Pitx3 deficiency produces decreased dopamine signaling and induces motor deficits in Pitx3(-/-) mice

Dopaminergic Input Regulates the Sensitivity of Indirect Pathway Striatal Spiny Neurons to Brain-Derived Neurotrophic Factor

Motor dysfunction in Parkinson's disease (PD) is closely linked to the dopaminergic depletion of striatal neurons and altered synaptic plasticity at corticostriatal synapses. Dopamine receptor D1 (DRD1) stimulation is a crucial step in the formation of long-term potentiation (LTP), whereas dopamine receptor D2 (DRD2) stimulation is needed for the formation of long-term depression (LTD) in striatal spiny projection neurons (SPNs). Tropomyosin receptor kinase B (TrkB) and its ligand brain-derived neurotrophic factor (BDNF) are centrally involved in plasticity regulation at the corticostriatal synapses. DRD1 activation enhances TrkB's sensitivity for BDNF in direct pathway spiny projection neurons (dSPNs). In this study, we showed that the activation of DRD2 in cultured striatal indirect pathway spiny projection neurons (iSPNs) and cholinergic interneurons causes the retraction of TrkB from the plasma membrane. This provides an explanation for the opposing synaptic plasticity changes observed upon DRD1 or DRD2 stimulation. In addition, TrkB was found within intracellular structures in dSPNs and iSPNs from Pitx3-/- mice, a genetic model of PD with early onset dopaminergic depletion in the dorsolateral striatum (DLS). This dysregulated BDNF/TrkB signaling might contribute to the pathophysiology of direct and indirect pathway striatal projection neurons in PD.

The Crucial Roles of Pitx3 in Midbrain Dopaminergic Neuron Development and Parkinson's Disease-Associated Neurodegeneration

The degeneration of midbrain dopaminergic (mDA) neurons, particularly in the substantia nigra pars compacta (SNc), is one of the most prominent pathological hallmarks of Parkinson's disease (PD). To uncover the pathogenic mechanisms of mDA neuronal death during PD may provide therapeutic targets to prevent mDA neuronal loss and slow down the disease's progression. Paired-like homeodomain transcription factor 3 (Pitx3) is selectively expressed in the mDA neurons as early as embryonic day 11.5 and plays a critical role in mDA neuron terminal differentiation and subset specification. Moreover, Pitx3-deficient mice exhibit some canonical PD-related features, including the profound loss of SNc mDA neurons, a dramatic decrease in striatal dopamine (DA) levels, and motor abnormalities. However, the precise role of Pitx3 in progressive PD and how this gene contributes to mDA neuronal specification during early stages remains unclear. In this review, we updated the latest findings on Pitx3 by summarizing the crosstalk between Pitx3 and its associated transcription factors in mDA neuron development. We further explored the potential benefits of Pitx3 as a therapeutic target for PD in the future. To better understand the transcriptional network of Pitx3 in mDA neuron development may provide insights into Pitx3-related clinical drug-targeting research and therapeutic approaches.

Developmental pathways linked to the vulnerability of adult midbrain dopaminergic neurons to neurodegeneration

The degeneration of dopaminergic and other neurons in the aging brain is considered a process starting well beyond the infantile and juvenile period. In contrast to other dopamine-associated neuropsychiatric disorders, such as schizophrenia and drug addiction, typically diagnosed during adolescence or young adulthood and, thus, thought to be rooted in the developing brain, Parkinson's Disease (PD) is rarely viewed as such. However, evidences have accumulated suggesting that several factors might contribute to an increased vulnerability to death of the dopaminergic neurons at an already very early (developmental) phase in life. Despite the remarkable ability of the brain to compensate such dopamine deficits, the early loss or dysfunction of these neurons might predispose an individual to suffer from PD because the critical threshold of dopamine function will be reached much earlier in life, even if the time-course and strength of naturally occurring and age-dependent dopaminergic cell death is not markedly altered in this individual. Several signaling and transcriptional pathways required for the proper embryonic development of the midbrain dopaminergic neurons, which are the most affected in PD, either continue to be active in the adult mammalian midbrain or are reactivated at the transition to adulthood and under neurotoxic conditions. The persistent activity of these pathways often has neuroprotective functions in adult midbrain dopaminergic neurons, whereas the reactivation of silenced pathways under pathological conditions can promote the survival and even regeneration of these neurons in the lesioned or aging brain. This article summarizes our current knowledge about signaling and transcription factors involved in midbrain dopaminergic neuron development, whose reduced gene dosage or signaling activity are implicated in a lower survival rate of these neurons in the postnatal or aging brain. It also discusses the evidences supporting the neuroprotection of the midbrain dopaminergic system after the external supply or ectopic expression of some of these secreted and nuclear factors in the adult and aging brain. Altogether, the timely monitoring and/or correction of these signaling and transcriptional pathways might be a promising approach to a much earlier diagnosis and/or prevention of PD.

Calcium-binding proteins typify the dopaminergic neuronal subtypes in the ventral tegmental area of zebra finch, Taeniopygia guttata

Calcium-binding proteins (CBPs) regulate neuronal function in midbrain dopamine (DA)-ergic neurons in mammals by buffering and sensing the intracellular Ca²⁺ , and vesicular release. In birds, the equivalent set of neurons are important in song learning, directed singing, courtship, and energy balance, yet the status of CBPs in these neurons is unknown. Herein, for the first time, we probe the nature of CBPs, namely, Calbindin-, Calretinin-, Parvalbumin-, and Secretagogin-expressing DA neurons in the ventral tegmental area (VTA) and substantia nigra (SN) in the midbrain of zebra finch, Taeniopygia guttata. qRT-PCR analysis of ventral midbrain tissue fragment revealed higher Calbindin- and Calretinin-mRNA levels compared to Parvalbumin and Secretagogin. Application of immunofluorescence showed CBP-immunoreactive (-i) neurons in VTA (anterior [VTAa], mid [VTAm], caudal [VTAc]), SN (compacta [SNc], and reticulata [SNr]). Compared to VTAa, higher Calbindin- and Parvalbumin-immunoreactivity (-ir), and lower Calretinin-ir were observed in VTAm and VTAc. Secretagogin-ir was highly localized to VTAa. In SN, Calbindin- and Calretinin-ir were higher in SNc, SNr was Parvalbumin enriched, and Secretagogin-ir was not detected. Weak, moderate, and intense tyrosine hydroxylase (TH)-i VTA neurons were demarcated as subtypes 1, 2, and 3, respectively. While subtype 1 TH-i neurons were neither Calbindin- nor Calretinin-i, ∼80 and ∼65% subtype 2 and ∼30 and ∼45% subtype 3 TH-i neurons co-expressed Calbindin and Calretinin, respectively. All TH-i neuronal subtypes co-expressed Parvalbumin with reciprocal relationship with TH-ir. We suggest that the CBPs may determine VTA DA neuronal heterogeneity and differentially regulate their activity in T. guttata.

The essential role of transcription factor Pitx3 in preventing mesodiencephalic dopaminergic neurodegeneration and maintaining neuronal subtype identities during aging

Pituitary homeobox 3 (Pitx3) is required for the terminal differentiation of nigrostriatal dopaminergic neurons during neuronal development. However, whether Pitx3 contributes to the normal physiological function and cell-type identity of adult neurons remains unknown. To explore the role of Pitx3 in maintaining mature neurons, we selectively deleted Pitx3 in the mesodiencephalic dopaminergic (mdDA) neurons of Pitx3fl/fl/DATCreERT2 bigenic mice using a tamoxifen inducible CreERT2/loxp gene-targeting system. Pitx3fl/fl/DATCreERT2 mice developed age-dependent progressive motor deficits, concomitant with a rapid reduction of striatal dopamine (DA) content and a profound loss of mdDA neurons in the substantia nigra pars compacta (SNc) but not in the adjacent ventral tegmental area (VTA), recapitulating the canonical neuropathological features of Parkinson's disease (PD). Mechanistic studies showed that Pitx3-deficiency significantly increased the number of cleaved caspase-3+ cells in SNc, which likely underwent neurodegeneration. Meanwhile, the vulnerability of SNc mdDA neurons was increased in Pitx3fl/fl/DATCreERT2 mice, as indicated by an early decline in glial cell line-derived neurotrophic factor (GDNF) and aldehyde dehydrogenase 1a1 (Aldh1a1) levels. Noticeably, somatic accumulation of α-synuclein (α-syn) was also significantly increased in the Pitx3-deficient neurons. Together, our data demonstrate that the loss of Pitx3 in fully differentiated mdDA neurons results in progressive neurodegeneration, indicating the importance of the Pitx3 gene in adult neuronal survival. Our findings also suggest that distinct Pitx3-dependent pathways exist in SNc and VTA mdDA neurons, correlating with the differential vulnerability of SNc and VTA mdDA neurons in the absence of Pitx3.

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.

miR-215 Targeting Novel Genes EREG, NIPAL1 and PTPRU Regulates the Resistance to E.coli F18 in Piglets

Previous research has revealed that miR-215 might be an important miRNA regulating weaned piglets’ resistance to Escherichia coli (E. coli) F18. In this study, target genes of miR-215 were identified by RNA-seq, bioinformatics analysis and dual luciferase detection. The relationship between target genes and E. coli infection was explored by RNAi technology, combined with E. coli stimulation and enzyme linked immunosorbent assay (ELISA) detection. Molecular regulating mechanisms of target genes expression were analyzed by methylation detection of promoter regions and dual luciferase activity assay of single nucleotide polymorphisms (SNPs) in core promoter regions. The results showed that miR-215 could target EREG, NIPAL1 and PTPRU genes. Expression levels of three genes in porcine intestinal epithelial cells (IPEC-J2) in the RNAi group were significantly lower than those in the negative control pGMLV vector (pGMLV-NC) group after E. coli F18 stimulation, while cytokines levels of TNF-α and IL-1β in the RNAi group were significantly higher than in the pGMLV-NC group. Variant sites in the promoter region of three genes could affect their promoter activities. These results suggested that miR-215 could regulate weaned piglets’ resistance to E. coli F18 by targeting EREG, NIPAL1 and PTPRU genes. This study is the first to annotate new biological functions of EREG, NIPAL1 and PTPRU genes in pigs, and provides a new experimental basis and reference for the research of piglets disease-resistance breeding.

Nurr1, Pitx3, and α7 nAChRs mRNA Expression in Nigral Tissue of Rats with Pedunculopontine Neurotoxic Lesion

Background and Objectives: The knowledge that the cholinergic neurons from pedunculopontine nucleus (PPN) are vulnerable to the degeneration in early stages of the Parkinson disease progression has opened new perspectives to the development of experimental model focused in pontine lesions that could increase the risk of nigral degeneration. In this context it is known that PPN lesioned rats exhibit early changes in the gene expression of proteins responsible for dopaminergic homeostasis. At the same time, it is known that nicotinic cholinergic receptors (nAChRs) mediate the excitatory influence of pontine-nigral projection. However, the effect of PPN injury on the expression of transcription factors that modulate dopaminergic neurotransmission in the adult brain as well as the α7 nAChRs gene expression has not been studied. The main objective of the present work was the study of the effects of the unilateral neurotoxic lesion of PPN in nuclear receptor-related factor 1 (Nurr1), paired-like homeodomain transcription factor 3 (Pitx3), and α7 nAChRs mRNA expression in nigral tissue. Materials and Methods: The molecular biology studies were performed by means of RT-PCR. The following experimental groups were organized: Non-treated rats, N-methyl-D-aspartate (NMDA)-lesioned rats, and Sham operated rats. Experimental subjects were sacrificed 24 h, 48 h and seven days after PPN lesion. Results: Nurr1 mRNA expression, showed a significant increase both 24 h (p < 0.001) and 48 h (p < 0.01) after PPN injury. Pitx3 mRNA expression evidenced a significant increase 24 h (p < 0.001) followed by a significant decrease 48 h and seven days after PPN lesion (p < 0.01). Finally, the α7 nAChRs nigral mRNA expression remained significantly diminished 24 h, 48 h (p < 0.001), and 7 days (p < 0.01) after PPN neurotoxic injury. Conclusion: Taking together these modifications could represent early warning signals and could be the preamble to nigral neurodegeneration events.

Influence of intranasal exposure of MPTP in multiple doses on liver functions and transition from non-motor to motor symptoms in a rat PD model

Besides the effects on the striatum, the impairment of visceral organs including liver functions has been reported in Parkinson's disease (PD) patients. However, it is yet unclear if liver functions are affected in the early stage of the disease before the motor phase has appeared. The aim of our present study was thus to assess the effect of intranasal administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in different doses on striatum and liver functions. Deterioration of non-motor activities appeared on single exposure to MPTP along with rise in striatum oxidative stress and decline in antioxidant levels. Decreases in dopamine, noradrenaline, and GABA and increase in serotonin were detected in striatum. Motor coordination was impaired with a single dose of MPTP, and with repeated MPTP exposure, there was further significant impairment. Locomotor activity was affected from second exposure of MPTP, and the impairment increased with third MPTP exposure. Impairment of liver function through increase in serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels was observed after first MPTP insult, and it worsened with second and third administrations. First administration of MPTP triggered systemic inflammation showing significant increase in inflammatory markers in the liver. Our data shows for the first time that an intranasal route of entry of MPTP affects liver from the non-motor phase of PD itself, occurring concomitantly with the reduction of striatal dopamine. It also suggests that a single dose is not enough to bring about progression of the disease from non-motor to locomotor deficiency, and a repeated dose is needed to establish the motor severity phase in the rat intranasal MPTP model.

Nox2 contributes to age-related oxidative damage to neurons and the cerebral vasculature

Oxidative stress plays an important role in aging-related neurodegeneration. This study used littermates of WT and Nox2-knockout (Nox2KO) mice plus endothelial cell–specific human Nox2 overexpression–transgenic (HuNox2Tg) mice to investigate Nox2-derived ROS in brain aging. Compared with young WT mice (3–4 months), aging WT mice (20–22 months) had obvious metabolic disorders and loss of locomotor activity. Aging WT brains had high levels of angiotensin II (Ang II) and ROS production; activation of ERK1/2, p53, and γH2AX; and losses of capillaries and neurons. However, these abnormalities were markedly reduced in aging Nox2KO brains. HuNox2Tg brains at middle age (11–12 months) already had high levels of ROS production and activation of stress signaling pathways similar to those found in aging WT brains. The mechanism of Ang II–induced endothelial Nox2 activation in capillary damage was examined using primary brain microvascular endothelial cells. The clinical significance of Nox2-derived ROS in aging-related loss of cerebral capillaries and neurons was investigated using postmortem midbrain tissues of young (25–38 years) and elderly (61–85 years) adults. In conclusion, Nox2 activation is an important mechanism in aging-related cerebral capillary rarefaction and reduced brain function, with the possibility of a key role for endothelial cells.

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.

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.

Transcription Factors: Potential Cell Death Markers in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disease with a long preclinical phase. The continuous loss of dopaminergic (DA) neurons is one of the pathogenic hallmarks of PD. Diagnosis largely depends on clinical observation, but motor dysfunctions do not emerge until 70%-80% of the nigrostriatal nerve terminals have been destroyed. Therefore, a biomarker that indicates the degeneration of DA neurons is urgently needed. Transcription factors are sequence-specific DNA-binding proteins that regulate RNA synthesis from a DNA template. The precise control of gene expression plays a critical role in the development, maintenance, and survival of cells, including DA neurons. Deficiency of certain transcription factors has been associated with DA neuron loss and PD. In this review, we focus on some transcription factors and discuss their structure, function, mechanisms of neuroprotection, and their potential for use as biomarkers indicating the degeneration of DA neurons.

Genetic analysis of PITX3 variants in patients with essential tremor

BACKGROUND

A clinical overlap between essential tremor (ET) and Parkinson's disease (PD) has prompted investigation whether these disorders share common genetic factors. The paired-like homeodomain transcription factor 3 gene (PITX3) has been shown to play an important role for the differentiation and survival of midbrain dopaminergic neurons in the substantia nigra pars compacta. The preferential degeneration of those dopaminergic neurons is the pathological hallmark in PD.

AIMS OF THE STUDY

The purpose of this study was to evaluate whether PITX3 variants are related to susceptibility of ET in Chinese Han population.

METHODS

Genetic analysis of two variants rs3758549 and rs4919621 of the PITX3 gene was conducted in 200 Chinese Han patients with ET and 426 controls.

RESULTS

We did not identify any statistically significant difference in either genotypic or allelic frequencies of variants between the ET patients and control cohort (all P > 0.05). Haplotype analysis of two variants in the PITX3 gene showed no potential association between the haplotypes and risk of ET (all P > 0.05).

CONCLUSIONS

Our data suggest that PITX3 variants rs3758549 and rs4919621 are not associated with ET in Chinese Han population.