Dopamine receptors in human embryonic stem cell neurodifferentiation

Defining the unknowns for cell therapies in Parkinson's disease

First-in-human clinical trials have commenced to test the safety and efficacy of cell therapies for people with Parkinson's disease (PD). Proof of concept that this neural repair strategy is efficacious is based on decades of preclinical studies and clinical trials using primary foetal cells, as well as a significant literature exploring more novel stem cell-derived products. Although several measures of efficacy have been explored, including the successful in vitro differentiation of stem cells to dopamine neurons and consistent alleviation of motor dysfunction in rodent models, many unknowns still remain regarding the long-term clinical implications of this treatment strategy. Here, we consider some of these outstanding questions, including our understanding of the interaction between anti-Parkinsonian medication and the neural transplant, the impact of the cell therapy on cognitive or neuropsychiatric symptoms of PD, the role of neuroinflammation in the therapeutic process and the development of graft-induced dyskinesias. We identify questions that are currently pertinent to the field that require further exploration, and pave the way for a more holistic understanding of this neural repair strategy for treatment of PD.

Human Embryonic Stem Cell-Derived Dopaminergic Grafts Alleviate L-DOPA Induced Dyskinesia

BACKGROUND

First-in-human studies to test the efficacy and safety of human embryonic stem cells (hESC)-derived dopaminergic cells in the treatment of Parkinson's disease (PD) are imminent. Pre-clinical studies using hESC-derived dopamine neuron transplants in rat models have indicated that the benefits parallel those shown with fetal tissue but have thus far failed to consider how ongoing L-DOPA administration might impact on the graft.

OBJECTIVE

To determine whether L-DOPA impacts on survival and functional recovery following grafting of hESC-derived dopaminergic neurons.

METHODS

Unilateral 6-OHDA lesioned rats were administered with either saline or L-DOPA prior to, and for 18 weeks following surgical implantation of dopaminergic neural progenitors derived from RC17 hESCs according to two distinct protocols in independent laboratories.

RESULTS

Grafts from both protocols elicited reduction in amphetamine-induced rotations. Reduced L-DOPA-induced dyskinesia preceded the improvement in amphetamine-induced rotations. Furthermore, L-DOPA had no effect on overall survival (HuNu) or dopaminergic neuron content of the graft (TH positive cells) but did lead to an increase in the number of GIRK2 positive neurons.

CONCLUSION

Critically, we found that L-DOPA was not detrimental to graft function, potentially enhancing graft maturation and promoting an A9 phenotype. Early improvement of L-DOPA-induced dyskinesia suggests that grafts may support the handling of exogenously supplied dopamine earlier than improvements in amphetamine-induced behaviours indicate. Given that one of the protocols will be employed in the production of cells for the European STEM-PD clinical trial, this is vital information for the management of patients and achieving optimal outcomes following transplantation of hESC-derived grafts for PD.

DNA methylation dynamics of long noncoding RNA during human fetal development

Aim: To determine whether the promoters of long noncoding RNAs (lncRNAs) undergo dynamic changes in DNA methylation during fetal development. Methods: ANOVA and the tissue specificity index were used to identify and validate tissue-specific methylation sites. Age-associated DNA methylation signatures were identified by applying the elastic net method. Results: The lncRNA methylome landscape was characterized in four types of fetal tissue and at three gestational time points, and specific characteristics relative to the tissue of origin and developmental age were identified. Higher levels of lncRNA methylation might be involved in tissue differentiation. LncRNAs harboring age-associated methylation signatures may participate in the fetal developmental process. Conclusion: This study provides novel insights into the role of lncRNA methylomes in fetal tissue specification and development.

Dopamine D3 Receptor Heteromerization: Implications for Neuroplasticity and Neuroprotection

The dopamine (DA) D3 receptor (D3R) plays a pivotal role in the control of several functions, including motor activity, rewarding and motivating behavior and several aspects of cognitive functions. Recently, it has been reported that the D3R is also involved in the regulation of neuronal development, in promoting structural plasticity and in triggering key intracellular events with neuroprotective potential. A new role for D3R-dependent neurotransmission has thus been proposed both in preserving DA neuron homeostasis in physiological conditions and in preventing pathological alterations that may lead to neurodegeneration. Interestingly, there is evidence that nicotinic acetylcholine receptors (nAChR) located on DA neurons also provide neurotrophic support to DA neurons, an effect requiring functional D3R and suggesting the existence of a positive cross-talk between these receptor systems. Increasing evidence suggests that, as with the majority of G protein-coupled receptors (GPCR), the D3R directly interacts with other receptors to form new receptor heteromers with unique functional and pharmacological properties. Among them, we recently identified a receptor heteromer containing the nAChR and the D3R as the molecular effector of nicotine-mediated neurotrophic effects. This review summarizes the functional and pharmacological characteristics of D3R, including the capability to form active heteromers as pharmacological targets for specific neurodegenerative disorders. In particular, the molecular and functional features of the D3R-nAChR heteromer will be especially discussed since it may represent a possible key etiologic effector for DA-related pathologies, such as Parkinson’s disease (PD), and a target for drug design.

Characterization and application of electrically active neuronal networks established from human induced pluripotent stem cell-derived neural progenitor cells for neurotoxicity evaluation

Neurotoxicity is mediated by a variety of modes-of-actions leading to disturbance of neuronal function. In order to screen larger numbers of compounds for their neurotoxic potential, in vitro functional neuronal networks (NN) might be helpful tools. We established and characterized human NN (hNN) from hiPSC-derived neural progenitor cells by comparing hNN formation with two different differentiation media: in presence (CINDA) and absence (neural differentiation medium (NDM)) of maturation-supporting factors. As a NN control we included differentiating rat NN (rNN) in the study. Gene/protein expression and electrical activity from in vitro developing NN were assessed at multiple time points. Transcriptomes of 5, 14 and 28 days in vitro CINDA-grown hNN were compared to gene expression profiles of in vivo human developing brains. Molecular expression analyses as well as measures of electrical activity indicate that NN mature into neurons of different subtypes and astrocytes over time. In contrast to rNN, hNN are less electrically active within the same period of differentiation time, yet hNN grown in CINDA medium develop higher firing rates than hNN without supplements. Challenge of NN with neuronal receptor stimulators and inhibitors demonstrate presence of inhibitory, GABAergic neurons, whereas glutamatergic responses are limited. hiPSC-derived GABAergic hNN grown in CINDA medium might be a useful tool as part of an in vitro battery for assessing neurotoxicity.

L-DOPA for Parkinson's disease-a bittersweet pill

3,4-dihydroxy-L-phenylalanine (L-DOPA) is the gold standard treatment for Parkinson's disease. It has earned that title through its highly effective treatment of some of the motor symptoms in the early stages of the disease but it is a far from perfect drug. The inevitable long-term treatment that comes with this chronic neurodegenerative condition raises the risk significantly of the development of motor fluctuations including disabling L-DOPA-induced dyskinesia. Being unsurpassed as a therapy means that understanding the mechanisms of dyskinesia priming and induction is vital to the search for therapies to treat these side effects and allow optimal use of L-DOPA. However, L-DOPA use may also have consequences (positive or negative) for the development of other interventions, such as cell transplantation, which are designed to treat or repair the ailing brain. This review looks at the issues around the use of L-DOPA with a focus on its potential impact on advanced reparative interventions.

Characterization of Dopaminergic System in the Striatum of Young Adult Park2-/- Knockout Rats

Mutations in parkin gene (Park2) are linked to early-onset autosomal recessive Parkinson's disease (PD) and young-onset sporadic PD. Park2 knockout (PKO) rodents; however, do not display neurodegeneration of the nigrostriatal pathway, suggesting age-dependent compensatory changes. Our goal was to examine dopaminergic (DAergic) system in the striatum of 2 month-old PKO rats in order to characterize compensatory mechanisms that may have occurred within the system. The striata form wild type (WT) and PKO Long Evans male rats were assessed for the levels of DAergic markers, for monoamine oxidase (MAO) A and B activities and levels, and for the levels of their respective preferred substrates, serotonin (5-HT) and ß-phenylethylamine (ß-PEA). The PKO rats displayed lower activities of MAOs and higher levels of ß-PEA in the striatum than their WT counterparts. Decreased levels of ß-PEA receptor, trace amine-associated receptor 1 (TAAR-1), and postsynaptic DA D2 (D2L) receptor accompanied these alterations. Drug-naive PKO rats displayed normal locomotor activity; however, they displayed decreased locomotor response to a low dose of psychostimulant methamphetamine, suggesting altered DAergic neurotransmission in the striatum when challenged with an indirect agonist. Altogether, our findings suggest that 2 month-old PKO male rats have altered DAergic and trace aminergic signaling.

Rs2459976 in ZW10 gene associated with congenital heart diseases in Chinese Han population

Congenital heart diseases (CHD) are a large group of prevalent and complex anatomic malformations of the heart, with the genetic basis remaining largely unknown. Since genes or factors associated with the differentiation of human embryonic stem (HES) cells would affect the development of all embryonic tissues, including cardiac progenitor cells, we postulated their potential roles in CHD. In this study, we focused on ZW10, a kinetochore protein involved in the process of proper chromosome segregation, and conducted comparative studies between CHD patients and normal controls matched in gender and age in Chinese Han populations. We identified three variations in the ZW10 gene, including rs2885987, rs2271261 and rs2459976, which all had high genetic heterozygosity. Association analysis of these genetic variations with CHD showed correlation between rs2459976 and the risk of CHD. We conclude that rs2459976 in the ZW10 gene is associated with CHD in Chinese Han populations.

Expansion of human midbrain floor plate progenitors from induced pluripotent stem cells increases dopaminergic neuron differentiation potential

Human induced pluripotent stem cells (hiPSCs) are invaluable to study developmental processes and disease mechanisms particularly in the brain. hiPSCs can be differentiated into mature and functional dopaminergic (DA) neurons. Having robust protocols for the generation of differentiated DA neurons from pluripotent cells is a prerequisite for the use of hiPSCs to study disease mechanisms, for drug discovery, and eventually for cell replacement therapy. Here, we describe a protocol for generating and expanding large numbers of homogeneous midbrain floor plate progenitors (mFPPs) that retain efficient DA neurogenic potential over multiple passages and can be cryobanked. We demonstrate that expanded mFPPs have increased DA neuron potential and differentiate more efficiently and rapidly than progenitors generated by standard protocols. In addition, this novel method results in increased numbers of DA neurons that in vitro show characteristic electrophysiological properties of nigrostriatal DA neurons, produce high levels of dopamine, and integrate into host mice when grafted in vivo. Thus, we describe a robust method for producing human mesencephalic DA neurons from hiPSCs.

Study of a fetal brain affected by a severe form of tyrosine hydroxylase deficiency, a rare cause of early parkinsonism

Tyrosine hydroxylase (TH) deficiency is an inborn error of dopamine synthesis. Two clinical phenotypes have been described. The THD "B" phenotype produces a severe encephalopathy of early-onset with sub-optimal L-Dopa response, whereas the "A" phenotype has a better L-Dopa response and outcome. The objective of the study is to describe the expression of key synaptic proteins and neurodevelopmental markers in a fetal brain of THD "B" phenotype. The brain of a 16-week-old miscarried human fetus was dissected in different brain areas and frozen until the analysis. TH gene study revealed the p.R328W/p.T399M mutations, the same mutations that produced a B phenotype in her sister. After protein extraction, western blot analyses were performed to assess protein expression. The results were compared to an age-matched control. We observed a decreased expression in TH and in other dopaminergic proteins, such as VMAT 1 and 2 and dopamine receptors, especially D2DR. GABAergic and glutamatergic proteins such as GABA VT, NMDAR1 and calbindin were also altered. Developmental markers for synapses, axons and dendrites were decreased whereas markers of neuronal volume were preserved. Although this is an isolated case, this brain sample is unique and corresponds to the first reported study of a THD brain. It provides interesting information about the influence of dopamine as a regulator of other neurotransmitter systems, brain development and movement disorders with origin at the embryological state. This study could also contribute to a better understanding of the pathophysiology of THD at early fetal stages.

Calcium signaling in human pluripotent stem cells

Human pluripotent stem cells provide new tools for developmental and pharmacological studies as well as for regenerative medicine applications. Calcium homeostasis and ligand-dependent calcium signaling are key components of major cellular responses, including cell proliferation, differentiation or apoptosis. Interestingly, these phenomena have not been characterized in detail as yet in pluripotent human cell sates. Here we review the methods applicable for studying both short- and long-term calcium responses, focusing on the expression of fluorescent calcium indicator proteins and imaging methods as applied in pluripotent human stem cells. We discuss the potential regulatory pathways involving calcium responses in hPS cells and compare these to the implicated pathways in mouse PS cells. A recent development in the stem cell field is the recognition of so called "naïve" states, resembling the earliest potential forms of stem cells during development, as well as the "fuzzy" stem cells, which may be alternative forms of pluripotent cell types, therefore we also discuss the potential role of calcium homeostasis in these PS cell types.

5-HT2 receptors mediate functional modulation of GABAa receptors and inhibitory synaptic transmissions in human iPS-derived neurons

Neural progenitors differentiated from induced pluripotent stem cells (iPS) hold potentials for treating neurological diseases. Serotonin has potent effects on neuronal functions through multiple receptors, underlying a variety of neural disorders. Glutamate and GABA receptors have been proven functional in neurons differentiated from iPS, however, little is known about 5-HT receptor-mediated modulation in such neuronal networks. In the present study, human iPS were differentiated into cells possessing featured physiological properties of cortical neurons. Whole-cell patch-clamp recording was used to examine the involvement of 5-HT₂ receptors in functional modulation of GABAergic synaptic transmission. We found that serotonin and DOI (a selective agonist of 5-HT_2A/C receptor) reversibly reduced GABA-activated currents, and this 5-HT_2A/C receptor mediated inhibition required G protein, PLC, PKC, and Ca²⁺ signaling. Serotonin increased the frequency of miniature inhibitory postsynaptic currents (mIPSCs), which could be mimicked by α-methylserotonin, a 5-HT₂ receptor agonist. In contrast, DOI reduced both frequency and amplitude of mIPSCs. These findings suggested that in iPS-derived human neurons serotonin postsynaptically reduced GABAa receptor function through 5-HT_2A/C receptors, but presynaptically other 5-HT₂ receptors counteracted the action of 5-HT_2A/C receptors. Functional expression of serotonin receptors in human iPS-derived neurons provides a pre-requisite for their normal behaviors after grafting.

Characterization of Transcriptional Repressor Gene MSX1 Variations for Possible Associations with Congenital Heart Diseases

Background

The human heart consists of several cell types with distinct lineage origins. Interactions between these cardiac progenitors are very important for heart formation. The muscle segment homeobox gene family plays a key role in the cell morphogenesis and growth, controlled cellular proliferation, differentiation, and apoptosis, but the relationships between the genetic abnormalities and CHD phenotypes still remain largely unknown. The aim of this work was to evaluate variations in MSX1 and MSX2 for their possible associations with CHD.

Methods

We sequenced the MSX1 and MSX2 genes for 300 Chinese Han CHD patients and 400 normal controls and identified the variations. The statistical analyses were conducted using Chi-Square Tests as implemented in SPSS (version 19.0). The Hardy-Weinberg equilibrium test of the population was carried out using the online software OEGE.

Results

Six variations rs4647952, rs2048152, rs4242182, rs61739543, rs111542301 and rs3087539 were identified in the MSX2 gene, but the genetic heterozygosity of those SNPs was very low. In contrast, the genetic heterozygosity of two variations rs3821949 near the 5’UTR and rs12532 within 3’UTR of the MSX1 gene was considerably high. Statistical analyses showed that rs3821949 and rs12532 were associated with the risk of CHD (specifically VSD).

Conclusions

The SNPs rs3821949 and rs12532 in the MSX1 gene were associated with CHD in Chinese Han populations.

Differentiation renders susceptibility to excitotoxicity in HT22 neurons

HT22 is an immortalized mouse hippocampal neuronal cell line that does not express cholinergic and glutamate receptors like mature hippocampal neurons in vivo. This in part prevents its use as a model for mature hippocampal neurons in memory-related studies. We now report that HT22 cells were appropriately induced to differentiate and possess properties similar to those of mature hippocampal neurons in vivo, such as becoming more glutamate-receptive and excitatory. Results showed that sensitivity of HT22 cells to glutamate-induced toxicity changed dramatically when comparing undifferentiated with differentiated cells, with the half-effective concentration for differentiated cells reducing approximately two orders of magnitude. Moreover, glutamate-induced toxicity in differentiated cells, but not undifferentiated cells, was inhibited by the N-methyl-D- aspartate receptor antagonists MK-801 and memantine. Evidently, differentiated HT22 cells expressed N-methyl-D-aspartate receptors, while undifferentiated cells did not. Our experimental findings indicated that differentiation is important for immortalized cell lines to render post-mitotic neuronal properties, and that differentiated HT22 neurons represent a better model of hippocampal neurons than undifferentiated cells.

Regulation of Human PAX6 Expression by miR-7

The paired box gene 6 (PAX6) is a powerful mediator of eye and brain organogenesis whose spatiotemporal expression is exquisitely controlled by multiple mechanisms, including post-transcriptional regulation by microRNAs (miRNAs). In the present study, we use bioinformatic predictions to identify three candidate microRNA-7 (miR-7) target sites in the human PAX6 3′ untranslated region (3′-UTR) and demonstrate that two of them are functionally active in a human cell line. Furthermore, transient transfection of cells with synthetic miR-7 inhibits PAX6 protein expression but does not alter levels of PAX6 mRNA, suggesting that miR-7 induces translational repression of PAX6. Finally, a comparison of PAX6 3′-UTRs across species reveals that one of the functional miR-7 target sites is conserved, whereas the second functional target site is found only in primates. Thus, the interaction between PAX6 and miR-7 appears to be highly conserved; however, the precise number of sites through which this interaction occurs may have expanded throughout evolution.

Insights into the structural biology of G-protein coupled receptors impacts drug design for central nervous system neurodegenerative processes

In the last few years, there have been important new insights into the structural biology of G-protein coupled receptors. It is now known that allosteric binding sites are involved in the affinity and selectivity of ligands for G-protein coupled receptors, and that signaling by these receptors involves both G-protein dependent and independent pathways. The present review outlines the physiological and pharmacological implications of this perspective for the design of new drugs to treat disorders of the central nervous system. Specifically, new possibilities are explored in relation to allosteric and orthosteric binding sites on dopamine receptors for the treatment of Parkinson's disease, and on muscarinic receptors for Alzheimer's disease. Future research can seek to identify ligands that can bind to more than one site on the same receptor, or simultaneously bind to two receptors and form a dimer. For example, the design of bivalent drugs that can reach homo/hetero-dimers of D2 dopamine receptor holds promise as a relevant therapeutic strategy for Parkinson's disease. Regarding the treatment of Alzheimer's disease, the design of dualsteric ligands for mono-oligomeric rinic receptors could increase therapeutic effectiveness by generating potent compounds that could activate more than one signaling pathway.