Cell replacement therapy for Parkinson's disease

Dopamine D3 Receptor in Parkinson Disease: A Prognosis Biomarker and an Intervention Target

Parkinson disease (PD) dementia, pathologically featured as nigrostriatal dopamine (DA) neuronal loss with motor and non-motor manifestations, leads to substantial disability and economic burden. DA therapy targets the DA D3 receptor (D3R) with high affinity and selectivity. The pathological involvement of D3R is evidenced as an effective biomarker for disease progression and DA agnostic interventions, with compensations of increased DA, decreased aggregates of α-synuclein (α-Syn), enhanced secretion of brain-derived neurotrophic factors (BDNF), attenuation of neuroinflammation and oxidative damage, and promoting neurogenesis in the brain. D3R also interacts with D1R to reduce PD-associated motor symptoms and alleviate the side effects of levodopa (L-DOPA) treatment. We recently found that DA D2 receptor (D2R) density decreases in the late-stage PDs, while high D3R or DA D1 receptor (D1R) + D3R densities in the postmortem PD brains correlate with survival advantages. These new essential findings warrant renewed investigations into the understanding of D3R neuron populations and their cross-sectional and longitudinal regulations in PD progression.

Characterization of Human-Induced Neural Stem Cells and Derivatives following Transplantation into the Central Nervous System of a Nonhuman Primate and Rats

Neural stem cells (NSCs) and derivatives are potential cellular sources to treat neurological diseases. In the current study, we reprogrammed human peripheral blood mononuclear cells into induced NSCs (iNSCs) and inserted GFP gene into the AAVS1 site for graft tracing. Targeted integration of GFP does not affect the proliferation and differentiation capacity of iNSCs. iNSC-GFP can be further differentiated into dopaminergic precursors (DAPs) and motor neuron precursors (MNPs), respectively. iNSCs were engrafted into the motor cortex and iNSC-DAPs into the striatum and substantia nigra (SN) of a nonhuman primate, respectively. The surviving iNSCs could respond to the microenvironment of the cortex and spontaneously differentiate into mature neurons that extended neurites. iNSC-DAPs survived well and matured into DA neurons following transplantation into the striatum and SN. iNSC-MNPs could also survive and turn into motor neurons after being engrafted into the spinal cord of rats. The results suggest that iNSCs and derivatives have a potential to be used for the treatment of neurological diseases.

Assessment of the anti-nociceptive effects of fetal ventral mesencephalic tissue allografts in a rat model of hemi-Parkinson's disease using fMRI

Extensive studies showed increased subjective pain sensitivity in Parkinson's disease (PD), which appeared to be partially reversed by dopaminergic (DA) treatment. Although cell replacement represents an attractive therapeutic strategy, its potential for PD-related hyperalgesia remains unclear. We investigated re-establishment of DA function via allografting exogenic DA cells on pain hypersensitivity in a rat model of PD. We evaluated the anti-nociceptive effects of fetal ventral mesencephalic (rVM) tissue allografts in PD rats after unilateral 6-OHDA-induced toxicity in the medial forebrain bundle. The drug -induced rotation test was used to validate the severity of the nigrostriatal lesion; von Frey and thermal pain tests were employed to evaluate nociceptive function. Nociception-induced cerebral blood volume (CBV) response was measured using a 4.7-T MR system. Finally, the immunohistochemical (IHC) studies were performed and the results were compared with the imaging findings from functional magnetic resonance imaging (fMRI). The grafts significantly improved drug-induced rotation behavior and increased mechanical and thermal nociceptive thresholds in PD rats. The elevation of CBV signals significantly recovered on the grafted striatum, whereas this effect was inhibited by the D2R antagonist eticlopride in each striatum. Quantitative IHC analysis revealed the transplantation markedly increased the numbers of tyrosine hydroxylase immunoreactive cells. Therefore, we concluded transplantation of rVM tissue results in anti-nociceptive effects and improves motor function. Moreover, in vivo CBV response confirmed the key role of D2R-mediated pain modulation. Therefore, we demonstrate fMRI as a reliable imaging index in evaluating the anti-nociceptive therapeutic effects of fetal rVM transplantation in the rat model of PD.

Dopamine inhibits the expression of proinflammatory cytokines of microglial cells through the formation of dopamine quinone in the mouse striatum

We previously reported that dopamine (DA) attenuated lipopolysaccharide (LPS)-induced expression of proinflammatory cytokines through the formation of DA quinone (DAQ) in murine microglial cell line BV-2 and primary murine microglial cells. To reveal whether DA inhibits the expression of proinflammatory cytokines of microglial cells through the formation of DAQ in the central nervous system (CNS), in this study, we examined the effect of DAQ on LPS-induced mRNA expression of proinflammatory cytokines in C57BL/6 mouse brain under two experimental conditions: 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration and l-dopa/carbidopa administration. Acute MPTP administration reduced the number of tyrosine hydroxylase-positive cells in the substantia nigra, and decreased the level of quinoprotein, an indicator of DAQ formation, in the striatum. Real-time RT-PCR analysis revealed that intraperitoneal administration of LPS increased the mRNA levels of proinflammatory cytokines, including tumor-necrosis factor-α and interleukin-1β, in the striatum. These increases were enhanced in MPTP-treated mice. On the other hand, l-dopa/carbidopa administration increased the level of quinoprotein, attenuated the LPS-induced mRNA expression of proinflammatory cytokines, and reduced the LPS-induced increase in the number of microglial cells in the striatum. These results suggest that DA attenuate the expression of proinflammatory cytokines in microglia through the formation of DAQ in the CNS.

Assessing Human Embryonic Stem Cell-Derived Dopaminergic Neuron Progenitor Transplants Using Non-invasive Imaging Techniques

PURPOSE

Human pluripotent stem cell (hPSC)-derived dopaminergic neuron progenitor cells (DAPCs) are a potential therapy for Parkinson's disease (PD). However, their intracranial administration raises safety concerns including uncontrolled proliferation, migration and inflammation. Here, we apply a bimodal imaging approach to investigate the fate of DAPC transplants in the rat striatum.

PROCEDURES

DAPCs co-expressing luciferase and ZsGreen or labelled with micron-sized particles of iron oxide (MPIOs) were transplanted in the striatum of RNU rats (n = 6 per group). DAPCs were tracked in vivo using bioluminescence and magnetic resonance (MR) imaging modalities.

RESULTS

Transgene silencing in differentiating DAPCs accompanied with signal attenuation due to animal growth rendered the bioluminescence undetectable by week 2 post intrastriatal transplantation. However, MR imaging of MPIO-labelled DAPCs showed that transplanted cells remained at the site of injection for over 120 days. Post-mortem histological analysis of DAPC transplants demonstrated that labelling with either luciferase/ZsGreen or MPIOs did not affect the ability of cells to differentiate into mature dopaminergic neurons. Importantly, labelled cells did not elicit increased glial reactivity compared to non-labelled cells.

CONCLUSIONS

In summary, our findings support the transplantation of hPSC-derived DAPCs as a safe treatment for PD.

From Transcriptome to Behavior: Intranasal Injection of Late Passage Human Olfactory Stem Cells Displays Potential in a Rat Model of Parkinson's Disease

Parkinson's disease (PD) is one of the most common neurodegenerative disorders, which is caused by the loss of dopaminergic (DAergic) neurons. Thus, cell replacement therapy (CRT) might be regarded as an alternative therapy to effectively treat motor functional defects in PD patients. Human olfactory ectomesenchymal stem cells (OE-MSCs) are a novel type of mesenchymal stem cells (MSCs) with a strong tendency to differentiate into DAergic neurons. However, there are various barriers to successful CRT including the proliferation capacity of stem cells at higher passage numbers as well as the route of stem cell delivery. In this regard, we aimed to explore the efficacy of late passage OE-MSC administration through the intranasal (IN) route in PD rat models. Herein, the proliferation capacity of OE-MSCs was compared at early and late passage numbers; then, the results were validated via RNA sequencing analysis. Subsequently, the efficacy of IN injection of late passage OE-MSC in PD models was evaluated. The results manifested the absence of noticeable differences in proliferation capacity and signaling pathways in OE-MSCs at early and late passage numbers. Moreover, it was found that the IN administration of OE-MSCs with a high passage number substantially increased the levels of DAergic markers and improved the motor function in rat models of PD. Overall, our findings suggested that OE-MSCs with a high passage number are a promising CRT candidate due to their fundamental potential to provide a large number of cells with an enormous proliferation capacity. Moreover, they exhibit the high efficiency of IN administration as a noninvasive route of late-passage OE-MSC delivery for CRT, particularly for PD.

Characterization of substantia nigra neurogenesis in homeostasis and dopaminergic degeneration: beneficial effects of the microneurotrophin BNN-20

Background

Loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) underlines much of the pathology of Parkinson’s disease (PD), but the existence of an endogenous neurogenic system that could be targeted as a therapeutic strategy has been controversial. BNN-20 is a synthetic, BDNF-mimicking, microneurotrophin that we previously showed to exhibit a pleiotropic neuroprotective effect on the dopaminergic neurons of the SNpc in the “weaver” mouse model of PD. Here, we assessed its potential effects on neurogenesis.

Methods

We quantified total numbers of dopaminergic neurons in the SNpc of wild-type and “weaver” mice, with or without administration of BNN-20, and we employed BrdU labelling and intracerebroventricular injections of DiI to evaluate the existence of dopaminergic neurogenesis in the SNpc and to assess the origin of newborn dopaminergic neurons. The in vivo experiments were complemented by in vitro proliferation/differentiation assays of adult neural stem cells (NSCs) isolated from the substantia nigra and the subependymal zone (SEZ) stem cell niche to further characterize the effects of BNN-20.

Results

Our analysis revealed the existence of a low-rate turnover of dopaminergic neurons in the normal SNpc and showed, using three independent lines of experiments (stereologic cell counts, BrdU and DiI tracing), that the administration of BNN-20 leads to increased neurogenesis in the SNpc and to partial reversal of dopaminergic cell loss. The newly born dopaminergic neurons, that are partially originated from the SEZ, follow the typical nigral maturation pathway, expressing the transcription factor FoxA2. Importantly, the pro-cytogenic effects of BNN-20 were very strong in the SNpc, but were absent in other brain areas such as the cortex or the stem cell niche of the hippocampus. Moreover, although the in vitro assays showed that BNN-20 enhances the differentiation of NSCs towards glia and neurons, its in vivo administration stimulated only neurogenesis.

Conclusions

Our results demonstrate the existence of a neurogenic system in the SNpc that can be manipulated in order to regenerate the depleted dopaminergic cell population in the “weaver” PD mouse model. Microneurotrophin BNN-20 emerges as an excellent candidate for future PD cell replacement therapies, due to its area-specific, pro-neurogenic effects.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02398-3.

Dopamine Neurons That Cotransmit Glutamate, From Synapses to Circuits to Behavior

Discovered just over 20 years ago, dopamine neurons have the ability to cotransmit both dopamine and glutamate. Yet, the functional roles of dopamine neuron glutamate cotransmission and their implications for therapeutic use are just emerging. This review article encompasses the current body of evidence investigating the functions of dopamine neurons of the ventral midbrain that cotransmit glutamate. Since its discovery in dopamine neuron cultures, further work in vivo confirmed dopamine neuron glutamate cotransmission across species. From there, growing interest has led to research related to neural functioning including roles in synaptic signaling, development, and behavior. Functional connectome mapping reveals robust connections in multiple forebrain regions to various cell types, most notably to cholinergic interneurons in both the medial shell of the nucleus accumbens and the lateral dorsal striatum. Glutamate markers in dopamine neurons reach peak levels during embryonic development and increase in response to various toxins, suggesting dopamine neuron glutamate cotransmission may serve neuroprotective roles. Findings from behavioral analyses reveal prominent roles for dopamine neuron glutamate cotransmission in responses to psychostimulants, in positive valence and cognitive systems and for subtle roles in negative valence systems. Insight into dopamine neuron glutamate cotransmission informs the pathophysiology of neuropsychiatric disorders such as addiction, schizophrenia and Parkinson Disease, with therapeutic implications.

Plastrum Testudinis Extracts Promote NSC Differentiation into Dopaminergic Neuron by Regulating the Interaction of TET1 and FoxA2

In recent years, stem cells have gained much attention for the treatment of neurodegenerative diseases. However, inducing neural stem cell directionally differentiation is a difficult problem in the treatment of Parkinson's disease (PD) by stem cell therapy. Plastrum Testudinis (PT) can enhance the number of TH-positive neurons in the PD rat brain substantia nigra, but the underlying mechanism has not been clarified. Here, we aimed at further investigating the mechanism by which PT can promote NSC differentiation into dopaminergic neurons. A rat model of PD was used for detecting the effect of PT on the rat brain substantia nigra in vivo. The results showed the expressions of tyrosine hydroxylase (TH) and TET1 enzyme were increased after treatment with PT. Consequently, Plastrum Testudinis extracts (PTEs) were used for inducing NSC differentiation into dopaminergic neurons ex vivo. During differentiation of NSCs induced by PTE, TH expression was increased, with a concomitant increase in both TET1 and FoxA2. Next, we performed coimmunoprecipitation analysis to examine the interaction between TET1 protein and FoxA2 protein. Our results show that PTE can increase the binding rate of TET1 and FoxA2. Thus, our findings show that PTE can increase the efficiency of NSCs to directionally differentiate into dopaminergic neurons and provide experimental evidence for PT in the treatment of Parkinson's disease.

Combination of Human Mesenchymal Stem Cells and Repetitive Transcranial Magnetic Stimulation Enhances Neurological Recovery of 6-Hydroxydopamine Model of Parkinsonian's Disease

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) has been in use for the treatment of various neurological diseases, including depression, anxiety, stroke and Parkinson's disease (PD), while its underlying mechanism is stills unclear. This study was undertaken to evaluate the potential synergism of rTMS treatment to the beneficial effect of human mesenchymal stem cells (hMSCs) administration for PD and to clarify the mechanism of action of this therapeutic approach.

METHODS

The neuroprotective effect in nigral dopamine neurons, neurotrophic/growth factors and anti-/pro-inflammatory cytokine regulation, and functional recovery were assessed in the rat 6-hydroxydopamine (6-OHDA) model of PD upon administration of hMSCs and rTMS.

RESULTS

Transplanted hMSCs were identified in the substantia nigra, and striatum. Enhancement of the survival of SN dopamine neurons and the expression of the tyrosine hydroxylase protein were observed in the hMSCs + rTMS compared to that of controls. Combination therapy significantly elevated the expression of several key neurotrophic factors, of which the highest expression was recorded in the rTMS + hMSC group. In addition, the combination therapy significantly upregulated IL-10 expression while decreased IFN-γ and TNF-α production in a synergistic manner. The treadmill locomotion test (TLT) revealed that motor function was improved in the rTMS + hMSC treatment with synergy.

CONCLUSION

Our findings demonstrate that rTMS treatment and hMSC transplantation could synergistically create a favorable microenvironment for cell survival within the PD rat brain, through alteration of soluble factors such as neurotrophic/growth factors and anti-/pro-inflammatory cytokines related to neuronal protection or repair, with preservation of DA neurons and improvement of motor functions.

Emerging regenerative medicine and tissue engineering strategies for Parkinson's disease

Parkinson's disease (PD) is the second most common progressive neurodegenerative disease, affecting 1-2% of people over 65. The classic motor symptoms of PD result from selective degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc), resulting in a loss of their long axonal projections to the striatum. Current treatment strategies such as dopamine replacement and deep brain stimulation (DBS) can only minimize the symptoms of nigrostriatal degeneration, not directly replace the lost pathway. Regenerative medicine-based solutions are being aggressively pursued with the goal of restoring dopamine levels in the striatum, with several emerging techniques attempting to reconstruct the entire nigrostriatal pathway-a key goal to recreate feedback pathways to ensure proper dopamine regulation. Although many pharmacological, genetic, and optogenetic treatments are being developed, this article focuses on the evolution of transplant therapies for the treatment of PD, including fetal grafts, cell-based implants, and more recent tissue-engineered constructs. Attention is given to cell/tissue sources, efficacy to date, and future challenges that must be overcome to enable robust translation into clinical use. Emerging regenerative medicine therapies are being developed using neurons derived from autologous stem cells, enabling the construction of patient-specific constructs tailored to their particular extent of degeneration. In the upcoming era of restorative neurosurgery, such constructs may directly replace SNpc neurons, restore axon-based dopaminergic inputs to the striatum, and ameliorate motor deficits. These solutions may provide a transformative and scalable solution to permanently replace lost neuroanatomy and improve the lives of millions of people afflicted by PD.

Dopamine D3 receptor: A neglected participant in Parkinson Disease pathogenesis and treatment?

Parkinson disease (PD) is a neurodegenerative disorder characterized by motor and non-motor symptoms which relentlessly and progressively lead to substantial disability and economic burden. Pathologically, these symptoms follow the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) associated with abnormal α-synuclein (α-Syn) deposition as cytoplasmic inclusions called Lewy bodies in pigmented brainstem nuclei, and in dystrophic neurons in striatal and cortical regions (Lewy neurites). Pharmacotherapy for PD focuses on improving quality of life and primarily targets dopaminergic pathways. Dopamine acts through two families of receptors, dopamine D1-like and dopamine D2-like; dopamine D3 receptors (D3R) belong to dopamine D2 receptor (D2R) family. Although D3R's precise role in the pathophysiology and treatment of PD has not been determined, we present evidence suggesting an important role for D3R in the early development and occurrence of PD. Agonist activation of D3R increases dopamine concentration, decreases α-Syn accumulation, enhances secretion of brain derived neurotrophic factors (BDNF), ameliorates neuroinflammation, alleviates oxidative stress, promotes neurogenesis in the nigrostriatal pathway, interacts with D1R to reduce PD associated motor symptoms and ameliorates side effects of levodopa (L-DOPA) treatment. Furthermore, D3R mutations can predict PD age of onset and prognosis of PD treatment. The role of D3R in PD merits further research. This review elucidates the potential role of D3R in PD pathogenesis and therapy.

Recent advances of induced pluripotent stem cells application in neurodegenerative diseases

Stem cell is defined by its ability to self-renewal and generates differentiated functional cell types, which are derived from the embryo and various sources of postnatal animal. These cells can be divided according to their potential development into totipotent, unipotent, multipotent andpluripotent. Pluripotent is considered as the most important type due to its advantageous capability to create different cell types of the body in a similar behavior as embryonic stem cell. Induced pluripotent stem cells (iPSCs) are adult cells that maintain the characteristics of embryonic stem cells because it can be genetically reprogrammed to an embryonic stem cell-like state via express genes and transcription factors. Such cells provide an efficient pathway to explorehuman diseases and their corresponding therapy, particularly, neurodevelopmental disorders. Consequently, iPSCs can be investigated to check the specific mutations of neurodegenerative disease due to their unique ability to differentiate into neural cell types and/or neural organoids. The current review addresses the different neurodegenerative diseases model by using iPSCs approach such as Alzheimer's diseases (AD), Parkinson diseases (PD),multiplesclerosis(MS) and psychiatric disorders. We also highlight the importance of autophagy in neurodegenerative diseases.

Nanodelivery of therapeutic agents in Parkinson's disease

Parkinson's disease (PD) as a motor disorder is pathologically featured by the loss of dopaminergic neurons of the substantia nigra compacta (SNc) and the consequent depletion of dopamine in the striatum. However, motor signs are detectable when the loss of dopaminergic striatal terminals exceeds to the dopaminergic neuronal degeneration in SN. Hence, recent evidences about the topological organization of the nigrostriatal system could provide novel insights about the progression of the neurodegenerative process as well as the correct application of the novel therapeutic strategies. Though dopaminergic drugs and different routes of administration have been proposed to treat PD, most of the effects are symptomatic with temporary effects resorting to invasive procedures to ameliorate the side effects. Since the blood-brain barrier (BBB) is the main obstacle for most of molecules to access to the brain, ongoing research is focused on halting the progression of PD through the use of those technologies that allow the effective delivery and diffusion of therapeutic molecules to the central nervous system for bypassing BBB and avoiding the side effects. In this context, nanotechnology is emerging as a promising tool for drug delivery. In fact, nanodelivery of restorative treatments in PD, such as gene therapy increased the effectiveness of neurotrophic factors for restoring the dopamine deficit and improving motor deficit in rodent models. Therefore, the present review is focused on the description and identification of the available nanotherapies developed in experimental models of PD which could suppose an important advance for controlled delivery of nanobioactive components into the brain and one more step for the clinical projection.

Dopaminergic precursors differentiated from human blood-derived induced neural stem cells improve symptoms of a mouse Parkinson's disease model

Autologous neural stem cells (NSCs) may offer a promising source for deriving dopaminergic (DA) cells for treatment of Parkinson's disease (PD).

Methods: By using Sendai virus, human peripheral blood mononuclear cells (PBMNCs) were reprogrammed to induced NSCs (iNSCs), which were then differentiated to dopaminergic neurons in vitro. Whole-genome deep sequencing was performed to search for mutations that had accumulated during the reprogramming and expansion processes. To find the optimal differentiation stage of cells for transplantation, DA precursors obtained at various differentiation time points were tested by engraftment into brains of naïve immunodeficient mice. At last, the safety and efficacy of iNSC-derived DA precursors were tested by transplantation into the striatum of immunodeficient PD mouse models.

Results: PBMNC-derived iNSCs showed similar characteristics to fetal NSCs, and were able to specifically differentiate to DA neurons with high efficiency in vitro. The sequencing data proved that no harmful SNVs, Indels and CNVs were generated during the reprogramming and expansion processes. DA precursors obtained between differentiation day 10 to 13 in vitro were most suitable for transplantation when a balanced graft survival and maturation were taken into account. Two weeks after transplantation of DA precursors into mouse PD models, the motor functions of PD mice started to improve, and continued to improve until the end of the experiments. No graft overgrowth or tumor was observed, and a significant number of A9-specific midbrain DA neurons were surviving in the striatum.

Conclusion: This study confirmed the efficacy of iNSC-derived DA precursors in a mouse PD model, and emphasized the necessity of genomic sequencing and vigorous safety assessment before any clinical translation using iNSCs.

Experimental Intrastriatal Applications of Botulinum Neurotoxin-A: A Review

Parkinson’s disease (PD) is one of the most frequent neurodegenerative disorders. Its main pathophysiological characteristic is the loss of dopaminergic neurons in the substantia nigra pars compacta followed by a lack of striatal dopaminergic input and a consequent disinhibition of tonically active cholinergic interneurons. The resulting striatal hypercholinism causes major motor symptoms in PD. Anticholinergic pharmacotherapies have antiparkinsonian effects on motor symptoms, but, due to systemic actions, also numerous severe side effects occur on a regular basis. To circumvent these side effects, a local anticholinergic therapy acting exclusively in the striatum would be reasonable. Botulinum neurotoxin-A (BoNT-A) is synthesized by Clostridium botulinum and blocks the release of acetylcholine from the presynaptic bouton. For several decades, BoNT-A has been used successfully for medical and cosmetic purposes to induce controlled paralyses of single muscles. Our group and others investigated the experimental treatment of striatal hypercholinism by the direct injection of BoNT-A into the striatum of rats and mice as well as of hemiparkinsonian animal models. This review gives an overview of the most important results of the experimental intrastriatal BoNT-A application, with a focus on hemiparkinsonian rats.

Nanoformulation: A Useful Therapeutic Strategy for Improving Neuroprotection and the Neurorestorative Potential in Experimental Models of Parkinson's Disease

Parkinson's disease (PD) is the second most frequent neurodegenerative disorder, but current therapies are only symptomatic. Experimental models are necessary to go deeper in the comprehension of pathophysiological mechanism and to assess new therapeutic strategies. The unilateral 6-hydroxydopamine (6-OHDA) lesion either in medial forebrain bundle (MFB) or into the striatum in rats affords to study various stages of PD depending on the evolution time lapsed. A promising alternative to address the neurodegenerative process is the use of neurotrophic factors; but its clinical use has been limited due to its short half-life and rapid degradation after in vivo administration, along with difficulties for crossing the blood-brain barrier (BBB). Tyrosine hydroxylase (TH) immunostaining revealed a significant decrease of the TH-immunopositive striatal volume in 6-OHDA group from rostral to caudal one. The loss of TH-ir neurons and axodendritic network (ADN) was higher in caudal sections showing a selective vulnerability of the topological distributed dopaminergic system. In addition to a remarkable depletion of dopamine in the nigrostriatal system, the administration of 6-OHDA into MFB induces some other neuropathological changes such as an increase of glial fibrillary acidic protein (GFAP) positive cells in substantia nigra (SN) as well as in striatum. Intrastriatal implantation of micro- or nanosystems delivering neurotrophic factor in parkinsonized rats for bypassing BBB leads to a significative functional and morphological recovery. Neurorestorative morphological changes (preservation of the TH-ir cells and ADN) along the rostrocaudal axis of caudoputamen complex and SN have been probed supporting a selective recovering after the treatment as well. Others innovative therapeutic strategies, such as the intranasal delivery, have been recently assessed in order to search the NTF effects. In addition some others methodological key points are reviewed.

Mechanisms and use of neural transplants for brain repair

Under appropriate conditions, neural tissues transplanted into the adult mammalian brain can survive, integrate, and function so as to influence the behavior of the host, opening the prospect of repairing neuronal damage, and alleviating symptoms associated with neuronal injury or neurodegenerative disease. Alternative mechanisms of action have been postulated: nonspecific effects of surgery; neurotrophic and neuroprotective influences on disease progression and host plasticity; diffuse or locally regulated pharmacological delivery of deficient neurochemicals, neurotransmitters, or neurohormones; restitution of the neuronal and glial environment necessary for proper host neuronal support and processing; promoting local and long-distance host and graft axon growth; formation of reciprocal connections and reconstruction of local circuits within the host brain; and up to full integration and reconstruction of fully functional host neuronal networks. Analysis of neural transplants in a broad range of anatomical systems and disease models, on simple and complex classes of behavioral function and information processing, have indicated that all of these alternative mechanisms are likely to contribute in different circumstances. Thus, there is not a single or typical mode of graft function; rather grafts can and do function in multiple ways, specific to each particular context. Consequently, to develop an effective cell-based therapy, multiple dimensions must be considered: the target disease pathogenesis; the neurodegenerative basis of each type of physiological dysfunction or behavioral symptom; the nature of the repair required to alleviate or remediate the functional impairments of particular clinical relevance; and identification of a suitable cell source or delivery system, along with the site and method of implantation, that can achieve the sought for repair and recovery.

Targeting the dopamine D3 receptor: an overview of drug design strategies

INTRODUCTION

Dopamine is a neurotransmitter widely distributed in both the periphery and the central nervous system (CNS). Its physiological effects are mediated by five closely related G protein-coupled receptors (GPCRs) that are divided into two major subclasses: the D1-like (D1, D5) and the D2-like (D2, D3, D4) receptors. D3 receptors (D3Rs) have the highest density in the limbic areas of the brain, which are associated with cognitive and emotional functions. These receptors are therefore attractive targets for therapeutic management.

AREAS COVERED

This review summarizes the functional and pharmacological characteristics of D3Rs, including the design and clinical relevance of full agonists, partial agonists and antagonists, as well as the capacity of these receptors to form active homodimers, heterodimers or higher order receptor complexes as pharmacological targets in several neurological and neurodegenerative disorders.

EXPERT OPINION

The high sequence homology between D3R and the D2-type challenges the development of D3R-selective compounds. The design of new D3R-preferential ligands with improved physicochemical properties should provide a better pharmacokinetic/bioavailability profile and lesser toxicity than is found with existing D3R ligands. It is also essential to optimize D3R affinity and, especially, D3R vs. D2-type binding and functional selectivity ratios. Developing allosteric and bitopic ligands should help to improve the D3R selectivity of these drugs. As most evidence points to the ability of GPCRs to form homomers and heteromers, the most promising therapeutic strategy in the future is likely to involve the application of heteromer-selective drugs. These selective ligands would display different affinities for a given receptor depending on the receptor partners within the heteromer. Therefore, designing novel compounds that specifically target and modulate D1R-D3R heteromers would be an interesting approach for the treatment of levodopa (L-DOPA)-induced dyskinesias.

Treating non-motor symptoms of Parkinson's disease with transplantation of stem cells

Parkinson's disease (PD) treatment-based research has focused on developing therapies for the management of motor symptoms. Non-motor symptoms do not respond to treatments targeting motor deficits, thus necessitating an urgent need to develop new modalities that cater to both motor and non-motor deficits. Stem cell transplantation is potentially therapeutic for PD, but the disease non-motor symptoms have been primarily neglected in such cell therapy regimens. Many types of stem cells are currently available for transplantation therapy, including adult tissue (e.g., bone marrow, placenta)-derived mesenchymal stem cells. The fact that mesenchymal stem cells can replace and rescue degenerated dopaminergic and non-dopaminergic cells suggests their potential for the treatment of motor as well as non-motor symptoms of PD, which is discussed in this article.

Targeting the vasculature to improve neural progenitor transplant survival

Neural progenitor transplantation is a promising therapeutic option for several neurological diseases and injuries. In nearly all human clinical trials and animal models that have tested this strategy, the low survival rate of progenitors after engraftment remains a significant challenge to overcome. Developing methods to improve the survival rate will reduce the number of cells required for transplant and will likely enhance functional improvements produced by the procedure. Here we briefly review the close relationship between the blood vasculature and neural progenitors in both the embryo and adult nervous system. We also discuss previous studies that have explored the role of the vasculature and hypoxic pre-conditioning in neural transplants. From these studies, we suggest that hypoxic pre-conditioning of a progenitor pool containing both neural and endothelial cells will improve engrafted transplanted neuronal survival rates.

Cell therapy for Parkinson's disease: Functional role of the host immune response on survival and differentiation of dopaminergic neuroblasts

Parkinson's disease (PD) is a neurodegenerative disorder, whose cardinal pathology is the loss of dopaminergic neurons in the substantia nigra. Current treatments for PD have side effects in the long term and do not halt disease progression or regenerate dopaminergic cell loss. Attempts to compensate neuronal cell loss by transplantation of dopamine-producing cells started more than 30 years ago, leading to several clinical trials. These trials showed safety and variable efficacy among patients. In addition to variability in efficacy, several patients developed graft-induced dyskinesia. Nevertheless, they have provided a proof of concept that motor symptoms could be improved by cell transplantation. Cell transplantation in the brain presents several immunological challenges. The adaptive immune response should be abolished to avoid graft rejection by the host. In addition, the innate immune response will always be present after transplanting cells into the brain. Remarkably, the innate immune response can have dramatic effects on the survival, differentiation and proliferation of the transplanted cells, but has been hardly investigated. In this review, we analyze data on the functional effects of signals from the innate immune system on dopaminergic differentiation, survival and proliferation. Then, we discussed efforts on cell transplantation in animal models and PD patients, highlighting the immune response and the immunomodulatory treatment strategies performed. The analysis of the available data lead us to conclude that the modulation of the innate immune response after transplantation can increase the success of future clinical trials in PD by enhancing cell differentiation and survival. This article is part of a Special Issue entitled SI: PSC and the brain.

PET Imaging of Serotonin Transporters With 4-[(18)F]-ADAM in a Parkinsonian Rat Model With Porcine Neural Xenografts

Parkinson's disease (PD) is a neurodegenerative disease characterized by a loss of dopaminergic neurons in the nigrostriatal pathway. Apart from effective strategies to halt the underlying neuronal degeneration, cell replacement now offers novel prospects for PD therapy. Porcine embryonic neural tissue has been considered an alternative source to human fetal grafts in neurodegenerative disorders because its use avoids major practical and ethical issues. This study was undertaken to evaluate the effects of embryonic day 27 (E27) porcine mesencephalic tissue transplantation in a PD rat model using animal positron emission tomography (PET) coupled with 4-[(18)F]-ADAM, a serotonin transporter (SERT) imaging agent. The parkinsonian rat was induced by injecting 6-hydroxydopamine into the medial forebrain bundle (MFB) of the right nigrostriatal pathway. The apomorphine-induced rotation behavioral test and 4-[(18)F]-ADAM/animal PET scanning were carried out following 6-OHDA lesioning. At the second week following 6-OHDA lesioning, the parkinsonian rat rotates substantially on apomorphine-induced contralateral turning. In addition, the mean striatal-specific uptake ratio (SUR) of 4-[(18)F]-ADAM decreased by 44%. After transplantation, the number of drug-induced rotations decreased markedly, and the mean SUR of 4-[(18)F]-ADAM and the level of SERT immunoreactivity (SERT-ir) in striatum were partially restored. The mean SUR level was restored to 71% compared to that for the contralateral intact side, which together with the abundant survival of tyrosine hydroxylase (TH) neurons accounted for functional recovery at the fourth week postgraft. In regard to the extent of donor-derived cells, we found the neurons of the xenografts from E27 transgenic pigs harboring red fluorescent protein (RFP) localized with TH-ir cells and SERT-ir in the grafted area. Thus, transplanted E27 porcine mesencephalic tissue may restore dopaminergic and serotonergic systems in the parkinsonian rat. The 4-[(18)F]-ADAM/animal PET can be used to detect serotonergic neuron loss in PD and monitor the efficacy of therapy.

Nanotechnology-mediated nose to brain drug delivery for Parkinson's disease: a mini review

Nose to brain delivery of neurotherapeutics have been tried by several researchers to explore the virtues of this route viz. circumvention of BBB, avoidance of hepatic metabolism, practicality, safety, ease of administration and non-invasiveness. Nanoparticle (NP) therapeutics is an emerging modality for the treatment of Parkinson's disease (PD) as it offers targeted delivery and enhances the therapeutic efficacy and/or bioavailability of neurotherapeutics. This review presents a concise incursion into the nanomedicines suitable for PD therapy delivered via naso-brain transport. Clinical signs of PD, its pathophysiology, specific genetic determinants, diagnosis and therapy involved have been hashed out. Properties of brain-targeting NPs, transport efficacy and various nanocarriers developed so far also been furnished. In our opinion, nanotechnology-enabled naso-brain drug delivery is an excellent means of delivering neurotherapeutics and is a promising avenue for researchers to develop new formulations for the effective management of PD.

Adult subventricular zone neural stem cells as a potential source of dopaminergic replacement neurons

Clinical trials engrafting human fetal ventral mesencephalic tissue have demonstrated, in principle, that cell replacement therapy provides substantial long-lasting improvement of motor impairments generated by Parkinson's Disease (PD). The use of fetal tissue is not practical for widespread clinical implementation of this therapy, but stem cells are a promising alternative source for obtaining replacement cells. The ideal stem cell source has yet to be established and, in this review, we discuss the potential of neural stem cells in the adult subventricular zone (SVZ) as an autologous source of replacement cells. We identify three key challenges for further developing this potential source of replacement cells: (1) improving survival of transplanted cells, (2) suppressing glial progenitor proliferation and survival, and (3) developing methods to efficiently produce dopaminergic neurons. Subventricular neural stem cells naturally produce a dopaminergic interneuron phenotype that has an apparent lack of vulnerability to PD-mediated degeneration. We also discuss whether olfactory bulb dopaminergic neurons derived from adult SVZ neural stem cells are a suitable source for cell replacement strategies.

Opposite effects of bone marrow-derived cells transplantation in MPTP-rat model of Parkinson's disease: a comparison study of mononuclear and mesenchymal stem cells

The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) animal model is a useful tool to study Parkinson's disease (PD) and was used in the present study to investigate the potential beneficial as well as deleterious effects of systemic bone-marrow mononuclear cell (BMMC) or mesenchymal stem cell (BM-MSC) transplantation. MPTP administration resulted in a breakdown of the blood-brain barrier and motor impairment in the open field test 24 h after surgery. Three and 7 days after receiving the lesion, the injured animals showed remaining motor impairment compared to the sham groups along with a significant loss of tyrosine hydroxylase-immunoreactive (TH-ir) cells in the substantia nigra pars compacta (SNpc). The MPTP-lesioned rats treated with BMMCs immediately after lesioning exhibited motor impairment similar to the MPTP-saline group, though they presented a significantly higher loss of TH-ir cells in the SNpc compared to the MPTP-saline group. This increased loss of TH-ir cells in the SNpc was not observed when BMMC transplantation was performed 24 h after MPTP administration. In contrast, in the MPTP animals treated early with systemic BM-MSCs, no loss of TH-ir cells was observed. BMMCs and BM-MSCs previously labeled with CM-DiI cell tracker were found in brain sections of all transplanted animals. In addition, cells expressing CD45, an inflammatory white blood cell marker, were found in all brain sections analyzed and were more abundant in the MPTP-BMMC animals. In these animals, Iba1+ microglial cells showed also marked morphological changes indicating increased microglial activation. These results show that systemic BMMC transplantation did not ameliorate or prevent the lesion induced by MPTP. Instead, BMMC transplantation in MPTP-lesioned rats accelerated dopaminergic neuronal damage and induced motor impairment and immobility behavior. These findings suggest that caution should be taken when considering cell therapy using BMMCs to treat PD. However, systemic BM-MSC transplantation that reaches the injury site and prevents neuronal damage after an MPTP infusion could be considered as a potential treatment for PD during the early stage of disease development.

Characterization of Porcine Ventral Mesencephalic Precursor Cells following Long-Term Propagation in 3D Culture

The potential use of predifferentiated neural precursor cells for treatment of a neurological disorder like Parkinson's disease combines stem cell research with previous experimental and clinical transplantation of developing dopaminergic neurons. One current obstacle is, however, the lack of ability to generate dopaminergic neurons after long-term in vitro propagation of the cells. The domestic pig is considered a useful nonprimate large animal model in neuroscience, because of a better resemblance of the larger gyrencephalic pig brain to the human brain than the commonly used brains of smaller rodents. In the present study, porcine embryonic (28–30 days), ventral mesencephalic precursor cells were isolated and propagated as free-floating neural tissue spheres in medium containing epidermal growth factor and fibroblast growth factor 2. For passaging, the tissue spheres were cut into quarters, avoiding mechanical or enzymatic dissociation in order to minimize cellular trauma and preserve intercellular contacts. Spheres were propagated for up to 237 days with analysis of cellular content and differentiation at various time points. Our study provides the first demonstration that porcine ventral mesencephalic precursor cells can be long-term propagated as neural tissue spheres, thereby providing an experimental 3D in vitro model for studies of neural precursor cells, their niche, and differentiation capacity.

Transplanting intact donor tissue enhances dopamine cell survival and the predictability of motor improvements in a rat model of Parkinson's disease

Primary cell transplantation is currently the gold standard for cell replacement in Parkinson's disease. However, the number of donors needed to treat a single patient is high, and the functional outcome is sometimes variable. The present work explores the possibility of enhancing the viability and/or functionality of small amounts of ventral mesencephalic (VM) donor tissue by reducing its perturbation during preparation and implantation. Briefly, unilaterally lesioned rats received either: (1) an intact piece of half an embryonic day 13 (E13) rat VM; (2) dissociated cells from half an E13 rat VM; or (3) no transplant. D-amphetamine- induced rotations revealed that animals receiving pieces of VM tissue or dissociated cells showed significant improvement in ipsilateral rotation 4 weeks post transplantation. By 6 weeks post transplantation, animals receiving pieces of VM tissue showed a trend for further improvement, while those receiving dissociated cells remained at their 4 week scores. Postmortem cell counts showed that the number of dopaminergic neurons in dissociated cell transplants was significantly lower than that surviving in transplants of intact tissue. When assessing the correlation between the number of dopamine cells in each transplant, and the improvement in rotation bias in experimental animals, it was shown that transplants of whole pieces of VM tissue offered greater predictability of graft function based on their dopamine cell content. Such results suggest that maintaining the integrity of VM tissue during implantation improves dopamine cell content, and that the dopamine cell content of whole tissue grafts offers a more predictable outcome of graft function in an animal model of Parkinson's disease.

Motoneuron replacement for reinnervation of skeletal muscle in adult rats

Reinnervation is needed to rescue muscle when motoneurons die in disease or injury. Embryonic ventral spinal cord cells transplanted into peripheral nerve reinnervate muscle and reduce atrophy, but low motoneuron survival may limit motor unit formation. We tested whether transplantation of a purified population of embryonic motoneurons into peripheral nerve (mean ± SE, 146,458 ± 4,011 motoneurons) resulted in more motor units and reinnervation than transplantation of a mixed population of ventral spinal cord cells (72,075 ± 12,329 motoneurons). Ten weeks after either kind of transplant, similar numbers of neurons expressed choline acetyl transferase and/or Islet-1. Motoneuron numbers always exceeded the reinnervated motor unit count. Most motor end plate were simple plaques. Reinnervation significantly reduced muscle fiber atrophy. These data show that the number of transplanted motoneurons and motoneuron survival do not limit muscle reinnervation. Incomplete differentiation of motoneurons in nerve and lack of muscle activity may result in immature neuromuscular junctions that limit reinnervation and function.

Clinical translation of stem cells in neurodegenerative disorders

Stem cells and their derivatives show tremendous potential for treating many disorders, including neurodegenerative diseases. We discuss here the challenges and potential for the translation of stem-cell-based approaches into treatments for Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis.

A synopsis on the role of tyrosine hydroxylase in Parkinson's disease

Parkinson's disease (PD) is a common chronic progressive neurodegenerative disorder in elderly people. A consistent neurochemical abnormality in PD is degeneration of dopaminergic neurons in substantia nigra pars compacta, leading to a reduction of striatal dopamine (DA) levels. As tyrosine hydroxylase (TH) catalyses the formation of L-dihydroxyphenylalanine (L-DOPA), the rate-limiting step in the biosynthesis of DA, the disease can be considered as a TH-deficiency syndrome of the striatum. Problems related to PD usually build up when vesicular storage of DA is altered by the presence of either α-synuclein protofibrils or oxidative stress. Phosphorylation of three physiologically-regulated specific sites of N-terminal domain of TH is vital in regulating its kinetic and protein interaction. The concept of physiological significance of TH isoforms is another interesting aspect to be explored further for a comprehensive understanding of its role in PD. Thus, a logical and efficient strategy for PD treatment is based on correcting or bypassing the enzyme deficiency by the treatment with L-DOPA, DA agonists, inhibitors of DA metabolism or brain grafts with cells expressing a high level of TH. Neurotrophic factors are also attracting the attention of neuroscientists because they provide the essential neuroprotective and neurorestorative properties to the nigrostriatal DA system. PPAR-γ, a key regulator of immune responses, is likewise a promising target for the treatment of PD, which can be achieved by the use of agonists with the potential to impact the expression of pro- and anti-inflammatory cytokines at the transcriptional level in immune cells via expression of TH. Herein, we review the primary biochemical and pathological features of PD, and describe both classical and developing approaches aimed to ameliorate disease symptoms and its progression.

Epigenetic engineering to reverse the Parkinson's expression state

Nature, initiation and ensuing cellular propagation mode of sporadic Parkinson's disease (comprising over 90% of all Parkinson's cases) remain open research questions. Accordingly, so does the best therapeutic avenue for addressing this debilitating disease that today affects an estimated 7-10 million people worldwide. Recently, we argued that sporadic Parkinson's be fundamentally characterized as a pathological deviation from normality in the expression program of a cell, the PD-state. Further, we suggested this generic cell state (not restricted to neurons) could be epigenetically locked-in. This raises the theoretical possibility of reverting a cell's PD-state to normality by appropriate epigenetic reprogramming. In here, we propose an in vitro relatively high throughput search for a cocktail of molecules that induces an epigenetic reversal of the PD-state. A generic multi-tissue PD-state phenotype appears to be a defect on mitochondrial bioenergetics. In the above search, we suggest utilizing a metabolic challenge as a preliminary screen for assessing, via improvement of energy metabolism, reversal of the PD-state.

Neural regeneration

Regeneration of the nervous system requires either the repair or replacement of nerve cells that have been damaged by injury or disease. While lower organisms possess extensive capacity for neural regeneration, evolutionarily higher organisms including humans are limited in their ability to regenerate nerve cells, posing significant issues for the treatment of injury and disease of the nervous system. This chapter focuses on current approaches for neural regeneration, with a discussion of traditional methods to enhance neural regeneration as well as emerging concepts within the field such as stem cells and cellular reprogramming. Stem cells are defined by their ability to self-renew as well as their ability to differentiate into multiple cell types, and hence can serve as a source for cell replacement of damaged neurons. Traditionally, adult stem cells isolated from the hippocampus and subventricular zone have served as a source of neural stem cells for replacement purposes. With the advancement of pluripotent stem cells, including human embryonic stem cells (hESCs) and human induced pluripotent stem cells (iPSCs), new and exciting approaches for neural cell replacement are being developed. Furthermore, with increased understanding of the human genome and epigenetics, scientists have been successful in the direct genetic reprogramming of somatic cells to a neuronal fate, bypassing the intermediary pluripotent stage. Such breakthroughs have accelerated the timing of production of mature neuronal cell types from a patient-specific somatic cell source such as skin fibroblasts or mononuclear blood cells. While extensive hurdles remain to the translational application of such stem cell and reprogramming strategies, these approaches have revolutionized the field of regenerative biology and have provided innovative approaches for the potential regeneration of the nervous system.

Cell transplantation and gene therapy in Parkinson's disease

Parkinson's disease is a progressive neurodegenerative disorder affecting, in part, dopaminergic motor neurons of the ventral midbrain and their terminal projections that course to the striatum. Symptomatic strategies focused on dopamine replacement have proven effective at remediating some motor symptoms during the course of disease but ultimately fail to deliver long-term disease modification and lose effectiveness due to the emergence of side effects. Several strategies have been experimentally tested as alternatives for Parkinson's disease, including direct cell replacement and gene transfer through viral vectors. Cellular transplantation of dopamine-secreting cells was hypothesized as a substitute for pharmacotherapy to directly provide dopamine, whereas gene therapy has primarily focused on restoration of dopamine synthesis or neuroprotection and restoration of spared host dopaminergic circuitry through trophic factors as a means to enhance sustained controlled dopamine transmission. This seems now to have been verified in numerous studies in rodents and nonhuman primates, which have shown that grafts of fetal dopamine neurons or gene transfer through viral vector delivery can lead to improvements in biochemical and behavioral indices of dopamine deficiency. However, in clinical studies, the improvements in parkinsonism have been rather modest and variable and have been plagued by graft-induced dyskinesias. New developments in stem-cell transplantation and induced patient-derived cells have opened the doors for the advancement of cell-based therapeutics. In addition, viral-vector-derived therapies have been developed preclinically with excellent safety and efficacy profiles, showing promise in clinical trials thus far. Further progress and optimization of these therapies will be necessary to ensure safety and efficacy before widespread clinical use is deemed appropriate.

Stem cell challenges in the treatment of neurodegenerative disease

Neurodegenerative diseases result from the gradual and progressive loss of neural cells and lead to nervous system dysfunction. The rapidly advancing stem cell field is providing attractive alternative options for fighting these diseases. Results have provided proof of principle that cell replacement can work in humans with Parkinson's disease (PD). However, three clinical studies of cell transplantation were published that found no net benefit, while patients in two of the studies developed dyskinesias that persisted despite reductions in treatment. Induced pluripotent stem cells (iPSC) have major potential advantages because patient-specific neuroblasts are suitable for transplantation, avoid immune reactions, and can be produced without the use of human ES cells (hESC). Although iPSCs have not been successfully used in clinical trials for PD, patients with amyotrophic lateral sclerosis (ALS) were treated with autologous stem cells and, though they had some degree of decline one year after treatment, they were still improved compared with the preoperative period or without any drug therapy. In addition, neural stem cells (NSCs), via brain-derived neurotrophic factor (BDNF), have been shown to ameliorate complex behavioral deficits associated with widespread Alzheimer's disease (AD) pathology in a transgenic mouse model of AD. So far, the FDA lists 18 clinical trials treating multiple sclerosis (MS), but most are in preliminary stages. This article serves as an overview of recent studies in stem cell and regenerative approaches to the above chronic neurodegenerative disorders. There are still many obstacles to the use of stem cells as a cure for neurodegenerative disease, especially because we still don't fully understand the true mechanisms of these diseases. However, there is hope in the potential of stem cells to help us learn and understand a great deal more about the mechanisms underlying these devastating neurodegenerative diseases.

Cell transplantation in Parkinson's disease: problems and perspectives

PURPOSE OF REVIEW

We review recent experiments conducted using embryonic tissue and stem cell transplants in experimental models of Parkinson's disease. We also highlight the challenges which remain to be met in order for cell therapy to become clinically effective and safe.

RECENT FINDINGS

The outcome of previous clinical transplantation trials was variable in terms of motor recovery. We discuss whether transplants can mitigate L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesias and consider the risk factors which predispose to graft-induced dyskinesias. In addition, we introduce Transeuro, a new European Union-funded multicenter consortium which plans to perform transplantation trials.Stem cells have emerged as an alternative source for the generation of dopaminergic precursors. We briefly outline progress made in the use of human embryonic stem cells and focus predominantly on the emerging field of induced pluripotency. We conclude by introducing the exciting and novel method of direct reprogramming which involves the conversion of fibroblasts to neurons without inducing a pluripotent state.

SUMMARY

The area of cell transplantation has been revitalized by the identification of parameters which predispose patients to graft-induced dyskinesias and by the emergence of novel methods of generating dopaminergic neurons. Hopefully, the Transeuro clinical trials will give further impetus and act as a stepping stone to future trials employing stem-cell-derived neurons.

Role of transcription factors in cell replacement therapies for neurodegenerative conditions

Parkinson's disease is the second most common neurological condition, behind dementia, for which there is currently no cure. A promising curative treatment approach is cell replacement therapy, which involves the introduction of new dopaminergic cells into a degenerative Parkinson's disease brain. The future progression of this field into a clinically viable treatment option is reliant on generating replacement dopaminergic cells. Furthermore, as the ability of transplanted dopaminergic neurons to form connections with host tissue is dependent on where the cells are derived from, the replacement dopaminergic cells will need to be phenotypically similar to substantia nigra dopaminergic neurons. This article focuses on how developmental transcription factors have been utilized to assist the progression of stem cell therapies for Parkinson's disease. Key transcription factor-mediated stages of substantia nigra dopaminergic neuronal development is described in the belief that a comprehensive understanding of this specific dopaminergic differentiation pathway is necessary for the progression of successful cell therapies for Parkinson's disease.

Progress and prospects: stem cells and neurological diseases

The central nervous system has limited capacity of regenerating lost tissue in slowly progressive, degenerative neurological conditions such as Parkinson's disease (PD), Alzheimer's disease or Huntington's disease (HD), or in acute injuries resulting in rapid cell loss for example, in cerebrovascular damage (for example, stroke) or spinal cord injury. Although the adult brain contains small numbers of stem cells in restricted areas, they do not contribute significantly to functional recovery. Transplantation of stem cells or stem cell-derived progenitors has long been seen as a therapeutic solution to repair the damaged brain. With the advent of the induced pluripotent stem cells technique a new and potentially better source for transplantable cells may be available in future. This review aims to highlight current strategies to replace lost cellular populations in neurodegenerative diseases with the focus on HD and PD and traumatic brain injuries such as stroke, discussing many of the technical and biological issues associated with central nervous system cell transplantation.

Treatment strategies for Parkinson's disease

Parkinson's disease (PD) is caused by progressive degeneration of dopamine (DA) neurons in the substantia nigra pars compacta (SNpc), resulting in the deficiency of DA in the striatum. Thus, symptoms are developed, such as akinesia, rigidity and tremor. The aetiology of neuronal death in PD still remains unclear. Several possible mechanisms of the degeneration of dopaminergic neurons are still elusive. Various mechanisms of neuronal degeneration in PD have been proposed, including formation of free radicals, oxidative stress, mitochondrial dysfunction, excitotoxicity, calcium cytotoxicity, trophic factor deficiency, inflammatory processes, genetic factors, environmental factors, toxic action of nitric oxide, and apoptosis. All these factors interact with each other, inducing a vicious cycle of toxicity causing neuronal dysfunction, atrophy and finally cell death. Considerable evidence suggests that free radicals and oxidative stress may play key roles in the pathogenesis of PD. However, currently, drug therapy cannot completely cure the disease. DA replacement therapy with levodopa (L-Dopa), although still being a gold standard for symptomatic treatment of PD, only alleviates the clinical symptoms. Furthermore, patients usually experience severe side effects several years after the L-Dopa treatment. Until now, no therapy is available to stop or at least slow down the neurodegeneration in patients. Therefore, efforts are made not only to improve the effect of L-Dopa treatment for PD, but also to investigate new drugs with both antiparkinsonian and neuroprotective effects. Here, the advantages and limitations of current and future therapies for PD were dicussed. Current therapies include dopaminergic therapy, DA agonists, MAO-B inhibitor, COMT inhibitors, anticholinergic drugs, surgical procedures such as pallidotomy and more specifically deep brain stimulation of the globus pallidus pars interna (GPi) or subthalamic nucleus (STN), and stem cell transplantation.

Parkinson's disease and growth factors - are they the answer?

The core loss of dopaminergic neurons in the substantia nigra in Parkinson's disease (PD) coupled to the therapeutic benefits of dopaminergic therapies in patients, simplifies the treatment strategy for this disease. In the context of neurotrophic factors, this distils down to the simple question as to whether a factor exists for these cells that can promote their survival in the face of the degenerative disease process. If such a factor exists, and GDNF seems a strong candidate, then one could anticipate that this treatment would be as effective as L-dopa therapy. However it would not be better than this, nor curative, given the extensive pathology in PD. To date a number of clinical trials have been undertaken in which GDNF has been directly delivered to the PD brain. In addition there have been studies in which neurturin (part of the GDNF family) has also been delivered to the CNS using a viral vector delivery system. These trials have produced mixed results. Importantly though, some patients have shown a sustained clinical response to this treatment which correlates with evidence of increased dopaminergic activity in the brain at the site of delivery using F-dopa PET as well as in a single post-mortem study. The challenge therefore is not whether this approach works, because it self-evidently does in some patients, but rather how we can do this more consistently.