Lmx1a encodes a rostral set of mesodiencephalic dopaminergic neurons marked by the Wnt/B-catenin signaling activator R-spondin 2

The biological functions and related signaling pathways of SPON2

Spondin-2 (SPON2), also referred to as M-spondin or DIL-1, is a member of the extracellular matrix protein family known as Mindin-F-spondin (FS). SPON2 can be used as a broad-spectrum tumor marker for more than a dozen tumors, mainly prostate cancer. Meanwhile, SPON2 is also a potential biomarker for the diagnosis of certain non-tumor diseases. Additionally, SPON2 plays a pivotal role in regulating tumor metastasis and progression. In normal tissues, SPON2 has a variety of biological functions represented by promoting growth and development and cell proliferation. This paper presents a comprehensive overview of the regulatory mechanisms, diagnostic potential as a broad-spectrum biomarker, diverse biological functions, involvement in various signaling pathways, and clinical applications of SPON2.

The Influence of Prenatal Exposure to Methamphetamine on the Development of Dopaminergic Neurons in the Ventral Midbrain

Methamphetamine, a highly addictive central nervous system (CNS) stimulant, is used worldwide as an anorexiant and attention enhancer. Methamphetamine use during pregnancy, even at therapeutic doses, may harm fetal development. Here, we examined whether exposure to methamphetamine affects the morphogenesis and diversity of ventral midbrain dopaminergic neurons (VMDNs). The effects of methamphetamine on morphogenesis, viability, the release of mediator chemicals (such as ATP), and the expression of genes involved in neurogenesis were evaluated using VMDNs isolated from the embryos of timed-mated mice on embryonic day 12.5. We demonstrated that methamphetamine (10 µM; equivalent to its therapeutic dose) did not affect the viability and morphogenesis of VMDNs, but it reduced the ATP release negligibly. It significantly downregulated Lmx1a, En1, Pitx3, Th, Chl1, Dat, and Drd1 but did not affect Nurr1 or Bdnf expression. Our results illustrate that methamphetamine could impair VMDN differentiation by altering the expression of important neurogenesis-related genes. Overall, this study suggests that methamphetamine use may impair VMDNs in the fetus if taken during pregnancy. Therefore, it is essential to exercise strict caution for its use in expectant mothers.

Development, wiring and function of dopamine neuron subtypes

The midbrain dopamine (mDA) system is composed of molecularly and functionally distinct neuron subtypes that mediate specific behaviours and are linked to various brain diseases. Considerable progress has been made in identifying mDA neuron subtypes, and recent work has begun to unveil how these neuronal subtypes develop and organize into functional brain structures. This progress is important for further understanding the disparate physiological functions of mDA neurons and their selective vulnerability in disease, and will ultimately accelerate therapy development. This Review discusses recent advances in our understanding of molecularly defined mDA neuron subtypes and their circuits, ranging from early developmental events, such as neuron migration and axon guidance, to their wiring and function, and future implications for therapeutic strategies.

Genes critical for development and differentiation of dopaminergic neurons are downregulated in Parkinson's disease

We performed transcriptome analysis using RNA sequencing on substantia nigra pars compacta (SNpc) from mice after acute and chronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment and from Parkinson's disease (PD) patients. Acute and chronic exposure to MPTP resulted in decreased expression of genes involved in sodium channel regulation. However, upregulation of pro-inflammatory pathways was seen after single dose but not after chronic MPTP treatment. Dopamine biosynthesis and synaptic vesicle recycling pathways were downregulated in PD patients and after chronic MPTP treatment in mice. Genes essential for midbrain development and determination of dopaminergic phenotype such as, LMX1B, FOXA1, RSPO2, KLHL1, EBF3, PITX3, RGS4, ALDH1A1, RET, FOXA2, EN1, DLK1, GFRA1, LMX1A, NR4A2, GAP43, SNCA, PBX1, and GRB10 were downregulated in human PD and overexpression of GFP tagged LMX1B rescued MPP⁺ induced death in SH-SY5Y neurons. Downregulation of gene ensemble involved in development and differentiation of dopaminergic neurons indicate their potential involvement in pathogenesis and progression of human PD.

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.

Transcriptional Profile of the Developing Subthalamic Nucleus

The subthalamic nucleus (STN) is a small, excitatory nucleus that regulates the output of basal ganglia motor circuits. The functions of the STN and its role in the pathophysiology of Parkinson's disease are now well established. However, some basic characteristics like the developmental origin and molecular phenotype of neuronal subpopulations are still being debated. The classical model of forebrain development attributed the origin of STN within the diencephalon. Recent studies of gene expression patterns exposed shortcomings of the classical model. To accommodate these findings, the prosomeric model was developed. In this concept, STN develops within the hypothalamic primordium, which is no longer a part of the diencephalic primordium. This concept is further supported by the expression patterns of many transcription factors. It is interesting to note that many transcription factors involved in the development of the STN are also involved in the pathogenesis of neurodevelopmental disorders. Thus, the study of neurodevelopmental disorders could provide us with valuable information on the roles of these transcription factors in the development and maintenance of STN phenotype. In this review, we summarize historical theories about the developmental origin of the STN and interpret the gene expression data within the prosomeric conceptual framework. Finally, we discuss the importance of neurodevelopmental disorders for the development of the STN and its potential role in the pathophysiology of neurodevelopmental disorders.

The Influence of Prenatal Exposure to Quetiapine Fumarate on the Development of Dopaminergic Neurons in the Ventral Midbrain of Mouse Embryos

The effects of second-generation antipsychotics on prenatal neurodevelopment, apoptotic neurodegeneration, and postnatal developmental delays have been poorly investigated. Even at standard doses, the use of quetiapine fumarate (QEPF) in pregnant women might be detrimental to fetal development. We used primary mouse embryonic neurons to evaluate the disruption of morphogenesis and differentiation of ventral midbrain (VM) neurons after exposure to QEPF. The dopaminergic VM neurons were deliberately targeted due to their roles in cognition, motor activity, and behavior. The results revealed that exposure to QEPF during early brain development decreased the effects of the dopaminergic lineage-related genes Tyrosine hydroxylase(Th), Dopamine receptor D1 (Drd1), Dopamine transporter (Dat), LIM homeobox transcription factor 1 alfa (Lmx1a), and Cell adhesion molecule L1 (Chl1), and the senescent dopaminergic gene Pituitary homeobox 3 (Pitx3). In contrast, Brain derived neurotrophic factor (Bdnf) and Nuclear receptor-related 1 (Nurr1) expressions were significantly upregulated. Interestingly, QEPF had variable effects on the development of non-dopaminergic neurons in VM. An optimal dose of QEPF (10 µM) was found to insignificantly affect the viability of neurons isolated from the VM. It also instigated a non-significant reduction in adenosine triphosphate formation in these neuronal populations. Exposure to QEPF during the early stages of brain development could also hinder the formation of VM and their structural phenotypes. These findings could aid therapeutic decision-making when prescribing 2nd generation antipsychotics in pregnant populations.

Prenatal Exposure to Gabapentin Alters the Development of Ventral Midbrain Dopaminergic Neurons

Background: Gabapentin is widely prescribed as an off-label drug for the treatment of various diseases, including drug and alcohol addiction. Approximately 83–95% of the usage of gabapentin is off-label, accounting for more than 90% of its sales in the market, which indicates an alarming situation of drug abuse. Such misuse of gabapentin has serious negative consequences. The safety of the use of gabapentin in pregnant women has always been a serious issue, as gabapentin can cross placental barriers. The impact of gabapentin on brain development in the fetus is not sufficiently investigated, which poses difficulties in clinical decisions regarding prescriptions.

Methods: The consequences effect of prenatal gabapentin exposure on the development of ventral midbrain dopaminergic neurons were investigated using three-dimensional neuronal cell cultures. Time-mated Swiss mice were used to isolate embryos. The ventral third of the midbrain was removed and used to enrich the dopaminergic population in 3D cell cultures that were subsequently exposed to gabapentin. The effects of gabapentin on the viability, ATP release, morphogenesis and genes expression of ventral midbrain dopaminergic neurons were investigated.

Results: Gabapentin treatment at the therapeutic level interfered with the neurogenesis and morphogenesis of vmDA neurons in the fetal brain by causing changes in morphology and alterations in the expression of key developmental genes, such as Nurr1, Chl1, En1, Bdnf, Drd2, and Pitx3. The TH + total neurite length and dominant neurite length were significantly altered. We also found that gabapentin could halt the metabolic state of these neuronal cells by blocking the generation of ATP.

Conclusion: Our findings clearly indicate that gabapentin hampers the morphogenesis and development of dopaminergic neurons. This implies that the use of gabapentin could lead to serious complications in child-bearing women. Therefore, caution must be exercised in clinical decisions regarding the prescription of gabapentin in pregnant women.

In vitro atlas of dorsal spinal interneurons reveals Wnt signaling as a critical regulator of progenitor expansion

Restoring sensation after injury or disease requires a reproducible method for generating large quantities of bona fide somatosensory interneurons. Toward this goal, we assess the mechanisms by which dorsal spinal interneurons (dIs; dI1-dI6) can be derived from mouse embryonic stem cells (mESCs). Using two developmentally relevant growth factors, retinoic acid (RA) and bone morphogenetic protein (BMP) 4, we recapitulate the complete in vivo program of dI differentiation through a neuromesodermal intermediate. Transcriptional profiling reveals that mESC-derived dIs strikingly resemble endogenous dIs, with the correct molecular and functional signatures. We further demonstrate that RA specifies dI4-dI6 fates through a default multipotential state, while the addition of BMP4 induces dI1-dI3 fates and activates Wnt signaling to enhance progenitor proliferation. Constitutively activating Wnt signaling permits the dramatic expansion of neural progenitor cultures. These cultures retain the capacity to differentiate into diverse populations of dIs, thereby providing a method of increasing neuronal yield.

The Effects of Prenatal Exposure to Pregabalin on the Development of Ventral Midbrain Dopaminergic Neurons

Pregabalin is widely used as a treatment for multiple neurological disorders; however, it has been reported to have the potential for misuse. Due to a lack of safety studies in pregnancy, pregabalin is considered the last treatment option for various neurological diseases, such as neuropathic pain. Therefore, pregabalin abuse in pregnant women, even at therapeutic doses, may impair fetal development. We used primary mouse embryonic neurons to investigate whether exposure to pregabalin can impair the morphogenesis and differentiation of ventral midbrain neurons. This study focused on ventral midbrain dopaminergic neurons, as they are responsible for cognition, movement, and behavior. The results showed that pregabalin exposure during early brain development induced upregulation of the dopaminergic progenitor genes Lmx1a and Nurr1 and the mature dopaminergic gene Pitx3. Interestingly, pregabalin had different effects on the morphogenesis of non-dopaminergic ventral midbrain neurons. Importantly, our findings illustrated that a therapeutic dose of pregabalin (10 μM) did not affect the viability of neurons. However, it caused a decrease in ATP release in ventral midbrain neurons. We demonstrated that exposure to pregabalin during early brain development could interfere with the neurogenesis and morphogenesis of ventral midbrain dopaminergic neurons. These findings are crucial for clinical consideration of the use of pregabalin during pregnancy.

Single-cell transcriptomics of human iPSC differentiation dynamics reveal a core molecular network of Parkinson's disease

Parkinson's disease (PD) is the second-most prevalent neurodegenerative disorder, characterized by the loss of dopaminergic neurons (mDA) in the midbrain. The underlying mechanisms are only partly understood and there is no treatment to reverse PD progression. Here, we investigated the disease mechanism using mDA neurons differentiated from human induced pluripotent stem cells (hiPSCs) carrying the ILE368ASN mutation within the PINK1 gene, which is strongly associated with PD. Single-cell RNA sequencing (RNAseq) and gene expression analysis of a PINK1-ILE368ASN and a control cell line identified genes differentially expressed during mDA neuron differentiation. Network analysis revealed that these genes form a core network, members of which interact with all known 19 protein-coding Parkinson's disease-associated genes. This core network encompasses key PD-associated pathways, including ubiquitination, mitochondrial function, protein processing, RNA metabolism, and vesicular transport. Proteomics analysis showed a consistent alteration in proteins of dopamine metabolism, indicating a defect of dopaminergic metabolism in PINK1-ILE368ASN neurons. Our findings suggest the existence of a network onto which pathways associated with PD pathology converge, and offers an inclusive interpretation of the phenotypic heterogeneity of PD.

Tcf4 Is Involved in Subset Specification of Mesodiencephalic Dopaminergic Neurons

During development, mesodiencephalic dopaminergic (mdDA) neurons form into different molecular subsets. Knowledge of which factors contribute to the specification of these subsets is currently insufficient. In this study, we examined the role of Tcf4, a member of the E-box protein family, in mdDA neuronal development and subset specification. We show that Tcf4 is expressed throughout development, but is no longer detected in adult midbrain. Deletion of Tcf4 results in an initial increase in TH-expressing neurons at E11.5, but this normalizes at later embryonic stages. However, the caudal subset marker Nxph3 and rostral subset marker Ahd2 are affected at E14.5, indicating that Tcf4 is involved in correct differentiation of mdDA neuronal subsets. At P0, expression of these markers partially recovers, whereas expression of Th transcript and TH protein appears to be affected in lateral parts of the mdDA neuronal population. The initial increase in TH-expressing cells and delay in subset specification could be due to the increase in expression of the bHLH factor Ascl1, known for its role in mdDA neuronal differentiation, upon loss of Tcf4. Taken together, our data identified a minor role for Tcf4 in mdDA neuronal development and subset specification.

Neurodevelopmental defects and neurodegenerative phenotypes in human brain organoids carrying Parkinson's disease-linked DNAJC6 mutations

Loss-of-function mutations of DNAJC6, encoding HSP40 auxilin, have recently been identified in patients with early-onset Parkinson's disease (PD). To study the roles of DNAJC6 in PD pathogenesis, we used human embryonic stem cells with CRISPR-Cas9-mediated gene editing. Here, we show that DNAJC6 mutations cause key PD pathologic features, i.e., midbrain-type dopamine (mDA) neuron degeneration, pathologic α-synuclein aggregation, increase of intrinsic neuronal firing frequency, and mitochondrial and lysosomal dysfunctions in human midbrain-like organoids (hMLOs). In addition, neurodevelopmental defects were also manifested in hMLOs carrying the mutations. Transcriptomic analyses followed by experimental validation revealed that defects in DNAJC6-mediated endocytosis impair the WNT-LMX1A signal during the mDA neuron development. Furthermore, reduced LMX1A expression during development caused the generation of vulnerable mDA neurons with the pathologic manifestations. These results suggest that the human model of DNAJC6-PD recapitulates disease phenotypes and reveals mechanisms underlying disease pathology, providing a platform for assessing therapeutic interventions.

Development of specialized sensory neurons engages a nuclear receptor required for functional plasticity

In this study, Rossillo and Ringstad sought to determine mechanisms that support the physiology and plasticity of BAG neurons, which are specialized neurons that sense the respiratory gas carbon dioxide (CO₂) and, in a context-dependent manner, switch from mediating avoidance of CO₂ to supporting CO₂ attraction in C. elegans. They used tandem ChIP-seq and cell targeted RNA-seq to identify gene targets of the transcription factor ETS-5, which is required for BAG development, and their functional screen of ETS-5 targets revealed that NHR-6, the sole C. elegans NR4A-type nuclear receptor, is required for BAG-mediated avoidance of CO₂ and regulates expression of a subset of BAG-specific genes.

During development, the nervous system generates neurons that serve highly specialized roles and, accordingly, possess unique functional attributes. The chemosensory BAG neurons of C. elegans are striking exemplars of this. BAGs sense the respiratory gas carbon dioxide (CO₂) and, in a context-dependent manner, switch from mediating avoidance of CO₂ to supporting CO₂ attraction. To determine mechanisms that support the physiology and plasticity of BAG neurons, we used tandem ChIP-seq and cell targeted RNA-seq to identify gene targets of the transcription factor ETS-5, which is required for BAG development. A functional screen of ETS-5 targets revealed that NHR-6, the sole C. elegans NR4A-type nuclear receptor, is required for BAG-mediated avoidance of CO₂ and regulates expression of a subset of BAG-specific genes. Unlike ets-5 mutants, which are defective for both attraction to and avoidance of CO₂, nhr-6 mutants are fully competent for attraction. These data indicate that the remarkable ability of BAGs to adaptively assign positive or negative valence to a chemosensory stimulus requires a gene-regulatory program supported by an evolutionarily conserved type of nuclear receptor. We suggest that NHR-6 might be an example of a developmental mechanism for modular encoding of functional plasticity in the nervous system.

Dose-Dependent and Subset-Specific Regulation of Midbrain Dopaminergic Neuron Differentiation by LEF1-Mediated WNT1/b-Catenin Signaling

The mesodiencephalic dopaminergic (mdDA) neurons, including the nigrostriatal subset that preferentially degenerates in Parkinson’s Disease (PD), strongly depend on an accurately balanced Wingless-type MMTV integration site family member 1 (WNT1)/beta-catenin signaling pathway during their development. Loss of this pathway abolishes the generation of these neurons, whereas excessive WNT1/b-catenin signaling prevents their correct differentiation. The identity of the cells responding to this pathway in the developing mammalian ventral midbrain (VM) as well as the precise progression of WNT/b-catenin action in these cells are still unknown. We show that strong WNT/b-catenin signaling inhibits the differentiation of WNT/b-catenin-responding mdDA progenitors into PITX3⁺ and TH⁺ mdDA neurons by repressing the Pitx3 gene in mice. This effect is mediated by RSPO2, a WNT/b-catenin agonist, and lymphoid enhancer binding factor 1 (LEF1), an essential nuclear effector of the WNT/b-catenin pathway, via conserved LEF1/T-cell factor binding sites in the Pitx3 promoter. LEF1 expression is restricted to a caudolateral mdDA progenitor subset that preferentially responds to WNT/b-catenin signaling and gives rise to a fraction of all mdDA neurons. Our data indicate that an attenuation of WNT/b-catenin signaling in mdDA progenitors is essential for their correct differentiation into specific mdDA neuron subsets. This is an important consideration for stem cell-based regenerative therapies and in vitro models of neuropsychiatric diseases.

Acquisition of the Midbrain Dopaminergic Neuronal Identity

The mesodiencephalic dopaminergic (mdDA) group of neurons comprises molecularly distinct subgroups, of which the substantia nigra (SN) and ventral tegmental area (VTA) are the best known, due to the selective degeneration of the SN during Parkinson’s disease. However, although significant research has been conducted on the molecular build-up of these subsets, much is still unknown about how these subsets develop and which factors are involved in this process. In this review, we aim to describe the life of an mdDA neuron, from specification in the floor plate to differentiation into the different subsets. All mdDA neurons are born in the mesodiencephalic floor plate under the influence of both SHH-signaling, important for floor plate patterning, and WNT-signaling, involved in establishing the progenitor pool and the start of the specification of mdDA neurons. Furthermore, transcription factors, like Ngn2, Ascl1, Lmx1a, and En1, and epigenetic factors, like Ezh2, are important in the correct specification of dopamine (DA) progenitors. Later during development, mdDA neurons are further subdivided into different molecular subsets by, amongst others, Otx2, involved in the specification of subsets in the VTA, and En1, Pitx3, Lmx1a, and WNT-signaling, involved in the specification of subsets in the SN. Interestingly, factors involved in early specification in the floor plate can serve a dual function and can also be involved in subset specification. Besides the mdDA group of neurons, other systems in the embryo contain different subsets, like the immune system. Interestingly, many factors involved in the development of mdDA neurons are similarly involved in immune system development and vice versa. This indicates that similar mechanisms are used in the development of these systems, and that knowledge about the development of the immune system may hold clues for the factors involved in the development of mdDA neurons, which may be used in culture protocols for cell replacement therapies.

EZH2 Is Essential for Fate Determination in the Mammalian Isthmic Area

The polycomb group proteins (PcGs) are a group of epigenetic factors associated with gene silencing. They are found in several families of multiprotein complexes, including polycomb repressive complex 2 (PRC2). EZH2, EED and SUZ12 form the core components of the PRC2 complex, which is responsible for the mono, di- and trimethylation of lysine 27 of histone 3 (H3K27Me3), the chromatin mark associated with gene silencing. Loss-of-function studies of Ezh2, the catalytic subunit of PRC2, have shown that PRC2 plays a role in regulating developmental transitions of neuronal progenitor cells (NPCs); from self-renewal to differentiation and the neurogenic-to-gliogenic fate switch. To further address the function of EZH2 and H3K27me3 during neuronal development, we generated a conditional mutant in which Ezh2 was removed in the mammalian isthmic (mid-hindbrain) region from E10.5 onward. Loss of Ezh2 changed the molecular coding of the anterior ventral hindbrain leading to a fate switch and the appearance of ectopic dopaminergic (DA) neurons. The correct specification of the isthmic region is dependent on the signaling factors produced by the Isthmic organizer (IsO), located at the border of the mid- and hindbrain. We propose that the change of cellular fate is a result of the presence of Otx2 in the hindbrain of Ezh2 conditional knock-outs (cKOs) and a dysfunctional IsO, as represented by the loss of Fgf8 and Wnt1. Our work implies that next to controlling developmental transitions, EZH2 mediated gene silencing is important for specification of the isthmic region by influencing IsO functioning and repressing Otx2 in the hindbrain.

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.

The Matricellular Protein R-Spondin 2 Promotes Midbrain Dopaminergic Neurogenesis and Differentiation

The development of midbrain dopaminergic (mDA) neurons is controlled by multiple morphogens and transcription factors. However, little is known about the role of extracellular matrix proteins in this process. Here we examined the function of roof plate-specific spondins (RSPO1-4) and the floor plate-specific, spondin 1 (SPON1). Only RSPO2 and SPON1 were expressed at high levels during mDA neurogenesis, and the receptor LGR5 was expressed by midbrain floor plate progenitors. Surprisingly, RSPO2, but not SPON1, specifically promoted the differentiation of mDA neuroblasts into mDA neurons in mouse primary cultures and embryonic stem cells (ESCs). In addition, RSPO2 was found to promote not only mDA differentiation, but also mDA neurogenesis in human ESCs. Our results thus uncover an unexpected function of the matricellular protein RSPO2 and suggest an application to improve mDA neurogenesis and differentiation in human stem cell preparations destined to cell replacement therapy or drug discovery for Parkinson disease.

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Rspo2 is dynamically expressed during midbrain dopaminergic neuron development

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RSPO2 promotes the dopaminergic differentiation of mouse neurons in culture

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RSPO2 increases dopaminergic neurogenesis and differentiation of human ESCs

Maternal Vitamin D Prevents Abnormal Dopaminergic Development and Function in a Mouse Model of Prenatal Immune Activation

Dysfunction in dopamine (DA) systems is a prominent feature in schizophrenia patients and may result from the abnormal development of mesencephalic (mes)DA systems. Maternal immune activation (MIA) and developmental vitamin D (DVD)-deficiency both induce schizophrenia-relevant dopaminergic abnormalities in adult offspring. In this study, we investigated whether maternal administration of the vitamin D hormone (1,25OHD, VITD) could prevent MIA-induced abnormalities in DA-related behaviors and mesDA development. We administrated the viral mimetic polyriboinosinic-polyribocytidylic (poly (I:C)) simultaneously with 1,25OHD and/or their vehicles, to pregnant mouse dams at gestational day 9. Maternal treatment with VITD prevented MIA-induced hypersensitivity to acute DA stimulation induced by amphetamine, whereas it failed to block prepulse inhibition deficiency in MIA-exposed offspring. MIA and VITD both reduced fetal mesDA progenitor (Lmx1a + Sox2+) cells, while VITD treatment increased the number of mature (Nurr1 + TH+) mesDA neurons. Single-cell quantification of protein expression showed that VITD treatment increased the expression of Lmx1a, Nurr1 and TH in individual mesDA cells and restored normal mesDA positioning. Our data demonstrate that VITD prevents abnormal dopaminergic phenotypes in MIA offspring possibly via its early neuroprotective actions on fetal mesDA neurons. Maternal supplementation with the dietary form of vitamin D, cholecalciferol may become a valuable strategy for the prevention of MIA-induced neurodevelopmental abnormalities.

Neural Stem Cell Grafts Promote Astroglia-Driven Neurorestoration in the Aged Parkinsonian Brain via Wnt/β-Catenin Signaling

neuronal phenotype. Wnt/β-catenin signaling antagonism abolished mDA neurorestoration and immune modulatory effects of NSC grafts. Our work implicates an unprecedented therapeutic potential for somatic NSC grafts in the restoration of mDA neuronal function in the aged Parkinsonian brain. Stem Cells 2018;36:1179-1197.

Tcf12 Is Involved in Early Cell-Fate Determination and Subset Specification of Midbrain Dopamine Neurons

The basic helix-loop-helix (bHLH) protein family has previously been shown to be involved in the development of mesodiencephalic dopaminergic (mdDA) neurons in the murine midbrain. Specifically, Ngn2 and Mash1 are known to have a role in the specification of neural progenitors in the ventricular zone (VZ) of the midbrain towards an mdDA neuronal cell-fate. Furthermore, other members of the bHLH protein family, the E-box factors, are expressed in the developing midbrain and are thought to have a role in neuronal differentiation. Here we show that the E-box factor Tcf12 is implicated in early and late development of mdDA neurons. Tcf12 is expressed in the midbrain and in young TH-expressing mdDA neurons throughout development. Tcf12^lox/lox;En1^cre/+ embryos, that lose Tcf12 at ~embryonic day (E)9 throughout the En1 expression domain, have a changed spatial expression of Lmx1a and Nurr1 and a consistent loss of rostral TH expression. Expression of the subset marker Ahd2 is initially delayed, but recovers during development, eventually showing an ~10% increase in AHD2-expressing cells at postnatal day (P)30. Tcf12^lox/lox;Pitx3^cre/+ embryos, that lose Tcf12 at ~E12 in post-mitotic mdDA neurons, show no effect on the amount of TH-expressing neurons in the developing midbrain. However, similar as to Tcf12^lox/lox;En1^cre/+ embryos, subset specification is delayed during development. Taken together, we have identified Tcf12 as a novel factor in mdDA neuronal development. It serves a dual function; one in early cell-fate commitment of neural progenitors and one late in subset specification.

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.

Molecular Programming of Mesodiencephalic Dopaminergic Neuronal Subsets

Dopamine neurons of the substantia nigra compacta (SNc) and ventral tegmental area (VTA) are critical components of the neuronal machinery to control emotion and movement in mammals. The slow and gradual death of these neurons as seen in Parkinson's disease has triggered a large investment in research toward unraveling the molecular determinants that are used to generate these neurons and to get an insight in their apparent selective vulnerability. Here, I set out to summarize the current view on the molecular distinctions that exist within this mesodiencephalic dopamine (mdDA) system and elaborate on the molecular programming that is responsible for creating such diversity.

Embryonic development of selectively vulnerable neurons in Parkinson's disease

A specific set of brainstem nuclei are susceptible to degeneration in Parkinson's disease. We hypothesise that neuronal vulnerability reflects shared phenotypic characteristics that confer selective vulnerability to degeneration. Neuronal phenotypic specification is mainly the cumulative result of a transcriptional regulatory program that is active during the development. By manual curation of the developmental biology literature, we comprehensively reconstructed an anatomically resolved cellular developmental lineage for the adult neurons in five brainstem regions that are selectively vulnerable to degeneration in prodromal or early Parkinson's disease. We synthesised the literature on transcription factors that are required to be active, or required to be inactive, in the development of each of these five brainstem regions, and at least two differentially vulnerable nuclei within each region. Certain transcription factors, e.g., Ascl1 and Lmx1b, seem to be required for specification of many brainstem regions that are susceptible to degeneration in early Parkinson's disease. Some transcription factors can even distinguish between differentially vulnerable nuclei within the same brain region, e.g., Pitx3 is required for specification of the substantia nigra pars compacta, but not the ventral tegmental area. We do not suggest that Parkinson's disease is a developmental disorder. In contrast, we consider identification of shared developmental trajectories as part of a broader effort to identify the molecular mechanisms that underlie the phenotypic features that are shared by selectively vulnerable neurons. Systematic in vivo assessment of fate determining transcription factors should be completed for all neuronal populations vulnerable to degeneration in early Parkinson's disease.

Mest/Peg1 Is Essential for the Development and Maintenance of a SNc Neuronal Subset

Mesodiencephalic dopaminergic (mdDA) neurons originate at the floor plate and floor plate-basal plate boundary of the midbrain ventricular zone. During development mdDA neurons are specified by a unique set of transcription factors and signaling cascades, to form the different molecular subsets of the mdDA neuronal population. In a time series micro-array study performed previously, mesoderm specific transcript (Mest) was found to be one of the most upregulated genes during early mdDA neuronal development. Here, we show that Mest transcript is expressed in the midbrain throughout development and becomes restricted to the substantia nigra (SNc) at late stages. In Mest KO animals mdDA neurons are progressively lost in the adult, mostly affecting the SNc, reflected by a 50% decrease of TH protein and DA release in the striatum and a reduction of climbing behavior. Analysis of Lrp6 KO embryos suggest a subtle opposite phenotype to the Mest KO, hinting toward the possibility that specific loss of mdDA neurons in Mest ablated animals could be due to affected WNT-signaling. Interestingly, the mdDA neuronal region affected by the loss of Mest remains relatively unaffected in Pitx3 mutants, suggesting that both genes are essential for the development and/or maintenance of different mdDA neuronal subsets within the SNc. Overall, the neuroanatomical and phenotypical consequences detected upon the loss of Mest, resemble the loss of SNc neurons and loss of movement control as seen in Parkinson’s Disease (PD), suggesting that the Mest mouse model may be used as a model-system for PD.

Expression and functional analysis of the Wnt/beta-catenin induced mir-135a-2 locus in embryonic forebrain development

BACKGROUND

Brain size and patterning are dependent on dosage-sensitive morphogen signaling pathways - yet how these pathways are calibrated remains enigmatic. Recent studies point to a new role for microRNAs in tempering the spatio-temporal range of morphogen functions during development. Here, we investigated the role of miR-135a, derived from the mir-135a-2 locus, in embryonic forebrain development.

METHOD

1. We characterized the expression of miR-135a, and its host gene Rmst, by in situ hybridization (ish). 2. We conditionally ablated, or activated, beta-catenin in the dorsal forebrain to determine if this pathway was necessary and/or sufficient for Rmst/miR-135a expression. 3. We performed bioinformatics analysis to unveil the most predicted pathways targeted by miR-135a. 4. We performed gain and loss of function experiments on mir-135a-2 and analyzed by ish the expression of key markers of cortical hem, choroid plexus, neocortex and hippocampus.

RESULTS

1. miR-135a, embedded in the host long non-coding transcript Rmst, is robustly expressed, and functional, in the medial wall of the embryonic dorsal forebrain, a Wnt and TGFβ/BMP-rich domain. 2. Canonical Wnt/beta-catenin signaling is critical for the expression of Rmst and miR-135a, and the cortical hem determinant Lmx1a. 3. Bioinformatics analyses reveal that the Wnt and TGFβ/BMP cascades are among the top predicted pathways targeted by miR-135a. 4. Analysis of mir-135a-2 null embryos showed that dorsal forebrain development appeared normal. In contrast, modest mir-135a-2 overexpression, in the early dorsal forebrain, resulted in a phenotype resembling that of mutants with Wnt and TGFβ/BMP deficits - a smaller cortical hem and hippocampus primordium associated with a shorter neocortex as well as a less convoluted choroid plexus. Interestingly, late overexpression of mir-135a-2 revealed no change.

CONCLUSIONS

All together, our data suggests the existence of a Wnt/miR-135a auto-regulatory loop, which could serve to limit the extent, the duration and/or intensity of the Wnt and, possibly, the TGFβ/BMP pathways.

Differences in the spatiotemporal expression and epistatic gene regulation of the mesodiencephalic dopaminergic precursor marker PITX3 during chicken and mouse development

Mesodiencephalic dopaminergic (mdDA) neurons are located in the ventral mesencephalon and caudal diencephalon of all tetrapod species studied so far. They are the most prominent DA neuronal population and are implicated in control and modulation of motor, cognitive and rewarding/affective behaviors. Their degeneration or dysfunction is intimately linked to several neurological and neuropsychiatric human diseases. To gain further insights into their generation, we studied spatiotemporal expression patterns and epistatic interactions in chick embryos of selected marker genes and signaling pathways associated with mdDA neuron development in mouse. We detected striking differences in the expression patterns of the chick orthologs of the mouse mdDA marker genes Pitx3 and Aldh1a1, which suggests important differences between the species in the generation/generating of these cells. We also discovered that the Sonic hedgehog signaling pathway is both, necessary and sufficient for the induction of ectopic PITX3 expression in chick mesencephalon downstream of WNT9A induced LMX1a transcription. These aspects of early chicken development resemble the ontogeny of zebrafish diencephalic DA neuronal populations, and suggest a divergence between birds and mammals during evolution.

Cell fate determination, neuronal maintenance and disease state: The emerging role of transcription factors Lmx1a and Lmx1b

LIM-homeodomain (LIM-HD) proteins are evolutionary conserved developmental transcription factors. LIM-HD Lmx1a and Lmx1b orchestrate complex temporal and spatial gene expression of the dopaminergic pathway, and evidence shows they are also involved in adult neuronal homeostasis. In this review, the multiple roles played by Lmx1a and Lmx1b will be discussed. Controlled Lmx1a and Lmx1b expression and activities ensure the proper formation of critical signaling centers, including the embryonic ventral mesencephalon floor plate and sharp boundaries between lineage-specific cells. Lmx1a and Lmx1b expression persists in mature dopaminergic neurons of the substantia nigra pars compacta and the ventral tegmental area, and their role in the adult brain is beginning to be revealed. Notably, LMX1B expression was lower in brain tissue affected by Parkinson's disease. Actual and future applications of Lmx1a and Lmx1b transcription factors in stem cell production as well as in direct conversion of fibroblast into dopaminergic neurons are also discussed. A thorough understanding of the role of LMX1A and LMX1B in a number of disease states, including developmental diseases, cancer and neurodegenerative diseases, could lead to significant benefits for human healthcare.

Establishing diversity in the dopaminergic system

Midbrain dopaminergic neurons (MbDNs) modulate cognitive processes, regulate voluntary movement, and encode reward prediction errors and aversive stimuli. While the degeneration of MbDNs underlies the motor defects in Parkinson's disease, imbalances in dopamine levels are associated with neuropsychiatric disorders such as depression, schizophrenia and substance abuse. In recent years, progress has been made in understanding how MbDNs, which constitute a relatively small neuronal population in the brain, can contribute to such diverse functions and dysfunctions. In particular, important insights have been gained regarding the distinct molecular, neurochemical and network properties of MbDNs. How this diversity of MbDNs is established during brain development is only starting to be unraveled. In this review, we summarize the current knowledge on the diversity in MbDN progenitors and differentiated MbDNs in the developing rodent brain. We discuss the signaling pathways, transcription factors and transmembrane receptors that contribute to setting up these diverse MbDN subpopulations. A better insight into the processes that establish diversity in MbDNs will ultimately improve the understanding of the architecture and function of the dopaminergic system in the adult brain.

Molecular mechanisms of dopaminergic subset specification: fundamental aspects and clinical perspectives

Dopaminergic (DA) neurons in the ventral mesodiencephalon control locomotion and emotion and are affected in psychiatric and neurodegenerative diseases, such as Parkinson's disease (PD). A clinical hallmark of PD is the specific degeneration of DA neurons located within the substantia nigra (SNc), whereas neurons in the ventral tegmental area remain unaffected. Recent advances have highlighted that the selective vulnerability of the SNc may originate in subset-specific molecular programming during DA neuron development, and significantly increased our understanding of the molecular code that drives specific SNc development. We here present an up-to-date overview of molecular mechanisms that direct DA subset specification, integrating our current knowledge about subset-specific roles of transcription factors, signaling pathways and morphogenes. We discuss strategies to further unravel subset-specific gene-regulatory networks, and the clinical promise of fundamental knowledge about subset specification of DA neurons, with regards to cell replacement therapy and cell-type-specific vulnerability in PD.

Mesodiencephalic dopaminergic neuronal differentiation does not involve GLI2A-mediated SHH-signaling and is under the direct influence of canonical WNT signaling

Sonic Hedgehog (SHH) and WNT proteins are key regulators in many developmental processes, like embryonic patterning and brain development. In the brain, SHH is expressed in a gradient starting in the floor plate (FP) progressing ventrally in the midbrain, where it is thought to be involved in the development and specification of mesodiencephalic dopaminergic (mdDA) neurons. GLI2A-mediated SHH-signaling induces the expression of Gli1, which is inhibited when cells start expressing SHH themselves. To determine whether mdDA neurons receive GLI2A-mediated SHH-signaling during differentiation, we used a BAC-transgenic mouse model expressing eGFP under the control of the Gli1 promoter. This mouse-model allowed for mapping of GLI2A-mediated SHH-signaling temporal and spatial in the mouse midbrain. Since mdDA neurons are born from E10.5, peaking at E11.0–E12.0, we examined Gli1-eGFP embryos at E11.5, E12.5, and E13.5, indicating whether Gli1 was induced before or during mdDA development and differentiation. Our data indicate that GLI2A-mediated SHH-signaling is not involved in mdDA neuronal differentiation. However, it appears to be involved in the differentiation of neurons which make up a subset of the red nucleus (RN). In order to detect whether mdDA neuronal differentiation may be under the control of canonical WNT-signaling, we used a transgenic mouse-line expressing LacZ under the influence of stable β-catenin. Here, we show that TH⁺ neurons of the midbrain receive canonical WNT-signaling during differentiation. Therefore, we suggest that early SHH-signaling is indirectly involved in mdDA development through early patterning of the midbrain area, whereas canonical WNT-signaling is directly involved in the differentiation of the mdDA neuronal population.