Pitx3 is required for motor activity and for survival of a subset of midbrain dopaminergic neurons

The absence of Pitx3 results in postnatal loss of dopamine neurons and is associated with an increase in the pro-apoptotic Bcl2 factor Noxa and cleaved caspase 3

Mesodiencephalic dopamine neurons (mdDA) of the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) play critical roles in regulating movement and motivation. Pitx3 is an essential transcription factor required for proper embryonic development and terminal differentiation of mdDA neurons. Although Pitx3 is expressed in every mdDA neuron, its ablation results only in the absence of the SNc, not the VTA. The developmental stage at which the loss of SNc first becomes apparent, as well as the underlying mechanism, remains elusive. Here, we demonstrate, using a Pitx3 knockout GFP knock-in mouse model, that this loss does not occur during embryogenesis but rather postnatally. Quantification of GFP expression revealed a significant reduction in the total number of dopamine neurons at postnatal day 3, but not at embryonic day 14.5, 155, and 18.5. Mechanistically this reduction is accompanied by an increase in the number of cleaved caspase 3-positive GFP neurons, suggesting apoptosis. In addition, RT-PCR performed on isolated GFP neurons, one day before the loss of dopamine neurons revealed a notable elevation in the expression of the pro-apoptotic BH3-only factor Noxa. Overexpression of Noxa in dopaminergic MN9D cells dose-dependently increases the level of cleaved caspase 3 and the number of propidium iodide-positive cells, indicating that Noxa expression is sufficient to induce cell death in dopamine cells. Additionally, Noxa expression in MN9D cells, combined with a Bax-inhibiting peptide, reduces the number of cleaved caspase 3-positive and propidium iodide-positive cells, further supporting apoptosis as the mechanistic form of cell death. Overall, our study provides insights into the cell death machinery implicated in the loss of dopamine neurons, which may hold relevance for diseases affected by the loss of dopamine neurons such as Parkinson's disease, where this is a hallmark feature.

Neurotoxic Effects of Atrazine on Dopaminergic System via miRNAs and Energy-Sensing Pathways

Atrazine (ATR, 2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) is a globally prevalent herbicide known to induce dopaminergic neurotoxicity at high concentrations. MicroRNAs (miRNAs), pivotal in regulating gene expression post-transcriptionally, play essential roles in neuronal differentiation, proliferation, and apoptosis. This study investigates the effects of ATR on the dopaminergic system and behavioral responses in rats, with a particular focus on critical dopaminergic proteins such as tyrosine hydroxylase (TH), nuclear receptor related-1 protein (NURR1), and α-synuclein. The results reveal that ATR exposure significantly reduces the expression of TH and NURR1, while elevating levels of α-synuclein. Through miRNA sequencing and proteomic analysis, we identify alterations in miRNA and protein profiles that are intricately linked to the development of the dopaminergic system. Notably, treatment with ATR results in a marked increase in AMPK levels concurrent with a decrease in miR-322-5p. The differentially expressed genes associated with ATR exposure primarily influence the dopaminergic system by engaging in critical pathways such as AMPK, mTOR, autophagy, FoxO, and HIPPO. This study underscores the neurotoxic impact of ATR on the dopaminergic system via miRNA regulatory mechanisms and energy-sensing pathways, including AMPK and SIRT1, providing a molecular foundation for developing strategies to prevent and treat neurotoxicity induced by ATR exposure.

State of the Art in Sub-Phenotyping Midbrain Dopamine Neurons

Dopaminergic neurons in the ventral tegmental area (VTA) and the substantia nigra pars compacta (SNpc) comprise around 75% of all dopaminergic neurons in the human brain. While both groups of dopaminergic neurons are in close proximity in the midbrain and partially overlap, development, function, and impairments in these two classes of neurons are highly diverse. The molecular and cellular mechanisms underlying these differences are not yet fully understood, but research over the past decade has highlighted the need to differentiate between these two classes of dopaminergic neurons during their development and in the mature brain. This differentiation is crucial not only for understanding fundamental circuitry formation in the brain but also for developing therapies targeted to specific dopaminergic neuron classes without affecting others. In this review, we summarize the state of the art in our understanding of the differences between the dopaminergic neurons of the VTA and the SNpc, such as anatomy, structure, morphology, output and input, electrophysiology, development, and disorders, and discuss the current technologies and methods available for studying these two classes of dopaminergic neurons, highlighting their advantages, limitations, and the necessary improvements required to achieve more-precise therapeutic interventions.

Inhibition of 7α,26-dihydroxycholesterol biosynthesis promotes midbrain dopaminergic neuron development

Dysregulated cholesterol metabolism has been linked to neurodegeneration. We previously found that free, non-esterified, 7α,(25R)26-dihydroxycholesterol (7α,26-diHC), was significantly elevated in the cerebrospinal fluid of patients with Parkinson's disease (PD). In this study we investigated the role of 7α,26-diHC in midbrain dopamine (mDA) neuron development and survival. We report that 7α,26-diHC induces apoptosis and reduces the number of mDA neurons in hESC-derived cultures and in mouse progenitor cultures. Voriconazole, an oxysterol 7α-hydroxylase (CYP7B1) inhibitor, increases the number of mDA neurons and prevents the loss of mDA neurons induced by 7α,26-diHC. These effects are specific since neither 7α,26-diHC nor voriconazole alter the number of Islet1+ oculomotor neurons. Furthermore, our results suggest that elevated 24(S),25-epoxycholesterol, which has been shown to promote mDA neurogenesis, may be partially responsible for the effect of voriconazole on mDA neurons. These findings suggest that voriconazole, and/or other azole CYP7B1 inhibitors may have implications in PD therapy development.

Dopamine-independent development and maintenance of mouse striatal medium spiny neuron dendritic spines

Striatal medium spiny neurons (MSNs) and striatal dopamine (DA) innervation are profoundly important for brain function such as motor control and cognition. A widely accepted theory posits that striatal DA loss causes (or leads to) MSN dendritic atrophy. However, examination of the literature indicates that the data from Parkinson's disease (PD) patients and animal PD models were contradictory among studies and hard to interpret. Here we have re-examined the potential effects of DA activity on MSN morphology or lack thereof. We found that in 15-day, 4- and 12-month old Pitx3 null mutant mice that have severe DA denervation in the dorsal striatum while having substantial residual DA innervation in the ventral striatum, MSN dendrites and spine numbers were similar in dorsal and ventral striatum, and also similar to those in normal mice. In 15-day, 4- and 12-month old tyrosine hydroxylase knockout mice that cannot synthesize L-dopa and thus have no endogenous DA in the entire brain, MSN dendrites and spine numbers were also indistinguishable from age-matched wild-type (WT) mice. Furthermore, in adult WT mice, unilateral 6-OHDA lesion at 12 months of age caused an almost complete striatal DA denervation in the lesioned side, but MSN dendrites and spine numbers were similar in the lesioned and control sides. Taken together, our data indicate that in mice, the development and maintenance of MSN dendrites and spines are DA-independent such that DA depletion does not trigger MSN dendritic atrophy; our data also suggest that the reported MSN dendritic atrophy in PD may be a component of neurodegeneration in PD rather than a consequence of DA denervation.

Mitochondrial Dysfunction and Parkinson's Disease: Pathogenesis and Therapeutic Strategies

Parkinson's disease (PD) is a common age-related neurodegenerative disorder whose pathogenesis is not completely understood. Mitochondrial dysfunction and increased oxidative stress have been considered as major causes and central events responsible for the progressive degeneration of dopaminergic (DA) neurons in PD. Therefore, investigating mitochondrial disorders plays a role in understanding the pathogenesis of PD and can be an important therapeutic target for this disease. This study discusses the effect of environmental, genetic and biological factors on mitochondrial dysfunction and also focuses on the mitochondrial molecular mechanisms underlying neurodegeneration, and its possible therapeutic targets in PD, including reactive oxygen species generation, calcium overload, inflammasome activation, apoptosis, mitophagy, mitochondrial biogenesis, and mitochondrial dynamics. Other potential therapeutic strategies such as mitochondrial transfer/transplantation, targeting microRNAs, using stem cells, photobiomodulation, diet, and exercise were also discussed in this review, which may provide valuable insights into clinical aspects. A better understanding of the roles of mitochondria in the pathophysiology of PD may provide a rationale for designing novel therapeutic interventions in our fight against PD.

Cell Type-Specific Nuclei Markers: The Need for Human Brain Research to Go Nuclear

Identifying and interrogating cell type-specific populations within the heterogeneous milieu of the human brain is paramount to resolving the processes of normal brain homeostasis and the pathogenesis of neurological disorders. While brain cell type-specific markers are well established, most are localized on cellular membranes or within the cytoplasm, with limited literature describing those found in the nucleus. Due to the complex cytoarchitecture of the human brain, immunohistochemical studies require well-defined cell-specific nuclear markers for more precise and efficient quantification of the cellular populations. Furthermore, efficient nuclear markers are required for cell type-specific purification and transcriptomic interrogation of archived human brain tissue through nuclei isolation-based RNA sequencing. To sate the growing demand for robust cell type-specific nuclear markers, we thought it prudent to comprehensively review the current literature to identify and consolidate a novel series of robust cell type-specific nuclear markers that can assist researchers across a range of neuroscientific disciplines. The following review article collates and discusses several key and prospective cell type-specific nuclei markers for each of the major human brain cell types; it then concludes by discussing the potential applications of cell type-specific nuclear workflows and the power of nuclear-based neuroscientific research.

Timing of MeCP2 Expression Determines Midbrain Dopamine Neuron Phenotype Specification

Midbrain dopamine (DA) neurons are associated with locomotor and psychiatric disorders. DA phenotype is specified in ancestral neural precursor cells (NPCs) and maintained throughout neuronal differentiation. Here we show that endogenous expression of MeCP2 coincides with DA phenotype specification in mouse mesencephalon, and premature expression of MeCP2 prevents in vitro cultured NPCs from acquiring DA phenotype through interfering NURR1 transactivation of DA phenotype genes. By contrast, ectopic MeCP2 expression does not disturb DA phenotype in the DA neurons. By analyzing the dynamic change of DNA methylation along DA neuronal differentiation at the promoter of DA phenotype gene tyrosine hydroxylase (Th), we show that Th expression is determined by TET1-mediated de-methylation of NURR1 binding sites within Th promoter. Chromatin immunoprecipitation assays demonstrate that premature MeCP2 dominates the DNA binding of the corresponding sites thereby blocking TET1 function in DA NPCs, whereas TET1-mediated de-methylation prevents excessive MeCP2 binding in DA neurons. The significance of temporal DNA methylation status is further confirmed by targeted methylation/demethylation experiments showing that targeted de-methylation in DA NPCs protects DA phenotype specification from ectopic MeCP2 expression, whereas targeted methylation disturbs phenotype maintenance in MeCP2-overexpressed DA neurons. These findings suggest the appropriate timing of MeCP2 expression as a novel determining factor for guiding NPCs into DA lineage.

Function of Excitatory Periaqueductal Gray Synapses in the Ventral Tegmental Area following Inflammatory Injury

Manipulating the activity of ventral tegmental area (VTA) dopamine (DA) neurons can drive nocifensive reflexes, and their firing rates are reduced following noxious stimuli. However, the pain-relevant inputs to the VTA remain incompletely understood. In this study, we used male and female mice in combination with identified dopamine and GABA neurons in the VTA that receive excitatory inputs from the periaqueductal gray (PAG), a nexus of ascending pain information. We tested whether PAG-VTA synapses undergo functional plasticity in response to a pain model using optical stimulation in conjunction with slice electrophysiology. We found that acute carrageenan inflammation does not significantly affect the strength of excitatory PAG synapses onto VTA DA neurons. However, at the PAG synapses on VTA GABA neurons, the subunit composition of NMDA receptors is altered; the complement of NR2D subunits at synaptic sites appears to be lost. Thus, our data support a model in which injury initially alters synapses on VTA GABA neurons. Over time, these alterations may increase tonic inhibition of VTA DA neurons leading to their reduced firing.

Maintenance of mitochondrial integrity in midbrain dopaminergic neurons governed by a conserved developmental transcription factor

Progressive degeneration of dopaminergic (DA) neurons in the substantia nigra is a hallmark of Parkinson’s disease (PD). Dysregulation of developmental transcription factors is implicated in dopaminergic neurodegeneration, but the underlying molecular mechanisms remain largely unknown. Drosophila Fer2 is a prime example of a developmental transcription factor required for the birth and maintenance of midbrain DA neurons. Using an approach combining ChIP-seq, RNA-seq, and genetic epistasis experiments with PD-linked genes, here we demonstrate that Fer2 controls a transcriptional network to maintain mitochondrial structure and function, and thus confers dopaminergic neuroprotection against genetic and oxidative insults. We further show that conditional ablation of Nato3, a mouse homolog of Fer2, in differentiated DA neurons causes mitochondrial abnormalities and locomotor impairments in aged mice. Our results reveal the essential and conserved role of Fer2 homologs in the mitochondrial maintenance of midbrain DA neurons, opening new perspectives for modeling and treating PD.

Mitochondrial dysfunction in dopaminergic neurons is a pathological hallmark of Parkinson’s disease. Here, the authors find a conserved mechanism by which a single transcription factor controls mitochondrial health in dopaminergic neurons during the aging process.

The Netrin-1 Receptor DCC Promotes the Survival of a Subpopulation of Midbrain Dopaminergic Neurons: Relevance for Ageing and Parkinson's Disease

Mechanisms that determine the survival of midbrain dopaminergic (mDA) neurons in the adult central nervous system (CNS) are not fully understood. Netrins are a family of secreted proteins that are essential for normal neural development. In the mature CNS, mDA neurons express particularly high levels of netrin‐1 and its receptor Deleted in Colorectal Cancer (DCC). Recent findings indicate that overexpressing netrin‐1 protects mDA neurons in animal models of Parkinson’s disease (PD), with a proposed pro‐apoptotic dependence function for DCC that triggers cell death in the absence of a ligand. Here, we sought to determine if DCC expression influences mDA neuron survival in young adult and ageing mice. To circumvent the perinatal lethality of DCC null mice, we selectively deleted DCC from mDA neurons utilizing DAT^cre/loxP gene‐targeting and examined neuronal survival in adult and aged animals. Reduced numbers of mDA neurons were detected in the substantia nigra pars compacta (SNc) of young adult DAT^cre/DCC^fl/fl mice, with further reduction in aged DAT^cre/DCC^fl/fl animals. In contrast to young adults, aged mice also exhibited a gene dosage effect, with fewer SNc mDA neurons in DCC heterozygotes (DAT^cre/DCC^fl/wt). Notably, loss of mDA neurons in the SN was not uniform. Neuronal loss in the SN was limited to ventral tier mDA neurons, while mDA neurons in the dorsal tier of the SN, which resist degeneration in PD, were spared from the effect of DCC deletion in both young and aged mice. In the ventral tegmental area (VTA), young adult mice with conditional deletion of DCC had normal mDA neuronal numbers, while significant loss occurred in aged DAT^cre/DCC^fl/fl and DAT^cre/DCC^fl/wt mice compared to age‐matched wild‐type mice. Our results indicate that expression of DCC is required for the survival of subpopulations of mDA neurons and may be relevant to the degenerative processes in PD.

Pitx genes in development and disease

Homeobox genes encode sequence-specific transcription factors (SSTFs) that recognize specific DNA sequences and regulate organogenesis in all eukaryotes. They are essential in specifying spatial and temporal cell identity and as a result, their mutations often cause severe developmental defects. Pitx genes belong to the PRD class of the highly evolutionary conserved homeobox genes in all animals. Vertebrates possess three Pitx paralogs, Pitx1, Pitx2, and Pitx3 while non-vertebrates have only one Pitx gene. The ancient role of regulating left-right (LR) asymmetry is conserved while new functions emerge to afford more complex body plan and functionalities. In mouse, Pitx1 regulates hindlimb tissue patterning and pituitary development. Pitx2 is essential for the development of the oral cavity and abdominal wall while regulates the formation and symmetry of other organs including pituitary, heart, gut, lung among others by controlling growth control genes upon activation of the Wnt/ß-catenin signaling pathway. Pitx3 is essential for lens development and migration and survival of the dopaminergic neurons of the substantia nigra. Pitx gene mutations are linked to various congenital defects and cancers in humans. Pitx gene family has the potential to offer a new approach in regenerative medicine and aid in identifying new drug targets.

Preclinical Efficacy and Safety of a Human Embryonic Stem Cell-Derived Midbrain Dopamine Progenitor Product, MSK-DA01

Parkinson's disease is characterized by the loss of dopaminergic neurons in the substantia nigra leading to disabling deficits. Dopamine neuron grafts may provide a significant therapeutic advance over current therapies. We have generated midbrain dopamine neurons from human embryonic stem cells and manufactured large-scale cryopreserved dopamine progenitors for clinical use. After optimizing cell survival and phenotypes in short-term studies, the cell product, MSK-DA01, was subjected to an extensive set of biodistribution, toxicity, and tumorigenicity assessments in mice under GLP conditions. A large-scale efficacy study was also performed in rats with the same lot of cells intended for potential human use and demonstrated survival of the grafted cells and behavioral amelioration in 6-hydroxydopamine lesioned rats. There were no adverse effects attributable to the grafted cells, no obvious distribution outside the brain, and no cell overgrowth or tumor formation, thus paving the way for a future clinical trial.

The PITX gene family as potential biomarkers and therapeutic targets in lung adenocarcinoma

ABSTRACT

The PITX gene family of transcription factors have been reported to regulate the development of multiple organs. This study was designed to investigate the role of PITXs in lung adenocarcinoma (LUAD).In this study, the transcriptional levels of the 3 identified PITXs in patients with LUAD were examined using the gene expression profiling interactive analysis interactive web server. Meanwhile, the immunohistochemical data of the 3 PITXs were obtained in the Human Protein Atlas website, and western blotting was additionally conducted for further verification. Moreover, the association between the levels of PITXs and the stage plot as well as overall survival of patients with LUAD was analyzed.We found that the mRNA and protein levels of PITX1 and PITX2 were higher in LUAD tissues than those in normal lung tissues, while those of PITX3 displayed no significant differences. Additionally, PITX1 and PITX3 were found to be significantly associated with the stage of LUAD. The Kaplan-Meier Plot showed that the high level of PITX1 conferred a better overall survival of patients with LUAD while the high level of PITX3 was associated with poor prognosis.Our study implied that PITX1 and PITX3 are potential targets of precision therapy for patients with LUAD while PITX1 and PITX2 are regarded as novel biomarkers for the diagnosis of LUAD.

Role and Mechanism of Vitamin A Metabolism in the Pathophysiology of Parkinson's Disease

Evidence shows that altered retinoic acid signaling may contribute to the pathogenesis and pathophysiology of Parkinson's disease (PD). Retinoic acid is the bioactive derivative of the lipophilic vitamin A. Vitamin A is involved in several important homeostatic processes, such as cell differentiation, antioxidant activity, inflammation and neuronal plasticity. The role of vitamin A and its derivatives in the pathogenesis and pathophysiology of neurodegenerative diseases, and their potential as therapeutics, has drawn attention for more than 10 years. However, the literature sits in disparate fields. Vitamin A could act at the crossroad of multiple environmental and genetic factors of PD. The purpose of this review is to outline what is known about the role of vitamin A metabolism in the pathogenesis and pathophysiology of PD. We examine key biological systems and mechanisms that are under the control of vitamin A and its derivatives, which are (or could be) exploited for therapeutic potential in PD: the survival of dopaminergic neurons, oxidative stress, neuroinflammation, circadian rhythms, homeostasis of the enteric nervous system, and hormonal systems. We focus on the pivotal role of ALDH1A1, an enzyme expressed by dopaminergic neurons for the detoxification of these neurons, which is under the control of retinoic acid. By providing an integrated summary, this review will guide future studies on the potential role of vitamin A in the management of symptoms, health and wellbeing for PD patients.

A Critical LRRK at the Synapse? The Neurobiological Function and Pathophysiological Dysfunction of LRRK2

Since the discovery of LRRK2 mutations causal to Parkinson's disease (PD) in the early 2000s, the LRRK2 protein has been implicated in a plethora of cellular processes in which pathogenesis could occur, yet its physiological function remains elusive. The development of genetic models of LRRK2 PD has helped identify the etiological and pathophysiological underpinnings of the disease, and may identify early points of intervention. An important role for LRRK2 in synaptic function has emerged in recent years, which links LRRK2 to other genetic forms of PD, most notably those caused by mutations in the synaptic protein α-synuclein. This point of convergence may provide useful clues as to what drives dysfunction in the basal ganglia circuitry and eventual death of substantia nigra (SN) neurons. Here, we discuss the evolution and current state of the literature placing LRRK2 at the synapse, through the lens of knock-out, overexpression, and knock-in animal models. We hope that a deeper understanding of LRRK2 neurobiology, at the synapse and beyond, will aid the eventual development of neuroprotective interventions for PD, and the advancement of useful treatments in the interim.

Carbon Fibers as a New Type of Scaffold for Midbrain Organoid Development

The combination of induced pluripotent stem cell (iPSC) technology and 3D cell culture creates a unique possibility for the generation of organoids that mimic human organs in in vitro cultures. The use of iPS cells in organoid cultures enables the differentiation of cells into dopaminergic neurons, also found in the human midbrain. However, long-lasting organoid cultures often cause necrosis within organoids. In this work, we present carbon fibers (CFs) for medical use as a new type of scaffold for organoid culture, comparing them to a previously tested copolymer poly-(lactic-co-glycolic acid) (PLGA) scaffold. We verified the physicochemical properties of CF scaffolds compared to PLGA in improving the efficiency of iPSC differentiation within organoids. The physicochemical properties of carbon scaffolds such as porosity, microstructure, or stability in the cellular environment make them a convenient material for creating in vitro organoid models. Through screening several genes expressed during the differentiation of organoids at crucial brain stages of development, we found that there is a correlation between PITX3, one of the key regulators of terminal differentiation, and the survival of midbrain dopaminergic (mDA) neurons and tyrosine hydroxylase (TH) gene expression. This makes organoids formed on carbon scaffolds an improved model containing mDA neurons convenient for studying midbrain-associated neurodegenerative diseases such as Parkinson's disease.

The antiparkinson drug ropinirole inhibits movement in a Parkinson's disease mouse model with residual dopamine neurons

The dopamine (DA) D2-like receptor (D2R) agonist ropinirole is often used for early and middle stage Parkinson's disease (PD). However, this D2-like agonism-based strategy has a complicating problem: D2-like agonism may activate D2 autoreceptors on the residual DA neurons in the PD brain, potentially inhibiting these residual DA neurons and motor function. We have examined this possibility by using systemic and local drug administration in transcription factor Pitx3 null mutant (Pitx3Null) mice that mimic the DA denervation in early and middle stage PD and in DA neuron tyrosine hydroxylase (TH) gene knockout (KO) mice that mimic the severe DA loss in late stage PD. We found that in Pitx3Null mice with residual DA neurons and normal mice with normal DA system, systemically injected ropinirole inhibited locomotion, whereas bilateral dorsal striatal-microinjected ropinirole stimulated movement in Pitx3Null mice; bilateral microinjection of ropinirole into the ventral tegmental area also inhibited movement in Pitx3Null mice; we further determined that ropinirole inhibited nigral DA neuron spike firing in WT mice. In contrast, both systemically and striatum-locally administered ropinirole increased movements in TH KO mice, but produced relatively more dyskinesia than L-dopa. Although requiring confirmation in non-human primates and PD patients, these data suggest that while activating D2-like receptors in striatal projection neurons and hence stimulating movements, D2-like agonists can inhibit residual DA neurons and cause akinesia when the residual DA neurons and motor functions are still substantial, and this motor-inhibitory effect disappears when almost all DA neurons are lost such as in late stage PD.

Beneficial effects of the phytocannabinoid Δ9-THCV in L-DOPA-induced dyskinesia in Parkinson's disease

The antioxidant and CB₂ receptor agonist properties of Δ⁹-tetrahydrocannabivarin (Δ⁹-THCV) afforded neuroprotection in experimental Parkinson's disease (PD), whereas its CB₁ receptor antagonist profile at doses lower than 5 mg/kg caused anti-hypokinetic effects. In the present study, we investigated the anti-dyskinetic potential of Δ⁹-THCV (administered i.p. at 2 mg/kg for two weeks), which had not been investigated before. This objective was investigated after inducing dyskinesia by repeated administration of L-DOPA (i.p. at 10 mg/kg) in a genetic model of dopaminergic deficiency, Pitx3^ak mutant mice, which serves as a useful model for testing anti-dyskinetic agents. The daily treatment of these mice with L-DOPA for two weeks progressively increased the time spent in abnormal involuntary movements (AIMs) and elevated their horizontal and vertical activities (as measured in a computer-aided actimeter), signs that reflected the dyskinetic state of these mice. Interestingly, when combined with L-DOPA from the first injection, Δ⁹-THCV delayed the appearance of all these signs and decreased their intensity, with a reduction in the levels of FosB protein and the histone pAcH3 (measured by immunohistochemistry), which had previously been found to be elevated in the basal ganglia in L-DOPA-induced dyskinesia. In addition to the anti-dyskinetic effects of Δ⁹-THCV when administered at the onset of L-DOPA treatment, Δ⁹-THCV was also effective in attenuating the intensity of dyskinesia when administered for three consecutive days once these signs were already present (two weeks after the onset of L-DOPA treatment). In summary, our data support the anti-dyskinetic potential of Δ⁹-THCV, both to delay the occurrence and to attenuate the magnitude of dyskinetic signs. Although further studies are clearly required to determine the clinical significance of these data in humans, the results nevertheless situate Δ⁹-THCV in a promising position for developing a cannabinoid-based therapy for patients with PD.

Animal models for preclinical Parkinson's research: An update and critical appraisal

Animal models of Parkinson's disease (PD) are essential to investigate pathogenic pathways at the whole-organism level. Moreover, they are necessary for a preclinical investigation of potential new therapies. Different pathological features of PD can be induced in a variety of invertebrate and vertebrate species using toxins, drugs, or genetic perturbations. Each model has a particular utility and range of applicability. Invertebrate PD models are particularly useful for high throughput-screening applications, whereas mammalian models are needed to explore complex motor and non-motor features of the human disease. Here, we provide a comprehensive review and critical appraisal of the most commonly used mammalian models of PD, which are produced in rats and mice. A substantial loss of nigrostriatal dopamine neurons is necessary for the animal to exhibit a hypokinetic motor phenotype responsive to dopaminergic agents, thus resembling clinical PD. This level of dopaminergic neurodegeneration can be induced using specific neurotoxins, environmental toxicants, or proteasome inhibitors. Alternatively, nigrostriatal dopamine degeneration can be induced via overexpression of α-synuclein using viral vectors or transgenic techniques. In addition, protein aggregation pathology can be triggered by inoculating preformed fibrils of α-synuclein in the substantia nigra or the striatum. Thanks to the conceptual and technical progress made in the past few years a vast repertoire of well-characterized animal models are currently available to address different aspects of PD in the laboratory.

Use of 3D Organoids as a Model to Study Idiopathic Form of Parkinson's Disease

Organoids are becoming particularly popular in modeling diseases that are difficult to reproduce in animals, due to anatomical differences in the structure of a given organ. Thus, they are a bridge between the in vitro and in vivo models. Human midbrain is one of the structures that is currently being intensively reproduced in organoids for modeling Parkinson’s disease (PD). Thanks to three-dimensional (3D) architecture and the use of induced pluripotent stem cells (iPSCs) differentiation into organoids, it has been possible to recapitulate a complicated network of dopaminergic neurons. In this work, we present the first organoid model for an idiopathic form of PD. iPSCs were generated from peripheral blood mononuclear cells of healthy volunteers and patients with the idiopathic form of PD by transduction with Sendai viral vector. iPSCs were differentiated into a large multicellular organoid-like structure. The mature organoids displayed expression of neuronal early and late markers. Interestingly, we observed statistical differences in the expression levels of LIM homeobox transcription factor alpha (early) and tyrosine hydroxylase (late) markers between organoids from PD patient and healthy volunteer. The obtained results show immense potential for the application of 3D human organoids in studying the neurodegenerative disease and modeling cellular interactions within the human brain.

The noradrenergic system is necessary for survival of vulnerable midbrain dopaminergic neurons: implications for development and Parkinson's disease

The cause of midbrain dopaminergic (mDA) neuron loss in sporadic Parkinson's disease (PD) is multifactorial, involving cell autonomous factors, cell-cell interactions, and the effects of environmental toxins. Early loss of neurons in the locus coeruleus (LC), the main source of ascending noradrenergic (NA) projections, is an important feature of PD and other neurodegenerative disorders. We hypothesized that NA afferents provide trophic support for vulnerable mDA neurons. We demonstrate that depriving mDA neurons of NA input increases postnatal apoptosis and decreases cell survival in young adult rodents, with relative sparing of calbindin-positive subpopulations known to be resistant to degeneration in PD. As a mechanism, we propose that the neurotrophin brain-derived neurotrophic factor (BDNF) modulates anterograde survival effects of LC inputs to mDA neurons. We demonstrate that the LC is rich in BDNF mRNA in postnatal and young adult brains. Early postnatal NA denervation reduces both BDNF protein and activation of TrkB receptors in the ventral midbrain. Furthermore, overexpression of BDNF in NA afferents in transgenic mice increases mDA neuronal survival. Finally, increasing NA activity in primary cultures of mDA neurons improves survival, an effect that is additive or synergistic in the presence of different concentrations of BDNF. Taken together, our results point to a novel mechanism whereby LC afferents couple BDNF effects and NA activity to provide anterograde trophic support for vulnerable mDA neurons. Early loss of NA activity and anterograde neurotrophin support may contribute to degeneration of vulnerable neurons in PD and other neurodegenerative disorders.

Lmx1b Influences Correct Post-mitotic Coding of Mesodiencephalic Dopaminergic Neurons

The Lim Homeobox transcription factor 1 beta (LMX1b) has been identified as one of the transcription factors important for the development of mesodiencephalic dopaminergic (mdDA) neurons. During early development, Lmx1b is essential for induction and maintenance of the Isthmic Organizer (IsO), and genetic ablation results in the disruption of inductive activity from the IsO and loss of properly differentiated mdDA neurons. To study the downstream targets of Lmx1b without affecting the IsO, we generated a conditional model in which Lmx1b was selectively deleted in Pitx3-expressing cells from embryonic day (E)13 onward. Supporting previous data, no significant changes could be observed in general dopamine (DA) marks, like Th, Pitx3and Vmat2 at E14.5. However, in depth analysis by means of RNA-sequencing revealed that Lmx1b is important for the mRNA expression level of survival factors En1 and En2 and for the repression of mdDA subset mark Ahd2 during (late) development. Interestingly, the regulation of Ahd2 by Lmx1b was found to be Pitx3 independent, since Pitx3 mRNA levels were not altered in Lmx1b conditional knock-outs (cKOs) and Ahd2 expression was also up-regulated in Lmx1b/Pitx3 double mutants compared to Pitx3 mutants. Further analysis of Lmx1b cKOs showed that post-mitotic deletion of Lmx1b additional leads to a loss of TH+ cells at 3 months age both in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). Remarkably, different cell types were affected in the SNc and the VTA. While TH+AHD2+ cells were lost the SNc, TH+AHD2- neurons were affected in the VTA, reflected by a loss of Cck expression, indicating that Lmx1b is important for the survival of a sub-group of mdDA neurons.

ALDH1A1 regulates postsynaptic μ-opioid receptor expression in dorsal striatal projection neurons and mitigates dyskinesia through transsynaptic retinoic acid signaling

Aldehyde dehydrogenase 1A1 (ALDH1A1), a retinoic acid (RA) synthase, is selectively expressed by the nigrostriatal dopaminergic (nDA) neurons that preferentially degenerate in Parkinson’s disease (PD). ALDH1A1–positive axons mainly project to the dorsal striatum. However, whether ALDH1A1 and its products regulate the activity of postsynaptic striatal neurons is unclear. Here we show that μ–type opioid receptor (MOR1) levels were severely decreased in the dorsal striatum of postnatal and adult Aldh1a1 knockout mice, whereas dietary supplement of RA restores its expression. Furthermore, RA treatment also upregulates striatal MOR1 levels and signaling and alleviates L-DOPA–induced dyskinetic movements in pituitary homeobox 3 (Pitx3)–deficient mice that lack of ALDH1A1–expressing nDA neurons. Therefore, our findings demonstrate that ALDH1A1–synthesized RA is required for postsynaptic MOR1 expression in the postnatal and adult dorsal striatum, supporting potential therapeutic benefits of RA supplementation in moderating L-DOPA–induced dyskinesia.

24(S),25-Epoxycholesterol and cholesterol 24S-hydroxylase (CYP46A1) overexpression promote midbrain dopaminergic neurogenesis in vivo

The liver X receptors Lxrα/NR1H3 and Lxrβ/NR1H2 are ligand-dependent nuclear receptors critical for midbrain dopaminergic (mDA) neuron development. We found previously that 24(S),25-epoxycholesterol (24,25-EC), the most potent and abundant Lxr ligand in the developing mouse midbrain, promotes mDA neurogenesis in vitro In this study, we demonstrate that 24,25-EC promotes mDA neurogenesis in an Lxr-dependent manner in the developing mouse midbrain in vivo and also prevents toxicity induced by the Lxr inhibitor geranylgeranyl pyrophosphate. Furthermore, using MS, we show that overexpression of human cholesterol 24S-hydroxylase (CYP46A1) increases the levels of both 24(S)-hydroxycholesterol (24-HC) and 24,25-EC in the developing midbrain, resulting in a specific increase in mDA neurogenesis in vitro and in vivo, but has no effect on oculomotor or red nucleus neurogenesis. 24-HC, unlike 24,25-EC, did not affect in vitro neurogenesis, indicating that the neurogenic effect of 24,25-EC on mDA neurons is specific. Combined, our results indicate that increased levels of 24,25-EC in vivo, by intracerebroventricular delivery in WT mice or by overexpression of its biosynthetic enzyme CYP46A1, specifically promote mDA neurogenesis. We propose that increasing the levels of 24,25-EC in vivo may be a useful strategy to combat the loss of mDA neurons in Parkinson's disease.

Evoked transients of pH-sensitive fluorescent false neurotransmitter reveal dopamine hot spots in the globus pallidus

Dopamine neurotransmission is suspected to play important physiological roles in multiple sparsely innervated brain nuclei, but there has not been a means to measure synaptic dopamine release in such regions. The globus pallidus externa (GPe) is a major locus in the basal ganglia that displays a sparse innervation of en passant dopamine axonal fibers. Due to the low levels of innervation that preclude electrochemical analysis, it is unknown if these axons engage in neurotransmission. To address this, we introduce an optical approach using a pH-sensitive fluorescent false neurotransmitter, FFN102, that exhibits increased fluorescence upon exocytosis from the acidic synaptic vesicle to the neutral extracellular milieu. In marked contrast to the striatum, FFN102 transients in the mouse GPe were spatially heterogeneous and smaller than in striatum with the exception of sparse hot spots. GPe transients were also significantly enhanced by high frequency stimulation. Our results support hot spots of dopamine release from substantia nigra axons.

Axin-2 knockdown promote mitochondrial biogenesis and dopaminergic neurogenesis by regulating Wnt/β-catenin signaling in rat model of Parkinson's disease

Wnts and the components of Wnt/β-catenin signaling are widely expressed in midbrain and required to control the fate specification of dopaminergic (DAergic) neurons, a neuronal population that specifically degenerate in Parkinson's disease (PD). Accumulating evidence suggest that mitochondrial dysfunction plays a key role in pathogenesis of PD. Axin-2, a negative regulator of Wnt/β-catenin signaling affects mitochondrial biogenesis and death/birth of new DAergic neurons is not fully explored. We investigated the functional role of Axin-2/Wnt/β-catenin signaling in mitochondrial biogenesis and DAergic neurogenesis in 6-hydroxydopamine (6-OHDA) induced rat model of PD-like phenotypes. We demonstrate that single unilateral injection of 6-OHDA into the medial forebrain bundle (MFB) potentially dysregulates Wnt/β-catenin signaling in substantia nigra pars compacta (SNpc). We used shRNA lentiviruses to genetically knockdown Axin-2 to up-regulate Wnt/β-catenin signaling in SNpc in parkinsonian rats. Genetic knockdown of Axin-2 up-regulates Wnt/β-catenin signaling by destabilizing the β-catenin degradation complex in SNpc in parkinsonian rats. Axin-2 shRNA mediated activation of Wnt/β-catenin signaling improved behavioural functions and protected the nigral DAergic neurons by increasing mitochondrial functionality in parkinsonian rats. Axin-2 shRNA treatment reduced apoptotic signaling, autophagy and ROS generation and improved mitochondrial membrane potential which promotes mitochondrial biogenesis in SNpc in parkinsonian rats. Interestingly, Axin-2 shRNA-mediated up-regulation of Wnt/β-catenin signaling enhanced net DAergic neurogenesis by regulating proneural genes (Nurr-1, Pitx-3, Ngn-2, and NeuroD1) and mitochondrial biogenesis in SNpc in parkinsonian rats. Therefore, our data suggest that pharmacological/genetic manipulation of Wnt signaling that enhances the endogenous regenerative capacity of DAergic neurons may have implication for regenerative approaches in PD.

Impaired neurite development associated with mitochondrial dysfunction in dopaminergic neurons differentiated from exfoliated deciduous tooth-derived pulp stem cells of children with autism spectrum disorder

Autism spectrum disorder (ASD) is a highly heterogeneous neurodevelopmental disorder characterized by impaired social interactions, restrictive interests, and repetitive stereotypic behaviors. Among the various mechanisms underlying the pathogenesis of ASD, dysfunctions of dopaminergic signaling and mitochondria have been hypothesized to explain the core symptoms of children with ASD. However, only a few studies focusing on the pathological association between dopaminergic neurons (DN) and mitochondria in ASD have been performed using patient-derived stem cells and in vitro differentiated neurons. Stem cells from human exfoliated deciduous teeth (SHED) are neural crest-derived mesenchymal stem cells present in the dental pulp of exfoliated deciduous teeth; these cells can differentiate into dopaminergic neurons (DN) in vitro. This study aimed to investigate the pathological association between development of DN and mitochondria in ASD by using SHED as a disease- or patient-specific cellular model. The SHED obtained from three children with ASD and three typically developing children were differentiated into DN, and the neurobiology of these cells was examined. The DN derived from children with ASD showed impaired neurite outgrowth and branching, associated with decreased mitochondrial membrane potential, ATP production, number of mitochondria within the neurites, amount of mitochondria per cell area and intracellular calcium level. In addition, impaired neurite outgrowth and branching of ASD-derived DN were not improved by brain-derived neurotrophic factor (BDNF), suggesting impairment of the BDNF signaling pathway in ASD. These results imply that intracerebral dopamine production may have decreased in these children. The earliest age at which deciduous teeth spontaneously exfoliate in humans, and SHED can be noninvasively collected, is approximately 6 years. Our results suggest that in vitro analysis of SHED-derived DN obtained from children with ASD provides neurobiological information that may be useful in determining treatment strategies in the early stages of ASD.

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Dental pulp stem cells of autistic patient differentiate into dopaminergic neurons.

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These neurons show impaired neurite development compared with those from controls.

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This impairment is associated with mitochondrial dysfunction.

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Dental pulp stem cells may help establish treatment strategies against autism.

Hyperactive Response of Direct Pathway Striatal Projection Neurons to L-dopa and D1 Agonism in Freely Moving Parkinsonian Mice

Dopamine (DA) profoundly stimulates motor function as demonstrated by the hypokinetic motor symptoms in Parkinson's disease (PD) and by the hyperkinetic motor side effects during dopaminergic treatment of PD. Dopamine (DA) receptor-bypassing, optogenetics- and chemogenetics-induced spike firing of striatal DA D1 receptor (D1R)-expressing, direct pathway medium spiny neurons (dSPNs or dMSNs) promotes movements. However, the endogenous D1R-mediated effects, let alone those of DA replacement, on dSPN spike activity in freely-moving animals is not established. Here we show that using transcription factor Pitx3 null mutant (Pitx3Null) mice as a model for severe and consistent DA denervation in the dorsal striatum in Parkinson's disease, antidromically identified striatonigral neurons (D1R-expressing dSPNs) had a lower baseline spike firing rate than that in DA-intact normal mice, and these neurons increased their spike firing more strongly in Pitx3Null mice than in WT mice in response to injection of L-dopa or the D1R agonist, SKF81297; the increase in spike firing temporally coincided with the motor-stimulating effects of L-dopa and SKF81297. Taken together, these results provide the first evidence from freely moving animals that in parkinsonian striatum, identified behavior-promoting dSPNs become hyperactive upon the administration of L-dopa or a D1 agonist, likely contributing to the profound dopaminergic motor stimulation in parkinsonian animals and PD patients.

Differential Synaptic Remodeling by Dopamine in Direct and Indirect Striatal Projection Neurons in Pitx3-/- Mice, a Genetic Model of Parkinson's Disease

In toxin-based models of Parkinson's disease (PD), striatal projection neurons (SPNs) exhibit dendritic atrophy and spine loss concurrent with an increase in excitability. Chronic l-DOPA treatment that induces dyskinesia selectively restores spine density and excitability in indirect pathway SPNs (iSPNs), whereas spine loss and hyperexcitability persist in direct pathway SPNs (dSPNs). These alterations have only been characterized in toxin-based models of PD, raising the possibility that they are an artifact of exposure to the toxin, which may engage compensatory mechanisms independent of the PD-like pathology or due to the loss of dopaminergic afferents. To test all these, we studied the synaptic remodeling in Pitx3^-/- or aphakia mice, a genetic model of PD, in which most of the dopamine neurons in the substantia nigra fail to fully differentiate and to innervate the striatum. We made 3D reconstructions of the dendritic arbor and measured excitability in identified SPNs located in dorsal striatum of BAC-Pitx3^-/- mice treated with saline or l-DOPA. Both dSPNs and iSPNs from BAC-Pitx3^-/- mice had shorter dendritic trees, lower spine density, and more action potentials than their counterparts from WT mice. Chronic l-DOPA treatment restored spine density and firing rate in iSPNs. By contrast, in dSPNs, spine loss and hyperexcitability persisted following l-DOPA treatment, which is similar to what happens in 6-OHDA WT mice. This indicates that dopamine-mediated synaptic remodeling and plasticity is independent of dopamine innervation during SPN development and that Pitx3^-/- mice are a good model because they develop the same pathology described in the toxins-based models and in human postmortem studies of advanced PD.SIGNIFICANCE STATEMENT As the only genetic model of Parkinson's disease (PD) that develops dyskinesia, Pitx3^-/- mice reproduce the behavioral effects seen in humans and are a good system for studying dopamine-induced synaptic remodeling. The studies we present here establish that the structural and functional synaptic plasticity that occur in striatal projection neurons in PD and in l-DOPA-induced dyskinesia are specifically due to modulation of the neurotransmitter dopamine and are not artifacts of the use of chemical toxins in PD models. In addition, our findings provide evidence that synaptic plasticity in the Pitx3^-/- mouse is similar to that seen in toxin models despite its lack of dopaminergic innervation of the striatum during development. Pitx3^-/- mice reproduced the alterations described in patients with advanced PD and in well accepted toxin-based models of PD and dyskinesia. These results further consolidate the fidelity of the Pitx3^-/- mouse as a PD model in which to study the morphological and physiological remodeling of striatal projection neurons by administration of l-DOPA and other drugs.

Conserved Upstream Regulatory Regions in Mammalian Tyrosine Hydroxylase

Tyrosine hydroxylase (Th) encodes the rate-limiting enzyme in catecholamine biosynthesis, and the regulation of its transcription is critical for the specification and maintenance of catecholaminergic neuron phenotypes. For many genes, regulatory genomic DNA sequences that are upstream of the proximal promoter control expression levels as well as region-specific expression patterns. The regulatory architecture of the genomic DNA upstream of the Th proximal promoter, however, is poorly understood. In this study, we examined the 11 kb upstream nucleotide sequence of Th from nine mammalian species and identified five highly conserved regions. Using cultured human cells and mouse olfactory bulb tissue, chromatin immunoprecipitation (ChIP) assays show that these conserved regions recruit transcription factors that are established regulators of Th transcription (such as NURR1, PITX3, FOXA2, MEIS2, and PAX6). This analysis also identified a conserved binding site for CTCF, and functional studies in cultured human cells and ChIP assays with mouse tissue show that CTCF is a novel regulator of Th transcription in the forebrain. Together, the findings in this study provide key insights into the upstream regulatory genomic architecture and regulatory mechanisms controlling mammalian Th gene transcription.

Parkinson's disease: experimental models and reality

Parkinson's disease (PD) is a chronic, progressive movement disorder of adults and the second most common neurodegenerative disease after Alzheimer's disease. Neuropathologic diagnosis of PD requires moderate-to-marked neuronal loss in the ventrolateral substantia nigra pars compacta and α-synuclein (αS) Lewy body pathology. Nigrostriatal dopaminergic neurodegeneration correlates with the Parkinsonian motor features, but involvement of other peripheral and central nervous system regions leads to a wide range of non-motor features. Nigrostriatal dopaminergic neurodegeneration is shared with other parkinsonian disorders, including some genetic forms of parkinsonism, but many of these disorders do not have Lewy bodies. An ideal animal model for PD, therefore, should exhibit age-dependent and progressive dopaminergic neurodegeneration, motor dysfunction, and abnormal αS pathology. Rodent models of PD using genetic or toxin based strategies have been widely used in the past several decades to investigate the pathogenesis and therapeutics of PD, but few recapitulate all the major clinical and pathologic features of PD. It is likely that new strategies or better understanding of fundamental disease processes may facilitate development of better animal models. In this review, we highlight progress in generating rodent models of PD based on impairments of four major cellular functions: mitochondrial oxidative phosphorylation, autophagy-lysosomal metabolism, ubiquitin-proteasome protein degradation, and endoplasmic reticulum stress/unfolded protein response. We attempt to evaluate how impairment of these major cellular systems contribute to PD and how they can be exploited in rodent models. In addition, we review recent cell biological studies suggesting a link between αS aggregation and impairment of nuclear membrane integrity, as observed during cellular models of apoptosis. We also briefly discuss the role of incompetent phagocytic clearance and how this may be a factor to consider in developing new rodent models of PD.

Striatal But Not Extrastriatal Dopamine Receptors Are Critical to Dopaminergic Motor Stimulation

Dopamine (DA) is required for motor function in vertebrate animals including humans. The striatum, a key motor control center, receives a dense DA innervation and express high levels of DA D1 receptors (D1Rs) and D2 receptors (D2Rs). Other brain areas involved in motor function such as the globus pallidus external segment (GPe) and the substantia nigra pars reticulata (SNr) and the motor cortex (MC) also receive DA innervation and express DA receptors. Thus, the relative contribution of the striatal and extrastriatal DA systems to the motor function has been an important question critical for understanding the functional operation of the motor control circuits and also for therapeutic targeting. We have now experimentally addressed this question in the transcription factor Pitx3 null mutant (Pitx3Null) mice that have an autogenic and parkinsonian-like striatal DA denervation and hence supersensitive motor response to DA stimulation. Using DA agonist unilateral microinjection-induced rotation as a reliable readout of motor stimulation, our results show that L-dopa microinjection into the dorsal striatum (DS) induced 5–10 times more rotations than that induced by L-dopa microinjection into GPe and SNr, while L-dopa microinjection into the primary MC induced the least number of rotations. Furthermore, our results show that separate microinjection of the D1R-like agonist SKF81297 and the D2R-like agonist ropinirole into the DS each induced only modest numbers of rotation, whereas concurrent injection of the two agonists triggered more rotations than the sum of the rotations induced by each of these two agonists separately, indicating D1R–D2R synergy. These results suggest that the striatum, not GPe, SNr or MC, is the primary site for D1Rs and D2Rs to synergistically stimulate motor function in L-dopa treatment of Parkinson’s disease (PD). Our results also predict that non-selective, broad spectrum DA agonists activating both D1Rs and D2Rs are more efficacious anti-PD drugs than the current D2R agonists.

Impact of Pitx3 gene knockdown on glial cell line-derived neurotrophic factor transcriptional activity in dopaminergic neurons

Pitx3 is strongly associated with the phenotype, differentiation, and survival of dopaminergic neurons. The relationship between Pitx3 and glial cell line-derived neurotrophic factor (GDNF) in dopaminergic neurons remains poorly understood. The present investigation sought to construct and screen a lentivirus expression plasmid carrying a rat Pitx3 short hairpin (sh)RNA and to assess the impact of Pitx3 gene knockdown on GDNF transcriptional activity in MES23.5 dopaminergic neurons. Three pairs of interference sequences were designed and separately ligated into GV102 expression vectors. These recombinant plasmids were transfected into MES23.5 cells and western blot assays were performed to detect Pitx3 protein expression. Finally, the most effective Pitx3 shRNA and a dual-luciferase reporter gene plasmid carrying the GDNF promoter region (GDNF-luciferase) were cotransfected into MES23.5 cells. Sequencing showed that the synthesized sequences were identical to the three Pitx3 interference sequences. Inverted fluorescence microscopy revealed that the lentivirus expression plasmids carrying Pitx3-shRNA had 40–50% transfection efficiency. Western blot assay confirmed that the corresponding Pitx3 of the third knockdown sequence had the lowest expression level. Dual-luciferase reporter gene results showed that the GDNF transcriptional activity in dopaminergic cells cotransfected with both plasmids was decreased compared with those transfected with GDNF-luciferase alone. Together, the results showed that the designed Pitx3-shRNA interference sequence decreased Pitx3 protein expression, which decreased GDNF transcriptional activity.

Early Postnatal but Not Late Adult Neurogenesis Is Impaired in the Pitx3-Mutant Animal Model of Parkinson's Disease

The generation of new neurons in the adult dentate gyrus has functional implications for hippocampal formation. Reduced hippocampal neurogenesis has been described in various animal models of hippocampal dysfunction such as dementia and depression, which are both common non-motor-symptoms of Parkinson's disease (PD). As dopamine plays an important role in regulating precursor cell proliferation, the loss of dopaminergic neurons in the substantia nigra (SN) in PD may be related to the reduced neurogenesis observed in the neurogenic regions of the adult brain: subventricular zone (SVZ) and dentate gyrus (DG). Here we examined adult hippocampal neurogenesis in the Pitx3-mutant mouse model of PD (aphakia mice), which phenotypically shows a selective embryonic degeneration of dopamine neurons within the SN and to a smaller extent in the ventral tegmental area (VTA). Proliferating cells were labeled with BrdU in aphakia mice and healthy controls from 3 to 42 weeks of age. Three weeks old mutant mice showed an 18% reduction of proliferating cells in the DG and of 26% in the SVZ. Not only proliferation but also the number of new neurons was impaired in young aphakia mice resulting in 33% less newborn cells 4 weeks after BrdU-labeling. Remarkably, however, the decline in the number of proliferating cells in the neurogenic regions vanished in older animals (8–42 weeks) indicating that aging masks the effect of dopamine depletion on adult neurogenesis. Region specific reduction in precursor cells proliferation correlated with the extent of dopaminergic degeneration in mesencephalic subregions (VTA and SN), which supports the theory of age- and region-dependent regulatory effects of dopaminergic projections. Physiological stimulation of adult neurogenesis by physical activity (wheel running) almost doubled the number of proliferating cells in the dentate gyrus of 8 weeks old aphakia mice to a number comparable to that of wild-type mice, abolishing the slight reduction of baseline neurogenesis at this age. The described age-dependent susceptibility of adult neurogenesis to PD-like dopaminergic degeneration and its responsiveness to physical activity might have implications for the understanding of the pathophysiology and treatment of non-motor symptoms in PD.

Pitx3 and En1 determine the size and molecular programming of the dopaminergic neuronal pool

Mesodiencephalic dopaminergic (mdDA) neurons are located in the ventral midbrain. These neurons form the substantia nigra (SNc) and the ventral tegmental area (VTA). Two transcription factors that play important roles in the process of terminal differentiation and subset-specification of mdDA neurons, are paired-like homeodomain transcription factor 3 (Pitx3), and homeobox transcription factor Engrailed 1 (En1). We previously investigated the single Pitx3KO and En1KO and observed important changes in the survival of mdDA neurons of the SNc and VTA as well as altered expression of pivotal rostral- and caudal-markers, Ahd2 and Cck, respectively. To refine our understanding of the regional-specific relationships between En1 and Pitx3 and their (combined) role in the programming mdDA neurons on the rostral-to-caudal axis, we created double En1tm1Alj/tm1Alj;Pitx3gfp/gfp (En1KO;Pitx3GFP/GFP) animals. Here we report, that in absence of En1 and Pitx3, only a limited number of mdDA neurons are present at E14.5. These mdDA neurons have a rudimentary dopaminergic cell fate, as they express Nurr1, Pbx3 and Otx2 but have lost their rostral or caudal subset identity. Furthermore, we report that the expression of Cck depends on En1 expression, while (in contrast) both Pitx3 and En1 are involved in the initiation of Ahd2 expression. Thus we reveal in this manuscript that regulated levels of Pitx3 and En1 control the size and rostral/caudal-identity of the mdDA neuronal population.

Xenopus pitx3 target genes lhx1 and xnr5 are identified using a novel three-fluor flow cytometry-based analysis of promoter activation and repression

BACKGROUND

Pitx3 plays a well understood role in directing development of lens, muscle fiber, and dopaminergic neurons; however, in Xenopus laevis, it may also play a role in early gastrulation and somitogenesis. Potential downstream targets of pitx3 possess multiple binding motifs that would not be readily accessible by conventional promoter analysis.

RESULTS

We isolated and characterized pitx3 target genes lhx1 and xnr5 using a novel three-fluor flow cytometry tool that was designed to dissect promoters with multiple binding sites for the same transcription factor. This approach was calibrated using a known pitx3 target gene, Tyrosine hydroxylase.

CONCLUSIONS

We demonstrate how flow cytometry can be used to detect gene regulatory changes with exquisite precision on a cell-by-cell basis, and establish that in HEK293 cells, pitx3 directly activates lhx1 and represses xnr5. Developmental Dynamics 246:657-669, 2017. © 2017 Wiley Periodicals, Inc.

MK-801 (Dizocilpine) Regulates Multiple Steps of Adult Hippocampal Neurogenesis and Alters Psychological Symptoms via Wnt/β-Catenin Signaling in Parkinsonian Rats

Adult hippocampal neurogenesis is directly involved in regulation of stress, anxiety, and depression that are commonly observed nonmotor symptoms in Parkinson's disease (PD). These symptoms do not respond to pharmacological dopamine replacement therapy. Excitotoxic damage to neuronal cells by N-methyl-d-aspartate (NMDA) receptor activation is also a major contributing factor in PD development, but whether it regulates hippocampal neurogenesis and nonmotor symptoms in PD is yet unexplored. Herein, for the first time, we studied the effect of MK-801, an NMDA receptor antagonist, on adult hippocampal neurogenesis and behavioral functions in 6-OHDA (6-hydroxydopamine) induced rat model of PD. MK-801 treatment (0.2 mg/kg, ip) increased neural stem cell (NSC) proliferation, self-renewal capacity, long-term survival, and neuronal differentiation in the hippocampus of rat model of PD. MK-801 potentially enhanced long-term survival, improved dendritic arborization of immature neurons, and reduced 6-OHDA induced neurodegeneration via maintaining the NSC pool in hippocampus, leading to decreased anxiety and depression-like phenotypes in the PD model. MK-801 inhibited glycogen synthase kinase-3β (GSK-3β) through up-regulation of Wnt-3a, which resulted in the activation of Wnt/β-catenin signaling leading to enhanced hippocampal neurogenesis in PD model. Additionally, MK-801 treatment protected the dopaminergic (DAergic) neurons in the nigrostriatal pathway and improved motor functions by increasing the expression of Nurr-1 and Pitx-3 in the PD model. Therefore, MK-801 treatment serves as a valuable tool to enhance hippocampal neurogenesis in PD, but further studies are needed to revisit the role of MK-801 in the neurodegenerative disorder before proposing a potential therapeutic candidate.

Neuronal survival in the brain: neuron type-specific mechanisms

Neurogenic regions of mammalian brain produce many more neurons that will eventually survive and reach a mature stage. Developmental cell death affects both embryonically produced immature neurons and those immature neurons that are generated in regions of adult neurogenesis. Removal of substantial numbers of neurons that are not yet completely integrated into the local circuits helps to ensure that maturation and homeostatic function of neuronal networks in the brain proceed correctly. External signals from brain microenvironment together with intrinsic signaling pathways determine whether a particular neuron will die. To accommodate this signaling, immature neurons in the brain express a number of transmembrane factors as well as intracellular signaling molecules that will regulate the cell survival/death decision, and many of these factors cease being expressed upon neuronal maturation. Furthermore, pro-survival factors and intracellular responses depend on the type of neuron and region of the brain. Thus, in addition to some common neuronal pro-survival signaling, different types of neurons possess a variety of 'neuron type-specific' pro-survival constituents that might help them to adapt for survival in a certain brain region. This review focuses on how immature neurons survive during normal and impaired brain development, both in the embryonic/neonatal brain and in brain regions associated with adult neurogenesis, and emphasizes neuron type-specific mechanisms that help to survive for various types of immature neurons. Importantly, we mainly focus on in vivo data to describe neuronal survival specifically in the brain, without extrapolating data obtained in the PNS or spinal cord, and thus emphasize the influence of the complex brain environment on neuronal survival during development.

Dopaminergic treatment weakens medium spiny neuron collateral inhibition in the parkinsonian striatum

The striatal medium spiny neurons (MSNs) are critical to both motor and cognitive functions. A potential regulator of MSN activity is the GABAergic collateral axonal input from neighboring MSNs. These collateral axon terminals are further under the regulation of presynaptic dopamine (DA) receptors that may become dysfunctional when the intense striatal DA innervation is lost in Parkinson's disease (PD). We show that DA D1 receptor-expressing MSNs (D1-MSNs) and D2 receptor-expressing MSNs (D2-MSNs) each formed high-rate, one-way collateral connections with a homotypic preference in both normal and DA-denervated mouse striatum. Furthermore, whereas the homotypic preference, one-way directionality and the basal inhibitory strength were preserved, DA inhibited GABA release at the D2-MSN→D2-MSN collateral synapse in a supersensitive manner in the DA-denervated striatum. In contrast, for D1-MSN-originated collateral connections, whereas D1 agonism facilitated D1-MSN→D1-MSN collateral inhibition in the normal striatum, this presynaptic D1R facilitation of GABA release was lost in the parkinsonian striatum. These results indicate that in the parkinsonian striatum, dopaminergic treatment can presynaptically weaken the D2-MSN→D2-MSN collateral inhibition and disinhibit the surrounding D2-MSNs, whereas the D1-MSN→D1-MSN collateral inhibition is weakened by the loss of the presynaptic D1 receptor facilitation, disinhibiting the surrounding D1-MSNs. Together, these newly discovered effects can disrupt the MSN circuits in the parkinsonian striatum and may contribute to dopaminergic treatment-induced aberrant motor and nonmotor behaviors in PD.NEW & NOTEWORTHY With the use of a large database, this study establishes that neighboring homotypic striatal spiny projection neurons have a 50% chance to form one-way collateral inhibitory connection, a substantially higher rate than previous estimates. This study also shows that dopamine denervation may alter presynaptic dopamine receptor function such that dopaminergic treatment of Parkinson's disease can weaken the surround inhibition and may reduce the contrast of the striatal outputs, potentially contributing to dopamine's profound motor and nonmotor behavioral effects.

Purification of functional human ES and iPSC-derived midbrain dopaminergic progenitors using LRTM1

Human induced pluripotent stem cells (iPSCs) can provide a promising source of midbrain dopaminergic (mDA) neurons for cell replacement therapy for Parkinson's disease (PD). However, iPSC-derived donor cells inevitably contain tumorigenic or inappropriate cells. To eliminate these unwanted cells, cell sorting using antibodies for specific markers such as CORIN or ALCAM has been developed, but neither marker is specific for ventral midbrain. Here we employ a double selection strategy for cells expressing both CORIN and LMX1A::GFP, and report a cell surface marker to enrich mDA progenitors, LRTM1. When transplanted into 6-OHDA-lesioned rats, human iPSC-derived LRTM1⁺ cells survive and differentiate into mDA neurons in vivo, resulting in a significant improvement in motor behaviour without tumour formation. In addition, there was marked survival of mDA neurons following transplantation of LRTM1⁺ cells into the brain of an MPTP-treated monkey. Thus, LRTM1 may provide a tool for efficient and safe cell therapy for PD patients.

Specificity of Pitx3-Dependent Gene Regulatory Networks in Subsets of Midbrain Dopamine Neurons

Dysfunction of midbrain dopaminergic (mDA) neurons is involved in Parkinson's disease (PD) and neuropsychiatric disorders. Pitx3 is expressed in mDA neuron subsets of the substantia nigra compacta (SNc) and of the ventral tegmental area (VTA) that are degeneration-sensitive in PD. The genetic network(s) and mode(s) of action of Pitx3 in these mDA neurons remain poorly characterized. We hypothesized that, given their distinct neuronal identities, Pitx3-expressing neurons of SNc and VTA should differ in their Pitx3-controlled gene expression networks and this may involve subset-specific co-regulators. Expression profiling of purified mDA neuronal subsets indicates that Pitx3 regulates different sets of genes in SNc and VTA, such as activating the expression of primary cilium gene products specifically in VTA neurons. Interaction network analysis pointed to the participation of differentially expressed Lhx/Lmo family members in the modulation of Pitx3 action in SNc and VTA mDA neurons. Conversely, global binding patterns of Pitx3 on genomic DNA of human dopaminergic cells revealed that Pitx3 is often co-recruited to regions that foster the formation of GATA-bHLH-BRN complexes, which usually involve Lmo co-regulatory proteins. We focused on Lmo3 for its preferential expression in SNc neurons and demonstrated that it functions as a transcriptional co-activator of Pitx3 by enhancing its activity on genomic regulatory elements. In summary, we defined the SN and VTA-specific programs of Pitx3-dependent gene expression and identified Lmo3 as a SN-specific co-regulator of Pitx3-dependent transcription.

Parkinson-like phenotype in insulin-resistant PED/PEA-15 transgenic mice

Neurological abnormalities, such as Parkinson-like disorders (PlD), are often co-morbidities of Type 2 Diabetic (T2D) patients, although the epidemiological link between these two disorders remains controversial. The PED/PEA-15 protein represents a possible candidate linking T2D and PD, because it is increased in subjects with T2D and is highly expressed in the brain. To test this hypothesis, we have analyzed the neurological and neurochemical phenotype of transgenic mice overexpressing PED/PEA-15 (tgPED). These mice develop impaired glucose tolerance and insulin resistance, accompanied by neurological features resembling PlD: feet clasping, slow and delayed locomotor movements in different behavioral tests in absence of clear cognitive deficits, ataxia or anxiety. Morphological analysis of the brains showed selective modifications of metabolic activity in the striatal region. In the same region, we have observed 26% decrease of dopamine fibers, confirmed by immunohistochemistry and Western Blot for tyrosine hydroxylase. Moreover, they also showed 48% reduction of dopamine levels in the striatum. Thus the tgPED mice may represent a genetic animal model of neurological disease linked to T2D.