Stem cells in the treatment of Parkinson’s disease

Abstract

Stem cells have been suggested as candidate therapeutic tools for neurodegenerative disorders, given their ability to give rise to the appropriate cell types after grafting in vivo. In this review I summarize some of the evidence currently available concerning two approaches for the treatment of Parkinson’s disease: (1) The generation of dopaminergic neurons from embryonic stem cells, multipotent stem cells, and neuronal progenitor cells for cell replacement therapy. (2) The engineering of multipotent stem cells to release glial cell-line derived neurotrophic factor, a potent neurotrophic factor for dopaminergic neurons, in a neuroprotective and neuroregenerative approach to the treatment of Parkinson’s disease.

Introduction

Parkinson’s disease (PD) is a very common neurodegenerative disorder that affects more than 2% of the population over 65 years of age. PD is characterized at a pathological level by a progressive degeneration and loss of: (1) nigrostriatal and mesolimbic dopaminergic neurons 57, 60, leading to tremor, rigidity, and hypokinesia, the classical symptoms of the disease 20, 62; (2) noradrenergic neurons of the locus coeruleus 51, 57, involved in the progression of the disease 51, 92a, dementia [158], and depression [25]; and (3) serotonergic neurons of the raphe obscurus and medial raphe [55], also involved in the symptoms of depression [65], often associated with PD. In addition to neuronal loss, a second prominent feature of PD is the formation of intracellular fibrillar inclusions, called Lewy bodies, which consist of abnormally accumulated proteins, including α-synuclein [131].

The cause of the selective degeneration of specific populations of neurons in PD is unknown, but it has been suggested that increased oxidative stress, mitochondrial dysfunction, and excytotoxic damage are involved in the physiopathology of the disease (reviewed in [106]). Because most of the cases of idiopathic PD are sporadic, it has been hypothesized that the convergence of predisposing genetic factors and environmental factors, including pesticides [11] and viruses [136], may play a causative role in PD [53]. Interestingly however, it has been the comparatively rare cases of familial PD that have recently provided new insights into the pathogenesis of PD. Mutations in two genes, α-synuclein [112] and ubiquitin carboxyl terminal hydroxylase-L1 [81] have been found to lead to autosomal dominant forms of PD, while mutations in the Parkin gene, thought to be an E3-ubiquitin-protein ligase [125], lead to autosomal recessive juvenile Parkinsonism without Lewy bodies [73]. Thus, all these findings suggest that abnormal ubiquitination of proteins, including α-synuclein and impaired proteosomal degradation are involved in the pathogenesis of PD [35].

Current approaches for the treatment of PD include symptomatic treatment with combined L-DOPA and carbidopa, which increase the synthesis and release of dopamine and are particularly effective at alleviating the akinesia and the rigidity during early stages of the disease. However, as the disease progresses, less dopamine neurons are available to synthesize dopamine, the effectiveness of the treatment decreases and L-DOPA-induced dyskinesia appears [103]. Another approach often combined with L-DOPA and carbidopa is deep brain stimulation of the globus pallidus and the subthalamic nuclei 104, 111, which is believed to relieve the motor symptoms by inducing a depolarization blockade of neurons in the area. Finally, positive results with intracaudate-putamen grafting of human embryonic mesencephalic tissue in PD patients 40, 58, 84, 105 have provided the foundation for the development of novel cell replacement strategies based on the grafting of neural stem cells, and these will be discussed here. In this review I will also focus on the use of neural stem cells to deliver neuroprotective molecules, which could be an alternative approach for directly transfer genes into the brain. This second approach, is an important therapeutic area because none of the treatments currently available for PD patients address the issue of preventing the progressive neuronal loss that takes place in PD.

Embryonic stem (ES) cells are pluripotent cells isolated from the inner cell mass of blastocysts, which give rise to all cells in the organism. These cells differ from zygotes in that they cannot give rise to extra-embryonic tissue (trophoblast and placenta). Similarly, multipotent stem cells are also able to self-renew, but they are believed to have a more restricted potential than ES cells, and are often defined by the organ from which they are derived. Neural stem cells (NSCs) were considered to be multipotent stem cells derived from the nervous system with capacity to self-renew and to give rise to cells belonging to all the three lineages in the nervous system, that is neurons, oligodendrocytes, and astrocytes. However, experimental evidence from different groups has indicated that multipotent stem cells are in fact not restricted to cell types specific to the tissue of origin, and that they are able to differentiate in response to local environmental clues from other tissues. For instance, when NSCs are exposed to early developmental signals following injection into blastocysts, or after morulae aggregation, they generate a very broad variety of tissues [28]. Also, when introduced in tissues where a specific cell type is lost, such as the bone marrow in irradiated mice, NSCs [15] or muscle progenitor cells [64] are thought to respond to appropriate local signals and give rise to blood cells. Similarly, hematopoietic stem cells have been found to generate myocytes 45, 54 and astrocytes [76], and bone marrow multipotent stem cells are able to give rise to cell lineages of the nervous system 36, 86, 120, 151. Moreover, oligodendrocyte progenitor cells, previously thought to be restricted to giving rise to only astrocytes and oligodendrocytes, have been recently reported to give rise to all neural lineages and to behave as NSCs after being exposed to platelet-derived growth factor (PDGF) for 5 days, PDGF and fetal calf serum (FCS) for 3 days, and basic fibroblast growth factor (bFGF) or epidermal growth factor (EGF) for 5 days [75]. Thus, it seems that more lineages than anticipated can be generated by multipotent stem cells after propagation in vitro (Fig. 1). It should be noted however that it is not known whether the in vitro amplification of the multipotent stem cells may have contributed to their reprogramming. If that is the case, the increased differentiation potential of the multipotent stem cells after culture would not reflect the actual properties of multipotent stem cells in vivo. Evidence in this direction has been recently reviewed and discussed by Anderson [4] and others to emphasize the importance of identifying markers to prospectively isolate and characterize the properties of uncultured tissue-specific multipotent stem cells.

The discovery that NSCs are present in and can be isolated from the developing and the adult mammalian brain 3, 47, 114 has stimulated an important revolution in our thinking about the brain, a structure long considered to be postmitotic and unable to regenerate. To date, adult multipotent stem cells have only been isolated from two sites of high-density cell division: the subventricular zone and the subgranular zone of the dentate gyrus in the hippocampus. The exact phenotype of multipotent NSCs in the periventricular region of adult brain is still a matter of debate and two cell types have been suggested to be the neural stem cell: a subpopulation of ependymal cells [68], or a subset of astrocyte-like cells in the subventricular zone 3, 26, 38, 49, 98, 99. Regardless of the in vivo phenotype, it seems clear that multipotent NSCs can be isolated from the nervous system, expanded in vitro and stored. Moreover, upon differentiation, they give rise to all cell lineages in the nervous system, adopting neural phenotypes appropriate to the brain region where they are grafted 48, 88, 91, 93, 118, 127, 128, 134, 150. In some cases NSCs are able to replace neurons undergoing targeted apoptotic degeneration 88, 130. All these findings have suggested that multipotent stem cells could be optimal candidates for cell replacement and have opened up the possibility of developing novel strategies for the treatment of neurodegenerative disorders [13].

In this review, I will focus on the possible use of stem cells as source material for transplantation in PD. I will also review the evidence suggesting that stem cells may also serve as sources of neuroprotective/neuroregenerative factors and prevent the loss and/or promote the regeneration of endogenous dopamine neurons.