Dyskinesias after neural transplantation in Parkinson's disease: what do we know and what is next?

Human iPSC-derived midbrain organoids functionally integrate into striatum circuits and restore motor function in a mouse model of Parkinson's disease

Rationale: Parkinson's disease (PD) is a prevalent neurodegenerative disorder that is characterized by degeneration of dopaminergic neurons (DA) at the substantia nigra pas compacta (SNpc). Cell therapy has been proposed as a potential treatment option for PD, with the aim of replenishing the lost DA neurons and restoring motor function. Fetal ventral mesencephalon tissues (fVM) and stem cell-derived DA precursors cultured in 2-dimentional (2-D) culture conditions have shown promising therapeutic outcomes in animal models and clinical trials. Recently, human induced pluripotent stem cells (hiPSC)-derived human midbrain organoids (hMOs) cultured in 3-dimentional (3-D) culture conditions have emerged as a novel source of graft that combines the strengths of fVM tissues and 2-D DA cells. Methods: 3-D hMOs were induced from three distinct hiPSC lines. hMOs at various stages of differentiation were transplanted as tissue pieces into the striatum of naïve immunodeficient mouse brains, with the aim of identifying the most suitable stage of hMOs for cellular therapy. The hMOs at Day 15 were determined to be the most appropriate stage and were transplanted into a PD mouse model to assess cell survival, differentiation, and axonal innervation in vivo. Behavioral tests were conducted to evaluate functional restoration following hMO treatment and to compare the therapeutic effects between 2-D and 3-D cultures. Rabies virus were introduced to identify the host presynaptic input onto the transplanted cells. Results: hMOs showed a relatively homogeneous cell composition, mostly consisting of dopaminergic cells of midbrain lineage. Analysis conducted 12 weeks post-transplantation of day 15 hMOs revealed that 14.11% of the engrafted cells expressed TH+ and over 90% of these cells were co-labeled with GIRK2+, indicating the survival and maturation of A9 mDA neurons in the striatum of PD mice. Transplantation of hMOs led to a reversal of motor function and establishment of bidirectional connections with natural brain target regions, without any incidence of tumor formation or graft overgrowth. Conclusion: The findings of this study highlight the potential of hMOs as safe and efficacious donor graft sources for cell therapy to treat PD.

Serotonergic imaging in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive degeneration of monoaminergic central pathways such as the serotonergic. The degeneration of serotonergic signaling in striatal and extrastriatal brain regions is an early feature of PD and is associated with several motor and non-motor complications of the disease. Molecular imaging techniques with Positron Emission Tomography (PET) have greatly contributed to the investigation of biological changes in vivo and to the understanding of the extent of serotonergic pathology in patients or individuals at risk for PD. Such discoveries provide with opportunities for the identification of new targets that can be used for the development of novel disease-modifying drugs or symptomatic treatments. Future studies of imaging serotonergic molecular targets will better clarify the importance of serotonergic pathology in PD, including progression of pathology, target-identification for pharmacotherapy, and relevance to endogenous synaptic serotonin levels. In this article, we review the current status and understanding of serotonergic imaging in PD.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

The quest of cell surface markers for stem cell therapy

Stem cells and their derivatives are novel pharmaceutics that have the potential for use as tissue replacement therapies. However, the heterogeneous characteristics of stem cell cultures have hindered their biomedical applications. In theory and practice, when cell type-specific or stage-specific cell surface proteins are targeted by unique antibodies, they become highly efficient in detecting and isolating specific cell populations. There is a growing demand to identify reliable and actionable cell surface markers that facilitate purification of particular cell types at specific developmental stages for use in research and clinical applications. The identification of these markers as very important members of plasma membrane proteins, ion channels, transporters, and signaling molecules has directly benefited from proteomics and tools for proteomics-derived data analyses. Here, we review the methodologies that have played a role in the discovery of cell surface markers and introduce cutting edge single cell proteomics as an advanced tool. We also discuss currently available specific cell surface markers for stem cells and their lineages, with emphasis on the nervous system, heart, pancreas, and liver. The remaining gaps that pertain to the discovery of these markers and how single cell proteomics and identification of surface markers associated with the progenitor stages of certain terminally differentiated cells may pave the way for their use in regenerative medicine are also discussed.

Stimulation of L-type calcium channels increases tyrosine hydroxylase and dopamine in ventral midbrain cells induced from somatic cells

Making high‐quality dopamine (DA)‐producing cells for basic biological or small molecule screening studies is critical for the development of novel therapeutics for disorders of the ventral midbrain. Currently, many ventral midbrain assays have low signal‐to‐noise ratio due to low levels of cellular DA and the rate‐limiting enzyme of DA synthesis, tyrosine hydroxylase (TH), hampering discovery efforts. Using intensively characterized ventral midbrain cells derived from human skin, which demonstrate calcium pacemaking activity and classical electrophysiological properties, we show that an L‐type calcium agonist can significantly increase TH protein levels and DA content and release. Live calcium imaging suggests that it is the immediate influx of calcium occurring simultaneously in all cells that drives this effect. Genome‐wide expression profiling suggests that L‐type calcium channel stimulation has a significant effect on specific genes related to DA synthesis and affects expression of L‐type calcium receptor subunits from the CACNA1 and CACNA2D families. Together, our findings provide an advance in the ability to increase DA and TH levels to improve the accuracy of disease modeling and small molecule screening for disorders of the ventral midbrain, including Parkinson's disease.

Gelator length precisely tunes supramolecular hydrogel stiffness and neuronal phenotype in 3D culture

The brain is one of the softest tissues in the body with storage moduli (G') that range from hundreds to thousands of pascals (Pa) depending upon the anatomic region. Furthermore, pathological processes such as injury, aging and disease can cause subtle changes in the mechanical properties throughout the central nervous system. However, these changes in mechanical properties lie within an extremely narrow range of moduli and there is great interest in understanding their effect on neuron biology. We report here the design of supramolecular hydrogels based on anionic peptide amphiphile nanofibers using oligo-L-lysines of different molecular lengths to precisely tune gel stiffness over the range of interest and found that G' increases by 10.5 Pa for each additional lysine monomer in the oligo-L-lysine chain. We found that small changes in storage modulus on the order of 70 Pa significantly affect survival, neurite growth and tyrosine hydroxylase-positive population in dopaminergic neurons derived from induced pluripotent stem cells. The work reported here offers a strategy to tune mechanical stiffness of hydrogels for use in 3D neuronal cell cultures and transplantation matrices for neural regeneration.

Imaging Transplantation in Movement Disorders

Cell replacement therapy with graft transplantation has been tested as a disease-modifying treatment in neurodegenerative diseases characterized by the damage of a predominant cell type, such as substantia nigra dopaminergic neurons in Parkinson's disease (PD) or striatal medium spiny projection neurons in Huntington's disease (HD). The results of these trials are mixed with success in preclinical and pilot open-label trials, which were not consistently reproduced in randomized controlled trials. Positron emission tomography (PET) and single photon emission computed tomography (SPECT) molecular imaging and functional magnetic resonance imaging allow the graft survival, and its relationship with the host tissues to be studied in vivo. In PD, PET with [¹⁸F]DOPA showed that graft survival does not necessarily correlate with the clinical improvement and PD patients with worse outcome had lower binding in the ventral striatum and a high serotonin ([¹¹C]DASB PET) to dopamine ([¹⁸F]DOPA PET) ratio in the grafted neurons. In HD, PET with [¹¹C]PK11195 showed the graft survival and the clinical responses may be related to the reactive activation of the host inflammatory/immune system. Findings from these studies have been used to refine study protocols and patient selection in current clinical trials, which includes identifying suitable candidates for transplantation using imaging markers and employing multiple and/or novel PET tracers to better assess graft functions and inflammatory responses to grafts.

Imaging Markers of Progression in Parkinson's Disease

Background

Parkinson's disease (PD) is the second-most common neurodegenerative disorder after Alzheimer's disease; however, to date, there is no approved treatment that stops or slows down disease progression. Over the past decades, neuroimaging studies, including molecular imaging and MRI are trying to provide insights into the mechanisms underlying PD.

Methods

This work utilized a literature review.

Results

It is now becoming clear that these imaging modalities can provide biomarkers that can objectively detect brain changes related to PD and monitor these changes as the disease progresses, and these biomarkers are required to establish a breakthrough in neuroprotective or disease-modifying therapeutics.

Conclusions

Here, we provide a review of recent observations deriving from PET, single-positron emission tomography, and MRI studies exploring PD and other parkinsonian disorders.

Discovery of Novel Cell Surface Markers for Purification of Embryonic Dopamine Progenitors for Transplantation in Parkinson's Disease Animal Models

Despite the progress in safety and efficacy of cell replacement therapy with pluripotent stem cells (PSCs), the presence of residual undifferentiated stem cells or proliferating neural progenitor cells with rostral identity remains a major challenge. Here we report the generation of a LIM homeobox transcription factor 1 alpha (LMX1A) knock-in GFP reporter human embryonic stem cell (hESC) line that marks the early dopaminergic progenitors during neural differentiation to find reliable membrane protein markers for isolation of midbrain dopaminergic neurons. Purified GFP positive cells in vitro exhibited expression of mRNA and proteins that characterized and matched the midbrain dopaminergic identity. Further quantitative proteomics analysis of enriched LMX1A⁺ cells identified several membrane-associated proteins including a polysialylated embryonic form of neural cell adhesion molecule (PSA-NCAM) and contactin 2 (CNTN2), enabling prospective isolation of LMX1A⁺ progenitor cells. Transplantation of human-PSC-derived purified CNTN2⁺ progenitors enhanced dopamine release from transplanted cells in the host brain and alleviated Parkinson's disease-related phenotypes in animal models. This study establishes an efficient approach for purification of large numbers of human-PSC-derived dopaminergic progenitors for therapeutic applications.

The serotonergic system in Parkinson's patients with dyskinesia: evidence from imaging studies

The purpose of review is to review the current status of positron emission tomography (PET) molecular imaging of serotonergic system in Parkinson’s patients who experience levodopa-induced (LIDs) and graft-induced dyskinesias (GIDs). PET imaging studies have shown that Parkinson’s disease is characterized by progressive loss of dopaminergic and serotonergic neurons. Parkinson’s patients who experienced LIDs and GIDs have an aberrant spreading of serotonergic terminals, which lead to an increased serotonergic/dopaminergic terminals ratio within the putamen. Serotonergic terminals convert exogenous levodopa into dopamine in a non-physiological manner and release an abnormal amount of dopamine without an auto-regulatory feedback. This results in higher swings in synaptic levels of dopamine, which leads to the development of LIDs and GIDs. The modulation of serotonergic terminals with 5-HT_1A and 5-HT_1B receptors agonists partially reduced these motor complications. In vivo PET studies confirmed that abnormal spreading of serotonergic terminals within the putamen has a pivotal role in the development of LIDs and GIDs. However, glutamatergic, adenosinergic, opioid systems, and phosphodiesterases 10A may also play a role in the development of these motor complications. An integrative multimodal imaging approach combining PET and MRI imaging techniques is needed to fully understand the mechanisms underlying the development of LIDs and GIDs.

PET Molecular Imaging Research of Levodopa-Induced Dyskinesias in Parkinson's Disease

Purpose of Review

To review the current status of positron emission tomography (PET) molecular imaging research of levodopa-induced dyskinesias (LIDs) in Parkinson’s disease (PD).

Recent Findings

Recent PET studies have provided robust evidence that LIDs in PD are associated with elevated and fluctuating striatal dopamine synaptic levels, which is a consequence of the imbalance between dopaminergic and serotonergic terminals, with the latter playing a key role in mishandling presynaptic dopamine release. Long-term exposure to levodopa is no longer believed to solely induce LIDs, as studies have highlighted that PD patients who go on to develop LIDs exhibit elevated putaminal dopamine release before the initiation of levodopa treatment, suggesting the involvement of other mechanisms, including altered neuronal firing and abnormal levels of phosphodiesterase 10A.

Summary

Dopaminergic, serotonergic, glutamatergic, adenosinergic and opioid systems and phosphodiesterase 10A levels have been shown to be implicated in the development of LIDs in PD. However, no system may be considered sufficient on its own for the development of LIDs, and the mechanisms underlying LIDs in PD may have a multisystem origin. In line with this notion, future studies should use multimodal PET molecular imaging in the same individuals to shed further light on the different mechanisms underlying the development of LIDs in PD.

Imaging the Nonmotor Symptoms in Parkinson's Disease

Parkinson's disease is acknowledged to be a multisystem syndrome, manifesting as a result of multineuropeptide dysfunction, including dopaminergic, cholinergic, serotonergic, and noradrenergic deficits. This multisystem disorder ultimately leads to the presentation of a range of nonmotor symptoms, now appreciated to be an integral part of the disease-specific spectrum of symptoms, often preceding the diagnosis of motor Parkinson's disease. In this chapter, we review the dopaminergic and nondopaminergic basis of these symptoms by exploring the neuroimaging evidence based on several techniques including positron emission tomography, single-photon emission computed tomography molecular imaging, magnetic resonance imaging, functional magnetic resonance imaging, and diffusion tensor imaging. We discuss the role of these neuroimaging techniques in elucidating the underlying pathophysiology of NMS in Parkinson's disease.

Foetal Cell Transplantation for Parkinson's Disease: Focus on Graft-Induced Dyskinesia

Transplantation of dopamine- (DA-) rich foetal ventral mesencephalic cells emerged as a promising therapy for Parkinson's disease (PD), as it allowed significant improvement of motor symptoms in several PD patients in open-label studies. However, double-blind clinical trials have been largely disappointing. The general agreement in the field is that the lack of standardization of tissue collection and preparation, together with the absence of postsurgical immunosuppression, played a key role in the failure of these studies. Moreover, a further complication that emerged in previous studies is the appearance of the so-called graft-induced dyskinesia (GID), in a subset of grafted patients, which resembles dyskinesia induced by L-DOPA but in the absence of medication. Preclinical evidence pointed to the serotonin neurons as possible players in the appearance of GID. In agreement, clinical investigations have shown that grafted tissue may contain a large number of serotonin neurons, in the order of half of the DA cells; moreover, the serotonin 5-HT1A receptor agonist buspirone has been found to produce significant dampening of GID in grafted patients. In this paper, we will review the recent preclinical and clinical studies focusing on cell transplantation for PD and on the mechanisms underlying GID.

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.

Molecular imaging of levodopa-induced dyskinesias

Levodopa-induced dyskinesias (LIDs) occur in the majority of patients with Parkinson's disease (PD) following years of levodopa treatment. The pathophysiology underlying LIDs in PD is poorly understood, and current treatments generate only minor benefits for the patients. Studies with positron emission tomography (PET) molecular imaging have demonstrated that in advanced PD patients, levodopa administration induces sharp increases in striatal dopamine levels, which correlate with LIDs severity. Fluctuations in striatal dopamine levels could be the result of the attenuated buffering ability in the dopaminergically denervated striatum. Lines of evidence from PET studies indicate that serotonergic terminals could also be responsible for the development of LIDs in PD by aberrantly processing exogenous levodopa and by releasing dopamine in a dysregulated manner from the serotonergic terminals. Additionally, other downstream mechanisms involving glutamatergic, cannabinoid, opioid, cholinergic, adenosinergic, and noradrenergic systems may contribute in the development of LIDs. In this article, we review the findings from preclinical, clinical, and molecular imaging studies, which have contributed to our understanding the pathophysiology of LIDs in PD.

Neuroimaging in Parkinson disease: from research setting to clinical practice

Over the past three decades, neuroimaging studies-including structural, functional and molecular modalities-have provided invaluable insights into the mechanisms underlying Parkinson disease (PD). Observations from multimodal neuroimaging techniques have indicated changes in brain structure and metabolic activity, and an array of neurochemical changes that affect receptor sites and neurotransmitter systems. Characterization of the neurobiological alterations that lead to phenotypic heterogeneity in patients with PD has considerably aided the in vivo investigation of aetiology and pathophysiology, and the identification of novel targets for pharmacological or surgical treatments, including cell therapy. Although PD is now considered to be very complex, no neuroimaging modalities are specifically recommended for routine use in clinical practice. However, conventional MRI and dopamine transporter imaging are commonly used as adjuvant tools in the differential diagnosis between PD and nondegenerative causes of parkinsonism. First-line neuroimaging tools that could have an impact on patient prognosis and treatment strategies remain elusive. This Review discusses the lessons learnt from decades of neuroimaging research in PD, and the promising new approaches with potential applicability to clinical practice.

Treating Parkinson's disease in the 21st century: can stem cell transplantation compete?

The characteristic and selective degeneration of a unique population of cells—the nigrostriatal dopamine (DA) neurons—that occurs in Parkinson’s disease (PD) has made the condition an iconic target for cell replacement therapies. Indeed, transplantation of fetal ventral mesencephalic cells into the DA-deficient striatum was first trialled nearly 30 years ago, at a time when other treatments for the disease were less well developed. Over recent decades standard treatments for PD have advanced, and newer biological therapies are now emerging. In the 21st century, stem cell technology will have to compete alongside other sophisticated treatments, including deep brain stimulation and gene therapies. In this review we examine how stem cell–based transplantation therapies compare with these novel and emerging treatments in the management of this common condition. J. Comp. Neurol. 522:2802–2816, 2014.

Serotonergic mechanisms responsible for levodopa-induced dyskinesias in Parkinson's disease patients

Levodopa-induced dyskinesias (LIDs) are the most common and disabling adverse motor effect of therapy in Parkinson's disease (PD) patients. In this study, we investigated serotonergic mechanisms in LIDs development in PD patients using 11C-DASB PET to evaluate serotonin terminal function and 11C-raclopride PET to evaluate dopamine release. PD patients with LIDs showed relative preservation of serotonergic terminals throughout their disease. Identical levodopa doses induced markedly higher striatal synaptic dopamine concentrations in PD patients with LIDs compared with PD patients with stable responses to levodopa. Oral administration of the serotonin receptor type 1A agonist buspirone prior to levodopa reduced levodopa-evoked striatal synaptic dopamine increases and attenuated LIDs. PD patients with LIDs that exhibited greater decreases in synaptic dopamine after buspirone pretreatment had higher levels of serotonergic terminal functional integrity. Buspirone-associated modulation of dopamine levels was greater in PD patients with mild LIDs compared with those with more severe LIDs. These findings indicate that striatal serotonergic terminals contribute to LIDs pathophysiology via aberrant processing of exogenous levodopa and release of dopamine as false neurotransmitter in the denervated striatum of PD patients with LIDs. Our results also support the development of selective serotonin receptor type 1A agonists for use as antidyskinetic agents in PD.

Dyskinesias in Parkinson's disease: views from positron emission tomography studies

Levodopa-induced dyskinesias (LIDs) and graft-induced dyskinesias (GIDs) are serious and common complications of Parkinson's disease (PD) management following chronic treatment with levodopa or intrastriatal transplantation with dopamine-rich foetal ventral mesencephalic tissue, respectively. Positron emission tomography (PET) molecular imaging provides a powerful in vivo tool that has been employed over the past 20 years for the elucidation of mechanisms underlying the development of LIDs and GIDs in PD patients. PET used together with radioligands tagging molecular targets has allowed the functional investigation of several systems in the brain including the dopaminergic, serotonergic, glutamatergic, opioid, endocannabinoid, noradrenergic and cholinergic systems. In this article the role of PET imaging in unveiling pathophysiological mechanisms underlying the development of LIDs and GIDs in PD patients is reviewed.

Sox-2 Positive Neural Progenitors in the Primate Striatum Undergo Dynamic Changes after Dopamine Denervation

The existence of endogenous neural progenitors in the nigrostriatal system could represent a powerful tool for restorative therapies in Parkinson's disease. Sox-2 is a transcription factor expressed in pluripotent and adult stem cells, including neural progenitors. In the adult brain Sox-2 is expressed in the neurogenic niches. There is also widespread expression of Sox-2 in other brain regions, although the neurogenic potential outside the niches is uncertain. Here, we analyzed the presence of Sox-2⁺ cells in the adult primate (Macaca fascicularis) brain in naïve animals (N = 3) and in animals exposed to systemic administration of 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine to render them parkinsonian (N = 8). Animals received bromodeoxyuridine (100 mg/kg once a day during five consecutive days) to label proliferating cells and their progeny. Using confocal and electron microscopy we analyzed the Sox-2⁺ cell population in the nigrostriatal system and investigated changes in the number, proliferation and neurogenic potential of Sox-2⁺ cells, in control conditions and at two time points after MPTP administration. We found Sox-2⁺ cells with self-renewal capacity in both the striatum and the substantia nigra. Importantly, only in the striatum Sox-2⁺ was expressed in some calretinin⁺ neurons. MPTP administration led to an increase in the proliferation of striatal Sox-2⁺ cells and to an acute, concomitant decrease in the percentage of Sox-2⁺/calretinin⁺ neurons, which recovered by 18 months. Given their potential capacity to differentiate into neurons and their responsiveness to dopamine neurotoxic insults, striatal Sox-2⁺ cells represent good candidates to harness endogenous repair mechanisms for regenerative approaches in Parkinson's disease.

Pluripotent stem cells for Parkinson's disease: progress and challenges

Parkinson's disease (PD) is a common debilitating neurodegenerative disease. The motor symptoms of PD are caused mainly by a progressive loss of dopaminergic neurons from the substania nigra, resulting in a loss of dopamine production. Current therapies are palliative and, in the long term, ineffective. In addition, some can result in significant clinical side effects. The relatively localized pathology of PD makes it an ideal candidate for cell replacement therapy. Initial efforts focused on fetal cell transplantation, and significant clinical benefit lasting more than 10 years has been reported in some cases. However, the approach is controversial and results have been inconsistent. Inherent limitations of this approach for widespread use are the limited availability and variability of transplant material. In contrast, the self-renewal and differentiation potential of human pluripotent stem cells (hPSCs) make them a promising alternative cell source for cell replacement therapy for PD. Efforts in the past decade have demonstrated that hPSCs can be induced to differentiate in culture to functional dopaminergic neurons. Studies in delivering these cells into PD animal models have demonstrated survival, engraftment, and behavioral deficit improvements. Several groups are developing these cells with clinical trials in mind. Here, we review the state of the technology and consider the suitability of current manufacturing processes, cell purity, and tumorgenicity for clinical testing.

Drug-induced dyskinesia in Parkinson's disease. Should success in clinical management be a function of improvement of motor repertoire rather than amplitude of dyskinesia?

BACKGROUND

Dyskinesia, a major complication in the treatment of Parkinson's disease (PD), can require prolonged monitoring and complex medical management.

DISCUSSION

The current paper proposes a new way to view the management of dyskinesia in an integrated fashion. We suggest that dyskinesia be considered as a factor in a signal-to-noise ratio (SNR) equation where the signal is the voluntary movement and the noise is PD symptomatology, including dyskinesia. The goal of clinicians should be to ensure a high SNR in order to maintain or enhance the motor repertoire of patients. To understand why such an approach would be beneficial, we first review mechanisms of dyskinesia, as well as their impact on the quality of life of patients and on the health-care system. Theoretical and practical bases for the SNR approach are then discussed.

SUMMARY

Clinicians should not only consider the level of motor symptomatology when assessing the efficacy of their treatment strategy, but also breadth of the motor repertoire available to patients.

Positron emission tomography imaging in neurological disorders

Positron emission tomography (PET) is a powerful tool for in vivo imaging investigations of human brain function. It provides non-invasive quantification of brain metabolism, receptor binding of various neurotransmitter systems, and alterations in regional blood flow. The use of PET in a clinical setting is still limited due to the high costs of cyclotrons and radiochemical laboratories. However, once these limitations can be bypassed, PET could aid clinical practice by providing a useful imaging technique for the diagnosis, the planning of treatment, and the prediction outcome in various neurological diseases.This review aims to explain the PET imaging technique and its applications in neurological disorders such as Parkinson’s disease, Huntington’s disease, multiple sclerosis, and dementias.

Cell fate analysis of embryonic ventral mesencephalic grafts in the 6-OHDA model of Parkinson's disease

Evidence from carefully conducted open label clinical trials suggested that therapeutic benefit can be achieved by grafting fetal dopaminergic (DAergic) neurons derived from ventral mesencephalon (VM) into the denervated striatum of Parkinson's disease (PD) patients. However, two double-blind trials generated negative results reporting deleterious side effects such as prominent dyskinesias. Heterogeneous composition of VM grafts is likely to account for suboptimal clinical efficacy.We consider that gene expression patterns of the VM tissue needs to be better understood by comparing the genetic signature of the surviving and functioning grafts with the cell suspensions used for transplantation. In addition, it is crucial to assess whether the grafted cells exhibit the DAergic phenotype of adult substantia nigra pars compacta (SNpc). To investigate this further, we used a GFP reporter mouse as source of VM tissue that enabled the detection and dissection of the grafts 6 weeks post implantation. A comparative gene expression analysis of the VM cell suspension and grafts revealed that VM grafts continue to differentiate post-implantation. In addition, implanted grafts showed a mature SNpc-like molecular DAergic phenotype with similar expression levels of TH, Vmat2 and Dat. However, by comparing gene expression of the adult SNpc with dissected grafts we detected a higher expression of progenitor markers in the grafts. Finally, when compared to the VM cell suspension, post-grafting there was a higher expression of markers inherent to glia and other neuronal populations.In summary, our data highlight the dynamic development of distinctive DAergic and non-DAergic gene expression markers associated with the maturation of VM grafts in vivo. The molecular signature of VM grafts and its functional relevance should be further explored in future studies aimed at the optimization of DAergic cell therapy approaches in PD.

Stem cells and regenerative therapies for Parkinson's disease

Currently the mainstay of Parkinson's disease (PD) therapy is the pharmacological replacement of the loss of the dopaminergic nigrostriatal pathway using drugs such as dopamine agonists and levodopa. Whilst these drugs effectively ameliorate some of the motor features of PD, they do not improve many of the nonmotor features that arise secondary to pathology outside of this system, nor do they slow the progressive neurodegeneration that is a characteristic of the disease. Regenerative therapies for PD seek to fill this therapeutic gap, with cell transplantation being the most explored approach to date. A number of different cell sources have been used in this therapeutic approach, but to date, the most successful has been the use of fetal ventral mesencephalic (VM) tissue that contains within it the developing nigral dopaminergic cells. Cell transplantation for PD was pioneered in the 1980-1990s, with several successful open-label trials of fetal VM transplantation in patients with relatively advanced PD. Whilst these findings were not replicated in two subsequent double-blind sham-surgery controlled trials, there were reasons to explain this outside of the one drawn at the time that these therapies are ineffective. Indeed all these studies have provided evidence that following the transplantation of fetal VM tissue, dopaminergic cells can survive long term, produce dopamine, and bring about clinical improvements in younger patients over many years. The use of fetal tissue, irrespective of its true efficacy, will never become a widely available therapy for PD for a host of practical and ethical reasons, and thus much work has been put in recently to exploring the utility of stem cells as a source of nigral dopaminergic neurons. In this respect, the advent of embryonic stem cell and induced pluripotent cells has heralded a new era in cell therapy for PD, and several groups have now demonstrated that these cells can form dopaminergic neurons which improve functional deficits in animal models of PD. Whilst encouraging, problems with respect to the immunogenicity and tumorigenicity of these cells means that they will need to be used in the clinic cautiously. Other regenerative therapies in PD have been tried over the years and include the use of trophic factors. This has primarily involved glial cell line-derived neurotrophic factor (GDNF) and again has produced mixed clinical effects, and in order to try and resolve this, a new trial of intraputamenal GDNF is now being planned. In addition, a new trial for platelet derived growth factor as a treatment for PD has just completed recruitment, and PYM50028 (Cogane) an oral agent shown in animal models to reduce the effects of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) lesioning by the induction of growth factors is currently under investigation in a multicentre Phase II trial. Overall, there are a number of promising new regenerative therapies being developed and tested in PD, although the true long-term efficacy of any of these in large numbers of patients is still not known.

Cell replacement therapy for Parkinson's disease: how close are we to the clinic?

Cell replacement therapy (CRT) offers great promise as the future of regenerative medicine in Parkinson´s disease (PD). Three decades of experiments have accumulated a wealth of knowledge regarding the replacement of dying neurons by new and healthy dopaminergic neurons transplanted into the brains of animal models and affected patients. The first clinical trials provided the proof of principle for CRT in PD. In these experiments, intrastriatal transplantation of human embryonic mesencephalic tissue reinnervated the striatum, restored dopamine levels and showed motor improvements. Sequential controlled studies highlighted several problems that should be addressed prior to the wide application of CRT for PD patients. Moreover, owing to ethical and practical problems, embryonic stem cells require replacement by better-suited stem cells. Several obstacles remain to be surpassed, including identifying the best source of stem cells for A9 dopaminergic neuron generation, eliminating the risk of tumor formation and the development of graft-induced dyskinesias, and standardizing dopaminergic cell production in order to enable clinical application. In this article, we present an update on CRT for PD, reviewing the research milestones, various stem cells used and tailored differentiation methods, and analyze the information gained from the clinical trials.

Clinical application of stem cell therapy in Parkinson's disease

Cell replacement therapies in Parkinson's disease (PD) aim to provide long-lasting relief of patients' symptoms. Previous clinical trials using transplantation of human fetal ventral mesencephalic (hfVM) tissue in the striata of PD patients have provided proof-of-principle that such grafts can restore striatal dopaminergic (DA-ergic) function. The transplants survive, reinnervate the striatum, and generate adequate symptomatic relief in some patients for more than a decade following operation. However, the initial clinical trials lacked homogeneity of outcomes and were hindered by the development of troublesome graft-induced dyskinesias in a subgroup of patients. Although recent knowledge has provided insights for overcoming these obstacles, it is unlikely that transplantation of hfVM tissue will become routine treatment for PD owing to problems with tissue availability and standardization of the grafts. The main focus now is on producing DA-ergic neuroblasts for transplantation from stem cells (SCs). There is a range of emerging sources of SCs for generating a DA-ergic fate in vitro. However, the translation of these efforts in vivo currently lacks efficacy and sustainability. A successful, clinically competitive SC therapy in PD needs to produce long-lasting symptomatic relief without side effects while counteracting PD progression.

In vivo imaging of the integration and function of nigral grafts in clinical trials

In vivo functional imaging has provided objective evidence for the integration and function of nigral grafts in the brains of patients with Parkinson's disease. Clinical trials with the use of positron emission tomography have shown that transplants of human dopamine-rich fetal ventral mesencephalic tissue can survive, grow, and release dopamine providing motor symptom relief, and also that they can restore brain activation related to movement. Positron emission tomography has aided in the elucidation of the pathophysiology of serious adverse effects, so-called graft-induced dyskinesias. With the use of newly established radioligands, positron emission tomography and single-photon emission computed tomography could help to improve Parkinson's patient selection in future clinical trials by selecting those with better predicted outcomes. Moreover, positron emission tomography could help monitoring postoperational inflammatory processes around the grafted tissue and the effect of immunosuppression. Recent evidence from positron emission tomography has provided insight of how ongoing extrastriatal serotonergic denervation may have relevance to nonmotor symptoms in transplanted Parkinson's disease patients indicating new cell therapy targets for a more complete relief of symptoms. Functional and structural magnetic resonance imaging techniques could help to better assess the integration of nigral graft with the host brain by assessing the restoration of brain activation during movement and of functional and structural connectivity. This knowledge should lead to the development of new, optimized in vivo imaging protocols that could help to better schedule, monitor, and modify the clinical outcomes of future human trials assessing the efficacy of fetal or stem cell therapy in Parkinson's disease.

Prospect of cell therapy for Parkinson's disease

The hallmark of Parkinson's disease is on-going degeneration of dopaminergic neurons in the substantia nigra, which may be due to various etiologies. Various approaches to alleviate symptoms are available, such as life-long pharmacological intervention, deep brain stimulation, and transplantation of dopaminergic neuron-containing fetal tissue. However, each of these approaches has a disadvantage. Several studies have shown that various kinds of stem cells, induced pluripotent stem cells, and other cells can differentiate into dopaminergic neurons and may be promising for treating Parkinson's disease in the future. Therefore, this review addresses those cells in terms of their prospects in cell therapy for Parkinson's disease. In addition, the need for safety and efficacy studies, various cell delivery modes and sites, and possible side effects will be discussed.

Neurotransplantation: lux et veritas, fiction or reality?

Neurotransplantation remains a much-debated frontier in contemporary neurosurgery and neuroscience, with roots dating to the late 19th century. Contemporary applications are far-reaching, and ongoing laboratory research and clinical trials seek to define the mechanisms at play in neurotransplant engraftment and growth, while advancing the field forward into the 21st century. Neural transplantation therapy remains an attractive idea for treating central nervous system (CNS) and peripheral nervous system (PNS) pathologies. Phase I and phase II clinical trials assessing safety and efficacy are currently underway for various disorders. The remainder of this review will focus on ongoing clinical trials and more recent research advances involving neural transplantation therapy for neuronal death, axonal injury, peripheral nerve lesions, and cancer. The field of neural transplantation, while promising, is not without ethical and scientific dilemmas; this review will conclude with a discussion of the challenges researchers and clinicians face as the field of neural transplantation moves forward.

Optimizing functional imaging protocols for assessing the outcome of fetal cell transplantation in Parkinson's disease

Clinical trials aiming to assess the safety and efficacy of fetal cell transplantation in Parkinson's disease rely on the hypothesis that the grafted tissue will survive and grow, restore striatal dopaminergic neurotransmission, improve the connectivity between striatum, thalamus and cortex and, thereby, produce long-lasting clinical improvement while avoiding the development of adverse effects. Although transplantation of human fetal ventral mesencephalic tissue has been reported as one of the most effective reparative therapies in Parkinson's disease patients to date, different studies have shown inconsistent results causing a paucity of new trials over the last decade. However, during this period, functional imaging alongside other scientific developments from clinical observations and animal work has significantly aided in understanding the mechanisms responsible for the success or failure of grafting human fetal tissue. Recent advances in functional imaging including both positron emission tomography and functional magnetic resonance imaging could be proven useful in vivo tools for the development and assessment of new clinically competitive trials. In this commentary we discuss how an optimized functional imaging protocol could assist new clinical trials using fetal cell transplantation in Parkinson's disease.