal medullary tissue into patients’ striatum [1-3], which resulted only moderate improvements. At the same time, researchers in Sweden performed transplantation of fetal ventral mesencephalic tissue from aborted fetuses [4, 5]. These early studies observed important and persistent improvement based on numerous clinical outcomes. Moreover, postmortem examination of the brains of PD patients, who received ventral mesencephalic tissue transplantation, showed sustained survival of the graft and re-innervation of the striatum [6]. With the lift of federal funding ban on using fetal tissue for research and therapy by President Clinton in 1993, United States also began clinical trials utilizing fetal ventral mesencephalic tissue [7, 8]. Unfortunately, not only the patients didn’t display any significant improvements following transplantation in these trials, they developed additional abnormal, involuntary movements (i.e., graft-induced dyskinesia), due to surgery, which was also observed in other trials.

Close examination of the transplantation studies using fetal ventral mesencephalic tissue revealed few noteworthy outcomes:

1. Younger patients with newly developed pathology showed significant improvements over older patients with severe PD pathology.

2. Some patients showed continued improvements 3-4 years after surgery, while they did not display any benefits during the first year, indicating that the improvement in clinical parameters may take a while to appear over time. Regardless, it is clear that patients respond differently to the transplants of dopaminergic neurons, making the clinical outcomes fluctuate considerably.

3. Preparation of the fetal tissues, as well as selection of patients for transplantation, varied significantly from center to center carrying out the clinical trials, further indicating the need for standardizing tissue preparation, patient selection and implantation site.

Compared to the aforementioned points, the use of fetal ventral mesencephalic tissue for grafting constitutes one of the biggest problems in cell based therapy for PD. It has been challenging to standardize the number and the quality of the fetal dopaminergic cells in graft preparations. Furthermore, the purity of the preparations also varies from batch to batch. Lastly, many ethical -and sometimes legal- issues surround fetal tissues/cells significantly limiting their clinical applicability. Do we have an alternative source that is free of these concerns/problems? The answer is yes, but not at the moment. With the isolation of human embryonic stem cells (hESCs) in 1998 and the introduction of human induced pluripotent stem cells (iPSCs) in 2007, stem cell derived dopaminergic neurons are at the top of everyone’s list when it comes to replacing degenerating neurons in PD. hESCs have been the primary source to produce dopaminergic neurons so far [9-11], but with the popularity and the advantages of iPSCs, the focus is more likely to shift to iPSC-derived dopaminergic neurons in future transplantation efforts.

Number of studies utilizing stem cell derived dopaminergic neurons in animal models of PD reported promising results over the years. However, we are far from using these cells in clinical trials. Many issues, such as long-term stability of the transplanted cells, sustained functional recovery, ability to re-innervate the host striatum, generation of GMP grade cells and long-terms safety especially with regards to tumor formation, remain to be determined. To be able to answer these concerns are critical for successful clinical translation of stem cell derived dopaminergic neurons. Nevertheless, the target is in front of everyone, and the field of regenerative medicine is moving at an incredible speed to reach it.  It should also be noted that an increasing number of novel therapeutic approaches (e.g., gene therapy and growth factor infusions) have been under development -in addition to cell transplantations- with the aim of restoring dopaminergic function in PD patients.

While we are looking ahead with the promise of stem cell derived dopaminergic neurons for future of cell-based therapy in PD, there are many lessons to be learnt from the early clinical trials using fetal ventral mesencephalic tissue. There is no question that fetal dopamine neurons will serve as a reference and a standard against stem cell derived neurons for future clinical trials, since we know that the transplants survived, re-innervated the striatum, and generated adequate symptomatic relief in some patients for more than a decade following surgery. For PD patients, who are interested in cell-based therapy now, the decision of whether to wait for clinical trials utilizing stem cell derived neurons or to proceed with currently available fetal tissue grafts remains a somewhat difficult question and should take into consideration the aforementioned strengths and weaknesses of each approach.



[1] Backlund EO, Granberg PO, Hamberger B, et al. Transplantation of adrenal medullary tissue to striatum in parkinsonism. First clini- cal trials. J Neurosurg 1985;62:169–173.

[2] Herrera-Marschitz M, Stromberg I, Olsson D, Ungerstedt U, Olson L. Adrenal medullary implants in the dopamine-denervated rat striatum. II. Acute behavior as a function of graft amount and location and its modulation by neuroleptics. Brain Res 1984;297:53–61.

[3] Madrazo I, Drucker-Colin R, Diaz V, Martinez-Mata J, Torres C, Becerril JJ. Open microsurgical autograft of adrenal medulla to the right caudate nucleus in two patients with intractable Parkinson’s disease. N Engl J Med 1987;316:831–834.

[4] Lindvall O, Brundin P, Widner H, et al. Grafts of fetal dopamine neurons survive and improve motor function in Parkinson’s dis- ease. Science 1990;247:574–577.

[5] Widner H, Tetrud J, Rehncrona S, et al. Bilateral fetal mesence- phalic grafting in two patients with parkinsonism induced by 1- methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). N Engl J Med 1992;327:1556–1563.

[6] Kordower JH, Rosenstein JM, Collier TJ, et al. Functional fetal nigral grafts in a patient with Parkinson’s disease: chemoanatomic, ultrastructural, and metabolic studies. J Comp Neurol 1996;370:203–230.

[7] Freed CR, Greene PE, Breeze RE, et al. Transplantation of embry- onic dopamine neurons for severe Parkinson’s disease. N Engl J Med 2001;344:710–719.

[8] Olanow CW, Goetz CG, Kordower JH, et al. A double-blind con- trolled trial of bilateral fetal nigral transplantation in Parkinson’s disease. Ann Neurol 2003;54:403–414.

[9] Lee SH, Lumelsky N, Studer L, Auerbach JM, McKay RD. Effi- cient generation of midbrain and hindbrain neurons from mouse embryonic stem cells. Nat Biotechnol 2000;18:675–679.

[10] Cho MS, Lee YE, Kim JY, et al. Highly efficient and large-scale generation of functional dopamine neurons from human embryonic stem cells. Proc Natl Acad Sci U S A 2008;105:3392–3397.

[11] Kawasaki H, Suemori H, Mizuseki K, et al. Generation of dopami- nergic neurons and pigmented epithelia from primate ES cells by stromal cell-derived inducing activity. Proc Natl Acad Sci U S A 2002;99:1580–1585.

Direct conversion or iPSCs: Do all roads lead to Rome?

During development, when cells are programmed to perform specific tasks via terminal differentiation, they don’t undergo fate changes and perform their specific tasks throughout life, a process referred to as phenotypic stability. The idea of reprogramming challenged this concept and introduced the notion that the genomic material from any given cell can be reprogrammed to move back in the developmental cascade and become more unrestricted in terms of what other cell types it could generate in the process. It was initially shown that through Somatic Cell Nuclear Transfer (SCNT), one can reprogram the nucleus of any somatic cell by injecting it into oocytes that were experimentally devoid of their own nucleus. Once the oocyte divides and begins the process of developing, pluripotent cells can be isolated to be used for many purposes (e.g., further differentiation into a specific cell type, such as neurons, expansion, etc.).

stem cells

Over the last decade, induced pluripotent stem cell (iPSC) technology revolutionized the way we now create pluripotent cells from differentiated, somatic cells. By ectopically expressing handful of genes -as low as 2 of them in some cases- differentiated cells can be pushed to express genes that are associated with a more undifferentiated state and eventually transform into “embryonic stem cell like” colonies. Numerous laboratories around the world is utilizing this technology to generate iPSCs for future cell-replacement therapy, studying disease mechanisms, testing drug toxicity and more. However, depending on the cell type that’s being differentiated from the iPSCs for these particular studies, one may not need to generate iPSCs and go straight to the last step of generating a particular cell type (e.g., neurons, beta endocrine cells, etc.) by using direct conversion.

describe the image

In direct conversion (also referred to as transdifferentiation), the ectopic expression of transcription factors converts somatic cells from one lineage to another without reverting the cell all the way back to a fully undifferentiated state. Direct conversion has been around a lot longer than the iPSC technology. For instance, conversion of fibroblasts into muscle cells by overexpression of MyoD [1] or conversion of lymphoid cells into macrophages by expressing Pu-1 [2] were among the first demonstrations that a single transcription factor could directly convert one cell type into another. Note that these initial studies achieved conversion from one cell type to another within the same lineage. More recently, using the iPSC approach, there have been studies that achieved direct conversion from one germ layer to that of another, such as fibroblasts to neurons or liver cells. Furthermore, there have been studies that utilized direct conversion in vivo, such as in vivo conversion of exocrine to endocrine pancreas cells [3], or in vivo conversion of fibroblasts into in functional myocytes in an infarct area [4]. Without a doubt, the success of recent experiments using direct conversion was inspired by Yamanaka’s discovery of iPSC technology, further indicating that the approach has importance beyond just the reprogramming of somatic cells into pluripotent cells.

In direct conversion, one cell type is transformed into another without going through the intermediate pluripotent state (i.e., iPSCs). In specific applications where the pluripotent state is not necessary, this is a welcome change, since the process of creating and maintaining iPSCs are labor intensive and costly. Furthermore, for cell-based replacement therapies, differentiation of cells from iPSCs and transplanting them into recipients carry additional risk factors, such as increased chance of tumor formation, while dealing with pluripotent cells. In the two examples stated above, in which the pancreatic exocrine cells were converted directly into endocrine cells or the resident fibroblasts in the injured heart that were converted into functional cardiomyocytes, there was no need for an intermediate pluripotent state to achieve the goal of transdifferentiation, and the process of conversion took place in vivo. By utilizing the right set of transcription factors, the cells transformed from one lineage into another and achieved the final result of repair and/or restoration of function following disease/injury. Even though, iPSCs would be necessary for many applications, for certain tasks, we can reach the same final aim by using direct conversion without the need for iPSCs. Furthermore, direct lineage conversion could provide important new sources of human cells for creating in vitro disease models and cell-based replacement therapies. While the use of direct conversion gains more popularity, it will be significant to carefully determine the fidelity of reprogramming and to develop methods for robustly and efficiently generating one specific human cell type from another, directly.


[1] Expression of a single transfected cDNA converts fibroblasts to myoblasts. Davis et al., (1987) Cell, 51, 987-1000

[2] Stepwise Reprogramming of B Cells into Macrophages. Xie et al., (2004) Cell, 117, 5, 663-676

[3] In vivo reprogramming of adult pancreatic exocrine cells to beta cells. Zhou et al., (2008) Nature 455, 627-632.

[4] Heart repair by reprogramming non-myocytes with cardiac transcription factors. Song et al., (2012) Nature, 485, 599–604.

The Promise of Induced Pluripotent Stem Cells

Human embryonic stem cells (hESCs) hold great potential for cell replacement therapies, where cells are lost due to disease and/or injury. During the last decade, we have witnessed the development of the induced pluripotent stem cell (iPSC) technology, which revolutionized the stem cell field, as well as the regenerative medicine. Since the source of the reprogrammed cells (e.g., fibroblasts, keratinocytes, etc.) as part of the generation of iPSCs is readily accessible, this opened up an entirely new chapter in regenerative medicine by significantly reducing immune rejection problems after transplantation. Furthermore, by taking somatic cells directly from patients, we now have the ability to create disease models (also know as “disease in a dish”) for many conditions that were not possible before. As of today, numerous diseases have been modeled using iPSCs and many researchers are exploring the possibility of utilizing iPSC-derived cells for cell replacement therapy.

One of the pioneers of the iPSC work, Shinya Yamanaka, was awarded the Nobel Prize in Physiology or Medicine (together with John B. Gurdon) this year. Considering that Dr. Yamanaka’s first study related to iPSCs was published in 2006, it’s easy to appreciate the enormous impact that this technology has created as substantiated by the decision of the Nobel Prize committee. If we look back at the chronological events, the initial report from the Yamanaka group using mouse fibroblast and 4 transcription factors (i.e., Oct4, Sox2, Klf4 and c-Myc) to create iPSCs was published in 2006 (Takahashi and Yamanaka, 2006), followed by the demonstration in 2007 (Takahashi et al., 2007) that a similar approach was applicable for human fibroblasts, and by introducing a defined set of transcription factors, human iPSCs can be generated. On the same day Yamanaka published his human iPSC paper, Jamie Thomson’s group also demonstrated the generation of iPSCs from human cells using a different set of factors (Yu et al., 2007).

describe the imageEven though, iPSCs can be generated reproducibly, the efficiency of the reprograming remains low (i.e., around 1% using the original method). Furthermore, the initial generation of iPSCs utilized either retroviruses or lentiviruses, which both integrate into the host genome and might cause insertional mutagenesis, collectively creating a risk for translational applications. However, the beauty of the iPSC technology is its simplicity and reproducibility. Since 2007, iPSCs have been in the central focus of stem cell research and regenerative medicine, and researchers throughout the world have made a number of improvements to Yamanaka’s original protocol. Today, we have safer and more efficient methods for generation human iPSCs for disease modeling or cell replacement therapies, and by the look of how much has been achieved so far, the technology is only going to get better and more proficient. Considering that human embryonic stem cell derived approaches have been successfully used at the clinical level for cell replacement therapies, there is no doubt that iPSCs will eventually prove to be the ultimate source for regenerative medicine in the future.